Norman Wagner University of Delaware
Gerald G. Fuller Stanford University
Jennifer Lewis University of Illinois, Urbana-Champaign
Ko Higashitani Kyoto University-Katsura
W1: Self-Assembly of Colloidal Crystals I
Monday PM, April 17, 2006
Room 2001 (Moscone West)
9:00 AM - **W1.1
On-chip Manipulation by Electric Fields: From Self-Assembling Particles to Self-Propelling Devices
Orlin Velev 1 , Suk Tai Chang 1 , Vesselin Paunov 2 Show Abstract
1 Department of Chemical and Biomolecular Engineering, North Carolina State University, Raleigh, North Carolina, United States, 2 Department of Chemistry, University of Hull, Hull United Kingdom
We have demonstrated previously how dielectrophoresis, particle interaction with external AC fields, can be used to manipulate and assemble colloidal particles on any size scale, from nanometer gold particles, to submicron sized latex, to millimeter sized droplets. The structures that can be assembled by these methods include, for example, microwires, switchable photonic crystals, and complex anisotropic supraparticles. We will demonstrate here that an additional level of complexity can be engineered to turn the particles into prototypes of self-propelling micromachines. We show how various types of miniature semiconductor diodes floating in water can propel themselves when an uniform alternating electric field is applied across the container. The millimeter-sized diodes generate electroosmotic force, which propels them in the direction of either the cathode or the anode depending on the surface charge of the particles. The electroosmotic velocity depends on the electrolyte concentration and pH of the solutions. The velocity of the particles, however, does not depend strongly on their size; the electrokinetic mobility could, in principle, be used to propel particles or devices on the microscale. The diode motility can be used to power rotating "gears" and can be controlled by light. Thus, diodes propelling in electric fields suggest rudimentary solutions to problems facing self-propelling microdevices, including harvesting power from external sources, internally controlled movement, and potential for a range of additional functions.
9:30 AM - W1.2
Utilizing the Electronic Industry's Tricks for Transistor Fabrication for Development of New Delivery Colloidal Vehicles for Nanomedicine Applications
Larken Euliss 1 4 , Christopher Welch 2 , Stephanie Gratton 1 4 , Benjamin Maynor 1 4 , Klaus Hahn 2 4 , Rudy Juliano 2 4 , Joseph DeSimone 1 3 4 Show Abstract
1 Chemistry, University of North Carolina Chapel Hill, Chapel Hill, North Carolina, United States, 4 Lineberger Comprehensive Cancer Center, University of North Carolina Chapel Hill, Chapel Hill, North Carolina, United States, 2 Pharmocology, University of North Carolina Chapel Hill, Chapel Hill, North Carolina, United States, 3 Chemical Engineering, North Carolina State University, Raleigh, North Carolina, United States
The delivery of therapeutic, detection and imaging agents for the diagnosis and treatment of cancer patients has improved dramatically over the years with the development of nano-carriers such as liposomes, micelles, dendrimers, biomolecules, polymer particles, and colloidal precipitates. While many of these carriers have been used with great success in vitro and in vivo, each suffers from serious drawbacks with regard to stability, flexibility, or functionality. To date, there has been no general particle fabrication method available that afforded rigorous control over particle size, shape, composition, cargo and chemical structure. By utilizing the method we has designed referred to as Particle Replication In Non-wetting Templates, or PRINT, we can fabricate monodisperse colloidal particles with simultaneous control over structure (i.e. shape, size, composition) and function (i.e. cargo, surface structure). Unlike other particle fabrication techniques, PRINT is delicate and general enough to be compatible with a variety of important next-generation cancer therapeutic, detection and imaging agents, including various cargos (e.g. DNA, proteins, chemotherapy drugs, biosensor dyes, radio-markers, contrast agents), targeting ligands (e.g. antibodies, cell targeting peptides) and functional matrix materials (e.g. bioabsorbable polymers or stimuli responsive matrices). PRINT makes this possible by utilizing low-surface energy, chemically resistant fluoropolymers as molding materials and patterned substrates to produce functional, harvestable, monodisperse polymeric particles.To demonstrate the potential and compatibility of PRINT for introducing “soft” molecular recognition moieties and/or valuable therapeutic agents into functional particles, we have encapsulated oligonucleotide and protein cargos within them to generate monodisperse “particle devices.” We have incorporated fragile biological cargos and recognition agents, i.e. DNA, proteins (fluorescently-labeled avidin (MW 68 kDa)), and small anti-cancer agents (doxorubicin) into PEG nanoparticles using the simple, mild and general PRINT technique. We have arguably generated DNA delivery vectors that are themselves first generation “synthetic viruses” (monodisperse populations of shape-specific particles containing DNA). Furthermore, these biomolecule-containing particles could be used as nanoscale, shape-specific biosensors or next-generation therapeutic agents. We were able to confirm the encapsulation of the oligonucleotides by observing fluorescence from monodisperse particles using confocal microscopy.PRINT has several distinct advantages over other vector fabrication techniques in that the particles are monodisperse and shape specific. In addition, no surfactants condensation agents, etc. are required.
