Crystal Layer Research Articles

Systematic improvement of excitonic features in hydrothermally synthesized WS2 due to post-synthesis drying

WS2 nanostructures were produced by mechanically exfoliating bulk WS2 powders synthesized through a solution-based hydrothermal route using WCl6 and thioacetamide (TAA) as reactants. An unoptimized post-synthesis recipe, which constitutes the last step, is the major hindrance in achieving a reproducible and scalable production strategy of layered bulk WS2 crystals. We have reported a comprehensive study on the crystal, morphological, and luminescence properties of WS2 material which have undergone post-synthesis treatment i.e. dried at mild temperature with varied dwelling times. The as-synthesized WS2 material which used to be in distorted 1T metallic phase, due to ionic intercalation released from the organic reactants, was found to evolve crystallographically into a stable 2H semiconducting phase with increased drying temperature and dwelling time. An in-depth optical analysis of the WS2 nanostructures through PL and PLE spectroscopy revealed substantial improvement in the intensity of A and B excitonic features for the sample dried at 80 °C for 24 h, indicating a reduction in the non-radiative defect centers in the sample under the post-synthesis conditions. We believe an effective post-synthesis treatment is needed for semiconducting 2H-WS2 synthesis through the solution route to improve its crystal (i.e. structural) and optical properties for potential electronic applications.

A Predictive Model for Monolayer‐Selective Metal‐Mediated MoS2 Exfoliation Incorporating Electrostatics

AbstractThe metal‐mediated exfoliation (MME) method enables monolayer‐selective exfoliation of van der Waals (vdW) crystals, improving the efficacy of large monolayer production. Previous physical models explaining monolayer‐selective MME propose that the main contributors to monolayer‐selectivity are vdW crystal/metal surface binding energy and/or vdW crystal layer strain resulting from lattice mismatch. However, the performance of some metals for MME is inconsistent with these models. Here, a new model is proposed using MoS2 as a representative vdW crystal. The model explains how the MoS2/metal interface electrostatics, in combination with strain, determines monolayer‐selectivity of MME by modulating the MoS2 interlayer energy. Monolayer MoS2/metal interfaces are characterized using in situ Raman spectroscopy and density functional theory calculations to estimate the electrostatics and strain of MoS2 in contact with different metals. The model successfully demonstrates the dependence of MME monolayer‐selectivity on the MoS2/metal interface electrostatics and highlights the significance of electrostatics in nanomaterial vdW interactions.

Plasmonic Modification of Epitaxial Nanostructures for the Development of a Highly Efficient SERS Platform

Epitaxy is the process of crystallization of monocrystalline layers and nanostructures on a crystalline substrate. It allows for the crystallization of various semiconductor layers on a finite quantity of semiconductor substrates, like GaAs, InP, GaP, InGaP, GaP, and many others. The growth of epitaxial heterostructures is very complicated and requires special conditions and the precise control of the growth temperature, the pressure in the reactor, and the flow of the precursors. It is used to grow epitaxial structures in lasers, diodes, detectors, photovoltaic structures, and so on. Semiconductors themselves are not suitable materials for application in surface-enhanced Raman spectroscopy (SERS) due to poor plasmonic properties in the UV/VIS range caused by missing free electrons in the conduction band due to the existing band gap. A plasmonic material is added on top of the nanostructured pattern, allowing for the formation of mixed photon–plasmon modes called localized surface plasmon-polaritons which stand behind the SERS effect. Typically, gold and silver are used as functional plasmonic layers. Such materials could be deposited via chemical or physical process. Attention has also been devoted to other plasmonic materials, like ones based on the nitrides of metals. The SERS performance of a functional surface depends both on the response of the plasmonic material and the morphology of the underlying semiconductor epitaxial layer. In the context of SERS, epitaxial growth allows for the fabrication of substrates with well-defined 3D nanostructures and enhanced electromagnetic properties. In this work, we described the possible potential plasmonic modification, composed of various coatings such as noble metals, TiN, and others, of well-developed epitaxial nanostructures for the construction of a new type of highly active SERS platforms. This abstract also highlights the role of epitaxial growth in advancing SERS, focusing on its principles, methods, and impact. Furthermore, this work outlines the potential of epitaxial growth to push the boundaries of SERS. The ability to design substrates with tailored plasmonic properties opens avenues for ultralow concentration detection.

