MS-8-P-6006 Switching behavior of single Ag-TCNQ nanowires: an in situ Transmission Electron Microscopy study

Unique properties, such as high density of charge carriers and electric field-induced switching behavior are found in one-dimensional nanostructures of metal-tetracyanoquinodimethane (M-TCNQ).[1] For the case of Ag-TCNQ, reversible phase transition can be induced by electric field applied along the TCNQ ∏-∏ stacking direction, resulting in resistive switching with an on-off ratio reported to be as high as 10⁴. A proposed mechanism suggests that, a partial neutral species of Ag and TCNQ form during the transition, provide additional conduction channels and increase the material conductivity substantially. Studies based on large-area Ag-TCNQ nanowires (NWs) provided useful information.[2] However, there is still lack of detailed studies from individual NWs, where the NW size and electric behavior can be correlated, and much more intrinsic qualities of a particular NW can be expected.
Using in situ Transmission Electron Microscopy (TEM), we are able to investigate the switching behaviors of individual Ag-TCNQ NWs. For the in situ study the NWs were grown directly on a Au TEM grid, as shown in Figure 1a. Figure 1b depicts one unit cell of the orthorhombic Ag-TCNQ phase II structure [3] along different axes. Typical TEM image and diffraction pattern from a single Ag-TCNQ NW are shown in Figure 1c and 1d. Generally, a NW growth direction along [100] is observed. In situ electric measurements were performed using an STM-TEM holder from Nanofactory Instruments AB in combination with the aberration-corrected Titan³ 80-300 microscope at the University of Erlangen-Nürnberg. The W tip and the Au TEM grid serve as the two electrodes as schematically shown in the inset of Figure 2a.
In our study, for up to 30 individual NWs with different sizes, phase transition and resistive switching have been successfully induced. The typical I-V behavior of a single NW is shown in Figure 2a: starting from a state with low conductivity, and then switched on once the electric field reaching a certain point. The differentiation of current over bias, shown as inset in lower right of Figure 2a, as well suggests a sudden increase in the NW conductivity. Together with the large current passing through the NW after switched on, Joule heating becomes an issue which can easily lead to the NW breaking down. Figure 2b shows a NW switched on and surviving a current rang up to ~100 nA. However, it broke down at the center under the same bias sweep, but with current compliance increased to ~1 µA. This failure indicates that, heat dissipation should be taken into consideration for achieving high performance devices.

Reference
[1] Potember P. S. et al., Appl. Phys. Lett., 34, 1979, 405.
[2] Xiao K. et al., Adv. Funct. Mater., 18, 2008, 3043.
[3] Shields L. J. Chem. Soc. Faraday Trans. 2, 1985, 1.