Effects of Pressure on Exciton Absorption and Emission in Strongly Quantum-Confined CsPbBr3 Quantum Dots and Nanoplatelets.

Soft lattices of metal halide perovskite (MHP) nanocrystals (NCs) are considered responsible for many of their optical properties associated with excitons, which are often distinct from other semiconductor NCs. Earlier studies of MHP NCs upon compression revealed how structural changes and the resulting changes in the optical properties such as the bandgap can be induced at relatively low pressures. However, the pressure response of the exciton transition itself in MHP NCs remains relatively poorly understood due to limitations inherent to studying weakly or nonconfined NCs in which exciton absorption peaks are not well-separated from the continuum interband transition. Here, we investigated the pressure response of the absorbing and emitting transitions of excitons using strongly quantum-confined CsPbBr3 quantum dots (QDs) and nanoplatelets (NPLs), which both exhibit well-defined exciton absorption peaks. Notably, the reversible vanishing and recovery of the exciton absorption accompanied by reversible quenching and recovery of the emission were observed in both QDs and NPLs, resulting from the reversible pressure modulation of the exciton oscillator strength. Furthermore, CsPbBr3 NPLs exhibited irreversible pressure-induced creation of trap states at low pressures (∼0.1 GPa) responsible for trapped exciton emission that developed on the time scale of ∼10 min, while the reversible pressure response of the absorbing exciton transition was maintained. These findings shed light on the diverse effects the application of force has on the absorbing and emitting exciton transitions in MHP NCs, which are important for their application as excitonic light emitters in high-pressure environments.

Ln2 (SeO3 )2 (SO4 )(H2 O)2 (Ln=Sm, Dy, Yb): A Mixed-Ligand Pathway to New Lanthanide(III) Multifunctional Materials Featuring Nonlinear Optical and Magnetic Anisotropy Properties.

Bottom-up assembly of optically nonlinear and magnetically anisotropic lanthanide materials involving precisely placed spin carriers and optimized metal-ligand coordination offers a potential route to developing electronic architectures for coherent radiation generation and spin-based technologies, but the chemical design historically has been extremely hard to achieve. To address this, we developed a worthwhile avenue for creating new noncentrosymmetric chiral Ln3+ materials Ln2 (SeO3 )2 (SO4 )(H2 O)2 (Ln=Sm, Dy, Yb) by mixed-ligand design. The materials exhibit phase-matching nonlinear optical responses, elucidating the feasibility of the heteroanionic strategy. Ln2 (SeO3 )2 (SO4 )(H2 O)2 displays paramagnetic property with strong magnetic anisotropy facilitated by large spin-orbit coupling. This study demonstrates a new chemical pathway for creating previously unknown polar chiral magnets with multiple functionalities.

Single-Ion Behavior in New 2-D and 3-D Gadolinium 4f7 Materials: CsGd(SO4)2 and Cs[Gd(H2O)3(SO4)2]·H2O.

The recent creation of 4f7 gadolinium materials has enabled vital studies of the free-ion properties of the Gd(III) cations. While the 8 S ground state in a trivalent Gd compound is, in principle, isotropic, it has been demonstrated that there is a residual orbital angular momentum affected by the crystal field and structural distortion in certain systems. By exploiting the atomistic control innate to material growth, we address a fundamental question of how the isotropic nature of Gd(III) is preserved in different dimensionalities of crystal structures. To achieve this, we designed two new trivalent Gd materials possessing two structurally distinct features, a 2-D CsGd(SO4)2 and a 3-D Cs[Gd(H2O)3(SO4)2]·H2O. The tunability of the structural dimension is facilitated by O-H---O hydrogen bonds. The structural divergence between the two compounds allows us to investigate each material individually and make a comparison between them regarding their physical properties as a function of lattice dimension. Our results demonstrate that structural dimensions have a negligible effect on the single-ion behavior of the materials. Magnetization measurements for the Gd(III) complexes yielded paramagnetic states with the isotropic spin-only nature. Specific heat data suggest that there is a lack of magnetic phase transition down to T = 1.8 K, and coupled lattice vibrations in the materials are attributable to strong covalent bonding characters of the (SO4)2- and H2O ligands. This work offers a pathway for retaining the single-ion property of Gd(III) while constructing the large spin magnetic moment S = 7/2 in large-scale extended frameworks.

Study of Integer Spin S = 1 in the Polar Magnet ß-Ni(IO3)2.

