Highlights

Nanocrystal superlattices: revealing a shape-induced orientation phase within 3D nanocrystal solids

 

Designing nanocrystal (NC) materials aims at obtaining superlattices that mimic the atomic structure of crystalline solids. In such atomic systems, spatially anisotropic orbitals determine the crystalline lattice type. Similarly, in NC systems the building block anisotropy defines the order of the final solid: here, the NC shape governs the final superlattice structure. Yet, in contrast to atomic systems, NC shape-anisotropy induces not only positional, but also orientational order, ranging from full rotational disorder to a stable, fixed alignment of all NCs. This orientational relation is of special interest, as it determines to what extent atomically coherent connections between NCs are possible, thereby enabling complete wave function delocalization within the NC solid.

In addition to predicting the final NC orientation and position structure, the realization of NC materials demands a controllable fabrication process such that the designed order can be produced. Generally, such highly ordered NC superstructures are achieved through solvent evaporation induced self‐assembly on hard substrates. For applications where the 2D nature of this substrates process is limiting, nonsolvent into solvent diffusion, a technique commonly used to grow single crystals of dissolved molecules, is an attractive option. However, the precise influence of self-assembly parameters on the final superlattice outcome remains unknown. In this work, the researchers posed two closely related questions regarding the design of novel free-standing NC materials: (i) how can the NC self-assembly process be controlled to yield highly ordered free-standing supercrystals and (ii) what is the detailed positional and orientational order within the NC solid? A multidisciplinary team of collaborators, including the Austrian Small Angle X-ray Scattering (SAXS) beamline at Elettra, approached this challenge by a combined experimental and computational strategy. First, the precise geometry of single Bi NCs was modeled from experimental SAXS patterns by shape-reconstruction algorithms (see Fig. 1a). Second, in-situscattering experiments were able to capture the full self-assembly process, ranging from single NCs in solution to micrometer sized NC SLs (see Fig. 1b).

Figure 1.  Small angle X-Ray scattering (SAXS) measurements, capturing the transition of Bi NCs from the stable solution (a) to the final NC superlattice (b).

Third, state-of-the-art hard-particle modelling confirmed and extended the experimental results, revealing a unique, unprecedented orientation phase within the NC solid (see Fig. 2). Thus, the researchers provide evidence that orientational and positional order within NC solids can be achieved without the help of surface ligands: the parallel alignment of atomically defined nanocrystal facets offers a new possibility for transferring atomic properties of the individual NCs to the entire supercrystal structure. The findings offer a recipe to push nanocrystal solids toward their full electronic potential and inspire more detailed studies toward orientation phases within colloidal solids.
This publication was selected for the ‘inside front cover’ at the journal “Advanced Materials”.

Figure 2.  Results of the hard-particle Monte Carlo simulations, which, for one, confirm the experimental results regarding the NC position order and, for the other, reveal an unprecedented orientation phase, induced by the NC shape.

Figure 3.  INSIDE FRONT COVER "The self‐assembly of 3D colloidal supercrystals built from faceted 20 nm Bi nanocrystals is studied by in situ synchrotron X‐ray scattering combined with Monte Carlo simulations. The assembly is driven by nonsolvent into solvent diffusion. In article number 1802078, Rainer T. Lechner and co‐workers reveal a unique orientation phase: the nanocrystals' shape induces 6 distinct global orientations within the cubic superlattice enabling parallel facet‐to‐facet alignment.. Adv. Mater. 30/2018, article 180278 inside cover page.

Retrieve article

A Shape­ Induced Orientation Phase within 3D Nanocrystal Solids;
Max Burian, Carina Karner, Maksym Yarema, Wolfgang Heiss, Heinz Amenitsch, Christoph Dellago, Rainer T. Lechner;
Advanced Materials, Vol. 30, 1802078 (2018). doi: 10.1002/adma.201802078

contact e-mail: rainer.lechner@unileoben.ac.at

Precise positioning of gold nanoparticles with DNA origami nanostructures

DNA is becoming a key player in self-assembly approaches towards advances in nanotechnology. The DNA origami method allows the design and assembly of nano objects which can serve as scaffolds for the arrangement of guest molecules. To fully exploit the placement precision of DNA origami templates we investigated gold nanoparticle (AuNP) positioning on DNA origami structures with SAXS. By these measurements we quantified the effect of attachment site and DNA linker type on the distance of two AuNPs on DNA origami in native solution conditions.

Nadrian Seeman pioneered the use of DNA as nanoscale construction material that is based on Watson-Crick base pairing and that has proven to enable the assembly of DNA objects of tailored shape and functionality (e.g. N.C. Seeman, “DNA in a material world”, Nature, 2003). The DNA origami technique – developed by Paul Rothemund (Nature, 2006) - relies on folding a ~ 8 kilo bases long single-stranded DNA scaffold into a desired shape by short, synthetic “staple” oligonucleotides. The addressability of the resulting DNA-object allows also for positioning of guest particles such as fluorophores or metal nanoparticles. To verify for correct assembly and to quantify the positioning accuracy of such DNA constructs in solution, we used small angle x-ray scattering (SAXS).

We measured the SAXS pattern of dimers and a trimer of nanoparticles connected to different attachment sites A, B, C on a DNA origami block using different linker types and of a helical arrangement of AuNPs on a DNA origami cylinder (Fig. 1a). The pair density distribution functions (PDDF) were obtained from the scattering data. The PDDFs of the dimers AB, AC and BC show different positions of the second maximum corresponding to different center-to-center distances of the AuNPs. (Fig. 1b) The PDDF of the trimer ABC is composed of the distances of the dimers that constitute the trimer. The AuNP distances obtained from fitting of the SAXS data and from the PDDF are in good agreement with the values estimated from the design of the assemblies.
 

Figure 1.  (a) Trimer ABC with gold nanoparticles attachment sites A, B, and C of a DNA origami block. (b) PDDF obtained from the scattering of the dimers AB, AC, and BC and trimer ABC (blue solid line, dashed line, dash-dot line, and black solid line, respectively). The TEM images of all three dimers and the trimer are shown. Adapted with permission from C. Hartl et al., Nano Lett. 18 (4), 2609-2615 (2018); DOI: 10.1021/acs.nanolett.8b00412. Copyright 2018 American Chemical Society.

The attachment sites consist of three DNA single strands with a specific sequence, protruding from the origami. For the attachment the surface of the AuNPs is covered with single stranded DNA of the complementary sequence. We probed three different linker types: (i) 15 bases protrusion (blue), (ii) 9 bases protrusion (orange), and (iii) 15 bases protrusion combined with AuNPs modified with DNA in an orientation that is expected to form a zipper configuration (green) (Fig. 2a). SAXS measurements of AuNPs sitting at opposite sides of the block (Fig. 2b) show that configuration (i) gives the largest and configuration (ii) the smallest center-to-center distance of the AuNPs. Sterical hindrance of long DNA single strands on the AuNPs or spacer oligonucleotides in configuration (iii) can prevent the AuNPs to be zipped tightly to the surface. In the configuration of dimers of nanoparticles sitting next to each other on the DNA origami block (Fig. 2c) center-to-center distances differ only slightly from each other with again configuration (i) giving the largest and (ii) the smallest AuNP distance. Possible influences could be repulsion due to long single stranded DNA covering the nanoparticles and effects of the flexibility of the different linker types. Due to the competing forces SAXS is crucial in order to establish the precise binding length of nanoparticles to DNA constructs.

The measurements further reveal that the PDDF of a helical arrangement of AuNPs provides information about the distances of neighboring AuNPs. From the neighbor distances conclusions about the geometry of the helix such as the radius can be drawn.

Altogether, we find, that SAXS measurements can give valuable information about AuNP distances of DNA origami mediated gold nanoparticle assemblies in solution conditions. This information can be used to tailor the assemblies to obtain the desired properties in changing environmental conditions.

Figure 2.  (a) Scheme of the three tested connector types: (i) A15 to T19 (blue), (ii) A9 to T8 (orange), and (iii) A15 to 3′ T19, zipper configuration (green). (b, c) PDDF for each of the three different connector types for dimers AC (b) and AB (c) are shown together with corresponding TEM images. Adapted with permission from C. Hartl et al., Nano Lett. 18 (4), 2609-2615 (2018); DOI: 10.1021/acs.nanolett.8b00412. Copyright 2018 American Chemical Society.

Retrieve article

Position Accuracy of Gold Nanoparticles on DNA Origami Structures Studied with Small-Angle X-ray Scattering ;
C. Hartl, K. Frank, H. Amenitsch, S. Fischer, T. Liedl and B. Nickel;
Nano Letters 18 (4), pp 2609–2615 (2018). doi: 10.1021/acs.nanolett.8b00412

Contact persons: nickel@lmu.de, tim.liedl@lmu.de

Mechanical and electrical properties of self-assembly-based porous organosilicate films

 

Abstract: The mechanical properties of SiO₂-based porous materials are important for various applications. Thus, we report on the effect of the replacement of Si–O–Si by Si–CH₂–Si groups on their mechanical and electrical properties. At a mass density of 0.87 g/cm^-3, the film containing Si–CH₂–Si groups has a higher Young’s modulus compared to the one with Si–O–Si functionalities being 6.6 GPa and 5.3 GPa, respectively. Concurrently, the introduction of the Si–CH₂–Si groups leads to a dielectric constant increase from 2.12 to 2.27.

SiO₂-based porous materials are used as a coating on solar cells, intermetal insulators in microchips, bone grafts for bone tissue regeneration, sensors for environmentally responsive materials, photonic devices, etc. In many of these applications, the mechanical stability of the porous material limits the device performance. As a result, the high Young’s modulus (YM) values reported for periodic mesoporous organosilicas (PMOs), SiO₂-based materials containing organic Si-CH₂-Si groups, seem promising. However, previous comparisons of PMO with other silica-based materials failed to control for the average bond connectivity of the matrix or the hydrophilicity which also affect the YM. Therefore, to compare the mechanical properties reliably, we prepared hydrophobic films which have a similar pore volume and matrix connectivity, and differ only in the presence of the Si-CH₂-Si groups. To ensure the integrity of the organic groups, the films were prepared by sol-gel processing. In the latter, surfactant self-assembly creates a template around which the organosilica molecules polycondensate. The subsequent thermal decomposition of the template leaves behind an organosilicate matrix with a pore architecture shaped by the template.

The pore structure of films templated by the surfactants BrijL4 and CTAC was investigated by grazing-incidence small-angle x-ray scattering (GISAXS). A narrow pore size distribution is deduced for the BrijL4-templated films due to the presence of a Debye-Scherrer ring (Fig. 1a) which was modelled by a monodisperse pore size distribution of non-overlapping spheres. In contrast, the CTAC-templated films have a wider pore size distribution and their scattering pattern (Fig. 1b) was modelled by a local monodisperse approximation size distribution and a Gaussian distribution function. Both of the patterns were modelled by assuming randomly oriented spheroid pores. The inferred pore shape results from the film’s unidirectional shrinkage along the axis normal to the surface during annealing. The shrinkage is unidirectional since in-plane the film adheres to the silicon substrate. As a result, the pore diameter parallel to the substrate is larger than that normal to the surface and, for the BrijL4-templated films, are 2.3 and 1.7 nm, respectively. Furthermore, the pore volume of the BrijL4-templated film calculated from the GISAXS experiment and adsorption porosimetry are very close, 37.6% and 38.1%, respectively. Since adsorption porosimetry probes only pores accessible to the adsorbate while GISAXS reveals also the closed pores, the similar pore volume excludes the presence of closed pores. For the CTAC-templated films, the pore sizes parallel and perpendicular to the surface are 3.5 and 2.5 nm, respectively. In this case, the pore volume calculated from the GISAXS analysis is 5% larger than the 35% porosity estimated by adsorption porosimetry. Therefore, micropores inaccessible for the adsorbate are present in the CTAC-templated film. Finally, the differences in the pore structure depend on the template but are not affected by the matrix chemistry.

Figure 1.  GISAXS patterns provide information on the pore shape and size: a) The Debye-Scherrer ring indicates a narrow pore size distribution in BrijL4-templated films b) diffuse scattering indicative of a less well-defined scatterer size in CTAC-templated films.


The replacement of some of the Si-O-Si bonds by Si-CH₂-Si results in a film with a higher YM, a higher dielectric constant as well as a higher matrix density (Fig. 2). Notably, at a film density of 0.86 g cm^-3, when the matrix connectivity is controlled, replacing some Si-O-Si by Si-CH₂-Si groups raises the YM by about 1.7 GPa which is significantly smaller than previously reported. We conclude that previously the effect of the organic bridging groups has been overestimated due to differences in the average bond connectivity of the matrices of the films being compared.

Figure 2.  The introduction of Si-CH₂-Si bonds raises the Young's modulus and dielectric constant in SiO₂-based materials at an equivalent density. Red: SiO₂-based film with Si-CH₂-Si groups, Blue: SiO₂-based film without Si-CH₂-Si groups.

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On the mechanical and electrical properties of self-assembly-based organosilicate porous films ;
M. Redzheb, S. Armini, T. Berger, M. Jacobs, M. Krishtab, K. Vanstreels, S. Bernstorff and P. Van Der Voort;
J. Mater. Chem. C 5, 8599-8607 (2017). doi: 10.1002/10.1039/C7TC02276J

contact e-mail: silvia.armini@imec.be

Combining modeling and in situ x-ray scattering to quantify confinement and desolvation in nanoporous carbon supercapacitors

 

A fundamental understanding of the mechanisms controlling ion transport and arrangement in carbon nanopores is essential to improve the performance of supercapacitors or devices for capacitive desalination. In situ small angle x-ray scattering represents an excellent tool to study global ion fluxes and local ion rearrangements in nanopores of carbon electrodes during charging and discharging. Using a novel data analysis strategy, we find that charge is most effectively stored in sites of the carbon structure with highest possible geometrical confinement, accompanied with partial desolvation.

Figure 1.  Scheme of the in situ SAXS experiments and the data evaluation strategy. Reproduced and adapted with permission from Prehal, C. et al., Nat. Energy 2, 16215 (2017). © Nature Publishing Group..


Storage and release of electric energy on a wide range of timescales are crucial for a sustainable energy management when implementing green technologies. This applies in particular to electric cars, microelectronics, or new forms of energy conversion. Today electric buses, aircraft doors, or systems that recover breaking energy from vehicles already utilize an ultrafast energy storage technology called electrical double-layer capacitors or supercapacitors. These systems reveal higher power densities and much longer cycle lifetimes (>1 Million) than batteries. Although the design is like a conventional electrochemical cell, the charge storage mechanism in supercapacitors is purely physical: If the supercapacitor is charged, electrons (or holes) attract cations (or anions) at the electrode- electrolyte interface forming an electric double-layer and thereby provide the capacitive behavior.

