**5. Conclusions**

In summary, though facing several critical problems, the optothermal Raman technique and micro-bridge method still show suitability and feasibility in energy transport characterization in 2D materials. Much pioneering work about the thermal non-equilibrium in different phonon branches has been reported. However, the physical model and data fitting used in the pioneering work still suffer grea<sup>t</sup> uncertainties and make the study

rather semi-quantitative. Secondly, past work has studied the thermal nonequilibrium in suspended 2D materials. However, for supported 2D materials, how the interface resistance between 2D material and substrate affects the *OP*-*AP* thermal nonequilibrium is unclear. Further work can be focused on the *OP*-*AP* thermal nonequilibrium in supported 2D materials.

**Author Contributions:** J.L. conceived and wrote the review; P.L. and H.Z. cooperated in bibliographic searches, studies. J.L. conceived and revised the paper. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Natural Science Foundation of Top Talent of SZTU (2019209) and the Guangdong Basic and Applied Basic Research Foundation (2020A1515110389).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** No new data were created or analyzed in this review. Data sharing is not applicable to this article.

**Acknowledgments:** J. Liu thanks Xinwei Wang of Iowa State University for the discussion.

**Conflicts of Interest:** The authors declare no conflict of interest.
