Tunable n-type and p-type doping of two-dimensional layered PdSe$_2$ via organic molecular adsorption
Palladium diselenide (PdSe$_2$) is a two-dimensional (2D) transition metal dichalcogenides (TMDs) semiconductor with desirable properties for nanoelectronics. Here, we demonstrate that 2D layered PdSe$_2$ adsorbed with two kinds of organic molecules, an electrophilic molecule tetracyano-p-quinodimethane (TCNQ) as electron acceptor and a nucleophilic molecule tetrathiafulvalene (TTF) as electron donor, can realize tunable p-type and n-type doping of 2D PdSe$_2$ by using first-principles density functional theory (DFT) calculations. We find that the TCNQ attracts electrons from PdSe$_2$ and introduces shallow acceptor states close to the valence band edge, resulting in p-type doping of PdSe$_2$, while the TTF donates electrons into PdSe$_2$ and introduces shallow donor states close to the conduction band edge, resulting in n-type doping of PdSe$_2$. Furthermore, such p-type and n-type doping of PdSe$_2$ can be efficiently controlled with the external electric field, interlayer distance and substrate thickness. Such effective bipolar doping of PdSe$_2$ via molecular adsorption would broaden its applications in nanoelectronics.