quantum dots (original) (raw)
Author: the photonics expert (RP)
Definition: microscopic structures confining charge carriers in three dimensions
Categories: 
photonic devices, 
quantum photonics
Related: quantum wells
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DOI: 10.61835/7fn Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn
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Contents
What Are Quantum Dots?
A quantum dot is a nanoscale structure, e.g. a semiconductor nanocrystal which can confine electrons or other carriers in all three spatial dimensions. As a result of this three-dimensional confinement, quantum dots exhibit discrete, atom-like electronic states rather than the continuous energy bands characteristic of bulk semiconductor materials. That corresponds to a delta-shaped density of states with no states in between the delta peaks.
Quantum dots are often referred to as “artificial atoms” due to the type of their electronic structure with discrete energy levels. In contrast to atoms, however, the electronic structure of quantum dots can be engineered by adjusting their size, shape, and composition. The confinement potential is defined by material interfaces and differences in band gap energy. Generally, the band gap of a quantum dot has to be lower than that of the surrounding medium.
A quantum dot is usually embedded in another semiconductor material or in a glass, but there are also colloidal quantum dots in a liquid (a colloidal suspension).
One often uses large ensembles of quantum dots, which exhibit some size distribution. That leads to a somewhat smeared-out density of states, i.e. to inhomogeneous broadening. However, there are also applications of single quantum dots, for example in single-photon sources.
Fabrication of Quantum Dots
Quantum dots can be fabricated by several techniques, each suited for different applications:
Epitaxial Growth
Quantum dots can be formed even unintentionally during semiconductor growth when quantum wells display thickness fluctuations. More deliberately, lattice-mismatched heteroepitaxy can induce self-assembled quantum dots through the Stranski–Krastanov growth mode. For example, indium arsenide (InAs) grown on gallium arsenide (GaAs) spontaneously forms nanoscale islands that act as QDs. These islands are later overgrown with the surrounding semiconductor (often GaAs), yielding buried dots. Typical dimensions range from 5–20 nanometers, with high surface densities (∼109–1011 dots per cm²). Such self-assembled dots are widely studied for optoelectronic devices.
Colloidal Quantum Dots
In wet-chemical synthesis, precursor compounds are heated in an organic solvent until supersaturated conditions favor nucleation and growth of nanocrystals. Careful control of reaction chemistry stabilizes the growth of nanocrystals such that these acquire specific sizes with narrow size distributions. These colloidal QDs, often capped with organic ligands for passivation, can be dispersed in inks and deposited on substrates by spin-coating, printing, or spray methods.
The major advantages of colloidal synthesis are scalability and low fabrication cost. Quantum dot films can be integrated into large-area devices, such as displays or lighting panels, at relatively low cost.
Lithographic and Electrostatic Techniques
In addition to epitaxial and chemical growth, lithographically patterned substrates or electrostatic gating in two-dimensional electron gases (2DEGs) allow highly controlled positioning and manipulation of QDs. Such approaches for deterministic placement are important for quantum computing and single-dot spectroscopy.
Applications
Quantum dots have a broad range of current and potential applications:
- Art & ancient applications: Historically, nanoscale semiconductor particles contributed to the colored appearances of certain stained glasses, where size-dependent plasmonic or band-gap effects determined hues — of course, without the users understanding the underlying physics.
- Quantum-dot lasers: Semiconductor laser diodes incorporating QDs offer distinct advantages such as reduced threshold current density, narrower linewidth, higher temperature stability, and potential for low-energy optical communication.
- Quantum-dot LEDs (QD-LEDs): Quantum dots can serve as emitters in light-emitting diodes (LEDs), where the emission wavelength can be tuned in fabrication through the QD dimensions. In down-conversion QD-LEDs, a conventional blue or UV LED excites a quantum dot film that re-emits red, green, or mixed colors, enabling efficient and high-quality white light or vivid color displays. Electroluminescent QD-LEDs, where QDs are directly injected with carriers, are also being pursued for next-generation flat-panel displays.
- Saturable absorbers: In semiconductor saturable absorber mirrors (SESAMs), QDs act as saturable absorbers with low saturation fluence, enabling passive mode locking in ultrafast lasers.
