Mar 12, 2009
Simple vapor-phase synthesis of pure nanoparticles
At present, vapour-phase-derived zinc (Zn) and cadmium (Cd) nanowires are usually formed at high evaporation temperatures, which may pose limitations to the choice of deposition substrates and to nanodevice fabrication and integration. In addition, it is well documented that the melting point of a solid material will be greatly reduced if it exists as a nanostructure due to the low dimension and large surface-to-volume ratio.
The thermal stability of Zn and Cd nanowires is of critical importance for interconnecting nanoscale-based functional devices and circuitry. Zn and Cd nanowires with clean surfaces can serve as an ideal subject for investigating their thermal properties. Therefore, it is necessary to further explore an extremely simple, versatile, catalyst-free and low-temperature vapour-phase approach to fabricating II-B metal (Zn and Cd) nanowires with clean surfaces on a large scale.
Researchers at Harbin Institute of Technology found that if the vapour pressure of the source material reaches 10–3 Torr at an elevated temperature then the vapour species diffuse to the low-temperature zone and grow into 1D nanostructures due to the pressure and temperature differences between the source and the deposition zone. Both the source and the deposition zone temperatures are lower than the melting point of the precursor.
The group describes its vacuum vapor deposition (VVD) approach as simple and environmentally friendly because it does not involve carrier gases and harmful chemical processes, and the purity of the as-prepared product is easily guaranteed.
Using the method, the team has synthesized single-crystal Zn and Cd nanowires and followed up the production by a series of XRD, SEM, TEM and HRTEM studies to characterize the material. It was found that these elementary species condensed at a favorable temperature region to produce one-dimensional nanostructures under the appropriate supersaturation degree.
The researchers have proposed a phenomenological growth mechanism of the so-called Zn (Cd) nanosponges that involves a vapour-solid mechanism mediated by agglomeration and a secondary nucleation phenomenon on the basis of the time-dependent experimental findings. The growth mechanism is considered to be versatile to elucidate the growth of various nanostructures through the VVD method. In addition, based on their observations, the researchers can imagine the feasibility that a Zn (Cd) nanowire could be locally welded to another Cd or Zn nanowire to form a metal-metal nanojunction under exposure of a high-energy electron beam.
In related work, highly oriented CuCl nanorod arrays and high-quality ultrawide selenium and tellurium nanobelts on silicon substrates were prepared via the VVD method. The group then went on to demonstrate that thin tellurium nanobelts can be transformed into helices upon exposure to an electron beam.
The researchers presented their work in Nanotechnology.
About the author
Prof. Gang Chen is the head of the Material Chemistry Teaching and Research Office at the Department of Applied Chemistry, Harbin Institute of Technology, China. His research interests are focused on the synthesis and characterization of novel low-dimensional thermoelectric materials, electrical transport properties and their applications. Dr Qun Wang is a lecturer at the Harbin Institute of Technology. Nan Zhou is a PhD student studying materials science at the Harbin Institute of Technology.