Xutang Tao

Bulk crystal growth of hybrid perovskite material CH3NH3PbI3

Organic–inorganic hybrid perovskite materials have been receiving considerable attention due to their promising applications in many optoelectronic fields. However, some of the fundamental properties of perovskite materials are still disputed, because most of them are derived from a thin-film state. To comprehend the intrinsic characteristics in a single crystal, herein we report, for the first time, the bulk crystal growth of CH3NH3PbI3. Single crystals of tetragonal CH3NH3PbI3 with dimensions of 10 mm × 10 mm × 8 mm were grown by a temperature-lowering method in HI solution. Studies in to the refinement and orientations of the CH3NH3PbI3 single crystal structure were conducted based on a high quality crystal. The absorption edge of a CH3NH3PbI3 single crystal was located at about 836 nm, indicating that the band gap of CH3NH3PbI3 is approximately 1.48 eV, which is close to the theoretical results and smaller than those derived from polycrystalline and thin-films. CH3NH3PbI3 crystal exhibits a relatively wide absorption (from 250 nm to 800 nm) and a relatively good thermal stability.

Dual-wavelength synchronously mode-locked Nd:CNGG laser

We have experimentally demonstrated a dual-wavelength synchronously mode-locked Nd:CNGG laser based on the semiconductor saturable absorber mirror technique. Mode locking was achieved simultaneously on two gain bands of the crystal that have a central wavelength separation of 2.4 nm. The fundamental mode-locked pulse train has a repetition rate of 88 MHz and pulse duration of 5 ps, with an average output power of approximately 90 mW. Autocorrelation measurements show that each of the synchronously mode-locked pulses consists of a train of quasi-periodic beat pulses with a 660 fs pulse width and a 0.63 THz repetition rate.

A highly selective colorimetric chemosensor for detecting the respective amounts of iron(II) and iron(III) ions in water

A novel colorimetric chemosensor containing terpyridine was synthesized. It showed high selectivity and sensitivity for Fe(II) and Fe(III) ions in neutral aqueous solution in the presence of other metal ions such as Cd2+, Cr3+, Zn2+, Co2+, Ni2+, Cu2+, Pb2+, Na+, Ca2+, Mg2+, K+ and Ag+. Upon the addition of Fe2+ or Fe3+, the sensor displayed a unique new peak around 567 nm in its absorption spectra, and the color of the solution changed from light yellow to light magenta. In particular, the respective amounts of Fe(II) and Fe(III) ions in the solution can be detected using the light-absorption ratio variation approach (LARVA).

Facile synthesis and systematic investigations of a series of novel bent-shaped two-photon absorption chromophores based on pyrimidine.

The synthesis, structure, and single- and two-photon spectroscopic properties of a series of pyrimidine-based (bent-shaped) molecules are reported. All these stable heterocyclic compounds are fully characterized, and exhibit intense single- and two-photon excited fluorescence (SPEF and TPEF) over a wide spectral range from blue to red, with the spectral peak position of the SPEF being basically the same as that of the TPEF. The well-conjugated pi-systems, observed from the crystal structure, indicate the charge transfer feature of the ground state. Meanwhile, the theoretical and experimental studies indicate that the charge transfer from donor to acceptor is greatly enhanced in the excited states and the different substituted donor groups on the pyrimidine have a large effect on the optical and electrochemical properties. Based on typical structure data and comprehensive spectral data, the following structure-property relationships can be determined: for such bent-shaped chromophores, the absorption and the fluorescence wavelength maximum of the SPEF and TPEF, and two-photon absorption cross sections show a similar trend with increasing electron-donating strength of the corresponding terminal group and the number of branches, while the average bond lengths of the pi-linkage and HOMO-LUMO energy levels show an inverse trend. Experimental data and theoretical calculation provide a coherent picture. With these findings, bent-shaped quadrupolar chromophores combining peak TPA cross sections (up to 2280 GM), broad TPA bands throughout the whole 700-900 nm range, and high fluorescence quantum yields could, thus, be obtained. Such compounds are of particular interest for TPEF microscopy, as well as optical data storage in the visible and NIR regions. A data recording experiment proved the potential application of these materials.

Simple synthesis of monodisperse, quasi-spherical, citrate-stabilized silver nanocrystals in water.

Monodisperse, quasi-spherical silver nanocrystals (Ag NCs) have been produced directly in water via adding the aqueous solution of a mixture of AgNO3, sodium citrate, and KI into the boiling aqueous solutions of ascorbic acid (AA). The AA is used to significantly accelerate reduction of AgNO3 in order to promote a very fast nucleation, and the KI is used to tailor the growth of the Ag NCs into a quasi-spherical shape via its preferential adsorption on the NC {111} facets. The major role of citrate is to stabilize the newly formed NCs, whereas it has a minor contribution to reduction of AgNO3. The synergy of the effects of AA, citrate, and KI can significantly narrow the size distributions of the Ag NCs obtained so and transform the NC shapes to be truly quasi-spherical.

