酞菁铜/金属薄膜界面电位与光二次谐波特性分析

Using the Kelvin probe and the optical second harmonic generation (SHG) measurement, the Space charged phenomenon and the non-linear optical effects at the interfaces of the coppertetra-tert-butyl phthalocyanine (CuttbPc) Langmuir-Blodgett (LB) film deposited on a metal (AlorAu) coated glass slide substrate were in vestigated. The surface potentials decreaseas the film thickness increases and eventually approaches a saturated value. The SHG has been detected although there is a centro-symmetric systemin the Cuttb Pcmolecular, and avery strong SH signal can be investigated at 1260 nm band for Cuttb Pc/Al samples. According to a proposed physical model for Cuttb PcLB film/metal, the nonlinear mechanism were analyzed by using electromagnetic wave theory. It is considered that the enhanced SH peak of CuttbPc/Al is attributed to the strong surface potential aroused by SCIEF at the interface, and shows that the production of SH signal is correlated closely with the electrostatic phenomena at the interface.     

Spectroscopic consideration of the surface potential built across phthalocyanine thin films on a metal electrode

The nonlinear optical properties of tert-butyl phthalocyanine copper Langmuir-Blodgett (CuttbPc LB) films and vacuum-evaporated phthalocyanine copper (CuPc) films deposited on a metal surface were investigated by second-harmonic generation (SHG) spectroscopy. At the organic/metal interface, a space charge field is formed due to the presence of excess charge injected from a metal electrode to the organic layer. Since the Pc molecule has D4h symmetry, an inversion center is present and the optical SH process is not allowed under the electric-dipole approximation. However, the space charge field at the interface directly influences the symmetric structure of the electrons in the Pc molecule. We investigated the contributions of the surface potential to the SHG using Pc LB and vacuum-evaporated films deposited on aluminum (Al) and gold (Au) metal electrodes, where a distinctive difference in the spectrum for the Pc films on the Al and Au surfaces was observed. The contribution of the surface potential was revealed based on the resonant conditions of the SH process, taking into account the electric-quadrupole transition and dc-field-induced electric-dipole transition.     

Reconsideration of second-harmonic generation from isotropic liquid interface: broken Kleinman symmetry of neat air/water interface from dipolar contribution

It has been generally accepted that there are significant quadrupolar and bulk contributions to the second-harmonic generation (SHG) reflected from the neat air/water interface, as well as common liquid interfaces. Because there has been no general methodology to determine the quadrupolar and bulk contributions to the SHG signal from a liquid interface, this conclusion was reached based on the following two experimental phenomena: the breaking of the macroscopic Kleinman symmetry and the significant temperature dependence of the SHG signal from the neat air/water interface. However, because the sum frequency generation vibrational spectroscopy (SFG-VS) measurement of the neat air/water interface observed no apparent temperature dependence, the temperature dependence in the SHG measurement has been reexamined and proven to be an experimental artifact. Here we present a complete microscopic analysis of the susceptibility tensors of the air/water interface, and show that dipolar contribution alone can be used to address the issue of the breaking of the macroscopic Kleinman symmetry at the neat air/water interface. Using this analysis, the orientation of the water molecules at the interface can be obtained, and it is consistent with the measurement from SFG-VS. Therefore, the key rationales to conclude significantly quadrupolar and bulk contributions to the SHG signal of the neat air/water interface can no longer be considered as valid as before. This new understanding of the air/water interface can shed light on our understanding of the nonlinear optical responses from other molecular interfaces as well.     

适合倍频光波导的多层LB膜结构研究

制备非中心对称的NMOB/CdA交替Y型累积LB膜,测量了它的二阶非线性光学特性.为了改善多层膜的有序性,研究在拉制NMOB/CdA多层膜时提高膜均匀性的两种方法,增强了它的二阶非线性光学特性.最后,制备光栅耦合的NMOB/CdA多层膜光波导,测量了它的Cerenkov相位匹配光学二次谐波产生.     

中心对称双酞菁铥LB膜二次谐波产生特性

利用二次谐波产生(SHG)方法研究了中心对称分子稀土夹心双酞菁铥(TmPc₂) Langmuir-Blodgett (LB)膜二阶非线性光学特性, 测量了二次谐波强度随入射基频光入射角的关系, 并对其二阶非线性产生机制进行了讨论. 实验结果表明, TmPc₂分子LB膜具有较好的二次谐波信号, 二次谐波信号强度的最大值在基频光入射角为45°的地方, 其二阶非线性极化率χ⁽²⁾和分子超极化率β分别为1.152×10^-8和1.905×10^-30 esu. 通过测量样品二次谐波信号的偏振特性, 并与理论分析相比较, 得出其二阶非线性起源于电四极子作用机制.     

