Time-resolved Scanning Tunneling Microscopy Research Articles

Nonlinear femtosecond laser induced scanning tunneling microscopy

We demonstrate ultrafast laser driven nonlinear scanning tunneling microscopy (STM), under ambient conditions. The design is an adaptation of the recently introduced cross-polarized double beat method, whereby z-polarized phase modulated fields are tightly focused at a tunneling junction consisting of a sharp tungsten tip and an optically transparent gold film as substrate. We demonstrate the prerequisites for ultrafast time-resolved STM through an operative mechanism of nonlinear laser field-driven tunneling. The spatial resolution of the nonlinear laser driven STM is determined by the local field intensity. Resolution of 0.3 nm-10 nm is demonstrated for the intensity dependent, exponential tunneling range. The demonstration is carried out on a junction consisting of tungsten tip and gold substrate. Nano-structured gold is used for imaging purposes, to highlight junction plasmon controlled tunneling in the conductivity limit.

Decoupling of the copper core in a single copperphthalocyanine molecule

Here, we show how a copper atom in a copperphthalocyanine (CuPc) molecule can be decoupled from its environment. This is realized by trapping the CuPc molecule between two adjacent nanowires that are 1.6 nm apart. Using low-temperature scanning tunnelling microscopy and spectroscopy, the structural and electronic properties of CuPc in the stable "molecular bridge" configuration have been studied. Constant current and differential conductivity maps are recorded to reveal the spatial variation of the electronic structure of the cores and the lobes of CuPc molecules. The core of CuPc molecule is dim at low voltages, but suddenly becomes bright at a voltage of 5 V. Time-resolved scanning tunnelling microscopy measurements show that some of the CuPc lobes are very stable, while other lobes are very dynamic.

A Window on Surface Explosions: Tartaric Acid on Cu(110)

Autocatalytic reaction mechanisms are observed in a range of important chemical processes including catalysis, radical-mediated explosions, and biosynthesis. Because of their complexity, the microscopic details of autocatalytic reaction mechanisms have been difficult to study on surfaces and heterogeneous catalysts. Autocatalytic decomposition reactions of S,S- and R,R-tartaric acid (TA) adsorbed on Cu(110) offer molecular-level insight into aspects of these processes, which until now, were largely a matter of speculation. The decomposition of TA/Cu(110) is initiated by a slow, irreversible process that forms vacancies in the adsorbed TA layer, followed by a vacancy-mediated, explosive decomposition process that yields CO2 and small hydrocarbon products. Initiation of the explosive decomposition of TA/Cu(110) has been studied by measurement of the reaction kinetics, time-resolved low energy electron diffraction (LEED), and time-resolved scanning tunneling microscopy (STM). Initiation results in a decrease...

Dynamics of Decanethiol Self-Assembled Monolayers on Au(111) Studied by Time-Resolved Scanning Tunneling Microscopy

We investigated the dynamics of decanethiol self-assembled monolayers on Au(111) surfaces using time-resolved scanning tunneling microscopy at room temperature. The expected ordered phases (β, δ, χ*, and φ) and a disordered phase (ε) were observed. Current-time traces with the feedback loop disabled were recorded at different locations on the surface. The sulfur end group of the decanethiolate molecule exhibits a stochastic two-level switching process when the molecule is adsorbed in a (local) β phase registry. This two-level process is attributed to the diffusion of the Au-thiolate complex between two adjacent adsorption sites. The irregular current jumps in the current-time traces recorded on the tails of decanethiolate molecules in the ordered β, δ, and χ* phases are ascribed to wagging of the alkyl tails. Finally, the disordered phase is characterized by even larger current jumps, which indicates that the tail of the decanethiolate flips up occasionally and makes contact with the tip. Our experiments reveal that the massive dynamics of the self-assembled monolayer is due to diffusion of decanethiol-Au complexes, rather than the diffusion of decanethiolate molecules.

Cobalt induced nanocrystals on Ge(001)

The deposition of several monolayers of cobalt on germanium (001) substrates results in the formation of two types of clusters: flat-topped and peaked nanocrystals. Scanning tunneling spectroscopy and helium ion microscopy measurements reveal that these nanocrystals contain cobalt. The shape evolution of the flat-topped and peaked nanocrystals as a function of their size is investigated with scanning tunneling microscopy. For small sizes the nanocrystals are compact. Beyond a critical size, however, the peaked nanocrystals exhibit an elongated shape, whilst the flat-topped nanocrystals remain compact. The shape transition of the peaked nanocrystals is driven by a competition between boundary and strain energies. For small sizes the boundary energy is the dominant term leading to a minimization of the peaked nanocrystal's perimeter, whereas at larger sizes the strain energy wins resulting in a maximization of the perimeter. On the top facet of the flat-topped nanocrystals one-dimensional structures are observed that are comprised of small square shaped units of about 1nm2. Time-resolved scanning tunneling microscopy measurements reveal that these square shaped units are dynamic at room temperature.

