Research
PMMA Building Blocks for Nonspherical Colloid–Based Photonic Crystals
Academic and Research Leadership Symposium, Nashville, TN, March 29, 2014.
Kenneth Lyons, Jr. 1, Chekesha M. Liddell Watson 1.
1 Cornell University, Ithaca, New York, USA
Colloidal building blocks for photonic crystals are being sought to achieve ‘molecular mimicry‘, where self–assembly directions are encoded by colloidal features such as functionalized surface inhomogeneity (patchy particles) or anisotropic morphology. The aim of the present work is to synthesize nonspherical poly(methyl methacrylate) (PMMA) particles. These colloids are easily refractive index matched for self–organization studies from monolayer to thin film to bulk using confocal microscopy. Dimers are formed using surfactant-free seeded emulsion polymerization. Spherical PMMA seed particles are crosslinked with ethyleneglycol–dimethacrylate (EGDMA). Upon heating, seeds swollen with monomer and crossslinker phase separate producing a secondary bulge which is polymerized to give the dimer shape. For polystyrene, the degree of fusion between lobes, relative radius ratio, and the angle of phase separation can be controlled through the monomer–to–polymer swelling ratio and the concentration of crosslinker. A hydrophilic layer adsorbed onto the hydrophobic surface of polystyrene seeds has been shown to produce uniform dimers in the submicron size regime. We report the colloidal shape and dispersity control from the addition of a hydrophilic layer on PMMA seeds to produce a range of asymmetric dimers < 2 μm in size. In addition to dimers, hemispheres and three-fourths cut–spheres display phase behavior with novel order–disorder transformations in simulation and in experiment using polystyrene however only a few layers can be studied due to strong light scattering. We hope to ultimately prepare such shapes in PMMA to study structure–optical property correlations in photonic materials.
Self–Assembly and Photonic Band Gaps for Square Bilayers of Cut–Spheres
1 Cornell University, Ithaca, New York, USA
Colloidal building blocks for photonic crystals are being sought to achieve ‘molecular mimicry‘, where self–assembly directions are encoded by colloidal features such as functionalized surface inhomogeneity (patchy particles) or anisotropic morphology. The aim of the present work is to synthesize nonspherical poly(methyl methacrylate) (PMMA) particles. These colloids are easily refractive index matched for self–organization studies from monolayer to thin film to bulk using confocal microscopy. Dimers are formed using surfactant-free seeded emulsion polymerization. Spherical PMMA seed particles are crosslinked with ethyleneglycol–dimethacrylate (EGDMA). Upon heating, seeds swollen with monomer and crossslinker phase separate producing a secondary bulge which is polymerized to give the dimer shape. For polystyrene, the degree of fusion between lobes, relative radius ratio, and the angle of phase separation can be controlled through the monomer–to–polymer swelling ratio and the concentration of crosslinker. A hydrophilic layer adsorbed onto the hydrophobic surface of polystyrene seeds has been shown to produce uniform dimers in the submicron size regime. We report the colloidal shape and dispersity control from the addition of a hydrophilic layer on PMMA seeds to produce a range of asymmetric dimers < 2 μm in size. In addition to dimers, hemispheres and three-fourths cut–spheres display phase behavior with novel order–disorder transformations in simulation and in experiment using polystyrene however only a few layers can be studied due to strong light scattering. We hope to ultimately prepare such shapes in PMMA to study structure–optical property correlations in photonic materials.
Self–Assembly and Photonic Band Gaps for Square Bilayers of Cut–Spheres
Symposium TT: Mesoscale Self-Assembly of Nanoparticles- Manufacturing, Functionalization, Assembly and Integration. Materials Research Society, San Francisco, CA, April 21 –25, 2014.
Angela Stelson 1, Erin Riley 1, Chekesha M. Liddell Watson 1.
1 Cornell University, Ithaca, New York, USA
Thermodynamic models of colloidal self–assembly with anisotropic basis particles combined with physical confinement of the colloid suspensions (Escobedo 2013) have identified a rich diversity of low–symmetry phases across systematically varied fill fractions and confinement heights which show promise for enhanced photonic bandgap properties. Experimental verification consistent with phase predictions have been realized using wedge confinement cells with synthesized colloidal particles of comparable shape (Riley 2010). Multi–dimensional photonic crystals have been investigated for their properties allowing electromagnetic field localization, switching, spontaneous emission suppression and negative refraction which are desired for applications including integrated optical circuitry, sub–wavelength focusing and planar lenses. For 2D slab structures of finite height, light is confined in the third dimension through index guiding. Restrictions in gap polarization and reductions in bandgap width have been reported as compared to infinite 2D structures, though improvements are seen in structures with reduced symmetry. Here, the self–assembly and photonic properties of the square bilayer phase of cut spheres with cut fraction χ=0.75 under confinement will be reported. The impact of dielectric contrast, dielectric fill fraction and variation in confinement height will be discussed for the direct and inverted square bilayer structures. Simulations indicate a large stable bandgap between the tenth and eleventh bands in the direct structure which is maximized at the dielectric contrast ε=12 (Silicon). In the inverted structure, two significant bandgaps between the fourth and fifth and the eighth and ninth bands were found.
Negative Refraction in Asymmetric Dimer –based Photonic Crystals
1 Cornell University, Ithaca, New York, USA
Thermodynamic models of colloidal self–assembly with anisotropic basis particles combined with physical confinement of the colloid suspensions (Escobedo 2013) have identified a rich diversity of low–symmetry phases across systematically varied fill fractions and confinement heights which show promise for enhanced photonic bandgap properties. Experimental verification consistent with phase predictions have been realized using wedge confinement cells with synthesized colloidal particles of comparable shape (Riley 2010). Multi–dimensional photonic crystals have been investigated for their properties allowing electromagnetic field localization, switching, spontaneous emission suppression and negative refraction which are desired for applications including integrated optical circuitry, sub–wavelength focusing and planar lenses. For 2D slab structures of finite height, light is confined in the third dimension through index guiding. Restrictions in gap polarization and reductions in bandgap width have been reported as compared to infinite 2D structures, though improvements are seen in structures with reduced symmetry. Here, the self–assembly and photonic properties of the square bilayer phase of cut spheres with cut fraction χ=0.75 under confinement will be reported. The impact of dielectric contrast, dielectric fill fraction and variation in confinement height will be discussed for the direct and inverted square bilayer structures. Simulations indicate a large stable bandgap between the tenth and eleventh bands in the direct structure which is maximized at the dielectric contrast ε=12 (Silicon). In the inverted structure, two significant bandgaps between the fourth and fifth and the eighth and ninth bands were found.
Negative Refraction in Asymmetric Dimer –based Photonic Crystals
MRS Symposium BB: Recent Advances in Optical, Acoustic, and Other Emerging Metamaterials. Materials Research Society, Boston, MA, November 25-30, 2012.
Erin Riley 1, Chekesha M. Liddell Watson 1.
1 Cornell University, Ithaca, New York, USA
Structures designed for negative refraction at specific optical frequencies are promising for sub–wavelength focusing, planar lenses and integrated optics. Photonic crystal approaches provide an alternative to high-loss metallic metamaterials. The 2D arrangements predominantly explored have been square and hexagonal crystals. Stronger light-matter interactions have been predicted for more complex geometries and crystal symmetry reduction can promote negative refraction effects. Advances in colloid synthesis enable a wide range of monodisperse nonspherical shapes as building blocks of photonic materials for anomalous refraction. For instance, dimers with controlled degrees of asymmetry and fusion of the constituent spheres have been self-assembled in experiments via gravitational sedimentation in a wedge-shaped confinement cell. The states formed can be described as centered-rectangular for in-plane oriented particles, quasi-2D hexagonal rotator, and hexagonal crystal for out-of-plane oriented particles. Here, negative refraction in the centered-rectangular monolayer case will be discussed for asymmetric dimer tilings. The effects of high-dielectric filling fraction, lobe symmetry, lobe fusion and dielectric contrast will be highlighted. For example, the analysis of equifrequency contours indicates negative refraction for the second photonic band when lobes are highly fused and moderately asymmetric and the particles are separated in air matrix. The infinitely extended 2D analogous structure supports right and left-handed negative refraction in the first two bands.
Quasi–2D Assembly of Peanut–shaped Colloids in Wedge Confinement
1 Cornell University, Ithaca, New York, USA
Structures designed for negative refraction at specific optical frequencies are promising for sub–wavelength focusing, planar lenses and integrated optics. Photonic crystal approaches provide an alternative to high-loss metallic metamaterials. The 2D arrangements predominantly explored have been square and hexagonal crystals. Stronger light-matter interactions have been predicted for more complex geometries and crystal symmetry reduction can promote negative refraction effects. Advances in colloid synthesis enable a wide range of monodisperse nonspherical shapes as building blocks of photonic materials for anomalous refraction. For instance, dimers with controlled degrees of asymmetry and fusion of the constituent spheres have been self-assembled in experiments via gravitational sedimentation in a wedge-shaped confinement cell. The states formed can be described as centered-rectangular for in-plane oriented particles, quasi-2D hexagonal rotator, and hexagonal crystal for out-of-plane oriented particles. Here, negative refraction in the centered-rectangular monolayer case will be discussed for asymmetric dimer tilings. The effects of high-dielectric filling fraction, lobe symmetry, lobe fusion and dielectric contrast will be highlighted. For example, the analysis of equifrequency contours indicates negative refraction for the second photonic band when lobes are highly fused and moderately asymmetric and the particles are separated in air matrix. The infinitely extended 2D analogous structure supports right and left-handed negative refraction in the first two bands.
Quasi–2D Assembly of Peanut–shaped Colloids in Wedge Confinement
MRS Symposium U: Colloidal Crystals, Quasicrystals, Assemblies, Jammings, and Packings. Materials Research Society, Boston, MA, November 25-30, 2012.
