**5. Conclusions**

The finding of a series of surprisingly high *T*c in FeSe-based materials is a significant breakthrough in the superconducting community and sets up a very interesting superconducting family. However, the microscopic mechanism of the enhancement of *T*c through multiple treatments is still elusive. Many kinds of effects have been proposed to explain the enhancement in *T*c such as coupling between the electrons and phonons of the substrate, the tensile strain effect introduced by lattice mismatch between FeSe film and the substrate, and the charge transfer, *etc*. More subtle experiments should be conducted to figure out which effect plays the decisive role. In addition, as FeSe thin films are very thin and are easily oxidized, the transport and magnetic properties are hard to measure. Therefore, new methods and more novel experimental tools should be developed to detect the intrinsic physical properties.

**Author Contributions:** Investigation, K.Z., J.W.; Y.S. writing—original draft preparation, K.Z.; J.W.; L.G.; J.-g.G.; writing—review and editing, K.Z.; J.W.; J.-g.G.; funding acquisition, J.-g.G.

**Funding:** This research was funded by the National Natural Science Foundation of China, gran<sup>t</sup> number 51922105 and 51772322, the National Key Research and Development Program of China, gran<sup>t</sup> number 2016YFA0300600 and 2017YFA0304700, and the Chinese Academy of Sciences, gran<sup>t</sup> number QYZDJ-SSW-SLH013.

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