Abstract: There are important applications of multiwalled carbon nanotubes (MWCNT) with deposited nanoparticles and several methods exist for synthesizing these nanocomposites. However, a simpler and more efficient method is desired in many cases. Here, we introduce a method where MWCNT were pre-mixed with sodium citrate, and using ultrasonication, shells of sodium citrate were formed on the nanotubes. These functionalized MWCNTs served as substrates for gold nanoparticle growth. When HAuCl4 was added to the reaction system, Au3+ was directly reduced at the surface of the MWCNT and gold nanoparticles were assembled along the MWCNT. Transmission electron microscopy (TEM) demonstrated that the density of the gold nanoparticle coating process depended on the amount of the carbon nanotubes when the sodium citrate concentration was keep unchanged. In addition, by controlling the concentration of sodium citrate and HAuCl4, the size of gold nanoparticles could be controlled. Scanning electron microscope (SEM), energy dispersive X-ray (EDX), and UV-vis were also used to characterize the MWCNT-Au nanocomposites. Compared with other methods, the procedure described here required only water solutions and there are no needs for high temperature steps, surfactants or organic solvents, resulting in a simple and fast method for efficient gold nanoparticle decoration of carbon nanotubes. (C) 2009 Elsevier B.V. All rights reserved.
Abstract: Here, we present a MEMS atomic force microscope sensor for use inside a transmission electron microscope (TEM). This enables direct in situ TEM force measurements in the nanonewton range and thus mechanical characterization of nanosized structures. The main design challenges of the system and sensor are to reach a high sensitivity and to make a compact design that allows the sensor to be fitted in the narrow dimensions of the pole gap inside the TEM. In order to miniaturize the sensing device, an integrated detection with piezoresistive elements arranged in a full Wheatstone bridge was used. Fabrication of the sensor was done using standard micromachining techniques, such as ion implantation, oxide growth and deep reactive ion etch. We also present in situ TEM force measurements on nanotubes, which demonstrate the ability to measure spring constants of nanoscale systems.
Abstract: Dimensionally Stable Anodes (DSA(r)) are used for industrial production of e.g. chlorine and chlorate. It is known that the superior electrocatalytical properties of DSA(r) is due to the large effective area of the porous coating. However, this knowledge is mainly found from in situ electrochemical measurements. Here, we used ex situ methods, AFM, TEM and gas porosimetry, for characterization at the nanoscale. The DSA(r) coating was found to consist of mono-crystalline grains with a size of 20-30 nm and with pores of about 10 nm in diameter. Using a simple geometrical model an effective area was calculated. For a typical coating thickness, an increase of about 1000 times in the effective surface area was found, which is consistent with in situ estimations. These results suggest that the dominating source of surface enlargement is due to nano-crystallinity.
Abstract: Self-assembly of complex, non-periodic nanostructures can only be achieved by using anisotropic building-blocks. The building blocks need to have at least four bonds pointing in separate directions [J. Comput. Theor. Nanosci. 3, 391 (2006)]. We have previously presented a method for the synthesis of such building-blocks using DNA-functionalized gold nanoparticles. Here, we report on the progress in the experimental realization of this scheme. The first goal, in a process to make programmable self-assembly building-blocks using nanoparticles, is the production of dimers with different DNA-functions on the two component particles. We report on the fabrication of anisotropically functionalized dimers of nanoparticles of two different sizes. As a result of their anisotropy, these demonstrator building blocks can be made to assemble into spherical structures.
Abstract: DNA self-assembly is a powerful route to the production of very small, complex structures. When used in combination with nanoparticles it is likely to become a key technology in the production of nanoelectronics in the future. Previously, demonstrated nanoparticle assemblies have mainly been periodic and highly symmetric arrays, unsuited as building blocks for any complex circuits. With the invention of DNA-scaffolded origami reported earlier this year (Rothemund P W K 2006 Nature 440 (7082) 297-302), a new route to complex nanostructures using DNA has been opened. Here, we give a short review of the field and present the current status of our experiments were DNA origami is used in conjunction with nanoparticles. Gold nanoparticles are functionalized with thiolated single stranded DNA. Strands that are complementary to the gold particle strands can be positioned on the self-assembled DNA-structure in arbitrary patterns. This property should allow an accurate positioning of the particles by letting them hybridize on the lattice. We report on our recent experiments on this system and discuss open problems and future applications.
Abstract: An important problem in nanotechnology is to develop a method for assembling complex, aperiodic, structures. While simple self-assembly will not be able to address this problem, programmable-, or algorithmic-, self-assembly is powerful enough to be a potential solution. Here, we address the question of how the basic properties of the constituent building blocks are related to the periodicity of the resulting assembly. By introducing the parameters unique structures, which gives a measure of the complexity of an assembly, and bond uniqueness, which gives a measure of how the building blocks fit together, we show how to quantify the structural quality of a general assembly system and present relations between the parameters. The introduced methods will be helpful when designing assembly systems to be used for direct fabrication of nanosystems or for nano-scaffolds and addressable arrays.
Abstract: Nanoparticles coated with single stranded DNA have been shown to efficiently hybridize to targets of complementary DNA. This property might be used to implement programmable (or algorithmic) self-assembly to build nanoparticle structures. However, we argue that a DNA coated nanoparticle by itself cannot be used as a programmable self-assembly building block since it does not have directed bonds. A general scheme for assembling and purifying nanoparticle eight-mers with eight geometrically well-directed bonds is presented together with some preliminary experimental work. (c) 2006 Elsevier B.V. All rights reserved.
Abstract: A capacitive force sensor for in situ nanoindentation experiments in TEM has been designed, manufactured and evaluated. The confined space of the TEM specimen holder restricts the size of the fabricated sensor to 2 turn x 1.5 mm x 2 mm to allow mounting. A unique feature of the sensor is an integrated fixture for interchangeable tips, e.g. diamond tips. The sensor is fabricated in silicon anodically bonded to glass and the device is formed by DRIE. To improve the control of spring thickness and circumvent problems during fabrication a SOI wafer and slightly altered design was used in conjunction to an improved process, which resulted in a yield near 100%. The sensor is characterized by a force application using a piezoelectric positioning system, an electrostatic evaluation and a resonance frequency test using a scanning laser doppler vibrometer. The capacitance is measured with an off-chip read-out circuit. The resonance frequency test yielded a spring constant of 750 N/m, which results in a sensitivity of 0.27 pF/0.1 mu N for small deflections. The evaluation shows that the force sensor is suitable for in situ nanoindentation for measurements in the range of 0-100 mu N. (C) 2005 Elsevier B.V. All rights reserved.
