Bulk Single Crystals Research Articles

Room temperature multiferroicity in a transition metal dichalcogenide

The coexistence of multiple ferroic orders, i.e., multiferroicity, is a scarce property to be found in materials. Historically, this state has been found mainly in 3-dimensional complex oxides, but so far, this state has still been elusive for the most widely studied and characterized family of 2-dimensional compounds, the transition metal dichalcogenides. In this study, we report the experimental realization of multiferroic states in this family of materials, at room temperature, in bulk single crystals of Te-doped WSe2. We observe the coexistence of ferromagnetism and ferroelectricity, evidenced in the presence of magnetization and piezoresponse force microscopy hysteresis loops. These findings open the possibility of widening the use and study of van der Waals-based multifunctional devices for nanoelectronics and spintronics applications.

Increasing the friction stress decreases the size dependence of strength in a family of face-centered cubic high- and medium-entropy alloy micropillars

Size effects (‘smaller is stronger’) are important aspects of the mechanical behavior of materials. Experimentally, they are usually probed by performing compression tests on micropillars of different sizes (cross-sectional areas). To overcome limitations associated with comparing different crystal structures and to better handle the influence of melting points in different metals, single crystal face centered cubic (fcc) micropillars of equiatomic binary, ternary, and quaternary medium-entropy alloy (MEA) subsystems of the quinary CrMnFeCoNi high-entropy alloy (HEA) were tested. All eight alloys investigated were single-phase solid solutions having the fcc crystal structure. Their melting temperatures varied only over a narrow range (1189-1462 °C). They exhibit a size-dependent critical resolved shear stress (CRSS ∝ d-n, where d is the pillar diameter, n is a power law exponent). The results show that, all else being equal, size effects scale inversely with friction stress. In contrast, there is no systematic dependence on configurational entropy, contrary to speculations in some earlier papers that solid solution strengthening would increase as the number of alloying elements increases. ‘Bulk’ CRSS values were estimated by extrapolating the measured CRSS values of pillars with diameters of approximately 1-8 μm to larger pillar sizes of 30 μm. Good agreement was found with available CRSS values of bulk single crystals. It is concluded that it is possible to obtain bulk CRSS values more reliably from micropillar tests than from the Taylor factor corrected yield strengths of bulk polycrystals.

Ohmic Contact to n-GaN Using RT-Sputtered GaN:O.

One of the key issues in GaN-based devices is the resistivity and technology of ohmic contacts to n-type GaN. This work presents, for the first time, effective intentional oxygen doping of sputtered GaN films to obtain highly conductive n+-GaN:O films. We have developed a novel and simple method to obtain these films. The method is based on the room temperature magnetron sputtering of a single crystal bulk GaN target doped with oxygen. The n+-GaN:O films exhibit a polycrystalline structure with a crack-free surface and a free electron concentration of 7.4 × 1018 cm3. Ohmic contact to GaN:Si with n+-GaN:O sub-contact layer achieves specific contact resistance of the order of 10-5 Ωcm2 after thermal treatment. The obtained results are very promising for the development of the technology of a whole new class of ohmic contacts to n-GaN.

Low-temperature resistivity upturn and weak antilocalization in layered Ta1.04Ru0.78Te4 bulk single crystal

Quantum corrections to conductivity, which reflect charge carriers' quantum behavior, are a significant topic in condensed state physics and device design. A resistivity upturn at low temperature or weak antilocalization due to quantum corrections has been often observed experimentally. However, the coexistence of the low-temperature resistivity upturn and weak antilocalization from quantum corrections in bulk single crystals is seldom reported. Here, we report the transport properties of bulk Ta1.04Ru0.78Te4 single crystals. The samples showed a metallic behavior with a resistivity upturn below ∼8.6 K, which may be the result of quantum correction to the resistivity. The magnetic field enhances the upturn feature. The weakly nonlinear Hall resistivity with a positive slope suggests a p-type and multiband feature for bulk Ta1.04Ru0.78Te4; the electron and hole concentrations and mobilities of the samples are very close to each other and have the same order of magnitude. The Ta1.04Ru0.78Te4 single crystals displayed small and positive magnetoresistance, and the 3 K magnetoresistance at 9 T was about 15%. A lack of overlap of Kohler's plot curves at different temperature implies the violation of Kohler's rule. At low temperature, the dip-like magnetoresistance at low field strengths suggests a weak antilocalization in the Ta1.04Ru0.78Te4 single crystal. A small phase coherence length implies weakened screening and enhancing electron–electron interaction effects. These results reveal the quantum transport properties of Ta1.04Ru0.78Te4 single crystals, which can be considered in the future device design.

