Excitons in monolayer transition metal dichalcogenides

Abstract

Excitons are quasiparticles formed due to electrostatic attraction between the electrons and the holes in a semiconductor. This Coulomb attraction is very strong in the mono- layers of Transition Metal Dichalcogenides (TMDs) mainly because of strong quantum confinement, reduced dielectric screening, and high effective mass of electrons and holes in these material systems. A 2D hydrogen atom is a simple model to describe confined excitons in these monolayer films. A more formal way to describe excitons in thin semi- conductors is through the Bethe-Salpeter formalism which describes these excitons as a superposition of the electronic states in momentum space. In order to understand exci- tons further, we explore the following excitonic features in this thesis: Probing intrinsic exciton linewidth: Monolayer TMDs are highly luminescent materials despite being sub-nanometer thick. This is due to the ultrashort radiative life- time of the strongly bound bright excitons hosted by these materials. The intrinsically short radiative lifetime results in a large broadening in the exciton band with a magnitude that is about two orders greater than the spread of the light cone itself. The situation calls for a need to revisit the conventional light cone picture. We present a modified light cone concept which places the light line as the generalized lower bound for allowed radia- tive recombination. A self-consistent methodology, which becomes crucial upon inclusion of large radiative broadening in the exciton band, is proposed to segregate the radiative and the nonradiative components of the homogeneous exciton linewidth. We estimate a fundamental radiative linewidth of 1:54 0:17 meV, owing purely to finite radiative lifetime in the absence of nonradiative dephasing processes. As a direct consequence of the large radiative limit, we nd a surprisingly large ( 0:27 meV) linewidth broadening due to zero-point energy of acoustic phonons. This obscures the precise experimental determination of the intrinsic radiative linewidth and sets a fundamental limit on the nonradiative linewidth broadening at T=0 K. Modulating exciton binding energy: Screening due to the surrounding dielectric medium reshapes the electron-hole interaction potential and plays a pivotal role in decid- ing the binding energies of strongly bound exciton complexes in quantum confined TMD monolayers. However, owing to strong quasiparticle band-gap renormalization in such systems, a direct quantification of estimated shifts in binding energy in different dielectric media remains elusive using optical studies. By changing the dielectric environment, we show a conspicuous photoluminescence peak shift at low temperature for higher energy excitons (2s,3s,4s,5s) in monolayer MoSe2, while the 1s exciton peak position remains unaltered a direct evidence of varying compensation between screening induced exciton binding energy modulation and quasiparticle band-gap renormalization. The estimated modulation of binding energy for the 1s exciton is found to be 58.6% (72.8% for 2s, 75.85% for 3s, and 85.6% for 4s) by coating an Al2O3 layer on top, while the correspond- ing reduction in quasiparticle band-gap is estimated to be 246 meV. Such direct evidence of large tunability of the binding energy of exciton complexes as well as the band-gap in monolayer TMDs holds promise of novel device applications. Enhancing exciton valley coherence time: In monolayer TMDs, valley coher- ence degrades rapidly due to a combination of fast scattering and inter-valley exchange interaction. This leads to a sub-picosecond valley coherence time, making coherent manip- ulation of exciton a highly formidable task. Using monolayer MoS2 sandwiched between top and bottom graphene, we demonstrate perfect valley coherence by observing 100% degree of linear polarization (DOLP) of excitons in steady state photoluminescence. This is achieved in this unique design through a combined effect of (a) suppression in exchange interaction due to enhanced dielectric screening, (b) reduction in exciton lifetime due to a fast inter-layer transfer to graphene, and (c) operating in the motional narrowing regime. We disentangle the role of the key parameters affecting valley coherence by using a com- bination of calculation (solutions of Bethe-Salpeter and steady-state Maialle-Silva-Sham equations) and choice of systematic design of experiments using four different stacks with varying screening and exciton lifetime. To the best of our knowledge, this is the first time where the valley coherence timescale has been significantly enhanced in monolayer semiconductors. Probing the role of motional narrowing in exciton valley coherence: We observe a strong effect of motional narrowing (regime of random phase cancellation) by observing a high DOLP from a defected MoS2 sample, as compared to a clean MoS2 sam- ple which shows relatively lower exciton DOLP. Similar observations hold good for both monolayer and bilayer MoS2 samples, which results from random phase cancellation in the exciton pseudospin in the motional narrowing regime. This highlights the counter- intuitive role of sample quality in the exciton DOLP measurements: a clean sample does not necessarily guarantee large exciton DOLP and vice versa. Generating highly luminescent, highly-polarized, ultra-narrow exciton peak : On generation, the excitons relax to the lowest energy 1s state by scattering with phonons through multiple possible pathways. We use a simple technique in which, by tuning the excitation laser wavelength, the excitons resonantly come down to the 1s state in a single- shot manner through scattering with a specific phonon mode. Using this technique in a monolayer WS2 sample sandwiched between few-layer graphene flakes, we obtain exciton peaks that are: (1) highly luminescent, (2) highly linearly polarized - demonstrating near- perfect valley coherence, and (3) ultra-narrow - due to a reduction in the inhomogeneous broadening. The lowest exciton linewidth obtained using this technique is 1:5 meV, which after deconvolution with the excitation laser gives an upper bound of 0:23 meV on the homogeneous linewidth of the exciton peak. We demonstrate the above features all the way from cryogenic temperature to room-temperature.