Superconducting joints for the 1.3 GHz persistent NMR magnet under JST-Mirai Program

Superconducting joint technologies connecting high-T c superconducting (HTS) tapes, such as REBCO-coated conductors and Bi2223 silver-sheathed tapes, have been developed for the fabrication of superconducting magnets generating 30.5 T with persistent current operation at 4.2 K under the JST-Mirai Program since 2017. These are indirect joints using a superconducting intermediate layer to join highly crystallized HTS layers of tapes. Thus far, high I c values of above 100 A at 77 K in the self-field have been achieved by both joints. These performances are sufficient for installation to the 30.5 T magnet, while efforts to further improve the reproducibility of high I c for joints connecting long-length HTS tapes have been continuously made. The current status of HTS joint technologies is summarized in this paper partly from the view point of material science.

Modeling and optimization of scintillating crystal layers for designing a three-layer phoswich Alpha-Beta-Gamma detector

Monitoring the level of radioactive contamination and the type of radiation in the environment is very important for the protection of facilities, employees, and people. Phoswich detector is one of the useful tools to identify different radiations and separate them from each other using different scintillating crystals. However, their design can be very diverse based on the type of particles analyzed and various parameters of scintillating crystals and needs optimization to have maximum efficiency. This work aims to optimize the scintillating crystal layers for the design of the three-layer alpha-beta-gamma phoswich detector based on Monte Carlo radiation transport calculations. The selection of the appropriate model for the type and thickness of each layer is based on the optimization of parameters such as energy accumulation, produced scintillation optical photons, attenuation of optical photons, and scintillation decay time. The results showed that the three scintillating layers of ZnS(Ag), CaF2(Eu), and LYSO(Ce) with optimal thicknesses of 25.01 μm, 3.50 mm, and 2.54 cm, respectively, have more suitable optical outputs for detecting common alpha-beta-gamma radionuclide emitters.

Preparation of a novel polymer suspensions with high stabilization, and their superior performances in high salinity

Amphoteric polymerhave attracted immense research interest in hydraulic fracturing.However, current fracturing fluids prepared using conventional suspensions have poor stability and cannot be adapted to field applications in highly mineralized oilfield blocks, and they cannot reuse highly mineralized oilfield wastewater. To solve the above problems, a new concentrated suspension was prepared as a fracturing fluid thickener. The crystal layer spacing of SMB-2 exhibited an improved value of d = 13.564 nm. Experimental findings indicate that SRS-2 exhibited rapid thickening in saline water within a 5-minute timeframe. The viscosity of SRS-2 in high-salinity solutions demonstrated an impressive viscosity retention rate of 96.8 %, highlighting its excellent salt resistance properties. The zeta potential was measured at -62.13 mV. Notably, there was no significant settlement observed even after 30 days, with a settling velocity of 9.2592 × 10−3 mm/s, underscoring its superior stability performance. Therefore, this new suspension thickener has good potential for applications as an alternative material in multiple fields, such as oil extraction and sewage treatment.

Ab initio investigations of two-dimensional carrier gas at interfaces in GaN/AlN and GaN/AlN/Al2O3 heterostructures

The formation of a two-dimensional electron gas (2DEG) at the GaN (0001)/AlN interface holds significant implications for GaN-based high-voltage and high-frequency (RF) devices. Due to the promising results provided by the addition of a thin layer of AlN in metal–oxide-semiconductor channel high-electron-mobility transistor devices, this interface can be found in both the access region and near the dielectric gate. Recent ab initio simulations shed light on the crucial role played by spontaneous and piezoelectric polarizations within polar GaN and AlN crystals in driving the formation of the 2DEG. This study explores the underlying mechanisms behind the 2DEG formation and investigates the impact of fixed charges and additional layers, like Al2O3, on the carrier concentration. Consistent with the literature, our findings highlight the predominant role of polarizations within III–V materials in the formation of the 2DEG. Moreover, we examine the influence of fixed charges on the AlN surface, revealing their ability to accumulate or deplete the 2DEG, while maintaining charge conservation through the emergence of a new two-dimensional charge gas on the AlN surface. Additionally, we explore the effects of incorporating a β-Al2O3 crystal layer on the GaN/AlN structure, finding that the 2DEG’s carrier density is reduced, yet not entirely eliminated, while a significant positive charge concentration at the AlN/Al2O3 interface pins the Fermi level. This comprehensive investigation contributes to our understanding of microscopic phenomena in III–V heterostructures, paving the way for future advancements and applications in power electronics.

