K. R. Balasubramaniam

In situ characterization of strontium surface segregation in epitaxial La0.7Sr0.3MnO3 thin films as a function of oxygen partial pressure

Using in situ synchrotron measurements of total reflection x-ray fluorescence, we find evidence of strontium surface segregation in (001)-oriented La0.7Sr0.3MnO3 thin films over a wide range of temperatures (25–900 °C) and oxygen partial pressures (pO2=0.15–150 Torr). The strontium surface concentration is observed to increase with decreasing pO2, suggesting that the surface oxygen vacancy concentration plays a significant role in controlling the degree of segregation. Interestingly, the enthalpy of segregation becomes less exothermic with increasing pO2, varying from −9.5 to −2.0 kJ/mol. In contrast, the La0.7Sr0.3MnO3 film thickness and epitaxial strain state have little impact on segregation behavior.

Electron tunneling characteristics on La0.7Sr0.3MnO3 thin-film surfaces at high temperature

We report on the electron tunneling characteristics on La0.7Sr0.3MnO3 (LSM) thin-film surfaces up to 580 °C in 10−3 mbar oxygen pressure, using scanning tunneling microscopy/spectroscopy (STM/STS). A thresholdlike drop in the tunneling current was observed at positive bias in STS, which is interpreted as a unique indicator for the activation polarization in cation-oxygen bonding on LSM cathodes. Sr-enrichment was found on the surface at high temperature using Auger electron spectroscopy, and was accompanied by a decrease in tunneling conductance in STS. This suggests that Sr-terminated surfaces are less active for electron transfer in oxygen reduction compared to Mn-terminated surfaces on LSM.

Epitaxial stabilization and structural properties of REMnO3 (RE=Dy,Gd,Sm) compounds in a layered, hexagonal ABO3 structure

REMnO3 (RE=Dy,Gd,Sm) films were deposited on (110) surfaces of single crystalline hexagonal (h-)YMnO3. These films adopted the metastable multiferroic h-REMnO3 structure instead of the stable perovskite structure. Sharp (hh0) diffraction peaks with narrow rocking curves were found for all films. The peak widths increased with increasing size of the rare-earth cation. The c-axis/a-axis lattice parameter decreased/increased monotonically with increasing rare-earth size for these epitaxial films. All films exhibited the following epitaxial relationship {110}REMnO3‖{110}YMnO3;⟨11¯0⟩REMnO3‖⟨11¯0⟩YMnO3. The single-phase hexagonal films were kinetically robust against back transformation to the stable perovskite structure even to thicknesses of 50nm.

Combinatorial substrate epitaxy: a new approach to growth of complex metastable compounds

A high-throughput processing-characterization method, called combinatorial substrate epitaxy (CSE), was developed that enables the investigation of epitaxial stabilization of metastable compositions in complex structures. To demonstrate the approach, we fabricated RE2Ti2O7 (RE = Dy, Gd, Sm, La) in a polymorphic structure for which RE = Dy, Gd, and Sm are metastable and Dy2Ti2O7 has not been previously observed. Dense sintered pellets of Sr2Nb2O7, which adopts the 110-layered perovskite (LP) structure, were prepared as substrates, polished flat, and characterized locally using electron backscatter diffraction (EBSD). Thin films of RE2Ti2O7 were deposited using pulsed laser deposition and were then characterized with EBSD. The EBSD patterns from all film–substrate pairs matched in a grain-by-grain fashion, which demonstrates that the films are in local epitaxial registry with the Sr2Nb2O7 grains over a wide spread of crystallographic orientations for the substrate surface. Furthermore, the EBSD patterns demonstrate that all RE2Ti2O7 films, whether stable or metastable in the bulk, adopt the 110-LP structure. Transmission electron microscopy was used to investigate more closely the metastable Sm2Ti2O7 films. The film–substrate interfaces are atomically smooth with relaxed epitaxial registry, indicating that the microcrystalline substrates can be treated as local single-crystal substrates and the metastable films are stable against back-transformation on strain relaxation. Electron diffraction patterns for Sm2Ti2O7 films are consistent with the monoclinic 110-LP unit cells. This work demonstrates that CSE allows for the growth of new materials that are thermodynamically and kinetically difficult to realize otherwise.