Atomic force microscopy of red-light photoreceptors using peakforce quantitative nanomechanical property mapping.

Atomic force microscopy (AFM) uses a pyramidal tip attached to a cantilever to probe the force response of a surface. The deflections of the tip can be measured to ~10 pN by a laser and sectored detector, which can be converted to image topography. Amplitude modulation or "tapping mode" AFM involves the probe making intermittent contact with the surface while oscillating at its resonant frequency to produce an image. Used in conjunction with a fluid cell, tapping-mode AFM enables the imaging of biological macromolecules such as proteins in physiologically relevant conditions. Tapping-mode AFM requires manual tuning of the probe and frequent adjustments of a multitude of scanning parameters which can be challenging for inexperienced users. To obtain high-quality images, these adjustments are the most time consuming. PeakForce Quantitative Nanomechanical Property Mapping (PF-QNM) produces an image by measuring a force response curve for every point of contact with the sample. With ScanAsyst software, PF-QNM can be automated. This software adjusts the set-point, drive frequency, scan rate, gains, and other important scanning parameters automatically for a given sample. Not only does this process protect both fragile probes and samples, it significantly reduces the time required to obtain high resolution images. PF-QNM is compatible for AFM imaging in fluid; therefore, it has extensive application for imaging biologically relevant materials. The method presented in this paper describes the application of PF-QNM to obtain images of a bacterial red-light photoreceptor, RpBphP3 (P3), from photosynthetic R. palustris in its light-adapted state. Using this method, individual protein dimers of P3 and aggregates of dimers have been observed on a mica surface in the presence of an imaging buffer. With appropriate adjustments to surface and/or solution concentration, this method may be generally applied to other biologically relevant macromolecules and soft materials.

Formation of mixed monolayers of silsesquioxanes and alkylsilanes on gold.

The formation of mixed monolayers of hydridospherosilsesquioxane clusters (H(8)Si(8)O(12)) and alkylsilanes (H(2n+1)C(n)SiH(3)) on Au has been investigated using X-ray photoelectron and reflection-absorption infrared spectroscopies and scanning tunneling microscopy. All of the techniques indicate the displacement of the majority of the siloxane clusters from the surface in favor of the alkylsilane.

Spatially anisotropic etching of graphite by hyperthermal atomic oxygen.

The spatially anisotropic kinetics involved in the chemical reaction between highly ordered pyrolytic graphite (HOPG) and a beam containing hyperthermal (approximately 8 km s(-1)) O((3)P) atomic oxygen and molecular oxygen yields unique surface morphologies. Upon exposure at moderate sample temperatures (298-423 K), numerous multilayer circular pits embedded in the reacted areas have been observed with the use of atomic force microscopy and scanning tunneling microscopy. These pits have diameters spanning nanometers to micrometers and depths from a few to tens of nanometers. The most striking characteristic of these pits is the convex curvature of the pit bottoms, where the highest point on the pit bottom is at the center and the lowest point occurs around the peripheral edge. Such structure arises by the interplay between kinetics of pit nucleation, the spatially anisotropic kinetics involved in the lateral and downward reactivity of HOPG, and the fluence of atomic oxygen. These kinetics, which are also influenced by the high reactivity of the translationally hot impinging oxygen atoms, govern the overall morphological evolution of the surface.

Chemical imaging of terrace-based active sites on gold.

Scanning tunneling microscopy data of a mixed monolayer comprised of a 40:60 ratio of H8Si8O12 and C6H13-H7Si8O12 clusters on gold are presented. The images display a composite monolayer surface with well-defined domain regions of the individual components. Holes present at face-centered cubic (fcc) sites of the starting Au/H7Si8O12 adsorbate layer indicate the location of active sites for impinging C6H13-H7Si8O12 clusters. Adsorption of a C6H13-H7Si8O12 cluster likely yields a mobile hydrogen atom available to recombine with and desorb an adjacent H8Si8O12 cluster. Hydrogen atom diffusion along substrate [121] directions is the proposed pattern formation mechanism of the mixed monolayer. Imaging of the spherosiloxane cluster domains identifies a novel terrace-based active site located in the fcc regions of the Au(111) 23 x square root3 surface reconstruction.

The differential reactivity of octahydridosilsesquioxane on Si(100)-2x1 and Si(111)-7x7: a comparative experimental study.

Scanning tunneling microscopy (STM), in conjunction with X-ray photoemission (XPS) and reflection-absorption infrared (RAIRS) spectroscopy, has been used to investigate the reaction of octahydridosilsesquioxane clusters (H(8)Si(8)O(12)) on the Si(100)-2x1 and Si(111)-7x7 surfaces. The clusters exhibit a markedly different reactivity upon exposure to the two clean silicon surfaces. STM data is presented that, in conjunction with XPS and RAIRS data, provides numerous constraints upon possible geometries for the chemisorbed clusters. The sum of the data is consistent with a dissociative reaction mechanism on Si(100)-2x1, resulting in cluster attachment to the surface via a single vertex. Conversely, data of Si(111)-7x7 subject to a saturation exposure of H(8)Si(8)O(12) is presented that is highly suggestive of cluster decomposition on the surface.

Formation of alkylsilane-based monolayers on gold.

The formation of monolayers of alkylsilanes on a gold surface is characterized by X-ray photoelectron and reflection-absorption infrared spectroscopies. The reaction occurs through the activation of multiple Si-H bonds. Reactivity of the newly synthesized systems to oxygen and water is reported.