James McLean

Chemically selective adsorption of molecular oxygen on GaAs(100)c(2×8)

The chemisorption sites of molecular oxygen on the technologically important As-rich GaAs(100)c(2×8) surface were imaged with scanning tunneling microscopy (STM). The oxygen atoms insert into the arsenic–gallium backbonds and, subsequently, replace the arsenic atoms in the dimer rows. The displaced arsenic atoms aggregate in clusters of increasing size forming metallic arsenic. The strongly electronegative oxygen atoms are initially attracted by the larger electron density at the arsenic atoms, but the reaction of the oxygen atoms with the gallium atoms is thermodynamically favored. This leads to a 100% chemical selectivity for oxygen insertion into the As–Ga backbonds and subsequent chemisorption of the oxygen atoms into the arsenic sites.

Relative reactivity of arsenic and gallium dimers and backbonds during the adsorption of molecular oxygen on GaAs(100)(6×6)

The chemisorption sites of molecular oxygen on the mixed GaAs(100)(6×6) surface were imaged at room temperature using scanning tunneling microscopy (STM). This surface is terminated by both gallium dimers and arsenic dimers, neither of which react with oxygen. Instead, the As–Ga backbonds are shown to react with O2 with 100% chemical selectivity. The reason for this selectivity is found in the interaction of the highly electronegative oxygen atoms with the higher electron density at the arsenic atoms. One oxygen atom displaces the attacked arsenic atom while the other oxygen atom bonds to two nearby gallium atoms, resulting in the thermodynamically most stable reaction products: metallic arsenic clusters and gallium oxide.

Localized excess negative charges in surface states of the clean Ga-rich GaAs(100)c(8×2)/4×2 reconstruction as imaged by scanning tunneling microscopy

Scanning tunneling microscopy images of the Ga-rich GaAs(100)c(8×2)/(4×2) surface exhibit vivid long-range patterns consisting of bright spots (“ghosts”) which are attributed to localized excess charge rather than atomic clusters. The nearly planar geometry of the sp2-hybridized gallium dimer atoms results in localized π states made up of a combination of the Ga pz orbitals. These states in the upper half of the band gap form the lowest unoccupied band. Surface or bulk defects lead to excess negative charge flowing into these localized states. Repulsion between the trapped negative excess charges leads to the observed “ghost” pattern.

Atomic structure determination for GaAs(001)-(6×6) by STM

Abstract We present a model structure for the GaAs(001)-(6×6) reconstructed surface based on high-resolution scanning tunneling microscopy images. This surface has previously also been referred to as (1×6), (2×6), (3×6) and (2×6)/(3×6) mixed, and contributes to the “4×6” mixed surface. Phase coexistence with the Ga-rich c(8×2) is used to determine the basic structure. Chemically selective halogen (Cl2) adsorption is used to distinguish Ga atoms from As atoms and to extract structural details that are not visible in scanning tunneling microscopy images of the clean surface. The structure is consistent with the electron-counting rule. Partial disorder appears in the structure. While some of the disorder is due to kinetic barriers to equilibrium, some is intrinsic to the structure.

CR-39 (PADC) Reflection and Transmission of Light in the Ultraviolet–Near-Infrared (UV–NIR) Range:

The spectral reflection (specular and diffuse) and transmission of Columbia Resin 39 (CR-39) were measured for incoherent light with wavelengths in the range of 200–2500 nm. These results will be of use for the optical characterization of CR-39, as well as in investigations of the chemical modifications of the polymer caused by ultraviolet (UV) exposure. A Varian Cary 5000 was used to perform spectroscopy on several different thicknesses of CR-39. With proper analysis for the interdependence of reflectance and transmittance, results are consistent across all samples. The reflectivity from each CR-39–air boundary reveals an increase in the index of refraction in the near-UV. Absorption observations are consistent with the Beer–Lambert law. Strong absorption of UV light of wavelength shorter than 350 nm suggests an optical band gap of 3.5 eV, although the standard analysis is not conclusive. Absorption features observed in the near infrared are assigned to molecular vibrations, including some that are new to the literature.