12 research outputs found
Stable Monolayer Transition Metal Dichalcogenide Ordered Alloys with Tunable Electronic Properties
From first-principles calculations,
we discover highly stable monolayer
transition metal dichalcogenides (TMDs) ternary alloys consisting
of group 5 and 7 transition metal elements. We show for Nb<sub>1ā<i>x</i></sub>Re<sub><i>x</i></sub>S<sub>2</sub>, Ta<sub>1ā<i>x</i></sub>Re<sub><i>x</i></sub>S<sub>2</sub> and their selenide counterparts that the 1H ordered alloy
structures for <i>x</i> ā¤ 0.5 are thermodynamically
stable, with formation energies an order of magnitude lower than currently
known TMD alloys such as Mo<sub><i>x</i></sub>W<sub>1ā<i>x</i></sub>S<sub>2</sub>, so that they could potentially be
synthesizable using chemical vapor deposition or exfoliation techniques.
This class of TMD alloys offer a wide tunable bandgap range of ā¼1
eV, displaying metallic to semiconducting behavior versus alloy composition.
Importantly, at <i>x</i> = 0.5, the alloys are valence isoelectronic
with MoS<sub>2</sub>. These stoichiometric compounds, Nb<sub>0.5</sub>Re<sub>0.5</sub>S<sub>2</sub>, Ta<sub>0.5</sub>Re<sub>0.5</sub>S<sub>2</sub>, and their selenide counterparts, exhibit band features similar
to MoS<sub>2</sub>, but possess significantly smaller bandgaps (ā¼1
to 1.2 eV). As a result, compared to MoS<sub>2</sub> and WS<sub>2</sub>, this class of alloy TMDs display enhanced absorbance in the visible
range of the solar spectrum where the solar spectral intensity is
the strongest. These ordered monolayer TMD alloys could open doors
for designing ultrathin solar absorbers with improved performance
Large Thermoelectricity via Variable Range Hopping in Chemical Vapor Deposition Grown Single-Layer MoS<sub>2</sub>
Ultrathin layers of semiconducting
molybdenum disulfide (MoS<sub>2</sub>) 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 MoS<sub>2</sub>, which is more
practical for wafer-scale applications, still remains unexplored.
Here, for the first time, we investigate these properties in grown
single layer MoS<sub>2</sub>. Microfabricated heaters and thermometers
are used to measure both electrical conductivity and thermopower.
Large values of up to ā¼30 mV/K at room temperature are observed,
which are much larger than those observed in other two-dimensional
crystals and bulk MoS<sub>2</sub>. 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 <i>S</i> ā¼ <i>T</i><sup>1/3</sup>, 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 MoS<sub>2</sub> and the high
thermopower value is of interest to future thermoelectronic research
and application
Evolution of Electronic Structure in Atomically Thin Sheets of WS<sub>2</sub> and WSe<sub>2</sub>
Geometrical confinement effect in exfoliated sheets of layered materials leads to significant evolution of energy dispersion in mono- to few-layer thickness regime. Molybdenum disulfide (MoS<sub>2</sub>) was recently found to exhibit indirect-to-direct gap transition when the thickness is reduced to a single monolayer. Emerging photoluminescence (PL) from monolayer MoS<sub>2</sub> opens up opportunities for a range of novel optoelectronic applications of the material. Here we report differential reflectance and PL spectra of mono- to few-layer WS<sub>2</sub> and WSe<sub>2</sub> that indicate that the band structure of these materials undergoes similar indirect-to-direct gap transition when thinned to a single monolayer. The transition is evidenced by distinctly enhanced PL peak centered at 630 and 750 nm in monolayer WS<sub>2</sub> and WSe<sub>2</sub>, respectively. Few-layer flakes are found to exhibit comparatively strong indirect gap emission along with direct gap hot electron emission, suggesting high quality of synthetic crystals prepared by a chemical vapor transport method. Fine absorption and emission features and their thickness dependence suggest a strong effect of Se p-orbitals on the d electron band structure as well as interlayer coupling in WSe<sub>2</sub>
Origin of Indirect Optical Transitions in Few-Layer MoS<sub>2</sub>, WS<sub>2</sub>, and WSe<sub>2</sub>
It has been well-established that
single layer MX<sub>2</sub> (M
= Mo, W and X = S, Se) are direct gap semiconductors with band edges
coinciding at the K point in contrast to their indirect gap multilayer
counterparts. In few-layer MX<sub>2</sub>, there are two valleys along
the ĪāK line with similar energy. There is little understanding
on which of the two valleys forms the conduction band minimum (CBM)
in this thickness regime. We investigate the conduction band valley
structure in few-layer MX<sub>2</sub> by examining the temperature-dependent
shift of indirect exciton photoluminescence peak. Highly anisotropic
thermal expansion of the lattice and the corresponding evolution of
the band structure result in a distinct peak shift for indirect transitions
involving the K and Ī (midpoint along Ī-K) valleys. We
identify the origin of the indirect emission and concurrently determine
the relative energy of these valleys
Efficient Carrier-to-Exciton Conversion in Field Emission Tunnel Diodes Based on MIS-Type van der Waals Heterostack
We
report on efficient carrier-to-exciton conversion and planar
electroluminescence from tunnel diodes based on a metalāinsulatorāsemiconductor
(MIS) van der Waals heterostack consisting of few-layer graphene (FLG),
hexagonal boron nitride (hBN), and monolayer tungsten disulfide (WS<sub>2</sub>). These devices exhibit excitonic electroluminescence with
extremely low threshold current density of a few pAĀ·Ī¼m<sup>ā2</sup>, which is several orders of magnitude lower compared
to the previously reported values for the best planar EL devices.
