Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session T10: Focus Session: Growth, Structure, Dynamics, and Function of Nanostructured Surfaces and Interfaces -- Oxides |
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Sponsoring Units: DMP Chair: Dan Dougherty, North Carolina State University Room: D221 |
Wednesday, March 23, 2011 2:30PM - 3:06PM |
T10.00001: Cross-sectional scanning tunneling microscopy and spectroscopy of fractured oxide surfaces and heterostructure interfaces Invited Speaker: Recently, interfaces between novel oxide materials have become a playground for manipulation of new functionalities. At interfaces, the broken symmetry and the spatially confined environment have been shown to modify the local interactions and generate wholly new electronic phases (e.g. magnetism, metallicity, superconductivity etc.) distinct from the composite bulk materials. However, to date our understanding of these interface driven phases is still limited. While there exists powerful spatially resolved tools for visualizing the chemical and magnetic structure of an interface, a direct observation of electronic behavior across the interface presents a major experimental challenge. After the success of creating flat fractured surfaces on Nb-doped SrTiO$_{3}$ (Nb:STO) accessible to scanning tunneling microscopy (STM) [1-3], we have further harnessed the high-sensitivity to electronic local density of states (LDOS) of the scanning tunneling spectroscopy (STS) in cross-sectional geometry to visualize complex oxide interface electronic properties. By extending XSTM/S to the interface between colossal magnetoresistant manganite La$_{2/3}$Ca$_{1/3}$MnO$_{3}$ (LCMO) and semiconducting Nb:STO, we were able to map the LDOS across the boundary to unambiguously visualize the interface by the location of the valence band and elucidate the fundamental issue of band alignment at a complex oxide heterointerface [4]. \\[4pt] [1] TeYu Chien \textit{et al.}, \textit{Appl. Phys. Lett.} \textbf{95}, 163107 (2009). \\[0pt] [2] Nathan P. Guisinger \textit{et al.}, \textit{ACS nano} \textbf{3}, 4132 (2009). \\[0pt] [3] TeYu Chien \textit{et al.}, \textit{J. Vac. Sci. Technol. B} \textbf{28}, C5A11 (2010). \\[0pt] [4] TeYu Chien \textit{et al.}, \textit{Phys. Rev. B} \textbf{82}, 041101(R) (2010). [Preview Abstract] |
Wednesday, March 23, 2011 3:06PM - 3:18PM |
T10.00002: Electrical-stress-induced transport and surface potential characterizations of metal/TiO$_{2}$/metal planar junctions Haeri Kim, Dong-Wook Kim Electric-field-induced resistive switching (RS) phenomena in metal oxides have attracted considerable research interest due to their potential use in nonvolatile memory device applications. Intensive investigations have revealed that coupled electron ion dynamics play a key role the RS mechanism. Metal/single crystal junction can be an ideal model system to study how the ionic drift and diffusion can affect the resistance. We investigated transport and local electrical properties of Pt/TiO$_{2}$ single crystal/Ti planar junctions with micron- sized gaps between the electrodes. Scanning Kelvin probe microscopy (SKPM) showed that negative (positive) electrical stress to the Pt electrodes significantly reduced (hardly affected) the Pt/TiO$_{2}$ contact resistance. The SKPM results also revealed that the electrical stress caused alteration of the local work function of TiO$_{2}$. The comparative investigations of the transport and SKPM results suggested that the electrical stress induced redistribution of ions, resulting in the change of the junction resistance. [Preview Abstract] |
Wednesday, March 23, 2011 3:18PM - 3:30PM |
T10.00003: ABSTRACT WITHDRAWN |
Wednesday, March 23, 2011 3:30PM - 3:42PM |
T10.00004: Theoretical study of Ge/BaTiO$_{3 }$ Interfaces Kurt Fredrickson, Alexander Demkov It has been shown (McKee \textit{et al.,} Phys. Rev. Lett. \textbf{81,} 3014 (1998), and R. McKee, \textit{et al.}, \textit{Science }\textbf{293}$, $468 (2001)) that perovskite oxides SrTiO$_{3}$ and BaTiO$_{3}$ (BTO) can be grown epitaxially on Si and Ge, respectively. It would be interesting to achieve the reverse, i.e. to grow for example, Ge on BTO. It is not clear, however, whether one can achieve wetting of BTO by Ge. Theoretically, the energy of the Ge (001) surface is estimated to be anywhere between 591 and 1700 erg/cm$^{2}$ and the surface energy of BTO is in the range of 1083-1496 erg/cm$^{2}$ depending on termination and environment. The missing piece of information is the energy of the Ge/BTO interface. We examine five possible Ge/BTO interface structures and calculate their energies using density functional theory to determine which one has the lowest energy, and whether wetting can be achieved. [Preview Abstract] |
