optical/electrical/mechanical properties at the nanoscale

optical/electrical/mechanical properties at the nanoscale

1. Optical Properties (size-tunable)

• Quantum confinement widens $E_g$ as $∝1/L²$ ⇒ absorption/emission blue-shifts as size shrinks (CdSe dots: red→blue with $5→2$ nm).
• Surface plasmon resonance in metal nanoparticles (Au, Ag) shifts with size/shape ⇒ colour tunable (colloidal gold red vs blue aggregates).
• Transparency: nano suspensions may be transparent if particles ≪λ (Rayleigh scattering $∝1/λ⁴$ small).

2. Electrical Properties

• Conductivity: may drop due to surface scattering (nanowire $ρ$ higher) or change from metal to semiconductor (thin film becomes gapped).
• Coulomb blockade: single-electron charging energy $e²/2C$ (~ $e²/(4π ε r)$) dominates at $r∼nm$ ⇒ current blocked until $V > e/2C$ (single-electron transistor).
• Ballistic transport: when $L$ < mean free path (~10 nm in Cu), electrons travel without scattering ⇒ quantized conductance $G=n·2e²/h$ in nanowire.

3. Mechanical Properties

• Strength/hardness: increases at nano (Hall-Petch: $strength∝1/√grain size$ down to ~10 nm, then softens)
  • Nanowires defect-free ⇒ near theoretical strength (~10 GPa for Au nanowire vs 0.1 GPa bulk).
• Elastic modulus may slightly decrease due to surface softness.
• Superplasticity: nanograined metals can deform 1000% without breaking at lower $T$.

4. Other Properties (quick)

• Thermal: melting point ↓ as $1/r$, thermal conductivity ↓ due to boundary scattering.
• Magnetic: superparamagnetism at <10 nm (single domain flips thermally).

5. Worked Summary

Bulk Si E_g=1.12 eV (IR) → Si quantum dot 2 nm E_g≈2.5 eV (green) due to confinement ⇒ electroluminescence.

Au nanowire 50 nm dia: resistivity ∼10× bulk due to surface scattering; but Au nanoparticle plasmon peak shifts from 520 nm (bulk-like) to 600 nm aggregated.

Nanograin Cu grain 20 nm: hardness 2× bulk (Hall-Petch).

6. Exam Points

• List optical (blue shift, plasmon), electrical (conductivity, Coulomb blockade), mechanical (strength ↑, Hall-Petch) changes — 5-7 marks, very frequent (this is the "properties at nanoscale" question).
• Give one example each — 5 marks, frequent.
• Mention $E_g∝1/L²$ and $S/V$ as reasons — 3-5 marks.