High-quality Crystals Research Articles

Equal-Bilayer MoSe2 Grown by a Nucleation-Etching Strategy with High Carrier Mobility.

Bilayer transition metal chalcogenides (TMDs) have gradually attracted a great deal of attention due to the higher density of states and carrier mobility than monolayer TMDs. Controlling the uniformity of the layer number is very crucial because it will intensively influence the physical properties. However, it is difficult to synthesize equal-bilayer (EB) TMDs with two identical layers via a normal layer-by-layer strategy. Most reported bilayer TMDs are not uniform and such unequal bilayers would introduce a sizable Schottky barrier, resulting in the low carrier mobility. Here, a nucleation-etching strategy is proposed to grow EB-MoSe2 by chemical vapor deposition (CVD), which breaks the limitations of normal layer-by-layer strategy. The second layer is preferentially formed beneath the first layer rather than above, and a different etching phenomenon is also observed, which occurs more preferentially at the overlapping grain boundary sites on the top layer. The obtained EB-MoSe2 flakes are 3R-stack with high crystal quality. Furthermore, the contact between EB-MoSe2 and metal electrodes is greatly improved, thereby EB-MoSe2 transistors exhibit an order of magnitude higher carrier mobility (104 cm2 V-1 s-1) than that of UEB-MoSe2 transistors (12 cm2 V-1 s-1). This value is also at a relatively high level compared with reported results. Our work offers a feasible strategy for the synthesis of EB-TMDs with high carrier mobility, which is meaningful for developing high-performance 2D optoelectronic devices.

Correlated photoluminescence blinking phenomenon on InGaN/GaN nanopillar structures

Light-emitting devices that take advantage of the wide bandgap characteristics of InGaN/GaN are widely used in the industry. However, inhomogeneities have been reported in their photoluminescence (PL) mapping at the nanometer and submicrometer scale, even in samples of high crystal quality. In addition, a blinking phenomenon (time variation of PL intensity) under photoexcitation has been reported in relation to these inhomogeneities. The reason why this blinking phenomenon occurs is still unclear; it has been observed in quantum dots and other single and multilayer quantum well structures. Nevertheless, there are very few publications on nanopillar InGaN quantum well samples, which are the focus of this research. Here, we report and analyze the behavior of the blinking phenomena on a nanopillar sample. We noticed that the blinking of the pillars is somehow synchronized on a long timescale among several spatially separated nanopillars. We demonstrated that the synchronization is not due to random intensity fluctuations. We suggest instead that the synchronization is caused by a nonlinear response of the quantum wells to the UV source. In other words, when the stimulation intensity surpasses a certain value, it triggers an ON/OFF state switch in the PL of some of the pillars. Even if preliminary, our study helps to provide clues to understanding the mechanism of the occurrence of the blink phenomenon.

Synthesis, Optical, and Magnetic Properties of the Mixed Chalcogenide Semiconductor Series Eu(II)2SiSexS4-x, Prepared via the Flux-Assisted Boron Chalcogen Mixture Method.

We report a detailed structural study of a series of five new quaternary Eu(II)-containing mixed chalcogenide phases, Eu2SiSe0.85S3.15, Eu2SiSe2.4S1.6, Eu2SiSe2.6S1.4, Eu2SiSe3.1S0.9, and Eu2SiSe4, synthesized using the flux-assisted boron chalcogen mixture (BCM) method. High-quality crystals were grown, and their crystal structures were determined by single-crystal X-ray diffraction. All members of the Eu2SiSexS4-x series crystallize in the monoclinic crystal system with space group P21/m, except Eu2SiSe4, which crystallizes in the P21 space group. The crystal structure of Eu2SiSexS4-x exhibits a complex three-dimensional network and is primarily composed of distorted trigonal prismatic EuQ6 polyhedra that are interconnected by SiQ4 tetrahedra. Polycrystalline powders were used for physical property measurements, including the magnetic susceptibility and UV-vis diffuse reflectance. Magnetic measurements indicated paramagnetic behavior with a slightly negative Weiss constant (θ = -13.54). The band gaps of the materials were determined from diffuse reflectance data, with optical band gaps estimated to be 2.04(2) eV, 1.98(2) eV, and 1.90(2) eV for Eu2SiSe0.85S3.15, Eu2SiSe2.6S1.4, and Eu2SiSe4, respectively. Band gap tuning was achieved by partially or fully replacing the S sites with Se.