9:45 AM - W1.3
Structure and Dynamics of Bbiphasic Colloidal Suspensions.
Ali Mohraz 1 , Eric Weeks 2 , Jennifer Lewis 1 Show Abstract
1 Materials Science and Engineering, Univeristy of Illinois at Urbana-Champaign, Urbana, Illinois, United States, 2 Physics, Emory University, Atlanta, Georgia, United States
We investigate the structure and dynamics of concentrated biphasic mixtures comprised of attractive and repulsive silica microspheres using confocal laser scanning microscopy. One population of colloids is rendered hydrophobic by chemically modifying their surface. These species flocculate when suspended in an index-matching (water-DMSO) solvent, while their unmodified (charge-stabilized) counterparts remain individually dispersed.By labeling the two microsphere populations with different fluorescent dyes, we can directly image the flocculated and dispersed phases independently. The structural and dynamical parameters salient to the mixture’s rheology are quantified as a function of total colloid volume fraction and different ratios of the two phases. These results will provide new insights into the development of concentrated colloidal inks for direct-write assembly of complex 3D structures.
10:00 AM - W1.4
Coulomb Blockade at Room Temperature in Nanoparticle Arrays Aligned on DNA Scaffolds
Gregory Kearns 1 2 , James Hutchison 1 2 Show Abstract
1 Chemistry, University of Oregon, Eugene, Oregon, United States, 2 Materials Science Institute, University of Oregon, Eugene, Oregon, United States
Nanoparticles are of great interest as components in new materials and devices due to their size dependent optical and electronic properties. Two significant challenges in developing nanoparticle-based devices are assembling nanoparticles in a useful way and bridging nanoscale assemblies to microscale electronics. We have developed methods to form linear arrays of 1.5 nm particles that can be easily integrated with the microscale in order to exploit the properties of the nanoscale components while allowing access to the devices through microscale contacts. Metal nanoparticles with core diameters of less than 2 nm exhibit Coulomb blockade at room temperature, which can be used to develop new electronic devices such as single electron transistors (SET). These devices offer several advantages over semiconductor-based transistors—(i) they are 1-2 orders of magnitude smaller than current state-of-the-art transistors, (ii) SETs are not hindered by electron tunneling that can lead to device heating and/or failure in semiconductor based transistors, and (iii) it is possible to use a greener, bottom-up assembly approach to develop complex structures.We have developed a convenient synthetic route to monodisperse 1.5 nm gold nanoparticles that can be functionalized with a wide range of ligand shells. Using these particles, we have shown that, in solution, nanoparticles can be organized into linear arrays using electrostatic interactions between the negatively charged backbone of DNA and the positively charged ligand shell of functionalized nanoparticles. These interactions result in extended linear chains of close-packed nanoparticles. Close-packing of the nanoparticles on the DNA scaffold allows precise control over interparticle spacing by the appropriate choice of ligand shell. In order to make useful devices of these arrays, we have been working to align DNA on surfaces prior to coating with nanoparticles in order to obtain parallel, linear arrays of nanoparticles. Using specially fabricated TEM grids composed of a silicon grid with thermally grown, electron transparent SiO2 windows, we have shown that we can make long-range (tens of microns) parallel arrays of nanoparticles on thermal SiO2 in a three step assembly process involving (i) silanization of the SiO2 surface, which promotes molecular combing of DNA and limits nonspecific adsorption of positively charged nanoparticles, (ii) molecular combing of DNA on the silanized surface, and (iii) nanoparticle assembly on the linear arrays of DNA. TEM analysis of these arrays shows that the arrays are parallel over the entire substrate and that high purity nanoparticles maintain their core size and spacing as determined by the thickness of the ligand shell. With these arrays we have developed electronic test structures that exhibit Coulomb blockade at room temperature.
