Class Of Layered Materials Research Articles

Microscopic understanding of the in-plane thermal transport properties of 2H transition metal dichalcogenides

Transition metal dichalcogenides (TMDs) are a class of layered materials that hold great promise for a wide range of applications. Their practical use can be limited by their thermal transport properties, which have proven challenging to determine accurately, both from a theoretical and experimental perspective. We have conducted a thorough theoretical investigation of the thermal conductivity of four common TMDs, MoSe2, WSe2, MoS2, and WS2, at room temperature, to determine the key factors that influence their thermal behavior. We analyze these materials using calculations performed with the program, anharmonic lattice dynamics and the Boltzmann transport equation formalism, as implemented in the temperature-dependent effective potentials method. Within this framework, we analyze the microscopic parameters influencing the thermal conductivity, such as the phonon dispersion and the phonon lifetimes. The aim is to precisely identify the origin of differences in thermal conductivity among these canonical TMD materials. We compare their in-plane thermal properties in monolayer and bulk form, and we analyze how the thickness and the chemical composition affect the thermal transport behavior. We showcase how bonding and the crystal structure influence the thermal properties by comparing the TMDs with silicon, reporting the cases of bulk silicon and monolayer silicene. We find that the interlayer bond type (covalent vs. van der Waals) involved in the structure is crucial in the heat transport. In two-dimensional silicene, we observe a reduction by a factor ∼15 compared to the Si bulk thermal conductivity due to the smaller group velocities and shorter phonon lifetimes. In the TMDs, where the group velocities and the phonon bands do not vary significantly passing from the bulk to the monolayer limit, we do not see as strong a decrease in the thermal conductivity: only a factor 2–3. Moreover, our analysis reveals that differences in the thermal conductivity arise from variations in atomic species, bond strengths, and phonon lifetimes. These factors are closely interconnected and collectively impact the overall thermal conductivity. We inspect each of them separately and explain how they influence the heat transport. We also study artificial TMDs with modified masses, in order to assess how the chemistry of the compounds modifies the microscopic quantities and thus the thermal conductivity. Published by the American Physical Society 2024

Predictions of delafossite-hosted honeycomb and kagome phases.

Delafossites, typically denoted by the formula ABO2, are a class of layered materials that exhibit a wide range of electronic and optical properties. Recently, the idea of modifying these delafossites into ordered kagome or honeycomb phases via strategic doping has emerged as a potential way to tailor these properties. In this study, we use high-throughput density functional theory calculations to explore many possible candidate kagome and honeycomb phases by considering dopants selected from the parent compounds of known ternary delafossite oxides from the inorganic crystal structure database. Our results indicate that while A-site in existing delafossites can host a limited range of elemental specifies, and display a low propensity for mixing or ordering, the oxide sub-units in the BO2 much more readily admit guest species. Our study identifies four candidate B-site kagome and fifteen candidate B-site honeycombs with a formation energy more than 50 meV f.u.-1 below other competing phases. The ability to predict and control the formation of these unique structures offers exciting opportunities in materials design, where innovative properties can be engineered through the selection of specific dopants. A number of these constitute novel correlated metals, which may be of interest for subsequent efforts in synthesis. These novel correlated metals may have significant implications for quantum computing, spintronics, and high-temperature superconductivity, thus inspiring future experimental synthesis and characterization of these proposed materials.

Carbon quantum dots-based fluorescent layered double hydroxide for targeted drug delivery application

Layered double hydroxides (LDHs) are a class of layered materials that have received great attention as drug delivery carriers due to their excellent properties such as biocompatibility, chemical stability, easy synthesis, anion exchange capacity, and controlled drug release. In this research, LDH nanosheets based on Ca and Al were synthesized by a coprecipitation method. The LDH nanosheets were in situ coated by polyethylene glycol (PEG). The LDH nanosheets were decorated with carbon quantum dots (CQDs) derived from citric acid by a hydrothermal method and targeted with folic acid. Doxorubicin hydrochloride (DOX) was used as a model drug to investigate the drug loading and release of the fluorescent LDH. XRD analysis showed that the content of PEG in the synthesis medium affects the crystallinity of the LDHs. FTIR analysis confirmed the successful attachment of CQDs and folic acid (FA) on LDH nanosheets. The fluorescence of the synthesized nanohybrid was influenced by the CQDs content. UV–Vis spectroscopy showed the DOX can be loaded on the nanohybrid with a loading efficiency of 32 %. The drug release of the nanohybrid is pH-dependent. The drug can be released up to 83 % within 12 h in the acidic environment while the release reaches 63 % in the physiological pH at the same time. The synthesized CQDs can be used as pH-dependent fluorescent and targeted nanocarriers in drug delivery systems.

