Showing papers on "Dielectric published in 2014"

Quantum spin Hall effect in two-dimensional transition metal dichalcogenides

Abstract: Quantum spin Hall (QSH) effect materials feature edge states that are topologically protected from backscattering. However, the small band gap in materials that have been identified as QSH insulators limits applications. We use first-principles calculations to predict a class of large-gap QSH insulators in two-dimensional transition metal dichalcogenides with 1T′ structure, namely, 1T′-MX 2 with M = (tungsten or molybdenum) and X = (tellurium, selenium, or sulfur). A structural distortion causes an intrinsic band inversion between chalcogenide- p and metal- d bands. Additionally, spin-orbit coupling opens a gap that is tunable by vertical electric field and strain. We propose a topological field effect transistor made of van der Waals heterostructures of 1T′-MX 2 and two-dimensional dielectric layers that can be rapidly switched off by electric field through a topological phase transition instead of carrier depletion.

Measurement of the optical dielectric function of monolayer transition-metal dichalcogenides: MoS 2 , Mo S e 2 , WS 2 , and WS e 2

Abstract: This chapter presents the complex in-plane dielectric function from 1.5 to 3 eV for monolayers of four transition metal dichalcogenides: MoSe2, WSe2, MoS2, and WS2. The results were obtained from optical reflection spectra using a Kramers–Kronig constrained variational analysis. From the inferred dielectric functions, we obtain the absolute absorbance of the monolayers. We also provide a comparison of the dielectric function for the monolayers with the respective bulk materials [1].

Highly aligned graphene/polymer nanocomposites with excellent dielectric properties for high-performance electromagnetic interference shielding.

Abstract: Nanocomposites that contain reinforcements with preferred orientation have attracted significant attention because of their promising applications in a wide range of multifunctional fields. Many efforts have recently been focused on developing facile methods for preparing aligned graphene sheets in solvents and polymers because of their fascinating properties including liquid crystallinity and highly anisotropic characteristics. Self-aligned in situ reduced graphene oxide (rGO)/polymer nanocomposites are prepared using an all aqueous casting method. A remarkably low percolation threshold of 0.12 vol% is achieved in the rGO/epoxy system owing to the uniformly dispersed, monolayer graphene sheets with extremely high aspect ratios (>30000). The self-alignment into a layered structure at above a critical filler content induces a unique anisotropy in electrical and mechanical properties due to the preferential formation of conductive and reinforcing networks along the alignment direction. Accompanied by the anisotropic electrical conductivities are exceptionally high dielectric constants of over 14000 with 3 wt% of rGO at 1 kHz due to the charge accumulation at the highly-aligned conductive filler/insulating polymer interface according to the Maxwell-Wagner-Sillars polarization principle. The highly dielectric rGO/epoxy nanocomposites with the engineered structure and properties present high performance electromagnetic interference shielding with a remarkable shilding efficiency of 38 dB.

A Review on Conduction Mechanisms in Dielectric Films

Abstract: The conduction mechanisms in dielectric films are crucial to the successful applications of dielectric materials. There are two types of conduction mechanisms in dielectric films, that is, electrode-limited conduction mechanism and bulk-limited conduction mechanism. The electrode-limited conduction mechanism depends on the electrical properties at the electrode-dielectric interface. Based on this type of conduction mechanism, the physical properties of the barrier height at the electrode-dielectric interface and the effective mass of the conduction carriers in dielectric films can be extracted. The bulk-limited conduction mechanism depends on the electrical properties of the dielectric itself. According to the analyses of bulk-limited conduction mechanisms, several important physical parameters in the dielectric films can be obtained, including the trap level, the trap spacing, the trap density, the carrier drift mobility, the dielectric relaxation time, and the density of states in the conduction band. In this paper, the analytical methods of conduction mechanisms in dielectric films are discussed in detail.

Dielectric Meta-Reflectarray for Broadband Linear Polarization Conversion and Optical Vortex Generation

Abstract: Plasmonic metasurfaces have recently attracted much attention due to their ability to abruptly change the phase of light, allowing subwavelength optical elements for polarization and wavefront control. However, most previously demonstrated metasurface designs suffer from low coupling efficiency and are based on metallic resonators, leading to ohmic loss. Here, we present an alternative approach to plasmonic metasurfaces by replacing the metallic resonators with high-refractive-index silicon cut-wires in combination with a silver ground plane. We experimentally demonstrate that this meta-reflectarray can be used to realize linear polarization conversion with more than 98% conversion efficiency over a 200 nm bandwidth in the short-wavelength infrared band. We also demonstrate optical vortex beam generation using a meta-reflectarray with an azimuthally varied phase profile. The vortex beam generation is shown to have high efficiency over a wavelength range from 1500 to 1600 nm. The use of dielectric resonato...