In order to store as many ions as possible, the electrodes are highly porous with typical specific surface area of several thousand square meters per Gramm of the material. The pores in such nanoporous carbon electrodes are not much larger than the (hydrated) ions themselves. Within the cross-linked network of pores, the ions have to share space with water molecules and ions of opposite charge. In this confined space large amounts of energy can be stored; yet ion transport could be hindered due to mutual blocking of ions with opposite charge, comparable to ion traffic jams. High energy densities therefore come along with low power densities. This subtle trade-off between power and energy needs to be understood on an atomistic level in order to improve the overall performance of supercapacitors.

In the present work we report a novel experimental and data analysis tool to increase the fundamental understanding of such phenomena. Combining in situ x-ray scattering and atomistic modeling we visualized ion electrosorption on a sub-nanometer scale and to benefit the further development of optimized electrode materials. In situ scattering experiments were carried out at the Austrian SAXS beamline at Elettra using a custom-built in situ supercapacitor cell. Different nanoporous carbon materials were used as electrode material and concentrated aqueous CsCl solutions as electrolyte. Installing a potentiostat at the beamline, 2D small-angle x-ray scattering (SAXS) patterns of the electrolyte-filled working electrode were recorded during charging and discharging. There are considerable changes of the time-dependent SAXS data, however, the complexity of the system makes their interpretation difficult. Therefore a novel data analysis approach, as visualized in Fig. 1, was introduced. First, a 3D pore model was generated from a simple ex situ SAXS measurement of the carbon electrode in air using the concept of Gaussian Random Fields (Fig. 1a and b). The pore model was then populated with a specific number of cations and anions associated to each voltage step and obtained from the in situ experiment (Fig. 1c). Using a Monte Carlo simulation, the equilibrium configurations of ions were determined and a subsequent Fast Fourier Transformation provided simulated scattering intensities for each cell voltage. These simulated patterns could be compared with real in situ measurements (Fig. 1d and e).
 
Using this analytical tool the ion positions can be tracked within the real space pore structure as a function of the applied voltage. Interestingly, ions do not just change their concentration within the electrode upon charging, but they also change their preferred positions within the nanopores. Defining a parameter called “degree of confinement” (DoC), the local ion rearrangement was investigated quantitatively. As a voltage is applied counter-ions preferably move into sites with high degree of confinement. This rearrangement is accompanied with a partial loss of the hydration shell each ion is carrying. As a major conclusion, ion charge was found to be stored in pore systems enabling the largest change of the ions’ DoC. By this way, the repulsive interaction between ions of the same charge is most effectively screened and ions can be packed most densely.

In situ SAXS, therefore, allows a direct prediction of the capacitive performance of nanoporous carbon electrodes. The developed method and insights are of great relevance also for other, related technologies dealing with ion electrosorption, like for instance capacitive seawater desalination.

Retrieve article

Quantification of ion confinement and desolvation in nanoporous carbon supercapacitors with modelling and in situ X-ray scattering;
C. Prehal, C. Koczwara, N. Jäckel, A. Schreiber, M. Burian, H. Amenitsch, M. A. Hartmann, V. Presser, O. Paris;
Nat. Energy 2, 16215 (2017). doi: 10.1038/nenergy.2016.215

Self-Assembly of the Cephalopod Protein Reflectin

 

Figure 1. a) A schematic of a GISAXS experiment for a RfA1 film. The incident X-rays are scattered by substrate-confined aggregated macromolecules, yielding a 2D pattern (data for RfA1 is shown). b) A 1D plot of the GISAXS intensity I(q) versus the scattering vector qy obtained at different times during the self-assembly / formation of a film from RfA1 nanoparticles. The measurements were obtained at 3 (blue), 7 (green), 11 (purple), and 23 (brown) min after initiation of assembly. The circles represent the experimental data, and the lines represent the simulated scattering intensity profiles. c) A 1D plot of the GISAXS intensity I(q) versus the scattering vector qy obtained for a RfA1 film before and after hydration. The orange circles represent the experimental data obtained before hydration, and the solid orange line represents the simulated scattering intensity profile. The red circles represent the experimental data obtained after hydration, and the solid red line represents the simulated scattering intensity profile. Figure 2. The hierarchical organization of RfA1 in solution and in films. When dispersed in solution, RfA1 aggregates into interacting nanoparticles with an asymmetric prolate (elongated) geometry. When self-assembled into films, the RfA1 nanoparticles still interact and adopt an oblate (flattened) geometry, which changes slightly upon the application of a stimulus (an increase in relative humidity leading to hydration).

Figure 3.  "Inside Back Cover" of Advanced Materials (doi: 10.1002/adma.201670270): X-rays emerge from a synchrotron to hit a protein-covered substrate. From the film, proteins emerge, which are aggregated into football-like nanoparticles, as described by A. A. Gorodetsky and co-workers on page 8405. The protein and the nanoparticles are present in cephalopod skin; these cells emerge from a purple squid and underpin its camouflage abilities.   Cephalopods (squids, octopus, cuttlefish) have captivated scientists due to their stunning camouflage displays. Recently, these animals have emerged as exciting sources of inspiration for the design of functional materials. Within this context, our group has investigated cephalopod structural proteins known as reflectins. We have studied the self-assembly, structural characteristics, and stimulus response of films from reflectin A1 (RfA1) with grazing-incidence small-angle X-ray scattering (GISAXS). RfA1 self-assembles into prolate nanoparticles in solution, which change shape both during film formation and upon application of an exogenous stimulus. Based on our measurements, we have elucidated the nanostructure of RfA1 films and obtained a rationale for their functionality. Due to the unique capabilities and characteristics of cephalopods, they have emerged as an exciting source of inspiration for the development of novel materials and technologies. Accordingly, our group has investigated cephalopod structural proteins known as reflectins, which possess an amino acid sequence consisting of characteristic conserved motifs separated by variable linker regions and containing a high percentage of charged and aromatic residues. We have discovered that films from the Doryteuthis pealeii RfA1 isoform function as stimuli-responsive reconfigurable infrared camouflage coatings, feature electrical properties that rival those of well-known artificial proton conductors, and support the proliferation and differentiation of neural stem cells. These indicate that both RfA1 and the extended reflectin protein family hold promise as advanced functional materials. Despite the technological potential of RfA1, a clear relationship between this protein’s high-order structure/ organization in films and in vitro functionality has remained elusive to date. Consequently, there exists an opportunity for further detailed structural characterization of RfA1-based ensembles, with the goal of better understanding their multifaceted functionality and exciting in vitro properties.

We studied the formation, nanostructure, and stimulus response of substrate-confined RfA1 using in situ grazing incidence small-angle x-ray scattering (GISAXS), for which the experimental setup is illustrated in Figure 1A. The integrated scattering intensity I(q) versus the horizontal scattering vector qy obtained at various times during the self-assembly of a representative RfA1 film is in turn shown in Figure 1B. To understand the emergence of structure during film formation, we monitored changes in the effective RfA1 nanoparticle geometry by extracting the radius of gyration RG and scattering intensity exponent P- which is related to the particle shape-from the scattering profiles collected at different time points after initiation of film assembly. At t =3 min, corresponding to an experiment on a nearly aqueous system, we calculated an RG of 830 Å, which was approximately twice the radius obtained in solution, which may indicate dimerization of RfA1 nanoparticles, and an exponent P of 1.5, indicative of an elongated spheroidal object. At the final time t =23 min (corresponding to complete loss of solvent), we found a nanoparticle radius RG of 910 Å and an exponent P of 2.1, similar to the value expected for an oblate or plate-like ellipsoid (P =2). The final geometry suggested that the nanoparticles had become flattened upon moving from solution to the interior of the film. Such substantial geometric changes probably resulted from a combination of factors, including the absence of solvation, presence of both lateral and vertical confinement, enhanced interparticle interactions due to crowding, and proximity-induced merger between neighbors. Altogether, our observations and analysis provided mechanistic insight into the formation and nanostructure of RfA1 films, enabling us to postulate a model for nanoparticle self-assembly, which is illustrated in Figure 2 . We used in situ GISAXS to evaluate the effect of a water-uptake on the nanostructure of our RfA1 films (water uptake is a prerequisite for protonic conductivity in RfA1 films and has been shown to change the films’ thickness and reflectance). Figure 1C illustrates the experimental and simulated scattering intensity profiles obtained for a typical RfA1 film before and after hydration. Before hydration (relative humidity of 50%), we extracted a nanoparticle radius RG of 920 Å and an exponent P of 2.0 (plate-like particle) from the simulated scattering profiles. This measurement indicated the RfA1 films were comprised of large oblate ellipsoids, in agreement with our observations during film formation. After hydration (relative humidity of ~90 %), we extracted a nanoparticle radius RG of 890 Å and an exponent P of 1.8 (elongated particle) from he simulated scattering profiles. This measurement indicated that the ellipticity of our nanoparticles had decreased, in agreement with expectations for their transition from a dry to a solvated environment. Altogether, our measurements revealed the nanostructure of RfA1 films in their hydrated state, as illustrated in Figure 2. Retrieve article Self-Assembly of the Cephalopod Protein Reflectin; K.L. Naughton,L. Phan, E.M. Leung, R. Kautz, Q. Lin, Y. Van Dyke, B. Marmiroli, B. Sartori, A. Arvai, S. Li, M.E. Pique, M. Naeim, J.P. Kerr, M.J. Aquino, V.A. Roberts, E.D. Getzoff, C. Zhu, S. Bernstorff and A.A. Gorodetsky; Advanced Materials, Vol. 28-38, pp. 8405-8412 (2016). doi: 10.1002/adma.201601666

A low temperature route towards hierarchically structured titania films for thin hybrid solar cells

"Back Cover" of Adv. Funct. Mater. 26, 7084 (2016), doi: 10.1002/adfm.201670259: P. Müller-Buschbaum and co-workers report the fabrication of dye-free hybrid solar cells based on hierarchically structured titania films and poly(3-hexylthiophene) at low temperature. This type of cells has unique advantages with respect to low cost and energy efficiency. The hierarchical structures of titania are beneficial for enhancing light harvesting in the hybrid solar cells, thereby improving device performance. Cover image by Christoph Hohmann, Nano-systems Initiative Munich (NIM). Figure 1. a) Schematic of the bottom part of hybrid solar cells based on a super-structured mesoporous titania film produced at low temperatures. b) Cross-section SEM image and c) 2D GISAXS data of a nano-imprinted mesoporous titania film.

Figure 2. a) PCE, b) Jsc, c) Voc, and d) FF normalized to the values that measured at zero incident angle for original solar cell. The pink triangles and the blue squares represent nano-imprinted and original solar cells, respectively. The dashed lines are guides to the eyes. Solar energy as a renewable energy source has drawn great attention over last decades due to the depletion and environmental impact of fossil fuels. Many technologies have been developed to harvest solar energy, among which solar cells are a promising technology to directly convert energy into electricity. To date, the photovoltaic market is dominated by silicon-based solar cells as they give reasonable high power conversion efficiencies. However, silicon solar cell panels are heavy, brittle and costly. Therefore, extensive research is dedicated to find alternative solar cell systems. Among them, dye-sensitized solar cells (DSSCs) feature low costs, use of an easy manufacturing process and efficiency values up to 12 %. Nevertheless, manufacturing of DSSCs requires high-temperatures typically above 400 ˚C, which challenges the fabrication of flexible devices. Moreover, the use of dye molecules complicates the DSSC manufacturing process and increases the energy payback time. Thus, fabrication of dye-free hybrid solar cells at low temperatures is a promising approach to optimize current DSSC technology. In the present work a low-temperature route for photovoltaic devices is realized by a combination of n-type titania films and p-type poly(3-hexylthiophene) (P3HT) in the active layer of the hybrid solar cells. In order to achieve more efficient titania photoanodes, artificial super-structures are introduced to mesoporous titania films at the low temperature. The sketch of the super-structured titania film integrated in to the bottom part of a hybrid solar cell is shown in Figure 1a. The hierarchically structured film is fabricated via the polymer template assisted sol-gel synthesis in combination with nano-imprint lithography (NIL).

In this approach, the sol-gel chemistry gives rise to the mesopores (the size is in the range of 10 to 20 nm), whereas the NIL provides periodic superstructure in submicrometer length scale on the top of the mesoporous titania films. The surface morphology is characterized by scanning electron microscopy (SEM) as illustrated in Figure 1b, where the artificial superstructures and film mesoporous nature are clearly demonstrated. The inner film morphology is probed with grazing incidence small-angle X-ray scattering (GISAXS) measurements. An example of the obtained 2D GISAXS data is depicted in Figure 1c. We find that NIL produces ordered superstructures efficiently and does not has any negative influence on the formation of mesopores, which are essential for excitin splitting. The nano-imprinted mesoporous titania films are filled with P3HT to form the active layer of hybrid solar cells. Through the study of the active layers, we find that more light is absorbed by the nano-imprinted active layers as compared to the original active layers (without super-structures). Finally, the dye-free hybrid solar cells are fabricated with our experimental route at low temperature to give a proof of practicability. The cells are measured at various angles of light incidence. The obtained power conversion efficiency (PCE) , short-circuit current density (Jsc), open-circuit voltage (Voc), and fill factor (FF) are shown in Figure 2 as function of different light incidence angles for nano-imprinted and original cells. In general, the angle-dependent PCE and Jsc of the nano-imprinted solar cells are higher than those of the reference solar cells. This phenomenon is caused by the super-structures enhancing light-harvesting in the active layer and patterned gold contacts increasing the light back-reflection. However, the Voc and FF remain almost constant, suggesting that they are not significantly affected by the artificial super-structures. Retrieve article A Low Temperature Route toward Hierarchically Structured Titania Films for Thin Hybrid Solar Cells; L. Song, A. Abdelsamie, C. J. Schaffer, V. Körstgens, W. Wang, T. Wang, E. D. Indari, T. Fröschl, N. Hüsing, T. Haeberle, P. Lugli, S. Bernstorff and P. Müller-Buschbaum; Adv. Funct. Mater. 26, 7084 (2016)). doi: 10.1002/adfm.201603867 Contact person: Peter Müller-Buschbaum, email: muellerb@ph.tum.de

Formation of swift heavy ion tracks on a rutile TiO₂ surface

Figure 1. (a) AFM image of overlapping ion tracks on a rutile TiO2 (001) surface (250 ion tracks per μm2, image height scale 4 nm, inset ×2 magnification). GISAXS maps of the irradiated surface acquired at (b) β = 0° and (c) β = 5°, where β is the angle formed by the surface tracks with the probing X-ray beam. The corresponding simulations of the GISAXS maps are shown as insets. The simulations are generated using the parameters of the fit. Figure 2. Cover of the Special issue on small-angle scattering of the Journal of Applied Crystallography, featuring in the lower left quarter GISAXS maps of a non-irradiated surface (upper left) and of surfaces irradiated with 50 (upper right), 250 (lower left) and 900 (lower right) ion tracks per micron2 obtained at β = 90o (see figure 6 of our paper cited below).

