Template assisted formation of micro- and nanotubular carbon nitride materials

Micro- and nanotubes of an amorphous carbon nitride material were synthesized by metathesis reactions between cyanuric chloride (C₃N₃Cl₃) and different nitrogen sources, such as Li₂(CN₂) or Li₃(BN₂). The intermediate formation of needle-shaped crystals of N(C₃N₃Cl₂)₃ was always observed in our reactions, and investigated with respect to their role as a template in the formation of tubes. Chemical analyses of the micro- and nanotubes reveal carbon to nitrogen ratios near 3:4, consistent with the suspected material C₃N₄. Synthesized carbon nitride materials were thermally stable up to 600 ^°C in inert atmosphere. They were inspected by a number of physical measurements, mainly using TEM, EDX and IR investigations.     

Double-exchange model study of multiferroic RMnO<Subscript>3</Subscript> perovskites

In this proceeding, recent theoretical investigations by the authors on the multiferroic RMnO₃ perovskites are briefly reviewed at first. Using the double-exchange model, the realistic spiral spin order in undoped manganites such as TbMnO₃ and DyMnO₃ is well reproduced by incorporating a weak next-nearest neighbor superexchange (~10% of nearest neighbor superexchange) and moderate Jahn-Teller distortion. The phase transitions from the A-type antiferromagnet (as in LaMnO₃), to the spiral phase (as in TbMnO₃), and finally to the E-type antiferromagnet (as in HoMnO₃), with decreasing size of the R ions, were also explained. Moreover, new results of phase diagram of the three-dimensional lattice are also included. The ferromagnetic tendency recently discovered in the LaMnO₃ and TbMnO₃ thin films is explained by considering the substrate stress. Finally, the relationship between our double-exchange model and a previously used J₁-J₂-J₃ model is further discussed from the perspective of spin wave excitations.     

Templated Grain Growth of Textured Piezoelectric Ceramics

Crystallographic texturing of polycrystalline ferroelectric ceramics offers a means of achieving significant enhancements in the piezoelectric response. Templated grain growth (TGG) enables the fabrication of textured ceramics with single crystal-like properties, as well as single crystals. In TGG, nucleation and growth of the desired crystal on aligned single crystal template particles results in an increased fraction of oriented material with heating. To facilitate alignment during forming, template particles must be anisometric in shape. To serve as the preferred sites for epitaxy and subsequent oriented growth of the matrix, the template particles need to be single crystal and chemically stable up to the growth temperature. Besides templating the growth process, the template particles may also serve as seed sites for phase formation of a reactive matrix. This process, referred to as Reactive TGG (RTGG), has been used to obtain highly oriented Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃, Sr_0.53Ba_0.47Nb₂O₆, and (Na_1/2Bi_1/2)TiO₃-BaTiO₃. Highly oriented Bi₄Ti₃O₁₂, Sr₂Nb₂O₇, CaBi₄Ti₄O₁₅, Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃, Sr_0.53Ba_0.47Nb₂O₆ and (Na_1/2Bi_1/2)TiO₃-BaTiO₃ ceramics have been produced by TGG. The resulting ceramics show texture levels up to 90%, and significant enhancements in the piezoelectric properties relative to randomly oriented ceramics with comparable densities. For example, piezoelectric coefficients of textured piezoelectrics are from 2 to 3 times higher than polycrystalline ceramics and as high as 90% of the single crystal values. In textured PMN-PT, a low field (< 5 kV/cm) piezoelectric coefficient (d ₃₃) of ～1600 pC/N was obtained with > 0.3% strain (at 50 kV/cm). The high field dielectric and electromechanical properties of textured perovskites are more hysteretic than those of single crystals, probably as a result of clamping by the residual template particles, residual random grains, the presence of non-ferroelectric second phases, and a wide orientation distribution. Lateral clamping of one grain by another may also be an important factor in fiber-textured samples. Means to further improve the quality of texture and thus properties of textured piezoelectric ceramics by TGG are presented.     

Direct growth of CdSe nanorods on ITO substrates by co-anchoring of ZnO nanoparticles and ethylenediamine

This work presents results from a study carried out on the Al/Cu₃BiS₃/Buffer/ZnO stacked layer, using high-resolution transmission electron microscopy (HRTEM). This system is used to fabricate solar cells with Al/Cu₃BiS₃/In₂S₃/ZnO and Al/Cu₃BiS₃/ZnS/ZnO structures. The conforming layers function as electrical contact, absorber layer, buffer layer, and optical window, respectively. The detailed results of Cu₃BiS₃ thin film investigation by HRTEM are presented. The Cu₃BiS₃ thin films are non-homogeneous and are strongly dependent on deposition conditions with grain size between 6.5 and 20?nm showing a nano-crystalline character. We found that the buffer layer of In₂S₃ grows in a polycrystalline structure, whereas the layer of ZnS reveals an amorphous structure. The performed study of these solar cells gives us significant information about their crystalline structure and allows us to visualize each of the constituting layers as well as of the Al/Cu₃BiS₃, Cu₃BiS₃/buffer, and buffer/ZnO interfaces. This study was correlated with electrical properties.     

