Single crystal growth and phase stability of photovoltaic grade ZnSiP2 by flux technique
ZnSiP2 is a potential optoelectronic material with possible application in lasers, LED's, photonic integrated circuits, and photovoltaics. The development of ZnSiP2 as a photovoltaic material could address the current technological challenge of implementing a monolithic top cell on silicon for tandem photovoltaics. In this work we present a detailed description of the growth of ZnSiP2 single crystals, which has enabled thorough optoelectronic characterization. A flux growth technique was used, under various conditions, to grow ZnSiP2 single crystals in Zn solution. The results of these growth experiments, along with analysis of previously determined phase diagrams, show that three secondary phases form as a result of the Zn flux growth technique: Zn3P2, Si, and the remaining Zn flux. Potential reasons for the formation of these particular phases are discussed, but their presence is found to be non-detrimental, and they can easily be removed. The resulting single crystals are high purity and enable the characterization of the fundamental optoelectronic properties of ZnSiP2.
Concerning growth of single crystal of the II-IV-V diamond-like compounds ZnSiP2, CdSiP2, ZnGeP2, and CdSnP2 and standard enthalpies of formation for ZnSiP2 and CdSiP2
Single crystals of the diamond-like tetragonal compounds ZnSiP2, CdSiP2 and CdSnP2 were grown by vapor transport and by precipitation from liquid Sn. ZnGeP2 crystals were grown directly from stoichiometric melts. The vapor transport experiments were carried out in the temperature range 800–1200°C in unsupported thick-walled quartz ampoules. These materials exhibit dissociation pressures in excess of 8 atm at 1000 °C. Total equilibrium decomposition pressures were determined in the temperature range 1000–1500 °K. Standard enthalpies of formation were estimated for ZnSiP2 and CdSiP2 and compared to a theoretical estimate. The dissociation pressure for ZnGeP2 increases rather sharply at the melting point (1025 °C). In contrast to ZnSiP2 and CdSiP2, the dissociation pressure is less than 4 atmospheres at 1000 °C and the gaseous decomposition product consists essentially of phosphorous in the temperature range 850–1150 °C.
An anionic shell shields a cationic core allowing for uptake and release of polyelectrolytes within core-shell responsive microgels.
To realize carriers for drug delivery, cationic containers are required for anionic guests. Nevertheless, the toxicity of cationic carriers limits their practical use. In this study, we investigate a model system of polyampholyte N-isopropylacrylamide (NIPAM)-based microgels with a cationic core and an anionic shell to study whether the presence of a negative shell allows the cationic core to be shielded while still enabling the uptake and release of the anionic guest polyelectrolytes. These microgels are loaded with polystyrene sulfonate of different molecular weights to investigate the influence of their chain length on the uptake and release process. By means of small-angle neutron scattering, we evaluate the spatial distribution of polystyrene sulfonate within the microgels. The guest molecules are located in different parts of the core-shell microgels depending on their size. By combining these scattering results with UV-vis spectroscopy, electrophoretic mobility and potentiometric titrations we gain complementary results to investigate the uptake and release process of polyelectrolytes in polyampholyte core-shell microgels. Moreover, Brownian molecular dynamic simulations are performed to compare the experimental and theoretical results of this model. Our findings demonstrate that the presence of a shell still enables efficient uptake of guest molecules into the cationic core. These anionic guest molecules can be released through an anionic shell. Furthermore, the presence of a shell enhances the stability of the microgel-polyelectrolyte complexes with respect to the cationic precursor microgel alone.