**7. Summary**

To sum up, this article uses the thermal conductivity of 2D materials as a research platform to discuss the most basic physical issues of heat conduction at the micro/nano scale. Compared with ten years ago, we have a certain understanding of the thermal conduction mechanism of two-dimensional materials. However, there is a long way to go, and there are still many problems in the study of the heat conduction of two-dimensional materials, but this can also point out the need for further research related to the heat conduction of two-dimensional materials.

Here are some issues that need to be considered: (1) There is no rigorous analytical solution for abnormal heat conduction in two-dimensional materials. Existing models for abnormal heat conduction are limited to two-dimensional lattices; (2) What is the thermal conductivity of graphene? There are significant differences between different experiments, and the calculation results of three-phonon scattering and four-phonon scattering are not self-consistent. With the current experimental measurement technology, the measurement of the intrinsic thermal conductivity of single-layer floating graphene seems to be an impossible task; (3) In the current theoretical framework, four-phonon scattering is often directly ignored because it is a type of phonon scattering behavior that only gradually appears in high-temperature regions. However, some theoretical studies have found that even at room temperature, the four-phonon dispersion process is caused by a large number of low-energy ZA mode phonons in single-layer graphene, and this cannot be ignored. Therefore, it is necessary to re-examine the thermal conduction behavior of two-dimensional materials with four-phonon scattering theory; (4) Although theoretical work has shown that even when the scale of the sample reaches the order of millimeters or even centimeters, there is still a scale effect of thermal conductivity; (5) Thermal conductivity measurements, such as the Raman method and thermal bridge method, inevitably have contact thermal resistance problems, which will greatly affect the experimental results. Although some research groups use the dual Raman laser method and electronic beam self-heating method to eliminate the influence of contact thermal resistance, harsh experimental conditions and expensive experimental equipment make it impossible for most research groups to carry out related experiments; (6) The commonly used interfacial thermal conductivity measurements, such as the TDTR and 3w methods, can provide micron-scale spatial resolution, but they can only be used to measure the interfacial thermal resistance of thin-film materials, and their in-plane spatial resolution is also limited to the spot size of the heating laser (usually ~micrometers). Therefore, it is necessary to develop a new measurement method, which needs a spatial resolution that can reach the nanometer scale and detect the interfacial thermal resistance information of two-dimensional materials.

**Author Contributions:** Writing—original draft preparation, M.H.K.; writing—review and editing, X.Z.; supervision, X.Z.; project administration, X.Z.; funding acquisition, X.Z. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Science Foundation CAREER Award, CBET-2145417, and National Science Foundation LEAPS Award, DMR-2137883.

**Data Availability Statement:** Not applicable.

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