Formation of Hybrid Perovskite Tin Iodide Single Crystals by Top‐Seeded Solution Growth

Hybrid perovskites have generated a great deal of interest because of their potential in photovoltaic applications. However, the toxicity of lead means that there is interest in finding a nontoxic substitute. Bulk single crystals of both cubic CH3NH3 SnI3 and CH(NH2)2 SnI3 were obtained by using the top-seeded solution growth method under an ambient atmosphere. Structural refinement, band gap, thermal properties, and XPS measurements of CH3NH3 SnI3 and CH(NH2)2 SnI3 single crystals are also reported in detail. These results should pave the way for further applications of CH3NH3 SnI3 and CH(NH2)2 SnI3.

Oriented Single-Crystal-to-Single-Crystal Phase Transition with Dramatic Changes in the Dimensions of Crystals

We report here a new polymorph of cocrystal CuQ2-TCNQ that shows an oriented single-crystal-to-single-crystal phase transition along its a-axis at ambient conditions. Upon mechanical stimulation, it converts into another polymorph accompanied by almost doubling its length and halving its thickness. Our crystallographic studies indicate the dramatic changes in crystal dimensions resulted from the prominent changes of molecular stacking patterns. A reasonable mechanism for the phenomenon was proposed on the basis of the structural, microscopic, and thermal analysis.

Elastic, dielectric, and piezoelectric properties of BaTeMo2O9 single crystal

BaTeMo2O9 is a noncentrosymmetric single crystal for nonlinear optical applications. In this letter, we report complete set of elastic, dielectric, and piezoelectric constants of BaTeMo2O9 single crystal determined by resonance technique and impedance analysis, the relative constants have been obtained and discussed. Our results show that BaTeMo2O9 crystal has excellent piezoelectric property (d34=30.25pC∕N, s44=36.46pm2∕N) and could be a promising candidate for piezoelectric applications.

MgTeMoO6: A neutral layered material showing strong second-harmonic generation

A noncentrosymmetric (NCS) compound, MgTeMoO6, has been synthesized using the high temperature solution method with TeO2–MoO3 mixture as a flux. MgTeMoO6 crystalizes in the orthorhombic space group P21212 (No. 18) with cell parameters a = 5.03780(10) A, b = 5.26910(10) A, c = 8.8985(2) A, and Z = 2. The compound exhibits a novel neutral layered structure consisting of asymmetric MgO6 octahedra, TeO4 polyhedra and MoO4 tetrahedra. Thermal stability measurements revealed an incongruent melting point of 682.35 °C. The crystal exhibits a very broad transmission range from 360 nm to 5.2 μm. Powder second-harmonic generation (SHG) measurements using 1064 nm radiation indicates that the crystal exhibits a strong SHG efficiency of ∼1.5 × KTiOPO4. Additional SHG measurements indicate the material is type-I phase-matchable. Furthermore, calculations on the local dipole moment indicates that the strong SHG response of MgTeMoO6 is mainly caused by the three types of NLO-active units (TeO4, MoO4 and MgO6).

Enhanced visible light photocatalytic activity of bismuth oxybromide lamellas with decreasing lamella thicknesses

BiOBr lamellas were synthesized at different reaction pH values via a hydrothermal process. X-ray diffraction, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, UV-vis diffuse reflectance spectroscopy and N2 sorption measurements were used to characterize the BiOBr samples. BiOBr samples have the same lamella structures and band gaps but different lamella sizes and thicknesses. Adjusting the pH of the reaction system tunes the BiOBr lamella thickness from 42 to 21 nm. The photodegradation efficiencies of the BiOBr lamellas for rhodamine B (RhB) and methylene blue (MB) in aqueous solution were examined. The degradation efficiency for RhB is much higher than that for MB. The decrease in BiOBr lamella thickness significantly enhances the photocatalytic activity for dye degradation, despite the decrease in exposed photoactive (001) facet percentage. Decreasing the lamella thickness from 42 to 21 nm yields a more than fourfold enhancement in photodegradation efficiency of BiOBr samples for RhB. The most important factor influencing the photocatalytic activity of the BiOBr samples is their lamella thickness, rather than the exposed (001) facet percentage. Thus, even for flaky semiconductors with high exposed photoactive facet contents, the influence of lamella thickness on photocatalytic activity should be preferentially considered.