Adsorption of 7-ethynyl-2,4,9-trithia-tricyclo[3.3.1.13,7]decane on ultra-thin CdS films

The adsorption mechanism for the new compound, 7-ethynyl-2,4,9-trithia-tricyclo[3.3.1.13,7]decane (7ETTD), on ultra-thin films (∼3 nm) of CdS is investigated. Multiple reflection absorption IR spectroscopy, in conjunction with inelastic electron tunneling spectroscopy, indicates that this compound forms a self-assembled monolayer adsorbed on the CdS surface via each molecule's trithia-adamantane anchor. Conductance-voltage data are recorded for tunnel junctions of the type Al/CdS/7ETTD/Pb over a temperature range of 4 K to room temperature and they indicate that the presence of the 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 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 the surface states on the CdS. The present work suggests that, because of the marked effect on the conductance of CdS ultra-thin films, 7ETTD and other similar compounds may be candidates for use in molecular electronic device fabrication.