Ultrafast dynamics at semiconductor and metal surfaces

A variety of important dynamical phenomena at metal and semiconductor surfaces are now being investigated with the use of new ultrafast measurement techniques involving lasers and nonlinear optics. Understanding of the rates and mechanisms for relaxation of optical excitations of the surface itself as well as those of adsorbates on the surface is providing new insight into surface chemistry, surface phase transitions, and surface recombination of charge carriers in semiconductors.     

Atomic resolution imaging of adsorbates on metal surfaces in air: iodine adsorption on pt(111)

The adsorption of iodine on platinum single crystals was studied with the scanning tunneling microscope (STM) to define the limits of resolution that can be obtained while imaging in air and to set a target resolution for STM imaging of metal surfaces immersed in an electrochemical cell. Two iodine adlattice unit cells of slightly different iodine packing density were clearly imaged: ( radical7 x radical7) R19.1 degrees -I, surface coverage ?(I) = 3/7; and (3 x 3)-I, ?(I) = 4/9. The three iodine atoms in the ( radical7 x radical7) unit cell form a regular hexagonal lattice interatomic distance d(I) = 0.424 nanometer, with two atoms adsorbed in threefold hollow sites and one atom adsorbed at an atop site. The (3 x 3) unit cell showed two different packing arrangements of the four iodine atoms exit. In one of the (3 x 3) structures, the iodine atoms pack to form a hexagonal lattice, d(I) = 0.417 nanometer, with three of the iodine atoms at twofold adsorption sites and one atom at an atop site. Another packing arrangement of iodine into the (3 x 3) unit cell was imaged in which the iodine atoms are not arranged symmetrically.     

Molecular transformations on single crystal metal surfaces

One of the primary objectives of modern surface chemistry of transition metals is the synthesis of surface compounds and complexes and the understanding of their reactivity, structure, and bonding. Such considerations are paramount for advancing understanding of catalysis, adhesion, organic thin-film growth, and electrocatalysis. On selected metals, particularly copper, silver, and gold, selective scission of X-H bonds (where X is oxygen, carbon, nitrogen, or sulfur) by surface-bound atomic oxygen occurs to form moderately stable species that can be isolated for further study. Selective oxidation reactions may occur heterogeneously by means of this novel oxygen- activated route. Furthermore, this selective chemistry offers a paradigm for synthesis of a wide variety of surface organometallic complexes, whose formation can be predicted from acid-base principles. These subjects are discussed in this article with emphasis on their role in catalytic oxidation cycles.     

NMR studies of simple molecules on metal surfaces

In recent years, improvements in the sensitivity of nuclear magnetic resonance have made it possible to detect progressively smaller numbers of nuclei. Experiments and studies previously thought to be impractical can now be undertaken, for example, the study of phenomena at surfaces. Nuclear magnetic resonance has been applied to study simple molecules (carbon monoxide, acetylene, and ethylene) adsorbed on metal surfaces (ruthenium, rhodium, palladium, osmium, iridium, and platinum). The metals, in the form of clusters 10 to 50 angstroms in diameter, supported on alumina, are typical of real catalysts. The experiments provide information about the bonding of the molecules to the metal, the structures the molecules assume after adsorption, the motion of molecules on the surface, the breakup of molecules induced by heating, and the products of such breakup.     

Atom counting at surfaces

Multiphoton resonance ionization has been combined with energetic ion bombardment to examine dopant concentrations ofindium on the surface of silicon. The results yield a linear relation between the indium concentration and the known bulk values and a detection limit of 9 parts per trillion, at a mass resolution exceeding 160. This measurement, which surpasses the limits of any previous surface analysis by a factor of 100, has been made possible with an experimental configuration that optimizes sampling and detection efficiency while reducing background noise to virtually zero. During the analysis, submonolayer quantities of the surface are removed, so that as few as 180 surface atoms may be counted.     

Organic molecules acting as templates on metal surfaces

The electronic connection of single molecules to nanoelectrodes on a surface is a basic, unsolved problem in the emerging field of molecular nanoelectronics. By means of variable temperature scanning tunneling microscopy, we show that an organic molecule (C90H98), known as the Lander, can cause the rearrangement of atoms on a Cu(110) surface. These molecules act as templates accommodating metal atoms at the step edges of the copper substrate, forming metallic nanostructures (0.75 nanometers wide and 1.85 nanometers long) that are adapted to the dimensions of the molecule.     

New mechanisms for chemistry at surfaces

It is becoming increasingly apparent that chemistry at surfaces, whether it be heterogeneous catalysis, semiconductors etching, or chemical vapor deposition, is controlled by much more than the nature and structure of the surface. Recent experiments that principally make use of molecular beam techniques have revealed that the energy at which an incident molecule collides with a surface can be the key factor in determining its reactivity with or on the surface. In addition, the collision energy of an incident particle has proven essential to the finding of new mechanisms for reaction or desorption of molecules at surfaces, collision-induced activation and collision-induced desorption. These phenomena are often responsible for the different surface chemistry observed under conditions of high reactant pressure, such as those present during a heterogeneous catalytic reaction, and of low pressure of reactants (< 10(-4) torr), such as those present in an ultrahigh vacuum surface science experiment. This knowledge of the microscopic origins of the effect of pressure on the chemistry at surfaces has allowed the development of a scheme to bypass the high-pressure requirement. Reactions that are normally observed only at high reactant pressures, and which are the ones most often of practical importance, can now be carried out in low-pressure, ultrahigh vacuum environments.     

