Crystal Layer Research Articles

Electrically reconfigurable MoO3/InSe van der Waals heterojunctions

Abstract Van der Waals (vdW) heterojunctions formed by stacking different layers of two-dimensional atomic crystals with atomically sharp interface and versatile band alignment have been considered as promising candidates for construction of nanoelectronic and nanophotonic devices. Here, we demonstrate the electrically reconfigurable behavior in MoO3/InSe vdW heterojunction with a type-III broken-gap band alignment. The electrical reconfigurability is enabled by the ambipolar transport property of InSe, which is achieved through the use of Pt as contact electrodes. By electrostatically doping the indium selenide (InSe), the reconfigurable MoO3/InSe heterojunctions can be converted between p-n and n+-n junctions. As a current rectifier, the MoO3/InSe heterojunction shows rectification ratios of ~ 107 and ~104 for forward and backward bias conditions, respectively. As a photodetector, the MoO3/InSe heterojunction shows stable photo-switching behavior with a ~105 on/off ratio and an ultralow dark current (≈10 fA).The response time is measured to be 500 μs and 300 μs for rise and fall processes, respectively. These results highlight the role of MoO3 with high electron affinity in construction of vdW heterostructure with type-III band alignment and provide a new platform for high performance optoelectronic devices.

Carbon dots induced homeotropic alignment in a negative dielectric nematic liquid crystal material

Abstract Recently, doping guest materials such as quantum dots (QDs) into liquid crystals (LCs) has been of great interest since their addition substantially enhances the properties of LC and opens new avenues for scientific advancement. Here, we report the induction of homeotropic alignment in cells without alignment layers of the negative dielectric nematic liquid crystal, N-(4-Methoxybenzylidene)-4-butylaniline (MBBA) by doping with carbon dots (CDs ~2.8±0.72 nm). The CDs-MBBA composites (CDs concentration: 0.002, 0.01, 0.03, 0.1 and 0.3 wt%) were investigated using optical polarising microscopy, electro-optical and dielectric techniques. Polarizing optical micrographs and voltage dependent optical transmission revealed the induced homeotropic alignment for all the composites under investigation. Interestingly, the least concentrated sample, 0.002 wt% exhibited partial homeotropic alignment. However, due to light leakage, the optical transmission value below threshold voltage was relatively higher than the rest of the composites. MBBA is a negative dielectric material, hence the application of a voltage across the cell was able to switch the alignment from a dark to a bright state for all composites. However, above a certain voltage (> threshold voltage), the bright state produced some instabilities. The value of dielectric permittivity was observed to decrease with increasing concentration, confirming the effect of CDs in producing homeotropic alignment in MBBA. Measurements as a function of temperature were conducted to examine the thermal stability of the induced alignment. The alignment was found to be stable throughout the nematic phase of MBBA. The induction of such alignment without conventional alignment (i.e., rubbing of polyimides) technique can be helpful in addressing the evolving display demands by making liquid crystal displays (LCDs) and other display devices cost effective.

Exploiting the Bragg Mirror Effect of TiO2 Nanotube Photonic Crystals for Promoting Photoelectrochemical Water Splitting

Exploiting the Bragg mirror effect of photonic crystal photoelectrode is desperately desired for photoelectrochemical water splitting. Herein, a novel TiO2 nanotube photonic crystal bi-layer structure consisting of a top nanotube layer and a bottom nanotube photonic crystal layer is presented. In this architecture, the photonic bandgap of bottom TiO2 nanotube photonic crystals can be precisely adjusted by modulating the anodization parameters. When the photonic bandgap of bottom TiO2 nanotube photonic crystals overlaps with the electronic bandgap of TiO2, the bottom TiO2 nanotube photonic crystal layer will act as a Bragg mirror, leading to the boosted ultraviolet light absorption of the top TiO2 nanotube layer. Benefiting from the promoted UV light absorption, the TiO2 NT-115-NTPC yields a photocurrent density of 1.4 mA/cm2 at 0.22 V vs. Ag/AgCl with a Faradic efficiency of 100%, nearly two times higher than that of conventional TiO2 nanotube arrays. Furthermore, incident photon-to-current conversion efficiency is also promoted within ultraviolet light region. This research offers an effective strategy for improving the performance of photoelectrochemical water splitting through intensifying the light–matter interaction.

