Current Research Projects:
1. Phase
Behavior and Phase Separation Kinetics of Polymer Dispersed Liquid
Crystal (PDLC) in Nanoscopic Geometries
The research is geared toward
creating a rational scientific basis for understanding the correlated
orientation of LC molecules, and the phase equilibrium and phase
separation kinetics of LC/polymer mixtures confined in nanoscopic
geometries by combining confocal Raman microscopy, electron microscopy,
atomic force microscopy, optical microscopy, X-ray scattering, and
computer simulations. These new fundamental insights will in turn help
guide the design of PDLC films in a more controllable manner, which will
likely lead to the development of electro-optical devices with desired
properties.
The phase separation kinetics of polymer dispersed liquid crystals
(PDLC) confined between two parallel, smooth walls are numerically
studied for the first time. The time evolutions of two order parameters
(i.e., composition order parameter and orientational order parameter)
are calculated by solving coupled time-dependent Ginzburg-Landau (TDGL)
model C equations. The ordering of LC is found to be accelerated as the
external confinement is enhanced (i.e., reduced separation distance
between two walls). The surface-induced structure formation of
polymer-dispersed liquid crystals (PDLCs) on a chemically patterned
substrate is also studied for the first time. The patterns on the
substrate are successfully transferred to the PDLC film, resulting in
alternating LC-rich and polymer-rich phases. This simple approach offers
a new means of organizing micrometer-sized LC domains into well-ordered
structures in a polymer matrix of PDLCs.
Recent Publications:
J. Wang, J. Xia, S. W. Hong, F. Qiu, Y. Yang, and Z. Lin*, "Pahse separation of polymer-dispersed liquid crystals on a chemically patterned substrate", Langmuir, 23, 7411 (2007). [PDF] (Cover Image in the July 3, 2007 Issue of Langmuir [PDF])
J. Xia, J. Wang, Z. Lin* , F. Qiu and Y. Yang, "Phase separation
kinetics of polymer dispersed liquid crystals confined between two
parallel walls", Macromolecules, 39, 2247 (2006).
[PDF]
2. Evaporation-Induced Self-assembly of Ordered
Structures from a Capillary-Held Solution
The use of spontaneous self-assembly as a lithography- and external
fields-free means to construct well-ordered, often intriguing structures
has received much attention due to the ease of producing complex,
large-scale structures with small feature sizes. These self-organized
structures promise new opportunities for developing miniaturized
optical, electronic, optoelectronic, and magnetic devices. One extremely
simple route to producing intriguing structures is the drying mediated
self-assembly of nonvolatile solutes (polymers, nanoparticles, and
colloids) through irreversible solvent evaporation of a sessile droplet
on a solid substrate. However, the flow instabilities within the
evaporating droplet often lead to non-equilibrium and irregular
dissipative structures, e.g., randomly organized convection patterns,
stochastically distributed multi-rings, and so on. Therefore, fully
utilizing evaporation as a simple tool for creating well-ordered
structures that have numerous technological applications requires
delicate control over the evaporative flux, solution concentration,
interfacial interaction between the solute and the substrate, etc.
The goal of this project is to develop a simple, one-step method for
fabricating nanostructured materials with or without hierarchical order
in a precisely controllable manner, dispensing with the need for
lithography techniques and external fields. To achieve this goal, three
specific objectives are proposed: (1) create ordered structures with
unprecedented regularity by controlling the flow of an evaporating
liquid in restricted geometries; (2) ＾synthesize￣ hierarchically odered
structures via the synergy of the drying-mediated self-assembly at the
microscopic scale and spontaneous self-assembly at the nanoscopic scale;
and (3) develop theoretical models to understand the mechanism of the
structure formation. We intend to produce highly regular structures with
homopolymers. Subsequently, we plan to design hierarchically ordered
structures consisting of diblock copolymers and quantum dots
self-assembled at the nanoscale that can serve as multifunctional
materials for potential applications in optical, optoelectronic, and
sensory materials and devices .