Abstract: The paper describes the use of an in-situ microscopy technique, which combines transmission electron microscopy (TEM) with scanning probe microscopy (SPM), to investigate the electrical and mechanical properties of individual silicon and germanium nanowires. Additionally, the formation of ordered arrays of size-monodisperse silicon and germanium nanowires within mesoporous silica powders and thin films using a supercritical fluid inclusion phase technique is described. In particular, we demonstrate ultra high-density arrays of germanium nanowires, up to 2 x 10(12) wires per square centimetre. These matric embedded nano-composite materials display unique optical properties such as intense room temperature ultraviolet and visible photoluminescence.
Abstract: Nanoscale gold contacts were investigated both experimentally and theoretically. Simulations of in situ processes in a new combined TEM-AFM microscope were performed by molecular dynamics and theoretical mechanics methods. Atomistic transformations of gold nanometer-sized wires (nanowires) between Au-probe and Au-surface were studied in processes both of loading-unloading and in the normal, lateral, diagonal and zigzag directions of the probe motion. Molecular dynamics was used for studies of "adhesion avalanche", shear and strain deformations. Theoretical mechanics was used for studies of jump-to-contact and jump-off-contact phenomena. Reorientations from (100) to (111) planes with formation of extended zigzag, vacancy cavities, a double-neck creation and a slip along the (110) plane with formation of twins and steps were observed. Deformation mechanisms were shown to depend on schemes of motions and on the ratio between the relative velocity of the probe and surface motion and the velocity of the defect relaxation.
Abstract: In this work we report how single crystal nanowires can be assembled into regular arrays using mesoporous thin films to define the architecture. Mesoporous thin films were prepared by a sol-gel method. These provide films of very regular structure and dimensions. The films produced in this way have almost single crystal like structures and can also exhibit strong epitaxy to the underlying silicon substrate. The films are subjected to a supercritical fluid (SCF) environment in which a precursor is decomposed to yield nanowires of metals, semiconductors or oxides. Using these SCF conditions, pore filling is complete and the products are nanowires which are single crystals and structurally aligned in one direction. The growth mechanism of the nanowires is described and size effects discussed.
Abstract: We apply molecular dynamics methods for simulation of in situ processes in new combined TEM/SPM technique. The atomic structure transformations of a gold nanobridge between two contacts are studied in processes of loading-unloading and friction cycles, vertical, lateral, diagonal and zigzag motion. In all cases only a singie-atom contact is broken at the final stage of deformations. The deformation process strongly depends on the velocity of fracture and schemes of motion.
Abstract: Here we demonstrate, for the first time experimentally, a nanopipette action for metals using multiwalled carbon nanotubes. The process relies on electromigration forces, created at high electron current densities, enabling the transport of material inside the hollow core of carbon nanotubes. In this way nanoparticles of iron were transported to and from electrically conducting substrates.
Abstract: A combined transmission electron microscopy-scanning tunneling microscopy (TEM-STM) technique has been used to investigate the force interactions of silicon and germanium nanowires with gold electrodes. The I(V) data obtained typically show linear behavior between the gold electrode and silicon nanowires at all contact points, whereas the linearity of I(V) curves obtained for germanium nanowires were dependent on the point of contact. Bistable silicon and germanium nanowire-based nanoelectromechanical programmable read-only memory (NEMPROM) devices were demonstrated by TEM-STM. These nonvolatile NEMPROM devices have switching potentials as low as 1 V and are highly stable making them ideal candidates for low-leakage electronic devices. (C) 2004 American Institute of Physics.
Abstract: Spatially distributed DNA oligomer arrays on Au(111) surfaces were created by one-step co-assembly of mixed monolayers of alkanethiol-conjugated DNA and mercaptohexanol (MCH). Tapping-mode AFM was used to visualize the distribution of DNA molecules on the surface and to study the mechanical properties of individual molecules. The DNA coating density increased nonlinearly with increasing mole fraction of DNA oligomer to MCH in the coating solution. For imaging in air, where surfaces are coated with a thin layer of water, the interaction between the AFM tip and the different structures on the monolayer varies between attractive and repulsive depending on the tapping amplitude, set-point ratio, and tip shape. It was found that both duplex and single-stranded DNA molecules extend approximately vertically upward from the surface.
Abstract: A scanning tunneling microscope (STM) with a compact, three-dimensional, inertial slider design is presented. Inertial sliding of the STM tip, in three dimensions, enables coarse motion and scanning using only one piezoelectric tube. Using the same electronics both for scanning and inertial sliding, step lengths of less than 5% of the piezo range were achieved. The compact design, less than 1 cm3 in volume, ensures a low mechanical noise level and enables us to fit the STM into the sample holder of a transmission electron microscope (TEM), while maintaining atomic scale resolution in both STM and TEM imaging. (C) 2003 American Institute of Physics.
Abstract: We report a "clean" and fast process, utilizing supercritical carbon dioxide, for producing ultrahigh densities, Up to 10(12) nanowires per square centimeter, of ordered germanium nanowires on silicon and quartz substrates. Uniform mesoporous thin films were employed as templates for the nucleation and growth of unidirectional nanowire arrays orientated almost perpendicular to a substrate surface. Additionally, these nanocomposite materials display room-temperature photoluminescence (PL), the energy of which is dependent on the diameter of the encased nanowires. The ability to synthesis ultrahigh-density arrays of semiconducting nanowires on-chip is a key step in future "bottom-up" fabrication of multilayered device architectures for nanoelectronic and optoelectronic devices.
