Mark J. Gallagher

Scanning tunneling microscopy study of the adsorption of C60 molecules on Si(100)‐(2×1) surfaces

Scanning tunneling microscopy has been used to investigate the adsorption of C60 molecules on Si(100)‐(2×1) surfaces at various deposition temperatures. At room temperature, submonolayers of C60 molecules experienced limited surface diffusion and randomly adsorbed to sites between dimer rows. At 650 °C, the C60 exhibited increased diffusion and preferentially adhered to step edges, and defects, or bonded to the top of the dimer rows. At low coverage, a strong interaction between the C60 and Si was evidenced by increased defect density which included the local atomic rearrangement in the vicinity of the C60 molecules. Higher coverage created a substantial concentration of defects which induced Si(100)‐c(4×4) reconstruction. At 750 °C, the C60 molecules decompose and react with the Si surface forming islands of SiC and C60–Six species.

Characterization of carbon nanotubes by scanning probe microscopy

Abstract Carbon nanotubes, fabricated by the Ebbesen-Ajayan method, were imaged using scanning tunneling microscopy (STM) and atomic force microscopy (AFM) in air and were compared to images obtained with high-resolution transmission electron microscopy (HRTEM). The HRTEM images revealed an abundance of elongated structures ranging in diameter from 3.0 to 30 nm, and with lengths of up to 0.8 μm. Many of the structures possessed several graphitic shells as if the tubes were nested one in the other. Reproducible images of the tubular structures, typically 20 nm in diameter and with a large variation in length, were obtained with both STM and AFM when the nanotubes were deposited on hydrogen-terminated Si(111), confirming that the nested structures observed with HRTEM do indeed have a tubular morphology. No single-walled, bare nanotubes or spherical fullerenes (typical of the Kratschmer-Huffman process) were observed.

Photon emission from gold surfaces in air using scanning tunneling microscopy

Photon emission was observed at the tunnel junction of a scanning tunneling microscope while scanning Au structures in air. Emission levels of about 4000 counts per second (cps) were routinely achieved with Au tips, allowing photon maps to be produced. The similarity between these photon maps and the topographic images of the Au samples are discussed.

High resolution images of single C60 molecules on gold (111) using scanning tunneling microscopy

Abstract The electronic interactions of fullerene molecules with metals, with other molecules, and with themselves are important to the chemical and conductive properties of these materials. We demonstrate high resolution scanning tunneling microscopy images of C 60 molecules condensed on epitaxial gold (111) films on mica, in which the C 60 molecules are isolated from each other. The C 60 molecules were locked in position to the gold substrate by an ordered layer of methyl isobutyl ketone. The images of the C 60 molecules exhibit intramolecular contrast indicating a significant electronic interaction with the gold substrate. Current versus voltage measurements show that both the C 60 and the thin film of methyl isobutyl ketone have conductances comparable to that of the gold substrate.

Improved atomic force microscope using a laser diode interferometer

The performance of an atomic force microscope using a laser diode interferometer has been improved to the point where its resolution is comparable to that of laser beam deflection systems. We describe the structure of this microscope, present a model that takes into account the main parameters associated with its operation, and demonstrate its sensitivity by showing images of a small area scan with atomic resolution as well as a large area scan in a stand‐alone configuration.

Buckyball-substrate interactions probed by STM and AFM

Abstract Submonolayers of buckyball molecules (C 60 ) on a gold substrate, deposited under ultrahigh-vacuum (UHV) conditions, have been imaged in air using scanning tunneling microscopy (STM) and atomic force microscopy (AFM). STM images show intramolecular contrasts within most individual C 60 molecules. AFM images of the C 60 molecules, often grouped in a hexagonal arrangement, show no atomic-scale features. A possible explanation for the presence or absence of the intramolecular contrasts in the images obtained with the STM and AFM, respectively, is given by a molecular orbital calculation.

Oxidation effects on cleaved multiple quantum well surfaces in air observed by scanning probe microscopy

Atomic force microscopy (AFM) and scanning tunneling microscopy (STM) of quantum well structures can give an independent method of measuring superlattice spacing and uniformity without having to resort to more involved techniques requiring intricate sample preparation. We present the first AFM images of cleaved InGaAs/InP multiple quantum wells and compare them with STM images taken of the same heterostructure. The images were stable in air for over a day. Based on our results, we propose that the mechanism for contrast in our images is due to an oxide layer that grows primarily on the InGaAs wells and not on the InP barriers. Both STM and AFM clearly resolve the individual wells of the heterostructure, although STM measured a larger corrugation than an AFM. STM also exhibited superior lateral resolution of about 2 nm while AFM had a lateral resolution of approximately 6 nm.

NANOSECOND TIME-SCALE SEMICONDUCTOR PHOTOEXCITATIONS PROBED BY A SCANNING TUNNELING MICROSCOPE

The high‐frequency response of scanning tunneling microscopy of a semiconductor is demonstrated by using the beat frequencies of the longitudinal modes of a HeNe laser at the tunneling junction. We present a comparison of the slow and fast optical response of photoexcited charge carriers in the layered structure semiconductors n‐type MoS2 and p‐type WSe2 using this method.

Fabrication of GaAs nanometer scale structures by dry etching

Nanometer-sized features as small as 400Ahave been fabricated in single-quantum-well GaAs/A1GaAs heterostructures for studies of quantum confinement effects in quantum dots. The features have been fabricated by dry-etching techniques using nanometer-sized etch masks by a novel surface deposition of colloidally-suspended spherical particles. SEM was used to examine the feature size.

Enhanced effects with scanning force microscopy

We present a general theory that describes the operation of scanning force microscopy in the contact force regime. We find that force derivatives along the surface of a sample produce images that can be dramatically enhanced relative to those of surface topography. For scanning tunneling microscopy atomic force microscopy (STM/AFM) and AFM configurations, the spring constant of the cantilever and the force derivatives perpendicular to the surface of the sample determine the enhancement, respectively.