Strain induced mobility modulation in single-layer MoS2 (original) (raw)

Phase transition, effective mass and carrier mobility of MoS2 monolayer under tensile strain

We report a computational study on the impact of tensile strain on MoS 2 monolayer. The transition between direct and indirect bandgap structure and the transition between semiconductor and metal phases in the monolayer have been investigated with tensile strain along all direction configurations with both x-axis and y-axis components ε xy (ε x and ε y ). Electron effective mass and the hole effective mass are isotropic for biaxial strain ε xy = ε x = ε y and anisotropic for ε xy with ε x / = ε y . The carrier effective mass behaves differently along different directions in response to the tensile strain. In addition, the impact of strain on carrier mobility has been studied by using the deformation potential theory. The electron mobility increases over 10 times with the biaxial strain: ε x = ε y = 9.5%. Also, the mobility decreases monotonically with the increasing temperature as ∼ T −1 . These results are very important for future nanotechnology based on two-dimensional materials.

Effect of strain on electronic and thermoelectric properties of few layers to bulk MoS2

The sensitive dependence of electronic and thermoelectric properties of MoS 2 on the applied strain opens up a variety of applications in the emerging area of straintronics. Using first principles based density functional theory calculations, we show that the band gap of few layers of MoS 2 can be tuned by applying i) normal compressive (NC), ii) biaxial compressive (BC), and iii) biaxial tensile (BT) strain. A reversible semiconductor to metal transition (S-M transition) is observed under all three types of strain. In the case of NC strain, the threshold strain at which S-M transition occurs increases with increasing number of layers and becomes maximum for the bulk. On the other hand, the threshold strain for S-M transition in both BC and BT strain decreases with the increase in number of layers. The difference in the mechanisms for the S-M transition is explained for different types of applied strain. Furthermore, the effect of strain type and number of layers on the transport properties are also studied using Botzmann transport theory. We optimize the transport properties as a function of number of layers and applied strain. 3L-and 2L-MoS 2 emerge as the most efficient thermoelectric material under NC and BT strain, respectively. The calculated thermopower is large and comparable to some of the best thermoelectric materials. A comparison between the feasibility of these three types of strain is also discussed. arXiv:1407.7522v1 [cond-mat.mtrl-sci]

Strain-engineering the Schottky barrier and electrical transport on MoS2

Nanotechnology

Strain provides an effective means to tune the electrical properties while retaining the native chemical composition of the material. Unlike three-dimensional solids, two-dimensional materials withstand higher levels of elastic strain making it easier to tune various electrical properties to suit the technology needs. In this work we explore the effect of uniaxial tensilestrain on the electrical transport properties of bi-and few-layered MoS2, a promising 2D semiconductor. Raman shifts corresponding to the in-plane vibrational modes show a redshift with strain indicating a softening of the in-plane phonon modes. Photoluminescence measurements reveal a redshift in the direct and the indirect emission peaks signaling a reduction in the material bandgap. Transport measurements show a substantial enhancement in the electrical conductivity with a high piezoresistive gauge factor of ~ 321 superior to that for Silicon for our bi-layered device. The simulations conducted over the experimental findings reveal a substantial reduction of the Schottky barrier height at the electrical contacts in addition to the resistance of MoS2. Our studies reveal that strain is an important and versatile ingredient to tune the electrical properties of 2D materials and also can be used to engineer high-efficiency electrical contacts for future device engineering.

Strain-engineering the Schottky barrier and electrical transport on MoS₂

Nanotechnology, 2020

Mobility enhancement and temperature dependence in top-gated single-layer MoS_{2}

Physical Review B, 2013

The deposition of a high-κ oxide overlayer is known to significantly enhance the room-temperature electron mobility in single-layer MoS2 (SLM) but not in single-layer graphene (SLG). We give a quantitative account of how this mobility enhancement is due to the non-degeneracy of the twodimensional electron gas system in SLM at accessible temperatures. Using our charged impurity scattering model [Ong and Fischetti, Phys. Rev. B 86, 121409 (2012)] and temperature-dependent polarizability, we calculate the charged impurity-limited mobility (µimp) in SLM with and without a high-κ (HfO2) top gate oxide at different electron densities and temperatures. We find that the mobility enhancement is larger at low electron densities and high temperatures because of finite-temperature screening, thus explaining the enhancement of the mobility observed at room temperature. µimp is shown to decrease significantly with increasing temperature, suggesting that the strong temperature dependence of measured mobilities should not be interpreted as being solely due to inelastic scattering with phonons. We also reproduce the recently seen experimental trend in which the temperature scaling exponent (γ) of µimp ∝ T −γ is smaller in top-gated SLM than in bare SLM. Finally, we show that a ∼ 37 percent mobility enhancement can be achieved by reducing the HfO2 thickness from 20 to 2 nm.

Effect of Tensile Strain on Performance Parameters of Different Structures of MoS2 Monolayer

2021

The present work is based on the computational study of MoS2 monolayer and effect of tensile strain on its atomic level structure. The bandgap for MoS2 monolayer, defected MoS2 monolayer and Silicon-doped monolayer are 1.82 eV (direct bandgap), 0.04 (indirect bandgap) and 1.25 eV (indirect bandgap), respectively. The impact of tensile strain (0-0.7 %) on the bandgap and effective mass of charge carriers of these MoS2 structures has been investigated. The bandgap decrease of 5.76 %, 31.86 % and 6.03 % has been observed in the three structures for biaxial strain while the impact of uniaxial strain is quite low. The impact of higher temperature on the bandgap under biaxial tensile strain has been also analyzed in this paper. These observations are extremely important for 2D material-based research for electronic applications.

