Darshana Wickramaratne

Tin Disulfide—An Emerging Layered Metal Dichalcogenide Semiconductor: Materials Properties and Device Characteristics

Layered metal dichalcogenides have attracted significant interest as a family of single- and few-layer materials that show new physics and are of interest for device applications. Here, we report a comprehensive characterization of the properties of tin disulfide (SnS2), an emerging semiconducting metal dichalcogenide, down to the monolayer limit. Using flakes exfoliated from layered bulk crystals, we establish the characteristics of single- and few-layer SnS2 in optical and atomic force microscopy, Raman spectroscopy and transmission electron microscopy. Band structure measurements in conjunction with ab initio calculations and photoluminescence spectroscopy show that SnS2 is an indirect bandgap semiconductor over the entire thickness range from bulk to single-layer. Field effect transport in SnS2 supported by SiO2/Si suggests predominant scattering by centers at the support interface. Ultrathin transistors show on-off current ratios >10(6), as well as carrier mobilities up to 230 cm(2)/(V s), minimal hysteresis, and near-ideal subthreshold swing for devices screened by a high-k (deionized water) top gate. SnS2 transistors are efficient photodetectors but, similar to other metal dichalcogenides, show a relatively slow response to pulsed irradiation, likely due to adsorbate-induced long-lived extrinsic trap states.

Gate tunable quantum oscillations in air-stable and high mobility few-layer phosphorene heterostructures

As the only non-carbon elemental layered allotrope, few-layer black phosphorus or phosphorene has emerged as a novel two-dimensional (2D) semiconductor with both high bulk mobility and a band gap. Here we report fabrication and transport measurements of phosphorene-hexagonal BN (hBN) heterostructures with one-dimensional edge contacts. These transistors are stable in ambient conditions for >300 h, and display ambipolar behavior, a gate-dependent metal?insulator transition, and mobility up to 4000 cm2 V?1 s?1. At low temperatures, we observe gate-tunable Shubnikov de Haas magneto-oscillations and Zeeman splitting in magnetic field with an estimated g-factor ?2. The cyclotron mass of few-layer phosphorene (FLP) holes is determined to increase from 0.25 to 0.31 me as the Fermi level moves towards the valence band edge. Our results underscore the potential of FLP as both a platform for novel 2D physics and an electronic material for semiconductor applications.

Electronic and thermoelectric properties of few-layer transition metal dichalcogenides

The electronic and thermoelectric properties of one to four monolayers of MoS2, MoSe2, WS2, and WSe2 are calculated. For few layer thicknesses, the near degeneracies of the conduction band K and Σ valleys and the valence band Γ and K valleys enhance the n-type and p-type thermoelectric performance. The interlayer hybridization and energy level splitting determine how the number of modes within kBT of a valley minimum changes with layer thickness. In all cases, the maximum ZT coincides with the greatest near-degeneracy within kBT of the band edge that results in the sharpest turn-on of the density of modes. The thickness at which this maximum occurs is, in general, not a monolayer. The transition from few layers to bulk is discussed. Effective masses, energy gaps, power-factors, and ZT values are tabulated for all materials and layer thicknesses.

Charge Density Waves in Exfoliated Films of van der Waals Materials: Evolution of Raman Spectrum in TiSe2

A number of the charge-density-wave materials reveal a transition to the macroscopic quantum state around 200 K. We used graphene-like mechanical exfoliation of TiSe(2) crystals to prepare a set of films with different thicknesses. The transition temperature to the charge-density-wave state was determined via modification of Raman spectra of TiSe(2) films. It was established that the transition temperature can increase from its bulk value to ~240 K as the thickness of the van der Waals films reduces to the nanometer range. The obtained results are important for the proposed applications of such materials in the collective-state information processing, which require room-temperature operation.

