The early stages of carbon nanotube nucleation are investigated using field ion/electron microscopy along with in situ local chemical probing of a single nanosized nickel crystal. To go beyond experiments, tight-binding Monte Carlo simulations are performed on oriented Ni slabs. Real-time field electron imaging demonstrates a carbon-induced increase of the number density of steps in the truncated vertices of a polyhedral Ni nanoparticle. The necessary diffusion and step-site trapping of adsorbed carbon atoms are observed in the simulations and precede the nucleation of graphene-based sheets in these steps. Chemical probing of selected nanofacets of the Ni crystal reveals the occurrence of Cn (n=1-4) surface species. Kinetic studies prove C2+ species are formed from C1 with a delay time of several milliseconds at 623 K. Carbon dimers, C2, must not necessarily be formed on the Ni surface. Tight-binding Monte Carlo simulations reveal the high stability of such dimers in the first layer beneath the surface.

Field ion microscopy combined with video techniques and chemical probing reveals the existence of catalytic oscillatory patterns at the nanoscale. This is the case when a rhodium nanosized crystalconditioned as a field emitter tip-is exposed to hydrogen and oxygen. Here, we show that these nonequilibrium oscillatory patterns find their origin in the different catalytic properties of all of the nanofacets that are simultaneously exposed at the tip's surface. These results suggest that the underlying surface anisotropy, rather than a standard reaction-diffusion mechanism, plays a major role in determining the self-organizational behavior of multifaceted nanostructured surfaces. Surprisingly, this nanoreactor, composed of the tip crystal and a constant molecular flow of reactants, is large enough for the emergence of regular oscillations from the molecular fluctuations.field ion microscopy ͉ heterogeneous catalysis ͉ nanopatterns ͉ nonequilibrium oscillations T he Belousov-Zhabotinskii reaction is probably the most famous example of an oscillating chemical reaction in the liquid phase (1, 2). However, studies of oscillations in heterogeneous catalysis begun in the 1970s (3, 4). One decade later, Ertl et al. demonstrated for the first time (5, 6) that oscillatory surface reactions are associated with the occurrence of specific pattern formations ranging from tens to hundreds of micrometers. More recently, oscillations have been discovered on the nanoscale in field electron and field ion microscopes by using video techniques (FEM and FIM, respectively) (7-10). At this length scale of tens of nanometers, self-sustained oscillatory patterns are observed about which little is known. This is certainly the case for the catalytic water production when exposing oxygen and hydrogen to a rhodium field emitter tip (11,12). Our purpose in the present article is to show that this nonequilibrium self-organizational behavior can be understood by taking into account the structural anisotropy of the crystalline tip that results in different catalytic properties on the various nanofacets. Moreover, we demonstrate that the external electric field, as applied in a FIM (of the order of 10 V/nm), promotes surface oxidation thus giving way to a feedback mechanism to explain self-sustained rate oscillations in the H 2 /O 2 /Rh system.The understanding of self-sustained oscillations at the macroscale was pioneered by Prigogine who showed that such phenomena are consistent with thermodynamics as long as the system is open and far from equilibrium (13). Such selforganization phenomena are understood in terms of reactiondiffusion processes, which also explains the mesoscopic patterns observed on oriented metal single-crystal surfaces (14, 15). However, the nanopatterns observed during a catalytic reaction in a FIM have a spatial scale smaller than typical diffusion lengths. Moreover, one may wonder how the molecular fluctuations that manifest themselves in such small systems (16, 17) affect the oscillations. Indeed, their regularity disappears...

A comprehensive theory for the adsorption of H2 and O2 on a nanometric rhodium field emitter tip is developed to describe the equilibrium properties, the adsorption−desorption kinetics, as well as its observed nonlinear reaction behavior and oscillatory states. The basis is a kinetic mean-field model for hydrogen, oxygen, and subsurface oxygen which takes into account the anisotropy of the tip’s surface. The resulting model reproduces the correct anisotropy, period and form of the oscillations, as well as the bistability diagram for a varying temperature, hydrogen pressure, and external electric field as observed in a field ion microscope.

We have studied oxygen interaction with Au crystals (field emitter tips) using time-resolved (atom-probe) field desorption mass spectrometry. The results demonstrate no adsorption to take place on clean Au facets under chosen conditions of pressures (p < 10(-4) m/bar) and temperatures (T = 300-350 K). Steady electric fields of 6 V/nm do not allow dissociating the oxygen molecule. The measured O2+ intensities rather reflect ionization of O2 molecules at critical distances above the Au tip surface. Certain amounts of Au-O2 complex ions can be found at the onset of Au field evaporation. Calculations by density functional theory (DFT) show weak oxygen end-on interaction with Au10 clusters (Delta E = 0.023 eV) and comparatively stronger interaction with Au1/Au(100) model surfaces (Delta E = 0.25 eV). No binding is found on {210} facets. Including (positive) electric fields in the DFT calculations leads to an increase of the activation energy for oxygen dissociation thus providing an explanation for the absence of atomic oxygen ions from the field desorption mass spectra.