Photoinduced Giant Dielectric Constant in Lead Halide Perovskite Solar Cells.

Abstract: Organic–inorganic lead trihalide perovskites have emerged as an outstanding photovoltaic material that demonstrated a high 17.9% conversion efficiency of sunlight to electricity in a short time. We have found a giant dielectric constant (GDC) phenomenon in these materials consisting on a low frequency dielectric constant in the dark of the order of e0 = 1000. We also found an unprecedented behavior in which e0 further increases under illumination or by charge injection at applied bias. We observe that e0 increases nearly linearly with the illumination intensity up to an additional factor 1000 under 1 sun. Measurement of a variety of samples of different morphologies, compositions, and different types of contacts shows that the GDC is an intrinsic property of MAPbX3 (MA = CH3NH3+). We hypothesize that the large dielectric response is induced by structural fluctuations. Photoinduced carriers modify the local unit cell equilibrium and change the polarizability, assisted by the freedom of rotation of MA. The ...

A family of oxide ion conductors based on the ferroelectric perovskite Na0.5Bi0.5TiO3

Abstract: Oxide ion conductors find important technical applications in electrochemical devices such as solid-oxide fuel cells (SOFCs), oxygen separation membranes and sensors. Na0.5Bi0.5TiO3 (NBT) is a well-known lead-free piezoelectric material; however, it is often reported to possess high leakage conductivity that is problematic for its piezo- and ferroelectric applications. Here we report this high leakage to be oxide ion conduction due to Bi-deficiency and oxygen vacancies induced during materials processing. Mg-doping on the Ti-site increases the ionic conductivity to ~0.01 S cm(-1) at 600 °C, improves the electrolyte stability in reducing atmospheres and lowers the sintering temperature. This study not only demonstrates how to adjust the nominal NBT composition for dielectric-based applications, but also, more importantly, gives NBT-based materials an unexpected role as a completely new family of oxide ion conductors with potential applications in intermediate-temperature SOFCs and opens up a new direction to design oxide ion conductors in perovskite oxides.

Field-Effect Transistors Built from All Two-Dimensional Material Components

Abstract: We demonstrate field-effect transistors using heterogeneously stacked two-dimensional materials for all of the components, including the semiconductor, insulator, and metal layers. Specifically, MoS2 is used as the active channel material, hexagonal-BN as the top-gate dielectric, and graphene as the source/drain and the top-gate contacts. This transistor exhibits n-type behavior with an ON/OFF current ratio of >106, and an electron mobility of ∼33 cm2/V·s. Uniquely, the mobility does not degrade at high gate voltages, presenting an important advantage over conventional Si transistors where enhanced surface roughness scattering severely reduces carrier mobilities at high gate-fields. A WSe2–MoS2 diode with graphene contacts is also demonstrated. The diode exhibits excellent rectification behavior and a low reverse bias current, suggesting high quality interfaces between the stacked layers. In this work, all interfaces are based on van der Waals bonding, presenting a unique device architecture where crystal...

Dielectric Screening of Excitons and Trions in Single-Layer MoS2

Abstract: Photoluminescence (PL) properties of single-layer MoS2 are indicated to have strong correlations with the surrounding dielectric environment. Blue shifts of up to 40 meV of exciton or trion PL peaks were observed as a function of the dielectric constant of the environment. These results can be explained by the dielectric screening effect of the Coulomb potential; based on this, a scaling relationship was developed with the extracted electronic band gap and exciton and trion binding energies in good agreement with theoretical estimations. It was also observed that the trion/exciton intensity ratio can be tuned by at least 1 order of magnitude with different dielectric environments. Our findings are helpful to better understand the tightly bound exciton properties in strongly quantum-confined systems and provide a simple approach to the selective and separate generation of excitons or trions with potential applications in excitonic interconnects and valleytronics.

