Dr. Mallik

Surface and Low Temperature Physics
Condensed Matter
Dr. Mallik

Certain polymers, and other smaller molecules, chemisorb on metal oxides via acid-base reactions to form covalent bridges or resonant bonds. It is important to understand the surface physics and chemistry of such systems from a fundamental standpoint, but also because they have important technological applications in aircraft construction, adhesion, corrosion, lubrication, and catalysis. We have used Multiple Reflection Absorption Infrared Spectroscopy (MRAIRS) and Inelastic Electron Tunneling Spectroscopy (IETS) to record spectra of quasi-monolayer films of molecules as large as polymers adsorbed on metal oxides to reveal information on the molecules? adsorbed geometrical configurations. Interpretation of the adsorption mechanisms for these systems depends on the ability to detect carbonyl, phosphoryl, phosphonyl, sulphonyl and other vibrational modes of surface-adsorbed species. Our results for carbonyl containing molecules show that in addition to stoichiometric and steric differences between the various adsorbates, the sample fabrication processes involved also affect the intensity of the associated spectral lines. Comparison of MRAIRS and IETS spectra in the past has largely ignored these constraints limiting the effectiveness of these techniques. Our work suggests that these two techniques, when used in tandem, can provide valuable information for a wider range of systems of adsorbates.

Surface coatings on glassy materials:
Common silica glass, SiO2, is one of the most widely used materials in the world. Various types of glass, either pure silica or those with metal or other inorganic ions incorporated in their amorphous structure, have had many uses over the centuries. Currently, silica glasses are used primarily for the production of various optical elements such as lenses, mirrors, plates, and diffraction gratings and printed circuit boards. Those doped with metal and/or inorganic ions are used in a diverse range of applications. These include, amongst many other things, the manufacture of optical filters, storage vessels, and fiber optics, the production of steel, and in the nuclear power industry for radiation screening. In many instances surface coatings are applied to the glasses for protection against erosion, or for optical filtering. We have fabricated ultra-thin (~ 1-2 nm) sputtered amorphous films of SiO2 and GeO2, characterized their topography and that of the underlying substrates using Scanning Tunneling Microscopy (STM) and Atomic Force Microscopy (AFM), and recorded their vibrational spectra with and without various adsorbates of commercial significance such as silane coupling agents. Our results reveal the nature of the bonding mechanism at the glass/coating interface, which cannot be obtained easily by other means. Currently we are studying different types of glasses.

Surface states of amorphous materials:
Over approximately the last two decades much work has been done to investigate the optical end electronic properties of various semiconducting materials as candidates for pholtovoltaic applications (in particular solar cells); they represent a renewable energy source, which has few detrimental effects on the environment. Currently, about 95% of commercially available photovoltaic cells are made from crystalline silicon wafers, similar to those in the computer chip industry. The remainder is primarily amorphous thin-film semiconductor materials such as rare-earth doped silicate and fluoride glasses, the so-called III-V materials (e.g., GaAs) and most recently the promising II-VI class of materials (e.g., CdTe, and CdSe). Semiconducting materials such as these are much cheaper to produce but, at present, their efficiency (at best approximately 7%) is vastly inferior to crystalline materials (as high as 18%). The efficiency is lower mainly because charge carriers or other impurities are absorbed at defect sites such as dangling bonds and unwanted impurities thus reducing the magnitude of the photocurrent.

Commercially viable manufacture of thin-film amorphous photovoltaic devices relies primarily on Chemical Vapor Deposition (CVD), Molecular Beam Epitaxy (MBE), and sputtering; the goal is to increase device efficiency by minimizing the number of defects created during the fabrication process. In order to do this, the various types of defects and their sources must be identified. Several workers have used Photoluminescence Spectroscopy (PLS), Raman Spectroscopy, Infrared Spectroscopy (IR) and other related techniques to study thin films of photovoltaic materials. These spectroscopies can record the energies and intensities of phonons, excitons and vibrational modes of the films. Many studies have been done to characterize the electronic and optical properties of amorphous hydrogenated silicon (a-Si:H), polycrystalline silicon, and their oxides. The chemical structure and composition has also been studied by a variety of spectroscopic techniques including Fourier Transform Infrared Spectroscopy (FTIR) infrared ellipsometry, neutron scattering, X-ray Photoelectron Spectroscopy (XPS), and Auger Electron Spectroscopy (AES). Our work focuses on our experience in the fabrication of ultra-thin (~ 1-2 nm) sputtered amorphous films of Si, Ge, and their oxides with and without various adsorbates. We have published their vibrational spectra using IETS and are now working on III-V and II-VI materials where we plan to identify adsorption mechanisms for surface coatings.