Wednesday, March 23, 2011 3:42PM - 3:54PM |
T10.00005: Strain relaxation in single crystal SrTiO$_{3}$ grown on Si (001) Miri Choi, Agham Posadas, Rytis Dargis, Dina Triyoso, David Theodore, Chih-Kang Shih, Alexander A. Demkov A layer of SrTiO$_{3}$ grown directly on Si may be used as a pseudo-substrate in perovskite deposition. As grown, SrTiO$_{3}$ is compressively strained, however, by subsequent annealing in oxygen at elevated temperature, a strain relieving SiO$_{x}$ buffer layer can be grown between the substrate and the perovskite layer. We perform a systematic study of strain relaxation in SrTiO$_{3}$ films grown on Si by molecular beam epitaxy as a function of the process conditions (annealing time, temperature, and oxygen partial pressure). Using a combination of X-ray diffraction, reflection high energy electron diffraction, and transmission electron microscopy we explore the oxidation and strain relaxation of SrTiO$_{3}$. We compare the kinetics of the buried oxide growth to that predicted by the conventional Deal-Grove model. An understanding of strain relaxation of SrTiO$_{3}$ on silicon can potentially be used to control the SrTiO$_{3}$ lattice constant for lattice matching with functional oxide overlayers. [Preview Abstract] |
Wednesday, March 23, 2011 3:54PM - 4:06PM |
T10.00006: X-ray 3D atomic imaging of Pt nanocrystals supported on SrTiO$_{3}$(001) Zhenxing Feng, Alexander Kazimirov, Michael Bedzyk Ultrathin metal or metal-oxide layers deposited onto oxide surfaces have wide applications in catalysis, chemical sensing and electronics. For sub-monolayer Pt deposited on the 2x1 SrTiO$_{3}$(001) surface, atomic-force microscopy shows the formation of nanoparticles and X-ray standing wave (XSW) atomic imaging shows that these nanoparticles are composed of Pt face-centered-cubic nanocrystals with cube-on-cube epitaxy coherent to the substrate unit cell. The phase sensitivity of the XSW allows for a direct measurement of the interface offset between the two unit cells along the c-axis. Different Pt coverages lead to differences in the observed XSW image of the interfacial structure, which is explained by the Pt-Pt interaction becoming stronger than the Pt-substrate interaction as the coverage is increased from 0.2 to 0.6 ML. Proposed atomic-scale interface models are based on a published double-layer TiO$_{2}$ terminated structure for the 2x1 SrTiO$_{3}$(001) surface and density functional theory. [Preview Abstract] |
Wednesday, March 23, 2011 4:06PM - 4:42PM |
T10.00007: Dynamics of early stage nano-oxidation by \textit{ in situ} UHV-TEM Invited Speaker: Environmental stability is one of the most important properties for materials exposed to air. As dimensions of engineered systems approach nanoscale, fundamental understanding of reactions with oxygen at this length scale is critical for environmental stability as well as for processing oxide nanostructures, where surface reactions are commonly utilized. The nanoscale stages of oxidation from the nucleation of the metal oxide to the formation of the thermodynamically stable oxide represent a scientifically challenging and technologically important terra incognito. The kinetics of early stage oxidation of Cu, Cu-Au and Cu-Ni alloys were visualized using in situ ultra-high vacuum transmission electron microscopy (UHV-TEM), where the initial oxidation stages can be observed in real-time under well-controlled surface conditions. We examined the dynamic responses of thin films to variations in thermodynamic variables such as temperature, oxygen pressure, strain, and crystallographic orientation. The kinetics of the nucleation and growth of three-dimensional oxide islands demonstrate that oxygen surface diffusion is the primary mechanism for oxide growth during initial oxidation in dry oxygen, and thus bears a striking resemblance to heteroepitaxy. Compared with the behavior of Cu films, the oxidation of Cu-Au alloys revealed more complexity. For example, the oxidation of (100)-oriented Cu-Au alloys with low Au content at $\sim $ 600C results in the formation of Cu2O oxide islands with a dendritic morphology and a non-uniform lateral distribution of Au around the islands. For Cu-Ni oxidation, the addition of Ni causes the formation Cu2O and/or NiO where the oxide type(s) and the relative orientation with the film depend on the Ni concentration, oxygen partial pressure and temperature. Evolution of the shape and size of the oxide islands can be quantitatively analyzed and provide fundamental insights into the complex kinetics and energetics of oxidation. Models based on surface orientation, strain development, and diffusion will be discussed to explain the formation of some of the novel oxide nano-structures. [Preview Abstract] |
