Quantum dots (QDs) are semiconductor-based nanomaterials with a wide range of applications. They are becoming a prominent platform in biomedical research, including drug delivery, medical diagnosis, and imaging. Apart from the medical industry, QDs are explored as an efficient structure capable of storing charge in solar cells as well as in consumer electronics such as displays and lighting.
Image Credit: Van Pympk/Shutterstock.com
What are Quantum Dots (QDs)?
QDs are semiconductor-based ultra-small nanocrystals that have extraordinary optical capabilities. Once fabricated, QDs perform like an “artificial” atom, with valence electrons that can be excited by lasers at precise frequencies. Early research on QDs was carried out by Alexei Ekimov in the 1980s.
QDs can be divided into 12 different categories based on their chemical makeup. These categories are dependent on the positions of the constituent elements in the periodic table. Group IV A QDs, for instance, are composed of elements with similar outer shell electronic structures. Examples include Carbon, Silicon, and Germanium. Apart from their structural similarities, these elements also exhibit metalloid and semiconducting electrical capabilities.
The chemical composition of most QDs is structured with a heavy metal core enveloped by a semiconductor shell. CdTe, PbSe, CdS, and ZnSe with a SiO2 shell are commonly found. These QDs have better surface structures and they provide higher quantum yield. Better quantum yield in material means a more efficient optical activity that can be harnessed to build devices.
Other forms of QDs include single-element semiconductors like Si QDs and QDs based on polymers. The latter has been shown to emit near-infrared radiation useful in biological diagnosis like in vivo imaging.
Characteristics of QDs
QDs are appealing for a range of applications because of their small particle size, customizable composition and characteristics, high brightness, high quantum yield, and light emission.
A major appeal of QDs can be attributed to their tunable optical characteristics. In contrast to bulk semiconductors, QDs behave like single atoms due to their extremely small size. The electron energy levels in ultra-small QDs are well separated as opposed to semi-continuous like in traditional semiconductors.
The small, atom-like form of QDs, leads to the "quantum confinement" phenomenon, where the band gap energy—which is the energy gap from the valence band to the conduction band—is relatively large. The energy of the emission photon rises with the size of the QDs, shortening the emission wavelength and increasing the energy of the emission photon, and vice versa. As a result, the optical performance of QDs is more size-dependent than material-dependent, which may be used to control the electromagnetic radiation of these QDs from the UV to the far infrared range by varying the particle size.
Additionally, QDs exhibit a strong "Stokes shift" phenomenon that results in a shorter-wavelength excitation spectrum far from the longer-wavelength emission spectrum, improving detection sensitivity and decreasing optical overlap in the process.
Recent applications of QDs
Many applications have benefited from the unique features of QDs. Some recent application developments are highlighted below:
- Medicine with QDs - The ability to investigate cell functions at the single molecule level, made possible by QDs, may greatly enhance the detection and treatment of diseases like cancer. QDs are used as active detectors in high-resolution cellular imaging, in which the fluorescence characteristics of the QDs are altered when reacting with the analyte. In passive mode, specific molecules, such as antibodies, have been connected to the QD surface. QDs are also explored as potential detectors of pathways linked to differentiating stem cells.
- QDs in solar cells - QDs have a number of advantages over other methods for producing solar cells that make them appealing. They can be produced in an energy-efficient room-temperature process and they can be made from affordable materials that don't require extensive purification. They can also be used with a variety of affordable and flexible substrate materials, like light plastics.
- Nanoscale graphene dots - The raw, planar form of a carbon nanotube known as graphene has emerged as one of the most intriguing candidate materials for nanoscale electronics. Researchers have demonstrated that a single graphene crystal, which is a graphene QD, may be used to create tiny transistors. In contrast to all other materials, graphene maintains its exceptional stability and conductivity even after being sliced into objects just one nanometer across.
- QDs made of perovskite - High photoluminescence quantum yields, variable emission color management, and solution processability are all characteristics of luminescent QDs. These are promising candidates for use in high-quality displays and lighting systems.
- QD displays and TVs - TV screens may be the application of QDs that is most well-known. Since 2015, after Samsung and LG introduced their QLED TVs, many new advancements have been explored and commercialized.
The remarkable qualities of QDs continue to advance many industries. As listed in the non-exhaustive list above, the medical, energy, and electronic sectors have gained substantial growth in the recent past due to continued research on QDs.
More from AZoQuantum: The James Webb Space Telescope Zooms in on Jupiter
Zhao, Shuangyi, and Zhigang Zang. 2022. "Advances in Perovskite Quantum Dots and Their Devices: A New Open Special Issue in Materials" Materials 15, no. 18: 6232. https://doi.org/10.3390/ma15186232
Byung-Ryool Hyun, Chin-Wei Sher, Yu-Wei Chang, Yonghong Lin, Zhaojun Liu, and Hao-Chung Kuo, “Dual Role of Quantum Dots as Color Conversion Layer and Suppression of Input Light for Full-Color Micro-LED Displays .” The Journal of Physical Chemistry Letters 2021 12 (29), 6946-6954
Liu, Chenxi and Wen, Mengying and Mai, Shihua and Ma, Yue and Duan, Qingfei and Zou, Wei and Liu, Hongsheng, Harnessing Nitrogen-Doped Graphene Quantum Dots for Enhancing the Fluorescence and Conductivity of the Starch-Based Film. Available at SSRN: https://ssrn.com/abstract=4205035
Saja Shahid Hussein, Ali G. Al-shatravi, Amin H. Al-khursan, Optical absorption from boron-containing quantum dot structures, Micro and Nanostructures, Volume 170, 2022, 207375, ISSN 2773-0123,
Sergey I. Pokutnyi, Andrzej Radosz, Electron states in perovskite quantum dots, Physica B: Condensed Matter, 2022, 414294, ISSN 0921-4526, https://doi.org/10.1016/j.physb.2022.414294.