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

vG

Homogeneous nucleation

• Energy barrier 2*

)3(

16

vGG

vGr

2*

Surface energy

Gibss free energy change

Nuclei

• formation favor:– 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– 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

HClHCHORhOHCHRhCl 32

3

2

333

ClNaCOHOHPdOHCONaPdCl 22)(2 3222322

OHPdHOHPd 222 2)(

ClClOHPtOHPtCl 3222

4 )( ClClOHPtOHClOHPt 222232 )()(

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– 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– e.g.

• Condensation of precursors– e.g.

• typical precursors: metal alkoxides or inorganic and organic salts

xEtOHOHOEtMOxHOEtM xx )()()( 424

OHOHOEtMOMOHOEt

OHOEtMOHOEtM

xxxx

xxxx

21414

44

)()()()(

)()()()(

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

water

Vigorous stirring

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:– 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

• micelles– 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– 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