Infrared Study of the Pressure-Induced Isostructural Metallic Transition in Mo0.5W0.5S2 (original) (raw)
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Pressure-dependent optical and vibrational properties of monolayer molybdenum disulfide
Nano letters, 2015
Controlling the band gap by tuning the lattice structure through pressure engineering is a relatively new route for tailoring the optoelectronic properties of two-dimensional (2D) materials. Here, we investigate the electronic structure and lattice vibrational dynamics of the distorted monolayer 1T-MoS 2 (1T′) and the monolayer 2H-MoS 2 via a diamond anvil cell (DAC) and density functional theory (DFT) calculations. The direct optical band gap of the monolayer 2H-MoS 2 increases by 11.7% from 1.85 to 2.08 eV, which is the highest reported for a 2D transition metal dichalcogenide (TMD) material. DFT calculations reveal a subsequent decrease in the band gap with eventual metallization of the monolayer 2H-MoS 2 , an overall complex structure−property relation due to the rich band structure of MoS 2 . Remarkably, the metastable 1T′-MoS 2 metallic state remains invariant with pressure, with the J 2 , A 1g , and E 2g modes becoming dominant at high pressures. This substantial reversible tunability of the electronic and vibrational properties of the MoS 2 family can be extended to other 2D TMDs. These results present an important advance toward controlling the band structure and optoelectronic properties of monolayer MoS 2 via pressure, which has vital implications for enhanced device applications.
Lecture Notes in Nanoscale Science and Technology, 2013
We review vibrational and electronic properties of single and a few layer MoS 2 relevant to understand their resonant and non-resonant Raman scattering results. In particular, the optical modes and low frequency shear and layer breathing modes show significant dependence on the number of MoS 2 layers. Further, the electron doping of the MoS 2 single layer achieved using top-gating in a field effect transistor renormalizes the two optical modes A 1g and E 1 2g differently due to symmetry-dependent electron-phonon coupling. The issues related to carrier mobility, the Schottky barrier at the MoS 2-metal contact pads and the modifications of the dielectric environment are addressed. The direct optical transitions for single layer-MoS 2 involve two excitons at K-point in the Brillouin zone and their stability with temperature and pressure has been reviewed. Finally, the Fermi-level dependence of spectral shift for a quasiparticle, called trion, has been discussed.
Composition-dependent Raman modes of Mo1-xWxS2 monolayer alloys
Two-dimensional (2D) transition-metal dichalcogenide alloys with tunable band gaps have promising applications in nanoelectronics and optoelectronics. Characterization of structures of 2D alloys, such as composition and atom mixing, is of fundamental importance to their applications. Here, we have conducted systematic Raman spectroscopic studies on Mo 1Àx W x S 2 monolayers (0 # x # 1). First-order Raman modes and second-order Raman modes have been observed in the range of 100-480 cm À1 in the 2D alloys. The out-of-plane A 1 0 modes and in-plane E 0 modes showed one-mode and two-mode behaviors, respectively. The broadening of A 1 0 and E 0 modes in the alloys has been observed. The disorder-related Raman peaks at $360 cm À1 were only observed in the 2D alloys but not in the two end materials. Modified random-element-isodisplacement (MREI) model has been adopted to successfully predict mode behaviors of A 1 0 and E 0 modes in the monolayer alloys. Further, composition-dependent A 1 0 and E 0 frequencies can be well fitted by the MREI model, giving composition-dependent force constants.
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.
The structural phases and vibrational properties of Mo 1−x W x Te 2 alloys
2D Materials, 2017
The structural polymorphism in transition metal dichalcogenides (TMDs) provides exciting opportunities for developing advanced electronics. For example, MoTe 2 crystallizes in the 2H semiconducting phase at ambient temperature and pressure, but transitions into the 1T semimetallic phase at high temperatures. Alloying MoTe 2 with WTe 2 reduces the energy barrier between these two phases, while also allowing access to the T d Weyl semimetal phase. The Mo 1−x W x Te 2 alloy system is therefore promising for developing phase change memory technology. However, achieving this goal necessitates a detailed understanding of the phase composition in the MoTe 2-WTe 2 system. We combine polarization-resolved Raman spectroscopy with X-ray diffraction (XRD) and scanning transmission electron microscopy (STEM) to study Mo 1−x W x Te 2 alloys over the full compositional range x from 0 to 1. We identify Raman and XRD signatures characteristic of the 2H, 1T , and T d structural phases that agree with density-functional theory (DFT) calculations, and use them to identify phase fields in the MoTe 2-WTe 2 system, including single-phase 2H, 1T , and T d regions, as well as a two-phase 1T + T d region. Disorder arising from compositional fluctuations in Mo 1−x W x Te 2 alloys breaks inversion and translational symmetry, leading to the activation of an infrared 1T-MoTe 2 mode and the enhancement of a double-resonance Raman process in 2H-Mo 1−x W x Te 2 alloys. Compositional fluctuations limit the phonon correlation length, which we estimate by fitting the observed asymmetric Raman lineshapes with a phonon confinement model. These observations reveal the important role of disorder in Mo 1−x W x Te 2 alloys, clarify the structural phase boundaries, and provide a foundation for future explorations of phase transitions and electronic phenomena in this system.