Atomic layer deposition of oxide thin films with metal alkoxides as oxygen sources

A chemical approach to atomic layer deposition (ALD) of oxide thin films is reported here. Instead of using water or other compounds for an oxygen source, oxygen is obtained from a metal alkoxide, which serves as both an oxygen and a metal source when it reacts with another metal compound such as a metal chloride or a metal alkyl. These reactions generally enable deposition of oxides of many metals. With this approach, an alumina film has been deposited on silicon without creating an interfacial silicon oxide layer that otherwise forms easily. This finding adds to the other benefits of the ALD method, especially the atomic-level thickness control and excellent uniformity, and takes a major step toward the scientifically challenging and technologically important task of replacing silica as the gate dielectric in the future generations of metal oxide semiconductor field effect transistors.     

Dynamics of molecular motion at single-crystal surfaces

Dramatic advances in our understanding of the motion of individual atoms and molecules at single-crystal surfaces have been made within the past 5 years. Recent experimental and theoretical studies of the interaction of nitric oxide with metal surfaces illustrate the depth of understanding now obtainable. General principles, applicable to a broader range of molecule-surface encounters, have begun to emerge out of the systematic and in-depth analyses of these and related studies.     

Atomic hydrogen on Mars: measurements at solar minimum

The Copernicus Orbiting Astronomical Observatory was used to obtain measurements of Mars Lyman-alpha (1215.671-angstrom) emission at the solar minimum, which has resulted in the first information on atomic hydrogen concentrations in the upper atmosphere of Mars at the solar minimum. The Copernicus measurements, coupled with the Viking in situ measurements of the temperature (170 degrees +/- 30 degrees K) of the upper atmosphere of Mars, indicate that the atomic hydrogen number density at the exobase of Mars (250 kilometers) is about 60 times greater than that deduced from Mariner 6 and 7 Lyman-alpha measurements obtained during a period of high solar activity. The Copernicus results are consistent with Hunten's hypothesis of the diffusion-limited escape of atomic hydrogen from Mars.     

Biomolecular interactions at phospholipid-decorated surfaces of liquid crystals

The spontaneous assembly of phospholipids at planar interfaces between thermotropic liquid crystals and aqueous phases gives rise to patterned orientations of the liquid crystals that reflect the spatial and temporal organization of the phospholipids. Strong and weak specific-binding events involving proteins at these interfaces drive the reorganization of the phospholipids and trigger orientational transitions in the liquid crystals. Because these interfaces are fluid, processes involving the lateral organization of proteins (such as the formation of protein- and phospholipid-rich domains) are also readily imaged by the orientational response of the liquid crystal, as are stereospecific enzymatic events. These results provide principles for label-free monitoring of aqueous streams for molecular and biomolecular species without the need for complex instrumentation.     

Interband electronic excitation-assisted atomic-scale restructuring of metal surfaces by nanosecond pulsed laser light

Interaction of high-power laser light with materials often causes irreversible damage of the near-surface region. It is shown that copper single-crystal surfaces can be patterned by laser light. Irradiation with green light produced adatoms and vacancies, which self-organized into nanoscale pyramids. This restructuring can be removed by annealing. In contrast to green light, infrared laser irradiation at equivalent absorbed energy density did not produce any structural change. This, for metallic systems, unforeseen spectral difference in laser light action points to a concerted process as the source for structural modification, which involves long-lived primary excitation of localized d-electrons through interband transition together with phonon excitation.     

Electro-optical evidence for the chelate effect at semiconductor surfaces

Monoamines and diamines dissolved in cyclohexane solution reversibly enhance the band-edge photoluminescence (PL) intensity of immersed n-type cadmium sulfide (n-CdS) and n-type cadmium selenide (n-CdSe) substrates through adsorption. The magnitude of the PL increase is used to estimate amine-induced contractions in the semiconductor's depletion width, and the dependence of the PL intensity on amine concentration provides an estimate of the adduct formation constant. Two diamines, ethylenediamine and o-phenylenediamine, exhibit unusually low reductions in depletion width and substantially larger adduct equilibrium constants relative to the other amines studied, consistent with chelation to surface Cd(2+) ions. These studies demonstrate that PL can be used as a contactiess, in situ technique for characterizing the steric and electronic landscape of semiconductor surfaces and for correlating molecular and surface chemistry.     

Alloying at surfaces by the migration of reactive two-dimensional islands

We have studied the formation kinetics of the copper-tin alloy bronze when tin is deposited on the (111) surface of copper at room temperature. Low-energy electron microscopy and atomic-resolution scanning tunneling microscopy reveal that bronze forms on the surface by a complicated, unanticipated cooperative mechanism: Ordered two-dimensional tin islands containing several hundred thousand atoms spontaneously sweep across the surface, leaving bronze alloys in their tracks. We propose that this process, driven by surface free energy, is a version of the "camphor dance" observed on liquid surfaces, and should be a general mechanism of surface alloying when surface diffusion is faster than exchange into the substrate.