Variable-temperature Scanning Tunneling Microscopy Research Articles

An ultrahigh vacuum fast-scanning and variable temperature scanning tunneling microscope for large scale imaging

We describe the design and performance of a fast-scanning, variable temperature scanning tunneling microscope (STM) operating from 80 to 700 K in ultrahigh vacuum (UHV), which routinely achieves large scale atomically resolved imaging of compact metallic surfaces. An efficient in-vacuum vibration isolation and cryogenic system allows for no external vibration isolation of the UHV chamber. The design of the sample holder and STM head permits imaging of the same nanometer-size area of the sample before and after sample preparation outside the STM base. Refractory metal samples are frequently annealed up to 2000 K and their cooldown time from room temperature to 80 K is 15 min. The vertical resolution of the instrument was found to be about 2 pm at room temperature. The coarse motor design allows both translation and rotation of the scanner tube. The total scanning area is about 8 x 8 microm(2). The sample temperature can be adjusted by a few tens of degrees while scanning over the same sample area.

Growth of iron on gold (7 8 8) vicinal surface: From nanodots to step flow

The growth morphology of iron on Au(7 8 8) has been investigated from 45 K to 430 K by use of variable temperature scanning tunneling microscopy. This system displays an ordered growth regime of a nanodots array from 70 K to 300 K (room temperature, RT). According to the temperature, the dots are found to grow on two different sites: below 250 K, they form rounded isolated islands above the steps where local traps act as nucleation centers; from 250 K to 300 K, they have the shape of platelets regularly disposed along the bottom of the step edges and located on face centered cubic area of the Au(7 8 8) surface reconstruction. Above RT, as a function of temperature, we find a progressive transition of the growth morphology, from ordered platelets to a step flow growth. A surface alloy formation induced by the high step density is the driving force of this growth mode transition.

Formation and local electronic structure of Ge clusters on Si(111)-7×7 surfaces

We report the formation and local electronic structure of Ge clusters on the Si(111)- 7 x 7 surface studied by using variable temperature scanning tunnelling microscopy (VT-STM) and low-temperature scanning tunnelling spectroscopy (STS). Atom-resolved STM images reveal that the Ge atoms are prone to forming clusters with 1.0 nm in diameter for coverage up to 0.12 ML. Such Ge clusters preferentially nucleate at the centre of the faulted-half unit cells, leading to the 'dark sites' of Si centre adatoms from the surrounding three unfaulted-half unit cells in filled-state images. Biasdependent STM images show the charge transfer from the neighbouring Si adatoms to Ge clusters. Low-temperature STS of the Ge clusters reveals that there is a band gap on the Ge cluster and the large voltage threshold is about 0.9 V.

Temperature and coverage driven condensation of pentacene on the Si(111)-–Ag surface

The adsorption and self-assembly behavior of pentacene molecules on theSi(111)-–Ag surface has been studied by variable-temperature scanning tunneling microscopy(STM). By cooling the surface from room temperature to 120 K we observed thecondensation of a two-dimensional molecular gas phase to a new ordered solid phase (P2).The structure models of this phase and the high-density brick-wall phase (P1) areproposed consistently based on a systematic investigation of the morphology,chirality and domain boundary structures of the two phases. The self-assemblymechanism of pentacene molecules driven by temperature and molecular coverage isdiscussed.

Al nanocluster arrays onSi(111)−7×7surfaces: Formation process and interactions among clusters

By means of variable-temperature scanning tunneling microscopy, we have investigated systematically the formation process of Al magic cluster arrays on the $\mathrm{Si}(111)\text{\ensuremath{-}}7\ifmmode\times\else\texttimes\fi{}7$ surface at high temperature in situ. It was found that the magic clusters form only when the Al coverage is over a critical value, $\ensuremath{\sim}0.08\ifmmode\pm\else\textpm\fi{}0.015$ ML. These clusters occupy preferentially on the faulted-half-unit cells of the $\mathrm{Si}(111)\text{\ensuremath{-}}7\ifmmode\times\else\texttimes\fi{}7$ surface, but this preference is coverage dependent. By analyzing systematically the spatial distribution of magic clusters below the saturation coverage, an attractive interaction between clusters was found, which prevents a triangular ordering of the nanocluster array.

