2009 NDNC Poster
Andrew Steigerwald et al
Title: Ultrafast Phonon Pulses for Materials Characterization and Modification in Diamond, A. Steigerwald, T. Wade,J. Qi, Y. Xu, A.B. Hmelo, N. Tolk, J. Davidson
Abstract: Coherent acoustic phonon (CAP) interferometry, or picosecond ultrasonics, is a method whereby short (~10 nm) strain waves are generated through the absorption of intense, ultrafast optical pulses. Rapid heating launches a strain wave consisting of longitudinal acoustic phonons, of center frequency and FWHM near 100 GHz, which travel normal to the surface into the bulk crystal. The CAP waves may be utilized in two general schemes, (a) as a characterization tool which measures depth-dependent optical and electronic properties, and (b) as a method to modify material properties on picosecond timescales. The latter is a particularly exciting proposition, with the potential to modify, for instance, the band gap of materials in a consistent and localized manner, on the timescale of picoseconds. Of particular interest is the use of materials which have high thermal conductivity, such as single or polycrystalline diamond, allowing phonon propagation over a large distance with minimal dispersion and attenuation. The CAP technique also offers the ability to study fundamental optomechanical properties of materials by evaluating the behavior of the propagating strain wave. Here we present results obtained from single crystal and CVD grown diamond samples, which exhibit marked differences according to their respective microstructures. We demonstrate the ability to monitor the acoustic wave to a depth of ten microns or more into single crystal diamond, and the ability to probe the diamond samples near the 5.5 eV band gap, allowing comparison between the electronic structure of single crystal and CVD grown diamond samples.
2008 Fall MRS Poster
Manoj Sridhar et al
Title: Thermal Bubble Nucleation in Nanochannels: Simulations and Strategies for Nanobubble Nucleation and Sensing, Manoj Sridhar, Dongyan Xu, Anthony Hmelo, Deyu Li and Leonard Feldman
Abstract: With continuing progress in the state of the art of nanofabrication it is now possible to conceive devices that may enable the experimental sensing of bubble nucleation in nanochannels, and the direct measurement of the nucleation rate in water and other fluids. In the present study the authors present the results of molecular dynamics simulations of thermal bubble nucleation in nano-confined argon and water systems using an isothermal-isobaric (NPT) ensemble to determine the conditions under which nanobubble nucleation may be expected. No bubbles were observed for either system under an external pressure of 0.01 - 0.1 MPa, even for temperatures much higher than the boiling temperature of the respective liquids at 0.1 MPa. The density of the nano-confined fluids at constant temperature is observed to be almost independent of external pressure on the system in the simulated pressure range, suggesting that the nano-confined liquids behave like liquids with low compressibility even at temperatures close to their superheat limit. To explain these observations, we hypothesize that bubble nucleation induces a pressure disturbance, which travels to the channel wall and reflects back to the nucleation site suppressing bubble nucleation as the characteristic pressure wave travel time is much shorter than the nucleation time. Our results suggest limits on the nanochannel length scale and conditions under which nanobubble nucleation can be expected.
The experimental sensing of bubble nucleation in a nanochannel reactor requires an advanced detection scheme. We report on the detailed characterization of an ultrasensitive fluidic device that has been used to detect the translocation of small particles through a sensing microchannel. The device connects a fluidic circuit to the gate of a metal-oxide-semiconductor field-effect transistor (MOSFET) and detects particles by monitoring the MOSFET drain current modulation instead of the modulation in the ionic current through the sensing channel. The minimum volume ratio of the particle to the sensing channel detected is 0.006%, which is about ten times smaller than the lowest detected volume ratio previously reported in the literature. This volume ratio is detected at a noise level of about 0.6% of the baseline MOSFET drain current, clearly showing the amplification effects from the fluidic circuits and the MOSFETs. We characterized the device sensitivity as a function of the MOSFET gate potential and show that its sensitivity is higher when the MOSFET is operating below its threshold gate voltage than when it is operating above the threshold voltage. In addition, we demonstrated that the device sensitivity linearly increases with the applied electrical bias across the fluidic circuit. We demonstrate the application of the device concept as a particle sensor for polystyrene and glass beads on a variety of micro/nano length scales.
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