Spin States of Cobalt Ions in the Bulk and on the Surface of LaCoO3 Probed by X-ray Absorption, Emission, and Photoelectron Spectra

We present X-ray photoelectron, Co {{L}_{{2,3}}} and O K X-ray absorption, as well as Co K{{beta }_{{1,3}}} X-ray emission spectroscopy results of studies of the spin states of trivalent cobalt ions in single-crystal cobaltite LaCoO3. We show that at room temperature, in the bulk of a LaCoO3 single crystal, Co3+ ions are in the low-spin state, while high-spin Co2+, high-spin Co3+, low-spin Co3+, and probably also intermediate-spin Co3+ ions are located on the surface.

Luminescence Properties of the Hexagonal Boron Nitride Epilayer

AbstractAs an emerging 2D ultrawide bandgap semiconductor, hexagonal boron nitride (h‐BN) is gaining significant attention for its superior properties and wide applications. Although optical properties of h‐BN have been partially revealed by using h‐BN bulk single crystal, luminescence properties of the h‐BN few‐layer are rare due to its poor crystallinity. In this work, the h‐BN epilayers have been synthesized on sapphire substrates by the submicron‐spacing vapor deposition method. The crystalline quality of the h‐BN epilayer is improved with the increase of the growth temperature, and the full width at half‐maximum of the Raman peak is only ≈13 cm−1. The low temperature cathodoluminescence spectra of h‐BN epilayers exhibit two strong deep ultraviolet (DUV) luminescence peaks at 5.49 and 5.35 eV, as well as a weaker and broader defect‐related emission band at ≈4.0 eV. The DUV emission band is ascribed to the recombination of bound excitons and its phonon replica. Based on the theoretically predicted energy levels, the 4.0 eV emission band is tentatively attributed to the radiative transition from C impurities occupying the B sites to the B vacancies. Moreover, the h‐BN‐based DUV photodetectors exhibit an outstanding performance, making it a promising prospect for optoelectronic applications.

Sizing Up Metal Halide Perovskite Photocatalysts: From Nano to Bulk

AbstractPhotocatalysts are developed in the pursuit of enabling more efficient chemical processes. Perovskites are uniquely situated for the role of a photocatalyst given their favorable optoelectronic properties and tailorable energetic features. While perovskites can be synthesized in a wide range of sizes, from quantum‐confined nanocrystals to bulk single crystals, research to date has primarily focused on perovskite nanocrystals. In this review, recent advancements in perovskite photocatalysts throughout various size regimes are outlined and emerging developments in the field are highlighted. Future directions for perovskite photocatalytic research, including a focus on larger dimensions, with more efficient photocatalysis enabled by the improvements discussed herein are proposed.

Crystal growth and infrared-to-visible upconversion luminescence in Nd-Yb-Tm: CaYAlO4 single crystal

Yb3+/Tm3+: CaYAlO4 (CYA) and Nd3+/Yb3+/Tm3+: CYA bulk single crystals were grown using the Czochralski (Cz) method. The polarized absorption, upconversion luminescence (UCL) spectra under 980 nm and 808 nm pumping, and fluorescence decay curves were measured as well as Judd-Ofelt (J-O) theory calculations were performed to investigate the sensitization effect of Yb3+ and Nd3+/Yb3+co-doping in Tm3+: CYA single crystal. Obvious enhancement was observed in Tm3+-activated crystals for the blue upconversion emission (∼474 nm) when doped with Yb3+ or co-doped with Nd3+/Yb3+. The energy transfer efficiency is 72.6% (Yb3+→Tm3+) and 71.7% (Nd3+→Yb3+/Tm3+) in Yb3+/Tm3+: CYA and Nd3+/Yb3+/Tm3+: CYA single crystals respectively. The gain cross-sections were calculated, and it can stated that the blue laser output derived from upconversion is possible around 474 nm related to the transition of 1G4→ 3H6 in Yb3+/Tm3+: CYA and Nd3+/Yb3+/Tm3+: CYA crystals.

