Constant Current Topographies Research Articles

Controlling photocurrent channels in scanning tunneling microscopy

We investigate photocurrents driven by femtosecond laser excitation of a (sub)-nanometer tunnel junction in an ultrahigh vacuum low-temperature scanning tunneling microscope (STM). The optically driven charge transfer is revealed by tip retraction curves showing a current contribution for exceptionally large tip-sample distances, evidencing a strongly reduced effective barrier height for photoexcited electrons at higher energies. Our measurements demonstrate that the magnitude of the photo-induced electron transport can be controlled by the laser power as well as the applied bias voltage. In contrast, the decay constant of the photocurrent is only weakly affected by these parameters. Stable STM operation with photoelectrons is demonstrated by acquiring constant current topographies. An effective non-equilibrium electron distribution as a consequence of multiphoton absorption is deduced by the analysis of the photocurrent using a one-dimensional potential barrier model.

The butterfly - a well-defined constant-current topography pattern on Si(001):H and Ge(001):H resulting from current-induced defect fluctuations.

Dangling bond (DB) arrays on Si(001):H and Ge(001):H surfaces can be patterned with atomic precision and they exhibit complex and rich physics making them interesting from both technological and fundamental perspectives. But their complex behavior often makes scanning tunneling microscopy (STM) images difficult to interpret and simulate. Recently it was shown that low-temperature imaging of unoccupied states of an unpassivated dimer on Ge(001):H results in a symmetric butterfly-like STM pattern, despite the fact that the equilibrium dimer configuration is expected to be a bistable, buckled geometry. Here, based on a thorough characterization of the low-bias switching events on Ge(001):H, we propose a new imaging model featuring a dynamical two-state rate equation. On both Si(001):H and Ge(001):H, this model allows us to reproduce the features of the observed symmetric empty-state images which strongly corroborates the idea that the patterns arise due to fast switching events and provides an insight into the relationship between the tunneling current and switching rates. We envision that our new imaging model can be applied to simulate other bistable systems where fluctuations arise from transiently charged electronic states.

Acetone Adsorption on Oxidized and Reduced TiO2(110): A Scanning Tunneling Microscope Study

Acetone adsorption on oxidized and reduced rutile TiO2(110) surfaces was examined in a vacuum with a scanning tunneling microscope. A vacuum-annealed TiO2 surface was oxidized with O2 and then exposed to acetone vapor at room temperature. The acetone−oxygen complex proposed in early studies appeared in constant-current topography. We assigned the complex to η2-2,2-propanediolates by comparing its topography with that of coadsorbed acetate. When the vacuum-annealed surface was directly exposed to acetone, mobile η1-acetone was observed on Ti-atom rows.

A versatile high resolution scanning tunneling potentiometry implementation

We have developed a new scanning tunneling potentiometry technique which can-with only minor changes of the electronic setup-be easily added to any standard scanning tunneling microscope (STM). This extension can be combined with common STM techniques such as constant current imaging or scanning tunneling spectroscopy. It is capable of performing measurements of the electrochemical potential with microvolt resolution. Two examples demonstrate the versatile application. First of all, we have determined local variations of the electrochemical potential due to charge transport of biased samples down to angstrom length scales. Second, with tip and sample at different temperatures we investigated the locally varying thermovoltage occurring at the tunneling junction. Aside from its use in determining the chemical identity of substances at the sample surface our method provides a controlled way to eliminate the influence of laterally varying thermovoltages on low-bias constant current topographies.

Theory of local electronic properties and finite-size effects in nanoscale open chains

The local electronic structure of nanoscale chains is investigated theoretically. We propose a mechanism to explain the even-odd oscillation observed in the length distribution of atom chains. We study the spatial peak structure as obtained by scanning-tunneling-microscopy constant-current topography as a function of the electron-electron interaction, band filling, and temperature. The site-dependent magnetic moment is also examined.

Reconstruction of the local density of states in Ag(111) surfaces using scanning tunneling potentiometry

We present scanning tunneling microscopy data on clean Ag(111) surfaces in UHV. Standing-wave patterns of the surface state are investigated to study the influence of a thermal step across the tunnel junction on constant current topographies. When constant current topographies are measured at bias voltages below $10\phantom{\rule{0.3em}{0ex}}\mathrm{mV}$ with temperature differences of more than a few kelvins between tip and sample, the apparent corrugation is drastically increased. Constant current topographies at low bias voltages and finite temperature differences do not reveal the true local density of states of the surface. The apparent corrugation is caused by a spatial varying thermovoltage superimposed to the bias voltage while scanning the sample's local density of states. With the help of scanning tunneling potentiometry, the correct local density of states of the sample can be reconstructed.

Evolution of thin Cu films on GaAs(110) towards atomically flat epitaxial overlayers

The morphology of thin copper films on cleaved GaAs(110) surfaces, deposited at 80K with subsequent annealing, has been investigated at room temperature by means of scanning tunneling microscopy. Constant-current topographies, taken during 24 days, that illustrate different stages of the film evolution towards an atomically flat, epitaxial metal overlayer are presented here. We compare our results with low-temperature grown silver films on GaAs(110) and find similar morphologies as predicted by the model of “electronic growth.” In contrast to the surface diffusion of Cu on Cu(111) single crystals and the kinetics of similarly prepared silver films on GaAs(110), a strongly increased film stability has been observed, which we ascribe to a high tensile strain in the metal overlayer.

