Distorted 1T Phase Research Articles

An embedded ReS2@MAPbBr3 heterostructure with downhill interfacial charge transfer for photocatalytic upgrading of biomass-derived alcohols to aldehydes and H2

Solar-driven selective upgrading of lignocellulosic biomass-derived alcohols to value-added chemicals and clean fuel hydrogen (H2) shows great potential for tackling the energy crisis and environmental pollution issues. Here, we construct a MAPbBr3/ReS2 heterostructure by embedding distorted tetragonal (1T) phase ReS2 nanoflowers into large-sized MAPbBr3 for green value-added utilization of biomass-derived alcohols. The embedded structure effectively enlarges the contact interface between the ReS2 and the MAPbBr3, and importantly, induces a strong built-in electric field aligned between the spatially well-defined MAPbBr3 and ReS2 nanoflowers. Moreover, the distorted 1T phase ReS2 with low work function well matches the energy band of MAPbBr3, forming a heterostructure with a downward band bending at the interface. These features empower the MAPbBr3/ReS2 photocatalyst with high capability to promote charge separation and expedite the surface redox reaction. Consequently, optimal BAD and H2 production rates of about 1220 μmol h−1 g−1 are realized over a MAPbBr3/ReS2 2% sample, which are approximately 9 times greater than those of blank MAPbBr3. This work demonstrates the great potential of constructing an embedded transition metal dichalcogenide@metal halide perovskites heterostructure with downhill interfacial charge transfer for photocatalytic upgrading of biomass-derived alcohols.

Correlating structural, electronic, and magnetic properties of epitaxial VSe2 thin films

The electronic and magnetic properties of transition metal dichalcogenides are known to be extremely sensitive to their structure. In this paper we study the effect of structure on the electronic and magnetic properties of mono- and bilayer $VSe_2$ films grown using molecular beam epitaxy. $VSe_2$ has recently attracted much attention due to reports of emergent ferromagnetism in the 2D limit. To understand this important compound, high quality 1T and distorted 1T films were grown at temperatures of 200 $^\text{o}$C and 450 $^\text{o}$C respectively and studied using 4K Scanning Tunneling Microscopy/Spectroscopy. The measured density of states and the charge density wave (CDW) patterns were compared to band structure and phonon dispersion calculations. Films in the 1T phase reveal different CDW patterns in the first layer compared to the second. Interestingly, we find the second layer of the 1T-film shows a CDW pattern with 4a $\times$ 4a periodicity which is the 2D version of the bulk CDW observed in this compound. Our phonon dispersion calculations confirm the presence of a soft phonon at the correct wavevector that leads to this CDW. In contrast, the first layer of distorted 1T phase films shows a strong stripe feature with varying periodicities, while the second layer displays no observable CDW pattern. Finally, we find that the monolayer 1T $VSe_2$ film is weakly ferromagnetic, with ~ $3.5 {\mu}_B$ per unit similar to previous reports.

Synthesis of Ultrathin Metallic MTe2 (M = V, Nb, Ta) Single-Crystalline Nanoplates.

Two-dimensional materials with intrinsic magnetism have recently drawn intense interest for both the fundamental studies and potential technological applications. However, the studies to date have been largely limited to mechanically exfoliated materials. Herein, an atmospheric pressure chemical vapor deposition route to ultrathin group VB metal telluride MTe2 (M = V, Nb, Ta) nanoplates with thickness as thin as 3nm is reported. It is shown that the resulting nanoplates can be systematically evolved from mostly thicker hexagonal domains to thinner triangular domains with an increasing flow rate of the carrier gas. X-ray diffraction and transmission electron microscopy studies reveal MTe2 (M = V, Nb, Ta) nanoplates are high-quality single crystals. High-resolution scanning transmission electron microscope imaging reveals the VTe2 and NbTe2 nanoplates adopt the hexagonal 1T phase and the TaTe2 nanoplates show a monoclinic distorted 1T phase. Electronic transport studies show that MTe2 single crystals exhibit metallic behavior. Magnetic measurements show that VTe2 and NbTe2 exhibit ferromagnetism and TaTe2 shows paramagnetic behavior. The preparation of ultrathin few-layered MTe2 nanoplates will open up exciting opportunities for the burgeoning field of spintronics, sensors, and magneto-optoelectronics.