Оксид галію — перспективний мультифункціональний матеріал четвертого покоління (огляд)

This work is devoted to the analysis and systematization of the main information on the properties of gallium oxide and materials based on it and their practical application, as well as the prospects for further research of the specified actual oxide material. A review of literature data concerns general properties and structure of gallium oxide Ga2O3, various methods to produce Ga2O3 thin films, nanostructures, bulk crystals, powders, the application of gallium oxide in various fields of science and technology, including semiconductor field, electronic engineering, optoelectronics, the creation of composite transparent materials, etc. In the last thirty years or so, thanks to the progress in growing large-volume, high-quality gallium oxide crystals, this material with an ultra-wide band gap and a high critical breakdown field has gained significant application in the manufacture of the latest power electronics and high-voltage electronic devices. Important experimental studies, in particular, in terms of developing methods of metallization, joining similar materials, connecting electrical contacts, for example, by soldering, require the study of the wetting of these oxide materials by metal melts and the contact interaction at the interphase boundaries. Data on surface phenomena, in particular the wetting of gallium oxide by metals, are practically absent in the literature, and this requires further additional research. Keywords: gallium oxide, physical properties, semiconductor, power electronics, optoelectronics, transparent composite materials.

Growth and magnetic properties of epitaxial thin films of the [formula omitted]-MAX phase (Mn[formula omitted]Sc[formula omitted])[formula omitted]GaC

i-MAX phases are quaternary variants of the nanolaminated MAX phases, with additional in-plane ordering of the M atoms. The combination of in-plane and out-of-plane ordering potentially gives rise to complex magnetic behaviour. The i-MAX phase (Mn2/3Sc1/3)2GaC has been synthesized in epitaxial thin film form on three different substrates, SiC-4H(001), MgO(111) and Al2O3(0001), by magnetron sputtering using elemental targets. Structural characterization by x-ray scattering and scanning transmission electron microscopy confirms the phase on all three substrates, although the highest crystal quality is obtained on SiC-4H(001). High-resolution images reveal the distinctive i-MAX structure, which is orthorhombic of space group Cmcm. Magnetic characterization reveals that the ground state is most likely antiferromagnetic. This confirms previous theoretical calculations which predicted an antiferromagnetic ground state and establishes the (Mn2/3Sc1/3)2GaC i-MAX phase as a potential candidate for antiferromagnetic spintronic applications.

Catalyst-free in-plane growth of high-quality ultra-thin InSb nanowires

InSb nanowires (NWs) show an important application in topological quantum computing owing to their high electron mobility, strong spin–orbit interaction, and large g factor. Particularly, ultra-thin InSb NWs are expected to be used to solve the problem of multiple sub-band occupation for the detection of Majorana fermions. However, it is still difficult to epitaxially grow ultra-thin InSb NWs due to the surfactant effect of Sb. Here, we develop an in-plane self-assembled technique to grow catalyst-free ultra-thin InSb NWs on Ge(001) substrates by molecular-beam epitaxy. It is found that ultra-thin InSb NWs with a diameter as small as 17 nm can be obtained by this growth manner. More importantly, these NWs have aspect ratios of 40–100. We also find that the in-plane InSb NWs always grow along the [110] and [11¯0] directions, and they have the same {111} facets, which are caused by the lowest-surface energy of {111} crystal planes for NWs grown with a high Sb/In ratio. Detailed structural studies confirm that InSb NWs are high-quality zinc blende crystals, and there is a strict epitaxial relationship between the InSb NW and the Ge substrate. The in-plane InSb NWs have a similar Raman spectral linewidth compared with that of the single-crystal InSb substrate, further confirming their high crystal quality. Our work provides useful insights into the controlled growth of in-plane catalyst-free III–V NWs.

