1. Definition and usage of quantum dots
Quantum dots are semiconductor nanoparticles with a diameter of 2–10 nm and an energy bandgap. When the particle size is reduced to 10 nm or less, the optical properties change, emitting light, because of quantum confinement effects. For nanoparticles, if the particle size decreases, electron motion is spatially restricted, thereby forming discrete electron energy levels . This leads to a decrease in the electronic density of states , resulting in a relatively wider energy bandgap than the bulk state (Fig 1). Once quantum dots are formed by the quantum confinement effects, their physical properties, including optical properties, differ from general bulk materials.
Fig. 1 Enlargement of energy levels in quantum dots due to the quantum confinement effect
(Reference: Wikimedia Commons)
Quantum dots produce lights of different wavelengths depending on their size, structure, and composition (Fig 2) and are, therefore, recognized as emerging next-generation optical materials for their excellent color purity. Along with excellent efficiency and stability, the emission wavelengths can be easily adjusted by changing the composition or size of the alloy particles. For these reasons, quantum dots can be used for quantum dot solar cells, quantum dot light-emitting diodes (QLEDs), photocatalysts, and biomarkers.
Fig. 2 Photoluminescence of quantum dots
(Reference: Wikimedia Commons)
2. Current development of quantum dots
Later termed quantum dots, colloidal quantum dots were first discovered in colloidal suspension by Ekimov and Louis Brus in the 1980s. The early material developed for quantum dots is cadmium (Cd), which features high electrical conductivity, ductility (thus, easily deformed), and light emission across the entire visible spectrum, so it has long received much attention from researchers. This element is highly suitable for display applications, but it has disadvantages, such as high-priced Cd precursors for quantum dots, ignitability, and toxicity harmful to the human body. Therefore, there are limits to the development of Cd-based quantum dots.
As alternatives to Cd-based quantum dots, II–VI semiconductor compounds, such as zinc phosphide (ZnS) and zinc selenide (ZnSe), and a III–V semiconductor, indium phosphide (InP), have drawn much attention. When InP-based quantum dots do not have core–shell structures, the quantum efficiency is very low. In addition, their covalent bonding nature makes it difficult to control the surface during synthesis and emit blue light. On the other hand, blue light-emitting zinc-based quantum dots, such as ZnS and ZnSe, are characterized by the possible emission of red light through the doping of transition metals. In particular, they have received attention as research on the synthesis of zinc-based quantum dots as an eco-friendly process has been recently undertaken.
3. ZnSe-based quantum dots
ZnSe is an intrinsic semiconductor with a bandgap of 2.70 eV at 25°C. Because the size of this bandgap corresponds to a wavelength of 460 nm, it can be applied as blue light emitting diodes (LEDs) (Fig 3). Diverse methods, such as the synthesis of colloids in a solvent, chemical/physical vapor deposition, and thermal decomposition, have been proposed to produce ZnSe. Different from conventional Cd-based quantum dots, it has been reported that ZnSe-based quantum dots can be synthesized without using organic solvents. Instead, deionized water is used as a safe solvent at relatively low temperature, so they are considered an eco-friendly alternative to organic synthesis.
Fig. 3 Photoluminescence of ZnSe/ZnS core-shell quantum dots and the schematic diagram of core-shell structure
4. Prospects and implications
In 2020, the global quantum dots market was estimated at around KRW 1–3 trillion. It is expected to increase between KRW 5 trillion and KRW 10 trillion by 2025 at a CAGR of 25%. By industry, the growth rate of the quantum dots market will be 25% and 27% in the medical device and display industries, respectively. Growth factors include the increasing demand in the display sector and growth potential in the medical/biotechnology sectors amid the COVID-19 pandemic. The growth will be observed by material (materials without toxic substances such as Cd), product (display/biolabeling/diagnostic products), and region (Asia and the United States).
Eco-friendly quantum dots currently subject to research as display/biotechnology materials include InP, ZnS, and ZnSe, and these binary or ternary quantum dots are chemically synthesized inorganic materials. These inorganic quantum dots are more economical than light-emitting materials such as organic light-emitting diodes (OLEDs) and have the advantages of stability, color reproducibility, and longer lifespans, suitable for display materials. In recent years, many reports have revealed the successful, eco-friendly synthesis of colloidal quantum dots without using harmful substances such as Cd or organic solvents. If mass production technology is developed, market competitiveness will be secured, accelerating their commercialization.
Hyun Seon Hong
Sungshin University, Department of Environment and Energy Engineering