Nanoparticles Lecture 2 郭修伯

Top-down Approaches milling or attrition thermal cycles 10 ~ 1000 nm; broad size distribution varied particle shape or geometry impurities for nanocomposites and nanograined bulk materials (lower sintering temperature)

Bottom-up Approaches Two approaches thermodynamic equilibrium approach generation of supersaturation nucleation subsequent growth kinetic approach limiting the amount of precursors for the growth confining in a limited space

Homogeneous nucleation Liquid, vapor or solid supersaturation temperature reduction metal quantum dots in glass matrix by annealing in situ chemical reactions (converting highly soluble chemicals into less soluble chemicals)

Homogeneous nucleation Driving force Fig 3.1

Homogeneous nucleation Surface energy Energy barrier Gibss free energy change

Nuclei formation favor: uniform nanoparticle size: high initial concentration or supersaturation low viscosity low critical energy barrier uniform nanoparticle size: same time formation abruptly high supersaturation -> quickly brought below the minimum nucleation concentration

Nuclei growth Steps size distribution growth species generation diffusion from bulk to the growth surface adsorption surface growth size distribution A diffusion-limited growth VS. a growth-limited processes

Diffusion-limited growth monosized nanoparticles how? Low/controlled supply growth species concentration increase the solution viscosity introduction a diffusion barrier

Metallic nanoparticles Reduction of metal complexes in dilute solution Diffusion-limited process maintaining Example: nano-gold particles chlorauric acid (2.5 x 10-4 M) 20 ml boiling solution+ sodium citrate (0.5%) 1 ml 100°C till color change + water to maintain volume uniform and stable 20 nm particles

Table 3.1

Other cases

Reduction reagents Affect the size and size distribution weak reduction reaction larger particles wider or narrower distribution (depends on “diffusion limited”) Affect the morphology type, concentration, pH value

Fig 3.10

Fig 3.12

Polymer stabilizer To prevent agglomeration surface interaction: surface chemistry of solid, the polymer, solvent and temperature Strong adsorbed stabilizers occupy the growth sites and reduce the growth rate A. Henglein, Chem. Mater. 10, 444 (1998). polyethyleneimine, sodium polyphosphate, sodium polyacrylate and poly(vinylpyrrolidone)

stabilizer concentration

temperature

Semiconductor nanoparticles Pyrolysis (熱裂解) of organometallic precursor(s) dissolved in anhydrate solvents at elevated temperatures in an airless environment in the presence of polymer stabilizer (i.e., capping material) Coordinating solvent Solvent + capping material phosphine + phosphine oxide (good candidate) controlling growth process, stabilizing the colloidal dispersion, electronically passivating the surface

Process discrete nucleation by rapid increase in the reagent concentration -> Ostwald ripening (熟成) during aging at increased temperature (large particle grow)-> size selective precipitation Ostwald ripening A dissolution-growth processes large particles grow at the expense of small particles produce highly monodispersed colloidal dispersions

Semiconductor nanocrystallites C.B. Murray (CdE, E=S, Se, Te), 1993 Dimethylcadmium (Me2Cd) + bis(trimethylsilyl) sulfide ((TMS)2S) or trioctylphosphine selenide (TOPSe) or Trioctylphosphine telluride (TOPTe) + solvent (Tri-n-octylphosphine, TOP) + capping material (tri-n-octylphosphine oxide, TOPO) before aging (440 ~ 460nm), after aging at 230-260°C (1.5~11.5 nm) Size-selective precipitation

Oxide nanoparticles Several methods principles: burst of homogeneous nucleation + diffusion controlled growth most commonly: sol-gel processing most studied: silica colloids

Sol-gel process Synthesis Ref inorganic and organic-inorganic hybrid materials colloidal dispersions powders, fibers, thin film and monolith(整塊) low temperature and molecular level homogeneity Ref Sol-Gel Science by Brinker and Scherer; Introduction to Sol-Gel Processing by Pierre; Sol-Gel Materials by Wright and Sommerdijk

Sol-gel process Hydrolysis Condensation of precursors e.g. Condensation of precursors typical precursors: metal alkoxides or inorganic and organic salts

