Far infrared and phase transitions in ferroelectric materials

Abstract

A number of far infrared absorption bands are found in molecular and hydrogen-bonded crystals when there are several formula units in the primitive cell. They are very sensitive to the crystal structure and constitute a convenient probe for looking at small changes in the structure, especially at low temperatures where there is a lack of X-Ray data. As well known examples, the case of LTT (Lithium Thallium Tartrate) where the absorption band observed at 21 cm^?1 at room temperature softens down to 8 cm^?1 at the Curie temperature (T_c = 12 K), and LiNH₄SO₄ which has a phase transition at 26 K, will be considered.

It is less known that some ferroelectric crystals can be studied as very thick single crystal plates (thickness t up to 10 mm) at 4 K, and they show in some cases a far IR transmission that is much higher than expected from the study of thin plates (i.e. t = 20 μm). In fact the transmission does not decrease very much when thickness is higher than some specific value t ₀/2. The crystal is not homogeneous at 4 K. A model with a far IR absorbing surface layer (thickness t ₀/2) and a transparent bulk is a good first approximation.

At some temperature T ₀ located between liquid helium and liquid nitrogen temperatures, the center of the bulk undergoes a phase transition from the absorbing phase into the transparent one. When temperature is still lowered, the transparent phase is extended towards the surface with an incommensurate phase between the surface and the bulk.     

Effect of substrate temperature on microstructure and optical properties of single-phased Ag2O film deposited by using radio-frequency reactive magnetron sputtering method

Using a radio-frequency reactive magnetron sputtering technique,a series of the single-phased Ag2O films are deposited in a mixture of oxygen and argon gas with a flow ratio of 2:3 by changing substrate temperature(T s).Effects of the T s on the microstructure and optical properties of the films are investigated by using X-ray diffractometry,scanning electron microscopy and spectrophotometry.The single-phased Ag2O films deposited at values of T s below 200℃ are(111) preferentially oriented,which may be due to the smallest free energy of the(111) crystalline face.The film crystallization becomes poor as the value of T s increases from 100℃ to 225℃.In particular,the Ag2O film deposited at T s = 225℃ loses the 111 preferential orientation.Correspondingly,the film surface morphology obviously evolves from a uniform and compact surface structure to a loose and gullied surface structure.With the increase of T s value,the transmissivity and the reflectivity of the films in the transparent region are gradually reduced,while the absorptivity gradually increases,which may be attributed to an evolution of the crystalline structure and the surface morphology of the films.     

Evolution of the structural and optical properties of silver oxide films with different stoichiometries deposited by direct-current magnetron reactive sputtering

Nitrogen doping of silver oxide(AgxO) film is necessary for its application in transparent conductive film and diodes because intrinsic AgxO film is a p-type semiconductor with poor conductivity.In this work,a series of AgxO films is deposited on glass substrates by direct-current magnetron reactive sputtering at different flow ratios(FRs) of nitrogen to O2.Evolutions of the structure,the reflectivity,and the transmissivity of the film are studied by X-ray diffractometry and sphectrophotometry,respectively.The specular transmissivity and the specular reflectivity of the film decreasing with FR increasing can be attributed to the evolution of the phase structure of the film.The nitrogen does not play the role of an acceptor dopant in the film deposition.     

Ordered porous TiO2 films obtained by freezing and the application in dye sensitized solar cells

TiO₂ films with ordered porous structure were prepared by freezing. By simply freezing wet TiO₂ film on a cold copper plate, ice crystals could grow from bottom of film. Removing such ice crystals then led to ordered, micro-sized pores in the films. With assistance of scanning electron microscopy and N₂ adsorption–desorption isotherms, micro-structural properties of the films were studied. Coarsening behavior of ice crystals was analyzed based on evolvement of the micro-structure. It was found that, both larger film thickness and longer freezing time were beneficial for the formation of ordered porous structure, which was caused by enhanced ice crystals growth. The films were then used to fabricate photoanode of dye sensitized solar cells, of which the photo-to-electric power conversion efficiency was evaluated. It was observed that, solar cell made of ordered porous TiO₂ film came out with 13% larger photo-current density comparing to that made of conventional doctor blading method, which is due to the enhanced light scattering by the ordered porous structure.     

