Vanadium oxide (VO2) is a semiconductor at a temperature cooler than 67°C, about the same as room temperature. At a temperature higher than 67°C, its crystalline structure changes, and this semiconducting material morphs into an electrically conducting metal with different magnetic and optical properties—a well-studied phase transition. Would VO2 particles billionths of a meter across have an effect on phase transition? Would such a phase transition occur at the same temperature in nanoparticles of VO2 as small as 100 nanometers (nm) or much smaller? What might be the mechanism behind these changes in properties and could there be applications?
To answer these questions as part of his thesis research for a Ph.D. degree in physics, Rene Lopez, now a postdoctoral scientist at Vanderbilt University in Tennessee, spent two years at ORNL tapping its unique collection of experts and equipment. “I could not have done this research anywhere else but ORNL because of its unusually large array of tools,” Lopez says, citing the ion implantation accelerator and the transmission electron microscope that he used for the work. He also relied on guidance from his collaborators in ORNL’s Solid State Division (SSD)—Lynn Boatner and Tony Haynes—as well as two professors from Vanderbilt, his thesis adviser Richard Haglud, and Len Feldman, who has collaborated with a number of SSD researchers over the years.
Lopez used one of ORNL’s ion implantation accelerators to implant ions of vanadium and oxygen in silicon dioxide (SiO2) substrates. A number of samples were made, each about the size of the end of a pen cap. “Then we cooked each sample in a furnace at 1000°C to induce the ions to form vanadium oxide nanoparticles,” Lopez says. “We found we could control the size of the nanoparticles by changing the cooking time. For samples annealed for only two minutes, we obtained round particles below 25 nm. For samples heated for longer times, up to an hour for some, the particles became elongated like bars, with the largest being 100 nm.” Lopez explained that the measurements of the particle size (in terms of a volume’s radius) were obtained by using a transmission electron microscope at ORNL’s High Temperature Materials Laboratory. Using X-ray analysis, he determined that the particles were not compressed inside the substrate, allaying the concern that pressure might affect phase transition in the nanoparticles.
Next, Lopez shone infrared laser light on various samples to determine if the optical properties are different for smaller VO2 nanoparticles compared with the larger particles. If the optical properties are different, that would mean that the magnetic and electrical properties are also different. Lopez and his collaborators determined that size matters when it comes to phase transition. “We found that the particles smaller than 35 nanometers transmitted much less infrared light than did the larger particles,” he says. “When we heated the samples having particles over 100 nm to 67°C using thermoelectric Peltier devices, we found that their optical properties were the same as ordinary samples of vanadium oxide. But the smaller particles did not switch from the semiconducting to the conducting metallic state until they were heated to 80°C and remain there even when cool down to room temperature.” In its semiconducting state, VO2 has a monoclinic structure in which half of the vanadium atoms are closer to the oxygen atoms and half are farther away than in the tetragonal structure of the metallic state, where the distances between the atoms are constant. “We believe that the phase transition is caused by the presence of suitable defects that are activated by the rising temperature,” Lopez says. “If the particles are much smaller, the probability that these suitable defects are present is lower. So, higher temperatures are required to activate defects that can cause a change of state.”
Lopez and his colleagues are searching for practical applications of this discovery. One possibility is to place VO2 nanocrystals in an optical fiber to create an optical switch that alternates between “on” and “off” through intentionally induced rapid changes in temperature. The properties of VO2 nanoparticles for such an application might be improved by doping them with tungsten ions, he notes. Another application might be a heat sensor that could keep solar electric cells from being heated up so much by the sun’s rays that their photoelectric properties are destroyed. Convinced that the Laboratory has much to offer, Lopez plans to conduct his postdoctoral research by dividing his time between Vanderbilt and ORNL. His future transitions should help the next phase of the Mexican physicist’s budding career in nanoscience.
