Second harmonic microscopy of monolayer MoS<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mrow /><mml:mn>2</mml:mn></mml:msub></mml:math>

Kumar, Nardeep; Najmaei, Sina; Cui, Qiannan; Ceballos, Frank; Ajayan, Pulickel M.; Lou, Jun; Zhao, Hui

doi:10.1103/physrevb.87.161403

Cited by 635 publications

(637 citation statements)

References 55 publications

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“…For reasons of consistency between different phases, our calculations on 2H use a nonprimitive rectangular unit cell whose axes align with zigzag and armchair directions of the structure. These special axes can be experimentally identified using second harmonic generation 35,36 , and possibly also using the intrinsic piezoelectricity predicted to exist in these materials 37 . Figure 1 shows the 2H structure within a rectangular unit cell having lattice constants a and b.…”

Section: Resultsmentioning

confidence: 99%

Structural phase transitions in two-dimensional Mo- and W-dichalcogenide monolayers

2014

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Mo-and W-dichalcogenide compounds have a two-dimensional monolayer form that differs from graphene in an important respect: it can potentially have more than one crystal structure. Some of these monolayers exhibit tantalizing hints of a poorly understood structural metal-to-insulator transition with the possibility of long metastable lifetimes. If controllable, such a transition could bring an exciting new application space to monolayer materials beyond graphene. Here we discover that mechanical deformations provide a route to switching thermodynamic stability between a semiconducting and a metallic crystal structure in these monolayer materials. Based on state-of-the-art density functional and hybrid Hartree-Fock/density functional calculations including vibrational energy corrections, we discover that MoTe 2 is an excellent candidate phase change material. We identify a range from 0.3 to 3% for the tensile strains required to transform MoTe 2 under uniaxial conditions at room temperature. The potential for mechanical phase transitions is predicted for all six studied compounds.

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Section: Resultsmentioning

confidence: 99%

Structural phase transitions in two-dimensional Mo- and W-dichalcogenide monolayers

2014

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“…For example, experiments like photoluminescence [25,170] and second harmonic generation [42,43] are strongly influenced by excitonic effects. The most common excitonic effects are a redshift in the optical gap (with respect to the quasiparticle gap) and, in some cases, a radical change in the optical spectra shape with respect to the independent particle spectra.…”

Section: Effective Charge Carrier Massesmentioning

confidence: 99%

“…The control of the valley polarization of the photogenerated electron-hole pairs paves the way for using MoS 2 in applications related to next-generation spin-and valleytronics [35,36,37,38,39]. Further studies dealing with charged exciton complexes (trions) [40,41] or with second harmonic generation have also been published [42,43].…”

Section: Introductionmentioning

confidence: 99%

Vibrational and optical properties of MoS2: From monolayer to bulk

Molina-Sánchez

Hummer

Wirtz

2015

Surface Science Reports

216

167

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Molybdenum disulfide, MoS 2 , has recently gained considerable attention as a layered material where neighboring layers are only weakly interacting and can easily slide against each other. Therefore, mechanical exfoliation allows the fabrication of single and multi-layers and opens the possibility to generate atomically thin crystals with outstanding properties. In contrast to graphene, it has an optical gap of 1.9 eV. This makes it a prominent candidate for transistor and opto-electronic applications. Single-layer MoS 2 exhibits remarkably different physical properties compared to bulk MoS 2 due to the absence of interlayer hybridization. For instance, while the band gap of bulk and multi-layer MoS 2 is indirect, it becomes direct with decreasing number of layers. In this review, we analyze from a theoretical point of view the electronic, optical, and vibrational properties of single-layer, few-layer and bulk MoS 2 . In particular, we focus on the effects of spinorbit interaction, number of layers, and applied tensile strain on the vibrational and optical properties. We examine the results obtained by different methodologies, mainly ab initio approaches. We also discuss which approximations are suitable for MoS 2 and layered materials. The effect of external strain on the band gap of single-layer MoS 2 and the crossover from indirect to direct band gap is investigated. We analyze the excitonic effects on the absorption spectra. The main features, such as the double peak at the absorption threshold and the high-energy exciton are presented. Furthermore, we report on the phonon dispersion relations of single-layer, few-layer and bulk MoS 2 . Based on the latter, we explain the behavior of the Raman-active A 1g and E 1 2g modes as a function of the number of layers. Finally, we compare theoretical and experimental results of Raman, photoluminescence, and optical-absorption spectroscopy.