Improving the Active Layer Crystallization Process by Promoting Acceptor Aggregation for High-Performance Near-Infrared Organic Photodetectors

Improving the Active Layer Crystallization Process by Promoting Acceptor Aggregation for High-Performance Near-Infrared Organic Photodetectors

Electric Structure and Thermoelectric Performance in Cs2O Crystal

In this work, first principles DFT calculations has been employed with anharmonic phonon scatter theory and Boltzmann transport method to perform a exhaustive study on the thermoelectric properties as electronic and phonon transport of layered Cs2O crystal. The results indicate that Cs2O crystal represents a flat-and-dispersive type band structure, which returns a high power factor. In the other hand, low lattice thermal conductivity is discovered in Cs2O semiconductor, combined with its high power factor, the Cs2O crystal is considered a promising thermoelectric material. It is demonstrated that p-type Cs2O could be optimized to exhibit outstanding thermoelectric performance with a maximum ZT value of 0.71 at 500K. Explored by density functional theory calculations, the high ZT value is due to its high Seebeck coefficient S, high electrical conductivity , and low lattice thermal conductivity .

Liquid Crystal Enables Extraordinarily Precise Tunability for a High‐Q Ultra‐Narrowband Filter Based on a Quasi‐BIC Metasurface

AbstractNotch filters usually involve high costs, great difficulties in processing, and very limited tunability. By coating a nematic liquid crystal (LC) layer onto a well‐designed quasi‐bound states in the continuum (quasi‐BICs) metasurface, this work designs and demonstrates a high‐Q tunable filtering system with precise tunability in the near‐infrared spectral range. Optimal structural parameters and the filtering performance are first determined by numerical simulations and then confirmed in experiments. The precise tunability is enabled by modifying the LC molecules’ principal axes with an applied voltage, where the least distinguishable central wavelength interval is smaller than 0.3 nm, and the largest Q factor extracted from experiments can be ≈256.1. Compared to most of the commercial notch filters that work in the visible region and have no tunability, this work achieves an ultra‐narrowband filtering system that features low manufacture difficulty and cost, and great control of light propagation. The proposed design also opens new avenues for developing BIC‐based high‐Q devices with enhanced signal‐to‐noise ratio and multifunctional properties.

Local Orientation Transitions to a Lying Helix State in Negative Dielectric Anisotropy Cholesteric Liquid Crystal

Orientation transitions in a cholesteric liquid crystal (CLC) layer with negative dielectric anisotropy, under the influence of a non-uniform spatially periodic electric field created using a planar system of interdigitated electrodes, were studied experimentally and numerically. In the interelectrode space, transitions are observed from a planar Grandjean texture, with the helix axis perpendicular to the layer plane, to states with a lying helix, when the helix axis is parallel to the layer plane and perpendicular to the electrode stripes. It was found that the relaxation time of the induced state in the Grandjean zones, corresponding to two or more half-turns of the helix, significantly exceeded the relaxation time for the first Grandjean zone with one half-turn. An analysis of experimentally observed and numerically simulated textures shows that slow relaxation to the initial state in the second Grandjean zone, as well as in higher-order zones, is associated with the formation of local topologically equivalent states. In these states, the helix has a reduced integer number of helix half-turns throughout the layer thickness or unwound into the planar alignment state.

Inert Liquid Exfoliation and Langmuir-Type Thin Film Deposition of Semimetallic Metal Diborides.

