Interface effects in ferroelectric PbTiO3 ultrathin films on a paraelectric substrate

Interface effects on the ferroelectric behavior of PbTiO3 ultrathin films deposited on a SrTiO3 substrate are investigated using an interatomic potential approach with parameters fitted to first-principles calculations. We find that the correlation of atomic displacements across the film-substrate interface is crucial for the stabilization of the ferroelectric state in films a few unit cells thick. We show that the minimum film thickness for the appearance of a spontaneous polarized domain state is not an intrinsic property of the ferroelectric film but depends on the polarizability of the paraelectric substrate. We also observe that the substrate displays an induced polarization with an unusual oscillatory behavior.     

Ab initio study of shear strain effects on ferroelectricity at PbTiO3 thin films

Ferroelectric thin film with the perovskite ABO₃ structure have been widely used in technology applications, e.g., actuators in MEMS/NEMS and nonvolatile random access memories (FeRAM). In order to clarify the effect of the shear strain on the ferroelectricity, the PbTiO₃ thin film as a typical one is chosen. The focus of this study is to put on the PbO-terminated (1? × ?1) and c(2? × ?2) surfaces and the TiO₂-terminated (1? × ?1) surface. Based on ab initio density functional theory calculations with the local density approximation, we have found out that in both the PbO and TiO₂-terminated (1? × ?1) models, the ferroelectricity in the PbO layers was enhanced under the positive shear strain while it was suppressed under the negative one. For the TiO₂ layers, the ferroelectricity was slightly enhanced and sharply suppressed under the positive and negative shear strains, respectively. In the PbO-terminated (2? × ?2) model, the AFE phase was suppressed by the FE phase under the positive shear strain while the opposite trend was found under the negative shear strain. For the PbO layers, the ferroelectricity was enhanced under the positive and negative shear strains. For the TiO₂ layers, the influence of the negative shear strain on the ferroelectricity was larger than that of the positive one. In addition, the ideal strength of the PbTiO₃ thin film with the different terminations was investigated as well.     

Ferroelectricity and tetragonality in ultrathin PbTiO3 films

The evolution of tetragonality with thickness has been probed in epitaxial c-axis oriented PbTiO3 films with thicknesses ranging from 500 down to 24 A. High resolution x ray pointed out a systematic decrease of the c-axis lattice parameter with decreasing film thickness below 200 A. Using a first-principles model Hamiltonian approach, the decrease in tetragonality is related to a reduction of the polarization attributed to the presence of a residual unscreened depolarizing field. It is shown that films below 50 A display a significantly reduced polarization but still remain ferroelectric.     

Stabilization of monodomain polarization in ultrathin PbTiO3 films

Using in situ high-resolution synchrotron x-ray scattering, the Curie temperature TC has been determined for ultrathin c-axis epitaxial PbTiO3 films on conducting substrates (SrRuO3 on SrTiO3), with surfaces exposed to a controlled vapor environment. The suppression of TC was relatively small, even for the thinnest film (1.2 nm). We observe that 180 degrees stripe domains do not form, indicating that the depolarizing field is compensated by free charge at both interfaces. This is confirmed by ab initio calculations that find polar ground states in the presence of ionic adsorbates.     

X-ray photoelectron diffraction study of ultrathin PbTiO3 films

Full hemispherical X-ray photoelectron diffraction (XPD) experiments have been performed to investigate at the atomic level ultrathin epitaxial c-axis oriented PbTiO₃ (PTO) films grown on Nb-doped SrTiO₃ substrates. Comparison between experiment and theory allows us to identify a preferential ferroelectric polarization state in a 60 ? -thick PTO film. Multiple scattering theory based on a cluster-model [ Phys. Rev. B , 075404 (2001)] is used to simulate the experiments.     

Synchrotron XPS and desorption study of the NO chemistry on a stepped Pt surface

The interaction of NO with Pt(4 1 0) was studied using high-energy resolution fast XPS and temperature programmed desorption/reaction mass spectroscopy. LEED studies show that the surface in the clean state restructures, which results in the formation of some larger {1 0 0} terraces. STM measurements show, that most terraces are small, ∼1 nm. Two different binding energy (BE) components were observed in the N 1s region of the core level spectra, both assigned to molecular forms of NO. NO dissociation starts between 350 and 400 K. This is a significantly higher temperature than previous literature reports suggested. This difference is thought to be caused by the restructuring of the surface used in our experiments. The reaction of NO with H₂, NH₃ and CO was also studied. The onset of these NO reduction reactions is determined by the NO_ad dissociation temperature (between 350 and 400 K) and NO_ad dissociation is the rate limiting step for all the reactions that were studied. Reaction with H₂ yields NH₃ below 600 K, but the selectivity shifts towards N₂ at higher temperatures. We did not find any indication that reaction between NO_ad and NH_3 ad proceeds via a special NO-NH₃ intermediate. A new surface species was detected during the reaction between NO and CO, both in the N 1s and the C 1s spectrum. It is tentatively assigned to either CN or CNO. The reactivity of NO on Pt(4 1 0) is compared with the reactivity that was observed for Pt(1 0 0) and other noble metal surfaces, such as Pd and Rh.     

