arXiv subjects

Goki Eda

Publications and source records attributed to Goki Eda.

At least 37 records · Page 2Linked to original sources

arXiv2019

Characterization of the second- and third-harmonic optical susceptibilities of atomically thin tungsten diselenide

We report the first detailed characterization of the sheet third-harmonic optical susceptibility, $χ_{s}^{(3)}$, of tungsten diselenide (WSe$_{2}$). With a home-built confocal microscope setup developed to study harmonics generation, we map the second- and third-harmonic intensities as a function of position in the sample, pump power and polarization angle, for single and few layers flakes of WSe$_{2}$. We register a value of $|χ_{s}^{(3)}| \approx$ 0.91 $\times$ 10$^{-28}$ m$^{3}$ V$^{-2}$ at a fundamental excitation frequency of $\hbarω$ = 0.8 eV, which is comparable in magnitude to the third-harmonic susceptibility of other group-VI transition metal dichalcogenides. The simultaneously recorded second-harmonic susceptibility is found to be $|χ_{s}^{(2)}| \approx$ 0.70 $\times$ 10$^{-19}$ m$^{2}$ V$^{-1}$ in very good agreement on the order of magnitude with recent reports for WSe$_{2}$, which asserts the robustness of our values for $|χ_{s}^{(3)}|$.

physics.optics↗

arXiv2018

Suppressed out-of-plane polarizability of free excitons in monolayer WSe$_{2}$

Monolayer semiconductors are atomically thin quantum wells with strong confinement of electrons in two-dimensional (2D) plane. Here, we experimentally study the out-of-plane polarizability of excitons in hBN-encapsulated monolayer WSe$_{2}$ in strong electric fields of up to 1.6 V/nm (16 MV/cm). We monitor free and bound exciton photoluminescence peaks with increasing electric fields at a constant carrier density, carefully compensating for unintentional photodoping in our double-gated device at 4K. We show that the Stark shift is < 0.4 meV despite the large electric fields applied, yielding an upper limit of polarizability α$_{z}$ to be ~ 10$^{-11}$ Dm/V. Such a small polarizability, which is nearly two orders of magnitude smaller than the previously reported value for MoS$_{2}$, indicates strong atomic confinement of electrons in this 2D system and highlights the unusual robustness of free excitons against surface potential fluctuations.

cond-mat.mes-hall↗

arXiv2018

Vapor-Liquid-Solid Growth of Monolayer MoS2 Nanoribbons

Chemical vapor deposition (CVD) of two-dimensional (2D) materials such as monolayer MoS2 typically involves the conversion of vapor-phase precursors to a solid product in a process that may be described as a vapor-solid-solid (VSS) mode. Here, we report the first demonstration of vapor-liquid-solid (VLS) growth of monolayer MoS2 yielding highly crystalline ribbon-shaped structures with a width of a few tens of nanometers to a few micrometers. The VLS growth mode is triggered by the reaction between molybdenum oxide and sodium chloride, which results in the formation of molten Na-Mo-O droplets. These droplets mediate the growth of MoS2 ribbons in the "crawling mode" when saturated with sulfur on a crystalline substrate. Our growth yields straight and kinked ribbons with a locally well-defined orientation, reflecting the regular horizontal motion of the liquid droplets during growth. Using atomic-resolution scanning transmission electron microscopy (STEM) and second harmonic generation (SHG) microscopy, we show that the ribbons are homoepitaxially on monolayer MoS2 surface with predominantly 2H- or 3R-type stacking. These findings pave the way to novel devices with structures of mixed dimensionalities.