Polyelectrolyte templating strategy for the fabrication of multilayer hemicyanine Langmuir-Blodgett films showing enhanced and stable second harmonic generation

Polyelectrolyte templating effectively suppresses the aggregation of cationic hemicyanine-based amphiphiles in monolayer Langmuir-Blodgett (LB) films leading to enhanced and stable optical second harmonic generation (SHG). The current study explores the impact of different polyelectrolytes (salts of poly(4-styrenesulfonic acid), deoxyribonucleic acid, and carboxymethylcellulose) on the mode of formation of multilayer LB films of the hemicyanine amphiphile and their SHG response. Pressure-area isotherms and Brewster angle microscopy reveal the impact of the polyelectrolyte complexation on the Langmuir films. Transfer ratios observed during film deposition, supported by electronic absorption spectra and atomic force microscope images of the multilayer LB films, suggest that the polyanions influence the deposition sequence, leading to significant variations in the SHG. Carboxymethylcellulose is identified as an optimal template that induces favorable z-type deposition, leading to the formation of stable multilayer films. These films exhibit the expected quadratic increase of SHG with the extent of deposition; significantly the film response is very stable under extended laser irradiation. It is proposed that structural adjustments of the sandwiched polymer layer lead to the observed deposition sequence and film stability. Polyelectrolyte templating is demonstrated to be a simple and effective strategy for the fabrication of multilayer LB films to elicit efficient quadratic nonlinear optical response.     

Interaction between nitric oxide and lipid-like DDPA LB film investigated with SHG and AFM

Interactions between Nitric oxide (NO) and DDPA Langmuir-Blodgett (LB) film are investigated with second harmonic generation (SHG) and atomic force microscopy (AFM). It has been found that the adsorption of NO molecules on DDPA LB film only changes the value of the second-order susceptibility of the DDPA molecule on film but not its orientation.     

Second harmonic generation phase measurements of Cr(VI) at a buried interface

Surface second harmonic generation (SHG) phase measurements are carried out on methyl ester-functionalized fused quartz/water interfaces in the presence and absence of Cr(VI). The experiments are performed at pH 7, room temperature, and a chromate concentration of 10(-4) M, which corresponds to monolayer Cr(VI) coverage. The liquid/solid interface is probed from the fused quartz side by directing the probe light field at 580 nm onto the interface together with an SHG reference signal at 290 nm that is collinear with the fundamental. The phase difference of the SHG signals generated at the interface in the presence and absence of Cr(VI) is 85 degrees, which is consistent with SHG resonance enhancement observed for the surface-bound Cr(VI) near 290 nm. The optical arrangement discussed here does not require vacuum technology or optics that compensate for the dispersion of the fundamental and the second harmonic E-fields in the two condensed-phase media. This approach is general and can be applied for analyzing thermodynamic and kinetic data derived from SHG measurements of physical and chemical processes occurring at any buried interface.     

An in situ second harmonic generation study of the electrochemical oxidation of silicon in fluoride media

The electrochemical oxidation of p-type Si in fluoride solutions has been studied by in situ second harmonic generation (SHG) with the SHG signal being recorded simultaneously with the cyclic voltammogram. The SHG signal is shown to change in tandem with the electrochemical response enabling the identification of transition points between different surface conditions such as hydrogen-terminated, hydrated oxide and oxide. Interpretation of the changes in SHG suggests that the initial response is due to the electric field-induced second harmonic (EFISH) due to the electric field gradient at the interface. It then becomes dominated by the variation in the resonant surface non-linear susceptibility as the changes in local bonding affect the response. SHG signals display a much greater sensitivity to surface conditions than the voltammetric response and allow the real-time identification of the potentials at which significant changes take place in the state of the surface.     

Enhanced optical and nonlinear optical responses in a polyelectrolyte templated Langmuir-Blodgett film

Optical and nonlinear optical properties like fluorescence and second harmonic generation (SHG) of molecular materials can be strongly influenced by the mode of assembly of the molecules. The Langmuir-Blodgett (LB) technique is an elegant route to the controlled assembly of molecules in ultrathin films, and complexation of ionic amphiphiles in the Langmuir film by polyions introduced in the aqueous subphase provides a simple and efficient access to further control, stabilization, and optimization. The monolayer LB film of the hemicyanine-based amphiphile, N-n-octadecyl-4-[2-(4-(N,N-ethyloctadecylamino)phenyl)ethenyl]pyridinium possessing a "tail-head-tail" structure, shows fluorescence as well as SHG response. The concomitant enhancement of both of these linear and nonlinear optical attributes is achieved through templating with the polyanion of carboxymethylcellulose. Brewster angle and atomic force microscopy reveal the influence of polyelectrolyte templating on the morphology of the Langmuir and LB films. Polarized absorption and fluorescence spectroscopy provide insight into the impact of complexation with the polyelectrolyte on the orientation and deaggregation of the hemicyanine headgroup leading to fluorescence and SHG enhancement in the LB film.     

