SWNTs. We have also investigated the effects of NO 2 and NH 3 on the electrical properties of mats of SWNT ropes made from as-grown laser ablation materials. In a 200-ppm NO 2 flow, the resistance of an SWNT mat is found to decrease from R ϭ 150 to 80 ohms (R before /R after ϳ 2) in ϳ10 min (Fig. 4A). In a 1% NH 3 flow, the resistance of a second SWNT mat increases from 120 to 170 ohms (R after /R before ϳ 1.5) in ϳ10 min (Fig. 4B). In these bulk SWNT samples, the molecular interaction effects are averaged over metallic and semiconducting tubes. Also, the inner tubes in SWNT ropes are blocked from interacting with NO 2 and NH 3 because the molecules are not expected to intercalate into SWNT ropes. This explains the small resistance change of bulk SWNT mats by gas exposure compared to that of an individual S-SWNT.The main feature of individual S-SWNT sensors, besides their small sizes, is that they operate at room temperature with sensitivity as high as 10 3 . An individual nanotube sensor can be used to detect different types of molecules. The selectivity is achieved by adjusting the electrical gate to set the S-SWNT sample in an initial conducting or insulating state. The fast response of a nanotube sensor can be attributed to the full exposure of the nanotube surface area to chemical environments. Thus, nanotube molecular wires should be promising for advanced miniaturized chemical sensors. Single-file diffusion, prevalent in many processes, refers to the restricted motion of interacting particles in narrow micropores with the mutual passage excluded. A single-filing system was developed by confining colloidal spheres in one-dimensional circular channels of micrometer scale. Optical video microscopy study shows evidence that the particle self-diffusion is non-Fickian for long periods of time. In particular, the distribution of particle displacement is a Gaussian function.Single-file diffusion (SFD) occurs when the individual pores of the medium are so narrow that the particles are unable to pass each other (1, 2). The sequence of particles remains unchanged over time, and thus, the basic principle of diffusion as a physical mixing process comes into question. The concept of SFD was originally introduced more than 40 years ago in biophysics to account for the transport of water and ions through molecular-sized channels in membranes (3); since then, in addition to biological systems (4, 5), SFD is also discussed in the context of interaction of Markov chains in statistics (6 ), the transportation of adsorbate molecules through zeolites (2), and charge-carrier migration in one-dimensional (1D) polymer and superionic conductors (7). Furthermore, SFD is also related to surface growth phenomena through some mapping (8).As the mutual passage of particles is prohibited in single-filing (SF) systems, the movements of individual particles are correlated, even at long time periods, because the displacement of a given particle over a long distance necessitates the motion of many other particles in the same direction. This...

We have studied the dewetting of thin liquid metal films (Au, Cu, Ni) on fused silica substrates which occurs after melting with a Q-switched laser pulse. Optical microscopy, scanning electron microscopy, and scanning near field acoustic microscopy reveal two distinctly different modes of the dewetting process: On one hand, we observe heterogeneous nucleation of "dry" circular patches, which grow in diameter during the melting period. On the other hand, an instability of the liquid film against the growth in amplitude of surface waves with a characteristic wavelength is observed, which we believe is the first observation of spinodal dewetting. In contrast, the final structure of the ruptured film depends on whether nucleation or spinodal dewetting is dominant. [S0031-9007(96) PACS numbers: 68.45. Gd, 47.20.Ma, 61.25.Mv, 68.15.+e Dewetting of metastable thin films from a solid substrate is currently a topic of great interest [1][2][3][4]. This is based not only on the applicational relevance, e.g., in thin film technology. Also from a fundamental point of view there are unsolved questions concerning the interpretation of experimentally observed phenomena with respect to the behavior predicted theoretically. Particularly interesting is the self-organized structure evolution in time and space. Often the basics of dewetting have been studied on liquid films because heterogeneous influences, i.e., from grain boundaries or stresses, associated with solid films are not as prominent or must not be considered. From the theoretical considerations, dewetting of a metastable liquid film can develop via two different mechanisms [5]: First, nucleation and growth of holes can take place. This process has already been studied in detail experimentally. Redon et al. [4] nucleated holes in thin films of alkanes on silicon wafers at temperatures above the glass transition and studied their growth as a function of the surface tension of the alkanes. In general the growth of the holes leads to accumulation of the material along the perimeter of the holes by building up an elevated rim around them. The second process which can lead to dewetting is based on an instability of the film against thermally activated surface waves. According to theory [5][6][7], this instability ruptures the film spontaneously and a characteristic wavelength of surface modulations with the minimal risetime should dominate for a given system. This characteristic wavelength should scale with the square of the liquid film thickness [5]. However, the experiments reported so far do not provide an unambiguous demonstration for this behavior. Polystyrene films dewetting from silicon surfaces were found to develop circular holes whose number density scaled with the film thickness [8]. This was taken as evidence for spinodal dewetting as the process responsible for their formation. However, no direct indication for unstable surface waves was found, which in a spinodal process should precede the breakup of the holes. Guerra et al. [9] observed rather ordered wave...