10:15 AM - W1.5
Dewetting Induced Formation of Nanoparticle Stripe Patterns.
Jiaxing Huang 1 2 , Franklin Kim 1 2 , Andrea Tao 1 2 , Stephen Connor 1 2 , Peidong Yang 1 2 Show Abstract
1 Department of Chemistry, UC-Berkeley, Berkeley, California, United States, 2 Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States
10:30 AM - W1.6
Generalized Rule for Order Formation by Colloidal Nanoparticles Adsorbed on a Substrate.
Minoru Miyahara 1 , Satoshi Watanabe 1 Show Abstract
1 Chemical Enginering, Kyoto University, Kyoto Japan
Ordered arrays of particles have been attracting much attention recently because of their unique functions especially as optical devices. We performed Brownian dynamics simulations of adsorption of electrostatically stabilized colloidal particles and their spontaneous order formation on a planar surface with countercharge, in which the particle–particle and particle–substrate interactions are modeled based on the DLVO theory. The results obtained are: (1) A hexagonally ordered array by adsorbed particles is found to form only if a controlling factor, the “average force” acting between adsorbed particles, exceeds a limiting value that is common to various ionic strengths; (2) A general concept extended from the Alder transition is proposed to give a comprehensive understanding of the characteristics of the “average force”, with which the controlling factor and resultant ordered structure can be predicted a priori for any combination of temperature, ionic strength and particle size. Further, large-scale simulations were performed to observe time-evolution of ordered domains and their merging processes, which exhibited two types of different behaviors depending upon the Debye lengths or ionic strengths: one type with easily merging domains and the other one with prevailing domain structures. The origin of the difference will be discussed in connection with the "average force" and the characteristics of the particle-substrate interactions.
10:45 AM - W1.7
Regulation of Atomic Alignment in Superlattices of Au and Ag Nanoparticles.
Keisaku Kimura 1 , Yang Yang 1 , Suhua Wang 2 , Seiichi Sato 1 , Hiroshi Yao 1 Show Abstract
1 Graduate School of Science, University of Hyogo, Hyogo Japan, 2 Department of Chemistry, National University of Singapore, Singapore Singapore
Intense research interest is focusing on the nano-sized substances such as surface modified gold and silver nanoparticles, nanodiscs and nanowires due to their notable electronic and optical properties based on the quantum size effect. Fabrication of macroscopic architecture using these nano-sized materials as a constructing block will be a next target in the coming nanotechnology era. There are many reports on the construction of two and three-dimensional superlattices made by metallic nanoparticles, because it will provide a peculiar performance based on collective electronic behaviors resulting from interparticle interactions between neighboring ordered particles. In order to accomplish an ultimate collective motion of electrons or excitation energy moving this ordered structure, we need atomic orientational as well as translational alignment in the superlattice. While regular lattices made of nanoparticles are prepared by spontaneous evaporation of organic solvent in most cases, we have developed a new class of superlattice at an air/water interface by virtue of the ionic or hydrogen bonding interactions under an equilibrium condition . It was found that water cluster exist interstitially in the superlattice spacing observed by FTIR at room temperature . At the same time, some arc patterns in the diffraction from atoms superimposed with the diffraction from superlattice were observed in the transmission electron diffraction (TED), suggesting atomic orientational arrangements to some extent in this small crystal. We will present more complete TED pattern that shows coexistence of atomic and superlattice orientational alignment in the crystal.Two metallic nanoparticles, gold and silver, were used to construct nanoparticle assemblies. For a typical preparation of gold nanoparticles at thiol to gold ratio = 3, 0.5 mmol of HAuCl4 aqueous solution was mixed with 1.5 mmol of MSA in 100-mL methanol. A freshly prepared 0.2 M ice-cold aqueous NaBH4 solution (25 mL) was then added under vigorous stirring. After further stirring for 1h, the precipitate was gathered by decantation and centrifugation. The precipitate was washed with water/methanol solution and then with pure methanol and ethanol. Finally, the sample was obtained as a solid powder by freeze-drying. The mean particle diameter was typically 3.5 nm (fwhm: 0.4 nm). The crystallization took place at an air/water interface in 4-10 days in the presence of an appropriate amount of hydrochloric acid in a sealed vial giving numerous micrometer-sized nanoparticle crystals with clear crystal habit [1,2]. The TED and TEM images were taken by a Hitachi H8100 transmission electron microscope. Samples were set on the amorphous carbon film on the Cu grid. The observed TED pattern is discussed along with the sample preparation treatment.. K.Kimura, S.Sato and H.Yao, Chem.Lett., 2001, 372-373.. S.Wang, H.Yao, S.Sato and K.KimuraJ.Amer.Chem.Soc. 126(24)(2004)7438-7439.
