In-situ STM studies of electrochemical underpotential deposition of Pb on Au(111)

A modified scanning tunnelling microscope (STM) has been used to observe in-situ deposition and stripping of an electrochemical film. With STM tip and sample immersed in an acid electrolyte, single atomic steps on Au(111) have been imaged during the deposition and stripping of a monolayer-thick, underpotential deposit (UPD) of Pb. Integration of the electrochemical current passed during the film deposition and evidence from the STM images themselves confirm monolayer coverage. Our images show enhanced film growth at step edges and defect sites. Observations of single plating and stripping cycles indicate that the Au substrate returns unaltered. Except for atomic resolution images of Au(111), which we have not yet achieved in an electrolyte, all types of Au surface features seen in air are reproduced under the electrolytic solution. The modifications made to our STM in order to perform in-situ electrochemical experiments are described.     

STM imaging of noble metal electrodes in solution under potentiostatic control

We have combined a three-electrode cell arrangement with a scanning tunnelling microscope (STM) to image the surfaces of polycrystalline noble metal electrodes under potentiostatic conditions. STM imaging was made in the potential range where a tip and the electrodes are ideally polarized. We have observed on a submicron scale that the electrochemically roughened Ag electrode surface relaxed over times of 40 min at ?0·16 V versus SCE in 0·1 m KCl. We have also imaged a dynamic process (formation and dissociation) of the submicron-scale reconstruction of the Au electrode subjected to electrochemical oxidation-reduction cycles. The results can be explained by the absorption of Cl^? ions and the surface diffusion of adatoms or clusters. STM imaging under potentiostatic control leads to the discovery of phenomena occurring at solid metal/electrolyte interfaces.     

Scanning tunnelling microscopy of 1-D and 2-D charge-density wave systems

We have used a new variable temperature scanning tunnelling microscope (STM) to study quasi-1D and 2-D charge-density wave (CDW) systems. The 1-D systems, typified by NbSe₃ and TaS₃, are of special interest since they exhibit unusual transport phenomena associated with moving CDW above a threshold electric field. In the case of NbSe₃, room temperature STM images show both major and subtle details of the lattice structure. At present, however, images taken below the Peierls transition temperature of T_P=144 K resolve major lattice details but are not sufficiently clear to resolve the CDW. On the other hand, for the fully gapped CDW system orthorhombic-TaS₃, the CDW modulation superimposed on the lattice structure and having the correct period of four times the S-S spacing of 3·3 Å, is observed below T_P=215 K. Above T_P, the main observable feature is the S-S spacing along the chains. STM measurements have also been performed on the 2-D CDW system 1T-TaS₂ in its incommensurate, nearly commensurate, fully commensurate and trigonal phases. For the nearly commensurate phase, STM images show uniform commensurability with a relatively low concentration of small, time-varying discommensurations in contrast to models pradicting a regular domain structure. In the trigonal phase, however, evidence is seen for the striped phase composed of long, nearly parallel discommensurations.     

STM and AFM investigations of high-Tc superconductors

We have studied the (001) surface of single crystal YBa₂Cu₃O_7-x high-T_c superconductors using scanning tunnelling microscopy (STM) and atomic force microscopy (AFM) at room temperature at ambient pressure. Both methods show flat terraces with steps which are multiples of the c-axis lattice constant (of 1·17 nm) high. Our results show that the bulk crystal structure extends to the surface and that the crystals were formed by island growth. Only occasionally tunnelling was possible with sample bias voltages below +1·0 V. We interpret the observed voltage dependence and the difficulty to get good STM images to be due to the presence of a less-conducting surface layer. Auger spectroscopy indicates that carbon is present at the surface, which is probably related to a contamination layer.     

Anomalous voltage dependence of tunnelling microscopy in WSe2

We present scanning tunnelling microscopy (STM) investigations of the layered semiconductor WSe₂. The tunnelling experiments were performed in air and under silicone oil with markedly different results. In air, atomic resolution images of the hexagonally structured surface could be obtained for sample-to-tip voltages of both negative and positive polarities, from ?1·5 to ?0·3 V for negative sample and from +0·6 to +1·6 V for positive sample, respectively. Under silicone oil, however, good atomic images could be seen for negative sample biases down to at least ?14 V, while for positive sample biases no difference with respect to the tunnelling in air was found.     

