The excellent spontaneous ultraviolet emission of GaN nanostructures grown on silicon substrates by thermal vapor deposition

In this study, GaN nanostructures were grown on p-Si (111) substrate by thermal chemical vapor deposition (TCVD). Ga vapor directly reacted with NH₃ solution in N₂ carrier gas flow of 2 L/min at different temperatures (950–1050 °C). The influence of NH₃ solution and growth temperature on the morphology, structure, optical and photoresponse properties of GaN nanostructures was investigated. Scanning electron microscopy images showed that the densities of the NWs varied with increasing temperature. The use of NH₃ solution and increased growth temperature improved the crystalline quality of GaN nanostructures. The photoluminescence (PL) spectra of nanostructures displayed a near band-edge (NBE) emission at around 363–367 nm. Higher growth temperature (1050 °C) resulted in a strong NBE emission with no yellow emission peak. With +5 V applied bias, the NWs metal–semiconductor–metal UV photodetector exhibited a high photocurrent of 1.6×10⁻³ A. The photocurrent to dark current contrast ratio was 120.     

Time‐resolved investigation of Cu(In,Ga)Se2 growth and Ga gradient formation during fast selenisation of metallic precursors

Ga segregation at the backside of Cu(In,Ga)Se₂ solar cell absorbers is a commonly observed phenomenon for a large variety of sequential fabrication processes. Here, we investigate the correlation between Se incorporation, phase formation and Ga segregation during fast selenisation of Cu–In–Ga precursor films in elemental selenium vapour. Se incorporation and phase formation are analysed by real‐time synchrotron‐based X‐ray diffraction and fluorescence analysis. Correlations between phase formation and depth distributions are gained by interrupting the process at several points and by subsequent ex situ cross‐sectional electron microscopy and Raman spectroscopy. The presented results reveal that the main share of Se incorporation takes place within a few seconds during formation of In–Se at the top part of the film, accompanied by outdiffusion of In out of a ternary Cu–In–Ga phase. Surprisingly, CuInSe₂ starts to form at the surface on top of the In–Se layer, leading to an intermediate double graded Cu depth distribution. The remaining Ga‐rich metal phase at the back is finally selenised by indiffusion of Se. On the basis of a proposed growth model, we discuss possible strategies and limitations for the avoidance of Ga segregation during fast selenisation of metallic precursors. Solar cells made from samples selenised with a total annealing time of 6.5 min reached conversion efficiencies of up to 14.2 % (total area, without anti‐reflective coating). The evolution of the Cu(In,Ga)Se₂ diffraction signals reveals that the minimum process time for high‐quality Cu(In,Ga)Se₂ absorbers is limited by cation ordering rather than Se incorporation. Copyright © 2014 John Wiley & Sons, Ltd.     

Structural and optical properties of InAs quantum dots grown by molecular beam epitaxy

In this work, we have studied the dependence of the size and luminescence of self-assembled InAs quantum dots (SAQDs) on the growth conditions. The SAQDs were grown on GaAs (1 0 0) substrates by molecular beam epitaxy (MBE). Their structural and optical properties were studied by atomic force microscopy (AFM), and photoluminescence spectroscopy (PL). The growth of the InAs SAQDs was in situ monitored by reflection high-energy electron diffraction (RHEED). The shape and size of the InAs SAQDs were significantly affected by the growth temperature and the arsenic over-pressure. We observe a decrease of the SAQDS density and an increase in their height by increasing the growth temperature, and/or decreasing the arsenic over-pressure. This is accompanied by a remarkable red-shift of the PL emission energy from 1.3 to .     

