Yuxing Yao

Synthesis and preliminary testing of molecular wires and devices.

Presented here are several convergent synthetic routes to conjugated oligo(phenylene ethynylene)s. Some of these oligomers are free of functional groups, while others possess donor groups, acceptor groups, porphyrin interiors, and other heterocyclic interiors for various potential transmission and digital device applications. The syntheses of oligo(phenylene ethynylene)s with a variety of end groups for attachment to numerous metal probes and surfaces are presented. Some of the functionalized molecular systems showed linear, wire-like, current versus voltage (I(V)) responses, while others exhibited nonlinear I(V) curves for negative differential resistance (NDR) and molecular random access memory effects. Finally, the syntheses of functionalized oligomers are described that can form self-assembled monolayers on metallic electrodes that reduce the Schottky barriers. Information from the Schottky barrier studies can provide useful insight into molecular alligator clip optimizations for molecular electronics.

Electron transport through single phenylene-ethynylene molecular junctions at low temperature

We present low-temperature electron transport measurements of individual phenylene–ethynylene molecular wires, connected to nanometer-spaced gold electrodes. Low-bias current–voltage (I–V) characteristics measured at 4.2K are stable and show irregular steps. After application of a large voltage, the low-bias I–V curves switch between different stable configurations, some of which show negative differential resistance (NDR). Similar behavior, including the NDR, has been observed in molecules irrespective of whether they contain a NO2 side group or not. We suggest that different I–V curves measured, including the NDR, could be due either to conformational changes in the molecules or a change in coupling of the molecular junction.

Evolution of Strategies for Self‐Assembly and Hookup of Molecule‐Based Devices

ABSTRACT: Strategies for self‐assembling molecule‐based devices are considered in terms of current chemical issues whose resolution appears critical to efficient connection and addressing of electronically active molecules between electrodes. We discuss issues related to the type and shape of the molecules, chemical bonding at junctions, molecular lengths and electrode gap matching, molecular alignment at electrodes, chemistry of deposited metal contacts, and the doping of molecular conductors. Examples of each of these aspects is given using fully conjugated molecules, constituted from rigid rod phenylene‐ethynylene units, self‐assembled onto metal and semiconductor surfaces.

Designing CMOS/molecular memories while considering device parameter variations

In recent years, many advances have been made in the development of molecular scale devices. Experimental data shows that these devices have potential for use in both memory and logic. This article describes the challenges faced in building crossbar array-based molecular memory and develops a methodology to optimize molecular scale architectures based on experimental device data taken at room temperature. In particular, issues in reading and writing such as memory using CMOS are discussed, and a solution is introduced for easily reading device conductivity states (typically characterized by very small currents). Additionally, a metric is derived to determine the voltages for writing to the crossbar array. The proposed memory design is also simulated with consideration to device parameter variations. Thus, the results presented here shed light on important design choices to be made at multiple abstraction levels, from devices to architectures. Simulation results, incorporating experimental device data, are presented using Cadence Spectre.

Nanowell device for the electrical characterization of metal–molecule–metal junctions

A nanowell device for the electrical characterization of metal–molecule–metal junctions was built using readily available processing tools and techniques. This device consisted of a nanoscale well, with a gold bottom, filled with a self-assembling monolayer of organic molecules, and capped with titanium and gold. Focused ion beam technology was used to fabricate the well with a width less than the grain size of gold. This nanowell improved the device performance dramatically by reducing the chances of pinhole formation in the self-assembling monolayer on the bottom gold electrode. Unlike some established characterization techniques, including conducting probe atomic force microscopy and scanning tunneling microscopy, the nanowell device has the potential for future circuit integration. The effectiveness of the device was confirmed by testing I–V characteristics of alkanethiols and oligomeric arylthiols. The alkanethiol current was exponentially dependent on chain length with a decay factor (β) that ranged...

Polarizabilities of Adsorbed and Assembled Molecules: Measuring the Conductance through Buried Contacts

We have measured the polarizabilities of four families of molecules adsorbed to Au{111} surfaces, with structures ranging from fully saturated to fully conjugated, including single-molecule switches. Measured polarizabilities increase with increasing length and conjugation in the adsorbed molecules and are consistent with theoretical calculations. For single-molecule switches, the polarizability reflects the difference in substrate−molecule electronic coupling in the ON and OFF conductance states. Calculations suggest that the switch between the two conductance states is correlated with an oxidation state change in a nitro functional group in the switch molecules.

Study of the room temperature molecular memory observed from a nanowell device

We tested the electrical characteristics of an oligo(phenylene ethynylene) (OPE) molecule with one nitro side group, an OPE with two nitro side groups, and an OPE with no nitro side groups in our nanowell device. The OPE molecule with nitro side group(s) showed switching behavior with memory as well as nonreversible negative differential resistance (NDR). Current-voltage (I‐V) characteristics showed a high conductivity state that switched to a low conductivity state upon the application of a threshold voltage. This low state held until the opposite threshold voltage was applied and the device switched back to the high conductivity state. The OPE with no nitro side groups did not show memory or NDR. In this work, we report the complete switching behavior observed including the device yield, average threshold voltage, and the average high to low current ratios.

Effects of Molecular Environments on the Electrical Switching with Memory of Nitro-Containing OPEs

An oligo(phenylene ethynylene) (OPE) molecule with a nitro side group has exhibited electrical switching with memory and thus has potential for use in molecular electronic devices. However, different research groups have reported different electrical behaviors for this molecule. In addition to variations among test structures, differences in local molecular environments could be partially responsible for the differences in the reported results. Thus, we tested four variations of a nitro-OPE/dodecanethiol monolayer in the same type of nanowell test device to study how the environment of the nitro-OPE affects the observed electrical behavior. We found that the density of the nitro-containing molecules in the device altered the observed electrical switching behavior. Further, we found a positive correlation between the disorder of the monolayer and the observed electrical switching behavior. This correlation is consistent with suggestions that nitro molecule switching may depend on a conformational change of the molecule, which may be possible only in a disordered monolayer.