Abstract: Nanowires are expected to play an important role in future electronic, optical devices and nanoelectromechanical devices. Measuring the electrical and mechanical properties of nanowires is however a difficult task due to their small dimensions. Here we report the use of an in situ microscopy technique, which combines transmission electron microscopy (TEM) with scanning probe microscopy (SPM), to investigate the electrical and mechanical properties of metallic and semiconductor nanowires.
Abstract: Metallic nanowires of cobalt, copper, and iron oxide magnetite (Fe3O4) have been synthesized within the pores of mesoporous silica using a supercritical fluid inclusion technique. The mesoporous matrix provides a means of producing a high density of stable, hexagonally ordered arrays of highly crystalline nanowires. The formation of the metal and metal oxide nanowires within the silica mesopores was confirmed by transmission electron microscopy (TEM), N-2 adsorption experiments, and powder X-ray diffraction (PXRD). The mechanism of nanowire formation within the mesopores appears to occur through the initial binding and coating of the pore walls with the metal atoms to form tubelike structures within the mesoporous template. The thickness of these tubes subsequently increases with further metal deposition until nanowires are formed. Additionally, the crystal structure of the cobalt nanowires formed within the mesoporous template can be readily changed by manipulating the density of the supercritical fluid phase.
Abstract: We present a method for combined far-field Raman imaging, topography analysis and near-field spectroscopy. Surface-enhanced Raman spectra of Rhodamine 6G (R6G) deposited on silver nanoparticles were recorded using a bent fibre aperture-type near-field scanning optical microscope (NSOM) operated in illumination mode. Special measures were taken to enable optical normal-force detection for control of the tip-sample distance. Comparisons between far-field Raman images of R6G-covered Ag particle aggregates with topographic images recorded using atomic force microscopy (AFM) indicate saturation effects due to resonance excitation.
Abstract: The paper describes the use of an in-situ microscopy technique, which combines transmission electron microscopy (TEM) with scanning probe microscopy (SPM), to investigate the electrical and mechanical properties of individual silicon and germanium nanowires. Additionally, the formation of ordered arrays of size-monodisperse silicon and germanium nanowires within mesoporous silica powders and thin films using a supercritical fluid inclusion phase technique is described. In particular, we demonstrate ultra high-density arrays of germanium nanowires, up to 2 x 10(12) wires per square centimetre. These matric embedded nano-composite materials display unique optical properties such as intense room temperature ultraviolet and visible photoluminescence.
Abstract: We have investigated force interactions between two gold samples using a combination of atomic force microscope (AFM) and a transmission electron microscope (TEM) (TEM-AFM). The size and shape of the tip and sample as well as size of contact area and interactions type (elastic-plastic) is observed directly. The force was measured by direct measurement of the displacement of the AFM tip. An anomalous high value of the jump-to-contact distance was found, which we interpret as due to an enhanced surface diffusion of gold atoms towards the tip-sample gap due to the van der Waals force, leading to an avalanche situation where the gap is quickly filled until the ordinary jump-to-contact distance. The contact radius at zero applied load were measured and compared with adhesion theories. The results were in the Maugis transition re-ion, between the limiting cases of the Derjaguin-Willer-Toporov (DMT) and the Johnson-Kendall-Roberts (JKR) models. (C) 2002 Elsevier Science B.V. All rights reserved.
Abstract: We report on the near-field imaging of silver nanoparticles using an aperture-type near-field microscope operated in illumination mode. The nanoparticles are imaged as interference patterns, due to far-field superposition of the optical fields emitted from the tip and elastically scattered from localized surface plasmons (SP). Aperture-type probe can thus be used to obtain information on the phase shift associated with localized SP coupling at the illumination wavelength. (C) 2002 American Institute of Physics.
Abstract: Magnetic particles of microscopic size can be created in the process of Ni, Fe and Co mechanically controllable break junctions fabrication and trapped between the electrodes by magnetic dipole forces. Tunneling between the protruding nanoparticle and the sample electrode shows clear distinctions from the usual junctions: heavy deviation of the current-distance I(z) dependence from the expected exponential behavior at electrode separations z below 4.0-4.5 Angstrom and on numerous occasions a sudden jump-like decrease of the tunnel current at z approximate to 1.5-2.0 Angstrom. Possible mechanisms behind observed anomalies including the short-range magnetic exchange coupling are discussed. (C) 2002 Elsevier Science B.V. All rights reserved.
Abstract: Nanowires are expected to play an important role in future electronic, optical devices and nanoelectromechanical devices. Measuring the electrical and mechanical properties of nanowires is however a difficult task due to their small dimensions. Here we report the use of an in-situ microscopy technique, which combines transmission electron microscopy (TEM) with scanning probe microscopy (SPM), to investigate the electrical and mechanical properties of metallic and semiconductor nanowires. Additionally, in this paper we describe a novel approach for synthesizing mesoporous silicas with tunable pore diameters, wall thickness and. pore spacings that can be used as templates for the assembly of semiconductor nanowire arrays. Silicon and germanium nanowires, with size monodisperse diameters, can readily be formed within the mesoporous silica matrix using a supercritical fluid inclusion technique. These nano-composite materials display unique optical properties such as intense room temperature ultraviolet and visible photoluminescence. The implication of these mesoporous nanowire materials for future electronic and opto-electronic devices is discussed.
Abstract: A long-standing problem for piezo-electric inertial motors is the decrease of efficiency and step length at lower temperatures. We have developed a piezoelectric vibration driver for positioning in a temperature range of 4 - 400K for Scanning Probe Microscopes (SPM). The vibrator consists of two parallel piezoceramic plates, which are fixed rigidly to the support at their centre, and to the endpieces at their ends. The vibrator is symmetrical to the three perpendicular symmetric axes. There is no considerable decrease of steps size in the novel piezoresonance device while cooling to a low temperature, because the decrease of piezomodule d(31) is compensated by an increase of the mechanical efficiency factor Q of the vibrator at low temperatures. The described vibrator can shift linear rails, rotate and/or shift cylindrical bodies.
