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Quantum dots (QDs) or semiconductor nanocrystals are semiconductor particles a few nanometres in size with optical and electronic properties that differ from those of larger particles via quantum mechanical effects. They are a central topic in nanotechnology and materials science.

When a quantum dot is illuminated by ultraviolet light, an electron can be excited from the valence band to the conduction band. The excited electron can then recombine with a hole in the valence band, releasing its energy as light (photoluminescence). The emitted color depends on the energy difference between discrete quantum states.

Introduction

A quantum dot can be defined as a semiconductor structure that confines electrons or holes in all three spatial dimensions, producing discrete energy levels. This confinement resembles a three-dimensional particle in a box model.

Because of this, quantum dots behave similarly to atoms and are often referred to as artificial atoms.^[1]^[2]

The electronic wave functions in quantum dots resemble those in real atoms, reinforcing their atomic-like behavior.^[3]

Quantum confinement

The defining property of quantum dots is the quantum confinement effect. As the size of the quantum dot decreases:

Energy levels become discrete
The effective band gap increases
Emission shifts toward shorter wavelengths (blue shift)

Larger quantum dots (≈5–6 nm) emit red/orange light, while smaller dots (≈2–3 nm) emit blue/green light.

The absorption and emission spectra correspond to transitions between quantized energy levels, similar to atomic spectra. This makes quantum dots highly tunable optical materials.

Optical and electronic properties

Quantum dots exhibit properties intermediate between bulk semiconductors and atoms. Their optoelectronic behavior depends strongly on size, shape, and composition.^[4]

Key features include:

Narrow emission spectra
Size-tunable fluorescence
High quantum yield
Discrete energy levels

The emission energy depends on parameters such as:

Dot size
Band gap energy
Effective electron and hole masses

Core–shell and heterostructures

Quantum dots are often engineered as core–shell nanostructures to improve optical performance.

In these systems:

A semiconductor core is surrounded by a shell with a larger band gap
Surface defects are passivated
Non-radiative recombination is reduced

There are four main types:

Type I
Inverse Type I
Type II
Inverse Type II

Core–shell structures allow tuning of emission wavelength and efficiency. However, lattice mismatch between materials can introduce strain, affecting performance.

Double-shell systems such as CdSe/ZnSe/ZnS improve:

Fluorescence efficiency
Stability against photo-oxidation

Surface passivation using ligands (e.g. oleic acid) further enhances stability, though it may reduce photoluminescence efficiency.

Production

Quantum dots can be produced using several methods:

Colloidal synthesis

A solution-based method where precursors decompose to form nanocrystals. Growth is controlled by temperature and monomer concentration.

This method enables:

Precise size control
Large-scale production
Monodisperse particles

Common materials include:

CdSe, CdS, PbS, PbSe
InAs, InP
Perovskite quantum dots

Plasma synthesis

A gas-phase method allowing control of size, composition, and doping.

Self-assembly

Quantum dots can form spontaneously due to lattice mismatch during epitaxial growth (Stranski–Krastanov mode).

Lithographic fabrication

Quantum dots can be defined using nanofabrication techniques and gate electrodes in semiconductor devices.

Applications

Quantum dots are widely used due to their tunable properties:

LEDs and displays (QLED technology)
solar cells
quantum computing and qubits
single-electron transistors
lasers and single-photon sources
medical imaging and biological labeling

Their ability to emit specific wavelengths makes them ideal for high-color-accuracy displays and optical devices.

Optical properties

Quantum dots have highly tunable optical behavior due to confinement of excitons.

An exciton consists of:

An excited electron
A hole in the valence band

These are bound by Coulomb interaction. When the dot size approaches the exciton Bohr radius, confinement increases the band gap energy.

As a result:

Smaller dots → higher energy emission
Larger dots → lower energy emission

Fluorescence lifetime also depends on size, with larger dots showing longer lifetimes.

Health and safety

Some quantum dots, particularly those containing cadmium, may pose health and environmental risks.

Toxicity depends on:

Size and composition
Surface chemistry
Environmental conditions

Under certain conditions (e.g. UV exposure), quantum dots can release toxic ions or generate reactive oxygen species.

Research continues into safer alternatives such as:

Carbon quantum dots
Cadmium-free nanocrystals

Table of contents (138 articles)

Index

Core theory

Foundations
Conceptual and interpretations
Mathematical structure and systems
Atomic and spectroscopy
Wavefunctions and modes
Quantum dynamics and evolution
Measurement and information
Quantum information and computing

Applications and extensions

Quantum optics and experiments
Open quantum systems
Quantum field theory
Statistical mechanics and kinetic theory
Condensed matter and solid-state physics
Plasma and fusion physics
Timeline
Advanced and frontier topics

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Physics:Quantum Semiconductor physics

Introduction

Quantum confinement

Optical and electronic properties

Core–shell and heterostructures

Production

Colloidal synthesis

Plasma synthesis

Self-assembly

Lithographic fabrication

Applications

Optical properties

Health and safety

See also

Table of contents (138 articles)

Index

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