Electronic state of C 60 monolayer on Ag(1 1 1) before and after Yb intercalation Hong-Nian Li a, * , Xiao-Xiong Wang a , Shao-Long He a , Kurash Ibrahim b , Hai-Jie Qian b , Run Su b , Jun Zhong b , M.I. Abbas b , Cai-Hao Hong b a Department of Physics, Zhejiang University, Hangzhou 310027, PR China b Laboratory of Synchrotron Radiation, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100039, PR China Received 12 October 2004; accepted for publication 29 April 2005 Available online 25 May 2005 Abstract C 60 monolayer was prepared on Ag(1 1 1) surface in ultra-high vacuum environment. The electronic state was inves- tigated by the synchrotron radiation photoemission technique. The results reveal the remarkable covalent component to the C 60 –Ag bonding, which was not observed in previously reported works. The bonding in the interface is primarily covalent with some ionic contribution. Two covalent bonding features were observed distinctly at 4.1 and 4.7 eV binding energy. These features are ascribed to the hybridization between the next highest occupied molecular orbital (HOMO-1) of C 60 and the Ag 4d state. The work function of the C 60 monolayer on Ag(1 1 1) is determined to be 4.63 ± 0.10 eV that is larger than that of the clean Ag(1 1 1) surface by 0.13 ± 0.02 eV. Yb intercalation was carried out to investigate the valence evolution of the monolayer system. The spectral weight at the Fermi level increases sub- stantially at low intercalation stage. After the filling of the lowest unoccupied molecular orbital (LUMO) band, the sample exhibits the semiconducting property for all the rest intercalation levels, and the electronic state departs from the simple band-filling scheme. The electrons may occupy either the hybridizing state or the Madelung-derived state. The above observations are drastically different from that for bulk Yb fullerides, indicating novel properties of rare- earth-metal intercalated C 60 monolayer. Ó 2005 Elsevier B.V. All rights reserved. Keywords: 1 ML C 60 /Ag(1 1 1); Bonding nature; Yb intercalation; Photoemission 1. Introduction C 60 monolayer on metal surfaces has spurred much interest in recent years due to its fun- damental meaning and possible technological 0039-6028/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2005.04.053 * Corresponding author. Tel.: +86 057 187 952880; fax: +86 057 187 951328. E-mail address: phylihn@public.zju.edu.cn (H.-N. Li). Surface Science 586 (2005) 65–73 www.elsevier.com/locate/susc