Won Il Park

Metalorganic vapor-phase epitaxial growth of vertically well-aligned ZnO nanorods

We report metalorganic vapor-phase epitaxial growth and structural and photoluminescent characteristics of ZnO nanorods. The nanorods were grown on Al2O3(00⋅1) substrates at 400u200a°C without employing any metal catalysts usually needed in other methods. Electron microscopy revealed that nanorods with uniform distributions in their diameters, lengths, and densities were grown vertically from the substrates. The mean diameter of the nanorods is as narrow as 25 nm. In addition, x-ray diffraction measurements clearly show that ZnO nanorods were grown epitaxially with homogeneous in-plane alignment as well as a c-axis orientation. More importantly, from photoluminescence spectra of the nanorods strong and narrow excitonic emission and extremely weak deep level emission were observed, indicating that the nanorods are of high optical quality.

ZnO nanorods: synthesis,characterization and applications

This paper presents a review of current research activities on ZnO nanorods (or nanowires). We begin this paper with a variety of physical and chemical methods that have been used to synthesize ZnO nanorods (or nanowires). There follows a discussion of techniques for fabricating aligned arrays, heterostructures and doping of ZnO nanorods. At the end of this paper, we discuss a wide range of interesting properties such as luminescence, field emission, gas sensing and electron transport, associated with ZnO nanorods, as well as various intriguing applications. We conclude with personal remarks on the outlook for research on ZnO nanorods.

Fabrication and electrical characteristics of high-performance ZnO nanorod field-effect transistors

We report on fabrication and electrical characteristics of high-mobility field-effect transistors (FETs) using ZnO nanorods. For FET fabrications, single-crystal ZnO nanorods were prepared using catalyst-free metalorganic vapor phase epitaxy. Although typical ZnO nanorod FETs exhibited good electrical characteristics, with a transconductance of ∼140nS and a mobility of 75cm2∕Vs, the device characteristics were significantly improved by coating a polyimide thin layer on the nanorod surface, exhibiting a large turn-ON/OFF ratio of 104–105, a high transconductance of 1.9μS, and high electron mobility above 1000cm2∕Vs. The role of the polymer coating in the enhancement of the devices is also discussed.

Random laser action in ZnO nanorod arrays embedded in ZnO epilayers

Random laser action with coherent feedback has been observed in ZnO nanorod arrays embedded in ZnO epilayers. The sample was fabricated by depositing a MgO buffer layer and followed by a layer of ZnO thin film onto a vertically well-aligned ZnO nanorod arrays grown on sapphire substrate. Under 355 nm optical excitation at room temperature, sharp lasing peaks emit at around 390 nm with a linewidth less than 0.4 nm has been observed in all directions. In addition, the dependence of the lasing threshold intensity on the excitation area is shown in good agreement with the random laser theory. Hence, it is demonstrated that random laser action can also be supported in ZnO nanorod arrays.

Time-resolved photoluminescence of the size-controlled ZnO nanorods

Size dependence of the time-resolved photoluminescence (TRPL) has been investigated for the ZnO nanorods fabricated by catalyst-free metalorganic chemical vapor deposition. The nanorods have a diameter of 35 nm and lengths in the range of 150 nm to 1.1 μm. The TRPL decay rate decreases monotonically as the length of the nanorods increases in the range of 150 to 600 nm. Decrease of the radiative decay rate of the exciton-polariton has been invoked to account for the results.

Heteroepitaxal fabrication and structural characterizations of ultrafine GaN/ZnO coaxial nanorod heterostructures

We report on heteroepitaxial fabrication and structural characterizations of ultrafine GaN/ZnO coaxial nanorod heterostructures. The coaxial nanorod heterostructures were fabricated by epitaxial growth of a GaN layer on ultrafine ZnO nanorods. Epitaxial growth and precise control of GaN overlayer thickness were obtained by low pressure metalorganic vapor-phase epitaxy. ZnO nanorods grown on Si and sapphire substrates using catalyst-free metalorganic chemical vapor deposition exhibited diameters as small as 7 nm. Furthermore, structural properties of the coaxial nanorod heterostructures were investigated using both synchrotron-radiation x-ray diffraction and high resolution transmission electron microscopy.

Quantum confinement effect in ZnO∕Mg0.2Zn0.8O multishell nanorod heterostructures

We report on photoluminescence measurements of Mg0.2Zn0.8O∕ZnO∕Mg0.2Zn0.8O multishell layers on ZnO core nanorods. Dominant excitonic emissions in the photoluminescence spectra show a blueshift depending on the ZnO shell layer thickness attributed to the quantum confinement effect in the nanorod heterostructure radial direction. Furthermore, near-field scanning optical microscopy clearly shows sharp photoluminescence peaks from the individual nanorod quantum structures, corresponding to subband levels.

Formation and photoluminescent properties of embedded ZnO quantum dots in ZnO/ZnMgO multiple-quantum-well-structured nanorods

ZnO∕Zn0.8Mg0.2O multiple-quantum-well (MQW) nanorods with a different number of periods and well widths were grown by catalyst-free metal-organic vapor phase epitaxy. Their optical and structural characteristics were investigated using photoluminescence, transmission electron microscopy, and field emission scanning electron microscopy. Unlike ZnO∕ZnMgO MQW thin films, it was observed that embedded quantum dots played a more important role in the optical characteristics of ZnO∕ZnMgO MQW nanorods than quantum confined Stark effect due to polarization field.

Catalyst-free growth of ZnO nanorods and their nanodevice applications

We review recent research activities on catalyst-free growth of ZnO nanorods and their nanodevice applications. Since ZnO nanomaterials add excellent chemical sensing characteristics due to their large surface-to-volume ratio to intrinsic ZnO properties including semiconducting and piezoelectric properties, a wide direct band gap energy, and a large exciton binding energy, they are expected to become one of the most ideal materials for future high performance nanodevice applications. There have been extensive research activities for synthesis of high quality ZnO nanomaterials for nanodevice applications. Among the numerous growth methods, we have focused on catalyst-free metal-organic chemical vapor deposition, since this enables us to grow vertically aligned ZnO nanorods on various substrates, including Si and glass, without any special substrate treatment. This catalyst-free approach yields high purity and single crystalline nanostructures, and does not require a subsequent catalyst removal process. Furthermore, we established techniques for fabricating ZnO nanorod heterostructures with composition modulations along either radial or axial direction, which can provide more opportunities for novel optical and electronic device development. Furthermore, we briefly describe our activities on ZnO nanorod device fabrication and evaluation, including field-effect transistors, biological molecule sensors, Schottky diodes, and light emitting devices. We also address the remaining scientific issues and technical challenges, required to fully understand the device characteristics and to realise integrated nanodevices.

Fabrication and photoluminescent properties of ZnO/ZnMgO quantum structure nanorods

We report the epitaxial growth of ZnO/ZnMgO nanorod quantum structures and their structural and photoluminescent characteristics. The nanorod quantum structures grown using low pressure, metal-organic vapor phase epitaxy (MOVPE), exhibit uniform width and length distributions. Typical diameters and lengths of nanorods were in the range of 20-70 nm and 0.5-2 /spl mu/m. During the nanorod growth, ZnO/ZnMgO superlattices with their periodic thicknesses of 46-145 /spl Aring/ were fabricated by depositing ZnO and ZnMgO layers alternatively. Furthermore, quantum confinement of carriers in the superlattice nanorods was observed using photoluminescence (PL) spectroscopy.