Properties Of Layered Materials Research Articles

Graphene-analogous structural MoS2 modification to improve electrochemical properties of Ni-rich layered oxide cathode material for lithium-ion batteries

Ni-rich cathode material attracts a substantial amount of attention due to its high specific capacity, superior energy density and low cost. Nonetheless, there are still a few intrinsic issues which need to be solved including rate performance, cycling stability, elevated-temperature performance, etc. In this paper, the graphene-analogous structural MoS2 with chemical stability and acceptable conductivity is purposefully introduced to modify LiNi0.8Co0.1Mn0.1O2 cathode material via a facile wet-chemical approach followed by low-temperature reaction. In addition, lithium-active MoS2 coating layer can provide an interconnected and stable tunnel for the insertion and extraction of lithium ions. Characterizations by X-ray diffraction, X-ray photoelectron spectroscopy, Raman, scanning electron microscopy and transmission electron microscopy demonstrate that MoS2 coating layer is uniformly deposited on the surface of LiNi0.8Co0.1Mn0.1O2 material. The electrochemical results indicate that the modified cathode material displays excellent structure stability, superior rate performance and outstanding cycling properties.

Plasma-Induced Phase Transformation of SnS2 to SnS

Layered van der Waals materials have recently attracted attention owing to their exceptional electrical and optical properties in thin layer form. One way to extend their utility is to form a heterostructure which combines various properties of layered materials to reveal intriguing behavior. Conventional heterostructure synthesis methods are difficult to develop and the heterostructure formed can be limited to a small area. Here, we investigate the phase transformation of SnS2 to SnS by removing sulfur atoms at the top surface using Ar plasma. By varying the plasma power and exposure time, we observed that SnS is subsequently formed on top of the mogul-like structure of SnS2. Since SnS is a p-type semiconductor and SnS2 is an n-type semiconductor, we naturally formed a vertical p-n junction. By using graphene at the top and bottom as transparent electrodes, a vertical p-n diode device is constructed. The device demonstrates good rectifying behavior and large photocurrent generation under white light. This method can be applied to large-area heterostructure synthesis using plasma via phase transformation of various metal dichalcogenides to metal monochalcogenides.

Selective Laser-Assisted Synthesis of Tubular van der Waals Heterostructures of Single-Layered PbI2 within Carbon Nanotubes Exhibiting Carrier Photogeneration.

The electronic and optical properties of two-dimensional layered materials allow the miniaturization of nanoelectronic and optoelectronic devices in a competitive manner. Even larger opportunities arise when two or more layers of different materials are combined. Here, we report on an ultrafast energy efficient strategy, using laser irradiation, which allows bulk synthesis of crystalline single-layered lead iodide in the cavities of carbon nanotubes by forming cylindrical van der Waals heterostructures. In contrast to the filling of van der Waals solids into carbon nanotubes by conventional thermal annealing, which favors the formation of inorganic nanowires, the present strategy is highly selective toward the growth of monolayers forming lead iodide nanotubes. The irradiated bulk material bearing the nanotubes reveals a decrease of the resistivity as well as a significant increase in the current flow upon illumination. Both effects are attributed to the presence of single-walled lead iodide nanotubes in the cavities of carbon nanotubes, which dominate the properties of the whole matrix. The present study brings in a simple, ultrafast and energy efficient strategy for the tailored synthesis of rolled-up single-layers of lead iodide (i.e., single-walled PbI2 nanotubes), which we believe could be expanded to other two-dimensional (2D) van der Waals solids. In fact, initial tests with ZnI2 already reveal the formation of single-walled ZnI2 nanotubes, thus proving the versatility of the approach.

