Academic literature on the topic '2H-MoS2'

Metallic 1T-phase MoS2 is a newly emerging and attractive catalyst since it has more available active sites and high carrier mobility in comparison with its widely used counterpart of semiconducting 2H-MoS2. Herein, 1T/2H-MoS2(N) (N: MoO3 nanowires were used to prepare 1T/2H-MoS2) was synthesized by using molybdenum trioxide (MoO3) nanowires as the starting material and applied in the photodegradation of antibiotic residue in water. Enhanced photocatalytic performance was observed on the obtained 1T/2H-MoS2(N), which was 2.8 and 1.3 times higher than those on 1T/2H-MoS2(P) (P: commercial MoO3 powder was used to prepare 1T/2H-MoS2) and 2H-MoS2, respectively. The active component responsible for the photodegradation was detected and a reaction mechanism is proposed.

Photocatalytic technology is widely used in water purification because of its environmental protection, high efficiency and energy saving. Therefore, it is extremely important for the selection and preparation of specific semiconductor materials used in the field of photocatalysis. In this work, 1T@2H MoS2 nanosheets were fabricated by simple hydrothermal method, and the photocatalytic property of as-prepared 1T@2H MoS2 were investigated by the photo-degradation of methylene blue (MB) water solutions under visible light irradiation via 2H MoS2.The results indicated that compared to 2H MoS2, the 1T@2H MoS2 exhibited more excellent photocatalytic degradation property. After 150 minutes of irradiation under visible light, 1T@2H MoS2 had a removal rate of 98% for MB, and 2H MoS2 eventually reached 19%. The enhancement photocatalytic property of 1T@2H MoS2 could be attributed to the reduced band gap energy of the hybrid-nanosheets and the increased in electron migration speed.

Following the successful experimental synthesis of single-layer metallic 1T-TaS2 and semiconducting 2H-MoS2, 2H-WSe2, we perform a first-principles study to investigate the electronic and interfacial features of metal/semiconductor 1T-TaS2/2H-MoS2 and 1T-TaS2/2H-WSe2 van der Waals heterostructures (vdWHs) contact. We show that 1T-TaS2/2H-MoS2 and 1T-TaS2/2H-WSe2 form n-type Schottky contact (n-ShC type) and p-type Schottky contact (p-ShC type) with ultralow Schottky barrier height (SBH), respectively. This indicates that 1T-TaS2 can be considered as an effective metal contact with high charge injection efficiency for 2H-MoS2, 2H-WSe2 semiconductors. In addition, the electronic structure and interfacial properties of 1T-TaS2/2H-MoS2 and 1T-TaS2/2H-WSe2 van der Waals heterostructures can be transformed from n-type to p-type Schottky contact through the effect of layer spacing and the electric field. At the same time, the transition from Schottky contact to Ohmic contact can also occur by relying on the electric field and different interlayer spacing. Our results may provide a new approach for photoelectric application design based on metal/semiconductor 1T-TaS2/2H-MoS2 and 1T-TaS2/2H-WSe2 van der Waals heterostructures.

Solar energy is an inexhaustible clean energy providing a key solution to the dual challenges of energy and environmental crises. Graphite-like layered molybdenum disulfide (MoS2) is a promising photocatalytic material with three different crystal structures, 1T, 2H and 3R, each with distinct photoelectric properties. In this paper, 1T-MoS2 and 2H-MoS2, which are widely used in photocatalytic hydrogen evolution, were combined with MoO2 to form composite catalysts using a bottom-up one-step hydrothermal method. The microstructure and morphology of the composite catalysts were studied by XRD, SEM, BET, XPS and EIS. The prepared catalysts were used in the photocatalytic hydrogen evolution of formic acid. The results show that MoS2/MoO2 composite catalysts have an excellent catalytic effect on hydrogen evolution from formic acid. By analyzing the photocatalytic hydrogen production performance of composite catalysts, it suggests that the properties of MoS2 composite catalysts with different polymorphs are distinct, and different content of MoO2 also bring differences. Among the composite catalysts, 2H-MoS2/MoO2 composite catalysts with 48% MoO2 content show the best performance. The hydrogen yield is 960 µmol/h, which is 1.2 times pure 2H-MoS2 and two times pure MoO2. The hydrogen selectivity reaches 75%, which is 22% times higher than that of pure 2H-MoS2 and 30% higher than that of MoO2. The excellent performance of the 2H-MoS2/MoO2 composite catalyst is mainly due to the formation of the heterogeneous structure between MoS2 and MoO2, which improves the migration of photogenerated carriers and reduces the possibilities of recombination through the internal electric field. MoS2/MoO2 composite catalyst provides a cheap and efficient solution for photocatalytic hydrogen production from formic acid.