Effects of Quantum and Dielectric Confinement on the Emission of Cs-Pb-Br Composites

The halide perovskite CsPbBr₃ belongs to the Cs-Pb-Br material system, which features two additional thermodynamically stable ternary phases, Cs₄PbBr₆ and CsPb₂Br₅. The coexistence of these phases and their reportedly similar photoluminescence (PL) have resulted in a debate on the nature of the emission in these systems. Herein, optical and microscopic characterizations are combined with an effective mass, correlated electron–hole model of excitons in confined systems, to investigate the emission properties of the ternary phases in the Cs-Pb-Br system. It is found that all Cs-Pb-Br phases exhibit green emission and the non-perovskite phases exhibit PL quantum yields orders of magnitude larger than CsPbBr₃. In particular, blue- and red-shifted emission for the Cs- and Pb-rich phases, respectively, are measured, stemming from embedded CsPbBr₃ nanocrystals (NCs). This model reveals that the difference in emission shift is caused by the combined effects of NC size and different band mismatch. Furthermore, the importance of including the dielectric mismatch in the calculation of the emission energy for Cs-Pb-Br composites is demonstrated. The results explain the reportedly limited blue shift in CsPbBr₃@Cs₄PbBr₆ composites and rationalize some of its differences with CsPb₂Br₅.     

Properties of GaAsN nanowires grown by magnetron-sputtering deposition

The effectiveness of magnetron-sputtering deposition in an Ar-N₂ plasma-forming mixture as a technique for the fabrication of arrays of GaAs_{1 ? x} N _x nanowires (NWs) with typical diameters from 10 to 200 nm and lengths up to 3000 nm is demonstrated. Dependence of the growth character of NW on the parameters of the synthesis process, such as the size of Au droplets, deposition rate, and crystallographic orientation of the surface and the temperature of the substrate, is investigated. Analysis of the dependence of NW height on their diameter demonstrates that growth occurs predominantly by the diffusion mechanism. The nitrogen content is kept stable for growth temperatures in the range 400–500°C, being as high as 2.7%. For the substrate temperatures in the range 530–600°C, an abrupt drop in the nitrogen content in the alloy takes place. In the photoluminescence (PL) spectra of the samples obtained, a red shift of the emission band is observed, which is related to an increase in the nitrogen content. The growth-temperature dependence of the position of the luminescence band and the nitrogen content is determined. It is found that the PL intensity of samples with GaAsN NWs containing 2.7% nitrogen increases by a factor of 5–10 as compared to samples with planar layers, which is explained by the absence of structural defects in NWs.     

Ultrastable and Reversible Fluorescent Perovskite Films Used for Flexible Instantaneous Display

All‐inorganic halide perovskite materials are regarded as promising materials in information display applications owing to their tunable color, narrow emission peak, and easy processability. However, the photoluminescence (PL) stability of halide perovskite films is still inferior due to their poor thermal stability and hygroscopic properties. Herein, all‐inorganic perovskite films are prepared through vacuum thermal deposition method to enhance thermal and hygroscopic stability. By intentionally adding extra bromide source, a structure of CsPbBr₃ nanocrystals embedded in a CsPb₂Br₅ matrix (CsPbBr₃/CsPb₂Br₅) is formed via an air exposure process, leading to impressive PL stability in ambient atmosphere. In addition, the as‐fabricated CsPbBr₃/CsPb₂Br₅ structure shows enhanced PL intensity due to the dielectric confinement. The CsPbBr₃/CsPb₂Br₅ structure film can almost reserve its initial PL intensity after four months, even stored in ambient atmosphere. The PL intensity for CsPbBr₃/CsPb₂Br₅ films vanishes at elevated temperature and recovers by cooling down in a short time. The reversible PL conversion process can be repeated over hundreds of times. Based on the reversible PL property, prototype thermal‐driven information display devices are demonstrated by employing heating circuits on flexible transparent substrates. These robust perovskite films with reversible PL characteristics promise an alternative solid‐state emitting display.     