Moderate strain induced indirect bandgap and conduction electrons in MoS2 single layers

npj 2D Materials and Applications, 2019

MoS2 single layers are valued for their sizeable direct bandgap at the heart of the envisaged electronic and optoelectronic applications. Here we experimentally demonstrate that moderate strain values (~2%) can already trigger an indirect bandgap transition and induce a finite charge carrier density in 2D MoS2 layers. A conclusive proof of the direct-to-indirect bandgap transition is provided by directly comparing the electronic and optical bandgaps of strained MoS2 single layers obtained from tunneling spectroscopy and photoluminescence measurements of MoS2 nanobubbles. Upon 2% biaxial tensile strain, the electronic gap becomes significantly smaller (1.45 ± 0.15 eV) than the optical direct gap (1.73 ± 0.1 eV), clearly evidencing a strain-induced direct to indirect bandgap transition. Moreover, the Fermi level can shift inside the conduction band already in moderately strained (~2%) MoS2 single layers conferring them a metallic character.

Structural stability of single-layer MoS2 under large strain

Journal of physics. Condensed matter : an Institute of Physics journal, 2015

Out-of-plane relaxation can introduce MoS2 in flexible electronic/optoelectronic devices, while under larger strain it is possible to frustrate the structure of MoS2. On the basis of first-principle calculations, the ideal tensile stress strain relations and failure mechanism of single-layer MoS2 structure under large strain is investigated. The instability of phonon modes near the K point results in the decrease of tensile stress under large strain. The relative out-of-plane movement of Mo atoms is found to contribute to the mechanism of the soft phonon mode.

Valley drift and valley current modulation in strained monolayer MoS2

Physical Review B

Elastic-mechanical deformations are found to dramatically alter the electronic properties of monolayer (ML) MoS 2 ; particularly, the low-energy Bloch bands are responsive to a directional strain. In this study, in-plane uniaxial deformation is found to drift the low-energy electron/hole valleys of strained ML-MoS 2 far away from K/K' points in the Brillouin zone (BZ). The amount of drift differs notably from hole to electron bands, where the conduction band minimum (CBM) drifts nearly 2 times more than the valence band maximum (VBM) in response to a progressively increasing strain field (0-10%). The resulting strain-induced valley asymmetry/decoherence can lift the momentum degeneracy of valley carriers at the K point, thereby affecting the low-energy valley excitations (K-valley polarization) in a strained ML-MoS 2 lattice. The quantum origin of this decoherent valley arises from the differences in the Bloch orbital wave functions of electron and hole states at the exciton band edges and their deformation under strain. A higher drift (>1.5 times) is noticed when strain is along the zigzag (ZZ) axis relative to the armchair (AC) axis, which is attributed to a faster decline in Young's modulus and Poisson's ratio (PR) along the ZZ direction. A similar valley drift only in the VBM of uniaxially strained ML-MoS 2 was reported in an earlier local density approximation (LDA) based density functional theory (DFT) study [Q. Zhang et al., Phys. Rev. B 88, 245447 (2013)], where a massive valley drift occurring at the CBM was fully overlooked. Moreover, the giant VBM drift reported therein is 6 times the drift observed in our DFT studies based on spin-orbit coupling (SOC) and Perdew-Burke-Ernzerhof generalized gradient approximation (PBE-GGA) functionals. The physical origin of valley drift has been ascertained in our thorough investigations. The robustness of our approach is substantiated as follows. With progressive increase in strain magnitude (0-10%), the band gap remains direct up to 2% uniaxial tensile strain, under SOC, which accurately reproduces the experimental strain-induced direct-to-indirect band gap transitions occurring at ∼2% strain. Based on LDA-DFT [Q. Zhang et al., Phys. Rev. B 88, 245447 (2013)], this crossover in band gap has been incorrectly reported to occur at a higher value of uniaxial strain of 4%. Moreover, the direct SOC band gap shows a linear redshift at a rate of 51-53 meV/(% of strain), under uniaxial tensile strain, which is in excellent quantitative agreement with experimentally observed rates in the redshift of direct excitonic transitions measured in several optical absorption and photoluminescence (PL) spectroscopy experiments. In addition, the Berry curvature (k) of electron/hole bands gets significantly modulated in strained ML-MoS 2 , where the intensity of the flux profile increases as a function of the magnitude of strain with an opposite drift around K/K', when strained along the ZZ/AC direction. A strong strain-valley coupling leads to an enhancement in the strength of spin-orbit induced spin splitting of bands at VBM/CBM, which is sizably enhanced (∼7 meV) simply by the strain-controlled orbital motions. Our findings are of prime importance in the valley physics of MoS 2. Besides, the important theoretical insights emerging from this work will trigger further experimental investigations on ML-MoS 2 to realize its novel technological potential in nanoelectronics, spintronics, and valleytronics.

Band-like transport in high mobility unencapsulated single-layer MoS2 transistors

Applied Physics Letters, 2013

Ultra-thin MoS 2 has recently emerged as a promising two-dimensional semiconductor for electronic and optoelectronic applications. Here, we report high mobility (>60 cm 2 /Vs at room temperature) field-effect transistors that employ unencapsulated single-layer MoS 2 on oxidized Si wafers with a low level of extrinsic contamination. While charge transport in the sub-threshold regime is consistent with a variable range hopping model, monotonically decreasing field-effect mobility with increasing temperature suggests band-like transport in the linear regime. At temperatures below 100 K, temperature-independent mobility is limited by Coulomb scattering, whereas, at temperatures above 100 K, phonon-limited mobility decreases as a power law with increasing temperature. V C 2013 AIP Publishing LLC. [http://dx.

Strain induced mobility modulation in single-layer MoS2 (original) (raw)

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