SpectroscopyNOW- Last Month's Most Accessed Feature: The heat is on: Thermal behaviour of MOFs

Are MOFs thermoelectric

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The first measurements of the thermoelectric behaviour of a nanoporous metal-organic framework (MOF), including infrared temperature measurements, have been carried out and could point the way to an entirely new class of materials for a wide range applications from heatsinks for computer chips and cameras to energy harvesting.

Thermoelectric devices are able to convert heat to electricity without resorting to moving parts and so could be used for either cooling or energy-harvesting applications. If thermoelectric MOFs were to exist, these materials could bring the design flexibility and improved performance capacity to this area. MOFs have a crystalline structure consisting of rigid organic molecules linked together through metal ions, which gives rise to many hybrid properties including nanoporosity, enormous surface area to volume ratio and high thermal stability. Those pores can, of course, be filled with other molecules to make "Guest@MOF" compounds, according to researchers at Sandia National Laboratories, USA. These guests can be used to impart entirely new properties to the MOF or fine tune existing ones for a given application simply by changing the guest.

Previously...

Previously, the team had demonstrated electrical conductivity in MOFs by feeding tetracyanoquinodimethane, TCNQ, into the pores, which made the researchers hopeful of observing thermoelectricity, although team member Mark Allendorf says this was by no means a given. “These results introduce MOFs as a new class of thermoelectric materials that can be tailored and optimized,” explains physicist François Léonard.“This discovery brings us a step closer to realizing the potential of MOFs in practical applications.” Allendorf adds that, “We found that not only is the material thermoelectric but also the Seeback coefficient exceeds that of the best thermoelectric materials, such as bismuth telluride."

Once they had filled the pores of their MOF with TCNQ, to make TCNQ@MOF, the researchers had to find a way to measure the anticipated thermoelectric properties. As such, Léonard, Alec Talin and Kristopher Erickson, created a thermoelectric device by connecting Peltier heaters and coolers to each end of a thin film of TCNQ@MOF to generate a tiny temperature gradient. They accurately measured the temperature gradient with an infrared camera whilst at the same time recording the voltage output. From the data they could obtain the Seebeck coefficient, the voltage per unit of temperature change. The thermal conductivity, which is needed to estimate the efficiency of thermoelectric energy conversion, was measured by Patrick Hopkins and Brian Foley from the University of Virginia using a laser technique. Time Dependent Thermal Reflectance or TDTR, in which a laser pulse is used to heat a metal film deposited on top of the material in question and to measure the reflectance. A second laser pulse delays by a short delta-t is then used to measure the change in reflectance. The metal reflectance is strongly dependent on the temperature, so by measuring delta-R you can measure delta-T, from which you can calculate the thermal conductivity using a model.

TCNQ@MOF has a high Seebeck coefficient and low thermal conductivity, which are two important prerequisites for efficient thermoelectricity. The Seebeck coefficient is on a par with that measured for bismuth telluride, a leading solid state thermoelectric material.

Brought to you by the letters MOF

The measurements have also given the researchers a clearer understanding of the electronic structure of TCNQ@MOF. Team members Catalin Spataru and Mike Foster conducted detailed electronic structure calculations and Reese Jones performed thermal conductivity simulations. "We were trying to understand the role of the guest molecule, TCNQ in this case, when it infiltrates the pore of a MOF," says Spataru. "Finding a representative configuration for the combined TCNQ@MOF system via computer simulations was particularly challenging, as we don't expect guest molecules to form an ordered structure." Nevertheless, the simulations revealed how charge transport occurs in the material and shows that the TCNQ@MOF is a p-type material.

The obvious next step, having revealed that latter fact, is to find a small molecule to combine with a MOF to create an n-type semiconductor but with similar properties to TCNQ@MOF. "Once we find that, we’ll be at the early stage of creating a full thermoelectric device," says said Léonard.

Efficient Guest@MOF materials could replace existing cooling technology in devices where size and weight matter, such as in the cameras mounted on satellites or in a more mundane situation to replace the noisy fans needed for cooling by high-power computer chips. Conversely, energy-harvesting thermoelectric devices would capture waste heat for countless sources to generate electricity. A thermoelectric device fitted to a car engine or exhaust system could grab the waste heat and use it to power the car's electronics. "Another potential application is using temperature gradients in the ground to power sensors in remote areas," adds Léonard. "Thermoelectrics could be quite ideal for this application, as you could set up a device and leave it to run for long periods of time."

"The next step is how do we make [them] better?" asks Allendorf. "The energy conversion is not competitive yet with solid state materials, but we think we can improve that with better electrical conductivity."

Improvements in MRIs, other image-detection applications on the horizon

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LIVERMORE, Calif. — Researchers at Sandia National Laboratories, along with collaborators from Rice University and the Tokyo Institute of Technology, are developing new terahertz detectors based on carbon nanotubes that could lead to significant improvements in medical imaging, airport passenger screening, food inspection and other applications.

