Ak Singh

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.

Pressure‐induced alpha‐omega transformation in titanium: Features of the kinetics data

The increase in the electrical resistance of the specimen, associated with the α→ω transformation in Ti, has been used to obtain the fraction ζ of the ω phase as a function of time under isobaric–isothermal conditions in the pressure range 4–9 GPa and at 300±3 K. In the entire pressure range ζ–t data fit an equation of the form: ζ=1−exp−(t/τ)n, where τ and n are constants at a given pressure. The results indicate that τ decreases rapidly and n slightly with increase in pressure. An analysis of τ–p data shows that (i) the activation free energy for the total process is very high at low pressures and decreases rapidly with increasing pressure, and (ii) the activation volume in the 6–9 GPa range is −4.3 cm3 mol−1. The activation enthalpy has been obtained by measuring τ at different temperatures, and the average value in the pressure range 5–8 GPa is 12±0.5 kCal mol−1.

The kinetics of pressure‐induced α→ω transformation in Ti

The kinetics of the pressure‐induced α→ω transformation in titanium is reported in the pressure range 5–9 G Pa. The occurrence of this transformation under shock loading has been discussed using the observed pressure dependence of the kinetics.

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.

Chemically exfoliated MoS2 layers: Spectroscopic evidence for the semiconducting nature of the dominant trigonal metastable phase

