Hyun-Wook Ra

Fabrication of ZnO Nanowires Using Nanoscale Spacer Lithography for Gas Sensors

Semiconductor metal-oxide nanowires have attracted strong research interest in the past few years because of their unique and superior properties as compared with the bulk materials. Among the various semiconductor metal-oxide nanowires, zinc oxide (ZnO) nanowires are considered to be one of the most important semiconductor nanomaterials for fabricating devices with applications such as optoelectronics, electronics, mechanics, and sensors due to their wide and direct bandgap energy (3.37 eV), large exciton binding energy (60 meV), and high thermal and mechanical stability. To date, the basic properties of nanowire devices have been investigated using electron-beam lithography, focused-ion-beam lithography, and dip-pen nanolithography. Although these techniques have made great contributions to nanowire technology, their use on a large scale still remains a significant challenge due to their high cost and low throughput. As part of the effort to address the challenge of producing ordered nanowire arrays, various methods of aligning nanowires using an electric field, microfluidic flow-assisted techniques, magnetic-field assembly, and the Langmuir– Blodgett method have been reported. However, these methods still suffer from inherent drawbacks, such as random placement, contamination, and general incompatibility with conventional silicon processing. Recently, direct patterning techniques using spacer lithography have been reported to avoid the problems associated with the alignment of the nanowires. Large-number-density nanowire arrays could fabricated by using these approaches with sizes of tens of nanometers and a lateral resolution of about 2 nm. These methods are expected to be more plausible and suitable for the large-scale manufacturing of nanowire devices. However, there are no reports that demonstrate the feasibility of the various applications based on semiconductor oxide nanowire devices fabricated by the spacer lithography methods. In this work, we first fabricated ZnO nanowire arrays by nanoscale spacer lithography (NSL), which is a technique that

Synthesis of ZnO nanowires on Si substrate by thermal evaporation method without catalyst: Structural and optical properties

Synthesis of ZnO nanowires was achieved on Si(100) substrate by the thermal evaporation of high purity metallic zinc powder without the use of any metal catalyst or additives. The diameter and length of the as-grown nanowires were in the range of 20–35 nm and few micrometers, respectively. The shapes and sizes of ZnO nanowires were dependent on the growth time. The high resolution transmission electron microscopy and selected area electron diffraction patterns indicated that the as-grown products are single crystalline with wurtzite hexagonal phase. Room temperature photoluminescence studies exhibited a strong UV emission and a suppressed green emission, confirming the good optical properties for the deposited nanowires.

The effect of grain boundaries inside the individual ZnO nanowires in gas sensing.

We present the gas sensing characteristics of the individual ZnO nanowires with single-crystalline and multiple grain boundaries (GBs) fabricated using bottom-up and top-down approaches, respectively. The sensor response of the individual ZnO nanowires with the multiple GBs was enhanced approximately three times as compared to that of single-crystalline ZnO nanowires due to well-known GB modulations. However, the response and recovery times of the individual ZnO nanowires with multiple GBs were much slower than those of the single-crystalline ZnO nanowire, indicating the presence of oxygen diffusion resistance to GBs due to the relatively fast surface kinetic reaction. Simplified kinetic diffusion modeling and experimental results could quantify the significant diffusion resistance of gas molecules into the GBs of the individual ZnO nanowires.

Ion bombardment effects on ZnO nanowires during plasma treatment

We present the effects of ion bombardment on ZnO nanowires caused by their exposure to an Ar inductively coupled plasma. The conductivity of the individual ZnO nanowire was increased in up to 3 orders of magnitude due to increase in both carrier concentration and mobility, with a substantial negative shift in the threshold gate voltage also being observed. The drastic changes in the electrical properties were attributed to the decrease in species adsorbed on the surface, as well as to the increase in oxygen vacancies near the surface caused by ion bombardment.

Robust and Multifunctional Nanosheath for Chemical and Biological Nanodevices

The contribution of advanced nanoscale chemical and biological devices to life science has been limited to a small number of nanomaterials, due to the absence of effective surface modification routes. Herein, we demonstrate a polymer-like nanosheath synthesized by nonthermal plasma technology (NPT) that can protect the core nanomaterial from the solution environment and provide a multifunctional platform for chemical and biological nanosensors. For ZnO nanowires (NWs) which are unstable in solution, we demonstrate that this nanosheath makes it possible for ZnO NW field-effect transistors to act as a pH sensor for 24 h and a biosensor for the real-time, label-free detection of liver cancer markers.

Effect of chemically reactive species on properties of ZnO nanowires exposed to oxygen and hydrogen plasma

We present a systematic study on the effect of oxygen and hydrogen plasma-generated reactive species on the properties of ZnO nanowires. Upon exposure to oxygen plasma, the electrical conductivity of an individual ZnO nanowire decreased with substantial changes in the surface chemistry, indicating a decrease in the number of donor-like defects and an increase in the number of electron-trapping species. In contrast, an individual ZnO nanowire exposed to hydrogen plasma showed a drastic increase in conductivity up to two orders of magnitude due to the incorporated hydrogen acting as a shallow donor inside the ZnO nanowires without a sputtering process.

The synthesis of single PdAu bimetallic nanowire: feasibility study for hydrogen sensing

Single PdAu bimetallic nanowires have been synthesized via the sequential processes of electrochemical deposition and dielectrophoresis (DEP). In the first step, Pd/Au grains on predefined Au electrodes were grown by electrochemical deposition and could assist effectively the formation of a single PdAu nanowire with a good directionality by the subsequent DEP process. The synthesized PdAu nanowires have an average of approximately 10 at.% of Pd, a good resistance of a few hundred Omega, diameters of 300 nm on average and lengths of up to 15 microm. Based on the single PdAu nanowire, hydrogen detection was demonstrated in the range from 100 to 2500 ppm.

Inductively Coupled Plasma Etching of Ta, Co, Fe, NiFe, NiFeCo, and MnNi with Cl2/Ar Discharges

Dry etching of the magnetic thin films such as Ta, Fe, Co, NiFe, NiFeCo, and MnNi was carried out in inductively coupled plasmas of Cl2/Ar mixture. All the magnetic materials went through a maximum etch rate at 25% Cl2. The effects of the ICP source power and the rf chuck power on the etch rate and the surface roughness were quite dependent of the materials. An ion-enhanced chemical etch mechanism was important for the magnetic films. The surface roughness of the etched samples was relatively constant of the rf chuck power up to 200 W, but a rougher surface at a higher rf power was obtained. Post-etch cleaning of the etched samples in de-ionized water reduced the chlorine residues substantially.

Submicron patterning of Ta, NiFe, and Pac-man type Ta/NiFe/Ta magnetic elements

Submicron patterning of Ta, NiFe, and Pac-man type magnetic elements of Ta/NiFe/Ta has been carried out in inductively coupled plasmas (ICPs) of Cl2/Ar. Etch behavior was quite dependent on materials and plasma parameters. An ion-enhanced etch mechanism played a critical role for desorption of metal chloride etch products. Side-wall contamination with etch products was observed at a higher Cl2 concentration (>50%). Compared to relatively damaged surfaces and profiles by the ion milling method, the ICP etching technique produced clear, smooth, and well-defined Pac-man type elements.