Nanoscale probing of transient carrier dynamics modulated in a GaAs–PIN junction by laser-combined scanning tunneling microscopy

The modulation of carrier dynamics in a GaAs-PIN junction after photoexcitation by an ultrashort-pulse laser was probed by shaken-pulse-pair-excited scanning tunneling microscopy (SPPX-STM), which enables nanoscale mapping of time-resolved STM images. The effect of the built-in potential on the carrier dynamics, diffusion and drift, which cannot be probed by the optical pump-probe technique, was successfully visualized in real space.

Time-resolved scanning tunnelling microscopy for molecular science

Time-resolved scanning tunnelling microscopy (STM) and its application in molecularscience are reviewed. STM can image individual atoms and molecules and thus is able toobserve the results of molecular processes such as diffusion, desorption, configurationswitching, bond-breaking and chemistry, on the atomic scale. This review will introducetime-resolved STM, its experimental limitations and implementations with particularemphasis on thermally activated and tunnelling current induced molecular processes.It will briefly examine the push towards ultrafast imaging. In general, resultsachieved by time-resolved STM demonstrate the necessity of both space andtime resolution for fully characterizing molecular processes on the atomic scale.

Study of dynamic processes on semiconductor surfaces using time-resolved scanningtunneling microscopy

The time resolution of a conventional scanning tunneling microscope can be improved bymany orders of magnitude by recording open feedback loop current–time traces.The enhanced time resolution comes, however, at the expense of the ability toobtain spatial information. In this paper, we first consider the Ge(111)-c(2 × 8) surface as an example of how surface dynamics can show up in conventional STM images.After a brief introduction to the time-resolved scanning tunneling microscopy technique, itscapabilities will be demonstrated by addressing the dynamics of a dimer pair of a Ptmodified Ge(001).

Laser-combined scanning tunnelling microscopy for probing ultrafast transientdynamics

The development of time-resolved scanning tunnelling microscopy (STM), in particular,attempts to combine STM with ultrafast laser technology, is reviewed with emphasis onobserved physical quantities and spatiotemporal resolution. Ultrashort optical pulsetechnology has allowed us to observe transient phenomena in the femtosecondrange, which, however, has the drawback of a relatively low spatial resolutiondue to the electromagnetic wavelength used. In contrast, STM and its relatedtechniques, although the time resolution is limited by the circuit bandwidth (∼100 kHz), enable us to observe structures at the atomic level in real space. Our purpose hasbeen to combine these two techniques to achieve a new technology that satisfies therequirements for exploring the ultrafast transient dynamics of the local quantumfunctions in organized small structures, which will advance the pursuit of futurenanoscale scientific research in terms of the ultimate temporal and spatial resolutions.

Oxygen adsorption-induced nanostructures and island formation on Cu{100}: Bridging the gap between the formation of surface confined oxygen chemisorption layer and oxide formation

Surface oxidation of Cu(100) has been investigated by variable temperature scanning tunneling microscopy and quantitative x-ray photoelectron spectroscopy as a function of O(2) pressure (8.0x10(-7) and 3.7x10(-2) mbar) at 373 K. Three distinct phases in the initial oxidation of Cu(100) have been observed: (1) the formation of the mixed oxygen chemisorption layer consisting of Cu(100)-c(2x2)-O and Cu(100)-(2sqrt[2]xsqrt[2])R45 degrees -O domains, (2) the growth of well-ordered (2sqrt[2]xsqrt[2])R45 degrees-O islands, and (3) the onset of subsurface oxide formation leading to the growth of disordered Cu(2)O. We demonstrate that the (2sqrt[2]xsqrt[2])R45 degrees-O reconstruction is relatively inert in the low pressure regime. The nucleation and growth of well-ordered two-dimensional Cu-O islands between two (2sqrt[2]xsqrt[2])R45 degrees-O domains is revealed by time-resolved scanning tunneling microscopy experiments up to 0.5 ML of oxygen. The formation of these islands and their nanostructure appear to be critical to the onset of further migration of oxygen atoms deeper into copper and subsequent Cu(2)O formation in the high pressure regime. The reactivity of each phase is correlated with the surface morphology and the role of the various island structures in the oxide growth is discussed.

Electronic properties and electron dynamics of theSi(001)(2×1)surface with C-defects

Both the time-resolved two-photon photoelectron spectroscopy and scanning tunneling microscopy are utilized for investigating the role of the C-defects in the electronic property and dynamics of the $p$-type $\mathrm{Si}(001)(2\ifmmode\times\else\texttimes\fi{}1)$ surface. It is found that the C-defect causes Fermi-level pinning when its density is more than 0.05 monolayer. The change in the inverse of the product of the bulk-to-surface electron transition rate $S$ and in the extinction lifetime $\ensuremath{\tau}$ of the surface excited electron is examined as a function of the C-defect density, and it was found that the change doubles when the C-defect density increases from 0 to 0.1 monolayer on the surface. This indicates that the C-defect plays an important (but limited) role in the surface carrier dynamics of Si(001).

