Fabrication and notable optical nonlinearities of ultrathin composite films derived from water-soluble Keggin-type polyoxometalates and water-insoluble phthalocyanine

Abstract

Composite films with the general formula (POM/CuTAPc)_n derived from water-soluble Keggin-type polyoxometalates (POMs = H₅PMo₁₀V₂O₄₀, H₄SiW₁₂O₄₀, H₃PMo₁₂O₄₀ and H₃PW₁₂O₄₀) and water-insoluble 4,9,16,23-copper tetraaminophthalocyanine (denoted CuTAPc) are successfully fabricated by a layer-by-layer self-assembly technique and systematically characterized. The structure of the polyoxometalate anions in the multilayers is kept intact; the deposition amounts of POM and CuTAPc remain constant in every adsorption cycle of the composite film assembly process. The nonlinear optical properties of the composite films were studied by a Z-scan technique at a wavelength of 532 nm and a pulse width of 7 ns. The results not only show that the composite films exhibit notable optical nonlinear self-defocusing behavior and a saturated absorption effect with the nonlinear optical absorption co-efficient β, refractive index n₂, and third-order NLO susceptibility χ⁽³⁾ of the films increasing with the increase in number of layers of the films, but also reveal importantly that the discrepancy of LUMO levels between CuTAPc and POMs is proportional to their third-order NLO response.