Sangen Zhao

Beryllium-free Li4Sr(BO3)(2) for deep-ultraviolet nonlinear optical applications

Nonlinear optical (NLO) materials are of great importance in laser science and technology, as they can expand the wavelength range provided by common laser sources. Few NLO materials, except KBe2BO3F2 (KBBF), can practically generate deep-ultraviolet coherent light by direct second-harmonic generation process, limited by the fundamental requirements on the structure-directing optical properties. However, KBBF suffers a strong layering tendency and high toxicity of the containing beryllium, which hinder the commercial availability of KBBF. Here we report a new beryllium-free borate, Li4Sr(BO3)2, which preserves the structural merits of KBBF, resulting in the desirable optical properties. Furthermore, Li4Sr(BO3)2 mitigates the layering tendency greatly and enhances the efficiency of second-harmonic generation by more than half that of KBBF. These results suggest that Li4Sr(BO3)2 is an attractive candidate for the next generation of deep-ultraviolet NLO materials. This beryllium-free borate represents a new research direction in the development of deep-ultraviolet NLO materials.

Beryllium-Free Rb3Al3B3O10F with Reinforced Interlayer Bonding as a Deep-Ultraviolet Nonlinear Optical Crystal

A new beryllium-free borate Rb3Al3B3O10F has been synthesized and characterized by single-crystal X-ray diffraction. It features a framework structure consisting of alveolate [Al3(BO3)3OF]∞ layers tightly bound via Al-O and Al-F bridged bonds, with the in-layer [BO3](3-) groups in nearly coplanar and aligned arrangement. This compound is transparent down to 200 nm and is phase-matchable with a powder second-harmonic generation efficiency of 1.2 times that of KH2PO4. Remarkably, it exhibits a strong interlayer bonding which is about one order larger than that of the benchmark KBe2BO3F2, thus no layering tendency was observed during the crystal growth. In addition, it is nonhygroscopic and thermally stable up to ∼1462 K. These attributes make Rb3Al3B3O10F a promising nonlinear optical crystal in the deep-ultraviolet region. First-principles calculations, combined with the anionic group theory, were adopted to rationalize the optical properties.

Deep-ultraviolet transparent phosphates RbBa2(PO3)5 and Rb2Ba3(P2O7)2 show nonlinear optical activity from condensation of [PO4](3-) units.

It is challenging to explore deep-ultraviolet (deep-UV) nonlinear optical (NLO) materials that can achieve a subtle balance between deep-UV transparency and high NLO activity. Known deep-UV NLO materials are almost exclusively limited to borates, except few newly discovered phosphates despite their small NLO activities. Here we report two asymmetric phosphates, RbBa2(PO3)5 (I) and Rb2Ba3(P2O7)2 (II), which feature [PO3]∞ chains and [P2O7](4-) dimers formed by condensation of [PO4](3-) units, respectively. Remarkably, I achieves the desired balance, with the shortest deep-UV absorption edge at 163 nm and the largest NLO activity of 1.4 × KDP (KH2PO4) in deep-UV NLO phosphates. According to first-principles calculations, the enhanced macroscopic SHG response of I can be attributed to the [PO3]∞ chains which exhibit significantly larger microscopic SHG coefficients as compared with the [P2O7](4-) dimers.

Designing a Beryllium-Free Deep-Ultraviolet Nonlinear Optical Material without a Structural Instability Problem

A beryllium-free deep-ultraviolet (deep-UV) nonlinear optical (NLO) material K3Ba3Li2Al4B6O20F is developed mainly by the element substitution of Be for Al and Li from Sr2Be2B2O7 that was considered as one of the most promising deep-UV NLO materials. K3Ba3Li2Al4B6O20F preserves the structural merits of Sr2Be2B2O7 and thus exhibits no layering growth tendency and possesses the optical properties required for deep-UV NLO applications, including deep-UV transparency, phase-matchability, and sufficiently large second-harmonic generation (1.5 × KH2PO4). Furthermore, it overcomes the structural instability problem of Sr2Be2B2O7, which is confirmed by the obtainment of large single crystals and phonon dispersion calculations. These attributes make it very attractive for next-generation deep-UV NLO materials. The substitution of Be for Al and Li in beryllium borates provides a new opportunity to design beryllium-free deep-UV NLO materials with good performance.

Tailored Synthesis of a Nonlinear Optical Phosphate with a Short Absorption Edge

A nonlinear optical phosphate Ba5P6O20 was rationally developed by a tailored synthetic approach based on the use of flexible [P3O10](5-) units. The phosphate exhibits a very short absorption edge of λ=167 nm, which is among the shortest known in phase-matchable phosphates. First-principles electronic structure analysis elucidated the origin of the changes in the optical properties, and specifically in the absorption edge, of the material. Such a tailored synthetic approach provides a new opportunity to design nonlinear optical materials with short absorption edges.

