PORTLAND, Ore. University researchers are using self-assembly techniques and chip-related chemistry to develop a process for mass producing tiny crystals called quantum dots.
Semiconductor nanocrystals promise a quantum leap over traditional optoelectronics due to their unique and size-tunable properties. Quantum dots measure a few nanometers in size and are already revolutionizing biological and environmental sensing due to their size-dependent luminescence.
Other applications include telecommunications, photovoltaics, lasers and quantum computing.
A research team at the University at Buffalo claims to have discovered a simple way to mass-produce quantum dots with extreme precision, in nearly any desirable size, using a technique based on self assembly and room-temperature chemistry.
"As quantum dots enter the commercial stage that requires mass production, techniques based on self assembly will offer a distinct advantage due to their simplicity and scalability," said Triantafillos Mountziaris, a professor of chemical and biological engineering at the university's School of Engineering and Applied Sciences.
"Our team has demonstrated such a scalable method for the controlled synthesis of luminescent zinc selenide quantum dots that exhibit size-dependent luminescence and excellent photostability."
The researchers employed chemical reactions similar to those used by the microelectronics industry to mass-produce thin films. The spontaneous reactions lead to particle formation even at room temperature.
"Our technique exploits such reactions and produces single-crystalline particles of almost uniform size," Mountziaris said.
The best feature of their method is the nanocrystals' size, and thus their color when they luminesce. That is determined by mixing up the correct ratios of the chemicals involved. The self assembly of the template, the conversion of the reactants to single-sized particles inside the template and their annealing into single crystals is automatic and occurs at room temperature.
The process works by using a microemulsion, resembling a dispersion of uniform-sized drops of oil in water. It is self-assembled by mixing precise amounts of a nonpolar liquid (heptane) with a polar version (formamide) using an
amphiphilic substance (a block copolymer) as a surfactant.
The tiny heptane droplets form numerous identical nanoreactors with a diameter of 40 nm each. The droplets are stabilized by the surfactant molecules that assemble at their interface with a hydrophobic "head" inside the heptane and two hydrophilic "tails" inside the surrounding formamide.
To form nanocrystals of a certain size, a precise amount of
diethylzinc was dissolved in the heptane before forming the
microemulsion. "We had no idea we would get such perfect crystals, and in such uniform sizes," Mountziaris said.