Introducing anchoring materials into cathodes for Li–S batteries has been demonstrated as an effective way to overcome the shuttle effect and enhance the cycling stability. In this work, the anchoring effects of 2H-MoS2 and 1T'-MoS2 monolayers for Li–S batteries were investigated by using density functional theory calculations. It was found that the binding energies of Li2S x absorbed on 1T'-MoS2 monolayer are in the range of 0.31–2.94 eV, which is much higher than on the 2H-phase. The 1T'-MoS2 monolayer shows stronger trapping ability for Li2S x than the 2H-MoS2 monolayer. The 1T'-MoS2 monolayer can be used as effective anchoring material in cathodes for Li–S batteries.

The weak transport charge efficiency and great band gap energy of layered MoS2 hamper its further commercial application. To overcome these deficiencies, we report a simple, controlled and handy hydrothermal process for realizing 2H MoS2 to 1T MoS2 transition with P source. Due to the more conductive ability and larger surface area, P-doped 1T@2H MoS2 nanosheets show an outstanding catalytic activity. Noticeably, P-doped 1T@2H MoS2 nanosheets with narrowed bandgap exhibits a remarkable optical photochemical performance. It fully eliminates 50 ml of 20 mg L–1 RhB in 70 minutes with outstanding recycling and structural stability by using 10 mg catalyst.

Abstract The geometric and electronic structure, partial (band decomposed) charge density, charge transfer, electron localization function and photocatalytic mechanism of the asymmetric 2H-MoS2/BiOCl Janus heterostructure were systematically studied with first-principles density functional theory. Our calculations showed that there exist several newly formed weak Bi-S bonds with shorter bond lengths between BiOCl and 2H-MoS2 which act as an electron transport bridge along the direction perpendicular to the heterojunction interface. This newly weak bonds lead to the formation of occupied shallow defect levels approximately 0.0–0.9 eV below the bottom of the conduction band. Electrons located at these defect levels can easily jump into the conduction band as a donor energy level under thermal fluctuations and simultaneously further promote the effective separation of photo-generated electron-hole pairs in the BiOCl. The photogenerated electrons located around Bi-atom layer in the conduction band of BiOCl transfer to the valence band of 2H-MoS2 around the S-atom layer through the interface of the asymmetric 2H-MoS2/BiOCl Janus heterostructure, which significantly reduce photo-generated holes in the 2H-MoS2 and electrons in the BiOCl. The large numbers of photogenerated electrons from the 2H-MoS2 cannot recombine with holes owing to lack of sufficient holes. They will move to the surface and greatly improve the hydrogen production activity in the 2H-MoS2. While the photogenerated holes from the BiOCl will significantly improve the ability of BiOCl to oxidize pollutant in the water owing to the absence of sufficient electrons. Our studies provide new way for the design of asymmetric Janus double-layer heterostructures with newly formed weak chemical bonding.

Molybdenum disulphide (MoS2) is emerging as one of the most attractive two-dimensional (2D) materials amongst the transition metal dichalcogenides (TMDs) group. MoS2 exhibits a thickness-dependent band gap that changes from indirect band gap of ~1.3 eV for bulk MoS2 to direct band gap of ~1.9 eV in monolayer form. Such indirect-to-direct gap transition due to quantum confi nement results in giant enhancement in photoluminescence quantum yield. Recent study demonstrated that the co-existence of 2H- and 1T-MoS2 synthesized by hydrothermal method exhibited superior electronic conductivity to those with 2H semiconducting phase. Metallic 1T phase of MoS2 is attracting much attention due to its high electronic conductivity and potential applications in supercapacitors, thermoelectric energy harvesting, and memristors. In this paper, we investigated a hybrid structure between 2H and 1T of MoS2 by a hydrothermal process and investigated its optical property via photoluminescence spectra with changes in pH values. Our results indicated that photoluminescence occurred in the visible light region and the band gap was determined to be ~1.96–2.50 eV. In addition, pH of the precursor solution strongly affected the fi nal structure of MoS2 nanocrystals, which in turn, affected their photoluminescence property wherein pH = 4–5 was identifi ed as optimum for 2H–1T-MoS2 synthesis.