cond-mat.mtrl-sci↗

arXiv2016

Topological Weyl phase transition in Mo$_x$W$_{1-x}$Te$_2$

Topological phases of matter exhibit phase transitions between distinct topological classes. These phase transitions are exotic in that they do not fall within the traditional Ginzburg-Landau paradigm but are instead associated with changes in bulk topological invariants and associated topological surface states. In the case of a Weyl semimetal this phase transition is particularly unusual because it involves the creation of bulk chiral charges and the nucleation of topological Fermi arcs. Here we image a topological phase transition to a Weyl semimetal in Mo$_x$W$_{1-x}$Te$_2$ with changing composition $x$. Using pump-probe ultrafast angle-resolved photoemission spectroscopy (pump-probe ARPES), we directly observe the nucleation of a topological Fermi arc at $x_c \sim 7\%$, showing the critical point of a topological Weyl phase transition. For Mo dopings $x < x_c$, we observe no Fermi arc, while for $x > x_c$, the Fermi arc gradually extends as the bulk Weyl points separate. Our results demonstrate for the first time the creation of magnetic monopoles in momentum space. Our work opens the way to manipulating chiral charge and topological Fermi arcs in Weyl semimetals for transport experiments and device applications.

cond-mat.mtrl-sci↗

arXiv2016

Discovery of a new type of topological Weyl fermion semimetal state in Mo$_x$W$_{1-x}$Te$_2$

The recent discovery of a Weyl semimetal in TaAs offers the first Weyl fermion observed in nature and dramatically broadens the classification of topological phases. However, in TaAs it has proven challenging to study the rich transport phenomena arising from emergent Weyl fermions. The series Mo$_x$W$_{1-x}$Te$_2$ are inversion-breaking, layered, tunable semimetals already under study as a promising platform for new electronics and recently proposed to host Type II, or strongly Lorentz-violating, Weyl fermions. Here we report the discovery of a Weyl semimetal in Mo$_x$W$_{1-x}$Te$_2$ at $x = 25\%$. We use pump-probe angle-resolved photoemission spectroscopy (pump-probe ARPES) to directly observe a topological Fermi arc above the Fermi level, demonstrating a Weyl semimetal. The excellent agreement with calculation suggests that Mo$_x$W$_{1-x}$Te$_2$ is the first Type II Weyl semimetal. We also find that certain Weyl points are at the Fermi level, making Mo$_x$W$_{1-x}$Te$_2$ a promising platform for transport and optics experiments on Weyl semimetals.

cond-mat.mtrl-sci↗

arXiv2016

Enhancing charge-density-wave order in 1T-TiSe2 nanosheet by encapsulation with hexagonal Boron Nitride

Layered transition metal dichalcogenides (TMDs) provide an ideal platform for exploring the effects of dimensionality on correlated electronic phases such as charge density wave (CDW) order. When TMDs are reduced in thickness to the 2-D limit, it is expected that the substrates will exert considerable influence on the electron states. Here we report a study of the charge density wave (CDW) state in 1T-TiSe2 nanosheets of different thicknesses when the sheets are encapsulated by hexagonal Boron Nitride (h-BN) or supported on SiO2 substrate. Our results show that dimensionality reduction results in an enhancement of CDW order and that disorder and substrate phonons tends to destroy CDW order, preventing observation of intrinsic CDW transition in ultrathin samples. Encapsulated 10 nm thick 1T-TiSe2 samples exhibit intrinsic CDW with transition temperature as high as 235 K. Our study points out that choosing the right substrate is important in the search for room temperature CDW materials.

cond-mat.mes-hall↗

arXiv2016

Controlling many-body states by the electric-field effect in a two-dimensional material

