Field-based manipulation of micro- and nano- particles in fluids: tunable macroscopic material properties and characterization of individual nanoparticles
Creators: name: Yuan, Wuhan
Publisher: name: No Publisher Supplied
Year: 2019

description: Field-based particle manipulation has provided us unique opportunities to remotely manipulate fluid-suspended micro- and nano- particles in a highly efficient and controllable manner. Specifically, an external electric/magnetic field can be used to control (1) the macroscopic physical properties of the materials and (2) the microscopic movement of micro- or nano- particles in a suspending fluid, both of which have given rise to a wide variety of novel applications.In terms of controlling the macroscopic material properties, we have investigated a novel type of smart material with anisotropic and tunable acoustic properties. To elaborate, the microstructure (e.g., particle orientation and chaining) of suspensions of non-spherical ferromagnetic particles can be controlled by an external magnetic field, making it possible to change the acoustic properties of the suspension. Here, we experimentally demonstrate that dilute suspensions of subwavelength-sized oblate-spheroidal nickel particles exhibit up to a 35% change in attenuation coefficient at MHz frequencies upon changing the direction of an external magnetic field, for particle volume fractions of only 0.5%. Comparison is made to suspensions of spherical particles, in which the attenuation is smaller and nearly isotropic. Optical transmission measurements and analysis of the characteristic timescales of particle alignment and chaining are also performed to investigate the reasons for this acoustic anisotropy. The alignment of the oblate-spheroidal particles is found to be the dominant mechanism for the anisotropic and tunable acoustic attenuation of these suspensions. A mathematical model has also been developed to describe the acoustic properties of the above-mentioned microstructured suspensions using effective medium theory as well as Eshelby’s analytical solutions for ellipsoidal inclusions.For individual micro- or nano- particles, we take advantage of the strong dependence between AC-field induced particle rotation and particle properties to effectively characterize individual particles. In detail, we utilize the contactless, high-throughput electro-orientation spectroscopy (EOS) technique to measure the electrical conductivity of Si nanowires with atomic-layer-deposition- (ALD-) induced Al2O3 layers in a statistically meaningful manner. Initial surface treatments with HF and chemical-induced oxide (SC1) before ALD passivation, were both investigated. We show that nanowire properties and variability are dominated by the surfaces, given that the ALD-induced Al2O3 layer changes the electrical conductivity by 5-orders-of-magnitudes while reducing the variation in conductivity by half. These changes in the nanowire conductivity are believed to be due to a combination of chemical passivation and the field-effect passivation introduced by the ALD treatment.Finally, we extend the EOS technique to enable characterization of nanowires with more complex structures. Specifically, we show that the EOS technique can determine the conductivity and dimensions of two distinct segments in individual Si nanowires with axially encoded dopant profiles. Our analysis combines experimental measurements and computational simulations to determine the electrical conductivity of the nominally undoped segment of two-segment Si nanowires, as well as the ratio of the segment lengths. We demonstrate the efficacy of our approach by comparing results generated by EOS with conventional four-point-probe measurements. Our work provides new insights into the control and variability of semiconductor nanowires for electronic applications and is a critical first step toward the high-throughput interrogation of complete nanowire-based devices.