Graphite is one of only a few layered materials that can be exfoliated into nanosheets with semimetallic properties, which limits the applications of nanosheet-based electrodes to material combinations compatible with the work function of graphene. It is therefore important to identify additional metallic or semimetallic two-dimensional (2D) nanomaterials that can be processed in solution for scalable fabrication of printed electronic devices. Metal diborides represent a family of layered non-van der Waals crystals with semimetallic properties for all nanosheet thicknesses. While previous reports show that the exfoliated nanomaterial is prone to oxidation, we demonstrate a readily accessible inert exfoliation process to produce quasi-2D nanoplatelets with intrinsic material properties. For this purpose, we demonstrate the exfoliation of three representative metal diborides (MgB2, CrB2, and ZrB2) under inert conditions. Nanomaterial is characterized using a combination of transmission electron microscopy, scanning electron microscopy, atomic force microscopy, IR, and UV-vis measurements, with only minimal oxidation indicated postprocessing. By depositing the pristine metal diboride nanoplatelets as thin films using a Langmuir-type deposition technique, the ohmic behavior of the networks is validated. Furthermore, the material decomposition is studied by using a combination of electrical and optical measurements after controlled exposure to ambient conditions. Finally, we report an efficient, low-cost approach for sample encapsulation to protect the nanomaterials from oxidation. This is used to demonstrate low-gauge factor strain sensors, confirming metal diboride nanosheets as a suitable alternative to graphene for electrode materials in printed electronics.

Color-Tunable Lead Halide Perovskite Single-Mode Chiral Microlasers with Exceptionally High glum.

Chiral microlasers hold great promise for optoelectronics from integrated photonic devices to high-density quantum information processing. Despite significant progress in lead-halide perovskite emitters, chiral lasing with high dissymmetry factors (glum) has not yet been realized. Here, we demonstrate chiral single-mode microlasers with exceptional stability and tunable emission across the visible range by combining CsPbClxBr3-x perovskite microrods (MRs) with a cholesteric liquid crystal (CLC) layer. The MRs lase via a whispering gallery mode (WGM) microcavity and confer chirality through the encapsulated CLC layer, thus exhibiting circularly polarized lasing with dissymmetry factors reaching 1.62. Importantly, we demonstrate wavelength-tunable high dissymmetry chiral lasers in a broad spectral range by tuning the halide composition and using CLC layers with the desired photonic bandgap (PBG). This facile approach to generate chiral lasing not only is applicable to semiconductor nano- and microcrystals but also paves the way for potential integration into nanoscale photonic devices.

Investigating Layered Topological Magnetic Materials as Efficient Electrocatalysts for the Hydrogen Evolution Reaction under High Current Densities

Despite considerable progress, high-performing durable catalysts operating under large current densities (i.e., >1000 mA/cm2) are still lacking. To discover platinum group metal-free (PGM-free) electrocatalysts for sustainable energy, our research involves investigating layered topological magnetic materials (semiconducting ferromagnets) as highly efficient electrocatalysts for the hydrogen evolution reaction under high current densities and establishes the novel relations between structure and electrochemical property mechanisms. The materials of interest include transition metal trihalides, i.e., CrCl3, VCl3, and VI3, wherein a structural unit, the layered structure, is formed by Cr (or V) atoms sandwiched between two halides (Cl or I), forming a tri-layer. A few layers of quantum crystals were exfoliated (~50−60 nm), encapsulated with graphene, and electrocatalytic HER tests were conducted in acid (0.5M H2SO4) and alkaline (1M KOH) electrolytes. We find a reasonable HER activity evolved requiring overpotentials in a range of 30–50 mV under 10 mA cm−2 and 400−510 mV (0.5M H2SO4) and 280−500 mV (1M KOH) under −1000 mA cm−2. Likewise, the Tafel slopes range from 27 to 36 mV dec−1 (Volmer–Tafel) and 110 to 190 mV dec−1 (Volmer–Herovsky), implying that these mechanisms work at low and high current densities, respectively. Weak interlayer coupling, spontaneous surface oxidation, the presence of a semi-oxide subsurface (e.g., O–CrCl3), intrinsic Cl (or I) vacancy defects giving rise to in-gap states, electron redistribution (orbital hybridization) affecting the covalency, and sufficiently conductive support interaction lowering the charge transfer resistance endow the optimized adsorption/desorption strength of H* on active sites and favorable electrocatalytic properties. Such behavior is expedited for bi-/tri-layers while exemplifying the critical role of quantum nature electrocatalysts with defect sites for industrial-relevant conditions.

