Quantum Dot Phosphors: A Nano-Fluorescent Material That Defied Convention

In our daily lives, displays for televisions, PCs, and smartphones have become indispensable tools. Displays create images by combining the three colors of light: blue, green, and red. The mainstream displays today are liquid crystal displays (LCDs). Their light source uses blue LEDs and phosphors that convert blue light into green and red. Displays are required to faithfully reproduce actual colors, but the range of colors that current displays can reproduce is insufficient. Quantum dot phosphors are attracting attention as a groundbreaking material to solve this problem. However, because these phosphors were a material that overturned industry conventions, it took time to gain trust.

Conventional inorganic phosphor materials that have been commercialized contain luminescent rare-earth or transition metal ions and are manufactured by firing powdered raw materials at high temperatures of 1000°C or more to create defect-free crystals. Firing at high temperatures causes crystal growth, resulting in particles ranging from a few microns to several tens of microns. When such micron-sized phosphors are crushed to a nano-size, their fluorescence intensity decreases significantly. This is because defects are generated on the surface and inside the phosphor particles, which inhibit luminescence. Thus, in the phosphor industry, the conventional wisdom was that 'the optimal particle size for phosphors is the micron size' and 'phosphors are made by firing solid raw materials at high temperatures.'

Quantum dot phosphors with sizes of 10 nm or less appeared in the 1990s. Quantum dots are created in a solution at temperatures below 300°C. A solution of raw materials is injected into a solvent containing surface-modifying molecules with high-boiling-point, long-chain alkyl chains, causing single-crystal nanoparticles of a uniform size to precipitate instantly. The surface-modifying molecules play two roles: they adsorb to the nanoparticle surface to suppress particle growth, and their alkyl chains create steric repulsion that prevents particle aggregation in the solvent, leading to stable dispersibility. Thus, because quantum dots are 'nano-sized' and 'made in a solution,' they were a material unthinkable under the aforementioned industry conventions.

Quantum dots are single crystals and do not contain defects within the crystal. However, the surface atoms lack adjacent atoms and are in a state of dangling bonds, which causes a decrease in luminescence efficiency. To solve this problem, a shell is epitaxially grown on the surface of the luminescent core. At this time, the crystal structures of the core and shell must be identical, and care must be taken to minimize the lattice constant mismatch as much as possible to prevent defects from forming at the core-shell interface. In quantum dots, an electron in the valence band is excited to the conduction band, and light with energy corresponding to the band gap (Eg) is emitted when the hole formed in the valence band and the electron in the conduction band recombine. In core/shell quantum dots, an electronic structure is designed where the Eg of the core is sandwiched by the Eg of the shell to make the core luminesce efficiently. The attractive luminescent properties of quantum dots include that 'the emission peak is single and sharp' and 'the emission wavelength (emission color) can be tuned by controlling the size and elemental composition.' Quantum dot phosphors for displays were initially CdSe/ZnS when research began, but the toxicity of Cd became a concern, leading to the subsequent development of InP/ZnS. Furthermore, new quantum dots such as perovskite materials CsPbX3 (X = Cl, Br, I) and nanocarbon materials (carbon dots) have also emerged and are continuing to evolve. Why not join us in our laboratory to research quantum dot phosphors?