Apr 25 2018
Researchers have demonstrated a new breakthrough, where tiny particles coated with organic dyes function as antennas to capture and convert previously missed sunlight into usable energy. This new development in the design and function of nanoparticles could result in highly efficient solar panels.
An erbium atom (red) in a nanocrystal emits visible, green light via a process known as upconversion that could lead to the development of improved solar cells that capture some previously missed solar energy. Scientists discovered that coating the particles with dyes (blue and purple molecules at right) can greatly enhance this light-converting property. (Image credit: Berkeley Lab)
Headed by scientists at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), the international team has shown that coating nanoparticles with organic dyes considerably improved their ability to capture and reemit the near-infrared light in the visible light spectrum, which can also prove useful for biological imaging.
Once the scientists interpreted the mechanism that allows the dyes on the tiny particles to work as antennas to collect a broad range of light, they effectively redesigned the nanoparticles to further increase the light-converting properties of the particles. The study has been reported online April 23 in Nature Photonics.
“These organic dyes capture broad swaths of near-infrared light,” said Bruce Cohen, a scientist at Berkeley Lab’s Molecular Foundry who headed the study together with Molecular Foundry researchers Emory Chan and P. James Schuck (now at Columbia University). The Molecular Foundry functions as a nanoscience research center.
Since the near-infrared wavelengths of light are often unused in solar technologies that focus on visible light and these dye-sensitized nanoparticles efficiently convert near-infrared light to visible light, they raise the possibility of capturing a good portion of the solar spectrum that otherwise goes to waste, and integrating it into existing solar technologies.
Bruce Cohen
The team observed that the organic dye itself amplifies the brightness of the reemitted near-infrared light about 33,000-fold, and when it interacts with the nanoparticles, its efficiency in converting light is increased by about 100 times.
Chan, Schuck, and Cohen had worked for about 10 years to design, develop, and explore the upconverting nanoparticles (UCNPs) employed in this work. These UCNPs are capable of absorbing near-infrared light and can efficiently change it to visible light—a unique property due to the combinations of lanthanide metal ions present in the nanocrystals. According to a study performed in 2012, while dyes on the UCNPs’ surface considerably improve the light-converting properties of particles, the mechanism continued to be a mystery.
“There was a lot of excitement and then a lot of confusion,” Cohen said. “It had us scratching our heads.”
While attempts were made by many researchers to replicate the study in the subsequent years, “Few people could get the published procedure to work,” added Chan. “The dyes appeared to degrade almost immediately upon exposure to light, and nobody knew exactly how the dyes were interacting with the nanoparticle surface.”
The unique combination of capabilities and expertise at the Molecular Foundry, which involved a mix of experiments and theoretical work, well-honed synthetic methods, and chemistry know-how, made the new study feasible, Chan noted. “It’s one of those projects that would be difficult to do anywhere else.”
Nicholas Borys, a Molecular Foundry project scientist, and David Garfield, a UC Berkeley Ph.D. student, headed experiments, which revealed a symbiotic effect between the lanthanide metals in the nanoparticles and the dye.
The dyes are close to the lanthanides in the particles and as a result, the presence of a dye state called “triplet” is enhanced. This triplet state subsequently transfers its energy to the lanthanides in a more efficient manner, and enables a more efficient conversion of numerous infrared units of light, called photons, into single photons of visible light.
The studies demonstrated that a match in the measurements of the particles’ light absorption and the dye’s light emission proved the presence of this triplet state, and assisted in informing the researchers regarding what was at work.
“The peaks (in dye emission and UCNP absorption) matched almost exactly,” Cohen said.
The researchers then discovered that when the concentration of lanthanide metals in the nanoparticles is increased from 22% to 52%, this triplet effect could be increased to enhance the light-converting properties of nanoparticles.
“The metals are promoting dyes to their triplet states, which helps to explain both the efficiency of energy transfer and the instability of the dyes since triplets tend to degrade in air,” Cohen said.
According to Schuck, the nanoparticles measuring approximately 12 nanometers, or billionths of meters, across, could possibly be applied to the solar cell surface so that these cells can capture more amount of light to convert into electricity.
“The dyes act as molecular-scale solar concentrators, funneling energy from near-infrared photons into the nanoparticles,” Schuck said. In the meantime, the particles themselves are highly transparent to visible light, which means other usable light would be allowed to pass through, Schuck noted.
Introducing the nanoparticles into cells to label cell components for optical microscopy analyses is another possible application. The nanoparticles could even be employed for deep-tissue imaging, for instance, or in optogenetics – a field that regulates cell activity by using light.
Cohen informed that researchers face certain barriers to overcome these applications because these applications are presently not stable and were examined in a nitrogen environment to prevent exposure to air.
More research and development has to be done to assess the potential protective coatings for the particles, for example, different types of polymers that function to encapsulate the particles. “We have even better designs in mind going forward,” he said.
The Molecular Foundry is a DOE Office of Science User Facility.
Scientists from the Korea Research Institute of Chemical Technology, UC Berkeley, the Kavli Energy NanoScience Institute at UC Berkeley, and Sungkyunkwan University in South Korea also participated in this study. The DOE Office of Science; the China Scholarship Council; the National Science Foundation; and the Ministry of Science, Information and Communication Technology, and Future Planning of South Korea supported the study.
Upconversion process of NaYF4 Nano Crystals Doped with Ytterbium and Erbium HD
ANIMATION: Energy transfers from a ytterbium atom (blue), which absorbs near-infrared light, to an erbium atom (red). The erbium atom then releases visible, green light. A study led by researchers at Berkeley Lab’s Molecular Foundry found a way to enhance this process, known as “upconversion,” by coating nanoparticles with dyes. Scientists hope to use this process to develop solar cells that capture and convert previously missed sunlight into usable energy. (Video credit: Andrew Mueller)