Recently, molecular fluorophores were shown to be formed in the bottom-up chemical synthesis, contributing to the emission of carbon dots (CDs), derived from a citric acid precursor. We applied an ammonothermal synthesis toward CDs, employing two reactants citric acid and supercritical ammonia functioning as both solvent and precursor. The resulting nanoparticles are identified as amorphous aggregates of molecular fluorophores based on citrazinic acid derivatives, which resemble many of the emission features typically reported to be characteristic for CDs. The aggregates absorb and emit at short and long wavelengths of the spectrum, a feature prior ascribed to intrinsic CD core and surface states, respectively. We identify three emission states: a high energy and a low energy aggregate state as well as an energy transfer state between both. Energy transfer is triggered only upon excitation within a narrow high energy spectral range, resulting in a characteristic bluegreen double emission. The high energy aggregate state exhibits a trapping mechanism elongating emission lifetime. To further analyze aggregated molecular fluorophores, we studied aqueous solutions and films of citrazinic acid and polyvinylpyrrolidone and demonstrated their concentration dependent optical behavior. Since fluorophore aggregates reproduce the emissive features of CDs, the contribution of sp 2 /sp 3 carbonized products and graphitic domains to the emission features of CDs must be carefully evaluated in future studies.

In this contribution, first synthesis of semiconducting ZnSiN 2 and ZnGeN 2 from solution is reported with supercritical ammonia as solvent and KNH 2 as ammonobasic mineralizer.T he reactions were conducted in custom-built highpressure autoclaves made of nickel-based superalloy.T he nitrides were characterized by powder X-ray diffractiona nd their crystal structures were refinedb yt he Rietveld method. ZnSiN 2 (a = 5.24637(4), b = 6.28025(5), c = 5.02228(4) , Z = 4, R wp = 0.0556) and isotypic ZnGeN 2 (a = 5.46677(10), b = 6.44640(12), c = 5.19080(10) , Z = 4, R wp = 0.0494) crystallize in the orthorhombic space group Pna2 1 (no. 33). The morphology and elemental composition of the nitrides were examined by electron microscopy and energy-dispersive X-ray spectroscopy (EDX). Well-definedsingle crystalswith adiameter up to 7 mmw ere grownb ya mmonothermal synthesis at temperatures between 870 and 1070 Ka nd pressures up to 230 MPa. Optical properties have been analyzed with diffuse reflectance measurements. The band gaps of ZnSiN 2 and ZnGeN 2 were determined to be 3.7 and 3.2 eV at room temperature, respectively.I ns itu X-ray measurements were performed to exemplarily investigate the crystallization mechanism of ZnGeN 2 .D issolution in ammonobasic supercritical ammonia between 570 and6 70 Kw as observed which is quite promising for the crystal growth of ternary nitrides under ammonothermalconditions.

Grimm-Sommerfeld analogous nitrides MgSiN , MgGeN , MnSiN , MnGeN , LiSi N and LiGe N (generally classified as II-IV-N and I-IV -N ) are promising semiconductor materials with great potential for application in (opto)electronics or photovoltaics. A new synthetic approach for these nitride materials was developed using supercritical ammonia as both solvent and nitride-forming agent. Syntheses were conducted in custom-built high-pressure autoclaves with alkali metal amides LiNH , NaNH or KNH as ammonobasic mineralizers, which accomplish an adequate solubility of the starting materials and promote the formation of reactive intermediate species. The reactions were performed at temperatures between 870 and 1070 K and pressures up to 230 MPa. All studied compounds crystallize in wurtzite-derived superstructures with orthorhombic space groups Pna2 (II-IV-N ) and Cmc2 (I-IV -N ), respectively, which was confirmed by powder X-ray diffraction. Optical bandgaps were estimated from diffuse reflectance spectra using the Kubelka-Munk function (MgSiN : 4.8 eV, MgGeN : 3.2 eV, MnSiN : 3.5 eV, MnGeN : 2.5 eV, LiSi N : 4.4 eV, LiGe N : 3.9 eV). Complementary DFT calculations were carried out to gain insight into the electronic band structures of these materials and to corroborate the optical measurements.

Nitrides represent an intriguing class of functional materials with a broad range of application fields. Within the past decade, the ammonothermal method became increasingly attractive for the synthesis and crystal growth of nitride materials. The ammonothermal approach proved to be eminently suitable for the growth of bulk III-nitride semiconductors like GaN, and furthermore provided access to numerous ternary and multinary nitrides and oxonitrides with promising optical and electronic properties. In this minireview, we will shed light on the latest research findings covering the synthesis of nitrides by this method. An overview of synthesis strategies for binary, ternary, and multinary nitrides and oxonitrides, as well as their properties and potential applications will be given. The recent development of autoclave technologies for syntheses at high temperatures and pressures, in situ methods for investigations of crystallization processes, and solubility measurements by ultrasonic velocity experiments is briefly reviewed as well. In conclusion, challenges and future perspectives regarding the synthesis and crystal growth of novel nitrides, as well as the advancement of autoclave techniques are discussed.

The first gallium-containing nitridosilicate CaGaSiN was synthesized in newly developed high-pressure autoclaves using supercritical ammonia as solvent and nitriding agent. The reaction was conducted in an ammonobasic environment starting from intermetallic CaGaSi with NaN as a mineralizer. At 770 K, intermediate compounds were obtained, which were subsequently converted to the crystalline nitride at temperatures up to 1070 K (70-150 MPa). The impact of other mineralizers (e.g., LiN , KN , and CsN ) on the product formation was investigated as well. The crystal structure of CaGaSiN was analyzed by powder X-ray diffraction and refined by the Rietveld method. The structural results were further corroborated by transmission electron microscopy, Si MAS-NMR, and first-principle DFT calculations. CaGaSiN crystallizes in the orthorhombic space group Cmc2 (no. 36) with lattice parameters a=9.8855(11), b=5.6595(1), c=5.0810(1) Å, (Z=4, R =0.0326), and is isostructural with CaAlSiN (CASN). Eu doped samples exhibit red luminescence with an emission maximum of 620 nm and FWHM of 90 nm. Thus, CaGaSiN :Eu also represents an interesting candidate as a red-emitting material in phosphor-converted light-emitting diodes (pc-LEDs). In addition to the already known substitution of alkaline-earth metals in (Ca,Sr)AlSiN :Eu , inclusion of Ga is a further and promising perspective for luminescence tuning of widely used red-emitting CASN type materials.