Time-resolved Scanning Tunneling Microscopy Research Articles

Photoassisted scanning tunneling microscopy

The combination of scanning tunneling microscopy (STM) with optical excitation adds new information to STM. A review is presented covering the work done on light-induced effects in STM during the past 15 years. Effects discussed include thermal effects, nonlinear effects, field enhancement at the STM tip, various effects on semiconductor surfaces, excitation of surface plasmons, detection of photoelectrons, spin-polarized tunneling, as well as light-induced nanomodifications, local optical spectroscopy, the use of ultrashort laser pulses for time-resolved STM, and the combination of STM and scanning near-field optical microscopy.

Effect of local environment on nanoscale dynamics at electrochemical interfaces: anisotropic growth and dissolution in the presence of a step providing evidence for a schwoebel-ehrlich barrier at solid/liquid interfaces.

The dynamics of nanoscale islands in the vicinity of a monoatomic step on a metal electrode surface under potential control have been investigated by potential pulse perturbation time-resolved scanning tunneling microscopy (P3 TR-STM). As the metastable islands decay, a clear anisotropy develops in the spatial distribution of islands, relative to the monoatomic step. Islands appear to be stabilized above the step, while a denuded region develops below the step over a distance of about 100 nm. Vacancy islands are observed to be more stable than adatom islands. These results provide evidence for an electrochemical Schwoebel-Ehrlich barrier to diffusion of atoms across the step.

Oxygen driven reconstruction dynamics of Ni(977) measured by time-lapse scanning tunneling microscopy

Time-lapse scanning tunneling microscopy (STM) has been used to observe the oxygen induced reconstruction behavior of Ni(977), a stepped metallic surface. Previous studies using helium atom diffraction resolved the macroscopic kinetics for the reversible step-doubling and -singling of this vicinal surface. Sequential STM imaging recorded at elevated temperature has now elucidated atomic-level mechanistic details for the merging of steps in the presence of small amounts of adsorbed oxygen, less than 2% of a monolayer. Point contact between neighboring steps decorated with chemisorbed oxygen facilitates rapid step coalescence by means of zippering. An optimal oxygen concentration of step edge saturation was found to enable the step merging to proceed most rapidly. Excess oxygen was found to hinder the coalescence of neighboring steps through the possible growth of overlayer structures on the terraces. At sufficiently high temperatures, the surface is driven back to single steps due to oxygen dissolution. The departure of oxygen from the surface through dissolution, as well as the associated presence of oxygen in the selvedge region, may both play a role in destabilizing the double steps. Local step density influences the coalescence behavior by defining the number of available step edge sites. The microscopic details made available by time-resolved STM imaging illuminate some of the mechanistic steps related to the initial stages of metallic oxidation, and the sensitivity of surface morphological transformations to local surface structure and adsorbate coverage.

Dynamics of Metastable Nanoscale Island Growth and Dissolution at Electrochemical Interfaces by Time-Resolved Scanning Tunneling Microscopy

The dynamics of nanoscale island growth, stability, and dissolution, accompanying the potential-induced phase transitions between the (22 × 3) and (1 × 1) structures of the Au(111) surface in 0.1 M HClO4 solution, have been investigated by potential pulse perturbation time-resolved scanning tunneling microscopy (P3 TR-STM). Starting from a potential at which the reconstructed (22 × 3) phase is stable, a short positive potential pulse briefly brings the electrode to a potential at which the (1 × 1) phase is stable. This pulse induces a perturbation that lifts the Au(111)−(22 × 3) reconstruction completely, resulting in nanoscale island formation on the surface. The nanoscale islands are metastable and dissolve with time. The initial average island area and the island decay rate are related to the pulse amplitude and duration. The higher and longer the pulse, the smaller the average size of the islands produced and the more slowly the islands decay. This result reveals a “voltammetric annealing” process tha...

Thermal vibrations of a two-dimensional vacancy island crystal in a strained metal film

Vacancy islands formed during room-temperature exposure to sulfur of a submonolayer Ag film on Ru(0001) order spontaneously to form a triangular lattice at an island area fraction just above 20%. A normal mode decomposition of the thermal vibrations of this vacancy island crystal, measured with time-resolved scanning tunneling microscopy, obtains Lamé coefficients in the range of 10 8 N m −2. We show that these estimates are consistent with stabilizing forces derived from long-range elastic interactions between the islands.

Identifying the forces responsible for self-organization of nanostructures at crystal surfaces

The spontaneous formation of organized surface structures at nanometre scales1,2 has the potential to augment or surpass standard materials patterning technologies. Many observations of self-organization of nanoscale clusters at surfaces have been reported1,2,3,4,5,6,7,8,9,10, but the fundamental mechanisms underlying such behaviour — and in particular, the nature of the forces leading to and stabilizing self-organization — are not well understood. The forces between the many-atom units in these structures, with characteristic dimensions of one to tens of nanometres, must extend far beyond the range of typical interatomic interactions. One commonly accepted source of such mesoscale forces is the stress field in the substrate around each unit1,11,12,13. This, however, has not been confirmed, nor have such interactions been measured directly. Here we identify and measure the ordering forces in a nearly perfect triangular lattice of nanometre-sized vacancy islands that forms when a single monolayer of silver on the ruthenium (0001) surface is exposed to sulphur at room temperature. By using time-resolved scanning tunnelling microscopy to monitor the thermal fluctuations of the centres of mass of the vacancy islands around their final positions in the self-organized lattice, we obtain the elastic constants of the lattice and show that the weak forces responsible for its stability can be quantified. Our results are consistent with general theories of strain-mediated interactions between surface defects in strained films.