Nanostructuring surfaces using swift heavy ions offers some unique possibilities owing to the deposition of a large amount of energy localized within a nanoscale volume surrounding the ion trajectory. Samples irradiated with different ion fluences were investigated using atomic force microscopy and grazing-incidence small-angle X-ray scattering. A detailed surface description was obtained even for the case of multiple ion track overlap. To fully exploit the possibilities of swift heavy ion (SHI) irradiation, the morphology of nanostructures formed after SHI impact has to be known in detail. In the present work the response of a rutile TiO2 (001) surface to grazing-incidence SHI irradiation is investigated. Surface ion tracks with the well-known intermittent inner structure were successfully produced using 23 MeV I ions. Samples irradiated with different SHI fluences were investigated using atomic force microscopy (AFM) and grazing-incidence small-angle X-ray scattering (GISAXS). With these two complementary approaches, a detailed description of the SHI impact sites, i.e. the ion tracks on the surface, can be obtained even for the case of multiple ion track overlap. In addition to the structural investigation of surface ion tracks, the change in stoichiometry of the rutile TiO2 (001) surface during swift heavy ion irradiation was monitored using in situ time-of-flight elastic recoil detection analysis (TOF-ERDA), and a preferential loss of oxygen was found.

SHI irradiations were performed at the "Ruđer Bošković" Institute (RBI) in Zagreb, Croatia, using a 23 MeV I beam. Surface modifications were investigated using tapping mode AFM. Complementary GISAXS analysis was carried out at the synchrotron radiation facility Elettra-Sincrotrone Trieste, on the SAXS beamline, using synchrotron radiation with wavelength λ=0.154 nm (photon energy of 8 keV). To investigate possible stoichiometric changes of the TiO2, in situ TOF-ERDA measurements were performed at the RBI using the same 23 MeV I beam. We have successfully demonstrated how the applied SHI fluence can be used for nanoscale patterning of the surface. We have investigated three irradiation regimes, namely non-overlapping ion tracks, overlapping ion tracks (figure 1) and multiple overlapping ion tracks. The successful characterization of the surface in all three different irradiation regimes constitutes the first and necessary step for exploiting surface patterning by grazing-incidence SHI irradiation. The preferential loss of oxygen from the rutile TiO2(001) surface during grazing-incidence SHI irradiation, monitored by in situ TOF-ERDA, opens up again the question of the composition of surface tracks. This surprising result clearly warrants further studies. Retrieve article Formation of swift heavy ion tracks on rutile TiO2 (001) surface ; Marko Karlušić, Sigrid Bernstorff, Zdravko Siketić, Branko Šantić, Ivančica Bogdanović-Radović, Milko Jakšić, Marika Schleberger and Maja Buljan; J. Appl. Cryst. 49, pp. 1704-1712 (2016). doi: 10.1107/S1600576716013704

Formation mechanism of CoFe₂O₄ magnetic spherical mesoporous assemblies

 

Figure 1. TEM images of the sample prepared at 80 °C at different times. Figure 2. SAXS patterns collected during the reaction at a) T = 80°C and at b) T = 50°C.The arrows indicate the q-range where we observe the growth of primary particles, N1, the aggregates N2, and the consumption of the intermediate lamellar phase, L. c) Schematic representation of the multistep hierarchical formation mechanism of CoFe2O4 mesoporous assemblies.

We have combined in-situ time-resolved SAXS, TEM and XRD techniques, to study the hierarchical mechanism of CoFe2O4 spherical mesoporous magnetic assemblies formation obtained with an eco-friendly, surfactant-assisted water-based precipitation approach. We found a lamellar (L) intermediate phase providing active sites for the formation of primary ferrite nanoparticles (N1), which in turn are seeds for the formation of secondary entities (N2) finally forming the stable spherical magnetic mesoporous assemblies. Mesoporous magnetic particles with high magnetization and surface area values are of particular interest for biomedicine, bioseparation, catalysis and adsorption. Our study proves that the application of an in situ time-resolved SAXS-XRD-TEM approach can provide a powerful and exhaustive tool to clearly visualize the peculiar mechanism leading to the formation of hierarchical organized andstable mesoporous structures obtained with an eco-friendly, surfactant-assisted water-based precipitation approach. Furthermore, we demonstrate the important, multifold role played by the surfactant (sodium dodecylsulphate, SDS) during primary particle formation and their subsequent assembly, affecting crystal size, shape, assembly as well as mesoporous size and pathway. To study the mechanism involved into the formation of the spherical assemblies, TEM and XRD analyses performed at different times of the reaction have been accompanied by in situ and time- resolved SAXS measurements. TEM and high resolution TEM (HRTEM) analyses on the sample prepared at 80 °C after different minutes of reaction (Fig. 1) show a great deal of hexagonal platelet crystals and at the same timeat their edges, and often at the corners, small assemblies of nanocrystals. The images show the evolution of the reaction; the number of the spherical assemblies increases and, at the same time, the hexagonal platelets are worn out gradually up to be almost completely consumed. XRD patterns allows to know the crystallographic phases: typical peaks ascribable to the CoFe2O4 phase coexist with peaks associable to metal hydroxides (β-Co(OH)2 or/and Fe(OH)2 ) and oxy-hydroxides phase (δ-FeOOH). The amount of CoFe2O4 with respect to hydroxides increases with reaction time and after 30 min the hydroxide-oxide transition is completed.

To understand the growth kinetics of the primary particles and their aggregation in mesoporous assemblies, in situ time-resolved SAXS measurements havebeen performed at two differenttemperatures (Fig. 2a and 2b). The evolution with time in SAXS patterns evidence the formation of three different entities that correspond to the formation of primary CoFe2O4 nanocrystals (N1), secondary nanoparticles made up of three primary nanoparticles (N2) grown on the lamellar phase edges, and CoFe2O4 spherical assemblies at the expenses of the lamellar phase (L) that gradually disappears (Fig. 2c). This is in agreement with TEM and XRD measurements that clearly show that the CoFe2O4 forms and grows on the hexagonal platelets starting from their corners or edges (the most reactive zones). In particular, the formation of the spinel phase probably derives from the reaction of iron(III) released from dissolution of δ-FeOOH in strongly alkaline media with the β-Co(OH)2 plates, which, as a consequence, are gradually consumed. The surfactant has a key role on the creation of mesopores, as confirmed by an experiment carried out in the absence of SDS . Our results suggest that the formation mechanism can be kinetically controlled by the reaction temperature, able to modify the speediness of the reaction, affecting both the size distribution of primary particles and the size of their mesoporous assemblies. The study of the formation mechanism involved into stable spherical mesoporous assemblies made up of primary nanoparticles is a prerequisite that could render this soft chemistry route highly appealing to other magnetic ferrites (MeIIFe2O4, Me= Mn, Fe, Ni, Zn, Cu), a versatile class of materials that find applications in a wide variety of fields. Retrieve article Hierarchical Formation Mechanism of CoFe2O4 Mesoporous Assemblies; C. Cannas, A. Ardu, A. Musinu, L. Suber, G. Ciasca, H. Amenitsch and G. Campi; ACS Nano 9, 7 (2015). doi: 10.1021/acsnano.5b02145

Response of the wurzite GaN surface to swift heavy ion irradiation

Figure 1. Front cover picture: AFM image of the GaN surface after 1° grazing incidence irradiation using 92 MeV Xe23+, fluence 1 × 1010 ions cm−2. From article 325304 by M. Karlušić et al. Figure 2. GaN surface irradiated at IRRSUD with 92 MeV Xe23+, grazing angle 1°, fluence= 1010/cm2 (a) AFM image and GISAXS spectra taken at different azimuthal angles with respect to orientation of the surface ion tracks: 0° (b), 2° (c), 10° (d).

The passage of a SHI through a solid material can result in permanent nanoscale damage called ion track. The most common description of the SHI track formation, the thermal spike model, suggests that the kinetic energy of the SHI projectile that is deposited as dense electronic excitation along the SHI trajectory, can lead to nanoscale melting of the material. Irradiation of a flat solid surface by SHI under grazing incidence angle can result in the formation of surface SHI tracks. These ion tracks can be observed directly using atomic force microscopy (AFM). However, to extract statistical information (average ion track length, length distribution etc.), structural investigations of this type are very time consuming. In the present work, we report the results of our investigations regarding SHI irradiation of wurzite GaN surface, and show that grazing incidence small angle X-ray scattering (GISAXS) can be utilized for acquiring an excellent statistics during short measuring times.. Wurzite GaN thin film samples were grown by low-pressure metalorganic vapor phase epitaxy on c-plane sapphire substrates. SHI irradiations were performed at the IRRSUD beamline at GANIL using 92 MeV Xe, and at the RBI using 23 MeV I. Surface modifications were investigated using tapping mode AFM. Complementary GISAXS analysis was carried out at the synchrotron facilities of Elettra-Sincrotrone Trieste, on the SAXS beamline, using synchrotron radiation with wavelength λ=0.154 nm (photon energy of 8 keV). To investigate possible stoichiometric changes of the GaN, in situ TOF-ERDA measurements were performed at the RBI using the same 23 MeV I beam. In contrast to previous works where nanohillocks were found within the surface ion track, the morphology of 92 MeV Xe ion tracks consists of both nanohillocks and nanoholes. A sample irradiated at high fluence with 92 MeV Xe, but still under conditions when ion tracks are not much overlapped, produces a strong GISAXS signal (Figure 1.). For lower energy irradiation using 23 MeV I, ion tracks consist only of nanoholes. TOF-ERDA measurements performed using 23 MeV I at the same grazing incidence angle of 1° show a significant loss of nitrogen already at the fluence of 2×1011/cm2..

While the hillocks are generally interpreted as a signature of molten material, the occurrence of holes indicates a loss of material. Very recently, it was shown that in case of another wide band gap material, silicon carbide (6H-SiC), grooves with a depth of ~ 0.3 nm instead of chains of nanohillocks appear when irradiated by SHI under grazing incidence angle. In a broader context, the observation of nitrogen loss reported here and the loss of silicon from the SiC surface upon SHI irradiation reported earlier opens up the question of the composition of SHI tracks.. The research leading to these results has received funding from the European Community’s Seventh Framework Programme (FP7/2007-2013) -CALIPSO under grant agreement nº 312284. Retrieve article Response of GaN to energetic ion irradiation: conditions for ion track formation; M. Karlušić, R. Kozubek, H. Lebius, B. Ban-d’Etat, R.A. Wilhelm, M. Buljan, Z. Siketić, F. Scholz, T. Meisch, M. Jakšić, S. Bernstorff, M. Schleberger and B. Šantić; J. Phys. D: Appl. Phys. 48, 325304 (2015). doi: 10.1088/0022-3727/48/32/325304

Effect of Additives on the Morphology of PCPDTBT:PC71BM Thin Films for Organic Photovoltaics

Figure 1. GISAXS patterns of PCPDTBT:PC71BM thin films (1:2.7) fabricated a) without and b) the use of 3 vol% ODT. The figure originally published in the ACS AuthorChoice open access publication referenced below. Figure 2. Effect of ODT on the nanometer-scaled morphology of PCPDTBT:PC71BM thin films for organic photovoltaics. a) The presence of ODT leads to enhanced lamellar polymer crystallization. B) Therefore, fullerene molecules must be expulsed from the film; a fullerene rich topping layer forms. The figure originally published in the ACS AuthorChoice open access publication referenced below.

One very successful way to realize polymer based organic solar cells makes use of a thin organic blend film consisting of a semi-conducting polymer and a fullerene derivative. These films allow not only for efficient large-scale application via printing or spraying techniques on flexible substrates but also allow for manufacturing semitransparent devices. Thus, light-weight, cost-efficient and optically tunable solar cells are available which open a vast range of applications such as in mobile devices or architecture. The most important issues yet to overcome are degradation, to enhance long-term stability and limited power conversion efficiency, to increase the device performance. A way to improve the photovoltaic efficiency is to add processing additives in the mutual polymer-fullerene solution before film application. Typically, high boiling point solvents are thereby added that influence the demixing process of the two materials which leads to the formation of an inter-penetrating domain network, a so-called bulk-heterojunction (BHJ), within the active layer. In order to systematically improve OPV performance and to understand degradation effects, it is necessary to understand the exact mechanism of how the solvent additive (SA) changes the BHJ morphology. In the present study, the exact mechanism of how the addition of 3 vol% SA in the processing solvent influences the nanometer-scaled BHJ morphology of films with different mixing ratios. Therefore, PCPDTBT:PC71BM thin films have been made from a chlorobenzene-based mutual solution with and without use of 3 vol% 1,8-octanedithiol (ODT) with different polymer-fullerene mixing ratios between 1:1.5 and 1:2.7. A combination of grazing incidence small and wide angle X-ray scattering (GISAXS and GIWAXS), X-ray reflectivity (XRR) and optical spectroscopy has been used to probe the vertical and lateral morphologies, whereby GISAXS and GIWAXS investigations were performed at the Austrian SAXS beamline of Elettra Sincrotrone de Trieste. Figure 1 shows exemplary GISAXS patterns of a film with a composition of 1:2.7 without and with use of ODT.

