Revealing the role of interfacial heterogeneous nucleation in the metastable thin film growth of rare-earth nickelate electronic transition materials.

Although rare-earth nickelates (ReNiO3, Re ≠ La) exhibit abundant electronic phases and widely adjustable metal to insulator electronic transition properties, their practical electronic applications are largely impeded by their intrinsic meta-stability. Apart from elevating the oxygen reaction pressure, heterogeneous nucleation is expected to be an alternative strategy that enables the crystallization of ReNiO3 at low meta-stability. In this work, the respective roles of high oxygen pressure and heterogeneous interface in triggering ReNiO3 thin film growth in the metastable state are revealed. ReNiO3 (Re = Nd, Sm, Eu, Gd and Dy) thin films grown on a LaAlO3 single crystal substrate show effective crystallization at atmospheric pressure without the necessity to apply high oxygen pressure, suggesting that the interfacial bonding between the ReNiO3 and substrates can sufficiently reduce the positive Gibbs formation energy of ReNiO3, which is further verified by the first-principles calculations. Nevertheless, the abrupt electronic transitions only appear in ReNiO3 thin films grown at high oxygen pressure, in which case the oxygen vacancies are effectively eliminated via high oxygen pressure reactions as indicated by near-edge X-ray absorption fine structure (NEXAFS) analysis. This work unveils the synergistic effects of heterogeneous nucleation and high oxygen pressure on the growth of high quality ReNiO3 thin films.

Electron-Doping Mottronics in Strongly Correlated Perovskite.

The discovery of hydrogen-induced electron localization and highly insulating states in d-band electron correlated perovskites has opened a new paradigm for exploring novel electronic phases of condensed matters and applications in emerging field-controlled electronic devices (e.g., Mottronics). Although a significant understanding of doping-tuned transport properties of single crystalline correlated materials exists, it has remained unclear how doping-controlled transport properties behave in the presence of planar defects. The discovery of an unexpected high-concentration doping effect in defective regions is reported for correlated nickelates. It enables electronic conductance by tuning the Fermi-level in Mott-Hubbard band and shaping the lower Hubbard band state into a partially filled configuration. Interface engineering and grain boundary designs are performed for Hx SmNiO3 /SrRuO3 heterostructures, and a Mottronic device is achieved. The interfacial aggregation of hydrogen is controlled and quantified to establish its correlation with the electrical transport properties. The chemical bonding between the incorporated hydrogen with defective SmNiO3 is further analyzed by the positron annihilation spectroscopy. The present work unveils new materials physics in correlated materials and suggests novel doping strategies for developing Mottronic and iontronic devices via hydrogen-doping-controlled orbital occupancy in perovskite heterostructures.

NaOH concentration effect on the oriented attachment growth kinetics of ZnS.

In this work, the crystal growth kinetics of ZnS nanoparticles coarsened under 100 degrees C with NaOH concentration from 2 to 8 M was investigated, aiming to study the role of NaOH concentration on the oriented attachment growth kinetics. It reveals that 2 M NaOH is sufficient to lead to two-stage growth kinetics of ZnS nanoparticles, resulting in pure and multistep oriented attachment growth characteristics in the first stage. When the NaOH concentration increases, the rate of crystal growth by oriented attachment mechanism increases, while the time period for crystal growth at the pure oriented attachment stage was similar. We suggest that the concentration of solute is critical to enhance the oriented attachment growth rate and achieve exclusively oriented attachment growth of nanoparticles at a large size scale.