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.     

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.     

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.     

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.     

Local suppression of ferroelectricity at PbTiO3 surface steps: a density functional theory study

Ab initio (first-principles) density functional theory (DFT) calculations are performed within the local density approximations (LDA) to investigate the ferroelectricity at PbTiO(3) surface steps consisting of (001) and (100) surfaces with a spontaneous polarization along [100]. For both the PbO- and TiO(2)-terminated surface steps, the [100] polarization is suppressed and the [001] polarization appears at their upper terraces, which results in a rotation of polarizations at the surface steps. The polarization rotation is induced by the local variation of the covalent Pb-O bond due to the charge redistribution at the surface steps. Furthermore, we investigate the interaction of the surface steps. Although surface steps with the same polarization configuration exhibit little interaction, steps of different types interact with each other strongly, suppressing the ferroelectricity, especially on the upper terrace.     

Effect of interstitial hydrogen impurities on ferroelectric polarization in PbTiO3

We investigate the electronic and atomic structure of interstitial hydrogen impurities in PbTiO3 and investigate their effect on ferroelectric polarization through first-principles total-energy calculations. Interstitial H in PbTiO3 is found to bond to oxygen and to act as a shallow donor impurity. At equilibrium, H does not much affect nearby Ti-O bonds and the H-O dipole increases the polarization. The barrier for reversing the defect dipole can give rise to imprint (extra stabilization of one of the two polarization modes).     

Unusual behavior of the ferroelectric polarization in PbTiO3/SrTiO3 superlattices

Artificial PbTiO3/SrTiO3 superlattices were constructed using off-axis rf magnetron sputtering. X-ray diffraction and piezoelectric atomic force microscopy were used to study the evolution of the ferroelectric polarization as the ratio of PbTiO3 to SrTiO3 was changed. For PbTiO3 layer thicknesses larger than the 3-unit cell SrTiO3 thickness used in the structure, the polarization is found to be reduced as the thickness is decreased. This observation confirms the primary role of the depolarization field in the polarization reduction in thin films. For the samples with ratios of PbTiO3 to SrTiO3 of less than one, a surprising recovery of ferroelectricity that cannot be explained by electrostatic considerations was observed.     

Spin and conductance-peak-spacing distributions in large quantum dots: a density-functional theory study

We use spin-density-functional theory to study the spacing between conductance peaks and the ground-state spin of 2D model quantum dots with up to 200 electrons. Distributions for different ranges of electron number are obtained in both symmetric and asymmetric potentials. The even/odd effect is pronounced for small symmetric dots but vanishes for large asymmetric ones, suggesting substantially stronger interaction effects than expected. The fraction of high-spin ground states is remarkably large.     

PbTiO3/PbZr0.3Ti0.7O3/PbTiO3夹心结构铁电薄膜子晶层的优化

用Sol-Gel法制备了Pb(1 x)TiO3/PbZr0.3Ti0.7O3/Pb(1 x)TiO3(PT/PZT/PT)夹心结构及PZT铁电薄膜,为了获得高质量的PT/PZT/PT夹心结构铁电薄膜,使用不同过量Pb配比(x)的PbTiO3 (PT)层进行制备,以获得优化的PT子晶层.X射线衍射和原子力显微镜分析结果表明PT层中过量Pb配比(x)对薄膜的微结构影响很大,只有PT层中Pb过量配比x＝0.10-0.15的薄膜为表面晶粒大小均匀致密的纯钙钛矿结构.X射线电子能谱对薄膜微区进行元素成分分析表明,对x=0.00的薄膜,在表面和界面处Pb明显的缺乏;而x=0.20时的薄膜,Pb则明显的过量.薄膜的铁电性能、疲劳特性和漏电流特性等电学性能与PT层中过量Pb配比(x)没有明显的变化趋势,但与薄膜的结晶性能密切相关.结晶性能较好的薄膜,其电学性能也较好.说明PT层中过量Pb配比(x)是通过影响PT子晶层自身的结晶,而影响整个薄膜的结晶行为,并进一步影响到整个薄膜的电学性能.因此,在其他工艺参数都相同时,PT层中合适的过量Pb配比应为x=0.10-0.15.优化的子晶层不仅能获得结晶性能较好的薄膜,而且薄膜的电学性能也好.     

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.     