Thermal stability of atomically flat metal nanofilms on metallic substrates

By means of variable temperature scanning tunneling microscope we studied the morphology and electronic structure of Pb films grown on Cu(1 1 1). Due to the spatial confinement of electrons, the islands display quantized energy levels. At 300 K, Pb forms 3D nanostructures with magic heights, that correspond to islands having a quantum well state (QWS) far from the Fermi energy. Below 100 K Pb grows in a quasi-layer-by-layer fashion. The QWS that develop in the films determine their total energy and, accordingly, their thermal stability. Films of particularly magic thickness are stable upon heating to 300 K.

Surface diffusion of Pb single adatoms on theSi(111)−(3×3)R30°−Pbsystem

The adsorption and diffusion of additional Pb adatoms on the $1∕3$ monolayer $\mathrm{Si}(111)\text{\ensuremath{-}}(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}\text{\ensuremath{-}}\mathrm{Pb}$ surface have been investigated by means of variable temperature scanning tunneling microscopy experiments. This surface experiences a reversible phase transition to a $(3\ifmmode\times\else\texttimes\fi{}3)$ symmetry at a critical temperature ${T}_{c}=86\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Scanning tunneling microscope (STM) images, acquired below and above ${T}_{c}$, have allowed us to identify the nature and adsorption sites of the additional Pb adatoms. A careful analysis of the diffusion, in which the influence of the STM in the measurements has been addresed and discarded, has been performed at temperatures well below room temperature but above ${T}_{c}$. In this way, the activation energy and the preexponential factor for the surface diffusion of single Pb adatoms on the high temperature $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ phase have been obtained. Attention has been paid to the role played by the underlying $(3\ifmmode\times\else\texttimes\fi{}3)$ phase at these temperatures.

Effect of Grain Boundaries on the Local Electronic Transport in Nanostructured Films of Colossal Magnetoresistive Manganites

We have studied the spatially resolved local electronic properties of nanostructured films of a colossal magnetoresistive (CMR) material by local conductance mapping (LCMAP) using a variable temperature scanning tunneling microscope (STM), as a function of temperature and magnetic field. The experiments were carried out on nanostructured (NS) films of rare-earth hole doped manganites like La0.67Sr0.33MnO3 (LSMO), (prepared by Chemical Solution Deposition (CSD)) having average grain sizes around 100 nm or less. Due to the presence of a large number of natural grain boundaries (GBs), these films exhibit interesting transport properties that are not observed in single crystals and epitaxial films of the same composition, such as low field magnetoresistance (LFMR), and significant MR at higher fields over the entire temperature range. Spatially resolved measurements of electronic properties, like the LCMAP, traces the contribution of the grain boundaries to the electronic transport in these NS films. It is found that the application of a low magnetic field enhances the DOS in the GB region, leading to the observed low field response of the MR.

Self‐organized nanotemplating on misfit dislocation networks investigated by scanning tunneling microscopy

Self-ordering growth of nanoarrays on strained metallic interfaces is an attractive option for preparing highly ordered nanotemplates. The great potential of this natural templating approach is that symmetry, feature sizes, and density are predicted to depend on the interfacial stress in these strained layers, which can be adjusted by changing the substrate-thin film composition, temperature, and adlayer coverage. This bottom-up approach of growing nanostructured two-dimensional ordered arrays of clusters on the misfit dislocation networks of strained metallic thin films and surfaces requires a detailed understanding of the nucleation and film-adsorbate interaction processes. Here we show how high resolution, large scale, variable temperature scanning tunneling microscopy imaging can improve our understanding of these self-assembly processes.

Chiral Ordering and Conformational Dynamics for a Class of Oligo-phenylene-ethynylenes on Au(111)

Adsorption structures formed from a class of planar organic molecules on the Au(111) surface under ultrahigh vacuum conditions have been characterized using scanning tunneling microscopy (STM). The molecules have different geometries, linear, bent, or three-spoke, but all consist of a conjugated aromatic backbone formed from three or four benzene rings connected by ethynylene spokes and functionalized at all ends with an aldehyde, a hydroxyl, and a bulky tert-butyl group. Upon adsorption, the molecules adopt different surface conformations some of which are chiral. For the majority of the observed adsorption structures, chirality is expressed also in the molecular tiling pattern, and the two levels of chirality display a high degree of correlation. The formation and chiral ordering of the self-assembled structures are shown to result from dynamic interchanges between a diffusing lattice gas and the nucleated islands, as well as from a chiral switching process in which molecules alter their conformation by an intramolecular rotation around a molecular spoke, enabling them to accommodate to the tiling pattern of the surrounding molecular structures. The kinetics of the conformational switching is investigated from time-resolved, variable temperature STM, showing the process to involve an activation energy of approximately 0.3 eV depending on the local molecular environment. The molecule-molecule interactions appear primarily to be of van der Waals character, despite the investigated compounds having functional moieties capable of forming intermolecular hydrogen bonds.