Magnetization studies on ErCo3-hydride and TmCo3-hydride

The compounds ErCo₃ and TmCo₃ are ferrimagnetic with Curie temperatures of 401 K and 370 K, respectively. These absorb hydrogen to form ErCo₃H_4.3 and TmCo₃H_3.3. From magnetization studies on these as well as other RCo₃ hydrides in the temperature interval 4.2 to 300 K, it is inferred that hydrogen absorption leads to a reduction in magnetic moment on cobalt and a weakening of the R-Co interaction (R = rare earth). Except in the case of the GdCo₃-hydride, saturation in magnetization is not achieved at 4.2 K in applied fields up to 21 kOe. This suggests the possibility of fanning of rare earth moments. The RCo₃-hydrides investigated earlier with R = Gd, Dy and Ho and the ErCo₃-hydride and TmCo₃-hydride all appear to be magnetically ordered at room temperature.     

Electrical and structural modifications of TZP

The stability range of the metastable tetragonal phase in the ZrO₂-Y₂O₃-TiO₂ and ZrO₂-Y₂O₃-Fe₂O₃ systems was investigated by XRD at room temperature. The solid solubility limit of TiO₂ was found to be as high as 20 mol%, while that of Fe₂O₃ does not exceed 0.8 mol%. Impedance measurements show a decrease of the total conductivity, bulk conductivity and grain boundary conductivity as a result of the TiO₂ and Fe₂O₃ addition. Dc-polarization measurements using the Hebb-Wagner technique were applied to determine the partial hole and electron conductivities of TiO₂ and Fe₂O₃ co-doped samples. These show a slightly higher hole conductivity as compared to pure TZP and a remarkably higher electron conductivity as compared to TiO₂ or Fe₂O₃ doped samples. The Hebb-Wagner curves are interpreted according to a model which considers the addition of mixed valence ions. The influence of the minority charge carriers on the charge-transfer resistance is investigated. Paper presented at the 4th Euroconference on Solid State Ionics, Renvyle, Galway, Ireland, Sept. 23–19, 1997.     

The Raman,Infrared and Proton NMR Spectra and the Structures of Dimethyl Chalcogenides

The vibrational bands of dimethyl chalcogenides (CH₃-X-CH₃) (X = O,S,Se,Te) have been assigned on the basis of C_2V symmetry by most of previous workers.^1–13) On the other hand, some ones have assigned D^3d to these molecules ^14–15) and to structurally related compounds; SiH₃-O-SiH₃ ¹⁶), CH₃-Hg-CH₃ ¹⁷), CF₃-Hg-CF₃ ¹⁸) and Cl₃Si-O-SiCl₃ ¹⁹). The symmetry of ethane(CH₃-CH₃) is obviously D_3d ²⁰⁾, because the vibrational spectra which indicate the lack of coincidence between Raman and infrared frequencies can be explained based upon D_3d symmetry, and the bent c c should theoretically be impossible. The CH₃-X-CH₃ molecules are considered as a derivative in which CH₃ group is connected to CH₃ in opposite direction along the z axis and x atom is inserted between the two CH₃ groups. Consequently, for a linear C-X-C model, the vibrational spectra of CH₃-X-CH₃ ^14,15) should be similar to those of CH₃-CH₃, except being added C-O-C stretching and bending bands. The obtained spectra of CH₃-X-CH₃ ^14,15) show a marked correspndence to that of ethane, and indicate that the rule of mutual exclusion holds between Raman and infrared frequencies. Therefore the symmetry of CH₃-X-CH₃ Seems to be D_3d, But mst of the previom papers ^1–13) have assigned C_2v to CH₃-X-CH₃ as mentioned above.     