11:30 AM - **W1.8
Functionalised Interfacial Particulate Coatings from Block Copolymer Micelles.
Simon Biggs 1 , Grant Webber 1 , Kenichi Sakai 1 , Steven Armes 2 Show Abstract
1 Institute of Particle Science and Engineering, University of Leeds, Leeds United Kingdom, 2 Chemistry, University of Sheffield, Sheffield United Kingdom
The facile production of highly functional particle coatings has many potential benefits across a wide range of established products in sectors such as pharmaceuticals, personal care, household and agricultural products. The key challenge is to produce a coating easily and at low cost. This provides significant challenges to both technology and manufacturing. In this paper, I will describe recent research into the production of novel particle coatings that potentially have a high degree of functionality but which can be produced very easily allowing for scale-up and manufacture. These coatings are based upon simple stimulus-responsive polymer materials that, themselves, have a high degree of potential variety. Allied to this is the possibility of 'tuning' the quality of the coating providing a wide variety of coating options.Results of recent research we have undertaken to characterise particulates coated by multilayers of stimulus responsive copolymer micelles will be described. These layers have been characterised by a combination of techniques including AFM, QCM-D, optical reflectometry, and zeta potential measurements.We will also discuss the potential applications of materials such as these in areas such as delivery agents, rheology modifiers, and capsule manufacture.
12:00 PM - W1.9
Patterning Colloidal Films via Evaporative Lithography.
Daniel Harris 1 2 , Angel Chan 1 2 , Jennifer Lewis 1 2 Show Abstract
1 Materials Science, University of Illinois, Urbana, Illinois, United States, 2 Materials Research Laboratory, University of Illinois, Urbana, Illinois, United States
We investigate evaporative lithography as a route for creating patterned colloidal films during drying. Specifically, films comprised of silica microsphere and polystyrene nanoparticle mixtures are patterned by placing a mask above the film surface to induce periodic variations between regions of free and hindered evaporation. Fluorescence and confocal microscopy coupled with surface profilometry measurements reveal that particles segregate laterally within the drying film, as fluid and entrained particles migrate towards regions of high evaporative flux. Such films exhibit remarkable pattern formation that can be regulated by carefully tuning initial film composition, mask geometry, and the separation distance between the mask and underlying film.
12:15 PM - W1.10
Directed Self-assembly of Virus Particles at Chemical Templates.
