Langmuir-Blodgett (LB) films
What are Langmuir-Blodgett (LB) films ?
When we drop a few spots of salad oil on clean surface of water, they become states of lens, and flow on the water surface. In the case of organic compounds such as acetic acid, on the other hand, they immediately dissolve into water, if a few spots of acetic acid are dropped on the water surface. So, how does it change in the case of organic compounds with both water-hated character such as salad oil (hydrophobic group) and water-loved character such as acetic acid (hydrophilic group) ? As seen in Fig. 1, if a balance of both parts is good on the water surface, the hydrophilic and hydrophobic parts are turn water and air, respectively, leading to the formation of thin films spread on the water surface.
And, if the water surface is sufficiently large, the thin films become the thickness of a molecule, resulting in the ultra-thin films which are termed monomolecular films. After the compression of the films by a constant pressure, we can transfer the monomolecular films to the solid substrate by dipping and raising the substrates such as glass, CaF2 and quartz, keeping the states of the condensed films at the air-water interface. Thus transferred films on the solid substrate are called Langmuir-Blodgett (LB) films.
Here, let us survey the process of the fabrication of the LB films. In Fig. 2, after making spreading solution by dissolving amphipathic molecules in organic solvent, the solution is spread on the air-water interface, finally leading to the formation of the monomolecular films in Fig. 2 (e) by a constant pressure after passing through the process from Fig. 2 (a) to (d).
In case that the surface on the solid substrate for the LB film deposition is hydrophobic, the monomolecular films can be transferred to the solid substrate only during the downward strokes, as seen in Fig. 3 (a). And, by combination and balance between the substrate surfaces and the film-forming materials, we can deposit the monomolecular films on the solid substrate during both upward and downward strokes, as is in Fig. 3 (b). In general, three kinds of deposition type is well known as follows:
(a) Films are transferred on the solid substrate only during downward stroke. (X-type of deposition) Thus fabricated LB films are called X-type films.
(b) Films are deposited on the substrate during upward and downward strokes. (Y-type of deposition). The LB films are termed Y-type films.
(c) Films are on the solid substrate only during upward stroke. (Z-type of deposition) The films are recognized as Z-type films.
Why do we do the basic research using LB films ?
While the reason depends on the stance of the individual studies, there are some clear motivations and merits of doing the research using LB films.
One is that unlike cast and spin coat films, the structure, composition and thickness of the LB films can be precisely controlled at the molecular level in the case of LB films, and that the structural characterization against the specific functions can be exactly carried out because the LB films are prepared by functional molecules which are categorized by hydrophobic, hydrophilic and chromophore parts. The other is that the intramolecular structures of the functional molecules in the monomolecular films may change drastically or slightly during the film fabrication because of using the specific interface composed of air and water whose interface is fairly different from the case of air-solid interface, and that due to its effect, the intermolecular interaction may also change, implying a possibility that the films with the unique structures are expected to be produced. The characteristics of the LB films are predicted to be quite different from those of cast and spin coat films.
Therefore, from the above viewpoints, the LB films are considered to be one of the research subjects to get the guidelines as to novel functions and structures of the films.
Characterization methods of structures of LB films
As is well known, “structures” of the LB films involve a broad meaning. Therefore, the following structures of the LB films should be synthetically examined. (1) Structure of deposition: X-, Y- and Z type film, (2) Orientation of molecules: Angle between the transition moment of each part of molecules and the substrate, (3) Formation of aggregates: J- and H-aggregates, (4) Conformation, (5) Packing: Hydrocarbon chain, (6) Intramolecular charge transfer and (7) Electronic states: Chromophore. The information on the structures of the LB films can be characterized by means of UV-visible absorption spectroscopy, fluorescence spectroscopy, FT-IR absorption spectroscopy, Raman spectroscopy and X-ray diffraction method.
Since the structural results obtained from only above spectroscopy are half of information about structures, we must also investigate morphology in the LB films. The reason is that in many cases, crystalline is formed by the self-assembly of the individual molecules (functional molecules) during the fabrication of the monomolecular films at an air-water interface. Therefore, we have to examine following structure as well. (8) Macroscopic morphology: Flat (roughness) and Island (crystalline), and (9) Microscopic morphology: Arrangement of molecules.
Atomic force microscopy (AFM), scanning near-field optical microscopy (SNOM), polarized light microscopy and florescence microscopy have been well used to check morphology in the LB films. Consequently, the structural characterization of the LB films should be carried out utilizing the above various tools.
2. Syed Arshad Hussain, Sekhar Chakraborty and Debajyoti Bhattacharjee; Nano Dimensional Hybrid Organo-clay Langmuir-Blodgett Films; Current Physical Chemistry 3(3) (2013)322 – 332