Multicomponents materials Sol-gel route ensure hetero-condensation reactions between different constituent precursors reactivity, electronegativity, coordination number, ionic radius precursor modification: attaching different organic ligands (e.g. reactivity: Si(OC2H5)4 < Si(OCH3)4) ) chemically modify the coordination state of the alkoxides multiple step sol-gel

Organic-inorganic hybrids Incorporating organic components into an oxide system by sol-gel processing co-polymerization co-condense trap the desired organic (or bio) components inside the network biocomponents-organic-inorganic hybrids

Sol-gel products Monodispersed nanoparticles temporal nucleation followed by diffusion-controlled growth complex oxides, organic-inorganic hybrids, biomaterials size = f(concentration, aging time) colloid stabilization: not by polymer steric barrier, by electrostatic double layer

Sol-gel example: silica Precursors: silicone alkoxides with different alkyl ligand sizes catalyst: ammonia solvent: various alcohols Vigorous stirring water

Vapor phase reactions Same mechanism as liquid phase reaction Elevated temperatures + vacuum (low concentration of growth) Collection on a down stream non-sticking substrate @ low temperature example: 2~3 nm silver particles may migrate and agglomerate

Vapor phase reactions Agglomerates: size affections large size spherical particles needle-like particle Au on (100) NaCl and (111) CaF substrate Ag on (100) NaCl substrate change in temperature and precursor concentration did not affect the morphology size affections reaction and nucleation temperature

Solid state phase segregation applications metals and semiconductor particles in glass matrix homogeneous nucleation in solids state metal or semiconductor precursors introduced to and homogeneously distributed in the liquid glass melt at high temperature glass quenching to room temperature glass anneal above the Tg solid-state diffusion and nanoparticles formed

Solid state phase segregation Glass matrix (or via sol-gel, polymerization): metallic ions Reheating (or UV, X-ray, gamma-ray): metallic atoms Nuclei growth by solid-state diffusion (slow!)

Solid state phase segregation

Heterogeneous nucleation A new phase forms on a surface of another material thermal oxidation, sputtering and thermal oxidation, Ar plasma and ulterior thermal oxidation associate with surface defects (or edges)

Heterogeneous nucleation

Kinetically confined synthesis Spatially confine the growth limited amount of source materials or available space is filled up groups liquid droplets in gas phase (aerosol & spray) liquid droplets in liquid (micelle & microemulsion) template-based self-terminating

Micelles or microemulsion surfactants or block polymers two parts: one hydrophilic and one hydrophobic self-assemble at air/aqueous solution or hydrocarbon/aqueous solution interfaces microemulsion dispersion of fine organic liquid droplets in an aqueous solution

Micelle CdSe nanoparticles by Steigerwald et al. surfactant AOT (33.3g) + heptane (1300ml)+ water (4.3ml) stirred -> microemulsion 1.0M Cd2+ (1.12 ml) + microemulsion Se(TMS)2 (210μl) + heptane (50ml) + microemulsion (syringe, 注射) formation of CdSe crystallites

Polymer nanoparticles Water-soluble initiator + surfactant + water + monomer monomer (large droplets, 0.5 ~ 10μm ) initiator polymerization nanoparticles (50 ~ 200nm)

Aerosol synthesis Characteristics process Regarded as top-down (maybe?) can be polycrystalline needs collection and redispersion process liquid precursor -> mistify -> liquid aerosol -> evaporation or reaction -> nanoparticles polymer particle 1~20 μm (from monomer droplets)

Size control by termination Termination by organic components or alien ion occupation

Spray pyrolysis Solution process metal (Cu, Ni …) and metal oxide powders converting microsized liquid droplets of precursor or precursor mixture into solid particles through heating droplets -> evaporation -> solute condensation -> decomposition & reaction -> sintering e.g. silver particle: Ag2CO3, Ag2O and AgNO3 with NH4HCO3 @ 400°C

Template-based synthesis Templates cation exchange resins with micropores zeolites silicate glasses ion exchange gas deposition on shadow mask (template)

Core-shell nanoparticles The growth condition control no homogeneous nucleation occur and only grow on the surface concentration control: not high enough for nucleation but high enough for growth drop wise addition temperature control

Semiconductor industry

Semiconductor industry