Noriyuki Nakatani

Room temperature gas sensing of p-type TeO2 nanowires

Tellurium dioxide (TeO2) nanowires with a tetragonal structure have been grown by thermally evaporating tellurium metal at 400°C in air. The nanowires produced have diameters ranging from 30to200nm and have lengths of several tens of micrometers. Gas sensors were fabricated using the obtained TeO2 nanowires. The sensing behavior to NO2, NH3, and H2S gases at room temperature showed typical characteristics of a p-type semiconductor. The results demonstrate the potential to develop TeO2 nanowire based gas sensors with low power consumption.

Ferroelectric Domain Structure of Tri-Glycine Sulfate Observed Using a Scanning Electron Microscope

Domain structures in the (010) plane are investigated in the secondary electron mode of S.E.M. The long axis of typical lenticular domains in crystals as grown is along the direction nearly perpendicular to the crystallographic c-axis. Fine lamellar structures appearing in annealed crystals are also along the same direction. The electron beam scanning causes a polarization reversal, and makes possible the direct observation of the dynamical behavior of the domain. The domain boundary has a fairly strong tendency to be perpendicular to the c-axis. The pointed end of the lenticular domain is the position where the domain boundary is caught.

Synthesis and Characterization of TeO2 Nanowires

TeO2 nanowires were successfully synthesized by a simple reactive thermal evaporation method using pure Te metal as the source material. The study on synthesis process indicates an optimal synthesis temperature of 400 °C. Structural characterization using X-ray diffraction and transmission electron microscopy shows that the TeO2 nanowires have a single phase tetragonal structure. Scanning electron microscopy observation demonstrates that each TeO2 nanowire starts to grow from a TeO2 nanopartice, which suggests a self-catalytic growth mechanism. The Fourier transform infrared spectrum of TeO2 nanowires shows a blue shift of 20 cm-1 in the axial stretching bands. The gas sensing measurements indicate that TeO2 nanowires have a p-type electrical conduction and can reversibly response to NO2 gas at room temperature.

Ferroelectric Domain Structure in TGS Just below the Curie Point after Heat Treatment

The domain structure immediately after heat treatment was investigated by the nematic liquid crystal method. Just below Tc very fine domain patterns narrower than 0.5 µm in width were observed. With time, the domain pattern gradually becomes coarse. Mean domain width grows in proportion to the time elapsed since the moment of Tc, and the rate of growth is almost independent of temperature. The evolution of the domain pattern is caused by the development of large domains accompanied by the vanishing of small domains, as well as by the contraction of crooked boundaries.

Microscopic Structure of Cleavage Surface of Ferroelectric Tri-Glycine Sulfate

The cleavage surface of TGS was examined by electron microscopy utilizing the low angle shadow decoration technique. The cleavage plane, in the unit cell, is the positive side of the layer of SO4-glycine 1. When the cleavage direction changes across the direction perpendicular to the (101) plane, the cleavage plane changes discontinuously by the amount b0/2, where b0 is the unit cell length along the b-axis. The step of b0 in height is bisected into smaller steps by cleavage along this direction. The elementary step formation, the inclination of the cleavage surface and the step bending at the domain boundary are interpreted synthetically by assuming that the bonding on the positive side of the crack front is more easily broken than that on the negative side by the cleavage along the c-axis.

Observation of Ferroelectric Domain Structure in TGS

Triglycine sulfate (TGS) is one of the most intensively investigated ferroelectrics. Though the domain structure is an important subject in the research of ferroelectricity, the domain in TGS is invisible by usual optical method. Many substitutive methods have been developed for the observation of static and dynamic domain structure in TGS. This review summarizes these methods mainly from a practical point of view. The methods referred here are the etching method, the powder deposition technique, the decoration method, the nematic liquid crystal (NLC) method, the scanning electron microscopy (SEM), the pyroelectric probe technique (PPT), the X-ray diffraction microscopy, the cleavage method, the scanning probe microscopy (SPM) and others.

Temperature-Independent Internal Bias Field in L-α-Alanine-Doped Triglycine Sulfate Crystal

The temperature dependence of the internal bias field in L-α-alanine-doped triglycine sulfate (LATGS) crystal was investigated by P-E hysteresis loops and i-E curves under application of a triangular field. We could not observe any changes in the internal bias field at the Curie point. The internal bias field is almost independent of the temperature from 25 to 55°C. The strict equivalence between the internal bias field and the external applied field is demonstrated by the phenomenological consideration.

Ferroelectric domain structure of X-ray-irradiated TGS

Domain patterns on the b-plane of X-ray-irradiated TGS crystals were examined by the rub etch method. The domain structure is not changed by the irradiation in the ferroelectric phase, and is reproduced after heat treatment. Crystals irradiated in the paraelectric phase show extremely fine domain patterns. The rate of evolution of the domain structure after heat treatment is reduced by the irradiation. These effects are explained by assuming that the microscopic defects formed by irradiation have polarity. The polarity of a defect is determined by the domain orientation at the defect site, and determines the direction of the bias field resulting from the defect.

Crystal Form of L-α-Alanine-Doped Triglycine Sulfate

Single crystals of ferroelectric TGS were grown from solutions with various concentrations of L-α-alanine, and the variation of crystal form with the alanine content was examined. Some distinctive features dependent on the alanine content were found. The doped crystal grows more rapidly in the +b side of the seed than in the -b side. Therefore, the mirror plane normal to the b-axis of TGS disappears with alanine doping. The degree of asymmetry about the (010) plane increases with increasing alanine content. The form of highly doped crystal clearly shows the polar symmetry 2. The degree of asymmetry has no relevance to the domain structure.

Distribution of internal bias field in L-α-alanine doped triglycine sulfate crystal

Distribution of internal bias field Eb was investigated in an LATGS crystal grown from TGS solution containing 10 mol% L-α-alanine. As the crystal was of single domain, the field Eb was oriented in one definite direction all over the crystal. The intensity of Eb distributed over a wide range of 30 to 500 kV/m or more, and distinctly depended on the crystal growth sector. The distribution showed the polar symmetry 2, similarly to the crystal form of LATGS. Generally speaking, Eb in the -b side of the seed crystal was more intense than in the +b side. The spontaneous polarization and the coercive field did not show a significant distribution in the crystal.