Construction of Interpenetrated Ruthenium Metal–Organic Frameworks as Stable Photocatalysts for CO2 Reduction

Poor stability has long been a major obstacle to the practical applications of metal-organic framework (MOF) photocatalysts. This problem can be overcome by the use of structural interpenetration. In this work, by modifying Ru metalloligands, we have rationally designed two Ru-polypyridine based MOFs (with non-interpenetrated and interpenetrated structures, respectively), both of which exhibit similar photocatalytic activities for CO2 photoreduction. Remarkably, the interpenetrated Ru-MOF possesses good photocatalytic durability and recyclability, and shows much higher thermal and photic stability in comparison with its non-interpenetrated counterpart. To the best of our knowledge, this is the first time that the stability of MOF photocatalysts was improved by using structural interpenetration.

A Photoferroelectric Perovskite-Type Organometallic Halide with Exceptional Anisotropy of Bulk Photovoltaic Effects.

Perovskite-type ferroelectrics composed of organometallic halides are emerging as a promising alternative to conventional photovoltaic devices because of their unique photovoltaic effects (PVEs). A new layered perovskite-type photoferroelectric, bis(cyclohexylaminium) tetrabromo lead (1), is presented. The material exhibits an exceptional anisotropy of bulk PVEs. Upon photoexcitation, superior photovoltaic behaviors are created along its inorganic layers, which are composed of corner-sharing PbBr6 octahedra. Semiconducting activity with remarkable photoconductivity is achieved in the vertical direction, showing sizeable on/off current ratios (>10(4) ), which compete with the most active photovoltaic material CH3 NH3 PbI3 . In 1 the temperature-dependence of photovoltage coincides fairly well with that of polarization, confirming the dominant role of ferroelectricity in such highly anisotropic PVEs. This finding sheds light on bulk PVEs in ferroelectric materials, and promotes their application in optoelectronic devices.

A new UV nonlinear optical material CsZn2B3O7: ZnO4 tetrahedra double the efficiency of second-harmonic generation.

A new noncentrosymmetric borate CsZn2B3O7 was synthesized by the solid-state reaction techniques. The crystals were obtained by flux method and are of block shape without layering tendency. Single-crystal X-ray diffraction analysis reveals that the crystal structure is composed of [Zn2BO5]∞ two-dimensional layers that are bridged by [B3O6](3-) groups to form a three-dimensional framework with one-dimensional channels occupied by Cs(+) cations along the a and c axes. Thermal analysis indicates that CsZn2B3O7 melts incongruently. UV-visible-near-IR diffuse reflectance spectrum gives a short absorption edge at 218 nm. CsZn2B3O7 is phase-matchable, with a powder second-harmonic generation (SHG) efficiency of 1.5 × KDP (KDP, potassium dihydrogen phosphate) at 1064 nm, based on the Kurtz-Perry method. These results show that CsZn2B3O7 may have prospects as a UV nonlinear optical material. Interestingly, the SHG efficiency of CsZn2B3O7 is about twice that of γ-KBe2B3O7, a structurally analogous alkaline and alkaline earth borate. First-principles calculations combined with atom-cutting analysis reveal that the ZnO4 tetrhedral groups in CsZn2B3O7 account for the SHG enhancement.

A sequentially switchable molecular dielectric material tuned by the stepwise ordering in diisopropylammonium trifluoromethanesulfonate

A novel switchable and tunable molecular dielectric material, diisopropylammonium trifluoromethanesulfonate (1), which undergoes two reversible second-order solid state phase transitions at 200 K (TcL) and 340 K (TcH) respectively, has been successfully synthesized and grown as bulk crystals. The differential scanning calorimetry (DSC) measurements, and dielectric and variable-temperature single-crystal X-ray diffraction studies confirmed the stepwise phase transitions. 1 exhibits a remarkable temperature-dependent dielectric behavior, which could be tuned through three distinctive states triggered by the temperature change. Temperature-dependent single crystal structural analyses of 1 reveal a collection of exceptionally distinct and synchronous molecular motions of both the cationic and anionic moieties in 1, and that the interesting stepwise ordering of cations and anions are mainly responsible for its switchable and tunable dielectric properties. All of this demonstrates its potential application as a switchable and tunable molecular dielectric material.

Hierarchical metal–organic framework nanoflowers for effective CO2 transformation driven by visible light

Although metal–organic frameworks (MOFs) have been considered as a new class of photocatalysts for CO2 photoreduction, more efforts need to be devoted to enhancing their catalytic activities. In this work, for the first time, we have developed metal–organic flower-like hierarchical nanostructures (nanoflowers), based on a bifunctional Ru-MOF, to act as stable and effective heterogeneous photocatalysts for visible-light driven CO2 photoreduction. The unique flower-like nanostructure remarkably enhances the photocatalytic activity of this material, which is almost 150% more active than that of solid micro-crystals. Meanwhile, the flower-like 3D hierarchical nanostructures also highly improve the stability of the nanosheets of the Ru-MOF, which greatly promotes the photostability and recyclability of the photocatalysts. We believe that our study will pave a new way to improve the photocatalytic performance of MOF based photocatalysts for CO2 reduction.