Academic literature on the topic 'Molybdenum Ditelluride'

We demonstrate an all-fiberized passively Q-switched erbium-doped fiber laser (EDFL) via evanescent field interaction between molybdenum ditelluride saturable absorber (SA) and guided mode of the D-shaped fiber. By integrating the few-layer molybdenum ditelluride prepared by CVD method onto the side-polished fiber, the SA can be realized by the strong interaction between the evanescent field of the waveguide and the nonlinear optical material. The proposed passively Q-switched EDFL could deliver output pulses at 1566 nm wavelength with pulse width of 5.03 μs, a repetition rate of 13.9 kHz, a pulse energy of 150.6 nJ, and an output power of 2.1 mW when pumped by a 980 nm laser diode of 180 mW.

Recently, transition metal dichalcogenides have come into the spotlight after they have been found to become direct semiconductor in the monolayer level. The paper presents the results of the study of the relaxers distribution in layers of amorphous molybdenum ditelluride obtained by high-frequency magnetron sputtering. According to the obtained values of relaxation parameters α and β, the transition from an asymmetric distribution to a symmetric distribution of relaxators at temperature T=283 K can be stated. The existence of maxima on the temperature dependence of relaxation times taumax=f(T) was found, which can be associated with phase transitions in the studied system. Keywords: molybdenum ditelluride, relaxers distribution, thin layers, phase transitions.

Transition metal chalcogenide molybdenum ditelluride (MoTe2) thin films have been electrosynthesized cathodically on indium tin oxide-coated (ITO) conducting glass substrates from ammonaical solution of H2MoO4 and TeO2. The electrode potential was varied while the bath temperature was maintained at 40±1 oC and deposition time of 30 minutes. Highly textured MoTe2 films with polycrystalline nature are observed by X-ray diffraction analysis. Compositional analysis by EDX gives their stoichiometric relationships. Scanning electron microscope (SEM) was used to study surface morphology and shows that the films are smooth, uniform and useful for device fabrication. The optical absorption spectra showed that the material has an indirect band-gap value of 1.91-2.04 eV with different electrode potential. Besides, the film exhibited p-type semiconductor behavior. Keywords: Molybdenum ditelluride; Thin films; Electrodepositon; Solar cell;

Abstract Flexible electronic and optoelectronic devices are highly desirable for various emerging applications, such as human-computer interfaces, wearable medical electronics, flexible display, etc. Layered two-dimensional (2D) material is one of the most promising types of materials to develop flexible devices due to its atomically thin thickness, which gives it excellent flexibility and mechanical endurance. However, the 2D material devices fabricated on flexible substrate inevitably suffer from mechanical deformation, which can severely affect device performances, resulting in function degradation and even failure. In this work, we propose a strain insensitive flexible photodetector based on MoS2/MoTe2 heterostructure on polyimide substrate, which provides a feasible approach to cancel unpredicted impacts of strain on the device performances. Specifically, the MoS2/MoTe2 heterostructure is deposited with 4 electrodes to form three independent devices of MoS2 FET, MoTe2 FET and MoS2/MoTe2 heterojunction. Among them, the MoS2/MoTe2 heterojunction is used as the photodetector, while the MoS2 FET is used as a strain gauge to calibrate the photo detection result. Such configuration is enabled by the Schottky barrier formed between the electrodes and the MoS2 flake, which leads to obvious and negligible photo response of MoS2/MoTe2 heterojunction and MoS2 FET, respectively, under low source-drain bias (ex. 10 mV). The experimental results show that the proposed mechanism can not only calibrate the photo response to cancel strain effect, but also successfully differentiate the wavelength (with fixed power) or power (with fixed wavelength) of light illumination.

