Electronically Coupled Heterojunctions Based on Graphene and Cu2−xS Nanocrystals: The Effect of the Surface Ligand

Optoelectronic devices combining single-layer graphene (SLG) and colloidal semiconducting nanocrystal (NC) heterojunctions have recently gained significant attention as efficient hybrid photodetectors. While most research has concentrated on systems using heavy metal-based semiconductor NCs, there is a need for further exploration of environmentally friendly nanomaterials, such as Cu_2-xS. Chemical ligands play a crucial role in these hybrid photodetectors, as they enable charge transfer between the NCs and SLG. This study investigates the photoresponse of an SLG/Cu_2-xS NCs heterojunction, comparing the effect of two short molecules-tetrabutylammonium iodide (TBAI) and 3,4-dimethylbenzenethiol (DMBT)-as surface ligands on the resulting structures. We have analysed charge transfer at the heterojunctions between SLG and the Cu_2-xS NCs before and after modification with TBAI and DMBT using Raman spectroscopy and transconductance measurements under thermal equilibrium. The photoresponse of two hybrid devices based on three layers of Cu₂₋_xS NCs, deposited in one case on SLG/Cu_2-xS/TBAI ("TBAI-only" device) and in the other on SLG/Cu_2-xS/DMBT ("DMBT + TBAI" device), with a TBAI treatment applied, for both, after each layer deposition, has been evaluated under 450 nm laser diode illumination. The results indicate that the TBAI-only device exhibited a significant increase in photocurrent (4 μA), with high responsivity (40 mA/W) and fast response times (<1 s), while the DMBT + TBAI device had lower photocurrent (0.2 μA) and responsivity (2.4 μA), despite similar response speeds. The difference is attributed to DMBT's π-π interactions with SLG, which enhances electronic coupling but reduces SLG's mobility and responsivity.