Transition Metal Dichalchogenides Research Articles

Shift current photovoltaic efficiency of 2D materials

Shift current photovoltaic devices are potential candidates for future cheap, sustainable, and efficient electricity generation. In the present work, we calculate the solar-generated shift current and efficiencies in 326 different 2D materials obtained from the computational database C2DB. We apply, as metrics, the efficiencies of monolayer and multilayer samples. The monolayer efficiencies are generally found to be low, while the multilayer efficiencies of infinite stacks show great promise. Furthermore, the out-of-plane shift current response is considered, and material candidates for efficient out-of-plane shift current devices are identified. Among the screened materials, MXY Janus and MX2 transition metal dichalchogenides (TMDs) constitute a prominent subset, with chromium based MXY Janus TMDs holding particular promise. Finally, in order to explain the band gap dependence of the PV efficiency, a simple gapped graphene model with a variable band gap is established and related to the calculated efficiencies.

Exciton absorption, band structure, and optical emission in biased bilayer graphene

Biased bilayer graphene (BBG) is a variable band gap semiconductor, with a strongly field-dependent band gap of up to $300 \, \text{meV}$, making it of particular interest for graphene-based nano-electronic and -photonic devices. The optical properties of BBG are dominated by strongly bound excitons. We perform ab initio density-functional-theory+Bethe-Salpeter-equation modelling of excitons in BBG and calculate the exciton band structures and optical matrix elements for field strengths in the range $30-300 \, \text{mV}/{\unicode{x212B}}$. The exciton properties prove to have a strong field dependence, with both energy ordering and dipole alignment varying significantly between the low and high field regions. Namely, at low fields we find a mostly dark ground state exciton, as opposed to high fields, where the lowest exciton is bright. Also, excitons preferentially align with a dipole moment opposite the field, due to the field-induced charge transfer in the ground state of BBG. However, in stronger fields, this alignment becomes energetically less favorable. Additionally, the bright excitons show particle- and light-like bands similar to monolayer transition metal dichalchogenides. Finally, we model the radiative lifetimes and emission properties of BBG, which prove to be strongly dependent on temperature in addition to field strength.

Growth of two-dimensional material black phosphorus guided by gold substrates and its growth mechanism

<p indent=0mm>Two-dimensional (2D) materials are the frontier research direction of material sciences and condensed matter physics in recent years. Among different kinds of 2D materials, black phosphorus (BP) has received extensive attention due to its unique band gap and excellent optical and electrical anisotropy properties. Compared with graphene and transition metal dichalchogenides (TMDCs), BP stands out for its unique properties with widely tunable direct bandgaps ranging from <sc>0.3–1.7 eV,</sc> covering the wavelength from ~600 to <sc>4000 nm,</sc> which fill in the gap between graphene <sc>(0 eV)</sc> and TMDCs (~1.0–2.5 eV) and is considered the ideal material for optoelectronic applications, such as light detection and light modulation. For example, photodetectors are basic devices for optoelectronics, as they transform the information stored in light into electric signals. BP based photodetectors not only stand out for their wide detection range reaching far infrared (IR) region, but also for their high responsivity and high gain, demonstrating their excellent performance for mid-IR detection. In order to achieve above-mentioned applications, few or even monolayer BP flakes or continuous BP films are required. However, how to control the preparation of thin layer of BP is one of the biggest challenges in this field. In this paper, we carried out a systematic study on the growth of BP on different substrates and found that Au can guide and promote the growth of BP, thereby controlling its growth. Based on the above findings, the growth of BP nanosheets on the Au-Si substrate is achieved by using a Si substrate deposited with Au thin film on top, and by designing a strategy with gradient temperature and in confined space. We observed that the BP nanosheets grown by this system exhibited different shapes such as ribbons and flakes, and their thicknesses can be reduced to be about <sc>51 nm</sc> with a lateral size of around <sc>10 µm.</sc> In addition, by systematically tuning the growth parameters in the system, we found that the growth temperatures and the amount of red phosphorous loading are the key parameters for the growth of BP. With certain temperatures and precursor loadings, we achieve the growth of BP nanosheets, while by-products would be obtained on the substrate with other parameters. In our previous work, we found that the temperature gradients play a crucial role in the yield, purity, and crystal quality of bulk BP growth. Therefore, further optimization of temperature distribution is of great importance for the growth of BP nanosheets. In addition, we observed in this paper that Au would guide the nucleation and growth of BP. Therefore, it is important to regulate Au-P compounds during growth of BP, as on the one hand, it can control the by-products during the reaction and on the other hand, it can also improve the controllability of BP nucleation. Based on this, we believe that optimization of the temperature distribution and the growth regulation of Au-P compounds are important research directions, as to precisely control and reduce the thickness of BP nanosheets to grow few layer BP. The growth system proposed in this paper improves the chemical vapor transport (CVT) growth of traditional bulk BP from three aspects: Substrate design, confined space design, and temperature gradient control, which have paved the way for the future controllability growth of BP nanosheets and have deepened the understanding of BP growth mechanism.

