Step and kink dynamics on Au(110) and Pb(111) studied with a high-speed STM

Self-assembly of acetate adsorbates drives atomic rearrangement on the Au(110) surface

Weak inter-adsorbate interactions are shown to play a crucial role in determining surface structure, with major implications for its catalytic reactivity. This is exemplified here in the case of acetate bound to Au(110), where the small extra energy of the van der Waals interactions among the surface-bound groups drives massive restructuring of the underlying Au. Acetate is a key intermediate in electro-oxidation of CO₂ and a poison in partial oxidation reactions. Metal atom migration originates at surface defects and is likely facilitated by weakened Au–Au interactions due to bonding with the acetate. Even though the acetate is a relatively small molecule, weak intermolecular interaction provides the energy required for molecular self-assembly and reorganization of the metal surface.

The efficiency of a catalyst relies on the stability of intermediates on its surface. Here, the authors find that van der Waals interactions between acetate adsorbates on Au(110) provide a small but necessary energy contribution to stabilize the acetate and drive restructuring of the Au surface.

The ReactorSTM: atomically resolved scanning tunneling microscopy under high-pressure, high-temperature catalytic reaction conditions

To enable atomic-scale observations of model catalysts under conditions approaching those used by the chemical industry, we have developed a second generation, high-pressure, high-temperature scanning tunneling microscope (STM): the ReactorSTM. It consists of a compact STM scanner, of which the tip extends into a 0.5 ml reactor flow-cell, that is housed in a ultra-high vacuum (UHV) system. The STM can be operated from UHV to 6 bars and from room temperature up to 600 K. A gas mixing and analysis system optimized for fast response times allows us to directly correlate the surface structure observed by STM with reactivity measurements from a mass spectrometer. The in situ STM experiments can be combined with ex situ UHV sample preparation and analysis techniques, including ion bombardment, thin film deposition, low-energy electron diffraction and x-ray photoelectron spectroscopy. The performance of the instrument is demonstrated by atomically resolved images of Au(111) and atom-row resolution on Pt(110), both under high-pressure and high-temperature conditions.

Fluctuations of Step Positions at KDP Crystal Faces

Applying atomic force microscopy in single string scanning mode, dependence of fluctuations of step shifts on KDP crystal faces on time was determined during crystallization from solution. It was shown that fluctuation amplitude grows proportionally to t1/4. On prism faces fluctuations at dissolving were bigger than at growth and bigger than on bipyramid faces. Amplitude of fluctuations was not dependent on distance between steps. Interpretation of experimental results and computation of elementary parameters of crystallization was done based on theory of V.V.Voronkov.

Fluctuations of Step Edges: Revelations About Atomic Processes Underlying Surface Mass Transport

Experimental advances in recent years make possible quantitative observations of step-edge fluctuations. By applying a capillary-wave analysis to these fluctuations, one can extract characteristic times, from which one learns about the mass-transport mechanisms that underlie the motion as well as the associated kinetic coefficients [1-3]. The latter do not require a priori insight about the microscopic energy barriers and can be applied to situations away from equilibrium. We have studied a large number of limiting cases and, by means of a unified formalism, the crossover between many of these cases[4]. Monte Carlo simulations have been used to corroborate these ideas. We have considered both isolated steps and vicinal surfaces; illustrations will be drawn from noble-metal systems, though semiconductors have also been studied. Attachment asymmetries associated with Ehrlich-Schwoebel barriers play a role in this behavior. We have adapted the formalism for nearly straight steps to nearly circular steps in order to describe the Brownian motion of single-layer clusters of adatoms or vacancies on metal surfaces, again in concert with active experimental activity [3,5]. We are investigating the role of external influences, particularly electromigration, on the fluctuations.

Dynamic interfaces in an organic thin film

Low-dimensional boundaries between phases and domains in organic thin films are important in charge transport and recombination. Here, fluctuations of interfacial boundaries in an organic thin film, acridine-9-carboxylic acid on Ag(111), have been visualized in real time and measured quantitatively using scanning tunneling microscopy. The boundaries fluctuate via molecular exchange with exchange time constants of 10-30 ms at room temperature, with length-mode fluctuations that should yield characteristic f(-1/2) signatures for frequencies less than approximately 100 Hz. Although acridine-9-carboxylic acid has highly anisotropic intermolecular interactions, it forms islands that are compact in shape with crystallographically distinct boundaries that have essentially identical thermodynamic and kinetic properties. The physical basis of the modified symmetry is shown to arise from significantly different substrate interactions induced by alternating orientations of successive molecules in the condensed phase. Incorporating this additional set of interactions in a lattice-gas model leads to effective multicomponent behavior, as in the Blume-Emery-Griffiths model, and can straightforwardly reproduce the experimentally observed isotropic behavior. The general multicomponent description allows the domain shapes and boundary fluctuations to be tuned from isotropic to highly anisotropic in terms of the balance between intermolecular interactions and molecule-substrate interactions.