Statistical-thermodynamic theory of ordering in a binary film deposited on the surface of a monocrystal

The ordering of a binary monolayer deposited on the surface of a monocrystal is studied. In the case when the structure of the film is equivalent to that of the crystal surface, the ordering in the film is qualitatively similar to that in the bulk of the sample: When the ordering energy is positive there exists an order-disorder phase transition at a certain temperature and for a certain concentration interval. If the structure of the film is different from the structure of the surface, then the film is ordered at all temperatures, independently of the sign of the ordering energy. In this case, in addition to the equilibrium ordered state in the region T < T_* (T_* is a certain temperature which is determined by the composition of the alloy and by the magnitude of the energy shift of the atoms of the film in the field of the substrate), there exists a metastable ordered state in which there is a jump-like change in the ordering at the point T_*, analogous to a first-order phase transition. Possible methods of experimental study of the ordering in films are discussed.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 53–57, May, 1987.     

脉冲偏压电弧离子镀TiO2薄膜的力学与光学性能

用脉冲偏压电弧离子镀技术在玻璃基片上制备均匀透明的TiO₂薄膜.利用X射线衍射仪、原子力显微镜、扫描电子显微镜、紫外-可见透射光谱仪和纳米压痕仪等手段,对不同脉冲负偏压下合成薄膜的相结构、微观结构、表面形貌、力学和光学性能进行表征.结果表明，沉积态薄膜为非晶态.脉冲负偏压对薄膜性能有明显的影响.随偏压的增加，薄膜厚度、硬度和弹性模量均先增大后减小，前者峰值出现在100—200 V负偏压范围，后两者则在250—350V范围.300 V负偏压时薄膜硬度最高，薄膜达到原子级表面光滑度，均方 关键词： 2薄膜')" href="#">TiO₂薄膜 脉冲偏压电弧离子镀 硬度 折射率     

Smart window coating based on nanostructured VO2 thin film

Smart window coating is fabricated by using vanadium dioxide (VO₂) thin film. The VO₂ thin film is nanocrystal structured and has low phase transition temperature which is about 35 °C. This kind of thin film is infrared-optically transparent in the semiconductor phase at low temperatures and highly reflective in the metallic phase at high temperatures. Based on the VO₂ thin film, a multilayered structure for smart window is designed and fabricated. The multilayered structure is optically transparent for visible light whether at low temperatures or high temperatures, and is transparent at low temperatures and opaque at high temperatures for infrared light, which is smart for adjusting infrared transmittance. This type of multilayered structure is potential to be applied to green smart windows to realize energy saving function.     

反应气压对直流磁控反应溅射制备的氧化银薄膜的结构和光学性质的影响

利用直流磁控反应溅射技术,通过调节反应气压(RP),在250 ℃衬底温度下制备了一系列氧化银 (Ag_xO) 薄膜,并利用X射线衍射谱、能量色散谱和分光光度计重点研究了RP对Ag_xO薄膜的结构和光学性质的影响. 研究结果表明,随着RP从0.5 Pa升高到3.5 Pa,薄膜明显呈现了从两相(AgO+Ag₂O)到单相(Ag₂O)结构再到两相(Ag₂O+AgO)结构的演变. 特 关键词： 氧化银薄膜 直流磁控反应溅射 X射线衍射谱 光学性质     

温度对ZnO/Al2O3(0001)界面的吸附、扩散及生长初期模式的影响

构建了一个ZnO沉积在α-Al₂O₃(0001)表面生长初期的模型，采用基于密度泛函理论的平面波超软赝势法进行了动力学模拟.发现在400，600和800℃的条件下界面原子有不同的扩散能力，因此温度对ZnO/α-Al₂O₃(0001)表面界面结构以及ZnO薄膜生长初期模式有决定性的影响.在整个ZnO吸附生长过程中，O原子的扩散系数大于Zn原子的扩散系数，O原子的层间扩散对薄膜的均匀生长起着重要作用.进一步从理论计算上证实了ZnO在蓝宝石(0001)上两种生长模式的存在，400℃左右生长模式主要是Zn螺旋扭曲生长，具有Zn六角平面对称特征，且有利于Zn原子位于最外表面.600℃左右呈现为比较规则的层状生长，且有利于O原子位于最外表面.模拟观察到在ZnO薄膜临近Al₂O₃基片表面处，Zn的空位缺陷明显多于O的空位缺陷. 关键词： 扩散 薄膜生长 2O₃(0001)')" href="#">α-Al₂O₃(0001) ZnO     

Quantum Cherenkov radiation at the motion of a small neutral particle parallel to the surface of a transparent dielectric

Quantum Cherenkov radiation and quantum friction at the motion of a small neutral particle parallel to the surface of a transparent dielectric with the refractive index n have been studied in a fully relativistic theory. Radiation appears at velocities above the threshold value, v > v _c = c/n. The friction force in the particle–plate configuration has been derived from the friction force in the plate–plate configuration under the assumption that one of the plates is significantly decharged. A decrease in the kinetic energy of the particle near the threshold velocity is due to its radiation and near the speed of light is determined by the heat power absorbed by the particle in the rest frame. The powers of quantum and classical Cherenkov radiation can be comparable in the relativistic case.     

