Santosh Pabba

Oriented nanomaterial air bridges formed from suspended polymer-composite nanofibers.

In a two-step method, carbon nanotubes, inorganic nanowires, or graphene sheets are connected between two anchor points to form nanomaterial air bridges. First, a recently developed method of forming directionally oriented polymer nanofibers by hand-application is used to form suspended composite polymer-nanomaterial fibers. Then, the polymer is sacrificed by thermally induced depolymerization and vaporization, leaving air bridges of the various materials. Composite fibers and bundles of nanotubes as thin as 10 nm that span 1 microm gaps have been formed by this method. Comparable bridges are observed by electrospinning solutions of the same nanomaterial-polymer composites onto micrometer-scale corrugated surfaces. This method for assembling nanomaterial air-bridges provides a convenient way to suspend nanomaterials for mechanical and other property determinations, and for subsequent device fabrication built up from the suspended nanosubstrates.

Side-by-side comparison of Raman spectra of anchored and suspended carbon nanomaterials.

Raman spectra of ordered carbon nanomaterials are quite sensitive to surface perturbations, including trace residues, structural defects and residual stress. This is demonstrated by a series of experiments with carbon nanotubes and graphene. Their spectra change due to subtle changes in preparation and attachment to the substrate and to each other. Differences are most clearly seen by forming a material into an air bridge and probing it in the air gap and at the anchor points. A monolayer graphene sheet, shows a larger disorder band at the anchor points than in the air gap. However, a bundle or rope of parallel-aligned single-wall nanotubes shows a larger disorder band in the gap than at the anchor points. For the graphene sheet the substrate surface deforms the graphene, leading to increases in the disorder band. For the rope, the close proximity of the nanotubes to each other appears to produce a larger stress than the rope resting on the substrate.

Fabrication of an insulated probe on a self-assembled metallic nanowire for electrochemical probing in cells

Conductive nanowire probes that are stiff enough too be inserted inside individual cells could provide unprecedented detail in real-time about the chemistry of living cells and cell compartments including cell membranes, cytosolic organelles and the nucleus. Localization also requires that the probe is insulated except at its end. To enable simple interconnection to an experimental apparatus, the nanowire would need to be attached to a larger platform, e.g. a MEMS device or cantilever (such as used in atomic force microscopes-AFM.) Such a device with 100 nm or less diameter can be fabricated with only a few processing steps by taking advantage of a previously reported technique for self-assembling metal alloy nanowires at selected locations and with desired orientations with respect to the surface [1]. In this report such a concept device and its fabrication process is reported. All steps have been individually demonstrated, although the complete fabrication from end-to-end has yet to be demonstrated at the time of this writing.

Nanotube suspension bridges directly fabricated from nanotube-polymer suspensions by manual brushing

Polymer fibers have been directly self-assembled into suspended bridges by manually brushing polymer dissolved in a volatile solvent across a microstructured surface. This process is extended by adding multiwall carbon nanotubes to the liquid polymer solution of poly (methyl methacrylate) (PMMA) in chlorobenzene. Suspended nanotube/polymer fibers with diameters between 20 nm and 20 microns (with bulk conductivities as large as 9.91 S/m) are created by brushing. Immersing the suspended fibers in acetone dissolves most of the polymer, leaving behind suspended bridges of nanotubes. The filamentary structures remain suspended following removal from the solvent bath and air drying. The nanotubes appear to be encased in a thin layer of polymer that assists in holding the bridge together. It is speculated that the residual coating is related to the organic functionalization that is added to the nanotubes to make them well dispersed and suspended in chlorobenzene.

Direct Write Fabrication of Polymer Fibers for Microscale Applications

A new direct write technique has been developed for processing viscous polymer solutions into suspended, three-dimensionally-oriented polymer fibers. This process has been successfully implemented to produce fibers from multiple materials including poly(methyl methacrylate) (PMMA), multi-wall carbon nanotube (MWNT)-doped PMMA, and biodegradable poly(L-lactic acid) (PLLA). Process characterization performed with PMMA suggests that fiber diameter is controllable through modulation of process geometry and/or polymer solution properties. Furthermore, suspended microchannels have been produced by employing directly written fibers as sacrificial structures.

Direct-Write Fabrication of Polymer Nanocomposite Fibers

The unique properties of carbon-nanotube (CNT)-doped polymers have generated several promising applications including gas sensors, high-strength/light-weight materials, and electromagnetic interference shielding. The ability to process CNT-doped materials into complex architectures may enable further advancement of these devices. We have developed a direct-write technique for processing CNT-doped poly(methyl methacrylate) (PMMA) into 3D arrays of precisely-positioned fibers with micro- and sub-microscale diameters. In this method, a programmable micromanipulator-controlled syringe was loaded with solvated CNT/PMMA and utilized to draw an array of freely-suspended solution filaments on a substrate in a “connect-the-dots” fashion. As the filaments are drawn, they are thinned by surface tension-driven necking as they dry and form solid fibers. The degree of thinning can be controlled by varying the viscosity of the solution, which acts to resist the necking while the volatile solvent evaporates and solidification occurs. Multiple fibers were drawn to investigate the effects of several factors on fiber diameter and process yield. These variables included fiber length (4, 8, and 18 mm), fiber drawing velocity (5 and 20 mm/s), polymer concentration in solution (22 and 24% by wt.), and CNT concentration in solution (0, 0.5, 1, and 1.5% by wt.), with the latter two of these variables strongly influencing solution viscosity. Measurement of the fibers via scanning electron microscopy (SEM) revealed several trends: Fiber diameter was not influenced by CNT concentration, but increased with increasing PMMA concentration (P 100 μm. Furthermore, fiber yield exceeded 75% for all tested solutions except for the lowest viscosity CNT-doped solution (24% PMMA/0.5% CNT, η=50.1 Pa*s), which experienced capillary breakup prior to solidification. The conductivities of direct-write PMMA/CNT fibers ranged from -7 to 0.15 S/m, with shorter fibers having higher conductivities (P