Sung-Wook Chung 1 , Chin Cheung 2 1 , Anju Chatterji 3 , Tianwei Lin 3 , John Johnson 3 , James DeYoreo 1 Show Abstract
1 Chemistry and Materials Science Directorate, Lawrence Livermore National Laboratory, Livermore, California, United States, 2 Department of Chemistry, University of Nebraska, Lincoln, Nebraska, United States, 3 Department of Molecular Biology, Scripps Research Institute, La Jolla, California, United States
Icosahedral viruses in solution are colloidal systems in which inter-particle potentials can be modulated by both site-directed mutagenesis and variation of solvent composition. Because individual surface sites can be engineered to bind specifically to self-assembled monolayers (SAMs), their assembly at surfaces can be directed by chemical templates. As a result, the physics of directed colloidal assembly can be explored in a system where the governing interactions are variable. Moreover, because other sites on viruses can be engineered to present catalytic, electronic, or optically active moieties, control over virus organization defines a route to nanoscale device fabrication. Here we report results using scanned probe nanolithography (SPN) to direct organization of viruses into 1D and 2D patterns and in situ AFM imaging to investigate the dynamics of organization as pattern geometry, inter-viral potential, virus flux, and virus-pattern interaction are systematically varied.As a model system, we chose Cowpea Mosaic Virus genetically engineered to present either cysteine (Cys) or histidine (His) tags at specific sites on the capsid surface. Atomically-flat gold substrates coated with SAMs of polyethylene glycol (PEG) terminated alkyl thiols were patterned with either maleimide (MA) or nickel-chelating nitrilotriacetic acid (Ni-NTA) terminated alkyl thiols using SPN. Template features had sizes ranging from 10-100nm and were separated by 50 to 1000nm. Virus attachment occurred either through a metal coordination complex between His-tags and Ni-NTA, or through a covalent bond between Cys and MA groups. AFM was then used to investigate the degree of ordering, packing geometry, assembly kinetics, and cluster-size distribution both on the templates as well as the surrounding PEG-terminated regions.We show that the degree of ordering depends on all parameters chosen: surface chemistry, virus concentration, PEG concentration, and feature size and separation. For example, as the solution PEG concentration is increased, which increases virus-virus attraction through hydrophobic effects, 2D arrays of viruses evolve from poorly-ordered, to well-ordered rhombohedral, and then hexagonally close packed assemblies and 1D patterns increase from one multiple rows of viruses in width. Disordered clusters form on PEG functionalized regions, but cluster growth dynamics is altered. Taking cues from previous work on both epitaxial and colloidal systems, we present a physical picture of virus assembly at templates in which the dominant factors are the ratio of virus flux to surface mobility and the strength of the virus-virus interaction, the latter being modulated by hydrophobic interactions and or/covalent bonds.This work was performed under the auspices of the U. S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under Contract No. W-7405-Eng-48.
12:30 PM - W1.11
Directed Adsorption of Polyelectrolyte- and Nucleic Base-Functionalized Colloids
Marianne Terrot 1 , Paula Hammond 1 Show Abstract
1 Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
The selective adsorption behavior of macromolecules, colloidal particles, and polyelectrolyte multilayers is of great importance to the production of nano and micro-scale features within organic thin films. We have previously demonstrated the ability of chemical surface templates to direct the deposition of polyelectrolyte multilayers; recently, this approach has been extended to polyelectrolyte-functionalized colloids, making possible the assembly of multi-component colloidal arrays via selective adsorption. Unlike lateral arrays of multilayer films, these colloidal assemblies can be efficiently and faithfully transferred from the original surface template directing their assembly to a new matrix better suited to the final application. In order to design more complex arrays of three or more components, we have sought to broaden the range of interactions used to guide adsorption and specifically, to seek out species capable of specific recognition. For this, we took our inspiration from nature and explored the ability of nucleotide base pairing to direct assembly; multiple hydrogen bonding such as that existing between adenine and uracil and cytosine and guanine is ideally suited to our work as the interaction is both highly specific and readily modulated via temperature and pH. RNA homopolymers were used both for functionalization of polystyrene latex colloids and for chemical patterning of surfaces; we have also synthesized base-terminated silanes for more robust functionalization of silicon surfaces and colloids. Finally, we explore the self-assembly in solution of microspheres asymmetrically functionalized with complementary bases.
W2: Self-Assembly of Colloidal Crystals II
Monday PM, April 17, 2006
Room 2001 (Moscone West)
2:30 PM - **W2.1
Adding Optical Functionality to 3D Self-Assembled Colloidal Crystals: Waveguides, Cavities, and Emitters
Paul Braun 1 Show Abstract
1 Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States
Three-dimensional photonic crystals have been postulated to exhibit a number of unique properties which may enable new paradigms for controlling light. However, photonic crystals formed through colloidal crystallization have limited application due to their lack of refractive index contrast and inherently periodic structure. New physics and new applications such as efficient light generation, negative refractive index properties, lasers, waveguides, and optical cavities require the incorporation of new materials and aperiodic defect structures within the colloidal crystal. I will present our results on the use of multiphoton writing, electrochemistry, and chemical vapor deposition to create highly functional 3D colloidal crystal based optical devices including 3-D waveguides with sharp bends, embedded optical cavities, and light emitters.
