Self Assembled Monolayer (ISAM) films present an opportunity to manufacture nonlinear optical devices in an economical and timely manner.  These films are created through alternate submersion of a substrate in an anionic and a cationic solution with an intermediate rinsing step. This creates a bilayer of material, which, through many dipping steps creates a film.

In order to achieve the second order nonlinear optical (NLO) properties desired from these films, noncentrosymmetric chromophores are incorporated into the structure of the films. The NLO activity is due to the structure of the chromophore and its orientation. Desired structural qualities include aromaticity and a net dipole moment. The orientation is crucial, because if the dipoles are not aligned in the film, there will be no second order NLO effects.

An approach to bypass the poling process is to create a superlattice of functionalized molecules for stable, ultrathin film growth. The Langmuir-Blodgett technique is one of the primary methods used to fabricate such structures; however, the technique is difficult and complicated. An alternative approach is to produce the superlattice by self-assembly. Self-assembled monolayers (SAMs) are structures of covalently bonded, precisely aligned chromophores. They are mechanically and thermally robust, they have excellent adhering properties, and uniform thicknesses can be fabricated. The nonlinearity can be improved by a subsequent ion exchange procedure.23 [1] NLO waveguides have been fabricated with SAMs of stilbazolium-based chromophores on fused quartz and glass substrates overcoated by polystyrene (PS)24 [2]or PMMA.25 [3]However, the SAM technique is limited by the chemical bonding required at each layer, leading to an increase in fabrication time (hrs. to days) due to the needed chemical reactions23,26 [1,4]

In contrast to these, the ISAM approach for ultrathin film growth is a simple and rapid procedure. The monolayers are formed by electrostatic attraction, rather than by chemical bonding, between oppositely charged organic polyelectrolytes: polyanions (negatively charged) and polycations (positively charged); see Fig. 4. Inorganic electrolytes can also be used, e.g., cationic nanoparticles of titanium dioxide TiO227 or of iron oxide Fe3O4 (i.e., magnetic thin films).28 [5]ISAMs have been implemented recently in biosensors, magnetic thin films, and LEDs.28,29 [5,6] In 1997, for the first time, the ISAM technique was implemented for nonlinear optical thin film growth.30[7]

 

Fig-1 Diagram of one ISAM PTOPDT/PSS bilayer. The cationic layer is PTOPDT, and the anionic layer is PSS.

ISAMs have the same advantages as LB films and covalent SAMs such as mechanical and thermal robustness, excellent adhering properties, uniform thickness, and tailored noncentrosymmetry and optical nonlinearity.23,31,32 In addition, the growth process is inexpensive, quick (1 monolayer in a few minutes), and fabrication is achieved at room temperature.30 ISAMs are insoluble in common organic solvents and strong acids23,31 The structures have long term stability, since their noncentrosymmetric structure is in ground state equilibrium; i.e., the chromophore alignment (nonlinearity) does not relax with time. Thicker films can be grown than those obtained by LB are possible.31 Complete control of detailed structure and thickness can be maintained easily.30 ISAMs are naturally self-limiting in thickness and uniform at the molecular level. Therefore, film thickness increases linearly with the number of bilayers and each bilayer has uniform thickness (~1.2 nm).30,33 The second harmonic intensity has dependence on fundamental intensity, on film thickness, and on polarization, also refractive index, and absorption coefficient.34 Furthermore, absorbance increases linearly with the number of bilayers.30

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