Nanoparticles

1
Nanoparticles

• Lecture 2
• ???

2
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)

3
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

4
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)

5
Homogeneous nucleation

• Driving force

Fig 3.1
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Homogeneous nucleation
Surface energy

• Energy barrier

Gibss free energy change
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Nuclei

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

8
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

9
Diffusion-limited growth

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

10
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
• 100C till color change water to maintain
  volume
• uniform and stable 20 nm particles

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Table 3.1
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Other cases
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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

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Fig 3.10
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Fig 3.12
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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)

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stabilizer concentration
18
temperature
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Semiconductor nanoparticles

• Pyrolysis (???) 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

20
Process

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

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Semiconductor nanocrystallites

• C.B. Murray (CdE, ES, 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
  230-260C (1.511.5 nm)
• Size-selective precipitation

22
Oxide nanoparticles

• Several methods
• principles burst of homogeneous nucleation
  diffusion controlled growth
• most commonly sol-gel processing
• most studied silica colloids

23
Sol-gel process

• Synthesis
• inorganic and organic-inorganic hybrid materials
  colloidal dispersions
• powders, fibers, thin film and monolith(??)
• 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

24
Sol-gel process

• Hydrolysis
• e.g.
• Condensation of precursors
• e.g.
• typical precursors metal alkoxides or inorganic
  and organic salts

25
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 lt
  Si(OCH3)4) )
• chemically modify the coordination state of the
  alkoxides
• multiple step sol-gel

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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

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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

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Sol-gel example silica

• Precursors
• silicone alkoxides with different alkyl ligand
  sizes
• catalyst
• ammonia
• solvent
• various alcohols

Vigorous stirring
water
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Vapor phase reactions

• Same mechanism as liquid phase reaction
• Elevated temperatures vacuum (low concentration
  of growth)
• Collection on a down stream non-sticking
  substrate _at_ low temperature
• example 23 nm silver particles
• may migrate and agglomerate

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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

31
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

32
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!)

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Solid state phase segregation
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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)

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Heterogeneous nucleation
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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

37
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

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Micelle

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

39
Polymer nanoparticles

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

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Aerosol synthesis

• Characteristics
• Regarded as top-down (maybe?)
• can be polycrystalline
• needs collection and redispersion
• process
• liquid precursor -gt mistify -gt liquid aerosol -gt
  evaporation or reaction -gt nanoparticles
• polymer particle 120 µm (from monomer droplets)

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Size control by termination

• Termination by organic components or alien ion
  occupation

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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 -gt evaporation -gt solute condensation
  -gt decomposition reaction -gt sintering
• e.g. silver particle Ag2CO3, Ag2O and AgNO3 with
  NH4HCO3 _at_ 400C

43
Template-based synthesis

• Templates
• cation exchange resins with micropores
• zeolites
• silicate glasses
• ion exchange
• gas deposition on shadow mask (template)

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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

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Semiconductor industry
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Semiconductor industry