Molecular order in Langmuir-Blodgett assembled films of an azobenzene amphiphile

Alternating multilayers of amphiphile, 4-{4-[Methyl-(4-octadecylcarbamoyl-butyl)-amino]-phenylazo}-benzenesulfonic acid (S-azo-C₁₈), and stearic acid were deposited on hydrophobized silica surfaces using the Langmuir-Blodgett technique. Ellipsometry, UV-visible spectroscopy, and second harmonic generation were used to characterize the films, demonstrating reproducible deposition and suggesting a well ordered film structure. Despite the appearance of order at the macroscopic level, neutron and X-ray scattering results unequivocally show that there is considerable disorder, with significant interpenetration between the stearic acid and S-azo-C₁₈ layers. These results suggest that molecular disorder in Langmuir-Blodgett deposited multilayer films may often go unrecognized mainly because the macroscopic probes based on these optical measurements are not adequate for determining the molecular level structural order.     

Phase change and optical band gap behavior of Ge-Te-Ga thin films prepared by thermal evaporation

The amorphous Ge_11.4Te_86.4Ga_2.2 chalcogenide thin films were prepared by thermal evaporation onto chemically cleaned glass substrates. Properties measurements include X-ray diffraction (XRD), Scanning electron microscopy (SEM), Differential scanning calorimetry (DSC), Four-point probe and VIS-NIR transmission spectra. The allowed indirect transition optical band gap and activation energy of samples were calculated according to the classical Tauc equation and Kissinger's equation, respectively. The results show that there is an amorphous-to-crystalline phase transition of Ge_11.4Te_86.4Ga_2.2 thin film. The investigated film has high crystallization temperature (∼200 °C) and activation energy (2.48 eV), indicating the film has good amorphous stability. The sheet resistance of the crystalline state is ∼10 Ω/and the amorphous/crystalline resistance ratio is about 10⁵. Besides, a wide optical band gap (0.653 eV) of Ge_11.4Te_86.4Ga_2.2 is obtained, indicating that the material possesses a low threshold current from amorphous-to-crystalline state for phase-change memory application.     

一种新型可溶喹吖啶酮衍生物LB膜的制备及其光谱特性

采用表面压-分子面积(π一A)等温曲线和紫外-可见吸收谱的方法研究了一种新型喹吖啶酮衍生物材料LB膜的制备及其光谱特性．实验表明,这种新型喹吖啶酮衍生物能够在水面上形成稳定的单分子膜,它与花生酸(AA)混合后不仅可以形成很好的单分子膜,而且可以较好地转移到固体基片上制备成LB膜多层膜．这种新型喹吖啶酮衍生物LB膜的紫外-可见吸收谱的吸收峰位较稀溶液发生了红移,这是由极性溶剂分子与其相互作用的结果．它在溶液和LB膜中都是以单体的形式存在．     

Langmuir-Blodgett films of palladium(II)-imidazole liquid crystals

The self-assembly of organic-inorganic compounds of palladium(II)-bis(N-long chain substituted imidazole) dichloride complexes has been investigated. The presence of a hydroxyl group at the 2-position of alkyl chain induced liquid crystal properties. Molecular ordering of Pd(II)-imidazole complexes upon mechanical compression at the air/water interface is studied by surface pressure-area (π-A) isotherms and Brewster angle microscope. Incorporation of lateral hydroxyl group in the alkyl chain enhances the stability of the films via intramolecular hydrogen bonding interactions. Langmuir-Blodgett (LB) films formed by these compounds were studied by various techniques to characterize their surface morphology, hydrophilic/hydrophobic property, surface functionalities and interlayer spacings. U-shaped interdigitated assembling, good crystallinity and in-air stability of these LB films were demonstrated.     