High-Sensitivity Metamaterial-Inspired Sensor for Microfluidic Dielectric Characterization

Abstract: A new metamaterial-inspired microwave microfluidic sensor is proposed in this paper. The main part of the device is a microstrip coupled complementary split-ring resonator (CSRR). At resonance, a strong electric field will be established along the sides of CSRR producing a very sensitive area to a change in the nearby dielectric material. A micro-channel is positioned over this area for microfluidic sensing. The liquid sample flowing inside the channel modifies the resonance frequency and peak attenuation of the CSRR resonance. The dielectric properties of the liquid sample can be estimated by establishing an empirical relation between the resonance characteristics and the sample complex permittivity. The designed microfluidic sensor requires a very small amount of sample for testing since the cross-sectional area of the sensing channel is over five orders of magnitude smaller than the square of the wavelength. The proposed microfluidic sensing concept is compatible with lab-on-a-chip platforms owing to its compactness.

Electrochemical dynamics of nanoscale metallic inclusions in dielectrics

Abstract: Nanoscale metal inclusions play an important role in solid-state dielectric devices. Here, the authors demonstrate that these inclusions can change their shape, size and position in response to an applied electric field, and that electrochemical processes can lead to metal cluster nucleation and growth.

The effect of the substrate on the Raman and photoluminescence emission of single-layer MoS2

Abstract: We quantitatively study the Raman and photoluminescence (PL) emission from single-layer molybdenum disulfide (MoS2) on dielectric (SiO2, hexagonal boron nitride, mica and the polymeric dielectric Gel-Film®) and conducting substrates (Au and few-layer graphene). We find that the substrate can affect the Raman and PL emission in a twofold manner. First, the absorption and emission intensities are strongly modulated by the constructive/destructive interference within the different substrates. Second, the position of the A1g Raman mode peak and the spectral weight between neutral and charged excitons in the PL spectra are modified by the substrate. We attribute this effect to substrate-induced changes in the doping level and in the decay rates of the excitonic transitions. Our results provide a method to quantitatively study the Raman and PL emission from MoS2-based vertical heterostructures and represent the first step in ad hoc tuning the PL emission of 1L MoS2 by selecting the proper substrate.