A spectroscopic study of some high-temperature superconductors using a low-temperature STM

We used a scanning tunnelling microscope (STM) to measure both the tunnel current, I, and the dynamic conductance, dI/dV, at 4·2 K for a number of high-transition temperature oxide superconductors. Large spatial variations in the tunnelling characteristics are observed. At low tunnel resistances, all samples show evidence of single electron tunnelling and incremental charging. Results on BiSrCaCu₂O_x show the coexistence of charging with Josephson coupling between grains within the sample. Results on both the Bi sample and a single crystal of YBa₂Cu₃O_6·5+x reveal possible energy gap (2A) values of 17 and 20 meV, respectively. A very sharp 5 meV gap, observed in ceramic samples of YBa₂Cu₃O_6·5+x and Y_0·5Al_0·05Ba₂Cu₃O_6·5+x, may indicate the presence of a lower temperature phase in these samples.     

STM imaging of the complete bacterial cell sheath of Methanospirillum hungatei

Using a large range scanning tunnelling microscope (STM) we have obtained images of the complete outer cell wall of the archaebacterium Methanospirillum hungatei, deposited on a graphite substrate and over-coated with an Au or Pt evaporated film. The width of the collapsed cylindrical sheath structure was found to be 5000 Å ± ***10%, which agrees closely with the value from previously published electron microscope (EM) studies. The double thickness of the collapsed sheath was found to be 160 Å ± 10%, which is about 20% smaller than that from the EM results. Higher resolution STM images taken on top of the collapsed sheaths show corrugations running perpendicular to the cylinder axis and having widths which are multiples of ～ 30 Å, the minimum period expected from EM studies. The height of the corrugations have a minimum value of about 4 Å. The expected 2-D crystalline structure was not seen in the STM images.     

Scanning probe microscopy of biological samples and other surfaces

Scanning probe microscopes derived from the scanning tunnelling microscope (STM) offer new ways to examine surfaces of biological samples and technologically important materials. The surfaces of conductive and semiconductive samples can readily be imaged with the STM. Unfortunately, most surfaces are not conductive. Three alternative approaches were used in our laboratory to image such surfaces. 1. Crystals of an amino acid were imaged with the atomic force microscope (AFM) to molecular resolution with a force of order 10^?8 N. However, it appears that for most biological systems to be imaged, the atomic force microscope should be able to operate at forces at least one and perhaps several orders of magnitude smaller. The substitution of optical detection of the cantilever bending for the measurement by electron tunnelling improved the reliability of the instrument considerably. 2. Conductive replicas of non-conductive surfaces enabled the imaging of biological surfaces with an STM with a lateral resolution comparable to that of the transmission electron microscope. Unlike the transmission electron microscope, the STM also measures the heights of the features. 3. The scanning ion conductance microscope scans a micropipette with an opening diameter of 0·04-0·1 μm at constant ionic conductance over a surface covered with a conducting solution (e.g., the surface of plant leaves in saline solution).     

Metal-induced reconstructions of the silicon(111) surface

We have used scanning tunnelling microscopy (STM) to study changes in the structure of the Si(111) surface induced by deposition of the group III metals In and Ga. For both metals, several different ordered reconstructions are seen as a function of coverage. The STM images provide new structural information on each of these reconstructions. With In metal deposition, we have seen the surface reconstructions √3×, √3, √31× √31, √7×√3 and 4×1 as the coverage increases. In the case of Ga on Si(111), we have studied structures that exist up to 0·7 ML. At 1/3 ML, there is a √3×√3 structure identical to that of In. Above 0·3 ML there is a different phase that may correspond to the (6·3times6·3) RHEED pattern reported in this coverage range. This surface tends to grow as triangular islands at higher coverages.     

Scanning tunnelling microscopy of biomacromolecules

When imaging biomacromolecules with a STM, coating of specimens with a conductive layer is a convenient preparation method which provides a good rate of success. Utilizing evaporated platinum/carbon as a coating film we have investigated two biomacromolecules of very different appearance. The first of these is the HPI-layer, a natural two-dimensional protein crystal with a period of 18 nm, which is found on the surface of the bacterium Deinococcus radiodurans. The second specimen is type IV collagen which forms long triple-helical strands approx. 1·5nm in diameter. The resulting STM pictures compare very well with electron microscopical images.     

Formation of large ordered domains in benzenethiol self-assembled monolayers on Au(1 1 1) observed by scanning tunneling microscopy

Benzenethiol (BT) self-assembled monolayers (SAMs) on Au(1 1 1) were prepared as a function of solution temperature after immersion in a 1 mM ethanol solution for 1 day. The surface structures of BT SAMs were examined by means of scanning tunneling microscopy (STM). Although BT molecules usually form disordered SAMs containing the Au adatom islands at room temperature, we found that they formed two-dimensional ordered SAMs containing a large size domain at a high solution temperature of 50 or 75 °C. High-resolution STM imaging revealed that BT SAMs on Au(1 1 1) formed at 50 °C have a (2×3√2)R23° packing structure. From our STM study, we revealed that two-dimensional ordered BT SAMs on Au(1 1 1) can be obtained by changing the solution temperature.     

