8 research outputs found
Large and Tunable Photothermoelectric Effect in Single-Layer MoS<sub>2</sub>
We study the photoresponse of single-layer MoS<sub>2</sub> field-effect
transistors by scanning photocurrent microscopy. We find that, unlike
in many other semiconductors, the photocurrent generation in single-layer
MoS<sub>2</sub> is dominated by the photothermoelectric effect and
not by the separation of photoexcited electronâhole pairs across
the Schottky barriers at the MoS<sub>2</sub>/electrode interfaces.
We observe a large value for the Seebeck coefficient for single-layer
MoS<sub>2</sub> that by an external electric field can be tuned between
â4 Ă 10<sup>2</sup> and â1 Ă 10<sup>5</sup> ÎŒV K<sup>â1</sup>. This large and tunable Seebeck coefficient
of the single-layer MoS<sub>2</sub> paves the way to new applications
of this material such as on-chip thermopower generation and waste
thermal energy harvesting
Fast and Broadband Photoresponse of Few-Layer Black Phosphorus Field-Effect Transistors
Few-layer black phosphorus, a new
elemental two-dimensional (2D)
material recently isolated by mechanical exfoliation, is a high-mobility
layered semiconductor with a direct bandgap that is predicted to strongly
depend on the number of layers, from 0.35 eV (bulk) to 2.0 eV (single
layer). Therefore, black phosphorus is an appealing candidate for
tunable photodetection from the visible to the infrared part of the
spectrum. We study the photoresponse of field-effect transistors (FETs)
made of few-layer black phosphorus (3â8 nm thick), as a function
of excitation wavelength, power, and frequency. In the dark state,
the black phosphorus FETs can be tuned both in hole and electron doping
regimes allowing for ambipolar operation. We measure mobilities in
the order of 100 cm<sup>2</sup>/V s and a current ON/OFF ratio larger
than 10<sup>3</sup>. Upon illumination, the black phosphorus transistors
show a response to excitation wavelengths from the visible region
up to 940 nm and a rise time of about 1 ms, demonstrating broadband
and fast detection. The responsivity reaches 4.8 mA/W, and it could
be drastically enhanced by engineering a detector based on a PN junction.
The ambipolar behavior coupled to the fast and broadband photodetection
make few-layer black phosphorus a promising 2D material for photodetection
across the visible and near-infrared part of the electromagnetic spectrum
Local Strain Engineering in Atomically Thin MoS<sub>2</sub>
Controlling the bandstructure through
local-strain engineering
is an exciting avenue for tailoring optoelectronic properties of materials
at the nanoscale. Atomically thin materials are particularly well-suited
for this purpose because they can withstand extreme nonhomogeneous
deformations before rupture. Here, we study the effect of large localized
strain in the electronic bandstructure of atomically thin MoS<sub>2</sub>. Using photoluminescence imaging, we observe a strain-induced
reduction of the direct bandgap and funneling of photogenerated excitons
toward regions of higher strain. To understand these results, we develop
a nonuniform tight-binding model to calculate the electronic properties
of MoS<sub>2</sub> nanolayers with complex and realistic local strain
geometries, finding good agreement with our experimental results
Photovoltaic and Photothermoelectric Effect in a Double-Gated WSe<sub>2</sub> Device
Tungsten diselenide (WSe<sub>2</sub>), a semiconducting transition
metal dichalcogenide (TMDC), shows great potential as active material
in optoelectronic devices due to its ambipolarity and direct bandgap
in its single-layer form. Recently, different groups have exploited
the ambipolarity of WSe<sub>2</sub> to realize electrically tunable
PN junctions, demonstrating its potential for digital electronics
and solar cell applications. In this Letter, we focus on the different
photocurrent generation mechanisms in a double-gated WSe<sub>2</sub> device by measuring the photocurrent (and photovoltage) as the local
gate voltages are varied independently in combination with above-
and below-bandgap illumination. This enables us to distinguish between
two main photocurrent generation mechanisms, the photovoltaic and
photothermoelectric effect. We find that the dominant mechanism depends
on the defined gate configuration. In the PN and NP configurations,
photocurrent is mainly generated by the photovoltaic effect and the
device displays a maximum responsivity of 0.70 mA/W at 532 nm illumination
and rise and fall times close to 10 ms. Photocurrent generated by
the photothermoelectric effect emerges in the PP configuration and
is a factor of 2 larger than the current generated by the photovoltaic
effect (in PN and NP configurations). This demonstrates that the photothermoelectric
effect can play a significant role in devices based on WSe<sub>2</sub> where a region of strong optical absorption, caused by, for example,
an asymmetry in flake thickness or optical absorption of the electrodes,
generates a sizable thermal gradient upon illumination
Elucidating the Methylammonium (MA) Conformation in MAPbBr<sub>3</sub> Perovskite with Application in Solar Cells
Hybrid organicâinorganic
perovskites, MAPbX<sub>3</sub> (X = halogen), containing methylammonium
(MA: CH<sub>3</sub>âNH<sub>3</sub><sup>+</sup>) in the large
voids conformed by the PbX<sub>6</sub> octahedral network, are the
active absorption materials in the new generation of solar cells.
CH<sub>3</sub>NH<sub>3</sub>PbBr<sub>3</sub> is a promising member
with a large band gap that gives rise to a high open circuit voltage.
