Novel Route for Enhancing Piezoelectricity of Ferroelectric Films: Controlling Nontrivial Polarization States in Pb(Zr, Ti)O3 Monodomain Superlattice Structure.

Artificial superlattice films made of Pb(Zr0.4Ti0.6)O3 and Pb(Zr0.6Ti0.4)O3 were investigated for their polarization states and piezoelectric properties theoretically and experimentally in this study. The developed theory predicts nontrivial polarization along neither [001] nor [111] directions in (111)-epitaxial monodomain superlattice films with uniform compressive strain. Such films were achieved via pulsed laser deposition. When the layer thickness is reduced to 3 nm, d33 becomes 128 ± 3.8 pm/V at 100 kV/cm and 71.3 ± 2.83 pm/V at 600 kV/cm, comparable to that of (111)-oriented Pb(Zr0.4Ti0.6)O3 or Pb(Zr0.6Ti0.4)O3 bulks and clearly exceeding that of the typical clamped films. The measurement agrees with the theoretical analysis, which reveals that the enhanced piezoelectricity is due to rotation of the nontrivial polarization. Furthermore, the theoretical study predicts an even larger d33 exceeding 300 pm/V for specific parameters in superlattice films with uniform tensile strain, which is promising for applications of microelectromechanical systems.

Unraveling the reasons behind lead phthalocyanine acting as a good absorber for near-infrared sensitive devices.

Lead phthalocyanine (PbPc) is well known to be used as a good near-infrared (NIR) light absorber for organic solar cells (OSCs) and photodetectors. The monoclinic and triclinic phases have been understood to absorb the visible and NIR regions, respectively, so far. In the present study, we demonstrated from the absorption spectra and theoretical analysis that the visible band considerably originates from not only the monoclinic but also the amorphous and triclinic phases, and revealed the exciton dynamics in the PbPc film from static/time-resolved photoluminescence (PL), which are first reported. By comparing the external quantum efficiency between PbPc- and ZnPc-based OSCs in relation to their structure, morphology, and optical (absorption and PL) characteristics, we unraveled the reasons behind the PbPc film used as a good absorber for NIR-sensitive devices.

Significant effect of Mg-pressure-controlled annealing: non-stoichiometry and thermoelectric properties of Mg2-δSi1-xSbx.

Non-stoichiometry related to such an element with high vapor pressure as Mg has been difficult to control, despite its significant influence on the thermoelectric properties of the material. Here, Mg non-stoichiometry and the thermoelectric properties of Mg2-δSi1-xSbx (0 ≤ x ≤ 0.60) are investigated systematically by tuning the Mg partial pressure during annealing. The range of Mg non-stoichiometry Δδ depends on x since the Sb dopant induces Mg vacancies, which are partly filled by Mg atoms in the gas phase according to an equilibrium reaction. The defect association between the Mg vacancies and the Sb dopant decreases Δδ at high Sb contents. The maximum Δδ of 0.016, which corresponds to a carrier concentration range Δn of 4.9 × 1020 cm-3, is obtained at x = 0.10. The Seebeck coefficient shows a universal relationship with the carrier concentration, which can be explained by the DFT-calculated band structure within the rigid band approximation. The carrier mobility is reduced significantly as a result of carrier scattering at the Mg-poor grain boundaries. The vacancy formation at the Mg-site, in addition to Si-site substitution for Sb, effectively suppresses the lattice thermal conductivity. The low thermal conductivity on the Mg-poor side increases the maximum zT, while a high energy conversion efficiency is obtained on the Mg-rich side, owing to the high electrical conductivity and resultant large zT at low temperatures. Mg non-stoichiometry is a key factor in tuning the thermoelectric properties of Mg2Si-based materials and thus its control is essential.

Charge screening strategy for domain pattern control in nano-scale ferroelectric systems.

Strain engineering is a widespread strategy used to enhance performance of devices based on semiconductor thin films. In ferroelectrics strain engineering is used to control the domain pattern: When an epitaxial film is biaxially compressed, e.g. due to lattice mismatch with the substrate, the film displays out-of-plane, often strongly enhanced polarization, while stretching the film on the substrate results in in-plane polarization. However, this strategy is of a limited applicability in nanorods because of the small rod/substrate contact area. Here we demonstrate another strategy, in which the polar axis direction is controlled by charge screening. When charge screening is maintained by bottom and top metallization, the nanorods display an almost pure c-domain configuration (polarization perpendicular to the substrate); when the sidewalls of the nanorods are metallized too, a-domain formation prevails (polarization parallel to the substrate). Simulations of the depolarization fields under various boundary conditions support the experimental observations. The employed approach can be expanded to other low-dimensional nano-scale ferroelectric systems.

Fabrication of Tetragonal Pb(Zr,Ti)O3 Nanorods by Focused Ion Beam and Characterization of the Domain Structure.

It has been widely revealed and discussed that the properties of ferroelectric nanostructures vary with their dimensionality and size. The mechanical substrate clamping and the depolarization field are considered as major factors, which cause their unique properties. In this paper, we fabricated tetragonal {100}-Pb(Zr, Ti)O3 rods with 100 nm - 4 µm widths on Nb-doped SrTiO3 substrates by using focused ion beam, and characterized their domain structure by synchrotron micro X-ray diffraction. It was found that the clapping angle in the a/c-domain structure became larger with decreasing the rod width, which indicates the significant reduction of substrate clamping by fabricating narrow rods.

Lung cancer incidence in middle-aged men estimated by low-dose computed tomography screening.

Lung cancer incidence in general middle-aged men estimated by low-dose computed tomography screening has been unknown. We conducted a longitudinal study to estimate the incidence of lung cancer in general middle-aged Japanese men based on smoking status and to determine the optimal targets for lung cancer screening. The subjects were 14,058 Japanese men aged 40-59 years (mean age, 48.3 years; median age, 49 years) who underwent twice or more low-dose computed tomography screenings at Nippon Telegraph and Telephone East Corporation Izu Hospital at the baseline examination during 2000-2007. Medical records of subjects with abnormal findings were collected and analyzed. The lung cancer incidence in men aged 40-59 or 50-59 years based on smoking status by person-years method, and relative risks of current- and former-smokers compared with non-smokers were calculated. Mean follow-up period was approximately 3 years, and total person-years were 45,152. The incidence was 24.4 per 100,000 person-years in men aged 40-59 years (95% confidence interval (95% CI) 13.6-43.6), and 56.5 in men aged 50-59 (95% CI 31.6-101). No statistically significances were observed among non-, current- and former-smokers. However, the relative risk of the current-smokers aged 50-59 years with 40 or more pack-years, compared with non-smokers was 6.31 (95% CI 1.27-31.3), and was statistically significant. No significant differences in the lung cancer incidence existed among any smoking status in the Japanese men aged 40-59 years. In this population, men aged 50-59 years with 40 and more pack-years could be an optimal target for lung cancer screening. The small number of incident lung cancers resulting in the incidence and relative risks with the wide 95% CI and the participants' demographic biases were the limitations of our study. Our screening recommendations should be made with more caution.