Size-selectivity and anomalous subdiffusion of nanoparticles through carbon nanofiber-based membranes.

A simulation is presented here that serves the dual functions of generating a nanoporous membrane replica and executing the Brownian motion of nanoparticles through the virtual membrane. Specifically, the concentration profile of a dilute solution of fluorescent particles in a stochastic and SiO(2)-coated carbon nanofiber (oxCNF), nanoporous membrane was simulated. The quality of the simulated profile was determined by comparing the results with experimental concentration profiles. The experimental concentration profiles were collected adjacent to the oxCNF membrane surface from time-lapse fluorescence microscopy images. The simulation proved ideal as an accurate predictor of particle diffusion-the simulated concentration profile merged with the experimental profiles at the inlet/exit surfaces of the oxCNF membrane. In particular, the oxCNF barrier was found to hinder the transport of 50 and 100 nm particles and transmembrane trajectories were indicative of anomalous subdiffusion; the diffusion coefficient was found to be a function of time and space.

Pulsed laser dewetting of nickel catalyst for carbon nanofiber growth.

We present a pulsed laser dewetting technique that produces single nickel catalyst particles from lithographically patterned disks for subsequent carbon nanofiber growth through plasma enhanced chemical vapor deposition. Unlike the case for standard heat treated Ni catalyst disks, for which multiple nickel particles and consequently multiple carbon nanofibers (CNFs) are observed, single vertically aligned CNFs could be obtained from the laser dewetted catalyst. Different laser dewetting parameters were tested in this study, such as the laser energy density and the laser processing time measured by the total number of laser pulses. Various nickel disk radii and thicknesses were attempted and the resultant number of carbon nanofibers was found to be a function of the initial disk dimension and the number of laser pulses.

Vertically aligned carbon nanofiber arrays: an electrical and genetic substrate for tissue scaffolding.

We present a discussion of the use of vertically-aligned carbon nanofibers (VACNFs) as nanoscale elements that directly interface to biological whole-cell systems. VACNFs are compatible with a large subset of microfabrication processes, thereby enabling their incorporation into mesoscale hybrid systems that provide addressability of the VACNFs as either bulk electrode material, or as individually addressed nanoelectrodes. These VACNF devices are compatible with cell cultures, and electrochemical addressability of nanofibers can be maintained for extended periods within cell cultures. We present results that demonstrate possible use of VACNF devices as electrical and genetic substrates for tissue scaffolding applications.

Formation of ultrasharp vertically aligned Cu-Si nanocones by a DC plasma process.

We report an effective method for the production of ultrasharp vertically oriented silicon nanocones with tip radii as small as 5 nm. These silicon nanostructures were shaped by a high-temperature acetylene and ammonia dc plasma reactive ion etch (RIE) process. Thin-film copper deposited onto Si substrates forms a copper silicide (Cu3Si) during plasma processing, which subsequently acts as a seed material masking the single-crystal cones while the exposed silicon areas are reactive ion etched. In this process, the cone angle is sharpened continually as the structure becomes taller. Furthermore, by lithographically defining the seed material as well as employing an etch barrier material such as titanium, the cone location and substrate topography can be controlled effectively.

Molecular transport in a crowded volume created from vertically aligned carbon nanofibres: a fluorescence recovery after photobleaching study.

Rapid and selective molecular exchange across a barrier is essential for emulating the properties of biological membranes. Vertically-aligned carbon nanofibre (VACNF) forests have shown great promise as membrane mimics, owing to their mechanical stability, their ease of integration with microfabrication technologies and the ability to tailor their morphology and surface properties. However, quantifying transport through synthetic membranes having micro- and nanoscale features is challenging. Here, fluorescence recovery after photobleaching (FRAP) is coupled with finite difference and Monte Carlo simulations to quantify diffusive transport in microfluidic structures containing VACNF forests. Anomalous subdiffusion was observed for FITC (hydrodynamic radius of 0.54 nm) diffusion through both VACNFs and SiO(2)-coated VACNFS (oxVACNFs). Anomalous subdiffusion can be attributed to multiple FITC-nanofibre interactions for the case of diffusion through the VACNF forest. Volume crowding was identified as the cause of anomalous subdiffusion in the oxVACNF forest. In both cases the diffusion mode changes to a time-independent, Fickian mode of transport that can be defined by a crossover length (R(CR)). By identifying the space-and time-dependent transport characteristics of the VACNF forest, the dimensional features of membranes can be tailored to achieve predictable molecular exchange.

Direct visualization of defect density waves in 2D.

A scanning tunneling microscopy investigation of the 1/3 of a monolayer alpha phase of Sn on Si(111) reveals a new low temperature phase, which is electronic and not structural. This phase consists of a one-dimensional incommensurate electronic wave that coincides with a periodic modulation of the population of the substitutional Si defects, i.e., a defect density wave.