We investigated on cyanine dye (HQ) in Langmuir and Langmuir-Blodgett (LB) films using UV-visible spectra and second-harmonic generation (SHG) methods. UV-visible spectra indicated a uniform film transfer. The significant SHG fringe signal as a function of angle of incidence for Z-type LB films observed respectively. The second-order susceptibility χ(2)zzz, refractive indexes at 532 nm as well as the average tilt angle ψ between the molecular (dipolar) axis and surface normal, obtained by means of fitting the SHG data, are 42 pM/V-1, 1.4841, 57° and 32 pM/V-1, 1.4775, 53° for LB films without and with Cd2+ ions, respectively. These high nonlinearities are due to resonant enhancement resulting from the proximity of the second harmonic wavelength to the absorption band of the films.
We investigated on cyanine dye (HQ) in Langmuir and Langmuir-Blodgett (LB) films by surface pressure-area (π-A)
isotherms, UV-visible and polarized UV-visible spectra as well as small angle x-ray diffraction (SAXD) measurement.
It is found from the isotherms that there is a critical point on Langmuir films near the area 0.8nm2/molecule for HQ LB
films with and without Cd2+ ions respectively and suggested that the facts should result from the phase transition due to
the change of molecular tilt angle on surface of sub-phase. UV-visible spectra indicated a uniform film transfer. The
orientation angle of HQ molecular chromophore in LB films was measured by polarized UV-visible absorption spectra,
and the result showed that the angle with respect to the substrate surface was 65 and 59 degree without and with Cd2+
ions respectively, which are agreement with those obtained from the surface pressure-area isotherm. It is suggested that
the arrangements of the HQ molecule in LB films incorporating Cd2+ ions was more compact than those without Cd2+.
The temperature dependence of pyroelectric coefficient in LB films of hemicyanine dyes with different thickness has been investigated. It was found that, in the range from 20 to 45°C, the pyroelectric coefficient was not almost changed for the film thickness less than 32 monolayers. However, the one was enhanced obviously with temperature increasing for the film thickness larger than 32 monolayers. The facts indicated that there is a critical thickness in the ferroelectric LB multilayer films. For the LB films of 20 and 30 monolayers, we can observe two peaks in the temperature dependence of pyroelectric coefficient (at 9 and/or 12°C) under heating and/or cooling processes. But, the heat-hysteresis was not found in 40 monolayers LB films. It is indicated that there are the heat-hysteresis and further confirmed the existence of phase transition in the LB films in the neighborhood of 10°C. It may be demonstrated primarily that the type of phase transition is the surface first-order ferroelectric-paraelectric phase transition by the pyroelectric coefficient measurement.
Pyroelectric properties in alternating DAEP/NC Langmuir-Blodgett (LB) films and effects of incorporating the barium cations on pyroelectric behaviors have been investigated. Its pyroelectric coefficient measured is found to be 58μC m-2 K-1at 300K. The dielectric properties have been measured and used to determine the figures of merit in thermal devices. Relative permittivity εr and dielectric loss values tanδ of pyroelectric films (in the range of 1KHz - 100KHz) are 2.34~1.96 and 0.08~0.04, respectively. These data give a maximum pyroelectric figure of merit of 150μC m-2 K-1 in the alternating LB films incorporating barium ions. The effects of different deposition on pyroelectric properties are also discussed in this paper. It is indicated that the alternating LB films is promising in the field of pyroelectricity and pyroelectric infrared detector.
Three organic dyes, diazostilbenes (HPNA), phenylhydrazone (PD) and hemicyanine (DAEP), all have large hyperpolarizability (beta) but different double bonds in their conjugated (pi) -electron systems (N equals N in HPNA, C equals N in PD and C equals C in DAEP). We investigated stability of LB multilayers of the three dyes and found that the second harmonic generation (SHG) intensities from those samples reduced significantly within a few tens of days. On the other hand, the Raman intensities from the powder samples of three dyes due to the stretching modes of the double bonds and the UV absorption band from the LB multilayers of the three dyes corresponding to the original chromophores also decreased with increasing time of UV irradiation, confirming the breakdown (cleavage) of the double bonds (photo-induced bleaching) in all the three dyes. The SHG, UV-vis absorption and Raman experiments deduced the same sequence of stability: N equals N the worst and C equals C the best, suggesting that photo- induced bleaching effect, instead of deterioration of the film frameworks, could possibly be responsible for the instability of the optical nonlinear LB films through a second-harmonic resonant effect.
The Langmuir-Blodgett (LB) films of hemocyanin dyes were prepared by LB technique. The films show pyroelectric effect. The pyroelectric coefficient measured by the method of integrated charges is 12 (mu) C m-2 K-1 at room temperature. The relationship between the orientation of DAEP in the films and temperature was studied by polarized FTIR spectroscopy and second-harmonic generation. The physical mechanism of pyroelectricity in the films was also discussed.
A novel bifat-chain amphiphilic molecule nitrogencrown (NC) was adopted as an inert material for fabrication of optical nonlinear Langmuir-Blodgett (LB) multilayers. Structural improvement in the Z-type mixed fullerene derivative (C60-Be)/NC LB multilayers samples was realized by insertion of the C60-Be molecules between two hydrophobic chains of the NC molecules. The relatively large third-order susceptibility (chi) (3)xxxx(- 3(omega) ;(omega) ,(omega) ,(omega) ) equals 2.9 multiplied by 10-19 M2V-2 (or 2.1 multiplied by 10-11 esu) was deduced by measuring third harmonic generation (THG) from the C60-Be samples. The second harmonic generation (SHG) intensity increased quadratically with the bilayer number (up to 116 bilayers) in Y-type hemicyanine (HEM)/NC interleaving LB multilayers due to improvement of the structural properties by insertion of the long hydrophobic tail of HEM molecules between two chains of NC molecules. The second-order susceptibility (chi) (2)zxx(-2(omega) ;(omega) ,(omega) ) equals 18 pM V-1 (or 4.35 multiplied by 10-8 esu) was obtained by measuring SHG from the HEM samples. The NC molecule has attractive features as a matrix material in fabrications of LB multilayers made from optically nonlinear materials with hydrophobic long tails or ball-like molecules.
We investigated the effects of deposition parameters, subphase additions and annealing on the optical linear (absorption) and nonlinear (second harmonic generation) properties of hemicyanine (HEM) Langmuir-Blodgett (LB) films via aggregate status variations. HEM molecules formed H- aggregates in LB films, characterized by a blue-shift of the absorption band relative to the monomeric spectra, leading to a reduction of the effective hyperpolarizability beta due to the second harmonic off-resonant effect. The spontaneous aggregation mainly occurred during compression on the Langmuir trough. The degree of H-aggregate could be reduced and beta could be enhanced by raising the compressional speed and/or lowering the deposition surface pressure. By adding KI in the subphase, the observed beta-enhancement and absorption band red-shift in HEM LB monolayers were attributed to H-aggregate dissociation and intramolecular charge transfer. The time-resolved fluorescence profiles of HEM LB multilayers showed that there mainly existed two species (monomer and dimer) with characteristic decay times in the order of 100 ps and 10 ps, respectively.
A new type of bifat-chain amphiphilic molecules, 1,10-bisteary1-4,6,13,15-tetraene-18-nitrogencrown-6 (NC), was adopted as a spacer for an optically nonlinear hemicyanine derivative (HD) dye to form interleaving Y-type LB multilayers. The second harmonic (SH) intensity increased quadratically with the bilayer number up to 116 bilayers due to improvement of the structural properties by insertion of the long hydrophobic tail of the HD between two fat-chains of NC.
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