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“…Different non-linear optical phenomena like second-, third-harmonic generation and four wave mixing (FWM) [5][6][7] can be quite strong in these materials [8][9][10][11][12][13][14][15]. However, the interpretation of the nonlinear optical response is strongly affected by electronic and phonon resonances [16][17][18][19], therefore the knowledge of the interplay between these resonances is desirable.…”

mentioning

confidence: 99%

Anomalous Nonlinear Optical Response of Graphene Near Phonon Resonances

et al. 2017

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In this work we probe the third-order non-linear optical property of graphene, hexagonal boron nitride and their heterostructure by the use of coherent anti-Stokes Raman Spectroscopy. When the energy difference of the two input fields match the phonon energy, the anti-Stokes emission intensity is enhanced in h-BN, as usually expected while for graphene a anomalous decrease is observed. This behaviour can be understood in terms of q coupling between the electronic continuum and a discrete phonon state. We have also measured a graphene/h-BN heterostructure and demonstrate that the anomalous effect in graphene dominates the heterostructure optical response.Two-dimensional materials like graphene, hexagonal boron nitride (h-BN) and heterostructures exibit novel physical properties and promisses different applications in electronics and photonics [1][2][3][4]. Different non-linear optical phenomena like second-, third-harmonic generation and four wave mixing (FWM) [5][6][7] can be quite strong in these materials [8][9][10][11][12][13][14][15]. However, the interpretation of the nonlinear optical response is strongly affected by electronic and phonon resonances [16][17][18][19], therefore the knowledge of the interplay between these resonances is desirable.Here we measured the third order optical emission by the degenerated four wave mixing emission of graphene, h-BN and their heterostructure near phonon resonances. We show that while the FWM signal in h-BN shows the expected enhancement, in graphene the signal is decreased exactly at the phonon resonance. These results are explained in terms of interference effects between the electronic continuum and discrete phonon states for these two different materials. We also show that this unusual effect in graphene dominates the optical response in the graphene/h-BN heterostructure.Four wave mixing is a third-order non-linear optical phenomena, where three frequencies are combined to generate a forth [7]. In this work we are restricted to the case of degenerate FWM, where two photons of frequency ω 1 combines with a photon of ω 2 at the material and generate the emission of another photon with frequency ω 4 . The energy conservation in this case is given by ω 4 = 2 ω 1 − ω 2 . Hendry et al. [8] have measured the FWM intensity in graphene as a function of the pump laser energy and its third order optical non linear optical property was characterized. Also different theoretical works have calculated the third-order optical conductivity of graphene [20][21][22][23], showing the importance of different physical quantities like Fermi energy or temperature. However these works did not treat the problem of the third order optical nonlinearity near phonon resonances. The so-called coherent anti-Stokes Raman spectroscopy (CARS) is a special case of FWM when the energy difference between ω 1 and ω 2 matches a phonon energy ( ω ph ), then ω 4 corresponds exactly to the anti-Stokes frequency in Raman scattering. In general, when the energy condition ω 1 − ω 2 = ω ph is satisfied, the ω ...

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Second harmonic microscopy of monolayer MoS2

Cited by 635 publications

References 55 publications

Structural phase transitions in two-dimensional Mo- and W-dichalcogenide monolayers

Structural phase transitions in two-dimensional Mo- and W-dichalcogenide monolayers

Vibrational and optical properties of MoS2: From monolayer to bulk

Anomalous Nonlinear Optical Response of Graphene Near Phonon Resonances

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