We tracked over time the conductance switching of single and bundled phenylene ethynylene oligomers isolated in matrices of alkanethiolate monolayers. The persistence times for isolated and bundled molecules in either the ON or OFF switch state range from seconds to tens of hours. When the surrounding matrix is well ordered, the rate at which the inserted molecules switch is low. Conversely, when the surrounding matrix is poorly ordered, the inserted molecules switch more often. We conclude that the switching is a result of conformational changes in the molecules or bundles, rather than electrostatic effects of charge transfer.

Using aryldiazonium salts that are air-stable and easily synthesized, we describe here a one-step, room-temperature route to direct covalent bonds between pi-conjugated organic molecules on three material surfaces: Si, GaAs, and Pd. The Si can be in the form of single crystal Si including heavily doped p-type Si, intrinsic Si, heavily doped n-type Si, on Si(111) and Si(100), and on n-type polycrystalline Si. The formation of the aryl-metal or aryl-semiconductor bond attachments was confirmed by corroborating evidence from ellipsometry, reflectance FTIR, XPS, cyclic voltammetry, and AFM analyses of the surface-grafted monolayers. A data-encompassing explanation for the mechanism suggests a diazonium activation by reduction at the open circuit potential, with aryl radical secondary products bonding to the surface. The synthetic details are included for preparing the surface-grafted monolayers and the precursor diazonium salts. This spontaneous diazonium activation reaction offers an attractive route to highly passivating, robust monolayers and multilayers on many surfaces that allow for strong bonds between carbon and surface atoms with molecular species that are near perpendicular to the surface.

Self-assembled monolayers (SAMs) of the nitro-substituted oligo(phenylene-ethynylene) (OPE) 4,4′-(diethynylphenyl)-2′-nitro-1-benzenethiolate on Au{111} were prepared, and the structures were characterized by multiple techniques, including infrared spectroscopy, ellipsometry, and X-ray photoelectron spectroscopy. Assembly of the nitro-OPE SAM, either via acidic hydrolysis of the thioacetate derivative or from the thiol in pure solvent, produces a well-ordered SAM with a ( 3 × 3) superlattice structure and an average molecular tilt of 32-39°from the surface normal. In comparison, SAMs prepared from the unsubstituted OPE show the same lattice structure and a similar tilt of ∼33°. In contrast, when the nitro-OPE SAM is assembled by hydrolysis of the thioacetate derivative under basic conditions, extensive redox reactions arise in which oxidation of the S atoms occurs with accompanying reduction of -NO2 to -NH2, apparently via intermediates including -NH(OH), to form mixed composition SAMs typically containing ∼30% of the amino-substituted molecule. Further, the nitro-OPE SAM, regardless of the preparation method, shows significant chemical instability under storage in air and/or light exposure. Since the nitro-OPE molecule and molecules with related structures are of considerable interest for molecular electronics applications, these results indicate that extreme diligence must be used in designing conditions for the fabrication of devices utilizing these SAMs.

Self-assembled monolayers (SAMs) of the isocyano derivative of 4,4'-di(phenylene-ethynylene)benzene (1), a member of the "OPE" family of "molecular wires" of current interest in molecular electronics, have been prepared on smooth, {111} textured films of Au and Pd. For assembly in oxygen-free environments with freshly deposited metal surfaces, infrared reflection spectroscopy (IRS) indicates the molecules assume a tilted structure with average tilt angles of 18-24 degrees from the surface normal. The combination of IRS, X-ray photoelectron spectroscopy, and density functional theory calculations all support a single sigma-type bond of the -NC group to the Au surface and a sigma/pi-type of bond to the Pd surface. Both SAMs show significant chemical instability when exposed to typical ambient conditions. In the case of the Au SAM, even a few hours storage in air results in significant oxidation of the -NC moieties to -NCO (isocyanate) with an accompanying decrease in surface chemical bonding, as evidenced by a significant increase in instability toward dissolution in solvent. In the case of the Pd SAM, similar air exposure does not result in incorporation of oxygen or loss of solvent resistance but rather results in a chemically altered interface which is attributed to polymerization of the -NC moieties to quasi-2D poly(imine) structures. Conductance probe atomic force microscope measurements show the conductance of the degraded Pd SAMs can diminish by approximately 2 orders of magnitude, an indication that the SAM-Pd electrical contact has severely degraded. These results underscore the importance of careful control of the assembly procedures for aromatic isocyanide SAMs, particularly for applications in molecular electronics where the molecule-electrode junction is critical to the operational characteristics of the device.