Extraordinary Atomic Mobility of Au{111} at 80 Kelvin: Effect of Styrene Adsorption

We describe how the presence of styrene, a weakly adsorbed molecule, dramatically restructures the Au{111} surface at temperatures as low as 80 K. The restructuring manifests itself both in mobility of step-edge atoms, as well as changes in the position of the herringbone reconstruction over time. These effects are explained in terms of the preferential adsorption sites of styrene allowing it to assist in atom detachment from step edges, as well as lowering of the energetic barrier for movement of the herringbone reconstruction. This work has important consequences for studies in which Au is used as a support for or as an electrical contact to molecules.

Distinctive fluctuations in a confined geometry

Spurred by recent theoretical predictions [Phys. Rev. E 69, 035102(R) (2004)10.1103/PhysRevE.69.035102; Surf. Sci. Lett. 598, L355 (2005)10.1016/j.susc.2005.09.023], we find experimentally using STM line scans that the fluctuations of the step bounding a facet exhibit scaling properties distinct from those of isolated steps or steps on vicinal surfaces. The correlation functions go as t0.15 +/- 0.03 decidedly different from the t0.26 +/- 0.02 behavior for fluctuations of isolated steps. From the exponents, we categorize the universality, confirming the prediction that the nonlinear term of the Kardar-Parisi-Zhang equation, long known to play a central role in nonequilibrium phenomena, can also arise from the curvature or potential-asymmetry contribution to the step free energy.

A fast response mechanism for insulin storage in crystals may involve kink generation by association of 2D clusters

Crystals that are likely rhombohedral of Zn-insulin hexamers form in the islets of Langerhans in the pancreases of many mammals. The suggested functions of crystal formation is to protect the insulin from proteases and increase the degree of conversion of soluble proinsulin. To accomplish these ends, crystal growth should be fast and adaptable to rate fluctuations in the conversion reaction. Zn-insulin crystals grow layer by layer. Each layer spreads by the attachment of molecules to kinks located at the layers' edges, also called steps. The kinks are thought to be generated either by thermal fluctuations, as postulated by Gibbs, or by 1D nucleation of new crystalline rows. The kink density determines the rate at which steps advance, and these two kink-generation mechanisms lead to weak near-linear responses of the growth rate to concentration variations. We demonstrate for the crystallization of Zn-insulin a mechanism of kink generation whereby 2D clusters of several insulin molecules preformed on the terraces between steps associate to the steps. This mechanism results in several-fold-higher kink density, a faster rate of crystallization, and a high sensitivity of the kinetics to small increases of the solute concentration. If the found mechanism operates during insulin crystallization in vivo, it could be a part of the biological regulation of insulin production and function. For other crystallizing materials in biological and nonbiological systems, this mechanism provides an understanding of the often seen nonlinear acceleration of the kinetics.

Sampling-time effects for persistence and survival in step structural fluctuations

The effects of sampling rate and total measurement time have been determined for single-point measurements of step fluctuations within the context of first-passage properties. Time dependent scanning tunneling microscopy has been used to evaluate step fluctuations on Ag(111) films grown on mica as a function of temperature (300-410 K) , on screw dislocations on the facets of Pb crystallites at 320 K , and on Al-terminated Si(111) over the temperature range 770-970 K . Although the fundamental time constant for step fluctuations on Ag and Al/Si varies by orders of magnitude over the temperature ranges of measurement, no dependence of the persistence amplitude on temperature is observed. Instead, the persistence probability is found to scale directly with t/delta t where delta t is the time interval used for sampling. Survival probabilities show a more complex scaling dependence, which includes both the sampling interval and the total measurement time t(m) . Scaling with t/delta t occurs only when delta t/ t(m) is a constant. We show that this observation is equivalent to theoretical predictions that the survival probability will scale as delta t/ L(z) , where L is the effective length of a step. This implies that the survival probability for large systems, when measured with fixed values of t(m) or delta t , should also show little or no temperature dependence.

