What Are Quantum Dots?
Quantum dots are such small particles of semiconductor material that their electron's behaviour within them is subject to three-dimensional quantum confinement. This isolation causes the energy states of electrons to be discretised within the quantum dots (like the energy states of atoms). That's why quantum dots are also called "artificial atoms".
Composition and Materials Used
Quantum dots usually have II-VI or III-V semiconductor layers like cadmium selenide (CdSe) and cadmium sulfide (CdS). These have band gaps that make quantum dots unusually sized in optical and electronic aspects. The quantum dots' surfaces also frequently are inked with organic or inorganic materials, which keep them chemically stable and alter their optical properties.
For better illustration of quantum dots' mechanism, think of them as tiny cages in which electrons fit into an extremely tiny cavity. If you place energy, like light or electricity, on the quantum dots, the electrons pick up the energy and rise to higher energy. When they fall back down to zero energy, they produce light of a particular wavelength. This is what makes quantum dots so powerful in luminescent and optoelectronic devices.
How Do Quantum Dots Work?
QDs are semiconductor nanoparticles where the quantum confinement effect is the working principle. This action keeps electrons and holes in a nano-sized box, changing their optical and electronic properties drastically.
Quantum Confinement
Quantum confinement describes the way that electron energy states split up when a material is condensed to the scale analogous to or smaller than the de Broglie wavelength of the electron wavefunction. It occurs because the electron wavefunction is locked up within nanostructures in spatial entanglement so that the electrons can only be at particular energies. Quantum dots are like three-dimensional "boxes" with electrons and holes locked inside a finite space and creating discrete energy levels.
Illustration of quantum confinement effect
Impact of Quantum Confinement on Optical and Electronic Properties
The quantum confinement effect significantly affects the optical and electronic behaviour of quantum dots:
Tunable Emission Wavelengths: The emission wavelength of quantum dots is proportional to their size. The smaller the quantum dot, the shorter its emission wavelength, and vice versa. That's because quantum dots' spacing at the energy level fluctuates as they get bigger, and their emission wavelengths change.
Size-tunable emission spectra of QDs
Optical Properties: Quantum dots are very efficient photoluminescent dots with a small spectral distribution due to quantum confinement. These properties are extremely desirable for quantum dots in display technology and bioimaging.
The quantum confinement effect doesn't only affect the underlying physical characteristics of quantum dots, it opens up an abyss of possibilities for using them in high-tech domains. You can also control quantum dots' size and shape, so their optical behaviour can be tailored to a specific application need.
How Does Quantum Dot Size Affect Performance?
Relationship Between Size and Emission Color
Quantum dots are the size and emission colour of a rhodium atom. For the most part, small quantum dots glow blue, big quantum dots glow red. It's because quantum dots of different sizes have bandgap energy variations that influence the wavelength of the reflected light. If a quantum dot's size decreases, for example, the bandgap energy increases and the emission wavelength is shifted towards shorter wavelengths – from red to blue.
Importance of Dimension (2D vs. 3D)
Quantum dots have very strong optical effects depending on their size. In 2D quantum dots, for example, the bandgap energy is very sensitive to change with size and gives highly customizable emission colours. But in 3D quantum dots this alteration is much smaller because electrons and holes are more evenly distributed in 3D systems, and thus sizes change the bandgap less. Therefore, 2D quantum dots are usually chosen for applications where a specific emission wavelength is needed, like bioimaging and display technology.
Practical Examples of Size-Dependent Applications
The quantum dot's size dependence is useful in a wide range of areas. In bioimaging, for example, quantum dots of different sizes can mark several biological targets at once, making multiplex imaging possible.
Quantum dots for biomedical applications
As in display technology, you can manipulate the size of quantum dots to constantly adjust the light to go from blue to red, making displays look better in terms of color.
This is also the case in solar cells, where the number of quantum dots can be regulated so as to maximise light absorption and energy conversion.
Not only do quantum dots' sizes influence the hue of their light, but also their effectiveness for other tasks. By finely regulating quantum dots' size, fast, flexible technological answers to problems of any kind can be found.
How Are Quantum Dots Synthesized?
The Synthesis Process
It is a process called colloidal synthesis, by which quantum dots are syringe-synthesized. It's done by heating precursors in a solution, which are broken down into monomers and then nitrated and grown in high temperatures, to form nanocrystals. Colloidal synthesis is very popular as it's cost-effective and easy to implement.
Colloidal synthesis of CdSe quantum dots
Key Factors in Synthesis
At the point of synthesis, temperature, precursors and reaction conditions are essential. Temperature directly affects the rate of precursor decomposition and the monomer formation, which then determines the quantum dots' size and shape. Selection and proportion of precursors also matter, as these will dictate the amount and purity of the monomers. It also depends on the solvent and composition of the reaction fluid which influence the optical behaviour and stability of quantum dots.
Importance of Controlling Size and Shape During Synthesis
Control over quantum dots' size and shape is critical to their use. This is done through regulation of size by changing reaction temperature, preamble concentration and reaction time. It's possible to maintain shape control by using the right stabilizers and solvents that do not result in aggregation and non-normal growth of quantum dots. Such balancing operations ensure optical consistency and high quantum yield so that quantum dots have wide applications potential for applications like bioimaging and solar cells.