Tellurite Glasses Handbook: Physical Properties and Data

Abstract: Introduction to and Formation of Tellurite Glasses Crystal Structure and General Properties of Tellurium Oxide (TeO2) Definition of Tellurite Glasses and Review of Early Research Preparation of Pure TeO2 Glass Glass-Forming Ranges of Binary TeO2 Glasses Glass-Forming Ranges of Multicomponent TeO2 Glasses Nonoxide-Tellurite Glasses Mixed Oxyhalide and Oxysulfate Tellurite Glasses General Physical Characteristics and Structure of Tellurite Glasses Structure and Bonding Nature of Tellurite Glasses New Tellurite Glasses Applications of Tellurite Glass and Tellurite Glass Ceramics References ELASTIC AND ACOUSTIC RELAXATION PROPERTIES Elastic Moduli of Tellurite Glasses Elastic Properties of Glass Experimental Techniques Elasticity Modulus Data of TeO2 Crystal Elastic Modulus Data of Pure TeO2 Glass Constants of Elasticity of Binary and Ternary Transition Metal Tellurite Glasses Comparison between MoO3 and V2O5 in Tellurite and Phosphate Glasses and K-V Relations Application of Makishima-Mackenzie Model to Pure TeO2, TeO2-V2O5, and TeO2-MoO3 Glasses Elastic Moduli and Vickers Hardness of Binary, Ternary, and Quaternary Rare-Earth Tellurite Glasses and Glass-Ceramics Quantitative Analysis of the Elasticity Moduli of Rare-Earth Tellurite Glasses Elastic Properties of Te Glasses Elastic Properties of New Tellurite Glasses Acoustic Relaxation Properties of Tellurite Glasses Introduction Ultrasonic Attenuation of Oxide-Tellurite Glasses at Low Temperature Properties of Ultrasonic Attenuation in Nonoxide-Tellurite Glasses Radiation Effect on Ultrasonic Attenuation Coefficient and Internal Friction of Tellurite Glasses Structural Analysis of Ultrasonic Attenuation and Relaxation Phenomena Correlations between Low-Temperature Ultrasonic Attenuation and Room-Temperature Elastic Moduli Acousto-Optical Properties of Tellurite Glasses Applications of Ultrasonics on Tellurite Glasses Introduction Ultrasonic Detection of Microphase Separation in Tellurite Glasses Debye Temperature of Oxide and Nonoxide-Tellurite Glasses References THERMAL PROPERTIES Thermal Properties of Tellurite Glasses Introduction Experimental Techniques of Measuring Thermal Properties of Glass Data of the Thermal Properties of Tellurite Glasses Glass Transformation and Crystallization Activation Energies Correlations between Glass Transformation Temperature and Structure Parameters Correlations between Thermal Expansion Coefficient and Vibrational Properties Tellurite Glass-Ceramics Nonoxide-Tellurite Glasses Thermal Properties of New Types of Tellurite Glass ELECTRICAL PROPERTIES Electrical Conductivity of Tellurite Glasses Introduction to Current-Voltage Drop and Semiconducting Characteristics of Tellurite Glasses Experimental Procedure to Measure Electrical Conductivity Theoretical Considerations in the Electrical Properties of Glasses DC Electrical-Conductivity Data of Tellurite Glasses at Different Temperatures AC Electrical Conductivity Data of Tellurite Glasses Electrical Conductivity Data of Tellurite Glass-Ceramics Electrical Conductivity Data of New Tellurite Glasses Dielectric Properties of Tellurite Glasses Introduction Experimental Measurement of Dielectric Constants Dielectric Constant Models Dielectric Constant Data of Oxide-Tellurite Glasses Dielectric Constant Data of Nonoxide-Tellurite Glasses Dielectric and Magnetic Data of New Tellurite Glasses and Ceramics References OPTICAL PROPERTIES Linear and Nonlinear Optical Properties of Tellurite Glasses in the Visible Region Introduction to Optical Constants Experimental Measurements of Optical Constants Theoretical Analysis of Optical Constants Linear Refractive Index Data of Tellurite Glasses Nonlinear Refractive Index Data of Tellurite Glasses Optical Applications of Oxide-Tellurite Glass and Glass-Ceramics (Thermal Luminescence Fluorescence Spectra) Optical Properties of New Tellurium Glass Optical Properties of Tellurite Glasses in the Ultraviolet Region Introduction-Absorption, Transmission, and Reflectance Experimental Procedure to Measure UV Absorption and Transmission Spectra Theoretical Absorption Spectra, Optical Energy Gap, and Tail Width UV Properties of Tellurite Glasses (Absorption, Transmission, and Spectra) UV Properties of New Tellurite Glasses Infrared and Raman Spectra of Tellurite Glasses Introduction Experimental Procedure to Identify Infrared and Raman Spectra of Tellurite Glasses Theoretical Considerations for Infrared and Raman Spectra of Glasses Infrared Spectra of Tellurite Glasses Raman Spectra of Tellurite Glasses Infrared and Raman Spectra of New Tellurite Glasses References Index

Exploring Strategies for High Dielectric Constant and Low Loss Polymer Dielectrics.

Abstract: Polymer dielectrics having high dielectric constant, high temperature capability, and low loss are attractive for a broad range of applications such as film capacitors, gate dielectrics, artificial muscles, and electrocaloric cooling. Unfortunately, it is generally observed that higher polarization or dielectric constant tends to cause significantly enhanced dielectric loss. It is therefore highly desired that the fundamental physics of all types of polarization and loss mechanisms be thoroughly understood for dielectric polymers. In this Perspective, we intend to explore advantages and disadvantages for different types of polarization. Among a number of approaches, dipolar polarization is promising for high dielectric constant and low loss polymer dielectrics, if the dipolar relaxation peak can be pushed to above the gigahertz range. In particular, dipolar glass, paraelectric, and relaxor ferroelectric polymers are discussed for the dipolar polarization approach.

Electromagnetic properties of Si–C–N based ceramics and composites

Abstract: Besides the excellent high-temperature mechanical properties, Si3N4 and SiC based ceramics containing insulating or electrically conductive phase are attractive for their tunable dielectric propert...

High Energy and Power Density Capacitors from Solution-Processed Ternary Ferroelectric Polymer Nanocomposites

Abstract: Concurrent improvements in dielectric constant and breakdown strength are attained in a solution-processed ternary ferroelectric polymer nanocomposite incorporated with two-dimensional boron nitride nanosheets and zero-dimensional barium titanate nanoparticles that synergistically interact to enable a remarkable energy-storage capability, including large discharged energy density, high charge-discharge efficiency, and great power density.

