Grafting of Cobaltic Protoporphyrin IX on Semiconductors toward Sensing Devices: Vibrational and Electronic High-Resolution Electron Energy Loss Spectroscopy and X-ray Photoelectron Spectroscopy Study

Cobaltic protoporphyrin IX (CoPP) was grafted on GaAs(100), Si(111), and highly oriented pyrolytic graphite, HOPG(0001), surfaces using different preparation conditions. Modified surfaces were characterized by X-ray photoelectron spectroscopy (XPS) and high-resolution electron energy loss spectroscopy (HREELS) recorded in vibrational and electronic domains. XPS confirms the adsorption of the porphyrin on all of the substrates through its main fingerprint, the photoelectron Co 2p region. HREELS spectra also attest to the presence of CoPP on the substrates. The presence of oxides is also clearly detected. Analysis of the corresponding cross sections confirms that vibration excitations are mostly produced via an electron impact mechanism. Analysis of the electronic domain allowed observation of the Q Soret, N, and C bands present in UV/vis spectra. Quantitative results for relative differential cross sections of vibrational and electronic excitations suggest that negative-ion resonances may play a role in the excitation.