Study of interfacial reactions and phase stabilization of mixed Sc, Dy, Hf high-k oxides by attenuated total reflectance infrared spectroscopy

Grazing angle attenuated total reflectance Fourier transform infrared spectroscopy is applied to study ultrathin film Hf⁴⁺, Sc³⁺ and Dy³⁺oxides, due to its high surface sensitivity. The (multi)metal oxides studied, are of interest as high-k dielectrics. Important properties affecting the permittivity, such as the amorphous or crystalline phase and interfacial reactions, are characterized.Dy₂O₃ is prone to silicate formation on SiO₂/Si substrates, which is expressed in DyScO₃ as well, but suppressed in HfDyO_x. Sc₂O₃, HfScO_x and HfO₂ were found to be stable in contact with SiO₂/Si. Deposition of HfO₂ in between Dy₂O₃ or DyScO₃ and SiO₂, prevents silicate formation, showing a buffer-like behavior for the HfO₂.Doping of HfO₂ with Dy or Sc prevents monoclinic phase crystallization. Instead, a cubic phase is obtained, which allows a higher permittivity of the films. The phase remains stable after anneal at high temperature.     

Phase transition control, melt growth of (Gd,RE)F3 single crystal and their luminescent properties

Rare-earth sesquifluorides with no absorption in visible spectral region, such as LaF₃, GdF₃, LuF₃, YF₃, ScF₃, are the topic of intense study as a host for luminescence materials. However, except Nd:LaF₃, they are not studied as a host for laser materials. The main obstacle troubling further study of GdF₃, LuF₃, YF₃, ScF₃ single crystal is the fact that there is first-order phase transition (LaF₃ type↔beta-YF₃ type for GdF₃, alpha-YF₃ type↔beta-YF₃ type for the rest) between the room and melting temperature.To prevent the phase transition, first of all, we have tried to make solid solution between GdF₃ and YF₃ in such a way that the average cation radii can be shifted to the size that does not have phase transition. Ce³⁺ perturbed luminescence was observed in the Ce- and Sr-codoped GdF₃-YF₃ system. Similar solid solution concept was applied to the combination between GdF₃ and YbF₃. The emission spectrum of Yb³⁺ that exhibits broad bands around 1 μm was observed. Room temperature up-conversion luminescence spectra of Pr³⁺-doped Gd_1−xYb_xF₃ were studied and visible emission from Pr³⁺ was obtained under infrared laser pumping in the Yb³⁺ broad absorption band at 935.5 nm.     

具有介孔结构的MnSiO3@Fe3O4@C纳米粒子的制备以及作为pH响应的T1-T2*双模MRI造影剂的研究应用

本文通过一个简单的、温和的方案制备了平均尺寸为120 nm,介孔结构的纳米粒子MnSiO₃@Fe₃O₄@C. 粒子的细胞毒性微小,可以用作T₁-T₂^*双模MRI造影剂. 酸性条件下MnSiO₃@Fe₃O₄@C释放出大量的Mn²⁺缩短T₁弛豫时间,提高成像分辨率. 超顺磁性的Fe₃O₄可以增强T₂对比成像,检测病变组织. 类似于肿瘤微环境/细胞器的酸性PBS(pH=5.0)中Mn²⁺的释放率达到31.66%,约为中性条件(pH=7.4)下的7倍. 释放的Mn²⁺通过内吞作用被细胞摄取,经肾脏排出,细胞毒性实验表明,MnSiO₃@Fe₃O₄@C具有低的细胞毒性,即使高浓度的200 ppm MnSiO₃@Fe₃O₄@C对HeLa细胞的毒性也相对较小. 对荷瘤小鼠静脉注射定量MnSiO₃@Fe₃O₄@C后,可以观察到一个快速增强的对比成像,给药24 h后,T₁MRI信号显著增强,达到132%,而T₂信号则明显降低至53.8%,活体MR成像证明了MnSiO₃@Fe₃O₄@C可以同时作为阳性和阴性造影剂. 此外,得益于介孔MnSiO₃优秀的酸敏感性,MnSiO₃@Fe₃O₄@C可以作为一种潜在的药物载体,实现肿瘤的诊疗一体化.     

Crystal field and exchange interactions in rare earth transition metal salen compounds containing bipyridine, pyrazole complexes

Crystal field parameters for Pr³⁺ in {[Ni(salen)Pr-(hfac)₃](H₂O)} (noted as NiPr) and {[Ni(salen)Pr(hfac)₃(pyr)]-(CHCl₃)} (noted as NiPrpyr) have been found from a fit to the thermal variation in the magnetic susceptibility of NiPr and NiPrpyr. The nature of exchange interaction in [Cu(salen)Pr(hfac)3(pyr)] (noted as CuPrpyr), {[{Cu(salen)Pr(hfac)₃}₂(pyz)](H₂O)₃} (noted as Cu₂Pr₂pyz) and {[{Cu(salen)Pr(hfac)₃}₂(bpy)]-(CHCl₃)₂} (noted as Cu₂Pr₂bpy, bpy=4,4_-bipyridine) have been found using the derived results for NiPr and NiPrpyr. All the exchange interactions give significant contribution to the thermal variation in magnetic susceptibility below 50 K. The contribution due to Pr-Cu interaction is positive while that of the Cu-Cu and Pr-Pr interactions are negative. The behaviors below 10 K for Pr-Cu and Pr-Pr are difficult to explain, and point to a possible change in structure of CuPrpyr, Cu₂Pr₂pyz and Cu₂Pr₂bpy below 10 K. The theoretical thermal variations in the magnetic specific heat of NiPr and NiPrpyr are computed and discussed.     

