Electric-field-tunable Molecular Doping In Germanane

Germanane,a new two-dimensional(2D)semiconductor material,has received great attention due to its novel electronic properties for applications of electronic and optoelectronic devices,such as field-effect transistors and p-n junctions.To increase the carrier concentration for reducing the Schottky barrier at the interface between metal and semiconductor or reproducing the electronic characteristics of the device,it is of great significance to achieve a safe and effective doping in germanane.In this thesis,the adsorption behaviors of tetrathiafulvalene(TTF)and tetrahydrocyanobenzene(TCNB)on germanane surface are studied through first-principles calculations based on density functional theory(DFT).The result shows that a safe and effective n-type or ptype molecular doping in germanane can be realized by adsorbing TTF or TCNB on the surface of germanane..For the case of TTF molecular adsorption,there is only a weak physical interaction between germanane and TTF molecule.After the adsorption of TTF molecule,the structural and the electronic properties of germanane have not changed obviously.However,TTF molecule can introduce an effective donor level into the band structure of germanane,which makes the adsorption system exhibiting a typical n-type doping characteristics.The energy difference for electronic excitation is 0.472 eV.More importantly,the intensity of this molecular doping effect can be linearly tuned by the external electric field ranged from-0.3V/? to 0.3V/? while the electronic structure of germanane is almost unchanged.In particular,when the external electric field of-0.3V/? is applied,the energy difference for exciting electron from highest occupied state to unoccupied state is only 0.085 eV.In addition,the charge transfer between the TTF molecule and the germanane substrate can also be effectively regulated by an external electric field.Especially under the electric field strength exceeding 0.15V/?,TTF molecule would even obtain electrons from the germanane substrate and overturn to be an electron acceptor.For TCNB molecular adsorption,it can induce an effective p-type doping effect in germanane.The energy difference for electron excitation is0.173 eV.Similar to the case of TTF,this p-type doping effect also can be linearly tuned by external electric field.In order to further explain the regulation effect,themechanism of doping effect and charge transfer under external electric field are revealed through equivalent capacitor model and the variety of energy levels for system.It is worthwhile to note that the effective molecular doping in germanane induced by TTF or TCNB adsorption would not affect the structural and the electronic properties of germanane.Importantly,this molecular doping effect can be linearly tuned by external electric field,which would be helpful for the development of related experiments in the future.