A comparison between thin film solar cells made from co‐evaporated CuIn1‐xGaxSe2 using a one‐stage process versus a three‐stage process

Until this day, the most efficient Cu(In,Ga)Se₂ thin film solar cells have been prepared using a rather complex growth process often referred to as three‐stage or multistage. This family of processes is mainly characterized by a first step deposited with only In, Ga and Se flux to form a first layer. Cu is added in a second step until the film becomes slightly Cu‐rich, where‐after the film is converted to its final Cu‐poor composition by a third stage, again with no or very little addition of Cu. In this paper, a comparison between solar cells prepared with the three‐stage process and a one‐stage/in‐line process with the same composition, thickness, and solar cell stack is made. The one‐stage process is easier to be used in an industrial scale and do not have Cu‐rich transitions. The samples were analyzed using glow discharge optical emission spectroscopy, scanning electron microscopy, X‐ray diffraction, current–voltage‐temperature, capacitance‐voltage, external quantum efficiency, transmission/reflection, and photoluminescence. It was concluded that in spite of differences in the texturing, morphology and Ga gradient, the electrical performance of the two types of samples is quite similar as demonstrated by the similar J–V behavior, quantum spectral response, and the estimated recombination losses. Copyright © 2014 John Wiley & Sons, Ltd.     

Systematic Study of the Growth of Aligned Arrays of α‐Fe2O3 and Fe3O4 Nanowires by a Vapor–Solid Process

Growth of aligned and uniform α‐Fe₂O₃ nanowire (NW) arrays has been achieved by a vapor–solid process. The experimental conditions, such as type of substrate, local growth and geometrical environment, gas‐flow rate, and growth temperature, under which the high density α‐Fe₂O₃ NW arrays can be grown by a vapor–solid route via the tip‐growth mechanism have been systematically investigated. The density of the α‐Fe₂O₃ NWs can be enhanced by increasing the concentration of Ni atoms inside the alloy substrate. The synthesized temperature can be as low as 400 °C. Fe₃O₄ NWs can be produced by converting α‐Fe₂O₃ NWs in a reducing atmosphere at 450 °C. The transformation of phase and structure have been observed by in situ transmission electron microscopy. The magnetic and field‐emission properties of the NWs indicate their potential applications in nanodevices.     

Ultrahigh Responsivity of Ternary Sb–Bi–Se Nanowire Photodetectors

High‐quality single‐crystalline ternary (Sb_1‐xBi_x)₂Se₃ nanowires (NWs) (x = 0–0.88) are synthesized by chemical vapor deposition. Nanowires with x from 0 to 0.75 are indexed as an orthorhombic structure. With increasing Bi incorporation ratio, (Sb_1‐xBi_x)₂Se₃ NWs exhibit remarkable photoresponsivities, which originate from growing surface Se vacancies and augmented oxygen chemisorptions. Notably, spectra responsivity and external quantum efficiency of an (Sb_0.44Bi_0.56)₂Se₃ NW photodetector reach as high as ≈8261.4 A/W and ≈1.6 × 10⁶ %, respectively. Those excellent performances unambiguously demonstrate that Sb–Bi–Se NWs are promising for the utilizations of high‐sensitivity and high‐speed photodetectors and photoelectronic switches.     

Synthesis and Room‐Temperature Ultraviolet Photoluminescence Properties of Zirconia Nanowires

This paper reports the synthesis of tetragonal zirconia nanowires using template method. An as‐prepared sample was characterized by scanning and transmission electron microscopy. It was found that the as‐prepared materials were tetragonal zirconia nanowires with average diameters of ca. 80 nm and length of over 10 μm. The Raman spectrum showed peaks at 120, 461, and 629 cm^–1, which are attributed to the E_g, E_g, and B_1g phonon modes of the tetragonal zirconia structure, respectively. The UV‐vis absorption spectrum showed an absorption peak at 232.5 nm (5.33 eV in photon energy). Photoluminescence (PL) spectra of zirconia nanowires showed a strong emission peak at ca. 388 nm at room temperature, which is attributed to the ionized oxygen vacancy in the zirconia nanowires system.     

