LETTER pubs.acs.org/NanoLett
Carrier Control of Graphene Driven by the Proximity Effect of Functionalized Self-assembled Monolayers Kazumichi Yokota,* Kazuyuki Takai, and Toshiaki Enoki Department of Chemistry, Tokyo Institute of Technology, 2-12-1-W4-1, Ookayama, Meguro-ku, Tokyo 152-8551, Japan
bS Supporting Information ABSTRACT: We demonstrated the carrier control of graphene by employing the electrostatic potential produced by several types of self-assembled monolayer (SAM) formed on SiO2 substrates. For single layer graphene on perfluoroalkylsilaneSAM, the stiffening of the Raman G-band indicates a large down shift of the Fermi level (∼0.8 eV) by accumulated hole carriers. Meanwhile, aminoarylsilane-SAM accumulated electron carriers, which compensate the hole carriers doped by adsorbed molecules under the ambient atmosphere, in graphene. The present results and their theoretical analysis reveal that the use of the dipole moments of SAM molecules can systematically modulate the electrostatic potential affecting graphene without destroying its intrinsic electronic structure and let us know that the proximity effect of the SAMs is a promising way in developing graphene-based solid state electronics. KEYWORDS: Graphene, self-assembled monolayers, carrier control, X-ray photoemission spectroscopy (XPS), Raman spectroscopy, density functional theory (DFT) calculation
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raphene, a single atomic layer of graphite, has attracted great interests in recent years due to the unconventional electronic properties such as extremely high carrier mobility, halfinteger quantum Hall effect, Klein tunneling, and so forth, which are attributed to its characteristic band structure with linear massless dispersions.14 In an effort to carry over these advantageous properties of graphene to electronic materials of the next generation and to progress graphene-based electronics,5 it is necessary to control the density and character of charge carrier on graphene without destroying its characteristic electronic structure. So far, the carrier doping to graphene has been achieved not only by electrical field modulation but also by gas phase molecular adsorption68 and contacts with metal and semimetal.9,10 However, the control of electronic properties for graphene on insulators would be preferable for solid state device applications, although the excessive interaction between graphene and these substrates seriously changes the intrinsic electronic structure of graphene.11 To overcome this drawback on the carrier control for graphene, we focused our attention on the use of self-assembled monolayers (SAMs) on SiO2 substrates by which the control of organic thin film transistor characteristics had been investigated.12 SiO2 substrates are compatible with the conventional Si-based electronics13 and are also used widely in the studies on graphene. On those substrates, silane coupling agents form SAMs which provide various merits such as chemical modifications of the surface and terminations of trap states in designing device architectures.14,15 Therefore, SAMs fabricated on SiO2 substrates are potential candidates to provide the desirable electronic properties to graphene and to develop graphene-based electronics devices, where careful r 2011 American Chemical Society
characterizations of the physical and chemical properties of those substrate surfaces, onto which graphene is transferred, are the important task. In this study, we demonstrated the systematic carrier control of graphene by using various SAMs on SiO2 substrates. X-ray photoemission spectroscopy (XPS) measurements enabled us not only to ensure the formation of SAMs but also to elucidate the chemical states and the electronic states of constituent molecules in SAMs. For the characterization of graphene, we conducted Raman spectroscopy, which is a powerful nondestructive technique for identifying the number of layers, structure and disorder, and doping level of graphene. Further theoretical investigations by using density functional theory (DFT) method elucidated the carrier doping mechanism of graphene on SAMs, and our results suggested that huge carrier modulations take place at graphene proximate to SAMs without destroying its intrinsic unique electronic structure. The schematic illustrations of sample preparations are shown in Figure 1. Sample substrates were 285 nm SiO2 on n-type Si(100) wafers. These substrates were exposed to O2 plasma for a hydrophilic treatment and their water contact angle, which was measured by putting 2 μL of water droplets onto the surface, decreased to
