S.J Pang

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.

Generation of two-dimensional ordered domains of gold mediated by pentacosadiynoic acid

Abstract Two-dimensional ordered domains of gold were obtained on the hydrophilic surface of polymerized 10,12-pentacosadiynoic acid (poly-PCDA) monolayer. Instead of the classical physical methods, such as scanning tunneling microscopy (STM) and low pressure evaporation, a ‘wet’ chemical method based on the Langmuir–Blodgett technique was employed. The structure was probed by STM and found to have a quasi-rectangular lattice, with the shorter lattice spacing constant at 0.37±0.02xa0nm, and the longer at 0.51±0.02xa0nm. The structure, which was stable within 24xa0h, contains at least 60 atoms within one domain. A possible model was also put forward based on the structural correlation between the gold atomic monolayer and the hydrophilic surface of the polymer substrate.

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.

Atomic structures of kinks at double-layer steps on Si(100) surfaces

We studied the kink structures at double-layer step by scanning tunneling microscopy and catalogued the kinks into two types: complex kinks (composed of minus kinks and plus kinks) and simple kinks (only minus or plus kinks). Both of the kinks create a local A terrace. The dimers in the local A terrace are buckled. For a complex kink, the buckling of the dimers in the local A terrace is determined by the interaction of the buckled dimers propagated from minus kinks and plus kinks. For a simple kink, the buckling of the dimers in the local A terrace is determined by the buckling of the rebonded dimers at step edges. The directions of the buckled dimers are determined by the tensile stress between dimer rows. The brightness of the outermost dimers in an upper terrace is caused by the buckling of the dimers and the charge transfer between the two atoms of the dimers.

Bond-responsive relaxation model: a microscopic property of a Si(111)-7×7 surface responding to the desorption of single adatoms

Abstract Within the tight-binding approximation, the binding energy of a condensed system can be expanded to individual atoms. These site-specific binding energy terms (SBEs) are calculated for a Si(111)-(7×7) surface. We find that the SBEs of the adatoms agree well with the desorption energies of the corresponding adatoms, in terms of both the relative values measured in experiments [Uchida et al., Phys. Rev. Lett. 70 (1993) 2040] and the absolute values estimated from the available results of ab initio calculations. On the basis of this result, a bond-responsive relaxation property of the substrate responding to the desorption of single adatoms can be deduced. That is, if an adatom is extracted, the relaxation energy of the substrate will be half of the bond-counting energy between the adatom and the substrate in the original (before extraction) configuration. This relaxation property is generalized to be a so-called bond-responsive relaxation model; its condition of applicability is discussed and a test case is presented.