Sharp Infrared Emission from Single‐Crystalline Indium Nitride Nanobelts Prepared Using Guided‐Stream Thermal Chemical Vapor Deposition

Single‐crystalline InN nanobelts have been synthesized using Au as the catalyst by a guided‐stream thermal chemical vapor deposition technique. The resultant InN nanobelts typically have widths ranging from 20 to 200 nm, a width to thickness ratio of 2–10, and lengths of up to several tens of micrometers. Structural analysis shows that these InN nanobelts have a wurtzite structure and exhibit a rectangular cross section with self‐selective facets, i.e., the nanobelts are enclosed only by ± (001) and ± (11?0) planes with [110] being the exclusive growth direction along their long axis. This facet selectivity can be understood by the differences in the surface energies of the different facets. Photoluminescence (PL) spectra of InN nanobelts show a sharp infrared emission peak at 0.76 eV with a full width at half maximum of 14 meV, narrower than the values reported for InN epilayers. The integrated PL intensity is found to increase linearly with the excitation power, which suggests that the observed PL can be attributed to direct band‐to‐band emission.     

Hierarchical Shelled ZnO Structures Made of Bunched Nanowire Arrays

The size‐ and morphology‐controlled growth of ZnO nanowire (NW) arrays is potentially of interest for the design of advanced catalysts and nanodevices. By adjusting the reaction temperature, shelled structures of ZnO made of bunched ZnO NW arrays are prepared, grown out of metallic Zn microspheres through a wet‐chemical route in a closed Teflon reactor. In this process, ZnO NWs are nucleated and subsequently grown into NWs on the surfaces of the microspheres as well as in strong alkali solution under the condition of the pre‐existence of zincate (ZnO₂^2–) ions. At a higher temperature (200 °C), three different types of bunched ZnO NW or sub‐micrometer rodlike (SMR) aggregates are observed. At room temperature, however, the bunched ZnO NW arrays are found only to occur on the Zn microsphere surface, while double‐pyramid‐shaped or rhombus‐shaped ZnO particles are formed in solution. The ZnO NWs exhibit an ultrathin structure with a length of ca. 500 nm and a diameter of ca. 10 nm. The phenomenon may be well understood by the temperature‐dependent growth process involved in different nucleation sources. A growth mechanism has been proposed in which the degree of ZnO₂^2–saturation in the reaction solution plays a key role in controlling the nucleation and growth of the ZnO NWs or SMRs as well as in oxidizing the metallic Zn microspheres. Based on this consideration, ultrathin ZnO NW cluster arrays on the Zn microspheres are successfully obtained. Raman spectroscopy and photoluminescence measurements of the ultrathin ZnO NW cluster arrays have also been performed.     

多层In_(0.55)Al_(0.45)As/A_(0.5)Ga_(0.5)As量子点的变温光致发光研究

测量了自组织多层In0.55Al0.45As/Al0.5Ga0.5As量子点的变温光致发光谱,同时观察到来自浸润层和量子点的发光,首次直接观察到了浸润层和量子点之间的载流子热转移.分析发光强度随温度的变化发现浸润层发光的热淬灭包括两个过程:低温时浸润层的激子从局域态热激发到扩展态,然后被量子点俘获;而温度较高时则通过势垒层的X能谷淬灭.利用速率方程模拟了激子在浸润层和量子点间的转移过程,计算结果与实验符合得很好     

Growth mechanisms of co‐evaporated kesterite: a comparison of Cu‐rich and Zn‐rich composition paths

