A. Mariscal

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

Two-dimensional transition metal dichalcogenide semiconductors have emerged as promising candidates for optoelectronic devices with unprecedented properties and ultra-compact footprints. However, the high sensitivity of atomically thin materials to the surrounding dielectric media imposes severe limitations on their practical applicability. Hence, to enable the effective integration of these materials in devices, the development of reliable encapsulation procedures that preserve their physical properties is required. Here, the excitonic photoluminescence (at room temperature and 10 K) is assessed on mechanically exfoliated WSe2 monolayer flakes encapsulated with SiOx and AlxOy layers by means of chemical and physical deposition techniques. Conformal coating on untreated and non-functionalized flakes is successfully achieved by all the techniques examined, with the exception of atomic layer deposition, for which a cluster-like oxide coating is formed. No significant compositional or strain state changes in the flakes are detected upon encapsulation, independently of the technique adopted. Remarkably, our results show that the optical emission of the flakes is strongly influenced by the stoichiometry quality of the encapsulating oxide. When the encapsulation is carried out with slightly sub-stoichiometric oxides, two remarkable phenomena are observed. First, dominant trion (charged exciton) photoluminescence is detected at room temperature, revealing a clear electrical doping of the monolayers. Second, a strong decrease in the optical emission of the monolayers is observed, and attributed to non-radiative recombination processes and/or carrier transfer from the flake to the oxide. Power- and temperature-dependent 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.

Europium monoxide nanocrystalline thin films with high near-infrared transparency

Nanocrystalline textured EuO thin films are prepared by an oxygen loss process from a pure Eu2O3 bulk ceramic target through pulsed laser deposition in vacuum at room temperature. X-ray diffraction spectra evidence a well-defined diffraction peak corresponding to the EuO phase textured along the (110) direction. Analysis of the XRD peak profile indicates that the films are nanocrystalline (average crystallite size of 11 nm) with a compressive residual strain. The formation of stoichiometric EuO is further confirmed by a strong signal from Eu2+ in the X-ray photoelectron spectra. The complex refractive index in the near infrared has been determined by spectroscopic ellipsometry and shows that the EuO films have a high transparency (k<10-3) and a refractive index of 2.1. A band-gap shift of 0.25 eV is found with respect to the EuO bulk. These films, deposited by an accessible and efficient method, open a new route to produce EuO films with optical quality, suitable for NIR optoelectronic components.