In this paper, near unity broadband absorption of Van der Waals semiconductors on a metallic substrate, and their photovoltaic performances in the visible spectrum are simulated. Ultrathin layered semiconductors such as Molybdenum disulfide (MoS2), Tungsten disulfide (WS2), Molybdenum di-selenide (MoSe2), Tungsten di-selenide (WSe2), Molybdenum ditelluride (MoTe2), and Tungsten ditelluride (WTe2) can create strong interference by damping optical mode in their multilayer form and increase light absorption at their heterojunctions with noble metals. From our simulation, it is observed that this absorbance can reach up to 94% when the semiconductors are placed on a gold substrate. The optimum thickness of these semiconductors in their heterostructures with gold is analyzed to create resonant absorption to generate the maximum amount of current density. The power conversion efficiency of the designed Schottky junction solar cells is calculated from their current density vs bias voltage characteristics that ranges from 1.57% to 6.80%. Moreover, the absorption coefficient, dark current characteristic, electric field intensity distribution in the device, and carrier generation rate during light illumination are presented with a view to characterizing and comparing among the parameters of TMDC based nanoscale solar cell.

In this thesis the layered, two-dimensional material MoTe2 is examined experimentally for its optoelectronic properties, using a field effect transistor device configuration. MoTe2 experiences a strong light matter interaction, which is highly dependent on the conditions of the measurement, and the wavelength of light used. Light is able to: produce a photocurrent in MoTe2, desorb adsorbates from the surface, and even controllably thin by a single layer at a time. A theoretical study on MoTe2 also provides insights on the source of some of these interesting light matter interactions. MoTe2 is found to be a fast and responsive photodetector when illuminated with red laser light in ambient conditions, with increases in current stemming from the photovoltaic effect. Due to the generated charge carriers from the photovoltaic effect, conductivity can increase by increasing the Fermi energy of the material, or by a photogating effect where excited charges are trapped and behave as an artificial gate for the field effect transistor. The mechanisms of charge trapping are experimentally investigated due to their prevalence in the photodetection mechanisms. A theoretical study points towards the existence of two types of trap states, in not just MoTe2 but all transition metal dichalcogenides, with shallow traps closer to the valence band edge (τ ~ 500 s) and deeper traps (τ ~ 1000 s), further away from the valence band edge. MoTe2, under the effects of higher energy photons from blue and green lasers, showed different photocurrent mechanisms to red light. From the increased energy of the photons, photo-desorption of adsorbates on the surface of MoTe2 occurred causing a decrease in the overall current, in a rarely seen photocurrent mechanism. Again, both shallow and deep traps are evident from the experimental measurements, with the shallow traps being removed when illuminated by higher energy photons. Finally, a humidity assisted photochemical layer-by-layer etching process was developed with an in-situ Raman spectroscopy system, able to thin MoTe2 by a single layer at a time with 200 nm spatial resolution. MoTe2 FETs were created with thinned channels to examine the effect of the thinning technique on optoelectronic properties. Some improvement in optoelectronic performance (higher responsivity, higher mobility) was seen for the thinned channel devices, with great improvement observed for monolayer MoTe2.