A deep knowledge of the crystal structure and, in particular, the
MA conformation inside the perovskite cage across the phase transitions
undergone below room temperature, seems essential to establish structureâproperty
correlations that may drive to further improvements. The presence
of protons requires the use of neutrons, combined with synchrotron
XRD data that help to depict subtle symmetry changes undergone upon
cooling. We present a consistent picture of the structural features
of this fascinating material, in complement with photocurrent measurements
from a photodetector device, demonstrating the potential of MAPbBr<sub>3</sub> in optoelectronics
Waveguide-Integrated MoTe<sub>2</sub><i>p</i>â<i>i</i>â<i>n</i> Homojunction Photodetector
Two-dimensional
(2D) materials, featuring distinctive electronic
and optical properties and dangling-bond-free surfaces, are promising
for developing high-performance on-chip photodetectors in photonic
integrated circuits. However, most of the previously reported devices
operating in the photoconductive mode suffer from a high dark current
or a low responsivity. Here, we demonstrate a MoTe2pâiân homojunction
fabricated directly on a silicon photonic crystal (PC) waveguide,
which enables on-chip photodetection with ultralow dark current, high
responsivity, and fast response speed. The adopted silicon PC waveguide
is electrically split into two individual back gates to selectively
dope the top regions of the MoTe2 channel in p- or n-types. High-quality reconfigurable MoTe2 (pâiân, nâiâp, nâiân, pâiâp) homojunctions are realized successfully, presenting rectification
behaviors with ideality factors approaching 1.0 and ultralow dark
currents less than 90 pA. Waveguide-assisted MoTe2 absorption
promises a sensitive photodetection in the telecommunication O-band
from 1260 to 1340 nm, though it is close to MoTe2âs
absorption band-edge. A competitive photoresponsivity of 0.4 A/W is
realized with a light on/off current ratio exceeding 104 and a record-high normalized photocurrent-to-dark-current ratio
of 106 mWâ1. The ultrasmall capacitance
of pâiân homojunction and high carrier mobility of MoTe2 promise
a high dynamic response bandwidth close to 34.0 GHz. The proposed
device geometry has the advantages of employing a silicon PC waveguide
as the back gates to build a 2D material pâiân homojunction directly and simultaneously
to enhance lightâ2D material interaction. It provides a potential
pathway to develop 2D material-based photodetectors, laser diodes,
and electro-optic modulators on silicon photonic chips
Strain Control of ExcitonâPhonon Coupling in Atomically Thin Semiconductors
Semiconducting
transition metal dichalcogenide (TMDC) monolayers
have exceptional physical properties. They show bright photoluminescence
due to their unique band structure and absorb more than 10% of the
light at their excitonic resonances despite their atomic thickness.
At room temperature, the width of the exciton transitions is governed
by the excitonâphonon interaction leading to strongly asymmetric
line shapes. TMDC monolayers are also extremely flexible, sustaining
mechanical strain of about 10% without breaking. The excitonic properties
strongly depend on strain. For example, exciton energies of TMDC monolayers
significantly redshift under uniaxial tensile strain. Here, we demonstrate
that the width and the asymmetric line shape of excitonic resonances
in TMDC monolayers can be controlled with applied strain. We measure
photoluminescence and absorption spectra of the A exciton in monolayer
MoSe<sub>2</sub>, WSe<sub>2</sub>, WS<sub>2</sub>, and MoS<sub>2</sub> under uniaxial tensile strain. We find that the A exciton substantially
narrows and becomes more symmetric for the selenium-based monolayer
materials, while no change is observed for atomically thin WS<sub>2</sub>. For MoS<sub>2</sub> monolayers, the line width increases.
These effects are due to a modified excitonâphonon coupling
at increasing strain levels because of changes in the electronic band
structure of the respective monolayer materials. This interpretation
based on steady-state experiments is corroborated by time-resolved
photoluminescence measurements. Our results demonstrate that moderate
strain values on the order of only 1% are already sufficient to globally
tune the excitonâphonon interaction in TMDC monolayers and
hold the promise for controlling the coupling on the nanoscale
Centimeter-Scale Synthesis of Ultrathin Layered MoO<sub>3</sub> by van der Waals Epitaxy
We
report on the large-scale synthesis of highly oriented ultrathin
MoO<sub>3</sub> layers using a simple and low-cost atmospheric pressure,
van der Waals epitaxy growth on muscovite mica substrates. By this
method, we are able to synthesize high quality centimeter-scale MoO<sub>3</sub> crystals with thicknesses ranging from 1.4 nm (two layers)
up to a few nanometers. The crystals can be easily transferred to
an arbitrary substrate (such as SiO<sub>2</sub>) by a deterministic
transfer method and be extensively characterized to demonstrate the
high quality of the resulting crystal. We also study the electronic
band structure of the material by density functional calculations.
Interestingly, the calculations demonstrate that bulk MoO<sub>3</sub> has a rather weak electronic interlayer interaction, and thus, it
presents a monolayer-like band structure. Finally, we demonstrate
the potential of this synthesis method for optoelectronic applications
by fabricating large-area field-effect devices (10 ÎŒm Ă
110 ÎŒm in lateral dimensions) and find responsivities of 30
mA W<sup>â1</sup> for a laser power density of 13 mW cm<sup>â2</sup> in the UV region of the spectrum and also as an electron
acceptor in a MoS<sub>2</sub>-based field-effect transistor