Brownian motion of steps on Si(111)

Determination of the Thermomigration Force on Adatoms

Thermal gradients in nanomaterials can cause surface mass transport phenomena. However, the atomic fluxes are challenging to quantify and the underlying atomic mechanisms are complex. Using low energy electron microscopy we have examined in operando, under a thermal gradient of 10^{4}  K/m, the thermomigration of supercooled Si(111)-1×1 advacancy islands. The islands move in the direction of the thermal gradient at 0.26±0.06  nm/s. This reveals that the adatoms move toward the cold region and the effective force exerted on Si adatoms is 1.4±0.4×10^{-8}  eV/nm. We quantify the heat of transport of Si atoms Q^{*}=1.2±0.4  eV and show that it corresponds to the combined effects of adatom creation at step edges and adatom diffusion on atomically flat terraces.

Dynamics of domain boundaries at metal-organic interfaces

Domain boundaries are a determining factor in the performance of organic electronic devices since they can trap mobile charge carriers. We point out the possibility of time-dependent motion of these boundaries and suggest that their thermal fluctuations can be a source of dynamic disorder in organic films. In particular, we study the C8-BTBT monolayer films with several different domain boundaries. After characterizing the crystallography and diversity of structures in the first layer of C8-BTBT on Au(111), we focus on quantifying the domain boundary fluctuations in the saturated monolayer. We find that the mean squared displacement of the boundary position grows linearly with time at early times but tends to saturate after about 7 s. This behavior is ascribed to confined diffusion of the interface position based on fits and numerical integration of a Langevin equation for the interface motion.

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.

Real-time imaging of surface evolution driven by variable-energy ion irradiation

We describe the design of a tandem instrument combining a low-energy electron microscope (LEEM) and a negative ion accelerator. This instrument provides video rate imaging of surface microtopography and the dynamics of its evolution during irradiation by energetic ions, at temperatures up to 1700 K. The negative ion beam is incident on the sample at normal incidence with impact energies selectable in the range 0-5 keV, and with current densities up to 30 muA/cm2 ( approximately 2 x 10(14)ions/cm2 s or approximately 0.2 ML/s). The LEEM operates at a base pressure in the 10(-9)Pa range. We describe the design and operating principles of the instrument and present examples of Pt(111) and Si(001) self-ion irradiation experiments.

Why is diffusion in metals and on metal surfaces universal?

Recent experiments that determine mass diffusion D(s) on the close packed surfaces of vacuum compatible metals are reviewed. The results turn out to be approximately universal when scaled to homologous temperatures T/T(m), with T(m) the melting temperature. Similar behaviour for vacancy-driven diffusion in bulk metals has been recognized for decades. Remarkably, the uncertainty with which this scaling occurs is only ∼10%. Possible origins of the universality are discussed.

Defect formation and kinetics of atomic terrace merging

Pairs of atomic scale terraces on a single crystal metal surface can be made to merge controllably under suitable conditions to yield steps of double height and width. We study the effect of various physical parameters on the formation of defects in a kinetic model of step doubling. We treat this manifestly nonequilibrium problem by mapping the model onto a 1D random sequential adsorption problem and solving this analytically. We also do simulations to check the validity of our treatment. We find that our treatment effectively captures the dynamic evolution and the final state of the surface morphology. We show that the number and nature of the defects formed is controlled by a single dimensionless parameter q . For q close to one we show that the fraction of defects rises linearly with epsilon identical with 1-q as 0.284epsilon . We also show that one can arrive at the final state faster and with fewer defects by changing the parameter with time.

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.

Atomic diffusion, step relaxation, and step fluctuations

We show that the dynamics of the pair correlation function in a step train can pinpoint the dominant relaxation mechanism occurring at a crystal surface. Evaporation-condensation and step-edge diffusion do not produce dynamical correlations between neighboring steps, while terrace diffusion may lead to correlations which fall off like a power law with distance and which are peaked at a characteristic time. We derive these results within a "real space" Langevin formalism which is based on diffusion kernels which are different for each mass transport process. We validate this formalism by reproducing the step fluctuation autocorrelation function. We then derive results on the pair correlation between different steps. Results for solvable limiting cases are summarized in Tables I and II of the paper. As an intermediate step in the analysis we also find expressions for the relaxation time tau(pq) of a mode of wave number q along the steps and wave number p perpendicular to the steps, which we also discuss and compare with prior work.