Single-nanocrystal fluorescence microscopy reveals that the immiscibility between PbBr2 and CH3NH3PbBr3 crystals imposes the limiting energetic barrier for nanocrystal conversion.
We report a reactive flux technique using the common reagent P 2 S 5 and metal precursors developed to circumvent the synthetic bottleneck for producing high-quality single-and mixed-metal two-dimensional (2D) thiophosphate materials. For the monometallic compound, M 2 P 2 S 6 (M = Ni, Fe, and Mn), phase-pure materials were quickly synthesized and annealed at 650 °C for 1 h. Crystals of dimensions of several millimeters were grown for some of the metal thiophosphates using optimized heating profiles. The homogeneity of the bimetallic thiophosphates MM′P 2 S 6 (M, M′ = Ni, Fe, and Mn) was elucidated using energy-dispersive X-ray spectroscopy and Rietveld refinement. The quality of the selected materials was characterized by transmission electron microscopy and atomic force microscopy measurements. We report two novel bimetallic thiophosphates, MnCoP 2 S 6 and FeCoP 2 S 6 . The Ni 2 P 2 S 6 and MnNiP 2 S 6 flux reactions were monitored in situ using variable-temperature powder X-ray diffraction to understand the formation reaction pathways. The phases were directly formed in a single step at approximately 375 °C. The work functions of the semiconducting materials were determined and ranged from 5.28 to 5.72 eV.
White-light broadband emission in the visible
range from the low-dimensional halide perovskites is commonly attributed
to structural distortions in lead bromide octahedra. In this paper,
we report Dion–Jacobson-phase two-dimensional (2D) lead bromide
perovskites based on short aromatic diammonium cations, p-phenylene
diammonium (pPDA), m-phenylene diammonium (mPDA), and two 1D compounds
templated by o-phenylene diammonium (oPDA). All of the compounds exhibit
white-light emission. Single-crystal X-ray diffraction analysis reveals
that the distortion of the Pb octahedra is influenced by the stereochemistry
of the cations and their interactions with the perovskite layers.
Solid-state 1H and 207Pb NMR spectroscopy analysis
further confirms this trend, whereby different 1H and 207Pb chemical shifts are observed for the pPDA and mPDA spacer
cations, indicating different hydrogen-bonding interactions and octahedral
distortions. Owing to the octahedral distortion, 2D (mPDA)PbBr4 compounds exhibit broader white-light emission than 2D (pPDA)PbBr4. Density functional theory calculations suggest that (pPDA)PbBr4 and (mPDA)PbBr4 are direct-band-gap semiconductors,
and they exhibit larger electronic band gaps and effective masses
than the Ruddlesden–Popper-phase (BA)2PbBr4. Among the films of these compounds, 2D (mPDA)PbBr4 shows
the best stability, which is attributed to stronger hydrogen-bonding
interactions in the material.
Heterotropic allosteric activation of protein function, in which binding of one ligand thermodynamically activates the binding of another, different ligand or substrate, is a fundamental control mechanism in metabolism and as such has been a long-aspired capability in protein design. Here we show that greatly increasing the magnitude of a protein’s net charge using surface supercharging transforms that protein into an allosteric ligand- and counterion-gated conformational molecular switch. To demonstrate this we first modified the designed helical bundle hemoprotein H4, creating a highly charged protein which both unfolds reversibly at low ionic strength and undergoes the ligand-induced folding transition commonly observed in signal transduction by intrinsically disordered proteins in biology. As a result of the high surface-charge density, ligand binding to this protein is allosterically activated up to 1,300-fold by low concentrations of divalent cations and the polyamine spermine. To extend this process further using a natural protein, we similarly modified Escherichia coli cytochrome b562 and the resulting protein behaves in a like manner. These simple model systems not only establish a set of general engineering principles which can be used to convert natural and designed soluble proteins into allosteric molecular switches useful in biodesign, sensing, and synthetic biology, the behavior we have demonstrated––functional activation of supercharged intrinsically disordered proteins by low concentrations of multivalent ions––may be a control mechanism utilized by Nature which has yet to be appreciated.
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