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The 2D semiconducting transition metal dichalcogenides (e.g., WS2) host strong coupling between various degrees of freedom leading to potential applications in next-generation device applications including optoelectronics. Such applications are strongly influenced by defects which can control both the optical and electronic properties of the material. We demonstrate the possibility to tailor the defect-related electronic states and the lattice dynamics properties of WS2 in their heterostructures with h-BN which host a strong interlayer coupling between the charge carriers in the WS2 layer and the phonons of h-BN. This coupling is observed to induce modifications to the interlayer phonons (manifested by their modified Raman-activity) and to the charge carrier mobilities in the WS2 layer (which results in creation of mid-gap energy states associated with many-body quasiparticle states). Our study also includes a detailed characterization of the defects through Raman measurements revealing an A_1g-type nature with differential resonance behavior for the modes that are related to defect scattering with respect to the other normal phonon modes of WS2., Comment: 22 pages, 5 figures, Supplemental material

Tailoring the quantum many-body interactions in layered materials through appropriate heterostructure engineering can result in emergent properties that are absent in the constituent materials thus promising potential future applications. In this article, we have demonstrated controlling the otherwise robust magnetic properties of transition metal phosphorus trisulphides (Mn/Fe/NiPS3) in their heterostructures with Weyl semimetallic MoTe2 which can be attributed to the Dzyaloshinskii Moriya (DM) interactions at the interface of the two different layered materials. While the DM interaction is known to scale with the strength of the spin-orbit coupling (SOC), we also demonstrate here that the effect of DM interaction strongly varies with the spin orientation/dimensionality of the magnetic layer and the low-energy electronic density of state of the spin-orbit coupled layer. The observations are further supported by a series of experiments on heterostructures with a variety of substrates/underlayers hosting variable SOC and electronic density of states., Comment: 35 pages, 5 figures (main text), supporting information

After the discovery of graphene, there have been tremendous efforts in exploring various layered two-dimensional (2D) materials for their potential applications in electronics, optoelectronics, as well as energy conversion and storage. One of such 2D materials, SnS2, which is earth abundant, low in toxicity, and cost effective, has been reported to show a high on/off current ratio, fast photodetection, and high optical absorption, thus making this material promising for device applications. Further, a few recent theoretical reports predict high electrical conductivity and Seebeck coefficient in its bulk counterparts. However, the thermal properties of SnS2 have not yet been properly explored, which are important to materialize many of its potential applications. Here, we report the thermal properties of SnS2 measured using the optothermal method and supported by density functional theory (DFT) calculations. Our experiments suggest very low in-plane lattice thermal conductivity (\k{appa} = 3.20 +- 0.57 W m-1 K-1) and cross-plane interfacial thermal conductance per unit area (g = 0.53 +- 0.09 MW m-2 K-1) for monolayer SnS2 supported on a SiO2/Si substrate. The thermal properties show a dependence on the thickness of the SnS2 flake. Based on the findings of our DFT calculations, the very low value of the lattice thermal conductivity can be attributed to low group velocity, a shorter lifetime of the phonons, and strong anharmonicity in the crystal. Materials with low thermal conductivity are important for thermoelectric applications as the thermoelectric power coefficient goes inversely with the thermal conductivity.

Transition metal dichalcogenides (TMDs) are layered materials which show excellent potential for nanoelectronic and optoelectronic applications. However, as many of the exciting features of these materials are controlled by the anharmonic effects, a proper understanding of the phonon properties and anharmonicity associated with these materials is essential for the proposed applications to be realized. We present here a comprehensive study on the phonon properties of two different TMDs; viz., MoS2 and MoTe2, as a function of temperature, laser excitations, and polarization, as well as flake-thickness. Our study includes the measurement of anharmonicity in the first-order and higher order Raman scattering processes. The variations in anharmonicity with the thickness of MoS2 and MoTe2 have been explained in terms of their phonon symmetries, electron-phonon coupling, and phonon-phonon interactions. Further, the effect of the underlying substrate on the anharmonic properties of the in-plane and out-of-plane phonons has also been estimated from the Raman measurements, thus elucidating the intrinsic phonon properties of the 2D layered materials., Comment: 58 pages (including Supplemental material)

Atomically thin monolayers of graphene show excellent electronic properties which have led to a great deal of research on their use in nanoscale devices. However, heat management of such nanoscale devices is essential in order to improve their performance. Graphene supported on hexagonal boron nitride (h-BN) substrate has been reported to show enhanced (opto)electronic and thermal properties as compared to extensively used SiO2/Si supported graphene. Motivated by this, we have performed temperature- and power-dependent Raman Spectroscopic measurements on four different types of (hetero)structures: (a) h-BN (BN), (b) graphene (Gr), (c) h-BN on graphene (BG), and (d) graphene encapsulated by h-BN layers from both top and bottom (BGB), all supported on SiO2/Si substrate. We have estimated the values of thermal conductivity (\k{appa}) and interfacial thermal conductance per unit area (g) of these four (hetero)structures to demonstrate the structure-activity (thermal) relationship. We report here the values of \k{appa} and g for h-BN supported on SiO2/Si as 280.0 +-58.0 Wm-1K-1 and 25.6+-0.4 MWm-2K-1, respectively. More importantly, we have observed an improvement in both thermal conductivity and interfacial thermal conductance per unit area in the heterostructures which ensures a better heat dissipation in devices. The \k{appa} and g of h-BN encapsulated graphene on SiO2/Si (BGB) sample was observed to be 850.0+-81.0 Wm-1K-1 and 105+-1 MWm-2K-1, respectively, as opposed to 600.0+-93.0 Wm-1K-1 and 1.15+-0.40 MWm-2K-1, respectively, for graphene on SiO2/Si substrate. Therefore, we propose that for graphene-based nanoscale devices, encapsulation with h-BN is a better alternative to address heat management issues.

Transition metal dichalcogenides (TMDs) are a class of widely studied 2D layered materials which exist in various polymorphs. The 1T' phase of MoTe2 is of prime importance as it has been reported to show quantum spin hall (QSH) behavior with a fairly large band-gap of ~ 60 meV, in contrast to most QSH materials known. It is noteworthy that though the monolayer 1T'-MoTe2 was initially predicted to show the QSH behavior, recent theoretical studies claim that the few-layered counterparts also exhibit higher order topological behavior. Besides, 1T'-MoTe2 also undergoes a hysteretic phase transition to the Td phase (which is a type-II Weyl semimetal) by breaking the inversion symmetry of the crystal. While the phase transition between these two topological phases is of utmost importance, its study has been mostly restricted to bulk single crystal flakes, thereby not sufficiently exploring the effect of dimensionality. We have studied the phase transition in 1T'-MoTe2 as a function of flake-thickness. Though our Raman studies show a suppression of the phase transition in the thin (thickness <10 nm) flakes [similar to the report Phys. Rev. B 97, 041410 (2018)], we have experimentally demonstrated the possibility of stabilizing the desired phase (1T' or Td) at room temperature by charge doping. Further, we have observed clear signatures of electron-phonon coupling in MoTe2, which evolves as a function of flake-thickness and charge doping., Comment: 50 pages, 24 figures (including Supplemental Material)