Low temperature transport study of the nitro molecules

Various research groups, including ours, have observed switching with memory behavior at room temperature from a monolayer of oligo(phenylene ethynylene) (OPE) molecules with a nitro sidegroup.$^{1,2}$. This switching behavior has the potential to be used in molecular electronic devices. However, the transport mechanisms of this ``nitro'' molecule are not well understood. Understanding the transport mechanisms of the nitro molecules may help identify the underlying cause of the switching behavior. We performed a systematic study of the transport characteristics of OPE molecules with and without a nitro side group in our nanowell test device$^{3}$ at various temperatures between 60K and 300K. We observed non-switching exponential current-voltage characteristics from OPEs without the nitro side group. The mechanism of transport was determined to be hopping with a transport barrier of 0.03$\pm $.01 V between 100K to 300K. Switching with memory behavior as well as non-switching exponential I-V characteristics were observed from the nitro molecules at various temperatures. The transport mechanism in switching devices was determined to be hopping with an activation barrier of 0.26$\pm $.08 V between 200K and 300K. However, a significantly lower activation barrier similar to that of OPEs without a nitro group was estimated for the nitro molecule devices that did not show any switching behavior.