Optical properties of the nanocomposite of molybdenum disulphide monolayers/cellulose nanofibrils

Studies on nanocomposites of cellulose nanofibrils (CNFs) and transition-metal dichalchogenides (TMD) monolayers are still rare in the literature. Among the TMDs, the molybdenum disulphide (MoS2) monolayers have been employed in a great variety of applications, such as sensors, photovoltaic devices, and transistors. The synthesis of the MoS2/CNF nanocomposite was performed through a liquid exfoliation process and the optical and morphological properties of MoS2/CNF films were characterized by confocal microscopy and UV–Vis absorption and the nanocomposite morphology was characterized by atomic force microscopy (AFM) and by field emission scanning electron microscopy (FESEM) techniques. UV–Vis measurements showed that the CNF addition caused a narrowing in the MoS2 flakes absorption band at 1.89 eV and 2.0 eV energy values, similar to those present in a MoS2 monolayer. Morphological analyses of the MoS2/CNF composite by FESEM indicate that the CNF addition reduces the MoS2 flakes anisotropy and corroborates the absorption narrowing bands. Stationary Photoluminescence (PL) and Second Harmonic Generation (SHG) were carried out by Confocal Microscopy indicating the presence of few layers composing the MoS2 flakes. The PL and SHG arise due to the liquid exfoliation process, which produces heterogenic flakes with randomic numbers of MoS2 layers. Results showed that the addition of up to 10% w/w of CNFs gives rise to a high fluorescence gain. However, the amount of SH regions increases as a function of the addition of CNFs above 10% w/w.

Pressure effect on the electronic, structural, and vibrational properties of layered 2H−MoTe2

Layered molybdenum dichalchogenides differ from the classic example of bilayer graphene with their unique electronic properties: the application of pressure can continuously tune electronic structure since the band gap is controlled by delicate interlayer interaction. Here, we have performed measurements of Raman scattering, synchrotron x-ray diffraction, electrical conductivity, and Hall coefficient combined with density functional theory calculations to synthetically study the pressure effect on $2H\ensuremath{-}{\mathrm{MoTe}}_{2}$. Both the experiments and calculations consistently demonstrate that ${\mathrm{MoTe}}_{2}$ undergoes a semiconductor-to-metallic (S-M) transition above 10 GPa. Unlike ${\mathrm{MoS}}_{2}$, the S-M transition is driven by the gradual tunability of electric structure and band gap without structural transition. The applied pressure also effectively enhances conductivity and carrier concentration while reducing the mobility, which makes ${\mathrm{MoTe}}_{2}$ more suitable for applications than most other transition-metal dichalchogenides and allows it to be applied in strain-modulated optoelectronic devices.

WS2/CoSe2 heterostructure: A designed structure as catalysts for enhanced hydrogen evolution performance

The development of hydrogen-evolving catalysts based on transition metal dichalchogenides (TMDs) is receiving a great attention for practical application of water-splitting devices and fuel cells due to their high electrocatalytic activity. Herein, we synthesized tungsten disulfide (WS2)/cobalt diselenide (CoSe2) hybrid catalyst for hydrogen evolution reaction (HER). CoSe2 films were first deposited via electrodeposition of Co and followed by selenization process. And, the surface of the synthesized CoSe2 films was covered with WS2 via combined process of sputtering and sulfurization. In acidic media, the WS2/CoSe2 heterostructure catalyst exhibited fast hydrogen evolution kinetics of onset potential and Tafel slope were at 95mV and 44mVdecade−1, respectively with the excellent electrocatalytic stability over 20h. WS2/CoSe2 heterostructure electrode demonstrates an excellent HER activity and long-term stability owing to their abundant active edge sites, and the strong chemical and electronic coupling between the CoSe2 and WS2.

