Poly(aminoimino)heptazine, otherwise known as Liebig's melon, whose composition and structure has been subject to multitudinous speculations, was synthesized from melamine at 630 degrees C under the pressure of ammonia. Electron diffraction, solid-state NMR spectroscopy, and theoretical calculations revealed that the nanocrystalline material exhibits domains well-ordered in two dimensions, thereby allowing the structure solution in projection by electron diffraction. Melon ([C(6)N(7)(NH(2))(NH)](n), plane group p2 gg, a=16.7, b=12.4 A, gamma=90 degrees, Z=4), is composed of layers made up from infinite 1D chains of NH-bridged melem (C(6)N(7)(NH(2))(3)) monomers. The strands adopt a zigzag-type geometry and are tightly linked by hydrogen bonds to give a 2D planar array. The inter-layer distance was determined to be 3.2 A from X-ray powder diffraction. The presence of heptazine building blocks, as well as NH and NH(2) groups was confirmed by (13)C and (15)N solid-state NMR spectroscopy using (15)N-labeled melon. The degree of condensation of the heptazine core was further substantiated by a (15)N direct excitation measurement. Magnetization exchange observed between all (15)N nuclei using a fp-RFDR experiment, together with the CP-MAS data and elemental analysis, suggests that the sample is mainly homogeneous in terms of its basic composition and molecular building blocks. Semiempirical, force field, and DFT/plane wave calculations under periodic boundary conditions corroborate the structure model obtained by electron diffraction. The overall planarity of the layers is confirmed and a good agreement is obtained between the experimental and calculated NMR chemical shift parameters. The polymeric character and thermal stability of melon might render this polymer a pre-stage of g-C(3)N(4) and portend its use as a promising inert material for a variety of applications in materials and surface science.

The easily tunable emission of halide perovskite nanocrystals throughout the visible spectrum makes them an extremely promising material for light-emitting applications. Whereas high quantum yields and long-term colloidal stability have already been achieved for nanocrystals emitting in the red and green spectral range, the blue region currently lags behind with low quantum yields, broad emission profiles, and insufficient colloidal stability. In this work, we present a facile synthetic approach for obtaining two-dimensional CsPbBr nanoplatelets with monolayer-precise control over their thickness, resulting in sharp photoluminescence and electroluminescence peaks with a tunable emission wavelength between 432 and 497 nm due to quantum confinement. Subsequent addition of a PbBr-ligand solution repairs surface defects likely stemming from bromide and lead vacancies in a subensemble of weakly emissive nanoplatelets. The overall photoluminescence quantum yield of the blue-emissive colloidal dispersions is consequently enhanced up to a value of 73 ± 2%. Transient optical spectroscopy measurements focusing on the excitonic resonances further confirm the proposed repair process. Additionally, the high stability of these nanoplatelets in films and to prolonged ultraviolet light exposure is shown.

Photocatalytic conversion of solar energy to fuels, such as hydrogen, is attracting enormous interest, driven by the promise of addressing both energy supply and storage. Colloidal semiconductor nanocrystals have been at the forefront of these efforts owing to their favourable and tunable optical and electronic properties as well as advances in their synthesis. The efficiency of the photocatalysts is often limited by the slow transfer and subsequent reactions of the photoexcited holes and the ensuing high charge recombination rates. Here we propose that employing a hydroxyl anion/radical redox couple to efficiently relay the hole from the semiconductor to the scavenger leads to a marked increase in the H2 generation rate without using expensive noble metal co-catalysts. The apparent quantum yield and the formation rate under 447 nm laser illumination exceeded 53% and 63 mmol g(-1) h(-1), respectively. The fast hole transfer confers long-term photostability on the system and opens new pathways to improve the oxidation side of full water splitting.

Solid-state nanopores bear great potential to be used to probe single proteins; however, the passage of proteins through nanopores was found to be complex, and unexpected translocation behavior with respect to the passage direction, rate, and duration was observed. Here we study the translocation of a model protein (avidin) through silicon nitride nanopores focusing on the electrokinetic effects that facilitate protein transport across the pore. The nanopore zeta potential zeta(pore) and the protein zeta potential zeta(protein) are measured independently as a function of solution pH. Our results reveal that electroosmotic transport may enhance or dominate and reverse electrophoretic transport in nanopores. The translocation behavior is rationalized by accounting for the charging states of the protein and the pore, respectively; the resulting translocation direction can be predicted according to the difference in zeta potentials, zeta(protein) - zeta(pore). When electrophoresis and electroosmosis cancel each other out, diffusion becomes an effective (and bias-independent) mechanism which facilitates protein transport across the pore at a significant rate.