Probing exciton species in atomically thin WS₂–graphene heterostructures

Abstract

We report on a detailed study of temperature-dependent photoluminescence spectroscopy of monolayer WS 2 grown on epitaxial graphene. The study reveals for the first time the intrinsic excitonic effects of WS 2 supported on one-and two-layer graphene (2LG) from room temperature to 83 K, with both excitons and trions evidenced in the entire temperature range and temperature-independent trion dissociation. All emission peaks exhibit a shift towards higher energy with decreasing temperature due to band gap renormalisation induced by electron-phonon interaction. A highly linear dependence of the photoluminescence with the excitation power is found for both WS 2 on oneand 2LG, confirming the excitonic nature of associated transitions and the high crystal quality of the WS 2 -graphene heterostructure.

Reduced Absorption Due to Defect-Localized Interlayer Excitons in Transition-Metal Dichalcogenide–Graphene Heterostructures

“…Our GW results are in agreement with previous calculations with a reported band gap reduction of ∼300–350 meV , compared to each pristine counterpart . They are also consistent with the experimental values of the quasi-particle band gap found in MoS 2 –Gr heterostructures, reported to be ∼2.0–2.2 eV, , and in WS 2 –Gr, which is in the range ∼2.0–2.3 eV. ,, …”

“…58 They are also consistent with the experimental values of the quasiparticle band gap found in MoS 2 −Gr heterostructures, reported to be ∼2.0−2.2 eV, 61,62 and in WS 2 −Gr, which is in the range ∼2.0−2.3 eV. 14,63,64 Next, we examine the excitonic properties of defected WS 2 − Gr and MoS 2 −Gr heterobilayers using the Bethe−Salpeter equation 54,55 within the Tamm−Dancoff approximation (see the SI). We show in Figure 2, top panel, the absorbance of WS 2 −Gr as well as its decomposition into intralayer graphene, intralayer TMDC, and interlayer contributions (see the SI for the case of MoS 2 −Gr).…”

Optical dielectric function of two-dimensional WS2 on epitaxial graphene

“…This concept has been fuelled by the recent rise of monolayer transition metal dichalcogenides (TMDC) [4][5][6], which have opened unprecedented possibilities in the development of new opto-electronic devices [7][8][9]. Among the almost infinite possible combinations of 2D materials, tungsten disulphide (WS 2 )/graphene heterostacks have recently attracted much interest due to their intriguing opto-spintronic properties [10][11][12][13][14], which allow to conceive hybrid TMDC/graphenebased ultrafast, ultrasensitive photodetectors [15][16][17]. The knowledge of the properties not only of the single constituent (mono)layers, but also of the whole heterostack, is an essential step towards the large-scale application of vdW heterostacks.…”

“…The main exciton-related absorption peaks of TMDCs in the visible range have been so far identified chiefly by means of reflectance-contrast spectroscopy [18]. Additionally, most of the studies about the optical properties of monolayer TMDC have been limited to the investigation of the photoluminescence peak and the related absorption peak [4,14,19,20]. A more precise spectroscopic characterization of the optical response of TMDC would however involve the measurement of their complex dielectric function ε := ε 1 − iε 2 , since this provides 'the most basic description of light-matter interactions in TMDC monolayers' [21].…”

Probing Nanoscale Schottky Barrier Characteristics at WSe2/Graphene Heterostructures via Electrostatic Doping

“…[39] Given the interface with the underlying graphene, electron transfer from graphene toward the WSe 2 is expected, resulting in negatively charged trions. [40,41]…”