M. Guzy

Layer-by-layer deposition and ordering of low-molecular-weight dye molecules for second-order nonlinear optics.

A combination of electrostatic interactions and covalent bonding is used to form films with low-molecular-weight chromophores by a layer-by-layer deposition process. Using a common, commercially available red dye, this deposition process results in noncentrosymmetric films (see scheme) that exhibit secondharmonic generation (red green), with (2) values as large as 11.3 10 9 esu, that is, six times that of quartz. K. E. Van Cott,* M. Guzy, P. Neyman, C. Brands, J. R. Heflin, H. W. Gibson, R. M. Davis . . . . . . . . . . . . . . . . 3236 ± 3238

In situ second harmonic generation measurements of the formation of ionically self-assembled monolayers

Recently, ionically self-assembled monolayers (ISAMs) of polymers with nonlinear optical chromophores have been shown to spontaneously exhibit a preferential ordering during the deposition process. This gives rise to a substantial second order nonlinear optical (NLO) response. Here, we use this second harmonic generation (SHG) to our advantage in the in situ study of the deposition of ISAMs. Upon immersion in the NLO-active polyelectrolyte solution, the SHG rises sharply over the first minute, demonstrating a rapid adsorption process. Immersion in the NLO-inactive partner polyelectrolyte leads to an instantaneous reduction in the SHG signal. We also show that the adsorption and ordering of a noncentrosymmetric nonlinear optical polymer increases with increasing PCBS concentration. These studies provide deeper understanding of the processes by which noncentrosymmetric order is formed in ISAM films and allows design of improved self-assembled nonlinear optical materials.

Second-order nonlinear optical responses of ionically self-assembled films: polycation variations and dianionic chromophores

Ionically self-assembled monolayer (ISAM) films have been shown to spontaneously produce noncentrosymmetric ordering that gives rise to a substantial second order nonlinear optical (NLO) response. Typically, the ISAM films for NLO response are an assemblage of bilayers of oppositely charged polymers whose thickness can be controlled through variation of pH and ionic strength of the immersion solutions. Ordinarily, the NLO-inactive polycation solution parameters are chosen the same as for the NLO-active polyanion solution. Here, we study the effects of varying the polycation solution parameters separately from the polyanion solution. We also investigate the effects of replacing the NLO-active polymer layers with layers of dianionic molecules. Films fabricated exclusively using polyelectrolytes contain some fraction of both randomly oriented and anti-parallel oriented chromophores. We have examined the incorporation of dianionic chromophores into ISAM films in order to increase the net polar orientation of the chromophores and reduce bilayer thickness.

Photovoltaic responses in ionically self-assembled nanostructures containing conjugated polymers and fullerenes

We use the technique of ironically self-assembled monolayers (ISAMs) to produce photovoltaic devices of well-controlled thickness and composition. The ISAM nanostructure fabrication method simply involves the alternate dipping of a charged substrate into aqueous cationic and anionic solutions at room temperature. We have employed several approaches to combine the tetrahydrothiophenium precursor of PPV with fullerenes and other organic materials .We apply modulation spectroscopy for the electro-optical characterization of the ISAM-devices. Analyzing the thickness dependence of the recorded photocurrent action spectra allows us to identify the photoactive region within the devices. The modulation frequency dependence of the photocurrent can be assigned to the influence of trapped charges taking part in the photovoltaic process. By utilizing the ability to control both thickness and composition of the organic layer at a nanometer level of precision, the composition and concentration of these defects has ben systematically varied.

Organic electro-optic films fabricated by hybrid covalent/ionic self-assembly

By alternating the deposition interaction between covalent and ionic binding, organic self-assembled films are fabricated with electro-optic coefficients of 20 pm/V. The rapidly deposited multilayer films exhibit excellent temporal and thermal stability.

In situ second harmonic generation measurements of the growth of nonlinear optical ionically self-assembled monolayers

Ionically self-assembled monolayers (ISAMs) have recently been shown to spontaneously exhibit a polar ordering that gives rise to a substantial second order nonlinear optical (NLO) response. Here, the deposition of ISAMs has been studied in situ via second harmonic generation (SHG). We show that the adsorption and ordering of a noncentrosymmetric nonlinear optical polymer is constant over a wide range of concentrations. Upon immersion in the NLO-active polyelectrolyte solution, the SHG rises sharply over the first minute. Immersion in the NLO-inactive partner polyelectrolyte leads to a reduction in the SHG signal. Furthermore, when the film is removed from the NLO-active solution and allowed to dry, the SHG increases rapidly as the water evaporates. These studies provide greater understanding of the processes by which noncentrosymmetric order is formed in ISAM films and allows design of improved self- assembled nonlinear optical materials.

Enhanced second order nonlinear optical susceptibilities in ionically self-assembled films incorporating dianionic molecules

Ionically self-assembled monolayer (ISAM) films have been recently shown to spontaneously produce noncentrosymmetric ordering that gives rise to a substantial second order nonlinear optical (NLO) response. Typically, the ISAM films for NLO response are an assemblage of bilayers of oppositely charged polymers whose thickness can be controlled through variation of pH and ionic strength of the immersion solutions. Here, we study the effects of replacing the NLO-active polymer layers with layers of dianionic molecules. Films fabricated exclusively using polyelectrolytes contain some fraction of both randomly oriented and anti-parallel oriented chromophores. The incorporation of dianionic molecules within the ISAM films affords greater χ (2) due to increased net polar orientation of the chromophores.

Hybrid covalent/ionic self-assembly of second order nonlinear optical films

A multilayer deposition scheme has been developed based on alternating covalent and ionic binding between successive layers to yield organic films with inherent polar order. Electro-optic coefficients of 20 pm/V have been observed

Hybrid covalent/ionic self-assembly of organic second-order nonlinear optical films

Ionically self-assembled monolayer (ISAM) films have been shown to spontaneously produce noncentrosymmetric ordering that gives rise to a substantial second order nonlinear optical (NLO response. Previously, the ISAM films for NLO response consisted of bilayers of oppositely charged polymers whose thickness can be controlled through variation of pH and ionic strength of the immersion solutions. Here, we present an approach that replaces the NLO-active polymer layers with layers of monomeric chromophores containing ionic and covalent bonding sites. We have examined the effect of the pH of the immersion solutions on the orientation the of monomeric chromophores and demonstrated bulk polar ordering with X(2) values six times larger than quartz.

In Situ Measurement of the Second Harmonic Signal of Adsorbing Nonlinear Optical Ionically Self-Assembled Monolayers.

Ionically self-assembled monolayers (ISAMs) have recently been shown to spontaneously exhibit a polar ordering that gives rise to substantial second order nonlinear optical response. The deposition of ISAMs has been studied in situ via second harmonic generation. This is a particularly sensitive probe of the growth of nanometer-thick films since the centrosymmetry of the immersion solutions, the substrate, and the container yields no SHG contribution from these bulk components. Upon immersion in the NLO-active polyelectrolyte solution, the SHG rises sharply over the first minute. When a film is immersed into salt water, the SHG decreases significantly only to be restored when the salt solution is replaced with deionized water.