3:00 PM - W2.2
Tunable Stopgap in Photonic Crystals Based on Homogeneous & Core-shell ZnS Colloids.
Ian Hosein 1 , Chekesha Liddell 1 Show Abstract
1 Materials Science & Engineering, Cornell University, Ithaca, New York, United States
3:15 PM - W2.3
Integration of Self-assembled Three-dimensional Photonic Crystals onto Structured Silicon Wafer.
Rudolf Zentel 1 , Jianghui Ye 1 , Sanna Arpianen 2 , Jouni Ahopelto 2 , Sergei Romanov 3 , Clivia Sotomayor Torres 3 Show Abstract
1 Chemistry, University Mainz, Mainz Germany, 2 , VTT Centre for Microelectronics, VTT Finland, 3 Tyndall National Institute, University College Cork, Cork Ireland
Advanced photonic circuits will need complex architectures, which can be achieved using functional platforms for positioning, shaping and coupling of high-quality two- and three-dimensional (2D and 3D) PhCs. In contrast to 2D PhCs, which can be built-in directly to, for example, SOI platforms by means of well-developed 2D nanolithography, integration of 3D PhCs poses a great challenge. Assembling 3D PhCs through wafer bonding was proved complicated and costly procedure, with no straightforward prospect of further integration. Alternative 3D nanolithography, which imply in-situ 3D patterning of polymer template followed by its infiltration by high refractive index semiconductor and completed by nanocomposite inversion, look more efficient but remains a costly process and requires sophisticated equipment. In our opinion, self-assembly of monodisperse microspheres into opal structures remains the favourite route towards templates for platform-integrated photonic materials, owing to its simplicity, flexibility and low cost. In particular, the preparation of a silicon-inverted PhC on a silicon platform for the telecommunication frequency range is practically important target. Accomplishing this task requires, first of all, realization of perfectly crystalline low refractive index 3D templates. The quality of colloidal crystal on structured silicon wafer will depend on the method of opal assembly, commensurability of opal lattice constant and pattern dimensions and spatial selectivity of opal growth. So far, opals have been crystallized in channel structures, whereby the deposition of opal on top of the wafer surface was mostly prevented by covering it with a slide or by making this surface hydrophobic. These methods are difficult to apply if the opal should be assembled on micrometer-size isolated areas. Combination of directed evaporation-induced self-assembly (DEISA) process and slowly stirring solution to keep the spheres in a levitated state (ADEISA), subsequently, suggested as a solution for assembling large spheres in channels.In this paper, we report the growth of high quality colloidal crystals from very large SiO2 spheres of 890 nm diameter in a drawing apparatus. The choice of sphere size is dictated by the need to match the telecommunication wavelength range and the high-order photonic bandgaps of the inverted Si-opal. Crystallization in moving meniscus allows easy managing of opal parameters by changing the withdrawal speed comparing to evaporation temperature control and can be implemented to growth of high quality opals onto complex structured silicon wafers. We focus on optimizing the opal crystallization conditions in confined geometries of complex topology and obtained silica opals with excellent quality, which is confirmed by optical and microscope studies.
3:30 PM - W2: SA co II
4:00 PM - **W2.4
Fabrication Techniques for Photonic Crystals
Pierre Wiltzius 1 Show Abstract
1 MS&E Department and Physics Department, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States
Photonic crystals are materials that allow the manipulation of light in new and unexpected ways. Colloidal self-assembly and multi-beam interference lithography are great tools to build crystals with interesting optical properties. Recent progress on making large area single crystals using colloidal self-assembly and holographic multi-beam interference techniques will be discussed. In addition, optical characterization and some device applications using photonic crystals will be presented.
4:30 PM - W2.5
Tuning the Optical Properties of Colloidal Thin Films Using Dual-frequency Liquid Crystal.