Abstract: Ultrathin Na films and islands on graphite realized prototype simple metal quantum wells with all valence electrons confined within boundaries well defined on the atomic scale. This is shown by angle-resolved photoemission and scanning tunneling microscopy data, which give unique information about electron dispersion, hole lifetimes, and Fermi wavelengths.
Abstract: STM and AFM combined with a transmission electron microscope (TEM) are powerful tools for direct investigation of structures, electronic properties, and interactions at the atomic and nanometer scale. Here, we report on three different designs of such TEM-SPM. In the first TEM-SPM design, a stepper motor performed the coarse approach. The advantage of this design was the strong pulling force that enabled notched metallic wires to be broken inside the TEM, which lead to clean sample surfaces. In the second construction a clutch combined with a one-dimensional inertial slider was added. The third type, with a new type of three-dimensional inertial slider, allowed lateral motion inside the TEM, which simplified the adjustment of tip location on the sample. Some new experimental results are included to demonstrate the capabilities of the TEM-STM and TEM-AFM.
Abstract: We use scanning tunneling microscopy (STM) and core and valence photoemission as well as low-energy electron diffraction to characterize recently discovered S/Cu(111) surface structures that appear at low coverage below ordering temperatures of around 230 K. At even lower coverage ordered local arrangements are observed near steps and dislocations. Of the laterally extending structures one is open. and honeycomb (hc) like, while three other structures (I,II,III) are more complicated. It is suggested that the structures can be explained as reordered (0001) planes of CuS. Surprisingly the open hc structure gives room for the Cu(111) surface state according to photoemission and scanning tunneling spectra. Core level spectra provide support for one of the models proposed for an earlier studied room-temperature structure [Cu(111)-( root 7x root7)R+/-19.1 degrees -S].
Abstract: Quench-condensed bismuth films of 3-5 nm thickness have been used as a cluster source to prepare Single Electron Transistors (SET) based on a single cluster with high charging energy. We used electron-beam defined shadow evaporation masks to pattern la nm wide constrictions in these films. By incremental depositions through these masks controlled by in situ sample conductance measurements, we obtained a SET geometry for clusters with, charging energies up to 90 meV Our experiment showed that the SET geometry can be achieved in every sample preparation ran, despite the apparent random nature of cluster formation in granular films. The resulting charging energy of the transistor varied from experiment to experiment. As value, however, was always higher than 10 meV.
Abstract: Nanobridges in thin 1-3 nm granular bismuth quench-condensed films with lengths and widths of 10 nm have been formed on top of an electrostatic gate made of oxidized aluminum. Periodic gate modulation of the Coulomb blockade-type I(V) curves of these nanoconstrictions has been observed in the temperature range 4.2-11 K, indicating that the charge of a single grain controlled the transport in the constriction. A reversible transition between two states with the same charging energy of 35 meV occurred at a temperature around 8 K, with up to 5 times enhanced conductivity at low temperatures. (C) 2000 Elsevier Science B.V. All rights reserved.
Abstract: We have investigated the conductance of gold point contacts using a scanning tunneling microscope (STM) inside a transmission electron microscope (TEM). Measuring the conductance of these point contacts as a function of radius, we could directly compare it with theories both in the ballistic regime (Sharvin) as well as in the diffusive regime (Maxwell). The width of the contacts were between a single atom and 20 nm. Using an interpolation formula (Wexler) between the two limits, we obtain a mean free path of 4 nm, which is about ten times shorter than the room-temperature bulk value. The low value indicates an enhanced scattering, which is not due to high temperature in the point contact, instead a large number of scattering centers is created during the point contact formation process.
Abstract: We study single-electron transport in quench-condensed bismuth films. By lateral confinement, we select a specific cluster of about 10(3) atoms and use tunneling barriers that appear naturally during thin-film formation. A remarkable reversible increase of the sample conductance up to 5 times was found as the temperature was lowered from 11 to 4 K. We attribute this effect to a spontaneous distortion of the cluster shape ansi discuss its relation to a phase transition predicted for free mettallic clusters.
Abstract: We have investigated methods for cleaning de-etched polycrystalline tungsten tips for scanning tunnelling microscopy (STM). The cleaning methods include Ar-ion sputtering, heating, chemical treatments and Ne-ion self-sputtering. We correlate transmission electron microscopy images of the tip, field-emission data from the tip and images of a clean Cu(111) surface to find an optimum procedure for STM imaging. Clean and sharp tips are made by sputtering, combined with careful heating by electron bombardment. We found that optimum sputtering was obtained either by use of a 4 keV Ar-ion gun for a few seconds or by self-sputtering with Ne ions for a few seconds or until decapitation occurs.
Abstract: We have observed local effects of the charge-density wave (CDW) in in situ Na-intercalated 1T-VSe2 using scanning tunneling microscopy and spectroscopy between 300 acid 60 K. Na intercalates nonuniformly, and divides the sample into intercalated (I) and nonintercalated (NI) areas. Below the CDW transition temperature these areas displayed totally different CDW's. The NI areas seemed unaffected by Na, and showed a 4X4 CDW that did not differ from the CDW in pure VSe2. The intercalated areas showed a strong octahedral CDW (lambda(1)= 0.99 nm, lambda(2)= 0.65 nm, angle 70 degrees, rotated 20 degrees with respect to the atomic lattice) occurring in three different orientations. Additional spots in Fourier space at low temperatures were interpreted to originate from Na ordering in a 1.3x 1.3 R45 degrees structure. Spectroscopy showed a shift of the V 3d-derived state below the Fermi level (E-F) upon intercalation. In the NI areas, a shift toward E-F (-60 meV compared to -80 meV in pure VSe2) was observed, while the I areas showed a shift away from E-F (-150 meV). The CDW energy gap was enlarged in the I areas (Delta approximate to 230 meV compared to Delta approximate to XO meV in pure VSe2), while it was never resolved in the NI areas. [S0163-1829(99)06511-X].
