Sep 9, 2011
Nanoparticles from empty space?
Nanoparticles can be synthesized using a variety of hard and soft templates, but integrating the smallest particles (<5 nm) with device structures remains a challenge. Bottom-up fabrication requires a multistep process involving template patterning; precursor infiltration; "development" using thermal, photolytic or chemical reduction methods; and possibly template removal. An ideal template material would enable synthesis of a range of particle sizes, possess a tunable chemical environment that stabilizes the nanoparticles once formed, and provide a three-dimensional support structure. Existing templates lack one or more of these properties, but scientists are busy exploring other options. Recently developed crystalline nanoporous materials known as metal-organic frameworks (MOFs) could serve as both highly ordered precursors for metal and semiconductor nanoparticles and as a stabilizing scaffold.
MOFs are crystalline materials composed of metal ions or clusters connected by organic ligands, which create a rigid, but porous framework. This structure is advantageous for nanoparticle synthesis because it provides a highly ordered arrangement of metal ions within a uniform redox environment. A MOF structure also offers great synthetic flexibility and allows the tailoring of space between metal ions and the surrounding chemical environment.
Tunable size distribution
A team of scientists from Sandia National Laboratories and the National Institute of Standards and Technology (NIST) has observed the formation of copper, indium and zinc oxide nanoparticles in real time under the beam of a transmission electron microscope. Narrow and tunable size distributions were obtained, comparable with those obtained from state-of-the-art methods.
Surprisingly, the group found that although metal ions in the MOF are often chemically bonded to oxygen, sometimes metal clusters rather than metal oxides were formed – a process that depends on the oxidation potential of the metal. The researchers also showed that the chemistry and structure of the MOF, as well as the electron beam properties, determine the size and morphology of the nanoparticles.
The results represent a first step towards the fabrication of nanoscale heterostructures using the highly controlled environment of the MOF pores as a scaffold or template.
More information can be found in the journal Nanotechnology.
About the author
The study was conducted by researchers in the Energy Nanomaterials Dept. at Sandia National Laboratories in Livermore, California, US. Ben Jacobs and Ron Houk are former postdoctoral fellows at Sandia and performed the electron microscopy experiments and MOF synthesis, respectively. They were assisted in the interpretation of the results by Bryan Wong, a computational chemist at Sandia, and Alec Talin, a project leader in the Center for Nanoscale Science and Technology at NIST. Mark Allendorf, a Distinguished Member of the Technical Staff at Sandia, guided the project and leads a research group focused on the development of MOFs for a variety of energy and national security applications. Sandia National Laboratories is a multi-program laboratory operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the US Department of Energy's National Nuclear Security Administration. With main facilities in Albuquerque, NM, and Livermore, CA, Sandia has major R&D responsibilities in national security, energy and environmental technologies, and economic competitiveness.