Dehomogenized Elastic Properties of Heterogeneous Layered Materials in AFM Indentation Experiments

Atomic force microscopy (AFM) is used to study mechanical properties of biological materials at submicron length scales. However, such samples are often structurally heterogeneous even at the local level, with different regions having distinct mechanical properties. Physical or chemical disruption can isolate individual structural elements but may alter the properties being measured. Therefore, to determine the micromechanical properties of intact heterogeneous multilayered samples indented by AFM, we propose the Hybrid Eshelby Decomposition (HED) analysis, which combines a modified homogenization theory and finite element modeling to extract layer-specific elastic moduli of composite structures from single indentations, utilizing knowledge of the component distribution to achieve solution uniqueness. Using finite element model-simulated indentation of layered samples with micron-scale thickness dimensions, biologically relevant elastic properties for incompressible soft tissues, and layer-specific heterogeneity of an order of magnitude or less, HED analysis recovered the prescribed modulus values typically within 10% error. Experimental validation using bilayer spin-coated polydimethylsiloxane samples also yielded self-consistent layer-specific modulus values whether arranged as stiff layer on soft substrate or soft layer on stiff substrate. We further examined a biophysical application by characterizing layer-specific microelastic properties of full-thickness mouse aortic wall tissue, demonstrating that the HED-extracted modulus of the tunica media was more than fivefold stiffer than the intima and not significantly different from direct indentation of exposed media tissue. Our results show that the elastic properties of surface and subsurface layers of microscale synthetic and biological samples can be simultaneously extracted from the composite material response to AFM indentation. HED analysis offers a robust approach to studying regional micromechanics of heterogeneous multilayered samples without destructively separating individual components before testing.

Controllable Growth and Formation Mechanisms of Dislocated WS2 Spirals.

Two-dimensional (2D) layered metal dichalcogenides can form spiral nanostructures by a screw-dislocation-driven mechanism, which leads to changes in crystal symmetry and layer stackings that introduce attractive physical properties different from their bulk and few-layer nanostructures. However, controllable growth of spirals is challenging and their growth mechanisms are poorly understood. Here, we report the controllable growth of WS2 spiral nanoplates with different stackings by a vapor phase deposition route and investigate their formation mechanisms by combining atomic force microscopy with second harmonic generation imaging. Previously not observed "spiral arm" features could be explained as covered dislocation spiral steps, and the number of spiral arms correlates with the number of screw dislocations initiated at the bottom plane. The supersaturation-dependent growth can generate new screw dislocations from the existing layers, or even new layers templated by existing screw dislocations. Different number of dislocations and orientation of new layers result in distinct morphologies, different layer stackings, and more complex nanostructures, such as triangular spiral nanoplates with hexagonal spiral pattern on top. This work provides the understanding and control of dislocation-driven growth of 2D nanostructures. These spiral nanostructures offer diverse candidates for probing the physical properties of layered materials and exploring new applications in functional nanoelectronic and optoelectronic devices.

Significantly improved mechanical properties of the self-assembled layered lignocelluloses material via grinding and hot pressing

Most of high strength wood-based materials used for decoration contain adhesives. In this study, layered lignocelluloses material was successfully fabricated via grinding and hot pressing methods. The as-prepared materials exhibit high mechanical properties without adhesive. Abundant furfurals are released during hot pressing to connect lignocelluloses, and layered structure is discovered in layered lignocelluloses material. Therefore, layered lignocelluloses material showed high strength mechanical properties which modulus of elasticity (MOE) and modulus of rupture (MOR) reach to 3119 ± 309 MPa and 18.87 ± 4.770 MPa, respectively. This research may provide an inexpensive, high strength and green material to the field of furniture in the future.

(Invited) The Impact of the Phase and Stacking of 2D Materials on Their Properties and Applications

The interesting and tunable properties of layered metal dichalcogenides (MX2) and other layered materials heavily depend on their phase and layer stacking. We discovered that chemical exfoliation of layered MX2 nanosheets and simultaneous conversion from the semiconducting 2H polymorph to the metallic 1T polymorph grants greatly enhanced activity for electrocatalytic hydrogen evolution reaction (HER). We have also developed layer-controlled vertical heterostructures of MX2 by van der Waals epitaxy that have strong electronic interactions across the heterojunctions. We explain how the layer stacking and physical properties of MX2 are influenced by screw dislocations. A one-to-one correlation of atomic force microscopy and high- and low-frequency Raman spectroscopy of many dislocated WSe2 nanoplates reveals variations in the number and shapes of dislocation spirals and different layer stackings that are determined by the number, rotation, and location of the dislocations. These lead to three class of behaviors: plates with triangular dislocation spirals have non-centrosymmetric stacking that gives rise to strong second-harmonic generation, plates with hexagonal dislocation spirals form the bulk 2H layer stacking commonly observed, and plates containing mixed dislocation shapes have intermediate non-centrosymmetric stackings with mixed properties. We also explain layer stackings also determine the etching behaviors of multilayer WSe2 materials that result in complex chemically derived kirigami structures.

