Nanoscale heat transport analysis by scanning thermal microscopy: from calibration to high-resolution measurements

Abstract

Scanning thermal microscopy (SThM) is a powerful technique for thermal characterization. However, one of the most challenging aspects of thermal characterization is obtaining quantitative information on thermal conductivity with nanoscale lateral resolution. We used this technique with the cross-point calibration method to obtain the thermal contact resistance, R_c, and thermal exchange radius, b, using thermo-resistive Pd/Si₃N₄ probes. The cross-point curves correlate the dependence of R_c and b with the sample's thermal conductivity. We implemented a 3ω-SThM method in which reference samples with known thermal conductivity were used in the calibration and validation process to guarantee optimal working conditions. We achieved values of R_c = 0.94 × 10⁶ ± 0.02 K W⁻¹ and b = 2.41 × 10⁻⁷ ± 0.02 m for samples with a low thermal conductivity (between 0.19 and 1.48 W m⁻¹ K⁻¹). These results show a large improvement in spatial resolution over previously reported data for the Wollaston probes (where b ∼ 2.8 μm). Furthermore, the contact resistance with the Pd/Si₃N₄ is ∼20× larger than reported for a Wollaston wire probe (with 0.45 × 10⁵ K W⁻¹). These thermal parameters were used to determine the unknown thermal conductivity of thermoelectric films of Ag₂Se, Ag_2−xSe, Cu₂Se (smooth vs. rough surface), and Bi₂Te₃, obtaining, in units of W m⁻¹ K⁻¹, the values of 0.63 ± 0.07, 0.69 ± 0.15, 0.79 ± 0.03, 0.82 ± 0.04, and 0.93 ± 0.12, respectively. To the best of our knowledge, this is the first time these microfabricated probes have been calibrated using the cross-point method to perform quantitative thermal analysis with nanoscale resolution. Moreover, this work shows high-resolution thermal images of the V_1ω and V_3ω signals, which can offer relevant information on the material's heat dissipation.