A Study of Charge Transport Phenomena and Nanoscale Investigation of the Modified CdS Surface

by Dolog, Ivan

The adsorption of 7-ethynyl-2,4,9-trithia-tricyclo[3.3.1.13,7]decane (7ETTD), polyaniline (PANI), triethoxysilane (TES), and poly(methyl methacrylate) (PMMA) on ultrathin sputtered amorphous CdS films has been investigated using inelastic electron tunneling spectroscopy (IETS), in conjunction with multiple reflection adsorption IR spectroscopy. Conductance“voltage data are recorded for tunnel junctions of the type Al/CdS/7ETTD/Pb over a temperature range of 4K to room temperature and they indicate that the presence of the adsorbed 7ETTD layer on the CdS dramatically modifies the conductance “ voltage behavior of the junctions. These measurements show that different conduction mechanisms, including tunneling and possibly hopping, are responsible for charge transfer through the junctions depending on current, temperature, and voltage. WKB fits to the data are used to determine tunnel barrier parameters (height and width) for Al/CdS/Pb junctions with and without adsorbed 7ETTD layers on the CdS. Analysis of the fits shows that tunneling occurs at low bias (less than ~0.2 V) but, at higher bias voltages, modification of the barrier parameters alone is insufficient to account for the observed conductance changes. A frontier orbital model is invoked which does offer a plausible explanation for these conductance changes. The model assumes bias-dependent coupling between HOMO and LUMO states of the adsorbed 7ETTD and surface states on the CdS. The present work suggests that, because of the marked effect on the conductance iv of CdS ultra-thin films, 7ETTD and other similar compounds may be candidates for use in molecular electronic device fabrication. It was found that PANI and TES do not adsorb strongly on amorphous CdS but can be used to modify conductance-voltage behavior. On the other hand, PMMA adsorbs strongly and shows promise as a material for use as a host matrix, in photovoltaic applications. Preliminary work is presented in which CdS nanoparticles size is estimated using surface enhanced Raman spectroscopy and Atomic Force Microscopy, and it has been determined that IETS and conductance-voltage measurement could be extended for the investigation of he CdS nanoparticles electronic properties.Further investigations into the surface properties of CdS are presented. Specifically, a robust technique, based on vertical, “z-lift”, manipulation of a negatively biased oscillating atomic force microscope tip, is extended to CdS and used to create raised columnar nanostructures with high aspect ratios (up to 40 nm high/150 nm wide) on amorphous CdS thin films. The nanostructures™ height 8“40 nm can be controlled and correlates with CdS film thickness. An in-house modified electric force microscope is used to record the associated surface charge distribution in the proximity of the nanostructures which is found to be opposite to that of the tip.