Electrical manifestation of dielectric non-centrosymmetry using Langmuir-Blodgett multilayers of ω-tricosenoic acid

Centrosymmetric (Y-type) and non-centrosymmetric (Z-type) Langmuir-Blodgett (LB) multilayers of ω-tricosenoic acid have been deposited as part of an M¦LB¦M structure and the electrical characteristics measured. The symmetry of the LB film was manifested in the electrical properties of the junction, with the behaviour of centrosymmetric LB films being independent of the applied bias sense and non-centrosymmetric LB films possessing a distinct bias sense dependence.     

A study of semiconducting properties of hydrogen containing passive films

Mott–Schottky and photoelectrochemical measurements were used to explore the effects of hydrogen and chloride ions on the electronic properties of the passive film on X70 micro-alloyed steel in a solution of 0.5 M NaHCO₃. Mott–Schottky analyses have shown that hydrogen increases the capacitance and donor density, and decreases the flat band potential and the space charge layer thickness of the passive film. The photocurrent of the film is remarkably increased by hydrogen. The effects of hydrogen become more pronounced with an increase in the hydrogen charging current densities. Hydrogen has no noticeable effect on the band gap energy E_g and the process by which hole-electron pairs are photo-generated in the film. The presence of chloride ions in the solution produces some similar effects on the electronic properties of the passive film to those observed with hydrogen, but reduces the photocurrent and increases the band gap energy of the film. No significant synergistic effects on the electronic properties of the passive film were observed in the presence of hydrogen and Cl⁻. These results provide very useful information for elucidating the mechanism by which hydrogen changes the properties of passive film and then promotes localized corrosion.     

Preparation and characterization of conducting poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) Langmuir-Blodgett film

The self-assembly of poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT-PSS) nanoparticles at an air/water interface was achieved by means of the electrostatic force between an octadecylamine (ODA) monolayer and PEDOT-PSS nanoparticles. A surface pressure (π)-area (A) isotherm and X-ray photoelectron spectroscopy of the composite film were used to confirm the electrostatic force between the SO₃⁻ group of PSS and the NH₄⁺ group of aliphatic amines. Monolayer and multilayer composite films of ODA/PEDOT-PSS and ODA-stearic acid (SA)/PEDOT-PSS were fabricated. These solid Langmuir-Blodgett films were investigated by the UV-Vis spectrum, atomic force microscopy, and X-ray diffraction method. It is observed that ODA-SA/PEDOT-PSS films had a higher film-forming capability than ODA/PEDOT-PSS films and an ordered multilayer structure was developed. The conductive properties of ODA-SA/PEDOT-PSS LB films were investigated in detail. Factors influencing the film conductivity such as the layer number and surface pressure were discussed and the conductive mechanism was also studied.     

Photopatterning of heterostructured polymer Langmuir-Blodgett films

Heterostructured polymer Langmuir-Blodgett (LB) film prepared by using poly(N-dodecylacrylamide-co-t-butyl 4-vinylphenyl carbonate) (p(DDA-tBVPC53)) and poly(N-neopentyl methacrylamide-co-9-anthrylmethyl methacrylate) (p(nPMA-AMMA10)) polymer LB films which can act as photogenerator layers were investigated. Patterns with a resolution of 0.75 μm were obtained on heterostructured polymer LB films composed of 4 layers of p(nPMA-AMMA10) LB film (top layers) and 40 layers of p(DDA-tBVPC53) LB film (under layers) on a silicon wafer by deep UV irradiation followed by development with 1% tetramethylammonium hydroxide aqueous solution. The sensitivity of the heterostructured polymer LB films was improved without loss of the resolution compared with p(DDA-tBVPC53) LB film. The etch resistance of the heterostructured polymer LB films was sufficiently good to allow patterning of a copper film suitable for photomask fabrication.     

