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 drop electrodes with n-alkanethiols, 1-CH3(CH2) n –1SH (n = 10, 11, 12, 14, 16, and 18) prior to the formation of intimate contact with hydrogen-terminated silicon (H-Si) and characterized by both solid-state electrical and electrochemical measurements. We have demonstrated that the current-voltage properties of these molecular junctions change with time and that diverse time-dependent progression “patterns” were observed between the systems of long-chain alkanethiols (C14SH, C16SH, and C18SH) and short-chain alkanethiols (C10SH, C11SH, and C12SH). It is remarkable that for mercury contact electrodes modified with long-chain alkanethiolate self-assembled monolayers (SAMs), the junctions became more rectifying and stabilized over a prolonged period (∼2 h). For short-chain counterparts, surprisingly, they initially became more rectifying, then changed to ohmic over a similar period. It was proposed that at the MS interface, short-chain alkanethiolate SAMs first reorganize to be more ordered before eventually collapse. Long-chain alkanethiolate SAMs, on the other hand, achieve a more uniform, oriented, and stable packing as time goes by. These novel findings shed light on “long-term” intermolecular interactions that drive molecular systems to undergo ultraslow reorganizations, which can be readily modulated by simply varying the precursor structures (e.g., chain length of n-alkanethiols).