The electronic properties of alkanethiolate [CH3(CH2)nS-, n = 9 and 11] and alkaneselenolate [CH3(CH2)nSe-, n = 9 and 11] self-assembled monolayers on Au{111} have been quantitatively compared. Simultaneously acquired apparent tunneling barrier height (ATBH) and scanning tunneling microscopy (STM) images reveal that alkanethiolate molecules have a lower barrier to tunneling, and therefore a higher conductance than alkaneselenolates of the same alkyl chain length. Molecular and contact conductance differences were elucidated by using observed STM topographic tunneling height differences between the analogous species. This apparent topographic difference combined with comparative ATBH data indicate that the observed decrease in conductance for alkaneselenolates compared to alkanethiolates originates exclusively from the Au-chalcogenide physical, chemical, and electronic contact.

Coexisting adsorbate phases in high-coverage decaneselenolate and dodecaneselenolate [CH 3 (CH 2 ) n Se, n ) 9 and 11] self-assembled monolayers on Au{111} have been characterized by scanning tunneling microscopy and consist of two types: a densely packed distorted hexagonal lattice incommensurate to the underlying gold substrate, as revealed by the observation of a moire ´pattern, and a commensurate linear missing-row structure. Examination of the nearest neighbor distances in the tightly packed lattice reveal two distinct repeat distances of 4.90 and 5.20 Å, which complements previous surface X-ray data. The linear missing row structure manifests in several variants of the ( 3 × 3 3)R30°unit cell differentiated by whether the molecules bind at 2-or 3-fold substrate sites. While the number of molecules within this unit cell is typically two, in some cases an additional alkaneselenolate molecule is located at a site one Au atom lower than the rest. The structural conclusions are supported by excellent agreement of experimental lattice parameters and those derived from molecular packing models. Comparison of the alkaneselenolate data with analogous structural phases reported for alkanethiolate monolayers on Au{111} shows that differences between the two systems can be understood on the basis that self-assembly is guided both by headgroup-headgroup as well as headgroup-substrate interactions.

We have investigated the interaction of vapor-deposited titanium and gold with a self-assembled monolayer (SAM) of 4-[4′-(phenylethynyl)-phenylethynyl]-benzenthiol, an unsubstituted oligo(phenylene-ethynylene), chemisorbed on a gold substrate, a typical SAM of interest for molecular electronics. Deposited titanium atoms are observed to react in a top-down fashion with the SAM molecules to form Ti–C bonds, destroying the monolayer structure. In contrast, deposited Au atoms undergo continuous penetration through the monolayer, even at high coverages, leaving the SAM “floating” on the Au substrate surface.

Vibrationally resonant sum-frequency generation (VR-SFG) and spectroscopic ellipsometry (SE) have been used to characterize self-assembled monolayer films of unsubstituted and mononitro-substituted oligo(phenylene−ethynylene) molecules on vapor-deposited Au substrates. When combined with quantum chemical calculations of the relevant transition moment directions, orientation distributions and electronic excitation spectra are obtained. The orientation distribution from VR-SFG is in good agreement with previous IR reflection studies, indicating both molecules are tilted from the surface normal by ∼30°. The calculated resonant hyperpolarizabilities are in good agreement with experimental spectra. The optical polarizability extracted from SE suggests strong intermolecular interactions, consistent with molecular exciton theory.