Nanomaterials can store all kinds of things, including energy, drugs and other cargo
A team of chemists led by Northwestern University’s William Dichtel has cooked up something big: The scientists created an entirely new type of nanomaterial and watched it form in real time — a chemistry first.
“Our work sets the stage for researchers interested in studying the fundamental properties of interesting materials and applied systems, such as solar cells, batteries, sensors, paints and drug delivery systems,” said Dichtel, the Robert L. Letsinger Professor of Chemistry at the Weinberg College of Arts and Sciences. “The findings have enormous implications for how chemists and materials scientists think about nanotechnology and their science in general.”
The researchers made covalent organic frameworks (COFs) that are stable — a major advancement. These strong, stiff polymers with an abundance of tiny pores are suitable for storing all kinds of things, including energy, drugs and other cargo. But what limits COFs from realizing these applications is that they are usually prepared as powdery substances that can’t be processed into useful forms.
In this study, the nanoparticles stay suspended in a liquid “ink,” creating a new nanomaterial called a COF colloid. This structure allows the unique materials to be processed into useful forms, such as films of arbitrary size and thickness.
Also, for the first time, the chemists demonstrated that the “cooking,” or heating up, of the ingredients for the nanomaterial can take place inside the imaging tool itself, in this case a powerful microscope called a transmission electron microscope. With this new technique, Dichtel and his team could investigate how molecules come together to form COF colloids.
The field of covalent organic frameworks is only a decade old, and much needs to be learned about how the porous polymers form and how to keep them stable. Dichtel is a leader in the young field, focused on bringing unprecedented functionality and improved stability to COFs.
The study, titled “Colloidal Covalent Organic Frameworks,” was published last week in the journal ACS Central Science. Dichtel is a corresponding author of the study.
The COF colloids are nanoparticles (approximately 50 nanometers in diameter, roughly the size of a virus) made from any number of building blocks in a predictable way. The COF colloids also feature small pores, whose size, shape and chemical groups can be designed precisely. (Each pore is approximately 2.5 nanometers wide, big enough to hold a variety of cargo.)
“This is about as close to useful ‘molecular LEGOs’ as I’ve seen,” Dichtel said. “Being able to keep these materials stable in solution is a major step forward towards taking advantage of their unique combination of properties.”
For the study, Dichtel teamed up with Nathan C. Gianneschi at the University of California, San Diego, who has developed cutting-edge analysis techniques. Gianneschi, a professor of chemistry and biochemistry, is also a corresponding author of the paper.
Dichtel and Gianneschi developed a new way to watch COF colloids form inside a transmission electron microscope, another major advancement.
The inability to observe reactions as they occur using electron microscopy has been a major limitation, Gianneschi said. Usually samples have to be dried or frozen to use this technique. The microscopy in this study opens the door to a new dimension: allowing scientists to initiate and observe materials as they form in real time.
“This is something that is routine on the macroscale, of course, but has eluded chemists, biologists and physicists at the nanoscale,” Gianneschi said.
Physicists have discovered radical new properties in a nanomaterial, opening new possibilities for highly efficient thermophotovoltaic cells that could one day harvest heat in the dark and turn it into electricity.
The research team from ANU/ARC Centre of Excellence CUDOS and the University of California Berkeley demonstrated a new artificial material, or metamaterial, that glows in an unusual way when heated.
The findings could drive a revolution in the development of cells which convert radiated heat into electricity, known as thermophotovoltaic cells.
“Thermophotovoltaic cells have the potential to be much more efficient than solar cells,” said Dr Sergey Kruk from the ANU Research School of Physics and Engineering.
“Our metamaterial overcomes several obstacles and could help to unlock the potential of thermophotovoltaic cells.”
Thermophotovoltaic cells have been predicted to be more than twice as efficient as conventional solar cells. They do not need direct sunlight to generate electricity, and instead can harvest heat from their surroundings in the form of infrared radiation.
They can also be combined with a burner to produce on-demand power or can recycle heat radiated by hot engines.
The team’s metamaterial, made of tiny nanoscopic structures of gold and magnesium fluoride, radiates heat in specific directions.
The geometry of the metamaterial can also be tweaked to give off radiation in specific spectral range, in contrast to standard materials that emit their heat in all directions as a broad range of infrared wavelengths. This makes the new material ideal for use as an emitter paired with a thermophotovoltaic cell.
The project started when Dr Kruk predicted the new metamaterial would have these surprising properties. The ANU team then worked with scientists at the University of California Berkeley, who have unique expertise in manufacturing such materials.
“To fabricate this material the Berkeley team were operating at the cutting edge of technological possibilities,” Dr Kruk said.
“The size of an individual building block of the metamaterial is so small that we could fit more than 12,000 of them on the cross-section of a human hair.”
The research is published in Nature Communications.
The key to the metamaterial’s remarkable behaviour is its novel physical property, known as magnetic hyperbolic dispersion.
Dispersion describes the interactions of light with materials and can be visualised as a three-dimensional surface representing how electromagnetic radiation propagates in different directions. For natural materials, such as glass or crystals the dispersion surfaces have simple forms, spherical or ellipsoidal.
The dispersion of the new metamaterial is drastically different and is hyperbolic in form. This arises from the material’s remarkably strong interactions with the magnetic component of light.
The efficiency of thermophotovoltaic cells based on the metamaterial can be further improved if the emitter and the receiver have just a nanoscopic gap between them. In this configuration, radiative heat transfer between them can be more than ten times more efficient than between conventional materials.