Origin of Photochemical Modification of the Resistivity of Ag(DMe-DCNQI)2 Studied by X-ray Absorption Fine Structure

Abstract

The Ag(DMe-DCNQI)2 (DMe-DCNQI = 2,5-dimethyl-N,N′-dicyanoquinonediimine, or DM) charge-transfer salt is a promising material for production by photolithography as it displays a unique photoinduced change in conduction. Photoproducts (β1 and γ) of Ag(DM)2 were investigated using the X-ray absorption fine structure (XAFS) technique in order to understand the origin of their conduction properties. In contrast to the metallic conduction exhibited by a pristine sample (α), β1 is a semiconductor, whereas γ is an insulator, even though the original Ag(DM)2 composition is maintained in both the β1 and the γ forms. A redox mechanism has been postulated to explain the photoinduced change in conduction. However, measurement of the Ag L3-edge XANES (X-ray absorption near-edge structure) did not provide any evidence of electron transfer from the DM radical anion to the Ag cation in the β1 and γ products. Ag K-edge extended X-ray absorption fine structure (EXAFS) data demonstrated that β1 is a mixture of the original pristine α phase and a newly found α′ phase, which has a shorter bond distance between the Ag cations and the N atoms of DM molecules. The conduction electrons remain in the DM column of α and α′, but the domain boundaries between α and α′ present an activation barrier for the conduction electrons to cross these boundaries, which provides a rationale for the semiconductive behavior of β1. The Ag K-edge EXAFS results showed that the γ photoproduct has a different local structure from α, with a shorter distance and smaller coordination number of the Ag−N bond. The DM radical anions in the γ compound form covalent bonds between themselves, resulting in a loss of the columnar structure. The complex local structure around Ag cations explains the insulating behavior of the γ compound.