| dc.description.abstract | Monolayers of transition metal dichalcogenides (TMDCs) host excitons, a bound state of
electron and hole. Excitonic many-body complexes dominate the optical spectra obtained
from this system and manipulating these complexes through external stimuli is intriguing.
In addition, due to the weak van der Waals interlayer interaction between these TMDC
materials, they can be stacked onto each other with a specific twist angle. This gave
rise to a new field named ‘twistronics.’ Several intriguing phenomena and validation
of quantum mechanical theories can be experimentally realized in these systems, giving
them the title of ‘condensed-matter quantum simulator.’
The first part of the thesis demonstrates controlled kinetic manipulation of the five particle excitonic complex (charged biexciton) in a monolayer WS2. This manipulation
is shown using a systematic dependence of the biexciton peak on excitation power, gate
voltage, and temperature using steady-state and time-resolved photoluminescence (PL).
With the help of a combination of the experimental data and a rate equation model, it
can be shown that the binding energy of the charged biexciton is less than the spectral
separation of its peak from the neutral exciton. Note that while the momentum direct
radiative recombination of the neutral exciton is restricted within the light cone, such
restriction is relaxed for a charged biexciton recombination due to the presence of near parallel excited and final states in the momentum space.
Many-body effects like exciton-exciton annihilation (EEA) have been widely explored
in TMDCs. However, a similar effect for charged excitons (or trions), that is, trion-trion
annihilation (TTA), is expected to be relatively suppressed due to repulsive like-charges
and has not been hitherto observed in such layered semiconductors. By a gate-dependent
tuning of the spectral overlap between the trion and the charged biexciton through
an “anti-crossing”-like behavior in monolayer WS2, an experimental observation of an
anomalous suppression of the trion emission intensity with an increase in gate voltage
is presented here. The results strongly correlate with time-resolved measurements and
are inferred as direct evidence of a nontrivial TTA resulting from non-radiative Auger
recombination of a bright trion and the corresponding energy resonantly promoting a
dark trion to a charged biexciton state. The extracted Auger coefficient for the process
is found to be tunable ten-fold through a gate-dependent tuning of the spectral overlap.
Excitonic states trapped in harmonic moir´e wells in twisted heterobilayers is an excel lent platform to study many-body interaction. However, the rigid twist angle primarily
governs the moir´e exciton potential, and its dynamic tuning thus remains a challenge
- a knob that would be highly desirable for both scientific exploration and device ap plications. Using moir´e trapped excitonic emission as a probe, here, a dynamic tuning
of moir´e potential in a WS2/WSe2 heterobilayer through two anharmonic perturbations
induced by the gate voltage and optical power is demonstrated. First, it is shown that,
dictated by the Poisson equation, a gate voltage can result in a local in-plane perturb ing field with odd parity around the high-symmetry points. This allows simultaneously
observing the first (linear) and second (parabolic) order Stark shift for the ground state
and first excited state respectively, of the moir´e trapped exciton - an effect opposite to
the conventional quantum-confined Stark shift. Depending on the degree of confinement,
the moir´e trapped excitons exhibit up to twenty-fold gate-tunability in the lifetime (100
ns to 5 ns). The second anharmonic tuning is demonstrated through exciton localization dependent dipolar repulsion, leading to an optical power-induced blueshift as high as 1
meV/µW.
In the last part, a twisted moir´e superlattice of WS2/WSe2 on nanopillars is explored.
The nanopillars are fabricated on metal lines. Due to strain induced by these nanopillars,
a drift of interlayer excitons (ILE) toward the center of these pillars is expected. As a
result, only the moir´e wells at the top of the nanopillars will get populated and can
emit. The trapped ILEs can induce strong dipolar repulsion, which in turn provides
us a knob to modulate the effective size of the strain well and the number of moir´e
pockets. A composite rate equation considering these excitons’ drift, diffusion, and
dipolar repulsion is solved numerically, giving a comprehensive view of the steady-state
excitonic concentration and modulation of the strain well. The simulation is in excellent
agreement with experimental data, supporting the claim | en_US |
