The long-term stability (gradual oxidation) of two structurally different organic monolayers, C6H5(CH2)3−Si (C3Ph−Si) and CH3(CH2)11−Si (C12−Si), covalently bonded to n-type silicon(111) was studied and correlated to the electrical performance of the thus formed mercury | monolayer | n-Si junctions. High-resolution XPS analysis of the O 1s region on freshly prepared samples identified physically adsorbed oxygen (Oad) and oxygen-containing species (SiO x ) bound to silicon at 532.2 and 533.5 eV, respectively. When left under ambient conditions, both bands increased in intensity as time progressed, while no change in the carbon content (C 1s peak) was observed. More importantly, upon aging, there was a noticeable increase in the electrical current flow of both and junctions; a considerably larger increase occurred in the former sample, which has less oxygen content than that of the latter junction (based on the O 1s(O−Si)/Si 2p peak area ratios). These results suggest that the slow oxidation of molecularly modified silicon may, in fact, dictate the electrical performance of the thus formed molecular diode junctions.

By preparing phenyl-terminated monolayers on hydrogen-terminated silicon (111), we show that their higher surface densities in comparison with n-alkyl monolayers improves their electrical properties (lower reverse-bias currents, higher effective barrier heights, and closer-to-unity ideality factors) when contacted using either mercury drop or thermally deposited gold electrodes. Consistent with these macroscopic results, the ballistic electron emission microscopy characterization shows a significant decrease in ballistic current and higher local barrier height for the phenyl-terminated monolayers, when compared with gold | n-alkyl monolayer | silicon junctions. We propose that increased intermolecular interaction through π–π stacking of the phenyl head-groups stabilizes the monolayer structure at the buried interface and inhibits the penetration of thermally deposited gold atoms.

Recent advances in creating rectifying gold|monolayer|silicon (Au-M-Si) junctions (namely, molecular silicon diodes) are reviewed. It is known that direct deposition of gold contacts onto molecular monolayers covalently bonded to silicon surfaces causes notable disruption to the junction structure, resulting in deteriorated performance and poor reproducibility that are unsuitable for practical applications. In the past few years, several new experimental approaches have been explored to minimize or eliminate such damage, including the "indirect" evaporation method and the pre-deposition of a protective "non-penetrating" metal. To enhance the interactions at the gold-monolayer interface, head-groups that allow bonding to gold are used to maintain the monolayer integrity. Construction of the device via flip-chip lamination and the modified polymer-assisted lift-off techniques also prohibits monolayer damage. Refining the fabrication and design techniques towards reliable molecular junctions is crucial if they are to be used in nanoelectronics for the purpose of miniaturization.

We demonstrate herein how to mechanically modulate the electrical properties of metastable molecular junctions, i.e., mercury−silicon junctions modified with "mobile" octadecanethiolate (C18) self-assembled monolayers (SAMs). By enlarging the mercury drop contact or changing its shape, the current density−voltage response of these molecular junctions vary remarkably from rectifying (off) to ohmic (on). More importantly, such switching behavior is reversible and reproducible when the shape of the mercury drop is changed from spherical to elliptical and vice versa (by pressing and releasing the mercury drop). Evaluation of the rectification ratio and effective barrier height of these molecular junctions enables determination of the threshold surface area of the mercury contact for the modulated electrical switching.

The effect of molecular dipoles on charge transport across organic monolayer-modified metal−semiconductor junctions has been investigated systematically. We have prepared a new set of organic monolayers with varied terminal derivatization on crystalline silicon to construct molecular junctions using mercury drops as the top contact electrode. Although the surface and structural characterization indicated the high quality and uniformity of all these monolayers, the junctions (Hg/R−Si�) showed a diverse electrical performance. Beyond taking the most common theoretical approach to analyze these molecular junctions, that is, applying the thermionic emission model (TE) to calculate the barrier height (Φ B ) and ideality factor (η), we have examined the contribution of the carrier generation−recombination (CGR) mechanism by fitting the experimental current−voltage curves. When η is close to unity, the charge transport across these molecular junctions is dominated by TE; for η values greater than unity, TE indeed remains the dominant current transport pathway, while CGR transport becomes significant.