The structures of inorganic fullerene-like (IF) MoS2 nanoparticles produced by arc discharge in water are reported in this paper. To adjust the chemistry and structure of IF nanoparticles, 2H–MoS2, graphite and composite 2H–MoS2/graphite rods were used as electrodes in the arc synthesis. In comparison to using MoS2 as both anode and cathode, mixed electrodes (graphite and MoS2) significantly increased the discharge current. Various IF-MoS2 nanoparticles were successfully produced by the water-based arc method, and their microstructures were studied using a transmission electron microscope equipped with an x-ray energy dispersive spectrometer. The IF–MoS2 nanoparticles commonly had a solid core wrapped with a few MoS2 layers and exhibit some differences in size and geometry. The IF-MoS2 nanoparticles were typically 5–30 nm in diameter as observed by transmission electron microscopy. Tiny IF-MoS2 nanoparticles (<10 nm) along with fragments of lamellar MoS2 were produced from arc discharge in water using both graphite and MoS2 electrodes. Carbon nano-onions and hybrid nanoparticles consisting of carbon and MoS2 were synthesized by using mixed electrodes of graphite and 2H–MoS2. The hybrid nanoparticles were MoS2 cores covered by a graphite shell. Our results show that the water-based arc method provides a simple tool for producing a variety of nanoparticles including some familiar and some new hybrid structures.

Abstract Monolayer molybdenum disulfide (MoS2) is one of the most studied two-dimensional (2D) transition metal dichalcogenides that is being investigated for various optoelectronic properties, such as catalysis, sensors, photovoltaics, and batteries. One such property that makes this material attractive is the ease in which 2D MoS2 can be converted between the semiconducting (2H) and metallic/semi-metallic (1T/1T′) phases or heavily n-type doped 2H phase with ion intercalation, strain, or excess negative charge. Using n-butyl lithium (BuLi) immersion treatments, we achieve 2H MoS2 monolayers that are heavily n-type doped with shorter immersion times (10–120 mins) or conversion to the 1T/1T′ phase with longer immersion times (6–24 h); however, these doped/converted monolayers are not stable and promptly revert back to the initial 2H phase upon exposure to air. To overcome this issue and maintain the modification of the monolayer MoS2 upon air exposure, we use BuLi treatments plus surface functionalization p-(CH3CH2)2NPh-MoS2 (Et2N-MoS2)—to maintain heavily n-type doped 2H phase or the 1T/1T′ phase, which is preserved for over two weeks when on indium tin oxide or sapphire substrates. We also determine that the low sheet resistance and metallic-like properties correlate with the BuLi immersion times. These modified MoS2 materials are characterized with confocal Raman/photoluminescence, absorption, x-ray photoelectron spectroscopy as well as scanning Kelvin probe microscopy, scanning electrochemical microscopy, and four-point probe sheet resistance measurements to quantify the differences in the monolayer optoelectronic properties. We will demonstrate chemical methodologies to control the modified monolayer MoS2 that likely extend to other 2D transition metal dichalcogenides, which will greatly expand the uses for these nanomaterials.

Dissertação (Mestrado) - Universidade Federal de Santa Catarina, Centro Tecnológico. Programa de Pós-Graduação em Ciência e Engenharia de Materiais, Florianópolis, 2012
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No presente trabalho foi desenvolvido um método de produção em escala aumentada de nanopartículas de dissulfeto de molibdênio (MoS2). Foi utilizada a rota hidrotérmica para sua produção, sendo projetado um forno de aquecimento resistivo com capacidade para 9 autoclaves. Estas foram confeccionadas em aço SAE 310, revestidas internamente com politetrafluoretileno (PTFE), tendo 120 ml de volume interno cada. Este forno conta com um assoalho vibratório capaz de manter as autoclaves sob agitação constante, alcançando até dois modos vibracionais fundamentais do conteúdo líquido no interior delas. As sínteses foram desenvolvidas na temperatura de 220°C em períodos de 6, 12 e 24 horas, tendo como reagentes o molibdato de amônio tetrahidratado, hidroxilamina cloridrato e tioureia, sendo a reação dada em água bidestilada. As amostras foram caracterizadas quanto ao seu rendimento químico, estrutura cristalina por Difração de Raios X (DRX), morfologia por Microscopia Eletrônica de Varredura por Emissão de Campo (FEG), Termogravimetria e Infravermelho por Transformada de Fourier (TG-IR) simultaneamente, Espectroscopia Raman, Interferometria Óptica e, com enfoque maior, Tribologia. Foram obtidas nanopartículas na morfologia de nanoflores com cerca de 200 nm de diâmetro com estrutura cristalina hexagonal lamelar (2H-MoS2) parcialmente amorfizadas. Estas formam dispersões relativamente estáveis com os três óleos sintéticos testados (fortemente polar, de média polaridade e apolar) e com água bidestilada. Obteve-se um rendimento médio de reação de 71,3% em relação ao Mo e foi verificado um aumento de 10% neste quando comparado com amostras estáticas, sem a aplicação da agitação por vibração. Desta forma, valores para uma produção industrial utilizando o sistema desenvolvido ficam em torno de 300 g/mês.

