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
| Material | Emission Range | Application |
|----------|---------------|-------------|
| CdSe/ZnS | 450–650 nm (visible) | Displays, biological imaging |
| InP/ZnS | 500–700 nm | Cd-free displays (Samsung) |
| PbS/PbSe | 800–2000 nm (NIR/IR) | Solar cells, IR detectors |
| Si QDs | 600–900 nm | Biocompatible imaging |
| Perovskite QDs | 400–800 nm | Displays, LEDs |
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.