Modular vertically-integrated skin patch for biosignal measurements

This paper presents research on a novel and modular vertically-integrated wearable skin patch for biosignal measurements. The flexible patch consists of a disposable skin-contacting electrode layer and a durable electronics part. The durable part is designed to include a printed re-chargeable battery, a solar cell, electronics for the measurement of electrocardiogram (ECG), galvanic skin response, acceleration, temperature and humidity and a covering visually appealing jewellery-like functional crystal layer for decoration and user interaction. The patch can store measurement data and transmit it to a mobile phone via a bluetooth low-energy radio. Integration process is developed for the vertical stacking that limits the skin-patch area to achieve a desirable form factor. The sensing electrodes are printed on stretchable thermoformable polyurethane substrate with vias through the substrate to couple skin-contacting electrodes to the upper functional layers. A removable adhesive layer between the disposable and durable parts is developed to enable separating the two parts after wear and then to couple the durable part with a new disposable part. The patch is tested on body for ECG sensing.

Separation performance evaluation of green crystallization technique: Model configuration and process design

Layer melt crystallization (LMC) is deemed a green separation technique and is widely used in the purification of crude chemical products. This work optimizes the separation of LMC based on process design and model configurations. Firstly, we monitor the central temperature with the crystal layer growth, where the temperature change can be divided into two stages. The main heat exchange and crystal growth occur in stage 1. Then we establish two kinetics models based on the temperature and concentration difference, to describe the features of crystal layer growth. Both kinetic models can successfully correlate the driving force and kinetic parameters. Further, the temperature effect and composition of the feed mixture on the separation performance are analyzed. The proposed separation coefficients accurately figure out the underlying correlations of the technical variables and target products. The optimized operating trajectory based on the model analyses and experiments is given to guide the separation process. Finally, four process intensification strategies are proposed to improve separation efficiency, where the incorporation of the stirring operation into LMC is more suitable for industrial production.

Revealing enhanced X-Ray radiation shielding of 2D layered materials and their laminar heterostructures

This paper demonstrates a new concept of using two-dimensional (2D) layered materials and their heterostructures for enhanced X-ray radiation shielding. This phenomenon is revealed by characterization on the X-ray shielding performances of several 2D materials with high atomic numbers (Z), including MoS2, antimonene (Sb), and MXene prepared as multi-layered and heterostructure films by assembly their few-layer sheets. Results showed considerable X-ray shielding enhancement of (40–50%) at 30 kVp for individual 2D multi-layered films compared with their bulk structures of these materials. Furthermore, when these multi-layered films were combined into laminar heterostructures structures (e.g., MoS2 + MXene), further enhancement of 60% was achieved. The mechanism of the observed X-ray shielding enhancement by these multi-layered 2D structures is not clear at this stage. It is postulated to be the result of an additional multiple scattering and reflections of photons between multiple layers of 2D crystals inside the film, which does not occur in their uniform bulk materials. The presented results suggest that multi-layered 2D materials with high atomic numbers (Z) and their laminar heterostructures could offer a new and promising strategy for designing of a new generation of Pb-free radiation-shielding materials, which is urgently needed across broad sectors.Graphical abstract

