"This new chemistry to make the tetrapods was fairly cheap, but we were looking for an even cheaper way," Wong said. "Sravani said, 'Let's get rid of this expensive phosphorus surfactant and just see what happens.' So she did, and these little things just popped out on the electron microscope screen."
Wong recalled the team's initial surprise. "We said, 'What is going on here? How do you go from four-legged nanojacks to these little balls?'"
He said it took the team more than a year to decipher the unusual formation mechanism that yielded the hollow, soft-shelled particles.
To make the particles, Gullapalli added three kinds of solid powder -- cadmium nitrate, selenium and a tiny amount of CTAB -- to an oil solvent. She then slowly heated the mixture while stirring. The cadmium nitrate melted first and formed tiny nanodroplets that cannot be seen with the naked eye.
"Nothing happens until the temperature continues to rise and the selenium melts," Gullapalli said. "The molten selenium then wraps around the cadmium nitrate droplet, and the cadmium nitrate diffuses out and leaves a hole where the droplet once was."
She said the cadmium selenide shell surrounding the hole is nanocrystalline and is enveloped in a soft outer shell of pure selenium.
When Gullapalli examined the lava dots with a transmission electron microscope, she found them to be bigger than standard quantum dots, about 15-20 nanometers in diameter. The holes were about 4-5 nanometers in diameter. She also noticed something peculiar: When sitting by themselves they appeared round, and when tightly packed, the shell appeared to become compressed, even though neighboring dots never came into actual contact with one another.
"That's one of the twists to this weird chemistry," Wong said. "The solvent forms its own surfactant during this process. The surfactant coats the particles and keeps them from touching each other, ev
|Contact: Mike Williams|