Thomas L. Sounart

Spatially-resolved analysis of nanoparticle nucleation and growth in a microfluidic reactor

Microfluidic systems provide a unique platform for investigation of fundamental reaction processes, which is critical to understanding how to control nanostructure synthesis on a production scale. We have examined the synthesis of cysteine-capped CdS quantum dot nanocrystals (CdS-Cys) between two interdiffusing reagent streams in a continuous-flow microfluidic reactor. Using spatially resolved photoluminescence imaging and spectroscopy of the microreactor, we have acquired kinetic and mechanistic data on the CdS-Cys nanoparticle nucleation and growth, and observed a binary shift in the particle emission spectrum from a higher (2.9 eV) to lower (2.5 eV) energy emission peak within the first second of residence time. Several reactor models have been tested against the spatially and spectrally resolved signals, which suggest that homogeneous reaction and particle nucleation are diffusion-limited and occur only at the boundary between the two laminar streams, while a slower activation process occurs on a longer (seconds) time scale. The results provide direct insight into the rapid processes that occur during crystallization in microfluidic mixing channels, and demonstrate the potential of using controlled microfluidic environments with spatially resolved monitoring to conduct fundamental studies of nanocrystal nucleation and growth.

Polarity and piezoelectric response of solution grown zinc oxide nanocrystals on silver

The crystal orientation and piezoelectric properties of solution grown ZnO nanorods on Ag films were measured by quantitative piezoelectric force microscopy (PFM). The polarity of the rods, important for many device applications, was determined to be oriented [0001] from the substrates. This indicates that the prevalence of the [0001] oriented crystals is dominated by the fastest growing direction in solution. The average value of the d33 piezoelectric coefficient was measured to be 4.41pm∕V, with a standard deviation of 1.73pm∕V among the 198 individual rods. For calibration and comparison, PFM measurements were also performed on single crystals of x-cut quartz, z-cut periodically poled and single domain LiNbO3, and z-cut ZnO. Repeated measurements on individual rods establish that the run-to-run variation of a single rod is similar to that of single crystal measurements on quartz and LiNbO3. Hence, the observed rod-to-rod variation is not due to measurement uncertainty. Potential origins of this rod-to-...

Frequency-dependent stability of parallel-plate electrostatic actuators in conductive fluids

We present an electromechanical stability analysis of passivated parallel-plate electrostatic actuators in conductive dielectric media and show that the pull-in instability can be eliminated by tuning the applied frequency below a design-dependent stability limit. A partial instability region is also obtained, where the actuator jumps from the pull-in displacement to another stable position within the gap. The results predict that the stability limit is always greater than the critical actuation frequency, and therefore any device that is feasible to actuate in a conductive fluid can be operated with stability over the full range of motion.

A Model for Electrostatic Actuation in Conducting Liquids

This paper presents a generalized model that describes the behavior of micromachined electrostatic actuators in conducting liquids and provides a guideline for designing electrostatic actuators to operate in aqueous electrolytes such as biological media. The model predicts static actuator displacement as a function of device parameters and applied frequency and potential for the typical case of negligible double-layer impedance and dynamic response. Model results are compared to the experimentally measured displacement of electrostatic comb-drive and parallel-plate actuators and exhibit good qualitative agreement with experimental observations. The model is applied to show that the pull-in instability of a parallel-plate actuator is frequency dependent near the critical frequency for actuation and can be eliminated for any actuator design by tuning the applied frequency. In addition, the model is applied to establish a frequency-dependent theoretical upper bound on the voltage that can be applied across passivated electrodes without electrolysis.

Characterization of silicon parallel-plate electrostatic actuator in partially conducting aqueous solution

In this paper we present experimental results on the behavior of a silicon parallel-plate electrostatic actuator operated in a partially conducting aqueous solution. First, an experimental setup based on a laser position sensor is described for dynamic measurement of out-of-plate motion of MEMS structures in liquids. This set up is used to characterize motion of a surface-micromachined, polysilicon piston actuator operated in water using high- frequency drive signals to avoid charge migration that screens actuation potential. Results show that besides actuator design and fluid properties, voltage-displacement characteristics and pull-in phenomenon of parallel-plate actuators also depend on drive frequency.