Exciton-Driven Ultrafast Enhancement of Quasiparticle Bandgap and Effective Mass in Monolayer MoS2

Abstract

We report an ultrafast increase of the quasi-particle bandgap and effective mass in photoexcited monolayer MoS2 on HOPG, utilizing extreme-ultraviolet time- and angle-resolved photoemission spectroscopy (XUV-trARPES). Combined with theoretical models, we attribute these compelling band renormalizations to the excitonic effects from bound electron-hole pairs.

Dynamic Quantum-State Renormalization and Effects of Competing Pathways on Carrier Relaxation in Semiconductor Nanoparticles

“…Following the results reported on the renormalization of the band-edge states in monolayer MoS 2 , , the photoexcitation changes the carrier screening experienced by each electron and hole in the WZ CdTe QWs. The screening for separate carriers and excitons will likely differ, and contributions from both are expected in both samples of the CdTe QWs as their Φ PL values are not unity.…”

“…This photoexcitation results in changes in electron screening, shielding, and electron–electron interactions in comparison to the unexcited semiconductor that alter the energetics of the VB and CB, and thus the effective masses of the carriers, in the spatial region of the NP sampled by the electron–hole pair, as depicted by the red bands in Figure d. As mentioned above, this photoinduced increase in the effective masses of the carriers is observed and well modeled in monolayer TMDCs, although the band structure for those systems leads to an increase in the band gap energy. , Since the energies of the quantum-confinement states depend on the effective masses of the carriers, the quantum-confinement states in the VB and CB energetically shift with photoexcitation, even with the presence of only a single electron–hole pair in each NP, as depicted in Figure e. Photoexcited electrons, holes, and excitons may also interact with the ions in a polar semiconductor nanocrystal through Fröhlich interactions. − The photoexcited electron and hole interact with the cations and anions in the crystal lattice, perturbing the crystal structure and coupling with longitudinal optical (LO) phonons. − The changes in the crystal structure that result from these Fröhlich interactions result in contrasting electronic band structures and effective masses for the CB and VB in the unexcited and excited semiconductor NPs. − ,,− These energetic shifts are grouped together as QSR.…”

“…In many regards, QSR is akin to the renormalization of the band-edge states characterized in the monolayer transition metal dichalcogenides (TMDCs). − Time- and angle-resolved photoemission spectroscopy experiments performed on room-temperature monolayer MoS 2 directly probed the electronic structure renormalization induced with low excitation fluences. , The band-edge states of the MoS 2 sample were observed to shift by ∼40 meV to higher energies at the K point, k = −1.29(1) Å –1 , upon photoexcitation with low fluences, followed by a recovery within 5.0(1.8) ps. The excitation energy at the K valley edge, k = −1.14(1) Å –1 , was observed to decrease by ∼100 meV, and this perturbation remained for much longer than that at the K point.…”

“…The excitation energy at the K valley edge, k = −1.14(1) Å –1 , was observed to decrease by ∼100 meV, and this perturbation remained for much longer than that at the K point. Supporting calculations indicate the photoexcitation and formation of excitons alters carrier screening in the semiconductor, and increases in the effective masses of the conduction band (CB) and valence band (VB) states, and a renormalization of their energies result. , The larger effective masses of the carriers result in a flattening of the MoS 2 monolayer bands and the observed increase in the band gap energy at the K point and the energetic decrease at the K valley edge.…”

Facet-Specific Electron Transfer in Pseudo-Two-Dimensional Wurtzite Cadmium Selenide Nanocrystals

“…In addition, it is the norm for TA data of semiconductor nanoparticles to have spectrally overlapping signals, both bleach and induced absorption, and these can complicate the measured temporal profiles of the features. Of particular importance here is recent theoretical and experimental results that indicate there is a prompt shifting of the quantum-confinement states that occurs with photoexcitation and the resultant changes in screening and shielding that alter the band structure and effective masses of the carriers. − We have termed this shifting as quantum-state renormalization (QSR), and we have shown QSR results in bleach and induced absorption throughout the spectral region of the semiconductor nanoparticles. , The contributions from the QSR appear during the excitation pulse in TA experiments, and the energies of the features can shift with time as the carriers relax to the band-edge states. Consequently, the temporal profiles of TA bleach signals often contain contributions from QSR that appear on ultrashort time scales as well as from the occupation of carriers in the different quantum-confinement states.…”