Magneto-spectroscopy of exciton Rydberg states in a CVD grown WSe2 monolayer

Abstract

The results of magneto-optical spectroscopy investigations of excitons in a CVD grown monolayer of WSe 2 encapsulated in hexagonal boron nitride are presented. The emission linewidth for the 1s state is of 4.7 meV, close to the narrowest emissions observed in monolayers exfoliated from bulk material. The 2s excitonic state is also observed at higher energies in the photoluminescence spectrum. Magneto-optical spectroscopy allows for the determination of the g-factors and of the spatial extent of the excitonic wave functions associated with these emissions. Our work establishes CVD grown monolayers of transition metal dichalcogenides as a mature technology for optoelectronic applications.Monolayers of semiconducting transition metal dichalcogenides (S-TMD), labelled as MX 2 , where M=W, Mo, Re or Zr and X=S, Se or Te for the most studied compounds, are direct band gap semiconductors with the band gap located at the two inequivalent K ± points of their hexagonal Brillouin zone 1 . They exhibit a large number of properties interesting both for fundamental science and for technology 2 . These properties include a band gap in the visible range 3 , a strong light matter coupling 4 , the presence of single photon emitters 5,6 , very strong excitonic effects 7-9 , possibilities of tuning the emission energy by Coulomb engineering 10,11 , and the opportunity of combining them to obtain type II band alignment 12 . The spin-orbit interaction in these materials is strong and splits the spin levels in the valence band by few hundreds of meV, and by few tens of meV in the conduction band 13 . This large splitting in the valence band defines two different excitons at the K ± points attached to the two spin-split bands, named A and B excitons 14 . Because of the lack of inversion symmetry, the two different valleys can be excited independently using circularly polarized light 15 , opening the possibility of initializing an exciton population in a given valley, or creating coherent superpositions of both valleys 16 .Best specimens of S-TMD monolayers are usually obtained by the mechanical exfoliation of bulk (natural or synthetic) crystals which leads to micrometer sized flakes, suitable for scientific investigations or for demonstrating a) Electronic mail: clement.faugeras@lncmi.cnrs.fr prototype devices, but preventing their use in realistic applications produced at an industrial scale. Growth techniques, mainly chemical vapor deposition (CVD) 17 and molecular beam epitaxy (MBE) 18,19 , have been developed to produce large scale monolayers of S-TMD suitable for electronics and optoelectronics applications, or to elaborate vertical/horizontal heterostructures 20,21 . In this letter, we demonstrate CVD grown S-TMD monolayers with optical quality comparable to state of the art exfoliated flakes. For that purpose, we have encapsulated it in between hexagonal boron nitride (h-BN) flakes in order to unveil its intrinsic properties. We show that the optical response of this CVD grown monolayer is comparable to the best exfoliated monol...

Rydberg series of dark excitons and the conduction band spin-orbit splitting in monolayer WSe2

“…Possible formations of many different excitonic states (including nontrivial indirect/inter valley excitons and excitonic complexes such as trions and biexcitons) account for the spectrum complexity which we disregard here, focusing our attention on neutral, bright and dark intravalley excitons. First, we examine the photoluminescence (PL) spectrum measured in the absence of magnetic field (B), and, in accordance to previous reports, recognize the characteristic sequence of emission peaks related to the Rydberg series of 1s , 2s , 3s , up to 4s states of bright excitons 24,25,[27][28][29] . The PL spectrum measured at B=0 shows also a weak transition related to the ground state, 1s , of dark exciton [14][15][16] .…”

“…In this paper, we report on low-temperature magneto-optical study of Rydberg series of excitonic states as they appear in a WSe 2 monolayer. Notably, in this representative example of a 2D semiconductor, the rich spectrum of excited excitonic states can be relatively easily traced with luminescence experiments 25,[27][28][29] . Profiting of this fact we brighten dark excitons in our sample by applying an in-plane magnetic field and uncover their s ( =1, up to 4) Rydberg series which appears in addition to a Rydberg spectrum of bright excitons 24,25 .…”

Band nesting and exciton spectrum in monolayer MoS2

“…Theoretical models and calculations of exciton excited s-series, experimentally accessible in single photon emission/absorption experiments 46,[68][69][70] , are usually based on model dispersion and screened Rytova-Keldysh interaction 71 or models using dielectric screening functions with various levels of sophistication 48,72,73 . State of art experiments, performed in magnetic fields allowing to identify signal from excited exciton states, were reported for different MX 2 compounds 14,47,48,[74][75][76] .…”

Reversible engineering of spin–orbit splitting in monolayer MoS2via laser irradiation under controlled gas atmospheres

“…Recent development in atomic-layered transition metal dichalcogenides (TMDs) with chemical formula MX2 (such as MoS2, MoSe2, WS2, and WSe2) demonstrate that [21,22] , TMDs are expected to be good candidates for spintronic technologies at room temperatures, due to their giant intrinsic SOC [20,23] . The spin-orbit splittings (charactering the strength of SOC) in TMDs monolayer induced by spin-orbit interaction are several hundred millielectron volts (meV) at the top of the valence band and a few to tens of meV for the bottom conduction band [22,24] . The origin of this dramatic interaction lies in the relative heavy elements in the TMDs materials and the involvement of the transition metal d orbitals [25] .…”