David E. Beck

Electronic contrast in scanning tunneling microscopy of Sn-Pt( 111) surface alloys

Abstract Scanning tunneling microscopy studies of mixed domain p(2×2) and ( 3 × 3 )R30° tin–platinum surface alloys are presented. It was found that the apparent height of Pt ensembles increases with a decreasing number of Sn next-neighbor atoms per Pt atom. This is explained by a reduction of the local density of states near the Fermi level at Pt sites due to the effects of alloying of Pt with Sn.

Laser Assisted Machining

The availability of high power cw carbon dioxide lasers with sufficient ruggedness, reliability and simplicity of operation for use in manufacturing facilities has led to the development of new machining methods. These methods currently involve the localized vaporization or melting of the material due to beam heating. (1) In the case of metallic materials beam heating is supplemented with burning enhanced by a flow of oxidizing gases at the point of impingement of the laser beam. In our research, we have developed a new and dif-ferent method of cutting with a laser, laser assisted hot spot machining (LAM), in which the laser is used to heat the volume of material directly in front of a single point cutting tool to a temperature less than its melting point.(2,3)The application of gas torch and induction heating to assist, in the turning of metals was first studied in the United States by Tour and Fletche(1949). Concurrently, Schmidt investigated the use of gas torch heating in milling. Although many advantages were reported such as reduction in power consumption tool life improvement and improvement in surface finish, hot-machining has not been perceived as a practical and economically viable method by industry. Recently, however, with the development of more intense heat sources such as the plasma-arc(5) and the laser, hot machining has become more attractive in specific applications as a metal removal technique.

Ultrahigh vacuum instrument that combines variable-temperature scanning tunneling microscopy with Fourier transform infrared reflection-absorption spectroscopy for studies of chemical reactions at surfaces

We describe the construction of an ultrahigh vacuum chamber that incorporates variable-temperature scanning tunneling microscopy (STM), Fourier transform infrared reflection-absorption spectroscopy (FT-IRAS), Auger electron spectroscopy, low-energy electron diffraction, and temperature programmed desorption, for studying structure and reactivity at surfaces. The chamber and manipulator design enables in situ sample preparation and analysis, and rapid access to several surface-analytical techniques by rotation only. This eliminates sample inconsistencies due to ex situ preparation or the necessity to run parallel experiments. Inclusion of FT-IRAS allows us to characterize surface species and identify adsorbates during studies using STM.

Tin-oxide overlayer formation by oxidation of Pt–Sn(111) surface alloys

Ordered (2×2) and (√3×√3)R30° Pt–Sn(111) surface alloys were oxidized by NO2 exposure at 400 K under ultrahigh vacuum conditions. The evolution of the surface morphology with annealing temperature was characterized by using low energy electron diffraction (LEED), scanning tunneling microscopy, Auger electron spectroscopy, and x-ray photoelectron spectroscopy. Both oxidized surface alloys form a SnOx overlayer that wets the substrate. However, the SnOx film does not completely cover the surface for the oxidized (2×2) surface alloy. For the oxidized (√3×√3)R30° surface alloy, an ordered (4×4) LEED pattern is formed upon flash annealing above 900 K. The formation of this ordered SnOx adlayer coincides with Sn segregation from the bulk to the interface region. A model for the (4×4) structure is discussed. The SnOx overlayer formed by oxidation of the (2×2) surface alloy is significantly less thermally stable than the oxidized (√3×√3)R30° surface alloy. Exothermic alloying of Sn with Pt may facilitate the decomposition of the oxide overlayers. Differences in the amount of subsurface tin and its segregation to the surface is proposed to explain the thermal stabilities of the oxide layers. The incompleteness of the SnOx overlayer and less subsurface tin for the oxidized (2×2) surface alloy is proposed to explain its significant lower thermal stability.Ordered (2×2) and (√3×√3)R30° Pt–Sn(111) surface alloys were oxidized by NO2 exposure at 400 K under ultrahigh vacuum conditions. The evolution of the surface morphology with annealing temperature was characterized by using low energy electron diffraction (LEED), scanning tunneling microscopy, Auger electron spectroscopy, and x-ray photoelectron spectroscopy. Both oxidized surface alloys form a SnOx overlayer that wets the substrate. However, the SnOx film does not completely cover the surface for the oxidized (2×2) surface alloy. For the oxidized (√3×√3)R30° surface alloy, an ordered (4×4) LEED pattern is formed upon flash annealing above 900 K. The formation of this ordered SnOx adlayer coincides with Sn segregation from the bulk to the interface region. A model for the (4×4) structure is discussed. The SnOx overlayer formed by oxidation of the (2×2) surface alloy is significantly less thermally stable than the oxidized (√3×√3)R30° surface alloy. Exothermic alloying of Sn with Pt may facilitate the deco...

Self-organized molecular-sized, hexagonally ordered SnOx nanodot superlattices on Pt(111)

Complete oxidation of the (√3×√3)R30° Sn/Pt(111) surface alloy or submonolayer amounts of Sn adatoms on Pt(111) under ultrahigh vacuum conditions, forms a highly ordered, lateral superlattice of SnOx islands on the Pt(111) substrate. The island superstructure exhibits a sharp (5×5) low energy electron diffraction pattern. Scanning tunneling microscopy images show islands arranged in a hexagonal lattice, uniformly distributed over the whole sample. This island array is thermally stable up to 1050 K. The coincidence of the island periodicity with a multiple of the supporting substrate, and the same hexagonal symmetry of islands and substrate, suggests a strong island–substrate interaction. We propose that the island formation results from the breakup of a strained SnOx adlayer.