Investigations of pH-dependent domain structure of lead arachidate Langmuir-Blodgett films by means of x-ray specular and diffuse scattering and atomic force microscopy

Limited propagation of lattice distortion in trilayer Langmuir-Blodgett films: correlation with mesoscopic structure

The structure of trilayer Langmuir-Blodgett (LB) films on oxidized silicon wafers has been investigated using grazing incidence X-ray diffraction at various incidence angles and atomic force microscopy (AFM). These films are formed by two behenic acid (BA) layers and a third monolayer of amphiphilic molecules having different architectures. These molecules have the same polar head and differ from each other by the chain, either saturated or unsaturated hydrogenated or semi-fluorinated. The structure of the first BA monolayer appears as unchanged in all cases, whereas a condensation of the second BA monolayer is evidenced when the third layer is not formed with the saturated hydrogenated chain. We interpret this condensation as resulting from the mismatch between the lattices of the second BA layer and the external monolayer, possibly associated with the formation of a new monolayer-air interface creating line tension effects. Line tension estimation has also been made from the size of the holes observed in the different LB films.

Crossover from layering to island formation in Langmuir-Blodgett growth: role of long-range intermolecular forces

Combined studies by atomic force microscopy, x-ray reflectivity, and Fourier transform infrared spectroscopy on transition-metal stearate (M-St, M = Mn, Co, Zn, and Cd) Langmuir-Blodgett films clearly indicate association of bidentate coordination of the metal-carboxylate head group to layer-by-layer growth as observed in MnSt and CoSt and partially in ZnSt. Crossover to islandlike growth, as observed in CdSt and ZnSt, is associated with the presence of unidentate coordination in the head group. Morphological evolutions as obtained from one, three, and nine monolayers (MLs) of M-St films are consistent with Frank van der Merwe, Stranski-Krastanov, and Volmer Weber growth modes for M=Mn/Co, Zn, and Cd, respectively, as previously assigned, and are found to vary with number (n) of metal atoms per head group, viz. n=1 (Mn/Co), n=0.75 (Zn), and n=0.5 (Cd). The parameter n is found to decide head-group coordination such that n=1.0 corresponds to bidentate and n=0.5 corresponds to unidentate coordination; the intermediate value in Zn corresponds to a mixture of both. The dependence of the growth mode on head-group structure is explained by the fact that in bidentate head groups, with the in-plane dipole moment being zero, intermolecular forces between adjacent molecules are absent and hence growth proceeds via layering. On the other hand, in unidentate head groups, the existence of a nonzero in-plane dipole moment results in the development of weak in-plane intermolecular forces between adjacent molecules causing in-plane clustering leading to islandlike growth.

Effect of water and air-water interface on the structural modification of Ni-arachidate Langmuir-Blodgett films

Nickel arachidate (NiA) Langmuir-Blodgett (LB) films have been deposited on hydrophilic Si(0 0 1) substrates by three (up-down-up) and five (up-down-up-down-up) strokes. During deposition, substrates were kept inside the water subphase for different times after each down stroke. Structural information of all the LB films have been obtained from X-ray reflectivity (XRR) studies. One and two symmetric monolayer (SML) was deposited on top of the asymmetric monolayer (AML) in three and five stokes respectively. All the preformed LB films were then used to go through the air-water interface with the same speed that was used at the time of film deposition. Structural information obtained from the XRR studies show that mainly the top layer density decreases after passing through the air-water interface but the layered structure remains the same. Information obtained from both the XRR and atomic force microscopy (AFM) studies suggest that molecules peeled from the top SML layer do not reincorporate with the LB film through tail-tail hydrophobic interaction. Our study shows that NiA LB film has better stability compared with cadmium arachidate LB film inside the water subphase without forming any out-of-plane molecular reorganization.