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Individual Ga-doped ZnO microwires covered by Au nanorods: wavelength-tunable incandescent-type light sources

Owing to their ultracompact physical sizes, highly localized coherent output, and efficient waveguiding, one dimensional (1D) components, such as nanowires (NWs), nanotubes, and microwires (MWs), have been considered as one of the most promising building blocks for fully integrated nano/microscale photonic and optoelectronic devices. ZnO has been recognized as a competent candidate for photoelectronic devices because of their excellent inherent electronic and optoelectronic properties. In the study of the research group from Nanjing University of Aeronautics and Astronautics (NUAA), individual ZnO MWs with controlled Ga-doping concentration (ZnO:Ga MWs) were successfully prepared in the synthesis process via chemical vapor deposition (CVD) by means of adjusting the Ga2O3 weight ratios in the precursor reaction mixtures, as well as corresponding the sizes of MWs.

Electrically driven strong light-emitting from individual ZnO:Ga MWs based devices were realized with tunable colors, and the emission region is localized towards the center of MWs, which can be treated as a typical incandescent filament lamp. They attached an individual ZnO:Ga MW to metal electrodes, directly on the substrate, and passed a current through the filaments to cause them lighting. The bright lighting is so intense that it can be clearly observed by naked eyes in normal indoor lighting conditions. Corresponding light-emitting can be derived from Ga-dopant induced impurity band and Joule heating effect, but cannot be compatible with thermoluminescence. By adjusting the length of MWs, lighting emitters can be tuned from elongated lighting sources to spot lighting sources. Meanwhile, owing to the absence of rectification characteristics, relevant electrical measurement results show that the alternating current-driven light-emitting functions excellently on the ZnO:Ga MWs. Specially, by incorporating Au nanorods with controlled sizes, the dominating emission peak wavelengths of single ZnO:Ga MW based incandescent-type filament light source can be redshifted into near-infrared spectral band. Relevant research results are published in Photonics Research, Vol.8, Issue 1, 2020(Zhipeng Sun, Mingming Jiang, Wangqi Mao, Caixia Kan, Chongxin Shan, Dezhen Shen. Nonequilibrium hot-electron-induced wavelength-tunable incandescent-type light sources[J]. Photonics Research, 2020, 8(1): 01000091).

In addition to the accurate control over the composition, band gap engineering, energy level, and doping level, this work proposes a novel scheme to construct wavelength-tunable incandescent-type light source from individual ZnO:Ga MWs prepared with Au nanorods decoration. By means of adjusting the aspect ratio of Au nanorods, the dominant peak wavelengths can be tuned across the visible to near-infrared spectral band. To investigate the modulation of Au-nanorod on the incandescent-type emission features, single ZnO:Ga MW covered by Au nanorods can provide a platform to achieve electrically driven the generation of hot electrons, which stay in a non-equilibrium energy distribution for the lifetimes well below a picosecond level. After relaxation nonradiatively, the generated non-equilibrium hot-electron can inject into neighboring semiconductors, leading to the formation of state-filling effect in the energy band of ZnO:Ga. Thereby, Au-nanorod plasmons induced the generation and injection of non-equilibrium hot electrons can be utilized to dominate the tuning emissions, instead of plasmons induced near-field coupling and lighting amplification. Consequently, the novel incandescent-type light sources may find potential applications in integrated optoelectronic devices, such as multicolor emission devices, and electric spasers.

Recently, creating light in small structures on the surface of a chip has enabled plenty of applications such as high-performance communication, low cost lighting and smart displays. It is crucial to develop fully integrated photonic circuits that do with light what is now done with electric currents in semiconducting integrated circuits. In modern integrated lighting sources industries, controlling the colors of light-emitting devices is a challenging task. Individual ZnO:Ga MWs prepared via Au nanorods deposition, while preliminary, opens up intriguing scientific questions, and possible applications of one dimensional linear, transparent, flexible displays and optical interconnects with electronic circuits. This kind of lighting emitters endow a new sense of the oldest and simplest artificial light source, the incandescent light bulb integrated onto a chip. Besides, the quantification of Ga-doping concentration in ZnO:Ga MWs is not accurate when the actual value of the Ga in ZnO is less than 1%. Thus, a straightforward identification of the distinction still remains elusive at this stage. Additionally, an effective approach to modify the electrical conductivities, as well as the EL emission wavelengths by means of Ga-dopant and another method still needs further exploration and practice.

Typical incandescent emitter composed of single Ga-doped ZnO microwire covered by Au nanorods fabricated. By adjusting the aspect ratios of Au nanorods, wavelength-tunable emissions were achieved, with the dominating peaks tuning from visible to near-infrared spectral regions.