Effects of Gd3+ doping on the microstructure and electrochemical properties of Li1.20[Mn0.54Ni0.13Co0.13]O2 as cathode for lithium-ion batteries

To enhance the electrochemical properties of the Mn-based lithium-rich layered cathode materials, Li1.20[Mn0.54−xGd x Ni0.13Co0.13]O2 (x = 0, 0.01, 0.02 and 0.03) were synthesized by using the carbonate co-precipitation method. The X-ray diffraction, scanning electron microscope, galvanostatic charge–discharge tests and electrochemical impedance spectroscopy techniques have been carried out to investigate the influences of the Gd3+ doping modification. And the results showed the lower cation mixing degree and superior electrochemical properties were obtained for the LMNCO by the Gd3+ doping. With the Gd3+ doping amount increasing, the capacity retentions after 100 cycles enhanced from 88.5 to 90.9% and then decrease to 86.7% with x = 0.01, 0.02 and 0.03, respectively. While the un-doped LMNCO only delivered the capacity retention of 85.1%. Besides, the discharge capacity of Li1.20[Mn0.52Gd0.02Ni0.13Co0.13]O2 was 29.4 mAh g−1 larger than that of un-doped sample at 5 C high rate. The electrochemical impedance spectroscopy results indicated that the Gd3+ doping modification could restrain the growth speed of charge transfer impedance of LMNCO during the charge and discharge process.

Studies of the Microstructure and Physical and Mechanical Properties of Layered Metal Composite Materials Based on a Molybdenum Matrix

A method of producing layered metal composite materials (layered MCM) based on a molybdenum matrix is examined; the microstructure and physical and mechanical properties of the produced layered MCM are investigated. Molybdenum foil with a thickness of 0.25 mm and metal composite powder based on a molybdenum matrix doped with silicon and boron are used as layers. The metal composite powder based on a molybdenum matrix is synthesized by a high-energy mechanochemical method. The layered metal composite material is produced by compacting the molybdenum foil and the metal composite powder by means of spark plasma sintering (SPS). Investigation of the microstructure of layered MCM samples after thermal treatment demonstrates that the material preserves the layered structure after a prolonged exposure to elevated temperatures. Preservation of a layered structure upon high-temperature treatment indicates a high thermal stability of the obtained composite material. The performed physical and mechanical tests allow us to make conclusions on a complete consolidation of the metal composite materials by means of spark plasma sintering. The layered MCM based on a molybdenum matrix are to be used in a production of assemblies and parts of for aeronautic and space flight vehicles.

Chemomechanical behaviors of layered cathode materials in alkali metal ion batteries

This review summarizes the recent progress in characterizing, understanding, and modifying the chemomechanical properties of layered oxide cathode materials.

Circularly polarized Raman study on diamond structure crystals

Circularly polarized Raman and/or photoluminescence (PL) analyses have recently been very important in studying physical properties of many layered materials that were either mechanically exfoliated or grown by chemical-vapor-deposition (CVD) on silicon substrates. Since silicon Raman signal is always accompanied by the circularly polarized Raman and/or PL signal from the layered materials, observation of proper circularly polarized Raman selection rules on silicon substrates would be extremely good indicator that the circularly polarized Raman and/or PL measurements on the layered materials were done properly. We have performed circularly polarized Raman measurements on silicon substrates and compared the results with the Raman intensities calculated by using Raman tensors of the diamond crystal structure. Our experimental results were in excellent agreement with the calculation. Similar circularly polarized Raman analysis done on germanium substrate also showed good agreement.