Revealing the Topology of Fermi-Surface Wave Functions from Magnetic Quantum Oscillations

The modern semiclassical theory of a Bloch electron in a magnetic field now encompasses the orbital magnetic moment and the geometric phase. These two notions are encoded in the Bohr-Sommerfeld quantization condition as a phase ($\lambda$) that is subleading in powers of the field; $\lambda$ is measurable in the phase offset of the de Haas-van Alphen oscillation, as well as of fixed-bias oscillations of the differential conductance in tunneling spectroscopy. In some solids and for certain field orientations, $\lambda/\pi$ are robustly integer-valued owing to the symmetry of the extremal orbit, i.e., they are the topological invariants of magnetotransport. Our comprehensive symmetry analysis identifies solids in any (magnetic) space group for which $\lambda$ is a topological invariant, as well as identifies the symmetry-enforced degeneracy of Landau levels. The analysis is simplified by our formulation of ten (and only ten) symmetry classes for closed, Fermi-surface orbits. Case studies are discussed for graphene, transition metal dichalchogenides, 3D Weyl and Dirac metals, and crystalline and $\mathbb{Z}_2$ topological insulators. In particular, we point out that a $\pi$ phase offset in the fundamental oscillation should \emph{not} be viewed as a smoking gun for a 3D Dirac metal.

Effect of pressure on crystal structure and superconductivity of NbSexTe2−x (x = 2, 1.5)

The high pressure structural and transport behaviors of transition metal dichalchogenides NbSexTe2−x (x = 2, 1.5) were investigated using powder X-ray diffraction and resistivity measurements. The structural results show all these title compounds remain stable under application of pressure up to 39 GPa. The lattice compressibilites were determined and show highly anisotropic behavior. A change in superconducting transition temperature dependent on composition was observed. The resistivity measurements performed under pressure for NbSe1.5Te0.5 revealed strong enhancement in transition temperature with Tc increasing from 3.35 K at ambient, to 4.4 K at 1.8 GPa, at a rate of 0.6 K/GPa.

Spin-dependent electron-phonon coupling in the valence band of single-layer WS2

The absence of inversion symmetry leads to a strong spin-orbit splitting of the upper valence band of semiconducting single layer transition metal dichalchogenides such as MoS$_2$ or WS$_2$. This permits a direct comparison of the electron-phonon coupling strength in states that only differ by their spin. Here, the electron-phonon coupling in the valence band maximum of single-layer WS$_2$ is studied by first principles calculations and angle-resolved photoemission. The coupling strength is found to be drastically different for the two spin-split branches, with calculated values of $\lambda_K=$0.0021 and 0.40 for the upper and lower spin-split valence band of the free-standing layer, respectively. This difference is somewhat reduced when including scattering processes involving the Au(111) substrate present in the experiment and the experimental results confirm the strongly branch-dependent coupling strength.

Exciton spectrum in two-dimensional transition metal dichalcogenides: The role of Diracness

The physics of excitons, electron-hole pairs that are bound together by their mutual Coulomb attraction, can to great extent be understood in the framework of the quantum-mechanical hydrogen model. This model has recently been challenged by spectroscopic measurements on two-dimensional transition-metal dichalchogenides that unveil strong deviations from a hydrogenic spectrum. Here, we show that this deviation is due to the particular relativistic character of electrons in this class of materials. Indeed, their electrons are no longer described in terms of a Schrödinger but a massive Dirac equation that intimately links electrons to holes. Dirac excitons therefore inherit a relativistic quantum spin-1/2 that contributes to the angular momentum and thus the exciton spectrum. Most saliently, the level spacing is strongly reduced as compared to the hydrogen model, in agreement with spectroscopic measurements and ab-initio calculations.

Strong enhancement of superconductivity at high pressures within the charge-density-wave states of2H−TaS2and2H−TaSe2

We present measurements of the superconducting and charge density wave critical temperatures (Tc and TCDW) as a function of pressure in the transition metal dichalchogenides 2H-TaSe2 and 2H-TaS2. Resistance and susceptibility measurements show that Tc increases from temperatures below 1 K up to 8.5 K at 9.5 GPa in 2H-TaS2 and 8.2 K at 23 GPa in 2H-TaSe2. We observe a kink in the pressure dependence of TCDW at about 4 GPa that we attribute to the lock-in transition from incommensurate CDW to commensurate CDW. Above this pressure, the commensurate TCDW slowly decreases coexisting with superconductivity within our full pressure range.

