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

Although rare-earth nickelates (ReNiO, 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 ReNiO at low meta-stability. In this work, the respective roles of high oxygen pressure and heterogeneous interface in triggering ReNiO thin film growth in the metastable state are revealed. ReNiO (Re = Nd, Sm, Eu, Gd and Dy) thin films grown on a LaAlO single crystal substrate show effective crystallization at atmospheric pressure without the necessity to apply high oxygen pressure, suggesting that the interfacial bonding between the ReNiO and substrates can sufficiently reduce the positive Gibbs formation energy of ReNiO, which is further verified by the first-principles calculations. Nevertheless, the abrupt electronic transitions only appear in ReNiO thin films grown at high oxygen pressure, in which case the oxygen vacancies are effectively eliminated 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 ReNiO thin films.