Lloyd A. Bumm

Are single molecular wires conducting

Molecular wire candidates inserted into “nonconducting” n-dodecanethiol self-assembled monolayers on Au{111} were probed by scanning tunneling microscopy (STM) and microwave frequency alternating current STM at high tunnel junction impedance (100 gigohms) to assess their electrical properties. The inserted conjugated molecules, which were 4,4′-di(phenylene-ethynylene)benzenethiolate derivatives, formed single molecular wires that extended from the Au{111} substrate to about 7 angstroms above and had very high conductivity as compared with that of the alkanethiolate.

Enhanced Raman Scattering by Molecules Adsorbed at the Surface of Colloidal Particles

The observation of SERS by molecules adsorbed at the surface of colloidal particles has led to analysis of classical electromagnetic field effects which contribute to enhancement of the Raman signals. Such effects alone can give rise to large enhancements which are sensitive to particle size, shape and optical constants. The theory is formulated in such a way that specific changes in Raman polarizability which occur upon adsorption can be incorporated, when these are known.

Control and placement of molecules via self-assembly

We use self- and directed assembly to pattern organic monolayers on the nanometre scale. The ability of the scanning tunnelling microscope to obtain both nanometre-scale structural and electronic information is used to characterize patterning techniques, to elucidate the intermolecular interactions that drive them and to probe the structures formed. We illustrate three successful approaches: (1) phase separation of self-assembled monolayers by terminal and internal functionalization, (2) phase separation of self-assembled monolayers induced by post-adsorption processing and (3) control of molecular placement by insertion into a self-assembled monolayer. These methods demonstrate the possibilities of patterning films by exploiting the intrinsic properties of the molecules. We employ these methods to prepare matrix-isolated samples to probe molecular electronic properties of single and bundled molecules.

Fluorescent in situ hybridization employing the conventional NBT/BCIP chromogenic stain

In situ hybridization techniques typically employ chromogenic staining by enzymatic amplification to detect domains of gene expression. We demonstrate the previously unreported near infrared (NIR) fluorescence of the dark purple stain formed from the commonly used chromogens, nitro blue tetrazolium (NBT) and 5-bromo-4-chloro-3-indolyl phosphate (BCIP). The solid reaction product has significant fluorescence that enables the use of confocal microscopy to generate high-resolution three-dimensional (3-D) imaging of gene expression.

Evolution of Strategies for Self‐Assembly and Hookup of Molecule‐Based Devices

ABSTRACT: Strategies for self‐assembling molecule‐based devices are considered in terms of current chemical issues whose resolution appears critical to efficient connection and addressing of electronically active molecules between electrodes. We discuss issues related to the type and shape of the molecules, chemical bonding at junctions, molecular lengths and electrode gap matching, molecular alignment at electrodes, chemistry of deposited metal contacts, and the doping of molecular conductors. Examples of each of these aspects is given using fully conjugated molecules, constituted from rigid rod phenylene‐ethynylene units, self‐assembled onto metal and semiconductor surfaces.

Molecularly Ordered Decanethiolate Self-Assembled Monolayers on Au(111) from in Situ Cleaved Decanethioacetate: An NMR and STM Study of the Efficacy of Reagents for Thioacetate Cleavage

The cleavage of decanethioacetate (C10SAc) has been studied by (1)H nuclear magnetic resonance (NMR) spectroscopy and scanning tunneling microscopy (STM) imaging of in situ prepared decanethiolate self-assembled monolayers (SAMs) on Au(111). Solutions of C10SAc (46 mM) and previously reported cleavage reagents (typically 58 mM) in CD(3)OD were monitored at 20 degrees C by NMR spectroscopy. Cleavage by ammonium hydroxide, propylamine, or hydrochloric acid was not complete within 48 h; cleavage by potassium carbonate was complete within 24 h and that by potassium hydroxide or 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) within 2 h. Similar cleavage rates were observed for phenylthioacetate. The degree of molecular ordering determined by STM imaging increased with increasing extent of in situ cleavage by these same reagents (2.5 mM C10SAc and 2.5 mM reagent in ethanol for 1 h, then 16 h immersion of Au/mica). Less effective cleavage reagents did not cleave the C10SAc sufficiently to decanethiol (C10SH) and gave mostly disordered SAMs. In contrast, KOH or DBU completely cleaved the C10SAc to C10SH and led to well-ordered SAMs composed of (square root(3) x square root(3))R30 degrees domains that are indistinguishable from SAMs grown from C10SH. Monolayer formation from thioacetates in the absence of cleavage agents is likely due to thiol or disulfide impurity in the thioacetates. Eliminating disulfide by using Bu(3)P as a sacrificial reductant also helped to produce good molecular order in the SAM. The methods presented here allow routine growth of molecularly ordered alkanethiolate SAMs from thioacetates using reagents of ordinary purity under ambient, benchtop conditions.

Spatially resolved surface enhanced second harmonic generation: Theoretical and experimental evidence for electromagnetic enhancement in the near infrared on a laser microfabricated Pt surface

Surface enhanced second harmonic generation (SESHG) has been observed for the first time from a Pt surface in a <10−3 Torr vacuum environment. Both ‘‘smooth,’’ mechanically polished Pt surfaces and rippled Pt microstructures prepared by laser microchemical etching in Cl2(g) were studied with the newly developed technique of SESHG imaging. The etching procedure and the behavior of the Pt surface under the SESHG imaging conditions is reported in detail. The rippled/smooth enhancement factor for SHG excited with 80 ps, 1064 nm pulses from a cw, mode‐locked Nd‐YAG laser focused to a 3.1 μm 1/e2 radius ranges from 4 to 17, in qualitative agreement with theoretical calculations for isolated Pt spheroids.

Probing Electronic Properties of Conjugated and Saturated Molecules in Self-Assembled Monolayers

ABSTRACT: We have measured the electron transport properties of single and bundled molecules imbedded in crystalline monolayers of alkanethiols self‐assembled on Au{111} surfaces. These measurements have been performed using scanning tunneling microscopy and microwave difference frequency alternating current scanning tunneling microscopy on molecules with saturated and fully conjugated chains. We have learned to separate the contributions of the molecules and of the tunneling junction. We have also found a means to determine the value of the contact conductance due to the choice of chemical connection to the substrate.

Development of the viscerocranial skeleton during embryogenesis of the sea lamprey, Petromyzon Marinus

Evolution of the skeleton was a key transition in early vertebrates. Lampreys lack a mineralized skeleton but possess cartilaginous neurocranial and viscerocranial elements. In lampreys, the visceral skeleton develops as a fused branchial basket supporting the pharynx. Here, we have adapted Alcian blue staining of lamprey cartilage and show this method results in cartilage fluorescence that we used to describe development of the branchial skeleton in Petromyzon marinus between 17 and 63 days of development. We show that skeletal rods develop from condensations of flattened discoidal chondrocytes and may involve cellular intercalation. Lamprey trabecular, parachordal, and subchordal cartilages consist of aggregations of polygonal chondrocytes positioned on the ventral and lateral surfaces of the notochord. We speculate that morphological differences relate to functional differences in the cartilage. We show that differentiated skeletal rods are derived from neural crest. Finally, we show how branchial muscles intercalate with skeletal rods of the branchial basket. Developmental Dynamics 238:3126–3138, 2009.

Small cavity nonresonant tunable microwave‐frequency alternating current scanning tunneling microscope

We have developed a broadly tunable microwave‐frequency alternating current scanning tunneling microscope which employs a cavity with no resonances in the frequency range used. This results in a low, flat background for spectroscopic measurements with this instrument.