High-speed Scanning Tunneling Microscopy Research Articles

An STM study on the diffusion of O atoms on a CO-covered Ru(0001) surface—The role of domain boundaries

We investigate tracer diffusion at the domain boundaries in an adsorption layer, an effect that corresponds to grain boundary diffusion in 3D polycrystalline solids. Experiments were performed on adsorbed O atoms on a Ru(0001) surface in a layer of CO molecules. The CO molecules form a (3x3)R30∘ structure which displays translational domains. High-speed scanning tunneling microscopy (STM) was used to image the motion of the O atoms. The data show that single O atoms preferentially move along the domain walls which in the STM movies appear as disordered, fluctuating stripes between the ordered domains. The diffusion coefficient of the O atoms is one order of magnitude higher than the diffusion coefficient in the ordered domains. By comparison with previous experiments on completely disordered CO layers, it is concluded that the diffusion is similarly promoted by the enhanced fluctuations in the disordered domain walls.

Real-Time Imaging of On-Surface Ullmann Polymerization Reveals an Inhibiting Effect of Adatoms.

Ullmann coupling is a widely used reaction for the on-surface growth of low-dimensional carbon nanomaterials. The irreversible nature of this reaction prevents the "self-healing" of defects, and a detailed knowledge of its mechanism is therefore essential to enable the growth of extended ordered structures. However, the dynamics of the Ullmann polymerization remain largely unexplored, as coupling events occur on a timescale faster than conventional scanning probe microscopy imaging frequencies. Here, we reveal the dynamics of these surface events using high-speed variable-temperature scanning tunneling microscopy (STM) (10 frames per second). Performing the measurements at the onset reaction temperatures provides an unprecedented description of the evolution of organometallic (OM) and covalent surface species during the Ullmann polymerization of para-dibromobenzene on Cu(110). Our results demonstrate the existence of an intermediate OM phase with Cu adatoms that inhibits the polymerization. These observations now complete the picture of the pathways of on-surface Ullmann polymerization, which includes the complex interplay of the phenylene moieties and metal atoms. Our work demonstrates the unique capability of high-speed STM to capture the dynamics of molecular self-assembly and coupling.

Diffusion of O Atoms on a CO-Covered Ru(0001) Surface─A Combined High-Speed Scanning Tunneling Microscopy and Density Functional Theory Study at an Enhanced CO Coverage

The diffusion of adsorbed O atoms on a CO-covered Ru(0001) surface has recently been explained by a “door-opening” mechanism which is driven by fluctuations in the CO layer. Here, we analyze how this mechanism changes at a higher CO coverage than the 0.33 monolayers applied in the previous study and, therefore, lower concentrations of empty sites. High-speed scanning tunneling microscopy was used to track the O atoms. A statistical model based on nearest-neighbor jumps and uniform hopping probabilities into the six equivalent directions of the Ru(0001) lattice describes the data well. Surprisingly, the hopping rates, which involve site exchanges with CO molecules, are higher than the site exchanges at 0.33 monolayers. Density functional theory calculations were performed to explain these observations. The configurations of the CO molecules around the adsorbed O atoms obtained at the higher CO coverage are disordered. Displacements of CO molecules cost less energy than displacements in the ordered (3×3)R30° structure of the previous study. In addition, the activation energies for the jumps of the O atoms are lowered by enhanced O/CO repulsions. The atomic processes are thus faster at the higher coverage, but the general features of the door-opening mechanism remain valid.

A multiscale wavelet algorithm for atom tracking in STM movies

High-speed scanning tunneling microscopy (STM) data have become available that provide movies of time-dependent surface processes. To track adsorbed atoms and molecules in such data automatic routines are required. We introduce a multiresolution wavelet particle detection algorithm for this purpose. To identify the particles, the images are decomposed by means of a discrete wavelet transform into wavelet planes of different resolutions. An ‘à trous’ low-pass filter is applied. The coefficients from the wavelet planes are filtered to remove noise. Wavelet planes with significant coefficients from the particles are multiplied, and the product is transformed into a binary particle mask. The precision of the method is tested with data sets of adsorbed CO molecules and O atoms on a Ru(0001) surface. The algorithm can safely detect and localize these particles with high precision, even in the presence of the enhanced noise characteristic for high-speed, constant-height STM data. By linking the particle positions, we obtain extended trajectories with a resolution of ∼0.5 Å or better allowing us to investigate the detailed motion of single atoms on a surface.

