Optical Properties and Band Gap of Single- and Few-Layer MoTe<sub>2</sub> Crystals

Ruppert, Claudia; Aslan, Özgür Burak; Heinz, Tony F.

doi:10.1021/nl502557g

Cited by 837 publications

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“…However, both modes appear as faint features in thick MoTe 2 flakes (considered as a bulk reference). This surprising observation, also reported recently on other MX 2 might be a consequence of the finite penetration depth of our laser due to the strong optical absorption of MoTe 2 [2,37] or may arise from a breakdown of the Raman selection rules due to resonance effects [10,14,40]. As predicted by group theory [12][13][14], the iX and oMX modes are not observed in monolayer MoTe 2 (see Figs.…”

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confidence: 86%

“…1) and shares many properties with the widely studied MoS 2 , MoSe 2 , WS 2 and WSe 2 crystals [2,12]. MoTe 2 has recently attracted particular interest due to its lower transport bandgap [33][34][35][36] and near-infrared emission [37,38]. Indeed, at room temperature, monolayer MoTe 2 exhibits a direct optical bandgap at 1.1 eV, whereas the bulk material has an indirect optical bandgap slightly below 1.0 eV [2,37,38].…”

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Unified Description of the Optical Phonon Modes in N-Layer MoTe₂

et al. 2015

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N -layer transition metal dichalcogenides provide a unique platform to investigate the evolution of the physical properties between the bulk (three dimensional) and monolayer (quasi two-dimensional) limits. Here, using high-resolution micro-Raman spectroscopy, we report a unified experimental description of the Γ-point optical phonons in N -layer 2H-molybdenum ditelluride (MoTe2). We observe a series of N -dependent low-frequency interlayer shear and breathing modes (below 40 cm −1 , denoted LSM and LBM) and well-defined Davydov splittings of the mid-frequency modes (in the range 100 − 200 cm −1 , denoted iX and oX), which solely involve displacements of the chalcogen atoms. In contrast, the high-frequency modes (in the range 200 − 300 cm −1 , denoted iMX and oMX), arising from displacements of both the metal and chalcogen atoms, exhibit considerably reduced splittings. The manifold of phonon modes associated with the in-plane and out-of-plane displacements are quantitatively described by a force constant model, including interactions up to the second nearest neighbor and surface effects as fitting parameters. The splittings for the iX and oX modes observed in N -layer crystals are directly correlated to the corresponding bulk Davydov splittings between the E2u/E1g and B1u/A1g modes, respectively, and provide a measurement of the frequencies of the bulk silent E2u and B1u optical phonon modes. Our analysis could readily be generalized to other layered crystals.Keywords: Two-dimensional materials, layered crystals, transition metal dichalcogenides, MoTe2, Raman spectroscopy, interlayer breathing and shear modes, force constants, Davydov splitting, surface effects.Introduction In the wake of graphene, a vast family of layered materials is attracting tremendous attention [1]. Now available in the form of N -layer crystals, the latter exhibit peculiar physical properties that complement the assets of graphene and offer exciting perspectives to design van der Waals heterostructures [1]. Semiconducting transition metal dichalcogenides (MX 2 , with M = Mo, W and X = S, Se, Te) are among the most actively investigated layered crystals [2]. Indeed, although bulk MX 2 exhibit indirect bandgaps, monolayer MX 2 are direct bandgap semiconductors [3,4] with remarkable spin, valley [5] and optoelectronic properties [6]. More Generally, N -layer MX 2 crystals provide an ideal platform to uncover the impact of symmetry breaking and interlayer interactions on the electronic, optical and vibrational properties, from the bulk (three-dimensional) to the monolayer (quasi two-dimensional) limit.In particular, in N -layer MX 2 , interlayer interactions result in a splitting of all the monolayer phonon modes [7][8][9][10][11][12][13][14][15][16] (see Table I). The latter effect is known as the Davydov splitting [17] and is closely related to the force constants that govern the vibrational properties of MX 2 [9]. The Davydov splitting has been previously studied in polyaromatic molecules [18], thin films [19], and bulk layered crystals, ...

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Unified Description of the Optical Phonon Modes in N-Layer MoTe₂

et al. 2015

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“…As it can be appreciated in Ref. 5, the edge states at the M point in MoS 2 have substantial component of S p z orbitals, which point along c-axis of the crystal. A similar band-structure, with the Te p z orbitals contributing to the maximum of the valence band in M point of the Brilloiun zone can be expected in MoTe 2 .…”

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Raman scattering of few-layers MoTe ₂

Grzeszczyk¹,

Gołasa²,

Zinkiewicz³

et al. 2016

2D Mater.