Noncollinear Magnetic Order in Two-Dimensional NiBr2 Films Grown on Au(111).

Metal halides are a class of layered materials with promising electronic and magnetic properties persisting down to the two-dimensional limit. While most recent studies focused on the trihalide components of this family, the rather unexplored metal dihalides are also van der Waals layered systems with distinctive magnetic properties. Here we show that the dihalide NiBr2 grows epitaxially on a Au(111) substrate and exhibits semiconducting and magnetic behavior starting from a single layer. Through a combination of a low-temperature scanning-tunneling microscopy, low-energy electron diffraction, X-ray photoelectron spectroscopy, and photoemission electron microscopy, we identify two competing layer structures of NiBr2 coexisting at the interface and a stoichiometrically pure layer-by-layer growth beyond. Interestingly, X-ray absorption spectroscopy measurements revealed a magnetically ordered state below 27 K with in-plane magnetic anisotropy and zero-remanence in the single layer of NiBr2/Au(111), which we attribute to a noncollinear magnetic structure. The combination of such two-dimensional magnetic order with the semiconducting behavior down to the 2D limit offers the attractive perspective of using these films as ultrathin crystalline barriers in tunneling junctions and low-dimensional devices.

Nacre Is a Liquid-Crystal Thermometer of the Oceans.

Nacre, or mother of pearl, is a biomaterial with a layered structure. In a recent geological study, researchers found that the width of the nacre layers depends on the formation temperature, which is determined by the ocean water temperature. A linear dependence of layer width with respect to temperature is understandable within the transient liquid-crystalline nature of incipient nacre. Thus, developing nacre is a liquid-crystal thermometer recording its formation temperature. A more complete understanding of nacre formation is of interest not only for biology and geology, in terms of biomineralization and paleoclimatology, but also for materials science: for reproducing nacre or fabricating synthetic analogues and also potentially for developing new classes of layered materials with layer spacings tunable by pH and temperature.

Effect of deformation rate on post-forming interfacial delamination in polymer laminated sheet metals

ABSTRACT The polymer laminated sheet metal (PLSM) is a class of layered materials which involves a sheet metal substrate, a polymer film and an adhesive layer between them. The PLSMs can be potentially used for sheet forming applications. Due to the incompatibility in mechanical and physical properties of the constituents, deformation of the PLSM could cause interfacial delamination between the metallic substrate and polymer adherends. This paper deals with the assessment of the effect of uniaxial elongation rate on post-forming interfacial delamination in PLSMs. For the polymer adherends used in this study, it is concluded that for large strains, higher deformation rate results in less amount of delamination. This behavior is attributed to the time-dependency of mechanical properties of the polymer adherend material. Abbreviations: PLSM: polymer laminated sheet metal; PSA: pressure-sensitive adhesive; BA: bright annealed; PE: polyethylene; PP: polypropylene; PVC: polyvinyl chloride

Layered material GeSe and vertical GeSe/MoS2 p-n heterojunctions

Group-IV monochalcogenides are emerging as a new class of layered materials beyond graphene, transition metal dichalcogenides (TMDCs), and black phosphorus (BP). In this paper, we report experimental and theoretical investigations of the band structure and transport properties of GeSe and its heterostructures. We find that GeSe exhibits a markedly anisotropic electronic transport, with maximum conductance along the armchair direction. Density functional theory calculations reveal that the effective mass is 2.7 times larger along the zigzag direction than the armchair direction; this mass anisotropy explains the observed anisotropic conductance. The crystallographic orientation of GeSe is confirmed by angleresolved polarized Raman measurements, which are further supported by calculated Raman tensors for the orthorhombic structure. Novel GeSe/MoS2 p-n heterojunctions are fabricated, combining the natural p-type doping in GeSe and n-type doping in MoS2. The temperature dependence of the measured junction current reveals that GeSe and MoS2 have a type-II band alignment with a conduction band offset of ~0.234 eV. The anisotropic conductance of GeSe may enable the development of new electronic and optoelectronic devices, such as high-efficiency thermoelectric devices and plasmonic devices with resonance frequency continuously tunable through light polarization direction. The unique GeSe/MoS2 p-n junctions with type-II alignment may become essential building blocks of vertical tunneling field-effect transistors for low-power applications. The novel p-type layered material GeSe can also be combined with n-type TMDCs to form heterogeneous complementary metal oxide semiconductor (CMOS) circuits.

Effects of reduced dimensionality on the properties of magnesium hydroxide and calcium hydroxide nanostructures.