Restructuring of Cobalt Nanoparticles Induced by Formation and Diffusion of Monodisperse Metal-Sulfur Complexes

Time-resolved scanning tunneling microscopy (STM) is used to investigate a massive sulfur-induced transformation of a homogeneous array of approximately 2 nm Co nanoparticles into a new cobalt sulfide phase. The underlying atomistic mass-transport process is revealed and, surprisingly, found to be mediated exclusively by the formation and detachment of monosized Co3S4 complexes at the perimeter of the Co nanoparticles. The process is followed by fast diffusion, agglomeration of the complexes, and subsequent crystallization into a cobalt sulfide phase.

What can We Probe using Femtosecond Time-Resolved STM?

Due to the size reduction in structures, the difference in the electronic properties, for example, caused by the structural nonuniformity in each element, has an ever more crucial influence on macroscopic functions of semiconductor devices. And the direct observation of the characteristics, which provides us with the basis for the macroscopic analysis of the results, is of great importance. Thus, for further advances, a method for exploring the transient dynamics of the local quantum functions in organized small structures is eagerly desired. However, it is extremely difficult to obtain spatial and temporal resolutions simultaneously on this scale, which requires a new method; namely, new microscopy. In this paper, we introduce the shaken-pulse-pair-excited scanning tunneling microscopy (SPPX-STM), which we have developed these years. SPPX-STM enables us to observe the dynamics of electronic structures with the ultimate spatial and temporal resolutions.

Adsorption and Dynamics of Long‐Range Interacting Fullerenes in a Flexible, Two‐Dimensional, Nanoporous Porphyrin Network

Herein, a detailed investigation of the adsorption and dynamics of C60 and C70 fullerenes hosted in a self-assembled, two-dimensional, nanoporous porphyrin network on a solid Ag surface is presented. Time-resolved scanning tunneling microscopy (STM) studies of these supramolecular systems at the molecular scale reveal distinct host-guest interactions giving rise to a pronounced dissimilar mobility of the two fullerenes within the porphyrin network. Furthermore, long-range coverage-dependent interactions between the all-carbon guests, which clearly affect their mobility and are likely mediated by a complex mechanism involving the Ag substrate and the flexible porphyrin host network, are observed. At increased fullerene coverage, this unprecedented interplay results in the formation of large fullerene chains and islands. By applying a lattice gas model with nearest-neighbor interactions and by evaluating the fullerene-pair distribution functions, the respective coverage-dependent guest-guest interaction energies are estimated.

Self-assembly via adsorbate-driven dislocation reactions.

Deposition of S onto a monolayer of Ag/Ru(0001) transforms the herringbone pattern of the clean Ag film into a strikingly regular array of 2D-vacancy islands [Nature (London) 397, 238 (1999)]]. Time-resolved scanning tunneling microscopy reveals that this nanometer-scale restructuring occurs by a cooperative mechanism involving the sequential formation of triangular regions with fcc and hcp stacking. Using a 2D Frenkel-Kontorova model, we can simulate the creation of these triangular building blocks via basic dislocation motions and reactions.

Electron transfer-induced dynamics of oxygen molecules on the TiO2(110) surface.

Diffusion of oxygen molecules on transition metal oxide surfaces plays a vital role for the understanding of catalysis and photocatalysis on these materials. By means of time-resolved scanning tunneling microscopy, we provide evidence for a charge transfer-induced diffusion mechanism for O2 molecules adsorbed on a rutile TiO2(110) surface. The O2 hopping rate depended on the number of surface donors (oxygen vacancies), which determines the density of conduction band electrons. These results may have implications for the understanding of oxidation processes on metal oxides in general.

Development of femtosecond time-resolved scanning tunneling microscopy

Development of femtosecond time-resolved scanning tunneling microscopy

Diffusive atom transport along step edges on Ag(1 1 1) at 295 K

The entropy-driven relaxation of a unique, non-equilibrium step edge configuration on the Ag(1 1 1) surface was observed using time-resolved STM imaging at room temperature. Using the Gibbs–Thomson relation, the relaxation process is quantitatively described as diffusive mass transport in terms of a gradient in the chemical potential along the monoatomic step edge. The STM data directly show that mass transport on Ag(1 1 1) is dominated by step edge diffusion at 295 K, and allow an estimate of the corresponding effective energy barrier. We obtain E eff=0.49±0.05 eV and compare this value with recent results on island diffusion studies.

Development of Time-Resolved Scanning Tunneling Microscopy in Femtosecond Range

A promising design for achieving the ultimate spatial and temporal resolution is proposed. With modulation of the time delay between two femtosecond laser pulses repeatedly illuminating a scanning tunneling microscope junction, the induced component of the tunneling current is detected as a function of the delay time. With this method, a high signal-to-noise ratio, stable measurement free from fluctuation of laser intensity, and removal of the thermal expansion effect can be achieved. Using the photovoltage effect of GaAs, the high performance of the newly developed system is demonstrated in the femtosecond range.

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.