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{{Quantum book backlink|Condensed matter and solid-state physics}}

'''Quantum dots''' ('''QDs''') or '''semiconductor nanocrystals''' are [[semiconductor]] particles a few [[nanometre]]s in size with [[optical]] and [[electronic]] properties that differ from those of larger particles via [[quantum mechanics|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 [[electron hole|hole]] in the valence band, releasing its energy as light ([[photoluminescence]]). The emitted color depends on the energy difference between discrete quantum states.
[[File:Quantum_dots_UV_fluorescence_yellow_bg.jpg|thumb|400px|Colloidal quantum dots irradiated with ultraviolet light. Differently sized quantum dots emit different colors due to [[quantum confinement]].]]
== 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'''.<ref>{{cite book |last1=Silbey |first1=Robert J. |last2=Alberty |first2=Robert A. |last3=Bawendi |first3=Moungi G. |title=Physical Chemistry |edition=4th |publisher=John Wiley & Sons |year=2005 |page=835}}</ref><ref>{{cite journal |last=Ashoori |first=R. C. |year=1996 |title=Electrons in artificial atoms |journal=Nature |volume=379 |pages=413–419 |doi=10.1038/379413a0}}</ref>

The electronic [[wave function]]s in quantum dots resemble those in real atoms, reinforcing their atomic-like behavior.<ref>{{cite journal |last1=Banin |first1=Uri |last2=Cao |first2=YunWei |last3=Katz |first3=David |last4=Millo |first4=Oded |title=Identification of atomic-like electronic states in indium arsenide nanocrystal quantum dots |journal=Nature |volume=400 |pages=542–544 |year=1999}}</ref>

== 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.<ref>{{cite journal |last1=Murray |first1=C. B. |last2=Kagan |first2=C. R. |last3=Bawendi |first3=M. G. |title=Synthesis and Characterization of Monodisperse Nanocrystals |journal=Annual Review of Materials Research |volume=30 |pages=545–610 |year=2000}}</ref>

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:

* [[Light-emitting diode|LEDs]] and displays (QLED technology)
* [[Quantum dot solar cell|solar cells]]
* [[quantum computing]] and qubits
* [[single-electron transistor]]s
* [[laser]]s and [[single-photon source]]s
* [[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 [[exciton]]s.

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

== See also ==
{{#invoke:PhysicsQC|tocHeadingAndList|Physics:Quantum basics/See also}}

= References =
{{reflist|3}}

{{Author|Harold Foppele}}
{{Sourceattribution|Physics:Quantum Semiconductor physics|1}}

Physics:Quantum Semiconductor physics - Revision history

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