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.
单根ZnO:Ga微米线发光，为白炽灯照明赋予新生
单根金纳米棒包裹Ga掺杂ZnO微米线：波长可调谐的荧光灯丝光源。通过调整金纳米棒的长径比，可以实现波长可调的发射，其主要发射峰从可见光到近红外光谱区域可调。
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。
以载玻片或者石英片做衬底，取一根ZnO:Ga微米线置于衬底上，以铟颗粒做电极，单根微米线在电驱动下会发出耀眼的可见光，发光区域局限于微米线的中间区域；且随着电压的增加发光强度增加；单根ZnO:Ga微米线荧光灯丝的发光在室内肉眼可见。单根ZnO:Ga微米线的发光类似于白炽灯钨丝灯丝光源光发射现象，因此，作者基于单根半导体微米线构筑了一种新型的荧光灯丝光源。有趣的是，随着单根ZnO:Ga微米线中Ga掺杂浓度增加，单根微米线荧光发光的中心波长在可见光波段范围内出现明显的红移。
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）。
发光中心波长可调谐的低维微纳光源在照明、显示、成像和诊断治疗等领域有着重要的应用价值和应用前景。而目前，实现对半导体微纳光源发光波长的有效调控需要半导体组分的精准调制、半导体能带工程和半导体微纳材料的有效掺杂调控等手段。该课题组在最近的研究中发现，采用纵横比可调的金纳米棒旋涂单根ZnO:Ga微米线（AuNRs@ZnO:Ga），可以将单根微米线荧光灯丝发光中心波长从可见光波段延伸至近红外波段，从而实现单根ZnO:Ga微米线基荧光灯丝光源发光特征的可调谐。在对单根AuNRs@ZnO:Ga微米线荧光灯丝光源的发光物理机制的研究中，研究人员发现金纳米棒局域表面等离激元在电驱动下有效激发主导了单根AuNRs@ZnO:Ga微米线的荧光发光特征，此外，单根AuNRs@ZnO:Ga微米线近红外荧光发光不是来自于金纳米棒局域表面等离激元对单根ZnO:Ga微米线荧光发光在近红外波段发光的选择性增强和放大。
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。
近年来，微纳米光源的开发使光电子器件得以在芯片上高度集成化，并广泛应用于高性能通信、低成本照明和智能显示器。而开发利用各种微纳光源器件成为研制集成光子和光电子电路中至关重要的一环。有效调控微纳光源器件的发光特征和发光品质是目前低维半导体光电子材料与器件研究工作中一项极具挑战性的任务。
该课题组近年来主要致力于低维微纳米材料的可控性生长以及相应光电器件的研究工作中，单根ZnO:Ga微米线的可控性掺杂、荧光灯丝光源的构筑、以及采用金属纳米结构调控单根微米线荧光灯丝发光特征。虽然该部分研究是初步的基础性研究，但它开启了基于单根微米线在电驱动下荧光灯丝发光器件的新的发光现象，同时为电驱动激发金属纳米结构局域表面等离激元的有效激发、金属纳米结构热电子产生的物理机制以及在界面处表面等离激元介入下的非辐射能量转移新方式等一系列科学问题，为日后可能应用于一维线性、透明、灵活的显示器和与电子电路提供了可行的实验方案。单根微米线基荧光灯丝光源为白炽灯照明这一最古老、最简单的低维微纳光源赋予了新的生命。此外，单根ZnO微米线中Ga-掺杂浓度的定量化与荧光灯丝发光中心波长之间的量化关系（在低维微纳米材料与结构中实际掺杂浓度值小于1%是不准确的，目前还没有比较精准的测试和表征手段）还有待于进一步的研究。同时，基于金属纳米结构包裹单根ZnO:Ga微米线在电驱动下表面等离激元激发的阈值条件、热电子震荡频率的精准度量等方面的科学问题仍需要进一步的探索和实践。