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Our recent research is focused on:

 1. Nanowires and nanobelts of semiconducting oxide: from materials, to properties and to devices (2000 - present).

 2. In-situ nanomeasurements on the mechanical, electrical and field emission properties of nanostructures (1997 - present).

 3. Dynamics of shape controlled nanocrystals and nanocrystals self-assembly (1995 - present).

1. Nanowires and nanobelts of semiconducting oxide: from materials, to properties and to devices (2000 - present).

    Recently a series of binary semiconducting oxide nanobelts (or nanoribbons), such as ZnO, In2O3, Ga2O3, CdO and PbO2 and SnO2 have been successfully synthesized in Dr. Wang’s laboratory by simply evaporating the source compound (Science, 209 (2001) 1947). The as-synthesized oxide nanobelts are pure, structurally uniform, single crystalline and most of them free from defects and dislocations; they have a rectangular-like cross-section with typical widths of 30~300 nm, width-to-thickness ratios of 5~10 and lengths of up to a few millimeters. The belt-like morphology appears to be a unique and common structural characteristic for the family of semiconducting oxides with cations of different valence states and materials of distinct crystallographic structures. The nanobelts are an ideal system for fully understanding dimensionally confined transport phenomena in functional oxides and building functional devices along individual nanobelts. This discovery has been reported by over 20 media and professional society journals. Dr. Wang’s group has recently applied the nanobelt materials to make the world’s first field effect transistor and single wire sensors.

    The latest breakthroughs is the success of first piezoelectric nanobelts and nanorings for applications as sensors, transducers and actuators in micro- and nano-electromechanical systems (Science, 303 (2004) 1348). Owing to the positive and negative ionic charges on the zinc- and oxygen-terminated ZnO basal planes, respectively, a spontaneous polarization normal to the nanobelt surface is induced. As a result, helical nanosprings/nanocoils are formed by rolling up single crystalline nanobelts. The mechanism for the helical growth is suggested for the first time to be a consequence of minimizing the total energy contributed by spontaneous polarization and elasticity. The nanobelts have widths of 10-60 nanometers and thickness of 5-20 nanometers, and they are free of dislocations. The polar surface dominated ZnO nanobelts and helical nanosprings are likely to be an ideal system for understanding piezoelectricity and polarization induced ferroelectricity at nano-scale.

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2. In-situ nanomeasurements on the mechanical, electrical and field emission properties of carbon nanotubes (1997 - present).

     Characterizing the physical properties of carbon nanotubes is limited not only by the purity of the specimen but also by the size distribution of the nanotubes. Traditional measurements relies on scanning probe microscopy. Based on transmission electron microscopy, Dr. Wang and his colleagues have developed a series of unique techniques for measuring the mechanical, electrical and field emission properties of individual nanotubes. The in-situ TEM technique developed by him is not only an imaging tool that allows a direct observation of the crystal and surface structures of nanocrystals, but also an in-situ apparatus that can be effectively used to carry nano-scale measurements ( Science , 283 (1999) 1513). Using a custom-built specimen stage, the quantum conductance of a carbon nanotube has been observed in-situ in TEM, confirming the ballistic conductance and no-heat dissipation across a defect-free nanotube first published by de Heer's group ( Science , 280 (1998) 1744). A nanobalance technique and a novel approach toward nanomechanics have been ( Phys. Rev. Letts . 85 (2000) 622). Their discoveries have attracted a great deal attention of the medium and professional community.

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3. Dynamics of shape controlled nanocrystals and nanocrystals self-assembly (1995 - present).

    Nanosize colloidal platinum (Pt) particles are potentially important in industrial catalysis. The selectivity and activities of Pt particles strongly depend on their sizes and shapes. Much effort has been devoted to synthesize smaller size Pt particles for increasing the surface to volume atom ratio. Searching for techniques which can produce monoshape Pt particles has attracted a lot of interest because the chemical activities of Pt between {100} and {111} facets have distinct differences. Dr. Wang's collaboration with Prof. M.A. El-Sayed had led to a new technique based on colloidal chemistry for controlling the shapes and sizes of Pt particles at room temperature [Science 272 (June 1996) 1924]. Following this development, the growth mechanism of shape controlled Pt nanocrystals was studied using in-situ transmission electron microscopy. The shape transformation and melting behavior of the Pt nanocrystals were revealed for the first time.

    The physical and chemical functional specificity of nanoparticles suggest that they are ideal building blocks for two- and three-dimensional cluster self-assembled superlattice structures in which the particles behave as well-defined molecular matter and they are arranged with long-range translational and orientational order. In 1996, Dr. Wang collaborating with the research group of Prof. R.L. Whetten obtained concrete experimental results demonstrating success of forming such superlattice structures using Au nanocrystals. Following this, Dr. Wang has concentrated on the preparation of size and shape controlled Ag and CoO nanocrystals. His group was the first to study the role of particle shape in determining the crystallography of 3-D assembling of nanocrystals and the structural stability and molecular bonding between nanocrystals. Dr. Wang's recent research has been focused on self-assembly of magnetic nanocrystals for ultrahigh density data storage media. His paper (Phys. Rev. Lett., 79 (No. 13) (1997) 2570-2573) won the 1998 Georgia Tech Sigma Xi Best Paper Award in a campus wide competition.

    Dr. Wang and his collaborators at IBM (H. Zeng and S. Sun) and University of Texas Arlington (J.P. Liu) have developed a process that incorporates FePt and Fe₃O₄ particles with different mass and radii ratio into binary assemblies (Nature, 420 (2002) 395-398). Controlled annealing results in metallic composites with magnetically hard and soft phase exchange coupled. The approach offers precise engineering control on the dimension of the components and their nanoscale interactions in the composite, rendering isotropic FePt-based nanocomposites with energy product value of 20 MGOe that exceeds the theoretical limit of 13 MGOe for single phase FePt. This paper is being published by Nature.

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