Abstract : In the present work was developed a method for scaling up the production of molybdenum disulfide (MoS2) nanoparticles. It was used a hydrothermal route to its production, being designed a resistive heating furnace with capacity for 9 autoclaves. They were designed in SAE 310 stainless steel, internally coated with polytetrafluoroethylene (PTFE), having 120 ml internal volume each. This furnace has a vibratory floor capable of maintaining the autoclaves under stirring, reaching until two fundamental vibrational modes of the liquid contents inside them. The syntheses were developed at temperature of 220 °C in periods of 6, 12 and 24 hours, with the reagents of ammonium molybdate tetrahydrate, hydroxylamine hydrochloride and thiourea, the reaction is given in bidistilled water. The samples were characterized according to their chemical yield, crystalline structure by X-ray Diffraction (XRD), morphology by Field Emission Scanning Electron Microscopy (FEG), Thermogravimetry and Fourier Transform Infrared (TG-IR) simultaneously, Raman Spectroscopy, Optical Interferometry and, with increased focus, Tribology. Nanoparticles were obtained on the morphology of nanoflowers about 200 nm diameter with lamellar hexagonal crystalline structure (2H-MoS2) partially amorphous. These form relatively stable dispersions with the three synthetic oils tested (strongly polar, medium polarity and nonpolar) and with bidistilled water. It was obtained an average reaction yield of 71.3% in relation to Mo and there was verified an increment of 10% in it in comparison with static samples, without the application of stirring by vibration. Thus, values for an industrial production using the developed system are around 300 g / month.

碩士
國立中正大學
光機電整合工程研究所
105
As the development of the semiconductor industry, two-dimensional materials such as MoS2, WSe2 have gain much attention by researchers. Among them, MoS2 is especially studied due to its unique properties. Utilizing its two dimensional property, energy gap structure, Halls effect, nonlinear optical property and fast carrier speed, MoS2 is the optimal material for optical detectors. In our study, 1.6M butyllithium solution in hexane is used to change the phase of MoS2 from 2H to 1T.1T phase MoS2 devices produce superior performance on transconductance, mobility, subthreshold over 2H phased MoS2 counterparts. Before the creation of the MoS2 devices, there are three prerequisites that need to be met. First is the ability to grow large area of MoS2 monocrystalline thin film. Second is being able to stably embed large area of MoS2. Third is fast and accurately identify the layer distribution of MoS2 thin film. Due to the fact that currently measuring technique such as Raman, AFM can’t analyze large area of MoS2 thin film in desired speed, we apply Hyperspectral imaging technique instead to achieve our goal. In this study, we apply CVD to grow few-layered MoS2 and apply chemically exfoliated MoS2 embedded chemically exfoliated MoS2 to change its phase. Hyperspectral imaging technique is then applied to identify the embedded MoS2 spectrum characteristic and build its spectrum database.