Brownian Motion of Dislocations in Thin Films

The motion of edge dislocations in a single monolayer film of Cu on Ru(0001) was studied by time-resolved scanning tunneling microscopy. The dislocations were observed to make rapid 1D random walks in the film. This dislocation motion is attributed to the equilibrium thermal exchange of atoms between the solid film and the adatom gas covering the film. These results highlight a fundamental difference between the dynamics of dislocations in thin films and the bulk, which is in principle important in understanding the mechanical properties of thin films. {copyright} {ital 1997} {ital The American Physical Society}

The Influence of Hydrogen on CVD-Growth on Si(111) Surfaces

Time-resolved scanning tunneling microscopy has been used to study the influence of atomic hydrogen on the growth behavior in silicon homoepitaxy and the etch attack of atomic hydrogen onto the silicon surface above 500 °C. In CVD of silicon on Si(111) using disilane (Si2H6) as gas phase precursor the presence of hydrogen on the surface plays an essential role in all stages of growth. At temperatures above 400 °C, for each Si atom deposited on the surface one H atom is adsorbed additionally. Under the influence of hydrogen the density of islands increases during nucleation on the bare substrate. The nucleation behavior in CVD, when quantitatively analyzed, differs distinctly from the one during MBE-growth. As opposed to MBE, the density of nuclei as a function of temperature and growth rate cannot be understood in terms of classical rate equation based nucleation theories. Under the influence of a closed hydrogen layer the topmost silicon layer is largely immobilized and lateral island coarsening can only take place after hydrogen desorption. At a high flux of disilane the growth rate is limited by this hydrogen desorption. The surface, which then has a hydrogen-coverage close to saturation, roughens linearly with coverage and completely decays into facets of the 〈110〉-zone. When the flux of the precursor gas is interrupted the surface flattens with a time constant determined by the hydrogen desorption. Atomic hydrogen is able to remove silicon from the surface visible in STM as step retraction. Above 500 °C we found a linear increase of the removal rate with temperature.

Molecularly resolved observation of anisotropic intermolecular force in a formate-ion monolayer on a TiO2 (110) surface by scanning tunneling microscopy

The diffusion kinetics of adsorbed formate ions have been observed on a TiO 2 (110) surface by scanning tunneling microscopy (STM) operated under ultrahigh vacuum. Formate ions in a desired area are removed without damage to the substrate, by rastering the monolayer with a high-biased STM tip. A patch of uncovered substrate is thus created in a (2 × 1)-formate monolayer. Time-resolved STM observation reveals the transport kinetics of individual formate ions filling the void. The anisotropic rate of migration suggests that a row of Ti ions exposed on the substrate presents a one-dimensional channel for formate transport. A significant anisotropy in formate-formate interaction is further postulated from the kinetics; a repulsive force between formate ions neighboring on a Ti row drives them to migrate, whereas an attractive force between formates on adjacent Ti-rows plays a secondary role in regulating overlayer structure. The present study shows how STM images document the anisotropy in intermolecular force regulating an adsorbate overlayer on a solid surface, by taking advantage of the relaxation of a nanometer-sized structure fabricated in the overlayer.

Direct observation of strain relaxation in iron layers on W(110) by time-resolved STM

Time-resolved,in-situ-applied STM has been used to study the epitaxial growth of iron on W(110) at room temperature. By this way, sequences of STM images show directly the atomistics of the growth process on the surface. The first layer of iron on W(110) grows pseudomorphically without a preferred growth direction. Beginning with the second layer, the islands grow anisotropically with preferred growth in the [001]-direction. The generation of an ordered two-dimensional dislocation network starts at a coverage of 1.4 pseudomorphic monolayers to relax the misfit of 9.4%. A direct correlation of the creation of misfit dislocations in the second layer and the nucleation of the third-layer islands was found. Together with the onset of strain relaxation, the growth mode abruptly changes from layer-by-layer to statistical growth. A quantitative statistical analysis of the data allows to exactly determine the onset of relaxation, the vertical location of the dislocation lines, and the lateral extension of an island that is necessary to induce the formation of dislocations.

Direct observation of strain relaxation in iron layers on W(1 1 0) by time-resolved STM

Direct observation of strain relaxation in iron layers on W(1 1 0) by time-resolved STM

Time-resolved scanning tunneling microscopy through tunnel distance modulation

The ability to perform time-resolved measurements of fast transient signals with a scanning tunneling microscope has been achieved through direct control of the tunneling distance on short time scales. Modulation of the tip-sample separation is provided by the use of a magnetostrictive tip driven by a local magnetic field coil. The technique offers a broadly applicable means of obtaining fast dynamical information in tunneling microscopy, and is demonstrated here on nanosecond time scales.

Luminescence properties and surface topography of porous silicon

We report optical and structural properties of porous silicon by steady-state and time-resolved photoluminescence (PL) and scanning tunneling microscopy (STM). The PL decay is non-exponential and the lifetimes are strongly dependent on temperature. Our experimental data of the recombination lifetime under variation of temperature, emission energy, and excitation density are discussed within the models of tail-state recombination, direct interband, and quantum-confined exciton recombination. Our optical and structural results support the model of quantum-confined carriers in crystalline Si structures.

The use of time-resolved scanning tunneling microscopy for the determination of microscopic reaction rates

Utilisation de la microscopie electronique a balayage pour evaluer la vitesse de la reaction a la surface d'une electrode lors de la dissolution ou du depot de cuivre