Metal hydroxides are a class of layered materials that contain two-dimensional metal hydroxide layers that can be isolated to form layered nanostructures. In this work, density functional theory (DFT) and self-consistent-charge density-functional tight-binding (SCC-DFTB) methods have been used to investigate the properties of magnesium hydroxide and calcium hydroxide nanostructures. The properties of single layer and multi layer structures with up to 10 metal hydroxide sheets and nanoparticles containing more than 2000 atoms have been calculated and compared with the bulk properties of these systems. The accuracy of the DFT methods employed and SCC-DFTB parameters developed in this study were validated against available experimental data. The results of the calculations indicate that significant differences exist between the properties of the nanostructures and the corresponding bulk values. In particular, the interlayer binding energies, electronic band gaps, and spectroscopic features are size-dependent and tend to converge to the bulk values as the size of the nanosystem is increased. The calculated binding energies and shear moduli show that all nanostructures are mechanically stable, in agreement with the experimental reports; although, their stabilities may be affected by the presence of intercalated species. Energy decomposition analyses reveal that the intralayer interactions in the investigated systems are predominantly electrostatic in nature, while the interlayer interactions are dominated by dispersion and polarization components. The results presented here quantify various properties of magnesium hydroxide and calcium hydroxide nanostructures, and could be used to understand the properties of other nanosystems containing layers of metal hydroxides in their structure.

Spin‐Structured Multilayers: A New Class of Materials for Precision Spintronics

Magnetoelectronic multilayer devices are widely used in today's information and sensor technology. Their functionality, however, is limited by the inherent properties of magnetic exchange or dipolar coupling which constrain possible spin configurations to collinear or perpendicular alignments of adjacent layers. Here, a deposition procedure is introduced that allows for a new class of layered materials in which complex spin structures can be accurately designed to result in a multitude of new and precisely adjustable spintronic and magnetoresistive properties. The magnetization direction and coercivity of each individual layer are determined by the deposition process in oblique incidence geometry and can be completely decoupled from neighboring layers. This applies for layers of any ferromagnetic material down to layer thicknesses of a few nm and lateral dimensions of a few 100 nm, enabling the design of efficient and compact magnetoelectronic devices, encompassing precision magnetoresistive sensors as well as layer systems with multiple addressable remanent states for magnetic memory applications.

Polyoxometalates Hosted in Layered Double Hydroxides: Highly Enhanced Catalytic Activity and Selectivity in Sulfoxidation of Sulfides

Layered double hydroxides (LDHs) are a class of layered materials with tunable interlayer galleries. In this paper, a series of Mg3Al–P2W17X (X = MnIII, FeIII, ZnII, CoII, CuII, NiII) have been prepared successfully by intercalating the polyoxometalate (POM) of [α2-P2W17O61(X·OH2)]n− ([P2W17X]n−, n = 7 or 8) into a Mg3Al–suberic precursor. Catalytic tests for H2O2-based sulfoxidation of various sulfides demonstrate that Mg3Al–P2W17Zn exhibits better catalytic efficiency and selectivity than pure POM or LDH precursors under mild conditions. The uniform and well-ordered dispersion of POM in the confined gallery of LDHs and the multiple interactions between POMs and LDHs contribute to the excellent catalytic performance. Moreover, Mg3Al–P2W17Zn is stable and can be easily separated from the reaction system. The recycled Mg3Al–P2W17Zn maintains both the intact structures of the POM anion and LDHs. The scaled-up experiment provides further support for its potential use for industrial applications.

Plane strain bifurcation and its relation to kinkband formation in layered materials

Abstract A bifurcation analysis was carried out for a class of layered materials whose constitutive behavior were described by the model of Christoffersen and Jensen, 1996. The problem was solved in uniaxial plane strain compression for compressible materials without volumetric plastic strain. The elastic–plastic response of the constituent materials is described by J 2 -deformation theory and J 2 -flow theory. Results indicate that bifurcation stresses vary smoothly with wavelength and are sensitive to the moduli ratio between constituent materials and the volume fractions. The results show that there is a transition in the bifurcation mode from an Euler type elastic bifurcation of a slender column made of a composite material to a surface bifurcation mode in the plastic regime for a wide plate. Furthermore, effects of residual stress were also incorporated and studied.

New Class of Layered Materials

A new class of layered materials has been prepared by alternate deposition of two dissimilar metals. Strong experimental evidence is shown that coherent structures with layer thicknesses approaching interatomic spacing can be prepared in this fashion. The experimental x-ray measurements are found to be in good qualitative and quantitative agreement with model calculations.