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(Nanowerk News) The fluorescence of carbon nanotubes is no longer a surprise. Finding a second level of fluorescence is surprising and potentially useful.
How does it work? Wait for it.
Rice University’s lab of Bruce Weisman, a chemistry professor who led the pioneering discovery of nanotube fluorescence in 2002, found that single-walled nanotubes emit delayed secondary fluorescence when triggered by a multi-step process in a solution. with dye molecules and dissolved oxygen.
The delay is only microseconds, but it is enough to be detected with some effort.
The complex process is described in detail by Weisman, lead author and student of Rice Ching-Wei Lin, and researcher Sergei Bachilo in Journal of the American Chemical Society (“Delayed fluorescence from carbon nanotubes through triplet excitons sensitized to singlet oxygen”).
The reaction begins when light excites a solution containing a dye called rose bengal. The dissolved oxygen molecules in the solution capture energy from the dye, forming energized O2. These then transfer their energy to the nanotubes, where excitons ?? quasiparticles made of electrons and electron holes ?? they are generated in their triplet state. With some added thermal energy, those excitons are promoted to a higher energy singlet state that emits the observed fluorescence.
?? For a number of years, we have observed interesting effects involving nanotubes and oxygen, ?? Weisman said. We have found a wide range of things that can happen, from physical effects such as this energy transfer or reversible quenching of fluorescence, to the triggering of chemical reactions between nanotubes and DNA. So this study was part of a larger exploration program.
Their ability to excite dissolved oxygen molecules prompted researchers to see how this would affect adjacent nanotubes, Weisman said.
?? We produce singlet oxygen by exciting a dye molecule with visible light, then oxygen deactivates the dye and energizes itself, ?? He said. This idea dates back decades in photophysics and is very conventional. What is unusual here is that the singlet oxygen interacts with the nanotube to directly produce excitations in the triplet state in the tube. Those three states have been pretty elusive.
?? Triplet states of organic molecules are the longest-lived excited states, ?? Weisman said. Their lifespan is orders of magnitude greater than singlet excited states, so they can hang around long enough to hit something else and undergo chemical reactions.
?? But because the triplet states of nanotubes do not emit light or absorb light directly very well, they are difficult to study and not much is known about them, ?? He said. What we have done is try to understand them a little better.
Activating the fluorescence still required an additional step. Just by random thermal agitation in their surroundings, these guys can sometimes be pushed up to the bright singlet state, and then they can tell you they’re there spitting out a photon, Weisman said.
Since the triplet state can last for about 10 microseconds, such upconverted emission is called delayed fluorescence.
The researchers had to find a way to detect the relatively weak effect between the nanotubes ?? bright primary fluorescence. ?? It was like trying to see a dark object right after being blinded by a bright camera flash, ?? Weisman said. ?? We had to come up with special equipment. ??
Is a device basically a fast mechanical shutter? covering the shortwave infrared spectrometer (SWIR) during bright flash and then pops up quickly, a kind of reversing camera that goes from covered to open in seven microseconds. The other device, he said, is a sensitive detector that is triggered with an electronic signal and measures how the weak emission fades over time. “These systems were both built by Ching-Wei, who is a formidable experimenter,” he said.
Weisman and colleagues employed nanotube fluorescence in medical imaging technologies and nanotube-based smart skin to measure deformation in surfaces, among other applications. He said the new discovery could finally find its way into optoelectronics and solar energy.
?? Isn’t there a direct passage where someone will read this and make a new, more efficient device, ?? Weisman said. ?? But this fundamental knowledge of processes and properties is the basis on which new technologies are built.
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