Reducing the metal-insulator transition temperature of VO2 nanowires by surface molecular adsorption-induced hole doping

Heteroelement doping is one of the most common techniques for decreasing or increasing the phase transition temperature (Tc) of VO2, but it generally suffers from a concomitant deterioration in the magnitude of the resistance change due to the lattice damage. Here, we report a straightforward and efficient technique to modify the metal-insulator transition (MIT) behavior of VO2 nanowires (NWs) by surface charge transfer produced by the adsorption of tetrafluorotetracyanoquinodimethane (F4TCNQ) molecules. It is anticipated that this surface molecule adsorption will maintain the steep MIT transition of VO2 NWs by preserving the pristine lattice without introducing substitutional disorder. An intriguing finding from the variable-temperature electrical measurements is that the Tc of VO2 NW adsorbed with F4TCNQ reduces by more than 25 K when compared to that of the pristine sample, and meanwhile the resulting amplitude in resistance remains similar to 4.5 orders of magnitude. The variable-temperature optical image observations provide additional confirmation of this result. According to first-principles calculations and crystal field analysis, there is a thermodynamically spontaneous charge transfer at the F4TCNQ/VO2 NW interface, signifying that holes transfer from F4TCNQ to the VO2 NWs and electrons transfer from the VO2 NWs to F4TCNQ. These hole carriers would lower the crystal stability energy by changing the V 3d orbital occupancy and weakening the electron-electron correlation. These factors facilitate the earlier occurrence of MIT in VO2 NWs (i.e., the decrease of Tc).