Step-by-step cooling of a two-dimensional Na gas on the Si„111…-„7 Ã 7… surface K. H. Wu, A. I. Oreshkin, Y. Takamura, Y. Fujikawa, and T. Nagao Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan T. Briere, V. Kumar, and Y. Kawazoe Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan R. F. Dou, J. F. Jia, and Q. K. Xue Institute of Physics, Chinese Academy of Sciences, Beijing 100080, China T. Sakurai Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan (Received 6 July 2004; published 15 November 2004) We report a temperature-dependent scanning tunneling microscopy study of Na adsorption on the Si111-7  7 surface by cooling the surface from room temperature to 80 K. While at room temperature Na atoms can diffuse freely on the surface to form a two-dimensional gas, their motion is confined within the 7  7 half-unit cells at 160 K. With further cooling to 80 K, Na atoms become localized within individual “basins.” The estimated diffusion barriers of Na atoms on the Si111-7  7 surface and nature of interaction among them agree well with our theoretical predictions. DOI: 10.1103/PhysRevB.70.195417 PACS number(s): 68.37.Ef, 68.03.Hj, 71.20.Dg, 82.45.Jn I. INTRODUCTION Alkali metals (AM) on semiconductor surfaces are model adsorption systems because of the prototypical nature of al- kali metals. The interface structure, electronic properties, and chemical bonds of various AM/semiconductor interfaces, particularly the AM-induced reconstructed surfaces such as the Si111-3  1-Na that form by annealing a surface at near monolayer AM coverage, 1,2 have been studied inten- sively during the past. However, the configuration and dy- namics of AM atoms on the as-deposited surface at initial coverage, which may be fundamentally more important, have been much less understood. On the other hand, there has been increasing interest in understanding the dynamics and clustering of various metals on the Si111-7  7 surface, aiming to the preparation of self-assembled metallic nanodots, 3–7 and the adsorption of AM atoms on the Si111-7  7 surface can serve as a model for these stud- ies. In the case of Na adsorption on the Si111-7  7 sur- face, in contrast to the previous dangling bond adsorption model in which Na atoms adsorb on top of the Si adatoms, our recent studies show that Na atoms adsorb in “basins” around the Si restatoms and that they are highly mobile to form a two-dimensional (2D) gas phase at room temperature and coverage lower than 0.08 monolayer (ML). 8 At room temperature, scanning tunneling microscopy (STM) cannot register individual Na atoms on the Si111- 7  7 surface due to their fast motion. As a result, in the STM images, only noisy features and periodic contrast changes due to an averaged charge transfer from moving Na atoms to the Si adatoms, are observed. In this article, we report a temperature-dependent STM study of Na adsorption on the Si111-7  7 surface. By decreasing the mobility of Na atoms at low temperature, the configuration and diffusion of individual Na atoms can be studied in detail by STM. The estimated diffusion barriers of Na atoms on the Si111- 7  7 surface and the nature of interaction among Na atoms agree very well with our theoretical predictions. Shown in Fig. 1 is the theoretical model for Na adsorption on the Si111-7  7 surface (see Ref. 8 for detail). The binding energies of a Na atom in different sites on the Si111-7  7 surface are listed in Table I. According to our first-principles calculations, the adsorption of Na on top of the Si adatoms or the rest atoms is highly unstable and thus can be excluded. The stable Na adsorption sites are several high-coordination sites located around the Si rest atoms, which form a hexagon or so-called “basin” 9 centered on the Si rest atom. There are three basins in each faulted half-unit FIG. 1. The theoretical model of Na adsorption on the Si111-7  7 surface. The lowest energy sites are the three sites (sites 2 and 4) surrounding the Si rest atom. The lines mark the three diffusion processes: diffusion inside the basin (gray hexagon), diffusion among different basins in the same half-unit cell (1-5-1), and diffusion across unit cell boundaries (3-6-9). PHYSICAL REVIEW B 70, 195417 (2004) 1098-0121/2004/70(19)/195417(4)/$22.50 ©2004 The American Physical Society 70 195417-1