Chiral molecular assemblies from a novel achiral amphiphilic 2-(heptadecyl) naphtha[2,3]imidazole through interfacial coordination

A novel amphiphilic compound 2-(heptadecyl) naphtha[2,3]imidazole (NpImC17) was synthesized, and its coordination with AgNO(3) in situ in the monolayer at the air/water interface and ex situ in the Langmuir-Blodgett (LB) film on solid substrate has been investigated. It has been found that interfacial coordination between NpImC17 and Ag(I) ion occurred both in the monolayer and in the LB film. It is interesting to note that the Ag(I)-coordinated ultrathin film became chiral although the ligand itself is achiral. It was suggested that the chirality of the Ag(I)-coordinated LB film was developed due to the formation of a helical coordination polymer through the interfacial coordination. To the best of our knowledge, this is the first report on the formation of chiral monolayer and LB films from an achiral molecule through interfacial coordination.     

Formation, structure, and morphology of triazole-based Langmuir-Blodgett films

The formation, morphology, and structure of two-dimensional Langmuir-Blodgett (LB) assemblies of octadecyltriazole (ODT)-based metal-containing oligomers presenting, in the case of iron, the spin-crossover phenomenon is studied with Brewster angle microscopy, IR dichroism, X-ray diffraction, and atomic force microscopy. Two processes occurring at the air-water interface are confirmed to dominate the mechanism of formation of these LB films, the instability of the coordination polymers at the air-water interface and recoordination of metal ions in the subphase at the interface during the LB deposition process. The Langmuir film allowing the LB film formation is mostly made of ODT. The films do present a lamellar structure in which the ODT molecules are tilted and incorporate coordinated isolated metal ions and oligomers of metal ions. The morphology of the LB films is globally flat but with a rather high roughness resulting from inhomogeneities related to phenomena occurring during the LB film formation. These observations are in agreement with the relative affinity of the metal ions with ODT and the relative stability of the coordination polymers at the air-water interface, which have been determined for the group Cu-Fe-Co-Ni.     

5,10,15,20—四（对—乙酯苯基）卟啉LB膜的结构

对卟啉类化合物LB膜的结构、电性质和气敏性的研究已见报道。本文利用膜天平和UV-Vis分光光度计研究了标题化合物在气-液界面上的成膜特性、分子间相互作用和LB膜的结构。5,10,15,20-四(对-乙酯苯基)卟啉(TPEPP)由5,10,15,20-四(对-氰苯基)卟啉和乙醇酯化得到。元素分析测定值与计算值相符。λ(CHCl_3,nm):421,515,550,590,645;ν(KBr压     

Self-assembled spiral nanoarchitecture and supramolecular chirality in Langmuir-Blodgett films of an achiral amphiphilic barbituric acid

An amphiphilic barbituric acid derivative was found to form stable monolayers showing a clear phase transition at the air/water interface. It is interesting to find that the deposited Langmuir-Blodgett (LB) films of the compound showed circular dichroism (CD) although the molecule itself was achiral. AFM measurements on the transferred one-layer LB film revealed that spiral nanoarchitectures were formed. It was further found that the supramolecular chirality of the LB films was related to symmetry breaking at the interface. Hydrogen bonding and the pi-pi stacking between the neighboring molecules resulted in chiral fibers which formed the spiral structures. To the best of our knowledge, this is the first report on the chirality of the molecular assemblies and spiral nanostructures formed through the air/water interface by achiral molecules.     

Effect of water and air-water interface on the structural modification of Ni-arachidate Langmuir-Blodgett films

Nickel arachidate (NiA) Langmuir-Blodgett (LB) films have been deposited on hydrophilic Si(0 0 1) substrates by three (up-down-up) and five (up-down-up-down-up) strokes. During deposition, substrates were kept inside the water subphase for different times after each down stroke. Structural information of all the LB films have been obtained from X-ray reflectivity (XRR) studies. One and two symmetric monolayer (SML) was deposited on top of the asymmetric monolayer (AML) in three and five stokes respectively. All the preformed LB films were then used to go through the air-water interface with the same speed that was used at the time of film deposition. Structural information obtained from the XRR studies show that mainly the top layer density decreases after passing through the air-water interface but the layered structure remains the same. Information obtained from both the XRR and atomic force microscopy (AFM) studies suggest that molecules peeled from the top SML layer do not reincorporate with the LB film through tail-tail hydrophobic interaction. Our study shows that NiA LB film has better stability compared with cadmium arachidate LB film inside the water subphase without forming any out-of-plane molecular reorganization.     

