Sharmila N. Shirodkar

Sensing behavior of atomically thin-layered MoS2 transistors.

Most of recent research on layered chalcogenides is understandably focused on single atomic layers. However, it is unclear if single-layer units are the most ideal structures for enhanced gas-solid interactions. To probe this issue further, we have prepared large-area MoS2 sheets ranging from single to multiple layers on 300 nm SiO2/Si substrates using the micromechanical exfoliation method. The thickness and layering of the sheets were identified by optical microscope, invoking recently reported specific optical color contrast, and further confirmed by AFM and Raman spectroscopy. The MoS2 transistors with different thicknesses were assessed for gas-sensing performances with exposure to NO2, NH3, and humidity in different conditions such as gate bias and light irradiation. The results show that, compared to the single-layer counterpart, transistors of few MoS2 layers exhibit excellent sensitivity, recovery, and ability to be manipulated by gate bias and green light. Further, our ab initio DFT calculations on single-layer and bilayer MoS2 show that the charge transfer is the reason for the decrease in resistance in the presence of applied field.

Thermal Expansion, Anharmonicity and Temperature‐Dependent Raman Spectra of Single‐ and Few‐Layer MoSe2 and WSe2

We report the temperature-dependent Raman spectra of single- and few-layer MoSe2 and WSe2 in the range 77-700 K. We observed linear variation in the peak positions and widths of the bands arising from contributions of anharmonicity and thermal expansion. After characterization using atomic force microscopy and high-resolution transmission electron microscopy, the temperature coefficients of the Raman modes were determined. Interestingly, the temperature coefficient of the A(2)(2u) mode is larger than that of the A(1g) mode, the latter being much smaller than the corresponding temperature coefficients of the same mode in single-layer MoS2 and of the G band of graphene. The temperature coefficients of the two modes in single-layer MoSe2 are larger than those of the same modes in single-layer WSe2. We have estimated thermal expansion coefficients and temperature dependence of the vibrational frequencies of MoS2 and MoSe2 within a quasi-harmonic approximation, with inputs from first-principles calculations based on density functional theory. We show that the contrasting temperature dependence of the Raman-active mode A(1g) in MoS2 and MoSe2 arises essentially from the difference in their strain-phonon coupling.

Characterization of few-layer 1T-MoSe2 and its superior performance in the visible-light induced hydrogen evolution reaction

Based on earlier results on the photocatalytic properties of MoS2, the 1T form of MoSe2, prepared by lithium intercalation and exfoliation of bulk MoSe2, has been employed for the visible-light induced generation of hydrogen. 1T-MoSe2 is found to be superior to both 2H and 1T MoS2 as well as 2H-MoSe2 in producing hydrogen from water, the yield being in the 60–75 mmol h−1 g−1 range with a turn over frequency of 15–19 h−1. First principles calculations reveal that 1T-MoSe2 has a lower work function than 2H-MoSe2 as well as 1T and 2H-MoS2, making it easier to transfer an electron from 1T-MoSe2 for the production of H2.

Charge‐Transfer Interaction between Few‐Layer MoS2 and Tetrathiafulvalene

Graphene has emerged to be a material of great interest because of its unique electronic structure and properties associated with its two-dimensional structure. Single-layer graphene is well known for properties such as quantum Hall effect, ambipolar electric field effect, and ballistic conduction of charge carriers. Graphene exhibits significant changes in electronic structure and properties on introduction of electrons or holes by electrochemical means. Such doping is reported to stiffen the Raman G band (frequency of the Raman band increases). Electron and hole doping can also be achieved by molecular charge transfer through interaction with electron donor and acceptor molecules, respectively. Molecular charge transfer with graphene has been investigated in detail by using Raman spectroscopy and other techniques. Charge-transfer interaction with an electron donor molecule like tetrathiafulvalene (TTF) softens the G band of graphene, whereas stiffening occurs upon interaction with an electron acceptor like tetracyanoethylene (TCNE), the Raman G band frequency will increase and decrease due to interaction with TCNE and TTF, respectively. These changes in the Raman G band are different from those found with electrochemical doping. We were interested to explore the interaction of electron donor and acceptor molecules with a two-dimensional layered material such as MoS2 to explore the occurrence of charge transfer, if any. With this purpose, we have studied the interaction of few-layer MoS2 material with TTF and TCNE. It is to be noted that MoS2 layers consist of Mo atoms sandwiched between two layers of chalcogen atoms, where the adjacent sheets are stacked by van der Waals interactions. We have observed the occurrence of charge transfer of fewlayer MoS2 material with TTF, but not with TCNE. Electronic absorption spectroscopic measurements indicate the formation of TTF radical cation by the interaction of TTF with few-layer MoS2 material, accompanied by the stiffening of the A1g mode of MoS2 in the Raman spectrum. This shift in the Raman A1g mode is opposite to that found in electrochemical doping. We have carried out detailed first-principle calculations to understand the results. The XRD pattern of the few-layer MoS2 material does not exhibit the (002) reflection, thus confirming the presence of only a few layers and the graphene-like nature of the material. The AFM images and the corresponding height profiles also confirm the presence of two to three layers with an average thickness of 2.44 nm. Figure 1 shows

Ab initiotight-binding Hamiltonian for transition metal dichalcogenides

The modeling of the electronic structure is the key to understanding layered transition-metal dichalcogenides (TMDCs) heterostructures. The authors present a full-range tight-binding hamiltonian for TMDCs by Wannier transformation of density functional theory results, which preserves both the orbital character and phase information. The tight-binding hamiltonians is expected to form the basis for further theoretical investigations of many-body physics and simulations for potential applications under external electric or magnetic fields in finite-size nanostructures, in either monolayer or heterostructure forms.