Conformal invariance and surface defects in the two-dimensional Ising model. Exact results

The surface critical behavior of the two-dimensional Ising model with homogeneous perturbations in the surface interactions is studied on the one-dimensional quantum version. A transfer-matrix method leads to an eigenvalue equation for the excitation energies. The spectrum at the bulk critical point is obtained using anL ^–1 expansion, whereL is the length of the Ising chain. It exhibits the towerlike structure which is characteristic of conformal models in the case of irrelevant surface perturbations (h _s /J _s 0) as well as for the relevant perturbationh _s =0 for which the surface is ordered at the bulk critical point leading to an extraordinary surface transition. The exponents are deduced from the gap amplitudes and confirmed by exact finite-size scaling calculations. Both cases are finally related through a duality transformation.     

Reconfigurable atom chip on a transparent ferrite-garnet film

We introduce a new choice of material for the creation of microscopic magnetic potentials for the trapping and guiding of ultracold neutral atoms. The potentials are created above a ferrimagnetic, transparent (BiYTmGd)₃(FeGa)₅O₁₂ film by patterning the magnetic-domain structure in the film with a magneto-optical recording method. Patterns with linewidth down to 2 μm have been achieved, enabling trap frequencies of the order of 100 kHz for ⁸⁷Rb atoms in the state |F=1, m_F=-1〉. The main advantages of the material are: 1) magnetic-field noise is suppressed due to the dielectricity of the material and the absence of electric currents, 2) trapped atoms can be addressed optically through the transparent film, and 3) the film can be repatterned, which enables different experiments with the same component.     

非晶硅光伏电池表面高效光陷阱结构设计

提出一种应用于非晶硅光伏电池表面的光陷阱结构,该结构主要由衍射光栅、低折射率MgF₂膜层、高折射率ZnS膜层及金属Ag反射镜组成.在标准测试条件(AM1.5,100 mW/cm²和25 ℃)下,运用严格耦合波理论,通过计算400-1000 nm波段内的1 μm厚非晶硅光伏电池的吸收光子数加权平均ξ_AM1.5,优化光陷阱结构的设计参数.结果表明:对于电池前表面减反射结构,在衍射光栅周期为800 nm、高度为160 关键词： 光陷阱结构 衍射光栅 减反射膜 非晶硅光伏电池     

Two- and three-dimensional waves in falling film flow in the nonlaminar flow regime: an NMR study

Nuclear magnetic resonance (NMR) velocity-encoded imaging results as well as propagators are presented for the nonlaminar flow regime of falling films. The film is generated by a continuous flow of silicon oil along a vertical poly(methyl methacrylate) plate. While the film remains purely laminar for a film Reynolds number Re_f=0.5, it exhibits laminar-wavy behavior for 1.0≤Re_f≤2.5. In this range, a laminar residual film can be distinguished from averaged waves near the surface of the film from measurements of the flow velocity ν₂ along the direction of gravity as a function of the coordinatex normal to the plate. The perpendicular velocity components, ν _x and ν _y are zero within the accuracy of the measurement, indicating that the wave motion is two-dimensional in the laminar-wavy case. For higher Re_f the waves are found to be three-dimensional, a straightforward division into a residual film and waves in ν _z is lost and the waves extend over the whole thickness of the film.     

Effects of the sputtering power on the crystalline structure and optical properties of the silver oxide films deposited using direct-current reactive magnetron sputtering

This paper reports that a series of silver oxide (Ag_xO) films are deposited on glass substrates by direct-current reactive magnetron sputtering at a substrate temperature of 250 oC and an oxygen flux ratio of 15:18 by modifying the sputtering power (SP). The Ag_xO films deposited apparently show a structural evolution from cubic biphased (AgO + Ag₂O) to cubic single-phased (Ag₂O), and to biphased (Ag₂O + AgO) structure. Notably, the cubic single-phased Ag₂O film is deposited at the SP = 105 W and an AgO phase with <220> orientation discerned in the Ag_xO films deposited using the SP > 105 W. The transmissivity and reflectivity of the Ag_xO films in transparent region decrease with the increase the SP, whereas the absorptivity inversely increases with the increase of the SP. These results may be due to the structural evolution and the increasing film thickness. A redshift of the films' absorption edges determined in terms of Tauc formula clearly occurs from 3.1 eV to 2.73 eV with the increase of the SP.     

