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Posted: Oct 18, 2012

Assembling functional nanowire yarns with light

(Nanowerk Spotlight) Nanoscale materials like quantum dots, carbon nanotubes, graphene, or nanowires, have intriguing properties, but unless they can be assembled in to larger structures it is difficult to take advantage of these properties. Figuring out how to assemble nanostructures into functional macroscale assemblies is one of the key challenges that nanoscientists around the world are faced with. This is akin to supramolecular chemistry except with nanomaterials. In the area of nanowires, this has led to researchers exploring various nanowire assembly techniques ranging from Langmuir Blodgett alignment to electrospinning.
Researchers at the University of Notre Dame, working with US Nano LLC, have now developed a novel approach for assembling nanowires into macroscopic yarns that consist of millions of nanowires bundled together. The team found that light can be used to charge inorganic semiconducting nanowires. Once charged, the nanowires can be manipulated with electric fields. They dubbed this effect LINA – Light Induced Nanowire Assembly.
"In LINA, photogenerated carriers induce a large dipole within individual wires to align them when in the presence of an electrostatic field," Masaru Kuno, a professor in the Department of Chemistry and Biochemistry at Notre Dame, explains to Nanowerk. "Resulting yarns have diameters on the order of 10 microns and have lengths that are currently as long as 25 cm. The process appears to be general and we have produced yarns from a variety of nanowire materials that include CdS, CdSe, CdTe, PbS, and PbSe. Furthermore, we are able to make mixed composition yarns (e.g. CdSe and CdTe). They can even be segmented, i.e. ABABAB... where A is one material and B is a second."
The nanowire yarns are photoconductive and are also polarization sensitive. They therefore retain the underlying properties of constituent wires. This may then lead to potential uses of such yarns in sensing and even in power generation applications.
"Given that the nanowire alignment process can be automated, we envision that very long threads can eventually be made," says Kuno. "This may ultimately lead to the creation of functional (woven) nanowire textiles."
nanowires
Low and high magnification SEM images of a CdTe nanowire yarn made using LINA. (Kuno Group, University of Notre Dame)
Louise E. Sinks, a Vice President at US Nano, who worked with Kuno's team, says that manipulating nanowires with electric fields can be as simple as using a static charge on a gloved hand to move around nanowires in solution, or as complicated as using a Van de Graaff generation to make nanowires based threads and fibers. "LINA is another tool to do so, and is extremely simple to implement. For many compositions of nanowires, ambient room light and residual static charges are enough to create cm long structures composed of millions of nanowires."
Kuno notes that LINA was a serendipitous discovery by Nattasamon Petchsang, a postdoctoral researcher in his laboratory. She found that nanowires she synthesized often aligned when she rubbed the sides of a plastic centrifuge tube with her gloves. This effect has been observed and reported before, but it was assumed that the nanowires acquire charge through triboelectric charging.
"In fact" Sinks points out, "the nanowires do acquire a slight positive or negative charge over time, which you can experimentally measure. But the important breakthrough we made was demonstrating that this charge is not the source of the electrostatic effect. Even if the nanowires are charged, you cannot manipulate them with static charge in the dark. From that point, we showed that the nanowires must be excited by a photon, and generate holes and electrons, and then you can manipulate them."
As the team reports in the October 2, 2012 online edition of Advanced Materials ("Light Induced Nanowire Assembly: The Electrostatic Alignment of Semiconductor Nanowires into Functional Macroscopic Yarns"), LINA differs from existing nanowire alignment approaches in that nanowires are deliberately excited with light to create photogenerated carriers. These electrons and holes then enable large induced dipoles to be formed within the wires when in the presence of an external electrostatic field. The large magnitude of the dipoles subsequently allows ready nanowire alignment and assembly.
After their initial discovery, the researchers just started to explore what type structures they could make – films, yarns, etc – and what properties these structures had. Experimenting with yarns, they were able to create a nanowires base thread that was 25 cm long. This is huge compared to the size of the nanowires (10 nm diameter, and 10 microns long) and the distance over which one might imagine that electrostatic interactions would be in effect.
A variety of applications arise from the materials that can be made with LINA and the team is now exploring how to fabricate functional nanowire textiles.
"For example, the yarns exhibit a strong photocurrent, and may be useful in making a solar cell thread," says Matthew P. McDonald, a graduate student in Kuno's group and one of the authors of the paper. "Since the LINA process yields macroscopic yarns, it should be possible to mate these materials with conventional fibers. This could lead to photovoltaic textiles that can be incorporated into clothing, tents, or boat sails allowing for energy collection during normal everyday activities."
"Figuring out how to make macrostructures of different types of nanowires that are assembled in a particular order is a big challenge," adds Sinks. "For example, in a solar thread, you need metallic nanowires for the electrodes and semiconducting wires for the photoactive layer. They need to be spatially separated for the photovoltaics to work. LINA can be used to generate heterostructures in certain circumstances, but other, not yet developed, assembly methods would be helpful."
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