J.W. Gerritsen

Macroscopic Hierarchical Surface Patterning of Porphyrin Trimers via Self-Assembly and Dewetting

The use of bottom-up approaches to construct patterned surfaces for technological applications is appealing, but to date is applicable to only relatively small areas (∼10 square micrometers). We constructed highly periodic patterns at macroscopic length scales, in the range of square millimeters, by combining self-assembly of disk-like porphyrin dyes with physical dewetting phenomena. The patterns consisted of equidistant 5-nanometer-wide lines spaced 0.5 to 1 micrometers apart, forming single porphyrin stacks containing millions of molecules, and were formed spontaneously upon drop-casting a solution of the molecules onto a mica surface. On glass, thicker lines are formed, which can be used to align liquid crystals in large domains of square millimeter size.

Real-time single-molecule imaging of oxidation catalysis at a liquid-solid interface

Many chemical reactions are catalysed by metal complexes, and insight into their mechanisms is essential for the design of future catalysts. A variety of conventional spectroscopic techniques are available for the study of reaction mechanisms at the ensemble level, and, only recently, fluorescence microscopy techniques have been applied to monitor single chemical reactions carried out on crystal faces and by enzymes. With scanning tunnelling microscopy (STM) it has become possible to obtain, during chemical reactions, spatial information at the atomic level. The majority of these STM studies have been carried out under ultrahigh vacuum, far removed from conditions encountered in laboratory processes. Here we report the single-molecule imaging of oxidation catalysis by monitoring, with STM, individual manganese porphyrin catalysts, in real time, at a liquid-solid interface. It is found that the oxygen atoms from an O2 molecule are bound to adjacent porphyrin catalysts on the surface before their incorporation into an alkene substrate.

Interlaboratory round robin on cantilever calibration for AFM force spectroscopy

Single-molecule force spectroscopy studies performed by Atomic Force Microscopes (AFMs) strongly rely on accurately determined cantilever spring constants. Hence, to calibrate cantilevers, a reliable calibration protocol is essential. Although the thermal noise method and the direct Sader method are frequently used for cantilever calibration, there is no consensus on the optimal calibration of soft and V-shaped cantilevers, especially those used in force spectroscopy. Therefore, in this study we aimed at establishing a commonly accepted approach to accurately calibrate compliant and V-shaped cantilevers. In a round robin experiment involving eight different laboratories we compared the thermal noise and the Sader method on ten commercial and custom-built AFMs. We found that spring constants of both rectangular and V-shaped cantilevers can accurately be determined with both methods, although the Sader method proved to be superior. Furthermore, we observed that simultaneous application of both methods on an AFM proved an accurate consistency check of the instrument and thus provides optimal and highly reproducible calibration. To illustrate the importance of optimal calibration, we show that for biological force spectroscopy studies, an erroneously calibrated cantilever can significantly affect the derived (bio)physical parameters. Taken together, our findings demonstrated that with the pre-established protocol described reliable spring constants can be obtained for different types of cantilevers.

High‐stability scanning tunneling microscope

We have constructed a scanning tunneling microscope for operation under UHV conditions (10−8 Pa). With this instrument topographic measurements can be made on metal and semiconductor surfaces by means of a scanning tip electrode, driven by piezoelectric ceramic elements. The maximum area covered ranges up to 4000×4000 A2 with a resolution better than 10 A laterally and 0.15 A rms perpendicular to the plane. Because of its compact design, the scan unit is very insensitive to vibrations and has a response time down to 0.3 ms. This allows a high scan rate to be used. In order to minimize temperature effects, special attention is paid to the geometry of the construction and the materials used, resulting in a drift ≤4 A/min along the surface and 0.5 A/min perpendicular.