Size-dependent detachment-limited decay kinetics of two-dimensional TiN islands on TiN(111)

In situ high-temperature (T(a)=1050-1250 K) scanning tunneling microscopy was used to determine the coarsening and decay kinetics of two-dimensional TiN adatom and vacancy islands on atomically smooth TiN(111) terraces. We report the first observation of an abrupt decrease in decay rates, irrespective of T(a), of adatom islands with areas less than a critical value of 1600 A(2). However, no decay rate transition was observed for vacancy islands. We attribute the size-dependent island decay behavior, which is consistent with detachment-limited kinetics, to anisotropic attachment and detachment barriers.

Continuum model for low temperature relaxation of crystal steps

High and low temperature relaxation of crystal steps are described in a unified picture, using a continuum model based on a modified expression of the step-free energy. Results are in agreement with experiments and Monte Carlo simulations of step fluctuations and monolayer cluster diffusion and relaxation. In an extended model where mass exchange with neighboring terraces is allowed, step transparency and a low temperature regime for unstable step meandering are found.

Renal angiomyolipoma: further immunophenotypic characterization of an expanding morphologic spectrum

BACKGROUND

Renal angiomyolipoma is a benign tumor histologically characterized by proliferation of spindle cells, epithelioid cells, and adipocytic cells in concert with many thick-walled blood vessels. To add further diagnostic confusion, an epithelioid cell-predominant variant of renal angiomyolipoma has recently been described. HMB-45 immunoreactivity correlates with ultrastructural striated organelles that closely resemble premelanosomes, although no evidence of melanogenesis has been documented in this tumor.

OBJECTIVE

To further characterize the immunophenotypic and ultrastructural profile of renal angiomyolipoma based on phenotypic cell type (epithelioid, spindle, and adipocytic cell).

DESIGN

Formalin-fixed, paraffin-embedded tissues from 27 renal angiomyolipomas and 8 renal cell carcinomas were immunostained with monoclonal antibodies to the melanoma-associated antigens HMB-45, HMB-50, NKI/C3 (CD63), and tyrosinase; the smooth muscle-related antigens calponin and muscle-specific actin (HHF-35); S100; and cytokeratin (CK). All renal angiomyolipomas were also immunostained with a polyclonal antibody to renin. Ultrastructural examination was performed on 9 selected cases.

RESULTS

All renal angiomyolipomas stained positive for HMB-45, HMB-50, NKI/C3, muscle-specific actin (HHF-35), and calponin. Overall, HMB-45, HMB-50, and NKI/C3 preferentially stained the epithelioid cells. Tyrosinase staining was present in 50% of the renal angiomyolipomas with adequate tissue for staining (12 of 24 cases); positive staining and intensity paralleled HMB-45, HMB-50, and NKI/C3. Muscle-specific actin (HHF-35) and calponin preferentially stained the spindle cells. The adipocytic cells stained positive for both melanoma-associated antigens and smooth muscle antigens. Epithelioid cells, spindle cells, and adipocytic cells were CK, S100, and renin negative. Ultrastructural findings paralleled immunohistochemical staining patterns. Premelanosome-like organelles and electron dense granules were more readily detected in the epithelioid cells within the tumor, whereas ultrastructural characteristics of smooth muscle cells were more easily found in the spindle cells. All renal cell carcinomas stained positive for CK, NKI/C3 staining was variable, and all were negative for HMB-45, HMB-50, smooth muscle actin (HHF-35), and calponin.

CONCLUSION

In renal angiomyolipoma, the epithelioid and spindle cells have preferential staining patterns for melanoma-associated antigens versus smooth muscle antigens, respectively. Positivity in renal angiomyolipoma for HMB-50, NKI/C3, and tyrosinase, in addition to HMB-45, provides evidence for the presence of different melanoma-associated gene products. Immunophenotypic overlap of the 3 histologically distinct renal angiomyolipoma cell populations suggests a common cell line, supporting a unitarian concept for renal angiomyolipoma. Ultrastructural characteristics of the 3 renal angiomyolipoma cell phenotypes parallel the immunophenotype, giving further support to a common cell line. Our study lends further credence to the perivascular epithelioid cell concept as proposed by Bonetti and colleagues.

Molecular mechanisms of crystallization and defect formation

Using the atomic force microscope (AFM) in situ during the crystallization of the protein apoferritin, we show that for this system the kink density along the steps is an equilibrium property that, multiplied by the frequency of molecular attachment, fully determines the propagation of growth steps. The intermolecular bond energy is 3.2k(B)T. Point defects are nonequilibrial and are caused by incorporation of impurity molecules, and replicate in subsequent layers due to the strain they cause. Using single-molecule manipulation with the AFM tip, the defects can be healed to restore the regular lattice.