Electronic structure of a 3, 4, 9, 10-perylene-tetracarboxylic-dianhydride thin film revealed by synchrotron-based resonant photoemission spectroscopy
Authors: Li Zhi-Hao  , Cao Liang  , Guo Yu-Xian  ,   School of Materials Science and Chemical Engineering, Anhui Jianzhu University, Hefei 230601, China;,   Anhui Province Key Laboratory of Condensed Matter Physics at Extreme Conditions, High Magnetic Field Laboratory, Chinese Academy of Sciences, Hefei 230031, China
Published: December 2020

The electronic structure of a 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) thin film is investigated in situ using synchrotron-based near edge X-ray absorption fine structure (NEXAFS) spectroscopy and resonant photoemission spectroscopy (RPES).The NEXAFS spectroscopy can monitor the electronic transitions from core level to unoccupied states.The C K-edge NEXAFS spectrum of the PTCDA thin film shows four distinct absorption peaks below 290 eV,which are attributed to the transitions from 1s core level of C-atoms in different chemical environments (perylene core C-atoms vs anhydride C-atoms) into lowest unoccupied molecular orbitals (LUMOs) with * symmetry. The RPES spectra are collected in the valence band region by sweeping photon energy across the C 1s * absorption edge.Three typical features of the C 1s signals excited by second-order harmonic X-ray,resonant photoemission and resonant Auger features are observed in RPES spectra,and are identified,relying on the development of kinetic energy of the emitted photoelectrons upon the change of incident photons energy.It is found that the C 1s signals excited by second-order harmonic X-ray are present at high kinetic energy side of spectrum.The kinetic energy of this feature shows photon energy dependence,that is,this feature shifts to higher kinetic energy by photon energy increasing twice.Resonant Auger peaks in RPES spectra are located on the low kinetic energy side with constant kinetic energy regardless the change of photon energy.The resonant Auger may originate from deeper molecular orbitals with binding energy large than 4.1 eV,suggesting that the resonant Auger decay process involved in deeper molecular orbitals occurs on a time scale comparable to C 1s core hole lifetime of 6 femtoseconds.Resonant enhancement of highest occupied molecular orbitals (HOMOs) derived valence band features or HOMO-1 and HOMO-2 derived resonant photoemission features in our case are lying between the C 1s signals and the resonant Auger signals.The Kinetic energy increases as the photon energy sweeps across the absorption edge,whereas their binding energy remains constant.In addition, the enhancements of two resonances show photon energy dependence that enhancement of HOMO-1 related resonance dominates over HOMO-2 related resonance at energies corresponding to perylene core C 1s to LUMOs transitions, whereas HOMO-2 related resonance becomes dominant at transitions from anhydride C 1s to LUMOs.This behavior can be related to the wavefunction character and symmetry of the frontier molecular orbitals.Clarifying each resonant feature in RPES spectra and their origin will pave the way for accurately determining the ultrafast charge transfer time at organic/electrode interfaces using synchrotron-based core hole clock technique implementation of RPES.