Xugang Xiong

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.

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.

Directed assembly of high density single-walled carbon nanotube patterns on flexible polymer substrates

We report an effective technique for the controlled assembly of single-walled carbon nanotubes (SWNTs) and demonstrate organized high density network architectures on soft polymeric substrates. We utilize the surface energy differential between a plasma treated (hydrophilic) parylene-C surface and a photoresist (hydrophobic) surface to create microscale patterns of SWNT networks on a 10 microm thick parylene-C substrate. The large scale fabrication of patterned SWNT structures presented is achieved by performing site-selective fluidic assembly of SWNTs. Electrically continuous nanotube network micro-arrays as small as 4 microm wide that are up to 1500 microm long with controlled separation have been fabricated by dissolving the photoresist after assembly. Electrical and mechanical characterization of nanotube networks on the flexible substrate in both static and dynamic modes indicates that the structure can handle both compressive and tensile deformations with no hysteresis. The technology presented has immediate applications in making thin film transistors, interconnects and sensors on flexible substrates.

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 controlled assembly of gold nanoparticles using a micromachined platform

By using optical lithographic procedures, the authors present a micromachined platform for large scale three dimensional (3D) assembly of gold nanoparticles with diameters of ∼50nm. The gold nanoparticles are formed into 3D low resistance bridges (two terminal resistance of ∼40Ω) interconnecting the two microelectrodes using ac dielectrophoresis. The thickness of the parylene interlevel dielectric can be adjusted to vary the height of the 3D platform for meeting different application requirements. This research represents a step towards realizing high density, three dimensional structures and devices for applications such as nanosensors, vertical integration of nanosystems, and characterization of nanomaterials.

A three dimensional Multi-Walled Carbon Nanotube based thermal sensor on a flexible Parylene substrate

We present the first design, fabrication and testing results from a three dimensional multi-walled carbon nanotube based thermal sensor fabricated on a flexible parylene-C substrate. Parylene-C is an inert, biocompatible, optically transparent, room temperature deposited polymer with a high mechanical strength, yet is rarely used as a flexible substrate. By utilizing a 2 mask process, we have manufactured a versatile microplatform for nanoscale assembly and then by utilizing dielectrophoretic assembly, incorporate MWNTs onto the platform in a 3D manner. The MWNTs are next encapsulated using a thin Parylene-C layer that acts as an environmental barrier and in addition keeps the MWNTs intact. The temperature Coefficient of Resistance of the MWNT sensor is measured to be between -0.21% and -0.66% per degree. The thermal sensor is compact, is very high density and could potentially be used for diverse temperature sensing applications such as in wearable textiles, on non planar surfaces and for in-vivo applications.

A Micromachined Platform for Three Dimensional Dielectrophoretic Assembly of Gold Nanoparticles for Nanodevices

This paper reports a novel technological approach for three-dimensional (3D) assembly of gold nanoparticles with an average diameter of 52 nm using dielectrophoresis (DEP). To realize the 3D assembly, the authors have designed and fabricated a versatile self-aligned micromachined platform which is applicable for assembling metallic nanoparticles and nanostructures. The assembly process is achieved at room temperature and is compatible with conventional semiconductor fabrication and large scale nanoassembly. The current-voltage curves obtained from the 3D gold nanoparticle bridges demonstrate that the assembly is functional with resistance values between -26 and 118 Ohms. This method has applications in making high density three-dimensional interconnects, vertically integrated nano sensors and for in-line testing of manufactured conductive nanoelements.

Highly Organized Carbon Nanotube-PDMS Hybrid System for Multifunctional Flexible Devices

We will present a method to fabricate a new class of hybrid composite structures based on highly organized multiwalled carbon nanotube (MWNT) and singlewalled carbon nanotube (SWNT) network architectures and a polydimethylsiloxane (PDMS) matrix for the prototype high performance flexible systems which could be used for many daily-use applications. To build 1–3 dimensional highly organized network architectures with carbon nanotubes (both MWNT and SWNT) in macro/micro/nanoscale we used various nanotube assembly processes such as selective growth of carbon nanotubes using chemical vapor deposition (CVD) and self-assembly of nanotubes on the patterned trenches through solution evaporation with dip coating. Then these vertically or horizontally aligned and assembled nanotube architectures and networks are transferred in PDMS matrix using casting process thereby creating highly organized carbon nanotube based flexible composite structures. The PDMS matrix undergoes excellent conformal filling within the dense nanotube network, giving rise to extremely flexible conducting structures with unique electromechanical properties. We will demonstrate its robustness under large stress conditions, under which the composite is found to retain its conducting nature. We will also demonstrate that these structures can be directly utilized as flexible field-emission devices. Our devices show some of the best field enhancement factors and turn-on electric fields reported so far.Copyright