Jul 30, 2010
Watching nanowires grow
Researchers at Cambridge University in the UK and IBM in New York have used a modified high-resolution transmission microscope to watch germanium nanowires grow during catalytic chemical vapour deposition. The technique clearly shows the nanowires growing in real-time with atomic lattice resolution. The method could be important for controlling how nanowires develop as they are being synthesized, something that is crucial for making real-world devices from these materials.
Stephan Hofmann's team obtained the results using environmental transmission electron microscopy, or ETEM. This technique combines high-resolution transmission electron microscopy and low-pressure chemical vapour deposition. "ETEM is an exciting and relatively new electron microscopy technique that allows us to study the catalytic growth of various nanostructures in real-time," explained team member Andrew Gamalski of Cambridge University.
The researchers used ETEM to image gold catalyst particles as they are exposed to digermane gas. As the gas comes into contact with the surface of the gold, it decomposes into germanium and hydrogen. Like a droplet on a glass surface, the liberated germanium completely wets the gold. After wetting the gold surface, the germanium then diffuses into the solid catalyst.
The microscope allowed the team to observe phase changes in a Au1–xGex alloy. It showed transient states that cannot be identified in any other way. The discovery is very interesting because it shows that the dynamics of nanoscale phase transformations can deviate from those of their bulk counterparts, Gamalski told nanotechweb.org.
"The most exciting part of this research is that we found a completely liquid Au-Ge phase existing at temperatures as low as 240 °C, which is more than 100 °C below the bulk melting, or eutectic, temperature of Au-Ge" he said.
Phase transitions in nanoscale systems
The discovery shows that germanium nanowires can be temporarily grown from a liquid catalyst particle well below the eutectic temperature. The research highlights the subtleties of phase transitions in nanoscale systems and in particular, how metastable phases may play a critical role in the early growth stages of germanium nanowires.
"The phase transition we observed in the Au-Ge system is very different from conventional phase transitions – often thought of as being induced by changes in temperature, pressure or volume," explained Gamalski. "Here, the phase change comes about by increasing the concentration of germanium. It is like how ice melts when salt is added."
Nanowires show promise for potential applications in nanoelectronics, photonics and sensors. However, such applications are being held back because it is still difficult to accurately control the structures of the wires as they grow. Electrical properties of nanowires depend on their geometry, so any kinks in the wires or changes in diameter will affect their electrical characteristics. Understanding and controlling the growth process to minimize such defects is thus critical for fabricating nanowire-based devices, states Gamalski.
"Our research also gives important insights into the state of the catalyst particles moments before nanowire growth begins," he added. "The physical and chemical state of the catalysts is very important because it largely determines the shape and orientation of the resulting wire."
The work was reported in Nano Letters.
About the author
Belle Dumé is contributing editor at nanotechweb.org