20 research outputs found
Valley Coherent Hot Carriers and Thermal Relaxation in Monolayer Transition Metal Dichalcogenides
We show room temperature valley coherence with in MoS2, MoSe2, WS2 and WSe2
monolayers using linear polarization resolved hot photoluminescence (PL), at
energies close to the excitation - demonstrating preservation of valley
coherence before sufficient scattering events. The features of the co-polarized
hot luminescence allow us to extract the lower bound of the binding energy of
the A exciton in monolayer MoS2 as 0.42 (+/- 0.02) eV. The broadening of the PL
peak is found to be dominated by Boltzmann-type hot luminescence tail, and
using the slope of the exponential decay, the carrier temperature is extracted
in-situ at different stages of energy relaxation. The temperature of the
emitted optical phonons during the relaxation process are probed by exploiting
the corresponding broadening of the Raman peaks due to temperature induced
anharmonic effects. The findings provide a physical picture of photo-generation
of valley coherent hot carriers, and their subsequent energy relaxation path
ways
Asymmetrically Encapsulated vertical ITO/MoS2/Cu2O photodetector with ultra-high sensitivity
Strong light absorption, coupled with moderate carrier transport properties,
makes two-dimensional (2-D) layered transition metal dichalcogenide (TMD)
semiconductors promising candidates for low intensity photodetection
applications. However, the performance of these devices is severely
bottlenecked by slow response with persistent photocurrent due to long lived
charge trapping, and nonreliable characteristics due to undesirable ambience
and substrate effects. Here we demonstrate ultra-high specific detectivity (D*)
of 3.2x10^14 Jones and responsivity (R) of 5.77x10^4 AW-1 at an optical power
density (P_op) of 0.26 Wm-2 and external bias (V_ext) of -0.5 V in an indium
tin oxide (ITO)/MoS2/copper oxide (Cu2O)/Au vertical multi-heterojunction
photodetector exhibiting small carrier transit time. The active MoS2 layer
being encapsulated by carrier collection layers allows us to achieve negligible
trap assisted persistent photocurrent and repeatable characteristics over large
number of cycles. We also achieved a large D*>10^14 Jones at zero external bias
due to the built-in field of the asymmetric photodetector. Benchmarking the
performance against existing reports in literature shows a pathway for
achieving reliable and highly sensitive photodetectors for ultra-low intensity
photodetection applications.Comment: Accepted in Small, Wile
Direct observation of giant binding energy modulation of exciton complexes in monolayer MoSe
Screening due to surrounding dielectric medium reshapes the electron-hole
interaction potential and plays a pivotal role in deciding the binding energies
of strongly bound exciton complexes in quantum confined monolayers of
transition metal dichalcogenides (TMDs). However, owing to strong
quasi-particle bandgap renormalization in such systems, a direct quantification
of estimated shifts in binding energy in different dielectric media remains
elusive using optical studies. In this work, by changing the dielectric
environment, we show a conspicuous photoluminescence (PL) peak shift at low
temperature for higher energy excitons (2s, 3s, 4s, 5s) in monolayer MoSe,
while the 1s exciton peak position remains unaltered - a direct evidence of
varying compensation between screening induced exciton binding energy
modulation and quasi-particle bandgap renormalization. The estimated modulation
of binding energy for the 1s exciton is found to be 58.6% (70.5% for 2s, 78.9%
for 3s, 85% for 4s) by coating an AlO layer on top, while the
corresponding reduction in quasi-particle bandgap is estimated to be 248 meV.
Such a direct evidence of large tunability of the binding energy of exciton
complexes as well as the bandgap in monolayer TMDs holds promise of novel
device applications.Comment: 19 pages including supplemental informatio
Substrate effects in high gain, low operating voltage SnSe2 photoconductor
High gain photoconductive devices find wide spread applications in low
intensity light detection. Ultra-thin layered materials have recently attracted
a lot of attention from researchers in this regard. However, in general, a
large operating voltage is required to obtain large responsivity in these
devices. In addition, the characteristics are often confounded by substrate
induced trap effects. Here we report multi-layer SnSe2 based photoconductive
devices using two different structures: (1) SiO2 substrate supported
interdigitated electrode (IDE), and (2) suspended channel. The IDE device
exhibits a responsivity of ~ 10^3 A/W and 8.66x10^4 A/W at operating voltages
of 1 mV and 100 mV, respectively - a superior low voltage performance over
existing literature on planar 2D structures. However, the responsivity reduces
by more than two orders of magnitude, while the transient response improves for
the suspended device - providing insights into the critical role played by the
channel-substrate interface in the gain mechanism. The results, on one hand,
are promising for highly sensitive photoconductive applications consuming
ultra-low power, and on the other hand, show a generic methodology that could
be applied to other layered material based photoconductive devices as well for
extracting the intrinsic behavior.Comment: 16 pages, 6 figures, Accepted in Nanotechnology (IOP
Photoresponse of atomically thin MoS2 layers and their planar heterojunctions
MoS2 monolayers exhibit excellent light absorption and large thermoelectric
power, which are, however, accompanied with very strong exciton binding energy
- resulting in complex photoresponse characteristics. We study the electrical
