Group VI Transition Metal Dichalcogenides Research Articles

(Invited) Spin-Valley Coupling in Monolayer Transition Metal Dichalcogenides

The monolayers of group VI transition metal dichalcogenides feature a valence band spin splitting with opposite sign in the two valleys located at corners of 1st Brillouin zone. This spin-valley coupling, particularly pronounced in tungsten dichalcogenides, can benefit potential spintronics and valleytronics with the important consequences of spin-valley interplay and the suppression of spin and valley relaxations. In this talk we discuss the spin-valley coupling in monolayers and multilayers WS2. We explored the interplay of spin and valley degrees of freedom with photocurrent experiments. We experimentally demonstrate that this giant spin-valley coupling, together with the valley dependent physical properties, may lead to rich possibilities for manipulating spin and valley degrees of freedom in these atomically thin 2D materials.

Environmental Changes in MoTe2 Excitonic Dynamics by Defects-Activated Molecular Interaction.

Monolayers of group VI transition metal dichalcogenides possess direct gaps in the visible spectrum with the exception of MoTe2, where its gap is suitably located in the infrared region but its stability is of particular interest, as tellurium compounds are acutely sensitive to oxygen exposure. Here, our environmental (time-dependent) measurements reveal two distinct effects on MoTe2 monolayers: For weakly luminescent monolayers, photoluminescence signal and optical contrast disappear, as if they are decomposed, but yet remain intact as evidenced by AFM and Raman measurements. In contrast, strongly luminescent monolayers retain their optical contrast for a prolonged amount of time, while their PL peak blue-shifts and PL intensity saturates to slightly lower values. Our X-ray photoelectron spectroscopy measurements and DFT calculations suggest that the presence of defects and functionalization of these defect sites with O2 molecules strongly dictate their material properties and aging response by changing the excitonic dynamics due to deep or shallow states that are created within the optical band gap. Presented results not only shed light on environmental effects on fundamental material properties and excitonic dynamics of MoTe2 monolayers but also highlight striking material transformation for metastable 2D systems such as WTe2, silicone, and phosphorene.

Lighten the Olympia of the Flatland: Probing and Manipulating the Photonic Properties of 2D Transition-Metal Dichalcogenides.

Following the adventures of graphene, 2D transition metal dichalcogenides (TMDs) have recently seized part of the territory in the flatland. Branched by different components of metals and chalcogenides, the families of 2D TMDs have grown rapidly, in which the semiconductive ones have shown colorful photonic properties. By tuning the atomic components and reducing the thickness or planar size of the layers, one can manipulate the optical performance of 2D TMDs, e.g., the intensity, angular momentum, and frequency of the emitted light, or toward ultrafast nonlinear absorption. As a powerful optical method, the Raman characteristics of 2D TMDs have been successfully used to explore their lattices and electronic structures. Along with the maturing of 2D TMDs, their hybrids play an important role. The unique photonic properties of 2D van der Waals heterostructures and 2D alloys are introduced here. Apart from the group VI TMDs, future prospects are identified to harness the optical properties of other 2D TMDs and the related investigations of their hybrids are underway.

An optical spectroscopic study on two-dimensional group-VI transition metal dichalcogenides.

The ultimate goal of making atomically thin electronic devices stimulates intensive research on layered materials, in particular the group-VI transition metal dichalcogenides (TMDs). Atomically thin group-VI TMD crystals with a 2H stacking order emerging as a family of intrinsic 2-dimensional (2D) semiconductors with a sizeable bandgap in the visible and near infrared range satisfy numerous requirements for ultimate electronics and optoelectronics. In addition, the characteristic inversion symmetry breaking presented in monolayer TMDs leads to non-zero but contrasting Berry curvatures and orbit magnetic moments at K/K' valleys located at the corners of the first Brillouin zone. These features provide an opportunity to manipulate electrons' additional internal degrees of freedom, namely the valley degree of freedom, making monolayer TMDs a promising candidate for the conceptual valleytronics. Besides, the strong spin-orbit interactions and the subsequent spin-valley coupling demonstrated in atomically thin group-VI TMDs open up potential routes towards quantum manipulation. In this tutorial review, we highlight recent advances in the optical study on electronic structures, vibrational properties, excitonic effects, valley dependent optical selection rules, and the interplay of valley, spin, and layer degrees of freedoms in this class of atomic 2D semiconductors including MoS2, MoSe2, WS2, and WSe2.