Variable-temperature independently driven four-tip scanning tunneling microscope

The authors have developed an ultrahigh vacuum (UHV) variable-temperature four-tip scanning tunneling microscope (STM), operating from room temperature down to 7 K, combined with a scanning electron microscope (SEM). Four STM tips are mechanically and electrically independent and capable of positioning in arbitrary configurations in nanometer precision. An integrated controller system for both of the multitip STM and SEM with a single computer has also been developed, which enables the four tips to operate either for STM imaging independently and for four-point probe (4PP) conductivity measurements cooperatively. Atomic-resolution STM images of graphite were obtained simultaneously by the four tips. Conductivity measurements by 4PP method were also performed at various temperatures with the four tips in square arrangement with direct contact to the sample surface.

Fluctuations and phase separation in Br/Pt(1 1 0)

The quasi-one-dimensional system c(2 × 2)-Br/Pt(1 1 0) is investigated by low-energy electron diffraction (LEED) spot-profile analysis and by variable-temperature scanning tunnelling microscopy (VT-STM). An order–disorder transition is found at 375 K. For 250 K < T < 375 K a long-range ordered c(2 × 2) phase is observed, but the spot profiles reveal strong correlated fluctuations along the close-packed row direction. At T = 50 K three different phases coexist in a glassy state: c(2 × 2), (2 × 1), and (3 × 2). The loss of long-range order and transition into a phase-separated state is a common feature predicted for strongly correlated systems. Some inconsistencies between theory and experiment in our previous interpretation of the (3 × 1)-Br/Pt(1 1 0) phase as a charge density wave (CDW) are resolved by identifying it as a fluctuating (3 × 2) CDW phase.

Dynamics of copper atoms on Si(1 1 1)-7 × 7 surfaces

This study investigated the dynamics of copper atoms adsorbed on Si(1 1 1)-7 × 7 surfaces between 300 K and 623 K using a variable-temperature scanning tunneling microscope (STM). The diffusion behavior of copper clusters containing up to ∼6 atoms into a particular half unit cell of the 7 × 7 reconstructed Si(1 1 1) surface was considered. The movements and the formation of copper clusters were tracked in detail. The activation energies and pre-exponential factors for various diffusion paths were estimated. Finally, the Cu-etching-Si process and the quasi-5 × 5 incommensurated phase of Cu/Si islands were discussed.

Defects in the(3×3)⇔(3×3)phase transition in thePb∕Si(111)system

The role played by point defects in the $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})\ensuremath{\Leftrightarrow}(3\ifmmode\times\else\texttimes\fi{}3)$ phase transition that takes place in the $1∕3$ monolayer-$\mathrm{Pb}∕\mathrm{Si}(111)$ system has been investigated by means of variable temperature scanning tunneling microscopy. In this system, it has been possible to grow exceptionally large defect-free regions coexisting with more defective ones. These defective regions have between 2% and 5% of point defects, which are mainly substitutional Si adatoms. Our experiments show that the point defects produce a local perturbation of the surface, but discard them as the fundamental driving force of the phase transition. By tracking exactly the same surface regions with atomic resolution while varying the sample temperature from $40\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}200\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, we have observed that substitutional point defects are not mobile throughout the phase transition and our statistical analysis of the STM images reveals that they are randomly placed from $40\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ to room temperature.

Diffusion pathways of hydrogen across the steps of a vicinal Si(001) surface

Hydrogen diffusion across ${D}_{B}$ steps on Si(001) surfaces is investigated by means of variable-temperature scanning tunneling microscopy and first-principles calculations. Experimentally, the hopping rate for diffusion from the step sites to the Si dimers of the upper terrace was found to be more than one order of magnitude higher than that for diffusion to the lower terrace. This clear preference, opposite to the trend for the respective binding energies, is explained by first-principles calculations that identify a metastable intermediate to be responsible for the unexpected lowering of the energy barrier for upward diffusion.