Recent Publications:
S.W. Hong, W. Jeong, H. Ko, M. Kessler, V.V. Tsukruk, and Z. Lin*, "Directed self-assembly of gradient concentric carbon nanotube rings", Advanced Functional Materials, 18 (2008) (in press)
M. Byun, S.W. Hong, L. Zhu, and Z. Lin*, "Self-assembling semicrystalline polymer into highly ordered, microscopic concentric rings by evaporation", Langmuir, 24, 3525 (2008) [PDF]
J. Xu, J. Xia and Z. Lin* , " Evaporation-induced self-assembly of nanoparticles from a sphere-on-flat geometry ", Angewandte Chemie International Edition, 46, 1860 (2007). [PDF]
S. W. Hong, J. Xia, and Z. Lin*, "Spontaneous formation of mesoscale polymer patterns in an evaporating bound solution", Advanced Materials, 19, 1413 (2007). [PDF]
S. W. Hong, J. Xia, M. Byun, Q. Zou, and Z. Lin*, "Mesoscale patterns formed by evaporation of a polymer solution in the proximity of a sphere on a smooth substrate: molecular weight and curvature effects", Macromolecules, 40, 2831 (2007). [PDF]
S. W. Hong, J. Xu, and Z. Lin*, "Template assisted formation of gradient concentric gold rings", Nano Letters, 6, 2949, (2006). [PDF]
S. W. Hong, S. Giri, V. S. Y. Lin, and Z. Lin*, "Simple route to gradient concentric metal and metal oxide rings", Chemistry of Materials, 18, 5164 (2006). [PDF]
J. Xu, J. Xia, S. W. Hong, Z. Lin* , F. Qiu and Y. Yang, "Self-assembly of gradient concentric rings via solvent evaporation from a capillary bridge", Physical Review Letters, 96, 066104 (2006). [PDF]
S. W. Hong, J. Xu, J. Xia, Z. Lin* , F. Qiu and Y. Yang, "Drying mediated pattern formation in a capillary-held organometallic polymer solution", Chemistry of Materials, 17 , 6223 (2005). [PDF]
3. Quantum Dots Tailored with Conjugated Polymers Confined at the
Nanoscale and Their Use in Solar Cells
Composites of quantum
dots/conjugate polymers (QD/CP) are of interest from the standpoint of
increased performance relative to either of the non-hybrid counterparts
with many applications envisioned in the areas of photovoltaic cells and
LEDs. They inherit decent mechanical strength from CPs and good
photostability and high conductivity from QDs. The QD/CP composites are
widely prepared by mixing these two components or by constructing a QD/CP
bilayer (only a small faction of excitons, i.e., the bound electron-hole
pairs, are able to diffuse to the interface where they are ionized) or
QD/CP alternating multilayer both physically or chemically. Thus, it is
difficult to control the detailed morphology and dispersion of QDs
within CPs. The interface between CP and QD, accomplished by stripping
ligand from QDs during film processing, is not well controlled, thereby
reducing the efficient electronic interactions between them. The
effective charge transfer, profoundly influenced by the quality of the
interface, is crucial for QD/CP composite for use in photovoltaic cells.
This implies that a bicontinuous and nanoscopic phase-separated mixture
of QD/CP is favorable for charge generation and transport, which is
currently difficult to realize by using a conventional blending
approach. On the other hand, for use in LED, it is important to
stabilize QDs in an appropriate host with retention of the fluorescence
emission. However, possible nanoparticle aggregation in composites often
limits the energy transfer pathway and leads to self-quenching of the
fluorescence of QDs.
In this context, placing the conjugated polymer (CP) in direct contact
with the quantum dot (QD) offers advantages over cases where QD
aggregation dominates. Such quantum dot-conjugated polymer
nanocomposites (QD-CP) possess a well-defined interface, thereby
significantly promoting the charge or energy transfer between these two
components. We intend to exploit nanoscopic geometries as unique
physical environments to control the conformation of CPs, which in turn
regulate the charge or energy transfer between QDs and CPs, and thus the
ultimate photophysical properties at the nanoscale. Three specific
research objectives will be pursued through this project: (1) prepare QD-CP
nanocomposites based on rational design; (2) explore the effect of
imposed external nanoscopic confinement on the photophysical properties
of QD-CP nanocomposites; (3) exploit QD-CP nanocomposites for use in
optoelectronic devices.
Recent Publications:
Z. Lin*, "Organic-inorganic nanohybrids through the direct tailoring of semiconductor nanocrystals with conjugated polymers", Chemistry-A European Journal, 14 (2008) (invited Concept; in press)
J. Wang and Z. Lin*, "Freestanding TiO2 nanotube arrays with ultrahigh aspect ratio via electrochemical anodization", Chemistry of Materials, 20, 1257 (2008) [PDF]
J. Xu, J. Wang, M. Mitchell, P. Mukherjee, M. Jeffries-EL, J. W. Petrich,and Z. Lin*, "Organic-inorganic nanocomposites prepared by grafting conjugated polymers onto quantum dots", Journal of the American Chemical Society 129, 12828 (2007). [PDF]
J. Xu, J. Xia, J. Wang, J. Shinar and Z. Lin*, "Quantum dots confined in nanoporous alumina membranes", Applied Physics Letters, 89, 133110 (2006). [PDF]