ABSTRACT
The stabilization of the B-site oxidation state in ABO3 perovskites using wet-chemical methods is a synthetic challenge, which is of fundamental and practical interest for energy storage and conversion devices. In this work, defect-controlled (Sr-deficiency and oxygen vacancies) strontium niobium(iv) oxide (Sr1-xNbO3-δ, SNO) metal oxide nanoparticles (NPs) were synthesized for the first time using a low-pressure wet-chemistry synthesis. The experiments were performed under reduced oxygen partial pressure to prevent by-product formation and with varying Sr/Nb molar ratio to favor the formation of Nb4+ pervoskites. At a critical Sr to Nb ratio (Sr/Nb = 1.3), a phase transition is observed forming an oxygen-deficient SrNbO3 phase. Structural refinement on the resultant diffraction pattern shows that the SNO NPs consists of a near equal mixture of SrNbO3 and Sr0.7NbO3-δ crystal phases. A combination of Rietveld refinement and X-ray photoelectron spectroscopy (XPS) confirmed the stabilization of the +4 oxidation state and the formation of oxygen vacancies. The Nb local site symmetry was extracted through Raman spectroscopy and modeled using DFT. As further confirmation, the particles demonstrate the expected absorption highlighting their restored optoelectronic properties. This low-pressure wet-chemical approach for stabilizing the oxidation state of a transition metal has the potential to be extended to other oxygen sensitive, low dimensional perovskite oxides with unique properties.
ABSTRACT
Rapid and reversible switching between amorphous and crystalline phases of phase-change material promises to revolutionize the field of information processing with a wide range of applications including electronic, optoelectronics, and photonic memory devices. However, achieving faster crystallization is a key challenge. Here, we demonstrate femtosecond-driven transient inspection of ultrafast crystallization of as-deposited amorphous Ge1Sb2Te4 and Ge1Sb4Te7 thin films induced by a series of 120 fs laser pulses. The snapshots of phase transitions are correlated with the time-resolved measurements of change in the absorption of the samples. The crystallization is attributed to the reiterative excitation of an intermediate state with subcritical nuclei at a strikingly low fluence of 3.19 mJ/cm2 for Ge1Sb2Te4 and 1.59 mJ/cm2 for Ge1Sb4Te7. Furthermore, 100% volumetric crystallization of Ge1Sb4Te7 was achieved with the fluence of 4.78 mJ/cm2, and also reamorphization is seen for a continuous stimulation at the same repetition rate and fluence. A systematic confirmation of structural transformations of all samples is validated by Raman spectroscopic measurements on the spots produced by the various excitation fluences.
ABSTRACT
We report here femtosecond laser-driven transient snapshots of ultrafast crystallization of Ge2Sb2Te5 films from its as-deposited amorphous phase, and the local structural change is validated by micro-Raman spectroscopy and x-ray diffraction. The decay time constant of â¼5 ps in transient spectra with a precise temporal resolution using 400 nm (pump) reveals about 68 volumetric percentage crystallization at a remarkably low fluence of 4.78 mJ·cm-2. This is attributed to reiterated excitation after a complete carrier relaxation and formation of a long-lasting transient phase at sub-threshold fluences. Furthermore, Raman spectra of irradiated spots confirm defective-octahedral modes at 110 and 160 cm-1 validating crystallization.
