Nanoparticles

What is important in NP fabrication

keeping size small and consistent, stabilising against aggregation

Why are NPs special

large SA/V ratio increasing surface energy and stronger VdW, dipole and capillary forces; confinement changes properties (eg ferromagnetic NPs become super-paramagentic)

What are the contributions to the energy required for new surface area gamma?

bond energy, stress, atomic reconfigurations, saturation of dangling bonds

Give the equation describing the energy required for new surface area due to breaking bonds

N_S * U_b/2, N_S=new surface atoms per m², U_b=bond energy

Give the equation describing the energy required for new surface area due to stress γ_s

~sigma = F/A

How does surface energy vary for a spherical NP

1/r

What are the volumetric forces on a NP

gravity, magnetic dipole force (on magnetic NP), Lorentz force, all related by r³

What are the surface forces on a NP and how does each vary with r (particle radius)

VdWs r, surface charge (dipole) r², surface bonds r², so VdW dominates for smaller radii

How is NP shape determined

growth occurs in lowest energy way, some crystal planes more energetically favourable dictating shape; 5-fold symmetry can occur unlike in macroparticles

Describe particle compression (Laplace pressure)

unsaturated surface bonds pull in to minimise energy; compressive strain shortens bonds (~1-5%) more at surface; can affect stability, Tm and more severe in smaller NPs

Describe particle expansion

longer lattice parameters can be caused by: high vacancy density, redox widening, reactions with environment, non-stoichiometry due to oxygen loss; can be greater effect than compression

Describe the structure dynamics of NPs

high mobility of surface atoms, poor thermal dissipation due to small volume

What is quasi-melting

below ~10nm NP can rapidly change shape with fluidity but atom movement not random; can fluctuate between morphologies, influenced by local heating (eg e beam in TEM)

How does melting temperature change with particle size

smaller particles have lower Tm as surface atoms are less tightly bound and more mobile; down to 100nm Tm stable, 100-10nm down by 10%, 10-2nm down by 50%

Plasmonics in metal NPs

incident light can generate plasmons (e excitations); incident e can generate strong light emission ‘cathodoluminescence’

Why do plasmons change the colour of NPs

when the plasmons oscillate in the NP at the same frequency as the light that wavelength is preferentially absorbed changing the colour of the NP

How do shape and size affect surface plasmons

both change the resonant frequency, shape/aspect ratio has greatest effect; different to SC where bandgap determines optics

What affects the NP colour

relative permittivity, free-electron metals give stronger effect as plasmon excitation has longer lifetime producing sharper resonance peak; Ag and Au are best