Spectroscopic study and visible luminescence of Tb3+ ions in CaF2 crystal co-activated with Yb3+

High quality crystals with composition of Ca0.95Tb0.05F2.05 and Ca0.85Tb0.05Yb0.1F2.15 were grown by Bridgman-Stockbarger technique. Yb3+ ions were used as a sensitizer for Tb3+ ions. Optical properties were measured, including the absorption spectrum, emission spectrum, and fluorescence decay curves. Judd-Ofelt theory was used to analyze the spectroscopic parameters. Fluorescence branch ratio, transition probability, radiative and fluorescence lifetimes, and quantum efficiency were determined. The emission cross section of Tb3+ ions in Tb:CaF2 and Tb/Yb:CaF2 crystals for the green emission at 544 nm, corresponding to the transitions of 5D4→7F5 were calculated to be 4.61x10-22 cm2 and 4.60x10-22 cm2, respectively. Our experiments have shown that doping with Yb3+ ions does not significantly affect the spectral properties of Tb3+ ions in CaF2 crystal. Quadratic dependence of the emission intensity of Tb3+ ion in Tb/Yb:CaF2 crystal on the intensity of a diode laser at 955 nm proved that the excitation of Tb3+ ion occurs due to the cooperative process of transferring the excitation from two Yb3+ ions to one neighboring Tb3+ ion. The Tb/Yb:CaF2 crystal can be considered as a promising up-converting medium for creating diode-pumped visible phosphorus and active laser media.

Protein crystallization and structure determination at room temperature in the CrystalChip.

The production of high-quality crystals is a key step in crystallography in general, but control of crystallization conditions is even more crucial in serial crystallography, which requires sets of crystals homogeneous in size and diffraction properties. This protocol describes the implementation of a simple and user-friendly microfluidic device that allows both the production of crystals by the counter-diffusion method and their in situ analysis by serial crystallography. As an illustration, the whole procedure is used to determine the crystal structure of three proteins from data collected at room temperature at a synchrotron radiation source.

Lead-Free All Inorganic Rubidium Copper Halide Rb2CuX3 (X = Cl, Br) for UVC Photodetector with Fast Response.

Recently, lead halide perovskites have shown great potential in the photodetection field. Unfortunately, the existence of toxic lead elements restricts their practical application. Herein, high-quality 1D lead-free crystals Rb2CuX3 (X = Cl, Br) are successfully synthesized in an acidic medium. Rb2CuCl3 and Rb2CuBr3 display violet photoluminescence emission peaks at 401 and 388nm with a high exciton lifetime of 12.78 and 59.67µs, respectively, which is due to self-trapped excitons (STE). Furthermore, a photodetector is fabricated to detect harmful UVC radiation with Rb2CuBr3 as a light absorber. The proposed photodetector has a responsivity of 0.92mAW-1 and a specific detectivity of 1.57 × 109 Jones with a fast response speed of 2.67/2.70 µs for rise/fall time, respectively. In addition, both the samples show good stability against open air, and continuous ultraviolet light (254nm), as well as good thermal stability which is necessary for device fabrication. Thus, the present study suggests that stable, non-toxic, and environment-friendly Rb2CuX3 (X = Cl, Br) can be a promising candidate for future UVC optoelectronic devices.

Fabrication of 0.6 eV Bandgap In0.69Ga0.31As Thermophotovoltaic Cells and Its System Demonstration

Thermophotovoltaic (TPV) is a promising energy conversion technology that can absorb the heat from a thermal radiator and transfer it into power. As the most significant energy converter, TPV cells need a narrower bandgap to realize a wider absorption spectrum range. In this work, the fabrication and characterization of single‐junction In0.69Ga0.31As TPV cells with a bandgap of 0.6 eV are presented. The main structure is grown on an InP substrate through metal‐organic chemical vapor deposition. Step‐graded InAsyP1−y buffer layers are used to mitigate the dislocations by relaxing the stress induced by lattice mismatch completely. Analysis of the composition, strain relaxation, layer tilt, and crystalline quality of each layer is demonstrated using triple‐axis X‐ray reciprocal space mapping and transmission electron microscopy. According to the tested results, each layer is found to be nearly fully relaxed and the InGaAs active layer grown on the buffer displays a high crystal quality. External quantum efficiency achieves 90% at 1100–1500 nm. Additionally, a TPV test platform is constructed to evaluate the cell performance. The maximum efficiency of the lattice‐mismatched TPV cell reaches 21.92% operating at a power density of 267.4 mW cm−2 and an emitter temperature of 1200 °C.

Nitrogen-doped Ga2O3 current blocking layer using MOCVD homoepitaxy for high-voltage and low-leakage Ga2O3 vertical device fabrication

In this Letter, a high-quality and high-resistivity nitrogen (N)-doped Ga2O3 current blocking layer (CBL) is grown utilizing metal-organic chemical vapor deposition homoepitaxial technology. By using nitrous oxide (N2O) as oxygen source for Ga2O3 growth and N source for doping and controlling the growth temperature, the grown CBL can effectively achieve high (∼1019 cm−3) or low (∼1017 cm−3) N doping concentrations, as well as high crystal quality. Furthermore, the electrical properties of the developed CBL are verified at the device level, which shows that the device using the CBL can withstand bidirectional voltages exceeding 3.5 kV with very low leakage (≤1 × 10−4 A/cm2). This work can pave the way for the realization of high-voltage and low-leakage Ga2O3 vertical devices, especially metal-oxide-semiconductor field effect transistors.

Gradient Mo1−xWxSe2 monolayer alloys: Synthesis and multifunctional applications

With the continuous and rapid development of electronic information technology, the demand for new materials with miniaturization, high integration, high performance, and multifunction is becoming increasingly urgent. However, it is difficult for existing photodetectors based on a single material to achieve multi-functionality while maintaining excellent performance, and the devices based on heterostructures are faced with complex preparation and compatibility problems. In this work, gradient Mo1−xWxSe2 monolayer alloys with high crystal quality have been synthesized by an evaporation-assisted chemical vapor deposition method. The composition of the synthesized Mo1−xWxSe2 monolayer alloys varies gradually with x ranging from 0 to 1. The Mo1−xWxSe2 based photodetector shows superior photoresponse performance in the visible wavelength with maximum responsivity of 26 A/W and external quantum efficiency up to 6320 % under 520 nm laser irradiation. Besides, the Mo1−xWxSe2 device is capable of high-resolution imaging, which can be used as image sensor. Moreover, nonvolatile memory function has also been realized in the device because the band gap of Mo1−xWxSe2 can be effectively adjusted by the gradient distribution of the component and the deep energy levels caused by Se vacancies can be modulated to shallow energy levels. A thrilling discovery is that the gradient Mo1−xWxSe2 alloy device can simulate the biological synapses successfully, including paired-pulse facilitation, short-term plasticity, long-term plasticity and learning process. The realization of high-performance photodetection, visualization, nonvolatile memory and synaptic simulation based on a single gradient crystal offers a new type of material to develop advanced optoelectronic devices with multiple functions.

Exciton dynamics in CsPbBr3 single crystal: LT splitting energy, exciton-polariton dispersion, and biexciton binding energy.

Metal halide perovskite materials (MHPs) are promising for several applications due to their exceptional properties. Understanding excitonic properties is essential for exploiting these materials. For this purpose, we focus on CsPbBr3 single crystals, which have higher crystal quality, are more stable, and have no Rashba effect at low temperatures compared to other 3D MHPs. We have estimated exciton energy positions, longitudinal-transverse splitting energy, and damping energy using low-temperature reflection spectra. Under high excitation intensity, two biexciton emissions (M-emission) and exciton-exciton scattering emission (P-emission) were observed. We assign the two M-emissions to the emission to the states of longitudinal and transverse excitons, i.e., ML and MT emissions. From the energy position of the MT emission, the biexciton binding energy has been estimated to be ∼2meV. By analyzing P-emission obtained from the back side of the sample, we have estimated the exciton binding energy to be 17.8-23.7meV. This estimation minimizes the influence of the wavenumber distribution in the scattering process. In addition, time-resolved transmittance measurements using pulsed white light have revealed the group velocity dispersion. Comparing experimental results with theoretical calculations using the Lorentz model clarifies that exciton dynamics in CsPbBr3 can be described with a simple Lorentz model. These insights enhance the understanding of exciton behavior and support the development of exciton-based devices using MHPs.

