Spin effects in InAs self-assembled quantum dots

We have studied the polarized resolved photoluminescence in an n-type resonant tunneling diode (RTD) of GaAs/AlGaAs which incorporates a layer of InAs self-assembled quantum dots (QDs) in the center of a GaAs quantum well (QW). We have observed that the QD circular polarization degree depends on applied voltage and light intensity. Our results are explained in terms of the tunneling of minority carriers into the QW, carrier capture by InAs QDs and bias-controlled density of holes in the QW

Revealing the Nature of Defects in α-Ag2WO4 by Positron Annihilation Lifetime Spectroscopy: A Joint Experimental and Theoretical Study

Electron–matter coupling is a fascinating way to tune and modify the properties of materials. In this work, we present a study on the formation and nature of vacancy-like defects in α-Ag2WO4 samples synthesized in a water or ethanol medium and subsequently submitted to electron beam irradiation at different exposure times. To understand the effects on the geometrical and electronic nature of the generated defects, the data obtained by positron annihilation lifetime spectroscopy were interpreted with the aid of first-principles calculations at the density functional theory level. To complement these results, X-ray diffraction, Raman spectroscopy, photoluminescence emissions, and field emission gun scanning electron microscopy techniques were also used. Based on the positron binding energy and the calculated and experimental positron lifetimes, the defect structure of the nonirradiated and irradiated samples was revealed. As a general feature, it was found that the defect structure is more complex for samples synthesized in ethanol than in water. In particular, the results show that all samples contain defects involving Ag vacancies and that the concentration of this type of defect increases with the irradiation time.This work was partly funded by the Fundação de Amparo à Pesquisa do Estado de São Paulo—FAPESP (grants nos. 2013/07296-2 and 2019/01732-1), the Financiadora de Estudos e Projetos—FINEP, Conselho Nacional de Desenvolvimento Cientifico e Tecnológico—CNPq (grants nos. 166281/2017-4 and 426634/2018-7), and the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior CAPES. J.A. acknowledges Universitat Jaume I (project UJI-B2019-30) and the Ministerio de Ciencia, Innovacio′n y Universidades (Spain) (project PGC2018094417-B-I00) for financially supporting this research. C.M. and A.S. thank the funding from the Agencia Nacional de Promoción Científica y Tecnológica—ANPCyT (Argentina) (PICT 2015-1832), the Comisión de Investigaciones Científicas de la Provincia de Buenos Aires—CICPBA (Argentina), and the Secretaría de Ciencia, Arte y Tecnología, UNCPBA (Argentina)

Effect of thermal annealing on the optical and structural properties of (311)B and (001) GaAsBi/GaAs single quantum wells grown by MBE

The effect of Furnace Annealing (FA) and Rapid Thermal annealing (RTA) on the structural and optical properties of GaAs1 − xBix/GaAs single quantum wells grown on (001) and (311)B substrates by molecular beam epitaxy was investigated. The structural properties were investigated by high-resolution x-ray diffraction (HR-XRD) and Transmission Electron Microscopy. The Bi concentration profiles were determined by simulating the HR-XRD 2θ−ω scans using dynamical scattering theory to estimate the Bi content, lattice coherence, and quality of the interfaces. The Bi composition was found to be similar for both samples grown on (001) and (311)B GaAs substrates. However, the simulations indicate that the Bi composition is not only limited in the GaAsBi quantum well (QW) layer but also extends out of the GaAsBi QW toward the GaAs barrier. Photoluminescence (PL) measurements were performed as a function of temperature and laser power for samples with a nominal Bi composition of 3%. PL spectra showed that (001) and (311)B samples have different peak energies at 1.23 eV and 1.26 eV, respectively, at 10 K. After RTA at 300 °C for 2 min, the PL intensity of (311)B and (001) samples was enhanced by factors of ∼2.5 and 1.75, respectively. However, for the (001) and (311)B FA samples, an enhancement of the PL intensity by a factor of only 1.5 times could be achieved. The enhancement of PL intensity in annealed samples was interpreted in terms of PL activation energies, with a reduction in the alloy disorder and an increase in the Bi cluster

Circular polarization in a non-magnetic resonant tunneling device

We have investigated the polarization-resolved photoluminescence (PL) in an asymmetric n-type GaAs/AlAs/GaAlAs resonant tunneling diode under magnetic field parallel to the tunnel current. The quantum well (QW) PL presents strong circular polarization (values up to -70% at 19 T). The optical emission from GaAs contact layers shows evidence of highly spin-polarized two-dimensional electron and hole gases which affects the spin polarization of carriers in the QW. However, the circular polarization degree in the QW also depends on various other parameters, including the g-factors of the different layers, the density of carriers along the structure, and the Zeeman and Rashba effects

Unconventional Magnetization Generated from Electron Beam and Femtosecond Irradiation on α-Ag2WO4: A Quantum Chemical Investigation

Novel magnetic metals and metal oxides that use both the spin and charge of an electron offer exciting technological applications. Their discovery could boost research on functional nanoscale materials. Here, for the first time, we report the magnetization of α-Ag2WO4 under electron beam and femtosecond laser irradiation. The formation and growth of silver oxides (AgO, Ag2O, and Ag3O4) and Ag nanofilaments can be observed on the surface of α-Ag2WO4 crystals. These features were also present in the composition of an extruded material and could open new avenues for surface magnetism studies. In order to understand these results, we used first-principles density functional theory calculations. This allowed us to investigate several potential scenarios for controlling magnetic properties. The effect of electron addition on the crystalline structures of α-Ag2WO4, Ag3O4, Ag2O, and AgO has been analyzed in detail. The creation of Ag and O vacancies on these compounds was also analyzed. Based on structural and electronic changes at the local coordination site of Ag, a mechanism was proposed. The mechanism illustrates the processes responsible for the formation and growth of metallic Ag and the magnetic response to electron beam irradiation