This thesis presents experimental as well as theoretical studies of three dimensional topological in-sulators (TIs) and transition metal dichalcogenides (TMDs). Raman spectroscopic measurements of these systems have been carried out as a function of pressure. For structural characterization as a function of pressure, x-ray diffraction measurements using synchrotron source have been pur-sued. The Þeld effect transistor devices fabricated from MoS2 are used in experiments to probe Raman evidence of electrostrictive and piezoelectric effects in multilayer and monolayer, respec-tively. In-situ photoluminescence (PL) and Raman measurements are performed as a function of laser-irradiation time to show healing of defects in MoS2. We provide an overview of our work on these systems chapter-wise. In Chapter-1, the brief introduction of the systems studied in this thesis has been provided. For topological insulators, the 2 topological index has been discussed for three dimensional strong TIs as well as weak TIs. Then, the various properties of MoS2 and MoTe2 such as electronic, vibrational, structural and photoluminescence are discussed in details. Chapter-2 includes a short introduction to Raman spectroscopy and x-ray diffraction. The tech-nical details of the high pressure experiments which include the alignment of the diamond anvil cell, gasket preparation, loading the pressure medium are discussed. This is followed by the discussions on the sample preparation as well as the electron beam lithography techniques. In Chapter-3, we present Raman spectroscopic measurements on a band insulator Sb2Se3 under high pressure upto 24.6 GPa. Different experimental techniques are being pursued to capture the surface electronic properties due to Dirac electrons and hence, thereby characterizing 2 topology of the system. The topological characterization of 2 TIs is solely based on the isolated quantum system of electrons, but in real materials they are coupled to other low energy excitations of the system. Depending upon the strength of the coupling with the environment, it can induce an electronic topological transition. In fact, it has been proposed [Phys. Rev. B 89, 205103 (2014)] that the increment of temperature gives rise to an increase in electron-phonon coupling and thereby driving a trivial insulator to a topological insulator. Here, we address the inverse situation, i.e., can we trace the topological signature due to electronic topological transition (ETT) from the environment or nonelectronic baths of the system rather than from the electronic system itself, experimentally as well as theoretically ? High pressure Raman spectroscopic measurements show a ETT transition at ∼2.5 GPa marked by a large softening of the low frequency Raman mode by ∼ 16% together with an anomalous increase of its linewidth by ∼ 200% within a narrow pressure range of 0 to 2.5 GPa. Our calcula-tions based on model Hamiltonian determined by projecting the electron-phonon coupling term onto the identity representation of the double group corresponding to the phonons of different irreducible representations (thereby ensuring the symmetry invariant Hamiltonian) captures the phonon anomalies qualitatively. It is convincing that the linewidth of phonon carries the non-trivial topological signature but there are few subtle things need to be considered. It is obvious from the linewidth that there is a band inversion at Fermi level but, it is not obvious that it probes strong topological 2 invariant unless the band gap closing and opening occur at odd number of time reversal invariant momentum (TRIM) points in the Brillouin zone and hence, the observed maximum in linewidth of an optical phonon mode is necessary but not sufÞcient to probe the 2 invariant. To the best of our knowledge, this is the Þrst report where vibrational properties through the linewidth of a Raman mode can capture the electronic topological transition of 2-type. In Chapter-4, we present Raman spectroscopic measurements on three strong topological insu-lators as a function of pressure. This chapter consists of two parts: In Part(I), we discuss the following (A) Bi2Te3; (B) Bi2Se3; and (C) Bi1Sb1Te1.25Se1.75. The stoichiometric materials of A2B3 type (Bi2Te3 and Bi2Se3) belong to the family of strong 2 TIs at ambient conditions. Previous high pressure Raman and xÐray diffraction studies showed that there is an isoÐstructural transition at low pressure regime in the range of 3 to 5 GPa where the parallel component of bulk modulus shows a kink. Earlier, this low pressure transition had been assigned to be an ETT based on the changes observed in structural parameters without a detailed exploration of electronic structure or direct evidence for change in electronic topology around the transition pressure. Here, we address this particular low pressure transition regime to clarify whether it is associated with the ETT or not ? We revisit this low pressure transition in the present work using high pressure Raman experi-ments along with Þrst-principles calculations on Bi2Se3 taken as a prototype of the family of A2B3 type TIs. We do not Þnd any change in electronic topology of the both types (ETT of 2Ðtype and ETT of LifshitzÐtype) as a function of pressure (P ≤ 8 GPa) by examining the density of states at Fermi level and the smallest electronic band gap as well as the 2 index and the surface Dirac conical electronic structure. The pressure derivatives of Raman modes show a clear change at 2.4 GPa, without the appearance of any new mode. Hence, the lowest pressure transition should be better termed as an isostructural transition, and not an ETT. Many authors assign this low pressure transition regime as an ETT for all the 3D-topological insulators just by observing the structural distortions and consequent changes in the phonon spectrum. The signature of ETT will reßect in the anomalies for the phonons, but the reverse is not true, i.e. it is not correct to assign an ETT prior to the detailed exploration of electronic band structure. In Part(II), we present Raman spectroscopic measurements on a weak topological insulator Bi1Se1 as a function of pressure. The essential thing in order to understand TIs is the concept of band inversion. What it means that band orderings of valence and conduction bands are changed in such a way that it cannot be connected adiabatically to the atomic limit of the system. When the bands are inverted, the phonons signatures will be useful in order to capture a transition to a band-inverted phase. If a weak topological insulator (WTI) can undergo a transition to a trivial insulator or to a strong topological insulator (STI) by inverting band structures at even or odd number of TRIMs respectively, the corresponding signatures will be reßected in the phonon spectrum. However, it is worthy to remind that WTIs have been identiÞed as strong/robust against the changes in the topology of the even number of Dirac cones at its surface as long as time reversal, translational invariance and (1) gauge symmetries are preserved on average. In our work, we have shown that with the application of hydrostatic