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

Abstract

Heteroelement doping is one of the most common techniques for decreasing or increasing the phase transition temperature (T_c) 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 T_c 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 ∼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 T_c).