Control of Crystal Structure and Orientation of Ni(salen) by Epitaxial Growth on Alkali Halide

Abstract

Thin films of N,N′-bis(salicylaldehydo)ethylenediaminato nickel(II) (Ni(salen)) are fabricated by vacuum epitaxy on (001) surfaces of KBr, KCl, and NaCl substrates and characterized by transmission electron microscopy. Two new monoclinic polymorphs of Ni(salen), β (a = 2.59 nm, b = 1.54 nm, c = 0.670 nm, β = 92.6°) and γ (a = 2.56 nm, b = 0.787 nm, c = 0.743 nm, β = 93.7°), are successfully identified by electron diffractions and high resolution images. The β form is produced on all three substrates by deposition at room temperature, while the γ form is produced on NaCl and KCl at 90 °C. The orientation of these polymorphs is controlled by lattice matching. Although the γ form is energetically favorable with better lattice matching to these substrates, the faster-growing β form is preferentially produced at lower substrate temperature. The polymorphic structure of the deposited film is thus governed by both the substrate surface structure and the growth temperature.