X-ray evidence for the absence of 90°-domains in polycrystalline PbTiO3 thin films with grains below a critical size

3 thin films is systematically studied by using X-ray diffraction (XRD). The PbTiO₃ thin films with different average grain sizes were prepared on various substrates by a sol-gel process. The films on NaCl and fused glass are randomly grain-oriented, while those on (111)Pt/Ti/SiO₂/Si are highly {100} cubic index grain-oriented . It is found from the XRD patterns of the films on NaCl that with decreasing average grain size from 230 to 80 nm, the intensities of high h index (h>l) peaks (hkl), such as (100), (110), (200), (201), (210), (211), etc., decrease rapidly and ultimately disappear, whereas another set of peaks (lkh), including (001), (002), (102), (112), etc., are still intense. This interesting result suggests that at grain size below a certain critical size an increasing number of grains no longer show 90°-domains, which is confirmed by TEM observations. Meanwhile, X-ray evidence of such a grain-size-related absence of 90°-domains is also found for PbTiO₃ films on Pt(111) and fused-glass substrates. The volume fractions of single-domain grains (without 90°-domains) in the films are estimated from their XRD patterns. By combining SEM and TEM investigations, the critical grain size for the formation of 90°-domains is further determined to be near 200 nm. Received: 19 December 1996/Accepted: 24 March 1997     

Dynamical study of phonons in PbTiO3

Lattice dynamical formalism, using rigid ion model, was applied to the crystal of PbTiO₃ in the tetragonal phase. The model includes long range Coulomb forces and short range axially symectric forces. The radial part of the short range force constants was assumed to be of Born-Meyer type. The parameters of the model were determined through a least squares analysis of zone center and a few zone boundary phonons. Dispersion curves for [100] and [001] directions are presented.     

Caloric characteristics of PbTiO3 in the temperature range of the ferroelectric phase transition

The heat capacity and thermal expansion of the PbTiO₃ ceramic sample have been measured in the temperature range 80?C970 K. The electrocaloric and barocaloric efficiencies of lead titanate in the ferroelectric phase transition range have been investigated by analyzing the experimental data in terms of the thermodynamic theory of phase transitions, the electrical equation of state P(T, E), the Pippard equation, and the S(T, p) diagram.     

Nanoscale retention‐loss dynamics of polycrystalline PbTiO3 nanotubes

We observed the nanoscale retention dynamics of polycrystalline PbTiO₃ nanotubes using piezoresponse force microscopy. We found that the retention loss of the nanodot domains on the nanotubes showed the stretched exponential relaxation behaviors with stretched exponential factor n being less than 1 (0.523 and 0.692), which are similar to the thin films. In addition, the nanodot domains showed a diverse relaxation time constant τ due to different remnant polarization of each dot domains. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

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     

Strain effect on the visible emission in PbTiO3 nanotubes: Template and wall-thickness dependence

We investigated the strain effect on temperature-dependent photoluminescence property in the clamped (with template) and free-standing (without template) PbTiO₃ (PTO) nanotubes. The wall-thickness of nanotubes was varied from 25 to 80 nm with the outer diameter fixed to 420 nm. While all nanotubes show sizable green/yellow emission, the temperature dependent shift of the emission energy is significantly suppressed in the clamped PTO nanotubes, which is attributed to the lattice strain driven by the template clamping. This clamping effect is more significant for thinner nanotubes. Even in the free-standing PTO nanotubes the temperature-dependence of emission is affected by the wall-thickness. Our finding is the clear manifestation of the template and geometrical shape effect on the optical property of the nanotubes.     

Electronic structure, energetics and geometric structure of carbon nanotubes: A density-functional study

Based on the local density approximation (LDA) in the framework of the density-functional theory, we study the details of electronic structure, energetics and geometric structure of the chiral carbon nanotubes. For the electronic structure, we study all the chiral nanotubes with the diameters between 0.8 and 2.0 nm (154 nanotubes). This LDA result should give the important database to be compared with the experimental studies in the future. We plot the peak-to-peak energy separations of the density of states (DOS) as a function of the nanotube diameter (D). For the semiconducting nanotubes, we find the peak-to-peak separations can be classified into two types according to the chirality. This chirality dependence of the LDA result is opposite to that of the simple π tight-binding result. We also perform the geometry optimization of chiral carbon nanotubes with different chiral-angle series. From the total energy as a function of D, it is found that chiral nanotubes are less stable than zigzag nanotubes. We also find that the distribution of bond lengths depends on the chirality.