Langmuir-Blodgett films as single-layer analogs of known organic/inorganic solid-state materials

Langmuir-Blodgett films of manganese octadecylphosphonate (Mn(O3PC18H37)·H2O) are prepared by depositing octadecylphosphonate monolayers from a MnCl2·4H2O containing subphase at a pH range of 5.2–5.5. The LB films possess crystalline order that is analogous to the bulk manganese organophosphonates. Pressure versus area isotherms suggest that the Mn2+ ions cross-link the phosphonate groups at the air/water interface. Two sharp IR bands at 978 cm−1 and 1088 cm−1 can be assigned to the symmetric and asymmetric PO32− stretching modes, respectively, and a H2O bending mode at 1608 cm−1 is also observed. XPS analysis demonstrates that the Mn to P ratio is 1 to 1 which is consistent with the bulk manganese phosphonates. X-ray diffraction yields a layer spacing of 48.5 Å. The in-plane structure is determined by transmission electron diffraction (TED). The combined diffraction data yield lattice parameters for the LB film based on an orthorhombic space group of a a = 5.7 Å, b = 48.5 Å and c = 4.9 Å. The in-plane order is analogous to that of the known bulk manganese organophosphonates. The shape and g-value of the EPR signals for the bulk manganese phenylphosphonate and the LB film are identical. Temperature dependent magnetic susceptibility of the LB film demonstrates short-range antiferromagnetic order with the coherence length of the interaction increasing as the temperature is lowered.