Effects of dielectric stoichiometry on the photoluminescence properties of encapsulated WSe2 monolayers

Abstract

Two-dimensional transition-metal-dichalcogenide semiconductors have emerged as promising candidates for optoelectronic devices with unprecedented properties and ultra-compact performances.However atomically thin materials are highly sensitive to surrounding dielectric media, which imposes severe limitations to their practical applicability. Hence for their suitable integration into devices, the development of reliable encapsulation procedures that preserve their physical properties are required.Here, the excitonic photoluminescence of monolayers is assessed, at room temperature and 10 K, on mechanically exfoliated WSe2 monolayer flakes encapsulated with SiOx and AlxOy layers employing chemical and physical deposition techniques. Conformal flakes coating on untreated -nonfunctionalized -flakes is successfully demonstrated by all the techniques except for atomic layer deposition, where a cluster-like oxide coating is observed. No significant compositional or strain state changes in the flakes are detected upon encapsulation by any of the techniques. Remarkably, our results evidence that the flakes' optical emission is strongly influenced by the quality of the encapsulating oxide -stoichiometry -. When the encapsulation is carried out with slightly sub-stoichiometric oxides two 2 remarkable phenomena are observed. First, there is a clear electrical doping of the monolayers that is revealed through a dominant trion -charged exciton -room-temperature photoluminescence. Second, a strong decrease of the monolayers optical emission is measured attributed to non-radiative recombination processes and/or carriers transfer from the flake to the oxide. Power-and temperaturedependent photoluminescence measurements further confirm that stoichiometric oxides obtained by physical deposition lead to a successful encapsulation, opening a promising route for the development of integrated two-dimensional devices.Two-dimensional (2D) semiconductor transition-metal-dichalcogenides (TMDCs) present unique physical properties, such as a native sizeable bandgap, relatively large in-plane carrier mobility up to 250 cm 2 /Vs, valley-dependent optoelectronics, strong light-matter interaction, and indirect-to-direct bandgap transition in monolayer crystals, which make them ideal candidates for ultra-compact optoelectronic, photonic and electronic applications [1][2][3][4][5] . Owing to strong spatial confinement of carriers and reduced dielectric screening of Coulomb interactions, electron and holes in monolayer TMDCs are tightly bound, thus forming exciton and multi-exciton complexes with binding energies up to 0.5-1 eV, more than one order of magnitude larger than in conventional III-V 2D quantum well nanostructures 6 .The simplest excitonic specie is the neutral exciton (X 0 ), which consists of an electron-hole pair bounded by Coulomb interaction. Singly charged excitons, or trions (X -), quasi-particles consisting of two electrons (holes) and a hole (electron), have been reported 7,8 with binding energies up to 20-40 meV relative to X 0 ...

Bandgap engineering of two-dimensional semiconductor materials

“…Overall, the latter was found to dominate, and a red-shift in the optical gap was observed. Figure 8a surveys experimentally obtained optical gaps under different dielectric environment for various TMDC, with optical gap tuning in the range of a few hundred of meV [165][166][167]176,179,180 . As for BP, it is commonly covered by capping layers of different materials, such as h-BN and sapphire.…”

“…It is clear that in a 2D semiconductor, the combination of dielectric screening by the material layer itself and that provided by its surrounding environment plays a major role in defining not only exciton-binding energies, but also the QP gaps 18,165,166 . However, it is known that the dielectric screening-induced shifts of these two effects are opposite; hence, their isolated effects are difficult to discern experimentally.…”

“…However, it is known that the dielectric screening-induced shifts of these two effects are opposite; hence, their isolated effects are difficult to discern experimentally. Indeed, early attempts to understand optical gaps under the influence of the surrounding dielectric environment were inconclusive 165,[167][168][169][170][171][172][173][174][175][176] . In these reports, either fused silica or oxides, such as SiO 2 , MgO, and sapphire, are used as substrates and/or capping layers.…”

Strain-Tunable Single Photon Sources in WSe2 Monolayers

“…The results reported in this work pave the way towards the exploitation of energy-tunable SPEs and emitters of entangled photon pairs 35 based on two dimensional crystals and it will stimulate the use of strain-fields to understand the origin of SPEs in 2D materials. For this last point, we anticipate that anisotropic strain fields delivered by micro-machined piezoelectric actuators 28,36 will have a key role where 2D materials can be incorporated upon integration in dielectric nanomembranes 37 .  (ns) g (2) (0)= 0.13 ± 0.06 g (2) (0)= 0.12 ± 0.07 g (2) (0)= 0.12 ± 0.06 by the low deconvoluted value g (2) ()~0.12, 0.13 and 0.12, respectively, which is independent of the applied field (i.e.…”

Strain-tuning of the optical properties of semiconductor nanomaterials by integration onto piezoelectric actuators

“…In principle, any encapsulating oxide can be employed for the nanomembrane as long as it is compatible with the rest of the processing in terms of etching selectivity. We have recently performed a systematic study on the effects of the oxide stoichiometry on the optical emission of the encapsulated flakes that should be taken into account to prevent electrical doping of the flakes and obtain similar emission as as-exfoliated uncapped flakes [49].…”