T hin-fi lm technology is widely implemented in numerous applications 1. Although fl at substrates are commonly used, we report on the advantages of using curved surfaces as a substrate. Th e curvature induces a lateral fi lm-thickness variation that allows alteration of the properties of the deposited material 2,3 . Based on this concept, a variety of implementations in materials science can be expected. As an example, a topographic pattern formed of spherical nanoparticles 4,5 is combined with magnetic multilayer fi lm deposition. Here we show that this combination leads to a new class of magnetic material with a unique combination of remarkable properties: Th e so-formed nanostructures are monodisperse, magnetically isolated, single-domain, and reveal a uniform magnetic anisotropy with an unexpected switching behaviour induced by their spherical shape. Furthermore, changing the deposition angle with respect to the particle ensemble allows tailoring of the orientation of the magnetic anisotropy, which results in tilted nanostructure material.

Hexagonally closed packed monolayers of colloids have found more and more applications, e.g. as lithographic masks. The monolayers are usually produced with the help of a self-organizing process where a suspension of colloids is applied to the desired substrate and left to dry. This method requires a good wettability and smoothness of the substrate, which severely limits the number of possible substrates. We present a new method for the application of colloid monolayers to almost any surface where these difficulties are circumvented. At first the monolayers are fabricated on glass substrates and afterwards floated off on a water surface. From there, they are transferred to the desired substrate. Examples where transferred monolayers were used as lithographic masks are shown on glass, indium tin oxide, and tungsten diselenide. The transfer of a colloid monolayer to a copper grid for transmission electron microscopy demonstrates the applicability of the technique to curved surfaces as well.

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We have developed a new method for observing cell/substrate contacts of living cells in culture based on the optical excitation of surface plasmons. Surface plasmons are quanta of an electromagnetic wave that travel along the interface between a metal and a dielectric layer. The evanescent field associated with this excitation decays exponentially perpendicular to the interface, on the order of some hundreds of nanometers. Cells were cultured on an aluminum-coated glass prism and illuminated from below with a laser beam. Because the cells interfere with the evanescent field, the intensity of the reflected light, which is projected onto a camera chip, correlates with the cell/substrate distance. Contacts between the cell membrane and the substrate can thus be visualized at high contrast with a vertical resolution in the nanometer range. The lateral resolution along the propagation direction of surface plasmons is given by their lateral momentum, whereas perpendicular to it, the resolution is determined by the optical diffraction limit. For quantitative analysis of cell/substrate distances, cells were imaged at various angles of incidence to obtain locally resolved resonance curves. By comparing our experimental data with theoretical surface plasmon curves we obtained a cell/substrate distance of 160 +/- 10 nm for most parts of the cells. Peripheral lamellipodia, in contrast, formed contacts with a cell substrate/distance of 25 +/- 10 nm.

We study closely packed crystalline structures formed by slow lateral compression of a colloidal suspension of hard spheres in a thin wedge. In addition to the known sequence of structural transitions, a buckling mechanism was recently proposed to maximize the packing fraction F between one and two layers. We here confirm this prediction experimentally and present the first evidence that for more than two layers, buckling, in this case of prism shaped arrays of particles, also takes place. This efficient mechanism may enhance F by up to 4% and dominates in major regions of the phase diagram.[ S0031-9007(97)

Flat gold nanostructures on inert substrates like glass or graphite were illuminated by single intensive laser pulses with fluences above the gold melting threshold. The liquid structures produced in this way are far from their equilibrium shape, and a dewetting process sets in. On a time scale of a few nanoseconds, the liquid contracted toward a sphere. During this contraction, the center of mass moved upward, which could lead to detachment of droplets from the surface due to inertia. The resulting velocities were on the order of 10 meters per second for droplets with radii in the range of 100 nanometers.When small droplets impinge on a surface, varying degrees of deposition can be observed, ranging from sticking to rebounding. Sticking is essential for ink-jet printing and in agricultural agents that function by sticking to leaves; rebounding is desirable in cases such as selfcleaning surfaces (1, 2).The physics of impacting droplets has been well studied (3-5), and various types of (macroscopic) droplet sources have been developed (6). The impact-rebound process can be described energetically as the transformation of the impinging drop_s kinetic energy (KE) into surface deformation energy, followed by the inverse process, which detaches the drop (7).We examined whether it is possible to begin with deformed droplets on a surface and observe only the transformation from surface deformation energy to KE, as indicated by droplets jumping off the surface. For this purpose, we used droplets in the submicrometer range, which are much smaller than those typically used in impact studies. Such droplets can readily be obtained in an energetically unfavorable pancakelike shape with a large surface-to-volume ratio by preparing nanostructures in the solid state (e.g., by evaporation) on a substrate that in equilibrium is not wetted by the deposited material. Upon melting the nanostructures with a short laser pulse, dewetting sets in, and under appropriate conditions, detachment of the resulting droplets can be observed.The gold nanostructures we used here were fabricated by colloidal lithography, in which a monolayer of monodisperse spherical particles (with diameters of 1.5 to 3 mm) serves as a deposition mask (8, 9) to produce flat gold triangles with side lengths between 400 and 800 nm (Fig. 1A). The thickness of the evaporated films ranged between 50 and 160 nm. After removal of the colloid mask, these triangular gold structures were irradiated with a frequencydoubled Neodymium Doped Yttrium Aluminum Garnet (Nd:YAG)-laser (wavelength l 0 532 nm, full-width at half maximum of 10 ns). Because the absorption length of the laser radiation is smaller than the thickness of the nanostructure, we have to consider the temperature distribution inside of the nanostructure. An estimate for the thermal diffusion lengths on the time scale of the laser pulse yields 1600 nm for solid gold and 900 nm for molten gold, which is well above the thickness of the structures used here (10). Thus, we can assume that the temperature stays almos...