Interplay between the deposition mode and microstructure in electrochemically deposited Cu thin films

The influence of the electrochemical potential and the deposition mode on the preferred orientation and morphology of crystallites and on the microstrain in electrochemically deposited copper thin films was investigated using a combination of cyclic voltammetry, chronoamperometry, X-ray diffraction, SEM and the diffraction of backscattered electrons. With increasing negative electrochemical potential, the deposition mode changed from the charge transfer controlled one to the diffusion controlled one. At the highest electrochemical potentials, copper deposition was accompanied by hydrogen reduction. In potentiostatically deposited thin films, the preferred orientation of crystallites changed from {111} in the charge transfer controlled deposition mode to {110} in the diffusion controlled one. The increase of the electrochemical potential in the charge transport controlled deposition mode caused an increase of the dislocation density. The increasing electrochemical potential in the diffusion controlled mode led to a decrease of the dislocation density. The copper deposition, which was accompanied by the hydrogen reduction, produced needle-like crystallites with the {100} texture and with the lowest dislocation density.     

Langmuir-Blodgett films of gold/silica core/shell nanorods

We report the preparation of Langmuir- and Langmuir-Blodgett films of mesoporous silica coated gold nanorods. The silica coating on the gold nanorods was found to prevent the aggregation of the plasmonic particles trapped at the air/water interface. Due to the small aspect ratio of the gold core and the presence of the silica shell, the orientational alignment of the nanorods in the Langmuir-Blodgett film is hindered. After particle deposition, no plasmon coupling was observed, which enables the design of the resulting film's optical property at the particle level. By using mesoporous silica as the shell material, the accessibility of the metal core's surface is preserved. Organic dye (Rhodamine 6G) was found to be able to penetrate into the mesoporous shell of the gold nanorods, resulting in a red shift of the longitudinal plasmon mode.     

Study of electrical polarization hysteresis in ferroelectric polyvinylidene fluoride films

Electric properties of polyvinylidene (PVDF) films fabricated using the Langmuir-Schaefer method have been studied. Films of different thickness were deposited on silicon substrates and analyzed using several techniques. X-ray diffraction (XRD) data showed that PVDF films crystallize at an annealing temperature above 130 °C. Polarization versus electric field (PE) ferroelectric measurements was done for samples prepared with electrodes. PE measurements show that the coercivity of the films increases as the maximum applied electric field increases. The coercivity dependence on the frequency of the applied electric field can be fitted as f^0.6. The results also show that the coercivity decreases with increasing the thickness of PVDF film due to the pinning effect.     

Effect of substrate temperature on ac conduction properties of amorphous and polycrystalline GaSe thin films

X-ray diffraction analysis of GaSe thin films used in the present investigation showed that the as-deposited and the one deposited at higher substrate temperature are in amorphous and polycrystalline state, respectively. The alternating current (ac) conduction properties of thermally evaporated films of GaSe were studied ex situ employing symmetric aluminium ohmic electrodes in the frequency range of 120-10⁵ Hz at various temperature regimes. For the film deposited at elevated substrate temperature (573 K) the ac conductivity was found to increase with improvement of its crystalline structure. The ac conductivity (σ_ac) is found to be proportional to (ω^s) where s < 1. The temperature dependence of ac conductivity and the parameter, s, is reasonably well interpreted by the correlated barrier-hopping (CBH) model. The maximum barrier heights W_m calculated from ac conductivity measurements are compared with optical studies of our previous reported work for a-GaSe and poly-GaSe thin films. The distance between the localized centres (R), activation energy (ΔE_σ) and the number of sites per unit energy per unit volume N(E_F) at the Fermi level were evaluated for both a-GaSe and poly-GaSe thin films. Goswami and Goswami model has been invoked to explain the dependence of capacitance on frequency and temperature.     

