Office: 128 Bard
Phone: 607.255.0159
Email: cml66@cornell.edu
The Liddell group research efforts focus on the development of colloid-based materials [using synthetic chemistry, surface modification, self-assembly and field-directed assembly] and on understanding the relationship between their structure and properties to provide engineering solutions for the micron and submicron length scales. We integrate fundamental science in the areas of inorganic and organic chemistry, interface science, materials science (principles for electronic, optical, magnetic, mechanical behavior of materials), condensed matter physics, and thermodynamics to design and fabricate materials for applications requiring low cost manufacture with performance tied to control of micro- and nanostructures. In particular, we address the limitations in achieving optimum photonic crystal materials for photonic technologies such as optical integrated circuits, light sources, photocatalysts, and solar cells. Photonic crystals [PCs, photonic bandgap (PBG) materials, periodic dielectrics] modify the dispersion relations and strongly affect the distribution of electromagnetic modes within dielectric materials, offering unique control of light; for example, enhanced or inhibited spontaneous emission of molecules and “super”-prism beam steering effects, with large angular dispersion (>50º) observed for small wavelength differences (~10nm).
Our work addresses the central challenge in the photonic crystal field, to develop inexpensive fine-scale periodic materials with sizable PBGs or large partial PBGs (stopbands) at visible and near-IR frequencies. Calculations indicate the materials design characteristics favorable for this achievement include a sub-micron lattice constant, a high refractive index contrast (high-n) between periodic regions, low optical absorption, and low filling fraction of high-n material (ordered porous structure). The crystal structure and shape and complexity of the photonic crystal basis also greatly influence the capability of photonic crystals to exhibit PBGs, by removing symmetry induced degeneracy in the photonic bands. For the past 15 years experimental achievements have been mainly limited to simple inverted face-centered cubic (FCC) structures templated from close-packed spheres due to [1] the lack of availability of non-spherical particles with consistent size and shape (CV<5%) in a variety of chemistries and [2] the lack of processes for particle orientation control which would lead to complex topologies and new lattice structures. Because of the spherical symmetry at lattice points, the inverted FCC structure exhibits weak light-matter interactions and fragile bandgaps (5% width as given by gap-midgap ratio) which close with minor structural disorder.
We create complex colloidal crystals from buildings blocks with diverse morphology [i.e., non-spherical, core-shell] and with functionality [i.e., fluorescent, magnetic]. Our experiments investigate the effects of colloid particle chemistry, shape, size, packing, orientation, and refractive index on photonic band structures and the resulting optical properties of photonic crystals with the ultimate goal of developing new functional (beyond passive waveguiding) photonic crystal structures. These studies are complemented by our fundamental interests in understanding colloidal self-assembly and the phase behavior of complex colloid shapes.
Synthesis and Characterization of New Monodisperse Colloidal Building Blocks with Well-Defined Shape and Diverse Functionality. Particle systems: hollow silica/FITC-dye peanut-shaped (fluorescent) particles, silica coated peanut-shaped iron oxide (magnetic), Cdot/ZnS coated polystyrene dimers [snowman-shaped] and mushroom-cap shaped particles (high refractive index, fluorescent), Ag coated ZnS spheres (catalytic, plasmonics), among others.
Self-Assembly. [1] Convective assembly is being investigated as an orientation selective crystallization process for a range of colloid shapes― for example, morphologies with a systematically varied degree of fusion between dimer lobes and radius ratio of the constituent lobes. [2] Electrosteric stabilization and convective assembly of colloidal crystals from ZnS@PS core-shell colloids. [3] Active PCs: co-assembly of non-spherical colloids into ordered photonic crystal structures, aided by fluorescent nanoparticle depletants (Cdots, with U. Wiesner). Modification of dye emission characteristics due to changes in the local density of photon states is being explored in the reduced symmetry photonic crystal system. Reflection, transmission, and optical diffraction measurements are used to probe the strength of light matter interactions for the crystals [1]-[3].
Phase Behavior Studies & Thermodynamic Modeling. We study the structure and dynamics of fluorescently labeled and index matched silica hollow peanut-shaped particles confined in 2D (with I. Cohen). Anisotropic colloidal building blocks of controlled size and shape form exotic phases and exhibit novel colloidal behavior in suspensions, compared to systems of monodisperse spheres. Phase structures examined by quantitative real-space analysis in the confocal microscope are compared with thermodynamic models from Monte Carlo simulations (with F. Escobedo).
Modeling Optical Behavior in Non-Spherical Systems: Calculations of Photonic Band Structures via Plane Wave Expansion Method. We model the effects of reduced symmetry, particle orientation, packing and refractive index contrast on the photonic band structure of the new phases derived from non-spherical colloid-based crystals (with J D Joannopoulos).
In addition to PC studies, our projects related to self-assembly of colloidal materials for other applications include:
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