A Colorado group is tackling one of the largest issues facing us with some of the smallest materials known to mankind.
Justin Sambur, Ph.D. is an Assistant Professor of Chemistry with the College of Natural Sciences at Colorado State University. He uses nanotechnology to investigate how to convert light into electricity. And ultimately provide low-cost energy to the planet, and beyond.
The 34-year old researcher focuses on the need to develop massively-scalable renewable energy technologies. Nanoscale materials are promising, low-cost light absorbers for solar energy conversion into chemical fuels or electricity, he said. But it is unclear how the chemical, electronic, and physical properties of individual nanomaterials affect their collective function.
That’s where his research begins.
Nanoscale solar power
His group develops new analytical tools to visualize device function on the nanoscale level and use this information to guide the development of high-efficiency devices.
Sambur uses a custom made a HORIBA SMS system – which is a standard Olympus IX-73 inverted microscope customized with multiple spectroscopy capabilities - to study ultra-thin semiconductor materials. These are two-dimensional graphene materials like MoS2 and MoSe2 (molybdenum disulfide and molybdenum diselenide).
“They're interesting for a lot of ultra-thin optoelectronic applications, things like LEDs, solar cells, and transistors,” he said. “When you make them, they're quite heterogeneous. They sometimes have different shapes, sizes and thicknesses. My group is really interested in understanding structure and function relationships between the materials themselves and their performance.”
Sambur and his team has developed some techniques with its HORIBA system to correlate the effects of exciting materials in various layer thicknesses. How does that scale, and how do the physical properties scale with the energy conversion efficiency? After all, the end game is the generation of large amounts of energy.
“We study them down to the monolayer or three atoms thick, and we find very interesting, nonlinear relationships between thickness and energy conversion efficiency,” he said.
The researchers consist of undergraduate, graduate and post-doctoral students. They believe the efficiencies have something to do with the electrical contacts that these ultra-thin materials make with the current collector of the substrate that it sits on.
The materials are so thin, the environment around it can have a huge impact on its properties. The group is beginning to change the materials in the electrical contacts because of the impact of the substrates, or conducting electrodes.
Sambur and his group are doing basic research, which hopefully will be studied by other scientists and eventually be commercialized.
“We're working with model systems, and trying to understand some basic key structure-function relationships that could be useful for solar energy conversion,” Sambur said.
They measure power conversion efficiency, which is at about in the neighborhood of 1 percent in these nanomaterials.
“But you could fix this in the future by making them somehow nanostructured, putting them on very high surface area materials.” he said.
That might take shape in the form of covering a building or a mountainside.
Since the light absorbing material is so thin, it has some properties that are different from the bulk material. In some cases, materials exhibit efficient carrier multiplication, where a cell that is excited with one photon produces two electrons. In effect, you get double the current in a cell.
“If we can begin to understand these materials and harness those type of physics, then you can have a next generation ultra-thin device,” Sambur said.
If it could be optimized, the ultrathin materials could be flexible and lightweight devices for niche applications, he said. It has advantages like if you were to wrap it around a pole, or in space applications. In fact, Sambur is funded by the U.S. Air Force because they're interested in very lightweight photovoltaics in space.
“We hope to reveal structure, function, and relationships for these devices that are going to either dictate how they're engineered, and also develop new materials,” he said. “If we find some scaling relationships that are useful with thickness as well as the chemistry of the material, we hope that it's going to inspire the way we design and synthesize new materials for solar energy conversion applications.”
TOM and Jerry
Sambur’s lab has two optical microscope setups. The researchers refer to them as TOM and Jerry. Jerry is a custom-built HORIBA Scientific multifunctional spectroscopy system. TOM to refers to The Other Microscope that is mainly used for single molecule imaging studies.
Jerry is a beast. It has multitasking capabilities, with multiple light sources, detectors and an optical microscope sprouting from its core. It features dedicated systems for spectroscopy and wide field imaging. The standard microscope spectroscopy (SMS) system is a platform allowing several types of analytical techniques, including Raman spectroscopy, photoluminescence, reflectance and photocurrent to be added to a any standard microscope, according to HORIBA Scientific Product Line Manager Francis Ndi.
“What makes it so great is we can really see our samples on our imaging camera and then we can do all sorts of spectroscopy measurements on specific areas on interest,” Sambur said. “We can switch between imaging and spectroscopy modes really quickly.”
TOM and Jerry sit in a microscope room in the Colorado State University chemistry facility on two eight by five-foot tables. The HORIBA microscope takes up an entire table.
Why the customization?
“We wanted to be able to excite our samples multiple different ways,” Sambur said. “Jerry is set up to do single molecule fluorescence microscopy, where we excite the sample from above. It's also set up to do mapping experiments, where we send the laser through the back of the microscope to excite a tiny spot on the sample and measure light emission or photocurrent generation from that tiny spot.”
The design team for the instrument layered additional functionality, which is made possible by the openness of the system. The confocal microscope and spectrometer was made by HORIBA’s Optical Spectroscopy Division, a systems integrator. It built the system as an open platform.
As Sambur’s team becomes familiar with Jerry, their own efficiency grows.
“All that flexibility, we're really leveraging it now,” he said.
HORIBA Scientific’s LabSpec 6 Spectroscopy software suite has been a critical component for Sambur’s researchers. He notes the software’s ease of use, and its triggering capability, where it can talk to other instruments.
“At any one time, our microscope is talking to five different instruments, and that's been really useful for us to do the things that we do,” he said.
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