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.

Colloidal quantum dots (CQDs) provide size-tunable optoelectronic properties, enabling broadband photodetection from the visible to the short-wave infrared (SWIR). However, CQD photodetector performance depends on different processes such as optical absorption, charge transport, and photogain mechanisms in CQD solids. Here, we investigate the impact of PbS CQD size on the electrical conductivity and photodetection performance of CQD films and hybrid graphene/CQD photodetectors. Three CQD sizes are studied: d~3.06 nm, d~3.78 nm, and d~5.27 nm, with exciton peaks at λ_e~935 nm, λ_e~1080 nm, and λ_e~1550 nm, respectively. Electrical measurements show that the largest CQDs exhibit higher conductivity. In contrast, photodetection measurements reveal that the largest-sized CQDs (~5.27 nm) produce the lowest photoresponse, both as bare CQDs and as hybrid graphene/CQD photodetectors. Spectral and power-dependent measurements show decreasing responsivity with increasing optical power, consistent with trap-mediated photoconductive gain. These results indicate that the CQD size can have a significant effect on the optoelectronic performance of CQD devices, requiring materials, interfaces, and design optimization to maintain high photodetector performance.