Classification of Nanostructured Materials: 0D (Quantum Dot)

Classification of Nanostructured Materials: 0D (Quantum Dot)

1. Dimensionality Classification (confinement)

Nanomaterials classified by how many dimensions are confined to $1$–$100$ nm (free in others):

Type	Confined Dimensions	Free Dimensions	Example	Quantum Confinement
0D	3 (all)	0	Quantum dot (dot ~2–10 nm), nanoparticle	3D confined — discrete levels (artificial atom)
1D	2	1	Nanowire, nanotube (diameter nano, length µm)	2D confined
2D	1	2	Nanosheet, graphene (thickness 0.34 nm), thin film	1D confined
3D	0 (?)	3 but nanostructured	Nanocrystalline bulk, porous nano	Grain boundaries nano

0D means zero dimensions free — particle confined in all three directions.

2. 0D — Quantum Dot (QD) Details

• Size: typically $2$–$10$ nm, contains $10^2$–$10^5$ atoms.
• Structure: semiconductor core (CdSe, InP, PbS) with shell (ZnS) and ligands; or metal dot.
• Electronic structure: fully quantized energy levels like particle in 3D box:

⇒ discrete like atom, hence artificial atom. Emission colour tunable by size (as earlier).

• Density of states: delta functions at discrete energies vs bulk continuous $∝√E$.

3. Properties of 0D

• Optical: size-tunable fluorescence (CdSe 2 nm blue, 6 nm red) with narrow emission, high quantum yield — used for QLED displays, bio-labels.
• Electronic: Coulomb blockade at room $T$ if small ($e²/2C ≫kT$).
• No conduction without hopping — isolated dot needs tunneling to neighbours.

4. Worked Contrast

Bulk CdSe E_g=1.74 eV (713 nm) continuous bands
QD 3 nm: E_g≈2.2 eV (564 nm green) discrete levels

TEM shows ~5 nm dots; absorption peak sharp vs bulk broad.

5. Exam Points

• Define 0D as confined in all three dimensions, give quantum dot example — 3-5 marks, very frequent (classification question).
• Mention discrete levels and $E∝1/R²$ — 3-5 marks, frequent.
• Give applications (displays, bio-labels) — 3 marks.
• Distinguish from 1D/2D — 3 marks.