Orientation-dependent growth mechanisms of graphene islands on Ir(111).

Using low-energy electron microscopy, we find that the mechanisms of graphene growth on Ir(111) depend sensitively on island orientation with respect to Ir. In the temperature range of 750-900 °C, we observe that growing rotated islands are more faceted than islands aligned with the substrate. Further, the growth velocity of rotated islands depends not only on the C adatom supersaturation but also on the geometry of the island edge. We deduce that the growth of rotated islands is kink-nucleation-limited, whereas aligned islands are kink-advancement-limited. These different growth mechanisms are attributed to differences in the graphene edge binding strength to the substrate.

Initial stages of FeO growth on Ru(0001).

We study how FeO wüstite films on Ru(0001) grow by oxygen-assisted molecular beam epitaxy at elevated temperatures (800­900 K). The nucleation and growth of FeO islands are observed in real time by low-energy electron microscopy (LEEM). When the growth is performed in an oxygen pressure of 10(−6) Torr, the islands are of bilayer thickness (Fe­O­Fe­O). In contrast, under a pressure of 10(−8) Torr, the islands are a single FeO layer thick. We propose that the film thickness is controlled by the concentration of oxygen adsorbed on the Ru. More specifically, when monolayer growth increases the adsorbed oxygen concentration above a limiting value, its growth is suppressed. Increasing the temperature at a fixed oxygen pressure decreases the density of FeO islands. However, the nucleation density is not a monotonic function of oxygen pressure.

Structure and magnetism in ultrathin iron oxides characterized by low energy electron microscopy.

We have grown epitaxial films a few atomic layers thick of iron oxides on ruthenium. We characterize the growth by low energy electron microscopy. Using selected-area diffraction and intensity-versus-voltage spectroscopy, we detect two distinct phases which are assigned as wüstite and magnetite. Spin-polarized low energy electron microscopy reveals magnetic domain patterns in the magnetite phase at room temperature.

Nanoscale periodicity in stripe-forming systems at high temperature: Au/W(110).

We observe using low-energy electron microscopy the self-assembly of monolayer-thick stripes of Au on W(110) near the transition temperature between stripes and the nonpatterned (homogeneous) phase. We demonstrate that the amplitude of this Au-stripe phase decreases with increasing temperature and vanishes at the order-disorder transition (ODT). The wavelength varies much more slowly with temperature and coverage than theories of stress-domain patterns with sharp boundaries would predict, and maintains a finite value of about 100 nm at the ODT. We argue that such nanometer-scale stripes should often appear near the ODT.

Evolution of a reactive surface via subsurface defect dynamics.

We find that the topography and composition of a reactive surface can evolve during epitaxy via motion of point and line defects within the material. We observe the response of a NiAl surface to an Al atom flux with low-energy electron microscopy. Initially, new NiAl layers grow as Al atoms exchange with bulk Ni atoms. When the surface is critically enriched in Al, condensation occurs at dislocations. They dissociate, move linearly, and leave tracks of altered composition and new atomic steps. We show how these dynamics depend on the identity and quantity of point defects near the surface.

Twin boundaries can be moved by step edges during film growth.

We track individual twin boundaries in Ag films on Ru(0001) using low-energy electron microscopy. The twin boundaries, which separate film regions whose close-packed planes are stacked differently, move readily during film growth but relatively little during annealing. The growth-driven motion of twin boundaries occurs as film steps advance across the surface--as a new atomic Ag layer reaches an fcc twin boundary, the advancing step edge carries along the boundary. This coupling of the microstructural defect (twin boundary) and the surface step during growth can produce film regions over 10 microm wide that are twin free.

Enhanced self-diffusion on Cu(111) by trace amounts of s: chemical-reaction-limited kinetics.

We find that less than 0.01 monolayer of S can enhance surface self-diffusion on Cu(111) by several orders of magnitude. The measured dependence of two-dimensional island decay rates on S coverage (theta(S)) is consistent with the proposal that Cu3S3 clusters are responsible for the enhancement. Unexpectedly, the decay and ripening are diffusion limited with very low and very high theta(S) but not for intermediate theta(S). To explain this result we propose that surface mass transport in the intermediate region is limited by the rate of reaction to form Cu3S3 clusters on the terraces.

Role of bulk thermal defects in the reconstruction dynamics of the TiO2(110) surface.

We use low-energy electron microscopy to show that changing the temperature of oxygen-deficient, rutile-structure crystals causes steps on the (110) surfaces to move. This motion occurs because the concentration of bulk oxygen vacancies changes with temperature, requiring that material be added to or subtracted from the surface. During cooling below a bulk-stoichiometry-dependent temperature, the surface reconstructs into a 1x2 structure in the regions surface steps have swept through, showing that the structural and compositional changes needed to form the 1x2 phase are facilitated by the surface-to-bulk mass flow.

Vacancies in solids and the stability of surface morphology.

Determining how thermal vacancies are created and destroyed in solids is crucial for understanding many of their physical properties, such as solid-state diffusion. Surfaces are known to be good sources and sinks for bulk vacancies, but directly determining where the exchange between the surface and the bulk occurs is difficult. Here we show that vacancy generation (and annihilation) on the (110) surface of an ordered nickel-aluminium intermetallic alloy does not occur over the entire surface, but only near atomic step edges. This has been determined by oscillating the sample's temperature and observing in real time the response of the surface structure as a function of frequency (a version of Angström's method of measuring thermal conductivity) using low-energy electron microscopy. Although the surface-exchange process is slow compared with bulk diffusion, the vacancy-generation rate nevertheless controls the dynamics of the alloy surface morphology. These observations, demonstrating that surface smoothing can occur through bulk vacancy transport rather than surface diffusion, should have important implications for the stability of fabricated nanoscale structures.

Raman scattering as a technique of measuring film thickness: interference effects in thin growing films.

Simultaneous Raman-scattering and interferometric-thickness measurements have been made during the growth of sodium sulfate films on Pt/10%Rh substrates. For the experimental geometry, the s-polarized Raman-scattered radiation exhibits oscillations in intensity as a function of film thickness, while no intensity oscillations are observed for the p-polarized Raman scattering. The experimental results are modeled by assuming the interference of the multiple reflections of the incident laser beam and the interference of the multiple reflections of the Raman-scattered radiation. The model predicts extreme oscillations in the Raman signal/film thickness relationship for the s-polarized Raman collection and only small oscillations for the p-polarized Raman collection, in qualitative agreement with the experimental results.