When SA is used, a clear lateral scattering pattern appears around the polymer Yoneda region. Detailed analysis reveals that this pattern arises from enhanced phase separation, causing also a doubling of small polymer domains on a length scale of a few ten nanometers. The phase separation is found to be driven by polymer crystallization as seen in the GIWAXS investigation. Since, by the crystallization process, fullerene molecules must be pushed out of the polymer matrix, a fullerene-rich topping layer forms on the blend films which is investigated using XRR. As a result, the mixing ratio between polymer and small molecule becomes independent of the blend composition within the bulk of the film. Combining these results, a full image of how ODT improves the power conversion capabilities of PCPDTBT:PC71BM thin films can be drawn as illustrated in Figure 2: The presence of solvent additive leads to a decrease of the solubility limit of fullerene molecules in the polymer matrix which leads to enhanced polymer crystallization and fullerene expulsion. Thereby polymer domains on a length scale of a few tens of nanometers form. These lead to both, improved charge carrier transport characteristics and enhanced light absorption, due to an enhanced degree of crystallinity. Furthermore, a fullerene-rich top layer forms. This layer acts as hole-blocking layer which decreases charge carrier recombination. All these effects contribute to an enhanced photovoltaic performance. Retrieve article Effect of Blend Composition and Additives on the Morphology of PCPDTBT:PC71BM Thin Films for Organic Photovoltaics; Christoph J. Schaffer, Johannes Schlipf, Efi Dwi Indari, Bo Su, Sigrid Bernstorff, and Peter Müller-Buschbaum; ACS Appl. Mater. Interfaces 7 (38), 21347-21355 (2015). doi: 10.1021/acsami.5b05939

Polymer/Nanocrystal Hybrid Solar Cells: Influence of Molecular Precursor Design on Film Nanomorphology, Charge Generation and Device Performance

Figure 1. Chemical structures of the used cadmium xanthate and the different alkyl moieties, TEM images of poly(3-hexylthiophene)/CdS hybrid layers prepared using a xanthate bearing a propyl (upper image) and a dimethylpentyl (bottom image) alkyl chain and the current/voltage characteristics of the corresponding hybrid solar cells (green: propyl, purple: dimethylpentyl). Figure 2. a) GIWAXS patterns of a polymer/Cd-butyl xanthate sample measured between 22.9 and 35° 2 theta during a heating run from room temperature to 200°C showing the evolution of the most intense peak of CdS at around 27° 2 theta. The increasing temperature (T) is indicated with a black arrow; b) Normalised integrated intensity of the GIWAXS patterns of the different samples plotted against time and temperature; c) GISAXS images of the ethyl and butyl xanthate samples at different temperatures during the heating run.

The morphological and optoelectronic properties of polymer/nanocrystal hybrid (pnh) solar absorber layers can be significantly influenced by small modifications of the chemical structures of the precursor molecules. Pnh solar cells are an interesting concept due to the incorporation of the attractive qualities of both organic and inorganic materials. In this study, polymer/nanocrystal films are prepared via thermal conversion of metal xanthates to metal sulfide nanocrystals directly in the polymer matrix without capping ligands for the nanocrystals. The effects of different alkyl chains of the metal xanthates on film nanomorphology and optoelectronic properties are investigated (Figure 1). As only little was known about the nanocrystal growth and the formation of the nanomorphologies during the heating step, this is thoroughly characterised on the nanometer scale by time resolved simultaneous GISAXS and GIWAXS (grazing incidence small and wide angle X-ray scattering) measurements performed at the SAXS beamline of Elettra Trieste (Figure 2). It is found that longer alkyl chains of the metal xanthates lead to a better mixing of the polymer and nanoparticle phase and to smaller domain sizes in the absorber layers, while using short ethyl moieties results in the biggest domain sizes. As revealed by time resolved GISAXS and GIWAXS measurements, the formation of these different morphologies is determined by agglomeration of the nanocrystals, while the nanocrystal size is similar in all the investigated samples. The agglomeration of the nanocrystals is influenced by growth temperature and growth kinetics of the nanocrystals. In addition, phase separation of polymer and metal xanthate phase in the precursor layer before the heating step plays an important role.

Moreover, the influences of the different nanomorphologies on the optoelectronic properties of the films are studied by microsecond and femtosecond transient absorption spectroscopy and a remarkable effect on device performance is revealed. While in finer mixed morphologies down to a certain domain size more efficient photoinduced charge generation was observed, the highest power conversion efficiencies were obtained in solar cells prepared using metal xanthates bearing propyl moieties, which have an absorber layer nanomorphology with medium sized domains. This originates from the fact that besides efficient charge generation also charge transport and recombination play a crucial role in the devices, which become more and more an issue when going to finer mixed morphologies.   Retrieve article Polymer/Nanocrystal Hybrid Solar Cells: Influence of Molecular Precursor Design on Film Nanomorphology, Charge Generation and Device Performance ; A.J. MacLachlan, T. Rath, U.B. Cappel, S.A. Dowland, H. Amenitsch, A.-C. Knall, C. Buchmaier, G. Trimmel, J. Nelson and S.A. Haque; Adv. Funct. Mater. 25, 409 (2015); doi: 10.1002/adfm.201403108

A Closer Look into 2-Step Perovskite Conversion with X-Ray Scattering

Figure 1. Crystal size distribution of the hybrid perovskite and its precursor PbI2 extracted from simulation of GISAXS data. Figure 2. Scheme depicting the assumed conversion mechanism from PbI2 precursor to perovskite. Figure 3. TOC of the original paper.

The morphology of the hybrid perovskite material CH3NH3PbI3-xClx was investigated using simultaneous GISAXS and GIWAXS. The conclusions on the crystallization mechanism helped to improve recently developed hybrid perovskite solar cells’ performance. The hybrid material CH3NH3PbI3-xClx with perovskite crystal structure has recently led to a revolution in thin film photovoltaics. Since its inception in 2009, solar cell power conversion efficiencies have risen up to 20°% now challenging conventional inorganic photovoltaic technologies as perovskite solar cells promise cheap production from abundant precursor materials. However, fundamental understanding of material properties and especially the correlation of crystallization dynamics and film morphology lack behind the rapid device development. Especially in the very simple planar cell architecture these cells suffer from performance losses due to unreproducible film morphologies. Thus, understanding and ultimately controlling the crystal morphology of perovskite thin films is of great importance to achieve a reproducible high photovoltaic performance. A very versatile approach for the synthesis of such hybrid perovskites is a 2-step technique, where the perovskite is obtained by conversion from a crystalline precursor film, lead iodide (PbI2), by dipping into a solution containing the organic cations. It has been reported before that optimization of the PbI2 layer is of utter importance for good performance in photovoltaic devices. We conducted the first successful grazing-incidence small angle X-ray scattering (GISAXS) measurements on thin films of a hybrid perovskite and its respective precursor while simultaneously monitoring the crystal phase with grazing-incidence wide angle X-ray scattering (GIWAXS). With the latter we confirmed the crystal structure for our thin films. In GISAXS we made use of the fact that an incident angle below the critical angle of the material yields surface-sensitive data while higher angles penetrate the entire film and give information on the bulk morphology. By horizontal line cuts we gather information on the lateral film morphology in the bulk and on the surface of PbI2 and perovskite thin films. Using a distorted wave Born approximation (DWBA) and assuming a local monodisperse domain distribution (LMA) we simulated the GISAXS data and extracted structure information.

We find a close correlation of crystal sizes in the precursor and the perovskite films. As during the conversion to perovskite these films usually exhibit a significant volume expansion due to the incorporation of CH3NH3 into the inorganic framework, it can be concluded that crystal growth is confined laterally. In fact, by a Williamson-Hall analysis on powder X-ray diffraction measurements certain amount of crystal strain is confirmed for the perovskite film. Furthermore, from our measurements it is possible to get insight into the distribution of crystal sizes (see Figure 1). Thus, it is apparent that not only does the ratio of smaller to larger crystals increase during conversion in favor of smaller crystals, but also that this effect is more severe inside the bulk. This leads to the following model of the conversion mechanism present: As the crystal growth is constrained laterally the volume expansion of the crystals during conversion of PbI2 to perovskite is mainly vertical. This, however, requires a mechanical redistribution in within the film, i.e. due to the strain larger crystals crack up into smaller units that accumulate closer to the substrate. On the surface, on the other hand, the concentration of organic compounds is higher and facilitates the complete reconstruction of the domains and smaller crystals are dissolved or incorporated into larger ones by Ostwald ripening. In conclusion, our results help to elucidate the origin of the morphology in such-way prepared hybrid perovskite thin films and thus aim in the improvement of synthesis methods for this type of material which is important for reproducible high photovoltaic performance. Retrieve article A Closer Look into Two-Step Perovskite Conversion with X-ray Scattering; J.Schlipf, P.Docampo, C.J.Schaffer, V.Körstgens, L.Bießmann, F.Hanusch, N.Giesbrecht, S.Bernstorff, T.Bein, P.Müller-Buschbaum; J. Phys. Chem. Lett. 6(7), 1265-1269 (2015). doi: 10.1021/acs.jpclett.5b00329

In operando morphology investigation of inverted bulk heterojunction organic solar cells by GISAXS

The correlation between morphology of the active layer and device performance of inverted polymer solar cells during continuous operation under illumination is revealed by in operando grazing incidence small angle X-ray scattering (GISAXS) and current−voltage (J-V) measurements. Consequently, the reason for enhanced stability is identified from the viewpoint of morphology. Solar power as an alternative energy source to replace depleting fossil fuels has received great attention over last decades. To date, main photovoltaic devices are based on silicon, which possess very high power conversion efficiency. However, silicon solar panels are costly, material-consuming and inflexible. Thus, organic solar cells have emerged as one of the most promising alternatives to silicon based devices due to their unique properties, such as low-cost, light, flexible and easy to manufacture. Nevertheless, short lifetimes are a major obstacle for the commercial breakthrough. Therefore, stabilizing organic photovoltaic devices is an urgent demand. Figure 1. (a) J-V curves of inverted P3HT:PCBM solar cells with different illumination time. They are recorded every 16 seconds until 240 min. Only the J-V curves are recorded at the same time as the GISAXS measurements are selected for illustration. (b) Time evolution of normalized photovoltaic parameters. The images are adapted from W. Wang et al., J. Mater. Chem. A 3, 8324 (2015) with permission from the Royal Society of Chemistry.

Figure 2. (a) Initial 2D GISAXS data of the P3HT:PCBM active layer in an inverted solar cell during operation under illumination. (b) Vertical line cuts and (c) Horizontal line cuts obtained from the 2D GISAXS data. Different line colors denote the illumination time of 0 min, 3 min, 10 min, 15 min, 30 min, 60 min, 120 min, 180 min and 240 min. In order to visualize the differences in the line cuts as a function of operation time, all the vertical or horizontal line cuts are plotted on top of each other. All the horizontal line cuts can be fitted with the same curve shown as a red line in (c). The images are adapted from W. Wang et al., J. Mater. Chem. A 3, 8324 (2015) with permission from the Royal Society of Chemistry. In the present work it is found that poly(3-hexylthiophene-2,5-diyl) (P3HT):phenyl-C61-butyric acid methyl ester (PCBM) bulk heterojunction solar cells with inverted geometry are highly stable. In order to explore the correlation between the morphological evolution and the solar cell performance during continuous operation under illumination, in operando GISAXS measurements are carried out. J-V sweeps are recorded simultaneously to the GISAXS study as shown in Figure 1a. Surprisingly, the power conversion efficiency (PCE) of the inverted solar cell is preserved to 75% as compared to its maximum value after 240 min continuous operations under illumination. By taking into account all the photovoltaic parameters, it is noticed that the open circuit voltage (VOC) and fill factor (FF) show quite similar decay behaviors as in the standard solar cell, whereas the short circuit current density (JSC) exhibits a much higher stability (in Figure 1b). Therefore, the relative stable photovoltaic performance mainly originates from the higher stability of JSC.

The in operando GISAXS measurements give the morphological evidence for this device stability. An example 2D GISAXS measurement is shown in Figure 2a. By analyzing the vertical and horizontal lines cuts of the in operando data, it is revealed that the P3HT:PCBM active layer in an inverted solar cell maintains a stable morphology, which does not alter during the device operation (Figure 2b and c). In contrast, in a standard solar cell P3HT domains increase in size along with illumination time, which is regarded as morphological degradation and which is responsible for the efficiency decay. Therefore, the stabilization of the P3HT:PCBM layer with an inverted geometry is directly correlated to the improved stability of the JSC and PCE as seen in our study for the first time Retrieve article In operando morphology investigation of inverted bulk heterojunction organic solar cells by GISAXS ; W. Wang, C. J. Schaffer, L. Song, V. Körstgens, S. Pröller, E. Dwi Indari, T. Wang, A. Abdelsamie, S. Bernstorff, P. Müller-Buschbaum; J. Mater. Chem. A 3, 8324 (2015). doi: 10.1039/c5ta01109d

Investigation of the thermal stability of nanoimprinted comb structures in a conjugated polymer and their application in hybrid solar cells

Figure 1. (a) SEM images of NIL-imprinted comb structures (periodicity: 500 nm) directly after the NIL process and after a temperature treatment at 195 oC; (b) GISAXS images of a NIL-structured PSiF-DBT film (periodicity: 180 nm) at 65, 105, 143 and 200 oC. The red boxes indicate the vertical and horizontal areas for integration. (c) Evolution of the out-of-plane scattering signal of a NIL-structured PSiF-DBT film during a heating run to 200 oC with a heating rate of 10 oC/min, approx. every 10th measurement is shown, the curves are shifted vertically for better visibility; (d) Temperature-dependent changes in out-of-plane intensity of the “line structure peak” at approx. qz = 0.9 nm-1; (e) Temperature-dependent changes in out-of-plane intensity of the wings of the Yoneda peak at approx. qz = 0.55 nm-1. Adapted with permission from S. Dunst et al, ACS Appl. Mater. Interfaces 6, 7633 (2014). Copyright (2014) American Chemical Society. Figure 2. (a) Schematic illustration of the NIL process for the preparation of comb structures and the infiltration process towards defined hybrid layers and chemical structures of the used polymer PSiF-DBT and the Cu and In xanthates; (b) SEM image of a cross section of a hybrid solar cell comprising a polymer/CIS absorber layer with a nanostructured interface (periodicity: 180 nm) prepared at 160 oC; (c) IV curves measured in the dark and under 100 mW/cm2 illumination of hybrid solar cells with flat and nanostructured interfaces prepared at 160 oC. Adapted with permission from S. Dunst et al, ACS Appl. Mater. Interfaces 6, 7633 (2014). Copyright (2014) American Chemical Society..

