Quantum Dots are semiconductor nanocrystals (2–10 nm diameter) that exhibit quantum confinement effects — confining electrons and holes in all three dimensions to produce size-tunable optical and electronic properties used in displays, solar cells, biological imaging, and single-photon sources for quantum computing.

Quantum Confinement

• When particle size approaches the exciton Bohr radius (~5 nm for CdSe), bulk band structure breaks down.
• Energy levels become discrete (like an atom) rather than continuous bands.
• Smaller dot → larger bandgap → bluer emission:
• 2 nm CdSe: Blue (~450 nm)
• 4 nm CdSe: Green (~530 nm)
• 6 nm CdSe: Red (~620 nm)
• Bandgap: $E_g \approx E_{g,bulk} + \frac{\hbar^2 \pi^2}{2 m^* r^2}$ (particle-in-a-box model)

Common QD Materials

QD Display Technology

• QD Enhancement Film (QDEF): QD film converts blue LED backlight to pure red and green — wider color gamut.
• QD-OLED: Samsung — blue OLED excites QD color converters for each sub-pixel.
• QD-LED (Electroluminescent): Direct electrical excitation of QDs — next generation, no OLED needed.

Synthesis

• Hot Injection: Precursors rapidly injected into hot coordinating solvent → uniform nucleation.
• Heat-Up: Gradual temperature ramp — more scalable for manufacturing.
• Size Control: Reaction time and temperature control diameter — narrow size distribution (< 5% σ) enables pure color emission.

Beyond Displays

• Solar Cells: Multi-exciton generation and tunable bandgap for tandem cells.
• Quantum Computing: Self-assembled InAs/GaAs QDs as single-photon sources.
• Biological Imaging: QD fluorophores — brighter, more stable than organic dyes.

Quantum dots are a textbook example of nanotechnology enabling tunable material properties — their size-dependent bandgap makes them the material platform of choice for next-generation displays, photovoltaics, and quantum information technologies.