Phase-separated semiconductors containing magnetic nanostructures are relevant systems for the realization of high-density recording media. Here, the controlled strain engineering of Ga$δ$FeN layers with Fe$_y$N embedded nanocrystals (NCs) \textit{via} Al$_x$Ga$_{1-x}$N buffers with different Al concentration $0<x_\mathrm{Al}<41$\% is presented. Through the addition of Al to the buffer, the formation of predominantly prolate-shaped $\varepsilon$-Fe$_3$N NCs takes place. Already at an Al concentration $x_\mathrm{Al}$\,$\approx$\,5\% the structural properties---phase, shape, orientation---as well as the spatial distribution of the embedded NCs are modified in comparison to those grown on a GaN buffer. Although the magnetic easy axis of the cubic $γ$'-Ga$_y$Fe$_{4-y}$N nanocrystals in the layer on the $x_\mathrm{Al} = 0\%$ buffer lies in-plane, the easy axis of the $\varepsilon$-Fe$_3$N NCs in all samples with Al$_x$Ga$_{1-x}$N buffers coincides with the $[0001]$ growth direction, leading to a sizeable out-of-plane magnetic anisotropy and opening wide perspectives for perpendicular recording based on nitride-based magnetic nanocrystals.
Single planar arrays of Ga(x)Fe(4-x)N magnetic nanocrystals embedded in GaN have been fabricated in an epitaxial process. The phase of the nanocrystals and their epitaxial relationship with the host matrix are studied $via$ high-resolution transmission electron microscopy and high-resolution x-ray diffraction. By changing the growth parameters and mode, the crystallographic phase and chemical composition of the nanocrystals can be varied on demand. In view of the different magnetic properties of the various phases, applications in room-temperature ferromagnetic as well as antiferromagnetic spi…
In the Fe-doped GaN phase-separated magnetic semiconductor GaFeN, the presence of embedded Ga$_y$Fe$_{4-y}$N nanocrystals determines the magnetic properties of the system. Here, through a combination of anomalous x-ray diffraction and diffraction anomalous fine structure, the local structure of Ga in self-assembled face-centered cubic (fcc) Ga$_y$Fe$_{4-y}$N nanocrystals embedded in wurtzite GaN thin layers is investigated in order to shed light onto the correlation between fabrication parameters, local structural arrangement and overall magnetic properties of the material system. It is found,…
The magnetic anisotropy of a planar array of Ga$_{x}$Fe$_{4-x}$N nanocrystals (NCs) embedded in a GaN host is studied by ferromagnetic resonance. X-ray diffraction and transmission electron microscopy are employed to determine the phase and distribution of the nanocrystals. The magnetic anisotropy is found to be primarily uniaxial with the hard axis normal to the NCs plane and to have a comparably weak in-plane hexagonal symmetry. The origin of the magnetic anisotropy is discussed taking into consideration the morphology of the nanocrystals, the epitaxial relations, strain effects and magnetic…
Primary crystallization in high Al-content metallic glasses is driven by nanometer-diameter regions with internal structure similar to fcc Al. Comparison of fluctuation electron microscopy (FEM) data to FEM simulations of fcc Al clusters dispersed in a dense-random packed matrix is used to extract the diameter and volume fraction of the ordered regions in a Al88Y7Fe5 base glass and in glasses with 1 at.% Cu substituted for Y or Al. The size and density of nanocrystals were measured as a function of isothermal annealing time for the same alloys. The volume fraction of crystalline material grows…
We report on the production of ordered assemblies of silicon nanostructures by means of irradiation of a Si(100) substrate with 1.2 keV Ar ions at normal incidence. Atomic Force and High-Resolution Transmission Electron microscopies show that the silicon structures are crystalline, display homogeneous height, and spontaneously arrange into short-range hexagonal ordering. Under prolonged irradiation (up to 16 hours) all dot characteristics remain largely unchanged and a small corrugation develops at long wavelengths. We interpret the formation of the dots as a result of an instability due to th…
We report on the metalorganic chemical vapor deposition (MOCVD) of GaN:Fe and (Ga,Fe)N layers on c-sapphire substrates and their thorough characterization via high-resolution x-ray diffraction (HRXRD), transmission electron microscopy (TEM), spatially-resolved energy dispersive X-ray spectroscopy (EDS), secondary-ion mass spectroscopy (SIMS), photoluminescence (PL), Hall-effect, electron-paramagnetic resonance (EPR), and magnetometry employing a superconducting quantum interference device (SQUID). A combination of TEM and EDS reveals the presence of coherent nanocrystals presumably FexN with t…