Preparation and characterization of the ZnO:Al/Fe65Co35/ZnO:Al multifunctional films

The ferromagnetic transparent conducting film is a multifunctional film which has high visible transmittance, low resistivity and room-temperature ferromagnetism, simultaneously. In this article, ferromagnetic transparent conducting ZnO:Al/Fe₆₅Co₃₅/ZnO:Al multilayer films were fabricated by inserting a middle magnetic Fe₆₅Co₃₅ layer into aluminum-doped zinc oxide (ZnO:Al) matrix using a magnetron sputtering apparatus at substrate temperature ranging from room temperature (RT) to 400^∘C. The total film thickness was about 400 nm and the middle Fe₆₅Co₃₅ alloy layer was 4 nm. The influences of substrate temperature (T _s ) on the structural, electrical, optical and magnetic properties of the multilayer films were systemically investigated. The results showed that the microstructure and performance of the composite multilayer films strongly depended on the substrate temperature. The present results also showed that the inserted middle Fe₆₅Co₃₅ alloy thin layer played an important role in providing the RT ferromagnetism and decreasing the resistivity of the multilayer films. Therefore, it is possible to obtain a multifunctional film material with the combination of good optical transparency, high electrical conductivity and RT ferromagnetism.     

Combinatorial study of a gold nanoparticle infusion process in a polymer film

A novel two-step process is described for infusion of gold nanoparticles (5–20 nm typical diameter) into a polymer film. The technique is demonstrated for the first time in a thermoplastic polyurethane elastomer (TPU). An amine-functional monomer, 2-(diethylamino)ethyl methacrylate, and a free-radical photoinitiator are infused into the surface of the TPU, followed by photopolymerization. An amine-functional semi-interpenetrating network (SIPN) is created within a shallow (~100 μm) surface layer. In the second step, a gold salt, HAuCl₄·3H₂O, is infused into the SIPN from a ternary solvent mixture, and redox reaction with the immobilized amine functional groups produces Au⁰ nanoparticles. Combinatorial processing is conducted to visualize the interdependent effects of two variables, monomer soak time (t ₁) and gold salt solution soak time (t ₂). Combinatorial infusion is accomplished by creating orthogonal gradients in t ₁ and t ₂ in a square TPU plate, allowing examination of sample color, particle size, and polydispersity over a wide range of parameter space. Small angle X-ray scattering (SAXS) is employed as non-invasive means to characterize the Au⁰ particles at three locations in the plate. SAXS measurements are validated by TEM analysis of Au⁰ particle size in a reference sample. A rationale is developed for changing particle size and polydispersity through variation of simple process parameters.     

Inversion of the nonlinear equations of reflection ellipsometry on film-substrate systems

We discuss an inversion procedure to determine the optical properties and film thickness of a transparent film on an absorbing substrate system. The determination of optical properties of the film and substrate are separated from that of the film thickness. An error analysis is presented for the special case of multiple-angle-of-incidence ellipsometry for the SiO₂-Si system at λ = 6328 Å.     

ZnO-SnO2透明导电薄膜的制备及性能研究

采用二步成胶工艺制备ZnO-SnO₂透明导电薄膜,应用X射线衍射、原子力显微镜、紫外-可见分光光度计、薄膜分析仪及四探针仪等对薄膜的结构、表面微观形貌、透过率和导电性能进行表征.结果表明,锌锡摩尔比为9/12,退火温度为500 ℃时,薄膜的透过率达90%,电阻率为3.15×10-3 Ω·cm.与其它工艺相比,二步成胶工艺所制备出的ZnO-SnO₂透明导电薄膜性能优异.     

Evolution of the fractal surface of amorphous lead zirconate-titanate films during crystallization

The recrystallization kinetics of amorphous lead zirconate-titanate films prepared by sol-gel technology are investigated experimentally using elastic scattering of light. Sequences of elastic dependences of the scattered light intensity are recorded directly during thermal annealing. The evolution of the morphology of the film surface during annealing is described in terms of the variation of their fractal dimensionalities D _s. The experimental dependences D _s(t) are compared with the results of a computer simulation of the phase transition kinetics in a thin plate (film). Fiz. Tverd. Tela (St. Petersburg) 41, 306–309 (February 1999)