Kullachate Muangnapoh1, Carlos Avendano1, Fernando Antonio Escobedo1, Chekesha M. Liddell Watson1.
1 Cornell University, Ithaca, New York, USA
Colloidal self–assembly has been demonstrated as a prime method for the fabrication of multidimensional photonic materials. Particle ‘shape programming’ can be combined with physical confinement of colloid suspensions to access and stabilize a rich diversity of quasi–2D transition structures as material templates for optical band gap and anomalous refraction properties. Here, wedge–cell confinement is employed to study colloidal phase behavior of hollow fluorescent silica dimers (peanut–shaped) as a function of confinement height. Five distinct transitions are discovered in the range from one to two layers of in–plane oriented dimers. Specifically, each configuration found is a degenerate crystal tiling of the corresponding sphere–based structure along the descriptive order sequence 1△ →1B→2▢→2△ where, △ and ▢ indicate layers with triangular and square symmetry, respectively. In this scheme the ‘buckled’ phase is indicated by 1B. Two distinct 2△ phases are determined for the dimer case, depending on the degree of out–of–plane tilted tiling. Mostly out–of–plane colloidal units in 2△I re-assemble to form two layers of predominantly in–plane lying dimers for 2△II. These arrangements can be predicted simply from closest packing arguments for incommensurate layer heights and are in agreement with findings from Monte Carlo simulations. Order parameters and distribution functions for positional and bond orientational order, voronoi constructions for detecting defects and number of nearest neighbors, and fast Fourier transforms (FFT) quantitatively characterize each phase from confocal image analysis. The experimental real time video microscopy and theoretical phase diagram will be presented.
Optical Characterization of Slab Photonic Crystals with Asymmetric Dimer Bases
1 Cornell University, Ithaca, New York, USA
Colloidal self–assembly has been demonstrated as a prime method for the fabrication of multidimensional photonic materials. Particle ‘shape programming’ can be combined with physical confinement of colloid suspensions to access and stabilize a rich diversity of quasi–2D transition structures as material templates for optical band gap and anomalous refraction properties. Here, wedge–cell confinement is employed to study colloidal phase behavior of hollow fluorescent silica dimers (peanut–shaped) as a function of confinement height. Five distinct transitions are discovered in the range from one to two layers of in–plane oriented dimers. Specifically, each configuration found is a degenerate crystal tiling of the corresponding sphere–based structure along the descriptive order sequence 1△ →1B→2▢→2△ where, △ and ▢ indicate layers with triangular and square symmetry, respectively. In this scheme the ‘buckled’ phase is indicated by 1B. Two distinct 2△ phases are determined for the dimer case, depending on the degree of out–of–plane tilted tiling. Mostly out–of–plane colloidal units in 2△I re-assemble to form two layers of predominantly in–plane lying dimers for 2△II. These arrangements can be predicted simply from closest packing arguments for incommensurate layer heights and are in agreement with findings from Monte Carlo simulations. Order parameters and distribution functions for positional and bond orientational order, voronoi constructions for detecting defects and number of nearest neighbors, and fast Fourier transforms (FFT) quantitatively characterize each phase from confocal image analysis. The experimental real time video microscopy and theoretical phase diagram will be presented.
Optical Characterization of Slab Photonic Crystals with Asymmetric Dimer Bases
MRS Symposium AAA: Synthesis, Fabrication, and Assembly of Functional Particles and Capsules. Materials Research Society, San Francisco, CA, April 12, 2012.
Erin Riley1, Chekesha M. Liddell Watson1.
1 Cornell University, Ithaca, New York, USA
The promise of integrated optical devices capable of electromagnetic field localization, switching, spontaneous emission suppression and negative refraction through engineered design has long inspired research on photonic crystals. However, the structural variability through colloidal self–assembly of spherical particles has been limited primarily to high symmetry 2– and 3–dimensional lattices. For finite height 2D slab structures (e.g., based on circular crosssection pillars), light is confined in the third dimension through index guiding. Restrictions in gap polarization and/or bandgap width have been reported. Stronger light–matter interactions have been predicted for more complex geometries. Recently, advances in the synthesis of monodisperse nonspherical colloids such as micron–sized dimers along with their controlled assembly in wedge cell confinement geometry have accessed new centered rectangular and unconventional plastic or rotator phases, among others. Here, the photonic bandgaps in the guided modes for direct and inverted monolayer slabs with in–plane aligned dimer building blocks will be reported. The impact of dielectric filling fraction, lobe asymmetry, degree of fusion between lobes and dielectric contrast on the optical properties will be discussed. Simulations indicate significant band gaps between the fifth and sixth, sixth and seventh, and seventh and eighth bands. The characteristic refraction as a function of structural parameters will also be presented.
Sea Urchin Inspired Photonic Solids
1 Cornell University, Ithaca, New York, USA
The promise of integrated optical devices capable of electromagnetic field localization, switching, spontaneous emission suppression and negative refraction through engineered design has long inspired research on photonic crystals. However, the structural variability through colloidal self–assembly of spherical particles has been limited primarily to high symmetry 2– and 3–dimensional lattices. For finite height 2D slab structures (e.g., based on circular crosssection pillars), light is confined in the third dimension through index guiding. Restrictions in gap polarization and/or bandgap width have been reported. Stronger light–matter interactions have been predicted for more complex geometries. Recently, advances in the synthesis of monodisperse nonspherical colloids such as micron–sized dimers along with their controlled assembly in wedge cell confinement geometry have accessed new centered rectangular and unconventional plastic or rotator phases, among others. Here, the photonic bandgaps in the guided modes for direct and inverted monolayer slabs with in–plane aligned dimer building blocks will be reported. The impact of dielectric filling fraction, lobe asymmetry, degree of fusion between lobes and dielectric contrast on the optical properties will be discussed. Simulations indicate significant band gaps between the fifth and sixth, sixth and seventh, and seventh and eighth bands. The characteristic refraction as a function of structural parameters will also be presented.
Sea Urchin Inspired Photonic Solids
MRS Symposium JJ: Biological Hybrid Materials for Life Sciences.
Materials Research Society, San Francisco, CA, April 25–29, 2011. Chekesha M. Liddell Watson11.
1 Cornell University, Ithaca, New York, USA
Anisotropic shape in biological and natural systems continues to inspire the development of structural complexity in synthetic materials. For instance, during gastrulation the shape of sea urchin embryos undergoes a dramatic systematic change that has been compared to particle models. Controlled deformation morphologies arise from the evacuation of swelling solvent and unreacted monomer through a mildly cross–linked skin layer as colloids contract during seeded polymerization. Deformation is driven by the pressure gradient across the porous shell surface. Isotropic shrinkage transitions to regimes where the inhomogeneous membrane shell flattens, curvature reversal occurs forming a depression, an invagination ensues and its tip deepens until tearing connects to a hollow or solid central core region. Here, to incrementally capture the morphology development a geometric model joining a hemisphere to a torus with varied inner and outer hemi–toroid radii is defined. The consequence of packing and transformation of colloidal building blocks from hemisphere through (mushroom cap) shaped particles to spheres for the optical properties of photonic crystals will be presented. Self–organizaiton of such nonspherical particles into photonic solids under confinement conditions will also be discussed.
Self–Assembly of Dimer-Based Photonic Crystals
1 Cornell University, Ithaca, New York, USA
Anisotropic shape in biological and natural systems continues to inspire the development of structural complexity in synthetic materials. For instance, during gastrulation the shape of sea urchin embryos undergoes a dramatic systematic change that has been compared to particle models. Controlled deformation morphologies arise from the evacuation of swelling solvent and unreacted monomer through a mildly cross–linked skin layer as colloids contract during seeded polymerization. Deformation is driven by the pressure gradient across the porous shell surface. Isotropic shrinkage transitions to regimes where the inhomogeneous membrane shell flattens, curvature reversal occurs forming a depression, an invagination ensues and its tip deepens until tearing connects to a hollow or solid central core region. Here, to incrementally capture the morphology development a geometric model joining a hemisphere to a torus with varied inner and outer hemi–toroid radii is defined. The consequence of packing and transformation of colloidal building blocks from hemisphere through (mushroom cap) shaped particles to spheres for the optical properties of photonic crystals will be presented. Self–organizaiton of such nonspherical particles into photonic solids under confinement conditions will also be discussed.
Self–Assembly of Dimer-Based Photonic Crystals
American Physical Society National Meeting, Dallas, TX, Mar. 21–25, 2011. Chekesha M. Liddell Watson1.
1 Cornell University, Ithaca, New York, USA
Recent advances in colloid synthesis to prepare monodisperse shape anisotropic particles provide the opportunity to address challenges related to structural diversity in ordered colloidal solids. In particular, computational simulations and mechanical models suggest that upon system densification nonspherical dimer colloids undergo disorder–order and order–order phase transitions to unconventional solid structures including, base-centered monoclinic crystals, degenerate aperiodic crystals, plastic crystal or rotator, etc. based on free energy minimization. The particle systems have notable analogy to molecular systems, where the shape of molecules and their packing density has been shown to critically influence structural phase behavior and lead to a rich variety of structures, both natural and synthetic. The materials engineering challenges have been in attaining sufficiently monodisperse (size uniformity) colloidal building blocks, as well as the lack of understanding and control of self–assembly processes for non–spherical colloids. This talk highlights our investigations of how particle shape programs the self–organization of colloidal structures. Methods including evaporation mediated assembly and confinement provide a platform to understand the formation of complex colloidal structures from non–spherical building blocks (silica-coated iron oxide, polystyrene, hollow silica shell). Optical property simulations for unconventional 2D and 3D structures with nonspherical particle bases will also be discussed.