Abstract: Via scanning tunneling microscopy and low energy electron loss diffraction we observe ordered low-temperature surface structures for S/Cu(111) at low-S coverage and characterize the system also with core level photoemission. The first structure to appear upon S deposition is honeycomblike with a large mesh [(root 43 x root 43) R +/- 7.5 degrees], a large nearest S-S distance (similar to 4 Angstrom) and low-ordering temperature (<170K). Islands of this structure are observed at low coverage (theta similar to 0.05) and at theta similar to 0.25 nearly all of the surface area is covered. [S0163-1829(99)07139-8].
Abstract: The charging energy is important for a mesoscopic body with small capacitance to the environment. Connecting the small island via low capacitance, high resistance tunnel junctions, the current through the resulting structure will depend upon the charge of the island - charge that can be applied via a gate electrode. For a normal metal island, the current varies periodically with the charge; the periodicity is the electron charge e. If the island is superconducting, the period, e or 2e, depends upon whether the superconducting energy gap is less than or larger than the charging energy. This parity effect can be used to study possible nodes in the gap function of a high-T-C, superconductor. The charge transport through a mesoscopic conductor can be controlled by a phase difference between superconducting contacts to the normal conductor. If the phase difference is applied via a superconducting loop connected to the contacts, the conductance varies periodically with the flux or the current through the loop and the period is the quantized flux unit. The oscillation amplitude is temperature dependent as expected from theoretical predictions.
Abstract: Scanning electrochemical microscopy and atomic force microscopy were used together with cyclic voltammetry and rotating ring disc experiments to study the initial oxidation of cobalt in alkaline solution. The results show that the mechanism of cobalt oxidation involves a solid state path as well as dissolution combined with precipitation. In the concentration region studied, 0.1-1 M NaOH, the solid state path seems to predominate. The AFM images clearly show a three-dimensional growth mechanism and that the initial oxidation starts at active sites at the surface.
Abstract: The growth of ex situ Tl2Ba2CuO6 films on single-crystal LaAlO3 was investigated using atomic force microscopy and X-ray diffraction. Most of the film surface consisted of flat areas separated by single or several unit-cell steps. This morphology suggests that the films grow predominantly by a step-flow mechanism. A Tl-2201 film grown using a precursor him deposited at 700 degrees C exhibited stacks of planes arranged in a screw-like manner on a flat background. Islands of 1-3 mu m size and a height of 2-20 unit cells were dominant structures on the surface. Possible causes for the formation of the screw-like structures are discussed.
Abstract: We have used an ultrahigh vacuum variable-temperature scanning tunneling microscope to study in situ Na intercalation in 1T-VSe2. At room temperature the clean surface showed large flat areas and a trigonal atomic arrangement. Tunneling spectroscopy revealed the known state at 100 mV below E-F. At 60 K the clean surface showed a 4 x 4 charge density wave and the spectra showed a CDW gap Delta approximate to 80 mV. When evaporating less than or equal to 1 ML Na at room temperature, the intercalated Na were distributed non-uniformly, giving bright areas of increased topographic height where Na was intercalated. In the intercalated material the VSe2 state was seen in the tunneling spectra, and was slightly shifted towards E-F. Preliminary spectroscopy of the intercalated material at 60 K showed two different types of spectra. One type showed the VSe2 peak clearly, but had no sign of the CDW gap, while the other type showed a gap structure but did not resolve the VSe2 state. Presently, we can not tell whether these different spectra originate from the differences in the intercalated and non-intercalated areas or not.
Abstract: We present and analyze theoretically measurements of the room temperature I - V characteristics of gold nanowires whose zero current conductance is quantized in units of 2e(2)/h. A faster than linear increase of current was observed at low voltages beginning from V-c similar or equal to 0.1V. We analyze the nonlinear behavior using a Luttinger liquid approach and show that it can be understood in terms of a dynamic Coulomb blockade effect.
Abstract: A technique was developed to fabricate and probe nanosize tunneling structures in thin metallic films. Using oblique evaporation through conventional undercut electron-beam lithographic masks, as the sample resistance was measured in situ, we defined constrictions with widths and lengths of about 10 nm in thin granular palladium films. The tunneling conductivity through a network of metallic grains was studied. Single electron tunneling transistor effects were registered. An electrostatic gate voltage at room temperature could clearly modulate Coulomb blockade offsets of the order of 0.1 V in the current-voltage curves. (C) 1998 American Institute of Physics. [S0003-6951(98)02250-5].
Abstract: When two metal electrodes are separated, a nanometer sized wire (nanowire) is formed just before the contact breaks. The electrical conduction measured during this retraction process shows signs of quantized conductance in units of G(0) = 2e(2)/h. Recent experiments show that the force acting on the wire during separation fluctuate, which is interpreted as due to atomic rearrangements, In this report we use a simple free electron model and show that the electronic contribution to the force fluctuations are comparable to the experimentally found values, about 2 nN.
Abstract: Three cryostats were designed and constructed where,the cooling is achieved without boiling in the main vessel: 1) smooth controlled raising of pressure of the cryoliquid inside the main vessel with SPM; 2) using the cooling cell where the cryoliquid boils at a temperature a little lower than that of the main vessel; 3) using a porous ceramic cryovessel for cooling the liquid by filtration and evaporation it through the 4 - 6 mm thick walls of the vessel.
Abstract: We have used an ultra-high vacuum variable temperature scanning tunnelling microscope to study the Cu(111) surface at temperatures from 90 K to 300 K. After the sample is heated to 900 K, adatoms enriched at the surface. Around these adatoms ring-formed standing-wave features can be seen in the local density of states (LDOS). When more than one of the adatoms were imaged it became evident that the adatoms preferred lateral distances in which they shared LDOS standing-wave maximas. This means that adatoms were positioned at multiples of half the Fermi wavelength (15 Angstrom) from each other. We ascribe the interaction that gave these results to the surface state electrons which form the LDOS standing waves. Furthermore the interaction was long-ranged (at least in the order of 80 Angstrom), oscillatory with the pair distance, and present at high temperatures since the adatoms stick to the surface above room temperature.
