Atomically resolved structure

K well-ordered chains running in the < 110 > direction separated by the atom resolved structure of the Cu(l 10) surface with a spacing between the rows of 0.36 nm (see line profile), (b) The spacing within the chains is 0.51 nm (see line profile), i.e. close to twice the Cu-Cu distance within the copper rows running in the < 110 > direction. (Reproduced from Refs. 16, 18). 

Since the most active catalytic sites are usually steps, kinks, and surface defects, atomically resolved structural information including atomic distribution and surface structure at low pressure, possible surface restructuring, and the mobility of adsorbate molecules and of the atoms of the catalyst surface at high temperature and high pressure is crucial to understanding catalytic mechanisms on transition metal surfaces. The importance of studying the structural evolution ofboth adsorbates... 

Block and co-workers [35] modified the atom probe to develop a method called pulsed-field desorption mass spectrometry (PFDMS), whereby a short high-voltage pulse desorbs all species present on the tip during a catalytic reaction. The repetition frequency of the field pulse controls the time for which the reaction is allowed to proceed. Hence, by varying the repetition frequency between desorption pulses in a systematic way, one can study the kinetics of a surface reaction [35], In fact, this type of experiment - where one focuses on a facet of desired structure, which may include steps and defects - comes close to one of the fundamental goals of catalyst characterization, namely studying a catalytic reaction on substrates of atomically resolved structure with high time resolution. 

Ihis approach, applicable when the atomic resolved structure of the target is unknown but predictable by means of the available experimental information, is able to handle the heterogeneous e q>erimental information on the ligands and their targets, and to synthesize and translate them into QSAR models. 

Summary. Scanning tunneling microscopy (STM) provides new possibilities to explore the link between the structure and the properties of thin oxide overlayers (passive films) formed electrochemically on well-defined metal surfaces. Passive oxide films protect many metals and alloys against corrosion. A better understanding of the growth mechanisms, the stability, and the degradation of passive films requires precise structural data. Recently, new results on the atomic structure of passive films have been obtained by STM. The important questions of crystallinity, epitaxy and the nature of defects have been addressed. Data on the structure of passive films on Ni, Cr, Fe, Al, and Fe-Cr alloys are reviewed with enq>hasis on atomically resolved structures. Ihe perspectives of future developments are discussed. 

The objective of this paper is to review the published data on ex-situ and in-situ STM of passivation of metals (Ni, Cr, Fe, Al) and alloys (Fe-Cr), with special emphasis on atomically resolved structures, and to discuss, on the basis of the reviewed data, the questions of crystalline versus amorphous character of passive films, the nature of the defects, the relation of ftie structure to the available chemical information, and the implications of the structural features in the stability and the breakdown of passive films. 

Atomically resolved structure of GaAs( 100) surfaces in electrolyte solutions... 

Part of a 15-nm long, 10 A tube, is given in Fig. 1. Its surface atomic structure is displayed[14], A periodic lattice is clearly seen. The cross-sectional profile was also taken, showing the atomically resolved curved surface of the tube (inset in Fig. 1). Asymmetry variations in the unit cell and other distortions in the image are attributed to electronic or mechanical tip-surface interactions[15,16]. From the helical arrangement of the tube, we find that it has zigzag configuration. 

Experimental methods in surface science are considered briefly in order to illustrate how experimental data and concepts that emerged from their application could be progressed through evidence from STM at the atom resolved level. They include kinetic, structural, spectroscopic and work function studies. Further details of how these methods provided the experimental data on which much of our present understanding of surfaces and their reactivity can be obtained from other publications listed under Further Reading at the end of this chapter. 

Figure 4.10 With increasing oxygen exposure at 295 K, the Mg(0001) surface consists of both hexagonal and square lattice structures the line profiles indicate repeat distances of 0.321 and 0.56 nm in the atom resolved hexagonal and square structures, respectively, the former being the most prevalent structure present. (Reproduced from Ref. 41).

Atom resolved studies were first reported by Ertl s group15 in the early 1990s for Cu(110)-K, indicating the development of (1 x 3) and (1 x 2) structures depending on the surface coverage. They were missing row structures with... 

What STM established first in 1991 for both Cu(110) K and Cu(110)-Cs systems was the localised nature of the reconstruction process and the atom resolved details of the complexity of the structural changes observed with increasing coverage.15 In 1993, Doyen et al.20 provided theoretical support for the experimental observations with both the Cu(110) and Au(110) surfaces. 

The chemisorption of CO at Pt(110) is one of the most extensively studied systems, which exhibits structural transformation induced by an adsorbate, with most experimental methods available in surface science being used. It was, however, the Aarhus group that provided atom resolved evidence, over the pressure range 10 9 103 mbar and temperature range 300 400 K, for a... 

Figure 10.6 STM images of the Ni(l 11) (5 /3 x 2)S phase and a model for the structure proposed to explain the decreased density of nickel within the islands, (a) 15.0 x 16.5 nm image showing the three possible domains of the (5 /3 x 2)S structure the brighter part of the image corresponds to an adlayer that has developed on top of a nickel island formed during H2S adsorption, (b) 1.8 x 2.9 nm atomically resolved image of the (5 /3 x 2)S structure, (c) Proposed clock structure for the (5 /3 x 2)S phase that accounts for the reduced nickel density in the sulfur adlayer. (Reproduced from Refs. 23 and 25).

Figure 10.9 STM images showing structural changes induced by sulfur adsorption. (a) Clean Au(lll) surface showing very regular herringbone pattern, (b) Close-up of the disordered herringbone pattern at low coverage of sulfur (<0.1ML). (c) Atomically resolved images of the Au atoms underlying approximately 0.3 ML sulfur adsorbed Au(lll). (Reproduced from Ref. 34).

Figure 3.18 (a) Ag(l 1 1) islands and pits surrounded by the Ag(l 1 l)-p(4x4)-0 structure. The white arrow points to the remnants of the surface oxide that are occasionally observed in the pits. The atomically resolved STM images show (b) the Ag(l 1 l)-p(4 x 4)-0 surface and (c) a small area of the (1 x 1) structure obtained on the island shown in (a) that appeared during CO oxidation. The black... 

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