- Photodetectors and photovoltaics: Quantum dots’ tunable absorption spectrum allows them to function as wavelength-selective, high-sensitivity photodetectors.
- Photovoltaics may in the future use QDs for multi-exciton generation and hot-carrier harvesting, offering the prospect of higher-efficiency solar cells.
- Quantum information science: Single quantum dots can act as deterministic single-photon sources or spin qubits, which can be used for quantum communication, quantum cryptography, and scalable quantum computing.
- Biomedical imaging: Colloidal QDs are used as fluorescent biomarkers, exploiting their bright, stable, size-tunable emission for tracking molecules and cells. Surface functionalization allows them to target specific biomolecules.
Technological Outlook
The most commercially advanced applications so far are in displays and lighting (QD-LED TVs, QD phosphors) and laser diodes. Research efforts continue into using QDs for quantum technologies, solar energy, and secure communications.
The field of quantum nanophotonics integrates QDs with tiny optical resonators, waveguides and plasmonic structures, aiming to exploit strong light–matter interactions at the nanoscale.
As methods for scalable fabrication, uniformity control, and environmental stability improve, quantum dots are expected to play an increasingly central role in optoelectronics, photonics, and quantum information processing.
Bibliography
| [1] | Y. Arakawa and H. Sakaki, “Multidimensional quantum well laser and temperature dependence of its threshold current”, Appl. Phys. Lett. 40 (11), 939 (1982); doi:10.1063/1.92959 |
|---|---|
| [2] | D. L. Huffaker et al., “1.3-μm room-temperature GaAs-based quantum-dot laser”, Appl. Phys. Lett. 73 (18), 2564 (1998); doi:10.1063/1.122534 |
| [3] | G. T. Liu et al., “Extremely low room-temperature threshold current density diode lasers using InAs dots in In_{0.15}Ga_{0.85}As quantum well”, Electron. Lett. 35, 1163 (1999); doi:10.1049/el:19990811 |
| [4] | H. Y. Liu et al., “High-performance three-layer 1.3-μm InAs–GaAs quantum dot lasers with very low continuous-wave room-temperature threshold currents”, IEEE Photon. Technol. Lett. 17 (6), 1139 (2005); doi:10.1109/LPT.2005.846948 |
| [5] | M. Scholz et al., “Non-classical light emission from a single electrically driven quantum dot”, Opt. Express 15 (15), 9107 (2007); doi:10.1364/OE.15.009107 |
| [6] | S. Qingjiang et al., “Bright, multicoulored light-emitting diodes based on quantum dots”, Nature Photon. 2, 717 (2008); doi:10.1038/nphoton.2007.226 |
| [7] | P. Morena et al., “Modeling of gain and phase dynamics in quantum dot amplifiers”, Opt. Quantum Electron. 40 (2-4), 217 (2008); doi:10.1007/s11082-008-9219-4 |
| [8] | K.-S. Cho et al., “High-performance crosslinked colloidal quantum-dot light-emitting diodes”, Nature Photon. 3, 341 (2009); doi:10.1038/nphoton.2009.92 |
| [9] | M. Toishi et al., “High-brightness single photon source from a quantum dot in a directional-emission nanocavity”, Opt. Express 17 (17), 14618 (2009); doi:10.1364/OE.17.014618 |
| [10] | W. W. Chow, M. Lorke and F. Jahnke, “Will quantum dots replace quantum wells as the active medium of chose in future semiconductor lasers?”, J. Sel. Top. Quantum Electron. 17 (5), 1349 (2011); doi:10.1109/JSTQE.2011.2157085 |
| [11] | F. Yue et al., “Stimulated emission from PbS-quantum dots in glass matrix”, Laser & Photon. Rev. 7 (1), L1 (2013); doi:10.1002/lpor.201200075 |
| [12] | H. Jung, N. Ahn and V. I. Klimov, “Prospects and challenges of colloidal quantum dot laser diodes” (review article), Nature Photonics 15, 643 (2021); doi:10.1038/s41566-021-00827-6 |
| [13] | G. L. Whitworth et al., “Solution-processed PbS quantum dot infrared laser with room-temperature tunable emission in the optical telecommunications window”, Nature Photonics 15, 738 (2021); doi:10.1038/s41566-021-00878-9 |
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