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We report on room-temperature Raman scattering measurements in few-layer crystals of exfoliated molybdenum ditelluride (MoTe2) performed with the use of 632.8 nm (1.96 eV) laser light excitation. In agreement with a recent study reported by G. Froehlicher et al 1 we observe a complex structure of the out-of-plane vibrational modes (A1g/A 1 ), which can be explained in terms of interlayer interactions between single atomic planes of MoTe2. In the case of low-energy shear and breathing modes of rigid interlayer vibrations, it is shown that their energy evolution with the number of layers can be well reproduced within a linear chain model with only the nearest neighbor interaction taken into account. Based on this model the corresponding in-plane and out-of-plane force constants are determined. We also show that the Raman scattering in MoTe2 measured using 514.5 nm (2.41 eV) laser light excitation results in much simpler spectra. We argue that the rich structure of the out-of-plane vibrational modes observed in Raman scattering spectra excited with the use of 632.8 nm laser light results from its resonance with the electronic transition at the M point of the MoTe2 first Brillouin zone.

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“…In the mono-or few-layer limit it is a direct-gap semiconductor, while the bulk has an indirect bandgap 5,17,18 of ∼ 1 eV. The size of the gap is similar to that of Si, making 2H−MoTe 2 particularly appealing for both purely electronic devices 19,20 and optoelectronic applications 21 .…”

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Engineering the Structural and Electronic Phases of MoTe₂ through W Substitution

et al. 2017

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MoTe2 is an exfoliable transition metal dichalcogenide (TMD) which crystallizes in three symmetries; the semiconducting trigonal-prismatic 2H−phase, the semimetallic 1T ′ monoclinic phase, and the semimetallic orthorhombic T d structure 1-4 . The 2H−phase displays a band gap of ∼ 1 eV 5 making it appealing for flexible and transparent optoelectronics. The T d−phase is predicted to possess unique topological properties 6-9 which might lead to topologically protected non-dissipative transport channels 9 . Recently, it was argued that it is possible to locally induce phasetransformations in TMDs 3,10,11,14 , through chemical doping 12 , local heating 13 , or electric-field 14,15 to achieve ohmic contacts or to induce useful functionalities such as electronic phase-change memory elements 11 . The combination of semiconducting and topological elements based upon the same compound, might produce a new generation of high performance, low dissipation optoelectronic elements. Here, we show that it is possible to engineer the phases of MoTe2 through W substitution by unveiling the phase-diagram of the Mo1−xWxTe2 solid solution which displays a semiconducting to semimetallic transition as a function of x. We find that only ∼ 8 % of W stabilizes the T d−phase at room temperature. Photoemission spectroscopy, indicates that this phase possesses a Fermi surface akin to that of WTe2 16 .The properties of semiconducting and of semimetallic MoTe 2 are of fundamental interest in their own right, but are also for their potential technological relevance. In the mono-or few-layer limit it is a direct-gap semiconductor, while the bulk has an indirect bandgap 5,17,18 of ∼ 1 eV. The size of the gap is similar to that of Si, making 2H−MoTe 2 particularly appealing for both purely electronic devices 19,20 and optoelectronic applications 21 . Moreover, the existence of different phases opens up the possibility for many novel devices and architectures. For example, controlled conversion of the 1T ′ −MoTe 2 phase to the 2H−phase, as recently reported 22 , could

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Optical Properties and Band Gap of Single- and Few-Layer MoTe₂ Crystals

Cited by 837 publications

References 51 publications

Unified Description of the Optical Phonon Modes in N-Layer MoTe₂

Unified Description of the Optical Phonon Modes in N-Layer MoTe₂

Raman scattering of few-layers MoTe ₂

Engineering the Structural and Electronic Phases of MoTe₂ through W Substitution

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Optical Properties and Band Gap of Single- and Few-Layer MoTe2 Crystals

Cited by 837 publications

References 51 publications

Unified Description of the Optical Phonon Modes in N-Layer MoTe2

Unified Description of the Optical Phonon Modes in N-Layer MoTe2

Raman scattering of few-layers MoTe 2

Engineering the Structural and Electronic Phases of MoTe2 through W Substitution

Contact Info

Product

Resources

About

Optical Properties and Band Gap of Single- and Few-Layer MoTe₂ Crystals

Unified Description of the Optical Phonon Modes in N-Layer MoTe₂

Unified Description of the Optical Phonon Modes in N-Layer MoTe₂

Raman scattering of few-layers MoTe ₂

Engineering the Structural and Electronic Phases of MoTe₂ through W Substitution