LOCAL ELECTRON TRANSPORT OF ORGANIC SEMICONDUCTING MONOLAYERS
Nanoscale devices are promising for the next generation of electronics, providing possibilities for greater hard drive storage capacity and higher computer execution speed. There are many strategies to produce such devices, molecular-scale electronics being a promising one. Transistors act as small switches that have a conducting channel controlled by a gate voltage. The gate is separated from the channel by a thin insulator. A potential design for a nanoscale transistor switch is to use a single monolayer of material that serves as both the insulator and the conducting channel. Self-assembled monolayers (SAMs) are excellent candidates to build these tiny transistors. The novel alkyl monolayer in my study is covalently bonded to the silicon substrate, providing the insulating layer. These alkyl chains can be functionalized with a conducting molecule to provide the conducting channel. There are many technical challenges to applying voltages and measuring currents on such small devices. There lacks an encompassing model to describe semiconducting SAMs. I used atomic force microscopy to study the metal-monolayer-semiconductor (MMS) system, which is formed by bringing a conductive tip in contact with the surface. More specifically, I measured the dependence of current on the length of the alkyl chains, and compared my findings to those from existing literature on similar alkyl based SAMs. I found that the resistance of alkyl monolayers increases exponentially as the number of carbons increases. After examining several competing models, I found that the tunneling model rather than the Schottky barrier model better describe the electron transport mechanism for the MMS system under investigation.