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Electronic structure and ultrafast charge transfer dynamics of phosphorous doped graphene layers on a copper substrate: a combined spectroscopic study

Graphene sheet layers were grown by chemical vapor deposition (CVD) under a polycrystalline copper substrate using methane (CH4) and triphenylphosphine (P(C6H5)3) as carbon and phosphorous precursors, respectively. The films obtained from the CH4 and P(C6H5)3 chemical precursors were labeled as G/Cu and GP/Cu, respectively. Electronic structure investigation was performed on these two graphene samples combining different spectroscopic techniques. Raman spectroscopy shows the presence of single and multilayers in G/Cu and GP/Cu, respectively. A blue shift of 30 cm−1 of the 2D band in the GP/Cu film with respect to G/Cu is evidence of the p-type doping of GP/Cu. X-ray photoelectron and reflection electron energy loss spectroscopy (REELS) confirm the bilayer formation in the GP/Cu film. REELS also shows that the presence of phosphorous does not open the electronic band gap of the graphene film. The work function of 4.3 eV for G/Cu and 4.8 eV for GP/Cu films were determined by ultraviolet photoelectron spectroscopy. The increase of the work function is attributed to the electron transfer to the Cu substrate. The partially unoccupied densities of states in phosphorous doped graphene (GP/Cu) were evaluated by X-ray photoabsorption spectroscopy. The core-hole clock approach using resonant Auger spectroscopy was employed for investigating the charge transfer dynamics around the P K-edge in GP/Cu. Ultrafast charge transfer delocalization on a time scale of femtoseconds was observed, demonstrating a strong electronic coupling between unoccupied states of the phosphorous and the conduction band of the copper substrate. The combined spectroscopic results suggest p-type doping in GP/Cu by the electron transfer mechanism.