Illustration of a nanohoop, or cycloparaphenylene, doped with nitrogen atoms. Image: Ramesh Jasti.When Ramesh Jasti began making tiny organic circular structures using carbon atoms, the idea was to create an improved version of carbon nanotubes for use in electrical and optical devices. He quickly realized, however, that his technique might have wider applications.
In a new paper, Jasti and five colleagues from the University of Oregon show that his nanohoops – known chemically as cycloparaphenylenes – can be made using a variety of atoms, not just carbon. They envision these circular structures, which can efficiently absorb and distribute energy, finding a place in solar cells, organic light-emitting diodes and as new sensors or probes for medicine.
The research, led by Jasti's doctoral student Evan Darzi, is described in a paper in ACS Central Science. The paper is a proof-of-principle for the process, which will have to wait for additional research to be completed before the full impact of these new nanohoops can be realized, Jasti said.
Barely 1nm in size, the nanohoops offer a new class of structures for use in electrical and optical devices, said Jasti. He was the first scientist to synthesize these types of molecules back in 2008, as a postdoctoral fellow at the Molecular Foundry at the Lawrence Berkeley National Laboratory.
"These structures add to the toolbox and provide a new way to make organic electronic materials," Jasti said. "Cyclic compounds can behave like they are hundreds of units long, like polymers, but be only six to eight units around. We show that by adding non-carbon atoms, we are able to move the optical and electronic properties around."
Nanohoops can help solve challenges related to materials with controllable band gaps – the energies lying between valance and conduction bands that are vital for designing organic semiconductors. Currently long materials such as those based on polymers make the best organic semiconductors.
"If you can control the band gap, then you can control the color of light that is emitted, for example," Jasti said. "In an electronic device, you also need to match the energy levels to the electrodes. In photovoltaics, the sunlight you want to capture has to match that gap to increase efficiency and enhance the ability to line up various components in optimal ways. These things all rely on the energy levels of the molecules. We found that the smaller we make nanohoops, the smaller the gap."
To prove that their approach could work, Darzi synthesized a variety of nanohoops using both carbon and nitrogen atoms to explore their behavior. "What we show is that the charged nitrogen makes a nanohoop an acceptor of electrons, and the other part becomes a donator of electrons," Jasti said.
"The addition of other elements like nitrogen gives us another way to manipulate the energy levels, in addition to the nanohoop size. We've now shown that the nanohoop properties can be easily manipulated and, therefore, these molecules represent a new class of organic semiconductors -- similar to conductive polymers that won the Nobel Prize in 2000," he said. "With nanohoops, you can bind other things in the middle of the hoop, essentially doping them to change properties or perhaps sense an analyte that allows on-off switching."
His initial work making nanohoop compounds was based entirely on carbon, with the idea of making nanohoops with different diameters and then combining them. But his group kept finding unique and unexpected electronic and optical properties.
Jasti brought his research from Boston University to the University of Oregon's Department of Chemistry and Biochemistry in 2014. He said the solar cell research being done by his colleagues in the Materials Science Institute, of which he is a member, was an important factor in his decision to move to the University of Oregon.
"We haven't gotten very far into the application of this," he said. "We're looking at that now. What we were able to see is that we can easily manipulate the energy levels of the structure, and now we know how to exchange any atom at any position along the loop. That is the key discovery, and it could be useful for all kinds of semiconductor applications."
This story is adapted from material from the University of Oregon, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.