Interlayer bonding and localized charge in MoSe2 and α-MoTe2

Wieting, Terence J.; Grisel, A.; Lévy, F.

doi:10.1016/0378-4363(80)90256-9

Cited by 39 publications

(38 citation statements)

References 12 publications

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“…Otherwise, the splitting is anomalous, as it has been reported for the iMX mode in bulk transition metal dichalcogenides [20].…”

Section: S2 Bulk Phonon Frequenciessupporting

confidence: 55%

“…In bulk MoTe 2 , we observe only a single feature at 26 cm −1 that shows similar intensities in the XX and XY configurations. The latter is assigned to the in-plane interlayer shear mode (LSM) with E 2g symmetry [20]. For N = 2, we observe a prominent feature at 28 cm −1 that shows strong extinction in the XY polarization and a fainter feature at 18 cm −1 whose intensity is similar in the XX and XY configurations.…”

mentioning

confidence: 74%

“…The LBM has B 2g symmetry in the bulk and is silent. As expected, we do not observe any interlayer mode for N = 1.In the mid-(100 − 200 cm −1 ) and high-frequency (200 − 300 cm −1 ) ranges, the Raman spectra of bilayer MoTe 2 displays four one-phonon features, which have previously been identified as originating from the following intralayer displacements: (i) the in-plane, outof-phase vibration of the Te planes, with E 1g symmetry in the bulk (iX mode, near 120 cm −1 ), (ii) the out-ofplane, out-of-phase vibration of the Te planes, with A 1g symmetry in the bulk (oX mode, near 170 cm −1 ), (iii) the in-plane vibration of the Mo and Te planes against each other, with E 2g symmetry in the bulk (iMX mode, near 230 cm −1 ), and (iv) the out-of-plane vibration of the Mo and Te planes against each other, with B 2g symmetry in the bulk, (oMX mode, near 290 cm −1 ) [13,20,39]. The bulk iX and oMX modes are predicted to be Raman inactive in a backscattering geometry and silent, respectively.…”

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confidence: 99%

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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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“…Otherwise, the splitting is anomalous, as it has been reported for the iMX mode in bulk transition metal dichalcogenides [20].…”

Section: S2 Bulk Phonon Frequenciessupporting

confidence: 55%

mentioning

confidence: 74%

mentioning

confidence: 99%

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

et al. 2015

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“…There are three major peaks observed in that range, which are due to first-order transitions at the Γ point of the Brillouin zone: the out-of-plane mode A 1g at ∼170 cm −1 , the in-plane mode E [25][26][27][28] In that mode, both Te atoms in each layer move at particular time in the same direction while the central Mo atom moves in the opposite direction (see Fig. 1).…”

Section: Resultsmentioning

confidence: 99%

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 Table I, we report our calculated BECs and Bader charges for the previously-introduced TMDs, alongside experimental data extracted from infrared spectra [12,[47][48][49][50]. These experiments only provide a measure of the magnitude of the BEC, and our calculated BECs must be compared accordingly.…”

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confidence: 99%

Origin of the counterintuitive dynamic charge in the transition metal dichalcogenides

et al. 2017

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We investigate the chemical bonding characteristics of the transition metal dichalcogenides based on their static and dynamical atomic charges within Density Functional Theory. The dynamical charges of the trigonal transition metal dichalcogenides are anomalously large, while in their hexagonal counterparts, their sign is even counterintuitive i.e. the transition metal takes the negative charge. This phenomenon cannot be understood simply in terms of a change in the static atomic charge as it results from a local change of polarization. We present our theoretical understanding of these phenomena based on the perturbative response of the system to a static electric field and by investigating the hybridization of the molecular orbitals near the Fermi level. Furthermore, we establish a link between the sign of the Born effective charge and the π-backbonding in organic chemistry and propose an experimental procedure to verify the calculated sign of the dynamical charge in the transition metal dichalcogenides. The expanding interest in the Transition Metal Dichalcogenides (TMDs) stems from their wide variety of applications, ranging from batteries to electronic devices [1-3]. Indeed, by adjusting either their chemical composition or the number of layers, one can tune their electronic, vibrational, and magnetic properties in a remarkable manner that cannot be imitated in other twodimensional and layered materials. In particular, the TMDs offer high carrier mobilities [4-6] and a straindependent indirect to direct band gap transition [7], that are crucial for future electronic and electro-optic applications [8,9]. Still, while the electronic properties of these materials are now relatively well-known [10], the character of their chemical bonds is, interestingly, quite diverse, and, to the best of our knowledge, not yet fully understood. For example, while ZrS 2 is reported as extremely ionic [11,12], MoS 2 and WS 2 are reported to possess both ionic and covalent characteristics [13,14]. Additionally, TiS 2 was recently reported as metallic and semiconducting, both experimentally and theoretically [15][16][17].In this Letter, we aim to provide a deeper understanding of the bonding characteristics and charge transfer in the TMDs thanks to Density Functional Theory (DFT) [18]. One common way to estimate the charge distribution within this theory is to partition the electronic density of the system into constituent atoms [19,20]. While conceptually simple, this notion of "static" charge is, unfortunately, ambiguous [21] and the corresponding charges cannot be measured experimentally. Contrarily, the Born effective charge (BEC) [22], arising from the change of dipole moment due to an atomic perturbation, is a physical observable as it corresponds to the dynamical charge response to a perturbation. It governs, for example, the splitting between the transverse optical (TO) and longitudinal optical (LO) vibrational modes [22]. It has been argued [23] that all the various operational charge definitions (including static and dyn...

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Interlayer bonding and localized charge in MoSe2 and α-MoTe2

Cited by 39 publications

References 12 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

Origin of the counterintuitive dynamic charge in the transition metal dichalcogenides

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Interlayer bonding and localized charge in MoSe2 and α-MoTe2

Cited by 39 publications

References 12 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

Origin of the counterintuitive dynamic charge in the transition metal dichalcogenides

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Product

Resources

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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