Comb-shaped nanostructures were prepared in a low band gap polymer using nanoimprint lithography (NIL) and their thermal stability was investigated using time resolved grazing incidence small angle x ray scattering (GISAXS). These measurements showed that the comb structures in the conjugated polymer are stable up to a temperature of 145 oC, which enabled us to apply them in nanostructured organic/inorganic hybrid solar cells. The nanostructured solar cells revealed improved power conversion efficiencies compared to flat bilayer devices. The absorber layer of organic/ inorganic hybrid solar cells consists of a mixture of a conjugated polymer and a nanostructured inorganic semiconductor. The nanostructured inorganic semiconductor can be present either in the polymer matrix as a random network of nanoparticles or, in a more defined way, as an ordered nanostructure. For obtaining high power conversion efficiencies (PCEs) of these devices, a large interface between polymer and nanoparticle phase, where charge separation of electrons and holes occurs, is beneficial. However, also continuous domains in both phases towards the respective electrode are needed for fast charge transport of electrons and holes to the electrodes in order to limit recombination. In this study, defined comb structures were prepared using NIL in the conjugated polymer PSiF-DBT, see Fig. 1a and 2a, and it was envisaged to use them in absorber layers of hybrid solar cells by filling the comb-shaped structure with copper indium sulphide. This can be realised by coating the nanostructured layer with a solution containing copper and indium xanthates, as schematically illustrated in Fig. 2a. The metal xanthates can be subsequently thermally converted at moderate temperatures to a copper indium sulphide layer covering the nanostructure.

For a successful preparation of nanostructured hybrid solar cells via this approach, the nanostructures have to be sufficiently thermally stable. Therefore, we performed a time resolved GISAXS study at the Austrian SAXS beamline at Elettra, which revealed a good thermal stability of the polymeric nanostructures. In the GISAXS images (see Fig. 1b), which were acquired at low temperatures, a semicircle-like chain of intensity maxima is visible, which is characteristic for periodic line structures oriented parallel to the x-ray beam. The evolution of the out-of-plane scattering signal shows that the most intense parts of this semicircle-like feature (peak at qz ~ 0.9 nm-1) are present up to a temperature of approx. 145 oC (Fig. 1c and d), which proves a good stability of the nanostructure up to this temperature. Moreover, at around 145 oC the intensity of the wings of the Yoneda peak at qz ~ 0.55 nm-1 (see Fig. 1e) increases, which suggests that the roughness of the polymer layer becomes higher at this temperature and the well-defined structure is lost. Time resolved grazing incidence wide angle x-ray scattering (GIWAXS) measurements pointed out that the formation of the copper indium sulphide nanocrystals from the metal xanthates starts shortly before the comb structures start to be unstable and that the formation of the nanocrystalline metal sulphide is completed at a temperature of about 150 - 160 oC. In a next step, nanostructured solar cells were prepared by coating the nanostructured polymer layers with the metal xanthate solution and subsequent annealing at 160 oC to form the copper indium sulphide. Indium tin oxide (ITO) as well as aluminium were used as electrodes. In Fig. 2b, a scanning electron microscopy (SEM) image of a cross section of a device with a nanostructured interface is presented, which proves that the comb-shaped polymer structure (appearing darker) is filled with copper indium sulphide (lighter areas). The current-voltage characteristics of the solar cells (Fig. 2c) exhibit a significantly improved PCE of the nanostructured device compared to a similarly prepared flat bilayer device. The increase in PCE (approx. 0.3% for a nanostructured device compared to 0.1-0.15% for a flat bilayer device) is based on a distinct improvement of the short circuit current in the nanostructured solar cells, which can be ascribed to the increased interface area.. In this study, we demonstrated that the preparation of nanostructured hybrid solar cells via this approach is generally feasible. To optimize the devices, it is envisaged to further reduce the periodicity of the nanoimprinted comb structures, which is 180 nm in the solar cells prepared in this study. Moreover, the temperature of the thermal conversion of the metal xanthates should be kept as low as possible in order to retain a well-defined nanostructure. Retrieve article Nanoimprinted Comb Structures in a Low Bandgap Polymer: Thermal Processing and Their Application in Hybrid Solar Cells; S. Dunst, T. Rath, A. Radivo, E. Sovernigo, M. Tormen, H. Amenitsch, B. Marmiroli, B. Sartori, A. Reichmann, A.-C. Knall and G. Trimmel; ACS Appl. Mater. Interfaces 6, 7633 (2014). doi: 10.1021/am5009425

Nanomaterial coatings: controlling and analyzing thin films of lipid cubic phase

Figure 1. Structures shown in order of increasing relative humidity (left to right): lamellar; gyroid LCP; diamond LCP. Figure 2. Top row: gyroid LCP with the (110) plane highlighted (left) and diamond LCP with the (111) plane highlighted (right). Bottom row: corresponding x ray scattering patterns with predicted x-ray reflections shown as small black circles. Figure adapted with permission from M. Rittman et al, Langmuir 29, pp. 9874-9880 (2013). Copyright 2013 American Chemical Society.

Abstract: We have studied new forms of a nanomaterial known as a LCP (Lipid Cubic Phase), preparing them as thin films rather than the bulk gels or liquids typically used. LCPs are of interest for biological research as cell membrane models, and for technological applications including drug delivery and nanomaterial production. Using its new thin film form, we show that we can use the surrounding humidity to control the nanostructure, suggesting a new type of huminity-responsive nanomaterial. We also show that we can induce order into the LCP, with the surface making it lie in a specific direction. Lipid cubic phases (LCPs) are complex three-dimensional nanostructures, containing networks of water channels billionths of a meter in size, separated by a lipid “bilayer” similar to a cell membrane. The material forms spontaneously when particular lipid molecules (typically plant-derived, low-cost, biocompatible and often edible) are mixed with water. These properties make them of interest for biological membrane research, and for technological applications such as in the production of high-performance platinum electrodes, or as vehicles for drug and food flavor delivery. Here, their structure can be made to respond to external stimuli such as temperature and acidity, for controlled or triggered release. LCPs are usually prepared in bulk gel or liquid forms, by mixing the lipid with the required amount of water. In our approach, we instead began with a thin film of lipid spread on a flat silicon surface. We introduced water by changing the humidity of the air above the sample. The lipid molecules absorb water from the air, and assemble themselves into a variety of different shapes shown in Fig. 1, as we gradually increase humidity: first, a “lamellar phase” forms, consisting of a stack of flat bilayers, on the left side; then, at relative humidity values above 83%, the molecules start to rearrange into an LCP known as the gyroid cubic phase; finally, at still higher humidity values, another LCP structure known as the diamond cubic phase appears. We know this by analyzing the film using a technique known as grazing-incidence small-angle x-ray scattering (GISAXS), which we carry out at the Austrian SAXS beamline at Elettra..

Because our LCP films are so thin – much thinner than a human hair – it only takes a few minutes for the water to penetrate into the lipid layer. In this way, we can use our thin film approach to make a new type of smart LCP material, which responds to humidity rather than temperature or acidity. A second new feature of LCPs studied as thin films is their orientation. Traditionally prepared gels or liquids contain many grains of LCP, millionths of a metre in size, all facing in different directions, like grains in a powder. These scatter x-rays equally in all directions, giving so-called “powder-like” x-ray scattering patterns consisting of rings. In contrast, when we prepared thin films of LCP samples, we found that the films were all aligned, with the same surface within the LCP sample lying parallel to the plane of the film throughout the sample. These surfaces in the gyroid and diamond LCP are technically referred to as the (110) and (111) surfaces, respectively, and are shown in Fig. 2, which also shows the scattering patterns from the two LCP structures; the fact that the x rays (white) give spots rather than rings shows that, rather than being powder-like, the samples are highly aligned; the small black circles show the predicted positions for gyroid and diamond LCPs aligned parallel to the (110) and (111) planes respectively, confirming the orientation. More recently, we have worked with professor W.T. Gozdz at the Polish Academy of Sciences, who has calculated the most favourable surface for the LCPs to adopt, and has shown that the (110) and (111) are indeed the orientations we would expect for these two LCP structures. For an x ray scattering scientist, an oriented sample is more like a single crystal and less like a powder, and the arrangement of spots gives much more structural information than the rings from a powder pattern. Over sixty years ago, the structure of DNA was solved by Crick and Watson, in a large part due to the information that could be obtained using x ray patterns from highly oriented DNA, produced by Rosalind Franklin. In the same way, at a more modest level, we hope that new structural insights into LCPs and the molecules they interact with will be gained using the highly oriented LCPs in our thin films.. Retrieve article Control and Analysis of Oriented Thin Films of Lipid Inverse Bicontinuous Cubic Phases Using Grazing Incidence Small-Angle X-ray Scattering; M. Rittman, H. Amenitsch, M. Rappolt, B. Sartori, B.M.D. O’Driscoll, and A.M. Squires; Langmuir 29, 9874-9880 (2013). doi: 10.1021/la401580y

Growth of LTA zeolite nanoclusters using a block copolymer template

Figure 1. SAXS data during the early stage of the LTA zeolite synthesis are described by a Guinier type analysis (for q<0.15 nm-1) as reported in the inset and a core-shell approach (for q>0.3 nm-1). Figure 2. Sketch of the self-assembly stages involved during the synthesis of block copolymer templated hybrid nano-particles. The initial association of aluminosilicate species into the corona region of the PDMS-PEO copolymer micelles (a) generates primary units with a core-shell morphology (b). Progressive aggregation process among these primary units leads to the formation of extended secondary fractal structures (c), while further cross-linking, fusion and rearrangement of the secondary particles leads to the formation of final submicron aggregates (d). In the inset (e) the time evolution of the SAXS intensity profile (at T=45°C) after the mixing of the main components is presented, while the scanning electron microscopy (SEM) image of the final nano-aggregates is reported in the inset (f).

Employment of highly branched copolymer micelles as core substrate to control the soft interactions at the boundary between organic and inorganic domains provides a steric stabilization that enhances the colloidal stability during LTA zeolite formation. Our results, while indicating an interesting possibility in substitution of more traditional templates for the zeolite synthesis, give important insights to the comprehension of the self-assembly processes involved in the development of organic-inorganic mesoporous nanoparticles and for alternative protocols in porous materials production. In recent years, special efforts have been devoted to develop strategies for the synthesis of supramolecular organic-inorganic nanostructures based on porous materials. The main goal of the different strategies is to achieve a synergy between the properties originating from the porous inorganic substrate and the properties of the involved organic components. Particularly stimulating is the study of alternative protocols for the assembly mechanism of such materials in which a macromolecular template drives the formation of nanostructures with peculiar final properties. Our investigation, performed at the Austrian SAXS beamline of Elettra, outline the self-assembly processes involved in the formation of sub-micron particles of zeolite LTA grown on a polydimethylsiloxane-b-polyethyleneoxide (PDMS-b-PEO) diblock copolymer used as templating agent. The combination of supramolecular interactions, together with the ability to control both the length scale and the structural morphologies, makes block copolymers particularly attractive templates in the synthesis of nonporous materials with new characteristic and properties. The early stage of the nanoparticles growth process, restricted to an initial time between 1-3 hours, was characterized by the incorporation of the (LTA zeolite) aluminosilicate components into the surface of the nanotemplate with the formation of primary units with a core-shell morphology, while the presence of aggregation processes among primary units led to the formation of extended secondary fractal structures (Fig. 1).

Further cross-linking, fusion and rearrangement of the secondary particles leads to the formation of final submicron aggregates as reported in Fig. 2, where the multistep mechanism of formation of the hybrid nano-clusters is proposed. The formation of large supramolecular assemblies at the late stage of the synthesis process was finally confirmed by scanning electron microscopy (SEM) experiments (Fig. 2f), that showed the presence of large spherical nanosized aggregates. The back-scattered SEM image of the system confirmed a condensation of the aluminosilicate components on the aggregates surfaces as proved by the energy dispersive x ray (EDX) microprobe analysis, while x ray diffraction (XRD) experiments indicated the formation of crystalline zeolite LTA, thus confirming the porous nature of the generated particles. Generally, the driving interaction regulating the structure formation in zeolites are difficult to understand, due also to the difficulty to follow (in situ) the time evolution in a multi-component complex environment. In this respect, the use of high molecular weight copolymer template seems to be an interesting possibility in substitution of more traditional templates, as the presence of micellar block copolymers provides a steric stabilization that induces an enhanced (transient) colloidal stability to the synthesis environment. The obtained results indicate how micellar block copolymers precursors offer, from a molecular point of view, favorable conditions for the self-assembly processes involved in the synthesis of hybrid matrices. The soft interaction involved in the sol–gel process as well as the high adaptability to the reaction conditions reveals the very promising properties that polymer based amphyphylic templates can offer in the design and construction of hybrid inorganic-organic functional materials based on zeolites. Retrieve article Self-assembly in poly(dimethylsiloxane)-poly(ethylene oxide) block copolymer template directed synthesis of Linde type A zeolite; L. Bonaccorsi, P. Calandra, M.A. Kiselev, H. Amenitsch, E. Proverbio and D. Lombardo; Langmuir 29, 7079 (2013). doi: 10.1103/10.1021/la400951s

Structure and morphology of magnetron sputtered W films

The structural and morphological details of magnetron sputtered tungsten (W) thin films as a function of the Ar working gas pressure and the sputtering power are presented. The crystal structure of the W films was examined with grazing incidence x-ray diffraction (GIXRD), while the morphology characterization was performed by x-ray reflectivity (XRR) and grazing incidence small-angle x-ray scattering (GISAXS). W films have an important role in a number of technological applications, including e.g. optic and near-infrared transition edge sensors, diffusion barriers in semiconductor interconnect layers, absorption masks in x-ray lithography or diffracting layers in x-ray mirrors. Recently, W films have also been used as coatings of plasma-facing surfaces in tokamaks. All these studies reported that the W film structure and morphology are crucial for the final properties of the device or functionality of the coating. Tungsten thin films exhibit two crystalline modifications: a thermodynamically stable body centred cubic (bcc) phase (α-W) and a metastable A15 phase (β-W). These two phases exhibit different properties. The lattice parameters of α-W and β-W are 3.16Å and 5.04 Å, respectively. The electrical resistivity of α-W is always lower than the electrical resistivity of β-W. The superconducting transition temperatures of the two phases are 15mK for α-W and between 1 and 4K for β-W. Moreover, the hardness of α-W and β-W films is different. The occurrence and stability of β-W in physical vapour deposition (PVD)-produced thin films are associated with the presence of oxygen in the deposition chamber, either as a controlled admixture or as a residual gas. Beyond the conditions for β-W formation, an amorphous tungsten film is formed. Tungsten films are often deposited by the magnetron sputtering technique which allows a high-rate, controlled, uniform deposition and thus lends itself to economic, large-area industrial applications. In addition, a variation of the sputtering conditions yields different film morphology which can be tailored to a number of physically and technologically interesting properties. With normal incidence sputter deposition, either compact W films with very smooth surface or W films with columnar morphology were reported previously. However, no comprehensive study of the relationship between phase composition and morphology of W thin films for normal incidence sputter-deposition geometry has been reported so far. We employed now a normal incidence sputter-deposition geometry in order to elucidate the effects of two magnetron deposition parameters—working gas (Ar) pressure in the deposition chamber pAr and W sputtering power PW—on the thin tungsten film structure and morphology. Both parameters, pAr and PW, affect the growth mechanism, and by changing them we studied the effects of the kinetics and flux of the W atoms on the final morphology and structure of the prepared W films. Furthermore, the deposition rate could influence the rate at which the impurities are incorporated into the growing film, and thus the crystal structure of the W film.

