We carry out interdisciplinary studies on the science and technology of novel nanostructured materials. Nanoscale materials rank among the most exciting new developments in modern science and engineering. They are appealing because they exhibit novel electronic and chemical properties that can be dramatically different from the same material in the bulk form. The nanomaterial systems we focus on are nanowires, carbon nanotubes and molecular wires. Our work starts from the synthesis of these novel materials and goes all the way to the fabrication, characterization and assembling of nanoscale devices and systems.

1. Controlled Synthesis, Fundamental Studies and Applications of Novel Nanowires

Nanowires are fascinating one-dimensional systems with nanometer-scale diameters and micro-scale lengths. They boast distinct advantages over other materials. My group has established a solid track record in synthesis of various nanowires including semiconducting nanowires (In₂O₃, SnO₂, CdO, GaN and InN) and transition metal oxide core-shell nanowires (YBCO, LCMO, PZT and Fe₃O₄). Our highly successful and versatile synthesis techniques provide us unique opportunities to carry out research at the forefront of this field.

1.1 Synthesis of nanowires We have developed several techniques with significant innovations for nanowires synthesis. For instance, we have developed a generic laser ablation method for the growth of metal oxide nanowires. Our innovation involves using a carefully tuned dose of oxygen mixed in argon as our carrying gas in a chemical vapor deposition system, where a target is ablated to supply the metal vapor. This approach has produced a variety of metal oxide nanowires such as In₂O₃ and SnO₂. In addition, we have developed a novel “nanocasting” technique to grow transition metal oxide core-shell nanowires by depositing the desired material onto a lattice-matching nanowire (e.g., MgO) template, as shown in the figure on the right. This technique has allowed us to produce YBCO, LCMO, PZT and Fe₃O₄ nanowires, which were previously unavailable.

1.2 Electronic transport studies The nanowire synthesis is followed by thorough and in-depth electronic transport studies, such as transistor / memory property characterization and spin-dependent studies. Here we use multilevel molecular memory as an example, which has been demonstrated for nonvolatile data storage up to three bits (eight levels) per cell, in contrast to the standard one-bit-per-cell (two levels) technology. In the demonstration, charges were precisely placed at up to eight discrete levels in redox active molecules self-assembled on a single-crystal semiconducting nanowire field effect transistor (shown on the right). Gate voltage pulses and current sensing were used for writing and reading operations, respectively. Charge storage stability was tested up to retention of 600 hours, as compared to the longest retention of a few hours previously reported for one-bit-per-cell molecular memories. Comparison between silicon and molecular devices shows that multilevel molecular memory enables low-power, ultra-dense and high-performance nonvolatile data storage to go beyond the silicon technology scaling limit.

1.3 Chemical and bio- sensing applications These novel nanowires are ideal candidates for chemical and bio- sensing applications due to their enormous surface-to-volume ratios. We have successfully demonstrated detection of NO₂ down to ppb concentrations using In₂O₃ nanowire transistors. These nanowires have also been used to detect low-density lipoprotein (LDL) and other bio species, which are important for health care and biomedical research. Our work will eventually lead to “smart chips” with integrated sensors for environmental or biomedical studies.

2. Synthesis and Device Applications of Single-Walled Carbon Nanotubes

Carbon nanotubes are sheets of graphite rolled into seamless cylinders with nanometer diameters and micron scale lengths. This one-dimensional system exhibits fascinating electronic and mechanical properties. Depending on their chiralities, nanotubes can be metallic, semimetallic or semiconducting. Nanotubes also possess remarkably high Young's moduli and tensile strength. Despite the utmost interest in nanotubes, previous studies have been hampered by a lack of control over the nanotube growth and the difficulty in wiring up individual nanotubes. Significant progress in controlling several aspects of nanotube growth has been recently made in our group. A novel chemical vapor deposition process has been developed to grow single-walled carbon nanotubes (SWNT) with controlled orientations (shown on the right), thus allowing the fabrication and integration of nanotubes devices in a way compatible with the semiconductor industry. Significant effort is being devoted to device studies and system integration of carbon nanotubes. Examples include nanotube transistors, complementary inverters, nanoscale signal processors, chemical sensors, and biosensors.

