**4. Conclusions**

In summary, uniformly distributed SnSe2 quantum dots were synthesized at room temperature using a facile and environment-friendly method. The UV-detector based on monolayer graphene and SnSe2 quantum dots was demonstrated. The device showed fast photoresponse time of ~0.31 s, and its photoresponsivity was up to 7.5 × 10<sup>6</sup> mAW−1. The n–n heterostructures between monolayer graphene and SnSe2 QDs improved the light absorption and the transportation of photocarriers, which have promising applications in optoelectronic devices.

**Author Contributions:** Data curation, X.L. and H.Z.; formal analysis, L.L., H.L., and K.T.; resources, S.R., W.Z., P.Y., and Z.S.; writing—review and editing, L.L.

**Funding:** This work was supported by Science and Technology Projects of Shenzhen (JCYJ20180305125000525, JCYJ20170302151033006, and JCYJ20160328144942069).

**Acknowledgments:** The authors thank Min Zhang from Peking University Shenzhen Graduate School for the help in the device fabrication and the Electron Microscope Center of Shenzhen University for the help in the TEM-EDS measurements.

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