Phyllosilicates as earth-abundant layered materials for electronics and optoelectronics: Prospects and challenges in their ultrathin limit

Phyllosilicate minerals are an emerging class of naturally occurring layered insulators with large bandgap energy that have gained attention from the scientific community. This class of lamellar materials has been recently explored at the ultrathin two-dimensional level due to their specific mechanical, electrical, magnetic, and optoelectronic properties, which are crucial for engineering novel devices (including heterostructures). Due to these properties, phyllosilicates minerals can be considered promising low-cost nanomaterials for future applications. In this Perspective article, we will present relevant features of these materials for their use in potential 2D-based electronic and optoelectronic applications, also discussing some of the major challenges in working with them.Comment: 29 pages, 4 figure

Structural and Optical Properties of Diluted Magnetic Ga1−xMnxAs–AlAs Quantum Wells Grown on High-Index GaAs Planes

We report on the structural and optical properties of Ga₁₋ᵪ Mn ᵪ As-AlAs quantum wells (QWs) with χ=0.1% grown by molecular beam epitaxy (MBE) on semi-insulating GaAs substrates with orientations (100), (110), (311)B and (411)B. Atomic force microscopy (AFM), X-ray diffraction (XRD) and photoluminescence (PL) techniques were used to investigate these QWs. AFM results have evidenced the formation of Mn-induced islands, which are randomly distributed on the surface. These islands tend to segregate for samples grown on (110) and (411)B planes, while no clear segregation was observed for samples grown on (100) and (311)B orientations. Results show that the PL line width increases with Mn segregation. XRD measurements were used to determine 2θ,d and cell parameters

Revealing localized excitons in WSe2/β-Ga2O3

We have investigated the optical and magneto-optical properties of monolayer (ML) WSe2 on flakes of β-Ga2O3 under high magnetic fields. Remarkably, sharp emission peaks were observed and associated with localized excitons related to point defects. A detailed study of low-temperature photoluminescence (PL) and magneto-PL under high perpendicular magnetic field up to 9 T was carried out. Several sharp emission peaks have shown valley g-factors values close to −4, which is an unusual result for localized excitons in WSe2. Furthermore, some PL peaks have shown higher g-factor values of ≈−7 and ≈−12, which were associated with the hybridization of strain localized dark excitons and defects. The reported results suggest that β-Ga2O3 is, indeed, a promising dielectric substrate for ML WSe2 and also to explore fundamental physics in view of possible applications in quantum information technology

Effect of growth techniques on the structural, optical and electrical properties of indium doped TiO2thin films

We have investigated the effect of the growth techniques on the structural, the electrically and optically active defects in Indium doped TiO2 thin films grown by pulsed laser deposition (PLD) and sputtering techniques. X-ray diffraction (XRD) and Raman spectroscopy patterns revealed both rutile and anatase phases for the sputtering samples. On the other hand, only the anatase phase was observed for the PLD samples. The photoluminescence (PL) spectra have unveiled several peaks which were explained by defect related optical transitions. Particularly, the PL bands are fully consistent with anatase/rutile TiO2 phases and the formation of In2O3 during the preparation of our samples. It was also observed that at −4 V reverse bias, the PLD samples have lower leakage currents (∼1.4 × 10−7 A) as compared to the sputtering samples (∼5.9 × 10−7 A). In addition, the PLD samples exhibited lower ideality factors and higher barrier heights as compared to those grown by sputtering. Finally, the Deep Level Transient Spectroscopy (DLTS) measurements have shown only one defect in the PLD samples whereas five defects have been detected in the sputtering samples. Therefore, our results provide strong evidence that the PLD technique is better suited for the growth of In-doped TiO2 thin films

Characterization of Na β alumina by nuclear magnetic resonance

Neste trabalho, estudamos a dependência com a temperatura dos tempos de relaxação nuclear T1 e T2 do 1H em policristais de Na &#946;\" alumina hidratada, no intervalo de temperatura de 255<T<540K, em 24,5MHz . Três amostras em pó foram selecionadas para medidas de RMN: duas puras e uma dopada com Fe (500ppm em peso). A presença de impurezas magnéticas nessas amostras foi observada através de espectros de Ressonância Paramagnética Eletrônica. Estudamos também a liberação de moléculas de água em função da temperatura pela técnica de TGA (Thermal Gravimetric Analysis). O sinal de RMN observado no intervalo de temperatura de 258<T<423K foi associado a moléculas de água fracamente ligadas enquanto que o sinal observado em temperaturas superiores foi associado a moléculas fortemente ligadas, provavelmente localizadas no plano de condução. Nossos resultados mostram que a hidratação o comportamento em função da temperatura dos tempos de relaxação nuclear são consideravelmente afetados pela presença de impurezas magnéticasProton NMR relaxation times T1 and T2 were measured on hydrated polycrystalline Na &#946; alumina in the temperature range 255<T<540K at 24.5MHz In order to get some feeling for the temperature at which water would leave the sample a thermal gravimetric analysis (TGA.) was also performed. Three powder samples, two pure and one iron doped (500ppm nominal), with particle diameters of 150-250micrômetros were selected for NMR measurements. Paramagnetic impurities were detected by EPR studies. Our results suggest that loosely bound water produces the NMR signal at temperatures lower than 423K while more tightly bound water, probably in the conduction planes, is predominant at higher temperatures. The relaxation time measurements show that magnetic impurities affect considerably the temperature dependenc