Thermometer Measures Small & Rapid Temperature Changes on the Nanoscale

A new micrometer-wide thermometer has been developed by researchers at Tokyo Institute of Technology (Tokyo Tech) and their collaborators to record small temperature changes.

(a) Video still showing a tightly focused laser beam making contact with the thermocouple. (b) Graph showing the thermocouple's response over time to different laser powers (3.6 and 1.8 mW) at different repetition rates, on glass and on the silicon nitride membrane (delta T: change in temperature, tau: time for temperature rise and decay). (Image credit: Scientific Reports)

This novel device is sensitive to heat produced by electron and optical beams and is capable of measuring small and quick temperature changes in real time. The thermometer can be used to inspect heat transport on the nano- and micro-scales, as well as in synchrotron radiation experiments and optical microscopy.

A device that can determine thermal behavior in real time and on the nanoscale is urgently required, because such a technology could possibly be applied in photo-thermal cancer treatment and also in advanced research on optical light harvesting and crystals, to name a few. In addition, there is a need for a nanoscale heat source and detector for upcoming development of advanced transistors that will be used for designing novel nanoscale devices.

A thermocouple is an example of an electrical device that contains two dissimilar electrical conductors, which form electrical junctions at varying temperatures. A temperature-dependent voltage is produced by a thermocouple which can be interpreted to determine temperature. Researchers at Tokyo Institute of Technology and their colleagues recently developed the micro-thermocouple, which is very important to researchers in many different fields.

This device features a nickel and gold thermocouple on a silicon nitride membrane and is miniaturized in such a way that the electrodes measure just 2.5 μm in width and the membrane measures only 30 nm in thickness. If such a system is to be used as a thermal characterization device, that is, a thermometer, it has to show responsiveness to temperature changes. The developed micro-thermocouple showed high sensitively to the heat produced by an electron beam and a laser. Most significantly, this thermocouple was used to measure small temperature changes for both types of heating.

The micro-thermocouple was prepared using an already developed miniaturization process, but important improvements were also made. In the proven technique, a cross pattern of metal stripes having widths of a few micrometers is produced to create a thermocouple. This technique was employed by the scientists at Tokyo Institute of Technology and their collaborators to produce a pattern on a nano-thin silicon nitride membrane, which not only improved the device sensitivity but also allowed it to respond faster. With this method, a thermometer that could measure small and rapid temperature changes was effectively created, with the measurements being carried out via the nano-thin silicon nitride membrane.

As described above, a nanoscale heat source, as well as a nanoscale detector, is required for a compact microscopy system. These requirements were effectively met by the team, who created a heat source with less than 1 μm diameter using the nano-thin membrane and a tightly focused electron or laser beam. Therefore, a nanoscale thermal microscopy system, together with the micro-thermocouple detector, was successfully realized. This system can be considered as a novel "toolbox" for studying heat transport behavior on the nano- and micro-scales, with many significant applications in a variety of fields.

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