Atomic structure and migration dynamics of MoS2/LixMoS2 interface

The performance of alkali-metal-ion batteries largely depends on the migration behavior of alkali metal ions in the electrodes. Probing the atomic structure of the reaction interface and the dynamic process during ion transport in the electrodes will help better understand the underlying electrochemical mechanisms and inspire rational electrode designs. In this study, by combining in situ transmission electron microscopy (TEM) and aberration-corrected scanning TEM (STEM), we track the reversible lithium ion transport in MoS2 nanostructures to reveal the atomic structure and dynamic behaviors of the reaction interface. We find that lithium ions insertion triggers complex phase transformations. Three different phases co-exist at the interface: a pristine 2H phase, a 1T phase with a shrank lattice constant of − 3.3% ( ± 2.3%), and a distorted 1T phase (called 1Tˊ phase) with an expanded lattice constant of 5.5% ( ± 2.5%). The atomically resolved Z-contrast image shows that the expanded 1Tˊ phase has distorted Mo arrangements. Furthermore, the lithium ions migration causes defects at the reaction front, and the diffusion on the surface is faster than that inside, forming a core-shell structure at the reaction interface. The diffusivity of lithium ions is directly measured to be ~1000–30,000 nm2/s, which is significantly higher than that of sodium insertion (~10–20 nm2/s). The atomic-scale observations of lithium-ion-migration-induced complex structural evolutions would help understand the properties of MoS2 nanostructures and shed light on the design of alkali-metal-ion batteries with general transition-metal dichalcogenide electrodes.

Origin of distorted 1T-phase ReS2: first-principles study

Group-VIIB transition metal dichalcogenides (TMDCs) are known to be stabilized solely in a distorted 1T phase termed as 1T″ phase, which is compared to many stable or metastable phases in other TMDCs. Using first-principles calculations, we study the structural origin of 1T″ phase group-VIIB TMDCs. We find that quasi 1D Peierls-like instability is responsible for the transition to the 1T″ phase ReS2 monolayer from the 1T′ phase, another distorted 1T phase. Two half-filled bands in 1T′-ReS2 make sharp peaks in the Lindhard function that prompt the charge density wave (CDW) phase with large band gap opening. Our calculations show that overlapping of the two bands in a broad energy range leads to robust CDW phase or stable 1T″ phase in group-VIIB TMDCs against compositional variation, which is in stark contrast to typical Peierls instability driven by a single band. Calculated total energy curve near the critical point exhibits the feature of the first-order Landau transition due to local chemical bonding. The structural stability of the 1T″ phase in group-VIIB TMDCs is thus guaranteed by two half-filled bands and local chemical bonding.

Hydrogen-induced structural transition in single layer ReS2

By performing density functional theory-based calculations, we investigate how structural, electronic and mechanical properties of single layer ReS2 can be tuned upon hydrogenation of its surfaces. It is found that a stable, fully hydrogenated structure can be obtained by formation of strong S-H bonds. The optimized atomic structure of ReS2H2 is considerably different than that of the monolayer ReS2 which has a distorted-1T phase. By performing phonon dispersion calculations, we also predict that the Re2-dimerized 1T structure (called 1T) of the ReS2H2 is dynamically stable. Unlike the bare ReS2 the 1T–ReS2H2 structure which is formed by breaking the Re4 clusters into separated Re2 dimers, is an indirect-gap semiconductor. Furthermore, mechanical properties of the 1T phase in terms of elastic constants, in-plane stiffness (C) and Poisson ratio (ν) are investigated. It is found that full hydrogenation not only enhances the flexibility of the single layer ReS2 crystal but also increases anisotropy of the elastic constants.

3D Anisotropic Thermal Conductivity of Exfoliated Rhenium Disulfide.

ReS2 represents a different class of 2D materials, which is characterized by low symmetry having 1D metallic chains within the planes and extremely weak interlayer bonding. Here, the thermal conductivity of single-crystalline ReS2 in a distorted 1T phase is determined at room temperature for the in-plane directions parallel and perpendicular to the Re-chains, and the through-plane direction using time-domain thermoreflectance. ReS2 is prepared in the form of flakes having thicknesses of 60-450 nm by micromechanical exfoliation, and their crystalline orientations are identified by polarized Raman spectroscopy. The in-plane thermal conductivity is higher along the Re-chains, (70 ± 18) W m-1 K-1 , as compared to transverse to the chains, (50 ± 13) W m-1 K-1 . As expected from the weak interlayer bonding, the through-plane thermal conductivity is the lowest observed to date for 2D materials, (0.55 ± 0.07) W m-1 K-1 , resulting in a remarkably high anisotropy of (130 ± 40) and (90 ± 30) for the two in-plane directions. The thermal conductivity and interface thermal conductance of ReS2 are discussed relative to the other 2D materials.

Tunable Ambipolar Polarization-Sensitive Photodetectors Based on High-Anisotropy ReSe2 Nanosheets.