Direct Bandgap Transition in Many-Layer MoS2 by Plasma-Induced Layer Decoupling

We report a robust method for engineering the optoelectronic properties of many-layer MoS2 using low-energy oxygen plasma treatment. Gas phase treatment of MoS2 with oxygen radicals generated in an upstream N2 -O2 plasma is shown to enhance the photoluminescence (PL) of many-layer, mechanically exfoliated MoS2 flakes by up to 20 times, without reducing the layer thickness of the material. A blueshift in the PL spectra and narrowing of linewidth are consistent with a transition of MoS2 from indirect to direct bandgap material. Atomic force microscopy and Raman spectra reveal that the flake thickness actually increases as a result of the plasma treatment, indicating an increase in the interlayer separation in MoS2 . Ab initio calculations reveal that the increased interlayer separation is sufficient to decouple the electronic states in individual layers, leading to a transition from an indirect to direct gap semiconductor. With optimized plasma treatment parameters, we observed enhanced PL signals for 32 out of 35 many-layer MoS2 flakes (2-15 layers) tested, indicating that this method is robust and scalable. Monolayer MoS2 , while direct bandgap, has a small optical density, which limits its potential use in practical devices. The results presented here provide a material with the direct bandgap of monolayer MoS2 , without reducing sample thickness, and hence optical density.

Zone-Folded Phonons and the Commensurate–Incommensurate Charge-Density-Wave Transition in 1T-TaSe2 Thin Films

Bulk 1T-TaSe2 exhibits unusually high charge density wave (CDW) transition temperatures of 600 and 473 K below which the material exists in the incommensurate (I-CDW) and the commensurate (C-CDW) charge-density-wave phases, respectively. The (13)(1/2) × (13)(1/2) C-CDW reconstruction of the lattice coincides with new Raman peaks resulting from zone-folding of phonon modes from middle regions of the original Brillouin zone back to Γ. The C-CDW transition temperatures as a function of film thickness are determined from the evolution of these new Raman peaks, and they are found to decrease from 473 to 413 K as the film thicknesses decrease from 150 to 35 nm. A comparison of the Raman data with ab initio calculations of both the normal and C-CDW phases gives a consistent picture of the zone-folding of the phonon modes following lattice reconstruction. The Raman peak at ∼154 cm(-1) originates from the zone-folded phonons in the C-CDW phase. In the I-CDW phase, the loss of translational symmetry coincides with a strong suppression and broadening of the Raman peaks. The observed change in the C-CDW transition temperature is consistent with total energy calculations of bulk and monolayer 1T-TaSe2.Bulk 1T-TaSe2 exhibits unusually high charge density wave (CDW) transition temperatures of 600 K and 473 K below which the material exists in the incommensurate (I-CDW) and the commensurate (C-CDW) charge-density-wave phases, respectively. The C-CDW reconstruction of the lattice coincides with new Raman peaks resulting from zone-folding of phonon modes from middle regions of the original Brillouin zone back to the Gamma point. The C-CDW transition temperatures as a function of film thickness are determined from the evolution of these new Raman peaks and they are found to decrease from 473K to 413K as the film thicknesses decrease from 150 nm to 35 nm. A comparison of the Raman data with ab initio calculations of both the normal and C-CDW phases gives a consistent picture of the zone-folding of the phonon modes following lattice reconstruction. In the I-CDW phase, the loss of translational symmetry coincides with a strong suppression and broadening of the Raman peaks. The observed change in the C-CDW transition temperature is consistent with total energy calculations of bulk and monolayer 1T-TaSe2.

Electronic and thermoelectric properties of van der Waals materials with ring-shaped valence bands