Semiconductive nanocrystals (NCs) can be self-assembled into ordered superlattices (SLs) to create artificial solids with emerging collective properties. Computational studies have predicted that properties such as electronic coupling or charge transport are determined not only by the individual NCs but also by the degree of their organization and structure. However, experimental proof for a correlation between structure and charge transport in NC SLs is still pending. Here, we perform X-ray nano-diffraction and apply Angular X-ray Cross-Correlation Analysis (AXCCA) to characterize the structu…
Single-photon emitters (SPEs) are at the basis of many applications for quantum information management. Semiconductor-based SPEs are best suited for practical implementations because of high design flexibility, scalability and integration potential in practical devices. Single-photon emission from ordered arrays of InGaN nano-disks embedded in GaN nanowires is reported. Intense and narrow optical emission lines from quantum dot-like recombination centers are observed in the blue-green spectral range. Characterization by electron microscopy, cathodoluminescence and micro-photoluminescence indic…
The realization of reliable single photon emitters operating at high temperature and located at predetermined positions still presents a major challenge for the development of solid-state systems for quantum light applications. We demonstrate single-photon emission from two-dimensional ordered arrays of GaN nanowires containing InGaN nano-disks. The structures were fabricated by molecular beam epitaxy on (0001) GaN-on-sapphire templates patterned with nanohole masks prepared by colloidal lithography. Low-temperature cathodoluminescence measurements reveal the spatial distribution of light emit…
Hybridization of semiconducting and magnetic materials into a single heterostructure is believed to be potentially applicable to the design of novel functional spintronic devices. In the present work we report epitaxial stabilization of four magnetically ordered iron oxide phases (Fe3O4, γ-Fe2O3, α-Fe2O3 and most exotic metastable ε-Fe2O3) in the form of nanometer sized single crystalline films on GaN(0001) surface. The epitaxial growth of as many as four distinctly different iron oxide phases is demonstrated within the same single-target Laser MBE technological process on a GaN semiconductor …
This paper studies short-range order (SRO) in the semiconductor alloy (GaN)$_{1-x}$(ZnO)$_x$. Monte Carlo simulations performed on a density functional theory (DFT)-based cluster expansion model show that the heterovalent alloys exhibit strong SRO because of the energetic preference for the valence-matched nearest-neighbor Ga-N and Zn-O pairs. To represent the SRO-related structural correlations, we introduce the concept of Special Quasi-ordered Structure (SQoS). Subsequent DFT calculations reveal dramatic influence of SRO on the atomic, electronic and vibrational properties of the (GaN)$_{1-x…
This document reports the x-ray powder diffraction main reflections (intensity threshold >= 100) for possible Fe-related phases forming during the metal-organic vapor phase epitaxy (MOVPE) growth of Fe in NH_3/H_2 mixture on wurtzite-GaN/sapphire. The 2θvalues are given for Cu Kα_1 radiation (1.5406 Å) in the range 25-100 deg (ordered by increasing 2θ). The GaN(000l) and Al_2O_3(000l) are also reported for reference.
A FePd alloy was nanostructured by severe plastic deformation following two different routes: ordered and disordered states were processed by high pressure torsion (HPT). A grain size in a range of 50 to 150 nm is obtained in both cases. Severe plastic deformation induces some significant disordering of the long range ordered L10 phase. However, Transmission Electron Microscopy (TEM) data clearly show that few ordered nanocrystals remain in the deformed state. The deformed materials were annealed to achieve nanostructured long range ordered alloys. The transformation proceeds via a first order…
Ultrathin GaN nanowires (NWs) are attractive to maximize surface effects and as building block in high-frequency transistors. Here, we introduce a facile route for the top-down fabrication of ordered arrays of GaN NWs with aspect ratios exceeding $10$ and diameters below $20\,$nm. Highly uniform thin GaN NWs are first obtained by using electron beam lithography to pattern a Ni/SiN$_x$ hard mask, followed by dry etching and wet etching in hot KOH. The SiN$_x$ is found to work as an etch stop during wet etching in hot KOH. Arrays with NW diameters down to $(33 \pm5)\,$nm can be achieved with a y…
