Enhanced vacuum ultraviolet photoemission from graphene nanoribbons

Photon-energy dependence of photoemission from seven-atoms-wide armchair graphene nanoribbons (GNRs) is studied experimentally and theoretically up to PLANCK CONSTANT OVER TWO PI omega=95 eV. A strong photon energy dependence of the normal emission from the valence band maximum (VB1) is observed, sharply peaked at PLANCK CONSTANT OVER TWO PI omega=12 eV. The detailed analysis of the light-polarization dependence of the photoemission from VB1 unambiguously characterizes the symmetry of the state. The experimental observations are analyzed based on ab initio one-step theory of photoemission. Off-normal emission is studied in detail and its relation to the standing-wave character of the valence band states is discussed. Excellent agreement with the earlier experiment (Senkovskiy et al 2018 2D Mater. 5 035007) is obtained. Rapid variations of the intensity with the ribbon-transverse photoelectron momentum are predicted from the ab initio theory, which are at variance with the prediction of the tight-binding rigid-wall model. These findings can help interpret angle-resolved photoemission measurements of similar systems. Moreover, the strong enhancement of the photoyield could trigger the GNR application as narrow-band photodetectors and contribute to the design of novel photocathodes for vacuum ultraviolet photodetection.