Abstract: We have used an ultra-high vacuum variable temperature scanning tunnelling microscope to study the Cu(111) surface at temperatures from 90 K to 300 K. After the sample is heated to 900 K, adatoms enriched at the surface. Around these adatoms ring-formed standing-wave features can be seen in the local density of states (LDOS). When more than one of the adatoms were imaged it became evident that the adatoms preferred lateral distances in which they shared LDOS standing-wave maximas. This means that adatoms were positioned at multiples of half the Fermi wavelength (15 Angstrom) from each other. We ascribe the interaction that gave these results to the surface state electrons which form the LDOS standing waves. Furthermore the interaction was long-ranged (at least in the order of 80 Angstrom), oscillatory with the pair distance, and present at high temperatures since the adatoms stick to the surface above room temperature.
Abstract: When two metal electrodes are separated, a nanometer-sized wire (nanowire) is formed just before the contact breaks. The electrical conduction measured during this retraction process shows signs of quantized conductance in units of G(o)=2e(2)/h. Recent experiments show that the force acting on the wire during separation fluctuates, which has been interpreted as being due to atomic rearrangements. In this paper we use a simple free-electron model, for two simple geometries, and show that the electronic contribution to the force fluctuations is comparable to the experimentally found values, about 2 nN, [S0163-1829(98)01816-5].
Abstract: Using scanning tunneling microscope (STM), we have fabricated steps of 4Hb-TaS2 with the height similar to 6 Angstrom and investigated the electronic and atomic structures on the two different layers near the step region at 4.2 K. The measured STM images and tunneling spectra revealed completely different atomic and electronic structures of the 1T and 1H type layers. The 1T type layers showed the typical root 13 x root 13 charge-density-wave (CDW) structures showing insulating behaviors, whereas the 1H type layers showed metallic behaviors and had the triangular atomic structure with a very weak 3x3 CDW superlattice at a low bias voltage and with a superposed root 13 x root 13 CDW superlattice at a high positive bias voltage. The bias dependent STM image on the surface of 1H layer can be explained by the energy dependent tunneling process between STM tip and a stack of metallic 1H layer and insulating 1T layer.
Abstract: A compact, less than 0.3 cm(3), scanning tunnelling microscope (STM) based on a new inertial slider design is presented. The STM is built of two concentric piezoelectric tubes, the inner one for scanning the tip and the outer one for inertial sliding of the sample to and from the tip. The coarse approach is made by inertial sliding of the sample along a plane that is non-perpendicular to the tip, while the sample surface is kept perpendicular to the tip throughout inertial sliding and image acquisitions. The compact design ensures a mechanical noise level of less than 0.1 Angstrom.
Abstract: When a scanning tunnelling microscope (STM) tip is driven into a metallic sample surface a nanometer sized wire (nanowire) is formed during the subsequent retraction. The electrical conduction measured during this retraction process shows signs of quantized conductance in units of 2e(2)/h. Due to the inherent non-reproducibility of the measured conductance curves a standard technique is to build histograms from a large number of curves. Such histograms, built with conductance experiments on gold nanowires at room temperature, show 3-4 peaks at integer values of 2e(2)/h, while in a low temperature mechanically controlled break junction study only the first peak is reported. In this work, histograms made up of thousands of consecutive curves at 4K are presented, showing up to 5 conductance peaks. An explanation for this discrepancy could be a higher nanowire temperature resulting from the higher retraction speed used in our measurements. However, a simple estimation, where we used macroscopic heat transport theory, resulted in a very low temperature increase, less than 1 mu k, ruling out this possibility. Thus, no significant difference with previous room temperature studies were observed, pointing to a conductance quantization that is the same at room and low temperature.
Abstract: In order to present a clear picture of the density of states at both sides of the Fermi level (E-F) in 1T-TaS2 near the nearly commensurate to commensurate transition (similar to 187 K) and at low temperatures, we have done a temperature-dependent tunneling spectroscopy study using a scanning tunneling microscope. We observed an abrupt transition from a charge-density-wave induced depletion to an opening of a deep pseudo gap near the E-F of the nearly commensurate to commensurate transition within similar to 1 K. The measured pseudo gap turned out to be deep enough to form localized states at E-F. In contrast to an inverse photoelectron spectroscopy study, our tunneling results indicate the gap structures of two Hubbard subbands resulting from band splitting of the Ta 5d band due to the electron correlation effect.
Abstract: We have investigated the charge-density-wave (CDW) formation associated with the periodic lattice distortion in 1T-VSe2 by using a low temperature scanning tunneling microscope (STM) at 4.2 K. To clarify the atomic and the superlattice structures, we used a layer etching technique to fabricate a step of 1T-VSe2 with a height of approximate to 6 Angstrom and investigated the atomic structures at 4.2 K on the newly etched region. The measured images showed a clear isotropic 4 x 4 charge-density-wave structure which was different from the previous results obtained by Coleman et al. The high resolution STM images at 4.2 K showed that the center of CDW was surrounded by three surface atoms from those images we were able to identify those atoms as Se. The surface Se atoms determined the STM pattern because of their proximity to the scanning tip.
Abstract: In situ Na intercalation of the layered compound VSe2 has been studied with photoemission and scanning tunneling microscopy. Core-level spectroscopy proves that Na deposited in UHV onto the VSe2 surface rapidly intercalates, leaving only small amounts at the surface. The scanning tunneling microscopy measurements show that the intercalated Na is not uniformly distributed between the VSe2 layers, but preferentially in two-dimensional islands. Thus the surface region is divided into intercalated and nonintercalated areas. Hole-like features in the intercalated areas are interpreted as locally missing Na.
Abstract: Conductance experiments on Bi nanowires at 4 K, obtained with a scanning tunneling microscope, are presented. The conductance of these Bi nanocontacts, formed between two Bi electrodes, exhibits plateaus at quantized values of G(0) = 2e(2)/h, remaining basically constant during electrode separations of about 50 nm. This is the first rime that such plateaus have been observed in semimetals. This histogram of conductance values, constructed with thousands of consecutive contact breakage conductance experiments. exhibits clear peaks at G(0) and 2G(0).