Molecular dynamics analysis on the indentation hardness of nano-twinned nickel

Molecular dynamics simulations were employed to perform the nanoindentation analysis of nanocrystalline nickel with a variety of crystal structures (single-crystal, polycrystalline, and nano-twinned) and twin layer thicknesses under an indenter of spherical shape. To understand the effects of crystal structure and twin spacing on nanoindentation responses, the indentation force, strain, atomic rearrangement, dislocation, temperature, hardness, Young's modulus and elastic recovery rate of the indentation region were analyzed. It was noted that the three types of nanocrystalline nickel's mechanical properties and elastic recovery rate were greatly affected by their crystal structures. The results show that the hardness of the nano-twinned nickel is negatively correlated with the thickness of the twin layer, and the maximum hardness is 18.1 GPa when the thickness of twin layer is 1.2 nm. For the three structures, in the width direction, the elastic recovery ratio of the nano-twin structure is the largest, and the elastic recovery ratio of the nanotwin structure in the depth direction is the smallest. In the single crystal structure, the compression area is concentrated under the indenter and distributed in a ' V ' shape, and in the polycrystalline and twin structures, the compression area is scattered. In the polycrystalline structure, the compression area were distributed below the indenter and at crystal boundaries. In the nano-twinned structure, the compression area were mainly distributed below the indenter.

Laser-Induced Periodic Surface Structures on Layered GaSe Crystals: Structural Coloring and Infrared Antireflection.

We study structural and morphological transformations caused by multipulse femtosecond-laser exposure of Bridgman-grown ϵ-phase GaSe crystals, a van der Waals semiconductor promising for nonlinear optics and optoelectronics. We unveil, for the first time, the laser-driven self-organization regimes in GaSe allowing the formation of regular laser-induced periodic surface structures (LIPSSs) that originate from interference of the incident radiation and interface surface plasmon waves. LIPSSs formation causes transformation of the near-surface layer to amorphous Ga2Se3 at negligible oxidation levels, evidenced from comprehensive structural characterization. LIPSSs imprinted on both output crystal facets provide a 1.2-fold increase of the near-IR transmittance, while the ability to control local periodicity by processing parameters enables multilevel structural color marking of the crystal surface. Our studies highlight direct fs-laser patterning as a multipurpose application-ready technology for precise nanostructuring of promising van der Waals semiconductors, whose layered structure restricts application of common nanofabrication approaches.

Polycarboxy/Sulfo Betaine-Calcium Phosphate Hybrid Materials with a Remineralization Potential.

Biomacromolecules control mineral formation during the biomineralization process, but the effects of the organic components' functionality on the type of mineral phase is still unclear. The biomimetic precipitation of calcium phosphates in a physiological medium containing either polycarboxybetaine (PCB) or polysulfobetaine (PSB) was investigated in this study. Amorphous calcium phosphate (ACP) or a mixture of octacalcium phosphate (OCP) and dicalcium phosphate dihydrate (DCPD) in different ratios were identified depending on the sequence of initial solution mixing and on the type of the negative functional group of the polymer used. The more acidic character of the sulfo group in PSB than the carboxy one in PCB determines the dominance of the acidic solid phases, namely, an acidic amorphous phase or DCPD. In the presence of PCB, the formation of ACP with acicular particles arranged in bundles with the same orientation was observed. A preliminary study on the remineralization potential of the hybrid material with the participation of PSB and a mixture of OCP and DCPD did not show an increase in enamel density, contrary to the materials based on PCB and ACP. Moreover, the latter showed the creation of a newly formed crystal layer similar to that of the underlying enamel. This defines PCB/ACP as a promising material for enamel remineralization.

Anisotropic Transport Behavior and X-ray Detection Based on Layered Organic Single Crystal

Anisotropic Transport Behavior and X-ray Detection Based on Layered Organic Single Crystal

Rethinking SnSe Thermoelectrics from Computational Materials Science.