Resolving atomic diffusion in Ru(0001)−O(2×2) with spiral high-speed scanning tunneling microscopy

An intermediate state in atomic diffusion processes in the $\mathrm{O}(2\ifmmode\times\else\texttimes\fi{}2)$ layer on Ru(0001) is resolved with spiral high-speed scanning tunneling microscopy (STM). The diffusion of atomic oxygen in the adlayer has been studied by density functional theory and STM. Transition state theory proposes a migration pathway for the diffusion in the oxygen adlayer. With spiral scan geometries---a new approach to high-speed STM---the oxygen vacancy mobility on the highly covered Ru(0001) surface is determined to be in the range of 0.1 to 1 Hz. Experimental evidence for the intermediate state along the oxygen diffusion pathway is provided in real space and real time.

Dynamics in the O(2 × 1) adlayer on Ru(0001): bridging timescales from milliseconds to minutes by scanning tunneling microscopy.

The dynamics within an O(2 × 1) adlayer on Ru(0001) is studied by density functional theory and high-speed scanning tunneling microscopy. Transition state theory proposes dynamic oxygen species in the reduced O(2 × 1) layer at room temperature. Collective diffusion processes can result in structural reorientations of characteristic stripe patterns. Spiral high-speed scanning tunneling microscopy measurements reveal this reorientation as a function of time in real space. Measurements, ranging over several minutes with constantly high frame rates of 20 Hz resolved the gradual reorientation. Moreover, reversible fast flipping events of stripe patterns are observed. These measurements relate the observations of long-term atomic rearrangements and their underlying fast processes captured within several tens of milliseconds.

Flexure Structural Scanner of Tip Scan Type for High-speed Scanning Tunneling Microscopy

Flexure Structural Scanner of Tip Scan Type for High-speed Scanning Tunneling Microscopy

Atoms on a surface quickly slip through crowds

High-speed scanning tunneling microscopy reveals recently identified atomic movements that hasten diffusion.

A path through a crowd

Surface Science Catalytic reactions on surfaces occur at pressures at which the surfaces are completely covered with adsorbed molecules. It would seem that this arrangement would interfere with reactants encountering one another through diffusion processes. Henβ et al. used high-speed scanning tunneling microscopy to follow the diffusion of oxygen atoms on a rubidium surface that was fully covered with carbon monoxide (CO) molecules (see the Perspective by Magnussen). Oxygen-atom diffusion was unexpectedly fast. A theoretical model revealed that CO diffusion appears to open pathways for oxygen-atom movement. Science , this issue p. [715][1]; see also p. [695][2] [1]: /lookup/doi/10.1126/science.aav4143 [2]: /lookup/doi/10.1126/science.aaw4900

Density fluctuations as door-opener for diffusion on crowded surfaces.

How particles can move on a catalyst surface that, under the conditions of an industrial process, is highly covered by adsorbates and where most adsorption sites are occupied has remained an open question. We have studied the diffusion of O atoms on a fully CO-covered Ru(0001) surface by means of high-speed/variable-temperature scanning tunneling microscopy combined with density functional theory calculations. Atomically resolved trajectories show a surprisingly fast diffusion of the O atoms, almost as fast as on the clean surface. This finding can be explained by a "door-opening" mechanism in which local density fluctuations in the CO layer intermittently create diffusion pathways on which the O atoms can move with low activation energy.

In Situ Video-STM Study of Adlayer Structure and Surface Dynamics at the Ionic Liquid/Au (111) Interface

The molecular arrangement and surface dynamics of Au (111) electrodes in 1-hexyl-3-methylimidazolium chloride ([HMIm][Cl]) and 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([HMIm][TFSA]) were investigated by in situ high-speed scanning tunneling microscopy (video-STM). These measurements provide direct insights into the potential-dependent adlayer structure of adsorbed [HMIm]+ cations and Cl– anions on the Au (111) surface. At low electrode charge densities highly dynamic structures with only local order are observed. After changing the potential in the negative direction, stripe-like adlayers structures are observed, which are attributed to [HMIm]+ in a planar adsorption geometry. These can be described by simple commensurate (√3 × 3) and a (√3 × √13) structures in the presence of Cl– and [TFSA]− counterions, respectively, indicating that the in-plane arrangement of the cations is affected by epitaxial effects and coadsorbed anions. Upon a further decrease in potential, a transition to a...

Potential and Adsorbate Effects in Electrodeposition: Lessons Learned from Atomic-Scale in Situ and Operando Studies

