Materials chemistry

Metal nanoparticles, quantum dots

Nanoscale dimensions reshape optical response: metal particles support localized plasmons, while semiconductor quantum dots show size-dependent electronic transitions.

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IntuitionIntuition: size changes what light can do

A gold colloid can look ruby-red rather than metallic yellow because its small particles collectively oscillate with the optical electric field. A semiconductor nanocrystal is different: confining an electron and a hole to a small domain raises the energy of its lowest optical transition, often shifting emission toward blue as the dot becomes smaller.

Illustrative plasmon absorption and size-dependent quantum-dot absorption and photoluminescence bands; curves are schematic, not reference spectra.

SchoolSchool: two kinds of nanoscale optical response

Definition: Localized surface plasmon resonance (LSPR)

A resonant collective oscillation of conduction electrons in a metal nanoparticle driven by incident light. Its wavelength and width depend on composition, shape, size, surroundings and interparticle coupling.

Definition: Quantum confinement

When a semiconductor domain approaches the exciton Bohr radius, boundary confinement changes its allowed electron and hole states. This is not simply a classical particle-size colour effect.

Contrasting mechanisms
Metal nanoparticle	Semiconductor quantum dot
Collective conduction-electron response	Discrete, confinement-shifted electron–hole transitions
Often strong absorption / scattering band	Band-edge absorption and photoluminescence

For a small metal sphere in a dielectric, a simplified quasistatic resonance condition is Re[ε(ω)] ≈ −2ε_m. This approximation assumes particle dimensions much smaller than the wavelength; retardation and multipoles matter for larger particles, and the surrounding medium can shift the resonance.

UndergraduateUniversity: confinement raises the optical gap

E(R) \approx E_{g,\mathrm{bulk}} + \frac{\hbar^2\pi^2}{2R^2}\left(\frac{1}{m_e^*}+\frac{1}{m_h^*}\right) - \frac{1.8e^2}{4\pi\varepsilon_0\varepsilon_r R}

This effective-mass, spherical-dot expression (Brus model) combines kinetic confinement of electron and hole with their Coulomb attraction. It is qualitative when the radius R is comparable to atomic bond lengths, dielectric screening is nonuniform, or surface states dominate; real spectra also include exciton fine structure and phonon broadening.

Example: A confinement trend

A CdSe dot is made smaller while composition and surface passivation are held comparable. Predict the direction of its first-exciton absorption shift.

Solution

The confinement term varies approximately as R⁻² and grows as R decreases. The optical transition therefore generally shifts to higher energy (shorter wavelength, a blue shift), subject to changes in surface chemistry and dielectric environment.

References

Electronic-electronic and electron-hole interactions in small semiconductor crystallites: The size dependence of the lowest excited electronic state · Louis E. Brus, 1984
Plasmonics: Fundamentals and Applications · Stefan A. Maier, 2007