Structure–Property Relationships in Transition Metal Dichalcogenide Bilayers under Biaxial Strains (original) (raw)
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Journal of Applied Physics, 2014
We characterize the electronic structure and elasticity of monolayer transitionmetal dichalcogenides MX 2 (M=Mo, W, Sn, Hf and X=S, Se, Te) with 2H and 1T structures using fully relativistic first principles calculations based on density functional theory. We focus on the role of strain on the band structure and band alignment across the series 2D materials. We find that strain has a significant effect on the band gap; a biaxial strain of 1% decreases the band gap in the 2H structures, by as a much 0.2 eV in MoS 2 and WS 2 , while increasing it for the 1T materials. These results indicate that strain is a powerful avenue to modulate their properties; for example, strain enables the formation of, otherwise impossible, broken gap heterostructures within the 2H class. These calculations provide insight and quantitative information for the rational development of heterostructures based on these class of materials accounting for the effect of strain.
Journal of Applied Physics, 2014
Tunability of the electronic properties of two-dimensional bilayer hetero structures of transitionmetal dichalcogenides (i.e., MX 2-M 0 X 0 2 with (M, M 0 ¼ Mo, W; X, X 0 ¼ S, Se) is investigated. Application of both strain and electric field is found to modify the band gap and carrier effective mass in the hybrid bilayers considered. The calculated results based on density functional theory suggest that the tensile strain considerably changes the band gap of semiconducting bilayers; it makes the band gap to be indirect, and later initiates the semiconductor-to-metal transition. Application of the external electric fields, on the other hand, shows asymmetric variation in the band gap leading to the closure of the gap at about 0.5-1.0 V/Å. Tuning of the band gap and carrier effective mass in such a controlled manner makes the hybrid bilayers of transition metal dichalcogenides to be promising candidates for application in electronic devices at nanoscale.
Physical Review B, 2012
Using the first-principles calculations, we explore the electronic structures of 2H-MX 2 (M = Mo, W; X = S, Se, Te). When the number of layers reduces to a single layer, the indirect gap of bulk becomes a direct gap with larger gap and the band curvatures are found to lead to the drastic changes of effective masses. On the other hand, when the strain is applied on the single layer, the direct gap becomes an indirect gap and the effective masses vary. Especially, the tensile strain reduces the gap energy and effective masses while the compressive strain enhances them. Furthermore, the much larger tensile stress leads to become metallic.
Electron Density and Its Relation with Electronic and Optical Properties in 2D Mo/W Dichalcogenides
Nanomaterials, 2020
Two-dimensional MX2 (M = Mo, W; X = S, Se, Te) homo- and heterostructures have attracted extensive attention in electronics and optoelectronics due to their unique structures and properties. In this work, the layer-dependent electronic and optical properties have been studied by varying layer thickness and stacking order. Based on the quantum theory of atoms in molecules, topological analyses on interatomic interactions of layered MX2 and WX2/MoX2, including bond degree (BD), bond length (BL), and bond angle (BA), have been detailed to probe structure-property relationships. Results show that M-X and X-X bonds are strengthened and weakened in layered MX2 compared to the counterparts in bulks. X-X and M-Se/Te are weakened at compressive strain while strengthened at tensile strain and are more responsive to the former than the latter. Discordant BD variation of individual parts of WX2/MoX2 accounts for exclusively distributed electrons and holes, yielding type-II band offsets. X-X BL ...