We present a facile and safe ligand exchange method for readily synthesized CuInSe2 (CIS) and CuIn(1-x)Ga(x)Se2 (CIGS) nanocrystals (NCs) from oleylamine to 1-ethyl-5-thiotetrazole which preserves the colloidal stability of the chalcopyrite structure. 1-ethyl-5-thiotetrazole as thermally degradable ligand is adapted for the first time for trigonal pyramidal CIS NCs (18 nm), elongated CIS NCs (9 nm) and CIGS NCs (6 nm). The exchanged NC solutions are spin-coated onto Si/SiO2 substrates with predefined gold electrodes to yield ordered NC thin films. These films are thermally annealed at 260 C to…
The charge ordered La$_{1/3}$Sr$_{2/3}$FeO$_{3-δ}$ (LSFO) in bulk and nanocrystalline forms are investigated using ac and dc magnetization, Mössbauer, and polarised neutron studies. A complex scenario of short range charge and magnetic ordering is realized from the polarised neutron studies in nanocrystalline specimen. This short range ordering does not involve any change in spin state and modification in the charge disproportion between Fe$^{3+}$ and Fe$^{5+}$ compared to bulk counterpart as evident in the Mössbauer results. The refinement of magnetic diffraction peaks provides magnetic momen…
Band-Like Transport and Cation Off-Centring in Ag/Bi-Based Solar Absorbers
Inorganic Chemistry Laboratory, University of Oxford（牛津大学无机化学实验室）；Graduate Institute of Photonics and Optoelectronics and Department of Electrical Engineering, National Taiwan University（台湾大学光电子学研究所及电气工程系）；Department of Materials, University of Oxford（牛津大学材料系）；Department of Chemistry, University of Manchester（曼彻斯特大学化学系）；Centre for Nano Science and Engineering, Indian Institute of Science（印度科学学院纳米科学与工程中心）；Department of Physics, University of Warwick（华威大学物理系）；Warwick Centre for Ultrafast Spectroscopy, University of Warwick（华威大学超快光谱学中心）；Catalonia Institute for Energy Research (IREC)（加泰罗尼亚能源研究所）；Facultat de Fisica, Universitat de Barcelona（巴塞罗那大学物理学院）；Cavendish Laboratory, University of Cambridge（剑桥大学卡文迪许实验室）；ISIS Neutron and Muon Source, Science and Technology Facilities Council, Rutherford Appleton Laboratory（科学和技术设施委员会卢瑟福阿普尔顿实验室ISIS中子和缪子源）；Institut Laue-Langevin（朗之万研究所）；School of Chemistry, University of Birmingham（伯明翰大学化学学院）；Departament d'Enginyeria Electrònica i Biomèdica, IN2UB, Universitat de Barcelona（巴塞罗那大学电子与生物医学工程系）；Institute for Solar Fuels, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH（柏林亥姆霍兹材料与能源研究所太阳能燃料研究所）；Yusuf Hamied Department of Chemistry, University of Cambridge（剑桥大学优素福·哈米德化学系）
Universal effect of ammonia pressure on synthesis of colloidal metal nitrides in molten salts
Pritzker School of Molecular Engineering, University of Chicago（芝加哥大学普里茨克分子工程学院）；Interdisciplinary Materials Science Program, Vanderbilt University（范德堡大学交叉材料科学项目）；Department of Chemistry and James Franck Institute, University of Chicago（芝加哥大学化学系和詹姆斯·弗兰克研究所）；Department of Chemistry, Northwestern University（西北大学化学系）；Center for Nanoscale Materials, Argonne National Laboratory（阿贡国家实验室纳米材料中心）；Department of Chemical and Biomolecular Engineering, Vanderbilt University（范德堡大学化学与生物分子工程系）
We report the evolution of crystallographic structure, magnetic ordering and electronic transport in thin films of full-Heusler alloy Co$_2$MnSi deposited on (001) MgO with annealing temperatures ($T_A$). By increasing the $T_A$ from 300$^\circ$C to 600$^\circ$C, the film goes from a disordered nanocrystalline phase to $B2$ ordered and finally to the $L2_1$ ordered alloy. The saturation magnetic moment improves with structural ordering and approaches the Slater-Pauling value of $\approx 5.0 μ_B$ per formula unit for $T_A$ = 600$^\circ$C. At this stage the films are soft magnets with coercive a…
A range of high quality Ga1-xMnxN layers have been grown by molecular beam epitaxy with manganese concentration 0.2 < x < 10%, having the x value tuned by changing the growth temperature (Tg) between 700 and 590 °C, respectively. We present a systematic structural and microstructure characterization by atomic force microscopy, secondary ion mass spectrometry, transmission electron microscopy, powder-like and high resolution X-ray diffraction, which do not reveal any crystallographic phase separation, clusters or nanocrystals, even at the lowest Tg. Our synchrotron based X-ray absorption near-e…
Two-dimensional (2D) transition metal dichalcogenides (TMDCs) have been the subject of sustained research interest due to their extraordinary electronic and optical properties. They also exhibit a wide range of structural phases because of the different orientations that the atoms can have within a single layer, or due to the ways that different layers can stack. Here we report the first study of direct-visualization of structural transformations in atomically-thin layers under highly non-equilibrium thermodynamic conditions. We probe these transformations at the atomic scale using real-time, …
