Thermal image of a lithographically fabricated gold nanodot under near-field thermal imaging.
By Amanda Biederman
NQPI editorial intern
Nanoscale heat localization and thermosensing has a number of potential applications including cancer therapeutics and nanoelectronics. Chemistry and Biochemistry professor and Nanoscale and Quantum Phenomena Institute member Hugh Richardson and Ph.D. student Susil Baral are working to develop this novel technology.
Dr. Hugh Richardson
In order to probe temperature at the nanoscale, the group generates two temperature-dependent emission peaks of a trivalent erbium ion. Richardson’s group developed a thermal sensor film of erbium ions for nanoscale temperature measurements in 2011 and used the technique for a number of successful projects. However, the technique is bound by one fundamental limitation, Baral said. They can only measure temperature in regions larger than the diffraction limit due to a light blurring effect.
“If we have really good optics, the best we can do is lambda (the wavelength of the excited light) divided by two,” Baral said. “So if you want to take a look at a 100 nm particle, or a 40 nm particle, or a 10 nm particle … you see a big 270 nm spot. You are missing the true picture.”
Richardson’s group is developing two strategies to overcome the diffraction limit effect. The first, which they began developing in 2014, is to use a smaller nanothermometer that is comparable to the size of the heater. Using this technology, the group can heat and measure temperature in a sub-diffraction regime. Baral said this technique is promising for biomedical applications, as the thermometer can be inserted into living cells.
Over the past year, the group has developed a second strategy to overcome the diffraction limit problem. This technique relies on near-field microscopy, meaning that the diffraction-limited region is excited and the emission from a sub-diffraction region of interest is isolated using a sharp metallic tip with a small opening (known as SNOM cantilever tip).
Baral said near-field techniques may not be ideal for biological applications because the needle can puncture cells. However, the technique may be of importance in thermal mapping of nanoelectronics and devices, which are susceptible to overheating.
“We have seen a lot of advances in phones and computers,” Baral said. “This is all because of work at the nanoscale.… You need a fundamental understanding of how things work and to understand the limitations first.”
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