Portland, Ore. - A research team from the State University of New York at Buffalo claims to have discovered a simple way to mass-produce semiconductor nanocrystals precisely, in nearly any desirable size, using a technique based on self-assembly and room-temperature chemistry.
Semiconductor nanocrystals promise to make a quantum leap over traditional optoelectronics due to their unique and size-tunable properties. Also known as quantum dots, these crystals, a few nanometers in size, are already revolutionizing biological and environmental sensing due to their size-dependent luminescence.
The crystals can also play a role in such technologies as telecommunications, photovoltaics, lasers and quantum computing.
"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 in 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 [ZnSe] quantum dots that exhibit size-dependent luminescence and excellent photostability," Mountziaris said. "We employ reactions between group-II alkyls and group-VI hydrides similar to those used by the microelectronics industry during the mass production of thin films. These reactions are spontaneous and lead to particle formation even at room temperature. Our technique exploits such reactions and produces single-crystalline particles of almost uniform size."
The best part about the method is that the size of the nanocrystals, and thus their color when they luminesce, is determined by merely 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 all done automatically, and at room temperature.
Mountziaris' co-researchers on the project were Paschalis Alexandridis, a professor of chemical and biological engineering; Athos Petrou, a professor of physics in the College of Arts and Sciences; Georgios Karanikolos, a graduate student in the Department of Chemical and Biological Engineering; and Grigorios Itskos, a graduate student in the Department of Physics.
A microemulsion, resembling a dispersion of uniform-sized tiny drops of oil in water, is self-assembled by mixing polar and nonpolar liquids-heptane and formamide, respectively-using an amphiphilic substance, a block copolymer, as a surfactant. The tiny heptane droplets form numerous identical nanoreactors, each 40 nanometers in diameter. 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, diethylzinc is dissolved in the heptane before forming the microemulsion. The concentration of this reactant in the heptane is calculated to yield a certain particle size in each nanoreactor upon complete conversion to ZnSe. The second reactant, hydrogen selenide, bubbles through the microemulsion, dissolves in the formamide and diffuses through the interface into the heptane nanoreactors that are floating freely in the formamide. There it spontaneously reacts with diethylzinc to form clusters of ZnSe. These clusters move around in the heptane droplet by diffusion and rapidly coalesce into a single particle by colliding with each other. The coalescence of these clusters is an exothermic process that releases enough energy to anneal the particles into perfect crystals.
"We had no idea we would get such perfect crystals, and in such uniform sizes. The self-annealing seems impossible at first, because you would typically need to raise the temperature to something like 1,000 degrees C to anneal bulk crystals of ZnSe. But apparently, the nanocrystals' small size also lowers their melting point, making the energy released by the coalescence sufficient to locally heat the nanoparticles and anneal them into single crystals," said Mountziaris. "The stability and uniformity in size are due to the very slow droplet-droplet interactions in the specific microemulsion. As a result, problems with nanocrystal aggregation are avoided."
After the synthesis, the team studied the optical properties of the nanocrystals using photoluminescence and absorption spectroscopy. They also spread the suspension on clean quartz wafers, and evaporated the liquids under vacuum, thus obtaining films of the block co-polymer with the ZnSe nanocrystals embedded into it. The nanocrystals were subsequently analyzed by X-ray diffraction and transmission electron microscopy.
The researchers have applied for a patent and are now pursuing details about their annealing hypothesis and just what other unseen mechanisms may be at work. They are also experimenting with ways to separate the nanocrystals from the emulsion, spread the emulsion out into a thin film and integrate the process with traditional semiconductor processing.
Mountziaris said this manufacturing method also allows the nanocrystals to be functionalized for specific applications as soon as they are formed-rather than requiring tedious and expensive post-manufacturing steps. Ideally, the molecules that adapt the nanocrystals to a specific application will be merely mixed in before they form.