pressure (as a tunable parameter) upto ∼ 7 GPa, there is no change in topological signature of Bi1Se1 by probing optical phonons using Raman spectroscopic measurements. Chapter-5 : It is not only the monolayer TMDs possessing tunable and electrical properties, they (TMDs) also exist in different bulk forms (polytypes) having different structure like 2H, 1T, 3R with different electrical and optical properties. Essentially the electrical and physical properties are not only limited to the dimension reduction or sample thickness, but also different crystal structures (polytypes) can also induce different electrical properties and high pressure technique is a powerful way to switch between different thermodynamically stable structural polytypes with-out introducing impurities unlike chemical doping. Recent high pressure Raman studies [Phys. Rev. Lett. 113, 036802 (2014) and J. Phys. Chem. C 118, 3230 (2014)] on 2H-MoS2 (P63/mmc) reveal that there is a onset of lateral shift of the adjacent S−Mo−S layers around ∼ 20 GPa lead-ing to a mixed phase of 2Hc (2H) and 2Ha structures with the 2Hc -phase being the dominant one [J. Phys. Chem. C 118, 3230 (2014)] and thereby changes the pressure coefÞcients of the Raman modes. Motivated by the results of high pressure research on MoS2, we investigated the high pressure induced different phase transitions in MoTe2 and MoSSe using Raman, XRD and Þrst-principles studies. In Part(A), we discuss the high pressure Raman spectroscopic studies of MoTe2 upto ∼ 29 GPa. We have observed a pressure induced semiconductor to semi-metal transition at ∼ 6 GPa and a Lifshitz transition at ∼ 18 GPa in 2H-MoTe2 by combining Raman measurements and Þrst-principles calculations. The frequencies of the Þrst order A1g and E12g Raman modes carry the signatures of semiconductor to semimetal and the Lifshitz transitions. The occurrence of a max-imum in the integrated area ratio of the A1g and E12g modes is mainly due to non-monotonous change in Raman tensor of E12g mode with pressure. DFT calculations of pressure effects on Ra-man active modes show that pressure inßuences the electron-phonon coupling of the A1g mode most strongly. In Part(B), we present high pressure Raman (upto ∼ 26 GPa) and XRD studies (upto ∼ 20 GPa) of MoSSe. All the three Mo-based TMDs (MoS2, MoSe2 and MoTe2) undergo semimetal transition and only for MoS2, iso-structural transition precedes the metallization phase. The rea-son possibly could be attributed to the d-electron propagation which favors the Mo-atoms on top of each other in metallic 2Ha-phase than semiconducting 2Hc (2H)-phase where Mo-atoms sit on top of S-atoms. However, in case of MoTe2 and MoSe2 the larger radius of chalcogen atoms increases the interlayer distance and thereby hinders the d-electron propagation mechanism, and hence favors the 2Hc -structural polytypes. The question we will address is that if we substitute one S-atom with the Se-atom, i.e. for MoSSe compound, what happens to pressure-induced phase transitions. We have shown that the substitution of S by Se reduces its crystal symmetry to P63mc (# 186) and an isostructural transition to a 2Ha-phase with the same space group takes place around 10.8 GPa, similar to the case of MoS2. We suggest that layer sliding transition completes around 18 GPa as reßected in our Raman data where few modes undergo a slope change in pressure-dependent frequency (ω) plot. We also observe a low pressure (P) transition around ∼ 2 GPa where Raman modes show a change in dω/dP, which is also reßected in c/a ratio variations obtained by synchrotron based x-ray diffraction experiments as a function of pressure. This low pressure transition at ∼ 2 GPa has not been reported so far for any of three MoX2 (X = S, Se and Te) compounds discussed before. Chapter–6 : Piezoelectric and electrostrictive materials are of signiÞcant interest in electrome-chanical devices to harvest energy as well as in a wide range of applications of sensors and ac-tuators. Here, we report in-situ Raman spectroscopic studies on multilayer MoS2 device to show that the observed changes in frequencies ( ω) of the A1g and E12g optical modes varies quadrati-cally as a function of applied electric Þeld ( ω ∝ E2DS ) due to the electrostrictive effects and to the best of our knowledge, this is the Þrst evidence of the observed electrostriction in multilayer MoS2. The observed ω are ∼ 1.6 cm−1 and 1.4 cm−1 for the A1g and E12g modes, respectively for the applied maximum electric Þeld of 45 kV/cm. The electrostrictive coefÞcient is ∼ 3.6 × 10−14 m2/V2, corresponding to ∼ 0.7 % in-plane uniaxial tensile strain. This value is ∼ 428 times higher than the most commonly used electrostrictive material PMN (PbMg1/3Nb2/3O3). We also carried out similar experiments on monolayer MoS2 device and the observed frequency of the A1g mode varies linearly ( ω ∝ EDS ) due to piezoelectric effect. The A1g mode hardens by ∼ 1.2 cm−1 with the applied electric Þled of 7.2 kV/cm. In Chapter–7, we present PL and Raman spectroscopic studies of monolayer MoS2 as a function of laser irradiation time. The two-dimensional monolayer MoS2 has enhanced Coulomb interactions than its bulk-counterpart. The change in the carrier concentration in the channel will lead to the renormalized binding energy for the relatively loosely bound trions and this fact will facilitate to modulate the PL spectrum of monolayer MoS2 as a function of channel doping. By depleting the excess electrons from the monolayer MoS2, we can not only destabilize the trion formation but also enhance radiative recombinations and hence, an increase in PL efÞciency. While extracting or adding electrons change the optical properties, it also renormalize phonons. To the best of our knowledge, there is no quantitative reported data on Raman study and the dynamics of enhanced trion as well as B-exciton PL emission as a function of laser irradiation time. Here, we show that exposing the monolayer MoS2 in air to a modest laser intensity for a brief period of time enhances simultaneously the PL intensity associated with both the trions and excitons, together with ∼ 3 to 5 times increase in the Raman intensity of Þrst and second order modes. The simultaneous increase of PL from trions and excitons cannot be understood based only on known-scenario of depletion of electron concentration in MoS2 by adsorption of O2 and H2O molecules. This is explained by laser induced healing of defect states resulting in reduction of non-radiative Auger processes. This laser healing is corroborated by an observed increase in intensity of both the Þrst order and second order 2LA(M) Raman modes by a factor of ∼ 3 to 5. The A1g mode hardens by ∼ 1.4 cm−1 whereas the E12g mode softens by ∼ 1 cm−1. The second order 2LA(M) Raman mode at ∼ 440 cm−1 shows an increase in wavenumber by ∼ 8 cm−1 with laser exposure. These changes are a combined effect of the change in electron concentrations and oxygen-induced lattice displacements. In Chapter–8, we have summarized our Þndings and highlighted few possibilities which can be pursued in future in order to have a better understanding of these systems.

