Chia-Ling Chen

Direct measurement of graphene adhesion on silicon surface by intercalation of nanoparticles

We report a technique to characterize adhesion of monolayered/multilayered graphene sheets on silicon wafer. Nanoparticles trapped at graphene-silicon interface act as point wedges to support axisymmetric blisters. Local adhesion strength is found by measuring the particle height and blister radius using a scanning electron microscope. Adhesion energy of the typical graphene-silicon interface is measured to be 151±28 mJ/m2. The proposed method and our measurements provide insights in fabrication and reliability of microelectromechanical/nanoelectromechanical systems.

Microfabricated multilayer parylene-C stencils for the generation of patterned dynamic co-cultures

Co-culturing different cell types can be useful to engineer a more in vivo-like microenvironment for cells in culture. Recent approaches to generating cellular co-cultures have used microfabrication technologies to regulate the degree of cell-cell contact between different cell types. However, these approaches are often limited to the co-culture of only two cell types in static cultures. The dynamic aspect of cell-cell interaction, however, is a key regulator of many biological processes such as early development, stem cell differentiation, and tissue regeneration. In this study, we describe a micropatterning technique based on microfabricated multilayer parylene-C stencils and demonstrate the potential of parylene-C technology for co-patterning of proteins and cells with the ability to generate a series of at least five temporally controlled patterned co-cultures. We generated dynamic co-cultures of murine embryonic stem cells in culture with various secondary cell types that could be sequentially introduced and removed from the co-cultures. Our studies suggested that dynamic co-cultures generated by using parylene-C stencils may be applicable in studies investigating cellular interactions in controlled microenvironments such as studies of ES cell differentiation, wound healing and development.

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.

Mechanical and electrical evaluation of parylene-C encapsulated carbon nanotube networks on a flexible substrate

Carbon nanotube networks are an emerging conductive nanomaterial with applications including thin film transistors, interconnects, and sensors. In this letter, we demonstrate the fabrication of single-walled carbon nanotube (SWNT) networks on a flexible polymer substrate and then provide encapsulation utilizing a thin parylene-C layer. The encapsulated SWNT network was subjected to tensile tests while its electrical resistance was monitored. Tests showed a linear-elastic response up to a strain value of 2.8% and nearly linear change in electrical resistance in the 0%–2% strain range. The networks’ electrical resistance was monitored during load-unload tests of up to 100 cycles and was hysteresis-free.

DNA-decorated carbon-nanotube-based chemical sensors on complementary metal oxide semiconductor circuitry.

We present integration of single-stranded DNA (ss-DNA)-decorated single-walled carbon nanotubes (SWNTs) onto complementary metal oxide semiconductor (CMOS) circuitry as nanoscale chemical sensors. SWNTs were assembled onto CMOS circuitry via a low voltage dielectrophoretic (DEP) process. Besides, bare SWNTs are reported to be sensitive to various chemicals, and functionalization of SWNTs with biomolecular complexes further enhances the sensing specificity and sensitivity. After decorating ss-DNA on SWNTs, we have found that the sensing response of the gas sensor was enhanced (up to approximately 300% and approximately 250% for methanol vapor and isopropanol alcohol vapor, respectively) compared with bare SWNTs. The SWNTs coupled with ss-DNA and their integration on CMOS circuitry demonstrates a step towards realizing ultra-sensitive electronic nose applications.

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.

The heterogeneous integration of single-walled carbon nanotubes onto complementary metal oxide semiconductor circuitry for sensing applications

A simple methodology for integrating single-walled carbon nanotubes (SWNTs) onto complementary metal oxide semiconductor (CMOS) circuitry is presented. The SWNTs were incorporated onto the CMOS chip as the feedback resistor of a two-stage Miller compensated operational amplifier utilizing dielectrophoretic assembly. The measured electrical properties from the integrated SWNTs yield ohmic behavior with a two-terminal resistance of approximately 37.5 kOmega and the measured small signal ac gain (-2) from the inverting amplifier confirmed successful integration of carbon nanotubes onto the CMOS circuitry. Furthermore, the temperature response of the SWNTs integrated onto CMOS circuitry has been measured and had a thermal coefficient of resistance (TCR) of -0.4% degrees C(-1). This methodology, demonstrated for the integration of SWNTs onto CMOS technology, is versatile, high yield and paves the way for the realization of novel miniature carbon-nanotube-based sensor systems.

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.

SWNT Based Nanosensors for Wireless Detection of Explosives and Chemical Warfare Agents

The detection of concealed explosives and chemical warfare agents is an urgent need to protect the safety of people and cities around the world. This requires the development of highly sensitive, portable, and stand-off sensors. Here, we present a wireless sensing unit based on single-walled carbon nanotubes (SWNTs) integrated with complementary metal oxide semiconductor (CMOS) circuitry, which can effectively detect trace explosives and chemical agents. The response of SWNT sensors to Dinitrotoluene (DNT) (a byproduct of TNT) and Dimethyl methylphosphonate (DMMP) (an analog of nerve agent sarin) vapors is improved dramatically after decoration with single stranded-DNA (ss-DNA). The response of carbon nanotube sensors to DNT and DMMP vapors is reversible and repeatable. The nanosensors integrated on a CMOS chip are tested with DMMP (DNT) vapors with concentrations varying from 1.57 to 130.49 ppm (9.41-45.73 ppm), and the corresponding change in resistance of the SWNT sensor varied from 7.5% to 27.5% (6.53-22.76%). The detected signal is processed by off-chip components on a circuitry board and is transmitted wirelessly to a computer. This versatile sensing system provides a promising platform to detect explosives and chemical agents at a trace level in a wireless manner and stand-off distance.

A Three Dimensional Thermal Sensor Based on Single-Walled Carbon Nanotubes

We present a novel three-dimensional thermal sensor based on Single-Walled Carbon Nanotubes (SWNTs) utilizing dielectrophoretic (DEP) assembly. The sensor is fabricated using a hybrid assembly technique combining top down (fabrication of the microplatform) and bottom up (DEP assembly) approaches. Encapsulating the structure with a thin (1mum) parylene layer protects it from the environment and also improves the contact resistance. Both single and multi finger assembly electrode structures have been utilized to manufacture the 3D thermal sensor and its thermal sensitivity is measured with a heated chuck. The resistances of the structures decrease more than 10% across a temperature range from 25degC to 65degC. The temperature coefficient of resistance for the SWNT-based thermal sensor is measured and ranged from -0.154 to -0.24% for the single electrode device and varied from -0.3 to - 0.57% for the multielectrode device.