Selected publications

 

1.	“Conductance of a Molecular Junction", M.A. Reed, C. Zhou, C.J. Muller, T.P. Burgin, and J.M. Tour, Science 278, 252 (1997).
2.	"Modulated Chemical Doping of Individual Carbon Nanotubes", C. Zhou, J. Kong, E. Yenilmez, H. Dai, Science 290, 1552 (2000).
3.	"Intrinsic Electric Properties of Individual Single-Walled Carbon Nanotubes with Small Band Gaps", C. Zhou, J. Kong and H. Dai, Phys. Rev. Lett. 84, 5604 (2000).
4.	"Reversible Electromechanical Characteristics of Carbon Nanotubes under Local Probe Manipulation", T.W. Tombler, C. Zhou, L. Alexseyev, J. Kong, H. Dai, L. Liu, C.S. Jayanthi, M. Tang, S. Wu, Nature 405, 769 (2000).
5.	"Nanotube Molecular Wires as Chemical Sensors", J. Kong, N. Franklin, C. Zhou, S. Peng, K. Cho, H. Dai, Science 287, 622 (2000).
6.	"Carbon Nanotube Field-Effect Inverters", X. Liu, R. Lee, J. Han, C. Zhou, Appl. Phys. Lett. 79, 3329 (2001).
7.	“Diameter-controlled Growth of Single-crystalline In2O3 Nanowires and Their Electronic Properties”, C. Li, D. Zhang, S. Han, X. Liu, T. Tang, and C. Zhou, Advanced Materials 15, 143 (2003).
8.	"Laser Ablation Synthesis and Electronic Transport Studies of Tin Oxide Nanowires" Z. Liu, D. Zhang, S. Han, C. Li, T. Tang, W. Jin, X. Liu, B. Lei, and C. Zhou, Advanced Materials 15, 1754 (2003).
9.	“Fabrication Approach for Molecular Memory Arrays”, C. Li, D. Zhang, X. Liu, S. Han, T. Tang, C. Zhou, W. Fan, J. Koehne, J. Han, M. Meyyapan, A.R. Rawlett, D.W. Price, J.M. Tour, Appl. Phys. Lett. 82, 645 (2003).
10.	“Multi-Level Molecular Memories” , C. Li, W. Fan, B. Lei, D. Zhang, S. Han, X. Liu, T. Tang, Z. Liu, S. Asano, M. Meyyapan, J. Han, and C. Zhou, Appl. Phys. Lett. 84, 1949 (2004).
11.	“Influence of Bis(terpyridine)-Fe(II) molecules on Charge Storage of a Nanowire Transistor”, C. Li, W. Fan, D. A. Straus, B. Lei, S. Asano, D. Zhang, J. Han, M. Meyyappan and C. Zhou, J. of Am. Chem. Soc. 126, 7750 (2004).
12.	"Single Crystalline Magnetite Nanotubes", Z. Liu, D. Zhang, S. Han, C. Li, B. Lei, W. Lu, J. Fang, and C. Zhou, J. of Am. Chem. Soc., in press (2004).
13.	"Generic Synthesis of Transition Metal Oxide Nanowires", S. Han, C. Li, Z. Liu, B. Lei, D. Zhang, W. Jin, X. Liu, T. Tang, and C. Zhou, NanoLetters 4, 1241 (2004).
14.	"Detection of NO2 down to ppb levels using individual and multiple In2O3 nanowire devices", D. Zhang, Z. Liu, C. Li, T. Tang, X. Liu, S. Han, B. Lei, and C. Zhou, NanoLetters 4, 1919 (2004).
15.	"Magnetite (Fe3O4) Core-Shell Nanowires: Synthesis and Magnetoresistance", D. Zhang, Z. Liu, S. Han, C. Li, B. Lei, M.P. Stewart, J.M. Tour, and C. Zhou, NanoLetters 4, 2151 (2004).