Probing Symmetry Properties of Few-Layer MoS<sub>2</sub> and h-BN by Optical Second-Harmonic Generation

Li, Yilei; Rao, Yi; Mak, Kin Fai; You, Yu‐Meng; Wang, Shuyuan; Dean, Cory; Heinz, Tony F.

doi:10.1021/nl401561r

Cited by 923 publications

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References 33 publications

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“…To apply an electric field along the armchair direction, the electrodes were designed to be parallel to or at 60°with respect to the sharply cleaved edges which form 60°or 120°corners. The direction was confirmed by SHG ( Supplementary Figs 3 and 4) 18,19 . Suspension, mechanical clamping and electrical contact were simultaneously achieved in the areas enclosed by yellow dashed lines in Fig.…”

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confidence: 61%

“…Transition-metal dichalcogenides (TMDCs) are ideal candidates as low-dimensional piezoelectric materials because of their structural non-centrosymmetry 7 . Although there has been extensive research interest in the unique properties originating from such symmetry breaking, including circular dichroism and second harmonic generation (SHG) [15][16][17][18][19] , experimental quantitative determination of the intrinsic piezoelectric properties of these two-dimensional crystals has yet to be demonstrated. Here, we report the observation of piezoelectricity in freestanding monolayer MoS 2 membranes.…”

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confidence: 99%

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Observation of piezoelectricity in free-standing monolayer MoS2

Zhu¹,

Wang²,

Xiao³

et al. 2014

Nature Nanotech

717

571

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Piezoelectricity allows precise and robust conversion between electricity and mechanical force, and arises from the broken inversion symmetry in the atomic structure [1][2][3] . Reducing the dimensionality of bulk materials has been suggested to enhance piezoelectricity 4 . However, when the thickness of a material approaches a single molecular layer, the large surface energy can cause piezoelectric structures to be thermodynamically unstable 5 . Transition-metal dichalcogenides can retain their atomic structures down to the single-layer limit without lattice reconstruction, even under ambient conditions 6 . Recent calculations have predicted the existence of piezoelectricity in these two-dimensional crystals due to their broken inversion symmetry 7 . Here, we report experimental evidence of piezoelectricity in a free-standing single layer of molybdenum disulphide (MoS 2 ) and a measured piezoelectric coefficient of e 11 = 2.9 × 10 -10 C m −1 . The measurement of the intrinsic piezoelectricity in such free-standing crystals is free from substrate effects such as doping and parasitic charges. We observed a finite and zero piezoelectric response in MoS 2 in odd and even number of layers, respectively, in sharp contrast to bulk piezoelectric materials. This oscillation is due to the breaking and recovery of the inversion symmetry of the two-dimensional crystal. Through the angular dependence of electromechanical coupling, we determined the two-dimensional crystal orientation. The piezoelectricity discovered in this single molecular membrane promises new applications in low-power logic switches for computing and ultrasensitive biological sensors scaled down to a single atomic unit cell 8,9 .Since its discovery in 1880, piezoelectricity has found a wide range of applications in actuation, sensing and energy harvesting. The rapidly growing demand for high-performance and miniaturized devices in micro-electro-mechanical systems (MEMS) and electronics 10-12 calls for nanoscale piezoelectric materials, motivating theoretical investigations into novel low-dimensional systems such as nanotubes and single molecules 13,14 . Transition-metal dichalcogenides (TMDCs) are ideal candidates as low-dimensional piezoelectric materials because of their structural non-centrosymmetry 7 . Although there has been extensive research interest in the unique properties originating from such symmetry breaking, including circular dichroism and second harmonic generation (SHG) [15][16][17][18][19] , experimental quantitative determination of the intrinsic piezoelectric properties of these two-dimensional crystals has yet to be demonstrated. Here, we report the observation of piezoelectricity in freestanding monolayer MoS 2 membranes. Interestingly, we found that this molecular piezoelectricity only exists when there are an odd number of layers in the two-dimensional crystal where inversion symmetry breaking occurs. We observed an angular dependence of the piezoelectric response in agreement with the three-fold symmetry of the crystal, and based...

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confidence: 61%

mentioning

confidence: 99%

Observation of piezoelectricity in free-standing monolayer MoS2

Zhu¹,

Wang²,

Xiao³

et al. 2014

Nature Nanotech

717

571

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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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“…The intriguing structural, electronic, and optical properties of many two-dimensional (2D) materials similar to graphene such as the transition-metal dichalcogenides [1][2][3], group IV elements [4,5], and group III-V based 2D materials [6][7][8] have motivated further research for new discoveries. Owing to its attractive intrinsic properties such as high carrier mobility, direct band gap, and superior optical properties, phosphorene has been widely in focus [9][10][11][12].…”

Section: Introductionmentioning

confidence: 99%

Optical properties of single-layer and bilayer arsenene phases

2016

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An extensive investigation of the optical properties of single-layer buckled and washboard arsenene and their bilayers was performed, starting from layered three-dimensional crystalline phase of arsenic using density functional and many-body perturbation theories combined with random phase approximation. Electron-hole interactions were taken into account by solving the Bethe-Salpeter equation, suggesting first bound exciton energies on the order of 0.7 eV. Thus, many-body effects were found to be crucial for altering the optical properties of arsenene. The light absorption of single-layer and bilayer arsenene structures in general falls within the visible-ultraviolet spectral regime. Moreover, directional anisotropy, varying the number of layers, and applying homogeneous or uniaxial in-plane tensile strain were found to modify the optical properties of two-dimensional arsenene phases, which could be useful for diverse photovoltaic and optoelectronic applications.

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Probing Symmetry Properties of Few-Layer MoS₂ and h-BN by Optical Second-Harmonic Generation

Cited by 923 publications

References 33 publications

Observation of piezoelectricity in free-standing monolayer MoS2

Observation of piezoelectricity in free-standing monolayer MoS2

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

Optical properties of single-layer and bilayer arsenene phases

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Probing Symmetry Properties of Few-Layer MoS2 and h-BN by Optical Second-Harmonic Generation

Cited by 923 publications

References 33 publications

Observation of piezoelectricity in free-standing monolayer MoS2

Observation of piezoelectricity in free-standing monolayer MoS2

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

Optical properties of single-layer and bilayer arsenene phases

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Product

Resources

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Probing Symmetry Properties of Few-Layer MoS₂ and h-BN by Optical Second-Harmonic Generation