AlN, GaN, AlxGa1 − xN nanotubes and GaN/AlxGa1 − xN nanotube heterojunctions

Semiconductor optoelectronic devices based on GaN and on InGaN or AlGaN alloys and superlattices can operate in a wide range of wavelengths, from far infrared to near ultraviolet region. The efficiency of these devices could be enhanced by shrinking the size and increasing the density of the semiconductor components. Nanostructured materials are natural candidates to fulfill these requirements. Here we use the density functional theory to study the electronic and structural properties of (10,0) GaN, AlN, Al_xGa_1 − xN nanotubes and GaN/Al_xGa_1 − xN heterojunctions, 0<x<1. The Al_xGa_1 − xN nanotubes exhibit direct band gaps for the whole range of Al compositions, with band gaps varying from 3.45 to 4.85 eV, and a negative band gap bowing coefficient of −0.14 eV. The GaN/Al_xGa_1 − xN nanotube heterojunctions show a type-I band alignment, with the valence band offsets showing a non-linear dependence with the Al content in the nanotube alloy. The results show the possibility of engineering the band gaps and band offsets of these III-nitrides nanotubes by alloying on the cation sites.     

Cross-sectional scanning tunneling microscopy and spectroscopy of strain in buried heterostructure lasers

Cross-sectional scanning tunneling microscopy and spectroscopy have been used to probe the unreconstructed (1 1 0) surface of a commercially available buried heterostructure laser in ultra high vacuum. Complex re-growth above the non-linear blocking layers is shown to induce tensile strain in the device. Spectroscopic measurements show an increase in both the density of filled valence band states and empty conduction band states as a result of the strain, with a particularly large increase at −3.1 V. Current imaging tunneling spectroscopy measurements show an increase in the tunneling current in to and out of the strained regions at both gap voltage polarities, consistent with the spectroscopy. Moving towards tensile strain, InP is known to maintain much the same bandgap, with the split-off level and lower lying states being drawn up towards the valence band edge, consistent with the data.     

Low-energy electron-excited nanoluminescence studies of GaN and related materials

We have used low-energy electron-excited nanoluminescence (LEEN) spectroscopy combined with ultrahigh vacuum surface analysis techniques to obtain electronic bandgap, confined state and deep-level trap information from III nitride compound semiconductor surfaces and their buried interfaces on a nanometer scale. Localized states are evident at GaN/InGaN quantum wells, GaN ultrathin films, AlGaN/GaN pseudomorphic heterostructures, and GaN/Al₂O₃ interfaces that are sensitive to the chemical composition, bonding and atomic structure near interfaces, and in turn to the specifics of the epitaxial growth process. Identification of electrically active defects in these multilayer nanostructures provides information to optimize interface growth and control local electronic properties.     

Studies on the resistance switching properties of the La0.5Ca0.5MnO3/Nb : SrTiO3 heterojunction

In this study, we have investigated the resistance switching behavior of the La_0.5Ca_0.5MnO₃/Nb:SrTiO₃ heterojunction. The junction shows a negative resistance switching ratio (ER) below 140 K. When , the ER goes from negative to positive with increasing bias voltage. When T>220 K, the junction shows a positive ER. This variation from a negative to a positive value indicates that the ER is determined primarily by two phenomena: (a) the negative ER value can be attributed to a disruption of the charge-ordered insulating domains in La_0.5Ca_0.5MnO₃ under large electric fields, and (b) the positive ER value at high temperatures is due to the modulation of the interface barrier width driven by the electrochemical migration of oxygen vacancies.     

Twofold spin-triplet pairing states and tunneling conductance in ferromagnet/ferromagnet/iron pnictide superconductor heterojunctions

By applying an extended eight-component Bogoliubov–de Gennes equation, we study theoretically the tunneling conductance in clean ferromagnet/ferromagnet/iron pnictide superconductor (FM/FM/iron-based SC) heterojunctions. Under the condition of noncollinear magnetizations, twofold novel Andreev reflections exist due to the existence of two bands in the SC, in which the incident electron and the two Andreev-reflected holes, belonging to the same spin subband, form twofold spin-triplet pairing states near the FM/iron-based SC interface. It is shown that the conversions of the conductance not only between the zero-bias peak and valley at zero energy but also between the peaks and dips at two gap energies are strongly dependent on both the interband coupling strength in the SC and the spin polarization in the FM. The qualitative differences from tunneling into a conventional s$s$-wave SC are also presented, which may help with experimentally probing and identifying the antiphase s$s$-wave pairing symmetry in the iron-based SC.     