Elton Graugnard 1 , Jeffrey King 1 , Swati Jain 1 , Yana Zhang-Williams 2 , Iam-Choon Khoo 2 , Christopher Summers 1 Show Abstract
1 Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States, 2 Department of Electrical Engineering, Pennsylvania State University, University Park, Pennsylvania, United States
Patterned 3D optical structures based on colloidal thin films, such as opals and inverse opals, offer unprecedented control over the propagation of light within materials, enabling effects such as slow light, negative refraction, and photonic band gaps. Inverse opals have been studied extensively due to the ability to form a complete photonic band gap for a high refractive index backbone. Concurrently, several groups have studied optical tuning by infiltrating inverse opals with non-linear optical materials, such as liquid crystals. To fully exploit the potential optical properties, devices whose optical properties can be dynamically controlled are necessary. Here, we report the combination of a high refractive inorganic backbone with a dual-frequency liquid crystal to create a hybrid film whose Bragg reflectance peak can be tuned over a large range, exhibiting both blue and red shifts about a central wavelength.Polymer colloidal thin films were formed by directed self-assembly of polystyrene spheres from 329 to 820 nm in diameter, and then sintered to increase the diameter of the contact points between neighboring spheres. The sintered films were infiltrated with amorphous titania, (refractive index 2.31 at 800 nm) using low temperature atomic layer deposition (ALD). The infiltrated films were ion-milled to expose the polystyrene spheres, which were then etched by immersing the films in toluene, creating lattices of air spheres within a dielectric backbone. These sintered inverse films were infiltrated at 60C with MLC2048, which is a multi-component liquid crystal (LC) with a frequency dependent polarization. By varying the frequency of an applied electric field, the Bragg peak position could be both blue and red shifted by controlling the orientation of the infiltrated LC, which dynamically controlled the refractive index contrast within the hybrid structure. The details of the fabrication process and the results of the optical tuning will be discussed, along with potential applications of these results.
4:45 PM - W2.6
Emission of TRITC Dye Encapsulated Silica-Nanoparticles in Non-Closepacked Photonic Crystals
Poorna Rajendran 1 , Erik Herz 1 , Ulrich Wiesner 1 , Chekesha Liddell 1 Show Abstract
1 Materials Science and Engineering, Cornell University, ithaca, New York, United States
The development of high performance light sources including low threshold microlasers and efficient light-emitting diodes has been one of the most promising applications of photonic crystals. Realizing such devices requires a spatially extended light source integrated into a periodic dielectric structure (photonic crystal) for effective photon confinement and control of spontaneous emission. Due to their broadband emission, which is easily made to coincide with the photonic crystal stopband, dyes are attractive probes of photonic bandgap effects (spectral redistribution, spatial filtering, amplified spontaneous emission, band-edge lasing) on the emission of optically active materials. Here, we investigate the modification of tetramethylrhodamine isothiocyanate (TRITC) dye emission characteristics due to changes in the local density of photon states within a photonic crystal. High-brightness, photostable core-shell nanoparticles (Cornell Dots ~30 nm in diameter) consisting of the TRITC dye encapsulated in a silica shell were co-assembled with polystyrene beads into photonic crystals using a convective approach. Dye incorporation through this hierarchical self-assembly process overcomes common problems in dye-opal composite systems such as fluorescence quenching at high dye concentration, reduced excited state lifetime and fluorescence quantum yield due to uncontrollable intermolecular interactions between the dye and its chemical environment, incomplete infiltration of interstitial pore space, and limited control of volume filling fraction in close-packed systems. The spectral position of the photonic crystal stopband was tuned, by varying the PS template bead size as well as the relative concentration of PS beads to dye incorporated nanoparticles. Large excesses of the fluorescent nanoparticles led to ordered non-closepacked morphologies as determined by scanning electron microscopy. Photoluminescence, transmission and reflectance measurements revealed inhibited spontaneous emission due to the existence of a photonic stopband with wavelength range comparable to that of the emission wavelength of the fluorescent nanoparticles. In addition, the refractive index contrast was enhanced to promote stronger light-matter interactions by removing the polymer sphere template using a plasma etch.
5:00 PM - W2.7
Plasmon-coupling in Self-assembled Colloidal Metal Nanoparticle Arrays.
R. de Waele 1 , A.F. Koenderink 1 , J.T. van Wijngaarden 1 , A. van Blaaderen 2 , A. Polman 1 Show Abstract
1 Center for Nanophotonics, FOM-institute AMOLF, Amsterdam Netherlands, 2 Soft Condensed Matter, Utrecht University, Utrecht Netherlands
5:15 PM - W2.8
Capillary Force Indexing of Colloidal Nanoparticles.
