Jiangping Dai

Significant improvements in InGaN/GaN nano-photoelectrodes for hydrogen generation by structure and polarization optimization.

The photoelectrodes based on III-nitride semiconductors with high energy conversion efficiency especially for those self-driven ones are greatly desirable for hydrogen generation. In this study, highly ordered InGaN/GaN multiple-quantum-well nanorod-based photoelectrodes have been fabricated by a soft UV-curing nano-imprint lithography and a top-down etching technique, which improve the incident photon conversion efficiency (IPCE) from 16% (planar structure) to 42% (@ wavelength = 400 nm). More significantly, the turn-on voltage is reduced low to −0.6 V, which indicates the possibility of achieving self-driven. Furthermore, SiO2/Si3N4 dielectric distributed Bragg reflectors are employed to further improve the IPCE up to 60%. And the photocurrent (@ 1.1 V) is enhanced from 0.37 mA/cm2 (original planar structure) to 1.5 mA/cm2. These improvements may accelerate the possible applications for hydrogen generation with high energy-efficiency.

Great enhancement in the excitonic recombination and light extraction of highly ordered InGaN/GaN elliptic nanorod arrays on a wafer scale.

A series of highly ordered c-plane InGaN/GaN elliptic nanorod (NR) arrays were fabricated by our developed soft UV-curing nanoimprint lithography on a wafer. The photoluminescence (PL) integral intensities of NR samples show a remarkable enhancement by a factor of up to two orders of magnitude compared with their corresponding as-grown samples at room temperature. The radiative recombination in NR samples is found to be greatly enhanced due to not only the suppressed non-radiative recombination but also the strain relaxation and optical waveguide effects. It is demonstrated that elliptic NR arrays improve the light extraction greatly and have polarized emission, both of which possibly result from the broken structure symmetry. Green NR light-emitting diodes have been finally realized, with good current-voltage performance and uniform luminescence.

Investigation of surface-plasmon coupled red light emitting InGaN/GaN multi-quantum well with Ag nanostructures coated on GaN surface

Surface-plasmon (SP) coupled red light emitting InGaN/GaN multiple quantum well (MQW) structure is fabricated and investigated. The centre wavelength of 5-period InGaN/GaN MQW structure is about 620 nm. The intensity of photoluminescence (PL) for InGaN QW with naked Ag nano-structures (NS) is only slightly increased due to the oxidation of Ag NS as compared to that for the InGaN QW. However, InGaN QW with Ag NS/SiO2 structure can evidently enhance the emission efficiency due to the elimination of surface oxide layer of Ag NS. With increasing the laser excitation power, the PL intensity is enhanced by 25%–53% as compared to that for the SiO2 coating InGaN QW. The steady-state electric field distribution obtained by the three-dimensional finite-difference time-domain method is different for both structures. The proportion of the field distributed in the Ag NS for the GaN/Ag NS/SiO2 structure is smaller as compared to that for the GaN/naked Ag NS structure. As a result, the energy loss of localized SP modes ...

Polarized Emission From InGaN/GaN Single Nanorod Light-Emitting Diode

InGaN/GaN single nanorod (NR) light-emitting diodes (LEDs) have successfully been fabricated by nanoimprint lithography and focused ion beam-induced deposition. The current-voltage characteristics of single NR LEDs show low leakage current of 2 × 10-12 A at a reverse bias of -5 V and turn-ON voltage of ~3.8 V. Linear polarization-oriented parallel to the c-axis with a degree of ~50% was discovered from the electroluminescence emission of single NR LEDs.

Improvement of color conversion and efficiency droop in hybrid light-emitting diodes utilizing an efficient non-radiative resonant energy transfer

Blue InGaN/GaN nanohole light-emitting diodes have been fabricated by soft UV-curing nanoimprint lithography, filling with CdSe/ZnS core/shell nanocrystals (NCs) as color conversion mediums. The excitonic recombination dynamics of hybrid nanohole light-emitting diodes were investigated by time-resolved photoluminescence, observing a significant reduction in the decay lifetime of excitons as a result of an efficient non-radiative resonant energy transfer, which leads to the improvement of color conversion and efficiency droop in these hybrid nanohole light-emitting diodes compared to hybrid nanocrystals/standard planar light-emitting diodes. The color-conversion efficiency and effective quantum yield of hybrid nanohole light-emitting diodes were nearly twice as much as those of hybrid standard light-emitting diodes. A model on the excitonic recombination process was proposed to explore this situation, explaining the advantages of non-radiative resonant energy transfer that avoiding energy loss associated w...

Optical polarization characteristics of c-plane InGaN/GaN asymmetric nanostructures

Highly ordered c-plane InGaN/GaN elliptic nanorod (NR) and nano-grating (NG) arrays were fabricated by our developed soft UV-curing nanoimprint lithography on a wafer. The polarized photoluminescence emission from these elliptic NR and NG arrays has been investigated both theoretically and experimentally. Considerable in-plane optical anisotropy, with a polarization ratio of 15% and 71% and a peak shift of 5.2 meV and 28.1 meV, was discovered from these c-plane InGaN/GaN elliptic NR and NG arrays, respectively. The k·p perturbation theory was adopted to explore this situation, simulating the transitions from conduction subbands to valence subbands and their corresponding optical momentum matrix elements at/around Γ point under the in-plane asymmetric strain. The good agreements of observed and simulated results demonstrate that the in-plane strain asymmetry is the essential cause of the optical polarization in this case, revealing the great potential to utilize strain effect to control the polarization of...