Thermal, dielectric, mechanical and structural behavior of 2-amino 4-methylpyridinium 4-nitrophenolate 4-nitrophenol bulk single crystal

Applications in both science and industry have received increased attention as a result of bulk single crystals with particular orientations. However, due to the instability of organic crystals at high temperatures and stress, there is an interest in growing good-quality bulk single crystals with stable thermal and mechanical properties. Here, the 2-amino 4-methylpyridinium 4-nitrophenolate 4-nitrophenol (2A4MPPP) crystal was prepared employing a single-wall ampoule and the vertical Bridgman technique. Structure and functional groups were determined by XRD, NMR, and FTIR studies. More importantly, detailed thermal and kinetic properties such as activation energy, frequency factor, rate constant, and Avrami exponent are discussed. The mechanical stability and dielectric studies are also demonstrated for the title compound. According to the single crystal XRD investigation, 2A4MPPP is a member of the orthorhombic crystal system with the Pna21 space group. Through the TGA and DTA analyses, it was confirmed that the compound starts to melt at 98 °C and complete melting occurs at 103.3 °C. The dielectric experiments reveal the crystal's poor dielectric constant and high-frequency dielectric loss. Vickers microhardness investigations show that grown 2A4MPPP belongs to the soft materials group. As a result of these findings, the 2A4MPPP crystal should be well suited for usage in thermomechanical, microelectronic, optical communications, and nonlinear optical applications.

Structure and spectroscopic properties of Er3+ doped LiNbO3 thin film grown by e-beam evaporation

Er3+ doped LiNbO3 (Er: LN) single crystal films with varied doping levels of Er3+ were grown on z-cut LiTaO3 bulk single crystals by using e-beam evaporation method. The crystallographic orientation, fluorescence properties, ions valence states, Er3+ concentration and Li2O content of films were studied. The main phase of the obtained films is determined to be LN from X-ray diffraction and Raman scattering studies, and the films are found to be orienting along (006). Enhancement in the visible and near-infrared luminescence is observed as the doping concentration of Er3+ is increased. The Li2O content of films is determined using the optical birefringence method and found to be at a constant value of 48.8 mol%, which is consistent with the Raman results. XPS results show that the Er concentration in the films is lower than that in the polycrystalline powder since e-beam evaporation has a low efficiency for heavier Er3+. Fluorescence intensity ratios estimated at different measurement temperatures suggest that the Er: LN films have excellent temperature sensitivity, which shows the potential of the films for optical thermometry applications.

Orientation dependence of the effect of short-range ordering on the plastic deformation of a medium entropy alloy

Multi-principal-element alloys (also known as medium and high entropy alloys) offer a much larger and richer design space than conventional alloys, providing opportunities for discovering new functionalities and their governing physics. Some of these alloys exhibit an outstanding combination of high strength and ductility, linked to the activation of various deformation modes triggered by low-energy stacking faults. However, a pressing question remains: Is the plasticity of medium- and high-entropy alloys governed only by stacking fault energy, or does atomic short-range order (SRO) play a role? To answer this, we investigated how SRO affects the deformation in single-crystalline NiCoCr, with previous contradictory findings. First, we established unique experimental evidence for SRO formation in bulk single crystals using high-energy synchrotron transmission X-Ray Diffraction. By tuning the degree of SRO by aging at high temperatures, twinning density and strain-induced martensitic phase transformation can be significantly increased in the [110] and [111] orientations under tension, increasing the tensile ductility; yet, no increase was observed along the [001] orientation due to lack of TWinning-Induced Plasticity (TWIP) or TRansformation-Induced Plasticity (TRIP), indicating a strong crystallographic orientation dependence. Our first-principles thermodynamic calculations unequivocally show SRO exists and governs the observed microstructural evolution and deformation hardening behavior. Here we find direct proof that SRO triggers a simultaneous TWIP and TRIP in NiCoCr, a rare microstructural evolution path. Our findings establish that the interplay of SRO and plasticity could be exploited to alter deformation modes and yield unprecedented mechanical response in medium- and high-entropy alloys.

Interface diagnostics: In-situ prediction of constitutional supercooling and backmelting by growth interface electromotive force

Constitutional supercooling and backmelting are common interface instability phenomena that occur during crystal growth and induce serious component inhomogeneity and defect proliferation; these flaws impede further crystalline devices. However, the interpretation and prevention of the above instabilities are extremely difficult because of the elusive melt convection behavior and extreme crystal growth environments. Herein, we use the growth velocity-related growth interface electromotive force (GEMF) to visualize the interface instability and trace the crystallization kinetics in situ, which can even predict the intrinsic unstable growth induced by components segregation (constitutional supercooling) and melt convection (backmelting). This factor further determines the real-time upper and lower limits of the critical growth velocity, squeezing a unique stable growth region. The predicted stable region complies well with the macro-tomography of the as-grown boules. Besides, 2D and 3D micro-mappings indicate ion segregation saturation due to the onset of constitutional supercooling. On this basis, a quantitative criterion for understanding and preventing critical interface instability is proposed. This GEMF-based method can help us understand ions migration and solidification under critical conditions, and further, be used to optimize the growth parameters of bulk single crystals and alloys, or manipulate the instability phenomena to fabricate natural unique microstructures.