Advances in small lasers

Abstract: The latest developments in laser miniaturization, including those based on metals and dielectrics, are reviewed and future challenges outlined. Small lasers have dimensions or modes sizes close to or smaller than the wavelength of emitted light. In recent years there has been significant progress towards reducing the size and improving the characteristics of these devices. This work has been led primarily by the innovative use of new materials and cavity designs. This Review summarizes some of the latest developments, particularly in metallic and plasmonic lasers, improvements in small dielectric lasers, and the emerging area of small bio-compatible or bio-derived lasers. We examine the different approaches employed to reduce size and how they result in significant differences in the final device, particularly between metal- and dielectric-cavity lasers. We also present potential applications for the various forms of small lasers, and indicate where further developments are required.

Efficient carrier transport in halide perovskites: theoretical perspectives

Abstract: Halide perovskites have recently been shown to exhibit excellent carrier transport properties. Density functional calculations are performed to study the electronic structure, dielectric properties, and defect properties of β-CH3NH3PbI3. The results show that Pb chemistry plays an important role in a wide range of material properties, i.e., small effective masses, enhanced Born effective charges and lattice polarization, and the suppression of the formation of deep defect levels, all of which contribute to the exceptionally good carrier transport properties observed in CH3NH3PbI3. Defect calculations show that, among native point defects (including vacancies, interstitials, and antisites), only iodine vacancy is a low-energy deep trap and non-radiative recombination centre. Alloying iodide with chloride reduces the lattice constant of the iodide and significantly increases the formation energy of interstitial defects, which explains the observed substantial increase in carrier diffusion length in mixed halide CH3NH3PbI2Cl compared to that in CH3NH3PbI3.

Back gated multilayer InSe transistors with enhanced carrier mobilities via the suppression of carrier scattering from a dielectric interface.

Abstract: The back gate multilayer InSe FETs exhibit ultrahigh carrier mobilities, surpassing all the reported layer semiconductor based electronics with the same device configuration, which is achieved by the suppression of the carrier scattering from interfacial coulomb impurities or surface polar phonons at the interface of an oxidized dielectric substrate. The room-temperature mobilities of multilayer InSe transistors increase from 64 cm(2)V(-1)s(-1) to 1055 cm(2)V(-1)s(-1) using a bilayer dielectric of poly-(methyl methacrylate) (PMMA)/Al2O3. The transistors also have high current on/off ratios of 1 × 10(8), low standby power dissipation, and robust current saturation in a broad voltage range.

Elastic strain engineering of ferroic oxides

Abstract: Using epitaxy and the misfit strain imposed by an underlying substrate, it is possible to elastically strain oxide thin films to percent levels—far beyond where they would crack in bulk. Under such strains, the properties of oxides can be dramatically altered. In this article, we review the use of elastic strain to enhance ferroics, materials containing domains that can be moved through the application of an electric field (ferroelectric), a magnetic field (ferromagnetic), or stress (ferroelastic). We describe examples of transmuting oxides that are neither ferroelectric nor ferromagnetic in their unstrained state into ferroelectrics, ferromagnets, or materials that are both at the same time (multiferroics). Elastic strain can also be used to enhance the properties of known ferroic oxides or to create new tunable microwave dielectrics with performance that rivals that of existing materials. Results show that for thin films of ferroic oxides, elastic strain is a viable alternative to the traditional method of chemical substitution to lower the energy of a desired ground state relative to that of competing ground states to create materials with superior properties.

Topological‐Structure Modulated Polymer Nanocomposites Exhibiting Highly Enhanced Dielectric Strength and Energy Density

Abstract: Dielectric materials with high electric energy densities and low dielectric losses are of critical importance in a number of applications in modern electronic and electrical power systems. An organic–inorganic 0–3 nanocomposite, in which nanoparticles (0-dimensional) are embedded in a 3-dimensionally connected polymer matrix, has the potential to combine the high breakdown strength and low dielectric loss of the polymer with the high dielectric constant of the ceramic fillers, representing a promising approach to realize high energy densities. However, one significant drawback of the composites explored up to now is that the increased dielectric constant of the composites is at the expense of the breakdown strength, limiting the energy density and dielectric reliability. In this study, by expanding the traditional 0–3 nanocomposite approach to a multilayered structure which combines the complementary properties of the constituent layers, one can realize both greater dielectric displacement and a higher breakdown field than that of the polymer matrix. In a typical 3-layer structure, for example, a central nanocomposite layer of higher breakdown strength is introduced to substantially improve the overall breakdown strength of the multilayer-structured composite film, and the outer composite layers filled with large amount of high dielectric constant nanofillers can then be polarized up to higher electric fields, hence enhancing the electric displacement. As a result, the topological-structure modulated nanocomposites, with an optimally tailored nanomorphology and composite structure, yield a discharged energy density of 10 J/cm3 with a dielectric breakdown strength of 450 kV mm–1, much higher than those reported from all earlier studies of nanocomposites.