ConspectusThe growing energy crisis and the adverse environmental impacts caused by carbon-based energy consumption have spurred the exploration of green and sustainable energy. Researchers have been devoted to developing thermoelectric technology that could directly and reversibly convert heat into electricity. By virtue of zero emissions, nonmoving parts, precise temperature control, and long service life, thermoelectrics exhibit broad application in power generation and refrigeration. Nevertheless, traditional narrow-bandgap thermoelectrics exhibit high performance within a narrow temperature range, limiting the overall energy conversion. Consequently, a selection rule for exploring advanced thermoelectrics was proposed: materials with wide-bandgap, crystals form, asymmetry, and anisotropic structure. According to the rules, we conducted much research and found some promising materials.As the lead-free, cost-effective, and stable thermoelectric candidates, layered SnSe crystals with wide-bandgap and covalent bonding have gained significant attention due to their ultralow thermal conductivity resulting from strong bonding anharmonicity, via strong polar covalent bonding, because of the electronegativity difference between the Sn and Se atoms. This was proved to be the result from the unique structure of layered SnSe crystals, a distorted rock-salt structure with high and anisotropic Grüneisen parameters. In this Account, we introduce and rethink our recent advancements in developing high-performance thermoelectric SnSe crystals from computational materials science, involving p- and n-type SnSe crystals, respectively. For p-type SnSe crystals, according to the complex valence band structures, we utilized the multiband synglisis via electronic structure calculations and multiband simulations to activate valence bands to participate in electrical transport of in-plane direction, achieving an ultrahigh power factor (PF) of ∼75 μW cm-1 K-2 at room temperature and an average figure-of-merit ZTave of ∼1.9 for Sn0.91Pb0.09Se. Besides, on the basis of defect chemistry, the characteristics of p-type SnSe crystals are determined by intrinsic Sn vacancies. We thus used point-defect calculations to achieve the lattice plainification, and we fixed the lattice intrinsic defects to weaken defect scattering of carriers along the in-plane direction, facilitating further a PF > 100 μW cm-1 K-2 and a ZT of ∼1.5 at room temperature for SnCu0.001Se. For n-type SnSe crystals, inspired by the anisotropic characteristics of the layered materials, we analyzed charge density and proposed the insight of 3D charge and 2D phonon transports and calculated the deformation potential to manipulate layered phonon-electron decoupling to achieve high performance, ultimately Pb-alloyed and Cl-doped SnSe (SnSe-Cl-PbSe) reaching a ZTave of ∼1.7 from 300 to 773 K. In the end, we offer potential perspectives on high-throughput calculations (HTC) and machine learning (ML), combined with our proposed insights, which could be a promising avenue for future thermoelectrics. By virtue of our theoretical and experimental understanding of thermoelectrics, integrating these insights as rules and descriptors for HTC and ML will accelerate the research and development of thermoelectrics. We want to share our recent works and latest perspectives in SnSe thermoelectrics, and we expect to inspire enthusiasm for innovative research on advanced thermoelectric materials and devices.

Metal localisation in gastropod shells: New insights from mass spectrometry techniques

Gastropod shells are calcified structures made of several crystal layers. They grow throughout the lifecycle of mollusks by integrating some of the chemical elements present in their environment, including metals. This characteristic means mollusks can be useful bioindicators of metal exposure. The present study aimed to better understand the role of layer composition on metal accumulation. To that end, the gastropods Radix balthica were collected in a French river adjacent to a municipal wastewater treatment plant. Microchemical metal analyses in the different shell layers were performed by Femtosecond-Laser Ablation Inductively Coupled Plasma Mass Spectrometry (Fs-LA-ICP-MS) and analyses of the molecular environment of the metals were performed by Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS). Strontium, Ba and Mn were well distributed within the whole shell and the high concentrations of these elements were found to be related to the aragonite structure of the shell. Copper, Ni, Pb and Zn were mostly present at the outer surfaces of the shell where the organic constituents were more concentrated. The analysis of metal distribution in shell layers could improve our understanding of the relationships between metal exposure and accumulation in mollusks, therefore providing evidences of their use as powerful integrated bioindicator of metal contamination.

Novel Piezo-active 2–1–2 Composites with Sets of Large Hydrostatic Parameters

A 2–1–2 composite with two single-crystal components is studied for the first time to highlight large hydrostatic piezoelectric coefficients dh * and gh *, figure of merit dh *gh *, and electromechanical coupling factor kh *. Each 2–1–2 composite contains layers of domain-engineered [011]-poled relaxor-ferroelectric (1–x)Pb(Zn1/3Nb2/3)O3 – xPbTiO3 single crystal (x = 0.0475–0.09) and heterogeneous layers being systems of aligned piezoelectric Li2B4O7 single crystal rods in polyethylene, and these rods have an elliptic cylindrical shape. New diagrams show the volume-fraction ranges wherein large values of dh *> 10−9 C/N, dh *gh *> 2.10−10 Pa−1 and kh *≈ 0.5–0.6 are observed.