Tutorial: Metalorganic chemical vapor deposition of β-Ga2O3 thin films, alloys, and heterostructures

β-phase gallium oxide (Ga2O3) is an emerging ultrawide bandgap (UWBG) semiconductor with a bandgap energy of ∼ 4.8 eV and a predicted high critical electric field strength of ∼8 MV/cm, enabling promising applications in next generation high power electronics and deep ultraviolet optoelectronics. The advantages of Ga2O3 also stem from its availability of single crystal bulk native substrates synthesized from melt, and its well-controllable n-type doping from both bulk growth and thin film epitaxy. Among several thin film growth methods, metalorganic chemical vapor deposition (MOCVD) has been demonstrated as an enabling technology for developing high-quality epitaxy of Ga2O3 thin films, (AlxGa1−x)2O3 alloys, and heterostructures along various crystal orientations and with different phases. This tutorial summarizes the recent progresses in the epitaxial growth of β-Ga2O3 thin films via different growth methods, with a focus on the growth of Ga2O3 and its compositional alloys by MOCVD. The challenges for the epitaxial development of β-Ga2O3 are discussed, along with the opportunities of future works to enhance the state-of-the-art device performance based on this emerging UWBG semiconductor material system.

Al0.64Ga0.36N channel MOSHFET on single crystal bulk AlN substrate

We report MOCVD-grown Al0.87Ga0.13N/Al0.64Ga0.36N metal-oxide-semiconductor-heterojunction-field-effect-transistors on single crystal bulk AlN substrate. As compared to control devices on AlN template, thermal impedance for devices on single crystal AlN decreased to 1/3 from 31 to 10 K mm W−1, comparable to SiC and copper heat-sinks. This represents a significant thermo-electric co-design advantage over other semiconductors. As a result, the peak drain saturation current increased from 410 to 610 mAmm−1. A 3-terminal breakdown field of 3.7 MV cm−1 was measured, which to date represents state-of-the-art performance for devices with similar Al x Ga1−x N-channel composition. This translates to a measured Baliga figure of merit of 460 MWcm−2.

High thermoelectric performance and anisotropy studies of n-type Mg3Bi2-based single crystal

Layered high-performing Mg3Bi2-based materials which made from nontoxic and earth-abundant elements have been considered as a promising thermoelectric candidate for low-grade energy recycling. High-quality compositionally controllable Mg3Bi2-based single crystals are grown using modified Bridgman method. The anisotropy of in- and out of plane in high-quality Mg3Bi1.49Sb0.5Te0.01 bulk single crystals are clearly characterized, record high ZT of ∼0.9 at 300 K as well as excellent ZTavg of 1.26 in the temperature range of 300-523 K are achieved along ab-plane, both of which rank as top values among the reported literature. Owing to the higher mobility (266 vs 189 cm2·V−1·s−1), the electrical conductivity along ab-plane is about 25% larger than that along c-axis at room temperature. Besides, Seebeck coefficient and lattice thermal conductivity are insensitive to the direction, resulting in anisotropy of thermoelectric performance but smaller than van der Waals layered materials. Compared with parent Mg3Bi1.5Sb0.5, both carrier concentration and mobility are significant improved with quantitative Te doping, leading to greatly enhanced power factor. With in-depth investigations of anisotropy, the results presented here will advance the fundamental understanding of Mg3Bi2-based system and suggest new ideas on design of the state-of-art thermoelectric materials with high performance.

Controlling of Localization by Elemental-substitution Effect in Layered BiCh2-based Compounds LaO1−xFxBiS2−ySey

We report transport properties for layered BiCh2-based (Ch = S, Se) superconductors LaO1−xFxBiS2−ySey (x = 0.2, 0.5, y = 0–1.05) and the observation of possible weak antilocalization (WAL). Electrical resistivity and Hall coefficients for the Se-poor samples increase with decreasing temperature. The increase becomes less pronounced with increasing Se concentration, indicating a loss of insulating behavior. Interestingly, the moderately Se-substituted samples exhibit metallic behavior in the high-temperature region and a weak increase in the resistivity in the low-temperature regions, which implies the existence of carrier localization. The heavily Se-substituted compounds show metallic behavior in the entire-temperature region. Magnetoresistance measurements indicate that WAL is realized in the heavily Se-substituted systems. The WAL behavior is weakened by the changes in F and Se concentrations. A crossover state of the WAL and weak localization (WL) emerges around the moderately F-doped and Se-free LaO0.8F0.2BiS2. The change of the resistivity and Hall coefficient by the F and Se substitution clearly correlates to the difference of the magnetoconductance. We propose that the BiCh2-based system is a good platform for studying the relationship between localization and superconductivity because those states are tunable by element substitutions with bulk single crystals.