Role of vacancy defects in Al doped ZnO thin films for optoelectronic devices

We report on the electrical, optical and photoluminescence properties of industry-ready Al doped ZnO thin films grown by physical vapor deposition, and their evolution after annealing under vacuum. Doping ZnO with Al atoms increases the carrier density but also favors the formation of Zn vacancies, thereby inducing a saturation of the conductivity mechanism at high aluminum content. The electrical and optical properties of these thin layered materials are both improved by annealing process which creates oxygen vacancies that releases charge carriers thus improving the conductivity. This study underlines the effect of the formation of extrinsic and intrinsic defects in Al doped ZnO compound during the fabrication process. The quality and the optoelectronic response of the produced films are increased (up to 1.52 and 3.73 eV) and consistent with the industrial device requirements.

Strain-Dependent Edge Structures in MoS2 Layers.

Edge structures are low-dimensional defects unavoidable in layered materials of the transition metal dichalcogenides (TMD) family. Among the various types of such structures, the armchair (AC) and zigzag (ZZ) edge types are the most common. It has been predicted that the presence of intrinsic strain localized along these edges structures can have direct implications for the customization of their electronic properties. However, pinning down the relation between local structure and electronic properties at these edges is challenging. Here, we quantify the local strain field that arises at the edges of MoS2 flakes by combining aberration-corrected transmission electron microscopy (TEM) with the geometrical-phase analysis (GPA) method. We also provide further insight on the possible effects of such edge strain on the resulting electronic behavior by means of electron energy loss spectroscopy (EELS) measurements. Our results reveal that the two-dominant edge structures, ZZ and AC, induce the formation of different amounts of localized strain fields. We also show that by varying the free edge curvature from concave to convex, compressive strain turns into tensile strain. These results pave the way toward the customization of edge structures in MoS2, which can be used to engineer the properties of layered materials and thus contribute to the optimization of the next generation of atomic-scale electronic devices built upon them.

(Invited) Phase and Stacking of 2D Materials in Heterostructures and Dislocated Nanoplates Impact Their Properties and Applications

The interesting and tunable properties of layered metal dichalcogenides (MX2) and other layered materials heavily depend on their phase and layer stacking. We discovered that chemical exfoliation of layered MX2 nanosheets and the simultaneous conversion from the semiconducting 2H polymorph to the metallic 1T polymorph grants greatly enhanced activity for electrocatalytic hydrogen evolution reaction (HER). We have also developed layer-controlled vertical heterostructures of MX2 by van der Waals epitaxy that have strong electronic interactions across the heterojunctions that can be useful for solar energy conversion. We further show and explain how the layer stacking and physical properties of MX2 are influenced by screw dislocations. A one-to-one correlation of atomic force microscopy and high- and low-frequency Raman spectroscopy of many dislocated WSe2 nanoplates reveals variations in the number and shapes of dislocation spirals and different layer stackings that are determined by the number, rotation, and location of the dislocations. These lead to three class of behaviors: plates with triangular dislocation spirals have non-centrosymmetric stacking that gives rise to strong second-harmonic generation, plates with hexagonal dislocation spirals form the bulk 2H layer stacking commonly observed, and plates containing mixed dislocation shapes have intermediate non-centrosymmetric stackings with mixed properties.

Tunable Mechano-responsive Color-Change Properties of Organic Layered Material by Intercalation

Summary Layered compounds generate host-guest composites with emergent properties. Organic layered materials have intercalation and dynamic natures originating from the layered structure and the organic compound, respectively. Our intention here is tuning the dynamic properties by intercalation. Here, we report on organic layered composites with tunable stimuli-responsive color-change properties by intercalation. A diacetylene monomer with a layered crystal structure accommodated the guest organic amines through intercalation. Subsequent polymerization formed the layered composites, consisting of the host polydiacetylene and the guest amines. The mechano-responsive color-change properties were systematically controlled by sorts of intercalated amines. In general, it is not easy to tune responsivity to mechanical stimuli, such as shear stress. The tunable mechano-responsive color-change properties realize memory and visualization of the accumulated shear stress, such as the number of rubbings. The intercalation approach shows potential for the development of tunable stimuli-responsive materials and their wide range of applications.