Properties of two-dimensional graphene-like materials

With the discovery of graphene, the interest in other two-dimensional (2D) layered materials became renewed. Owing to their variety, since they can range from being metallic to semiconducting or insulating in nature, transition metal dichalchogenides have been one of the most studied families of materials. Furthermore, it has been shown that they are unstable against folding, forming nanotubes and fullerene-like nanoparticles. Due to their weak interlayer bonding, they are perfect candidates for thinning into monolayer flakes. Their ability to form structures of different dimensionalities opens up a broad range of potential applications and research possibilities. While nanotubes and nanoparticles of these materials have already been implemented in industry, mainly as lubricants, the various potential applications of their 2D counterparts are still under investigation. So far, their prospects for implementation are mainly associated with the bandgap transformation from indirect to direct as their thickness decreases to single layers, opening up dramatic enhancement of photoluminescence quantum yield. With many more interesting properties of 2D layered materials being discovered, it can be expected that the field will grow significantly in the future.

Pi-Extended 2D-Networks of Heteroaromatic Compounds - Towards Two-Dimensional Nanostructured Materials

Graphene and other two-dimensional (2D) materials such as inorganic transition metal dichalchogenides (TMDs) represent key nanostructural materials for use in a range of functional devices including gas and biosensors. The functionalization of nanomaterials is a critical factor for tailoring their surface properties, enhancing their potential applicability in optoelectronics and catalysis to name but a few. Porphyrins can easily be used for the construction of π-extended systems using well-established synthetic chemistry methodologies. [1] These range from organometallic C-C bond forming reactions to introduce tailored substituents at the porphyrin periphery and the construction of porphyrin arrays connected conjugated linkers, to porphyrin tape-like oligomers through oxidative meso-meso and meso-β-fusing reactions. The selective functionalization of these heteroaromatic systems now opens the possibility to combine them with carbon-based aromatic systems to yield mixed π-extended systems with unique photophysical properties. Present studies reveal the emergence of a surface chemistry of such systems where specific reactions, e.g., metallation, remetallation and C-C–coupling can be induced on surfaces. [2] Surface/macrocycle interactions alter the chemical properties of porphyrins and the adsorption of tailored macrocycles onto 2D surfaces could generate novel platform systems for, e.g., the attachment of biomarkers for use as biosensors. Additionally, TMDs, e.g., MoS2, represent a growing area of research due to their fascinating electronic and optoelectronic properties. [3] Through chemical modification of these surfaces it is hoped that attachment of large heteroaromatic systems will further tailor these materials for functional use.

Indirect-to-direct band gap crossover in few-layer MoTe₂.

We study the evolution of the band gap structure in few-layer MoTe2 crystals, by means of low-temperature microreflectance (MR) and temperature-dependent photoluminescence (PL) measurements. The analysis of the measurements indicate that in complete analogy with other semiconducting transition metal dichalchogenides (TMDs) the dominant PL emission peaks originate from direct transitions associated with recombination of excitons and trions. When we follow the evolution of the PL intensity as a function of layer thickness, however, we observe that MoTe2 behaves differently from other semiconducting TMDs investigated earlier. Specifically, the exciton PL yield (integrated PL intensity) is identical for mono and bilayer, decreases slightly for trilayer, and it is significantly lower in the tetralayer. The analysis of this behavior and of all our experimental observations is fully consistent with mono and bilayer MoTe2 being direct band gap semiconductors with tetralayer MoTe2 being an indirect gap semiconductor and with trilayers having nearly identical direct and indirect gaps. This conclusion is different from the one reached for other recently investigated semiconducting transition metal dichalcogenides for which monolayers are found to be direct band gap semiconductors, and thicker layers have indirect band gaps that are significantly smaller (by hundreds of meV) than the direct gap. We discuss the relevance of our findings for experiments of fundamental interest and possible future device applications.

Intrinsic disorder in graphene on transition metal dichalcogenide heterostructures.

Semiconducting transition metal dichalchogenides (TMDs) are a family of van der Waals bonded materials that have recently received interest as alternative substrates to hexagonal boron nitride (hBN) for graphene, as well as for components in novel graphene-based device heterostructures. We elucidate the local structural and electronic properties of graphene on TMD heterostructures through scanning tunneling microscopy and spectroscopy measurements. We find that crystalline defects intrinsic to TMDs induce substantial electronic scattering and charge carrier density fluctuations in the graphene. These signatures of local disorder explain the significant degradation of graphene device mobilities using TMD substrates, particularly compared to similar graphene on hBN devices.