We find that the crystalline properties and nanoporosity vary systematically with the deposition conditions. Depositions at low Ar pressures (<5mTorr) and high powers (>40 W) result in compact and smooth layers with only α-W crystallites. By reducing the sputtering power (20 W), along with stable α-W, a metastable β-W phase occurs. For an even lower power (10 W), the W film becomes amorphous and exhibits a columnar morphology accompanied by a 25% reduced layer’s mass density compared with that of bulk tungsten. A similar columnar morphology was also found for films deposited at higher Ar pressures (>5mTorr), and a moderate sputtering power of 20W. However, the columns in high-pressure films consist of the metastable β-W phase. The cross-section diameter of the columnar voids in the amorphous sample is 3 nm, while that in the high-pressure samples is approximately 4–6 nm. The highest mass density reduction, of up to 50%, is observed for the highest pressure of 20mTorr. Moreover, the morphology of W films deposited at high Ar pressures exhibits a depth dependence: smaller columns (and voids) closer to the Si substrate tend to increase in size towards the surface. On the other hand, in the amorphous-like sample we observed a 3.5 nm surface layer, which we attributed to the oxygen-rich W or WO3 phase. The formation of a metastable β-W phase in some films can be related and understood based on the rate at which oxygen incorporates into the film during the growth process. We systematically found β-W in the films deposited with lower flux of W atoms. This is a strong argument that for lower fluxes the residual oxygen in the deposition chamber has a higher probability to affect the growth process and in that way to stabilize the β-W. The phase composition of the films is closely related to their columnar morphology or porosity. We found β-W or amorphous W, without stable α-W, in those films which develop columnar morphology. This type of morphology is due to the self-shadowing effects when deposited atoms arrive at non-normal directions to the substrate and with reduced kinetic energy. This work shows the dramatic effects of the deposition conditions on the structure and morphology of tungsten films, and the powerfulness of small-angle x-ray methods in the characterization of thin films. The results presented here are useful in optimizing the process parameters to obtain W films with desired properties. Retrieve article Structure and morphology of magnetron sputtered W films studied by x-ray methods; K. Salamon, O. Milat, N. Radic, P. Dubcek, M. Jercinovic and S. Bernstorff; Journal of Physics D 46 (9), (2013) 095304. 10.1088/0022-3727/46/9/095304

Growth of anisotropic Ge quantum dot lattices in alumina matrix

Figure 1. Structural properties of the films. (a), (b) GISAXS maps measured for parallel and perpendicular direction of the probing x-ray beam with respect to the main direction of the adparticle diffusion (the insets show the corresponding simulations), and (c), (d) the corresponding TEM images of the films cross-sections. (e) GISAXS map of a film deposited under standard isotropic conditions. (f) Structure of the anisotropic QD lattice. It is distorted along the y axis with respect to the ideal BCT lattice, which is shown by light symbols. (g) and (h) Simulations of the QD ordering found by GISAXS analysis. [M. Buljan et al. J. Appl. Cryst. 46, 709 (2013)].   Figure 2. Electrical properties of the films. (a) Geometry used for the electrical conductivity measurements (above), and frequency-dependent measurements of the electrical conductivity for the perpendicular (┴) and parallel (||) geometry (below). (b) A three-dimensional model of the QD arrangement and separations. The shortest edge-to-edge distances between QDs (in nm) are indicated. (c) and (d) The most probable path of the charge carriers for the perpendicular and parallel geometries, respectively. Only two characteristic layers (A and B) are shown for clarity; the consideration is valid for the entire multilayer stack. [M. Buljan et al. J. Appl. Cryst. 46, 709 (2013)].

An anisotropic lattice of Ge quantum dots embedded in amorphous alumina was produced by magnetron sputtering deposition. A specific deposition geometry with oblique incidence of Ge and Al2O3 adparticles was used to achieve the anisotropy. The observed quantum dot ordering is explained by a combination of directional diffusion of Ge and Al2O3 adparticles and a shadowing process which occurs during deposition as a result of the specific surface morphology. The prepared material shows a strong anisotropy of the electrical conductivity in different directions parallel to the sample surface. Simple processes for the preparation of regularly ordered lattices of semiconductor quantum dots (QDs) embedded in dielectric amorphous matrices play an important role in various nanotechnology applications. Of particular interest are QD lattices with properties that differ significantly in different directions parallel to the material surface. In our past work we have analysed the preparation and ordering properties of isotropic Ge QD lattices in different amorphous matrices produced by magnetron sputtering deposition. The regularity in the QD positions was achieved by a self-assembly process during the films growth, which is based on surface morphology influence on Ge nucleation places. However, our previous depositions were all performed using a standard substrate stage which rotates during the deposition. This ensures the homogeneity of the films, but the regular ordering of QDs appears in domains, randomly rotated around the surface normal. In our most recent work we examined the ordering in ten period (Ge+Al2O3)/Al2O3 multilayer prepared also by magnetron sputtering deposition but under specific deposition geometry. We used small sputtering targets, a close distance between the substrate and targets, and a fixed substrate stage (held at a temperature of 573 K) during deposition. Under such conditions, the Ge and Al2O3 adparticles coming to the substrate from the sputtering targets have a preferential diffusion direction and a non-vanishing in-plane (tangential) component of their velocities. The growth of the Ge QDs and alumina matrix is not isotropic under such conditions due to the combination of directional diffusion of adparticles and shadowing effects caused by Ge QDs.

The result of such deposition is formation of an anisotropic lattice of Ge QDs embedded in amorphous alumina matrix. The details of the QD ordering properties in these films were investigated at the SAXS beamline of the synchrotron Elettra. Different directions of the probing x-ray beam with respect to the preferential diffusion direction of adparticles were used. Two specific cases, i.e. grazing incidence small angle x-ray scattering (GISAXS) maps taken with the x-ray beam set parallel and perpendicular (|| and ┴) to the diffusion direction are demonstrated in Fig. 1a and Fig. 1b. The different arrangement of Bragg spots in them demonstrates clearly the anisotropy in the material structure. The anisotropy occurs along the diffusion direction of the Al2O3 adparticles. The anisotropy is also nicely visible in the TEM cross-sections of the film (Fig. 1c and Fig. 1d), where the correlation direction of the QD ordering is different for the parallel and perpendicular cross-sections. The GISAXS map of the film deposited under standard, isotropic conditions and using of rotational stage (Fig. 1e) is symmetrical and looks the same for all probing x-ray directions due to the existence of randomly distributed domains. The numerical analysis of the GISAXS maps shows, that the resulting ordering in the system is a distorted body centred tetragonal (BCT) lattice, tilted toward alumina target, which is schematically illustrated in Fig. 1f and Fig. 1h. The prepared materials show also a strong anisotropy in their electrical properties. The conductivities measured in directions parallel and perpendicular to the main diffusion direction differ by an order of magnitude (Fig. 2a). Such a big difference is explained by the structural properties of the films. More precisely, the conductivity in QD-based materials is highly influenced by the hoping probability between neighbouring QDs, and it decreases exponentially with the QD separation. The QD separations for various directions, found by GISAXS analysis, are shown in Fig. 2b. Due to difference in QD separations along the paths used for conductivity measurements (Fig. 2c and Fig. 2d), the large anisotropy in the electrical transport properties occurs. Retrieve article Growth of a three‐dimensional anisotropic lattice of Ge quantum dots in an amorphous alumina matrix; M. Buljan, O. Roshchupkina, A. Šantić, V. Holý, C. Baehtz, A. Mücklich, L. Horák, V. Valeš, N. Radić, S. Bernstorff and J. Grenzer; J. Appl. Cryst. 46, 709-715 (2013). 10.1107/S0021889813008182

 

Control of lipid structuring by trans- and cis-fatty acids 

Figure 1. The effect of temperature on the structure of a system consisting of monoelaidin and oleic acid in the presence of excess water. The monoelaidin/oleic acid weight ratio was fixed. (A) The contour x ray diffraction plot displays the occurrence of different interesting structures: non-viscous and viscous liquid crystals (gel-like samples), (B) the structural analysis (the unit cell parameters as a function of temperature) are presented (H 2: inverse hexagonal phase, Fd3m: cubic inverse micellar phase, and L 2: isotropic inverse micellar phase).   Figure 2. Temperature-dependence behavior of monoelaidin at different loaded amounts of oleic (A) and elaidic acids (B) in the presence of excess water. The dashed/dotted curves indicate the approximate boundaries between the different structures.

Oleic and elaidic acids are the two most abundant fatty acids in food products and vegetable oils. Synchrotron high resolution Small and Wide Angle X ray Scattering (SAXS/WAXS) investigations were performed to study their influence on a monoglyceride membrane structure. Exploring the effect of these two fatty acids on biological membrane structures is of great interest due to the implications of their daily consumption in vital biological processes related to health and disease, and their role in designing gel-like formulations for controlling the release of drugs or functional foods. The traditional Mediterranean diet supplemented with olive oil is associated with beneficial health effects. In European countries such as Greece and Italy and in the Middle East the intake of olive oil is high and is linked in different regions to a relatively reduced blood pressure and a reduced risk of developing coronary heart disease, a reduced breast cancer, and a low level of plasma cholesterol. Various studies suggested that the consumption of olive oil, which is rich in oleic acid (monounsaturated fatty acid with the natural cis configuration), is strongly associated with positive health effects. This explains the interest in recent years in introducing to the market oleic acid-rich foods and producing synthetic oleic acid derivatives that can be used as antitumoral and antihypertensive drugs. The past decade has witnessed a tremendous interest in understanding why the consumption of oleic acid-rich diet is important to our health and wellness. In contrast to the traditional Mediterranean diet, different industrial food products in the market are rich with trans fatty acids (unsaturated fatty acids with the non-natural trans configuration), which are produced during the well-known process of partial hydrogenation of unsaturated oils in food industry, and their dietary is associated with various negative health effects including an increased risk of coronary heart disease and cancer, and increase in body weight. This explains the growing interest in introducing to the market new food products with low and even zero trans fatty acids content. A first positive trend has been set by the food industry, e.g., in the UK, where according the latest National Diet and Nutrition Survey (2011) the trans fatty acids are now only found at low levels in foods indicating that average trans fats intake was less than 2 gr. per day for all age groups, which falls below the maximum WHO recommendations.

In this study, synchrotron SAXS was applied for studying the effect of two of the most abundant fatty acids in cis (oleic acid) and trans (elaidic acid) fat dietary intake on the structure at the nanoscale level of monoglycerides, which are unique lipids displaying a rich variety of structures at different water content and temperatures. The monoglycerides are ingredients in different food products and are interesting as they form biologically relevant intermediates during fat digestion and metabolism. Noteworthy, the difference in the molecular shape of the investigated fatty acids (elaidic acid has a rod-like molecular structure; whereas oleic acid has a more cone-like shape) has a significant impact on the lipid structuring. The experimental findings show that both fatty acids lead to a variety of diverse structures when mixed with the monoglyceride monoelaidin in the presence of excess water (monoelaidin is a lipid derived from elaidic acid). The phase transition boundaries and stability can be easily controlled by the amount of the added fatty acid and by changing the temperature. Clearly, oleic acid due to its cone-like molecular structure induces stronger interfacial membrane curvature as compared to elaidic acid, and provides the membrane with a higher flexibility. Our results are in a good agreement with previous studies suggesting that oleic acid is more efficient than elaidic acid in modifying the properties of lipid membranes. For instance, it was suggested that the reduction in the blood pressure is linked to the membrane structure that can be modified by oleic acid to regulate the biological activity. Fig. 1 shows an example on the effect of oleic acid on controlling the nanostructure of monoelaidin in the presence of excess water. The effect of adjusting the loaded amounts of oleic and elaidic acids on the temperature-dependent structural behaviour of monoelaidin in the presence of excess water is illustrated in Fig. 2. It is fascinating to obtain these unique tuneable structures by rather simple means and to highlight the role of oleic and elaidic acids on model membrane structuring. Retrieve article Control of lipid structuring by trans- and cis-fatty acids; A. Yaghmur, B. Sartori and M. Rappolt, Langmuir 28, 10105 (2012). doi: 10.1021/la3019716

Highly Luminescent Metal-Organic Frameworks Through Quantum Dot Doping

The incorporation of highly luminescent core–shell quantum dots (QDs) within a metal–organic frame- work (MOF) is achieved through a one-pot method. Through appropriate surface functionalization, the QDs are solubilized within MOF-5 growth media. This permits the incorporation of the QDs within the evolving framework during the reaction. The resulting QD@MOF-5 composites are characterized using small-angle X-ray scattering/diffraction, which proofed the undistorted incorporation of the ODs into the MOF structure. Such structures showed the synergistic combination of luminescent QDs and the controlled porosity of MOF-5 in the QD@MOF-5 composites demonstrated within a prototype molecular sensor that can discriminate on the basis of molecular size. To date, the synthesis of host–guest luminescent MOFs has only been achieved by post impregnation with organic dyes or through a two-step procedure functionalizing a ceramic microparticlewith QDs and then using it as a seed to grow MOF-5.A one-step approach is considered better suited to industrial processes when compared with multistep procedures.The emission properties of QD@MOF composites have critical advantages over organic counterparts, in terms of long-term stability, quantum yield, and precise control of the emission wavelength.Maintaining the optical quality of QDs through processing into QD@MOF-5 composites is a critical challenge that must be overcome in order to utilize such MOF materials in the still unexplored fields of photocatalysis, photovoltaics, and optoelectronics. The one-pot methodology of growing QD@MOF-5 composites reported in this work has the potential to result in significant inhomogeneity of the QD distribution within the framework, as well as to cause distortion of the original MOF-5 framework crystal lattice. These features could result in undesired deterioration of both the QD optical properties and the framework microstructure. A campaign was therefore launched to screen the experimental variables that influence both the retention of optical properties and the framework crystal structure of the composites. By careful optimization of these variables the desired result has been obtained.

