Quantum dot technology
Quantum Dots
Colloidal quantum dots (QDs) are nanoscale semiconductor crystals with unique optical properties that obey quantum mechanics. They are typically 2-10 nm in diameter and composed of about 10-50 atoms. Generally, due to the quantum size effect, the band gap can be tuned depending on the size of the colloidal nanocrystal, resulting in characteristic emission properties that depend on the grain size, converting the spectrum of incident light into energy of different frequencies. As the grain size decreases, the band gap expands, and the emission wavelength moves toward the shorter wavelength side (Figure 1). Considering its electronic state, it is also known as an artificial atom because its properties are closer to those of an atom than to a bulk material.
Figure 1 (A) Quantum dot size vs. energy level. The energy level splitting is due to the quantum size effect; as the nanocrystal size decreases, the semiconductor band gap increases. (B) Structure of CdSe core-type and CdSe/ZnS core/shell-type quantum dots.
Quantum dots not only have tunable emission wavelengths in the visible to infrared regions and narrow spectral half-widths but also exhibit high quantum efficiency while absorbing light at a wide range of wavelengths. Materials can be designed whose chemical, physical, electrical, and optical properties can be tuned by manipulating their energetic state and charge interactions through composition, structure, surface state, crystallinity, and ligands. Their applications range from biological imaging, illumination, and displays to solar cells, security tags, sensors, and quantum information technology, and they are being actively researched and developed for use in a wide variety of applications.1-7,25,30,48
Quantum dots can be fabricated on substrates using lithography, MOCVD, MBE, or other methods of processing in a vacuum environment or vapor phase growth, or colloidal particles by liquid phase synthesis under mild conditions. The former method yields highly crystalline QDs that are precisely controlled and applied to high-performance lasers and integrated circuits. The latter method is relatively easy to synthesize in large quantities and is expected to be used for large-area applications by low-cost printing methods.
The tunable surface chemistry of colloidal quantum dots enables the conjugation of QDs with small molecules, antibodies, and proteins, which, coupled with the excellent optical properties of the QDs, can be used for biosensors, fluorescent labeling, and biological imaging. Surface ligands also play an important role in determining the optoelectronic properties, stability, dispersibility in solvents, functionalization, and charge transport properties of QDs in thin films. Ligand selection during the synthesis process as well as ligand exchange techniques in the liquid and solid phases after synthesis and deposition have been extensively investigated.49,50
Common quantum dots are semiconductors made of cadmium and lead, but the use of these heavy metals is regulated in many applications. Therefore, cadmium-free quantum dots are being developed that retain the same brightness and stability as their conventional counterparts. There is also an active search for synthesis methods that can produce large quantities of laboratory-grade quality at low cost and with good reproducibility. Although the use of quantum dots in markets where small amounts of materials are required for bio-imaging and other applications has been prevalent, their use in displays, lasers, and other electronics fields has begun in earnest with the recent development of high-performance and mass-production technologies.
Perovskite quantum dots are a type of direct bandgap semiconductor material characterized by high photoluminescence quantum yield (PLQY) and luminescence intensity and exhibit tunable narrow emission with symmetric emission peaks by changing the composition. It is expected to replace conventional Cd-based quantum dots as a material for displays, X-ray and UV sensors, and light-emitting materials.40 It is also expected to be used as a QD solar cell material, with PCEs exceeding 16% obtained through advanced ligand control technology and device engineering.46,47 Sigma-Aldrich's products are also available in a wide range of applications. Sigma-Aldrich products are characterized by high brightness, narrow FWHM (≤ 20-25 nm), and high PLQY (≥ 60-80%) and are available in organic-inorganic hybrid perovskites (905062) and all-inorganic perovskites represented by CsPbX3 (X = Cl, Br).
Core-Shell Quantum Dots
The luminescence properties of quantum dots result from the recombination of electron-hole pairs (exciton decay). Exciton decay can also occur without radiation, but the fluorescence quantum yield is reduced. To improve the properties and brightness of semiconductor nanocrystals, there is a method of growing a shell of a different semiconductor material with a larger band gap around it. Shell-coated quantum dots increase the quantum yield by inactivating the radiation-free recombination sites and thus provide high processing properties in a variety of applications. The resulting quantum dots are called core-shell quantum dots and have been widely studied as a means of tuning the photophysical properties of quantum dots.31-33
CuInS2/ZnS Core-Shell Quantum Dots (1 mg/mL in toluene)
CuInS2 is a direct bandgap semiconductor suitable for solar absorption because it does not contain elements such as Cd, Pb, Se, and As and has low environmental and health impact.36,37 CuInS2/ZnS is a quantum dot that exhibits superior quantum efficiency and long-term stability by coating its core with a ZnS shell. It has applications in LED and solar cell development as well as bio-imaging.
Oleylamine functionalized
Oleic acid functionalized
CdSe/ZnS ZnS Core-Shell Quantum Dots
Core-type quantum dots
Core-type quantum dots are single-composition nanocrystals with uniform internal composition. Chalcogenide compounds (selenium compounds or sulfides) of metals such as cadmium and zinc are typical examples. Their photoluminescence and electroluminescence properties can be finely tuned by varying the crystal size.
Graphene quantum dots
Ag2S, Ag2Se Quantum Dots
Silver chalcogenide quantum dots (Ag2X, X = S, Se, Te) generally have a narrow band gap and near-infrared emission properties.53 They have low cytotoxicity and are attracting attention for applications in bioimaging, nanocarriers, chemical/biosensors, as well as optoelectronics such as infrared sensors and solar cells.54,55
PbS (lead sulfide) quantum dots absorb photons up to the near-infrared region and re-emit them in the near-infrared region. The emission peak is tuned in the range of 900 to 1600 nm by reducing the nanoparticle size to 3 to 7 nm. Their excellent photoabsorption and photoelectric properties make them suitable for near-infrared (NIR) image sensors, infrared LEDs, and infrared photovoltaic cells42 (not only single junction but also in combination with other cells with different absorption wavelengths to build tandem or multi-junction solar cells to improve conversion efficiency).
Emission type
Absorption type
References
To continue reading please sign in or create an account.
Don't Have An Account?