To understand complex physics of a system with strong electron electron interactions, it is ideal to control and monitor its properties while tuning an external electric field applied to the system. Indeed, complete electric field control of many body states in strongly correlated electron systems is fundamental to the next generation of condensed matter research and devices. However, the material must be thin enough to avoid shielding of the electric field in bulk material. Two-dimensional materials do not experience electrical screening, and their charge carrier density can be controlled by gating. 1T TiSe2 is a prototypical 2D material that shows charge density wave(CDW) and superconductivity in its phase diagram, presenting several similarities with other layered systems such as copper oxides, iron pnictides, crystals of rare-earth and actinide atoms. By studying 1T TiSe2 single crystals with thicknesses of 10 nm or less, encapsulated in 2D layers of hexagonal boron nitride, we achieve unprecedented control over the CDW transition temperature, tuned from 170 K to 40 K, and over the superconductivity transition temperature, tuned from a quantum critical point at 0 K up to 3 K. Electrically driving TiSe2 over different ordered electronic phases allows us to study the details of the phase transitions between many-body states. Observations of periodic oscillations of magnetoresistance induced by the Little Parks effect show that the appearance of superconductivity is directly correlated to the spatial texturing of the amplitude and phase of the superconductivity order parameter, corresponding to a 2D matrix of superconductivity. We infer that this superconductivity matrix is supported by a matrix of incommensurate CDW states embedded in the commensurate CDW states. Our results show that spatially modulated electronic states are fundamental to the appearance of 2D superconductivity.

cond-mat.str-el↗

arXiv2016

Evidence for Fast Interlayer Energy Transfer in MoSe2/WS2 Heterostructures

Strongly bound excitons confined in two-dimensional (2D) semiconductors are dipoles with a perfect in-plane orientation. In a vertical stack of semiconducting 2D crystals, such in-plane excitonic dipoles are expected to efficiently couple across van der Waals gap due to strong interlayer Coulomb interaction and exchange their energy. However, previous studies on heterobilayers of group 6 transition metal dichalcogenides (TMDs) found that the exciton decay dynamics is dominated by interlayer charge transfer (CT) processes. Here, we report an experimental observation of fast interlayer energy transfer (ET) in MoSe2/WS2 heterostructures using photoluminescence excitation (PLE) spectroscopy. The temperature dependence of the transfer rates suggests that the ET is Förster-type involving excitons in the WS2 layer resonantly exciting higher-order excitons in the MoSe2 layer. The estimated ET time of the order of 1 ps is among the fastest compared to those reported for other nanostructure hybrid systems such as carbon nanotube bundles. Efficient ET in these systems offers prospects for optical amplification and energy harvesting through intelligent layer engineering.

cond-mat.mes-hall↗

arXiv2016

Valence band electronic structure evolution of graphene oxide upon thermal annealing for optoelectronics

We report valence band electronic structure evolution of graphene oxide (GO) upon its thermal reduction. Degree of oxygen functionalization was controlled by annealing temperatures, and an electronic structure evolution was monitored using real-time ultraviolet photoelectron spectroscopy. We observed a drastic increase in density of states around the Fermi level upon thermal annealing at ~600 oC. The result indicates that while there is an apparent band gap for GO prior to a thermal reduction, the gap closes after an annealing around that temperature. This trend of band gap closure was correlated with electrical, chemical, and structural properties to determine a set of GO material properties that is optimal for optoelectronics. The results revealed that annealing at a temperature of ~500 oC leads to the desired properties, demonstrated by a uniform and an order of magnitude enhanced photocurrent map of an individual GO sheet compared to as-synthesized counterpart.

cond-mat.mtrl-sci↗

arXiv2016

Engineering bandgaps of monolayer MoS2 and WS2 on fluoropolymer substrates by electrostatically tuned many-body effects

Intrinsic electrical and excitonic properties of monolayer transition metal dichalcogenides are studied on CYTOP fluoropolymer substrates with greatly suppressed unintentional doping and dielectric screening. Ambipolar transport behavior is observed in monolayer WS2 by applying solid states backdates. The excitonic properties of monolayer MoS2 and WS2 are determined by intricate interplays between the bandage renormalization, Pauli blocking and carrier screening against carrier doping.

cond-mat.mtrl-sci↗

arXiv2015

Halide-Assisted Atmospheric Pressure Growth of Large WSe2 and WS2 Monolayer Crystals