Interfacial reactions in PZT/Pd/PZT sandwich structures

The interfacial reactions of palladium foil and lead zirconate-titanate (PZT) were studied using samples with a sandwich structure in the temperature range 1373–1523 K and under conditions where no lead is lost to the environment. The interfacial reactions were analysed using scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction and wavelength-dispersive X-ray spectroscopy analysis. The density of the PZT powder phase increased with increasing temperature and, when sintered above 1373 K, reached over 95% of the theoretical density of PZT. The weight loss of pellets was less than 0·8% when sintered below 1523 K. The degree of interfacial reactions became more severe with increasing temperature, as indicated by an expanding reacted region. The reaction at the PZT side of the Pd/PZT interface involved the decomposition of PZT into a monoclinic ZrO₂ phase, PbO and a lower x-value Pb(Zr_xTi₁__x)O₃ composition. Three distinguishable microstructures exist on the Pd side when sintered below 1473 K: a thin layer of PbPd₃ phase, a Pd–Pb solid solution zone and an unreacted region. Only the cubic PbPd₃ eutectic structure was found when sintered above 1473 K. The oxidation of palladium occurred during interfacial reactions, expedited by increasing temperature and resulting in the formation of the tetragonal PdO phase and the hexagonal PbPdO₂ phase. A model for the overall reaction is proposed involving decomposition of the PZT, migration of PbO and diffusion of Pb into Pd foil.     

Observation of removal of an Fmoc protecting group by scanning tunneling microscopy

We demonstrate here by imaging successive surface reactions in self-assembled monolayers (SAMs) on Au(1 1 1) at molecular scale with a scanning tunneling microscope (STM): (i) SAM matrices formation with 1-octanethiol on Au(1 1 1) in ethanol, (ii) insertion of N-Fmoc-aminooctanethiol into the SAM matrices in ethanol, and (iii) removal of the Fmoc protecting group with tris(2-aminoethyl)amine (TAEA). The total reaction is formation of SAMs containing a small amount of NH₂ terminated molecules in the CH₃ terminated SAM matrices. After the reaction of the protecting group with TAEA, STM imaging revealed the decrease in heights of the inserted molecules on average. We attributed this observation to removal of the protecting group by taking account of a convolution of electronic and topographic contributions to observed STM heights. Apparent areas of the terminal groups, however, became larger on removal. The increase in the areas was attributed to water adsorption to the NH₂ terminal group under air.     

Characterization of the interaction between bovine serum albumin and doxycycline by chemiluminescence and molecular docking

The interaction of bovine serum albumin with doxycycline was investigated using chemiluminescence and molecular docking. Doxycycline at concentrations from 1.0 to 2.5 × 10³ pmol · L^?1 quenched the chemiluminescence from the luminol–bovine serum albumin system. The data were analyzed using a chemiluminescence mathematic model for protein–ligand interaction, log[(I₀ – I)/I] = logK + nlog[D]. The binding constant of bovine serum albumin with doxycycline was 3.36 × 10⁵ L · mol^?1 at 298 K with one binding site. The binding constant, enthalpy change, entropy change, and binding free energy change showed that the bonding of doxycycline to bovine serum albumin was spontaneous and enthalpy driven via hydrogen bonding and van der Waals forces. Further molecular docking analysis substantiated that doxycycline was well positioned in the pocket at the subdomain IIA (site I) of bovine serum albumin with a binding constant of 3.31 × 10⁵ L · mol^?1. Doxycycline served as both a hydrogen acceptor and donor and mainly interacted with the Arg217 residue through four hydrogen bonds with an average length of 2.55 Å. The chemiluminescence mechanism of doxycycline on luminol–bovine serum albumin system was evaluated, showing that a ternary complex of luminol–bovine serum albumin–doxycycline was formed with luminol and doxycycline at sites III and I of bovine serum albumin.     

