Bret N. Flanders

Directional growth of polypyrrole and polythiophene wires

This work establishes an innovative electrochemical approach to the template-free growth of conducting polypyrrole and polythiophene wires along predictable interelectrode paths up to 30 μm in length. These wires have knobby structures with diameters as small as 98 nm. The conductivity of the polypyrrole wires is 0.5±0.3 S cm−1; that of the polythiophene wires is 7.6±0.8 S cm−1. Controlling the growth path enables fabrication of electrode-wire-target assemblies where the target is a biological cell in the interelectrode gap. Such assemblies are of potential use in cell stimulation studies.

Single-step growth and low resistance interconnecting of gold nanowires

We present a single-step, electrochemical approach to the growth and low contact resistance interconnecting of gold nanowires with targeted points on lithographic electrodes. Electron diffraction studies indicate that these nanowires are composed of face-centred cubic crystalline gold, and that the crystal structure is invariant along the wire lengths. Four-point resistance determinations of these electrode?nanowire?electrode assemblies consistently yield resistances of <50??, and the contributions from the electrode?wire contacts are of the order of 10??. Atomic force microscopy was used to depict the structurally integrated nature of the electrode?wire contacts. This feature underlies the low electrode?wire contact resistances.

Reproducible interconnects assembled from gold nanorods

By using cleanroom-based lithographic procedures to produce identical electrode arrays, we have fabricated dielectrophoretic nanowires that vary in their conductance by ±10%. Transmission electron microscopy established the presence of interconnect segments composed of densely aggregated nanoparticles and of individual nanorods lying in the current-carrying path. The current-voltage profiles of these interconnects exhibited barriers to charge transport at temperatures less than ∼225K; furthermore, their conductances increased exponentially with temperature with an activation energy comparable to the nanorod charging energy. These results indicate that the Coulomb blockade associated with individual nanorods in the interconnects is the primary conductance-limiting feature.

One-step synthesis of graphene via catalyst-free gas-phase hydrocarbon detonation

A one-step, gas-phase, catalyst-free detonation of hydrocarbon (C2H2) method was developed to produce gram quantities of pristine graphene nanosheets (GNs). The detonation of C2H2 was carried out in the presence of O2. The molar ratios of O2/C2H2 were 0.4, 0.5, 0.6, 0.7 and 0.8. The obtained GNs were analyzed by XRD, TEM, XPS and Raman spectroscopy. The GNs are crystalline with a (002) peak centered at 26.05° (d = 0.341 nm). TEM shows that the GNs are stacked in two to three layers and sometimes single layers. An increase in the size of the GNs (35-250 nm) along with a reduction in defects (Raman I(D)/I(G) ~ 1.33-0.28) and specific surface area (187-23 m(2) g(-1)) was found with increasing O2 content. The high temperature of the detonation, ca. 4000 K, is proposed as the cause of graphene production rather than normal soot. The method allows for the control of the number of layers, shape and size of the graphene nanosheets. The process can be scaled up for industrial production.

Directed growth of single-crystal indium wires

Tailored electric fields were used to direct the dendritic growth of crystalline indium wires between lithographic electrodes immersed in solutions of indium acetate. Determination of the conditions that suppress sidebranching on these structures has enabled the fabrication of arbitrarily long needle-shaped wires with diameters as small as 370nm. Electron diffraction studies indicate that these wires are crystalline indium, that the unbranched wire segments are single-crystal domains, and that the predominant growth direction is near ⟨110⟩. This work constitutes a critical step towards the use of simply prepared aqueous mixtures as a convenient means of controlling the composition of submicron, crystalline wires.

Solvent intermolecular polarizability response in solvation

Polarizability response spectroscopy, a two-color optical Kerr effect method, has been developed and employed to study solvent intermolecular polarizability responses to photoexcited solutes. Here, we report solvent intermolecular polarizability responses in (dipolar) solvation. The time-resolved nonresonant polarizability signals are analyzed in the frequency domain where they are fit to a functional form representing diffusive reorientational, interaction-induced, and librational motions. Diffusive reorientational motion of CHCl3 was preferentially driven following photoexcitation of Coumarin 153 while interaction-induced motion was mainly driven in CH3CN solutions. The mechanism for selective solvent responses involves the relative orientation of the solvent dipole and most polarizable molecular axes and their interaction strength to the solute dipole.

