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MEETING NEWS Elizabeth Zubritsky reports from the Systems—Squaw Valley, Calif.
Seventh International Conference on Micro Total Analysis
Tailoring semiconductor nanocrystals For those who aren’t experts, synthesizing nanocrystals can seem like more of an art than a science, which is unfortunate because the physical properties of nanocrystals strongly depend on their size and shape. To better control the process, Emory Chan, Richard Mathies, and Paul Alivisatos at the University of California, Berkeley, and the Lawrence Berkeley National Laboratory replaced the traditional in-flask protocols with a chip-based microreactor that can finetune the reaction parameters quickly and reproducibly to make crystals with specific characteristics. “We thought that by scaling down the reaction to microfluidic volumes, we could better control all the conditions,” says Chan, a graduate student in both the Mathies and Alivisatos groups. And because chips are automated and can accommodate multiple streams of fluid, he adds, “We could [perform] very complex reactions that would be tedious to do in a flask.” The microreactors are fabricated in borofloat glass wafers, which can withstand the punishment of high temperatures and strong reagents such as selenium tributyl phosphine (Nano Lett. 2003, 3, 199–201). This means that the researchers can retain many of the materials and methods of traditional reactions. “If you want really high-quality nanocrystals, you really have to do it in air-free, water-free conditions at high temperature, or else you get very defective nanocrystals that have poor optical properties,” Chan explains. Some adjustments to the usual protocols were needed, though. For example, the surfactants for making semiconductor nanocrystals are usually solid at room temperature, so the researchers had to devise a strategy to keep them fluid enough to move through microfluidic channels. 12 A
Two separate serpentine microreactor channels are etched into a single chip. In a typical reaction, a plug of organometallic precursor solution is injected into the 4.7-µL channel (65 cm long, 150 µm wide, and 47 µm deep) using a syringe pump. The precursor plug decomposes to form CdSe nanocrystals as it flows through the channel, the temperature of Inject Dilute Dilute
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Schematic of the chip showing (a) the 4.7-µL channel and (b) the 12.5-µL mixing channel. Dotted lines indicate boundaries of the heated regions.
which is controlled by integrated resistive heaters. The second channel has a volume of 12.5 µL (105 cm long, 200 µm wide, and 57 µm deep) and mixes a second fluid with the precursors before the reaction. The plugs are typically about 100 µL—much larger than the volumes of the channels—which ensures that any diffusion at the ends of the plugs affects only a small portion of the material. It also means that the chips run in a continuous-flow mode. The reactions last anywhere from 10 s to 5 min, the same as in flask reactions, so the microreactors don’t save time in that sense, Chan says. But once a chip has been set up, you can run many reac-
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tions in a short amount of time. “I normally run [about] six reactions in 3– 4 h, which is unheard of for doing a flask reaction,” he notes. As the nanocrystals exit the microreactor, they are mixed with a stream of cold solution that stops the chemical reaction and dilutes the crystals. Then they enter a capillary flow cell where they are characterized by on-line fluorescence spectroscopy. For semiconductor crystals, the size of the particles is monitored by looking at the peak wavelength, and the quality is quickly checked by looking at the fwhm intensity, which depends on the distribution of crystal sizes. Chan and colleagues report fwhm values in the low 30s. “Anything below 30 nm is really good,” he says. “So we’re decent.” The real excitement, though, is the reproducibility the researchers achieved. To control the size and shape of the nanocrystals, the researchers varied the temperature, flow rate, and concentration of the precursor solution. For nanodots, raising the temperature from 180 to 210 °C increased the crystal diameter from 2.4 to 2.7 nm. Reducing the flow rate from 3.0 to 1.5 µL/min yielded larger crystals, as did raising the concentration of precursor from 1:1 to 5:1. But nanodots are “old school”, Chan says, because “there’s only a limited number of things you can do with spheres.” To make interesting devices, you need nanoscale building blocks of various shapes. So he made nanorods, increasing the temperature and the concentration of precursor to produce longer crystals, and cadmium telluride tetrapods, which have four arms in tetrahedral symmetry—a shape that might be useful for making a fourterminal transistor or a junction. And this is just the beginning, Chan predicts, because “chips have an unprecedented ability to control and manipulate reactions as they occur.”
