Taking nanomaterials to a new level of structural complexity, Harvard scientists have determined how to introduce kinks into arrow-straight nanowires, transforming them into zigzagging two- and three-dimensional structures with correspondingly advanced functions.
Among possible applications, the authors say, the new technology could foster a new nanoscale approach to detecting electrical currents in cells and tissues.
“We are very excited about the prospects this research opens up for nanotechnology,” said Lieber, Mark Hyman Jr. Professor of Chemistry in Harvard’s Faculty of Arts and Sciences. “For example, our nanostructures make possible integration of active devices in nanoelectronic and photonic circuits, as well as totally new approaches for extra- and intracellular biological sensors. This latter area is one where we already have exciting new results, and one we believe can change the way much electrical recording in biology and medicine is carried out.”
Lieber and Tian’s approach involves the controlled introduction of triangular “stereocenters” – essentially, fixed 120-degree joints – into nanowires, structures that have previously been rigidly linear. These stereocenters, analogous to the chemical hubs found in many complex organic molecules, introduce kinks into 1-D nanostructures, transforming them into more complex forms.
The researchers were able to introduce stereocenters as nanowires, which are self-assembled. The researchers halted growth of the 1-D nanostructures for 15 seconds by removing key gaseous reactants from the chemical brew in which the process was taking place, replacing these reactants after joints had been introduced into the nanostructures. This approach resulted in a 40 percent yield of bent nanowires, which can then be purified to achieve higher yields.
“The stereocenters appear as kinks, and the distance between kinks is completely controlled,” said Tian, a research assistant in Harvard’s Department of Chemistry and Chemical Biology. “Moreover, we demonstrated the generality of our approach through synthesis of 2-D silicon, germanium, and cadmium sulfide nanowire structures.”
The ability to control and modulate the composition doping crystal structure and morphology of semiconductor nanowires during the synthesis process has allowed researchers to explore various applications of nanowires. However, despite advances in nanowire synthesis, progress towards the ab initio design and growth of hierarchical nanostructures has been limited. Here, we demonstrate a ‘nanotectonic’ approach that provides iterative control over the nucleation and growth of nanowires, and use it to grow kinked or zigzag nanowires in which the straight sections are separated by triangular joints. Moreover, the lengths of the straight sections can be controlled and the growth direction remains coherent along the nanowire. We also grow dopant-modulated structures in which specific device functions, including p–n diodes and field-effect transistors, can be precisely localized at the kinked junctions in the nanowires.
Nanowires Are not Damaging the Brain
In separate but nanowire related research: Nanowire [are showing] Biocompatibility in the Brain – Looking for a Needle in a 3D Stack
We investigated the brain-tissue response to nanowire implantations in the rat striatum after 1, 6, and 12 weeks using immunohistochemistry. The nanowires could be visualized in the scar by confocal microscopy (through the scattered laser light). For the nanowire-implanted animals, there is a significant astrocyte response at week 1 compared to controls. The nanowires are phagocytized by ED1 positive microglia, and some of them are degraded and/or transported away from the brain.
“Together with other findings and given that the number of microglial cells decreased over time, the results indicate that the brain was not damaged or chronically injured by the nanowires,” Christelle Prinz concludes.