Self–Assembled Structures from Nonspherical C–60 Hexagonal Discs and Rods
1 Cornell University, Ithaca, New York, USA
Recent advances in colloid synthesis to prepare monodisperse shape anisotropic particles provide the opportunity to address challenges related to structural diversity in ordered colloidal solids. In particular, computational simulations and mechanical models suggest that upon system densification nonspherical dimer colloids undergo disorder–order and order–order phase transitions to unconventional solid structures including, base-centered monoclinic crystals, degenerate aperiodic crystals, plastic crystal or rotator, etc. based on free energy minimization. The particle systems have notable analogy to molecular systems, where the shape of molecules and their packing density has been shown to critically influence structural phase behavior and lead to a rich variety of structures, both natural and synthetic. The materials engineering challenges have been in attaining sufficiently monodisperse (size uniformity) colloidal building blocks, as well as the lack of understanding and control of self–assembly processes for non–spherical colloids. This talk highlights our investigations of how particle shape programs the self–organization of colloidal structures. Methods including evaporation mediated assembly and confinement provide a platform to understand the formation of complex colloidal structures from non–spherical building blocks (silica-coated iron oxide, polystyrene, hollow silica shell). Optical property simulations for unconventional 2D and 3D structures with nonspherical particle bases will also be discussed.
Self–Assembled Structures from Nonspherical C–60 Hexagonal Discs and Rods
MRS Symposium MM: Evaporative Self Assembly of Polymers, Nanoparticles, and DNA. Materials Research Society, San Francisco, CA, April 7, 2010.
Ian D. Hosein1, R. Fischer1, Chekesha M. Liddell Watson1.
1 Cornell University, Ithaca, New York, USA
Self–assembly of submicron particles into colloidal crystal structures offers a rapid, tunable and scalable process for creating spatially periodic templates for nano–fabrication, micro–lens arrays and photonic crystals. Theoretical calculations have shown that colloidal crystals from non–spherical particles may allow robust and complete photonic bandgaps to open at lower refractive index contrasts, enabling a wider range of materials to be accessible for fabrication. Reprecipitation of fullerene (C–60) in ‘good’ and poor solvent mixtures produces uniform anisotropic particles in quantities amenable to particulate thin film formation. In the present work, 2D structures from C–60 discs and rods with hexagonal cross-section were fabricated via convective and confinement assembly methods. The structures were examined using scanning electron and optical microscopy. Photonic band calculations of corresponding model packing arrangements show that stable gaps open between several optical bands, for both transverse electric (TE) and transverse magnetic (TM) polarizations of light. The gaps were optimized by adjusting the particle shape and filling fraction of high refractive index material.
Mesocrystals from Peanut– and Mushroom Cap–Shaped Colloids
1 Cornell University, Ithaca, New York, USA
Self–assembly of submicron particles into colloidal crystal structures offers a rapid, tunable and scalable process for creating spatially periodic templates for nano–fabrication, micro–lens arrays and photonic crystals. Theoretical calculations have shown that colloidal crystals from non–spherical particles may allow robust and complete photonic bandgaps to open at lower refractive index contrasts, enabling a wider range of materials to be accessible for fabrication. Reprecipitation of fullerene (C–60) in ‘good’ and poor solvent mixtures produces uniform anisotropic particles in quantities amenable to particulate thin film formation. In the present work, 2D structures from C–60 discs and rods with hexagonal cross-section were fabricated via convective and confinement assembly methods. The structures were examined using scanning electron and optical microscopy. Photonic band calculations of corresponding model packing arrangements show that stable gaps open between several optical bands, for both transverse electric (TE) and transverse magnetic (TM) polarizations of light. The gaps were optimized by adjusting the particle shape and filling fraction of high refractive index material.
Mesocrystals from Peanut– and Mushroom Cap–Shaped Colloids
MRS Symposium MM: Evaporative Self Assembly of Polymers, Nanoparticles, and DNA. Materials
Materials Research Society, San Francisco, CA, April 7, 2010.
Ian D. Hosein1, Chekesha M. Liddell Watson1.
1 Cornell University, Ithaca, New York, USA
Calculations suggest that photonic bandgap structures with nonspherical bases promote defect resilient properties and wide bandgaps at low refractive index contrasts. However, the realization of complex ordered structures in three–dimensions has remained challenging and it is not clear that phases which form spontaneously using common methods will exhibit the desired optical properties. In the present work, we explore the self–assembly of polystyrene dimer– and spherocylinder shaped colloids via controlled drying on glass and silicon substrates. 3D monoclinic colloidal crystal structures are determined from scanning electron microscopy images of sections prepared using focused ion–beam (FIB) milling. Full photonic band gaps between the eighth and ninth bands are found for a systematic range of colloidal dimer shapes explored with respect to the degree of constituent lobe fusion and radius ratio. The pseudogap between bands 2 and 3 for spherocylinder–based monoclinic crystals is also probed using normal incidence reflection spectroscopy.
Confinement–Controlled Ordering of Colloids with Simultaneous Isotropic and Anisotropic Cross–section
1 Cornell University, Ithaca, New York, USA
Calculations suggest that photonic bandgap structures with nonspherical bases promote defect resilient properties and wide bandgaps at low refractive index contrasts. However, the realization of complex ordered structures in three–dimensions has remained challenging and it is not clear that phases which form spontaneously using common methods will exhibit the desired optical properties. In the present work, we explore the self–assembly of polystyrene dimer– and spherocylinder shaped colloids via controlled drying on glass and silicon substrates. 3D monoclinic colloidal crystal structures are determined from scanning electron microscopy images of sections prepared using focused ion–beam (FIB) milling. Full photonic band gaps between the eighth and ninth bands are found for a systematic range of colloidal dimer shapes explored with respect to the degree of constituent lobe fusion and radius ratio. The pseudogap between bands 2 and 3 for spherocylinder–based monoclinic crystals is also probed using normal incidence reflection spectroscopy.
Confinement–Controlled Ordering of Colloids with Simultaneous Isotropic and Anisotropic Cross–section
Symposium OO: Hierarchical Self Assembly of Functional Materials - From Nanoscopic to Mesoscopic Length Scales. Materials Research Society, San Francisco, CA, April 6, 2010.
Erin Riley1, Chekesha M. Liddell Watson1.
1 Cornell University, Ithaca, New York, USA
The pursuit to determine the realizable structures formed by anisotropic particles is foundational to colloidal solutions for correlating structure and properties for photonics. Confinement has proven to be a powerful structure directing technique for molecular–based materials including block copolymers, where complex morphologies are induced by surface separations on the length scale of the microphase domains. A mildly expanded range of phase stability has also been demonstrated for spherical colloids under wedge–shaped confinement geometry. In addition to hexagonal (hex) and square symmetry, rhombic and buckled structures have been reported in the transition regions between one and two particle layers. Here, we investigate the promotion of greater structural diversity utilizing nonspherical colloids and confinement. Particularly, the mushroom–cap shaped particles employed in this study have features of both isotropic and anisotropic colloids. Results from fast confocal microscopy imaging of particles’ gravitational sedimentation in a wedge cell into a range of mono– and bilayer structures under 2D and quasi–2D confinement will be described. Particle tracking routines enable quantitative analysis of positional and orientational correlations in the order. The sequence of phases tracked with increasing confinement height includes those cited for spheres, as well as the more complex rotator and orientation–dependent phases observed for the class of short rod–like building blocks (i.e., major axis re–orients with respect to the substrate). Closest packing considerations provide rationale for the observed 1hex–1buckled–1rotator (oblique)–2square–2hex–2rotator (oblique) phase behavior with height. Photonic band structure simulations of the experimental particle arrangements will be presented to characterize the optical properties.
Glassy Dislocation Relaxation in Colloidal Peanut Crystals
1 Cornell University, Ithaca, New York, USA
The pursuit to determine the realizable structures formed by anisotropic particles is foundational to colloidal solutions for correlating structure and properties for photonics. Confinement has proven to be a powerful structure directing technique for molecular–based materials including block copolymers, where complex morphologies are induced by surface separations on the length scale of the microphase domains. A mildly expanded range of phase stability has also been demonstrated for spherical colloids under wedge–shaped confinement geometry. In addition to hexagonal (hex) and square symmetry, rhombic and buckled structures have been reported in the transition regions between one and two particle layers. Here, we investigate the promotion of greater structural diversity utilizing nonspherical colloids and confinement. Particularly, the mushroom–cap shaped particles employed in this study have features of both isotropic and anisotropic colloids. Results from fast confocal microscopy imaging of particles’ gravitational sedimentation in a wedge cell into a range of mono– and bilayer structures under 2D and quasi–2D confinement will be described. Particle tracking routines enable quantitative analysis of positional and orientational correlations in the order. The sequence of phases tracked with increasing confinement height includes those cited for spheres, as well as the more complex rotator and orientation–dependent phases observed for the class of short rod–like building blocks (i.e., major axis re–orients with respect to the substrate). Closest packing considerations provide rationale for the observed 1hex–1buckled–1rotator (oblique)–2square–2hex–2rotator (oblique) phase behavior with height. Photonic band structure simulations of the experimental particle arrangements will be presented to characterize the optical properties.
Glassy Dislocation Relaxation in Colloidal Peanut Crystals
American Physical Society (APS) Symposium D12, Colloidal Crystals, Suspensions and Films, 2010 APS March Meeting, Portland, OR, March 15–19, 2010.
Sharon Gerbode1, D. Ong1, U. Agarwal1, Chekesha M. Liddell Watson1.