Studies of Metal Dichalcogenide Formation Using Electrochemical ALD

Group VI transition metal dichalcogenides (TMDCs) are graphene-like 2D layered semiconductors that exhibit anisotropic conductivity, which favors conductivity along the basal planes.1 In particular, MoS2, MoSe2 and WSe2 have received special interests for their potential toward photoelectrochemical and photoelectronic applications. These materials have been shown to be viable hydrogen evolution reaction (HER) catalysts in photoelectrochemical water-splitting devices.1-2 Applications in solar energy conversion are feasible due to the direct band gaps these materials possess, which have energies range from near-IR to visible. They are also highly stable against photocorrosion due to the d-d transition that occur upon photoexcitation.3 Among the TMDCs, MoSe2 synthesis and characterization is currently the main focus in this group. In photovoltaic (PV) devices that commonly use Mo as the back contact, having MoSe2 as the intermediate layer between the PV absorber and the Mo back contact may create a more favorable ohmic-type contact that reduces resistive losses.4 A lower resistivity ohmic contact helps to reduce carrier recombination and improves conversion efficiency of a PV device.Formation of MoSe2 nanofilms by electrochemical atomic layer deposition (E-ALD) is currently being investigated by this group. E-ALD can be used to grow high quality nanofilms by alternating precursor solutions through a flow cell to deposit the desired elements up to one atomic layer at a time. Such fine control is possible by utilizing the underpotential deposition (UPD) of elements on each other. Pure Mo electrodeposition is hampered by the presence of HER, but “induced co-deposition” is possible by electrodepositing Mo in the presence of other metal ions.5 In E-ALD of MoSe2, UPD Se plays the role of this other metal ion. Electrochemical studies for growing MoSe2 by E-ALD as well as characterization of preliminary MoSe2 films will be presented.1. Tang, H.; Dou, K. P.; Kaun, C. C.; Kuang, Q.; Yang, S. H., MoSe2 nanosheets and their graphene hybrids: synthesis, characterization and hydrogen evolution reaction studies. J Mater Chem A 2014, 2 (2), 360-364.2. (a) Velazquez, J. M.; Saadi, F. H.; Pieterick, A. P.; Spurgeon, J. M.; Soriaga, M. P.; Brunschwig, B. S.; Lewis, N. S., Synthesis and hydrogen-evolution activity of tungsten selenide thin films deposited on tungsten foils. J Electroanal Chem 2014, 716 (0), 45-48; (b) Jaramillo, T. F.; Jorgensen, K. P.; Bonde, J.; Nielsen, J. H.; Horch, S.; Chorkendorff, I., Identification of active edge sites for electrochemical H-2 evolution from MoS2 nanocatalysts. Science 2007, 317 (5834), 100-102.3. Pathak, V. M.; Patel, K. D.; Pathak, R. J.; Srivastava, R., Improved photoconversion from MoSe2 based PEC solar cells. Sol Energ Mat Sol C 2002, 73 (2), 117-123.4. Wada, T.; Kohara, N.; Nishiwaki, S.; Negami, T., Characterization of the Cu(In,Ga)Se-2/Mo interface in CIGS solar cells. Thin Solid Films 2001, 387 (1-2), 118-122.5. (a) Chandra, S.; Sahu, S. N., Electrodeposited semiconducting molybdenum selenide films. I. Preparatory technique and structural characterisation. Journal of Physics D: Applied Physics 1984, 17 (10), 2115; (b) Delphine, S. M.; Jayachandran, M.; Sanjeeviraja, C., Pulsed electrodeposition and characterization of molybdenum diselenide thin film. Mater Res Bull 2005, 40 (1), 135-147.