Wednesday, March 23, 2011 4:42PM - 4:54PM |
T10.00008: Controlled surface reorganization of complex oxides by laser MBE Mikhail Kareev, B. Gray, Jian Liu, E.J. Moon, J. Chakhalian We report on the fabrication of ultra-thin layers of complex oxide perovskites, which display a variety of high-order surface reconstructions. In order to obtain the observed complex surface reconstructions (e.g. 6x2, 4x2, 4x4, etc.), nearly stoichiometric complex oxide material are found to be re-arranged into specific combinations of long-ordered periodic structures. We examine details of homo- and heterogeneous growth of SrTiO3 (STO) and LaNiO3 respectively on TiO2 terminated and mixed TiO2/SrO termination STO substrates by the combination of high-pressure RHEED and AFM to investigate mechanisms behind the high order surface reconstruction. J.C. was supported by DOD-ARO under the Contract No. 0402-17291 and NSF Contract No. DMR-0747808. [Preview Abstract] |
Wednesday, March 23, 2011 4:54PM - 5:06PM |
T10.00009: Quantum confinement effects in nanocrystals of SnO$_{2}$ in MgO matrix M.B. Sahana, C. Sudakar, A. Dixit, J.S. Thakur, R. Naik, V.M. Naik We have studied the nanocrystal formations of SnO$_{2 }$in$_{ }x$SnO$_{2}$-(1-$x)$MgO composite thin films ($x$ =0 to 1 and thickness 0.5 to 1 $\mu $m) prepared by metal-organic decomposition method. We find a direct relationship between the size of SnO$_{2}$ nanocrystals and the annealing temperature. Similarly, higher concentration of Mg in $x$SnO$_{2}$-(1-$x)$MgO leads to smaller size nanocrystals of SnO$_{2}$. Under the controlled choice of composition and annealing conditions, the bandgap of SnO$_{2}$ can be continuously increased from 3.89 eV to 4.5 eV thus providing a generic approach for tuning the bandgap in nanocomposite systems over a wide range of energy. We discuss this behavior in terms of the quantum confinement effect arising from particle size being comparable to the order of Bohr radius of the material. [Preview Abstract] |
Wednesday, March 23, 2011 5:06PM - 5:18PM |
T10.00010: Detection of Dielectric Trap States in Hafnium Oxide By Single Electron Tunneling Force Spectroscopy Dustin Winslow, Jon Johnson, Clayton Williams Atomic scale detection and imaging of electronic trap states in dielectric films has recently been demonstrated.\footnote{J.P. Johnson et al, Nanotechnology \textbf{20} (2009) 055701} Standard methods typically provide characterization over a much larger scale. Single Electron Tunneling Force Spectroscopy has been employed to measure the energy levels of trap states in HfO$_{2}$ with sub-nanometer spatial resolution. Analysis of individual spectra obtained at different locations shows variation in the density of defect states. When multiple spectra taken from 40 different locations are averaged, a broad peak 0.3 eV below the conduction band is observed, which agrees with data obtained over large areas by standard measurements.\footnote{G. Ribes et al, IEEE Trans. Dev. Mat. Reliability \textbf{6}, 132 (2006).} Additional peaks, not seen by the standard methods, are also observed. The method will be described and the data discussed. [Preview Abstract] |
Wednesday, March 23, 2011 5:18PM - 5:30PM |
T10.00011: Charge Injection and Relaxation in HfO$_{2}$ Films Measured by Single Electron Tunneling Force Spectroscopy Clayton Williams, Dustin Winslow, Jon Johnson Detection and imaging of individual trap states in dielectric materials with atomic scale spatial resolution has been recently demonstrated.\footnote{J.P. Johnson et al, Nanotechnology \textbf{20 }(2009) 055701.} Spectroscopic measurements on HfO$_{2}$ films by Single Electron Tunneling Force Spectroscopy have now shown evidence of both reversible and irreversible tunneling to and from these electronic trap states. The irreversibility is small near the middle of the band gap, becoming larger at $\sim $ 0.7 eV below the conduction band and 1.3 eV above the valence band. The irreversibilty of tunneling is likely due to charge relaxation. The evidence of charge relaxation in the film and a description of this new nanometer scale spectroscopic capability will be presented. The possible mechanisms by which the relaxation takes place will also be described. [Preview Abstract] |
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