1 Cornell University, Ithaca, New York, USA
Previous studies of dislocations in crystals of colloidal dimers have revealed unusual restrictions on dislocation glide. In the current study, we induce defect formation in such crystals using an optically manipulated spherical intruder particle dragged through an otherwise pure dimer crystal grain. We find that the relaxation response of the perturbed crystal changes as a function of the size of the grain. For small grains, the crystal relaxes via unrestricted dislocation glide, while in larger grains, other slower relaxation mechanisms are utilized. Furthermore, we have uncovered a two–stage defect relaxation process in crystals of dimers, reminiscent of relaxation in glassy systems, in which an initial fast glide response is followed by a slower relaxation process where dislocations hop between caged configurations. We find that the relaxation decay of dislocations is consistent with the combination of a fast exponential decay followed by a slow logarithmic decay characterized by a timescale 5 orders of magnitude longer than that of the exponential decay. Together these results reveal an interesting new class of materials possessing crystalline order but whose defects are characterized by glassy behavior.
Dislocation Interactions in Crystals of Colloidal Dimers
1 Cornell University, Ithaca, New York, USA
Previous studies of dislocations in crystals of colloidal dimers have revealed unusual restrictions on dislocation glide. In the current study, we induce defect formation in such crystals using an optically manipulated spherical intruder particle dragged through an otherwise pure dimer crystal grain. We find that the relaxation response of the perturbed crystal changes as a function of the size of the grain. For small grains, the crystal relaxes via unrestricted dislocation glide, while in larger grains, other slower relaxation mechanisms are utilized. Furthermore, we have uncovered a two–stage defect relaxation process in crystals of dimers, reminiscent of relaxation in glassy systems, in which an initial fast glide response is followed by a slower relaxation process where dislocations hop between caged configurations. We find that the relaxation decay of dislocations is consistent with the combination of a fast exponential decay followed by a slow logarithmic decay characterized by a timescale 5 orders of magnitude longer than that of the exponential decay. Together these results reveal an interesting new class of materials possessing crystalline order but whose defects are characterized by glassy behavior.
Dislocation Interactions in Crystals of Colloidal Dimers
83rd American Chemical Society (ACS) Colloid and Surface Science Symposium, Columbia University, NY, Jun. 14–19, 2009.
Stephanie Lee1, Shareon Gerbode1, D. Ong1, Chekesha M. Liddell Watson1.
1 Cornell University, Ithaca, New York, USA
Recent studies of densely packed colloidal dimers have demonstrated that when confined to lie in a monolayer, the particles form a ‘degenerate crystal’ (DC). In this structure, the dimer lobes order into a triangular lattice, much like close–packed spheres, while the connections between lobe pairs are randomly oriented, uniformly populating the three crystalline directions of the underlying lattice. In such crystals, geometric frustrations resulting from interlocking dimers limit the motion of crystal defects, leading to unusual dislocation interactions. In particular, obstacles formed by certain dimer orientations block dislocation glide. Defect transport beyond such obstacles has been experimentally observed to proceed via dislocation reactions, which utilize multiple dislocations to switch the direction of motion. In this talk, I will discuss the implications of such cooperative defect motion on the mechanical response to local perturbations introduced by moving impurities through the crystal.
Quasi– Two–Dimensional (2D) Ordering of Nonspherical Colloids Under Confinement
1 Cornell University, Ithaca, New York, USA
Recent studies of densely packed colloidal dimers have demonstrated that when confined to lie in a monolayer, the particles form a ‘degenerate crystal’ (DC). In this structure, the dimer lobes order into a triangular lattice, much like close–packed spheres, while the connections between lobe pairs are randomly oriented, uniformly populating the three crystalline directions of the underlying lattice. In such crystals, geometric frustrations resulting from interlocking dimers limit the motion of crystal defects, leading to unusual dislocation interactions. In particular, obstacles formed by certain dimer orientations block dislocation glide. Defect transport beyond such obstacles has been experimentally observed to proceed via dislocation reactions, which utilize multiple dislocations to switch the direction of motion. In this talk, I will discuss the implications of such cooperative defect motion on the mechanical response to local perturbations introduced by moving impurities through the crystal.
Quasi– Two–Dimensional (2D) Ordering of Nonspherical Colloids Under Confinement
Symposium BB: Material Systems and Processes for Three-Dimensional Micro- and Nanoscale Fabrication and Lithography. Materials Research Society Symposium (MRS), San Francisco, CA, Apr. 15, 2009.
Erin Riley1, Esther Fung1, Stephanie Lee1, Itai Cohen1, Chekesha M. Liddell Watson1.
1 Cornell University, Ithaca, New York, USA
Fabrication of photonic crystals has been intensely pursued because of their optical properties, such as localization of the electromagnetic field energy and negative refractive index, and the possibility of exploiting these properties in integrated optical devices. Many engineered materials have been made using self–assembled spherical constituents as templates; however, stronger light–matter interactions have been predicted for a range of more complex geometries. For example, simulations have shown that large and robust photonic band gaps may be achieved by lowering the crystal symmetry using asymmetric scattering units [1]. In addition, structures lacking traditional crystalline order have recently been discovered to exhibit band gaps [2]. Until recently, difficulty in synthesizing monodisperse colloids of controlled shapes has limited the exploration of the structures formed by anisotropic particles. In the present work, micron–sized dimers of various degrees of asymmetry were self–assembled by gravitational sedimentation in a wedge–shaped confinement cell [3]. Unconventional phases including plastic (or rotator) crystals and degenerate crystals were examined by confocal and optical image analysis. The structures were characterized using positional and orientational correlation functions as well as order parameters. Transformations as a function of cell height were compared with thermodynamic models from Monte Carlo simulations. Photonic band structure simulations based on ideal and experimental particle arrangements for the 2D and quasi–2D structures formed will be discussed. [1] Y. Xia, B. Gates and Z.–Y. Li, Self–assembly approaches to three–dimensional photonic crystals, Adv. Mater., 2001, 13, 409. [2] K. Edagawa, S. Kanoko and M. Notomi, Photonic amorphous diamond structure with a 3D photonic band gap, Phys. Rev. Lett., 2008, 100, 013901. [3] S. H. Lee, S. J. Gerbode, B. S. John, A. K. Wolfgang, F. A. Escobedo, I. Cohen and C. M. Liddell, Synthesis and assembly of nonspherical hollow silica colloids under confinement, J. Mater. Chem., 2008, 18, 4912.
Long–range attractions in antiferromagnetic peanut–shaped particles
1 Cornell University, Ithaca, New York, USA
Fabrication of photonic crystals has been intensely pursued because of their optical properties, such as localization of the electromagnetic field energy and negative refractive index, and the possibility of exploiting these properties in integrated optical devices. Many engineered materials have been made using self–assembled spherical constituents as templates; however, stronger light–matter interactions have been predicted for a range of more complex geometries. For example, simulations have shown that large and robust photonic band gaps may be achieved by lowering the crystal symmetry using asymmetric scattering units [1]. In addition, structures lacking traditional crystalline order have recently been discovered to exhibit band gaps [2]. Until recently, difficulty in synthesizing monodisperse colloids of controlled shapes has limited the exploration of the structures formed by anisotropic particles. In the present work, micron–sized dimers of various degrees of asymmetry were self–assembled by gravitational sedimentation in a wedge–shaped confinement cell [3]. Unconventional phases including plastic (or rotator) crystals and degenerate crystals were examined by confocal and optical image analysis. The structures were characterized using positional and orientational correlation functions as well as order parameters. Transformations as a function of cell height were compared with thermodynamic models from Monte Carlo simulations. Photonic band structure simulations based on ideal and experimental particle arrangements for the 2D and quasi–2D structures formed will be discussed. [1] Y. Xia, B. Gates and Z.–Y. Li, Self–assembly approaches to three–dimensional photonic crystals, Adv. Mater., 2001, 13, 409. [2] K. Edagawa, S. Kanoko and M. Notomi, Photonic amorphous diamond structure with a 3D photonic band gap, Phys. Rev. Lett., 2008, 100, 013901. [3] S. H. Lee, S. J. Gerbode, B. S. John, A. K. Wolfgang, F. A. Escobedo, I. Cohen and C. M. Liddell, Synthesis and assembly of nonspherical hollow silica colloids under confinement, J. Mater. Chem., 2008, 18, 4912.
Long–range attractions in antiferromagnetic peanut–shaped particles
American Chemical Society 83rd American Chemical Society (ACS) Colloid and Surface Science Symposium, Raleigh, NC, June 15–18, 2008.
Stephanie Lee1, Chekesha M. Liddell Watson1.
1 Cornell University, Ithaca, New York, USA
Anisotropic colloids of exotic size, shape, and properties have recently received growing interest as new methods for producing such particles have emerged. Simulations predict that these colloids can exhibit a structural diversity quite different than that of their spherical counterparts. Despite this evidence however, few examples of such complex assemblies have yet to be shown experimentally. Here, we describe the synthesis and ordering of peanut–shaped α–Fe2O3/SiO2 core–shell particles with a long–range attractive potential in 2D. The hematite core is prepared by aging a condensed ferric hydroxide gel under hydrothermal conditions, while a rhodamine–functionalized silica shell is grown onto the particle surface via sol-gel chemistry. An unusual canted antiferromagnetism in the hematite cores gives rise to a magnetic dipole moment oriented perpendicular to each particle’s long axis. Characterization of the peanut–shaped colloids and their monolayer structure is conducted via confocal microscopy, electron microscopy, SQUID, and x–ray powder diffraction.