Effects of strontium on the surface acoustic wave properties of Sm-modified PbTiO3 ceramics

The Sm-modified lead titanate ceramics with a composition of (Pb(0.88-x)Sr(x)Sm(0.08))(Ti(0.98)Mn(0.02))O(3); x = 0.05-0.25 were prepared by conventional mixed-oxide method. Surface acoustic wave (SAW) properties, including phase velocity, electromechanical coupling coefficient and temperature coefficient of frequency, were measured. The experiments successfully showed that Sr additive is helpful to obtain higher phase velocity and high electromechanical coupling coefficient. The SAW properties of our samples (V(p),k(2)) are better than some commercially-made PZT and PT samples.     

S在Pt皮肤Pt3Ni(111)面吸附的第一性原理研究

采用基于密度泛函理论的第一性原理方法,计算并分析了S原子在 Pt皮肤Pt3Ni(111)面不同位置的吸附特性.结果表明：S原子在Pt皮肤Pt3Ni的fcc位吸附最强,吸附能为5. 49 eV;与S原子在纯净Pt(111)表面的吸附相比,S原子在Pt皮肤Pt3Ni表面相应吸附位置的吸附能变小、与近邻的Pt原子形成的S-Pt键变长,表明掺杂的Ni会减小相应位点S原子的吸附能,降低体系对S原子的吸附能力,进而减弱S吸附对体系催化能力的影响;态密度分析发现, S原子的吸附使得Pt基催化剂的催化活性降低,主要是S的2p电子引起的;这些结果将为后续研究Pt基合金电极抗S中毒效果以及探究S原子吸附后Pt3Ni的活性位提供依据.     

Polarization effects in photoisomerization of azo dyes in polymeric films

The reversible cis-trans photoisomerization of disperse red 1 (DR1) in PMMA thin films has been demonstrated previously by using the Attenuated Total Reflection (ATR) method. In this communication photoisomerization of DR1 is shown to be strongly polarization sensitive. This new property of dye doped polymeric films could lead to practical applications in integrated optics. A simple molecular interpretation is given. Illumination of a DR1 sample by polarization fringes produces a grating of molecular orientation able to diffract a probe beam.     

纳米结构Pt膜上CO吸附的异常红外效应机理研究

CO吸附在经方波电势处理后的纳米结构Pt金属膜电极上可观察到异常红外效应（AIREs）.在实验基础上用粒子间的相互作用和电子-空穴衰减机理分析、模拟了AIREs，表明Pt金属表面的纳米岛状结构增强了CO分子间及CO分子与纳米结构表面的相互作用，电子-空穴衰减引起的能量转移和粒子间相互作用的增强可以导致异常红外吸收谱线变化特征的出现. 关键词： 异常红外效应 粒子相互作用 电子-空穴衰减     

The evolution of polarization inside ultrathin PbTiO3 films: a theoretical study

How to control the material properties by manipulating the unitcell thickness is crucial for applications of ferroelectric ultrathin films. To understand the polarization behaviour of ultrathin PbTiO₃ (PTO) films grown on SrTiO₃ (STO) substrate, we have systematically explored the strength and direction of polarization in each unitcell layer, using density functional theory combined with Born effective charge method. Strikingly, we find that the polar state of ultrathin PTO films is a composite result depending not only on thickness but also on boundary condition, initial polarization direction, etc. Besides, we also studied the surface effect on the polarization in the thicker PTO films for comparison with the ultrathin ones, which suggests that the surface effect is basically confined in a small range (3–5 unitcells thick at surface region) no matter what kinds of surface terminations and polarization directions.     

Structural and dielectric properties of Er substituted sol-gel fabricated PbTiO3 thin films

Er substituted lead titanate thin films were processed via chemical solution deposition. Their structural properties were investigated by means of atomic force microscopy, X-ray diffractometry and micro-Raman spectroscopy. The Curie temperature, T_C, was obtained from dielectric measurements. The results show that the grain size, tetragonality factor, c/a, E(1TO) transverse optical phonon frequency and the ferroelectric-to-paraelectric phase transition are affected by Er doping. Particularly the lattice tetragonality and Curie temperature of Er substituted films are found to be substantially lower in comparison to undoped specimen. In contrast, the E(1TO) phonon frequency is only slightly reduced by Er doping. The Results obtained for pure and Er substituted specimens are discussed in terms of combined effects of grain size, stress development and doping on the structural characteristics. PACS 68.55.-a; 77.80.Bh; 78.30.Hv     

Polarization effects on the line-strength calculations of Er3+-doped LiNbO3

The polarized optical absorption of Er³⁺-doped LiNbO₃ has been obtained and analyzed within the framework of the Judd-Ofelt theory including polarization dependences. These results are compared with those obtained under unpolarized conditions. Fluorescence lifetimes for different Er³⁺ concentrations have also been measured in the temperature range 10–300 K.     

