Introduction to Oil-Soluble Quantum Dots
Oil-soluble quantum dots are quantum dots that have been hydrophobically etched to their surface, allowing them to be stable in nonpolar or organic solvents. Oil-soluble quantum dots, in contrast to water-soluble quantum dots, have a superior stability and optical character because of their hydrophobic surface ligands or organic molecules.
Structure and Surface Modification
The core of oil-soluble quantum dots is typically made of semiconductors with II-VI group (CdSe, CdTe), III-V group (InP), or IV-VI group (PbS, PbSe). They require surface modifications for their oil solubility and the usual surface modifications are:
Hydrophobic Ligand Treatment: The surface of oil-soluble quantum dots are generally coated with hydrophobic ligands like alkyl chains (octylamine, oleic acid) and long-chain phospholipids (oleic acid, hexadecylamine). These amorphous molecules can coordinate with the metal ions on the quantum dot's surface, which make them oil-soluble.
Organic Small Molecule Modification: Long-chain organic small molecules are often used as well to make quantum dots' surfaces smoother such as stearic acid and hexadecyltrimethylammonium chloride. They disperse easily in nonpolar or low-polar solvents.
Multilayer Shell Coating: Oil-soluble quantum dots can also be covered with multiple layers of shells, such as ZnS or CdS to make them stronger and more optically stable. In oil-soluble media, quantum dots with such shells have excellent chemical and optical stability.
Key Properties of Oil-Soluble Quantum Dots
Oil-soluble quantum dots share similar optical and electrical properties with water-soluble quantum dots but have some unique characteristics due to different surface modifications:
Good Oil Solubility: Oil-soluble quantum dots are stably dispensed in nonpolar or weakly polar organic solvents like toluene, hexane, chloroform, etc. They bind to the surface of their molecular envelope, and the centre of the quantum dots is closed off from the polar environments.
High Optical Quality: Oil-soluble quantum dots offer excellent quantum yield, fluorescence emission that is bright and narrow bandwidth and adjusts emission wavelength across a wide spectral range which can be used in various displays and luminescent devices.
Great Stability: The hydrophobic ligands help to keep oil-soluble quantum dots chemically and thermally stable in organic solvents and nonpolar atmospheres against oxidation and degradation.
How Oil-Soluble Quantum Dots Can Be Prepared
It's possible to make oil-soluble quantum dots in different ways, with different technical directions based on different materials and requirements. Here are some popular techniques for oil-soluble quantum dots:
Solvothermal method: This process is often used to make oil-soluble carbon quantum dots. For instance, the surfactant is mixed with a high-boiling point low-polarity solvent, heated to 120-250 °C, reduced to room temperature and eluted with diatomaceous earth as the medium and petroleum ether and ethyl acetate as the eluents.
Hydrothermal method: This is applicable to oil-soluble PbSe quantum dots. With just a few hydrothermal reactions, we can obtain oil-soluble PbSe quantum dots of single shape and small size distribution.
Colloid chemistry: This is a common method to create oil-soluble quantum dots in organic solvents. Colloidal chemistry is generally done by reactions in organic solvents to generate the quantum dots you need.
Pyrolysis technique: pyrolysis, for instance, is applicable to citric acid, glutathione and oleylamine as starting material, to make oil-soluble photocatalytic carbon quantum dots.
Phase transfer process: The phase transfer technique is the process of water phase quantum dots to oil phase quantum dots. A phase transfer agent is added through fast stirring and the quantum dot precipitate is separated. We can then obtain the oil-soluble quantum dot solid, after rinsing and drying.
Exchange of ligand: With the help of a surface loaded with special oil-soluble ligands on quantum dots, oil-soluble quantum dots can be synthesized in high solubility and high stability.
These methods have their own advantages and disadvantages. The specific method selected depends on the desired quantum dot type, size, optical properties and application field.
Application Fields
Oil-soluble quantum dots, due to their excellent optical properties and hydrophobic structure, are widely used in various applications in displays, optoelectronic devices, nanomaterial composites, and oil-based media:
Quantum Dot Displays (QLED)
Because of the brightness and purity of oil-soluble quantum dots' emission spectrum, oil-soluble quantum dots are important for quantum dot QLED displays in terms of colour accuracy and energy savings. Oil-soluble quantum dots are highly soluble in organic light sources and can be applied as a color layer to OLEDs, QLEDs, etc.
Optoelectronic Devices
Oil-soluble quantum dots are commonly found in optoelectronics, in solar cells and photosensitive sensors, particularly heterojunction photovoltaics and photodetectors. Quantum dots' band structure and high photoelectric conversion rate are the advantages of quantum dots in thin-film solar cells and nanophotovoltaics.
Nanocomposite Materials
Oil-soluble quantum dots are usable as glow-in-the-dark fillers, added to polymers or oil matrix to make composites. Quantum dots' optical nature makes the composites very suitable for luminescent coatings, lights, and ornaments.
Biological Labeling
Although oil-soluble quantum dots are generally not suitable for direct application in aqueous biological environments, they can be adapted for use in nonpolar biological systems through surface coatings. In some special cases, oil-soluble quantum dots can be modified with amphiphilic molecules for specific biological environments.
References
- Ko Y H, Jalalah M, Lee S J, et al. Super ultra-high resolution liquid-crystal-display using perovskite quantum-dot functional color-filters[J]. Scientific reports, 2018, 8(1): 12881.
- Zhao X Y, Zhang X S, Liu X, et al. Improving efficiency of silicon solar cells by integrating SiO2-coated lead-free Cs3Bi2Br9 perovskites quantum dots as luminescence down-shifting layer[J]. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2024, 682: 132887.