Three–Dimensional Crystals from Polystyrene Asymmetric Dimer–Shaped Colloids
1 Cornell University, Ithaca, New York, USA
Anisotropic colloids of exotic size, shape, and properties have recently received growing interest as new methods for producing such particles have emerged. Simulations predict that these colloids can exhibit a structural diversity quite different than that of their spherical counterparts. Despite this evidence however, few examples of such complex assemblies have yet to be shown experimentally. Here, we describe the synthesis and ordering of peanut–shaped α–Fe2O3/SiO2 core–shell particles with a long–range attractive potential in 2D. The hematite core is prepared by aging a condensed ferric hydroxide gel under hydrothermal conditions, while a rhodamine–functionalized silica shell is grown onto the particle surface via sol-gel chemistry. An unusual canted antiferromagnetism in the hematite cores gives rise to a magnetic dipole moment oriented perpendicular to each particle’s long axis. Characterization of the peanut–shaped colloids and their monolayer structure is conducted via confocal microscopy, electron microscopy, SQUID, and x–ray powder diffraction.
Three–Dimensional Crystals from Polystyrene Asymmetric Dimer–Shaped Colloids
American Chemical Society 82nd ACS Colloid & Surface Science Symposium, Raleigh, NC, June 15–18, 2008.
Ian D. Hosein1, Stephanie Lee1, Chekesha M. Liddell Watson1.
1 Cornell University, Ithaca, New York, USA
Self–assembly of submicron nonspherical particles into colloidal crystal structures offers a rapid process for creating complex spatially periodic templates for nano–fabrication, micro–lens arrays and photonic crystals. In particular, theoretical calculations have shown that photonic bandgap structures with non–spherical shaped bases promote defect resilient properties and wide bandgaps at lower refractive index contrasts. This makes a wider range of material chemistries appropriate for fabrication as compared to the simple inverted face–centered cubic structures from close–packed spheres. The realization of such complex structures in three–dimensions has remained challenging. In the present work, polystyrene dimer–shaped colloids with systematically tuned particle morphology– degree of fusion (DOF) between dimer lobes and degree of asymmetry between lobe radii– were assembled into 3D colloidal crystals via controlled drying on silicon substrates. The crystal structures were determined from scanning electron microscopy images on sections prepared using focused ion–beam milling (FIB). Optical laser diffraction was performed to enable structure–optical property correlations. Highly fused particles, i.e., DOF below 0.37, produced rotator structures (plastic crystals characterized by positional order and orientational randomness), while particles with a low degree of fusion between lobes resulted in true crystalline structures. Both observations were consistent with theoretical calculations and simulations in the literature. The diffraction properties of the structures were modeled using the optical Bragg equation. Additionally, inverted germanium (physical vapor deposition, PVD) and alumina (atomic layer deposition, ALD) structures from the complex templates were demonstrated.
Degenerate Crystal Phases in Nonspherical Silica Colloids
1 Cornell University, Ithaca, New York, USA
Self–assembly of submicron nonspherical particles into colloidal crystal structures offers a rapid process for creating complex spatially periodic templates for nano–fabrication, micro–lens arrays and photonic crystals. In particular, theoretical calculations have shown that photonic bandgap structures with non–spherical shaped bases promote defect resilient properties and wide bandgaps at lower refractive index contrasts. This makes a wider range of material chemistries appropriate for fabrication as compared to the simple inverted face–centered cubic structures from close–packed spheres. The realization of such complex structures in three–dimensions has remained challenging. In the present work, polystyrene dimer–shaped colloids with systematically tuned particle morphology– degree of fusion (DOF) between dimer lobes and degree of asymmetry between lobe radii– were assembled into 3D colloidal crystals via controlled drying on silicon substrates. The crystal structures were determined from scanning electron microscopy images on sections prepared using focused ion–beam milling (FIB). Optical laser diffraction was performed to enable structure–optical property correlations. Highly fused particles, i.e., DOF below 0.37, produced rotator structures (plastic crystals characterized by positional order and orientational randomness), while particles with a low degree of fusion between lobes resulted in true crystalline structures. Both observations were consistent with theoretical calculations and simulations in the literature. The diffraction properties of the structures were modeled using the optical Bragg equation. Additionally, inverted germanium (physical vapor deposition, PVD) and alumina (atomic layer deposition, ALD) structures from the complex templates were demonstrated.
Degenerate Crystal Phases in Nonspherical Silica Colloids
Symposium BB: Material Systems and Processes for Three–Dimensional Micro– and Nanoscale Fabrication and Lithography. Materials Research Society Symposium (MRS), San Francisco, CA, March 24–28, 2008.
Stephanie Lee1, Sharon Gerbode1, Itai Cohen1, Chekesha M. Liddell Watson1.
1 Cornell University, Ithaca, New York, USA
Colloids are often used as models to elucidate fundamental principles in nucleation processes, phase transitions, and crystal growth behavior. Practically speaking, however, colloidal self–assembly is also an attractive means of forming the complex structures required in various photonic, optoelectronic, memory, and sensing applications. While progress has been made in these areas with spherical colloids, similar efforts that employ anisotropic shapes remain scarce primarily because nonspherical particles of sufficient monodispersity are more difficult to produce. In the present work, we describe a two–dimensional colloidal system that organizes into a phase (known as a (degenerate crystal)) predicted only by computer simulations to date. Our building block consists of micron–sized, nonspherical silica dumbbells synthesized via a sol–gel templating procedure. Such particles crudely approximate two fused spheres and may be considered as analogues to diatomic molecules on a colloidal length scale. Using confocal microscopy, the degenerate crystal (DC) is characterized for structure using both its center–of–mass (COM) as well as individual lobe positions. The results show good agreement with simulations in that (1) COMs were found to decorate aperiodic sites on a Kagome lattice while (2) individual lobes occupied sites on a triangular lattice. Analysis of the DC phase by correlation functions, orientation mapping, and Voronoi constructions will be presented.
Magnetically–Ordered Monolayers of ‘Dumbbell’–shaped Colloids
1 Cornell University, Ithaca, New York, USA
Colloids are often used as models to elucidate fundamental principles in nucleation processes, phase transitions, and crystal growth behavior. Practically speaking, however, colloidal self–assembly is also an attractive means of forming the complex structures required in various photonic, optoelectronic, memory, and sensing applications. While progress has been made in these areas with spherical colloids, similar efforts that employ anisotropic shapes remain scarce primarily because nonspherical particles of sufficient monodispersity are more difficult to produce. In the present work, we describe a two–dimensional colloidal system that organizes into a phase (known as a (degenerate crystal)) predicted only by computer simulations to date. Our building block consists of micron–sized, nonspherical silica dumbbells synthesized via a sol–gel templating procedure. Such particles crudely approximate two fused spheres and may be considered as analogues to diatomic molecules on a colloidal length scale. Using confocal microscopy, the degenerate crystal (DC) is characterized for structure using both its center–of–mass (COM) as well as individual lobe positions. The results show good agreement with simulations in that (1) COMs were found to decorate aperiodic sites on a Kagome lattice while (2) individual lobes occupied sites on a triangular lattice. Analysis of the DC phase by correlation functions, orientation mapping, and Voronoi constructions will be presented.
Magnetically–Ordered Monolayers of ‘Dumbbell’–shaped Colloids
Gordon Research Conference (GRC), Thin Films & Crystal Growth Mechanisms, Mount Holyoke College, South Hadley, MA, June 24–29, 2007.
Stephanie Lee1, Itai Cohen1, Chekesha M. Liddell Watson1.
1 Cornell University, Ithaca, New York, USA
Anisotropic colloids of controlled size, shape and properties have received growing interest recently as new methods for producing such particles have emerged. In theory, these colloids may form phases and exhibit behavior quite different than those associated with their spherical counterparts. In the present work, monodispersed hematite (α-Fe2O3) particles coated with fluorescent silica were synthesized with a ‘dumbbell’ shape. The core hematite exhibits canted antiferromagnetism at 293K, and as a consequence forms an ordered monolayer when sedimented. This monolayer is orientationally ordered, fully dense and close–packed with the lobes of the dumbbells located on triangular lattice sites. Using confocal microscopy, the structure evolution was observed to occur through a chaining intermediate state. The interparticle interactions and magnetic field effects will be discussed.
Crystalline Monolayers from Convectively Self–Assembled Non-Spherical Colloids
1 Cornell University, Ithaca, New York, USA
Anisotropic colloids of controlled size, shape and properties have received growing interest recently as new methods for producing such particles have emerged. In theory, these colloids may form phases and exhibit behavior quite different than those associated with their spherical counterparts. In the present work, monodispersed hematite (α-Fe2O3) particles coated with fluorescent silica were synthesized with a ‘dumbbell’ shape. The core hematite exhibits canted antiferromagnetism at 293K, and as a consequence forms an ordered monolayer when sedimented. This monolayer is orientationally ordered, fully dense and close–packed with the lobes of the dumbbells located on triangular lattice sites. Using confocal microscopy, the structure evolution was observed to occur through a chaining intermediate state. The interparticle interactions and magnetic field effects will be discussed.
Crystalline Monolayers from Convectively Self–Assembled Non-Spherical Colloids
American Chemical Society 81st ACS Colloid and Surface Science Symposium, Colloidal Assembly I: Particles Session, Newark, DE, Jun. 24–27, 2007.
Ian D. Hosein1, Chekesha M. Liddell Watson1.
1 Cornell University, Ithaca, New York, USA
Self–assembly of submicron particles into colloidal crystal structures offers a rapid, tunable and scalable process for creating spatially periodic templates for nano–fabrication, micro–lens arrays and photonic crystals. Theoretical calculations have shown that colloidal crystals from non–spherical particles could allow robust and complete photonic bandgaps to open at lower refractive index contrasts, allowing a wider range of materials to be accessible for fabrication. In the present work, 2D structures with a high degree of positional and orientational order from mushroom–cap, pear and peanut shaped colloids were fabricated via convective assembly. Structure–optical property correlations were made using SEM and optical diffraction spectroscopy. The ordered assembly process will be explained with respect to particle concentration, solvent surface tension at the drying front, and geometric packing efficiency. The structures were modeled using the diffraction grating equation. Structural phases were examined with thermodynamic models from Monte Carlo simulations. Transitions between isotropic and ordered phases were followed using the simulated osmotic pressure versus density curve and by visualization of the equilibrated system from the simulations.
