Metallic Functionalization of CdSe 2D Nanoplatelets and Its Impact on Electronic Transport

We explore the gold functionalization of 2D CdSe nano-platelets (NPL) as a possible way to tune their electronic and transport properties. We demonstrate that the size and location of the gold tip can be controlled using light and temperature. The Au tip-CdSe NPL hybrid presents a large rise of the conductance compared to the pristine semiconductor (i.e., without gold functionalization). The role of the semiconductor in this transport remains unclear and needs to be better understood. We hypothesize four mechanisms: (i) a reduction of the band gap energy due to the formation of a gold-selenium compound, (ii) a charge transfer between the metal and the semiconductor leading to an increase in carrier concentration, (iii) a change in the inter-nanoparticle tunnel barrier height, and (iv) a simple percolation process between the metallic grain. X-ray photoelectron spectroscopy (XPS) shows that the CdSe NPL are unaffected by oxidation and that gold is in the Metallic state Au. We consequently exclude the formation of a narrow band gap Au2Se phase as the possible mechanism leading to the observed rise of conductance. Moreover, Kelvin probe force microscopy and XPS give evidence for an increase in work function upon gold tipping, which can be interpreted in terms of a shift of the Fermi level toward the valence band maximum. As hole conduction in CdSe NPLs is very unlikely to occur, we rather favor the hypothesis that the strong increase in conduction is largely driven by percolation between the metallic tips as the main mechanism responsible for transport in this hybrid system.