Observation of nanoscale 180 degrees stripe domains in ferroelectric PbTiO3 thin films

We report the observation of periodic 180 degrees stripe domains below the ferroelectric transition in thin films. Epitaxial PbTiO3 films of thickness d=1.6 to 42 nm on SrTiO3 substrates were studied using x-ray scattering. Upon cooling below T(C), satellites appeared around Bragg peaks indicating the presence of 180 degrees stripe domains of period Lambda=3.7 to 24 nm. The dependence of Lambda on d agrees well with theory including epitaxial strain effects, while the suppression of T(C) for thinner films is significantly larger than that expected solely from stripe domains.     

Raman-spectroscopy study of PbTiO3 thin films grown on Si substrates by metalorganic chemical vapor deposition

Raman spectra have been investigated in PbTiO₃ thin films grown on Si by metalorganic chemical vapor deposition. A large grazing-angle scattering technique was taken to measure the temperature dependence of Raman spectra below room temperature. All Raman modes in the thin films are assigned and compared with those in the bulk single crystal, a newA ₁(TO) soft mode at 104 cm^–1 was recorded which satisfies the Curie-Weiss relation ² =A(T _c –T). Intensities of theA ₁(1TO) andE(1TO) modes were anomalously strengthened with increasing temperature. Raman modes for the thin films exhibit remarkable frequency downshift and upshift which is related to the effect of internal stress.     

Polarization dependent chemistry of ferroelectric BaTiO3(001) domains

Recent works suggest that the surface chemistry, in particular the presence of oxygen vacancies, can affect the polarization in a ferroelectric material. This should, in turn, influence the domain ordering driven by the need to screen the depolarizing field. Here we show using density-functional theory that the presence of oxygen vacancies at the surface of BaTiO(3)(001) preferentially stabilizes an inward pointing, P-, polarization. Mirror electron microscopy measurements of the domain ordering confirm the theoretical results.     

Characterization of the crystallization behaviors in the PbTiO3 thin films on Si substrates by an infrared spectroscopy technique

PbTiO₃ thin films on silicon substrates derived from a modified sol–gel technique are characterized by IR reflectance spectroscopy for the first time. Seven infrared (IR) reflectance peaks modes have been observed in the crystallized perovskite PbTiO₃ thin films and are assigned to the corresponding phonon modes. Comparisons between the IR reflectance spectra of PbTiO₃ thin films obtained by different annealing processes, i.e., rapid thermal annealing (RTA) and conventional thermal annealing (CTA), have also been carried out. It is observed that the frequencies of most peaks in the RTA-derived PbTiO₃ films are lower than that in the CTA-derived films.     

Stress effect and evidence of ferroelectric weakening in highly c-axis-oriented PbTiO3 thin films

3 were successfully grown on Pt-coated SrTiO₃ single-crystal substrates by metalorganic chemical vapor deposition (MOCVD) and were investigated by using X-ray diffraction (XRD), scanning electron microscopy (SEM) and Raman spectroscopy. The as-deposited thin films were found to be highly (001)-oriented with an average grain size of about 0.3 μm. Both a decrease of the tetragonality and a frequency downshift of the long-wavelength optical phonons were observed and attributed to the effect of compressive stress in the thin films. However, Raman scattering studies estimated a stress value of 2.6 GPa, which is much larger than the value of 0.75 GPa obtained from the XRD analyses. Raman spectroscopic studies also confirmed the grain-size-related disorder feature in the as-grown PbTiO₃ thin films. Structural investigations implied the weakening of ferroelectricity in the heteroepitaxial ferroelectric thin films. Received: 1 April 1997/Accepted: 14 July 1997     

Interface effects on Gd induced disordering of Co films on Pt(111)

Amorphization of epitaxial Co thin films grown on top of a Pt(111) surface has been studied by surface X-ray diffraction after deposition of Gd overlayers. The results indicate strong differences of the disordering process depending on the thickness of the Co film. First basic difference is that thick Co films (15 atomic layers) are only partially amorphized by 4 atomic layers of Gd on top of them, whereas thinner Co films (5 atomic layers) are completely disordered by just 2 atomic layers of Gd. Moreover, amorphization by Gd overlayers induces different stress relaxation processes in both cases. For 15 atomic layers thick Co films a preferential amorphization of the more strained Co grains is observed, leading to an effective relaxation of about ? 0.5% of the in-plane lattice parameter during amorphization, approaching its relaxed value. On the contrary, for 5 atomic layers thick Co films, the initial steps of disordering are accompanied by a stronger increase of the in-plane lattice constant, by about 1.4%, typical of Co–Pt interface alloy formation, followed by a complete amorphization. Furthermore, the magnetic characterization, carried out by magneto-optical Kerr effect and resonant magnetic surface X-ray diffraction, strongly supports that the amorphization of thin Co films is changing the nature of the Co/Pt interface. In particular, as Gd overlayers are deposited, and the amorphization proceeds, the structural disordering of the Co/Pt interface flips its characteristic perpendicular magnetic anisotropy toward in-plane orientation before the complete magnetic depolarization of the interface Pt atoms is reached. All these results confirm a marked dependence of amorphization processes on film thickness, which can be related to the enhanced influence of the nearby film/substrate interface.