Thermoelectric properties of tetradymite-like layered materials in the pseudoternary system Sb2Te3–GeTe–Bi2Te3

The Hall coefficient, resistivity, and thermoelectric power of quaternary tetradymite-like layered materials in the pseudoternary system Sb2Te3–GeTe–Bi2Te3 have been measured in the temperature range 100–800 K. The results demonstrate that all of the samples studied in the Sb2Te3–GeTe–Bi2Te3 system are p-type and have high hole concentration due to point defects. Plots of lnσ against 1/T in the intrinsic region were used to evaluate the band gap (ΔE) of two materials: GeSb3.91Bi0.03Te6.91(ΔE = 0.22 eV) and GeSbBiTe4 (ΔE = 0.197 eV).

Rapid visualization of grain boundaries in monolayer MoS2 by multiphoton microscopy

Grain boundaries have a major effect on the physical properties of two-dimensional layered materials. Therefore, it is important to develop simple, fast and sensitive characterization methods to visualize grain boundaries. Conventional Raman and photoluminescence methods have been used for detecting grain boundaries; however, these techniques are better suited for detection of grain boundaries with a large crystal axis rotation between neighbouring grains. Here we show rapid visualization of grain boundaries in chemical vapour deposited monolayer MoS2 samples with multiphoton microscopy. In contrast to Raman and photoluminescence imaging, third-harmonic generation microscopy provides excellent sensitivity and high speed for grain boundary visualization regardless of the degree of crystal axis rotation. We find that the contrast associated with grain boundaries in the third-harmonic imaging is considerably enhanced by the solvents commonly used in the transfer process of two-dimensional materials. Our results demonstrate that multiphoton imaging can be used for fast and sensitive characterization of two-dimensional materials.

New insights into low temperature properties of Li-rich layered cathode materials

Abstract Since Li-ion batteries made of Li-rich layered materials might work at extreme environments, understanding the low temperature properties of Li-rich layered materials is of great importance. In this article, the low temperature properties and corresponding mechanism of Li-rich layered materials with a great amount of activated Li 2 MnO 3 are unraveled through a comparison of the low temperature characteristics of Li-rich layered materials with a small amount of activated Li 2 MnO 3 versus the former. Our findings indicate that in the electrode with a large amount of activated Li 2 MnO 3, the sluggish kinetics of the electrode reaction at low temperatures suppresses greatly the Mn 4+ /Mn 3+ reduction reaction, leading to much smaller initial discharge capacity and much better cyclic performance compared with situations at room temperature. Moreover, the increasingly suppressed Mn 4+ /Mn 3+ reduction reaction stop the discharge voltage fading, which might mitigate the layered-to-spinel phase transformation.

Corrosion Resistance and Mechanical Properties of Layered Structural Material Based on Aluminum Alloy and Fiberglass Thin Sheets

Within Russia, a course has been set for development of the Arctic as a transport zone and organization of full-scale recovery of natural resources, primarily hydrocarbons. One problem of economic and everyday development of the Arctic region is creation of materials and structures capable of operating in harsh climatic conditions. Materials used under these conditions should be distinguished not only by good mechanical properties, but also by improved corrosion resistance connected with the high moisture content of the region and presence of salt mist caused by the presence of the sea. There is great potential for use in structural objects of transport engineering of layered composite materials based on a metal matrix reinforced with various nonmetallic fibers, but for reliable operating capacity it is necessary to select components not having an unfavorable effect on each other. Data are provided in this article from research for preparing new structures of layered aluminum-fiberglass with improved corrosion resistance.

Rehabilitation of the Perdew-Burke-Ernzerhof generalized gradient approximation for layered materials

The structural and energetic properties of layered materials propose a challenge to density functional theory with common semilocal approximations to the exchange-correlation. By combining the most-widely used semilocal generalized gradient approximation (GGA), Perdew--Burke--Ernzerhof (PBE), with the revised Vydrov--Van Voorhis non-local correlation functional (rVV10), both excellent structural and energetic properties of 28 layered materials were recovered with a judicious parameter selection. We term the resulting functional as PBE+rVV10L with "L" denoting for layered materials. Such combination is not new, and involves only refitting a single global parameter, however the resulting excellency suggests such corrected PBE for many aspects of theoretical studies on layered materials. For comparison, we also present the results for PBE+rVV10 where the parameter is determined by the 22 interaction energies between molecules.