Showing papers by "Wei Ji published in 2006"

Selective nontemplated adsorption of organic molecules on nanofacets and the role of bonding patterns.

Abstract: A key element of functionalizing nanocrystals with organic molecules is the nontemplated selective adsorption of different molecules on different facets. Here we report scanning-tunneling-microscopy images of perylene-3,4,9,10-tetracarboxylic-dianhydride and 2,5-dimethyl-N,N 0 -dicyanoquinonediimine on silver, demonstrating selective adsorption on different facets. We also report first-principles calculations that account for the data and show that bonding, which controls selectivity, occurs via the end atoms, while the molecule’s midregion arches away from the substrate. The results are also consistent with data that have been interpreted in terms of bonding via the midregion.

Selective analysis of molecular states by functionalized scanning tunneling microscopy tips.

Abstract: Selective analysis of molecular states in scanning tunneling microscopy (STM) has so far been achieved in a few cases by tuning the bias range of the STM in high-resolution measurements. Correspondingly, perylene adsorbed in a close-packed monolayer on Ag(110) is imaged mainly through the pi states of the molecule. By contrast, functionalizing the STM tip with a perylene molecule leads to a mismatch between the energy levels of the STM tip and the molecule adsorbates and, instead, images only the metal states of the underlying silver surface. The observation opens a route for better energy selectivity in electron transport measurements through organic interfaces.

Role of lateral alkyl chains in modulation of molecular structures on metal surfaces.

Abstract: We use low energy electron diffraction, scanning tunneling microscopy, first-principles densityfunctional theory, and molecular mechanics calculations to analyze the adsorption and growth of quinacridone derivatives (QA) with alkyl chains of 4 and 16 carbon atoms on a Ag(110) substrate. Surprisingly, we find that the alkyl chains determine the orientation of the molecular overlayers. While the interaction of QA and the Ag substrate is primarily due to chemical bonding of oxygen to the silver substrate, determining the molecular orientation and preferred adsorption site, the intermolecular arrangement can be adjusted via the length of alkyl chains. We are thus able to fabricate uniform QA films with very well controlled physical properties.

Electron core-hole interaction and its induced ionic structural relaxation in molecular systems under x-ray irradiation.

Abstract: The experiments on perylene-3,4,9,10-tetracarboxylic-3,4,9,10-dianhydride and 16FCuPc on Ag(111) surfaces by the normal incidence x-ray standing wave are poorly reproduced by first-principles ground state calculations but well reproduced by considering initial- and final-state effects, which include the response of the valence electrons to screen a core hole created by x-ray photoemission and further induced ionic structural relaxation. This study shows that the initial- and final-state effects due to screening are of great importance in characterizing molecules on metal surfaces in the normal incidence x-ray standing wave experiments.

Understanding and controlling the weakly interacting interface in perylene/Ag(110)

Abstract: The weakly interacting interface, perylene on Ag(110), has been systematically investigated in detail with molecular-beam epitaxy and low-energy-electron diffraction (LEED), low-temperature-scanning-tunneling microscopy (STM), and first-principles calculations. The overall process of perylene monolayer growth has been clearly discussed by LEED and STM. The monolayer structure is sensitive to growth conditions because perylene has a poor self-assembly ability on Ag(110). However, we achieved the control of the formation of the uniform perylene monolayer composed of only one superstructure. The locally distributed metastable structures produced on certain growth conditions were also discussed.

Structural and Conductance Transitions of Rotaxane Based Nanostructures and Application in Nanorecording

Abstract: Rotaxane-based molecules have been intensively studied for use as functional molecular electronic devices and as a medium for nanorecording. By means of both I-V, Raman and conductive contact atomic force microscopic measurements of rotaxane H1 and H2 molecular thin films and scanning tunneling microscopy studies of individual H2 molecules, we demonstrate conclusively that the conductance transition originates from single molecules and correlates with changes in molecular conformation. This provides the best evidence to date that the conductance transition arises from motion of the cyclobis(paraquat-p-phenylene) ring along the molecular backbone. Taking advantage of the switching property, we have achieved reproducible nanometer-scale recording (dots) on rotaxane Langmuir-Blodgett (LB) films via voltage application using a scanning tunneling microscope (STM) probe. Stable, reversible conductance transitions and memory effects were observed in solid state rotaxane-based H1 LB thin films; while the thin films of rotaxane H2 molecule can be successfully written, erased, re-written, and re-erased. The stable and controllable conductance switching makes the rotaxane an attractive candidate for nanorecording. Furthermore, these results suggest that controlling the rotaxane conformation, its electronic structure, and decoration of the molecule, are strongly needed to strive the practical applications to the molecular electronic devices.

Processing of an atomically smooth Ge(001) surface on a large scale

Abstract: An atomically smooth Ge(001) surface on a large scale is obtained by deposition of submonolayer Ge on a Ge(001) surface at 300 °C, which repairs the missing dimer defects produced during the enhanced energy ion bombardment and annealing of the substrate The Ge(001) samples are characterized by scanning tunnelling microscopy (STM) before and after the submonolayer Ge deposition The ion bombardment/subsequent annealing processing and STM tip induced defects are also investigated in detail

Growth of Ge hexagonal meshwork films on Si(111)-7 × 7

Abstract: Ge growth on a Si(111)-7 x 7 surface at room temperature was studied by in situ scanning tunnelling microscopy. The Ge hexagonal meshwork film composed of Ge nanoclusters located on the centre of the half-unit-cells of Si(111)-7 x 7 grows in the 'monolayer mode': the first layer is 0.22 nm thick, and then increases in thickness by 0.11 nm, forming the 0.33 nm bilayer. This growth mode leads to a simple stacking sequence. However, the Stranski-Krastanov growth mode was still found for the thick meshwork films.