Journal of Physical Chemistry C, 2022
Following the rise of interest in the properties of transition metal dichalcogenides, many experimental techniques were employed to research them. However, the temperature dependencies of optical transitions, especially those related to band nesting, were not analyzed in detail for many of them. Here, we present successful studies utilizing the photoreflectance method, which, due to its derivative and absorption-like character, allows investigating direct optical transitions at the high-symmetry point of the Brillouin zone and band nesting. By studying the mentioned optical transitions with temperature from 20 to 300 K, we tracked changes in the electronic band structure for the common transition metal dichalcogenides (TMDs), namely, MoS2, MoSe2, MoTe2, WS2, and WSe2. Moreover, transmission and photoacoustic spectroscopies were also employed to investigate the indirect gap in these crystals. For all observed optical transitions assigned to specific k-points of the Brillouin zone, their temperature dependencies were analyzed using the Varshni relation and Bose–Einstein expression. It was shown that the temperature energy shift for the transition associated with band nesting is smaller when compared with the one at high-symmetry point, revealing reduced average electron–phonon interaction strength.
Following the rise of interest in the properties of transition metal dichalcogenides, many experimental techniques were employed to research them. However, the temperature dependencies of optical transitions, especially those related to band nesting, were not analyzed in detail for many of them. Here, we present successful studies utilizing the photoreflectance method, which, due to its derivative and absorption-like character, allows investigating direct optical transitions at the high-symmetry point of the Brillouin zone and band nesting. By studying the mentioned optical transitions with temperature from 20 to 300 K, we tracked changes in the electronic band structure for the common transition metal dichalcogenides (TMDs), namely, MoS 2 , MoSe 2 , MoTe 2 , WS 2 , and WSe 2. Moreover, transmission and photoacoustic spectroscopies were also employed to investigate the indirect gap in these crystals. For all observed optical transitions assigned to specific k-points of the Brillouin zone, their temperature dependencies were analyzed using the Varshni relation and Bose−Einstein expression. It was shown that the temperature energy shift for the transition associated with band nesting is smaller when compared with the one at high-symmetry point, revealing reduced average electron−phonon interaction strength.
Electrical, electronic and optical properties of MoS2 & WS2
2017
ELECTRICAL, ELECTRONIC and OPTICAL PROPERTIES OF MoS2 & WS2 by Weitao Tang Two dimensional materials such as graphene, boron nitride and transition metal dichalcogenide (TMDCs) monolayers have arisen as a new class of materials with unique properties at monolayer thickness. Their electrical, electronic and optical properties are of great importance for a variety of applications in optoelectronics as light emitters, detectors, and photovoltaic devices. This work focuses on MoS2 and WS2, which are two important members of the TMDC class of materials. The properties of monolayer MoS2 and WS2 are investigated as well as the properties of bulk MoS2 and WS2 to provide an understanding of their significant difference. A detailed investigation of the electrical and electronic properties including temperature dependent resistivity, contact resistance, band structure and electronic excitation are discussed in this work. The temperature dependence of the energy gap for monolayer MoS2 and WS2 i...
Piezoreflectance and Raman Characterization of Mo1− xWxS2 Layered Mixed Crystals
Solid State Phenomena, 2011
A systematic optical characterization of a series of Mo 1-x W x S 2 (0 ≤ x ≤ 1) layered mixed crystals grown by chemical vapour transport method were carried out by using piezoreflectance (PzR) and Raman scattering measurements. From a detailed lineshape fit of the PzR spectra over an energy range from 1.6 to 5.0 eV, the energies of the band-edge excitonic and higher lying interband transitions were determined accurately. The transition energies and their splittings vary smoothly with the tungsten composition x indicating that the nature of the band structure is similar for the Mo 1-x W x S 2 series compounds. The peaks of the two dominant first-order Raman-active modes, g A 1 and 1 2 g E , and several second-order bands are observed in the range of 250-450 cm -1 . The peaks corresponding to g A 1 mode show a one-mode type behavior, while the peaks of 1 2 g E mode demonstrate two-mode type behavior for the entire series of Mo 1-x W x S 2 crystals. These behaviors were discussed on the basis of the lattice vibrational properties of 2H-MoS 2 and 2H-WS 2 compounds.