Active–Inactive Molten Salt Synthesis of Li- and Mn-Rich Layered Oxide Single Crystals as Cathode Materials for All-Solid-State Batteries

Active–Inactive Molten Salt Synthesis of Li- and Mn-Rich Layered Oxide Single Crystals as Cathode Materials for All-Solid-State Batteries

Role of Ag Addition on the Microscopic Material Properties of (Ag,Cu)(In,Ga)Se2 Absorbers and Their Effects on Losses in the Open‐Circuit Voltage of Corresponding Devices

ABSTRACTAg alloying of Cu(In,Ga)Se2 (CIGSe) absorbers in thin‐film solar cells leads to improved crystallization of these absorber layers at lower substrate temperatures than for Ag‐free CIGSe thin films as well as to enhanced cation interdiffusion, resulting in reduced Ga/In gradients. However, the role of Ag in the microscopic structure–property relationships in the (Ag,Cu)(In,Ga)Se2 thin‐film solar cells as well as a correlation between the various microscopic properties of the polycrystalline ACIGSe absorber and open‐circuit voltage of the corresponding solar cell device has not been reported earlier. In the present work, we study the effect of Ag addition by analyzing the differences in the various bulk, grain‐boundary, optoelectronic, emission, and absorption‐edge properties of ACIGSe absorbers with that of a reference CIGSe absorber. By comparing thin‐film solar cells with similar band‐gap energies ranging from about 1.1 to about 1.2 eV, we were able to correlate the differences in their absorber material properties with the differences in the device performance of the corresponding solar cells. Various microscopic origins of open‐circuit voltage losses were identified, such as strong Ga/In gradients and local compositional variations within individual grains of ACIGSe layers, which are linked to absorption‐edge broadening, lateral fluctuations in luminescence‐energy distribution, and band tailing, thus contributing to radiative VOC losses. A correlation established between the effective electron lifetime, average grain size, and lifetime at the grain boundaries indicates that enhanced nonradiative recombination at grain boundaries is a major contributor to the overall VOC deficit in ACIGSe solar cells. Although the alloying with Ag has been effective in increasing the grain size and the effective electron lifetime, still, the Ga/In gradients and the grain‐boundary recombination in the ACIGSe absorbers must be reduced further to improve the solar‐cell performance.

Off-axis viewing angle control technology applied to displays

ABSTRACT With the development of display technology and the expansion of application fields, such as vehicle/airborne displays, medical imaging displays, industrial control displays, etc., spatial constraints of these displays often result in viewers observing them at specific off-axis viewing angles. Therefore, the frequently using demands of off-axis viewing angles lead to increasing requirements for the viewing angle performance of displays. A method to enhance the off-axis viewing angle effect is proposed in this paper by attaching a designed liquid crystal cell on the surface of the display to control the emission direction. When an appropriate driving voltage is applied to the liquid crystal cell, the liquid crystal molecules align in a specific periodic arrangement, creating an optical effect similar to micro/nano structured light control devices. Different driving voltages induce distinct modulations of light within the liquid crystal layer. When no driving voltage is applied, the frontal viewing angle effect of the display is optimal, suitable for vertical viewing by a single individual, as an on-axis viewing mode. When a driving voltage is applied, certain off-axis viewing angle properties such as brightness is increased and colour shift is reduced, making it suitable as an off-axis viewing mode. In practical use, individuals can freely switch between these two display modes based on their actual needs.

High-precision alignment techniques for realizing an ultracompact electromagnetic calorimeters using oriented high-z scintillator crystals

Electromagnetic calorimeters used in high-energy physics and astrophysics rely heavily on high-Z inorganic scintillators, such as lead tungstate (PbWO4 or PWO). The crystalline structure and lattice orientation of inorganic scintillators are frequently underestimated in detector design, even though it is known that the crystalline lattice strongly modifies the features of the electromagnetic processes inside the crystal. A novel method has been developed for precisely bonding PWO crystals with aligned atomic planes within 100 μrad, exploiting X-ray diffraction (XRD) to accurately measure miscut angles. This method demonstrates the possibility to align a layer of crystals along the same crystallographic direction, opening a new technological path towards the development of next-generation electromagnetic calorimeters.

