Seeing a buried interface with elemental specificity

Surfaces and interfaces are of fundamental importance in a wide variety of scientific and technological contexts, ranging from biology and life processes, to geological phenomena such as cloud formation and mineral chemistry, and modern electronics.  In the case of examples like water interacting with the sand on a beach, the process is particularly difficult to study because the interface isn’t just between air and the water, but rather between two different structures of condensed matter – in this case, the water and the sand. These processes are often extremely sensitive to conditions, so understanding the conditions in detail is hugely important.  Unfortunately, while the interface is important, it makes up just a small fraction of a material, so scientists have to devise methods to enhance the relative contribution of the interface to their measurements so they don’t just detect the rest of the material.
At present, one of the most common way to study buried interfaces is to use a technique called optical second harmonic generation.  This technique is only sensitive to materials when there is a break in symmetry, usually just at a surface or an interface.  While this technique is powerful, it lacks the elemental specificity and sensitivity associated with x-ray spectroscopy. Thus, a technique which can probe buried interfaces with the advantages of x-ray spectroscopy would be a welcome addition.
At the FERMI FEL, beamline EIS-TIMEX, a novel approach combining FEL radiation to perform second harmonic generation on a buried interface has been demonstrated.  The experiment could only be performed at FERMI because of the high coherence of the source.  The sample studied consisted of boron layered on top of a polymer, creating a simple example of an organic-inorganic junction and was directly compared to a boron-vacuum interface.  These are shown in Figure 1.  The photon energy of the FERMI FEL was varied throughout the experiment and the signal was measured at the B K-edge (~185 eV), providing a probe of the electronic structure of boron at the interface of a polymer as well as a boron-vacuum interface, depending on the sample.
 

Figure 1.  Schematic of the experiment for generating second harmonic generation from (a) boron-vacuum and (b) boron-Parylene-N, with Parylene-N being a polymer. 

This method allowed the difference between the boron and polymer interface as compared to the boron-vacuum interface. As compared to the traditional x-ray absorption method, x-ray second harmonic generation showed a large difference between the two samples (Figure 2).  Based on extensive calculations, it was determined that the distance between the boron and the neighboring polymer to be approximately 1.9 Ångstroms (10^-10 meters). This is information that is not easily accessible by any other technique due to the disordered nature of the interface.
The simple boron and polymer sample serves as a proof of concept of what can be studied with this technique. The experiment opens the possibility of studying all sorts of technologically and naturally importance interfaces including computer chips, solar cells, and catalysts.  All sorts of interfaces can now be understood in a way that previously wasn’t possible!

Figure 2.  Second harmonic generation spectra of the boron-vacuum and boron–Parylene-N interfaces. The SHG spectrum of the boron-vacuum (dark red triangle) and boron-Parylene-N (dark blue circle) interfaces, shown along with the linear x-ray absorption of the boron film (light red) and boron Parylene N multilayer film (light blue). The arrows indicate the appropriate y-axis. The differences in the x-ray absorption spectra are believed to be due to the differences in the background and attempts to correct for it.

 

This research was conducted by the following research team:

Craig P. Schwartz¹, Sumana L. Raj², Sasawat Jamnuch³, Chris J. Hull^1,2, Paolo Miotti^4,5, Royce K. Lam^1,2, Dennis Nordlund⁶, Can B. Uzundal², Chaitanya Das Pemmaraju⁷, Riccardo Mincigrucci⁸, Laura Foglia⁸, Alberto Simoncig⁸, Marcello Coreno^9,10, Claudio Masciovecchio⁸, Luca Giannessi^8,10, Luca Poletto⁴, Emiliano Principi⁸, Michael Zuerch^2,11,12,13, Tod A. Pascal^3,14,15, Walter S. Drisdell^1,16, and Richard J. Saykally^1,2

¹ Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, USA
² Department of Chemistry, University of California, Berkeley, California, USA
³ ATLAS Materials Science Laboratory, Department of NanoEngineering and Chemical Engineering, University of California, San Diego, La Jolla, California, USA
⁴ Institute of Photonics and Nanotechnologies, National Research Council of Italy, Padova, Italy
⁵ Department of Information Engineering, University of Padova, Padova, Italy
⁶ Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, California, USA
⁷ Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, Menlo Park, California, USA
⁸ Elettra-Sincrotrone Trieste S.C.p.A., Trieste, Italy
⁹ ISM-CNR, Istituto di Struttura della Materia, LD2 Unit, Trieste, Italy
¹⁰ Istituto Nazionale di Fisica Nucleare, Laboratori Nazionali di Frascati, Frascati, Italy
¹¹ Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, USA
¹² Institute for Optics and Quantum Electronics, Abbe Center of Photonics, University of Jena, Jena, Germany
¹³ Fritz Haber Institute of the Max Planck Society, Berlin, Germany
¹⁴ Materials Science and Engineering, University of California San Diego, La Jolla, California, USA
¹⁵ Sustainable Power and Energy Center, University of California San Diego, La Jolla, California, USA
¹⁶ Joint Center for Artificial Photosynthesis, Lawrence Berkeley National Laboratory, Berkeley, California, USA

Contact persons:

Richard Saykally, email: