High-resolution X-ray Diffraction Research Articles

High-pressure phase transition in clinochlore: IIa polytype stabilization

Abstract Structural variations of natural clinochlore with pressure have been studied by in situ single-crystal X-ray diffraction (XRD) in a diamond-anvil cell in the pressure range 0–20 GPa at room temperature. High-resolution XRD data allowed for the identification of a polytypic phase transition at about 9 GPa. Around 4.32(5) GPa, the unit-cell parameters exhibited a significant deviation from linear behavior, particularly the c and β values, abruptly interrupted when the phase transition occurs. XRD patterns showed a drastic reduction of diffuse scattering due to the stabilization of the high-pressure structure, suggesting that the atomic reorganization of the layers led to a disorder reduction. The phase transition showed complete reversibility during the experiment. Ab initio structural refinements identified the transition as polytypic, from the initial IIb-4 triclinic polytype (space group C1) to the IIa-1 monoclinic structure (space group C2/m), with unit-cell parameters a = 5.2058(6) Å, b = 9.0208(4) Å, c = 13.560(7) Å, β = 97.34(3)°. The latter was theoretically derived in the 1960s as the least stable chlorite polytype but has never been observed in natural chlorites. The phase transition also has a significative effect on the bulk modulus, with a reduction from K0 = 81.2(13)t to K0 = 56.0(6) GPa for the high-pressure structure. An isothermal run at 600 K from ambient pressure to 14 GPa showed the same phase transition at 7.8(5) GPa. Its occurrence at lower pressures suggests a negative P/T slope for the transition. Therefore, at high-temperature and high-pressure conditions compatible with impact phenomena, the polytypic phase transition could prevent chlorite from early destabilization and dehydration.

Synthesis, Structure, and Properties of Nontrivial Iridium(III) Complexes Based on Anthracene-Decorated Benzimidazole Ligand.

Reactions of iridium trichloride hydrate with bulky 2-(9-anthracenyl)-1-phenyl-benzimidazole (anbi) in the presence of N-donor ligands afforded a number of unique noncyclometalated complexes, while attempts to prepare a common μ-chloro-bridged bis-cyclometalated dimer systematically gave a monocyclometalated complex cis-[Ir(C,N-anbi)(N-anbi)Cl2] instead. The obtained complexes were characterized by 1H NMR, high-resolution mass spectrometry, single-crystal and powder X-ray diffraction, UV-vis spectroscopy, and cyclic voltammetry. The noncyclometalated complexes fac-[Ir(N-anbi)(N^N)Cl3)], where N^N are 4,4'-disubstituted 2,2'-bipyridines, are octahedral and contain the anthracene and 2,2'-bipyridine units in a close cofacial arrangement. These complexes were found to be exceptionally inert to the chloride ligand exchange even in the presence of silver triflate, forming a rare trinuclear Ir-μ-Cl3-Ag-μ-Cl3-Ir structure instead. In the monocyclometalated complex, the Ir(III) ion is pentacoordinated in a rare square-pyramidal geometry, where the bulky anthracene fragment is involved in the steric shielding of the metal center. This is in line with the results of gas-phase density functional theory calculations, demonstrating that the experimentally observed structure is energetically most preferable. The monocyclometalated complex is deeply colored due to intense charge-transfer absorption bands in the range 450-650 nm with ε = 2000-5000 M-1 cm-1, superior to the noncyclometalated complexes. The synthesis, structures, and properties of the new complexes are discussed in the context of the related mono-, bis-, and noncyclometalated iridium(III) compounds.

Controllable growth of large 1T-NbTe2 nanosheets on mica by chemical vapor deposition and its magnetic properties

Two-dimensional (2D) magnetic materials have recently attracted broad attention for their potential technological applications. Here, we report controllable growth of NbTe2 nanosheets on mica substrate by atmospheric chemical vapor deposition. Different from the SiO2/Si substrates, dangling bonds on the surface of the mica are absent, which induces low nucleation density and higher lateral growth rate. As a result, the lateral size of NbTe2 nanosheets on mica substrates can be significantly larger than those on SiO2/Si substrates. Large single crystal NbTe2 nanosheets with a lateral size of up to 151 µm were fabricated on a mica. High-resolution transmission electron microscope image, XRD patterns and Raman spectra reveal the NbTe2 nanosheets adopt the single crystal with 1T phase. Furthermore, the NbTe2 nanosheets on mica demonstrate ferromagnetic properties with a Curie temperature of up to 162 K. Our work paves the way for the atmospheric chemical vapor deposition growth and applications of many more 2D magnets.

Engineering wafer scale single-crystalline Si growth on epitaxial Gd2O3/Si(111) substrate using radio frequency sputtering for silicon on insulator application

Silicon on Insulator (SOI) technology has been the promising solution for the On-chip low-power mixed-signal systems. However, the traditional smart-cut method for SOI wafer fabrication is costly. Hence, in this work, we present an optimized methodology to fabricate a wafer scale single-crystalline Si on epitaxial Gd2O3/Si(111) substrate (SOXI) using a low-cost, high volume manufacturable (HVM)-friendly Radio Frequency magnetron sputtering technique for SOI application. The grown Si and Gd2O3 layers are physically characterized using high-resolution X-ray Diffraction and high-resolution Transmission Electron Microscopy (HRTEM). The Grazing Incidence X-ray Diffraction, Pole Figure, and HRTEM imaging confirm the formation of an epitaxial Top-Si layer on the epitaxial-Gd2O3/Si(111) substrate. Hence, we demonstrate a low-cost, HVM-friendly technique for the fabrication of SOXI wafers.

Cryogenic Digital Image Correlation as a Probe of Strain in Iron-Based Superconductors

Abstract Uniaxial strain is a powerful tuning parameter that can control symmetry and anisotropic electronic properties in iron-based superconductors. However, accurately characterizing anisotropic strain can be challenging and complex. Here, we utilize a cryogenic optical system equipped with a high-spatial-resolution microscope to characterize surface strains in iron-based superconductors using the digital image correlation method. Compared with other methods such as high-resolution X-ray diffraction, strain gauge, and capacitive sensor, digital image correlation offers a non-contact, full-field measurement approach, acting as an optical virtual strain gauge that provides high spatial resolution. The results measured on detwinned BaFe2As2 are quantitatively consistent with the distortion measured by X-ray diffraction and neutron Larmor diffraction. These findings highlight the potential of cryogenic digital image correlation as an effective and accessible tool for probing the isotropic and anisotropic strains, facilitating the application of uniaxial strain tuning in the study of quantum materials.

Investigation of the growth rate on optical and crystal quality of InGaAs/AlGaAs multi-quantum wells and InGaAs single layer grown by molecular beam epitaxy (MBE)

The impact of growth rate on the optical and crystal quality of InGaAs/AlGaAs multi-quantum wells (MQWs) and InGaAs single layer grown by molecular beam epitaxy (MBE) is studied. Photoluminescence (PL) and high-resolution X-ray diffraction (HRXRD) are applied for evaluation of the optical, crystal quality and interfaces smoothness. The room-temperature PL integral intensity increases by 220 % when the growth rate decreases from 2 to 1 Å/s. HRXRD analysis reveals the growth rate of 1 Å/s improves crystal and interface quality of MQWs, as well as significantly enhances PL intensity. But further reduce growth rate to 0.5 Å/s, the PL integral intensity decreases, instead. At lower growth rate, more impurities can incorporate into InGaN layer and act as nonradiative recombination centers. Growth rate can effectively control crystal quality, interfaces smoothness and impurity content and thus determine the optical quality of InGaAs/AlGaAs MQWs.

Impact of composition of AlGaInAs confining barriers on emission of InAs quantum dots embedded in AlGaAs/GaAs dot-in-a-well heterostructures

The emission of InAs quantum dots (QDs) grown on Al0.30 Ga0.70As/GaAs heterostructures and integrated into additional capping/buffer quantum wells (QW), known as dot-in-a-well (DWELL) structures, has been investigated. Two different AlGaInAs confining barriers (CBs) and buffer layers (BLs) are compared. The first QD structure includes the Al0.30 Ga0.70As CB (#1) and the In0.15Ga0.85As BL. The second QD structure consists of the Al0.40Ga0.45In0.15As CB (#2) and the In0.25Ga0.75As BL. Comparison of photoluminescence (PL) spectra has revealed that the ground state (GS) emission band in the structure #2 with Al0.40Ga0.45In0.15As CB is characterized by the lower peak energy, smaller PL linewidth (more homogeneous QD sizes) and higher GS emission intensity compared to that parameters in the structure #1 with Al0.30 Ga0.70As CB. The smaller potential barriers at the Al0.40Ga0.45In0.15As CB/QD interfaces in #2 lead to faster thermal decrease in the GS PL intensity at high temperatures compared with that in #1. High-resolution X-ray diffraction (HR-XRD) method was used for the study of the QD structures with the aim of monitoring the sizes and compositions of QDs and QWs. Numerical simulation of HR-XRD scans has shown that the material compositions of QDs and QW in #2 with Al0.40Ga0.45In0.15As CB has not changed in the process of QD structure growth at high temperatures compared to those in #1. The obtained results are interesting for further improvement of InAs/GaAs QD structures for telecommunication technology and optoelectronic applications.

Energetic Methylene-Bridged Furoxan-Triazole/Tetrazole Hybrids.

Design and synthesis of new energetic materials retains its urgency in chemistry and materials science. Herein, rational construction and regioselective synthesis of a series of energetic compounds comprising of a methylene-bridged combination of 1,2,5-oxadiazole and nitrogen-rich azoles (1,2,4-triazole and tetrazole) enriched with additional explosophoric functionalities (nitro and azo moieties) is presented. All target materials were thoroughly characterized using IR and multinuclear NMR (1H,13C,14N,15N) NMR spectroscopy, high-resolution mass spectrometry, X-ray diffraction,and differential scanning calorimetry. All synthesized energetic substances showed good thermal stability (up to 239oC) and low mechanical sensitivity, while their performance reached or exceeded the level of TNT.

Optical Characterization of the Interband Cascade LWIR Detectors with Type-II InAs/InAsSb Superlattice Absorber.

The long-wave infrared (LWIR) interband cascade detector with type-II superlattices (T2SLs) and a gallium-free ("Ga-free") InAs/InAsSb (x = 0.39) absorber was characterized by photoluminescence (PL) and spectral response (SR) methods. Heterostructures were grown by molecular beam epitaxy (MBE) on a GaAs substrate (001) orientation. The crystallographic quality was confirmed by high-resolution X-Ray diffraction (HRXRD). Two independent methods, combined with theoretical calculations, were able to determine the transitions between the superlattice minibands. Moreover, transitions from the trap states were determined. Studies of the PL intensity as a function of the excitation laser power allowed the identification of optical transitions. The determined effective energy gap (Eg) of the tested absorber layer was 116 meV at 300 K. The transition from the first light hole miniband to the first electron miniband was red-shifted by 76 meV. The detected defects' energy states were constant versus temperature. Their values were 85 meV and 112 meV, respectively. Moreover, two additional transitions from acceptor levels in cryogenic temperature were determined by being shifted from blue to Eg by 6 meV and 16 meV, respectively.

Wide & deep learning for predicting relative mineral compositions of sediment cores solely based on XRF scans, a case study from Pleistocene Paleolake Olduvai, Tanzania

This study develops a method to use deep learning models to predict the mineral assemblages and their relative abundances in paleolake cores using high-resolution XRF core scan elemental data and X-ray diffraction (XRD) mineralogical results from the same core taken at coarser resolution. It uses the XRF core scan data along with published mineralogical information from the Olduvai Gorge Coring Project (OGCP) 2014 sediment cores 1A, 2A, and 3A from Paleolake Olduvai, Tanzania. Both regression and classification models were developed using a Keras deep learning framework to assess the predictability of mineral assemblages with their relative abundances (in regression models) or at least the mineral assemblages (in classification models) using XRF core scan data. Models were created using the Sequential class and Functional API with different model architectures. The correlation matrix of element ratios calculated from XRF element intensity records from the cores and XRD-derived mineralogical information was used to select the most useful features to train the models. 1057 training data records were used for the models. Lithological classes were also used for some models using Wide & Deep neural networks since those combine the benefits of memorization and generalization for mineral prediction. The results were validated using 265 validation data records unseen by the model and discuss the accuracy of models using six test records. The optimized Deep Neural Network (DNN) classification model achieved over 86% binary accuracy while the regression models were also able to predict the relative mineral abundances of samples with high accuracies. Overall, the study shows the efficacy of a carefully crafted Deep Learning (DL) model for predicting mineral assemblages and abundances using high-resolution XRF core scan data.

Analysis of Lattice Damage in 4H-SiC Epiwafers Implanted with High Energy Al Ions at Elevated Temperatures

4H-SiC wafers with 12 µm epilayers were blanket implanted to a depth of 12 µm with 5 x 1016 cm-3 Al ions via Tandem Van de Graaff accelerator located at Brookhaven National Laboratory with energy range of 13.8 to 65.7 MeV at room temperature, 300 °C and 600 °C. High resolution X-ray diffraction measurements reveal the implanted layers are characterized by tensile strains. However, the dynamic annealing process reduces the level of tensile strains as the temperature of implantation is increased. Analysis indicates that the implant temperature of 600 °C is not sufficient to minimize lattice damage due to implantation and a higher implantation temperature will be required. This preliminary experiment will guide the optimization of implantation conditions for fabricating superjunction devices.

Quantification of microstructural homogeneity in indium arsenide epilayers by X-ray diffraction

In this article a complete procedure to investigate thin semiconductor plates (epitaxial layers), including high-resolution X-ray diffraction measurements, mathematical modelling of both crystalline structure and crystalline microstructure and computations to approximate solving inverse problems, is proposed and described in detail. The method is successfully applied to estimate crystalline homogeneity of a square indium-arsenide plate epitaxially-grown on gallium-arsenide substrate. To this end, the specimen is tested in nine areas around points forming a square grid. It is demonstrated that whole specimen may be regarded as a single large crystalline grain consisting of crystallites separated by small-angle boundaries. The crystallites occur as rode-like cuboids elongated in the direction perpendicular to the plate surface, with different areas of the sample and with base sizes not much differing. The mean-absolute second-order strain is very small and almost constant in the whole sample. The first-order strain also appears and, effectively, the structure of the crystalline layer is tetragonal with unit-cell parameters being smaller parallelly and larger perpendicularly to the layer surface and varying slightly in the layer. The results are presented in tables and figures and commented.

Direct determination of POSS cage structure in a self-assembled ionic bridged silsesquioxane by using the total pair distribution function G(r) and density functional theory.

In the present study, both short-range and long-range structural features of an ionic bridged silsesquioxane, specifically one containing the 1,4-diazoniabicyclo[2.2.2]octane chloride group (ISSQ), were elucidated. This ionic silsesquioxane was synthesized via direct polycondensation of a bridged organosilane precursor, without any additional functionalization step. Si-O-Si cage structures typical of Polyhedral Oligomeric Silsesquioxanes (POSS) were identified. The average interatomic distances of the POSS cages, including the open T8 cage and the T12 cage for the ISSQ, as well as the T8 cage for a commercially available pendant POSS were determined. It is the first report of the interatomic distance determination of POSS cage; achieved by using total pair distribution function G(r) values obtained through high-resolution synchrotron X-ray diffraction combined with density functional theory (DFT) calculations. The application of DFT was crucial for accurately assigning X-ray peaks and verifying structural details. Furthermore, the analysis of X-ray diffraction peaks and the examination of crystalline domains via transmission electron microscopy enabled the proposal of a hexagonal arrangement of Si-O-Si cages over long ranges within the ionic bridged silsesquioxane. This proposed arrangement highlights a distinctive structural organization that could impact the material's properties and applications.

Al-Si Order and Chemical Composition Model across Scapolite Solid Solutions with Evidence from Rietveld Structure Refinements

Scapolite forms solid solutions between the end members marialite, Na4[Al3Si9O24]Cl = Me0, and meionite, Ca4[Al6Si6O24]CO3 = Me100. Al-Si order and chemical composition models are proposed for the scapolite solid solutions. These models predict the chemical composition, Al-Si order, and average <T–O> distances between Me0–Me100. These models are based on the observed order of clusters and on two solid solutions that meet at Me75 coupled with predicted chemical compositions and <T–O> distances. The [Na4·Cl]3+ and [NaCa3·CO3]5+ clusters are ordered between Me0–Me75, whereas the clusters [NaCa3·CO3]5+ and [Ca4·CO3]6+ are disordered from Me75–Me100. To confirm the structural model, the crystal structure of 27 scapolite samples between Me6–Me93 has been obtained using synchrotron high-resolution powder X-ray diffraction (HRPXRD) data and Rietveld structure refinements. The structure was refined in space group P42/n for all the samples. The <T–O> distances indicate that the T1 (=Si), T2 (=Al), and T3 (=Si) sites are completely ordered at Me37.5, where the 1:1 ratio of [Na4·Cl]3+:[NaCa3·CO3]5+ clusters are ordered and gives rise to antiphase domain boundaries (APBs) based on Cl-CO3 order instead of Al-Si order. The presence of APBs based on Cl-CO3 order and cluster order indicate that neither space group P42/n nor I4/m are correct for the structure of scapolite, but the lower symmetry space group P42/n is a good approximation for modeling the average structure of scapolite. The complete Al-Si order at Me37.5 changes in a regular and predictable manner toward the end members: Me0, Me75, and Me100. The observed unit cell and several structural parameters show a discontinuity at Me75, where the series is divided into two. There is no structural evidence to support any phase transition in the scapolite series. The T1 site contains only Si from Me0–Me37.5; from Me37.5–Me100, Al atoms enter the T1 site and the <T1–O> distance increases linearly to Me100.

(Invited) Influence of the Semiconductor Thin Film Morphologies on Dntt Organic Electronic Devices Performances

Interest in organic semiconductors has grown enormously in the last few years, indeed their use in microelectronics is continuously rising up in a large range of fields, spanning from academic to industrial new applications [1]. They provide a large number of interesting properties that have pushed a gradual replacement of classical amorphous silicon-based devices with organic ones [2], as they promise to be the best candidates as drivers of the necessary green and digital transition and become the building blocks in a huge number of future applications.In most devices, particularly in OTFTs, the quality and properties of the interface between semiconductor and dielectric are of fundamental importance for the device electrical characteristics [3-4]. Dinaphtho [2,3-b:2′,3′-f] thieno [3,2-b] thiophene (DNTT)-based OTFTs are among the best-known devices as they provide good performances in terms of charge mobility, good stability as well as easy processing and they are particularly suitable to study deeper the role of insulator/semiconductor interface [5-6]. It has been widely reported that the best performances of the small molecule organic based transistors, in terms of charge mobility as well as stability, are obtained when the molecules align with their long axis almost perpendicular to the growth surface. The DNTT molecules deposited on SiO2, align to form a homogeneous film, which results to be highly crystalline and provides good transport properties. However, in DNTT-based OTFTs different insulator surfaces, above which the active layer is grown, can provide different properties in terms of morphology and surface energy so that the electrical performances of the resulted devices can span in a wide range [7].In this work, we have studied the growth of DNTT semiconductor thin films on two different dielectric materials relating the ordered DNTT thin film structure to the OTFTs' electrical characteristics. OTFTs have been fabricated with Bottom Gate Top Contact (BGTC) configuration using thermal SiO2 or a double layer of SiO2/CytopTM as gate dielectrics. Semiconductor thin films, with thicknesses ranging from 3 nm to 50 nm, were detailed analyzed with high resolution AFM and XRD measurements (Fig. 1a, b, c, d) for different growth conditions to understand the morphologies of the films starting by the very first ordered layers of DNTT semiconductors. Our measurements show that DNTT molecules on SiO2 and Cytop pack into ordered structures placing themselves with their long axis perpendicular to the plane of the substrate (edge-on). However, in the case of DNTT on SiO2, a portion of ordered molecules can lie horizontally on the substrate (face-on). This causes an increase of defective states for the majority carriers in the conductive channel of the transistor, with a consequent reduction of the field-effect mobility (Fig. 1a, b). Numerical simulations were performed to analyze the effects of different semiconductor film morphologies on the electrical characteristics of the OTFTs. We also show that the morphological semiconductor differences lead to better performances for organic phototransistors based on DNTT/Cytop interface [8].[1] C. Wang et al., Chemical reviews 2012, 112 (4), 2208-2267.[2] Y. Bonnassieux et al., Flexible and Printed Electronics 2021, 6 (2), 023001.[3] H. I. Un et al., Advanced Science 2019, 6 (20), 1900375.[4] H. Sirringhaus et al., Advanced materials 2014, 26 (9), 1319-1335.[5] H. Chang et al., Org. Electron., 2015, 22, 86-91.[6] Q. Wang et al., ACS applied materials & interfaces 2018, 10 (26), 22513-22519.[7] M. Geiger et al., Advanced Materials Interfaces 2020, 7 (10), 1902145.[8] S. Calvi, et al., Org. Electron., 2022, 102, 106452. Figure 1

(Invited) Development of Single Crystal Nbn Superconductor on Wide Bandgap GaN By Sputtering and Thermal Annealing

The field of quantum computing is rapidly evolving, and Niobium Nitride (NbN) superconductors have emerged as a pivotal component in the field of quantum computing due to their unique properties. NbN possesses relatively higher superconducting transition temperature (Tc = 15-17 K) as compared to traditional superconductors, making it a promising material for various applications, including superconductor quantum computers, single photon detectors, and hot electron bolometers [1]. The investigation of NbN's superconductivity has been a subject of extensive research over the past few decades. Given that the properties of NbN are intricately linked to its crystal structure, lattice constant, and nitrogen content, precise control of the structural and electrical characteristics of NbN films is essential for the realization of NbN based quantum devices. The (111) planes of δ-NbN exhibit relatively small lattice mismatches with group-III nitride semiconductors (~0.2% for AlN and ~2.7% for GaN) [2], suggesting the potential for integrating crystalline NbN superconductors with nitride semiconductors on a single wafer. Notably, N-polar AlGaN/GaN high electron mobility transistors have been successfully fabricated on NbN films, operating under Tc with a negative differential resistance [3], indicating that the integration of nitride semiconductors and superconductors can be harnessed to create Josephson junctions. To achieve high-quality all-nitride NbN/group-III nitride/NbN Josephson junctions, a comprehensive understanding of the NbN/GaN heterointerfaces is crucial. Despite limited reports on the epitaxial growth of NbN films on nitride semiconductors, little is known about the structural and transport properties of such NbN films. Previously, MBE growth of NbN films have been demonstrated on GaN substrate, showing superconducting critical temperatures exceeding 10 K [4]. In this presentation, our efforts on the development of single crystal NbN thin films on wide bandgap GaN substrate using cost efficient sputtering technique followed by a subsequent high-temperature annealing will be presented. Commercially available single-crystalline GaN (0001)/sapphire templates as well as differently oriented (c and a-planes) sapphire substrates are used as the substrates for the sputtered deposition of NbN films at 20 °C. After the deposition, a high temperature annealing was performed separately at 950 °C for 30 mins under Ar as well as N2 atmosphere. While the as-deposited NbN film on GaN did not exhibit a discernible peak in the high-resolution XRD measurement, a high-intensity, single crystal (111)-oriented NbN film emerged prominently after annealing in both N2 and Ar environments. This suggests a notable enhancement in the crystalline quality of the superconducting material attributable to the thermal annealing process. Although the crystallinity of the NbN layer improves post-annealing, there is an associated increase in grain size and surface roughness. Preliminary findings indicate a reduction in the critical temperature of the NbN film from 12.82 K to 8.69 K after high-temperature annealing, potentially linked to the incorporation of impurities and crystallographic defects during this process. Further investigations using atomic resolution transmission electron microscopy, secondary ion mass spectrometry and X-ray photoelectron spectroscopy will be performed to investigate the influence of annealing duration, temperature, and conditions on the critical temperature, impurity