A trigonal phase existing only as small patches on chemically exfoliated few layer, thermodynamically stable 1H phase of MoS2 is believed to influence critically properties of MoS2 based devices. This phase has been most often attributed to the metallic 1T phase. We investigate the electronic structure of chemically exfoliated MoS2 few layered systems using spatially resolved (≤120 nm resolution) photoemission spectroscopy and Raman spectroscopy in conjunction with state-of-the-art electronic structure calculations. On the basis of these results, we establish that the ground state of this phase is a small gap (~90 meV) semiconductor in contrast to most claims in the literature; we also identify the specific trigonal (1Tʹ) structure it has among many suggested ones. 2D transition metal dichalcogenides have emerged as a viable alternative to graphene with extraordinary properties and potential applications. Molybdenum disulfide (MoS2) is undoubtedly the preeminent member in the family for applications in transparent and flexible electronics. While the usual crystallographic form of MoS2 is the hexagonal 1H phase, MoS2 exhibits a number of trigonal polymorphic forms, such as 1T, 1T′, 1T′′ and 1T′′′ (see Fig. 1), distinguished by small distortions. These metastable states can be kinetically formed as small patches embedded in the majority 1H phase during chemical exfoliation, which is an attractive, easily scalable route to obtain one or few layer MoS2 in substantial quantities . Even mechanically exfoliated MoS2 may have small quantities of these metastable forms, influencing its material and device properties. The stable 1H form has been extensively studied and its electronic properties are well understood as a semiconductor with a large (1.9 eV) band gap. Unfortunately, electronic structures of different polymorphic MoS2 are not known, though it may potentially limit or enhance the applicability of two dimensional MoS2 devices by its presence within 1H MoS2 samples. It has been generally assumed that the metastable phase is of 1T form and metallic in nature. This presumed metallic nature is considered to be the cause of some novel beneficial device properties as well. For example, metallic 1T phase is believed to be responsible for the very high energy and power densities in supercapacitors and also for the remarkably high hydrogen evolution reaction efficiency achieved using chemically exfoliated MoS2. Whatever little is known of electronic structures of metastable phases is primarily from theoretical calculations that present contradictory views, ranging from being metallic (1T phase) to normal insulator (for 1T′ phase) or even ferroelectric insulator (1T′′′ phase). Surprisingly, direct structural investigations, based mostly on TEM and STM, also lack any agreement between different reports with the crystal structure of the metastable patches of MoS2 being variously ascribed to the 1T form, 19,26 the distorted 1T′′ form with a 2a×2a superstructure, the 1T′ form with a zigzag chain-like clustering of Mo atoms, and also the distorted 1T′′′ with a √3a×√3a superstructure where a trimerization of Mo atoms takes place (see Fig.1). Thus, the hope to understand the true electronic structure of this important phase of MoS2 via an experimental determination of its geometric structure in conjunction with electronic structure calculations has not been realized so far. The main difficulty in experimentally probing this metastable phase of MoS2 is that it exists only in small patches in the 1H matrix of few layer MoS2. Photoemission spectroscopy is the only direct probe of the electronic structure due to its inherent extreme surface sensitivity. However, the usual practice of photoemission spectroscopy does not have the required spatial resolution to enhance the relative contribution from the microscopic patches of the metastable phase. Therefore, we have carried out spatially resolved photoemission investigation with a ~120 nm photon beam diameter to directly determine the electronic structure of this metastable phase of MoS2 and conclusively establish that this elusive phase is actually a small gapped (~90 meV) semiconductor in sharp contrast to the dominant belief of it being metallic. We use state-of-the-art electronic structure calculations to provide evidence that this phase corresponds to the 1T′ structure, supported by micro-Raman experiments. The minority phase of MoS2 was extensively stabilized on conducting indium doped tin oxide (ITO) substrates using the well-known organolithium route. The mechanically exfoliated pure sample of MoS2 and the one after chemical treatment to stabilize the metastable polymorphic form are referred to as A and B, respectively in this manuscript. Raman scattering is a powerful and sensitive tool to detect presence of different phases of any given material and has been extensively used for low dimensional materials in recent times, providing vibrational fingerprints for different phases. Therefore, we characterized samples A and B using a micro-Raman probe with a spatial resolution of ~1 μm, with only three representative spectra shown for each sample. Raman spectra of the pure 1H sample (Fig. 2a) exhibit two peaks at 383 cm and 408.5 cm due to E2g and A1g modes, respectively, consistent with earlier reports. For sample B (Fig. 2b), we observe three additional peaks at 156 cm, 227 cm and 330 cm, referred to as J1, J2 and J3 modes respectively, in the literature 21,32 and associated with the formation of the metastable trigonal phase. We could not find any spot on this sample with only signals of the metastable phase with no signature of the 1H phase, clearly indicating that the patch size of this chemically induced MoS2 phase is smaller than 1 μm. Scanning Photo Electron Microscopy (SPEM) measurements were performed to understand the electronic structure of the chemically exfoliated system. Fig. 3a shows a typical photoelectron spectrum in the Mo 3d region with two narrow peaks at binding energies of 229.3 eV (Mo 3d5/2) and 232.4 eV (Mo 3d3/2) and a broad, low intensity feature at ~226.5 eV due to S 2s contributions. We compare this with a typical spectrum obtained from the sample B. Clearly, peaks in the spectrum of the mixed phase are specifically broadened on the lower binding energy side. Since the intensity of Mo 3d5/2 peak from pure 1H appears almost entirely between 228.7 and 230.0 eV in Fig. 3a, the additional intensity from sample B between 228 eV and 228.7 eV must arise from polymorphic forms of MoS2 other than 1H. This shift in the binding energy of the metastable form allows us to map its presence in the form of an image by scanning the sample through the focused photon beam and plotting the relevant Mo 3d5/2 intensity. In the SPEM imaging mode, first we carried out a detailed mapping of Mo 3d5/2 intensity over the entire 228-230 eV binding energy window, covering contributions from both phases and thus, imaging the distribution of MoS2 without any reference to its polymorphic forms on the ITO substrate as shown in Fig. 3b. Then, we plot in Fig. 3c an image of the ratio of intensities corresponding to energy windows, I and II, shown in Fig. 3a, corresponding to Mo 3d5/2 intensities arising primarily from the metastable phase and the 1H phase, respectively. The contrast in the intensity ratio, I/II, being independent of topographic features, reveals the relative abundance of the metastable phase in the sample B with the dark blue regions corresponding to the metastable MoS2 rich region. We point out that almost all spots imaged in Fig. 3c contain signals of both the 1H and the metastable phase, explicitly checked by recording Mo 3d5/2 spectra from over 30 different spots; there were only few spots that corresponded to the pure 1H phase and no spot that had contribution only from the metastable phase. Coupled with the fact that there is a considerable intensity contrast even with the present photon spot size, this implies that the typical size of metastable patches is in the order of ≤120 nm, but not very much smaller. We identified a region with the largest contribution from the metastable phase, marked by the rectangular window in Fig. 3c; the intensity of the Mo 3d5/2 from the metastable phase was essentially uniform over this region. Thereafter, we carried out a detailed spectroscopic investigation with the photon beam focused at the center of this window to maximize contributions from the metastable phase that we are interested in. The Mo 3d spectrum obtained from this spot is shown in Fig. 3d. We have decomposed this spectrum in terms of contributions from the 1H phase and the metastable phase using a least squared-error approach with the spectral feature of the 1H component as determined from measurements of sample A and that for the metastable phase approximated by a Lorentzian line convoluted with a Gaussian function with full width at the half maximum, FWHM, representing the life-time and the resolution broadenings. The resulting components, also shown in Fig. 3d, establish the dominance of the metastable phase at this spot on the sample, with the area ratio (~2.8) of the two components providing a quantitative measure of the relative abundance of the two phases. We also note that the electronic binding energy difference (~0.7 eV) between the two components is in agreement with earlier publications. 23 There have been suggestions in the literature that the presence of Li ions on such chemically exfoliated samples and consequent charge doping may influence the formation of a specific metastable state. Therefore, we scanned carefully the binding energy region corresponding to Li 1s level with a high counting statistics and found no evidence of any presence of Li in our samples. This shows that the sample preparation method, that involves repeated washing of the chemically exfoliated samples first with hexane and then with water is effective in removing all traces of

Electron-hole asymmetry in the electron-phonon coupling in top-gated phosphorene transistor

Using in-situ Raman scattering from phosphorene channel in an electrochemically top-gated field effect transistor, we show that its phonons with A

Raman anomalies as signatures of pressure induced electronic topological and structural transitions in black phosphorus: Experiments and theory

_g

The kinetics of pressure‐induced fcc‐bcc transformation in ytterbium

symmetry depend much more strongly on concentration of electrons than that of holes, while the phonons with B

Pressure-dependent semiconductor to semimetal and Lifshitz transitions in 2H-MoTe2: Raman and first-principles studies

_g

Thermoelectric power of YBa2Cu3O7−δ under pressure up to 9 GPa

symmetry are insensitive to doping. With first-principles theoretical analysis, we show that the observed electon-hole asymmetry arises from the radically different constitution of its conduction and valence bands involving