Room temperature negative differential resistance (NDR) has been measured through individual organic molecules on degenerately doped Si(100) surfaces using ultrahigh vacuum scanning tunneling microscopy (STM). For styrene molecules on n-type Si(100), NDR is observed only for negative sample bias because positive sample bias leads to electron stimulated desorption. By replacing styrene with a saturated organic molecule (2,2,6,6-tetramethyl-1-piperidinyloxy), electron stimulated desorption is not observed at either bias polarity. In this case, NDR is observed only for negative sample bias on n-type Si(100) and for positive sample bias on p-type Si(100). This unique behavior is consistent with a resonant tunneling mechanism via molecular orbitals and opens new possibilities for silicon-based molecular electronic devices and chemical identification with STM at the single-molecule level.

Ultrahigh vacuum scanning tunneling microscopy is employed for the nanofabrication and characterization of atomically registered heteromolecular organosilicon nanostructures at room temperature. In the first fabrication step, feedback controlled lithography (FCL) is used to pattern individual 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) free radical molecules at opposite ends of the same dimer row on the Si(100)-2×1:H surface. In atomic registration with the first pattern, FCL is subsequently applied for the removal of a single hydrogen atom. The resulting dangling bond templates the spontaneous growth of a styrene chain that is oriented along the underlying dimer row. The styrene chain growth is bounded by the originally patterned TEMPO molecules, thus resulting in a heteromolecular organosilicon nanostructure. The demonstration of multistep FCL suggests that this approach can be widely used for fundamental studies and fabricating prototype devices that require atomically registered organic molecules mounted on silicon surfaces.

The ultrahigh vacuum scanning tunnelling microscope was used to probe charge transport through two different organic monolayers adsorbed on the Si(100) substrate at room temperature. I–V measurements were taken on monolayers of 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) and cyclopentene for degenerately doped n-type and p-type substrates. Initial I–V measurements for transport through the TEMPO monolayer exhibited a suppression of negative differential resistance (NDR) relative to previously reported charge transport through isolated molecules. I–V measurements were also performed on isolated cyclopentene molecules and cyclopentene monolayers. Similarly to TEMPO monolayers, the cyclopentene monolayers exhibited attenuated NDR behaviour relative to isolated molecules. The observed NDR suppression suggests that the high packing density of organic monolayers influences charge transport through molecule–semiconductor junctions.

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A soft lithographic microcontact printing (μCP) procedure is successfully applied for the first time to form densely packed organosilane self-assembled monolayers (SAMs) on the surface of ITO (Sn-doped In2O3) coated glass via a thermally activated deposition process. Hot microcontact printing (HμCP) enables localized transfer with 1.0−40 μm feature sizes of dense docosyltrichlorosilane (CH3(CH2)20CH2SiCl3 = DTS) monolayer patterns onto ITO, which reacts sluggishly under conventional μCP conditions. X-ray reflectivity measurements yield a thickness of 12.1 ± 0.1 Å and a surface roughness of 2.8 ± 0.1 Å for HμCP printed DTS films, which is well within the range for self-assembled monolayer formation, while the weak reflected intensity from conventionally prepared DTS films indicates a poorly organized monolayer structure. Noncontact mode AFM studies reveal that HμCP creates uniform SAMs over a wide area with excellent line edge resolution, while the original patterns are poorly transferred by conventional μCP, presumably due to the slow Si−O bond formation. Cyclic voltammetry of 1,1‘-ferrocenedimethanol solutions using HμCP-derived, DTS SAM coated ITO working electrodes evidences good barrier properties, consistent with dense films. The DTS SAM patterns can be imaged by fabricating organic light-emitting diode (OLED) heterostructures on the patterned ITO. The DTS SAM role as a hole injection blocking layer enables patterned luminescence from the hot contact printed ITO surfaces.