P.J. Neyman

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.

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.

Efficiency optimization in ionically self-assembled thin film polymer light-emitting diodes

We present detailed studies of polymer light emitting diodes fabricated from ionically self-assembled monolayer thin films. The ionically self-assembled monolayer (ISAM) films are created with a new thin film fabrication technique that allows detailed structural and thickness control at the molecular level. The ISAM fabrication method simply involves the dipping of a charged substrate alternately into polycationic and polyanionic aqueous solutions at room temperature. Importantly, the ISAM technique yields exceptionally homogeneous, large area films with excellent control of total film thickness. Our studies concentrate on improving the performance of ISAM light emitting diodes that include poly(para-phenylene vinylene) (PPV). The individual thickness of each monolayer and the interpenetration of adjacent layers can be precisely manipulated through the parameters of the electrolyte solutions. The effects of the pH and ionic strength of the immersion solutions, the total film thickness, and the PPV thermal conversion parameters on the photoluminescence and electroluminescence yields have been systematically studied. Through the ISAM process we can also deposit well-defined thicknesses of different polymers at the indium tin oxide and aluminum electrode interfaces. The interface layers are found to affect the electroluminescence efficiency.

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.

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

Ionically self-assembled monolayer (ISAM) films spontaneously assemble with a noncentrosymmetric ordering that gives rise to a substantial second order nonlinear optical (NLO) response with exceptional temporal and thermal stability. Typically, polar ISAM films are made from two oppositely-charged polyelectrolytes with an ionic, conjugated NLO chromophore attached as a side-chain to one of the polymers. The second order susceptibility of such a system is diminished due to competing dipole alignment at opposing ends of each polyelectrolyte layer and by randomized chromophore orientation within thicker layers. Significant enhancements in NLO response have been achieved by replacing the NLO-active polyelectrolyte with a monomeric chromophore that has reactive functionality and ionic moieties on opposite ends of the molecule, as illustrated in Figure 1 for the case of Procion Red MX-5B [1]. The growth of multilayers through alternating mechanisms of covalent coupling and electrostatic adsorption results in highly polar chromophore ordering with χ(2) values as large as 30× 10−9 esu, fifteen times that of quartz. Variation of the solution pH allows one to turn the reactive coupling on and off, verifying the important role of the alternating adsorption mechanisms in producing a film with bulk polar order. Quadratic growth of the second harmonic generation (SHG) intensity with the number of layers demonstrates that the bulk polar order exists through large (>50) numbers of bilayers. Since the solution cells, aqueous solutions, and glass susbtrates are all amorphous and exhibit negligible SHG, in situ SHG measurements allow real-time measurement of the growth of a single, polar monolayer onto the substrate from the solution. These measurements demonstrate that the covalent formation of the chromophore monolayer is complete in less than two minutes, allowing for relatively rapid buildup of thick multilayer films. In addition, we will discuss the effects of the glass-film and film-air interfaces on the observed SHG signals. Each of these produces a large SHG signal that causes a plot of the square root of the SHG versus the number of bilayers to intercept the ordinate axis well-above the origin but does not affect the slope of this plot, which determines χ(2). However, this does demonstrate that care must be taken in determining the χ(2) values of selfassembled films comprising small numbers of layers.

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.