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, electronics, and energy conversion and storage applications. Our research covers nanomaterials growth, device fabrication/characterization, and application development. These projects are in close collaboration with academia, industry, and government labs.
Our nanomaterials synthesis work is focused on preparing high-aspect ratio nanowires (NWs). A major effort is on exploring new methods to grow nanowires deterministically on solid substrates with controlled diameter, length, and orientation (particularly in free-standing vertical orientation) for device applications. 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. Another effort is on large-quantity synthesis of NWs with hydrothermal method. NW materials such as ZnO, Bi2Te3, MnO2, etc. have been prepared for various 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. Most fabrication processes employ a bottom-up method using massive arrays of vertically aligned CNTs and NWs on patterned substrates. The electronic, physical, and chemical properties and device performance are studied with electrochemistry, I-V measurements, optical spectroscopy, electron microscopy, 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 through collaborations.
(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 on the vertical CNF array is being explored as a multi-functional neural electrical interface to provide topographical, mechanical, chemical, and electrical support of neural network. The modification of the surface with conductive polymers further improves the biocompatibility as investigated with neuronal cell culture. Electrical stimulation/recording with rat hippocampal slices has shown much improved efficiency, indicating a more intimate neural electrical interface than planar microelectrode arrays.
(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, transistors, and chemical/biochemical sensors. We are currently working on further evaluation and optimization of both materials properties and processes.
(4) Energy conversion and storage: The large surface area of CNTs and NWs is attractive for the development of new solar cells, supercapacitors, and lithium ion batteries. Particularly, a TiO2 film deposited on the vertically aligned CNF array by MOCVD has shown an interesting nanoneedle structure. Such core-shell materials have been demonstrated as a novel dye-sensitized solar cell architecture. The fundamental understanding of energy conversion through materials and interface modification is being pursued in a collaboration involving researchers from all three major universities in Kansas, which is recently supported by the NSF EPSCoR program.
Li, Y.; Syed, L. U.; Liu, J.; Hua, D.; Li, J.*, Label-free electrochemical impedance detection of kinase and phosphatase activities using carbon nanofiber nanoelectrode arrays. Analytica Chimica Acta Accepted. (http://dx.doi.org/10.1016/j.aca.2012.07.027)
Liu, J.; Essner, J.; Li, J.*, Hybrid Supercapacitor Based on Coaxially Coated Manganese Oxide on Vertically Aligned Carbon Nanofiber Arrays. Chem. Mater. 2010, 22, (17), 5022-5030.
J. Liu, Y-T Kuo, K.J. Klabunde, C. Rochford, J. Wu J, and J. Li, Novel Dye-Sensitized Solar Cell Architecture Using TiO2-Coated Vertically Aligned Carbon Nanofiber Arrays. ACS Applied Materials & Interfaces 2009, 1(8), 1645-1649.
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.
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.
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.