Projects Synthesis Materials Fabrication Devices Characterization Theory
Numerical Approach to Dielectrophoretic Assembly   FRET  
Superconductivity in CNT   Band gap engineering  
Density Functional Theory   Surface Acoustic Waves (SAW)  
Numerical Approach to Dielectrophoretic Assembly
Nanoparticle assembly is currently of great interest as nanoparticles are considered as a fundamental building block in the fabrication of viable nanodevices. Various methods have been developed for the assembly of nanoparticles. Dielectrophoresis (DEP) utilizes differences in the electric polarizability of materials and the surrounding aqueous media. This phenomenon can be exploited to place suitable nanoparticles in-between the narrow gap of electrodes, enabling us to form electrical connections, expandable to a large area assembly. We investigate in order to quantify the different forces appropriate for controlled manipulation of discrete ZnO particles at the sub-micron levels as necessary for device applications. These manipulating forces are derived from AC electrokinetic phenomena and include various forces such as DEP, electro thermal force, electro osmotic force, and Stokes drag force on the fluid and buoyancy, Brownian motion directly on immerged particles. Consequent study on electric field and flow field distribution for better understanding and optimizing conditions for the real experiment are performed.
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FRET
FRET (Fluorescence Resonance Energy Transfer) is a technique for measuring interactions between two proteins in vivo. In this technique, two different fluorescent molecules (fluorophores) are genetically fused the two proteins of interest. Regular (non-FRET) fluorescence occurs when a fluorescent molecule (fluorophore) absorbs electromagnetic energy of one wavelength (the excitation frequency) and re-emits that energy at a different wavelength (the emission frequency).
Conceptually, one can imagine each fluorophore to have a two-peaked spectrum in which the first peak is the excitation peak, and the second is the emission peak. For the combined FRET effect, the emission peak of the donor must overlap with the excitation peak of the acceptor. In FRET, light energy is added at the excitation frequency for the donor fluorophore, which transfers some of this energy to the acceptor, which then re-emits the light at its own emission wavelength. The net result is that the donor emits less energy than it normally would (since some of the energy it would radiate as light gets transferred to the acceptor instead), while the acceptor emits more light energy at its excitation frequency (because it is getting extra energy input from the donor fluorophore).
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Superconductivity in CNT
Low dimensionality is generally considered as a necessary ingredient for high superconducting transition temperatures. Surprisingly, perhaps, systems based on graphite[1 - 3] have received little attention in this context. Introducing metal atoms between the carbon layers can tune the interlayer spacing and charging of the graphite host through a variety of electronic ground states. One such ground state is superconductivity[3], which is not present in pure graphite. In reference 7 the discovery of superconductivity in the intercalation compounds C6Yb and C6Ca, with transition temperatures of 6.5 and 11.5 K, respectively is reported. These critical temperatures are unprecedented in graphitic systems and have not been explained by a simple phonon mechanism for the superconductivity. This discovery has already stimulated several proposals for the superconducting mechanism[4 - 6] that range from coupling by way of the intercalant phonons through to acoustic plasmons. It also points towards the potential of superconductivity in systems such as carbon nanotubes. However to investigate this phenomenon, catalyst free semiconducting carbon nanotubes must be prepared. Our goal in this project is to fabricate a superconducting device from Yb doped singlewall carbon nanotubes.
References:
1. Dresselhaus, M. S. & Dresselhaus, G. Adv. Phys. 51, 1-186 (2002).
2. Zabel, H. & S olin, S. A. Graphite Intercalation Compounds I (Springer, Berlin, 1990). (Ibid II, 1992).
3. Enoki, T., Masatsugu, S. & Morinobu, E. Graphite Intercalation Compounds and Applications (OxfordUniv. Press, Oxford, 2003).
4. Csanyi, G., Littlewood, P. B., Nevidomskyy, A. H., Pickard, C. J. & Simons, B. D. Electronic structure of the superconducting graphite intercalates. Nature Phys. 1, 42-45 (2005).
5. Mazin, I. I. http://xxx.soton.ac.uk/PS cache/cond-mat/pdf/0504/0504127.pdf (April 2005).
6. Calandra, M.& Mauri, F. http://xxx.soton.ac.uk/PS cache/cond-mat/pdf/0506/0506082.pdf (June 2005).
7. Thomas E. Weller, Mark Ellerby, Siddharth S. Axena, Robert P. Smith & Neal T. Skipper, Nature phys. 1, 39 (2005)
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Band gap engineering
The process of controlling or altering the band gap of a material by controlling the composition of certain semiconductor alloys, is known as bandgap engineering. Bandgap engineering depends on which materials are being ‘engineered’ but typically one considers the bandgap of a material and decides how it might be modified by doping the material or making a three-component material by combining the starting material with a new one. For example consider InN, which has a bandgap of .78eV, and GaN, which has a bandgap of 3.37eV. In fact, by making InGaN composites one can engineer the bandgap to be as low as .7 (very low Ga content) to as high as 3.4 eV (higher Ga to In ratio).
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Density Functional Theory
Density Functional Theory will be employed to investigate crystal structure and physical properties of materials. To study transition metal oxides which are wellknown for their strong correlation of localized 3d electrons, simple approximation Local Density Approximation (LDA) is not appropriate any more and should be replaced by LDA+U or Dynamical Mean-Field Theory(DMFT).
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Surface Acoustic Waves (SAW)
A SAW is a generated effect of an acoustic wave on piezoelectric material. The acoustic wave is the transformed wave from electric signal to wave signal. A SAW is generated by an Interdigitated transducer (IDT).
As seen in the figure above, an IDT consists of a series of parallel planar metal electrodes, which are periodically spaced on plain-polished surface of a piezoelectric substrate. If an AC voltage is applied to the IDT, a harmonic deformation is generated. This in turn leads to the launching of elastic surface waves radiating out of the transducer in both directions normal to the electrodes. This acoustic wave is concentrated on the substrate surface and therefore can interact with conduction electrons of a semiconductor material on substrate surface.
The SAW propagation creates a retrograde elliptic motion of the material particles near the surface. This motion is illustrated in the figure below.
Due to this property, material on the substrate can be moved. SAW propagation can also be used for charge pumping. Please visit the devices page to see how SAW can be used in a sensor.
References:
1. http://www.ieee-uffc.org/freqcontrol/tutorials/Reindl_2002_files/frame.htm
2. Sensors and Actuators B, 113 389-397 (2006)
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