C. Brands

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

Thermal stability and immersion solution dependence of second-order nonlinear optical ionically self-assembled films

Polymeric films fabricated from ionically self-assembled monolayers (ISAMs) spontaneously from in a noncentrosymmetric structure requisite for a nonzero second order nonlinear optical (NLO) susceptibility, (chi) (2), without the need for electric field poling. ISAM NLO films exhibit excellent long-term temporal stability of (chi) (2), having shown no decay over a period of nearly three years. They are also remarkably stable at elevated temperatures. While (chi) (2) decreases by 20 percent as the temperature is raised to 150 degrees C, total recovery of the susceptibility is observed upon cooling, demonstrating that the decrease is not due to an irreversible randomization of the chromophore alignment. The thickness, orientational order, and NLO response are found to be strongly dependent on the pH and ionic strength of the solutions form which the films are deposited. The largest (chi) (2) values are observed in films with the smallest bilayer thickness. This suggests that polar orientation is obtained primarily at the interfaces between adjacent layers rather than throughout a full monolayer.

Photovoltaic cells based on ionically self-assembled nanostructures

We use the technique of ionically 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 poly(para-phenylene-vinylene) (PPV) with fullerenes and other organic materials. We apply modulation spectroscopy for the electro-optical characterization of the ISAM-devices. The modulation frequency dependence of the photocurrent can be assigned to the influence of trapped charges taking part in the photovoltaic process.

Photocarrier generation quantum yield for ionically self-assembled monolayers

We have fabricated photovoltaic cells from organic donor/acceptor couples arranged as ionically self assembled monolayers. Making use of the morphological control on the nanometer level we were able to study the influence of the device structure and layer composition on the quantum yield of charge carrier photogeneration. The photoluminescence quenching and the photocurrent spectra reveal sample composition, charge carrier photogeneration and transport properties independently.

Interface effects in plasmon-enhanced second- harmonic generation from self-assembled multilayer films

We have compared the plasmonic enhancement of second-order nonlinear optical (NLO) properties in two types of ionic self-assembled multilayer (ISAM) films combined with Ag nanoparticles fabricated using nanosphere lithography (NSL). The light-concentrating properties of the Ag particles lead to a marked increase in the NLO efficiencies of thin ISAM films. The induced enhancement is found to be much larger in conventional ISAM films than in films made with the hybrid covalent ISAM technique (HCISAM), even though the latter have a significantly larger intrinsic bulk second-order nonlinear susceptibility (χ(2)). The plasmonic enhancement of NLO effects is shown to be primarily an interface effect due to the short decay length of the plasmon modes. The importance of interface effects in the films has been investigated by surrounding thin ISAM and HCISAM films with NLO-inactive buffer layers, which confirmed the important role played by the interfaces, particularly for the HCISAM films.

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.

Photoluminescence studies on energy migration in multilayer organic photovoltaic devices based on ionically self-assembled monolayers

We report the fabrication of thin organic layers and photovoltaic devices made from them. Building thin layers of organic materials via the method of ionically self-assembled monolayers provides control over the layer thickness and composition of multilayer structures on a nanometer scale. This allows to accurately dope a photoluminescent host material with energy or charge accepting guests, changing the emissive character of the pure photoluminescent host film to a predominantly non-emissive, charge generating structure. We show that by varying the concentration of the guest Copper phthalocyanine and C60(OH)2 in poly- (para-phenylene-vinylene) we can measure the energy migration as well as dissociation of the exciton and can determine the lifetime and the diffusion radius of the exciton. Increasing the number of dopands in the host material, the photoluminescence emission spectra shift and decrease in intensity reflecting a decrease in the number of excitons transferring to neighboring chains or conjugation segments. For high dopand concentrations the recombination of excitons only happens on the same chain as the generation. Building a device to achieve the optimal guest/host ratio for optimal exciton dissociation is one important step in the design of high efficiency photovoltaic devices.

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.