Due to their outstanding properties for optoelectronic and versatile electronic applications, the atomically thin layers of transition-metal dichalcogenide (TMDC) materials have demonstrated a potential candidacy to succeed its analog silicon-based technology. Hence, the elucidation of the most important features of these materials is indispensable. In this study, we provide a theoretical elucidation of the structural, electronic, elastic, and optical characteristics of TMDCs. The study has been carried out by elucidating the material in its two particular forms, namely, bulk and two-dimensional (2D) layered (monolayer). The theoretical investigation was carried out within the framework of the density functional theory (DFT) method using first-principles calculations. The Perdew−Burke−Ernzerhof (PBE) variant of the generalized gradient approximation (GGA) scheme, as performed in the Quantum Espresso package, is used. Van der Waals density functional effects, involving the nonlocal correlation part from the rVV10 and vdW-DF2 methods, were treated to remedy the lack of the long-range vdW interaction. An illustration of the performance of both rVV10 and vdW-DF2 functionalities, with the popular PBE correlations, is elucidated. The Born stability criterion is employed to assess structural stability. The obtained results reveal an excellent stability of both systems. Furthermore, the theoretical results show that band-gap energy is in excellent agreement with experimental and theoretical data. Pugh's rule suggested that both the bulk and MoS 2-2D layered systems are ductile materials. The refractive indices obtained herein are in good agreement with the available theoretical data. Moreover, the theoretical results obtained with the present approach demonstrate the ductility of both systems, namely, the bulk and the MoS 2-2D layered. The results obtained herein hold promise for structural, elastic, and optical properties and pave the way for potential applications in electronic and optoelectronic devices.
We report first principles calculations of the electronic structure of monolayer 1H-MX2 (M = Mo, W; X = S, Se, Te), using the pseudopotential and numerical atomic orbital basis sets based methods within the local density approximation. Electronic band structure and density of states calculations found that the states around the Fermi energy are mainly due to metal d states. From partial density of states we find a strong hybridisation between metal d and chalcogen p states below the Fermi energy. All studied compounds in this work have emerged as new direct band gap semiconductors. The electronic band gap is found to decrease as one goes from sulphides to the tellurides of both Mo and W. Reducing the slab thickness systematically from bulk to monolayers causes a blue shift in the band gap energies, resulting in tunability of the electronic band gap. The magnitudes of the blue shift in the band gap energies are found to be 1.14 eV, 1.16 eV, 0.78 eV, 0.64, 0.57 eV and 0.37 eV for MoS2, WS2, MoSe2, WSe2, MoTe2 and WTe2, respectively, as we go from bulk phase (indirect band gap) to monolayer limit (direct band gap). This tunability in the electronic band gap and transitions from indirect to direct band make these materials potential candidates for the fabrication of optoelectronic devices.
Strain-engineering in two-dimensional transition metal dichalcogenide alloys
Journal of Applied Physics
Two-dimensional (2D) transition metal dichalcogenides (TMDs) are attractive semiconductors for use in electronic, optoelectronic, and spintronic devices. This study examines how the electronic properties of 2D TMDs can be tuned for specific applications through a combination of alloying and applying strain. Group VIB TMDs (MoS2, MoSe2, WS2, and WSe2) are alloyed by mixing in the metal or chalcogen sublattices. Density functional theory is used to model the structures of the alloys at varying compositions and examine the electronic structure of the alloys under biaxial tensile and compressive strain. Alloying results in the continuous monotonic tuning of the direct bandgap between the limits of the pure components, with low bowing coefficients for all alloys. Applying strain results in a transition of the bandgap from direct to indirect at low values of tensile strain and higher values of compressive strain. Strain can also be used to increase or decrease the bandgap with low compres...
