Highlights

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.

Retrieve article

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