The quality of the red QD@MOF-5 samples was assessed by comparing synchrotron small angle X-ray scattering (SAXS) patterns taken at the Austrian SAXS beamline for the composite with a control sample of un-doped MOF-5 powder. The diffraction results confirm the crystalline order of the framework matrix surrounding the QDs. The intense and sharp diffraction peaks at 2 theta = 6.9°({200}plane, d = 12.8 Å) and 9.7°({220}plane, d = 9.1 Å) indicate long-range modular arrangement of large pores, indicative of a typical MOF-5 cubic lattice; the diffraction of the QD@MOF-5 sample is consistent with the diffraction pattern of the control powder. In addition, the relative intensity of the low-angle reflections and the low intensity of the 13.8° peak ({400}plane, d = 6.4 Å) indicate that the crystals are of high crystallinity and present very limited framework interpenetration, respectively. The analysis demonstrates that although the QD diameter (8.3 nm) is ten times bigger than the MOF-5 cavities (0.8 nm), the overall microstructural arrangement of the framework matrix has not been significantly distorted. It is important to note here that no diffraction from the embedded QDs has been detected, given their low volume concentration within the crystals. In synthesis, the structural identity of the MOF matrix has been preserved in the QD@MOF-5 sample, thus indirectly indicating that the QDs sit within the MOF-5 crystals with very little distortion of the framework microstructure. Retrieve article Highly Luminescent Metal-Organic Frameworks Through Quantum Dot Doping D. Buso, J. Jasieniak, M.D.H. Lay, P. Schiavuta, P. Scopece, J. Laird, H. Amenitsch, A.J. Hill, P. Falcaro; small 8 (1) 80-88 (2012) . 10.1002/smll.201290003

Avoiding the Raft: Losartan´s Affinity to Fluid Bilayers

Figure 1. A possible scenario for losartan plasma membrane interactions is presented in panel E. Due to the denser lipid packing in the cholesterol rich rafts, losartan is likely to be excluded from this area, and preferentially found in the more fluid plasma membrane regions. Here losartan can accumulate and finally reach the AT1 receptor site.

We examined losartan’s action in different biomimetic membrane models. Losartan is an angiotensin II receptor antagonist mainly used for the regulation of high blood pressure, and structural details on its incorporation into the biomembrane interface have been studied by small angle X-ray scattering. We found that losartan tends to avoid cholesterol-rich membrane domains. Losartan is an angiotensin II receptor antagonist mainly used for the regulation of high blood pressure. Since it was anticipated that losartan reaches the receptor site via membrane diffusion, the impact of losartan on model membranes has been investigated by small angle X-ray scattering. For this purpose 2–20 mol% losartan was incorporated into dimyristoyl-phosphatidylcholine (DMPC) and palmitoyl-oleoyl-phosphatidylcholine (POPC) bilayers and into their binary mixtures with cholesterol in the concentration range of 0 to 40 mol%. Effects of losartan on single component bilayers are alike. Partitioning of losartan into the membranes confers a negative charge to the lipid bilayers that causes the formation of unilamellar vesicles and a reduction of the bilayer thickness by 3-4%. Analysis of the structural data resulted in an estimate for the partial area of losartan, ALos ~ 40 Å2. In the presence of cholesterol, differences between the effects of losartan on POPC and DMPC are striking. Membrane condensation by cholesterol is retarded by losartan in POPC. This contrasts with DMPC, where an increase of the cholesterol content shifts the partitioning equilibrium of losartan towards the aqueous phase, such that losartan gets depleted from the bilayers from 20 mol% cholesterol onwards.

This indicates (i) a chain-saturation dependent competition of losartan with lipid-cholesterol interactions, and (ii) the insolubility of losartan in the liquid ordered phase of PCs. Consequently, losartan’s action is more likely to take place in fluid plasma membrane patches rather than in domains rich in cholesterol and saturated lipid species such as in membrane rafts.   Figure 2.  Structural results overview: Schematic illustration of POPC (A) and DMPC (B) bilayer structure alterations induced by losartan. In both cases the up-take of losartan leads to membrane unbinding (losartan concentration xLos = 0.2). At very high cholesterol concentrations losartan still finds shelter in the POPC-Chol bilayer (C), whereas it gets expelled from DMPC–Chol membranes (D) (xChol = 0.4). Retrieve article Losartan's affinity to fluid bilayers modulates lipid-cholesterol interactions; A. Hodzic, P. Zoumpoulakis, G. Pabst, T. Mavromoustakos, M. Rappolt; Phys. Chem. Chem. Phys. 14, 4780 (2012); 10.1039/c2cp40134g

A new mechanism for mesostructure formation of ordered mesoporous carbons (OMCs) was investigated in situ

Figure 1.  A) A typical SAXS pattern for the circular hexagonal structure in an AAM host, indexed in the circular hexagonal (p6mm) unit cell (B) with a lattice spacing of 15 nm. The squares show the reflections from the mesophases in the AAM pores while the circles show reflections from a top layer on the membrane. Reprinted with permission from Schuster et al. J. Am. Chem. Soc. 134, 11136 (2012).

A new mechanism for mesostructure formation of ordered mesoporous carbons (OMCs) was investigated at the Austrian SAXS beamline at Elettra with in situ small-angle x ray scattering (SAXS) measurements: thermally induced selfassembly. Unlike the well-established evaporation-induced selfassembly (EISA), the structure formation for organic−organic self-assembly of an oligomeric resol precursor and the blockcopolymer templates Pluronic P123 and F127 does not occur during evaporation but only by following a thermopolymerization step at temperatures above 100 °C. Ordered mesoporous carbon in bulk or powder form is commonly synthesized either by hard templating, where periodic mesoporous silica is filled with carbon precursors followed by carbonization and removal of the silica, or by soft templating, using the self-assembly of soluble carbon precursors with liquid crystalline phases of surfactants acting as soft templates. The examples for mesoporous carbon thin films or phases still embedded in alumina membrane (AAM) hosts are limited to soft-templating methods. Hardtemplating methods for ordered mesoporous carbon based on porous silica templates have not yet been implemented for these morphologies, which is mainly attributed to weak adhesion of the resulting carbon material to the substrate after etching of the silica template. While the final carbon structure obtained via hard templating is controlled by the solid template, the final structures made by soft templating are much more sensitive to experimental conditions, such as concentrations, temperature or humidity during structure formation. Therefore, the understanding and control of structure formation processes with soft-templating methods concerning mesostructural symmetry, morphology, and orientation of the desired  mesoporous carbon phases are essential, especially for syntheses in confined environments. In situ grazing incidence small-angle x ray scattering (GISAXS) for characterization of thin films and in situ small-angle x ray scattering (SAXS) of AAM/OMC composites are powerful tools to investigate structural changes during all steps of structure formation and processing. The self-assembly mechanisms for OMC materials made by softtemplating have not yet been investigated in detail. For OMC systems, mainly for the popular resol-Pluronic system, the structure formation was mostly described as an EISA process similar to the one found for mesostructured metal oxides (e.g. silica or titania) followed by a thermopolymerization step to cross-link the precursor oligomers. In our study, different OMC phases (2D-hexagonal and orthorhombic as thin films and cubic and circular hexagonal in AAM hosts) were obtained by organic−organic selfassembly of a preformed oligomeric resol precursor and the triblock copolymer templates Pluronic P123 and F127, respectively.

The thermopolymerization step was investigated in detail with in situ grazing incidence small-angle x ray scattering (GISAXS, for films) and in situ SAXS (for AAMs). A typical SAXS pattern and a scheme of the corresponding unit cell for the circular hexagonal structure in an AAM are presented in Fig. 1. The processes in the thermally induced structure formation for this sample are illustrated in Fig. 2. After heating for 15 min at 130 °C, the first reflections related to a circular hexagonal structure start to appear. A diffuse ring attributed to worm-like phases is also visible, thus some parts are oriented randomly, while others already show the final orientation. Upon further heating the intensity of the reflection spots increases, and the structure becomes completely circular hexagonal. This shows that unlike in the case of mesostructured metal oxides, the structure formation in these systems does not occur during evaporation of the solvent but during the thermopolymerization step and should, therefore, rather be called thermally induced self-assembly. As a remarkable consequence, the mesostructure is not fixed but still flexible and can be controlled during this step. Moreover, we find that higher thermopolymerization temperatures result in increased unit cell parameters, caused by swelling of the liquid crystal structures of the block copolymer templates. The new mechanism discovered here offers additional opportunities for mesostructure control. We have demonstrated the influence of different temperatures during this thermally induced self-assembly on the final mesostructure, and we suppose that the change of other synthesis parameters, such as the vapor atmosphere, will also show significant effects and should thus be subject of further studies. Figure 2.  In situ SAXS of the structure formation during thermopolymerization at 130 °C in AAMs. After heating for 15 min at 130 °C, the first reflections related to a circular hexagonal structure start to appear (indicated with squares), which then increase in intensity until the structure becomes completely circular hexagonal. Reprinted with permission from Schuster et al. J. Am. Chem. Soc. 134, 11136 (2012). Acknowledgement The research leading to these results has received funding from the European Community’s Seventh Framework Programme (FP7/2007-2013) under grant agreement n° [226716] (ELISA). Retrieve article In Situ SAXS Study on a New Mechanism for Mesostructure Formation of Ordered Mesoporous Carbons: Thermally Induced Self-Assembly; J. Schuster, R. Köhn, M. Döblinger, A. Keilbach, H. Amenitsch and T. Bein; J. Am. Chem. Soc. 134, 11136 (2012). 10.1021/10.1021/ja208941s Hot Templating (Editors' Choice); P. Szuromi; Science 337, 270 (2012). 10.1126/science.337.6092.270-d

How to prepare sphere- and rod-like ZnO particles?

Figure 1. "nonclassical growth mechanism” concept for the self-assembling mechanism.

We propose a growth mechanism that follows a “nonclassical crystallization” concept for the self-assembling mechanism of approximately 10-nmsized building units into peanut-like ZnO and/or microsphere-like hydrozincite particles. Zinc oxide (ZnO) is a very interesting inorganic material because of its specific chemical, surface and microstructural characteristics. ZnO particles can be grown in many different nano- and micro-scale forms. The morphological diversity influences the particles’ properties, thus suitable particles can be used for various novel applications. On the other hand, the formation mechanism and the understanding of the crucial parameters concerning the control of the particles’ growth and morphology are still a great challenge for an increasing number of researchers. The primary hindrance to such investigations has been the absence of appropriate techniques to probe the in-situ growth of nanocrystals. Furthermore, we were surprised to find no investigation where the combination of an insitu technique and an ex-situ technique was used to study the growth mechanism, starting with the first building units up to final micro-sized particles. Finally, from a fundamental chemical viewpoint, we still lack an understanding of ZnO particle growth, starting from molecular precursors to complexation reactions of the precipitation in many cases. The above-mentioned challenges were a great motivation in our research. We have undertaken an in-depth study of several of these long-held notions using a combination of time-resolved experiments

involving the SAXS beamline at the synchrotron Elettra and ex-situ electron-microscopy (TEM and FE-SEM) techniques. The particles were prepared by the precipitation of zinc nitrate with urea. Depending on the reaction conditions, ZnO, hydrozincite, or a mixture of both phases was detected in our system. The condensation and complexation reactions led to the formation of nanoparticle building units up to a size of 10 nm. Afterwards, the nanoparticles immediately self-assembled into micro-sized particles. The molecular precursors and complexation reactions of the formation process were numerically predicted in the frame of the partial-charge model (PCM). We proposed a growth mechanism that follows a “nonclassical crystallization” concept for the self-assembling mechanism of approximately 10-nm sized building units into peanut-like ZnO and/or microsphere-like hydrozincite particles as presented in Figure 1. The influence of the reaction conditions on the particles’ formation kinetics and the phase composition were also defined. Retrieve article The growth mechanism of zinc oxide and hydrozincite: a study using electron microscopies and in situ SAXS; M. Bitenc, P. Podbršček, P. Dubček, S. Bernstorff, G. Dražić, B. Orel and Z. Crnjak Orel; Cryst.Eng.Comm. 14, 3080 (2012). 10.1039/C2CE06134A

Structural Characterization of MOF-5 crystals allowing for dynamic positioning in a magnetic field

The cobalt framework composite obtained responds efficiently to magnetic stimuli. A luminescent functionality is added, showing that multifunctional MOF devices can be prepared. This new generation of adaptive material is tested as a position-controlled molecular sensor. Metal organic frameworks (MOF) are very promising ultra-porous materials for a variety of significant applications, such as sensing, detecting, gas storage and separation catalysis and drug delivery. The high surface area in the thousands of square meter per gram, and the controlled pore size and pore size distribution of MOFs are relevant features for the fabrication of devices that rely on highly controlled transport properties materials. In addition, recent investigations describe MOFs as adaptive materials because they respond to a variety of stimuli (e.g. molecular and environmental). Despite the interesting intrinsic properties of these ultra-porous materials, different strategies are currently under investigation to achieve spatial control of MOF position on a variety of substrates.

The different proposed routes involve controlled crystal sizes for subsequent growth, surface functionalization, soft lithography and seeding approaches through heterogeneous nucleation. This latter approach has been shown to allow exogenous functionality to be coupled with the properties intrinsic to MOFs. In the present study we utilise functionality gained through synthesis of MOF with magnetic nanoparticles in order to control the location of MOF crystal growth and to dynamically position 3-D MOF forms for use in reconfigurable engineering devices. X-ray diffraction patterns taken at the Austrian SAXS beamline revealed the MOF-5 structure for the composite material. Retrieve article Dynamic Control of MOF-5 Crystal Positioning Using a Magnetic Field; P. Falcaro, F. Normandin, M. Takahashi, P. Scopece, H. Amenitsch, S. Costacurta, C.M. Doherty, J.S. Laird, M.D.H. Lay, F. Lisi, J.A. Hill, D. Buso; Advanced Materials 23 (34) 3901-3906 (2011). 10.1002/adma.201101233

Evolution of the protein corona of lipid gene vectors as a function of plasma concentration

Figure 2. Top panels: 1D SDS-PAGE gel of human plasma proteins obtained from (a) DOTAP cationic liposome-protein complexes and (b) DOTAP/DNA lipoplex-protein complexes following incubation at different plasma concentrations. Bottom panels: schematic presentation of the evolution of the protein corona that forms around both CLs (c) and lipoplexes (d) upon exposure to plasma. Passing from low to high plasma concentrations the protein corona of CLs is made of both low-affinity and competitive-binding proteins whose relative abundance changes (c) while the protein corona of lipoplexes changes in abundance but not in composition (d). Adapted with permission from (Langmuir, 2011, 27 (24), pp 15048–15053). Copyright (2011) American Chemical Society.

