Structural studies of Langmuir–Blodgett films containing rare-earth metal cations

Comparative structural study of gadolinium stearate Langmuir–Blodgett (LB) films formed by monolayer deposition from either aqueous gadolinium acetate or gadolinium chloride solutions have been carried out. Structure of the films was characterized by X-ray diffraction, Fourier transform infrared spectroscopy, high-energy electron diffraction, atomic force microscopy and scanning electron microscopy. It was found that when subphase pH had a value at which all monolayer stearic acid molecules were ionized and bound with Gd3+ cations (pH>5), the LB films deposited from gadolinium acetate and gadolinium chloride subphases had substantially different structure. The gadolinium stearate LB films formed with gadolinium acetate subphase were highly ordered and consisted of hexagonal layers with unit cell parameter a≈4.8 Å and interlayer spacing d≈49 Å. LB films formed with GdCl3 subphase under the same conditions (salt concentration, pH) were characterized by a rather disordered structure where along with hexagonal phase, a monoclinic phase with the interlayer parameter d≈40 Å was present. The reason of this phenomenon is discussed. The data obtained indicate that the control of multivalent metal cations complexes formation in the subphase and at the monolayer surface can be an instrument for optimization, the conditions to form metal-containing LB film with regulated structure and properties.