The article reflects on the process of time-domain thermoreflectance, its principles, and its applications.
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Introduction to Time-Domain Thermoreflectance
Over the last twenty years, ultrafast laser-based time-domain thermoreflectance (TDTR) has developed as a dependable, efficient, and adaptable approach for measuring the thermal characteristics of a diverse variety of substantial and thin-film materials and their junctions.
The time-domain thermoreflectance (TDTR) approach, in particular, has been used to measure substances with thermal conductivities spanning from a high-end of 2000 W.m-1.K-1 (like diamond and graphite) to a low end of 0.03 W.m-1.K-1 (e.g., disordered WSe2 sheets). TDTR has also been used to test the heat capacity of novel materials and the thermal conductivity of diverse solid/solid interfaces.
TDTR is suitable for a broad range of materials and specimen morphologies. The ability to concentrate the laser dots and have a good signal-to-noise ratio allows for maximum capacity imaging of heat capacity as a factor of location.
As TDTR is a non-contact visual approach, it may be linked topically to specimens in visual cryostats or high-temperature microscopy levels, as well as samples subjected to other harsh circumstances, such as the high-stress situation of a diamond anvil cell. The method entails depositing a metal film transducer on the sample of interest, warming the transducer with an optical signal, and measuring changes in reflectance of the sensor for temperature measurement.
What is Meant by Thermal Conductivity?
Thermal conductivity K is an essential physical property that gauges a material's capacity to transmit heat from high to low degrees. A material's thermal conductivity is normally sensitive to temperature and might be orthogonally dependant, i.e., anisotropic.
Because of geometric restrictions, the thermal conductivity of a thin coating differs greatly from that of its bulk equivalent when its depth is less than the mean free pathways or frequencies of its heat transports (electrons or phonons). As a result, the thermal properties of most crystallographic nanomaterials are thickness-dependent and asymmetrical, despite the fact that their bulk counterparts' thermal properties are symmetric.
Why Monitoring Thermal Properties Is Important
Evaluating thermal characteristics of materials is important not only for comprehending the transport mechanisms of energy carriers but also for potential implementation in communication and power systems. Since the 1950s, extensive attempts have been undertaken to characterize heat capacity and heat transfer resistance in bulk counterparts.
Substances with high-quality thermal properties are required in numerous applications such as the aerospace industry, industrial production facilities, and most notably in materials science, hence monitoring such features is critical. Most current thermal conductivity measuring methods, however, lack the positioning accuracy required to quantify the temperature gradient/difference or thermal flow across length scales less than tens of micrometers.
Advantages of the TDTR Approach
TDTR offers numerous benefits over other heat capacity measuring techniques as a pump-probe approach. It necessitates minimum sample pre-treatment and does not necessitate the precise construction of electric warmers or thermocouples, and it operates as well under normal ambient circumstances or via a vacuum chamber opening. Significant efforts have been made to develop both the TDTR technology and its applications in the temperature and phonon characteristic assessment of many types of materials.
Basic Principle of TDTR
The thermoreflectance, which changes with temperature and is measured by TDTR, is a measure of thermal characteristics. When the temperature increase is minimal, samples are often covered with a thin metal film functioning as a sensor, whose optical reflectance varies linearly with temperature.
A mode-locked Ti: sapphire laser oscillator emits a series of 150 fs pulses at an 80-MHz sampling frequency, with spectra centered around 800 nm in a typical TDTR system. To avoid the laser beam from bouncing back into the synthesizer, a broadband Faraday photonic insulator is fitted at the oscillator's output. For TDTR measurements, a half-wave plate positioned before the isolator can be utilized to modify the laser power.
An electromechanical delay device slows the probe beam concerning the pump beam. A high-speed photodiode sensor collects the redirected probe laser and turns the photons into electrical impulses.
Researchers from California, USA have published their latest research in the Journal of Applied Physics, focusing on the capability of detecting semiconductor materials without the usage of metallic transducers using standard TDTR apparatus. The coating of a thin metal transducer on the material to be examined is required for TDTR. Both the pump and probe lasers have impinged straight on the semiconductor specimen in the innovative transducer less time-domain reflectivity measurements.
A framework for both transport and temperature reactions to laser energy activation and detection with finite dimension and non-zero-layer thickness has been proposed. The output from actual measurements of germanium and silicon samples was accurately described by the model. The best-fit surface recombination velocity for germanium was found to be much higher than the published value. By varying the values of each variable separately, the errors for the recombination rate and the surface recombination speed were determined.
Limitations of TDTR
TDTR is strong and trustworthy, but it has numerous limits which provide enormous prospects for future enhancements of the TDTR approach. Because TDTR is an optical-based pump-probe technology, the sample surface must be optically smooth for the probe beam to be specularly mirrored into the sensor.
According to one theory, the diffusely dispersed probe light is most likely altered by thermoelastic processes and would provide an incorrect signal detected by the detector. The spectrum of heat capacity that TDTR can measure is restricted by the modulation rates and laser spot diameters that can be used. TDTR cannot yet be utilized to determine the thermal conductivity of monolayer or few-layer two-dimensional (2D) materials.
In short, TDTR has opened up vast opportunities in the field of material sciences; however, research needs to be focused on paving a way to overcome its limitations.
References and Further Reading
Linesis, 2022. Time-domain thermoreflectance (TDTR). [Online] Available at:https://www.linseis.com/en/methods/tdtr-time-domain-thermoreflectance/
Warkander, S., & Wu, J. 2022. Transducerless time domain reflectance measurement of semiconductor thermal properties. Journal of Applied Physics, 131(2), 025101. Available at: https://aip.scitation.org/doi/10.1063/5.0069360
Kwon, H., Perez, C., Park, W., Asheghi, M., & Goodson, K. E. 2021. Thermal Characterization of Metal–Oxide Interfaces Using Time-Domain Thermoreflectance with Nanograting Transducers. ACS Applied Materials & Interfaces, 13(48). 58059-58065. Available at: https://pubs.acs.org/doi/abs/10.1021/acsami.1c12422
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