Structure of Polymer Ultra-Thin Films Studied by
Novel Optical Techniques and Fluorescence Spectroscopy
1. Fluorescence Probe Method
Fundamental photophysical processes such as energy transfer and electron transfer take place in a nanometric space. When a fluorescent group (fluorescence probe) is incorporated into a polymer system, the fluorescence from the probe provides information on the nano-structure of the system. We have investigated the layered structure of polymer LB films using this method. For example, the figure depicts the fluorescence spectra before and after themal relaxation of the LB film. Initially, the two layers labled by phenanthrene (excitation energy donor, blue layer in the figure) and anthracene (energy acceptor, red one) were separated ca. 4 nm by the unlabeled layers. At this stage, the strong fluorescence from phenenthrene was observed due to the low excitation energy transfer rate from phenenthrene to anthracene (blue curve). After annealing above the glass transition temperature, the phenanthrene and anthracene layers interpenetrated due to the structural relaxation, resulting in the increase of the anthracene fluorescence component (red curve in the figure). Thus, the fluorescece method is one of powerful tools to probe nanometric structures.
2. Surface Plasmon Resonance Spectroscopy
Surface plasmon resonance is very useful phenomenon for probing thin films and their adsorption kinetics on surfaces. The figure (left) shows the Kretschmann configuration optics in which a thin gold film is deposited on the bottom of prism. The oscillation of free electrons at the metal surface propagates with an intrinsic wave vector along the air(dielectric)-metal interface, when it is excited by momentum-matching with the tangential component of the photon incident through the prism. Since the matching is obtained at a particular angle of incidence, this resonant excitation is observed as a steep dip in the reflectivity curve as shown in the figure (right). This resonant angle is greatly dependent on the thickness and reflactive index of the adsorbate on the gold film. Therefore, the thickness of an ultra-thin film deposited on gold can be estimated from the angular shift. In our laboratory, the structure of layer-by-layer self-assembled films is investigated using this method.
3. Brewster Angle Microscopy
p-Polarized light is not reflected by an interface when it is incident at the Brewster angle (53.1 degree for the air-water interface). A dielectric object on the surface breaks the Brewster condition, resulting in the reflection of the p-polarized light. The advantage of BAM is that it can directly observe polymer monolayer on a water surface. We have revealed the characteristic properties of polymer monolayers originated from low degree of freedom due to the restriction in two dimensions.Back to page top
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