This thesis describes the fabrication and characterization of two-dimensional transition dichalcogenides (2D TMDs) nanolayers for various applications in electronic and opto-electronic devices applications. In Chapter 1, crystal and optical structure of TMDs materials are introduced. Many TMDs materials reveal three structure polytypes (1T, 2H, and 3R). The important electronic properties are determined by the crystal structure of TMDs; thus, the information of crystal structure is explained. In addition, the detailed information of photon vibration and optical band gap structure from single-layer to bulk TMDs materials are introduced in this chapter. In Chapter 2, detailed information of physical properties and synthesis techniques for molybdenum disulfide (MoS2), tungsten disulfide (WS2), and molybdenum ditelluride (MoTe2) nanolayers are explained. The three representative crystal structures are trigonal prismatic (hexagonal, H), octahedral (tetragonal, T), and distorted structure (Tʹ). At room temperature, the stable structure of MoS2 and WS2 is semiconducting 2H phase, and MoTe2 can reveal both 2H (semiconducting phase) and 1Tʹ (semi-metallic phase) phases determined by the existence of strains. In addition, the pros and cons of the synthesis techniques for nanolayers are discussed. In Chapter 3, the topic of synthesized large-scale MoS2, WS2, and MoTe2 films is considered. For MoS2 and WS2 films, the layer thickness is modulated from single-layer to multi-layers. The few-layer MoTe2 film is synthesized with two different phases (2H or 1Tʹ). The all TMDs films are fabricated using two-step chemical vapor deposition (CVD) method. The analyses of atomic force microscopy (AFM), high-resolution transmission electron microscopy (HRTEM), photoluminescence (PL), and Raman spectroscopy confirm that the synthesis of high crystalline MoS2, WS2, and MoTe2 films are successful. The electronic properties of both MoS2 and WS2 exhibit a p-type conduction with relatively high field effect mobility and current on/off ratio. In Chapter 4, vertically-stacked few-layer MoS2/WS2 heterostructures on SiO2/Si and flexible polyethylene terephthalate (PET) substrates is presented. Detailed structural characterizations by Raman spectroscopy and high-resolution/scanning transmission electron microscopy (HRTEM/STEM) show the structural integrity of two distinct 2D TMD layers with atomically sharp van der Waals (vdW) heterointerfaces. Electrical transport measurements of the MoS2/WS2 heterostructure reveal diode-like behavior with current on/off ratio of ~ 104. In Chapter 5, optically uniform and scalable single-layer Mo1-xWxS2 alloys are synthesized by a two-step CVD method followed by a laser thinning. Post laser treatment is presented for etching of few-layer Mo1-xWxS2 alloys down to single-layer alloys. The optical band gap is controlled from 1.871 to 1.971 eV with the variation in the tungsten (W) content, x = 0 to 1. PL and Raman mapping analyses confirm that the laser-thinning of the Mo1-xWxS2 alloys is a self-limiting process caused via heat dissipation to SiO2/Si substrate, resulting in fabrication of spatially uniform single-layer Mo1-xWxS2 alloy films.

碩士
國立交通大學
電子研究所
106
Because of lack of dangling bonds at the contact interface, transition metal dichalcogenides (TMDs) encounter a significant challenge of reducing contact resistance. Heterophase edge-contacted structures, which form chemical bonds at the lateral interface, are promising solutions for the contact problem of TMD materials. Because of the similar ground-state energy of 2H and 1T’ phases of MoTe2, MoTe2 with a smaller phase transition barrier is considered as a highly potential candidate for realizing TMD devices with a heterophase contact. In this thesis, we successfully demonstrated heterophase edge-contacted MoTe2 back-gate transistors on a SiO2 substrate. We provided several methods for synthesizing 1T’-phase MoTe2 by sputtering amorphous MoTe2 films on the SiO2 substrate, such as control of synthesis temperature, atmosphere treatment, distilled water immersion, oxygen treatment and diluted HF treatment. By using the diluted HF treatment method, 1T’-phase MoTe2 was selectively formed at designated source/drain region treated by diluted HF while 2H-phase MoTe2 was formed at the channel region without treatment. Palladium (Pd) and nickel (Ni) was deposited by electron beam evaporation as the contact metal. 1T’-MoTe2/ Pd interface is an ohmic-like contact because of their metallic properties. The major contact barrier that influences devices performance is the heterophase interface. To investigate the properties of heterophase interface of MoTe2, both the traditional metal top-contacted structure and heterophase edge-contacted structure were simulated by the density functional theory (DFT) using the Vienna Ab initio simulation package (VASP). Schottky barrier height was extracted by two improved layer-decomposed density of states methods. The in-plane averaged electric potential method was used to evaluate the tunnel barrier at the interface. The traditional metal top-contacted structures have large tunnel barriers at the interface. By contrast, the heterophase edge-contacted structure shows a lower tunnel barrier and a comparable Schottky barrier height. The results suggest that the reduction of tunnel barrier at the interface is the main advantage of using the heterophase edge-contacted structure.

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.