Measurements of stretch lengths of gold mono-atomic wires covered with 1,6-hexanedithiol in 0.1 M NaClO4 with an electrochemical scanning tunneling microscope

Stretch lengths of pure gold mono-atomic wires have been studied recently with an electrochemical scanning tunneling microscope (STM). Here, we will report a study of stretch lengths of gold mono-atomic wires with and without 1,6-hexanedithiol (HDT) using the STM break-junction method. First, the stretch length was measured as a function of electrode potentials of a bare Au(1 1 1) substrate and a gold STM tip in a 0.1 M NaClO₄ aqueous solution. Second, a self-assembled monolayer (SAM) was fabricated on an Au(1 1 1) substrate by dipping the substrate into a 1 mM HDT ethanol solution. At last, we measured the stretch length of gold mono-atomic wires on a substrate covered with the SAM in place of the bare Au(1 1 1) substrate. We compared the electrode potential dependence of the stretch lengths of gold mono-atomic wires covered with and without HDT. We will discuss the effect of the electrode potential on the stretch lengths by taking account of electrocapillarity of gold mono-atomic wires.     

Two-dimensional ordering of pentafluorobenzenethiol self-assembled monolayers on Au(1 1 1) prepared by ambient-pressure vapor deposition

Formation and surface structures of pentafluorobenzenethiol (PFBT) self-assembled monolayers (SAMs) on Au(1 1 1) prepared by ambient-vapor phase deposition were examined by means of scanning tunneling microscopy (STM) as a function of deposition temperature. PFBT SAMs formed at room temperature have disordered phases with bright aggregated domains, which are very similar to benzenethiol SAMs. As deposition temperature increases to 50 °C, partially ordered domains and large aggregated domains appeared. High-resolution STM clearly revealed that PFBT SAMs formed at 75 °C were composed of long-range, two-dimensional (2D) ordered domains, which can be described as a c(2×√3) structure. The results of this study clearly demonstrate that deposition temperature is a crucial factor for obtaining PFBT SAMs on Au(1 1 1) with a high degree of structural order.     

Design and operation of a variable temperature scanning tunnelling microscope

A new variable temperature STM has been developed which utilizes two concentric piezoelectric tubes; an inner scanning tube, and an outer thermal compensation tube which also provides for inertial translation of the sample into tunnelling range. With this design, continuously variable temperature operation is demonstrated for the first time in an STM. Also, by eliminating all mechanical components such as springs, levers and gears, which normally couple directly to the tunnelling gap in other designs, atomic resolution operation is demonstrated in which no vibration isolation is necessary. During operation, the inside of the scanning tube is maintained at ground potential while the feedback signal is electronically summed to the scanning voltages applied to the outer quadrants. In addition to shielding the sensitive tunnelling circuit, this mode of operation enables one to electronically balance out mechanical imperfections of the scanning tube. To date, this new STM has been operated over the 77–400 K temperature range with the observed thermal drift as low as 1 Å/h and 10 Å/K. Another useful feature of this new design is the ability to reposition a sample to within 200 Å of the same location after it has been translated macroscopic distances (several mm) out of tunnelling range.     

Imaging an optical disc by the combined use of scanning tunnelling microscopy and scanning electron microscopy

We present the data obtained by scanning tunnelling microscopy combined with scanning electron microscopy of the digitally encoded structure on a stamper used to fabricate optical discs. The combination allows us to focus the STM tip on a preselected spot with a precision of ?0·3 μm. The data show the superiority of STM for a more detailed characterization of shape, width, length, height and fine structure appearing on the sample. We also show the influence of tip shape on STM resolution. Simultaneous use of both microscopes is possible but high electron doses produce an insulating layer of contaminants thick enough to make STM operation impossible.     

Scanning tunnelling microscopy of tobacco mosaic virus on evaporated and sputter-coated palladium/gold substrates

We present scanning tunnelling microscope (STM) images of untreated tobacco mosaic virus (TMV) deposited on thermally evaporated and on sputter-coated palladium/gold 40:60 (Pd/Au) substrates, and imaged under ordinary atmospheric conditions. The TMV imaged on both evaporated and sputter-coated substrates was consistently several times wider than the known diameter of the virus. TMV on evaporated Pd/Au became overcoated with Pd/Au material during sample preparation and appeared elevated in STM images, whereas TMV on sputter-coated Pd/Au appeared as depressions. When naked TMV were intentionally overcoated with Pd/Au, the STM images were found to be similar to those for TMV on evaporated Pd/Au.     

In situ,atomic scale observation of electrode topography and reactions

The topographical changes occurring on a Au(111) electrode during simple electrochemical reactions have been followed with an STM capable of operating in situ, i.e. in the electrochemical environment, and of yielding atomic scale resolution. Changes in the surface structure induced by adsorption and desorption of Cl^? ions could be observed and the effects of subjecting the sample to an oxidation-reduction cycle could be monitored.