Chemical vapor deposition (CVD) of two-dimensional (2D) tungsten dichalcogenide crystals requires steady flow of tungsten source in the vapor phase. This often requires high temperature and low pressure due to the high sublimation point of tungsten oxide precursors. We demonstrate atmospheric pressure CVD of WSe2 and WS2 monolayers at moderate temperatures (700 ~ 850 oC) using alkali metal halides (MX where M= Na or K and X=Cl, Br or I) as the growth promoters. We attribute the facilitated growth to the formation of volatile tungsten oxyhalide species during growth, which leads to efficient delivery of the precursor to the growth substrates. The monolayer crystals were found to be free of unintentional doping with alkali metal and halogen atoms. Good field-effect transistor (FET) performances with high current on/off ratio ~10 7, hole and electron mobilities up to 102 and 26 cm2 V 1 s-1 for WSe2 and electron mobility of ~14 cm2 V-1 s-1 for WS2 devices were achieved.

cond-mat.mtrl-sci↗

arXiv2014

Spin-Orbit Proximity Effect in Graphene

The development of a spintronics device relies on efficient generation of spin polarized currents and their electric field controlled manipulation. While observation of exceptionally long spin relaxation lengths make graphene an intriguing material for spintronics studies, modulation of spin currents by gate field is almost impossible due to negligibly small intrinsic spin orbit coupling (SOC) of graphene. In this work, we create an artificial interface between monolayer graphene and few-layers semiconducting tungsten disulfide (WS2). We show that in such devices graphene acquires a SOC as high as 17meV, three orders of magnitude higher than its intrinsic value, without modifying any of the structural properties of the graphene. Such proximity SOC leads to the spin Hall effect even at room temperature and opens the doors for spin FETs. We show that intrinsic defects in WS2 play an important role in this proximity effect and that graphene can act as a probe to detect defects in semiconducting surfaces.

cond-mat.mes-hall↗

arXiv2014

Large Thermoelectricity via Variable Range Hopping in Chemical Vapor Deposition Grown Single-layer MoS2

Ultrathin layers of semiconducting molybdenum disulfide (MoS2) offer significant prospects in future electronic and optoelectronic applications. Although an increasing number of experiments bring light into the electronic transport properties of these crystals, their thermoelectric properties are much less known. In particular, thermoelectricity in chemical vapor deposition grown MoS2, which is more practical for wafer-scale applications, still remains unexplored. Here, for the first time, we investigate these properties in grown single layer MoS2. Micro-fabricated heaters and thermometers are used to measure both electrical conductivity and thermopower. Large values of up to ~30 mV/K at room temperature are observed, which are much larger than those observed in other two dimensional crystals and bulk MoS2. The thermopower is strongly dependent on temperature and applied gate voltage with a large enhancement at the vicinity of the conduction band edge. We also show that the Seebeck coefficient follows S~T^1/3 suggesting a two-dimensional variable range hopping mechanism in the system, which is consistent with electrical transport measurements. Our results help to understand the physics behind the electrical and thermal transports in MoS2 and the high thermopower value is of interest to future thermoelectronic research and application.

cond-mat.mes-hall↗

arXiv2014

Charge transport in ion-gated mono-, bi-, and trilayer MoS2 field effect transistors

Charge transport in MoS2 in the low carrier density regime is dominated by trap states and band edge disorder. The intrinsic transport properties of MoS2 emerge in the high density regime where conduction occurs via extended states. Here, we investigate the transport properties of mechanically exfoliated mono-, bi-, and trilayer MoS2 sheets over a wide range of carrier densities realized by a combination of ion gel top gate and SiO2 back gate which allows us to achieve high charge carrier (>10^13) density. We discuss the gating properties of the devices as a function of layer thickness and demonstrate resistivities of as low as 1 kΩ for monolayer and 420Ω for bilayer devices at 10 K. We show that from the capacitive coupling of the two gates, quantum capacitance can be roughly estimated to be on the order of 1 μF/cm^2 for all devices studied. Temperature dependence of the carrier mobility in the high density regime indicates that short-range scatterers limit charge transport at low temperatures.