response to scanning photo-excitation on MoS2 monolayer (1L) and bilayer (2L)
devices, and also on monolayer/bilayer (1L/2L) planar heterojunction and
monolayer/few-layer/multi-layer (1L/FL/ML) planar double heterojunction devices
to unveil the intrinsic mechanisms responsible for photocurrent generation in
these materials and junctions. Strong photoresponse modulation is obtained by
scanning the position of the laser spot, as a consequence of controlling the
relative dominance of a number of layer dependent properties, including (i)
photoelectric effect (PE), (ii) photothermoelectric effect (PTE), (iii)
excitonic effect, (iv) hot photo-electron injection from metal, and (v) carrier
recombination. The monolayer and bilayer devices show peak photoresponse when
the laser is focused at the source junction, while the peak position shifts to
the monolayer/multi-layer junction in the heterostructure devices. The
photoresponse is found to be dependent on the incoming light polarization when
the source junction is illuminated, although the polarization sensitivity
drastically reduces at the monolayer/multi-layer heterojunction. Finally, we
investigate laser position dependent transient response of photocurrent to
reveal trapping of carriers in SiO2 at the source junction is the critical
factor to determine the transient response in 2D photodetectors, and also show
that, by systematic device design, such trapping can be avoided in the
heterojunction devices, resulting in fast transient response. The insights
obtained will play an important role in designing fast 2D TMDs based
photodetector and related optoelectronic and thermoelectric devices.Comment: Nanoscale, 201
Strong Single- and Two-Photon Luminescence Enhancement by Nonradiative Energy Transfer across Layered Heterostructure
The strong light-matter interaction in monolayer transition metal
dichalcogenides (TMDs) is promising for nanoscale optoelectronics with their
direct band gap nature and the ultra-fast radiative decay of the strongly bound
excitons these materials host. However, the impeded amount of light absorption
imposed by the ultra-thin nature of the monolayers impairs their viability in
photonic applications. Using a layered heterostructure of a monolayer TMD
stacked on top of strongly absorbing, non-luminescent, multi-layer SnSe2, we
show that both single-photon and two-photon luminescence from the TMD monolayer
can be enhanced by a factor of 14 and 7.5, respectively. This is enabled
through inter-layer dipole-dipole coupling induced non-radiative Forster
resonance energy transfer (FRET) from SnSe2 underneath which acts as a
scavenger of the light unabsorbed by the monolayer TMD. The design strategy
exploits the near-resonance between the direct energy gap of SnSe2 and the
excitonic gap of monolayer TMD, the smallest possible separation between donor
and acceptor facilitated by van der Waals heterojunction, and the in-plane
orientation of dipoles in these layered materials. The FRET driven uniform
single- and twophoton luminescence enhancement over the entire junction area is
advantageous over the local enhancement in quantum dot or plasmonic structure
integrated 2D layers, and is promising for improving quantum efficiency in
imaging, optoelectronic, and photonic applications
Nature of carrier injection in metal/2D semiconductor interface and its implications to the limits of contact resistance
Monolayers of transition metal dichalcogenides (TMDCs) exhibit excellent
electronic and optical properties. However, the performance of these
two-dimensional (2D) devices are often limited by the large resistance offered
by the metal contact interface. Till date, the carrier injection mechanism from
metal to 2D TMDC layers remains unclear, with widely varying reports of
Schottky barrier height (SBH) and contact resistance (Rc), particularly in the
monolayer limit. In this work, we use a combination of theory and experiments
in Au and Ni contacted monolayer MoS2 device to conclude the following points:
(i) the carriers are injected at the source contact through a cascade of two
potential barriers - the barrier heights being determined by the degree of
interaction between the metal and the TMDC layer; (ii) the conventional
Richardson equation becomes invalid due to the multi-dimensional nature of the
injection barriers, and using Bardeen-Tersoff theory, we derive the appropriate
form of the Richardson equation that describes such composite barrier; (iii) we
propose a novel transfer length method (TLM) based SBH extraction methodology,
to reliably extract SBH by eliminating any confounding effect of temperature
dependent channel resistance variation; (iv) we derive the Landauer limit of
the contact resistance achievable in such devices. A comparison of the limits
with the experimentally achieved contact resistance reveals plenty of room for
technological improvements.Comment: Accepted in Physical Review
High field carrier transport in graphene: Insights from fast current transient
In this work, we observe gate tunable negative differential conductance (NDC) and current saturation in single layer and bilayer graphene transistor at high source-drain field, which arise due to the interplay among (1) self-heating, (2) hot carrier injection, and (3) drain induced minority carrier injection. The magnitude of the NDC is found to be reduced for a bilayer, in agreement with its weaker carrier-optical phonon coupling and less efficient hot carrier injection. The contributions of different mechanisms to the observed results are decoupled through fast transient measurements with nanosecond resolution. The findings provide insights into high field transport in graphene. (C) 2012 American Institute of Physics. http://dx.doi.org/10.1063/1.4754103