Fig. 5
From: Room temperature multi-phonon upconversion photoluminescence in monolayer semiconductor WS2

Upconversion photoemission process in monolayer WS2 at 7 K. a The PL spectrum recorded for WS2/hBN(136 nm)/SiO2/Si at 7 K. Black, green, blue and orange arrows point out excitation energies used in UPC experiment. b Examples of UPC spectra at 7 K excited with energies indicated by coloured arrows in a. c The dependence of the upconversion integrated intensity on the energy difference of the neutral exciton (X) and the excitation photon energy (EX–Eex)