Sivasubramanian Somu

A Nanoparticle Convective Directed Assembly Process for the Fabrication of Periodic Surface Enhanced Raman Spectroscopy Substrates

Surface enhanced Raman scattering was discovered over 30 years ago, when it was noted that the usually weak molecular Raman scattering cross section was increased by orders of magnitude in the vicinity of metal surfaces. [ 1 , 2 ] Despite the early reports of single-molecule detection, [ 3 ] the promise of the technique as the basis for portable chemical sensors has not been fully realized. The reason for this gap between the science and the engineering of SERS lies in the formidable nanofabrication challenges it poses, which is the need to prepare large numbers of very small yet highly controlled “hot spots” as the sensing device. The need for “hot spots” arises because the SERS enhancement is composed of an electromagnetic effect and a chemical (or resonance) enhancement, with the electromagnetic effect being responsible for the majority of the enhancement. The “hot spots” are the manifestations of this fi eld enhancement, occurring only for select plasmonic materials. [ 4 ] Modeling studies of the electromagnetic effect indicate that dimerized plasmonic metal structures offer a signifi cantly higher electromagnetic fi eld enhancement than isolated structures, with the maximum fi elds occurring in the gap between the structures. [ 5 , 6 ] The fi eld enhancement dependence on the gap size is highly nonlinear: it becomes signifi cant when the gap is less than ∼ 10 nm, and then rises steeply with decreasing gap size. The importance of the small gap size on signal strength has been confi rmed experimentally. [ 7 , 8 ] While the SERS effect may be very high in a localized volume of the order of a few nm 3 , the probability that target analyte molecules adsorb there is very small. [ 9 ] Consequently, practical SERS-based sensors require the engineering of a very large number of such “hot spots” over areas that are at least a few mm 2 . Small interparticle spacing with a large number of metal particles has been realized for SERS experiments performed in nanoparticle solutions, which have been “activated” by electrolyte-induced aggregation. [ 10 ] However, solution-based

Monopole antenna arrays for optical trapping, spectroscopy, and sensing

We introduce a nanoplasmonic platform merging multiple modalities for optical trapping, nanospectroscopy, and biosensing applications. Our platform is based on surface plasmon polariton driven monopole antenna arrays combining complementary strengths of localized and extended surface plasmons. Tailoring of spectrally narrow resonances lead to large index sensitivities (S∼675 nm/RIU) with record high figure of merits (FOM∼112.5). These monopole antennas supporting strong light localization with easily accessible near-field enhanced hotspots are suitable for vibrational nanospectroscopy and optical trapping. Strong optical forces (350 pN/W/μm2) are shown at these hotspots enabling directional control with incident light polarization.

Large scale directed assembly of nanoparticles using nanotrench templates

The authors describe a general high throughput directed assembly technique to address some of the challenges to enable high rate∕high volume nanomanufacturing. The directed assembly of colloidal particles using an applied electric field shows the ability of precise control of nanoparticles by controlling assembly voltage, time, and geometric design of templates. The results show that single nanoparticle lines as small as 10nm wide and 100000nm long over a 2.25cm2 area as well as other nanoparticle structures can be fabricated using electrophoresis. This approach offers a simple, robust, and fast means of directed assembly of nanoelements for many applications.

Directed assembly of gold nanoparticle nanowires and networks for nanodevices

Alternating electric field is used to assemble gold nanoparticle nanowires from liquid suspensions. The effects of electrode geometry and the dielectrophoresis force on the chaining and branching of nanowire formation are investigated. The nanowire assembly processes are modeled using finite element calculations, and the particle trajectories under the combined influence of dielectrophoresis force and viscous drag are simulated. Nanoparticle nanowires with 10nm resolution are fabricated. The wires can be further oriented along an externally introduced flow. This work provides an approach towards rapid assembly and organization of ultrasmall nanoparticle networks.

Environmental Life Cycle Assessment of a Carbon Nanotube-Enabled Semiconductor Device

Carbon nanotubes (CNTs) demonstrate great promise in a variety of electronic applications due to their unique mechanical, thermal, and electrical properties. Although commercialization of CNT-enabled products is increasing, there remains a significant lack of information regarding the health effects and environmental impacts of CNTs as well as how the addition of CNTs may affect the environmental profile of products. Given these uncertainties, it is useful to consider the life cycle environmental impacts of a CNT-enabled product to discover and potentially prevent adverse effects through improved design. This study evaluates the potential application of CNT switches to current cellular phone flash memory. Life cycle assessment (LCA) methodology is used to track the environmental impacts of a developmental nonvolatile bistable electromechanical CNT switch through its fabrication, expected use, and end-of-life. Results are reported for environmental impact categories including airborne inorganics, land use, and fossil fuels, with the largest contributions from gold refining processes and electricity generation. First-order predictions made for the use and end-of-life (EOL) stages indicate that the CNT switch could provide potential improvements to reduce environmental burden during use, although CNT release could occur through existing EOL processes.

