Atmospheric Oxygen Binding and Hole Doping in Deformed Graphene on a SiO2 Substrate Sunmin Ryu,†,| Li Liu,‡,| Stephane Berciaud,‡,⊥ Young-Jun Yu,§ Haitao Liu,‡ Philip Kim,§ George W. Flynn,*,‡ and Louis E. Brus*,‡ Department of Applied Chemistry, Kyung Hee University, Yongin, Gyeonggi 446-701, Korea and ‡ Department of Chemistry and § Department of Physics, Columbia University, New York, New York 10027, United States ABSTRACT Using micro-Raman spectroscopy and scanning tunneling microscopy, we study the relationship between structural distortion and electrical hole doping of graphene on a silicon dioxide substrate. The observed upshift of the Raman G band represents charge doping and not compressive strain. Two independent factors control the doping: (1) the degree of graphene coupling to the substrate and (2) exposure to oxygen and moisture. Thermal annealing induces a pronounced structural distortion due to close coupling to SiO2 and activates the ability of diatomic oxygen to accept charge from graphene. Gas ﬂow experiments show that dry oxygen reversibly dopes graphene; doping becomes stronger and more irreversible in the presence of moisture and over long periods of time. We propose that oxygen molecular anions are stabilized by water solvation and electrostatic binding to the silicon dioxide surface. KEYWORDS Graphene, Raman spectroscopy, scanning tunneling microscopy (STM), chemical doping, ripple, oxygen †
raphene, a single sheet of graphite, has been the subject of intensive research owing to its potential application in electrical devices,1 ﬂexible and transparent electrodes,2 ultrathin membranes,3 and various nanocomposites.4 Recent reports of efﬁcient chemical growth5,6 and band gap tuning in double-layered graphene7 have expanded our ability to synthesize and control graphene for these applications.8 Initial reports of thickness-dependent chemical reactivity,9,10 photochemical/electrochemical reactivity,11 basal plane functionalization,10-13 and intercalant-induced chemical doping,14 have recently appeared. Purposeful graphene modiﬁcation and systematic processing, however, require a deeper understanding of graphene chemistry than is presently available. Single atomic layer graphene is a unique two-dimensional electronic material. With all atoms on the surface, its properties are strongly inﬂuenced by the supporting substrate15-19 and the local molecular environment.20 While graphene shows atomic-level ﬂatness on mica21 and h-BN22 substrates, it shows different degrees of local structure and corrugation when deposited on SiO2/Si substrates, or when suspended across a trench.23,24 These structural deviations from planarity are believed to strongly affect electronic properties25 and chemical reactivity.26 * To whom correspondence should be addressed. E-mail: (G.W.F) gwf1@ columbia.edu; (L.E.B.) email@example.com. | ⊥
These authors contributed equally to this study.
Present afﬁliation: IPCMS (UMR 7504), Universite de Strasbourg and CNRS, ´ F-67034 Strasbourg, France. Received for review: 08/21/2010 Published on Web: 11/11/2010 © 2010 American Chemical Society
Adsorption on SiO2/Si substrates is of central importance for technology, and such adsorption signiﬁcantly modiﬁes charge transport properties and Raman spectra.18,19,27 Several independent studies have reported a signiﬁcant stiffening (shift to higher energy) of the Raman G and 2D bands in thermally annealed, supported graphene;9,28,29 however, the basic cause of this shift remains uncertain. To control and purposefully modify graphene for application, these issues need to be clearly understood. Molecular O2 exhibits a rich variety of chemical interactions with aromatic molecules30,31 and carbon nanotubes,32 and adsorbed O2 is a well-known, effective hole-dopant for these species.30,32-34 In general, distortion of aromatic π systems from planarity creates a stronger interaction with O2.31 The present study reveals...