Energy Scaling of Compositional Disorder in Ternary Transition‐Metal Dichalcogenide Monolayers
Advanced Electronic Materials, 2021
to play a crucial role as essential building blocks in future high-tech devices. [1-8] Already in the late 1960s, early studies on the structural and electronic properties of bulk TMDs had started and are detailed in a report by Wilson and Yoffe. [9] TMDs are layered materials which combine a transition metal (M: Mo, W, Ti, Zr, etc.) and a chalcogen (X: S, Se, or Te) in the general formula MX 2 with one layer of M atoms sandwiched between two layers of X atoms. [10] Group VIB TMDs with a 2H structural phase (e.g., MoSe 2) are the most explored representatives of such systems. They are characterized by an intrinsic bandgap within the visible and near-infrared regions. [11] Furthermore, the material system exhibits a transition from an indirect to a direct bandgap located at the K or K′ points of the hexagonal Brillouin zone, linked to a decrease in the number of layers from the bulk crystal to a monolayer. [12,13] Moreover, such materials also possess a strong spin-orbit interaction due to the presence of a relatively heavy transition metal along with huge exciton binding energies [14-16] resulting from a strong Coulomb interaction and a lack of dielectric screening. These properties lead to a valence-band splitting that strongly affects Alloying semiconductors are often used to tune the material properties desired for device applications. The price for this tunability is the extra disorder caused by alloying. In order to reveal the features of the disorder potential in alloys of atomically thin transition-metal dichalcogenides (TMDs) such as Mo x W 1−x Se 2 , the exciton photoluminescence is measured in a broad temperature range between 10 and 200 K. In contrast to the binary materials MoSe 2 and WSe 2 , the ternary system demonstrates non-monotonous temperature dependences of the luminescence Stokes shift and of the luminescence linewidth. Such behavior is a strong indication of a disorder potential that creates localized states for excitons and affects the exciton dynamics responsible for the observed non-monotonous temperature dependences. A comparison between the experimental data and the results obtained by Monte Carlo computer simulations provides information on the energy scale of the disorder potential and also on the shape of the density of localized states created by disorder. Statistical spatial fluctuations in the distribution of the chemically different material constituents are revealed to cause the disorder potential responsible for the observed effects. A deeper understanding of the disorderinduced effects is vital for prospective TMD alloy-based devices.
First-principles study of strain-induced charge polarization in a molybdenum disulfide monolayer
Scientia Iranica, 2017
In the presence of elastic planar strain distributions, electronic properties of molybdenum disul de (MoS 2) monolayer are investigated within Density Functional Theory (DFT) calculations as implemented in SIESTA package. Three types of planar strain are considered with some di erent intensity values, and uniaxial strain along the armchair and zigzag directions as well as biaxial strain. We present a systematic study of the strained MoS2 monolayer by focusing on the calculation of Total Density Of State (TDOS), Partial Density Of State (PDOS), electron charge density, and electrostatic potential using post processing tools. In most cases, the states due to Mo atoms have dominant association in the TDOS close to the Fermi level of MoS2 monolayer under strain. As a consequence of the strain, S atom takes electron from Mo atom and becomes negatively charged. In addition, the tensile and compressive strains introduce the charge polarization in two opposite directions per three types of strain for both sheets, which is in line with the experimental study. As another important result, the strain-induced charge polarization is proportional to the intensity value of strain.
Biaxial strain tuning of the optical properties of single-layer transition metal dichalcogenides
npj 2D Materials and Applications
Since their discovery, single-layer semiconducting transition metal dichalcogenides have attracted much attention, thanks to their outstanding optical and mechanical properties. Strain engineering in these two-dimensional materials aims to tune their bandgap energy and to modify their optoelectronic properties by the application of external strain. In this paper, we demonstrate that biaxial strain, both tensile and compressive, can be applied and released in a timescale of a few seconds in a reproducible way on transition metal dichalcogenides monolayers deposited on polymeric substrates. We can control the amount of biaxial strain applied by letting the substrate expand or compress. To do this, we change the substrate temperature and choose materials with a large thermal expansion coefficient. After the investigation of the substrate-dependent strain transfer, we performed microdifferential spectroscopy of four transition metal dichalcogenides monolayers (MoS 2 , MoSe 2 , WS 2 , WSe 2 ) under the application of biaxial strain and measured their optical properties. For tensile strain, we observe a redshift of the bandgap that reaches a value as large as 95 meV/% in the case of single-layer WS 2 deposited on polypropylene. The observed bandgap shifts as a function of substrate extension/compression follow the order MoSe 2 < MoS 2 < WSe 2 < WS 2 . Theoretical calculations of these four materials under biaxial strain predict the same trend for the material-dependent rates of the shift and reproduce well the features observed in the measured reflectance spectra.