Fabrication, characterization, properties and theoretical analysis of ceramic/PVDF composite flexible films with high dielectric constant and low dielectric loss

Abstract: Calcium barium zirconate titanate (Ba0.95Ca0.05Zr0.15Ti0.85O3, BCZT) ceramic particles were prepared by a conventional solid-state method. BCZT powders were modified by dopamine through a chemical coating method. The composite flexible films based on dopamine@BCZT and polyvinylidene fluoride were fabricated via a solution casting method. The microstructure and morphology were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, high-resolution transmission electron microscopy, and field emission scanning electron microscopy. A precision impedance analyzer and a dielectric withstand voltage test were used to test the dielectric constant, loss tangent, and breakdown strength. TEM results showed that dopamine was uniformly coated on the surface of BCZT particles with an average thickness of 20 nm. SEM results showed that the ceramic particles were dispersed homogeneously in the matrix. The dielectric constant increased with the increase of BCZT contents, while the loss tangent remained constant in the frequency range of 103 to 105 Hz. Different theoretical models were employed to predict the effective dielectric constants of the composite films, and the estimated results were compared with the experimental data. Weibull distribution was used to analyze the dielectric breakdown strength, and the results showed that the breakdown strength decreased then stayed over 60 kV mm−1.

Impact of different dopants on the switching properties of ferroelectric hafniumoxide

Abstract: The wake-up behavior of ferroelectric thin film capacitors based on doped hafnium oxide dielectrics in TiN-based metal–insulator–metal structures is reported. After field cycling a remanent polarization up to 40 µC/cm2 and a high coercive field of about 1 MV/cm was observed. Doping of HfO2 by different dopants with a crystal radius ranging from 54 pm (Si) to 132 pm (Sr) was evaluated. In all cases, an improved polarization–voltage hysteresis after wake-up cycling is visible. For smaller dopant atoms like Si and Al stronger pinching of the polarization hysteresis appeared with increasing dopant concentration and proved to be stable during cycling.

Self-consistent hybrid functional for condensed systems

Abstract: A self-consistent scheme for determining the optimal fraction of exact exchange for full-range hybrid functionals is presented and applied to the calculation of band gaps and dielectric constants of solids. The exchange-correlation functional is defined in a similar manner to the PBE0 functional, but the mixing parameter is set equal to the inverse macroscopic dielectric function and it is determined self-consistently by computing the optimal dielectric screening. We found excellent agreement with experiments for the properties of a broad class of systems, with band gaps ranging between 0.7 and 21.7 eV and dielectric constants within 1.23 and 15.9. We propose that the eigenvalues and eigenfunctions obtained with the present self-consistent hybrid scheme may be excellent inputs for ${G}_{0}{W}_{0}$ calculations.

Field-Effect Transistors Based on Few-Layered alpha-MoTe_2

Abstract: Here we report the properties of field-effect transistors based on few layers of chemical vapor transport grown alpha- MoTe_2 crystals mechanically exfoliated onto SiO_2. We performed field-effect and Hall mobility measurements, as well as Raman scattering and transmission electron microscopy. In contrast to both MoS_2 and MoSe_2, our MoTe_2 field-effect transistors (FETs) are observed to be hole-doped, displaying on/off ratios surpassing 106 and typical sub-threshold swings of ~ 140 mV per decade. Both field-effect and Hall mobilities indicate maximum values approaching or surpassing 10 cm^2/Vs which are comparable to figures previously reported for single or bi-layered MoS_2 and/or for MoSe_2 exfoliated onto SiO_2 at room temperature and without the use of dielectric engineering. Raman scattering reveals sharp modes in agreement with previous reports, whose frequencies are found to display little or no dependence on the number of layers. Given that both MoS_2 is electron doped, the stacking of MoTe_2 onto MoS_2 could produce ambipolar field-effect transistors and a gap modulation. Although the overall electronic performance of MoTe_2 is comparable to those of MoS_2 and MoSe_2, the heavier element Te should lead to a stronger spin orbit-coupling and possibly to concomitantly longer decoherence times for exciton valley and spin indexes.