Non–Spherical Based Colloidal Crystals from Asymmetric–Dimer Shaped Polymer Mesoparticles
1 Cornell University, Ithaca, New York, USA
Self–assembly of submicron particles into colloidal crystal structures offers a rapid, tunable and scalable process for creating spatially periodic templates for nano–fabrication, micro–lens arrays and photonic crystals. Theoretical calculations have shown that colloidal crystals from non–spherical particles could allow robust and complete photonic bandgaps to open at lower refractive index contrasts, allowing a wider range of materials to be accessible for fabrication. In the present work, 2D structures with a high degree of positional and orientational order from mushroom–cap, pear and peanut shaped colloids were fabricated via convective assembly. Structure–optical property correlations were made using SEM and optical diffraction spectroscopy. The ordered assembly process will be explained with respect to particle concentration, solvent surface tension at the drying front, and geometric packing efficiency. The structures were modeled using the diffraction grating equation. Structural phases were examined with thermodynamic models from Monte Carlo simulations. Transitions between isotropic and ordered phases were followed using the simulated osmotic pressure versus density curve and by visualization of the equilibrated system from the simulations.
Non–Spherical Based Colloidal Crystals from Asymmetric–Dimer Shaped Polymer Mesoparticles
Symposium AA: Three–Dimensional Nano– and Microphotonics. Material Research Society Symposium (MRS), San Francisco, CA, April 9–13, 2007.
Ian Hosein1, Chekesha M. Liddell Watson1.
1 Cornell University, Ithaca, New York, USA
Self-assembly of submicron particles into colloidal crystal structures offers a rapid, tunable and scalable process for creating spatially periodic templates for fabricating three–dimensional photonic crystals. Theoretical calculations have shown that colloidal crystals from non–spherical particles could allow robust and complete photonic bandgaps to open up at lower refractive index contrasts, making a diversity of materials accessible for producing photonic bandgaps. In the present work, 2D structures with a high degree of positional and orientational order from asymmetric–dimer shaped colloids were fabricated via convective assembly. Structure–optical property correlations were made using SEM and optical diffraction spectroscopy. The ordered assembly process will be explained with respect to particle concentration, solvent surface tension at the drying front, and geometric packing efficiency. The 2D structures were modeled using the diffraction grating equation and a volume average refractive index, and compared to spherical based structures to investigate improvements in light–matter interaction. Structural phases were examined with thermodynamic models from Monte Carlo simulations. Transitions between isotropic and ordered phases were followed using the simulated osmotic pressure versus density curve and by visualization of the equilibrated system from the simulations. Photonic band calculations were also performed for several asymmetric dimer crystals, with 2D and 3D complete photonic bandgaps predicted at refractive index values lower than those for sphere–based inverted opal crystals.
Two–Dimensional Phase Behavior of Colloidal Peanuts
1 Cornell University, Ithaca, New York, USA
Self-assembly of submicron particles into colloidal crystal structures offers a rapid, tunable and scalable process for creating spatially periodic templates for fabricating three–dimensional photonic crystals. Theoretical calculations have shown that colloidal crystals from non–spherical particles could allow robust and complete photonic bandgaps to open up at lower refractive index contrasts, making a diversity of materials accessible for producing photonic bandgaps. In the present work, 2D structures with a high degree of positional and orientational order from asymmetric–dimer shaped colloids were fabricated via convective assembly. Structure–optical property correlations were made using SEM and optical diffraction spectroscopy. The ordered assembly process will be explained with respect to particle concentration, solvent surface tension at the drying front, and geometric packing efficiency. The 2D structures were modeled using the diffraction grating equation and a volume average refractive index, and compared to spherical based structures to investigate improvements in light–matter interaction. Structural phases were examined with thermodynamic models from Monte Carlo simulations. Transitions between isotropic and ordered phases were followed using the simulated osmotic pressure versus density curve and by visualization of the equilibrated system from the simulations. Photonic band calculations were also performed for several asymmetric dimer crystals, with 2D and 3D complete photonic bandgaps predicted at refractive index values lower than those for sphere–based inverted opal crystals.
Two–Dimensional Phase Behavior of Colloidal Peanuts
2007 American Physical Society (APS) March Meeting, Denver, CO, March 5–9, 2007.
Stephanie Lee1, Sharon Gerbode1, A. Wolfgang1, B. S. John1, Itai Cohen1, Fernando Escobedo1, Chekesha M. Liddell Watson1.
1 Cornell University, Ithaca, New York, USA
While the phase behavior of spherical colloidal suspensions has been well studied, the ordering of non-spherical colloidal particles remains a largely unex plored yet important problem. In this talk we will describe ongoing studies of one very simple extension of the spherical particle: the colloidal peanut. These peanuts have an aspect ratio that makes them comparable to dimer particles. Confining the colloidal peanuts to two dimensions, we find that the suspension can undergo a phase transition from a liquid to an ordered phase in which each individual peanut lobe resides on a triangular lattice site. The lobe packing is very similar to the hexagonally close packed crystalline arrangement formed by spheres in 2D. Unlike their spherical counterparts, however, the colloidal peanuts are not isotropic, and in particular, each peanut has a specific orientation, or director. In this talk we will describe the correlations between defects in the underlying triangular lattice and the local director field. We will also report on our measurements of long–range director correlations, and if time permits, we will describe ongoing work relating to phases formed by peanut particles with different aspect ratios.
Mushroom–Cap and Snowman–Shaped Colloid–Based Mesocrystals for Light Controls
1 Cornell University, Ithaca, New York, USA
While the phase behavior of spherical colloidal suspensions has been well studied, the ordering of non-spherical colloidal particles remains a largely unex plored yet important problem. In this talk we will describe ongoing studies of one very simple extension of the spherical particle: the colloidal peanut. These peanuts have an aspect ratio that makes them comparable to dimer particles. Confining the colloidal peanuts to two dimensions, we find that the suspension can undergo a phase transition from a liquid to an ordered phase in which each individual peanut lobe resides on a triangular lattice site. The lobe packing is very similar to the hexagonally close packed crystalline arrangement formed by spheres in 2D. Unlike their spherical counterparts, however, the colloidal peanuts are not isotropic, and in particular, each peanut has a specific orientation, or director. In this talk we will describe the correlations between defects in the underlying triangular lattice and the local director field. We will also report on our measurements of long–range director correlations, and if time permits, we will describe ongoing work relating to phases formed by peanut particles with different aspect ratios.
Mushroom–Cap and Snowman–Shaped Colloid–Based Mesocrystals for Light Controls
80th American Chemical Society (ACS) Colloid and Surface Science Symposium, Colloidal Nanoscience and Technology Session, Boulder, CO, June 18–21, 2006.
Stephanie Lee1, Ian Hosein1, M. Ghebrebrhan1, B. S. John1, J. D. Joannopoulos1, Fernando Escobedo1, Chekesha M. Liddell Watson1.
1 Cornell University, Ithaca, New York, USA
Structural colors, not originating from pigments but rather from the interaction of light with features having the same size as the wavelength, can be observed in many living systems including butterfly wings, peacock feathers, and the brilliant blue facial skin of certain primates. Examination of the microscopic structure of these tissues reveals attributes of a photonic crystal system– a color specific mirror formed by fine–scale periodic arrays composed of nonspherical building blocks. Analogous engineered materials with periodicity in the micron and submicron size regime have been difficult to achieve. In this work, the synthesis and self–assembly of nonspherical mushroom–cap and snowman–shaped colloids into 2D and 3D structures is reported. Nonspherical particles were synthesized by seeded emulsion polymerization utilizing the dynamic swelling method and were assembled under the influence of lateral capillary forces and convective flow of suspension medium. Assemblies were deposited layer–by–layer by controlling particle concentration. 3D structure visualization as well as band structure and absorption/reflection spectra modeling were utilized to characterize light matter interactions and to correlate the effects with structure. Phases were examined using SEM image analysis and were compared with thermodynamic models from Monte Carlo simulations. Transitions between isotropic and ordered phases were followed using the simulated osmotic pressure versus density curve and by visualization of the equilibrated system from the simulations. Photonic band calculations were performed for several asymmetric dimer crystals and complete photonic band gaps were predicted at refractive index values lower than those for sphere–based inverted opal crystals.
Synthesis and Assembly of Nonspherical Iron Oxide–Silica Colloidal Building Blocks
1 Cornell University, Ithaca, New York, USA
Structural colors, not originating from pigments but rather from the interaction of light with features having the same size as the wavelength, can be observed in many living systems including butterfly wings, peacock feathers, and the brilliant blue facial skin of certain primates. Examination of the microscopic structure of these tissues reveals attributes of a photonic crystal system– a color specific mirror formed by fine–scale periodic arrays composed of nonspherical building blocks. Analogous engineered materials with periodicity in the micron and submicron size regime have been difficult to achieve. In this work, the synthesis and self–assembly of nonspherical mushroom–cap and snowman–shaped colloids into 2D and 3D structures is reported. Nonspherical particles were synthesized by seeded emulsion polymerization utilizing the dynamic swelling method and were assembled under the influence of lateral capillary forces and convective flow of suspension medium. Assemblies were deposited layer–by–layer by controlling particle concentration. 3D structure visualization as well as band structure and absorption/reflection spectra modeling were utilized to characterize light matter interactions and to correlate the effects with structure. Phases were examined using SEM image analysis and were compared with thermodynamic models from Monte Carlo simulations. Transitions between isotropic and ordered phases were followed using the simulated osmotic pressure versus density curve and by visualization of the equilibrated system from the simulations. Photonic band calculations were performed for several asymmetric dimer crystals and complete photonic band gaps were predicted at refractive index values lower than those for sphere–based inverted opal crystals.