Photonic defect modes in cholesteric liquid crystal resonators with embedded isotropic layers

Photonic defect modes are explored as a viable alternative to standard photonic band edge modes in photonic crystal applications, especially due to their typically high Q-factors and local density of states. For example, they can be used in nonlinearity enhancement, lasing, and cavity quantum electrodynamics. However, they are strongly dependent on any structural change and need to be well-controlled to ensure the desired resonance frequency. Here, we present a study of the photonic defect modes that appear in a structure where a layer of isotropic material is embedded between two layers of cholesteric liquid crystal (CLC), using full electrodynamics numerical simulations. We present typical transmission spectra and electric field profiles of selected defect modes and then analyze the influence of geometrical and material parameters on the eigenfrequencies and Q-factors of the modes within and around the photonic bandgap, including refractive indices and thicknesses of isotropic and liquid crystal layers, and different anchoring orientations at the boundaries of the isotropic defect layer. Additionally, a connection of such defect modes to previously extensively analyzed twist defect modes is given. Eigenmodes in asymmetric resonators are also presented, where CLC layers surrounding the intermediate isotropic layer are not equally thick, enabling biasing of specific directional light emission. More generally, this work aims to contribute to the understanding and design capability in topological soft matter photonics where defect mode lasing could be realized in CLC geometries with different singular and solitonic topological defect structures.

Preparation and Characterization of Multi-Network Crosslinked Gels for Preventing Spontaneous Combustion of Coal

ABSTRACT In order to improve the inhibition performance of a retardant in various stages of coal spontaneous combustion, this study selected guar gum (guar) mixed with sodium tetraborate to form a multi-web gel. Then, polyethylene glycol (PEG) was added for the insertion and modification to improve the physical inhibition effect. Furthermore, we compounded the composite guar/PEG with the chemical retardant 2-hydroxyphosphonoacetic acid (HPAA) to prepare a composite retardant with the effect of full-stage inhibition. Multi-web gel composite inhibitor with a full-stage inhibition effect was prepared. Response surface analysis was used to determine the optimal solution. Scanning electron microscopy tests revealed that the surface folds of PEG/guar increased and the water retention capacity was greatly enhanced. The plugging wind experiment results show that the limiting air pressure of the gel material is 249 Pa, which can match the downhole plugging wind work. Using XRD experiments, the diffraction angle of PEG/guar was narrowed from 18.20° to 17.64°, the crystal layer spacing was increased from 4.8685 nm to 5.0218 nm, and PEG was successfully intercalated into the guar. Using Fourier-transform infrared spectroscopy, it was found that the content of free hydroxyl groups and aliphatic groups became lower, and the content of carbonyl groups and ether bonds increased, which greatly enhanced the antioxidant capacity of coal. Using thermogravimetric analysis, the characteristic temperature points of single and composite inhibitors were examined macroscopically. The results showed that the inhibition treatment could obviously chelate the transition metal ions in the coal samples, which greatly inhibited the process of coal oxidation and achieved the purpose of inhibiting the spontaneous combustion of coal.

Layered heterogeneous structures integrated device for multiplication, division arithmetic unit and multiple-physical sensing

A layered heterogeneous structure (LHS), consisting of silver, liquid crystal, and nonlinear dielectric layers, is proposed to realize functions of computing and sensing. By leveraging the optical Tamm state, the intrinsic absorption principle of liquid crystal, and nonlinear effects, the design of an integrated device capable of passive multiplication and division operations, along with high-performance multi-physical quantity sensing functionalities, is achieved. The given LHS exhibits Janus properties, with different physical functions manifested depending on the direction of electromagnetic wave (EW) propagation. During forward propagation of EWs, the LHS displays high and sharp absorptivity peaks at 774.8 and 1517.6 nm. The relationship between the two peaks approximates a frequency multiplication factor of 1.960, enabling signal multiplication. Furthermore, the two absorptivity peaks at different wavelengths facilitate the sensing of serum creatinine solution concentration and external pressure, with sensitivity (S), quality factors (Q), and figure of merit (FOM) of 266.76 μmol L−1/nm and 213.33 GPa/nm, as well as 248.76 and 348.22, 84.1 L (μmol)−1 and 49.06 GPa−1, respectively. During backward propagation of EWs, absorptivity peaks with distinct resolutions are observed at 1423 and 2809 nm, with a multiple relationship between them of 1.974, enabling frequency doubling for signal division. Additionally, the absorptivity curve facilitates temperature sensing over a wide range from 257 to 347 K. Owing to the unique temperature S of liquid crystal, different sensitivities and resolutions are observed at 257 to 297 K and 307 to 347 K, with S of 1.015 and 0.686 K/nm, and corresponding Q and FOM of 21.57 and 12.576, 0.076 K−1 and 0.003 04 K−1, respectively.

