**1. Introduction**

Graphene-based electronic and optoelectronic devices have attracted extensive attention [1–3]. Mueller et al. demonstrated a vertical incidence metal–graphene–metal photodetector with an external responsivity of 6.1 mAW−<sup>1</sup> at 1.55 μm [4]. The photoresponsivity was limited by the low absorption of the graphene. Quantum dots (QDs) can break this limitation. They can act as light absorption spots. The photo-induced carriers in them can transfer into the graphene film, and the charges in the graphene film transport to the electrodes quickly. Thus, the responsivity of a graphene-based device is improved [5–7]. Cheng et al. showed a phototransistor based on graphene and graphene QDs with a photoresponsivity of up to 4 × 10<sup>10</sup> mAW−1, but the response time was 10 s [8]. Sun et al. constructed an infrared photodetector based on graphene and PbS QDs with a responsivity of up to 10<sup>10</sup> mAW−<sup>1</sup> and a response time of 0.26 s [9]. Sun et al. demonstrated a UV phototransistor based on graphene and ZnSe/ZnS core/shell QDs. Its responsivity was up to 10<sup>6</sup> mAW−<sup>1</sup> and the response time was 0.52 s [10].

In order to fabricate the QD solution with uniform distribution, the wet chemical method was commonly used. Some organic solvents, such as toluene or pyridine, were used in the process [9,10]. The chemical groups can cap the surface of the QDs and modify their charge transfer property, thus influencing the photo responsivity of the device. Synthesizing QDs using facile and green methods and utilizing them in optoelectronic devices are active areas of research.

Two-dimensional transition-metal dichalcogenides (TMDCs) have been applied in fluorescent imaging [11], biological sensing [12], and photocatalytic [13] due to their unique optoelectronic properties. Tin diselenide (SnSe2) is a semiconductor in the TMDCs family. SnSe2 QDs can be used in fast and highly responsive phototransistors since they have a tunable bandgap and high quantum efficiency. In this paper, SnSe2 QDs were fabricated via sonication and a laser ablation process. The deionized water was used as a solvent, and there were no organic chemicals introduced in the process. It was a facile and environmentally-friendly method. The phototransistor based on monolayer graphene and SnSe2 quantum dots was demonstrated. The photoresponse time was ~0.31 s, and the photoresponsivity was up to 7.5 × 10<sup>6</sup> mAW−1. The n–n heterostructures between monolayer graphene and SnSe2 quantum dots enhance the light absorption and the generation of photocarriers. The photocarriers can transfer quickly from SnSe2 QDs to graphene, thus improving the photoresponsivity of the device.