Using a reference dye, we estimate the EL quantum efficiency to be
ā¼1% at low current density limit, which is of the same order
of magnitude as photoluminescence quantum yield at the equivalent
excitation rate. Our observations reveal that the efficiency of our
devices is not limited by carrier-to-exciton conversion efficiency
but by the inherent exciton-to-photon yield of the material. The device
characteristics indicate that the light emission is triggered by injection
of hot minority carriers (holes) to n-doped WS<sub>2</sub> by FowlerāNordheim
tunneling and that hBN serves as an efficient hole-transport and electron-blocking
layer. Our findings offer insight into the intelligent design of van
der Waals heterostructures and avenues for realizing efficient excitonic
devices
Dynamic Structural Evolution of MetalāMetal Bonding Network in Monolayer WS<sub>2</sub>
Layered
transition metal dichalcogenides (TMDs) exist in a range
of crystal phases with distinct electronic character. Some crystal
phases are known to exhibit unique in-plane anisotropy characterized
by a periodic distortion of the lattice and a formation of metalāmetal
bonding network. Here, we report in situ observation of dynamic structural
evolution in the one-dimensional zigzag chains of single layer WS<sub>2</sub> induced by electron beam irradiation. Metastable zigzag chains
of tungsten atoms are found to undergo reorganization of metalāmetal
bonds, resulting in emergence of tetramer clusters and zigzag chains
with a new orientation. Our first-principles calculations reveal a
small (ā¼0.1 eV per formula unit) activation energy barrier
for monolayer WS<sub>2</sub> zigzag chain reorientation and a metastable
transition state in the form of tetramer clusters. We further show
that local tetramer clusters can be induced and stabilized by local
electronic charging effects. The formation of local tetramer clusters
indicate that this dynamic structural evolution is not a charge density
wave phenomenon; we find instead that these lattice changes are a
response to electronic instabilities that weaken the WāS bonds
in the zigzag phase. Our findings shed light on the origin of structural
instabilities and phases in two-dimensional materials, and constitute
a step further toward their potential uses in phase change applications
Conducting MoS<sub>2</sub> Nanosheets as Catalysts for Hydrogen Evolution Reaction
We
report chemically exfoliated MoS<sub>2</sub> nanosheets with
a very high concentration of metallic 1T phase using a solvent free
intercalation method. After removing the excess of negative charges
from the surface of the nanosheets, highly conducting 1T phase MoS<sub>2</sub> nanosheets exhibit excellent catalytic activity toward the
evolution of hydrogen with a notably low Tafel slope of 40 mV/dec.
By partially oxidizing MoS<sub>2</sub>, we found that the activity
of 2H MoS<sub>2</sub> is significantly reduced after oxidation, consistent
with edge oxidation. On the other hand, 1T MoS<sub>2</sub> remains
unaffected after oxidation, suggesting that edges of the nanosheets
are not the main active sites. The importance of electrical conductivity
of the two phases on the hydrogen evolution reaction activity has
been further confirmed by using carbon nanotubes to increase the conductivity
of 2H MoS<sub>2</sub>
Evidence for Fast Interlayer Energy Transfer in MoSe<sub>2</sub>/WS<sub>2</sub> 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 MoSe<sub>2</sub>/WS<sub>2</sub> heterostructures using photoluminescence excitation (PLE)
spectroscopy. The temperature dependence of the transfer rates suggests
that the ET is FoĢrster-type involving excitons in the WS<sub>2</sub> layer resonantly exciting higher-order excitons in the MoSe<sub>2</sub> 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
Transport Properties of Monolayer MoS<sub>2</sub> Grown by Chemical Vapor Deposition
Recent
success in the growth of monolayer MoS<sub>2</sub> 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<sub>2</sub>.
The devices show low temperature mobilities up to 500 cm<sup>2</sup> V<sup>ā1</sup> s<sup>ā1</sup> 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 optical phonons as a limiting factor is discussed
Heterointerface Screening Effects between Organic Monolayers and Monolayer Transition Metal Dichalcogenides
The
nature and extent of electronic screening at heterointerfaces
and their consequences on energy level alignment are of profound importance
in numerous applications, such as solar cells, electronics <i>etc.</i> The increasing availability of two-dimensional (2D)
transition metal dichalcogenides (TMDs) brings additional opportunities
for them to be used as interlayers in āvan der Waals (vdW)
heterostructuresā and organic/inorganic flexible devices. These
innovations raise the question of the extent to which the 2D TMDs
participate actively in dielectric screening at the interface. Here
we study perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) monolayers
adsorbed on single-layer tungsten diselenide (WSe<sub>2</sub>), bare
graphite, and Au(111) surfaces, revealing a strong dependence of the
PTCDA HOMOāLUMO gap on the electronic screening effects from
the substrate. The monolayer WSe<sub>2</sub> interlayer provides substantial,
but not complete, screening at the organic/inorganic interface. Our
results lay a foundation for the exploitation of the complex interfacial
properties of hybrid systems based on TMD materials