Graphene Phototransistors Sensitized by Cu2–x Se Nanocrystals with Short Amine Ligands

Recent attempts to employ colloidal semiconductor nanocrystals (NCs) as the sensitizing materials for hybrid NC–graphene phototransistors have provided a new effective photosensing platform. However, most of these devices are based on NCs containing either lead or cadmium, which would not be the most preferred material candidates for commercialization. Here, we demonstrate the use of colloidal Cu2–x Se NCs that do not contain lead or cadmium as the sensitizers for NCs–graphene hybrid visible phototransistors. Because the long olyelamine ligands originally attached on Cu2–x Se NCs are known to impede electronic process between NCs and graphene, the long ligands are replaced with short amines including octylamine, hexylamine, and butylamine. It is found that the NCs layer with shorter amine ligands yields a more prominent n-doping effect on graphene under illumination, which results in a systematic negative shift in Dirac voltage. More importantly, this leads to devices with larger photocurrent and larger light responsivity. Consequently, from Cu2–x Se NC–graphene hybrid phototransistors attached with butylamine ligands, responsivity as high as 2600 A/W and photocurrent gain as high as 36 000 are achieved at an optical power of 5 × 10–8 W, which are expected be even larger at lower optical powers.