Mechanism of very large scale assembly of SWNTs in template guided fluidic assembly process.

Very large scale patterned single-walled carbon nanotube (SWNT) networks were fabricated using a newly developed template guided fluidic assembly process. A mechanism for SWNT assembly and their control is described here. To maximize the directed assembly efficiency of SWNTs toward a wafer level SWNT deposition, Si or SiO(2) substrate was pretreated with precisely controlled SF(6), O(2), and Ar plasma. Chemical and physical properties of the surface were characterized using several surface characterization techniques to investigate and control the mechanism of SWNT assembly. We found that hydrophilic chemical groups such as hydroxides were created on the silicon or silicon oxide surface through the controlled plasma treatment and fluidic SWNT dip-coating process. Also we found that nanoscale rough surface structures formed during the plasma treatment significantly increased the number of dangling bonds and hydroxide functional groups on the surface. These combined chemical and physical enhancements that attract SWNTs in the aqueous solution enable us to build highly organized and very large scale SWNT network architectures effectively in various dimensions and geometries.

Directed Assembly of Polymer Blends Using Nanopatterned Templates

The direct assembly of polymer blends on chemically functionalized surfaces is shown to produce a variety of nonuniform complex patterns. This method provides a powerful tool for easily producing nonuniform patterns in a rapid (30 s), one-step process with high specificity and selectivity for a variety of applications, such as nanolithography, polymeric optoelectronic devices, integrated circuits, and biosensors.

Topological transitions in carbon nanotube networks via nanoscale confinement.

Efforts aimed at large-scale integration of nanoelectronic devices that exploit the superior electronic and mechanical properties of single-walled carbon nanotubes (SWCNTs) remain limited by the difficulties associated with manipulation and packaging of individual SWNTs. Alternative approaches based on ultrathin carbon nanotube networks (CNNs) have enjoyed success of late with the realization of several scalable device applications. However, precise control over the network electronic transport is challenging due to (i) an often uncontrollable interplay between network coverage and its detailed topology and (ii) the inherent electrical heterogeneity of the constituent SWNTs. In this article, we use template-assisted fluidic assembly of SWCNT networks to explore the effect of geometric confinement on the network topology. Heterogeneous SWCNT networks dip-coated onto submicrometer wide ultrathin polymer channels become increasingly aligned with decreasing channel width and thickness. Experimental-scale coarse-grained computations of interacting SWCNTs show that the effect is a reflection of a topology that is no longer dependent on the network density, which in turn emerges as a robust knob that can induce semiconductor-to-metallic transitions in the network response. Our study demonstrates the effectiveness of directed assembly on channels with varying degrees of confinement as a simple tool to tailor the conductance of the otherwise heterogeneous network, opening up the possibility of robust large-scale CNN-based devices.

Scalable nanotemplate assisted directed assembly of single walled carbon nanotubes for nanoscale devices

The authors demonstrate precise alignment and controlled assembly of single wall nanotube (SWNT) bundles at a fast rate over large areas by combining electrophoresis and dip coating processes. SWNTs in solution are assembled on prepatterned features that are 80nm wide and separated by 200nm. The results show that the direction of substrate withdrawal significantly affects the orientation and alignment of the assembled SWNT bundles. I-V characterization is carried out to demonstrate electrical continuity of these assembled SWNT bundles.

Three-Dimensional Crystalline and Homogeneous Metallic Nanostructures Using Directed Assembly of Nanoparticles

Directed assembly of nano building blocks offers a versatile route to the creation of complex nanostructures with unique properties. Bottom-up directed assembly of nanoparticles have been considered as one of the best approaches to fabricate such functional and novel nanostructures. However, there is a dearth of studies on making crystalline, solid, and homogeneous nanostructures. This requires a fundamental understanding of the forces driving the assembly of nanoparticles and precise control of these forces to enable the formation of desired nanostructures. Here, we demonstrate that colloidal nanoparticles can be assembled and simultaneously fused into 3-D solid nanostructures in a single step using externally applied electric field. By understanding the influence of various assembly parameters, we showed the fabrication of 3-D metallic materials with complex geometries such as nanopillars, nanoboxes, and nanorings with feature sizes as small as 25 nm in less than a minute. The fabricated gold nanopillars have a polycrystalline nature, have an electrical resistivity that is lower than or equivalent to electroplated gold, and support strong plasmonic resonances. We also demonstrate that the fabrication process is versatile, as fast as electroplating, and scalable to the millimeter scale. These results indicate that the presented approach will facilitate fabrication of novel 3-D nanomaterials (homogeneous or hybrid) in an aqueous solution at room temperature and pressure, while addressing many of the manufacturing challenges in semiconductor nanoelectronics and nanophotonics.