Anomalous phase transitions in one-dimensional organo-metal halide   perovskite nanorods grown inside porous silicon nanotube templates
  One-dimensional organo-metal halide Perovskite (CH3NH3PbI3) nanorods whose diameter and length are dictated by the inner size of porous silicon nanotube templates have been grown, characterized and compared to bulk perovskites in the form of microwires. We have observed a structural phase transition for bulk perovskites, where the crystal structure changes from tetragonal to orthorhombic at about 150K, as opposed to small diameter one-dimensional perovskite nanorods, of the order of 30-70 nm in diameter, where the phase transition is inhibited and the dominant phase remains tetragonal. Two major experimental techniques, infrared absorption spectroscopy and photoluminescence, were utilized to probe the temperature dependence of the perovskite phases over the 4-300K temperature range. Yet, different characteristics of the phase transition were measured by the two spectroscopic methods and explained by the presence of small, tetragonal inclusions embedded in the orthorhombic phase. The inhibition of the phase transition is attributed to the large surface area of these one-dimensional perovskite nanorods, which gives rise to a large stress that, in turn, prevents the formation of the orthorhombic phase. The absence of phase transition enables the measurement of the tetragonal bandgap energy down to low temperatures. 
Neta Arad-Vosk, Naama Rozenfeld, Roberto Gonzalez-Rodriguez, Jeffery   L. Coffer and Amir Saar
cond-mat.mes-hall