Retraction Speed Research Articles

CoPt3 nanoparticles adsorbed on SiO2: a GISAXS and SEM study

ABSTRACTUltra-thin CoPt3 nanoparticle films have been prepared on SiO2 surfaces using a Langmuir-Blodgett (LB) deposition technique. The structural properties of the overlayers have been investigated by grazing-incidence small-angle x-ray scattering (GISAXS) and high-resolution scanning electron microscopy (SEM) for the first time. Self-assembly of the nanoparticles is found and with GISAXS an average particle-particle distance of (8.23 ± 0.06) nm is determined, in good agreement with the SEM results. A particle correlation length of (22.3 ± 1.2) nm was derived which is shown to be independent of the surface coverage. The latter quantity may be controlled by choice of a suitable retraction speed during the LB step.

Periodic mass shedding of a retracting solid film step

A semi-infinite, uniform film on a substrate tends to contract from the edge to reduce the surface energy of the system. This work studies the two-dimensional retraction of such a film step, assuming that the film evolves by capillarity-driven surface diffusion. It is found that the retracting film edge forms a thickened ridge followed by a valley. The valley sinks with time and eventually touches the substrate. The ridge then detaches from the film. The new film edge retracts to form another ridge accompanied again by a valley, and the mass shedding cycle is repeated. This periodic mass shedding is simulated numerically for contact angle α between 30 and 180°. For smaller α, a small-slope late-time solution is found that agrees with the numerical solution for α=30°. Thus, the complete range of α is covered. The long-time retraction speed and the distance traveled per cycle agree quantitatively with experiments.

Formation of Si nanowire by atomic manipulation with a high temperature scanning tunneling microscope

The formation of Si nanowire during indentation of the scanning tunneling microscope tip onto the Si(111) surface was investigated by changing the sample bias, the temperature, and the tip retraction speed. The wire length at room temperature is in the order of 1 nm, however, the wire was elongated remarkably with either increasing temperature or bias voltage while keeping a positive sample bias. The wire was also elongated when the tip speed was decreased. The typical length was ∼14 nm at 481 °C, +2.0 V and 320 nm/s. In order to explain these results, we proposed a simple model for the wire formation by taking into account the surface diffusion and electromigration effects.

Conductance quantization histograms of gold nanowires at 4 K

Driving a scanning tunneling microscope (STM) tip into a metallic surface and pulling it out afterwards results in the formation of a nanometer-sized wire (nanowire) between the electrodes. The electrical conduction measured during this process shows signs of quantized conductance in units of ${\mathrm{G}}_{0}$=${2\mathrm{e}}^{2}$/h. Due to the inherent irreproducibility of the measured conductance curves, the standard technique has been to construct histograms with a few hundred selected curves. These histograms, for gold nanowires at room temperature, have shown three to four peaks at integer values of ${2\mathrm{e}}^{2}$/h, while in a low-temperature mechanically controlled break junction study, with statistics using only 65 curves, only the first peak has been reported. A proposed explanation for this apparent experimental discrepancy has been a higher nanowire temperature arising from the higher retraction speed used in scanning tunneling microscopy measurements. However, our simple estimation using macroscopic heat transport theory produces a very low temperature rise, less than 1 \ensuremath{\mu}K. In this work, an improved statistical study is presented, where histograms built with thousands of consecutive gold contact breakage experiments at 4 K show up to seven conductance peaks. Thus, no significant differences with our previous room-temperature (RT) studies are observed, pointing to a conductance quantization behavior that is the same at low, intermediate, and high (RT) temperatures.

Jump to contact and neck formation between Pb surfaces and a STM tip

We present a study of the interaction of a W STM tip and the (110) and (111) surfaces of Pb. Atomic resolution has been obtained at room temperature on Pb(110) and up to 330 K on Pb(111). At higher temperatures the surfaces can jump to mechanical contact with the STM tip, resulting in the formation of a connecting neck of Pb between tip and surface. As the tip is retracted, the neck elongates and finally breaks. The dependence of the maximum neck size on the temperature and the tip retraction speed indicates that surface diffusion is responsible for the neck build-up. When the surface is partially oxidized the maximum neck size is reduced. We derive a scaling relation between the maximum neck size, the retraction speed and the surface diffusion coefficient. With this relation and the temperature dependence of the maximum neck size we obtain activation energies for the neck build-up of 1.3 and 0.9 eV respectively for necks on Pb(110) and Pb(111). When a neck breaks, either a crater or a hillock is left on the surface.

Jump to contact, neck formation, and surface melting in the scanning tunneling microscope.

With a scanning tunneling microscope the Pb (110) surface was studied from room temperature to temperatures where surface melting occurs. At room temperature scanning tunneling microscopy images of Pb(110) can be recorded with atomic resolution. At higher temperatures we observe a jump to contact by the surface, resulting in the formation of a connecting neck of Pb between the surface and the tip. As the tip is retracted, the neck elongates and finally breaks. The dependence of the average neck height just before rupture on temperature and retraction speed suggests that mobile adatoms are responsible for the growth of the neck.

3D-surface reconstruction of intravascular ultrasound images using personal computer hardware and a motorized catheter control.

A system for three-dimensional (3D) presentation of intravascular ultrasound (US) images was designed. A standard hardware configuration based on personal computer equipment was used for acquisition and processing of image data. Pullback imaging with the US catheter was controlled by a specially designed motor assembly and performed either in equidistant 1-10 mm steps or at a constant retraction speed. Curvature of the vessel was documented on biplane digital subtraction angiograms and the US images were arranged according to the vessel shape in the angiograms. 3D reconstruction appears essential for spatial orientation of intravascular US and may be helpful in the planning of vascular interventions for a better appreciation of the extent and morphology of vascular lesions.

Effect of Load on Leg Movement and Step Coordination of the Stick Insect Carausius Morosus

A treadwheel mounted on a torque motor was used to study changes in step coordination as a function of the magnitude and direction of external forces resisting and assisting leg retraction. The apparent contradiction in the literature concerning the shift in the posterior extreme point is shown to depend upon the magnitude of the load. External forces resisting leg retraction generally shift step end-points rostrally and those assisting leg retraction shift them caudally, shifting leg movement so that the active muscles can provide greater force. However, this primary trend reverses for external forces equivalent to or less than the weight of the body. Together, these results indicate that the decision to end stance does not simply depend upon the leg's loading but rather on its efficiency in contributing propulsion. As expected, loads resisting leg retraction decrease retraction speed and increase step period. They also decrease protraction speed, increase swing duration and shift phase relationships of adjacent ipsilateral legs away from alternation towards a metachronal pattern. Moderate to large assisting loads evoke similar changes: they increase swing duration less but shift the phase of middle and rear legs more than the corresponding resisting loads. Thus, by itself, a reduction in the force required during retraction does not transform the gait from metachronal to tripod. The mean phase of contralateral leg pairs is unaffected by load: these legs continue to alternate.

Maximum Altitude Sailplane Winch-Launch Trajectories

SummaryA class of dynamic sailplane winch-launch trajectory problems for maximum altitude is formulated. Numerical solutions are presented for representative values of maximum allowed lift coefficient and cable acceleration. The optimal trajectory is shown to consist of a climb-dive-climb sequence. An inequality constraint on cable retraction speed is also imposed and results in more realistic trajectories.