Electrodeposition has become an important tool in nanoscience and -technology. Examples range from metallization processes in the microelectronics industry to self-organized growth of nanomaterials. Because the growth behavior and the resulting deposit morphology can be varied over a wide range by the electrochemical parameters and by suitably chosen additives, electrodeposition often provides superior control as compared to rivaling methods or even enables deposition of structures which are unobtainable by other means. On the other hand, precisely the strong dependence on potential and solution composition poses a problem as they are largely not understood. Development of electrodeposition or electroless plating processes therefore is still largely empirical, specifically for the deposition of structures with well-defined nanoscale morphology. For knowledge-based strategies better understanding of the atomic–scale processes is required. In particular, it is necessary to clarify how the potential-dependent interface structure and the presence of surface-active species influence processes such as diffusion on the planar surface and across steps, nucleation events, and the ion transfer itself. Such data can be obtained by state-of-the-art surface-sensitive methods, which enable in situ studies of the interface structure and dynamics or even operando investigations at growth rates approaching those in technological processes. Together with ab initio theory calculations they provide first insights into the complex effects of potential and coadsorbates on the dynamics of species at the surface of the deposit during the growth (e.g. metal adatoms). I will specifically present studies of Cu and Au single crystal electrodes by synchrotron-based X-ray scattering techniques and high-speed scanning tunneling microscopy, focusing on the influence of halides on surface dynamics and metal growth. The obtained results reveal very pronounced and partially unexpected effects, which can lead to crossover between qualitatively different growth regimes. The relevance of these observations for electroplating, prospects for operando studies of technologically interesting cases, as well as remaining challenges and future directions will be discussed.

Potential-Dependent Adlayer Structure and Dynamics at the Ionic Liquid/Au(111) Interface: A Molecular-Scale In Situ Video-STM Study.

Room-temperature ionic liquids are of great current interest for electrochemical applications in material and energy science. Essential for understanding the electrochemical reactivity of these systems are detailed data on the structure and dynamics of the interfaces between these compounds and metal electrodes, which distinctly differ from those in traditional electrolytes. In situ studies are presented of Au(111) electrodes in 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([BMP][TFSA]) by high-speed scanning tunneling microscopy (video-STM). [BMP][TFSA] is one of the best-understood air and water stable ionic liquids. The measurements provide direct insights into the potential-dependent molecular arrangement and surface dynamics of adsorbed [BMP](+) cations in the innermost layer on the negatively charged Au electrode surface. In particular, two distinct subsequent transitions in the adlayer structure and lateral mobility are observed with decreasing potential.

In situ video-scanning tunneling microscopy studies of the structure and dynamics of Cl adlayers on Au(1 0 0) electrodes

The chloride adlayer on Au(100) electrodes in 0.01M HCl solution was studied on the atomic scale by in situ high-speed scanning tunneling microscopy. In the potential regime 0–0.4VAg/AgCl a hitherto not reported defective c(2×2) structure was observed. In addition to characteristic straight domain boundaries oriented along the [010] and [011] directions of the Au substrate, unusual chain-like adsorbate structures were found. These defects can exhibit open structures with locally lower coverage as well as more close-packed arrangements, in which the adsorbates deviate from the lattice sites and have a 15% reduced nearest neighbor spacing. Dynamic fluctuations within the adlayer occur on the time scale of seconds, much slower than in Cl adlayers on Cu(100). The increase in defect density observed toward more positive potentials suggests a change of the adsorbate–adsorbate interactions with potential.

Surface Dynamics of Lead Adsorbates at the Cu(100)–Electrolyte Interface

Direct quantitative studies of the dynamic behavior of individual cationic adsorbates at an electrochemical interface have been performed for Pb on Cu(100) electrode surfaces in Cl-containing electrolyte. Using in situ high-speed scanning tunneling microscopy, the motion of the Pb adsorbates on the c(2 × 2)-Cl covered Cu surface was monitored and analyzed by statistical models. Contrary to Pb on Cu(100) under ultrahigh vacuum conditions, the tracer diffusion of isolated adsorbates can be described by simple activated hopping between neighboring sites within the c(2 × 2) lattice, with an activation energy that increases linearly by 0.5 eV/V with potential. This surprising behavior, which is opposite in sign to that expected for a cation and similar to that for adsorbed sulfide at this interface, suggests a decisive influence of the surrounding Cl coadsorbates on the Pb diffusion. Furthermore, attractive interactions of ∼20 meV were found between Pb adsorbates on nearest and next-nearest neighbor sites of the c(2 × 2) lattice. The effective diffusion barriers are increased in the vicinity of neighboring Pb, resulting in a mutual stabilization of the adsorbates.

In Situ Video‐STM Studies of Adsorbate Dynamics at Electrochemical Interfaces

The dynamic behavior of individual adsorbates at electrochemical interfaces was studied directly by in situ high-speed scanning tunneling microscopy, using sulfur adsorbed on Cu(100) electrodes in 0.01 M HCl solution as an example. By dosing from diluted Na(2)S solutions S(ad) coverages of a few percent can be prepared, with the sulfur adsorbates occupying positions within the c(2x2) lattice of coadsorbed chloride. S(ad) tracer diffusion occurs via hopping between neighboring c(2x2) lattice sites at considerably higher rates than those of sulfur on Cu(100) under UHV conditions, indicating a pronounced influence of the electrochemical environment on the adsorbate surface dynamics. The diffusion barrier linearly increases by 0.5 eV per V with potential and is strongly affected by neighboring S(ad) and surface defects. The S(ad)-S(ad) interactions extend over approximately 7 A. They are repulsive between nearest-neighbor and attractive between next-nearest-neighbor sites, respectively, and result in significantly reduced diffusion barriers. S(ad) on the upper terrace side of steps are transiently trapped and exhibit lower diffusion rates, leading to the formation of small metastable p(2x2) domains. Attractive interactions between S(ad) and domain boundaries in the c(2x2) adlayer result in boundary pinning as well as transient trapping and enhanced diffusion of S(ad) along the boundary.