Aggregation of non-amphiphilic bathophenanthroline in the restricted geometry of Langmuir-Blodgett films with two different matrices

The behavior of binary mixed Langmuir monolayers from the mixture of non-amphiphilic bathophenanthroline (BPH) and behenic acid (BA)/poly(methyl methacrylate) (PMMA) spread on aqueous subphase was investigated on the basis of the analysis of surface pressure-average area per molecule (π-A) isotherms complemented with UV-vis absorption spectroscopy and scanning electron microscopy. In addition, the miscibility of the components in the two investigated mixed systems (BPH/BA and BPH/PMMA) was also tested by using additivity and surface phase rules. The plots of area per molecule versus mole fraction suggest that BPH and BA are immiscible, whereas BPH and PMMA mixtures show non-ideal behavior at low surface pressures and complete miscibility or immiscibility at higher surface pressures. Spectroscopic study reveals that J-type of aggregates is formed in the mixed films. Scanning electron microscopic study supports the presence of aggregates.     

Sputter deposition of semicrystalline tin dioxide films

Tin dioxide is emerging as an important material for use in copper indium gallium diselenide based solar cells. Amorphous tin dioxide may be used as a glass overlayer for covering the entire device and protecting it against water permeation. Tin dioxide is also a viable semiconductor candidate to replace the wide band gap zinc oxide window layer to improve the long-term device reliability. The film properties required by these two applications are different. Amorphous films have superior water permeation resistance while polycrystalline films generally have better charge carrier transport properties. Thus, it is important to understand how to tune the structure of tin dioxide films between amorphous and polycrystalline. Using X-ray diffraction (XRD) and Hall-effect measurements, we have studied the structure and electrical properties of tin dioxide films deposited by magnetron sputtering as a function of deposition temperature, sputtering power, feed gas composition and film thickness. Films deposited at room temperature are semicrystalline with nanometer size SnO₂ crystals embedded in an amorphous matrix. Film crystallinity increases with deposition temperature. When the films are crystalline, the X-ray diffraction intensity pattern is different than that of the powder diffraction pattern indicating that the films are textured with (101) and (211) directions oriented parallel to the surface normal. This texturing is observed on a variety of substrates including soda-lime glass (SLG), Mo-coated soda-lime glass and (100) silicon. Addition of oxygen to the sputtering gas, argon, increases the crystallinity and changes the orientation of the tin dioxide grains: (110) XRD intensity increases relative to the (101) and (211) diffraction peaks and this effect is observed both on Mo-coated SLG and (100) silicon wafers. Films with resistivities ranging between 8 mΩ cm and 800 mΩ cm could be deposited. The films are n-type with carrier concentrations in the 3 × 10¹⁸ cm^− 3 to 3 × 10²⁰ cm^− 3 range. Carrier concentration decreases when the oxygen concentration in the feed gas is above 5%. Electron mobilities range from 1 to 7 cm²/V s and increase with increasing film thickness, oxygen addition to the feed gas and film crystallinity. Electron mobilities in the 1-3 cm²/V s range can be obtained even in semicrystalline films. Initial deposition rates range from 4 nm/min at low sputtering power to 11 nm/min at higher powers. However, deposition rate decreases with deposition time by as much as 30%.     

Structure and electrical properties of sputtered lithium cobaltite thin films

Li_xCoO_y films with x<1 and y>2 have been prepared by radio-frequency (rf) sputtering from high temperature (HT) LiCoO₂ targets. Their structures have been examined with high resolution electron microscopy. Conductivities have been studied between 77 and 400 K. The electrochemical behaviour of film electrodes have been investigated with Li/LiClO₄-PC/Li_xCoO_y cells. The annealed films consist of nanocrystalline domains with amorphous boundaries. Electrical conductivities appear to arise from variable-range hopping (VRH) of holes. The films form good electrodes with operating potentials between 2.7 and 3.8 V. The observations have been discussed on the basis of a tentative and heuristic molecular orbital based energy band diagram.     

Preparation and characterization of Bi2Ti2O7 thin films by chemical solution deposition technique

Transparent and crack-free Bi₂Ti₂O₇ thin films with strong (111) orientation were successfully prepared on n-Si(100) by chemical solution deposition (CSD) using bismuth nitrate and titanium butoxide as starting materials. The structural properties were studied by X-ray diffraction. The dielectric constant at 100 kHz at room temperature was 118 and loss factor was 0.074, for a 0.4-μm-thick film annealed at 500°C for 30 min. The leakage current density was 4.06×10⁻⁷ A/cm² at an applied voltage of 15 V.     

