Zhanwen Xiao

AFM observations of self-assembled lambda DNA network on silanized mica

Atomic force microscopy (AFM) was used to explore self-assembly behavior of λ-DNA molecules adsorbed to bare mica and aminosilanized mica surfaces. AFM experiments show that λ-DNA molecules can be hardly adsorbed on bare mica surface, but can be strongly adsorbed and can be self-assembled into DNA network on silanized mica surface. Interestingly, a new neural-like DNA network structure was observed on the mica surface derivatized by N-(2-aminoethyl)-3-aminopropyl-trimethoxysilane, differing quite from the mesh network structure on the mica surface modified by aminopropylytriethoxy silane (APS). Our experimental results can offer the possibility for constructing DNA-based nanostructures on solid surface. The formation of the neural-like network was also discussed in this article.

Scanning Tunneling Microscopy Observation of Electron Standing Waves on Au(111) Film with a Superconducting Tip

Using low-temperature scanning tunneling microscopy (LT-STM) with normal metal tungsten tips and superconducting niobium tips, we have observed in real space the formation of electron standing waves by the scattering of surface state electrons at surface defects and step edges on a Au (111) surface. From constant-current STM images of the Au(111) surface observed at low temperature with both tips, we have observed the strong effects of tip materials on the interference of the two-dimensional (2D) electron waves. Compared with using tungsten tips, enhanced electron standing waves have been clearly observed using niobium tips at 4.2 K. A simple model has been proposed to explain these observed effects.

Local density of electronic states in MgB2 studied by low temperature STM and STS: direct evidence for a multiple-gap superconductor

Abstract Scanning tunneling microscopy and spectroscopy (STM/STS) experiments on a high-density sintered MgB 2 surface were performed at 4.2 K, using a low temperature STM. The intrinsic superconducting density of states (DOS) was obtained in the tunneling spectra exhibiting the BCS-shaped characteristic with a metallic background. A double-gap structure, with the gap values of Δ L =9.5 meV and Δ S =4.0 meV, was clearly observed in the local tunneling spectra. These values give a BCS ratio (2 Δ / k B T C ) of 5.6 and 2.4, respectively, well above and well below the BCS limit. The result supports the multiple-gap model in MgB 2 in the clean limit. The proximity effect observed in the conductance spectra and the origin of the varied reported energy gap values of MgB 2 were also discussed in this study.

Influence of Silicon Surface Structure on Long Deoxyribonucleic Acid Molecule Alignment

We investigated the effects of silicon surface structure on the alignment of double-stranded lambda deoxyribonucleic acid (λ-DNA) molecules using atomic force microscopy (AFM). It has been found that the reproducible λ-DNA alignment strongly depends on the nature of the silicon surface charge, uniform distribution of positive charge sites and the roughness of silicon surface. AFM experiments demonstrate that it is difficult for λ-DNA molecules to adsorb on a bare SiO2/Si substrate, but it is easy for the λ-DNA molecules to be well aligned on the aminosilanized SiO2/Si surface. Compared with mica, the SiO2/Si substrate offers a significant advantage in providing a well-defined amino-terminated self-assembled monolayer silane film. Our AFM experiments also show that the rough SiO2/Si surface has a negative effect on λ-DNA alignment.

Controlled assembly of DNA nanostructures on silanized silicon and mica surfaces for future molecular devices

In order to establish key technology for future molecular devices, we have explored the assembly behaviour of λ-deoxyribonucleic acid (DNA) molecules adsorbed on silanized mica and silanized oxide silicon surfaces by using atomic force microscopy (AFM). AFM experiments show that λ-DNA molecules can be hardly adsorbed on untreated mica and oxidized silicon surfaces, but can be strongly adsorbed onto aminosilanized mica and oxidized silicon surfaces. Importantly, DNA molecules can be assembled into linear DNA alignment, and can also self-assemble into various network structures on the silanized surfaces. Our experimental observations have demonstrated the feasibility of assembling DNA-based nanostructures by varying surface chemistry of substrates, and offer useful clues in constructing DNA-based nanodevices for nanoelectronics and biomolecular computation as well as quantum computation.

Direct observation of electron standing waves with superconducting Nb tips on Au(111) using low temperature scanning tunnelling microscopy

Using a low temperature scanning tunnelling microscope with normal metal tungsten tips and superconducting niobium tips, we have observed in real space the formation of electron standing waves by the scattering of surface state electrons at surface defects on Au(111) surfaces. In the constant-current STM images of Au(111) observed at low temperature with both tips, standing waves are clearly seen emanating from step edges and point defects. From the observed structure of the standing waves, the wavenumber of the surface electrons can be obtained as a function of the electron energy. In comparison with the cross-sectional profiles (height versus distance) measured across the standing waves, it can be seen that the amplitude of the standing waves obtained with the niobium tip at 4.2 K is about three times larger than that with the tungsten tip. We have also proposed a simple model to explain these observed effects.

Compositional analysis of the MgB2 surface using AES

Abstract High-density sintered MgB 2 surface was characterized by Auger electron spectroscopy in combination with sputter depth profiling. The compositional changes of the MgB 2 surface were investigated in real time during the course of Ar + ion sputtering. The as-polished MgB 2 surface was found to be Mg-rich and oxidized, especially near the grain boundaries. Therefore, careful protection of MgB 2 from ambient atmosphere is required for its practical applications. The composition of the sputtered MgB 2 subsurface was compared with that of the as-polished MgB 2 surface and found to be Mg deficient, especially on the top of crystallites. The detailed compositional analysis of the MgB 2 surface in this paper may help in elucidating the reported varied results obtained in surface-sensitive scanning tunneling spectroscopy measurements.

Local density of electronic states in MgB2 studied by scanning tunneling microscopy

A high-density sintered MgB2 surface has been investigated at 4.2 K by low-temperature scanning tunneling microscopy and spectroscopy (LT-STM/STS). The intrinsic superconducting density of states (DOS) is determined in most typically found spectra, exhibiting the BCS-shaped characteristic with a metallic background. A double-gap structure, with the gap values of ΔL=9.5 meV and ΔS=4.5 meV, is identified in the intrinsic local conductance spectra. These values give a BCS ratio (2Δ/κBTC) of 5.6 and 2.6, i.e., respectively, well above and well below the BCS limit. Our results presented here support the multiple-gap model in MgB2 in the clean limit.