Michael Gordon 1 , David Peyrade 1 Show Abstract
1 Laboratoire des Technologies de la Microelectronique, LTM-CNRS, Grenoble France
Going beyond the microelectronics-only frontier is quickly becoming a necessary mindset in the fabrication and large-scale implementation of nano-objects into useful devices and structures. For example, colloidal synthesis techniques can now provide a wealth of different nanoscale materials with well controlled shape and size dispersion (2-100 nm). However, large-scale assembly and precise localization of many nano-objects at the same time is still rather challenging. In this work, we use capillary forces to direct the self-assembly of Au nanoparticles (NPs) into lithographic patterns during colloidal solution evaporation (single droplets and large baths). Particle indexation was studied in real-time using dark-field optical microscopy of evaporating droplets to understand how NP flow within the drop, particle congregation at the tri-phase contact line, and intermittent contact line pinning affect the organization process. This visualization demonstrated that good pattern filling requires a large NP flux from the liquid interior toward the drop edge and that the contact line must advance smoothly over the pattern as the colloidal solution de-wets. As such, indexation was seen to be extremely sensitive to the template material (photoresist, SiO2), surface treatment (controlling the contact angle), and evaporation speed/direction. By optimizing the evaporation process, NPs from 10-200 nm were organized into a wide variety of lithographic features such as holes (round/square/triangular), trenches, and large open areas. Furthermore, it was possible to control pattern filling at the single particle level over extended areas (100x100 μm^2). Careful adjustment of the hole size and shape allowed well-organized arrays (50x50 matrix) of 1, 2, 3, or 4 (non-touching) particle groups within one hole to be created. The lithographic template layer was also removable after indexation by plasma treatment without damaging NP organization - opening up a new route to particle arrays for application to nanowire growth (templates) or optical detection of single molecules. This talk will discuss particulars of the evaporation process, indexation, and plasmon spectroscopy conducted on individual groups of 1-4 spherical Au NPs sitting on Si/SiO2, to demonstrate that the number of particles/hole (1, 2, 3, or 4 NPs) can be distinguished in far-field.
5:30 PM - **W2.9
Ionic Colloidal Crystals from Oppositely Charged Particles.
Alfons van Blaaderen 1 Show Abstract
1 Physics, Utrecht University, Utrecht Netherlands
Concentrated dispersions of monodisperse colloidal particles are both interesting as model system to study fundamental condensed matter questions like freezing and melting and because their self-organization can be used to make regular 3D (metallo-)dielectric structures that can be used in advanced (photonic) applications. Moreover, the process of colloidal crystallization into 3D periodic lattices can be manipulated relatively easily by using external fields and studied quantitative on a single particle level with light microscopy. In this talk we will show how colloidal crystals can be grown from binary dispersions of oppositely charged spheres and manipulated with external fields. Previously, it was considered not possible to grow equilibrium phases from such systems because of heteroaggregation. Our new findings make it possible to grow large (binary) crystals of large particles (e.g., 2 micron), which is interesting for photonic applications. Moreover, ionic crystallization and glass formation can now be modelled with colloids, if the Debye screening length is made to be sufficiently large. However, the fact that the interparticle interactions are screened Coulomb also gives interesting new opportunities that are not possible with atomic salts. The fact that the colloids together with their double layer are charge-neutral relieves the strong restrictions on stoichiometry that need to be met for ionic crystals. Thus, we have observed CsCl type crystals of 2 micron sized particles that were not equally charged (e.g., + 125e, -75e). For applications it is important that CsCl crystals could also be obtained from mixtures of oppositely charged micron sized PMMA and silica spheres, despite the large density difference. For a size ratio between large (L) and small (S) spheres of 0.31 we have in addition seen: LS6, LS8, NaCl and NiAs type crystals. Some of the crystal types we observed have, as far as we know, never been seen before. We investigated the stability of the different crystal types also by Madelung calculations and computer simulations. Finally, we show that relatively small external electric fields can be used to melt the ionic colloidal crystals. At low volume fractions of the oppositely charged spheres an interesting out-of-equilibrium phase transition takes place whereby the particles that move in the same direction organize themselves in ‘lanes’ that are oriented in the field direction. At higher volume fractions the particles get jammed into structures that resemble wavy patterns perpendicular to the field direction.