MaCayla J. Caso

The self-assembly of nanoparticle colloids into large-area monolayers with long-range order is a grand challenge in nanotechnology. Using acoustic energy, i.e., acoustic annealing, to improve the crystal quality of self-assembled colloidal monolayers is a new solution to this challenge, but the characterization of the capillary waves driving the annealing process is lacking. We use a laser Doppler vibrometer and optical diffraction to uncover the frequency-dependent effects of capillary waves on the real-time self-assembly of submicrometer diameter polystyrene nanospheres at an air-water interface. Our study unambiguously demonstrates that low-frequency, e.g., sub-100 Hz, capillary waves are key to improving the long-range order of colloidal monolayers on an air-water interface. Furthermore, we demonstrate how a simple immersion transducer can generate capillary waves and how transducer placement and design affect vibrational spectra. Lastly, we show that frequency-shift keying of a high-frequency focused transducer provides a straightforward method of exciting low-frequency capillary waves that are effective at forming colloidal monolayers with excellent crystal quality, exhibited by grains over 3.5 cm².

Photocatalytic advanced oxidation processes (AOPs) promise a chemical-free route to energy-efficient degradation of waterborne micropollutants if long-standing mass transfer and light management issues can be overcome. Herein, we developed a dual-porous photocatalytic system consisting of a mesoporous (i.e., 2-50 nm pores) TiO₂ (P25) photocatalyst supported on macroporous (i.e., >50 nm pores) fused quartz fibers (P25/QF). Our reusable photocatalytic AOP reduces chemical consumption and exhibits excellent energy efficiency, demonstrated by degrading various pharmaceutical compounds (acetaminophen, sulfamethoxazole, and carbamazepine) in natural waters with electrical energy per order (E_EO) values of 4.07, 0.96, and 1.35 kWh/m³, respectively. Compared to the conventional H₂O₂/UVC AOP, our photocatalytic AOP can treat water without chemical additives while reducing energy consumption by over 2800%. We examine these improvements based on mass transport and optical (UVA and UVC) transmittance and demonstrate that the enhancements scale with increasing flow rate.

Colloidal lithography (CL) is a promising method for large-area fabrication of nanohole and nanodot arrays with applications in optical biosensing, separations, and magnetic data storage. However, reducing the diameter of the polystyrene sphere mask by plasma etching unavoidably increases their coefficient of variation (CV) and deforms their shape, thereby limiting the pitch-to-hole-diameter ratio of the resulting nanohole array to less than 3:1 and the minimum hole size to 200 nm with a 10% or better CV. We show that tilt-rotate evaporation colloidal lithography (TRE-CL) breaks the trade-off between hole diameter and polydispersity by leveraging glancing angle evaporation, not plasma etching, to adjust the hole size. TRE-CL allows pitch-to-hole-diameter ratios as high as 7:1 and nanohole diameters down to 60 nm while maintaining a nearly constant CV below 10% and hole circularity above 91%. We transfer these hole arrays into ultrathin Si3N4 films to form nearly-monodisperse microsieves for separation applications. Furthermore, we extend TRE-CL to fabricate adhesion-layer-free plasmonic Au nanodot arrays down to 70 nm in diameter with 10% CV.