The valence band of a variety of few-layer, two-dimensional materials consists of a ring of states in the Brillouin zone. The energy-momentum relation has the form of a ‘Mexican hat’ or a Rashba dispersion. The two-dimensional density of states is singular at or near the band edge, and the band-edge density of modes turns on nearly abruptly as a step function. The large band-edge density of modes enhances the Seebeck coefficient, the power factor, and the thermoelectric figure of merit ZT. Electronic and thermoelectric properties are determined from ab initio calculations for few-layer III-VI materials GaS, GaSe, InS, InSe, for Bi2Se3, for monolayer Bi, and for bilayer graphene as a function of vertical field. The effect of interlayer coupling on these properties in few-layer III-VI materials and Bi2Se3 is described. Analytical models provide insight into the layer dependent trends that are relatively consistent for all of these few-layer materials. Vertically biased bilayer graphene could serve as an experimental test-bed for measuring these effects.The valence band of a variety of few-layer, two-dimensional materials consist of a ring of states in the Brillouin zone. The energy-momentum relation has the form of a “Mexican hat” or a Rashba dispersion. The two-dimensional density of states is singular at or near the band edge, and the band-edge density of modes turns on nearly abruptly as a step function. The large band-edge density of modes enhances the Seebeck coefficient, the power factor, and the thermoelectric figure of merit ZT. Electronic and thermoelectric properties are determined from ab initio calculations for few-layer III–VI materials GaS, GaSe, InS, InSe, for Bi2Se3, for monolayer Bi, and for bilayer graphene as a function of vertical field. The effect of interlayer coupling on these properties in few-layer III–VI materials and Bi2Se3 is described. Analytical models provide insight into the layer dependent trends that are relatively consistent for all of these few-layer materials. Vertically biased bilayer graphene could serve as an experimental test-bed for measuring these effects.

All-metallic electrically gated 2H-TaSe2 thin-film switches and logic circuits

We report the fabrication and performance of all-metallic three-terminal devices with tantalum diselenide thin-film conducting channels. For this proof-of-concept demonstration, the layers of 2H-TaSe2 were exfoliated mechanically from single crystals grown by the chemical vapor transport method. Devices with nanometer-scale thicknesses exhibit strongly non-linear current-voltage characteristics, unusual optical response, and electrical gating at room temperature. We have found that the drain-source current in thin-film 2H-TaSe2–Ti/Au devices reproducibly shows an abrupt transition from a highly resistive to a conductive state, with the threshold tunable via the gate voltage. Such current-voltage characteristics can be used, in principle, for implementing radiation-hard all-metallic logic circuits. These results may open new application space for thin films of van der Waals materials.

Synthesis of Atomically Thin

Atomically thin molybdenum disulfide (MoS2) triangles and hexagrams were prepared by a two-step growth ambient pressure chemical vapor deposition (APCVD) process. Molybdenum Trioxide (MoO3) nanobelts, a few microns in length and width, were prepared using a hydrothermal technique and utilized as the starting material. High temperature treatment of the MoO3 nanobelts followed by a rigorous sulfurization via APCVD processing provided different morphologies of MoS2 monolayers and bilayer (BL) sheets. Triangle and hexagram morphologies were characterized using Raman spectroscopy, photoluminescence (PL) measurements, scanning electron microscopy and atomic force microscopy (AFM). The regrowth step in the CVD process was proven to be ideal in enlarging the grain size. PL and Raman spectroscopy and AFM results confirmed the presence of monolayer and BL regions in the regrowth growth process. Triangle and hexagram domains are observed to be cooperatively nucleating and coalescing together to form large-area layers. Furthermore, the electrical transport properties of the synthesized MoS2 layers were studied. Electron mobility based on back gated field effect transistors was measured to be approximately 0.02 cm2/V. S.

{\bf MoS}_{\bf 2}

Tin sulfides can exist in a variety of phases and polytypes due to the different oxidation states of Sn. A subset of these phases and polytypes take the form of layered 2D structures that give rise to a wide host of electronic and optical properties. Hence, achieving control over the phase, polytype, and thickness of tin sulfides is necessary to utilize this wide range of properties exhibited by the compound. This study reports on phase-selective growth of both hexagonal tin (IV) sulfide SnS2 and orthorhombic tin (II) sulfide SnS crystals with diameters of over tens of microns on SiO2 substrates through atmospheric pressure vapor-phase method in a conventional horizontal quartz tube furnace with SnO2 and S powders as the source materials. Detailed characterization of each phase of tin sulfide crystals is performed using various microscopy and spectroscopy methods, and the results are corroborated by ab initio density functional theory calculations.