Physics:Quantum Semiconductor physics: Difference between revisions
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'''Quantum dots''' ('''QDs''') or '''semiconductor nanocrystals''' are | <div style="display:flex; gap:24px; align-items:flex-start; max-width:1200px;"> | ||
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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. | |||
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[[File:Quantum_dots_UV_fluorescence_yellow_bg.jpg|thumb|280px|Quantum Semiconductor physics.]] | |||
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== Introduction == | == 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 | 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> | 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 | The electronic wave functions 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 == | == Quantum confinement == | ||
| Line 19: | Line 36: | ||
* Energy levels become discrete | * Energy levels become discrete | ||
* The effective | * The effective band gap increases | ||
* Emission shifts toward shorter wavelengths (blue shift) | * Emission shifts toward shorter wavelengths (blue shift) | ||
| Line 28: | Line 45: | ||
== Optical and electronic properties == | == Optical and electronic properties == | ||
Quantum dots exhibit properties intermediate between bulk semiconductors and atoms. Their | 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: | Key features include: | ||
| Line 95: | Line 112: | ||
Quantum dots are widely used due to their tunable properties: | 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. | Their ability to emit specific wavelengths makes them ideal for high-color-accuracy displays and optical devices. | ||
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== Optical properties == | == Optical properties == | ||
Quantum dots have highly tunable optical behavior due to confinement of | Quantum dots have highly tunable optical behavior due to confinement of excitons. | ||
An exciton consists of: | An exciton consists of: | ||
| Line 112: | Line 129: | ||
* A hole in the valence band | * A hole in the valence band | ||
These are bound by Coulomb interaction. When the dot size approaches the exciton | These are bound by Coulomb interaction. When the dot size approaches the exciton Bohr radius, confinement increases the band gap energy. | ||
As a result: | As a result: | ||
| Line 122: | Line 139: | ||
== Health and safety == | == Health and safety == | ||
Some quantum dots, particularly those containing | Some quantum dots, particularly those containing cadmium, may pose health and environmental risks. | ||
Toxicity depends on: | Toxicity depends on: | ||
| Line 129: | Line 146: | ||
* Environmental conditions | * Environmental conditions | ||
Under certain conditions (e.g. UV exposure), quantum dots can release toxic ions or generate | 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: | Research continues into safer alternatives such as: | ||
Latest revision as of 22:00, 20 May 2026
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
See also
Table of contents (198 articles)
Index
Full contents
References
- ↑ Silbey, Robert J.; Alberty, Robert A.; Bawendi, Moungi G. (2005). Physical Chemistry (4th ed.). John Wiley & Sons. p. 835.
- ↑ Ashoori, R. C. (1996). "Electrons in artificial atoms". Nature 379: 413–419. doi:10.1038/379413a0.
- ↑ Banin, Uri; Cao, YunWei; Katz, David; Millo, Oded (1999). "Identification of atomic-like electronic states in indium arsenide nanocrystal quantum dots". Nature 400: 542–544.
- ↑ Murray, C. B.; Kagan, C. R.; Bawendi, M. G. (2000). "Synthesis and Characterization of Monodisperse Nanocrystals". Annual Review of Materials Research 30: 545–610.
Source attribution: Physics:Quantum Semiconductor physics