| dc.description.abstract | Layered transition metal dichalcogenides (TMDCs) host a variety of strongly bound
exciton complexes that control the optical properties in these materials. Apart from
spin and valley, layer index provides an additional degree of freedom in a few-layer-thick
lm. While in the 1H monolayer TMD inversion symmetry is broken, and the reflection
symmetry is maintained but, in the bilayer, it is reversed. Trions are excitonic species
with a positive or negative charge, and thus, unlike neutral excitons, the
flow of trions can
generate a net detectable charge current. Trions under favourable doping conditions can
be created in a coherent manner using resonant excitation. The neutral biexciton (bound
state of two excitons) can assemble further to create a charged state with another electron
or hole. Generally, in W-based TMDs these ve-particle quinton states dominate the
population density and this can also be engineered to produce photocurrent at cryogenic
temperature.
In the firrst work, we show that in a few-layer TMDC lm, the wave functions of the
conduction and valence-band-edge states contributing to the K(K0) valley are spatially
con ned in the alternate layers - giving rise to direct (quasi-)intralayer bright exciton and
lower-energy interlayer dark excitons. Depending on the spin and valley con figuration,
the bright-exciton state is further found to be a coherent superposition of two layer-
induced states, one (E type) distributed in the even layers and the other (O type) in the
odd layers. The intralayer nature of the bright exciton manifests as a relatively weak
dependence of the exciton binding energy on the thickness of the few-layer lm, and
the binding energy is maintained up to 50 meV in the bulk limit - which is an order of
magnitude higher than conventional semiconductors. Fast Stokes energy transfer from
the intralayer bright state to the interlayer dark states provides a clear signature in
the layer-dependent broadening of the photoluminescence peak and plays a key role in
the suppression of the photoluminescence intensity observed in TMDCs with thickness
beyond a monolayer.
In the second work, we show that bilayer WS2 exhibits a quantum con ned Stark
effect (QCSE) that is linear with the applied out-of-plane electric field, in contrast to a
quadratic one for a monolayer because of the contrasting symmetries between monolayer
and bilayer. The interplay between the unique layer degree of freedom in the bilayer
and the field-driven partial interconversion between intralayer and interlayer excitons
generates a giant tunability of the exciton oscillator strength. This makes bilayer WS2 a
promising candidate for an atomically thin, tuneable electro-absorption modulator at the
exciton resonance, particularly when stacked on top of a graphene layer that provides
an ultrafast nonradiative relaxation channel. By tweaking the biasing confi guration,
we further show that the excitonic response can be largely tuned through electrostatic
doping, by efficiently transferring the oscillator strength from neutral to charged exciton.
In the third and fourth work, we demonstrate interlayer charge transport from top
few-layer graphene to bottom monolayer graphene, mediated by a coherently formed
trion state using a few-layer graphene/monolayer WS2/monolayer graphene vertical het-
erojunction. This is achieved by using a resonant excitation and varying the sample
temperature. The resulting change in the WS2 bandgap allows us to scan the excitation
around the exciton-trion spectral overlap with high spectral resolution. By correlating
the vertical photocurrent and in situ photoluminescence features at the heterojunction as
a function of the spectral position of the excitation, we show that (1) trions are anoma-
lously stable at the junction even up to 463 K due to enhanced doping, and (2) the
photocurrent results from the ultrafast formation of a trion through exciton-trion coher-
ent coupling, followed by its fast interlayer transport. Further, the resonant photocurrent
thus generated can be effectively controlled by a back gate voltage applied through the
incomplete screening of the bottom monolayer graphene, and the photocurrent strongly
correlates with the gate dependent trion intensity, while the non-resonant photocurrent exhibits only a weak gate dependence. We estimate a sub-100 fs switching time of the
device.
In the final work, we have used the pulsed laser excitation to create the quinton
states in monolayer WS2 while resonantly exciting the exciton and trion states at low
temperature. Strong light absorption by the charged biexciton under spectral resonance,
coupled with its charged nature, makes it intriguing for photodetection - an area that is
hitherto unexplored. Using the high built-in vertical electric eld in an asymmetrically
designed few-layer graphene encapsulated 1L-WS2 heterostructure, here we report, for
the rst time, a large, highly nonlinear photocurrent arising from the strong absorption
by two charged biexciton species under zero external bias (self-powered mode). Time-
resolved measurement reveals that the photoresponse is ultra-fast, on the order of sub-5
ps. By using single- and two-color photoluminescence excitation spectroscopy, we show
that the two biexcitonic peaks originate from bright-dark and bright-bright exciton-trion
combinations.
The possibility of electrical manipulation and detection of a charged exciton (trion)
before its radiative recombination makes it promising for excitonic devices. The demon-
stration of coherent formation, high stabilization, vertical transportation, and electrical
detection of trions marks a step toward room-temperature trionics. Following the same
the ve-particle charged quinton can also be efficiently generated and electrically de-
tected. They can be used in electrical detection of constituting bright and dark states
and quantum manipulation of the coupled spin-valley physics. Such innate nonlinearity
in the photocurrent due to its biexcitonic origin, coupled with the ultra-fast response
due to swift inter-layer charge transfer exempli fies the promise of manipulating many-
body effects in monolayers. Also, the findings are prospective toward highly tunable,
atomically thin, compact, and light on chip, re-confi gurable components and promising
for several applications such as higher harmonics generation of the modulating signal,
receiver design in microwave photonics and visible light communication, square-law cir-
cuits, and also in nonlinear next generation optoelectronics. | en_US |
