Excitonic complexes in semiconducting monolayers and their twisted heterojunctions

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