Metastable Metal-Monolayer-Semiconductor Junctions: Diverse Chain-Length-Dependent and Ultraslow Electrical Progression

Molecularly tunable metal-semiconductor (MS)junctions have been fabricated by modifying mercury dropelectrodes withn-alkanethiols, 1-CH3(CH2)n-1SH (n= 10, 11,12, 14, 16, and 18) prior to the formation of intimate contact withhydrogen-terminated silicon (H-Si???) and characterized by bothsolid-state electrical and electrochemical measurements. We havedemonstrated that the current-voltage properties of these molecularjunctions change with time and that diverse time-dependentprogression"patterns"were observed between the systems oflong-chain alkanethiols (C14SH, C16SH, and C18SH) and short-chain alkanethiols (C10SH, C11SH, and C12SH). It is remarkablethat for mercury contact electrodes modified with long-chainalkanethiolate self-assembled monolayers (SAMs), the junctionsbecame more rectifying and stabilized over a prolonged period (similar to 2 h). For short-chain counterparts, surprisingly, they initiallybecame more rectifying, then changed to ohmic over a similar period. It was proposed that at the MS interface, short-chainalkanethiolate SAMsfirst reorganize to be more ordered before eventually collapse. Long-chain alkanethiolate SAMs, on the otherhand, achieve a more uniform, oriented, and stable packing as time goes by. These novelfindings shed light on"long-term"intermolecular interactions that drive molecular systems to undergo ultraslow reorganizations, which can be readily modulated bysimply varying the precursor structures (e.g., chain length ofn-alkanethiols).