Unique Properties: Comparison of bulk and nanomaterials

Unique Properties: Comparison of bulk and nanomaterials

1. What Changes at Nanoscale — General Trend

When size reduces from bulk (µm–mm) to nano (1–100 nm), many properties (not just one) shift due to surface dominance and confinement:

Property	Bulk Behaviour	Nanomaterial (≤100 nm) Trend	Reason
Mechanical strength/hardness	moderate, dislocations move	much higher strength, super-hard / super-plastic depending	small grains block dislocations, fewer defects
Melting point	fixed (e.g., Au 1064°C)	decreases sharply (<100 nm)	surface atoms weakly bound, $T_m∝1/r$
Electrical conductivity	bulk metal high	may decrease (scattering) or become size-tunable semiconductor (quantum dot)	confinement opens gap
Optical absorption/emission	fixed band gap ⇒ fixed colour	tunable colour with size (quantum dot)	$E_g∝1/L²$
Magnetic	ferromagnetic bulk	may become superparamagnetic (<10 nm, thermal flips)	single domain, $kT$ vs anisotropy
Chemical reactivity / catalysis	low surface per mass	hugely enhanced (Au becomes catalyst)	surface atoms many, dangling bonds
Thermal conductivity	bulk value	often reduced (phonon boundary scattering)	mean free path comparable to size

2. Example — Gold

• Bulk: yellow, inert, $T_m=1064°C$, conductor.
• Nano 10 nm: red, catalytic, $T_m≈900°C$, plasmon peak shifts with size.

3. Why Bulk Laws Fail

Bulk uses statistical averages over $10^{23}$ atoms; nano has few atoms, surface dominates, quantum levels discrete ($E_n=h²n²/8mL²$ as in particle in box) — continuum approximations break.

4. Exam Points

• List comparison table with 4–5 properties (mechanical, thermal, optical, electrical, magnetic) — 5-7 marks, very frequent (the main bulk vs nano question).
• Explain surface and confinement reasons — 5 marks, frequent.
• Give gold example — 3 marks.