Metal-based electronics remain one of the longstanding goals of researchers to achieve ultra-fast and radiation-hard electronic circuits. Generally, metals are primarily used as passive conductors in modern electronics and do not play an active role. Nanoscale materials with distinctive size-dependent properties provide opportunities to achieve new device functionalities. Ta-based di-chalcogenides, particularly 1T-TaS2 and 2H-TaSe2, which form layered structures and exhibit charge density waves (CDW), are promising in this context. CDW is a macroscopic state shown by materials with reduced dimensions, for example, one-dimensional and layered two-dimensional crystals. It results from the modulation in the electronic charge arising due to a periodic modulation in the crystal lattice. 1T-TaS2 exhibits one of the strongest known CDW characteristics enabling temperature-dependent distinct resistivity phases. The nearly commensurate (NC) to the incommensurate (IC) CDW phase transition that usually occurs at 353 K and can be driven electrically at room temperature is of high practical interest. However, resistivity switching during this phase transition is weak (< 2) and cannot be modulated by an external gate voltage – limiting its widespread usage. Using a back-gated 1T-TaS2/2H-MoS2 heterojunction, we show resistivity switching up to 17.3, which is ~14.5-fold higher than standalone TaS2. We demonstrate a low barrier electrical contact between a TaS2 source and a MoS2 channel, promising “all-2D” flexible electronics. Additionally, we show that the usual resistivity switching in TaS2 due to different phase transitions is accompanied by a surprisingly strong modulation in the Schottky barrier height (SBH) at the TaS2/MoS2 interface – providing an additional knob to control the degree of the phase-transition-driven resistivity switching by an external gate voltage. In particular, the commensurate (C) to triclinic (T) CDW phase transition increases the SBH owing to a collapse of the Mott gap in TaS2. The change in SBH allows us to estimate an electrical Mott gap opening of ~71 ± 7 meV in the C phase of TaS2. The results show a promising pathway to externally control and amplify the CDW induced resistivity switching. Further, we achieve gate- and light-controlled negative differential resistance (NDR) characteristics in an asymmetric 1T-TaS2/2H-MoS2 T-junction by exploiting the electrically driven CDW phase transition of TaS2. The device operation is purely governed by majority charge carriers, making it distinct from typical tunneling-based NDR devices, thus avoiding the bottleneck of weak tunneling efficiency in van der Waals heterojunctions. Consequently, we achieve a peak current density over 10^5 nA μm^(-2), which is about two orders of magnitude higher than that obtained in typical layered material-based NDR implementations. An external gate voltage and photo-gating can effectively tune the peak current density. The device characteristics show a peak-to-valley current ratio (PVCR) of 1.06 at 290 K, increasing to 1.59 at 180 K. To exploit the low thermal conductivity of 1T-TaS2 and 2H-TaSe2 in a local heater structure, we insert 2H-TaSe2 in between TaS2 and MoS2 layers, thereby forming a triple-layered 1T-TaS2/2H-TaSe2/2H-MoS2 T-junction. TaSe2 acts as a buffer layer preventing the CDW-induced SBH modulation at TaS2/MoS2 interface. This will allow efficient thermionic switching of carriers resulting from sharp temperature rise in the junction due to electrically driven TaS2 phase transitions. Interestingly, the device can toggle between the current increment and NDR characteristics by simply changing the biasing conditions. At TaS2 biasing, the heterostructure device shows a current increment by a factor of 3 at 300 K, which gets enhanced up to ~10^3 at 77 K, beneficial for various switching circuits and sensing applications. However, under TaSe2 biasing, the device exhibits NDR characteristics with a PVCR of 1.04 and 1.10 at 300 K and 77 K, respectively. The external back-gate voltage can effectively tune the current enhancement factor and NDR. The devices mentioned above are robust against ambiance-induced degradation, and the characteristics repeat in multiple measurements over more than six months. Conventional metals, in general, do not exhibit strong photoluminescence. However, we found that 2H-TaSe2 exhibits a surprisingly strong optical absorption and photoluminescence resulting from inter-band transitions. We use this perfect combination of electrical and optical properties in several optoelectronic applications. We show a seven-fold enhancement in the photoluminescence intensity of otherwise weakly luminescent multi-layer MoS2 through non-radiative resonant energy transfer from TaSe2 transition dipoles. Using a combination of scanning photocurrent and time-resolved photoluminescence measurements, we also show that the hot electrons generated by light absorption in TaSe2 have a relatively long lifetime, unlike conventional metals, making TaSe2 an excellent hot-electron injector. Finally, we show a vertical TaSe2/MoS2/graphene photodetector demonstrating a responsivity greater than 10 AW^(-1) at 0.1 MHz - one of the fastest reported photodetectors using MoS2. The findings will boost device applications that exploit CDW phase transitions, such as ultra-broadband photodetection, negative differential conductance, thermal sensors, fast oscillator, and threshold switching in neuromorphic chips. These functionalities will enable the implementation of active metal-based circuits.