In situ STM of pulsed laser nanostructured deposits: First stages of film formation

In the synthesis of nanostructured thin films the characterization of the growth processes plays a fundamental role for the control of the film and surface properties. Moreover when the deposition technique is based on the production and the assembling of nanoparticles/clusters the characterization of the precursor size distribution is of fundamental importance. We have designed a pulsed laser deposition (PLD) apparatus for the production of nanostructured thin films and surfaces, connected to a UHV variable temperature scanning tunneling microscope (STM). The whole system is devoted to the synthesis and in situ study of nanostructured and nanoporous functional metal and metal oxide films and surfaces. We have deposited W nanoparticles produced by a few hundreds laser pulses in order to investigate the initial mechanisms of the film growth. Different deposition conditions have been explored by controlling the laser generated plasma expansion through a background gas in the PLD chamber. STM measurements have been performed on W thin films deposited on different substrates to study both the size distribution and the aggregation of the precursors on the surface. Although substrate effects must be taken into account, the control of the background gas pressure and of the target-to-substrate distance allows to produce surfaces with different morphologies. This opens the possibility to tailor the material properties through the control of the size and deposition energy of the building nano-units.

The behavior of Co atoms on Si (111)-7 × 7 surfaces at low temperatures

The behavior of Co atoms on Si (111)-7 × 7 surfaces at low temperatures was studied by using a variable-temperature scanning tunneling microscopy (VT-STM). Co atoms deposited on Si (111)-7 × 7 surfaces are randomly adsorbed at 100 K. Co atoms start to react with adatoms of Si (111)-7 × 7 surfaces at temperatures between 126 K and 130 K. The reaction transfers the bright dots of Co atoms to dark dots under the STM observation of negative bias. Analysis of the reaction occurrence sites and comparing with the results of room temperature deposition shows that the Co atoms tend to diffuse and react with the adatoms of Si (111)-7 × 7 surfaces at the center sites of unfaulted half unit cell (UHUC) at higher temperatures.

Self-assembly of trimetallic nitride template fullerenes on surfaces studied by STM

Trimetallic nitride template fullerenes have been deposited onto a variety of substrates in order to elucidate the substrate–fullerene interactions. We have investigated self-assembled island formation and molecular detail of Er 3N@C 80 and Sc 3N@C 80 on Ag/Si(1 1 1), Au(1 1 1)/mica, Si(1 1 1), and Si(0 0 1) using variable temperature scanning tunnelling microscopy (STM). At room temperature, the fullerenes self-assemble into monolayer-high hexagonal close-packed islands on Ag-passivated Si(1 1 1) whereas annealing at elevated temperatures (250–300 °C) is necessary for the self-assembly of close-packed islands on Au(1 1 1). Intra-molecular resolution of the fullerenes has been achieved at liquid nitrogen temperature on Ag/Si(1 1 1) and already at room temperature on Si(0 0 1), when the rotation of the fullerenes is frozen. Whereas the bonding between the fullerenes and Si surfaces is mainly covalent, it appears to be mainly van-der-Waals on the other surfaces.

Decay of multilayer holes on SrTiO 3(0 0 1)

We have studied the decay kinetics of nanoscale multilayer holes on SrTiO 3(0 0 1) surfaces, using variable temperature scanning tunneling microscopy. We have performed real time observation of the decay of multilayer holes with diameters of 10 nm order at 750 °C. We have found that the hole decays, filling layer by layer from the bottom while expanding the periphery of the hole. We have performed numerical simulations of hole decay based on a step flow model. The observed decay kinetics is found to be diffusion limited with local mass conservation.

Real-Space Direct Visualization of the Layer-Dependent Roughening Transition in Nanometer-Thick Pb Films

By means of variable-temperature scanning tunneling microscopy and spectroscopy we studied the thickness-dependent roughening temperature of Pb films grown on Cu(111), whose electronic structure and total energy is controlled by quantum well states created by the spatial confinement of electrons. Large scale STM images are employed to quantify the layer population, i.e., the fraction of the surface area covered by different Pb thicknesses, directly in the real space as a function of temperature. The roughening temperature oscillates repeatedly with bilayer periodicity plus a longer beating period, mirroring the thickness dependence of surface energy calculations. Conditions have been found to stabilize at 300 K Pb films of particular magic thicknesses, atomically flat over microns.