Atomically thin 2D-layered transition-metal dichalcogenides have been studied extensively in recent years because of their intriguing physical properties and promising applications in nanoelectronic devices. Among them, ReSe2 is an emerging material that exhibits a stable distorted 1T phase and strong in-plane anisotropy due to its reduced crystal symmetry. Here, the anisotropic nature of ReSe2 is revealed by Raman spectroscopy under linearly polarized excitations in which different vibration modes exhibit pronounced periodic variations in intensity. Utilizing high-quality ReSe2 nanosheets, top-gate ReSe2 field-effect transistors were built that show an excellent on/off current ratio exceeding 10(7) and a well-developed current saturation in the current-voltage characteristics at room temperature. Importantly, the successful synthesis of ReSe2 directly onto hexagonal boron nitride substrates has effectively improved the electron motility over 500 times and the hole mobility over 100 times at low temperatures. Strikingly, corroborating with our density-functional calculations, the ReSe2-based photodetectors exhibit a polarization-sensitive photoresponsivity due to the intrinsic linear dichroism originated from high in-plane optical anisotropy. With a back-gate voltage, the linear dichroism photodetection can be unambiguously tuned both in the electron and hole regime. The appealing physical properties demonstrated in this study clearly identify ReSe2 as a highly anisotropic 2D material for exotic electronic and optoelectronic applications.

Structural Properties and Phase Transition of Na Adsorption on Monolayer MoS2.

First-principles calculations are performed to investigate the structural stability of Na adsorption on 1H and 1T phases of monolayer MoS2. Our results demonstrate that it is likely to make the stability of distorted 1T phase of MoS2 over the 1H phase through adsorption of Na atoms. The type of distortion depends on the concentration of adsorbed Na atoms and changes from zigzag-like to diamond-like with the increasing of adsorbed Na atom concentrations. Our calculations show that the phase transition from 1H-MoS2 to 1T-MoS2 can be obtained by Na adsorption. We also calculate the electrochemical properties of Na adsorption on MoS2 monolayer. These results indicate that MoS2 is one of potential negative electrodes for Na-ion batteries.

The In-Plane Anisotropy of WTe2 Investigated by Angle-Dependent and Polarized Raman Spectroscopy.

Tungsten ditelluride (WTe2) is a semi-metallic layered transition metal dichalcogenide with a stable distorted 1T phase. The reduced symmetry of this system leads to in-plane anisotropy in various materials properties. We have systemically studied the in-plane anisotropy of Raman modes in few-layer and bulk WTe2 by angle-dependent and polarized Raman spectroscopy (ADPRS). Ten Raman modes are clearly resolved. Their intensities show periodic variation with sample rotating. We identify the symmetries of the detected modes by quantitatively analyzing the ADPRS results based on the symmetry selection rules. Material absorption effect on the phonon modes with high vibration frequencies is investigated by considering complex Raman tensor elements. We also provide a rapid and nondestructive method to identify the crystallographic orientation of WTe2. The crystallographic orientation is further confirmed by the quantitative atomic-resolution force image. Finally, we find that the atomic vibrational tendency and complexity of detected modes are also reflected in the shrinkage degree defined based on ADPRS, which is confirmed by corresponding density functional calculation. Our work provides a deep understanding of the interaction between WTe2 and light, which will benefit in future studies about the anisotropic physical properties of WTe2 and other in-plane anisotropic materials.

In-Plane Anisotropy in Mono- and Few-Layer ReS2 Probed by Raman Spectroscopy and Scanning Transmission Electron Microscopy.

Rhenium disulfide (ReS2) is a semiconducting layered transition metal dichalcogenide that exhibits a stable distorted 1T phase. The reduced symmetry of this system leads to in-plane anisotropy in various material properties. Here, we demonstrate the strong anisotropy in the Raman scattering response for linearly polarized excitation. Polarized Raman scattering is shown to permit a determination of the crystallographic orientation of ReS2 through comparison with direct structural analysis by scanning transmission electron microscopy (STEM). Analysis of the frequency difference of appropriate Raman modes is also shown to provide a means of precisely determining layer thickness up to four layers.

Dissulfeto de molibdênio, um material multifuncional e surpreendente

The aim of this work is to review the chemical and physical properties of layered molybdenum disulfide. The three polymorphic/polytypic modifications of the compound were found, the polytypes 2H (molybdenite) and 3R are semiconductors while the polymorph 1T is an electronic conductor. 2H-MoS2 has several important industrial applications as hydrotreatment catalysts, energy storage devices, solar cells, solid lubricants, among others. When intercalated, the 2H phase changes to a distorted 1T phase, producing unstable intercalation compounds that can be exfoliated in solution, producing single layers and consequently nanocomposites. The direct synthesis of the 1T phase produces stable intercalation compounds. Recently molybdenum disulfide was prepared as nanotubes and fulerene-like structures that bring new insights in the investigation of this important material.