Synthesis and Assembly of Nonspherical Iron Oxide–Silica Colloidal Building Blocks
Symposium W: Colloidal Materials - Synthesis, Structure, and Applications. Material Research Society Symposium (MRS), San Francisco, CA April 16–20, 2006.
Stephanie Lee1, Ian Hosein1, M. R. Buckley1, Itai Cohen1, Chekesha M. Liddell Watson1.
1 Cornell University, Ithaca, New York, USA
Anisotropic colloidal building blocks of controlled size and shape are of fundamental importance since they have been predicted not only to form new phases and exhibit novel colloidal behavior, but also to enable advances in emerging technology fields. For example, calculations have shown that nonspherical colloids in a face–centered cubic packing arrangement produce complete photonic bandgaps by lifting symmetry induced degeneracies in the band structure of photonic crystals. The resulting bandgaps are more tolerant to disorder arising from self-assembly processes than those associated with crystals composed of spherical building blocks. While research on spherical colloids with narrow size distribution has matured, methods resulting in monodispersed nonspherical particles appropriate in size for photonic crystal applications are relatively scarce and assembly is just beginning to be explored. In the present work, monodispersed hematite Fe2O3 particles of various morphology (peanuts, rods, and ellipsoids) are synthesized from condensed ferric hydroxide gel and coated with a shell of fluorescent silica. A sonochemical approach was employed to control the silica shell thickness. The core was subsequently converted to magnetite Fe3O4 by heat treatment in a reducing atmosphere. Ordered structures were obtained using magnetic field–directed assembly parallel and perpendicular to the substrate. Hollow, fluorescent silica shells were formed by selectively etching the core–shell particles with concentrated hydrochloric acid. Using confocal microscopy, the shear–inducing ordering of the hollow anisotropic shells in confined colloidal solutions was also investigated. Colloidal phases were determined as a function of major parameters (i.e., shear rate and volume fraction) for each particle morphology system. The potential of these systems as photonic crystals with active, nonspherical bases will be discussed.
Tunable Stopgap in Photonic Crystals Based on Homogeneous and Core–shell ZnS Colloids
1 Cornell University, Ithaca, New York, USA
Anisotropic colloidal building blocks of controlled size and shape are of fundamental importance since they have been predicted not only to form new phases and exhibit novel colloidal behavior, but also to enable advances in emerging technology fields. For example, calculations have shown that nonspherical colloids in a face–centered cubic packing arrangement produce complete photonic bandgaps by lifting symmetry induced degeneracies in the band structure of photonic crystals. The resulting bandgaps are more tolerant to disorder arising from self-assembly processes than those associated with crystals composed of spherical building blocks. While research on spherical colloids with narrow size distribution has matured, methods resulting in monodispersed nonspherical particles appropriate in size for photonic crystal applications are relatively scarce and assembly is just beginning to be explored. In the present work, monodispersed hematite Fe2O3 particles of various morphology (peanuts, rods, and ellipsoids) are synthesized from condensed ferric hydroxide gel and coated with a shell of fluorescent silica. A sonochemical approach was employed to control the silica shell thickness. The core was subsequently converted to magnetite Fe3O4 by heat treatment in a reducing atmosphere. Ordered structures were obtained using magnetic field–directed assembly parallel and perpendicular to the substrate. Hollow, fluorescent silica shells were formed by selectively etching the core–shell particles with concentrated hydrochloric acid. Using confocal microscopy, the shear–inducing ordering of the hollow anisotropic shells in confined colloidal solutions was also investigated. Colloidal phases were determined as a function of major parameters (i.e., shear rate and volume fraction) for each particle morphology system. The potential of these systems as photonic crystals with active, nonspherical bases will be discussed.
Tunable Stopgap in Photonic Crystals Based on Homogeneous and Core–shell ZnS Colloids
Symposium W: Colloidal Materials – Synthesis, Structure, and Applications. Material Research Society Symposium (MRS), San Francisco, CA, April 16–20, 2006.
Ian Hosein1, Chekesha M. Liddell Watson1.
1 Cornell University, Ithaca, New York, USA
ZnS–based colloids are desirable building blocks for photonic crystals due to their high refractive index, low absorption in the visible wavelength regime, and capability of being synthesized as monodispersed particles over a wide range of sizes. However, highly ordered colloidal crystal films of solid ZnS, polystyrene–ZnS core–shell, and hollow ZnS colloids have been difficult to obtain, hindering the investigation of the unique optical properties in these systems. The challenges associated with the assembly process include 1) the low surface charge density of ZnS, which renders the colloids unstable and induces flocculation and sedimentation of colloidal aggregates and 2) the high density of ZnS, compared to the conventional PS and silica colloids, which promotes rapid settling from solution. In the present work, highly ordered ZnS-based colloidal crystals from homogeneous, core–shell, and hollow building blocks (200nm–500nm) were prepared via electrosteric colloid stabilization combined with a convective assembly technique. The polyelectrolyte stabilized colloids assembled into FCC arrays with the (111) face perpendicular to the substrate. Structure–optical property correlations were made using SEM, TEM and UV–Vis–NIR spectroscopy. Controlled film growth from 1 to 10 layers, with film thickness of several micrometers was achieved. Optical spectra showed (111) stopgaps along with pronounced higher order peaks. The spectral position of the photonic stopgap was predicted using a volume average refractive index of the particle and the surrounding air matrix. Tunable optical properties were obtained for core–shell ZnS–based photonic crystals by varying the core size and shell thickness.
On–Chip Planar Solenoid Inductors Using Magnetite (Fe3O4) Nanorod Cores For High Frequency Applications
1 Cornell University, Ithaca, New York, USA
ZnS–based colloids are desirable building blocks for photonic crystals due to their high refractive index, low absorption in the visible wavelength regime, and capability of being synthesized as monodispersed particles over a wide range of sizes. However, highly ordered colloidal crystal films of solid ZnS, polystyrene–ZnS core–shell, and hollow ZnS colloids have been difficult to obtain, hindering the investigation of the unique optical properties in these systems. The challenges associated with the assembly process include 1) the low surface charge density of ZnS, which renders the colloids unstable and induces flocculation and sedimentation of colloidal aggregates and 2) the high density of ZnS, compared to the conventional PS and silica colloids, which promotes rapid settling from solution. In the present work, highly ordered ZnS-based colloidal crystals from homogeneous, core–shell, and hollow building blocks (200nm–500nm) were prepared via electrosteric colloid stabilization combined with a convective assembly technique. The polyelectrolyte stabilized colloids assembled into FCC arrays with the (111) face perpendicular to the substrate. Structure–optical property correlations were made using SEM, TEM and UV–Vis–NIR spectroscopy. Controlled film growth from 1 to 10 layers, with film thickness of several micrometers was achieved. Optical spectra showed (111) stopgaps along with pronounced higher order peaks. The spectral position of the photonic stopgap was predicted using a volume average refractive index of the particle and the surrounding air matrix. Tunable optical properties were obtained for core–shell ZnS–based photonic crystals by varying the core size and shell thickness.
On–Chip Planar Solenoid Inductors Using Magnetite (Fe3O4) Nanorod Cores For High Frequency Applications
Symposium II: Fabrication and Characterization Methods for Novel Magnetic Nanostructures. Material Research Society Symposium (MRS), Boston, MA, November 28 – December 2, 2005.
Ian Hosein1, J. Kim1, W. Ni, Y. Song1, E. C. Kan, Chekesha M. Liddell Watson1.
1 Cornell University, Ithaca, New York, USA
The on–chip passive inductor is a key microwave integrated circuit element. Despite of numerous efforts to incorporate ferromagnetic (FM) materials to improve the performance and area efficiency, inductors with FM materials have relatively poor performance at high frequency due to the Eddy current loss (ECL) and ferromagnetic resonance (FMR) in the FM material. To decrease these losses, patterning the FM film into small segments has been common [1], but lithography limit and fabrication complexity hinder its wide applicability. Considering ECL, resistive loss, and easy/hard axis orientation in FMR, we adopt magnetite (Fe3O4) nanorods for magnetic cores of on–chip inductors. The nanorods have a mono–dispersed size of 0.5 μm diameter and 2 μm length with high resistivity [2]. The silica coating on the outer nanorod surface provides electrical isolation that further reduces ECL at high frequencies. Various core patterns, including pie, vertical bars, multi–ring, and cylinders are additionally defined for the solenoid cores. External magnetic fields are applied during nanorod deposition to form different easy/hard axis orientations to investigate the range for FMR, which are also independently characterized by a vibrating sample magnetometer. We use analytical calculations and electromagnetic simulation [3] in the design phase to estimate the inductor performance The fabrication starts from a normal Si substratewith both top and bottom 500nm–thick nitride layers for final membrane structures. A PECVD oxide of 5mm is deposited, followed by the electroplating of 30nm Cr and 1mm Au layers for the signal lines. Magnetite nanorods are deposited from its water mixture. The drying process is accompanied with applied magnetic field to align nanorods to a specific direction. A 100nm SiO2 layer is evaporated to fix the nanorods on the PECVD oxide. The core patterns are then completed by a lift–off process. With the S–parameter measurements and the open–short–through de–embedding procedure [4] between 400MHz and 40GHz, the quality factor (Q–factor), inductance and resistance as a function of frequency are obtained. We have found improvement in both inductance and the Q–factor up to 15GHz with magnetite nanorod cores. In comparison, typical integrated solenoid inductors using FM thin films cannot achieve performance enhancement for frequencies over 3GHz. References: [1] Y. Zhuang,M.Vroubel,B. Rejaei, and J.N. Burghartz, IEDM02,475 (2002) [2] T. Sugimoto, M. M. Khan, A. Muramatsu, Colloids Surf. 1993, A70, 167. [3] HFSS Electromagnetic Simulation Tools, Ansoft, Release, 9.0, 2003. [4] C. Chen, and M. J. Deen, Trans, on MTT, vol. 49, No.5, May 2001, pp.1004–1005. Jinsook Kim, Cornell University, School of Electrical and Computer Engineering, 323 Phillips Hall, Ithaca, NY, 14853, USA, Tel: 607–254–8842, FAX: 607–254–3508.
