ABSTRACT
Two-dimensional (2D) magnets such as chromium trihalides CrX3 (X = I, Br, Cl) represent a frontier for spintronics applications and, in particular, CrCl3 has attracted research interest due its relative stability under ambient conditions without rapid degradation, as opposed to CrI3. Herein, mechanically exfoliated CrCl3 flakes are characterized at the atomic scale and the electronic structures of pristine, oxidized, and defective monolayer CrCl3 phases are investigated employing density functional theory (DFT) calculations, scanning tunneling spectroscopy (STS), core level X-ray photoemission spectroscopy (XPS), and valence band XPS and ultraviolet photoemission spectroscopy (UPS). As revealed by atomically resolved transmission electron microscopy (TEM) and energy dispersive X-ray (EDX) analysis, the CrCl3 flakes show spontaneous surface oxidation upon air exposure with an extrinsic long-range ordered oxidized O-CrCl3 structure and amorphous chromium oxide formation on the edges of the flakes. XPS proves that CrCl3 is thermally stable up to 200 °C having intrinsically Cl vacancy-defects whose concentration is tunable via thermal annealing up to 400 °C. DFT calculations, supported by experimental valence band analysis, indicate that pure monolayer (ML) CrCl3 is an insulator with a band gap of 2.6 eV, while the electronic structures of oxidized and Cl defective phases of ML CrCl3, extrinsically emerging in exfoliated CrCl3 flakes, show in-gap spin-polarized states and relevant modifications of the electronic band structures.
ABSTRACT
Via experiments performed by varying the doping level of single-layer mechanically exfoliated MoS2via post exfoliation thermal annealing in the 200-300 °C temperature range, we study the power dependent room temperature (RT) photoluminescence (PL), which is dominated by A type excitons. The PL spectral yields of the as-exfoliated and annealed samples show sub-linear behaviour as a function of the excitation laser power. The PL signal of the 200 °C annealed sample is dominated by the charged exciton (trion) related peak, while the PL signal of the 300 °C annealed sample is dominated by the neutral exciton related peak, and the PL spectral weight of excitons is tunable in this temperature range. The PL signal increase due to annealing and the intensity ratio of the A type excitons are related, showing a hyperbolic tangent trend. We directly quantitatively demonstrate the one-to-one correlation of the RT resonant Raman (632.8 nm) integrated spectral intensity with the corresponding PL spectral yield, providing experimental evidence of the exciton-phonon coupling effect. The in-plane E2g1 Raman mode exhibits strong coupling with A excitons, while the out-of-plane A1g Raman mode does not. This is an indication of the in-plane spatial symmetry of the A excitons.
