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

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.

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.

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因具有超紧凑的物理尺寸、高度局域化的相干输出和有效的波导调制等功能,低维功能纳米结构,如纳米线、纳米管和微米线等,被认为是完全可用于构筑高度集成的微纳米尺度光子和光电子器件最有前途的元器件之一。氧化锌因其拥有优异的光学和光电特性,较高的激子束缚能和宽带隙等特点,成为典型的第三代半导体候选材料之一。南京航空航天大学姜明明教授课题组采用化学气相沉积(CVD)实验方法,通过调节反应源中Ga2O3高纯粉末的质量比,成功地制备了Ga掺杂浓度可调的单根ZnO (ZnO:Ga)微米线。此外,单根ZnO:Ga微米线的尺寸、结构和形貌同样也可以通过反应源的质量比、载气的流量和反应温度等实验件实现可调,比如横截面为四边形的微米线,横截面为六边形的微米线、微米带等。单根微米线的直径在5 - 30 μm,长度达2 cm,在宏观条件下可直接操作。


通过对单根ZnO:Ga微米线荧光发光物理机制的研究发现,单根微米线的I-V属于典型的欧姆接触特性,且随着微米线长度的改变发光区的长度、发光区域的位置同样可以调控。单根ZnO:Ga微米线荧光灯丝光发射现象不需要传统半导体发光器件的结区构筑,比如p-n结,金-半的Schottky结等结构;同样也不能用钨丝灯发光的物理机制来解释。这是因为单根微米线所能承载的温度很低,不可能像钨丝那样产生几千度的高温;且随着环境温度的降低,单根ZnO:Ga微米线的发光出现明显的增强;随微米线两端电压的增加,发光中心波长基本不发生任何的红移或者蓝移现象;甚至在交流电驱动下,单根ZnO:Ga微米线同样可以发光。特别地,在单根ZnO:Ga微米线表面旋涂一层金纳米棒之后,可实现调制单ZnO:Ga微米线基荧光灯丝发光的中心波长,甚至红移至近红外光谱波段。相关研究成果发表在Photonics Research 2020年第8卷第1期上,并被主编选为封面文章(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)。


单根微米线在经Ga掺杂和金纳米棒包裹之后拥有较为优异的导电能力,同时兼具较高的结晶质量。在微米线两端电压达到一定值时,焦耳热效应导致的热量限域在微米线的中间区域产生热点(hottest spot),加载在微米线两端的电压等同于直接施加在热点上形成强电场(场强高达106 V/m)。单根AuNRs@ZnO:Ga微米线在电驱动下能激发金纳米棒表面等离激元,经非辐射形式衰减时将产生热电子(金纳米棒表面电子的集体振荡频率接近金体材料的等离子体频率),在金纳米棒-ZnO:Ga微米线接触界面处能够直接注入到ZnO:Ga导带中。经弛豫之后,在ZnO:Ga能级结构中形成态填充(state-filling effect)。因此,金纳米棒等离子体激元诱导的非平衡热电子的产生和注入能够用来主导和调控单根AuNRs@ZnO:Ga微米线荧光灯丝光源的发光特征。此外,由于这种单根半导体微纳米线基新型白炽灯式荧光灯丝光源没有传统半导体电致发光器件所需的结区耗尽层,也不需要像钨丝灯式光源发光需要相对苛刻的真空条件,且能够在交流电驱动下发光,因此有望在多色光发射器件、电驱动隧穿发光器件以及电抽运等离激子激光器等微纳尺度集成光电器件中得到应用。



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