The effective unit of interest in cell-nanomaterial interactions is not the nanoparticle itself but the particle and its hard corona of associated proteins from plasma or other bodily fluids. Here we investigate the compositional evolution of the protein corona of 1,2-dioleoyl-3-trimethylammonium propane (DOTAP) cationic liposomes (CLs) and DOTAP/DNA lipoplexes over a wide range of plasma concentrations (2.5%-80%). The composition of the hard corona of lipoplexes is quite stable, but that of CLs does evolve considerably. As a consequence, the biological identity of lipid gene vectors whose surfaces are entirely lipidic may change dramatically as the amount of protein in the environment changes.         Recently, it has been shown that the effective unit of interest in the cell-nanomaterial interaction is not the nanoparticle itself but the particle and its hard corona of associated proteins from plasma or other bodily fluids. This corona of proteins at the surface of the particle is sufficiently long-lived that actually the entity is “seen” and processed by living cells. This is a key issue that has broad implications for in vitro – in vivo extrapolations and will determine the future road map of nanomedicine and perhaps impact the overall field of nanoscience. Here we investigate the compositional evolution of the protein corona of 1,2-dioleoyl-3-trimethylammonium propane (DOTAP) cationic liposomes (CLs) and DOTAP/DNA lipoplexes as a function of increasing plasma concentration. According to recent findings, CLs are excellent model systems of lipid nanoparticles (LNP) in which a DNA/polycation core is coated with a lipid envelope. This study allowed us to elucidate more quantitatively the degree to which the protein corona of lipid gene vectors can change, depending on the biological environment. To better investigate the protein corona-nanoparticle complex, we performed a preliminary physicalchemical characterization of both DOTAP CLs and DOTAP/DNA lipoplexes. SAXS data collected at the SAXS beamline at Elettra are reported in Fig. 1: Fig. 1a shows the SAXS pattern of DOTAP CLs characteristic of uncorrelated bilayers (e.g., unilamellar vesicles) and Fig. 1b shows the SAXS pattern of DOTAP/DNA lipoplexes characteristic of a multilamellar structure. Combined sizing, ζ-potential, and SAXS data well support the accepted model of the DNA-induced restructuring of CLs upon lipoplex formation. Taken together, these results confirm the recent suggestion that lipoplexes are hybrid structures with the lipid surface partially decorated by negatively charged DNA chains.

To focus on the evolution of the protein corona that forms around both CLs and lipoplexes upon exposure to plasma, one-dimensional (1D) Sodium Dodecyl Sulphate/PolyAcrylamide Gel Electrophoresis (SDS/PAGE) experiments were performed. Fig. 2 shows 1D SDS/PAGE gel results in which DOTAP CLs and DOTAP/ DNA lipoplexes were incubated in plasma, over a wide range of plasma concentrations (2.5%-80%). With increasing plasma concentration, the protein pattern for DOTAP CLs changes considerably (Fig. 2a), whereas for lipoplexes the intensity of the protein bands seem to increase monotonously with increasing plasma concentration (Fig. 2b). The identities of the proteins were determined by mass spectrometry analysis of selected bands cut from the gels reported in Figure 2a,b. We observed that the protein corona of CLs is made of both low-affinity and competitivebinding proteins whose relative abundance changes with the plasma concentration (Fig. 2c). On the other side, passing from low to high plasma concentrations, the protein corona of lipoplexes changes in abundance but not in composition (Fig. 2d). Such effects may be so striking that the biological identity of lipid gene vectors with DNA cargo confined in the interior space (e.g., LNP) may change dramatically as the amount of protein in the environment changes. Therefore, the evolution of the protein corona passing from in vitro to in vivo conditions is severely affected by the presence of DNA. This aspect should be carefully considered for the rational design of lipid gene vectors. Synchrotron SAXS experiments have revealed the structure of DOTAP CLs and DOTAP/DNA lipoplexes on the nanoscale. Such a nanostructure would have a deep impact on the adsorption of plasma proteins, which is to a large extent an electrostatically driven phenomenon.     Figure 1.  (a) SAXS pattern of DOTAP cationic liposomes. (b) SAXS pattern of DOTAP/DNA lipoplexes. Bragg peaks arise from the multilayered lipid membrane/DNA structure. The black arrow indicates the DNA peak arising from the 1D DNA-DNA in-plane correlation. Adapted with permission from (Langmuir, 2011, 27 (24), pp 15048–15053). Copyright (2011) American Chemical Society. Retrieve article Evolution of the Protein Corona of Lipid Gene Vectors as a Function of Plasma Concentration; G. Caracciolo, D. Pozzi, A.L. Capriotti, C. Cavaliere, P. Foglia, H. Amenitsch and A. Laganà; Langmuir 27, 15048 (2011). 10.1021/la202912f

Bottom-up approach towards titanosilicate mesoporous pillared planar nanochannels for nanofluidic

Nanofluidics transport in lab-on-chip devices requires nanochannels that are difficult to fabricate since they require challenging top-down technological approaches. We present a bottom-up, scalable, low-cost and robust alternative to construct large areas of mesofluidic Pillared Planar Nanochannels (PPNs). Microscopy images displayed in Fig. 1 are representative of the whole system, and show the typical PPN layers made of dense and mesoporous materials organized into vertical nanopillars supporting a continuous roof. Such conformation is homogeneously present all over the coated substrate. They have been obtained upon combining self-assembly of block copolymer, nanostructured sol-gel coatings and highly controlled liquid deposition processing. The mesoporous structure contains two types of porosity. The first one is the fully open and accessible inter-pillar porosity with characteristic dimensions adjustable between 20 and 400 nm, which is necessary to allow facile diffusion despite the double layer effect. The second one is present inside the pillars and the roof, and is composed of less than 10 nm pores, which aim at increasing the surface area. Both layer structures have been characterised by Grazing Incidence Small Angle X-ray Scattering. Pillar ordering can be assessed by GISAXS analysis as shown in Fig. 2 for F127-templated titanosilicate systems. The intense Bragg diffraction signal at qy = 0.14 nm-1, together with its first harmonic at qy = 0.28 nm-1, is associated to both (01)p and (02)p diffractions of the highly ordered 2D lateral hexagonal structure formed by the pillar array with a corresponding inter-pillar distance of 51 nm (d(01) = 44 nm). The latter dimensions can be found on TEM and SEM images whatever the type of inorganic material used for impregnation. The fact that the (01)p diffraction has a vertical tail (diffraction rod) extending only in the z direction, suggests a high degree of vertical alignment of the pillars. For the F127-templated titanosilicate layers, the GISAXS diagram (Fig. 2) exhibits, together with the previous (01)p diffractions of the pillars at low q, the characteristic (110)m, (101)m and (1-10)m diffraction points of the Im3m mesostructure, revealing that the nanoporous network is composed of a contracted body centre cubic arrangement of pores with extended domains having the [110] direction normal to the surface.

The lateral periodicity is found to be 14.5 nm, which corresponds to the structure obtained for plain continuous films, for which pores sizes were deduced from ellipsometry porosimetry to be around 6 nm. The high order structure is clearly observed in SEM and TEM, see the corresponding images in Fig.s 1c)-f). Interestingly, the SEM image c) reveals that the pillars do not have the same texture as the roof. The TEM image of Fig.s 1f, 1g, shows that the pillars are actually hollow cylinder (tubes) below the roof. These Pillared Planar Nanochannel showed the ability to vehicle fluids in the inter-pillars porosity through natural capillary forces, for which the classical Washburn model of diffusion is verified. In addiction PPNs are compatible with lithography techniques, such as deep X-ray lithography, for the production of complex designs and thus demonstrating to be ideal candidates for micro/nano fluidic applications. Figure 1.  Pillared Planar Nanochannels layers with various chemical compositions, morphologies, and pillar and roof inner structures. a) and b) are SEM (profile cut) and TEM (top view) images respectively of a dense TiO2 PPN. Images c) to f) were taken from a similar system for which the material is composed of mesoporous 10%SiO2-90%TiO2. SEM images c) to e) display profile cuts, where one can easily observe the pillars supporting a roof of different thicknesses (obtained with various sol-gel deposition condition), but all bearing the same ordered Im3m mesoporosity induced by the F127.  TEM images f) and g) are top views of the layer and reveal a clear emptiness of the pillars. The latter are organised into hexagonal organisation. Scale bars = 50 nm for a), b), and f); 100 nm for c), d), and e); and 5 nm for g). Figure 2. GISAXS pattern of Im3m mesoporous (F127 templated) 10%SiO2-90%TiO2 mesofluidic Pillared Planar Nanochannels. Retrieve article Bottom-up approach toward titanosilicate mesoporous pillared planar nanochannels for nanofluidic applications; M. Faustini, M. Vayer, B. Marmiroli, M. Hillmyer, C. Sinturel, H. Amenitsch and D. Grosso; Chem. Mater. 22, 5687 (2010). 10.1021/cm101502n

Growth of spatially ordered Ge nanoclusters in an amorphous matrix on rippled substrates

A rippled substrate highly influences the arrangement of Ge quantum dots in a matrix; it ensures a substantial improvement in the regularity of their ordering and a narrowing of their size-distribution. The grazing incidence small angle x-ray scattering (GISAXS) technique was shown to be amazingly efficient in the determination of the above mentioned structural properties and the characterization of the self-assembly process. Recently we have developed a method for the production of self-assembled Ge QDs in amorphous silica matrix. The method is based on diffusion-induced nucleation combined with the effect of surface morphology. Ge QDs were created by the deposition of (Ge+SiO2)/SiO2 multilayer films on a flat Si(111) substrate at an elevated temperature. The benefits of the method are a simple and efficient production of the material, a relatively narrow size distribution as well as a rather regular arrangement of the dots in the matrix. The main problem with the produced material is that the regular ordering appeared in small domains randomly rotated with respect to the normal to the film surface. Our latest investigation however solved this problem and resulted in a further improvement of the material properties. Instead of flat substrates, we have now used periodically corrugated-rippled substrates. These substrate surfaces exhibit a very small periodicity (10-20 nm) and they can be easily produced on large surface areas by ion erosion.

Our GISAXS measurements showed that the QDs appear in the valleys between theripples, so they follow the substrate morphology. The regularity of the quantum dots positions is nicely visible in the GISAXS maps of the films. The GISAXS map of the rippled substrate prior to deposition shows only two lateral streaks stemming from the periodicity of the ripples. After the deposition of a 5 bi-layer film a beautiful GISAXS intensity distribution is obtained showing that the arrangement of the formed QDs follows the morphology of the rippled substrate. Increasing the number of deposited layers to ten, the lateral features in the GISAXS images become slightly broader, showing a small increase in the QD position disorder with increasing number of layers. Retrieve article Growth of spatially ordered Ge nanoclusters in an amorphous matrix on rippled substrates; M. Buljan, J. Grenzer, A. Keller, N. Radić, V. Valeš, S. Bernstorff, T. Cornelius, H.T. Metzger, and V. Holý, Physical Review B, Vol. 82 (12), pp. 125316 (2010).. doi: 10.1103/PhysRevB.82.125316

Wake Me Up! Anaesthesia, a Membrane Mediated Loss of Sensation?

Most probably everyone reading this article has already encountered several times in his life one of the most important, if not “the” most important drugs of human mankind: Anaesthetics. Without them modern surgery would just not be possible, and even simple medical interventions as those carried out by your dentist would become unbearable. Yet, despite their daily successful application for more than 160 years, we do not know how these drugs act on the molecular level. The quest for molecular targets boils down to two possibilities. Either anaesthetics directly act on ion channels of the central nervous system, or they change the biophysical properties of nerve membranes such that neurotransmission signals are affected. Arguments can be found in favour of one or the other target and the controversy on this issue is almost as old as the clinical use of anaesthetics. We set out to study a mechanism that in a way integrates both views. Membrane proteins such as ion channels are under a constant field of lateral pressure, which is caused by the collective properties of the lipid membrane. When they open up and act as passive channels for the exchange of ions, they have to do this against the lateral pressure field, which costs some work. The idea is simple, yet very powerful. If an anaesthetic drug inserts into the membrane it will change its lateral pressure field, such that the work that the protein needs to perform for opening the channel is different. Thus, it couples via membrane properties mechanically to the opening probability of ion channels. It is very difficult to determine the lateral pressure field experimentally. However, it is possible to measure its integral parameters, such as the membrane thickness, lateral area per lipid or bending rigidity. Because effects can be expected to be small, in particular at clinically relevant drug concentration, ultra-high structural resolution as provided by the SAXS beamline at Elettra is of need.

Nerve membranes were mimicked by vesicles composed of the phospholipid palmitoyl oleoyl phosphatidylcholine (POPC) to which well defined amounts of ketamine was added. Diffraction patterns were analysed in terms of a full q-range model both in terms of the membrane thickness and area per lipid. Interestingly, neither of these parameters showed significant changes in the concentration range of 0 – 8 mol% ketamine. Therefore, we performed molecular dynamics (MD) simulations in order to trace the effects to molecular details. Moreover, MD simulations allow to derive lateral pressure fields in membranes. The MD simulations confirmed our experimental observations. However, at the same time we found significant changes to the pressure field (Fig. 1). In particular, the results demonstrated that ketamine locates preferentially close to the lipid/water interface and shifts the lateral pressure field toward the centre of the membrane. These results allowed us to calculate the consequent inhibition of ion channels applying simple protein models (Fig. 2). The half value (IC50), where 50% of the ion channels are inhibited was found to be either at 2 mol% or 18 mol%, respectively, depending on the protein geometry. It can be estimated that this corresponds to ketamine concentrations of 0.08 µM and 0.9 µM, respectively, in the blood. These values compare favourably with concentrations of ketamine in clinical applications. Results strongly encourage a further exploitation of membrane mediated effects of anaesthetics, which we think may lead to novel ways for designing membrane active anaesthetic drugs. Retrieve article Membrane-mediated effect on ion channels induced by the anesthetic drug ketamine; H. Jerabek, G. Pabst, M. Rappolt, and T. Stockner; J. Am. Chem. Soc. 132, 7990 (2010). 10.1021/ja910843d

Last Updated on Tuesday, 14 May 2019 17:05