Daihua Zhang

In2O3 nanowires as chemical sensors

We present an approach to use individual In2O3 nanowire transistors as chemical sensors working at room temperature. Upon exposure to a small amount of NO2 or NH3, the nanowire transistors showed a decrease in conductance up to six or five orders of magnitude and also substantial shifts in the threshold gate voltage. These devices exhibited significantly improved chemical sensing performance compared to existing solid-state sensors in many aspects, such as the sensitivity, the selectivity, the response time, and the lowest detectable concentrations. Furthermore, the recovery time of our devices can be shortened to just 30 s by illuminating the devices with UV light in vacuum.

Electronic transport studies of single-crystalline In2O3 nanowires

Single-crystalline In2O3 nanowires were synthesized and then utilized to construct field-effect transistors consisting of individual nanowires. These nanowire transistors exhibited nice n-type semiconductor characteristics with well-defined linear and saturation regimes, and on/off ratios as high as 104 were observed at room temperature. The temperature dependence of the conductance revealed thermal emission as the dominating transport mechanism. Oxygen molecules adsorbed on the nanowire surface were found to have profound effects, as manifested by a substantial improvement of the device performance in high vacuum. Our work paved the way for In2O3 nanowires to be used as nanoelectronic building blocks and nanosensors.

Doping dependent NH3 sensing of indium oxide nanowires

NH3 gas sensing properties of In2O3 nanowires were carefully studied. Change of conductance in opposite directions was observed with different nanowire sensors. We suggest that this differential response is caused by various doping concentrations in the semiconducting In2O3 nanowires. In addition, we have also investigated a “gate-screening effect” exhibited in our nanowire chemical sensors at high NH3 concentrations, which is induced by adsorbed NH3 molecules working as charge traps. Both the doping-dependent response and the gate-screening effect will be especially valuable and helpful for understanding the detailed sensing mechanism of semiconducting metal oxide materials.

Fabrication approach for molecular memory arrays

We present an approach to tackle long-standing problems in contacts, thermal damage, pinhole induced short circuits and interconnects in molecular electronic device fabrication and integration. Our approach uses metallic nanowires as top electrodes to connect and interconnect molecular wires assembled on electrode arrays in crossbar architectures. Using this simple and reliable approach, we have revealed intriguing memory effects for several different molecular wires, and demonstrated their applications in molecular memory arrays. Our approach has great potential to be used for fast screening of molecular wire candidates and construction of molecular devices.

Multilevel memory based on molecular devices

Multilevel molecular memory devices were proposed and demonstrated for nonvolatile data storage up to three bits (eight levels) per cell, in contrast to the standard one-bit-per-cell (two levels) technology. In the demonstration, charges were precisely placed at up to eight discrete levels in redox active molecules self-assembled on single-crystal semiconducting nanowire field-effect transistors. Gate voltage pulses and current sensing were used for writing and reading operations, respectively. Charge storage stability was tested up to retention of 600 h, as compared to the longest retention of a few hours previously reported for one-bit-per-cell molecular memories.

Synthesis and characterization of single-crystal indium nitride nanowires

InN nanowires were synthesized and characterized using a variety of techniques. A two-zone chemical vapor deposition technique was used to operate the vapor generation and the nanowire growth at differential temperatures, leading to high-quality single-crystalline nanowires and growth rates as high as 4–10 μm/h. Precise diameter control was achieved by using monodispersed gold clusters as the catalyst. Photoluminescence and Raman studies have been carried out for the InN nanowires at room temperature. Devices consisting of single nanowires have been fabricated to explore their electronic transport properties. The temperature dependence of the conductance revealed thermal emission as the dominating transport mechanism.

Nanowire transistors with ferroelectric gate dielectrics: Enhanced performance and memory effects

Integration of ferroelectric materials into nanoscale field-effect transistors offers enormous promise for superior transistor performance and also intriguing memory effects. In this study, we have incorporated lead zirconate titanate (PZT) into In2O3 nanowire transistors to replace the commonly used SiO2 as the gate dielectric. These transistors exhibited substantially enhanced performance as a result of the high dielectric constant of PZT, as revealed by a 30-fold increase in the transconductance and a 10-fold reduction in the subthreshold swing when compared to similar SiO2-gated devices. Furthermore, memory effects were observed with our devices, as characterized by a counter-clockwise loop in current-versus-gate-bias curves that can be attributed to the switchable remnant polarization of PZT. Our method can be easily generalized to other nanomaterials systems and may prove to be a viable way to obtain nanoscale memories.

Synthesis, Electronic Properties, and Applications of Indium Oxide Nanowires

Abstract: Single‐crystalline indium oxide nanowires were synthesized using a laser ablation method and characterized using various techniques. Precise control over the nanowire diameter down to 10 nm was achieved by using monodisperse gold clusters as the catalytic nanoparticles. In addition, field effect transistors with on/off ratios as high as 104 were fabricated based on these nanowires. Detailed electronic measurements confirmed that our nanowires were n‐type semiconductors with thermal emission as the dominating transport mechanism, as revealed by temperature‐dependent measurements. Furthermore, we studied the chemical sensing properties of our In2O3 nanowire transistors at room temperature. Upon exposure to a small amount of NO2 or NH3, the nanowire transistors showed a decrease in conductance of up to five or six orders of magnitude, in addition to substantial shifts in the threshold gate voltage. Our devices exhibit significantly improved chemical sensing performance compared to existing solid‐state sensors in many aspects, such as the sensitivity, the selectivity, the response time and the lowest detectable concentrations. We have also demonstrated the use of UV light as a “gas cleanser” for In2O3 nanowire chemical sensors, leading to a recovery time as short as 80 seconds.

Chemical gating of In2O3 nanowires by organic and biomolecules

In2O3 nanowire transistors were used to investigate the chemical gating effect of organic molecules and biomolecules with amino or nitro groups. The nanowire conductance changed dramatically after adsorption of these molecules. Specifically, amino groups in organic molecules such as butylamine, donated electrons to In2O3 nanowires and thus led to enhanced carrier concentrations and conductance, whereas molecules with nitro groups such as butyl nitrite made In2O3 nanowires less conductive by withdrawing electrons. In addition, intrananowire junctions created by partial exposure of the nanowire device to butyl nitrite were investigated, and pronounced rectifying current–voltage characteristics were obtained. Furthermore, chemical gating by low-density lipoprotein cholesterol, the offending agent in coronary heart diseases, was also observed and attributed to the amino groups carried by the bio species.

Controlled growth of gallium nitride single-crystal nanowires using a chemical vapor deposition method

Chemical vapor deposition (CVD) using gold nanoparticles as the catalyst to grow high-quality single-crystal gallium nitride nanowires was developed. This method enables control over several important aspects of the growth, including control of the nanowire diameter by using monodispersed gold clusters, control of the nanowire location via e-beam patterning of the catalyst sites, and control of the nanowire orientation via epitaxial growth on a -plane sapphire substrates. Our work opens up new ways to use GaN nanowires as nanobuilding blocks.