Yangjun Xing

Optimizing Single-Molecule Conductivity of Conjugated Organic Oligomers with Carbodithioate Linkers

In molecular electronics, the linker group, which attaches the functional molecular core to the electrode, plays a crucial role in determining the overall conductivity of the molecular junction. While much focus has been placed on optimizing molecular core conductivity, there have been relatively few attempts at designing optimal linker groups to metallic or semiconducting electrodes. The vast majority of molecular electronic studies use thiol linker groups; work probing alternative amine linker systems has only recently been explored. Here, we probe single-molecule conductances in phenylene-ethynylene molecules terminated with thiol and carbodithioate linkers, experimentally using STM break-junction methods and theoretically using a nonequilibrium Greens function approach. Experimental studies demonstrate that the carbodithioate linker augments electronic coupling to the metal electrode and lowers the effective barrier for charge transport relative to the conventional thiol linker, thus enhancing the conductance of the linker-phenylene-ethynylene-linker unit; these data underscore that phenylene-ethynylene-based structures are more highly conductive than originally appreciated in molecular electronics applications. The theoretical analysis shows that the nature of sulfur hybridization in these species is responsible for the order-of-magnitude increased conductance in carbodithioate-terminated systems relative to identical conjugated structures that feature classic thiol linkers, independent of the mechanism of charge transport. Interestingly, in these systems, the tunneling current is not dominated by the frontier molecular orbitals. While barriers >k(B)T to produce the low beta values seen in our experiments. Taken together, these experimental and theoretical studies indicate a promising role for carbodithioate-based connectivity in molecular-scale electronics applications involving metallic and semiconducting electrodes.

TiO2/LiCl-Based Nanostructured Thin Film for Humidity Sensor Applications

A simple and straightforward method of depositing nanostructured thin films, based on LiCl-doped TiO(2), on glass and LiNbO(3) sensor substrates is demonstrated. A spin-coating technique is employed to transfer a polymer-assisted precursor solution onto substrate surfaces, followed by annealing at 520°C to remove organic components and drive nanostructure formation. The sensor material obtained consists of coin-shaped nanoparticles several hundred nanometers in diameter and less than 50 nm thick. The average thickness of the film was estimated by atomic force microscopy (AFM) to be 140 nm. Humidity sensing properties of the nanostructured material and sensor response times were studied using conductometric and surface acoustic wave (SAW) sensor techniques, revealing reversible signals with good reproducibility and fast response times of about 0.75 s. The applicability of this nanostructured film for construction of rapid humidity sensors was demonstrated. Compared with known complex and expensive methods of synthesizing sophisticated nanostructures for sensor applications, such as physical vapor deposition (PVD) and chemical vapor deposition (CVD), this work presents a relatively simple and inexpensive technique to produce SAW humidity sensor devices with competitive performance characteristics.

An STM study of the pH dependent redox activity of a two-dimensional hydrogen bonding porphyrin network at an electrochemical interface.

Studying electron transfer reactions of porphyrin molecules is important for a wide range of applications including biology, molecular devices, artificial photosynthesis, information storage, and fuel cells. It is known that porphyrins adsorbed in a self-assembled monolayer at an electrochemical interface may lose their electrochemical activity. However, the mechanism of the suppressed electrochemical activity is not clear. In this article, the electrochemical behavior of the two-dimensional network structures of 5,10,15,20-tetrakis(4-carboxylphenyl)-21H,23H-porphyrin (TCPP) molecules, formed via intermolecular hydrogen bonding on Au(111), was investigated by electrochemical scanning tunneling microscopy (EC-STM). Three types of domains, including a square network with molecules trapped inside, square packing, and hexagonal close-packing structures have been observed under various pH conditions. The difference in STM contrast between oxidized and reduced TCPP allows the slow electrochemical reduction of adsorbed TCPP to be visualized by STM. For the first time, the pH dependent reduction of porphyrins was imaged by EC-STM, revealing the mechanism of porphyrin slow reduction at electrochemical interfaces. TCPP reduction can be accelerated either by tuning the working electrode potential to a more negative value or by lowering the H(+) concentration. A redox reaction model was proposed based on the pH dependent reduction of TCPP to elucidate the fundamental aspects of porphyrin redox reactions.

The Single-Molecule Conductance and Electrochemical Electron-Transfer Rate Are Related by a Power Law

This study examines quantitative correlations between molecular conductances and standard electrochemical rate constants for alkanes and peptide nucleic acid (PNA) oligomers as a function of the length, structure, and charge transport mechanism. The experimental data show a power-law relationship between conductances and charge transfer rates within a given class of molecules with the same bridge chemistry, and a lack of correlation when a more diverse group of molecules is compared, in contrast with some theoretical predictions. Surprisingly, the PNA duplexes exhibit the lowest charge-transfer rates and the highest molecular conductances. The nonlinear rate-conductance relationships for structures with the same bridging chemistries are attributed to differences in the charge-mediation characteristics of the molecular bridge, energy barrier shifts and electronic dephasing, in the two different experimental settings.

Single‐Molecule Sensing of Environmental pH—an STM Break Junction and NEGF‐DFT Approach

Sensors play a significant role in the detection of toxic species and explosives, and in the remote control of chemical processes. In this work, we report a single-molecule-based pH switch/sensor that exploits the sensitivity of dye molecules to environmental pH to build metal-molecule-metal (m-M-m) devices using the scanning tunneling microscopy (STM) break junction technique. Dyes undergo pH-induced electronic modulation due to reversible structural transformation between a conjugated and a nonconjugated form, resulting in a change in the HOMO-LUMO gap. The dye-mediated m-M-m devices react to environmental pH with a high on/off ratio (≈100:1) of device conductivity. Density functional theory (DFT) calculations, carried out under the non-equilibrium Greens function (NEGF) framework, model charge transport through these molecules in the two possible forms and confirm that the HOMO-LUMO gap of dyes is nearly twice as large in the nonconjugated form as in the conjugated form.