DNA分子在气液界面的组装相变特性及其LB膜结构研究

对十八胺与DNA在气液界面上组装及其相变过程进行了研究，利用AFM观察了不 同压力下转移的DNA复合LB膜结构。发现在低表面压时，DNA复合单分子膜表现为技 术发散的分形结构；随着压力的升高，DNA复合膜逐渐由紧密的网状排布结构变为 团聚的块状和团簇结构。表明通过调节膜压，可使膜内DNA分子的构象发生大的变 化，从而生成具有特定形态的二维纳米图案。这种具有特殊形态和结构的DNA LB膜 可望为合成新型生物纳米结构有序功能体系提供模板。     

Alignment of liquid crystals using Langmuir‒Blodgett films of unsymmetrical bent-core liquid crystals

ABSTRACT

The properties of the thin films of liquid crystal (LC) molecules can be governed easily by external fields. The anisotropic structure of the LC molecules has a large impact on the electrical and optical properties of the film. The Langmuir monolayer (LM) of LC molecules at the air–water interface is known to exhibit a variety of surface phases which can be transferred onto a solid substrate using the Langmuir?Blodgett (LB) technique. Here, we have studied the LM and LB films of asymmetrically substituted bent-core LC molecules. The morphology of LB film of the molecules is found to be a controlling parameter for aligning bulk LC in the nematic phase. It was found that the LB films of the bent-core molecules possessing defects favour the planar orientation of nematic LC, whereas the LB films with fewer defects show homeotropic alignment. The defect in LB films may introduce splay or bend distortions in the nematic near the alignment layer which can govern the planar alignment of the bulk LC. The uniform layer of LB film facilitates the molecules of nematic to anchor vertically due to a strong van der Waals interaction between the aliphatic chains leading to a homeotropic alignment.     

Increased interfacial thickness of the NaF, NaCl and NaBr salt aqueous solutions probed with non-resonant surface second harmonic generation (SHG)

Specific ion effects on the nonlinear optical response from the water molecules at the air/sodium halide solution interfaces are measured using non-resonant surface second harmonic generation (SHG). Procedures have been developed to monitor and remove the impurities in the salt solution samples to ensure measurement of small changes in the SHG signal. Quantitative polarization analysis of the measured SHG data indicated that the average orientation of the interfacial water molecules changed only slightly around 40 degrees with the increase of the bulk concentration of the three sodium halides, namely NaF, NaCl and NaBr, from that of the neat air/water interface. The observed significant SHG signal increase with the bulk salt concentration is attributed to the overall increase of the thickness of the interfacial water molecular layer, following the order of NaBr > NaCl approximately NaF. The absence of the electric-field-induced SHG (EFISHG) effect indicated that the electric double layer at the salt aqueous solution interface is much weaker than that predicted from the molecular dynamics (MD) simulations. These results provided quantitative data to the specific anion effects on the interfacial water molecules of the electrolyte aqueous solution, not only for the larger and more polarizable Br(-) anion, but also for the smaller and less polarizable F(-) and Cl(-) anions.     

一个对称萘酞菁的LB膜及其二阶非线性光学性质的研究

用水平提拉法制备了四叔丁基萘酞菁锌的LB膜。利用表面压-面积曲线、紫外-可见吸收光谱、小角X-射线衍射等表征了LB膜的结构,结果表明该化合物可以形成较高质量的具有很好层状结构的多层LB膜,在膜中分子大环平面垂直于基片平面并且以分子平面并不完全重合的H-聚集体存在。采用透射SHG方法测量了相应LB膜的二阶非线性光学性质,结果显示在实验条件下该化合物LB膜的宏观二阶非线性系数以及分子超极化率分别为X     

Polymer-surfactant layered heterostructures by electropolymerization of phenosafranine in Langmuir-Blodgett films

Langmuir-Blodgett (LB) films of the water-soluble dye phenosafranine (PS) have been prepared by its adsorption from aqueous dye solution to an arachidic acid (AA) monolayer at the air-water interface. Atomic force microscopy (AFM) images of the LB films revealed the effect of change in pH of deposition on the degree of complexation of AA with the PS dye. Well-defined circular islands and holes were observed which disappeared with the increase in pH. Polarized absorption studies indicated that the dye molecules are oriented uniaxially with their long axis titled at a constant angle to the surface normal of the LB film. Within the restricted geometry of the LB film, the PS dye was electropolymerized to form a two-dimensional film of poly(phenosafranine) sandwiched between arachidic acid layers. The film was characterized by IR spectroscopy, cyclic voltammetry, and AFM. X-ray diffraction studies reveal the presence of a layer structure in the AA-PS LB film before and after polymerization. The polymer film showed highly anisotropic electrical conductivity of ca. 10 orders of magnitude. This indicates the formation of two-dimensional polyPS layers between arachidic acid layers resulting in a layered heterostructure film having alternate conducting and insulating regions. Also, the conductivity of the polyPS prepared from LB film was found to be approximately 2.5 times higher than the conductivity of polyPS prepared by solution polymerization method.