Probing excitonic states in suspended two-dimensional semiconductors by photocurrent spectroscopy.

A R Klots,K A Velizhanin,D Caudel,J Yan,D Prasai,A K M Newaz,D G Mandrus,N H Tolk,H Krzyzanowska,K I Bolotin,Junhao Lin,B L Ivanov,Bin Wang,S T Pantelides,A Burger,N J Ghimire

doi:10.1038/srep06608

Abstract

The optical response of semiconducting monolayer transition-metal dichalcogenides (TMDCs) is dominated by strongly bound excitons that are stable even at room temperature. However, substrate-related effects such as screening and disorder in currently available specimens mask many anticipated physical phenomena and limit device applications of TMDCs. Here, we demonstrate that that these undesirable effects are strongly suppressed in suspended devices. Extremely robust (photogain > 1,000) and fast (response time < 1 ms) photoresponse allow us to study, for the first time, the formation, binding energies, and dissociation mechanisms of excitons in TMDCs through photocurrent spectroscopy. By analyzing the spectral positions of peaks in the photocurrent and by comparing them with first-principles calculations, we obtain binding energies, band gaps and spin-orbit splitting in monolayer TMDCs. For monolayer MoS2, in particular, we obtain an extremely large binding energy for band-edge excitons, Ebind ≥ 570 meV. Along with band-edge excitons, we observe excitons associated with a van Hove singularity of rather unique nature. The analysis of the source-drain voltage dependence of photocurrent spectra reveals exciton dissociation and photoconversion mechanisms in TMDCs.

Highlights

Chemical, Biological & Materials Engineering, University of Oklahoma, Norman, Probing excitonic states in suspended two-dimensional semiconductors by photocurrent spectroscopy
Despite rapid progress in understanding the electronic and optical properties of TMDCs1, important fundamental questions remain unanswered: 1) What types of excitons exist in transition-metal dichalcogenides (TMDCs) and what are their binding energies? While calculations predict a plethora of excitonic states with extremely large binding energies[26,27], experimental progress has been hampered by large broadening of the excitonic peaks in the available samples[5,26]
2) How do substrate-related effects perturb the intrinsic properties of monolayer TMDCs? there are indications that the presence of a substrate can cause strong carrier scattering[28,29] and affect exciton energies through screening[30]

Summary

NANOSCIENCE AND TECHNOLOGY

Correspondence and requests for materials should be addressed to K.I.B. Two-dimensional confinement, high effective carrier mass and weak screening lead to strong electron-electron interactions and dominance of tightly bound excitons in the optical properties of 1L-TMDCs5–14 These extraordinary properties make TMDCs ideal platform for studying many anticipated phenomena including quantum-, valley- and spin-Hall effects[6,15,16], superconductivity in monolayer MoS217,18 and many-body effects[12,13,19]. We investigate the photoconversion and photogain mechanisms in monolayer TMDCs. By controlling the source-drain voltage, we observe different dissociation pathways for A/B- and C-excitonic states, demonstrate photogain of the order of 1000 with response times faster than 1 ms, and investigate the mechanism of this photogain. Upon illumination with power P, n increases by Dn~ðP=hvÞaðhvÞDt, where a is the absorption coefficient, D is the photoconversion probability (the probability of generating an unbound photocarrier by an absorbed photon), and t is the photocarrier lifetime[35]

LG Ln

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