Multiferroic and magnetoelectric nature of GaFeO3, AlFeO3 and related oxides

Abstract GaFeO 3 , AlFeO 3 and related oxides are ferrimagnetic exhibiting magnetodielectric effect. There has been no evidence to date for ferroelectricity and hence multiferroicity in these oxides. We have investigated these oxides as well as oxides of the composition Al 1− x − y Ga x Fe 1+ y O 3 ( x =0.2, y =0.2) for possible ferroelectricity by carrying out pyroelectric measurements. These measurements establish the occurrence of ferroelectricity at low temperatures below the Neel temperature in these oxides. They also exhibit significant magnetoelectric effect. We have tried to understand the origin of ferroelectricity based on non-centrosymmetric magnetic ordering and disorder by carrying out first-principles calculations.

Phase Transitions of AlFeO3 and GaFeO3 from the Chiral Orthorhombic (Pna21) Structure to the Rhombohedral (R3̅c) Structure

AlFeO(3) and GaFeO(3), which crystallize in a chiral orthorhombic (Pna2(1)) structure, transform to a rhombohedral (R3c) structure when subjected to ball-milling. There is a distinct difference between the transformations of AlFeO(3) and GaFeO(3). AlFeO(3) first transforms to an orthorhombic P2(1)2(1)2(1) structure followed by its transformation to the R3c structure, while GaFeO(3) goes directly to the R3c structure. The transformations have been characterized by X-ray diffraction and Raman spectroscopy. Magnetic properties of Pna2(1) and the transformed phases show significant differences. It is noteworthy that partial substitution of aluminum by gallium in AlFeO(3) as in Al(0.5)Ga(0.5)FeO(3) eliminates the intermediate P2(1)2(1)2(1) phase, causing direct transformation of the Pna2(1) structure to the R3c structure. All of the transformations are thermodynamically first-order associated with significant changes in volume. We have used first-principles simulations to determine the pressure-dependent properties of AlFeO(3) and GaFeO(3) in orthorhombic and corundum structures and have estimated the critical pressures for the structural phase transition between the two structures. On the basis of this information, we also comment on the differences in the behavior of AlFeO(3) and GaFeO(3) under ball-milling.

Excitation intensity dependence of photoluminescence from monolayers of MoS2 and WS2/MoS2 heterostructures

A detailed study of the excitation dependence of the photoluminescence (PL) from monolayers of MoS2 and WS2/MoS2 heterostructures grown by chemical vapor deposition on Si substrates has revealed that the luminescence from band edge excitons from MoS2 monolayers shows a linear dependence on excitation intensity for both above band gap and resonant excitation conditions. In particular, a band separated by ~55 meV from the A exciton, referred to as the C band, shows the same linear dependence on excitation intensity as the band edge excitons. A band similar to the C band has been previously ascribed to a trion, a charged, three-particle exciton. However, in our study the C band does not show the 3/2 power dependence on excitation intensity as would be expected for a three-particle exciton. Further, the PL from the MoS2 monolayer in a bilayer WS2/MoS2 heterostructure, under resonant excitation conditions where only the MoS2 absorbs the laser energy, also revealed a linear dependence on excitation intensity for the C band, confirming that its origin is not due to a trion but instead a bound exciton, presumably of an unintentional impurity or a native point defect such as a sulfur vacancy. The PL from the WS2/MoS2 heterostructure, under resonant excitation conditions also showed additional features which are suggested to arise from the interface states at the heteroboundary. Further studies are required to clearly identify the origin of these features.

1H and 1T polymorphs, structural transitions and anomalous properties of (Mo,W)(S,Se)2 monolayers: first-principles analysis

Among the 1H and 1T structures exhibited by monolayers of transition metal dichalcogenides, the group VI compounds MX2 (M = Mo, W and X = S, Se) largely occur in the 1H form. Recently, transformation of the 1H form to the 1T form with metallic electronic structure at high temperatures was demonstrated in MoS2 with Re substitution and electron irradiation by Lin et al (2014 Nat. Nanotechnology 9 391). Here, we use first-principles calculations to determine the energy landscape associated with the 1H to 1T phase transition, predict novel 1T structures and relate the observed by Lin et al (2014 Nat. Nanotechnology 9 391) intermediate structures to structural instabilities of the 1T structure of MX2 compounds. We show that the metallic centrosymmetric 1T (c 1T ) structure of these compounds is unstable with respect to dimerization or trimerization of metal atoms, leading to a competing metallic 1T form and ferroelectric semiconducting 1T form respectively. While the former is a more stable 1T form of MoSe2, WS2 and WSe2, the latter is a more stable 1T form of MoS2 exhibiting rich ferroelectric dipolar domain structure. In the vicinity of metal-semiconductor transitions, their semiconducting forms are shown to exhibit an anomalous response to electric fields. To facilitate the experimental verification of these subtle features of the 1T forms of MX2 monolayers, we present comparative analysis of their vibrational properties, and identify their Raman and infra-red spectroscopic signatures.

Composition-dependent photoluminescence and electronic structure of 2-dimensional borocarbonitrides, BC X N (x = 1, 5)

Layered borocarbonitrides BCN and BC5N with a wide difference in composition have been prepared by the urea route. These 2D materials show a significant difference in the photoluminescence spectra, with BCN and BC5N showing maxima at 340 and 410 nm (3.61 and 3.0 eV), besides exhibiting different electrical resistivities. First-principles calculations show that BCN and BC5N are associated with different band gaps, the gap of the carbon-rich composition being lower. The change in the electronic structure and properties is related to the composition of BC X N i.e. the ordering of the graphene and BN domains.