What Factors Affect Quantum Dot Performance?
The behaviour of quantum dots is controlled by surface defects and passivation, environmental stability, ligands and coatings. The following is a summary of those parameters:
Surface Defects and Passivation
Surface defects also play a significant role in quantum dots' performance. Because quantum dots are usually nanometres in size, the surface atoms are many times more numerous than the internal ones, and therefore coordinatively unsaturated. This creates floating bonds and defects in the surface. These defects might attract electrons or holes, increasing non-radiative recombination and thus reducing quantum yield and optical performance. This problem is frequently resolved by surface passivation, which encapsulates the defects in the surface with organic or inorganic ligands that prevent non-radiative recombination, enhancing quantum dots' optical quality and stability.
Environmental Stability
The environment (humidity, temperature, light) influences the stability of quantum dots in particular. Quantum dots are oxidised under air or water conditions and thereby produce surface defects that degrade their optical performance. Other things like temperature change can also cause lattice instability or grain growth of quantum dots, which then impact their optical properties. Therefore, quantum dots must have a higher environmental stability which can be accomplished by encapsulation, the choice of ligands, and by optimisation of the synthesis process.
Role of Ligands and Coatings
Coatings and ligands contribute to quantum dot stability and function. When a ligand is chosen, surface energy can be lowered, surface defects are minimized, and quantum dots are better distributed and solubilized. Coating technologies can also make quantum dots more environmentally stable, shielding them from air and moisture. With alloying or shell coating, for instance, quantum dots can be made optically stable and corrosion resistant.
Quantum dots are subject to various effects such as surface imperfections, ambient environments, ligands and coatings. Surface passivation, environmental stability, and fine-tuning of ligands and coatings can maximise quantum dots' optical performance and application capability.
What Are the Applications of Quantum Dots?
Displays (QLED TVs)
Quantum dots are most well known in display technology such as QLED (Quantum Dot Light Emitting Diode) displays. The QLED uses quantum dots as color convertors and the resulting colour space, brightness and energy efficiency are also better. It has since been adopted in luxury TVs, monitors and smartphones, which make the visual experience more realistic.
Solar Cells
Quantum dots are used in solar cells to mainly optimise photoelectric conversion. Quantum dots also demonstrate thermodynamic efficiencies up to 66 per cent making them a potential candidate for next generation high efficiency solar cells. And quantum dots can be applied to building-integrated products like smart windows, bringing even more energy savings.
Semiconductor-nanostructure-based next-generation solar cells
Medical Imaging
For medical applications, quantum dots are used in bioimaging and diagnostics because they are fluorescent and extremely bright. They could also be used as fluorescent probes for imaging cells and tissues, early cancer diagnosis and drug delivery. Quantum dots' multicolor image is one reason molecular imaging technology can monitor multiple signalling channels simultaneously.
How Can Quantum Dots Be Further Developed?
Current Limitations of Quantum Dot Technology
Quantum dots are promising in a variety of areas, but there remain substantial constraints. In the biomedical field, for instance, the toxicity of quantum dots and the brittleness of metals and glass constrain their use in vivo. Moreover, they are limited by issues like low bi-specificity, low water solubility, surface chemistry complexity and lack of control over biodistribution to target tissues that prevent their application widely. Additionally, the low quantum yield, the lack of known metabolic pathways, and the cost of synthesis are all factors that hamper quantum dots' adoption in reality.
Ongoing Research and Development
Researchers are taking various research and development initiatives to get around these limitations. Quantum yield and toxicity reduction is one example, with the advancements in quantum dot synthesis techniques and surface functionalizations being studied. In the biomedical realm, researchers are looking at graphene quantum dots to overcome toxicity, and new functionalisation strategies to handle a range of loading needs. Moreover, it's being sought to make quantum dots more stable over the long term in biological buffers and for better clearance in vivo.
Potential Breakthroughs and Future Trends
A future of quantum dots is likely to yield some breakthroughs. For instance, quantum dots could in the biomedical field be used for better cancer diagnosis, treatment and prevention. As more research goes on, the fine tuning of quantum dots' size, shape and surface chemistry will only make them better. Furthermore, carbon quantum dots (CQDs) are an attractive alternative because they are very biocompatible and non-toxic.
The evolution of quantum dots will also have an advantage in collaboration with other futuristic technologies including quantum computers and AI. Combining with these technologies, quantum dots will play a more important role in many more fields. In short, even with these limitations, quantum dot technology will make more breakthroughs and use cases in the future, as long as research continues and technology evolves.
References
- Jagtap, S., et al. "A review on the progress of ZnSe as inorganic scintillator." Opto-Electronics Review 27.1 (2019): 90-103.
- Wen, L., et al. "Aptamer-modified semiconductor quantum dots for biosensing applications." Sensors 17.8 (2017): 1736.
- He, X., and Nan M. "An overview of recent advances in quantum dots for biomedical applications." Colloids and Surfaces B: Biointerfaces 124 (2014): 118-131.
- Kango, S., et al. "Semiconductor–polymer hybrid materials." Organic-Inorganic Hybrid Nanomaterials (2015): 283-311.
- Kamat, Prashant V. "Quantum dot solar cells. The next big thing in photovoltaics." The journal of physical chemistry letters 4.6 (2013): 908-918.