Valley-splitting and valley-dependent inter-Landau-level optical transitions in monolayerMoS2quantum Hall systems

The valley dependent optical selection rules in recently discovered monolayer group-VI transition metal dichalcogenides (TMDs) make possible optical control of valley polarization, a crucial step towards valleytronic applications. However, in presence of Landaul level(LL) quantization such selection rules are taken over by selection rules between the LLs, which are not necessarily valley contrasting. Using MoS$_{2}$ as an example we show that the spatial inversion-symmetry breaking results in unusual valley dependent inter-LL selection rules, which directly locks polarization to valley. We find a systematic valley splitting for all Landau levels (LLs) in the quantum Hall regime, whose magnitude is linearly proportional to the magnetic field and in comparable with the LL spacing. Consequently, unique plateau structures are found in the optical Hall conductivity, which can be measured by the magneto-optical Faraday rotations.

Spin-orbit-coupled quantum wires and Majorana fermions on zigzag edges of monolayer transition-metal dichalcogenides

Majorana fermions, quantum particles with non-Abelian exchange statistics, are not only of fundamental importance, but also building blocks for fault-tolerant quantum computation. Although certain experimental breakthroughs for observing Majorana fermions have been made recently, their conclusive dection is still challenging due to the lack of proper material properties of the underlined experimental systems. Here we propose a new platform for Majorana fermions based on edge states of certain non-topological two-dimensional semiconductors with strong spin-orbit coupling, such as monolayer group-VI transition metal dichalcogenides (TMD). Using first-principles calculations and tight-binding modeling, we show that zigzag edges of monolayer TMD can host well isolated single edge band with strong spin-orbit coupling energy. Combining with proximity induced s-wave superconductivity and in-plane magnetic fields, the zigzag edge supports robust topological Majorana bound states at the edge ends, although the two-dimensional bulk itself is non-topological. Our findings points to a controllable and integrable platform for searching and manipulating Majorana fermions.

The Study of Spin‐Valley Coupling in Atomically Thin Group VI Transition Metal Dichalcogenides

In the hunt for ultimately thin electronic devices, atomically thin layers of group VI transition metal dichalcogenides (TMDCs) are recognized as ideal 2D materials after the success of graphene. Monolayer TMDCs feature nonzero but contrasting Berry curvatures and valence-band spin splitting with opposite sign at inequivalent K and K' valleys located at the corners of the 1st Brillouin zone. These features raise the possibility of manipulating electrons' valley and spin degrees of freedom by optical and electric means, which subsequently makes monolayer TMDCs promising candidates for spintronics and valleytronics applications.

Magnetoelectric effects and valley-controlled spin quantum gates in transition metal dichalcogenide bilayers

In monolayer group-VI transition metal dichalcogenides, charge carriers have spin and valley degrees of freedom, both associated with magnetic moments. On the other hand, the layer degree of freedom in multilayers is associated with electrical polarization. Here we show that transition metal dichalcogenide bilayers offer an unprecedented platform to realize a strong coupling between the spin, valley and layer pseudospin of holes. Such coupling gives rise to the spin Hall effect and spin-dependent selection rule for optical transitions in inversion symmetric bilayer and leads to a variety of magnetoelectric effects permitting quantum manipulation of these electronic degrees of freedom. Oscillating electric and magnetic fields can both drive the hole spin resonance where the two fields have valley-dependent interference, making an interplay between the spin and valley as information carriers possible for potential valley-spintronic applications. We show how to realize quantum gates on the spin qubit controlled by the valley bit.

