Impact of substrate induced band tail states on the electronic and optical properties of MoS2

Abstract

Substrate, environment, and lattice imperfections have a strong impact on the local electronic structure and the optical properties of atomically thin transition metal dichalcogenides. We find by a comparative study of MoS2 on SiO2 and hexagonal boron nitride (hBN) using scanning tunneling spectroscopy (STS) measurements that the apparent bandgap of MoS2 on SiO2 is significantly reduced compared to MoS2 on hBN. The bandgap energies as well as the exciton binding energies determined from all-optical measurements are very similar for MoS2 on SiO2 and hBN. This discrepancy is found to be caused by a substantial amount of band tail states near the conduction band edge of MoS2 supported by SiO2. The presence of those states impacts the local density of states in STS measurements and can be linked to a broad red-shifted photoluminescence peak and a higher charge carrier density that are all strongly diminished or even absent using high quality hBN substrates. By taking into account the substrate effects, we obtain a quasiparticle gap that is in excellent agreement with optical absorbance spectra and we deduce an exciton binding energy of about 0.53 eV on SiO2 and 0.44 eV on hBN.

Direct measurement of key exciton properties: Energy, dynamics, and spatial distribution of the wave function

“…A large portion of the research on excitonic phenomena in TMDCs adopts optical spectroscopic techniques [10,12,[14][15][16][18][19][20][21], which only access bright excitonic transitions with near-zero momentum transfer. Although techniques such as time-resolved THz spectroscopy also allow probing optically dark excitons via internal quantum transitions [13], finite-momentum excitons that lie outside the radiative light cone remain inaccessible to such methods.…”

Bose‐Einstein‐Kondensat aus Exzitonen?

“…Meist befindet sich der kristall auf einem isolierenden substrat und ist entweder mit einer isolierenden Deckschicht (Abbildung 6) oder Vakuum beziehungsweise Luft umgeben. Dies ermöglicht die gezielte Beeinflussbarkeit der Bindungsenergie der Exzitonen durch die dielektrische Umgebung [12,13].…”

Light-field and spin-orbit-driven currents in van der Waals materials

“…Due to their strong spin-orbit coupling, monolayer TMDs inherently intertwine angular momentum, out-of-plane spin, and crystal momentum degrees of freedom, such that under polarized optical excitation directed spin and charge currents can emerge [17][18][19][20][21]. Directly after a pulsed photoexcitation, the presence of a large density of (photogenerated) charge carriers can alter the Coulomb screening in monolayer TMDs [22][23][24][25][26][27][28][29], such that both the quasi-particle band gap and the excitonic binding energies are renormalized on femtosecond time scales. The renormalization effects are based upon an interplay of excitation-induced dephasing, phasespace filling and the screening of Coulomb interaction in combination with ultrafast non-radiative relaxation and recombination processes [22][23][24][25][26]30].…”