incorporation and defect formation in NbN films deposited with different thickness.In continuation of our recent exploration into depositing crystalline superconducting NbN films on III-nitride semiconductors, this presentation will also focus on our recent advancement of ultrawide bandgap semiconductor materials such as the growth of high-quality quality β-Ga2O3 films and its ternary (AlxGa1-x)2O3 alloys, using metalorganic (MOCVD) and low-pressure (LPCVD) chemical vapor deposition growth techniques. Ga2O3 and (AlxGa1-x)2O3 have emerged as a promising semiconductor material platform for applications in high-power electronics and ultraviolet optoelectronics due to their excellent chemical and thermal stability, as well as high breakdown field strengths. The structural and electrical characteristics of Si-doped β-Ga2O3 and (AlxGa1-x)2O3 thin films will be presented. Additionally, we will present recent findings on achieving phase-pure β-(AlxGa1-x)2O3 films with a record-high Al composition (< 99%) [5]. Our research efforts into understanding the orientation-dependent Al incorporation in β-(AlxGa1-x)2O3 thin films and the determination of band offsets at β-(AlxGa1-x)2O3/β-Ga2O3 heterostructures will be presented. Furthermore, the presentation will touch upon the heteroepitaxial development of other phases of Ga2O3 and (AlxGa1-x)2O3, including α, γ, and κ. Acknowledgment: Dr. Bhuiyan acknowledges the support and guidance of his PhD advisor- Dr. Hongping Zhao at The Ohio State University for the MOCVD growth and characterization of β-Ga2O3 and (AlxGa1-x)2O3 films conducted in Zhao's group. Reference: Yamashita et al., Phys. Rev. Appl. 8, 054028 (2017).Kobayashi et al., Appl. Phys. Express 13, 061006 (2020).Yan et al., Nature 555, 183 (2018).G. Wright et al., APL Mater. 10, 051103 (2022).Bhuiyan et. al, Phys. Status Solidi RRL 17, 2300224 (2023). Figure 1

The Halogen Bond to Ethers - Prototypic Molecules and Experimental Electron Density.

Halogen bonds to dialkyl ether molecules have remained largely unexplored. We here address the synthesis and the structural chemistry of the first halogen-bonded noncyclic alkyl ethers, combining 1,4-diiodotetrafluorobenzene and the prototypic or commonly used ethers dimethyl ether, tetrahydrofuran, and methyl-tert-butyl ether as halogen acceptors. Two different structural motifs based on moderately strong halogen bonds were obtained: Discrete trimolecular aggregates are formed, and unexpected halogen-bonded supramolecular chain adducts feature oxygen-bifurcated halogen bonds with 1:1 donor:acceptor ratio. Both structure types may be selectively obtained even for the same ether by adjusting the stoichiometry in the crystallization experiments. The geometric features of the etheric oxygen center were found to be flexible, in contrast to the almost linear geometry about the halogen donor atom. A high-resolution X-ray diffraction experiment on the extended adduct of dimethyl ether allowed us to study the electronic details of the acceptor-bifurcated I···O···I halogen bonds. The electron density in the bond critical points and derived properties such as the Laplacian indicate essentially electrostatic interactions and explain the geometrical flexibility of ethers in halogen bonds. Our studies demonstrate the great versatility of ethers as halogen bond acceptors, that can occur in many geometrical arrangements and whose contribution to nature's structural designs should not be underestimated.

Molybdenum and Vanadium-Codoped Cobalt Carbonate Nanosheets Deposited on Nickel Foam as a High-Efficient Bifunctional Catalyst for Overall Alkaline Water Splitting.