A paper in Nano Letters journal, "Carbon Nanotube Terahertz Detector," debuted in the May 29 edition of the publication's "Just Accepted Manuscripts" section. The paper describes a technique that uses carbon nanotubes to detect light in the terahertz frequency range without cooling.

Historically, the terahertz frequency range — which falls between the more conventional ranges used for electronics on one end and optics on another — has presented great promise along with vexing challenges for researchers, said Sandia's François Léonard, one of the authors.

"The photonic energy in the terahertz range is much smaller than for visible light, and we simply don't have a lot of materials to absorb that light efficiently and convert it into an electronic signal," said Léonard. "So we need to look for other approaches."

Terahertz technology offers hope in medicine and other applications

Researchers need to solve this technical problem to take advantage of the many beneficial applications for terahertz radiation, said co-author Junichiro Kono of Rice University. Terahertz waves, for example, can easily penetrate fabric and other materials and could provide less intrusive ways for security screenings of people and cargo. Terahertz imaging could also be used in food inspection without adversely impacting food quality.

Perhaps the most exciting application offered by terahertz technology, said Kono, is as a potential replacement for magnetic resonance imaging (MRI) technology in screening for cancer and other diseases.

"The potential improvements in size, ease, cost and mobility of a terahertz-based detector are phenomenal," he said. "With this technology, you could conceivably design a hand-held terahertz detection camera that images tumors in real-time, with pinpoint accuracy. And it could be done without the intimidating nature of MRI technology."

Researchers at Sandia National Laboratories, Rice University and the Tokyo Institute of Technology developed a terahertz detector using several nanoscopic-sized tubes, creating a macroscopic thin film that contains a mix of metallic and semiconducting carbon nanotubes.

(Photo Credit: Dino Vournas, Sandia National Laboratories)

Carbon nanotubes may help bridge the technical gap

Sandia, its collaborators and Léonard, in particular, have been studying carbon nanotubes and related nanomaterials for years. In 2008, Léonard authored The Physics of Carbon Nanotube Devices, which looks at the experimental and theoretical aspects of carbon nanotube devices.

Carbon nanotubes are long, thin cylinders composed entirely of carbon atoms. While their diameters are in the 1- to 10-nanometer range, they can be up to several centimeters long. The carbon-carbon bond is very strong, so it resists any kind of deformation.

The scientific community has long been interested in the terahertz properties of carbon nanotubes, said Léonard, but virtually all of the research to date has been theoretical or computer-model based. A handful of papers have investigated terahertz sensing using carbon nanotubes, but those have focused mainly on the use of a single or single bundle of nanotubes.

The problem, Léonard said, is that terahertz radiation typically requires an antenna to achieve coupling into a single nanotube due to the relatively large size of terahertz waves. The Sandia, Rice University and Tokyo Institute of Technology research team, however, found a way to create a small but visible-to-the-naked eye detector, developed by Rice researcher Robert Hauge and graduate student Xiaowei He, that uses carbon nanotube thin films without requiring an antenna. The technique is thus amenable to simple fabrication and represents one of the team's most important achievements, Léonard said.

"Carbon nanotube thin films are extremely good absorbers of electromagnetic light," he explained. In the terahertz range, it turns out that thin films of these nanotubes will soak up all of the incoming terahertz radiation. Nanotube films have even been called "the blackest material" for their ability to absorb light effectively.

The researchers were able to wrap together several nanoscopic-sized tubes to create a macroscopic thin film that contains a mix of metallic and semiconducting carbon nanotubes.

"Trying to do that with a different kind of material would be nearly impossible, since a semiconductor and a metal couldn't coexist at the nanoscale at high density," explained Kono. "But that's what we've achieved with the carbon nanotubes."

The technique is key, he said, because it combines the superb terahertz absorption properties of the metallic nanotubes and the unique electronic properties of the semiconducting carbon nanotubes. This allows researchers to achieve a photodetector that does not require power to operate, with performance comparable to existing technology.

A clear path to performance improvement

The next step for researchers, Léonard said, is to improve the design, engineering and performance of the terahertz detector.

For instance, they need to integrate an independent terahertz radiation source with the detector for applications that require a source, Léonard said. The team also needs to incorporate electronics into the system and to further improve properties of the carbon nanotube material.

"We have some very clear ideas about how we can achieve these technical goals," said Léonard, adding that new collaborations with industry or government agencies are welcome.

"Our technical accomplishments open up a new path for terahertz technology, and I am particularly proud of the multidisciplinary and collaborative nature of this work across three institutions," he said.

In addition to Sandia, Rice and Tokyo Tech, the project received contributions from researchers taking part in NanoJapan, a 12-week summer program that enables freshman and sophomore physics and engineering students from U.S. universities to complete nanoscience research internships in Japan focused on terahertz nanoscience.

This photograph depicts the terahertz detector developed by researchers at Sandia National Laboratories, Rice University and the Tokyo Institute of Technology. The terahertz radiation is captured by a carbon nanotube thin film contacted by two gold electrodes.

(Photo Credit: Rice University)