Electrical control of neutral and charged excitons in a monolayer semiconductor

Monolayer group-VI transition metal dichalcogenides have recently emerged as semiconducting alternatives to graphene in which the true two-dimensionality is expected to illuminate new semiconducting physics. Here we investigate excitons and trions (their singly charged counterparts), which have thus far been challenging to generate and control in the ultimate two-dimensional limit. Utilizing high-quality monolayer molybdenum diselenide, we report the unambiguous observation and electrostatic tunability of charging effects in positively charged (X(+)), neutral (X(o)) and negatively charged (X(-)) excitons in field-effect transistors via photoluminescence. The trion charging energy is large (30 meV), enhanced by strong confinement and heavy effective masses, whereas the linewidth is narrow (5 meV) at temperatures <55 K. This is greater spectral contrast than in any known quasi-two-dimensional system. We also find the charging energies for X(+) and X(-) to be nearly identical implying the same effective mass for electrons and holes.

Structural and Optical Characterization of Tungsten Diselenide Crystals Grown by DVT Technique

WSe2 is a member of groupVI Transition Metal Dichalcogenides (TMDCs) and has been observed to be a highly stable semiconducting material. It has been grown in crystal form using a direct vapour transport technique in present case. The grown WSe2 crystals were characterized for the structural properties using X-ray diffraction technique (XRD). The hexagonal structure was confirmed through this analysis. Using the data of XRD, various parameters like crystalline size, lattice parameters, micro strain, dislocation density, unit cell volume, unit cell density, growth fault and deformation fault probability etc were found. It was seen that the micro strain, dislocation density and growth &amp; deformation fault probabilities were found to be very low in WSe2 crystals. The grown WSe2 crystals were characterized for optical properties using UV-VIS-NIR spectroscopy. The absorption spectra of WSe2 grown in present case revealed the fact that WSe2 posses direct band gap around 1.38eV. Layered di-chalcogenides WSe2 (or MoSe2) are used for desired applications such as in photo-conversion devices.

Intervalley Scattering and Localization Behaviors of Spin-Valley Coupled Dirac Fermions

We study the quantum diffusive transport of multivalley massive Dirac cones, where time-reversal symmetry requires opposite spin orientations in inequivalent valleys. We show that the intervalley scattering and intravalley scattering can be distinguished from the quantum conductivity that corrects the semiclassical Drude conductivity, due to their distinct symmetries and localization trends. In immediate practice, it allows transport measurements to estimate the intervalley scattering rate in hole-doped monolayers of group-VI transition metal dichalcogenides (e.g., molybdenum dichalcogenides and tungsten dichalcogenides), an ideal class of materials for valleytronics applications. The results can be generalized to a large class of multivalley massive Dirac systems with spin-valley coupling and time-reversal symmetry.

Intrinsic spin Hall effect in monolayers of group-VI dichalcogenides: A first-principles study

Using first-principles calculations within density functional theory, we investigate the intrinsic spin Hall effect in monolayers of group-VI transition-metal dichalcogenides MX2 (M = Mo, W and X = S, Se). MX2 monolayers are direct band-gap semiconductors with two degenerate valleys located at the corners of the hexagonal Brillouin zone. Because of the inversion symmetry breaking and the strong spin-orbit coupling, charge carriers in opposite valleys carry opposite Berry curvature and spin moment, giving rise to both a valley- and a spin-Hall effect. The intrinsic spin Hall conductivity (ISHC) in p-doped samples is found to be much larger than the ISHC in n-doped samples due to the large spin-splitting at the valence band maximum. We also show that the ISHC in inversion-symmetric bulk dichalcogenides is an order of magnitude smaller compared to monolayers. Our result demonstrates monolayer dichalcogenides as an ideal platform for the integration of valleytronics and spintronics.

