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Research Overview
Research in the Li laboratory is in the interdisciplinary field of nanoscience and nanotechnology with an emphasis on the development of micro-/nano- devices for analytical, biomedical, and electronic applications. Our research covers nanomaterials growth, device fabrication/characterization, and application development. These projects are in close collaboration with academic, industrial, and government partners.
Nanomaterials Growth
Our nanomaterials synthesis work is focused on preparing high-aspect ratio nanowires (NWs) to achieve many one-dimensional nanoscale materials properties. A big portion of the efforts is on exploring new methods to grow nanowires deterministically on solid substrates with controlled diameter, length, and orientation (particularly in free-standing vertical orientation). The nanowire materials include carbon nanotubes (CNTs), carbon nanofibers (CNFs), semiconducting inorganic crystalline nanowires (s-NWs), and metallic nanowires (m-NWs). The methods include thermal chemical vapor deposition (CVD), plasma enhanced chemical vapor deposition (PECVD), and electrochemical deposition. Besides tuning the processing conditions, each system will be also optimized with proper catalysts to enhance the selective growth of specific NW materials. Substrate engineering is another route to achieve the lattice match required for epitaxial growth of inorganic NWs. The combination of catalysis, surface modification, and processing optimization presents a very active on-going research area for nanomaterials growth.
Device Fabrication/Characterization
To characterize and make use of the intrinsic properties of individual NWs, micro- and nanofabrication techniques have to be employed to make connections with NWs in well-defined configurations. This becomes exceedingly challenging due to the lack of methods to manipulate individual nanomaterials and the presence of fundamental interface problems at the nanoscale. The development of new integration methods becomes the critical pathway to such device applications. We employ conventional solid-state micro-/nano- fabrication techniques including lithography, CVD/PVD, plasma and wet chemical etching, sputtering, and chemical mechanical polishing. In addition, nonconventional methods such as soft-lithography, imprinting, templating, electrochemical etching/deposition, and chemical functionalization are investigated for various applications. A bottom-up process has been developed previously to fabricate massive arrays of vertically aligned NWs, which enables the study of individual NWs. The electronic, physical, and chemical properties and device performance are under study with four-probe I-V measurements, spectroscopy, electron microscopy, electrochemistry, and scanning probe microscopy. For biomaterials and biomedical devices, experiments involving molecular biochemistry, cell/tissue culture, and in-vivo animal experiments are carried in our lab or in collaboration with other partners.
Application Development
(1) Biosensors: Inlaid CNF nanoelectrode arrays are employed as electronic sensors. The exposed tip of CNFs is selectively functionalized with oligo- nucleotides, antibodies, or peptides for the development of electrochemical or impedance-based sensors to detect nucleic acids, antigens, and kinase activities. In another configuration, the inlaid nanoelectrode array is fabricated in a microfluidic channel as a highly effective dielectrophoresis device for bioparticle trapping and sensing. An integrated biochip for bacteria detection is under development in collaboration with industrial partners.
(2) Biomedical devices: Vertically aligned CNFs are used as a brush-like electrode to interface with tissues. A conductive polymer coated vertical CNF array is been explored as a multi-functional neural electrical interface to provide topographical, mechanical, chemical, and electrical support of neural network. Applications as other implantable biomedical devices requiring long-term stability is under investigation.
(3) Solid-state devices: Novel integration and fabrication methods are developed for applications of CNTs, CNFs, and inorganic NWs as on-chip integrated circuit interconnects, thermal interface materials, and transistors. We are currently working on the further evaluation and optimization of both materials and processes.
(4) Energy sources: The large surface area of the 3D structure of vertically aligned CNFs is studied for the development of supercapacitors and lithium ion storage. The semiconducting NW arrays with the similar configurations are explored for solar cell and photodiodes, which may combine the advantages of high excitation efficiency and high conductivity.
Selected Publications
B. T. D. Nguyen-Vu, H. Chen, A. M. Cassell, R. J. Andrews, M. Meyyappan, and J. Li, “Vertically-Aligned Carbon Nanofiber Architecture as a Multifunctional 3D Neural Electrical Interface”, IEEE Trans. Biomed. Eng., 2007, 54(6), 1121-1128.
Q. Ngo, A. M. Cassell, A. J. Austin, J. Li, S. Krishnan, M. Meyyappan, and C. Y. Yang, “Characteristics of Vertically Aligned Carbon Nanofibers for Interconnect Via Applications”, IEEE Electron Device Letters, 2006, 27(4), 221-224.
B. Nguyen-Vu, H. Chen, A. M. Cassell, R. J. Andrews, M. Meyyappan, and J. Li, “Vertically Aligned Carbon Nanofiber Arrays: an Advance toward Electrical-Neural Interfaces”, Small, 2006, 2(1), 89-94.
Jun Li, Jessica E. Koehne, Alan M. Cassell, Hua Chen, Hou Tee Ng, Qi Ye, Wendy Fan, Jie Han, and M. Meyyappan, “Inlaid Multi-walled Carbon Nanotube Nanoelectrode Arrays for Electroanalysis”, Electroanalysis, 2005, 17(1), 15-27.
Q. Ngo, B. A. Cruden, A. M. Cassell, G. Sims, M. Meyyappan, J. Li, and C. Yang, “Thermal Interface Properties of Cu-filled Vertically Aligned Carbon Nanofiber Arrays”, NanoLett., 2004, 4(12), 2403-2407.
P. Nguyen, H. T. Ng, T. Yamada, M. K. Smith, J. Li, J. Han, and M. Meyyappan, “Direct Integration of Metal Oxide Nanowire in Vertical Field-Effect Transistor”, NanoLett, 2004, 4(4), 651-657.
H.T. Ng, B. Chen, J. Li, J. Han, and M. Meyyappan, “Optical Properties of Single Crystalline ZnO Nanowires on m-Sapphire”, Appl. Phys. Lett., 2003, 82 (13), 2023-5.
H. T. Ng, J. Li, M. K. Smith, P. Nguyen, A. Cassell, J. Han, and M. Meyyappan, Growth of Epitaxial Nanowires at the Junctions of Nanowalls, Science, 2003, 300, 1249.
J. Li, H. T. Ng, A. Cassell, W. Fan, H. Chen, Q. Ye, J. Koehne, J. Han, and M. Meyyappan, “Carbon Nanotube Nanoelectrode Array for Ultrasensitive DNA Detection”, Nanolett., 2003, 3(5), 597-602.
J. Li, Q. L. Ye Q, A. M. Cassell, H.T. Ng, R. Stevens, J. Han, and M. Meyyappan, “Bottom-up Approach for Carbon Nanotube Interconnect”, Appl. Phys. Lett., 2003, 82 (15), 2491-3.
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