Earth abundant kesterite solar cells have achieved 7–10% cell efficiency mostly by processes that separate the film deposition and the annealing into two sequential steps. In contrast, co‐evaporation onto a high‐temperature substrate, demonstrating previous success in chalcopyrite (Cu(In,Ga)Se₂) solar cells, allows real‐time composition control. Chalcopyrite research widely supports the model that Cu‐rich growth conditions assist grain growth, and subsequently, the endpoint composition can be adjusted back to Cu‐poor via monitoring the surface emissivity of the film. On the basis of the same intentions, the recent development of co‐evaporated kesterite (Cu₂ZnSnSe₄) adapts the concept and achieves 9.2% efficiency. To understand the effect of growth strategies, this study examines the phase evolution, grain morphology, and device performance in Cu‐rich growth and other strategies (Zn‐rich and close‐to‐stoichiometric). By characterizing films obtained from interrupted depositions and also interpreting the variation in surface emission during growths, this study found a subtle hindrance in the reaction of Cu_xSe_y and ZnSe possibly caused by the volatile nature of SnSe_x. The hindrance explains why, distinctive from chalcopyrite, little difference in grain size is observed between kesterite films made by Cu‐rich versus Zn‐rich growth at these deposition rates. At last, a Zn‐rich growth 9.1% device, certified by the National Renewable Energy Laboratory, is presented, which equals the performance of the previously‐reported Cu‐rich growth device. At the present stage, we believe the Cu‐rich and Zn‐rich growth share equal promise for the optimization of kesterite solar cells. Copyright © 2013 John Wiley & Sons, Ltd.     

Amorphous lead titanate: a new wide‐band gap semiconductor with photoluminescence at room temperature

This paper describes a new amorphous wide‐band gap semiconductor with photoluminescence (PL) at room temperature. The amorphous PbTiO₃ was prepared by a sol–gel‐like process in powder and thin film form. The optical property and the PL behaviour showed a direct relation to the amorphous structure. The PL peak energy can be controlled by the change of the exciting surge energy. Copyright © 2000 John Wiley & Sons, Ltd.     

Charge‐Compensated Compound Defects in Ga‐containing Thermoelectric Skutterudites

Heavy doping changes an intrinsic semiconductor into a metallic conductor by the introduction of impurity states. However, Ga impurities in thermoelectric skutterudite CoSb₃ with lattice voids provides an example to the contrary. Because of dual‐site occupancy of the single Ga impurity charge‐compensated compound defects are formed. By combining first‐principle calculations and experiments, we show that Ga atoms occupy both the void and Sb sites in CoSb₃ and couple with each other. The donated electrons from the void‐filling Ga (Ga_VF) saturate the dangling bonds from the Sb‐substitutional Ga (Ga_Sb). The stabilization of Ga impurity as a compound defect extends the region of skutterudite phase stability toward Ga_0.15Co₄Sb_11.95 whereas the solid–solution region in other directions of the ternary phase diagram is much smaller. A proposed ternary phase diagram for Ga‐Co‐Sb is given. This compensated defect complex leads to a nearly intrinsic semiconductor with heavy Ga doping in CoSb₃ and a much reduced lattice thermal conductivity (κ_L) which can also be attributed to the effective scattering of both the low‐ and high‐frequency lattice phonons by the dual‐site occupant Ga impurities. Such a system maintains a low carrier concentration and therefore high thermopower, and the thermoelectric figure of merit quickly increases to 0.7 at a Ga doping content as low as 0.1 per Co₄Sb₁₂ and low carrier concentrations on the order of 10¹⁹ cm^?3.     

Photoluminescent properties of ZnO thin films grown on SiO2/Si(100) by metal-organic chemical vapor deposition

We report on the photoluminescent (PL) properties of ZnO thin films grown on SiO₂/Si(100) substrates using low pressure metal-organic chemical vapor deposition. The growth temperature of the films was as low as 400°C. From the PL spectra of the films at 10–300 K, strong PL peaks due to free and bound excitons were observed. The origin of the near bandedge emission peaks was investigated measuring temperature-dependent PL spectra. In addition, the Zn O films demonstrated a stimulated emission peak at room temperature. Upon illumination with an excitation density of 1 MW/cm², a strong, sharp peak was observed at 3.181 eV.     