Abstract: We have studied the atomic and electronic structures of the two different layers in 4Hb-TaS2 prepared by a layer-by-layer etching technique using a scanning tunneling microscope at 4.2 K, One layer (1T) showed the typical root 13x root 13 charge-density-wave structure, whereas the other layer (1H) had at low bias a triangular atomic structure with weakly superposed 3x3 and at high positive bias a root 13x root 13 charge-density-wave structure originating from the lower 1T layer due to tunneling through the top 1H layer. The bias-dependent-intensity of this charge-density-wave structure was shown to be consistent with room-temperature measurements, showing that this is a real intrinsic property of the material. Measured tunneling spectra of each layer at 4.2 K showed a metallic 1H layer and an insulating 1T layer with an opening of wide-gap structures at the Fermi level.
Abstract: Conductance experiments on Bi nanowires at 4K, obtained with a Scanning Tunneling Microscope, are presented. The conductance of these Bi nanocontacts, formed between two Bi electrodes, exhibits plateaus at quantized values of G(0)=2e(2)/k, remaining basically constant during electrode separations of about 50 nm. This is the first time that such plateaus have been observed in semimetals. The histogram of conductance values, constructed with thousands of consecutive contact breakage conductance experiments, exhibits clear peaks at 1 and 2 G(0). In addition, current-voltage characteristics obtained during the contact breakage are presented, showing preliminar evidence of conductance quanta transitions due to the change in the applied bias voltage.
Abstract: Tl2Ba2CuO6 (Tl-2201) thin films were synthesised on single crystal LaAlO3 and SrTiO3 substrates and on bicrystal LaAlO3 by an ex-situ process. Depending on thallium content and processing conditions, the films were orthorhombic or tetragonal. T-c was in the range 30 - 92 K while J(c) reached 4.10(6)A/cm(2) at 12 K. By oxygen depletion, T-c was increased by 15 - 20 K for some films.
Abstract: Using an ultra high vacuum (UHV) scanning tunneling microscope where the sample is cooled while the tip remains close to room temperature, we measured the thermoelectric voltage induced by the large thermal gradient between the tip anti sample. Using tunneling thermometry technique we studied the change of electronic structures of 1T-TaS2 front room temperature to 40 K. At the temperature for the nearly commensurate to commensurate transition, we found an abrupt change of the thermoelectric power from a very small value to about 0.15 mV/K.
Abstract: We have used scanning tunneling spectroscopy to measure temperature dependent tunneling spectra of 1T-TaS2 from room temperature to similar to 40 K. An abrupt transition from the charge-density-wave induced broad depletion to an opening of a deep pseudo gap was observed within similar to 1 K of the nearly commensurate to commensurate transition temperature. This pseudo gap was deep enough to form localized states at the Fermi level. Our tunneling results therefore suggest, in contrast to inverse photoelectron spectroscopy, that the band splitting of the Ta 5d band is due to the electron correlation effect.
Abstract: The surface of polyimide (PI) films for aligning liquid crystals were studied by means of atomic force microscopy (AFM). The surface of the unrubbed PI films consisted of polymer clusters of different sizes which are randomly distributed over the film area. After rubbing, however, these polymer clusters formed long chains along the rubbing direction. The cluster chains were separated with periodicity about 100 nm for weak rubbing strength. Deeper grooves are also present, separated with periodicity about 2 mu m that could be related to the microstructure of the fibres from the rubbing cloth. On increasing the rubbing strength further on, the cluster chains coalesce into wider ones. The rubbed PI films show optical retardation, which was increased rapidly with increased the rubbing strength and reached a constant value of about 1.4nm.
Abstract: We have studied the atomic and electronic structures of 4Hb-TaS2, which has alternating layers of the 1T and 1H type, at room temperature and 77 K, using a scanning tunneling microscope. Using a layer-by-layer etching technique, we fabricated staircases with alternating layers of the 1T and 1H type. The T-type layers showed the typical root 13 X root 13 charge-density-wave structures, whereas the H-type layers had the triangular atomic structure at both temperatures. The measured tunneling spectra of each layer at 77 K showed entirely different characteristics; the 1H layer remained in the metallic state, whereas the 1T layer showed an insulating behavior with a wide opening of the energy gap at the Fermi level at 77 K.
Abstract: Planarized layers of YBa2Cu3O7 (YBCO) were made by etching trenches in SrTiO3 (STO) substrates laser depositing a YBCO film mechanically polishing film down to the substrate surface. These structures exhibited critical temperatures (T-c) of 88 K and a critical current density (J(c)) of 10(6) A/cm(2) at 77 K. The planarized surface was smooth, with a maximum height difference between the YBCO STO of 20 nm. The surfaces were used as templates for epitaxial growth of multilayer insulators of STO and PrBa2Cu3O7 (PBCO) and top YBCO layers. Complete crossovers, free of superconducting shorts, with T-c of 86 K and critical current density (J(c)) of 2x10(5) A/cm(2) were made.
Abstract: A new method is introduced to study transport in a mesoscopic sample. The electric field from an STM-tip is used to locally influence a mesoscopic object. We detect abrupt changes in the mesoscopic signal both as a function of the tip position and tip-sample voltage. They could be interpreted as due to spatial shifts or changes in the activity of scattering centers in the sample.
Abstract: We report a technique for the fabrication of sharp and straight step edges on LaAlO3 (LAO) substrates by ion milling. An electron beam lithography defined amorphous carbon film was used as an etch mask. It had very low ion milling rate and was easily prepared and removed. Atomic force microscopy was used to determine the step profile. YBa2Cu3O7 step edge junctions fabricated at the LAO steps show promising results. An I(c)R(n) product of 1 mV was obtained at 30 K. A Fraunhofer-like magnetic field dependence of I(c) was obtained up to +/-2 PHI0. One weak link or possibly identical weak links in series for these step edge junctions were observed from the current-voltage (I-V) curves as well as from the magnetic field dependence of the I-V curves.
