J. A. DeRose

Gold grown epitaxially on mica: conditions for large area flat faces

Abstract We have grown Au epitaxially on green and ruby muscovite mica in UHV for use as substrates for STM and AFM in solution and air. We have optimized the process to obtain flat, large areas (micrometers squared with a corrugation which rarely exceeded 10 nm from maximum to minimum height).

Scanning tunneling microscopy and atomic force microscopy studies of biomaterials at a liquid–solid interface

We have compared the scanning tunneling microscopy (STM) and atomic force microscopy (AFM) using a clean gold surface under electrochemical potential control, finding that the STM usually yields higher resolution, although this may be a consequence of better production methods for STM tips. New methods for tethering DNA molecules to substrates have yielded many excellent AFM images of DNA, even under water. The highest resolution has been obtained with specially sharpened tips, but recent work suggests that this may not be necessary. Control of contamination, and tip–sample interactions (i.e., adhesion) are also important factors. We describe a scheme of magnetic control of AFM cantilevers which should overcome the mechanical instability that places a lower limit on contact force. This should permit active control of interaction forces with the instrument operated in water. We illustrate local force measurements made under water with examples of the measurements of the microelastic properties of bone and ...

Potentiostatic deposition of DNA for scanning probe microscopy.

We describe a procedure for reversible adsorption of DNA onto a gold electrode maintained under potential control. The adsorbate can be imaged by scanning probe microscopy in situ. Quantitative control of a molecular adsorbate for microscopy is now possible. We found a potential window (between 0 and 180 mV versus a silver wire quasi reference) over which a gold (111) surface under phosphate buffer is positively charged, but is not covered with a dense adsorbate. When DNA is present in these conditions, molecules adsorb onto the electrode and remain stable under repeated scanning with a scanning tunneling microscope (STM). They become removed when the surface is brought to a negative charge. When operated at tunnel currents below approximately 0.4 nA, the STM yields a resolution of approximately 1 nm, which is better than can be obtained with atomic force microscopy (AFM) at present. We illustrate this procedure by imaging a series of DNA molecules made by ligating a 21 base-pair oligonucleotide. We observed the expected series of fragment lengths but small fragments are adsorbed preferentially.

Initial stage of underpotential deposition of Pb on reconstructed and unreconstructed Au(111)

Abstract We have studied the initial stage of Pb nucleation and growth on both reconstructed and unreconstructed Au(111) from 5.7mM Pb(NO 3 ) 2 + 50mM HClO 4 using scanning tunneling microscopy. We have found that Pb clusters grow preferentially along the 23 × √3 reconstruction of Au(111), a result that is similar to studies of metal deposition on reconstructed Au(111) in ultrahigh vacuum. A superperiodic structure of monolayer Pb was also observed and can be explained as a result of the rotation of the hexagonal-close packed Pb monolayer relative to the Au lattice. The coverage and average diameter of the growing Pb clusters were found to increase in two steps which correlate with the double peak in the voltammogram from underpotential deposition.

A technique for stable adhesion of DNA to a modified graphite surface for imaging by scanning tunneling microscopy

A procedure has been developed in which mercurated DNA is covalently bonded to a modified graphite surface. This technique allows for stable adhesion of the DNA which enables one to image it with the scanning tunneling microscope. The method is outlined in this paper and images are presented of a partially modified graphite surface and one completely modified with DNA attached. The DNA images demonstrate B‐helix periodicity.

Comparative scanning probe microscopy study of the surface morphology of Au films grown from the vapor onto glass, fused silica, and muscovite mica

In a previous paper [J. A. DeRose, T. Thundat, L. A. Nagahara, and S. M. Lindsay, Surf. Sci. 256, 102 (1991)], the conditions which can be used to grow Au films epitaxially on muscovite mica to obtain large area, flat faces are reported. Since then, it has been reported that Au films grown on an amorphous substrate [J. Hwang and M. A. Dubson, J. Appl. Phys. 72, 1852 (1992)] (nonepitaxy) have shown to be at least as smooth as those on mica. In this article, the results from scanning tunneling microscopy analysis of Au films grown on mica, glass cover slips, and fused silica are compared. By comparing the rms surface roughness (z data standard deviation, SD) values for each film type, we find that the mica films show the smoothest surfaces, i.e., smallest rms surface roughness values. We then attempt to explain these results by taking into account the accepted model of film growth from the vapor (physical vapor deposition) and the surface structure of bare mica, glass (optical microscope slide cover slip), ...

Studies of the electrical properties of large molecular adsorbates

The scanning tunneling microscope (STM) was used to investigate the conductivity of organic aggregates on gold electrodes. DNA fragments can form very stable aggregates in the presence of tris(hydroxymethyl)aminomethane buffer salt. With a submonolayer coverage, the tip can be moved back and forth between ‘‘clean’’ gold (in contact with the electrolyte) and an organic adsorbate patch. In these conditions, a systematic variation of the current voltage (I–V) characteristics indicates that the electrical characteristics are not dominated by a contaminant particle on the STM tip. The dependence of image contrast on tip bias was also studied over the range +0.3 to −0.3 V. It is found that: (a) the image contrast does not depend strongly on tip bias; (b) the I–V curves over what appears to be ‘‘clean’’ gold under the electrolyte are similar to those observed over clean gold maintained in an inert atmosphere; (c) the I–V curves over an adsorbate patch are diode‐like; (d) the I–V curves show no sign of a large vo...

Electrochemical deposition of nucleic acid polymers for scanning probe microscopy

We have studied gold and graphite as substrates for electrochemical deposition of DNA fragments for scanning tunneling microscope imaging. We obtain the most reproducible results with gold grown epitaxially onto mica. Gold can react with chlorine ions, and with the tris(hydroxymethyl) aminomethane buffer salt we use. However, under potentiostatic or galvanostatic control, the surface is modified in quite characteristic ways when DNA fragments are present. We present results for positive electrodes where DNA appears to react predictably only in the presence of salts which co‐adsorb with it. Although very reproducible coverage of the electrode is possible, the salt complicates interpretation of the images.

Atomic force and scanning tunneling microscopy observations of whisker crystals and surface modification on evaporated gold films

Abstract We present atomic force microscope (AFM) images of whisker crystals interacting with the AFM probe tips for whiskers grown out of the surface of epitaxially made Au(111) on mica substrates. AFM images of broken-off whiskers lying on these gold substrates are presented as well. AFM and scanning tunneling microscopy (STM) have also been used to observe (and create, in the case of the STM) stress-induced steps on the gold-on-mica surface. We have also observed these stress-induced steps (with AFM) after applying an external stress to the gold-on-mica substrate.