6,332 publications from this institution
The ability to manipulate matter on the nanometer scale is a requirement for the full development of nanotechnology. Scanning probe microscopies show great promise in this respect and recent progress in the area is discussed. for example surface modification, pattern formation (e.g. the 150 nm high Harvard University symbol in the figure), and the manipulation of nanostructures. magnified image
The growth of cavities along grain interfaces was analyzed by the combined processes of grain boundary diffusion and plastic dislocation creep in the adjoining grains. It is shown that the coupling between the processes can be expressed in terms of a parameter L, which has the dimensions of length and which is a function of material properties, temperature and applied stress; L decreases with increasing temperature and stress and has values in the range of 0.25 to 25 ..mu..m for various pure metals when stressed to 10/sup -3/ x shear modulus at 0.5 T/sub m/. The contribution of dislocation creep to the cavity growth rate is shown to be negligible when L is comparable to or larger than the cavity spacing, but significant interactions occur, leading to growth rates very much in excess of those predicted on the basis of boundary diffusion alone, when L is comparable to or smaller than the cavity size. The coupling occurs because extensive dislocation creep allows local accomodation of matter diffused into the grain boundary from the cavity walls. The cavity growth rate is analyzed by formulating a new variational principle that governs combined processes of grain boundary diffusion and non-linear viscous creep, and by implementing this principle through the finite-element method to obtain numerical solutions. Results for the cavity growth rate are presented for a wide range of ratios of L to cavity spacing, and of cavity radius to spacing. Also, results are presented for the total growth time of cavities from an initial size to final coalescence.
Carbon nanotubes are potentially ideal atomic force microscopy probes because they can have diameters as small as one nanometer, have robust mechanical properties, and can be specifically functionalized with chemical and biological probes at the tip ends. This communication describes methods for the direct growth of carbon nanotube tips by chemical vapor deposition (CVD) using ethylene and iron catalysts deposited on commercial silicon-cantilever-tip assemblies. Scanning electron microscopy and transmission electron microscopy measurements demonstrate that multiwalled nanotube and single-walled nanotube tips can be grown by predictable variations in the CVD growth conditions. Force-displacement measurements made on the tips show that they buckle elastically and have very small (</= 100 pN) nonspecific adhesion on mica surfaces in air. Analysis of images recorded on gold nanoparticle standards shows that these multi- and single-walled carbon nanotube tips have radii of curvature of 3-6 and 2-4 nm, respectively. Moreover, the nanotube tip radii determined from the nanoparticle images are consistent with those determined directly by transmission electron microscopy imaging of the nanotube ends. These molecular-scale CVD nanotube probes have been used to image isolated IgG and GroES proteins at high-resolution.
A ‘generic’ system consisting of bonded layers of different materials and illustrations of various failure modes are shown in Figure 1.1. Arguably, the two dominant technological applications with this type of geometry are microelectronic devices and protective coatings in extreme environments (e.g., thermal barrier coatings). These applications involve layers with very disparate properties and are subjected to rather aggressive external stimuli. Cracking happens either between the layers (interface delamination or debonding) or within a layer (tunneling or channeling cracks). Delamination can occur regardless of whether the stresses are tensile or compressive, while buckling-driven delamination occurs only in layers experiencing compressive stress, and channeling or tunneling cracks require tensile stress in the layers.