Two-Dimensional Roughening of Adsorbate Islands in Thermodynamic Equilibrium

Equilibrium fluctuations of islands of adsorbed O atoms on Ru(0001) were investigated by scanning tunneling microscopy (STM), density functional theory calculations (DFT) and Monte Carlo (MC) simulations. Very ramified (2 x 2)-O islands were observed by high-speed STM that point to complex interactions between the O atoms. The DFT calculations show that, in addition to pairwise attractive interactions between third-nearest neighbors, a repulsive three-body interaction exists between these. MC simulations that include three-body interactions reproduce the observed ordering behavior.

Video STM Studies of Adsorbate Diffusion at Electrochemical Interfaces

Direct in situ studies of the surface diffusion of isolated adsorbates at an electrochemical interface by high-speed scanning tunneling microscopy (video STM) are presented for sulfide adsorbates on Cu(100) in HCl solution. As revealed by a quantitative statistical analysis, the adsorbate motion can be described by thermally activated hopping between neighboring adsorption sites with an activation energy that increases linearly with electrode potential by 0.50 eV per V. This can be explained by changes in the adsorbate dipole moment during the hopping process and contributions from coadsorbates.

In-situ atomic-scale studies of the mechanisms and dynamics of metal dissolution by high-speed STM

The fundamental atomic-scale processes during active metal dissolution—the local removal/addition of atoms at atomic kinks in the steps on the crystal surface—were directly studied by novel instrumental methods based on in-situ scanning tunneling microscopy (STM), using the dissolution of Cu(100) in 0.01 M HCl solution as an example. Direct observation of these rapid dynamic processes with a novel high-speed electrochemical STM (Video-STM), capable of acquiring up to 25 atomic resolution images per second, reveals that metal dissolution proceeds at a single type of structurally well-defined kinks. The reactivity of these kinks is tentatively explained by the coordination of Cu surface atoms and Cl adsorbates at these sites. The kinks nucleate predominantly at outer terraces corners and can condense into larger facets, resulting in an apparent ‘collective’ local dissolution or growth of terraces. Quantitative data on the kink dynamics were obtained by an alternative approach (TOW-STM) and show pronounced local dissolution/redeposition fluctuations at the individual kinks even at the onset of Cu dissolution with average kink propagation and reaction rates at kinks in the range 10 3 and 10 5 atoms s −1, respectively.

Identification of Alcohol Dehydration Sites on an Oxide Surface by Scanning Tunneling Microscopy

High-speed scanning tunneling microscopy (STM), temperature-programmed desorption (TPD), Auger electron spectroscopy and low-energy electron diffraction (LEED) were used to characterize the (001) surface of monoclinic γ-WO3, and to determine the structure and reactivity of active sites on this surface. Cleavage along the WO3(001) plane followed by heating in O2 to remove carbonaceous impurities and subsequent reduction above 775 K in ∼10-5 Torr of O2 resulted in a c(2 × 2) reconstruction that is visible in LEED and atomic resolution STM images. The catalytic activity of the c(2 × 2) surface was probed through exposure to a series of alcohols. No difference in the sticking coefficient for 1-propanol, 2-propanol and 2-methyl-2-propanol (tert-butyl alcohol) was detectable. As the surface temperature was increased during TPD, desorption of unreacted alcohol and water was seen at temperatures less than 600 K, independent of the alcohol. The water desorption is attributed to deprotonation of the adsorbed alcohols to form alkoxides; at this stage STM images show terraces covered with adsorbates with no preference for the adsorbates to occupy step or other defect sites. At higher temperatures the alkoxide all desorbs as alkenes: no dehydrogenation products were observed, indicating that the c(2 × 2) surface displays only dehydration activity under these conditions. The alkene desorption peak temperature decreases from primary to tertiary alcohol (1-propanol → tert-butyl alcohol), indicating that desorption is limited by the rate of C−O bond scission of the adsorbed alkoxide. STM images are presented in which the alkoxide intermediates are resolved simultaneously with atomic resolution of the WO3(001)-c(2 × 2) substrate. These demonstrate that the sites for oxidative dehydration of the alcohol molecules are the exposed 5-fold coordinated W6+ ions. Further, it is demonstrated that the alkoxide can be removed from the surface with the STM tip to reveal the structure of the underlying adsorption sites.