Magnetic Property Characterization of Magnetite (Fe3O4) Nanorod Cores For Integrated Solenoid RF Inductors
1 Cornell University, Ithaca, New York, USA
The on–chip passive inductor is a key microwave integrated circuit element. Despite of numerous efforts to incorporate ferromagnetic (FM) materials to improve the performance and area efficiency, inductors with FM materials have relatively poor performance at high frequency due to the Eddy current loss (ECL) and ferromagnetic resonance (FMR) in the FM material. To decrease these losses, patterning the FM film into small segments has been common [1], but lithography limit and fabrication complexity hinder its wide applicability. Considering ECL, resistive loss, and easy/hard axis orientation in FMR, we adopt magnetite (Fe3O4) nanorods for magnetic cores of on–chip inductors. The nanorods have a mono–dispersed size of 0.5 μm diameter and 2 μm length with high resistivity [2]. The silica coating on the outer nanorod surface provides electrical isolation that further reduces ECL at high frequencies. Various core patterns, including pie, vertical bars, multi–ring, and cylinders are additionally defined for the solenoid cores. External magnetic fields are applied during nanorod deposition to form different easy/hard axis orientations to investigate the range for FMR, which are also independently characterized by a vibrating sample magnetometer. We use analytical calculations and electromagnetic simulation [3] in the design phase to estimate the inductor performance The fabrication starts from a normal Si substratewith both top and bottom 500nm–thick nitride layers for final membrane structures. A PECVD oxide of 5mm is deposited, followed by the electroplating of 30nm Cr and 1mm Au layers for the signal lines. Magnetite nanorods are deposited from its water mixture. The drying process is accompanied with applied magnetic field to align nanorods to a specific direction. A 100nm SiO2 layer is evaporated to fix the nanorods on the PECVD oxide. The core patterns are then completed by a lift–off process. With the S–parameter measurements and the open–short–through de–embedding procedure [4] between 400MHz and 40GHz, the quality factor (Q–factor), inductance and resistance as a function of frequency are obtained. We have found improvement in both inductance and the Q–factor up to 15GHz with magnetite nanorod cores. In comparison, typical integrated solenoid inductors using FM thin films cannot achieve performance enhancement for frequencies over 3GHz. References: [1] Y. Zhuang,M.Vroubel,B. Rejaei, and J.N. Burghartz, IEDM02,475 (2002) [2] T. Sugimoto, M. M. Khan, A. Muramatsu, Colloids Surf. 1993, A70, 167. [3] HFSS Electromagnetic Simulation Tools, Ansoft, Release, 9.0, 2003. [4] C. Chen, and M. J. Deen, Trans, on MTT, vol. 49, No.5, May 2001, pp.1004–1005. Jinsook Kim, Cornell University, School of Electrical and Computer Engineering, 323 Phillips Hall, Ithaca, NY, 14853, USA, Tel: 607–254–8842, FAX: 607–254–3508.
Magnetic Property Characterization of Magnetite (Fe3O4) Nanorod Cores For Integrated Solenoid RF Inductors
50th MMM (Magnetism and Magnetic Materials) Conference, San Jose, CA, October 30 – November 3, 2005.
Y. Song1, Ian Hosein1, W. Ni1, J. Kim1, E. C. Kan1, Chekesha M. Liddell Watson1.
1 Cornell University, Ithaca, New York, USA
The on–chip magnetic solenoid inductors with Fe3O4 magnetite nanorod (MN) cores are fabricated and characterized up to 40 GHz. By vibrating–sample magnetometer measurements, the magnetic property of MN as a magnetic core for a solenoid inductor is investigated. In addition, high–frequency characterization with scattering parameter measurements is performed to estimate the high–frequency performance of the solenoid inductors with the MN cores.
Preparation and Properties of ‘Raspberry’–type Colloidal Building Blocks of ZnS and Fluorescent Core–Shell Silica Nanoparticles
1 Cornell University, Ithaca, New York, USA
The on–chip magnetic solenoid inductors with Fe3O4 magnetite nanorod (MN) cores are fabricated and characterized up to 40 GHz. By vibrating–sample magnetometer measurements, the magnetic property of MN as a magnetic core for a solenoid inductor is investigated. In addition, high–frequency characterization with scattering parameter measurements is performed to estimate the high–frequency performance of the solenoid inductors with the MN cores.
Preparation and Properties of ‘Raspberry’–type Colloidal Building Blocks of ZnS and Fluorescent Core–Shell Silica Nanoparticles
79th American Chemical Society (ACS) Colloid and Surface Science Symposium, Clarkson University, Potsdam, NY, June 12–15, 2005.
Stephanie Lee1, Ian Hosein1, V.L. Anderson1, V. Crockett1, W. W. Webb1, Ulrich Wiesner1, Chekesha M. Liddell Watson1.
1 Cornell University, Ithaca, New York, USA
A synthetic route to incorporate high–brightness core–shell silica nanoparticles (CU Dots 20–30nm) onto high–refractive index (n) ZnS colloids in a ‘raspberry’ configuration is demonstrated. Motivation for this work lies in the promise of phenomena such as enhanced non–linear optical properties leading to ultrafast switching in 3–D photonic crystals with active light–emitting sources. Previous approaches involved either infiltration of a low–n colloidal template with quantum dots or surface modification of such colloids with dye molecules. In the former case tunability at the single particle level is lost, while in the latter, the index contrast is insufficient to promote bandgaps. For the present work, CU Dots and a thin layer of ZnS are co–condensed onto ZnS colloids via the thermal decomposition of thioacetamide in the presence of metal salt. The fluorescent high–n building blocks (200nm–2m) exhibit complete and uniform ‘monolayer’ coverage without a decrease in monodispersity, and are suitable for assembly into 3–D structures. Incorporating nanoparticles in the raspberry configuration also provides potential for brightness enhancement by avoiding quenching of neighboring dye molecules. The preparation and characterization of these colloids will be highlighted along with preliminary optical measurements of their assembly, including fluorescence microscopy, excitation–emission fluorescence spectroscopy and transmission spectroscopy.
Supernatant Controlled Synthesis of Monodispersed Zinc Sulfide Spheres and Multimers
1 Cornell University, Ithaca, New York, USA
A synthetic route to incorporate high–brightness core–shell silica nanoparticles (CU Dots 20–30nm) onto high–refractive index (n) ZnS colloids in a ‘raspberry’ configuration is demonstrated. Motivation for this work lies in the promise of phenomena such as enhanced non–linear optical properties leading to ultrafast switching in 3–D photonic crystals with active light–emitting sources. Previous approaches involved either infiltration of a low–n colloidal template with quantum dots or surface modification of such colloids with dye molecules. In the former case tunability at the single particle level is lost, while in the latter, the index contrast is insufficient to promote bandgaps. For the present work, CU Dots and a thin layer of ZnS are co–condensed onto ZnS colloids via the thermal decomposition of thioacetamide in the presence of metal salt. The fluorescent high–n building blocks (200nm–2m) exhibit complete and uniform ‘monolayer’ coverage without a decrease in monodispersity, and are suitable for assembly into 3–D structures. Incorporating nanoparticles in the raspberry configuration also provides potential for brightness enhancement by avoiding quenching of neighboring dye molecules. The preparation and characterization of these colloids will be highlighted along with preliminary optical measurements of their assembly, including fluorescence microscopy, excitation–emission fluorescence spectroscopy and transmission spectroscopy.
Supernatant Controlled Synthesis of Monodispersed Zinc Sulfide Spheres and Multimers
Symposium DD: Organic and Nanocomposite Optical Materials. Material Research Society Symposium (MRS), Boston, MA, November 28 – December 3, 2004.
Y. Song1, Chekesha M. Liddell Watson1.
1 Cornell University, Ithaca, New York, USA
Uniform zinc sulfide spheres and multimers ranging in size from ~ 90 nm to 1.0 micron were produced in large quantities by adding varying amount of supernatant in a secondary nucleation process. The particle morphology was investigated using scanning and transmission electron microscopy. Smaller particles exhibited increased surface roughness in the nitrate system. Characterization by x–ray and electron diffraction showed that the particles were built up from nanocrystallites. The relationship between the particle size, porosity, and refractive index was studied by modeling UV–Vis spectra using the Mie scattering method. Monodispersed zinc sulfide spheres and multimers in the size range from 100 nm to 600 nm can be used as high refractive index building blocks for photonic crystals with band gaps covering the entire visible spectrum as well as portions of the near–IR and UV regions.
1 Cornell University, Ithaca, New York, USA
Uniform zinc sulfide spheres and multimers ranging in size from ~ 90 nm to 1.0 micron were produced in large quantities by adding varying amount of supernatant in a secondary nucleation process. The particle morphology was investigated using scanning and transmission electron microscopy. Smaller particles exhibited increased surface roughness in the nitrate system. Characterization by x–ray and electron diffraction showed that the particles were built up from nanocrystallites. The relationship between the particle size, porosity, and refractive index was studied by modeling UV–Vis spectra using the Mie scattering method. Monodispersed zinc sulfide spheres and multimers in the size range from 100 nm to 600 nm can be used as high refractive index building blocks for photonic crystals with band gaps covering the entire visible spectrum as well as portions of the near–IR and UV regions.