Spectro-microscopy characterization of CoPc-Au(111) interface

Abstract

Abstract Co-phthalocyanine molecules have wide range of optoelectronics and sensor applications. Motivated by the importance of this material, we studied the adsorption of CoPc thin to thick films on Au(111) surface by using ultraviolet photoelectron spectroscopy (UPS) coupled with angle resolved UPS (AR-UPS) and scanning tunnelling microscopy (STM). Molecular spectral emissions are found to be at 5.13, 7.94, and 10.32 eV below the Fermi level. Observed UPS and AR-UPS measurements show both changes and shifting in valence band features. The work functions of clean Au(111) surface and thick CoPc film are found circa 5.32 eV and 5.25 eV, respectively. Both the work function variation and shifting of molecular features dictate strong adsorbate-substrate interaction. AR-UPS measurements further illustrates that CoPc molecules lie flat on the Au(111) surface. Our STM results disclose that at sub-monolayer (Sub-ML, 0.4–0.8 ML) coverage and room temperature, CoPc molecules tend to adsorb in random fashions; which reveal a strong interaction at the CoPc-Au(111) interface. However, at the ML coverage and elevated temperature (~520K), long range pattern with the oblique structure is observed. Beyond ML coverage, existence of mixed phases consisting of well arranged patterns and molecular clusters have been observed. Overall, our STM results show that depending on the molecular thickness, CoPc molecules form three types of organized oblique, rectangular and square geometries which is consistent with the findings of CuPc and CoPc on Au(111) surface. Our findings may seek potential use in the organic molecular based electronic devices.