Directional growth of metallic and polymeric nanowires

This work delineates the mechanism by which directional nanowire growth occurs in the directed electrochemical nanowire assembly (DENA) technique for growing nanowires on micro-electrode arrays. Indium, polythiophene, and polypyrrole nanowires are the subjects of this study. This technique allows the user to specify the growth path without the use of a mechanical template. Nanowire growth from a user-selected electrode to within +/- 3 microm of the straight line path to a second electrode lying within a approximately 140 degrees angular range and a approximately 100 microm radius of the selected electrode is demonstrated. Theory for one-dimensional electrochemical diffusion in the inter-electrode region reveals that screening of the applied voltage is incomplete, allowing a long range voltage component to extend from the biased to the grounded electrode. Numerical analysis of two-dimensional multi-electrode arrays shows that a linear ridge of electric field maxima bridges the gap between selected electrodes but decays in all other directions. The presence of this anisotropic, long range voltage defines the wire growth path and suppresses the inherent tip splitting tendency of amorphous polymeric materials. This technology allows polythiophene and polypyrrole to be grown as wires rather than fractal aggregates or films, establishing DENA as an on-chip approach to both crystalline metallic and amorphous polymeric nanowire growth.

DIRECTED ELECTROCHEMICAL NANOWIRE ASSEMBLY: PRECISE NANOSTRUCTURE ASSEMBLY VIA DENDRITIC SOLIDIFICATION

The electrode-nanowire-target architecture, where the target is a second electrode or, say, a biological cell, is critical to fundamental experiments and high performance devices in low dimensional charge transport and nanomaterials based bio-instrumentation. The relatively new technique of directed electrochemical nanowire assembly, which is based on the diffusion limited process of dendritic solidification, permits the single step fabrication of electrode-nanowire-target assemblies. Hence, this technique is reviewed here in order to assess its current state and to elucidate aspects where further study of the underlying solidification process would be likely to expand its capabilities and applications.

Optical damage threshold of Au nanowires in strong femtosecond laser fields

Ultrashort, intense light pulses permit the study of nanomaterials in the optical non-linear regime. Non-linear regimes are often present just below the damage threshold thus requiring careful tuning of the laser parameters to avoid melting the materials. Detailed studies of the damage threshold of nanoscale materials are therefore needed. We present results on the damage threshold of gold (Au) nanowires when illuminated by intense femtosecond pulses. These nanowires were synthesized via the directed electrochemical nanowire assembly (DENA) process in two configurations: (1) free-standing Au nanowires on tungsten (W) electrodes and (2) Au nanowires attached to fused silica slides. In both cases the wires have a single-crystalline structure. For 790 nm laser pulses with durations of 108 fs and 32 fs at a repetition rate of 2 kHz, we find that the free-standing nanowires melt at intensities close to 3 TW/cm2 (194 mJ/cm2) and 7.5 TW/cm2 (144 mJ/cm2), respectively. The Au nanowires attached to silica slides melt at slightly higher intensities, just above 10 TW/cm2 (192 mJ/cm2) for 32 fs pulses. Our results can be explained with an electron-phonon interaction model that describes the absorbed laser energy and subsequent heat conduction across the wire.

The directed-assembly of CdS interconnects between targeted points in a circuit

We demonstrate the one-step dielectrophoretic assembly and interfacing of individual interconnects from populations of 3.7 nm CdS nanoparticles between targeted points in a circuit. We further show that the nanoparticles fuse into bulk CdS during the fabrication process. This finding is significant because it establishes a critical step towards the fabrication of structurally continuous semiconducting interconnects from nanoscopic building blocks.