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NREL Records Progress Toward Understanding of Nanotubes

December 17, 2015

Scientists at the Energy Department's National Renewable Energy Laboratory (NREL) have made progress towards answering a fundamental question about single-walled carbon nanotubes: do absorbed photons create only neutral excitations (excitons) or are some free charges created also?

Jaehong Park, Obadiah Reid, Jeffrey Blackburn, and Garry Rumbles are with NREL's Chemistry and Nanoscience Center. Their research findings, "Photoinduced spontaneous free-carrier generation in semiconducting single-walled carbon nanotubes," were published in the journal Nature Communications.

Exposing a nanotube to photons creates an exciton, which is a bound electron-hole pair that transports energy. Research continues into nanotubes, which have the potential to be used in photovoltaics, telecommunications, biological imaging, and solar fuels. Before that can happen, researchers need a greater understanding of how light interacts with nanotubes. Previous experiments have found that photoexcitation can also create unbound charges. However, in those experiments it was unclear whether charges were created as a result of an artifact in the experiment, or whether the photons actually create a charge directly.

"To understand how nanotubes would work in a device, you actually need to know very precisely when you photoexcite a nanotube, what are you creating?" Rumbles said. "Are you creating excitons? Are you creating unbound charges? How many of each? Those are crucial things that you have to understand before you actually can know what's happening in your device."

The NREL scientists designed a sample and an experiment that aimed to keep the results as free of artifacts as possible. Using samples containing only one specific type of semiconducting carbon nanotube, suspended in a low-dielectric toluene solution, and exciting with low light levels, charges were detected that lived for hundreds of nanoseconds, a time that far exceeds previous reports.

"For many years experiments performed on nanotubes sometimes used poorly defined samples," Blackburn said, "and now the field has evolved to have these really ultra-pure nanotube samples. It's important to understand how all of those impurities in previous experiments contributed to those results."

Ongoing research at NREL will build upon these findings and attempt to control whether the nanotubes produce either neutral excitations for light-emitting applications, or charges for solar-harvesting applications.

The research was supported by the Department of Energy's Basic Energy Sciences Solar Photochemistry Program.