Enhanced Photonic Properties of Thin Opaline Films as a Consequence of Embedded Nanoparticles. D E Whitehead, M Bardosova and M E Pemble Tyndall National Institute, University College Cork Ireland
Introduction: Photonic crystals consist of monodispersed close packed spheres. There can be made from designer particles. Nanoparticles may be readily coated with ashell core shell particles. Three-dimensional nanostructures (modified opals) may then be prepared via the self assembly of core-shell nanoparticles. The intercore distance is dictated by the thickness of the silica shells and thus may be tailored to suit the application.
Motivation for study of core shell particles: Versatility tailored functionality Stabilizes particle against photo degradation. Size dependent, novel optical properties Reactions within the core forming hollow shells Slow release of drugs
Optical Properties of Metal Nanoparticles Nanoparticles of noble metals display very interesting optical properties due to the presence of surface plasmons, i.e. oscillations of conduction electrons in resonance with the alternating electric field of incident electromagnetic radiation. The frequency of the surface plasmon depends on the nature of the metal, but also on the size and shape of the nanoparticles, among other parameters.
Gold nanoparticles 20nm gold spheres not spherical 15nm spheres protected with 1nm thick silica shell
Surface functionalization and shell growth 3-Mercaptopropyl trimethoxy silane 3-aminopropyl trimethoxy silane Vitreophilic surface primed and then a thin shell is formed using sodium silicate (5-10nm thick) Stöber growth
Embedded gold nanoparticles Growth after 5 dayssilica seeds start to nucleate out of solution Correct conditions of gold core coating by Oswald ripening and then Stöber growth. Particles are 165nm in diameter.
Rhodamine B isothiocyanate with a coupling agent 3- aminopropyltrimethoxysilane. A schematic representation of the resulting product is shown. The functionalised dye is then utilized in a standard Stöber synthesis to produce doped silica cores. A seeded technique is then used to grow a silica shell around the cores to the desired particle diameter. The functionalised dye product was also reacted to a silica core producing a shell of doped silica around a silica core. A schematic representation of the dye molecule attached to the opal surface. Type A Type B
Stöber Method High concentration of catalyst, low amounts of water Induce the condensation onto the seed particles Si-OR + H 2 O Si-OH + ROH (1) Si-OR + Si-OH Si-O-Si + ROH (2) Si-OH + Si-OH Si-O-Si + H 2 O (3) where R is an alkyl group of the form C x H 2x+1
Tuning the shell thickness Amount of TEOS to be added to reach particle diameter V 2 = V 1 RAuSiO 2 3-1 RAu V 2 = Volume of TEOS V 1 = Volume of a seed particle R = Radius
Assembly techniques: Self assembly settling in a tube Controlled evaporationthickness controlled by temperature and volume fraction Langmuir Blodgett assembly
Surface functionalisation for LB deposition 3-(trimethoxysilyl)propyl methacrylate O O CH 2 CH 2 CH 2 Renders surface hydrophobic Si OMe OMe OMe
Model 1222D2 Modular Combination LB Trough http://www.nima.co.uk
Pressure-Area (π-a) Isotherm
A B monolayers 370nm and 300 nm 1 μm
10 Layers: Surface modified silica spheres 10 Layers: Surface modified silica spheres with a 100nm core containing dye
Digital camera image of gold core silica shell opal settled in a glass tube 3 cm
Optical data of gold core shell particles. Intensity (a.u.) 100 90 80 70 60 50 40 30 20 10 0 Plasmon 525nm 300 500 700 Reflectance Transmittance Wavelength (nm)
Reflectance data of gold core - silica shell particles. Wavelength (nm) 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 High film quality as evidenced by the large number of F-P peaks 0 350 400 450 500 550 600 650 700 750 800 Intensity (a.u.)
Calculation of thickness from the F-P peaks Fringe order (p) 10 8 6 4 2 0 y = 3228.4x - 0.4671 0 0.001 0.002 0.003 0.004 Thickness = 3228.4/ d-spacing = 22 layers
SEM images of colloidal crystal thin films
SEM image showing gold spheres
SEM of primary ZnS particles
SEM of CdS particles. Acidity controls the surface roughness Strict temperature control for low dispersity.
Reflectance data of ZnS spheres with a diameter of 180nm. 16 14 Intensity (a.u.) 12 10 8 6 4 2 0 200 300 400 500 600 700 800 Wavelength (nm)
Photoluminescence of ZnS photonic crystal thin film with diameter of 165nm. 60 Excitation wavelength 363.5 nm 50 Intensity (a.u.) 40 30 20 10 0 300 350 400 450 500 550 Wavelength (nm)
Conclusions: Core shell colloidal particles are very versatile. Permit the combination of physical and optical properties Surface chemistry can be fine-tuned allowing for different methods of self-assembly Many potential applications possible
Acknowledgements: Science foundation Ireland Worawut Khunsin for PL measurements