Single Cs adatoms at Si(001)2×1 surfaces: a local surface photovoltage imaging study

Local surface photovoltage (SPV) and current imaging, using a scanning tunneling microscope (STM), were utilized in order to elucidate the effect of a single adsorbed cesium atom on the local electronic structure of p- and n-type Si(001)2×1 surfaces, in its vicinity. The Cs adatom gives rise to a small depression at negative sample bias constant current topography (CCT) image. The opposite sides of two adjacent Si dimers appear as protrusions. SPV and current imaging show that the Fermi level is ‘softly’ pinned at the surface in the location of these protrusions, possibly because of a downshift and/or broadening of the π ∗ band and its screening of the Cs positive charge.

The lost symmetry of Ge(111)-c(2×8)

Using the scanning tunneling microscope in the constant current topography mode, we observe, for different bias voltages, that not only the two rest-atoms but also the two adatoms in the unit cell of the Ge(111)-c(2×8) reconstruction are not equivalent. Furthermore, earlier assumed mirror symmetry in the unit cell is lost in agreement with recent calculations [N. Takeuchi et al., Phy. Rev. Lett. 69, 648 (1992)]. The asymmetry is much more pronounced in the local density of electronic states than in the real shift of atomic coordinates. By increasing the tunneling current up to 40 nA, asymmetric features are strongly enhanced to the extent that observed protrusions can hardly be associated with atomic positions.

Tunneling characteristics at atomic resolution on close-packed metal surfaces

We have observed atomic resolution in constant-current topography along close-packed directions on Cu(111), Au(110) (1x2) and Ag(111) surfaces in UHV. Novel experimental information obtained from tunneling I-V and I-s characteristics observed under “atomic resolution” conditions is presented. Our results are discussed in the light of current theoretical models. We suggest that foreign material in the tip apex plays an important role in the imaging mechanism.

Real-time STM investigation of the initial stages of oxygen interaction with Si(100)2 × 1

Using a fast STM we have recorded constant-current topographies during oxygen adsorption on Si(100)2 X 1 at room temperature. At an oxygen pressure of 2 × 10 -8 mbar the development of the surface was followed up to an exposure of a few tens of langmuirs. Numerous atomic processes were identified. For example, bridges are formed between rows of dimers leading to an antiphase buckling in both rows over a length of 100 Å, complete dimers disappeared, protruding features on the scale of a 2 × 1 unit cell appeared atop and between dimers. During adsorption a clustering of the deep positions was obvious. The hitherto unknown complexity of adsorption processes is discussed in terms of the local atomic and electronic structure and of current atomic models of O/Si(100)2×1.

Atomic resolution analysis with scanning tunneling microscopy

A brief overview is given on the use of scanning tunneling microscopy for the identification of surface species. The commonly applied theoretical models are summarized and some experimental details are given. As typical results constant current topographies of clean Si(111)7×7, the Ag/Si(111)7×7 and K/Si(111)7×7 systems are shown and discussed. Differences in the local density of states on clean and Ag-covered parts of Si(111)7×7 can be visualized by current images. The method is particularly suitable for an atomically resolved analysis of single-component adsorbate systems, where the adsorbate can be uniquely distinguished from the substrate.

Scanning tunnelling microscopy and spectroscopy on Cu(111) and Au(111)

SUMMARYWe have studied Cu(111) and Au(111) by means of scanning tunnelling microscopy and spectroscopy. The constant current topographies showed flat parts as well as regions with a high density of monoatomic steps (in particular on Au(111)). Local I/U characteristics have been determined at a fixed sample‐tip distance in the range of −10 V≤U≤10 V. They show a linear behaviour near the Fermi level and a nearly exponential dependency for larger values of U. Neither an influence of the sp‐like surface states or an onset due to d electron contributions of the sample could be observed.

Atomic nature of reconstructed Si(110).

Using scanning tunneling microscopy (STM) we have measured constant-current topographies of reconstructed Si(110). While low-energy-electron diffraction (LEED) showed a 4\ifmmode\times\else\texttimes\fi{}5 pattern with dominant 2\ifmmode\times\else\texttimes\fi{}1 contributions, in STM a 2\ifmmode\times\else\texttimes\fi{}5 periodicity of atomic building blocks is observed. These building blocks are arranged in the form of chains along [\ifmmode\bar\else\textasciimacron\fi{}110]-like directions and show a tendency to pairing, which accounts for the 4\ifmmode\times\else\texttimes\fi{}5 LEED results. The 2\ifmmode\times\else\texttimes\fi{}1 reconstruction, which was previously ascribed to a high-temperature phase of Si(110), is explained by a reduction of chain length caused by the generation of defects with increasing temperature.

Scanning tunneling spectroscopy on Si

An overview is given of various methods for applying tunneling microscopy to obtain information on the electronic structure of surfaces. Results are presented for clean Si(111)7×7 and for a Si(111)7×7 surface on which 1 3 monolayer Ag was deposited. The interrelation between constant current topographies and current images is discussed. It is shown that the current characteristics obtained at a fixed sample-tip distance contain both influences of the local density of states and the transmission probability of the tunneling electrons. Information on the local density of states is not only of fundamental importance but is needed for an accurate interpretation of the constant current topographies and may be used to study the electronic effects upon metal atom adsorption.