To address issues of global energy sustainability, it is essential to develop highly efficient bifunctional transition metal-based electrocatalysts to accelerate the kinetics of both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). Herein, the heterogeneous molybdenum and vanadium codoped cobalt carbonate nanosheets loaded on nickel foam (VMoCoCOx@NF) are fabricated by facile hydrothermal deposition. Firstly, the mole ratio of V/Mo/Co in the composite is optimized by response surface methodology (RSM). When the optimized composite serves as a bifunctional catalyst, the water-splitting current density achieves 10 mA cm-2 and 100 mA cm-2 at cell voltages of 1.54 V and 1.61 V in a 1.0 M KOH electrolyte with robust stability. Furthermore, characterization is carried out using field emission scanning electron microscopy-energy dispersive spectroscopy (FESEM-EDS), high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Density functional theory (DFT) calculations reveal that the fabricated VMoCoCOx@NF catalyst synergistically decreases the Gibbs free energy of hydrogen and oxygen-containing intermediates, thus accelerating OER/HER catalytic kinetics. Benefiting from the concerted advantages of porous NF substrates and clustered VMoCoCOx nanosheets, the fabricated catalyst exhibits superior electrocatalytic performance. This work presents a novel approach to developing transition metal catalysts for overall water splitting.

Pinpointing lattice-matched conditions for wurtzite ScxAl1−xN/GaN heterostructures with x-ray reciprocal space analysis

Using comprehensive x-ray reciprocal space mapping, we establish the precise lattice-matching composition for wurtzite ScxAl1−xN layers on (0001) GaN to be x = 0.14 ± 0.01. 100 nm thick ScxAl1−xN films (x = 0.09–0.19) were grown in small composition increments on c-plane GaN templates by plasma-assisted molecular beam epitaxy. The alloy composition was estimated from the fit of the (0002) x-ray peak positions, assuming the c-lattice parameter of ScAlN films coherently strained on GaN increases linearly with Sc-content determined independently by Rutherford backscattering spectrometry [Dzuba et al., J. Appl. Phys. 132, 175701 (2022)]. Reciprocal space maps obtained from high-resolution x-ray diffraction measurements of the (101¯5) reflection reveal that ScxAl1−xN films with x = 0.14 ± 0.01 are coherently strained with the GaN substrate, while the other compositions show evidence of relaxation. The in-plane lattice-matching with GaN is further confirmed for a 300 nm thick Sc0.14Al0.86N layer. The full-width-at-half-maximum of the (0002) reflection rocking curve for this Sc0.14Al0.86N film is 106 arc sec and corresponds to the lowest value reported in the literature for wurtzite ScAlN films.

Influence of low-temperature cap layer thickness on the structure and luminescence of InGaN/GaN multiple quantum wells

In order to effectively regulate the luminescence performance of MQW and enhance its optical quality, it is crucial to investigate the InGaN/GaN MQW's structure and luminescence properties. In this study the focus is how they are affected by low-temperature cap (LT-cap) layer's thickness which is grown above each InGaN well layer during growth process of InGaN/GaN multiple quantum well (MQW) samples in MOCVD system. This was achieved by analyzing high resolution X-ray diffraction (HRXRD) spectra, electroluminescence (EL) spectra, temperature-dependent photoluminescence (TDPL) spectra, and micro-area fluorescence imaging of these samples. The results show that changes in LT-cap layer's thickness even have no significant impact on some structural parameters of MQW, such as the thickness of the well layer, but have an influence on the In component of the well layer. Due to the existence of LT-cap layer, dissociation of InGaN can be effectually reduced. In addition, the augmentation of LT-cap layer's thickness will make polarization effect of the QW sample more remarkable, so that the blue shift of the EL peak with the augmentation of current injection increases. The change of LT-cap layer's thickness will also influence distribution of the tail states of the quantum wells, which leads to a different localization states for injected carriers. As LT-cap layer becomes getting thicker, the material's internal quantum efficiency (IQE) tends to decrease, which may result from an increase in non-radiative recombination centers.