Interfacial charge transfer in carbon nitride heterojunctions monitored by optical methods

Solar energy conversion is inciting tremendous research efforts in many fields due to the vast potential of sunlight as a sustainable energy source. For solar energy to become widely used and become a major component of our energy mix, energy storage on large scales must be addressed and the components used must be abundant. Artificial photosynthesis to produce solar fuels holds promise as a way to convert solar energy into storable energy. Organic photocatalysts have rapidly established themselves as a viable alternative to inorganic systems. Organic photocatalyst can be prepared from inexpensive precursors and offer a synthetic versatility and tunability that can be exploited to improve efficiencies. Carbon nitride (CN_x) has emerged as a leading organic photocatalyst with advantageous chemical and photo stabilities. Recombination of photogenerated electrons and holes limit the efficiency of CN_x materials below levels necessary to become a viable energy production system. To improve the efficiency and key characteristics such as light harvesting, charge carrier lifetime, and interfacial rate of charge transfer, a second material is put in contact with CN_x to form a heterojunction. While there are many examples of heterojunctions improving the photocatalytic activity beyond that of the isolated CN_x, we are still lacking the deep understanding of charge carrier dynamics necessary to rationalize the improvements and design optimal junctions. This review covers the studies of CN_x heterojunctions that have used optical methods to monitor the charge carrier dynamics. Time-resolved photoluminescence (TRPL) is the most common technique used and there are many examples that have used transient absorption spectroscopy (TAS) to probe the charge carrier dynamics. However, attempting to link the lifetime change to the activity differences does not yield a clear trend. It is likely that the reactive charges are not consistently being monitored and is obscuring the expected correlations. Both shorter and longer charge carrier lifetimes can be observed with both TRPL and TAS techniques and can be interpreted as arising from interfacial charge separation. Even when the same materials are used in the junction there is no consistency in observing a shorter or longer lifetime. The holistic view of charge carrier dynamics in CN_x heterojunctions presented here intends to identify overarching themes from a wide range of CN_x-containing systems and help take stock of where our current understanding stands. More specific spectral assignments and linking the observed lifetimes to certain photophysical or photochemical processes are needed to build models to help us understand the links between the charge carrier dynamics and the activity. These are crucial to develop general strategies that will lead to optimal CN_x heterojunctions.     

一步法制备铁氧化物/氮改性氧化石墨/碳纳米管异质结及其用于催化活化过一硫酸氢钾降解亚甲基蓝模型染料（英文）

随着较差的生物相容性和更高毒性有机染料的应用,如酚类化合物和抗生素,水污染和食品污染变得极其严重.这不仅危害人类健康,而且严重污染自然环境.过硫酸盐去污技术利用自由基活化降解过程,成为处理一系列污染物非常有效的方法;然而设计具有多功能性的高性能催化剂仍然面临着巨大的挑战.因此,本文借鉴铁基材料、氮改性石墨和碳纳米管独特的物化性质,以尿素、铁盐、氧化石墨、碳纳米管为原材料,通过一步水热法成功制备了三维多功能铁氧化物/氮改性氧化石墨/碳纳米管异质结,用作活化过一硫酸氢钾复合盐以降解有机模型污染物亚甲基甲蓝(MB),研究了高级氧化法(AOPs)作用机理和优化反应条件.XRD、红外光谱、SEM和XPS结果表明,铁氧化物通过物理静电作用力和化学键结合力已经被牢牢固定在了氮修饰的氧化石墨结构框架内.当加入了碳纳米管之后,它会与石墨形成类似于互穿聚合物网络的结构,从而具有三维材料的优点,且提升电子转移电导率,使得催化剂的结构和性能有了很大的改善.此外,优化了降解系统、PMS负载量、初始有机污染物浓度和催化剂用量等因素.结果表明,处于催化剂/PMS系统时,亚甲基蓝可以在12min之内有效地完全降解,可归结于碳、氮以及主要活性物质铁氧化物之间的协同作用.基于数据拟合分析,污染物氧化降解系统与拟一阶动力学相符合,其速率常数约为0.33 min~(-1).淬灭实验证明,硫酸根自由基和羟基自由基是主要的反应活性物种.这种同时富含铁/氮分级的多孔碳骨架异质结物质不仅可用作过渡金属催化剂,而且为制备其他异质结提供参考,以用于超级电容器、储能材料、电催化剂等领域     

Design of AlInN on silicon heterojunctions grown by sputtering for solar devices

AlInN alloys offer great potential for photovoltaics thanks to their wide direct bandgap covering the solar spectrum from the infrared (0.7 eV – InN) to the ultraviolet (6.2 eV – AlN), and their superior resistance to high temperatures and high-energy particles. We report the design of AlInN-on-silicon heterojunctions grown by radio-frequency sputtering to explore their potential for low-cost devices. Particularly, we study the influence of AlInN bandgap energy, thickness and carrier concentration, silicon surface recombination, interface defect density and wafer quality, on the photovoltaic properties of the junction. The effect of introducing an anti-reflective coating is also assessed. Optimized AlInN-on-Si structures show a conversion efficiency of 23.6% under 1-sun AM1.5G illumination. In comparison with silicon homojunctions, they own an improved responsivity at wavelengths below 500 nm. These results make AlInN-on-Si heterojunctions a promising technology for solar devices with impact in space applications. Experimental results on novel AlInN-on-Si solar cells are also presented.     

Ferromagnet-superconductor nanocontacts grown by focused electron/ion beam techniques for current-in-plane Andreev Reflection measurements

Superconductor-ferromagnetic in-plane nanocontacts have been created with focused-electron/ion-beam-induced deposition techniques for studies of Andreev Reflection. The final resistance of the nanocontact is tuned during the growth by in situ resistance measurements. The results show that Co nanodeposits grown with focused electron beam have large spin polarization (∼35%), making this nanomaterial of great potential for use in Spin Electronics applications. The experiments have also allowed the determination of the superconducting gap of the W-based nanodeposits grown with focused ion beam.