Microstructural characterization of AlxGa1−xN/GaN high electron mobility transistor layers on 200 mm Si(111) substrates

Realization of high crystal quality GaN layers on silicon substrates requires a great control over epitaxy as well as detailed characterization of the buried defects and propagating dislocations within the epilayers. In this Letter, we present the microstructural characterization of AlxGa1−xN/GaN high electron mobility transistor (HEMT) structures epitaxially grown on 200 mm Si (111) substrates with superlattice (SL) buffers. The HEMT stack is also composed of an intermediate thick AlxGa1−xN layer sandwiched between the short-period and long-period AlxGa1−xN/AlN SLs. Structural and morphological characteristics of the GaN-based epilayers are studied to assess the quality of the HEMT stack grown on such 200 mm diameter substrates. The detailed microstructural uniformity of the epilayers is addressed by the transmission electron microscopy (TEM) technique. In depth scanning TEM with electron energy loss spectroscopy (EELS) investigations have been carried out to probe the microstructural quality of the HEMT stack comprising of such short- and long-period AlxGa1−xN/AlN SLs. The EELS-plasmon maps are utilized to address the sharp interface characteristics of the SL layers as well as the top active p-GaN/AlxGa1−xN/GaN layers. This work highlights the capability of high-resolution TEM as a complementing characterization method to produce reliable AlxGa1−xN/GaN HEMTs on such a large diameter silicon substrate.

Surface gradient structure induced by Cu ion implantation optimizes the degradation behavior of biodegradable Zn–Cu–Li alloy

Surface modification is a crucial method for regulating the degradation behavior of biomedical zinc-based alloys in consideration of clinical safety. In this work, Cu ion implantation was introduced for Zn–4Cu–0.02Li. The results indicated that a gradient modified layer formed, which was composed of three regions, namely, a 70-nm element implanted layer, a 600-nm nanocrystalline layer, and a 2-μm columnar crystal layer. The modified layer reduced corrosion within one month and transformed the localized corrosion into the uniform corrosion mode. Its hierarchical degradation process induced the controlled degradation. Additionally, Cu ion implantation enhanced the inhibitory effect on S. aureus.

High-Efficiency Circularly Polarized Phased Array Based on 5 μm-Thick Nematic Liquid Crystals: Design, Over-The-Air Calibration, and Experimental Validation

This paper presents a systematic investigation and demonstration of a K-band circularly polarized liquid-crystal-based phased array (LCPA), including the design, over-the-air (OTA) in-array calibration, and experimental validation. The LCPA contains 16 phase-shifting radiating channels, each consisting of a circularly polarized stacked patch antenna and a liquid-crystal-based phase shifter (LCPS) based on a loaded differential line structure. Thanks to its slow-wave properties, the LCPS exhibits a maximum phase-shifting range of more than 360° with a figure of merit of 78.3(° )·dB−1 based on a liquid crystal layer with a thickness of only 5 μm. Furthermore, an automatic OTA calibration based on a state ergodic method is proposed, which enables the extraction of the phase–voltage curve of every individual LCPA channel. The proposed LCPA is manufactured and characterized with a total profile of only 1.76 mm, experimentally demonstrating a scanned circularly polarized beam from −40° to +40° with a measured peak gain of 12.5 dBic and a scanning loss of less than 2.5 dB. The bandwidth of the LCPA, which satisfies the requirements of port reflection (|S11|) < −15 dB, an axial ratio (AR) < 3 dB, beam squinting < 3°, and a gain variation < 2.2 dB, spans from 25.5 to 26.0 GHz. The total efficiency is about 34%, which represents a new state of the art. The use of the demonstrated low-profile LCPA to support circularly polarized scanning beams, along with the systematic design and calibration methodology, holds potential promise for a variety of millimeter-wave applications.