NANO LETTERS
Reversible Passivation of Silicon Dangling Bonds with the Stable Radical TEMPO
2003 Vol. 3, No. 10 1431-1435
Jason L. Pitters,† Paul G. Piva,† Xiao Tong,† and Robert A. Wolkow*,†,‡ National Institute for Nanotechnology, Edmonton, Alberta, Canada, and Department of Physics, UniVersity of Alberta, Edmonton, Alberta, Canada Received April 24, 2003; Revised Manuscript Received July 22, 2003
ABSTRACT TEMPO, 2,2,6,6-tetramethylpiperidinyloxy, was used to passivate dangling bonds on hydrogen-terminated Si(100) and Si(111) surfaces. TEMPO reacts with the dangling bond through a radical coupling reaction of the oxygen and silicon atoms. STM images reveal that the TEMPOpassivated Si is stable under typical imaging conditions and also protects the surface from reaction with styrene. TEMPO may also be desorbed from the surface by scanning at elevated voltages (greater than −3.5 V) and provides the ability to selectively remove TEMPO to prepare single dangling bonds on an otherwise passivated surface.
Reactions of organic compounds with silicon surfaces have received substantial attention in recent years because of the attraction of incorporating the richness of organic chemistry within semiconductor science and technology. Reactions with clean silicon surfaces in ultrahigh vacuum (UHV) have received the majority of attention to date and have been reviewed in several papers.1-4 A related area of interest, and the inspiration for this work, involves organic molecular reactions on hydrogen-terminated silicon (H-Si) surfaces, which are relatively stable in air as well as various organic solvents.5 H-Si surfaces may be prepared in buffered HF or by direct reaction of a clean Si wafer with H atoms, in vacuum. Recently, it has been demonstrated that styrene molecules react with single dangling bonds on the HSi(100) surface via a “self-directed” growth process, resulting in contiguous lines composed of multiple styrene molecules aligned with the dimer rows of the Si substrate.6 Subsequent studies have shown that various alkenes and aldehydes undergo similar self-directed nanostructure growth processes on the H-Si(100) surface.7,8 While these processes provide a means for the creation of complex nanostructures of variable function, orientation, growth extent, and controlled absolute position, a number of practical refinements to the technique are yet required. One specific area requiring further work is the production of defect-free H-Si(100) surfaces. The UHV prepared H-Si(100) surface inevitably contains dangling bonds (DBs) (i.e., voids in the H atom layer), and in this paper, we discuss * Corresponding author. E-mail:
[email protected] † National Institute for Nanotechnology. ‡ University of Alberta. 10.1021/nl034258+ CCC: $25.00 Published on Web 09/11/2003
© 2003 American Chemical Society
the passivation of these residual DBs. These residual dangling bonds can act as centers for undesired reactions, leading to uncontrolled line growth on the surface, limiting the ability to create desired surface patterns and also hampering detailed kinetic studies of the self-directed growth process. Although our aim was to passivate DBs on the H-Si(100) surface, we first performed passivation experiments on the HSi(111) surface. H-Si(111) was chosen as a test surface for the H-Si(100) surfaces because of its ease of preparation and also due to the initial experimental setup. Therefore, a significant amount of the experimental data is presented from this surface. Also, performing reactions on both H-Si(111) and H-Si(100) surfaces allows for a test of generality of the passivation reaction. DBs do not occur naturally on the wet chemically prepared H-Si(111) surface and thus were created through electron-stimulated desorption of hydrogen from the surface. The available reactions for termination of DBs are somewhat limited. Reaction with alkenes or aldehydes, for example, while extinguishing the silicon radical, lead to carbon radical formation, thereby failing to pacify the site. While amines attach at DBs via a dative bond, bonding is too weak to survive at room temperature. Ideally, a radicalradical coupling reaction would be employed, but that in turn is challenging, as the incoming radical must react exclusively at DBs. Also, it may be difficult to generate and deliver suitable radicals to the surface. The most straightforward approach, exposing the surface to H atoms, has the unfortunate limitation that incoming H has some probability of abstracting a surface H to create volatile H2 and a DB, making it difficult to achieve DB densities below a few
Scheme 1: TEMPO, 2,2,6,6-tetramethylpiperidinyloxy, Reacting with a Dangling Bond on the H-Si(111) Surfacea
a
The radical coupling reaction passivates the dangling bond.
percent. An attractive candidate, 2,2,6,6-tetramethylpiperidinyloxy, also known as TEMPO, is shown in Scheme 1. TEMPO, a nitroxide, is a stable organic radical that has an unpaired electron at the oxygen atom. The stability of the radical arises from the nature of the nitrogen and oxygen bond along with the absence of beta hydrogen atoms. Because of their unique properties, nitroxide radicals have been extensively studied and have been used in various fields.9-11 Here we report the results of the reaction of TEMPO with DBs on the H-Si(100) and H-Si(111) surfaces. Using scanning tunneling microscopy (STM) it is shown that TEMPO reacts selectively with DBs. No reaction occurs at hydrogen-terminated sites. It is further shown that TEMPO can be reversibly desorbed from the surface using the STM tip, restoring the DBs to their normal reactive state. It is also demonstrated that TEMPO binds with sufficient stability to protect DBs from reaction with styrene. The experiments were performed on both hydrogenterminated Si(100)-2 × 1 and Si(111)-1 × 1 surfaces. A Si(100) crystal, As-doped (0.005 Ω‚cm), was cleaned by repeated flashing to 1250 °C and H-terminated at 330 °C by exposure to atomic hydrogen created at a tungsten filament.12 The H-Si(111) surface was prepared by the established method of etching of a silicon crystal (phosphorusdoped, 1 Ω‚cm) in degassed 40% aqueous NH4F solution.13 Dangling bonds on the H-Si(111) surfaced were created by electron stimulated desorption using the STM tip as a field emission source.14 On the H-Si(100) surface, residual dangling bonds typically exist at the level of a few percent. All images were collected at room temperature in UHV (