Synthesis and characterization of BaZrO3-doped YBa2Cu3O7−δ microtapes with improved critical current densities

We have recently demonstrated that through a sol–gel route, superconductor crystallization in the presence of simple biopolymers results in a drastic alteration of morphology, producing technologically useful nanowires and porous architectures. Morphological control is of the utmost importance to bulk high-temperature superconductors, as grain boundaries act as weak links in limiting the achievable critical current density (J_c). Here we show that, as expected, the incorporation of nanoparticulate barium zirconate (BaZrO₃) species into a biopolymer-mediated synthetic protocol for YBa₂Cu₃O_7?δ (Y123) leads to a significantly improved in-field J_c compared to that observed in a sample without BaZrO₃ additions. To ameliorate degradation of the BaZrO₃ species in this protocol, we demonstrate that by drawing the precursor sol into fibers, a microtape architecture is able to be formed, leading to lengthy, anisotropic structures having enhanced J_c through the retention of the BaZrO₃ species.     

Band structure and high pressure study of Rh3Sc, Rh3Y and Rh3La

The electronic structure of the Rhodium based intermetallic compounds (A₃B) such as Rh₃Sc, Rh₃Y and Rh₃La are studied by the Self Consistent Tight Binding Linear Muffin Tin Orbital (TB-LMTO) method. In the present work, an attempt has been made to understand why the compounds namely Rh₃Y and Rh₃La crystallize in hexagonal structure, rather than the cubic structure, where as some of the similar rhodium based A₃B compounds namely Rh₃Ti, Rh₃Zr, Rh₃Hf, Rh₃V, Rh₃Nb, Rh₃Ta and Rh₃Sc are found to stabilize in cubic structure. In this work a prediction has been made about the structural phase transition in Rh₃Y and Rh₃La, from Hexagonal phase to Cubic phase. A report of the lattice constant, bulk moduli, cohesive energy and electronic specific heat coefficient is made and is compared with the available experimental data. Band structure and density of states histograms are also plotted. An electronic topological transition is predicted in Rh₃La, which may lead to the changes in the Fermi surface topology and hence changes the physical properties of Rh₃La.     

First principles study of Ca in BaTiO3 and Bi0.5Na0.5TiO3

BaTiO₃–Bi_0.5Na_0.5TiO₃ is one of the promising candidates as a high-temperature relaxor with a high Curie temperature and several preferred dielectric characteristics. It has been found experimentally for a long time that adding calcium to BaTiO₃–Bi_0.5Na_0.5TiO₃ improves its temperature characteristic of the capacitance [J. Electron. Mater. 39, 2471]. In this study, Calcium (Ca) defects in perovskite BaTiO₃ and Bi_0.5Na_0.5TiO₃ have been studied based on first-principles calculations. In both BaTiO₃ and Bi_0.5Na_0.5TiO₃, our calculations showed that Ca atom energetically prefers to substitute for the cations, that is Ba, Bi, Na and Ti, depending on the growth conditions. In most cases, Ca predominantly substitutes on the A-site without providing additional electrical carriers (serve as either neutral defects or self-compensating defects). The growth conditions where Ca can be forced to substitute for B-site (with limited amount) and the conditions where Ca can be forced to serve as an acceptor are identified. Details of the local structures, formation energies and electronic properties of these Ca defects are reported.     

Optical amplification properties of Dy3+-doped Gd2SiO4, Lu2SiO5 and YAl3(BO3)4 single crystals

Comparative investigation of the optical amplification properties of dysprosium doped Gd₂SiO₅, Lu₂SiO₅ and YAl₃(BO₃)₄ single crystals was performed in a pump-and-probe experiment. High power laser pulses at 475 nm were used as the pump source in order to strongly populate the ⁴F_9/2 level of the Dy³⁺ ions due to ground state absorption. Low signal beam cw radiation at 574 nm was used as the probe beam to stimulate the emission associated with the ⁴F_9/2→⁶H_13/2 electronic transition of the Dy³⁺ ions. The process was modelled as a three levels system, and their populations were analysed and simulated in order to study the gain dynamics. Positive optical gain was observed and compared in these crystals. These results confirmed that among the systems studied the Dy³⁺-doped YAl₃(BO₃)₄ single crystal can be considered as a good candidate to develop an optical amplifier employing the ⁴F_9/2→⁶H_13/2 transition at around 574 nm which is the first step to consider as laser active media.     

