Self-Oscillated Growth Formation of Standing Ultrathin Nanosheets out of Uniform Ge/Si Superlattice Nanowires

Self-oscillation is an intriguing and omnipresent phenomenon that governs a broad range of growth dynamics and formation of nanoscale periodic and delicate heterostructures.A self-oscillating growth phenomenon of catalyst droplets,consuming surface-coating a-Si/a-Ge bilayer,is exploited to accomplish a high-frequency alternating growth of ultrathin crystalline Si and Ge(c-Si/c-Ge)nano-slates,with Ge-rich layer thickness of 14–19 nm,embedded within a superlattice nanowire structure,with pre-known position and uniform channel diameter.A subsequent selective etching of the Ge-rich segments leaves a chain of ultrafine standing c-Si nanosheets down to~6 nm thick,without the use of any expensive high-resolution lithography and growth modulation control.A ternary-phase-competition model has been established to explain the underlying formation mechanism of this nanoscale self-oscillating growth dynamics.It is also suggested that these ultrathin nanosheets could help to produce ultrathin fin-channels for advanced electronics,or provide size-specified trapping sites to capture and position hetero nanoparticle for high-precision labelling or light emission.

超薄三维径向结叠层高开启电位(1.15V)光阴极用于太阳能制氢

a-Si:H薄膜中存在的无序结构和缺陷会大幅减小载流子扩散长度.因此,传统的二维平面叠层光伏器件需要在有效的载流子收集(较短的载流子分离距离)和足够的光吸收(较厚的光吸收层厚度)之间平衡.三维a-Si:H径向结薄膜光伏器件可以很好地解决传统平面结中这一难题,它是直接在气-液-固法生长的竖直站立的硅纳米线框架上构建.三维硅米线框架具有很强的光捕获能力,允许使用十分薄的光吸收层(<100 nm),有助于解除光学吸收长度与电学载流子收集路径之间的耦合.薄的吸收层有助于改善光致衰减,提升对材料质量的容忍程度,降低对高品质、成本贵的材料的需求.本文提出了将先进的叠层设计与独特的三维硅纳米线框架相结合的新策略,设计了具有高开启电位的光电阴极,以推动太阳能制氢的应用进程.提出了径向结叠层薄膜光电阴极,其直接在气-液-固法生长的硅纳米线上构筑,由两个径向堆叠的p-i-n结组成.外层p-i-n结的吸收层为非晶硅,用于吸收短波长的光;内层p-i-n结的吸收层则为非晶硅锗合金,用于吸收长波长的光.随机分布的硅纳米线框架具有优异的陷光效应,可允许使用非常薄的非晶硅(~50 nm)和非晶硅锗(~40 nm)吸收层.在100 mW/cm^(2)的太阳光照射下,该径向结叠层光阴极在覆盖2.5 nm厚的Pt催化剂后,在0.1 mol/L磷酸钾水溶液(KPi,pH=7)电解液中具有1.15 V vs.RHE的高开启电位,零偏压下的光电流为2.98 mA/cm^(2),外加偏压下的光电转换效率为1.72%.综上,本文展现了三维径向结叠层技术的独特潜力,有利于发展下一代高开启电位、高效、低成本、耐用、可大规模制备的光阴极.

Fabrication of Anti-reflecting Si Nano-structures with Low Aspect Ratio by Nano-sphere Lithography Technique

Nano-structured photon management is currently an interesting topic since it can enhance the optical absorption and reduce the surface reflection which will improve the performance of many kinds of optoelectronic devices, such as Si-based solar cells and light emitting diodes. Here, we report the fabrication of periodically nano-patterned Si structures by using polystyrene nano-sphere lithography technique. By changing the diameter of nano-spheres and the dry etching parameters, such as etching time and etching power, the morphologies of formed Si nano-structures can be well controlled as revealed by atomic force microscopy.A good broadband antireflection property has been achieved for the formed periodically nano-patterned Si structures though they have the low aspect ratio(<0.53). The reflection can be significantly reduced compared with that of flat Si substrate in a wavelength range from 400 nm to 1200 nm. The weighted mean reflection under the AM1.5 solar spectrum irradiation can be as low as 3.92% and the corresponding optical absorption is significantly improved, which indicates that the present Si periodic nano-structures can be used in Si-based thin film solar cells.

Energy transfer process between Eu^(3+) and wide-band-gap SnO_2 nanocrystals in silica films studied by photoluminescence excitation and time-resolved photoluminescence techniques

Eu3+ions embedded in silica thin films codoped with SnO2 nanocrystals were fabricated by sol–gel and spin-coating methods.SnO2 nanocrystals with controllable sizes were synthesized through precisely controlling the Sn concentrations.The influences of doping and annealing conditions on the photoluminescence intensity from SnO2 nanocrystals are systematically investigated.The effective energy transfer from the defect states of SnO2nanocrystals to nearby Eu3+ions has revealed by the selective photoluminescence excitation spectra.The efficiency of the Forster resonance energy transfer is evaluated by the time-resolved photoluminescence measurements,which is about 29.1%based on the lifetime tests of the SnO2emission.

Si nanocrystals-based multilayers for luminescent and photovoltaic device applications

Low dimensional Si materials have attracted much attention because they can be developed in many kinds of new-generation nano-electronic and optoelectronic devices, among which Si nanocrystals-based mul- tilayered material is one of the most promising candidates and has been extensively studied. By using multilayered structures, the size and distribution of nanocrystals as well as the barrier thickness between two adjacent Si nano- crystal layers can be well controlled, which is beneficial to the device applications. This paper presents an over- view of the fabrication and device applications of Si nanocrystals, especially in luminescent and photovoltaic devices. We first introduce the fabrication methods of Si nanocrystals-based multilayers. Then, we systematically review the utilization of Si nanocrystals in luminescent and photovoltaic devices. Finally, some expectations for further development of the Si nanocrystals-based photonic and photovoltaic devices are proposed.

Annealing effect on optical and electronic properties of silicon rich amorphous silicon-carbide films

A series of Si-rich amorphous silicon carbide （a-SiC：H） thin films were deposited in conventional plasma enhanced chemical vapor deposition system with various gas ratio R = [CH4]/[SiH4]. The microstructural, optical and electronic properties of as-deposited films were investigated in this study. It was found that optical band gap was linearly proportional to carbon content in the films and it could be controlled in a range of 1.8-2.4 eV by changing the gas ratio, R. Both dark and photo conductivities in room temperature were decreased with the increasing of carbon content in the films, and the photosensitivity reached as high as 104 for the film with the optical band gap of 1.96 eV. The as-deposited samples were subsequently annealed at the temperatures of 900℃ and 1000℃. The formation of nanocrystalline silicon （nc- Si） dots in amorphous silicon carbide （a-SiC） host matrix was shown. The dark conductivity was enhanced by five orders of magnitude after annealing compared with that of as-deposited films. The result of temperature-dependent conductivity suggested that the property of carrier transport was dominated by conduction process between the extended states. Furthermore, room temperature electroluminescence （EL） was achieved from nc-Si/SiC system and the possible mechanism of radiative recombination mechanism was discussed.