Influence of electrodeposition potential and heat treatment on structural properties of CdTe films

A modified Butler-Volmer model is developed to predict the potential of perfect stoichiometry (PPS) for electrodeposition. CdTe thin films are deposited in an acidic solution; their electrodeposition mechanism is investigated using cyclic voltammetry. Calculated and experimental PPS values exhibit good agreement. At PPS, well-connected granular CdTe thin films can be deposited; these are predicted to be intrinsic, and slightly p-type due to cadmium vacancies. Type conversion occurs only because of defect redistribution and local defect reactions after annealing; the converted n-type layer exhibits lower resistivity and higher mobility. A film annealed at 350 °C exhibits excellent crystallization.     

Electrodeposition and characterization of nanocrystalline CoNiFe films

Nanocrystalline Co₄₅Ni₁₀Fe₂₄ films have been fabricated using cyclic voltammetry technique from the solutions containing sulfate, then characterized by scanning electron microscopy, X-ray diffraction and vibrating sample magnetometer. Meanwhile, Electrochemical Impedance Spectroscopy technique has been employed to probe into the nucleation/growth behavior of Co₄₅Ni₁₀Fe₂₄ films. The results show that, the obtained Co₄₅Ni₁₀Fe₂₄ film possesses low coercivity of 973.3 A/m and high saturation magnetic flux density of 1.59 × 10⁵ A/m. Under the experimental conditions, the nucleation/growth process of Co₄₅Ni₁₀Fe₂₄ films is mainly under activation control. With the increase of the applied cathodic potential bias, the charge transfer resistance for CoNiFe deposition decreases exponentially.     

Unipolar pulse electrodeposition of nickel hexacyanoferrate thin films with controllable structure on platinum substrates

Unipolar pulse electrodeposition was used to prepare nickel hexacyanoferrate (NiHCF) films on platinum substrates with controllable structure. The unipolar pulse waveforms consisted of an applied cathodic potential during the on-period and the open-circuit potential during the off-period. The effects of the pulse deposition parameters on the electrochemical properties of NiHCF films were investigated by potential cycling in K⁺-containing electrolytes. The morphology and composition of NiHCF films were characterized using scanning electron microscopy and energy dispersive X-ray spectroscopy. The charge transfer and the stability of the film electrodes were also examined by cyclic voltammetry (CV). Experimental results show that the cathodic pulse potential is the primary factor influencing the composition of the NiHCF films. The structure of NiHCF films prepared with a pulse potential of 0.7 V SCE appears close to that of the “insoluble” form with a single reversible CV peak at lower voltage and a unit cell with a stoichiometry close to KNi₄^II[Fe^III(CN)₆]₃. These films exhibit good stability and high dynamics for charge transport. The application of more negative cathodic potential pulses produces less dense and robust NiHCF films, with a structure closer to the “soluble” form and an approximate stoichiometry K₈Ni₄^II[Fe^II(CN)₆]₄.     

基于SPM的花生酸LB膜的拉膜工艺及结构表征的研究

通过扫描探针显微镜（SPM）直接观察沉积在基片表面的花生酸LB（Langmuir-Blodgett）薄膜不同范围尺度下的微观结构.研究在经过表面两亲性（亲水性或疏水性）处理的基片上,在相同的制膜条件下,改变其拉膜沉积方式,对花生酸LB膜样品的表面结构、薄膜均匀性和缺陷等的影响.结果显示,花生酸LB膜在不同拉膜沉积方式下,薄膜表面将形成不同的分子自组装形态;改变基片表面的亲水性强弱也直接影响LB膜的表面形貌的均匀性,可能通过选择恰当表面处理和沉积方式来获得平整度更高,缺陷更少的LB膜.