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J. Phys. Chem. C 2008, 112, 2116-2120
Subnanometer Imaging of Adsorbate-Induced Electronic Structure Perturbation on Silicon Surfaces N. P. Guisinger, N. L. Yoder, S. P. Elder, and M. C. Hersam* Department of Materials Science and Engineering, Northwestern UniVersity, EVanston, Illinois 60208-3108 ReceiVed: October 5, 2007; In Final Form: NoVember 28, 2007
Room-temperature scanning tunneling microscopy is utilized to explore the consequences of a single covalent bond formed between an organic molecule, 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO), and the Si(111)-7 × 7 surface at the atomic scale. Upon binding, both topographic imaging and spectroscopic techniques reveal significant charge rearrangement within the substrate that is delocalized from the organic adsorbate. With scanning tunneling spectroscopy, the spatial extent of this charge transfer is directly visualized and determined to extend up to 2 nm from the molecule predominately within half of the Si(111)-7 × 7 unit cell. Analysis of individual differential tunneling conductance spectra suggests that the charge transfer is mediated by the back-bonds of the silicon substrate.
The study of organically functionalized semiconductor surfaces has become an increasingly active field of research due to its fundamental significance and technological relevance. Of particular interest is semiconductor-based molecular electronics,1-4 which seeks to exploit the electronic properties of organic molecules in conjunction with conventional microelectronic devices. Additionally, organic molecules covalently bound to semiconductor surfaces have been proposed as the active elements in chemical and biological sensors.5-7 With this motivation, several research efforts have focused on investigating the fundamental surface chemistry for organic molecules on various semiconductor substrates.8-11 However, to realize the full potential of organically functionalized semiconductors in nanoelectronic and molecular sensing applications, the resulting electronic property changes in both the organic molecule and the underlying substrate upon chemisorption should be characterized and understood with submolecular spatial resolution. The ultrahigh vacuum (UHV) scanning tunneling microscope (STM) has played a key role in elucidating both the mechanisms of adsorption as well as the resulting properties of these adsorbate-semiconductor systems. We show here that chemisorption of an organic free radical to the Si(111)-7 × 7 surface results in a relatively long-range charge reordering within the substrate that extends up to 2 nm away from the molecular adsorbate and encompasses an area including nine unreacted dangling bonds. Topographic STM images reveal that the charge transfer is most significant from the adjacent center adatom in the neighboring half of the unit cell to the region of TEMPO adsorption. Utilizing atomic scale imaging and scanning tunneling spectroscopy (STS), we explore and visualize the extent of charge transfer associated with a single organosilicon covalent bond as a function of distance and energy. The dimer-adatom-stacking-fault (DAS) model proposed by Takayanagi et al. is the generally accepted description of the Si(111)-7 × 7 surface.12,13 The 7 × 7 surface reconstruction induces significant charge transfer between the dangling bonds, resulting in an inequivalent electron occupancy for the various * Corresponding author. E-mail:
[email protected]. Northwestern University, 2220 Campus Drive, Room 1017A, Evanston, Illinois 60208-3108. Telephone: +1-847-491-2696. Fax: +1-847-491-7820.
reactive sites. The rest atom and corner hole dangling bonds are fully occupied due to donated charge from the adatom dangling bonds during the surface reconstruction.14,15 However, the adatoms are further inequivalent as the center adatom transfers more charge to its two neighboring rest atoms compared to the corner adatom, which transfers charge to a single neighboring rest atom.15 The spatial and electronic variations of the Si(111)-7 × 7 surface lend itself to the fundamental investigation of numerous surface chemistries. Previous studies have included the adsorption of various metal atoms16-22 as well as the attachment of organic molecules via [4 + 2]-like and [2 + 2]-like cycloaddition,23 dissociative reactions,10,11,24-34 and dative bonding.23,35 Several of these molecules tend to dissociate at relatively low temperatures, while the majority initiate adsorption at adatomrest atom pair sites on the surface. Site-selective adsorption is commonly reported and is typically preferential to a center adatom-rest atom pair.36 In this paper, a room-temperature UHV STM is used to study the covalent binding of the nitroxyl free radical TEMPO to the Si(111)-7 × 7 surface. All experiments were performed at room temperature using a home-built UHV STM operating at a base pressure of 4 × 10-11 Torr.37 Samples were prepared in an isolated UHV preparation chamber to help preserve the vacuum integrity of the microscope chamber, which is coupled via a gate valve. The silicon substrate was commercially purchased (Virginia Semiconductor, Fredricksburg, VA) and degenerately doped n-type (resistivity
