Chung-Liang Cheng

Enhancement of band gap emission stimulated by defect loss.

Defect radiation has been always considered as the most important loss for an emitter based on band gap emission. Here, we propose a novel approach which goes against this conventional wisdom. Based on the resonance effect between the surface plasmon of metal nanoparticles and defect emission, it is possible to convert the useless defect radiation to the useful excitonic emission with a giant enhancement factor. Through the transfer of the energetic electrons excited by surface plasmon from metal nanoparticles to the conduction band of the emitter, the band gap emission can be greatly enhanced, while the defect emission can be suppressed to noise level.

Giant enhancement of band edge emission based on ZnO/TiO 2 nanocomposites

Enhancement of band edge emission of ZnO nanorods up to a factor of 120 times has been observed in the composite consisting of ZnO nanorods and TiO(2) nanoparticles, while the defect emission of ZnO nanorods is quenched to noise level. Through a detailed investigation, it is found that the large enhancement mainly arises from fluorescence resonance energy transfer between the band edge transition of ZnO nanorods and TiO(2) nanoparticles. Our finding opens up new possibilities for the creation of highly efficient solid state emitters.

Raman scattering and field-emission properties of RuO2 nanorods

We report Raman scattering and field emission properties of rutile RuO2 nanorods obtained by metalorganic chemical vapor deposition. The RuO2 nanorods have lengths up to several micrometers and diameters in the range of 10–50 nm. The nanosize dependencies of the peak shift and the broadening of the three first-order Raman modes agree well with those calculated on the basis of the phonon confinement model. The unique geometrical features of RuO2 nanorods exhibit a strong effect on field enhancement (β∼1153), which results in a low threshold field (Eth∼4.9V∕μm) defined at the beginning of emission. The low turn-on field for driving a current of 10μA∕cm2 is about 10.3V∕μm, which is comparable with amorphous carbon film. Our results indicate that RuO2 nanorods provide an excellent alternative for field emitter due to several advantages, including nanometer structure, natural conductor, enhanced resistance to oxidation, and long-term stability.

Selectively enhanced band gap emission in ZnO/Ag 2 O nanocomposites

A new composite consisting of ZnO nanorods decorated with Ag(2)O nanoparticles has been synthesized and characterized. It is found that the band gap emission of ZnO nanorods can be greatly enhanced by about 10 times, while the defect emission can be suppressed to the detection limit, simultaneously. The ratio between the band gap and defect emission reaches to an enhanced factor of about 600 times. The underlying mechanism is attributed to the combined effects of surface modification, band alignment, as well as charge transfer. Our approach provided here can be extended to many other semiconductors for creating nanocomposites with novel optical properties.

Residual thermal strain in thick GaN epifilms revealed by cross-sectional Raman scattering and cathodoluminescence spectra

Strain can significantly alter the physical properties of a solid. We demonstrate that in a thick GaN epilayer there exists a residual thermal strain along the growth direction. This result is clearly revealed by cathodoluminescence spectra, in which the band gap of the GaN film decreases with distance away from the epifilm?substrate interface. This result is further confirmed by Raman scattering spectra in which the phonon modes show a red shift along the growth direction. Our finding is important for the understanding and application of nitride semiconductors.

White-light electroluminescence from ZnO nanorods/polyfluorene by solution-based growth

We report bright white-light electroluminescence (EL) from a diode structure consisting of a ZnO nanorod (NR) and a p-type conducting polymer of poly(fluorine) (PF) fabricated using a hydrothermal method. ZnO NRs are successfully grown on an organic layer of PF using a modified seeding layer. The EL spectrum shows a broad emission band covering the entire visible range from 400 to 800 nm. White-light emission is possible because the ZnO-defect-related emission from the ZnO NR/PF heterostructure is enhanced to become over thousand times stronger than that from the usual ZnO NR structure. This strong green-yellow emission associated with the ZnO defects, combined with the blue PF-related emission, results in the white-light emission. Enhancement of the ZnO-defect emission is caused by the presence of Zn(OH)(2) at the interface between the ZnO NRs and PF. Fourier transform infrared spectroscopy reveals that the absorption peaks at 3441, 3502, and 3574 cm(-1) corresponding to the OH group are formed at the ZnO NR/PF heterostructure, which confirms the enhancement of defect emission from the ZnO NR/PF heterostructure. The processing procedure revealed in this work is a convenient and low-cost way to fabricate ZnO-based white-light-emitting devices.

Photoelastic effect in ZnO nanorods

A novel phenomenon called the photoelastic effect had been observed in ZnO nanorods, along with a number of intriguing anomalies. With increasing excitation power, it was found that the A1 (LO) phonon exhibited a red-shift in frequency, on top of a blue-shift in the photoluminescence (PL) peak energy. In addition, the temperature-dependent photoluminescence spectra behaved quite differently under high and low excitation power. All our results can be accounted for by the photoelastic effect, in which the built-in surface electric field was screened by photoexcited electrons and holes. Through the converse piezoelectric effect, the internal strain was therefore altered. Our results make possible a new thrust for manipulating the physical properties of ZnO nanorods, and should prove very useful in the application of optoelectric devices.

Enhancement of band-edge emission induced by defect transition in the composite of ZnO nanorods and CdSe/ZnS quantum dots

A new and general approach to enhance band-edge emission at the expense of defect emission in a semiconductor nanocomposite is proposed. The underlying mechanism is based on the resonance effect between defect transition and band-to-band excitation and transfer of excited electrons between conduction band edges. With our approach, it is possible to convert defect loss into bandgap emission. As an example, we demonstrate that the bandgap emission of ZnO nanorods can be enhanced by as much as 30 times when they are compounded with CdSe/ZnS nanoparticles.

Giant white and blue light emission from Al2O3 and ZnO nanocomposites

A new and general approach enabling us to amplify not only the bandgap emission of ZnO nanorods but also the defect emission of Al(2)O(3) is proposed. The light intensity of the band edge emission of ZnO nanorods can be improved by as much as 19 times after the decoration of Al(2)O(3) layers. Moreover, white light emission arising from Al(2)O(3) defects in ZnO/Al(2)O(3) nanostructures also shows a large enhancement factor of 12 times. Our new strategy offers an alternative possibility to create strong white and blue light-emitting devices.

Studies of Stokes shift in InxGa1−xN alloys

InGaN ternary alloys have been studied with photoluminescence, photoluminescence excitation spectroscopy, scanning electron microscopy, and cathodoluminescence spectroscopy. The relatively large Stokes shift observed in the photoluminescence and photoluminescence excitation spectroscopy has been found to be consistent with previous results reported in the literature. By correlating our experimental findings and others reported in the literature with those of scanning electron microscopy and cathodoluminescence spectroscopy, we conclude that the physical origin of the Stokes shift in InGaN ternary alloy system is primarily due to the effects of alloy composition fluctuations. A plausible model responsible for the observed Stokes shift is proposed.