J.N Gao

Large area dimer vacancy array on the Si(100) surface studied by scanning tunneling microscope

Abstract Large area dimer vacancy arrays can be formed on the Si(100) surface when Si atoms are deposited on the Si(100)-2×1 surface followed by quenching from 1200°C. The dimer vacancy lines (DVLs) of the dimer vacancy array run perpendicular to the dimer rows and the basic building cells of the DVLs are: (i) a cluster of two missing dimers; (ii) a complex of one missing dimer and a cluster of two missing dimers. Small isolated islands with DVLs like that of the Si(100) surface are observed on the surface. The precursors of the islands are investigated. The formation mechanism of the island is that the interaction between the stress field of the dimers on the Si(100) surface and that of the island causes the dimer vacancies in the dimer rows of the island and the attraction between the dimer vacancies in adjacent dimer rows aligns the dimer vacancies.

Scanning tunneling microscope study of polyacrylonitrile-based carbon fibers

Scanning tunneling microscopy (STM) was used to characterize the surface topography of polyacrylonitrile (PAN)-based carbon fibers before and after electrochemical treatment, stretch resistance test, and high-temperature treatment. A new kind of spiral structures was found, which was not only on the surface but also in the inner layer. The spiral structure of the fibers was caused by the spinning process. The fiber structure contained the shape of the precursor. There were some large cracks in the carbon fibers after the stretching resistance test. The large cracks can result in carbon fiber breaking under certain stress conditions. The difference in the structures of the carbon fibers before and after the high-temperature treatment was determined. om

Regular defects on the Si(111)-(7×7) surface studied by scanning tunneling microscopy

Abstract Two kinds of regular defects have been observed in our experiments. One kind is caused by losing or inserting adatom rows between two domain boundaries. The other is caused by losing or inserting adatom rows between two domain boundaries, while one domain shifts relative to the other along the short diagonal of the (7×7) unit cell. The former usually has simple shapes and a period of 7 a , while the latter has complex shapes and a period of 14 a , where a is the length of the primitive lattice vector of the Si(111) surface. The shapes of the defects are decided by three factors: (i) the nucleation sites of the two domains, (ii) the growth rates of the two domains, and (iii) the strong interaction between the dimers and adatoms located at the domain boundaries.

Site-specific binding energy terms on Si(111)-7×7 surface a reflection of local binding behavior of surface atoms

Abstract The binding energy of a condensed system in the form of the semi-empirical tight-binding (SETB) approximation is expanded in individual terms, each corresponding to an atom. We find that these binding energy terms agree surprisingly well with the site-specific measurement of adatom binding energies in atom manipulation experiments [Uchida et al., Phys. Rev. Lett. 70 (1993) 2040]. This approach may provide an effective tool for analyzing local binding properties of surface atoms.

Nanometer-scale regular grooves and ridges created with scanning tunnelling microscope

Abstract The scanning tunnelling microscope (STM) has been employed to fabricate grooves by extracting Si atoms out of the Si(111)-7×7 surface and deposit the Si atoms back onto the Si(111)-7×7 surface at room temperature. The deposited Si atoms can form a straight ridge under controlled conditions. The width of the ridge can be controlled better than 2 nm. Ridges can only be formed after the tip extracts enough atoms out of the Si(111)-7×7 surface. If the tip is clean, no atoms will be deposited from the tip under the depositing conditions. This suggests that the deposited atoms are silicon atoms extracted out of the Si(111)-7×7 surface. The deposition mechanism is discussed.

Formation and atomic structures of double-layer steps on Si(100) surfaces studied by scanning tunneling microscope

Abstract In this paper, we used an ultra-high-vacuum scanning tunneling microscope (STM) to study the step structures of the Si(100) surfaces and found that double-layer steps can exist even at a low miscut angle (∼0.5°) from the (100) direction. The double-layer B steps are caused by the shrinkage of the span of the single-layer steps, and basically run in a zigzag manner with many kinks. The second layer atoms are rebonded at the step edges, and the rebonded atoms form asymmetric dimers that make the outmost dimers on the upper terrace brighter. The asymmetric dimers propagate from the kink site and form local c-4×2 structures. Their related atomic structure models are discussed.

Study on surface and interface structures of nanocrystalline silicon by scanning tunneling microscopy

The scanning tunneling microscope has been employed to study the morphology, atomic surface structures, and grain interface of hydrogenated nanocrystalline silicon (nc-Si:H) before and after hydrofluoric acid (HF) etching. It was found: (1) The nc-Si:H films were composed of many different sizes of grains and these grains were composed of many finer grains. (2) There were line structures on the surface of the fine grains and loop structures at the grain boundaries without HF treatment. After etching, two more structures were observed: loop structures on the surface of fine grains and spider bonding structures besides the interface of fine grains. (3) The loop structures found at the grain boundaries was larger and more irregular than those on the grain surfaces. Line structures were similar to crystal silicon, but the distance between lines was enlarged. Considering the experimental results, a discussion was made about the formation mechanism of these atomic structures.