cond-mat.mes-hall↗

arXiv2014

Nonlinear Photoluminescence in Atomically Thin Layered WSe2 Arising from Diffusion-Assisted Exciton-Exciton Annihilation

We studied multi-exciton dynamics in monolayer WSe2 using nonlinear photoluminescence (PL) spectroscopy and Monte Carlo simulations. We observed strong nonlinear saturation behavior of exciton PL with increasing excitation power density, and long-distance exciton diffusion reaching several micrometers. We demonstrated that the diffusion-assisted exciton-exciton annihilation model accounts for the observed nonlinear PL behavior. The long-distance exciton diffusion and subsequent efficient exciton-exciton annihilation process determined the unusual multi-exciton dynamics in atomically thin layered transition metal dichalcogenides.

cond-mat.mes-hall↗

arXiv2014

Photocarrier relaxation in two-dimensional semiconductors

Two-dimensional (2D) crystals of semiconducting transition metal dichalcogenides (TMD) absorb a large fraction of incident photons in the visible frequencies despite being atomically thin. It has been suggested that the strong absorption is due to the parallel band or "band nesting" effect and corresponding divergence in the joint density of states. Here, we show using photoluminescence excitation spectroscopy that the band nesting in mono- and bilayer MX$_2$ (M = Mo, W and X = S, Se) results in excitation-dependent characteristic relaxation pathways of the photoexcited carriers. Our experimental and simulation results reveal that photoexcited electron-hole pairs in the nesting region spontaneously separate in the $k$-space, relaxing towards immediate band extrema with opposite momentum. These effects imply that the loss of photocarriers due to direct exciton recombination is temporarily suppressed for excitation in resonance with band nesting. Our findings highlight the potential for efficient hot carrier collection using these materials as the absorbers in optoelectronic devices.

cond-mat.mtrl-sci↗

arXiv2014

Transport properties of monolayer MoS$_2$ grown by chemical vapour deposition

Recent success in the growth of monolayer MoS$_2$ via chemical vapor deposition (CVD) has opened up prospects for the implementation of these materials into thin film electronic and optoelectronic devices. Here, we investigate the electronic transport properties of individual crystallites of high quality CVD-grown monolayer MoS$_2$. The devices show low temperature mobilities up to 500 cm$^2$V$^{-1}$s$^{-1}$ and a clear signature of metallic conduction at high doping densities. These characteristics are comparable to the electronic properties of the best mechanically exfoliated monolayers in literature, verifying the high electronic quality of the CVD-grown materials. We analyze the different scattering mechanisms and show, that the short-range scattering plays a dominant role in the highly conducting regime at low temperatures. Additionally, the influence of phonons as a limiting factor of these devices is discussed.

cond-mat.mes-hall↗

arXiv2013

Lattice dynamics in mono- and few-layer sheets of WS2 and WSe2

Thickness is one of the fundamental parameters that define the electronic, optical, and thermal properties of two-dimensional (2D) crystals. Phonons in molybdenum disulfide (MoS2) were recently found to exhibit unique thickness dependence due to interplay between short and long range interactions. Here we report Raman spectra of atomically thin sheets of WS2 and WSe2 in the mono- to few-layer thickness regime. We show that, similar to the case of MoS2, the characteristic and modes exhibit stiffening and softening with increasing number of layers, respectively, with a small shift of less than 3 cm-1 due to large mass of the atoms. Thickness dependence is also observed in a series of multiphonon bands arising from overtone, combination, and zone edge phonons, whose intensity exhibit significant enhancement in excitonic resonance conditions. Some of these multiphonon peaks are found to be absent only in monolayers. These features provide a unique fingerprint and rapid identification for monolayer flakes.

cond-mat.mtrl-sci↗