Liquid-Phase Exfoliation of 3D Metal-Organic Frameworks into Nanosheets.

Traditionally, the acquisition of 2D materials involved the exfoliation of layered crystals. However, the anisotropic bonding arrangements within 3D crystals indicate they are mechanically reminiscent of 2D counterparts and could also be exfoliated into nanosheets. This report delineates the preparation of 2D nanosheets from six representative 3D metal-organic frameworks (MOFs) through liquid-phase exfoliation. Notably, the cleavage planes of exfoliated nanosheets align perpendicular to the direction of the minimum elastic modulus (Emin) within the pristine 3D frameworks. The findings suggest that the in-plane and out-of-plane bonding forces of the exfoliated nanosheets can be correlated with the maximum elastic modulus (Emax) and Emin of the 3D frameworks, respectively. Emax influences the ease of cleaving adjacent layers, while Emin governs the ability to resist cracking of layers. Hence, a combination of large Emax and small Emin indicates an efficient exfoliation process, and vice versa. The ratio of Emax/Emin, denoted as Amax/min, is adopted as a universal index to quantify the ease of mechanical exfoliation for 3D MOFs. This ratio, readily accessible through mechanical experiments and computation, serves as a valuable metric for selecting appropriate exfoliation methods to produce surfactant-free 2D nanosheets from various 3D materials.

Preparation of polymer dispersed liquid crystal doped with bilayer nanofiber membranes and phase‐change microcapsule coating for enhanced infrared modulation

AbstractThe effective manipulation and regulation of infrared (IR) radiation represent significant advancements in the realm of liquid crystal applications. In this investigation, the synergistic integration of nanofiber membranes and phase change microencapsulated film with polymer dispersed liquid crystal (PDLC) layers was employed for IR modulation. To obtain the excellent electro‐optical properties of PDLC composite films, the ratio of the PDLC system, the thickness of the nanofiber membrane, and the concentration of phase‐change microcapsules were meticulously regulated, and the polymer network morphology of the PDLC films was analyzed to achieve a high contrast ratio (162.72) of PDLC composite films. The fabricated PDLC composite devices show excellent IR shielding efficiency of 15% in the presence of doped nanofiber membranes and phase change microencapsulated films. Notably, patterned nanofiber layers and phase‐change microcapsule coating were obtained by template electrospinning, resulting in patternable PDLC composite film devices.

Molecular dynamics study on phase change properties and their nano-mechanism of sugar alcohols: Melting and latent heat

Sugar alcohol phase change materials have gained considerable attention for heat storage in recent years owing to their higher thermal conductivity and larger latent heat compared to traditional paraffin materials. However, knowledge about the reason of large latent heat and latent heat difference among diverse sugar alcohols as well as the mechanism of melting process remains elusive. In this study, molecular dynamics simulation was employed to investigate the melting process and latent heat of four sugar alcohols (glycerol, erythritol, arabinitol, and mannitol). Melting points and latent heat of fusion were determined first, different melting temperatures were observed under different one-dimensional heat conduction directions in several sugar alcohols. It was found that the strength of intermolecular interactions within molecular crystal layers has a positive correlation with the anisotropic melting tendencies, which can be more intuitively reflected by the difference in the number of hydrogen bonds within the molecular layers. In addition, the decomposition of the latent heat of fusion revealed that the latent heat is mainly composed of van der Waals and Coulombic contributions, where the interaction between hydroxyl groups accounts for a major part. Hydrogen bond analysis demonstrated that the latent heat of fusion is partly due to the breakage of hydrogen bonds during the phase transition of melting. It was also noted that the differences in the hydrogen bonding lifetimes between different sugar alcohols may be related to the magnitude of latent heat of fusion. Erythritol and mannitol which coincidentally happened to have an even number of carbon atoms can fulfill the properties of better PCMs compared to glycerol and arabinitol, which have an odd number of carbons. Those results will pave the way for better design optimization and performance evaluation of sugar alcohols as phase change materials.