|PhD, Chemical Engineering, 2005|
North Carolina State UniversityM.S., Chemical Engineering, 2003
North Carolina State University
B.S., Chemical Engineering, 2001
Research Focus: Engineered Deposition of Functional Coatings from Micro-and Nanoparticles Using Convective Assembly
I defended my dissertation research on Nov. 18, 2005, and since then have working for the past few months as a short term post doc through the end of February. I have been wrapping up the AR coatings for solar cells project which is an extension of the initial AR materials work done in collaboration with Dr. Yeon Hwang. This particular work, which we are preparing for submission was begun by Emily Hon, an REU last summer who has stayed in the same capacity while attending classes at NCSU.
In early March, I will packing up my things, selling my house here in the Raleigh, NC area and moving to Santa Barbara, CA where I will begin my post doctoral fellowship with the Zasadzinski group at the UCSB department of chemical engineering. My research there will be focusing on the exploration of different types of metal/organic colloids specifically designed for drug delivery, targeted therapies, and other biomedical applications.
For a brief description of basic aspects of my graduate thesis work please read the descriptions below and refer to the publications listed below.
My project is devoted to the study of colloidal deposition in the interest of fabricating colloidal crystals, metallic nanocoatings, antireflective films and other materials. In view of future technological applications of micro- and nanoparticle coatings and structures, it is important to develop assembly procedures that are fast, inexpensive and easy to control. Current methods of evaporation induced self assembly, or convective assembly of particle coatings such as colloidal crystals and nanoparticle thin films can take hours or days (via dipcoating), and additionally require comparatively large volumes of slowly evaporating colloidal suspensions.We have developed a technique for making thin films of structured micro- and nanoparticle particle layers by dragging a meniscus with constant velocity. The advantages of this technique are improved process speed and efficiency and reduced material consumption relative to standard dip coating techniques. Ordered coatings of larger than few square centimeters are deposited in minutes from aqueous suspension volumes of approximately 10 microliters. The governing mechanism of particle deposition is convective assembly at high volume fractions. Working at high volume fractions permits faster deposition speeds based on the principles of convective assembly. A variety of colloidal systems have been studied: latex colloidal crystals, metallic nanoparticle coatings, and ferritin protein coatings (a collaborative effort with Professor Plamen Atanossov’s group at University of New Mexico).
Using the monodisperse polystyrene latex microspheres as a model system (see Figure 1), we studied the relationship between deposition process parameters and coating structure. Ordered arrays of microspheres, commonly referred to as colloidal crystals, form by reduction of free volume as the solvent evaporates. The structural transitions from monolayer to multilayers that we observed in drying liquid films followed the packing transitions reported earlier by Pansu et al. [1-2] for particle infiltration into a gap between two solid surfaces. The deposition process can be controlled by several parameters: the speed of the meniscus movement (akin to withdrawal speed in dipcoating), the particle concentration, and evaporative flux of the solvent. Varying these parameters provides the means to control and tune nanocoating structure and properties. We used a modified version of the volumetric flux relationship proposed by Dimitrov and Nagayama  to relate coating properties to process parameters. We observed that fluctuations in the packing symmetry and layer thickness occur at meniscus withdrawal speeds that are incommensurate with the assembly speed of specific layers as they deposit from solvent to substrate.
Figure 1: The schematic on top depicts the layering transitions reported for monodisperse spheres confined in a small steadily, increasing gap between solid surfaces [1-2]. In our system, the top surface has been replaced with a liquid-air interface, but we see the same behavior for monodisperse latex spheres. The images shown below are optical micrographs of 1.1 µm polystyrene latex coatings deposited using our technique. As the assembling film thickness increases, the colloidal crystals showed the same hexagonal to square packing transitions as observed by Pieranski. The different colors of each layer are due to optical interference of the transmitted light. The colored polygons are to aid the eye.
Metallic nanoparticle coatings followed the behavior observed in the latex crystals, save that they were not ordered due to the strong VDW interactions coupled with the lack of steric protecting ligands. Spectrophotometry showed that these gold nanoparticle coatings exhibited red shifted surface plasmon absorbance (relative to native gold nanoparticle suspensions) due to the degree of aggregation upon deposition. The coatings also exhibited tunable properties. The reflective intensity could be tuned simply by adjusting the speed of deposition of the coatings which governed the surface coverage of gold.
Figure 2: By the same process as for latex crystals, we can make coatings of gold nanoparticles of varying degrees of surface coverage thickness simply by varying the speed of deposition, vw. Note that the thick aggregated gold stripes decrease in spacing as deposition speed is decreased from 20% (B) to 10 % (C) to 2% (D) of the maximum deposition speed, 211 µm/s, until a uniform, aggregate layer results. Currently we are investigating a simple theory for explaining the striping phenomena, which appears to be a transport limited process. There is a definite correlation to the ring formation commonly seen in evaporating droplets.
In addition to further work with aqueous suspensions, we are currently exploring the potential of this technique for particle deposition using low viscosity, highly wetting and environmentally benign solvents such as supercritical and liquid carbon dioxide.
1. Pansu, B.; Pieranski, P.; Strzelecki, L. Thin colloidal crystals: A series of structural transitions. J. Phys-Paris 1983, 44, 531.
2. Pansu, B.; Pieranski, Pi.; Pieranski, Pa. Structures of thin layers of hard spheres: High pressure limit. J. Phys-Paris 1984, 45, 331.
3. Dimitrov, A. S.; Nagayama, K. Steady-state unidirectional convective assembling of fine particle into two-dimensional arrays. Chem. Phys. Lett. 1995, 243, 462.
Prevo, B. G.; Hon, E. W.; Velev, O. D. Assembly and characterization of colloid-based antireflective coatings on multicrystalline silicon solar cells. Journal of Materials Chemistry 2007, 17, 791–799.
Prevo, B. G.; Kuncicky, D. M.; Velev, O. D. Engineered deposition of coatings from nano- and micro-particles: A brief review of convective assembly at high volume fraction. Colloids and Surfaces A: Physicochemical and Engineering Aspects 2007, 311, 2–10.