Micromagnetic understanding of evolutions of antiferromagnetic domains in NiO

Antiferromagnetic domain formations and behaviors are long-standing implicit questions in magnetism and are becoming more important in recently emerging antiferromagnetic spintronics. In this paper, we experimentally investigate and numerically simulate evolutions of antiferromagnetic domains in NiO of a bulk single crystal under magnetic fields. With appropriate magnetic and crystallographic potential energy considerations, we extend a micromagnetic framework with a simulation algorithm which reproduces well the domain patterns and furnishes detailed understanding of the domain evolutions. The present work provides an important theoretical methodology to predict and understand behaviors of micromagnetic structures in antiferromagnets.

Sankaranarayanan-Ramasamy (SR) technique L- threonine lithium chloride single crystal: Unidirectional growth, optical and mechanical studies

L-Threonine Lithium Chloride (LTLC), an effective semi-organic NLO material, was successfully produced as a bulk single crystal using the Sankaranarayanan Ramasamy (SR) process. According to a study of a single crystal's X-ray diffraction, LTLC crystallises in an orthorhombic system with space group P212121. From the powder XRD method, unidirectional development along the plane (110) was established. The optical absorption spectra reveals that LTLC has a broad bandgap of 3.75 eV and is very transparent over the whole visible region. On the magnified crystal, photoconductivity measurements show positive photoconductivity. According to the photoluminescence investigation, LTLC emits green light with a wavelength of 535 nm. Vickers microhardness testing was done to determine the crystal's mechanical strength. At different temperatures and frequencies, the dielectric constant and dielectric loss were calculated. In addition to the density functional theory (DFT) calculations such as frontier molecular orbital’s (FMOs), analysis was carried out. Utilizing Kurtz's Nd:YAG Laser, powder testing is another method used to assess the produced crystal's SHG activity.

Deconvoluting Cr states in Cr-doped UO2 nuclear fuels via bulk and single crystal spectroscopic studies

Cr-doped UO2 is a leading accident tolerant nuclear fuel where the complexity of Cr chemical states in the bulk material has prevented acquisition of an unequivocal understanding of the redox chemistry and mechanism for incorporation of Cr in the UO2 matrix. To resolve this, we have used electron paramagnetic resonance, high energy resolution fluorescence detection X-ray absorption near energy structure and extended X-ray absorption fine structure spectroscopic measurements to examine Cr-doped UO2 single crystal grains and bulk material. Ambient condition measurements of the single crystal grains, which have been mechanically extracted from bulk material, indicated Cr is incorporated substitutionally for U+4 in the fluorite lattice as Cr+3 with formation of additional oxygen vacancies. Bulk material measurements reveal the complexity of Cr states, where metallic Cr (Cr0) and oxide related Cr+2 and Cr+32O3 were identified and attributed to grain boundary species and precipitates, with concurrent (Cr+3xU+41-x)O2-0.5x lattice matrix incorporation. The deconvolution of chemical states via crystal vs. powder measurements enables the understanding of discrepancies in literature whilst providing valuable direction for safe continued use of Cr-doped UO2 fuels for nuclear energy generation.

Sonication-assisted liquid exfoliation and size-dependent properties of magnetic two-dimensional α-RuCl3

Originating from the hexagonal arrangement of magnetic ions in the presence of strong spin orbit coupling, α-RuCl3 is considered as model system for the Kitaev-Heisenberg model. While the magnetic properties of α-RuCl3 have been studied in bulk single crystals or micromechanically-exfoliated nanosheets, little is known about the nanosheets’ properties after exfoliation by techniques suitable for mass production such as liquid phase exfoliation (LPE). Here, we demonstrate sonication-assisted LPE on α-RuCl3 single crystals in an inert atmosphere. Coupled with centrifugation-based size selection techniques, the accessible size- and thickness range is quantified by statistical atomic force microscopy. Individual nanosheets obtained after centrifugation-based size selection are subjected to transmission electron microscopy to confirm their structural integrity after the exfoliation. The results are combined with bulk characterisation methods, including Raman and x-ray photoelectron spectroscopy, and powder diffraction experiments to evaluate the structural integrity of the nanosheets. We report changes of the magnetic properties of the nanomaterial with nanosheet size, as well as photospectroscopic metrics for the material concentration and average layer number. Finally, a quantitative analysis on environmental effects on the nanomaterial integrity is performed based on time and temperature dependent absorbance spectroscopy revealing a relatively slow decay (half-life of ∼2000 h at 20 °C), albeit with low activation energies of 6–20 kJ mol−1.