Hanhong Chen

Integrated ZnO nanotips on GaN light emitting diodes for enhanced emission efficiency

Enhancement of light extraction from an integrated ZnO nanotips/GaN light emitting diode (LED) is demonstrated. The device is composed of a GaN LED with a Ga-doped ZnO (GZO) transparent conductive layer and ZnO nanotips grown on GZO for light extraction. The light output power of a ZnO nanotips/GZO/GaN LED exhibits 1.7 times enhancement, in comparison with a conventional Ni∕Au p-metal LED. The higher emission efficiency is attributed to the enhanced light transmission and scattering in the ZnO∕GaN multilayer.

Dye-sensitized solar cells using ZnO nanotips and Ga-doped ZnO films

Ga-doped ZnO (GZO) transparent conducting films and well-aligned ZnO nanotips were sequentially grown on a glass substrate using metal–organic chemical vapor deposition (MOCVD). The morphology control of ZnO from dense films to nanotips was realized through temperature-modulated growth. The ZnO nanotips/GZO structure was sensitized with dye N719 to form photoelectrochemical cells. It is found that the power conversion efficiency linearly increases with the length of ZnO nanotips. For the 1.0 cm2 dye-sensitized solar cell built from 4.8 µm ZnO nanotips, a peak incident photo-to-current conversion efficiency of 79% (at ~530 nm) and a power conversion efficiency of 0.77% under the illumination of one sun-simulated sunlight were achieved. UV light harvesting directly by ZnO was observed. The I–V characteristics of the cells were analyzed using a one-diode equivalent circuit model.

A ZnO Nanostructure-Based Quartz Crystal Microbalance Device for Biochemical Sensing

We report a ZnO-nanostructure-based quartz crystal microbalance (nano-QCM) device for biosensing applications. ZnO nanotips are directly grown on the sensing area of a conventional QCM by metalorganic chemical vapor deposition (MOCVD). Scanning electron microscopy (SEM) shows that the ZnO nanotips are dense and uniformly aligned along the normal to the substrate surface. By using superhydrophilic nano-ZnO surface, more than tenfold increase in mass loading sensitivity of the nano-QCM device is achieved over the conventional QCM. The ZnO nanotip arrays on the nano-QCM are functionalized. The selective immobilization and hybridization of DNA oligonucleotide molecules are confirmed by fluorescence microscopy of the nano-QCM sensing areas.

Properties of ZnO Nanotips Selectively Grown by MOCVD

We report the catalyst-free growth of ZnO nanotips by metalorganic chemical vapor deposition (MOCVD) on various substrates, including c-sapphire, (100) Si, titanium, glass and SiO2. Structural, optical, and electrical properties of ZnO nanotips are investigated. ZnO nanotips are found to be single crystalline and oriented along the c-axis normal to the growth plane. The nanotips exhibit dominant free excitonic transition and enhanced luminescence efficiency with negligible deep-level emission. Controllable in situ Ga doping during MOCVD growth reduces the resistivity of ZnO nanotips. Selective growth of ZnO nanotips has been achieved on patterned Ti/r-Al2O3, SiO2/r-Al2O3, and silicon-on-sapphire (SOS) substrates. It provides the potential to integrate ZnO nanotips and ZnO epitaxial films on a single patterned substrate for various device applications.

Ultrafast transport in dye sensitized ZnO nanotips investigated by terahertz spectroscopy

Transient far-infrared conductivity of as-grown and dye-sensitized ZnO nanotips is measured using time-resolved terahertz spectroscopy. We reveal that the sensitization procedure dramatically affects the transport properties of ZnO nanotips.

SU‐FF‐T‐246: Novel ZnO Nanotips: Potential Nanomaterials for New Generation of Radiation Detectors

Purpose: Current dosimetry technology is limited by large detector size (ion chambers), complex handling processes (TLDs), energy dependent and limited dynamic range (films), and thermal noise and low sensitivity (narrow bandgap semiconductor-based dosimeters). Wide bandgap materials have lower background carrier concentration and better temperature stability. ZnO is an emerging wide bandgap semiconductor with a direct bandgap Eg∼3.30 eV at room temperature. ZnO is biocompatible and significantly more radiation-hard than Si, GaAs, or GaN. Thus, nanoscale ZnO is particularly suitable for long term in vivo therapeutic X-ray and γ-ray measurements. Here, we report our preliminary investigations of ZnO nanotips for potential medical dosimetry applications. Method and Materials: High-quality ZnO nanotips were grown on Si substrates by metal-organic chemical vapor deposition. The as-grown ZnO nanotip samples (n=20) were irradiated using 6 MV, 15 MV, 9 MeV, and 20 MeV photon and electron beams on a VARIAN CLINAC 21EX. The irradiated samples were analyzed using X-ray diffraction (XRD), field emission scanning electron microscopy (SEM), photoluminescence spectroscopy (PLS), and sheet resistance measurements to characterize the radiation effects. Results: Irradiated ZnO nanotips were found to exhibit identical morphology and functionality as the as-grown ones without observable lattice damage or defect. XRD data showed no change in the intensity or the full width at half maximum (FWHM) of the peaks. The nanotips maintained the primary c-axis orientation, further peaks other than ZnO (002) – (004) doublet and substrate Si (400) were not observed, indicating absence of secondary phase formation or change in structure. Particularly, they maintained good electrical, optical, and crystal properties, the key to make X-ray and γ-ray detectors based on ZnO nanotips. Conclusion: ZnO nanotips are a promising new generation of nanomaterials with great potentials in nano-electronics, nano-photonics, high-resolution real-time in vivo dosimetry, integrated dose measurement by implantation, and microbeam calibration and profiling.