Th3Ni2B2N3 a possible new superconducting compound: insights from electronic band structure

La₃Ni₂B₂N₃, which is similar to YNi₂B₂C and related borocarbides, was earlier studied by the electronic structure calculations [D.J. Singh, W.E. Pickett, Phys. Rev. B 51 (1995) 8668.], and was predicted to be a 3-D metal. In search of new compounds of the borocarbide and related families to get higher T_C, we have studied the compound Th₃Ni₂B₂N₃, by the first principles full potential electronic structure calculations by the linear augmented plane wave method. We get similar band structure for Th₃Ni₂B₂N₃ as found for La₃Ni₂B₂N₃, and the various atom-split component density of states show similar properties. The total electron density of states at Fermi level has been increased to about 92 states per Ry per f.u. as compared to 57 states per Ry per f.u. in La₃Ni₂B₂N₃. The main increase is due to the increased hybridization of the 5f states as seen by the more prominent low energy tail in the Th-component density of states. Based on this enhancement we predict Th₃Ni₂B₂N₃ to be a high temperature superconductor with a T_c in excess of 30 K.     

Topological insulator nanostructures: Materials synthesis, Raman spectroscopy, and transport properties

Nanostructured topological insulator materials such as ultrathin films, nanoplates, nanowires, and nanoribbons are attracting much attention for fundamental research as well as potential applications in low-energy dissipation electronics, spintronics, thermoelectrics, magnetoelectrics, and quantum computing due to their extremely large surface-to-volume ratios and exotic metallic edge/surface states. Layered Bi₂Se₃ and Bi₂Te₃ serve as reference topological insulator materials with a large nontrivial bulk gap up to 0.3 eV (equivalent to 3600 K) and simple single-Dirac-cone surface states. In this mini-review, we present an overview of recent advances in nanostructured topological insulator Bi₂Se₃ and Bi₃Te₃ from the viewpoints of controlled synthesis and physical properties. We summarize our recent achievements in the vapor-phase synthesis and structural characterization of nanostructured topological insulator Bi₂Se₃ and Bi₂Te₃, such as nanoribbons and ultrathin nanoplates.We also demonstrate the evolution of Raman spectra with the number of few-layer topological insulators, as well as the transport measurements that have succeeded in accessing the surface conductance and surface state manipulations in the device of topological insulator nanostructures.     

Effective silicon surface passivation by atomic layer deposited Al2O3/TiO2 stacks

In this work atomic layer deposition of Al₂O₃ and TiO₂ has been used to obtain dielectric stacks for passivation of silicon surfaces. Our experiments on n‐ and p‐type silicon wafers deposited by thin Al₂O₃/TiO₂ stacks show that a considerably improved passivation is obtained compared to the Al₂O₃ single layer. For Al₂O₃ films thinner than 20 nm the emitter saturation current density decreases with increasing TiO₂ thickness. Especially the passivation of ultrathin (～5 nm) Al₂O₃ is very effectively enhanced by TiO₂ due to a decreased interface defect density as well as an increased fixed negative charge in the stacks. Hence, the thin Al₂O₃/TiO₂ stacks developed in this work can be used as a passivation coating for Si‐based solar cells. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Lifetime of metastable magnesium atoms in superfluid helium

The metastable 3s3p³P_{0, 1, 2} states of the magnesium atom immersed into superfluid helium have been investigated. Absorption-fluorescence measurements were carried out to monitor the population of the³P₀,³P₁ and³P₂ level as a function of time. The population of these levels was found to decrease exponentially with a constant of =15±2 ms. This is about three times as long as the vacuum lifetime of the³P₁ level. In the resonant excitation band of the 3s3p³P states to the 3s4s³S state a blueshift of 70 nm compared to the emission and a large broadening were detected. The³P₂ and³P₀ states are not at all metastable any more. Additionally the weak intercombination transition of the³P₁ state to the¹S₀ ground state was investigated by monitoring this emission line as a function of time and of wavelength. The experiments resulted in the same exponential decay time as the excitation measurement. This outcome indicates a rather effective fine structure mixing of the considered Mg states in superfluid helium. Moreover, this raises the question whether common atomic quantum numbers are conserved and the selection rules are still valid.