Effect of organic acid additives on sulfite oxidation in a calcium-based slurry

The forced oxidation of sulfite ions was conducted to obtain high-quality gypsum in the flue gas desulfurization system. The effects of formic, lactic, acrylic, acetic, propionic, and hexanoic acids added to calcium-based slurry on sulfite oxidation were investigated in this study. Sulfite oxidation was inhibited when the pKa value of organic acids was small or the carbon chain was long. In the identical slurry pH 6, the SO4 2- fractions of slurries with formic and lactic acids, which have the smallest pKa values, were lower than half that of the additive-free slurry. The addition of hexanoic acid with the longest carbon chain showed a similar SO4 2- fraction to that of additive-free slurry. These results indicate that the inhibitory effect on sulfite oxidation is more expressed by the acidity of organic acids. The SO4 2- fraction in the slurry affects the growth and quality of gypsum crystals. The slurry with acetic acid presented the highest SO4 2- fraction and resulted in the formation of high-quality gypsum crystals. The findings of this study can contribute to the selection of organic acid additives with high desulfurization efficiency and the production of high-quality gypsum.

Top-down fabrication of Si nanotube arrays using nanoimprint lithography and spacer patterning for electronic and optoelectronic applications

Silicon (Si) nanotube arrays are expected to be a promising material for application in electronics and optoelectronics due to their large specific surface area with axially hollow spaces, whereas realizing smooth surfaces, high aspect ratios, and controlled dimensions remains a challenge. Moreover, it is also necessary to estimate various aspects of Si nanotubes such as their surface damage and anti-reflective properties. Here, we demonstrate a top-down fabrication of Si nanotube arrays with wall thicknesses of ∼40 to ∼10 nm using nanoimprint lithography (NIL) and spacer patterning. The Bosch process yields the nanotubes with smooth surfaces, long lengths (∼1000 nm), and no noticeable distortion or deformation. Raman scattering and electron spin resonance (ESR) measurements show their high crystal quality with a low density of surface dangling-bond defects. Furthermore, a significant enhancement of the anti-reflection effect of nanotubes and its dimension dependence is demonstrated and investigated by UV–Vis–NIR spectrophotometry as well as numerical simulations.

Growth and spectroscopy characteristics of Er, Ce:BGSO crystal: A promising material for eye-safe laser applications

Er, Ce co-doped BGSO single crystals were successfully grown by the Bridgman method. The growth parameters, including temperature gradient and growth rate, were optimized to achieve high-quality crystals. The crystal structure and morphology were characterized using X-ray diffraction and scanning electron microscopy. The optical absorption properties of Er, Ce co-doped BGSO single crystals were studied using UV–Vis absorption spectroscopy. The absorption spectrum revealed the absorption edge and the presence of absorption bands related to the presence of Ce dopant, providing valuable insights into the energy transfer processes occurring within the crystal lattice. Furthermore, the luminescence mechanism of BGSO crystals was analyzed using the Judd-Ofelt (J-O) theory, with detailed investigation into the role of ligands in spectral parameters. The photoluminescence spectra and emission decay time of Er, Ce co-doped BGSO single crystals were characterized. A comparative analysis with Er, Ce:BGSO crystals was conducted to investigate the sensitization mechanism of Ce ions on Er emission at 1.5 μm, demonstrating their potential for use as eye-safe laser materials in various applications.

Super High-k Unit-Cell-Thick α-CaCr2O4 Crystals.

High-dielectric-constant (high-k) insulators are indispensable components to integrate semiconductors into metal-oxide-semiconductor field-effect transistors with sub-10 nm channel length, where the equivalent oxide thickness (EOT) of high-k insulator needs to be decreased to subnanometer scale. The traditional insulators, including Al2O3, SiO2, and HfO2, fit well with the existing silicon industry but suffer from serious degeneration of insulating properties, such as large leakage currents caused by high-density borders and interface traps, when their thicknesses are reduced to a few nanometers. Here, we synthesize a high-quality nonlayered ultrathin α-CaCr2O4 crystal down to unit-cell thickness (∼1.2 nm) by an elements slow-supply chemical vapor deposition (CVD) method. The unit-cell-thick α-CaCr2O4 crystals show a super high dielectric constant of 87.34, which is over 20 times higher than that of well-known layered insulator h-BN and corresponds to an EOT below 1 nm. Furthermore, it has a high breaking strength (39 GPa) and excellent stability. This strategy can also be used to fabricate other ultrathin ternary oxides, such as high-k ultrathin FeNb2O6 crystals, demonstrating the universality of the CVD method.

