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Terahertz spectroscopy at nanoscale resolution

Researchers at Brown University, Providence, Rhode Island (US), have brought laser terahertz emission microscopy (LTEM) — a powerful technique to characterize the performance of solar cells and study a wide variety of materials — to the nanolevel. 

How nanolevel LTEM works

“Laser terahertz emission microscopy — or LTEM — is a technique where electrons in a material are accelerated by femtosecond laser pulses, which causes emission of terahertz pulses,” explains postdoctoral researcher Pernille Klarskov Pedersen, who is the lead author of the paper “Nanoscale Laser Terahertz Emission Microscopy,” published in the journal ACS Photonics. 

The emitted terahertz pulses give an indication of the carrier mobility in the material. “In our experiment, we have pushed LTEM to the nanoscale by using an atomic force microscope (AFM) that provides a sharp metal needle that is tapered down to 20 nm,” Pedersen reports. The needle is able to confine both the incident femtosecond laser pulses as well as the emitted terahertz pulses below the metal tip, due to plasmonic field enhancement of the metal tip, according to the expert. Furthermore, the AFM needle serves as an antenna for the radiated terahertz light, if it has the right length — in this case 80 µm. “Having such a long and sharp AFM probe enabled us to perform LTEM with a spatial resolution similar to the diameter of the metal tip, i.e. 20 nm,” she says.

The key advantage of the novel technique is that it is able to give a direct measure of the carrier mobility without being in direct contact with the sample and that the spatial resolution is roughly 1000 times better than for conventional LTEM.

Understanding electrical properties of nano-components

Pedersen says today’s fabrication technology is indeed capable of providing components and even circuits with features on the nanoscale to meet the demand for better and better, yet smaller devices. Characterizing the electrical properties of a device, however, becomes trickier and more impractical the smaller the sample is. And performing LTEM at nanoscale now opens for new possibilities to characterize and understand the electrical properties of nano-components.  

Challenges in developing nano-LTEM

The researchers report that when they combined the AFM and LTEM technologies, a number of new challenges appeared. “Most significantly,” says Pederson, “the signal-to-noise [ratio] of the emitted terahertz signal decreased when we started looking at emission from smaller volume, so when we reached 20 nm, everything had to be perfectly optimized in terms of optical alignment of the AFM probe as well as the terahertz detection, while any electrical noise had to be eliminated.”
Once the team had the experiment running and achieved the 20 nm spatial resolution, understanding the details about the coupling between the terahertz emission from their sample and the AFM probe took quite a lot of additional experiments, where they varied every possible parameter, such as incident polarization, laser power, out-coupled polarization, tip-sample separation etc., together with simulations and calculations in order to draw the final conclusion.

Key applications for the new nanoscale LTEM technique

“Nanoscale LTEM is useful for anything where one is interested in knowing about the electrical properties with nanometer resolution,” Pedersen notes, adding that characterization of semi-conductor devices, such as integrated circuits and solar cells with features on the nanoscale, are obvious applications for the new LTEM technique. Furthermore, studies of nano-structures, such as carbon nanotubes, nano-wires and quantum dots, can reveal new details about their physical properties at the nanoscale, where yet many things are unknown.

Impact on the future of perovskites

The pioneering LTEM technique at nanoscale could have an impact particularly on the design of the next generation of solar cells based on the perovskites, the material du jour. Recently, after all, remarkable attention has been drawn to perovskites as new and promising materials for solar cells that would have both significantly improved efficiency as well as far reduced production costs, compared with current technologies. “Still,” Pedersen remarks, “the grain structure of the perovskites is limiting for the electron transport across the material together with its tendency to degrade over time in ambient air.” Much research is currently dedicated to overcoming those challenges, and the expert believes that the novel technique she and her colleagues have developed “can help this process by mapping out the carrier mobility over the grain structure.”

Next steps

“In the future, we will work towards emphasizing our technique as useful tool for the electronics industry with a constant demand for smaller and better devices, but also for studying the underlying physics of carriers in new materials and nanostructures, where yet many details are still unknown,” Pedersen concludes.

Written by Sandra Henderson, research editor Solar Novus Today

Labels: Terahertz spectroscopy,Laser Terahertz Emission Microscopy,LTEM,Brown University,perovskite solar cells,Pernille Klarskov Pedersen

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