Functionalization of Liquid‐Exfoliated Two‐Dimensional 2H‐MoS2

AbstractLayered two‐dimensional (2D) inorganic transition‐metal dichalchogenides (TMDs) have attracted great interest as a result of their potential application in optoelectronics, catalysis, and medicine. However, methods to functionalize and process such 2D TMDs remain scarce. We have established a facile route towards functionalized layered MoS2. We found that the reaction of liquid‐exfoliated 2D MoS2, with M(OAc)2 salts (M=Ni, Cu, Zn; OAc=acetate) yielded functionalized MoS2–M(OAc)2 materials. Importantly, this method furnished the 2H‐polytype of MoS2 which is a semiconductor. X‐ray photoelectron spectroscopy (XPS), diffuse reflectance infrared Fourier transform spectroscopy (DRIFT–IR), and thermogravimetric analysis (TGA) provide strong evidence for the coordination of MoS2 surface sulfur atoms to the M(OAc)2 salt. Interestingly, functionalization of 2H‐MoS2 allows for its dispersion/processing in more conventional laboratory solvents.

Functionalization of liquid-exfoliated two-dimensional 2H-MoS2.

Layered two-dimensional (2D) inorganic transition-metal dichalchogenides (TMDs) have attracted great interest as a result of their potential application in optoelectronics, catalysis, and medicine. However, methods to functionalize and process such 2D TMDs remain scarce. We have established a facile route towards functionalized layered MoS2 . We found that the reaction of liquid-exfoliated 2D MoS2 , with M(OAc)2 salts (M=Ni, Cu, Zn; OAc=acetate) yielded functionalized MoS2 -M(OAc)2 materials. Importantly, this method furnished the 2H-polytype of MoS2 which is a semiconductor. X-ray photoelectron spectroscopy (XPS), diffuse reflectance infrared Fourier transform spectroscopy (DRIFT-IR), and thermogravimetric analysis (TGA) provide strong evidence for the coordination of MoS2 surface sulfur atoms to the M(OAc)2 salt. Interestingly, functionalization of 2H-MoS2 allows for its dispersion/processing in more conventional laboratory solvents.

Multiflake Thin Film Electronic Devices of Solution Processed 2D MoS2 Enabled by Sonopolymer Assisted Exfoliation and Surface Modification

The solvent-assisted exfoliation of transition metal dichalchogenides (TMDs) is a promising method for preparing scalable quantities of two-dimensional nanomaterials dispersed in a liquid phase. However, low concentrations and the restacking/aggregation of TMD layers remain challenges to the solution based preparation of large-area electronic devices. Here we present advances in the exfoliation and solution processing of 2D MoS2 that are subsequently leveraged to prepare and electronically probe homogeneous multiflake thin-film devices. We report that sonopolymer, formed when using 1,2-dichlorobenzene (DCB) as a solvent, plays a critical role in affording stable dispersions (up to 0.5 mg mL–1) of few-layer MoS2 flakes in the 2H phase after only 6 h of low-power sonication. After removing the sonopolymer using a washing procedure, alkyl-trichlorosilane surfactants were used to prevent the restacking of 2D MoS2 layers and create stable dispersions with concentrations as high as 85 mg mL–1. In spin-coated multiflake thin films as thin as 20 nm, electron transport parallel to the substrate was quantifiable over channel lengths of 50 μm owing to the homogeneous film formation. By further varying the alkyl-trichlorosilane chain length we show that a trade-off between dispersibility (film homogeneity) and electronic insulation from the surfactant leads to a maximum (multiflake) electron mobility of 0.02 cm2 V–1 s–1 using hexyl-trichlorosilane modified MoS2 in the direction perpendicular to the substrate as measured by space-charge limited current devices.

The charge bag mechanism of superconductivity in carrier injected charge density wave systems

A new mechanism is proposed for superconductivity (SC) arising in carrier injected charge density wave (CDW) insulators. It is demonstrated that an attractive interaction between the injected charge carriers is predominantly generated by the exchange of the CDW-amplitude mode, which is a high frequency optic phonon of the lattice distorted CDW state. This attractive interaction then produces pairing and hence superconductivity, whose transition temperature ( T c) is enhanced due to an increase in the cutoff frequency as well as the density of states at the Fermi level. It is argued that the model can successfully explain the high temperature superconductivity observed in the perovskite compound Ba 1− x K x BiO 3. An understanding of the enhancement of T c on intercalation in certain layered transition metal dichalchogenides can also be achieved in terms of this charge bag model.