Polar magnetic materials exhibiting appreciable asymmetric exchange interactions can potentially host new topological states of matter such as vortex-like spin textures; however, realizations have been mostly limited to half-integer spins due to rare numbers of integer spin systems with broken spatial inversion lattice symmetries. Here, we studied the structure and magnetic properties of the S = 1 integer spin polar magnet ß-Ni(IO3)2 (Ni2+, d8, 3F). We synthesized single crystals and bulk polycrystalline samples of ß-Ni(IO3)2 by combining low-temperature chemistry techniques and thermal analysis and characterized its crystal structure and physical properties. Single crystal X-ray and powder X-ray diffraction measurements demonstrated that ß-Ni(IO3)2 crystallizes in the noncentrosymmetric polar monoclinic structure with space group P21. The combination of the macroscopic electric polarization driven by the coalignment of the (IO3)- trigonal pyramids along the b axis and the S = 1 state of the Ni2+ cation was chosen to investigate integer spin and lattice dynamics in magnetism. The effective magnetic moment of Ni2+ was extracted from magnetization measurements to be 3.2(1) µB, confirming the S = 1 integer spin state of Ni2+ with some orbital contribution. ß-Ni(IO3)2 undergoes a magnetic ordering at T = 3 K at a low magnetic field, µ0H = 0.1 T; the phase transition, nevertheless, is suppressed at a higher field, µ0H = 3 T. An anomaly resembling a phase transition is observed at T ≈ 2.7 K in the Cp/T vs. T plot, which is the approximate temperature of the magnetic phase transition of the material, indicating that the transition is magnetically driven. This work offers a useful route for exploring integer spin noncentrosymmetric materials, broadening the phase space of polar magnet candidates, which can harbor new topological spin physics.

Spin and Orbital Effects on Asymmetric Exchange Interaction in Polar Magnets: M(IO3)2 (M = Cu and Mn).

Magnetic polar materials feature an astonishing range of physical properties, such as magnetoelectric coupling, chiral spin textures, and related new spin topology physics. This is primarily attributable to their lack of space inversion symmetry in conjunction with unpaired electrons, potentially facilitating an asymmetric Dzyaloshinskii-Moriya (DM) exchange interaction supported by spin-orbital and electron-lattice coupling. However, engineering the appropriate ensemble of coupled degrees of freedom necessary for enhanced DM exchange has remained elusive for polar magnets. Here, we study how spin and orbital components influence the capability of promoting the magnetic interaction by studying two magnetic polar materials, α-Cu(IO3)2 (2D) and Mn(IO3)2 (6S), and connecting their electronic and magnetic properties with their structures. The chemically controlled low-temperature synthesis of these complexes resulted in pure polycrystalline samples, providing a viable pathway to prepare bulk forms of transition-metal iodates. Rietveld refinements of the powder synchrotron X-ray diffraction data reveal that these materials exhibit different crystal structures but crystallize in the same polar and chiral P21 space group, giving rise to an electric polarization along the b-axis direction. The presence and absence of an evident phase transition to a possible topologically distinct state observed in α-Cu(IO3)2 and Mn(IO3)2, respectively, imply the important role of spin-orbit coupling. Neutron diffraction experiments reveal helpful insights into the magnetic ground state of these materials. While the long-wavelength incommensurability of α-Cu(IO3)2 is in harmony with sizable asymmetric DM interaction and low dimensionality of the electronic structure, the commensurate stripe AFM ground state of Mn(IO3)2 is attributed to negligible DM exchange and isotropic orbital overlapping. The work demonstrates connections between combined spin and orbital effects, magnetic coupling dimensionality, and DM exchange, providing a worthwhile approach for tuning asymmetric interaction, which promotes evolution of topologically distinct spin phases.

Synthesis of Stoichiometric SrTiO3 and Its Carrier Doping from Air-Stable Bimetallic Complexes.

We demonstrate the synthesis of perovskite oxide SrTiO3 with ideal cation stoichiometry and homogeneous La doping using air-stable Sr-Ti and La-Ti bimetallic peroxo complexes with a 1:1 cation ratio. Phase-pure SrTiO3, La2Ti2O7, and LaTiO3 were successfully synthesized by thermal decomposition of those complexes. La-doped SrTiO3 was obtained by mixing the Sr-Ti and La-Ti complexes in an acid solution, followed by thermal decomposition. La-doped SrTiO3 showed systematic chemical trends in the lattice constant and electrical conduction. Not only those bulk polycrystals but also cation-stoichiometric SrTiO3 epitaxial thin films were grown with an atomically flat surface from the Sr-Ti complex.

Crystal structure and Hirshfeld surface analysis of a new di-thio-glycoluril: 1,4-bis-(4-meth-oxy-phen-yl)-3a-methyl-tetra-hydro-imidazo[4,5-d]imidazole-2,5(1H,3H)-di-thione.

In the title di-thio-glycoluril derivative, C19H20N4O3S2, there is a difference in the torsion angles between the thio-imidazole moiety and the meth-oxy-phenyl groups on either side of the mol-ecule [C-N-Car-Car = 116.9 (2) and -86.1 (3)°, respectively]. The N-C-N bond angle on one side of the di-thio-glycoluril moiety is slightly smaller compared to that on the opposite side, [110.9 (2)° cf. 112.0 (2)°], probably as a result of the steric effect of the methyl group. In the crystal, N-H⋯S hydrogen bonds link adjacent mol-ecules to form chains propagating along the c-axis direction. The chains are linked by C-H⋯S hydrogen bonds, forming layers parallel to the bc plane. The layers are then linked by C-H⋯π inter-actions, leading to the formation of a three-dimensional supra-molecular network. Hirshfeld surface analysis and two-dimensional fingerprint plots were used to investigate the mol-ecular inter-actions in the crystal.