The controlled synthesis of materials by methods that permit their assembly into functional nanoscale structures lies at the crux of the emerging field of nanotechnology. Although only one of several materials families is of interest, carbon-based nanostructured materials continue to attract a disproportionate share of research effort, in part because of their wide-ranging properties. Additionally, developments of the past decade in the controlled synthesis of carbon nanotubes and nanofibers have opened additional possibilities for their use as functional elements in numerous applications. Vertically aligned carbon nanofibers (VACNFs) are a subclass of carbon nanostructured materials that can be produced with a high degree of control using catalytic plasma-enhanced chemical-vapor deposition (C-PECVD). Using C-PECVD the location, diameter, length, shape, chemical composition, and orientation can be controlled during VACNF synthesis. Here we review the CVD and PECVD systems, growth control mechanisms, catalyst preparation, resultant carbon nanostructures, and VACNF properties. This is followed by a review of many of the application areas for carbon nanotubes and nanofibers including electron field-emission sources, electrochemical probes, functionalized sensor elements, scanning probe microscopy tips, nanoelectromechanical systems (NEMS), hydrogen and charge storage, and catalyst support. We end by noting gaps in the understanding of VACNF growth mechanisms and the challenges remaining in the development of methods for an even more comprehensive control of the carbon nanofiber synthesis process.
We demonstrate the integration of vertically aligned carbon nanofiber (VACNF) elements with the intracellular domains of viable cells and controlled biochemical manipulation of cells using the nanofiber interface. Deterministically synthesized VACNFs were modified with either adsorbed or covalently-linked plasmid DNA and were subsequently inserted into cells. Post insertion viability of the cells was demonstrated by continued proliferation of the interfaced cells and long-term (> 22 day) expression of the introduced plasmid. Adsorbed plasmids were typically desorbed in the intracellular domain and segregated to progeny cells. Covalently bound plasmids remained tethered to nanofibers and were expressed in interfaced cells but were not partitioned into progeny, and gene expression ceased when the nanofiber was no longer retained. This provides a method for achieving a genetic modification that is non-inheritable and whose extent in time can be directly and precisely controlled. These results demonstrate the potential of VACNF arrays as an intracellular interface for monitoring and controlling subcellular and molecular phenomena within viable cells for applications including biosensors, in-vivo diagnostics, and in-vivo logic devices.
The penetration and residence of vertically aligned carbon nanofibers (VACNF) within live cell matrices is demonstrated upon substrates that incorporate spatially registered indices to facilitate temporal tracking of individual cells. Penetration of DNA-modified carbon nanofibers into live cells using this platform provides efficient delivery and expression of exogenous genes, similar to "microinjection"-styled methods, but on a massively parallel basis. Spatially registered indices on the substrate allow one to conveniently locate individual cells, facilitating temporal tracking of gene expression events. We describe fabrication and use of this gene delivery platform which consists of arrays of individual carbon nanofibers at 5-µm pitch within numerically indexed, 100-µm square grid patterns. Fabrication of these devices on silicon substrates enables mass production of 100 devices (5 mm 2 ) per wafer, with each device providing over 800,000 nanofiber-based "needles" for cellular impalement and gene delivery applications.
We report experimental evidence showing a direct correlation between the alignment of carbon nanofibers (CNFs) prepared by plasma-enhanced chemical-vapor deposition and the location of the catalyst particle during CNF growth. In particular, we find that CNFs that have a catalyst particle at the tip (i.e., growth proceeds from the tip) align along the electric-field lines, whereas CNFs with the particle at the base (i.e., growth proceeds from the base) grow in random orientations. We propose a model that explains the alignment process as a result of a feedback mechanism associated with a nonuniform stress (part tensile, part compressive) that is created across the interface of the catalyst particle with the CNF due to electrostatic forces. Furthermore, we propose that the alignment seen recently in some dense CNF films is due to a crowding effect and is not directly the result of electrostatic forces.
We have investigated the ͑ p 3 3 p 3 ͒ to ͑3 3 3͒ phase transition in the a phase of Sn͞Ge(111) with variable temperature STM at temperatures between 30 and 300 K. Point defects in the Sn film stabilize localized regions of the ͑3 3 3͒ phase, where the size is characterized by a temperature dependent length (exponential attenuation). The inverse of the attenuation length is a linear function of temperature showing that the phase transition occurs at 70 K. At low temperature a density wave mediated defect-defect interaction realigns the defects to be in registry with the ͑3 3 3͒ domains.Structural phase transitions belong to a group of phenomena, which are strongly related to surface symmetry and its lowering. In many cases a prediction of the order and universality class of an anticipated phase transition can be made based solely on the knowledge of the space group of the surface or adsorbate structure [1]. This idealized picture is rarely achieved in the real world, where defects and imperfections in the surface break the symmetry. Various types of surface phase transitions have been reported in the literature [2,3], and even though an atomistic picture of the phase nucleation process has remained elusive, it is generally believed that this process involves defects and impurities [4,5]. Since the energy differences between different phases on a surface are usually very small, a slight perturbation of this energy balance, coupled with the broken symmetry induced by an imperfection, can affect not only the transition temperature but also the temperature dependence of the order parameters near the critical point. For example, consider a system with a charge density wave (CDW) instability. Electrons in the normal state will screen charged impurities, producing an attenuated CDW or Friedel oscillation near the impurity sites. The ion cores follow the local charge rearrangement and, consequently, the normal-state symmetry is broken locally and short-range CDW order develops. The electronic response to the external perturbation [6], x͑q, T ͒, depends on the temperature, leading to the intriguing proposition that defects will in general affect the evolution of long-range ordering. Tosatti and Anderson concluded that "a CDW can be regarded as unattenuated Friedel oscillations" [7].There is indeed evidence that imperfections have a strong influence on surface phase transitions. For example, the anticipated second-order phase transition on a Si(100) surface from a ͑2 3 1͒ to a c͑4 3 2͒ structure is not sharp [8]. This behavior was qualitatively reproduced by Monte Carlo simulations based on an Ising spin model [9,10]
In this paper we present the fabrication and initial testing results of high aspect ratio vertically aligned carbon nanofiber (VACNF)-based electrochemical probes. Electron beam lithography was used to define the catalytic growth sites of the VACNFs. Following catalyst deposition, VACNF were grown using a plasma enhanced chemical vapor deposition process. Photolithography was performed to realize interconnect structures. These probes were passivated with a thin layer of SiO2, which was then removed from the tips of the VACNF, rendering them electrochemically active. We have investigated the functionality of completed devices using cyclic voltammetry (CV) of ruthenium hexammine trichloride, a highly reversible, outer sphere redox system. The faradaic current obtained during CV potential sweeps shows clear oxidation and reduction peaks at magnitudes that correspond well with the geometry of these nanoscale electrochemical probes. Due to the size and the site-specific directed synthesis of the VACNFs, these probes are ideally suited for characterizing electrochemical phenomena with an unprecedented degree of spatial resolution.
Vertically aligned carbon nanofiber (VACNF) electrode arrays were tested for their potential application in recording neuro-electrophysiological activity. We report, for the first time, stimulation and extracellular recording of spontaneous and evoked neuroelectrical activity in organotypic hippocampal slice cultures with ultramicroelectrode VACNF arrays. Because the electrodes are carbon-based, these arrays have potential advantages over metal electrodes and could enable a variety of future applications as precise, informative, and biocompatible neural interfaces.
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