Using chemical vapor deposition, researchers at the University of Wisconsin-Madison have made nanometer-scale lead sulfide wires that sprouted into treelike constructs that could develop into forests of improved photodetectors and highly efficient photovoltaic devices.
Importantly, Song Jin and his colleagues made the nanowire constructs without resorting to a catalyst material, which heretofore had been necessary to prompt the wires out of one-dimensionality. The group reported its findings in the May 1 edition of Science Express.
Scanning electron microscope images show a PbS “pine tree” nanowire structure with several branches (left) and an example of “tree on tree” morphology that occasionally is observed (right). Images courtesy of AAAS/Science.
The scientists synthesized the nanostructures using PbCl2 and elemental sulfur under independent flows of argon and hydrogen. Optimizing the flow of hydrogen enabled them to reproducibly create intricate structures that resemble pine trees. Images acquired with a scanning electron microscope from LEO Electron Microscopy Ltd. (now part of Carl Zeiss SMT AG) showed that the trunks are 40 to 350 nm thick and up to several hundred microns long. Each tree has four sets of epitaxial branches that spiral up the length of the trunk. The branches are tens of microns long and do not begin new branchlets of their own.
The researchers examined the nanowire structures with a Philips transmission electron microscope, finding screw dislocations in the trunks. No such dislocations were found on the branches themselves, however. They propose that screw dislocation is driving the one-dimensional crystal growth without the help of a catalyst to form the trunk nanowires, whereas the branches are still formed with the help of a catalyst. The stresses and strains of the dislocation in the trunk led to the twisting of the crystal lattice and to the rotation of the epitaxial branches.