Morphology and surface topology of YBa2Cu3O7-x crystals; theory and STM observations

A network analysis according to the periodic bond chain (PBC) theory of Hartman and Perdok, which predicts the macroscopical morphology and some aspects of the microscopic surface topography, is presented for the high Tc superconductor YBa2Cu3O7-x. We find that growth forms will be characterised by four possible crystal face with a morphological importance (MI) given by MI{001} ≈ 8 MI{010} ≫ MI{103} > MI{110}. Scanning tunneling microscopy (STM) measurements of the microscopica surface structures on the {001} crystal surface support the network analysis and yield further information of the growth process. Among the results are the first observations of small {103} facets occuring on the {001} surface.

Synthesis and self-assembly of giant porphyrin discsElectronic supplementary information (ESI) available: experimental procedures and characterization data, NMR- and UV/Vis-titration data. See http://www.rsc.org/suppdata/cc/b4/b401324g/

A giant porphyrin disc (M(w)= 15 kDa) has been synthesized and its self-assembly behaviour at an interface studied by liquid STM which reveals the presence of huge domains (>400 x 400 nm2) of very well ordered and molecularly resolved columnar stacks.

Scanning tunnelling microscopy study of polypyrrole films and of glucose oxidase as used in a third-generation biosensor

Abstract We have studied the surface morphology of different forms of the organic conductor polypyrrole, which serves as the environment for immobilization of the redox enzyme glucose oxidase. Scanning tunnelling microscopy (STM) images of polypyrrole films, obtained electrochemically on highly oriented pyrolytic graphite, STM images of polypyrrole microtubules and STM images of glucose oxidase molecules adsorbed on a gold facet are presented and discussed. The conclusions from this study are: (i) the polypyrrole films, prepared chemically or electrochemically, exhibit disordered non-crystalline structure; (ii) the polypyrrole surface corrugations and the dimensions of the glucose oxidase molecule are of the same order of magnitude, allowing strong adsorption of the enzyme to the conducting polymer. On the basis of these data, a model for the direct interaction between glucose oxidase and polypyrrole in the biosensor is proposed.

Tip for scanning tunneling microscopy made of monocrystalline, semiconducting, chemical vapor deposited diamond

A tip for scanning tunneling microscopy (STM) was fabricated from semiconducting, chemical vapor deposited (CVD) diamond, epitaxially grown on a natural diamond substrate. Extremely high, p‐type conductivity was realized by heavy boron doping. A sharp tip was obtained by conventional diamond polishing in such a way that the ultimate tip (radius<12 nm) is situated in the electrically conductive CVD layer. Atomic resolution on graphite surfaces could easily be obtained under normal operating conditions for STM in air. The feasibility of using the diamond tip to create nanostructures on surfaces was also investigated.

Small metallic particles studied by scanning tunneling microscopy

Abstract Low-temperature Scanning Tunneling Microscopy (STM) and spectroscopy have been used to study single small metallic particles. The dominating feature of the current-voltage characteristic of the double tunnel junction formed by STM tip, particle, and conducting substrate, is the Coulomb staircase. In this paper we describe some deviations from the standard Coulomb staircase. The first deviation, a variation in time of the characteristics, we ascribe to trapping and detrapping of electrons in the oxide tunnel barrier in the environment of the particle. The second deviation, observed for the ligand stabilized clusters Pt 309 , consists of fine structure on the Coulomb staircase which might be ascribed to a quantum size effect.

Scanning tunneling microscopy observations of metallic clusters Pd561 and Au55 and the implications of their use as a well defined tip

We investigated ligand stabilized clusters Au55(PPh3)12Cl6 and Pd561(phen)38±2O≊200 on gold and graphite substrates with scanning tunneling microscopy (STM) under atmospheric conditions. We were able to image dense layers of clusters as well as free‐lying clusters. Clusters are frequently picked up by the tip. This results in a tip with a well defined shape and electronic structure. Analysis of images of clusters scanned with another cluster attached to the tip can yield direct information on the tunnel distance. We found a tunnel distance of 24±5 A.