Xiaowei Yao

Scanning tunneling microscopy and spectroscopy of individual C60 molecules on Si(100)-(2×1) surfaces

Abstract Individual C 60 molecules chemisorbed on Si(100)-(2 × 1) surfaces have been studied by scanning tunneling microscopy and spectroscopy. Chemisorption was realized by annealing the samples with room-temperature deposited adsorbates to 600°C. Spectroscopic results on individual adsorbates reveal a transition of their electronic structure from that of a near-free adsorbate to that of SiC, as the adsorbate-substrate interaction increases.

The (3×2) phase of Ba adsorption on Si(001)-2×1

Abstract The initial stages and surface structures of the (3×2) phase of Ba adsorption on an Si(100)-2×1 surface held at 900°C have been studied by low-energy electron diffraction, Auger electron spectroscopy, and ultra-high vacuum scanning tunneling microscopy (STM). At low coverages ( 1 6 ML) , the Ba atoms form atomic chains across the Si dimer rows by occupying valley bridge sites, as well as on fourfold sites by replacing Si dimers and exhibiting a local (3×2) phase, with the 3×-phase along the Si dimer row direction. Two different configurations for the (3×2) phase, namely, mono- and dimer-Ba models, are proposed based on the STM images. Below a coverage of 1 6 ML , the (3×2) phase is formed by single Ba atoms at fourfold sites by replacing original Si dimers. For a Ba coverage of 1 3 ML, the (3×2) phase is formed by buckled Ba dimers, as revealed by high-resolution STM images.

Intramolecular features of individual C60 molecules on Si(100) observed by STM

Abstract Intramolecular features (IMF) of a variety of individual C 60 molecules adsorbed on an Si(100)-(2 × 1) surface have been imaged by scanning tunneling microscopy under ultrahigh vacuum conditions. Features of individual C 60 molecules clearly show the local density of states superimposed on their cage structure. Both physisorbed (pre-annealed) and chemisorbed (post-annealed) species have been imaged on the same surface, exhibiting characteristics that depend on their bonding nature. Intramolecular features of a physisorbed C 60 molecule and of two chemisorbed molecules are presented.

Growth of silicon nitride by scanned probe lithography

Scanning probe lithography has been used for the first time to grow silicon nitride nanostructures on silicon substrates. The lithography was performed by an atomic force microscope (AFM) placed in an evacuated chamber with a partial pressure of annhydrous ammonia. The silicon nitride nanostructures were grown by negatively biasing the silicon tip with respect to the sample. By changing the environment of the AFM, both silicon oxide and silicon nitride can be grown and subsequently processed.

Barium adsorption on Si(100)-(2×1) at room temperature: a bi-polar scanning tunneling microscopy study

Abstract The initial stages of barium adsorption on Si(100)-(2×1) at room temperature has been studied by ultrahigh vacuum scanning tunneling microscopy (STM) under both positive and negative sample-bias imaging conditions. Two distinct adsorption sites have been identified by the high-resolution STM images. It was found that, with the substrate held at room temperature, barium atoms can occupy both valley-bridge sites in the trough between silicon dimers and silicon-vacancy sites. It is possible to image the barium atoms at valley-bridge sites with both negative and positive sample bias, revealing filled and empty surface states, respectively. For barium atoms adsorbed at vacancy sites, however, it is only possible to obtain filled-state images, and imaging with positive sample bias will induce the removal of the atom, possibly transferred to the tip, revealing a missing silicon dimer below.

V-shaped metallic-wire cantilevers for combined atomic force microscopy and Fowler-Nordheim imaging

A method for fabricating V-shaped cantilevers from a flattened Pt/Ir metal wire for combined atomic force microscopy and Fowler-Nordheim imaging is described. These novel cantilevers have been found to be more robust then conventional ones used for scanning capacitance and magnetic force microscopy as their conductivity is maintained even after a large number of surface scans. The use of a V-shaped geometry improves on earlier single-beam geometries by reducing rms imaging noise. Characterization of these cantilevers and combined atomic force microscopy and Fowler-Nordheim images are reported.

Stability and superstructure of squarylium dye TSQ Langmuir–Blodgett films

The stability of monolayer Squarylium Dye TSQ in air-subphase interface was studied. Langmuir-Blodgett films of this dye were . . deposited on mica and on hydrophobic silicon 100 wafers and a contact mode atomic force microscope AFM was used to characterize the structures of these films. It was found that the stability of a TSQ monolayer in the air-subphase interface was increased substantially when cadmium chloride was added to the subphase. Although TSQ dye does not have a long alkyl tail typically required to form a LB film, AFM images of these films and their 2D Fourier Transform analyses show that TSQ LB films possess a triclinic superstructure. . Based on the analyses of pressure-area p-A isotherms and AFM images of these films, models of TSQ molecules packing on the subphase were developed. q 1998 Published by Elsevier Science S.A.

Observation of interface band bending on GaAs/AlAs heterostructures by scanning tunneling microscopy

The electronic structure at the interface of GaAs/AlAs multilayers grown by molecular beam epitaxy is investigated on the (110) surface using scanning tunneling microscopy. The valence band bending, which is produced by an interface dipole layer, is observed from cross-sectional profiles exhibiting spike structures. It is found that the transition region of the AlAs/GaAs interface (3.0 - 4.0 nm) is smaller than that of the GaAs/AlAs interface (4.0 - 5.0 nm). Similar spike structures showing a transition region of 3.5 - 4.5 nm are also observed at the GaAs/Al0.6Ga0.4As interface.