Abstract: A compact scanning probe microscope for operation in air and liquid is described. The probe techniques implemented are scanning tunnelling microscopy and scanning ion conductance microscopy. The software, electronics, mechanical construction and some representative measurements will be presented here. The compact and concentric microscope head is built around a commercial piezoelectric inchworm motor. The scanner is a standard piezo tube. An analogue feedback system is used for taking images, while digital control of the probe-sample distance is used for other experiments, such as measurements of current-voltage characteristics. A Macintosh personal computer is used for control and presentation of data. A simple method to make scanning tunnelling microscope tips suitable for electrochemical use is described. The microscope has a high resonance frequency (9.6 kHz), low noise (0.01 nm Hz-1/2 at 10 Hz), low thermal drift (less than 0.1 nm min-1), and high acoustical noise suppression. The current-distance-dependency of the scanning ion conductance microscope was found to be linear.
Abstract: A new method is introduced to study electron transport on the mesoscopic scale. A scanning tunnelling microscope (STM) tip is used both to form a point-contact potential probe to a thin film and to affect scattering centres in its vicinity. We detect abrupt changes in the voltage with this probe as a function of both tip position and tip-sample voltage. These changes could be interpreted as due to spatial shifts of scattering centres in the film surface.
Abstract: An ex situ process has been developed to produce thin superconducting Tl2Ba2CaCu2O8 films. The properties of films grown on different substrates using different annealing regimes were studied. Critical temperatures of 103-107 K were measured on films prepared in a broad range of annealing temperatures on SrTiO3, LaAlO3, and Y-ZrO2 substrates. A critical current density, J(c), of 2 x 10(6) A/cm2 at 77 K was measured on LaAlO3. Film morphology was studied by SEM, AFM, and STM.
Abstract: We have used a dilution refrigerator cooled STM for investigating the vortex state in lead-bismuth alloy 4000 angstrom films. The vortices were recognized by the decreasing dynamic conductance near the gap voltage when the instrument was operated in its normal feed back mode. A clean surface suitable for tunneling was maintained by a 75 angstrom layer of gold deposited on top of the sample film. The temperature was kept at 80 mK and magnetic fields between 0 and 0.2 T were used.
Abstract: The early stages of growth of high quality YBa2Cu3O7-delta (YBCO) films grown on (001) Y-ZrO2 (YSZ) substrates by pulsed laser deposition have been studied using a combination of atomic force microscopy and transmission electron microscopy. A one unit cell thick YBCO layer and relatively large CuO particles formed in the initial stages. Additional YBCO grew on top of the first layer in the form of one or a few unit cell high c-axis oriented islands about 30 nm in diameter. The rounded islands subsequently coalesced into faceted domains. Elongated Y2BaCuO5 particles nucleated after the first layer of YBCO. A highly textured BaZrO3 layer formed between the YSZ and the YBCO with a cube-on-cube dominant orientation relationship with respect to the YBCO film.
Abstract: A scanning tunneling microscope suitable for very low temperatures has been designed, and preliminary testing has been carried out. In order to improve cooling and temperature uniformity the instrument is arranged for operation immersed in the He-3-He-4 mixture inside the mixing chamber of a small dilution refrigerator. A discussion of the specific problems present in the design of this kind of an instrument is given as well as a description of our design. Special attention is given to the vacuum sealing and vibration-damping solutions required.
Abstract: Using an STM in air, we have studied three different electropolished Ti surfaces. One sample was analyzed without further treatment. The second was thermally oxidized in air at 500-degrees-C. The third was exposed to an argon glow discharge and then oxidized in pure oxygen. The samples were characterized by Auger spectroscopy prior to STM measurements. Electropolished and thermally oxidized samples showed a granular structure, with a low corrugation between 2 and 10 nm. At a larger scale, the glow discharge treated sample showed a high corrugation approximately 100 nm.
Abstract: High-quality YBCO thin films deposited on yttria-stabilized zirconia substrates by laser ablation have been studied by scanning tunneling microscopy in air. The films have been grown at different substrate temperatures, and the measured critical current density (J(c)) and surface resistance (R(s)) have been shown to vary with deposition temperature; where the J(c) decreased with deposition temperature, the R(s) increased. These variations were related to a change in the film surface morphology, showing screw dislocations for the films with lower deposition temperature, while at higher temperatures these dislocations were absent.
Abstract: High quality superconducting YBa2Cu3O7-delta (YBCO) thin films were fabricated with ex situ co-evaporation on SrTiO3 and in situ laser deposition on (001) MgO substrates. The co-evaporated films gave a superconducting transition temperature, T(c) approximately 90 K, and a critical current density, j(c) approximately 5 x 10(5) A/cm2 at 77 K after an ex situ post anneal at 890-degrees-C. For laser deposited films, T(c) was above 90 K, and j(c), about 10(6)A/cm2 at 77 K. Two kinds of Josephson weak links were developed. The first kind consisted of a 0.8-mu-m or wider constriction in a co-evaporated YBCO film. A lift-off stencil was used to pattern the film before the post anneal. The second kind of weak link utilized the inherent grain boundaries in the laser deposited YBCO film. A 70-130 nm wide strip in Al was deposited across a 4-6-mu-m wide YBCO bridge. After a 10-20 min long anneal at 300-450-degrees-C, the Al had diffused into the film. We believe that the diffusion occurs mainly along the grain boundaries. By the proximity effect, the normal metal weakened the superconductivity at the grain boundary, and a weak link was formed with I(c)R(N) approximately 0.9 mV at 77 K. Dry an wet etching were also used to pattern some structures.
Abstract: We briefly present a method for the parameterization of assembly systems derived from their ability to form unique structures. The concept of bond uniqueness is introduced and we show how it influences the number of unique structures that a programmable, or algorithmic, self-assembly system can create. Further, we argue that programmable self-assembly systems create embedded, additional computation that is reflected in the complexity of the generated structures and show how this complexity is related to the bond uniqueness of the building blocks. A brief introduction to sticky graphs, a mathematical tool for modeling self-assembly systems, is given. From the theoretical discussions it becomes clear that building blocks for programmable self-assembly need to have at least four distinct, geometrically separated bonds. A scheme for the production of building blocks with well-directed bonds for programmable self-assembly using DNA-nanoparticles is presented. The introduced procedure is a completely bottom-up approach and can be used to produce quite advanced PSA building blocks like nanoparticle eight-mers with eight bonds. Initial experiments are presented.
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