Fullerene/thiol-terminated molecules.

A series of fullerene-terminated oligo(phenylene ethynylene) (OPEs) have been synthesized for potential use in electronic or optoelectronic device monolayers. Electronic properties such as the energy levels and the distribution of HOMOs and LUMOs of fullerene-terminated OPEs have been calculated using the ab initio method at the B3LYP/6-31G(d) level. The calculations have revealed the concentration of frontier orbitals on the fullerene cage and a narrow distribution of HOMO-LUMO energy gaps. Ultraviolet photoelectron spectroscopy and inverse photoemission spectroscopy studies have been performed to further examine the electronic properties of the fullerene-terminated OPEs on gold surfaces. The obtained broad photoelectron spectra suggest that there are strong intermolecular interactions in the fullerene self-assembled monolayers, and the small bandgap (approximately 1.5 eV), determined by the photoelectron spectroscopy, indicates the unique nature of the fullerene-terminated OPEs in which the C(60) moiety can be connected to the Au surface through the conjugated OPE backbone.

Reversible photo-switching of single azobenzene molecules in controlled nanoscale environments.

We drive reversible photoinduced switching of single azobenzene-functionalized molecules isolated in tailored alkanethiolate monolayer matrices on Au{111}. We designed molecular tethers to suppress excited-state quenching from the metal substrate and formed rigid assemblies of single tethered azobenezene molecules in the domains of monolayer to limit steric constraints and tip-induced and stochastic switching effects. Single molecules were reversibly photoisomerized between trans and cis conformations by cycling exposure to visible and UV light. Trans and cis conformations were imaged as high (2.1 +/- 0.3 A) and low (0.7 +/- 0.2 A) protrusions in STM images and were assigned to the on and off states of the molecule, respectively.

Metal-free silicon-molecule-nanotube testbed and memory device.

Work from several laboratories has shown that metal nanofilaments cause problems in some molecular electronics testbeds. A new testbed for exploring the electrical properties of single molecules has been developed to eliminate the possibility of metal nanofilament formation and to ensure that molecular effects are measured. This metal-free system uses single-crystal silicon and single-walled carbon nanotubes as electrodes for the molecular monolayer. A direct Si-arylcarbon grafting method is used. Use of this structure with pi-conjugated organic molecules resulted in a hysteresis loop with current-voltage measurements that are useful for an electronic memory device. The memory is non-volatile for more than 3 days, non-destructive for more than 1,000 reading operations and capable of more than 1,000 write-erase cycles before device breakdown. Devices without pi-conjugated molecules (Si-H surface only) or with long-chain alkyl-bearing molecules produced no hysteresis, indicating that the observed memory effect is molecularly relevant.

Reversible bistable switching in nanoscale thiol-substituted oligoaniline molecular junctions.

Single molecular monolayers of oligoaniline dimers were integrated into sub-40-nm-diameter metal nanowires to form in-wire molecular junctions. These junctions exhibited reproducible room temperature bistable switching with zero-bias high- to low-current state conductance ratios of up to 50, switching threshold voltages of approximately +/-1.5 V, and no measurable decay in the high-state current over 22 h. Such switching was not observed in similarly fabricated saturated dodecane (C12) or conjugated oligo(phenylene ethynylene) (OPE) molecular junctions. The low- and high-state current versus voltage was independent of temperature (10-300 K), suggesting that the dominant transport mechanism in these junctions is coherent tunneling. Inelastic electron tunneling spectra collected at 10 K show a change in the vibrational modes of the oligoaniline dimers when the junctions are switched from the low- to the high-current state. The results of these measurements suggest that the switching behavior is an inherent molecular feature that can be attributed to the oligoaniline dimer molecules that form the junction.

Fabrication of carbon nanotube-molecule-silicon junctions.

A hybridization strategy for the covalent connection of single wall carbon nanotubes to silicon surfaces via oligo(phenylene ethynylene)s has been demonstrated using an orthogonal bisdiazonium functionalization protocol.

Orthogonally functionalized oligomers for controlled self-assembly.

The synthesis of molecules terminated with complementary thiol-protecting groups is described. The target compounds contain functionalities on one end known to form self-assembled monolayers on metal surfaces while at the other end an intact thioacetate is present whereby self-assembly may again occur after an in situ deprotection. Self-assembly data is reported for selected compounds to assess their efficacy in surface adhesion.

NanoCell electronic memories.

NanoCells are disordered arrays of metallic islands that are interlinked with molecules between micrometer-sized metallic input/output leads. In the past, simulations had been conducted showing that the NanoCells may function as both memory and logic devices that are programmable postfabrication. Reported here is the first assembly of a NanoCell with disordered arrays of molecules and Au islands. The assembled NanoCells exhibit reproducible switching behavior and two types of memory effects at room temperature. The switch-type memory is characteristic of a destructive read, while the conductivity-type memory features a nondestructive read. Both types of memory effects are stable for more than a week at room temperature, and bit level ratios (0:1) of the conductivity-type memory have been observed to be as high as 10(4):1 and reaching 10(6):1 upon ozone treatment, which likely destroys extraneous leakage pathways. Both molecular electronic and nanofilamentary metal switching mechanisms have been considered, though the evidence points more strongly toward the latter. The approach here demonstrates the efficacy of a disordered nanoscale array for high-yielding switching and memory while mitigating the arduous task of nanoscale patterning.