2H/1T-1T' molybdenum disulfide(MoS2) are typical phase existed during the crystal or thin materials preparation. In this work, by controlling the atmosphere during the CVD thin film preparation, 2H/1T-1T' MoS2 are separately growth. Additionally, by z-scan technique, the phase dependent optical nonlinearity of MoS2 was observed and investigated.

The maybe superconductive and optobistable 2H MoS2 compound was first studied by a photoacoustic spectroscopic method at room temperature. Our PAS measurements were performed on a conventional PAS set with particular protection against thermal and vibrational disturbances. The incident light from a 250-W sodium lamp through a monochromator fell on the MoS2 cleavage surface of an ~50-μm thick sample, which was stuck to a piezoelectric ceramic sensor, in the measuring wavelength range from 540 to 740 nm at room temperature. We obtained three strong peaks at the absorption edge, namely, 595 nm (2.084 eV), 648 nm (1.913 eV), and 680 nm (1.823 eV), which agree well with respect to their positions and shapes with that obtained by Frindt and Yoffe1 at 77 K by an ordinary method. We verified these results further on electrolyte electromodulation spectra and photomodulation reflectance measurements. These two peaks at 595 and 648 nm can be identified by calculating the absorption spectrum by the well-known transmission formula T = (1 – R)exp(–al) or by, more formally, Fermi’s golden rule through Loudon’s scattering matrix elements from the band structure of 2H-MoS2 schemed in the paper of Evans and Young.2 The peak appearing at 680 nm is probably due to the impurity activities.

The Chevrel phase compounds including ternary compounds(1) had been well investigated in view to their high critical temperature superconductivity since seventies. In this concern it is reminiscent to call us to examine the structure of the excitonic spectra of MoS compounds, with important conception in mind in Little and Ginzburg sense.(2) In this paper we report we are able to reproduce the exciton spectra of 2H MoS2 at the absorption edge obtained by B.L. Evans and P. A. Young by conventional method at 77K(3), while ours by more sensitive photoacoustic spectroscopy method but at room temperature. It is easy to obtain the exciton spectra of the superconductive MoS compounds in studies of high Tc superconductive MoS cpmpounds at or near room temperature on this method.