High quality and sensitivity phononic crystal channel drop filter to detect ethyl lactate in mixtures of ethyl lactate and 2-butoxy ethanol

This paper introduces a new sensing approach utilizing a solid-solid phononic crystal (PnC) channel drop filter structure for the detection of varying molar fractions of Ethyl lactate within a mixture of Ethyl lactate and 2-butoxy ethanol. The sensor features a two-dimensional PnC constructed from poly methyl methacrylate as the background material, incorporating a regular arrangement of circular Tungsten columns. The design integrates a bus waveguide linked to two interconnected ring resonators, which are coupled to a drop waveguide. Each ring resonator is equipped with three strategically positioned pillars near the coupling region, allowing for the accommodation of varying concentrations of Ethyl lactate. The sensor’s performance is significantly influenced by the ring resonators and the integrated pillars. It identifies specific resonance frequencies that shift in response to changes in Ethyl lactate concentrations within the resonators. The transmission resonance frequency exhibits valuable sensitivity to these concentration variations, reflecting the unique sound velocities and mass densities associated with each level of Ethyl lactate. The effectiveness of the proposed sensor is validated through coupled mode theory, demonstrating a close match with the device’s observed behavior. The measured frequency range spans from 1.972 MHz to 1.979 MHz with a step size of 1 Hz. Notably, the sensor displays considerable characteristics, including an average quality factor of 90,613, an average sensitivity of 3816 Hz, an average figure of merit of 172, an average signal-to-noise ratio of 137, an average resolution of 22 Hz, an average damping ratio of 0.56 × 10− 5, and an average limit of detection of 34 × 10− 5. These results underscore the potential of the channel drop filter for the accurate detection of Ethyl lactate concentrations with high quality factor and sensitivity. The sensor can effectively identify variations in Ethyl lactate levels, which are prevalent in various applications, including pharmaceuticals and food additives.

Sublimation and Recrystallization of CsPbBr3 Constructing a Perovskite-Carbon Interface to Facilitate Hole Transfer in Printable All-Inorganic Perovskite Solar Cells.

All-inorganic perovskite solar cells (APSCs) are a promising photovoltaic technology due to their unique physical and optical properties. For printable all-inorganic perovskite solar cells, the key factors limiting their photoelectric conversion performance are the crystallization of perovskite and the hole collection capability at the perovskite-carbon interface. In this study, leveraging the high-temperature tolerance of CsPbBr3 perovskite, the sublimation and recrystallization processes were controlled, significantly improving the crystalline quality of the perovskite and suppressing the generation of the nonphotovoltaic CsPb2Br5 phase. Furthermore, a continuous distribution of high-quality CsPbBr3 crystals was achieved at the interface between the carbon electrode and the zirconium oxide layer, ensuring efficient collection of photogenerated holes. The photoelectric conversion efficiency of the printable mesoscopic all-inorganic perovskite solar cell was increased from 5.04 to 8.34% (with a record value of 10.04%) and achieved an ultrahigh open-circuit voltage of 1.54 V due to the significant improvement in the crystal quality of CsPbBr3. This study proposes a novel strategy to enhance the photovoltaic conversion performance of carbon-based all-inorganic perovskite solar cells by suppressing the presence of nonphotovoltaic phases.

SbSeI for high-efficient photocatalytic degradation of multiple pollutants

Photocatalytic degradation is an effective technology for degrading water pollution that plays a significant role in environmental remediation. Ternary 2D ternary V-VI-VIIA semiconductors are ideal candidates for photocatalytic degradation of pollutants due to effective light absorption and high charge carrier mobility. In this work, high-quality SbSeI crystals were prepared using the chemical vapor transport (CVT) method and their photocatalytic degradation performance for multiple pollutants was studied. SbSeI exhibits excellent photocatalytic performance in the degradation of potassium dichromate (Cr (VI)), rhodamine B (RhB), tetracycline hydrochloride (TC-HCl) and methyl orange (MO). More than 98% of Cr (VI) and RhB can be removed after irradiation with an Xe lamp for 10 min and 40 min, respectively. The capture experiments and electron spin resonance results indicated that ·O2− plays a major role in reducing Cr (VI), while h+ plays a primary role in the degradation of MO, RhB and TC-HCl. Interestingly, the degradation rate of Cr (VI) is 1.3 times higher than that of a single pollutant system, and the degradation rate of RhB is 1.6 times higher, due to the enhanced separation and utilization of holes and electrons. The results demonstrate that SbSeI is a potential photocatalytic degradation material.