Highly Photoluminescent and Environmentally Stable Perovskite Nanocrystals Templated in Thin Self‐Assembled Block Copolymer Films

Ordered nanostructured crystals of thin organic–inorganic metal halide perovskites (OIHPs) are of great interest to researchers because of the dimensional‐dependence of their photoelectronic properties for developing OIHPs with novel properties. Top‐down routes such as nanoimprinting and electron beam lithography are extensively used for nanopatterning OIHPs, while bottom‐up approaches are seldom used. Herein, developed is a simple and robust route, involving the controlled crystallization of the OIHPs templated with a self‐assembled block copolymer (BCP), for fabricating nanopatterned OIHP films with various shapes and nanodomain sizes. When the precursor solution consisting of methylammonium lead halide (MAPbX₃, X = Br^?, I^?) perovskite and poly(styrene)‐block‐poly(2‐vinylpyridine) (PS‐b‐P2VP) is spin‐coated on the substrate, a nanostructured BCP is developed by microphase separation. Spontaneous crystallization of the precursor ions preferentially coordinated with the P2VP domains yields ordered nanocrystals with various nanostructures (cylinders, lamellae, and cylindrical mesh) with controlled domain size (≈40–72 nm). The nanopatterned OIHPs show significantly enhanced photoluminescence (PL) with high resistance to both humidity and heat due to geometrically confining OIHPs in and passivation with the P2VP chains. The self‐assembled OIHP films with high PL performance provide a facile control of color coordinates by color conversion layers in blue‐emitting devices for cool‐white emission.     

Zero‐Dimensional Perovskite Nanocrystals for Efficient Luminescent Solar Concentrators

Luminescent solar concentrators (LSCs) are able to efficiently harvest solar energy through large‐area photovoltaic windows, where fluorophores are delicately embedded. Among various types of fluorophores, all‐inorganic perovskite nanocrystals (NCs) are emerging candidates as absorbers/emitters in LSCs due to their size/composition/dimensionality tunable optical properties and high photoluminescence quantum yield (PL QY). However, due to the large overlap between absorption and emission spectra, it is still challenging to fabricate high‐efficiency LSCs. Intriguingly, zero‐dimensional (0D) perovskites provide a number of features that meet the requirements for a potential LSC absorber, including i) small absorption/emission spectral overlap (Stokes shift up to 1.5 eV); ii) high PL QY (>95% for bulk crystal); iii) robust stability as a result of its large exciton binding energy; and iv) ease of synthesis. In this work, as a proof‐of‐concept experiment, Cs₄PbBr₆ perovskite NCs are used to fabricate semi‐transparent large‐area LSCs. Cs₄PbBr₆ perovskite film exhibits green emission with a high PL QY of ≈58% and a small absorption/emission spectral overlap. The optimized LSCs exhibit an external optical efficiency of as high as 2.4% and a power conversion efficiency of 1.8% (100 cm²). These results indicate that 0D perovskite NCs are excellent candidates for high‐efficiency LSCs compared to 3D perovskite NCs.     

Synthesis and photoluminescence properties of the 4H-SiC/SiO2 nanowires

4H-SiC/SiO2 nanowires are synthesized and the temperature-dependent photoluminescence （PL） properties of the nanowires are studied. Their structure and chemical composition are studied by scanning electron microscopy （SEM）, transmission electron microscopy （TEM）, X-ray diffraction （XRD）, and Raman spectra. At room temperature, an ul- traviolet PL peak and a green PL band are observed. From the PL spectrum measured in the temperature range from 80 K to 300 K, the free excition emission, donor bound excition emission and their multiple-phonon replicas have been observed in ultraviolet region, and their origins have been identified. Moreover, it has been found that the temperature dependence of the free exciton peak position can be described by standard expression, and the thermal activation en- ergy values extracted from the temperature dependence of the free exciton and bound exciton peak integral intensity are about 40 meV and 181 meV, respectively.