Highly conductive group VI transition metal dichalcogenide films by solution-processed deposition

A new soluble synthetic route was developed to fabricate thin films of layered structure transition metal dichalcogendies, MoS2 and WS2. High-quality thin films of the dichalcogenides were prepared using new soluble precursors, (CH3NH3)2MS4 (M = Mo, W). The precursors were dissolved in organic solvents and spun onto substrates via both single- and multistep spin coating procedures. The thin films were formed by the thermal decomposition of the coatings under inert atmosphere. Structural, electrical, optical absorption, thermal, and transport properties of the thin films were characterized. Surface morphology of the films was analyzed by atomic force microscopy and scanning electron microscopy. Highly conductive and textured n-type MoS2 films were obtained. The measured room temperature conductivity ∼50 Ω−1 cm−1 is substantially higher than the previously reported values. The n-type WS2 films were prepared for the first time using solution-processed deposition. WS2 displays a conductivity of ∼6.7 Ω−1 cm−1 at room temperature.

Tellurium depletion electrical effects in MoTe2-x single crystals doped with bromine

Electrical conductivity and thermoelectric power measurements have been performed on n-type MoTe2-single crystals in a wide temperature range (125–770 K). Stoichiometric samples were prepared by vapour transport with bromine as transport reagent. The experimental results which have been performed along a direction perpendicular to the c-axis, are analysed with the model already used for p-type MoTe2-x single crystals grown in a molten tellurium bath without any contaminating reagent. A bromine donor level which is located near the bottom of the conduction band is found and takes into account all the new experimental results. The semiconductor is compensated and the random potential due to the charged lacunar sites and bromine impurities induces a broadening of this level into a narrow band for the compound with the larger tellurium depletion. This band contributes to the conduction in the form of thermally activated hopping. The schematic band structure deduced from transport measurements is in good agreement with the electronic band structure of the group VI transition metal dichalcogenides with trigonal prism coordination.

Use of molybdenum telluride as a substrate for the imaging of biological molecules during scanning tunnelling microscopy

Scanning tunnelling microscopy was used to image biological molecules including supercoiled deoxyribonacetic acid and specific retrovirus enzymes, the reverse transcriptases of the avian myeloblastosis virus, the moloney murine leukaemia virus and the human immunodeficiency virus. Measurements were carried out on graphite and Group VI transition metal dichalcogenide layered crystals. Images obtained with graphite could not be unequivocally interpreted and attachment appears to occur solely at surface defect sites. The layered crystal MoTe2 shows different imaging properties. The bimolecules are clearly visible, distributed over the semiconductor surface, and the molecular shapes and dimensions show good correlation with structure predictions.

Rapid solid-state synthesis of materials from molybdenum disulphide to refractories

LAYERED molybdenum and tungsten dichalcogenides have stimulated considerable interest as lubricants1,2, battery cathodes3–5, and catalysts6–8. Pure, crystalline group VI transition-metal dichalcogenides are normally prepared by intermittently grinding and heating the elements at >900 °C for several days. Low-temperature solution routes to these materials have also been explored9–11. In those studies, exchange (metathesis) reactions between transition-metal halides and alkali-metal sulphides or covalent sulphiding agents in polar organic solvents were found generally to yield finely divided products at close to ambient conditions. Here, in contrast, we consider the factors that influence reactions between transition-metal halides and alkali-metal chalcogenides in the solid state. These highly energetic reactions, driven by the formation of very stable product species, provide a powerful method for the rapid synthesis of materials normally prepared at high temperatures over long periods of time. Other advantages of these solid-state reactions include control of reaction conditions and of product particle sizes. Although our study focuses on group VI transition-metal dichalcogenides, analogous metathesis reactions can be used for rapid syntheses, initiated at low temperatures, of many other technologically important materials.

ChemInform Abstract: DETAILED PHOTOCURRENT SPECTROSCOPY OF THE SEMICONDUCTING GROUP VIB TRANSITION METAL DICHALCOGENIDES

AbstractDie Photostromspektren von Einkristallen der Dichalcogenide i M082, WS, , WSeZ und Mose, werden als Funktion der Kristallorientierung und der Oberflächenmorphologie sowie der Polarisation und des Einfallswinkels der Strahlung gemessen und mit einer revidierten Bandkantenanalyse‐ausgewertet sowie die Übergangsenergien abgeschätzt.