Bonnell, Dawn A

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2000 - 200918
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  • Publication
    Imaging mechanism of piezoresponse force microscopy of ferroelectric surfaces
    (2002-03-15) Kalinin, Sergei V.; Bonnell, Dawn A
    In order to determine the origin of image contrast in piezoresponse force microscopy (PFM), analytical descriptions of the complex interactions between a small tip and ferroelectric surface are derived for several sets of limiting conditions. Image charge calculations are used to determine potential and field distributions at the tip-surface junction between a spherical tip and an anisotropic dielectric half plane. Methods of Hertzian mechanics are used to calculate the response amplitude in the electrostatic regime. In the electromechanical regime, the limits of strong (classical) and weak (field-induced) indentation are established and the relative contributions of electroelastic constants are determined. These results are used to construct ‘‘piezoresponse contrast mechanism maps’’ that correlate the imaging conditions with the PFM contrast mechanisms. Conditions for quantitative PFM imaging are set forth. Variable-temperature PFM imaging of domain structures in BaTiO3 and the temperature dependence of the piezoresponse are compared with Ginzburg-Devonshire theory. An approach to the simultaneous acquisition of piezoresponse and surface potential images is proposed.
  • Publication
    Local Phenomena in Oxides by Advanced Scanning Probe Microscopy
    (2005-05-01) Kalinin, Sergei V.; Shao, Rui; Bonnell, Dawn A
    In the last two decades, scanning probe microscopies (SPMs) have become the primary tool for addressing structure and electronic, mechanical, optical, and transport phenomena on the nanometer and atomic scales. Here, we summarize basic principles of SPM as applied for oxide materials characterization and present recent advances in high-resolution imaging and local property measurements. The use of advanced SPM techniques for solutions of material related problems is illustrated on the examples of grain boundary transport in polycrystalline oxides and ferroelectric domain imaging and manipulation. Future prospects for SPM applications in materials science are discussed.
  • Publication
    Micromagnetic and Magnetoresistance Studies of Ferromagnetic La0.83Sr0.13MnO2.98 Crystals
    (2000-11-27) Popov, Guerman; Kalinin, Sergei V; Alvarez, Rodolfo A.; Greenblatt, Martha; Bonnell, Dawn A
    Magnetic force microscopy (MFM) and atomic force microscopy (AFM) were used to investigate the surface topography and micromagnetic structure of La0.83Sr0.13MnO2.98 single crystals with colossal magnetoresistance (CMR). The crystals were grown by fused salt electrolysis and characterized by chemical analysis, X-ray diffraction, magnetic and transport measurements. The crystals are rhombohedral (R 3 c). Magnetic and transport measurements indicate that the ferromagnetic ordering at 310 K is associated with an insulator-metal transition at the same temperature. A maximum negative magnetoresistance (-62 %) is observed at 290 K in an applied magnetic field of 5 T. The magnetoresistance increases in magnitude sharply (1.8 %), comparing to the rest of the change, with increasing magnetic field up to 20 G, and then it increases slowly with increasing field. MFM and AFM were used to study the (110) surface as well as a number of unspecified surfaces. Surface topography of an as-grown crystal exhibits well-developed surface corrugations due to extensive twinning. The corrugation angle at twin boundaries can be related to the unit cell parameters, surface and twinning planes. Magnetic force microscopy images show that magnetic domain boundaries are pinned to the crystallographic twins; a small number of unpinned boundaries are observed. The statistical analysis of domain boundary angle distribution is consistent with cubic magnetocrystalline anisotropy with easy axis along [100] directions for this material. Unusual magnetization behavior in the vicinity of topological defects on the surface is also reported. MFM contrast was found to disappear above the ferromagnetic Curie temperature; after cooling a new magnetic structure comprised of Bloch walls of opposite chiralities developed.
  • Publication
    Consequence of Nanometer-Scale Property Variations to Macroscopic Properties of CrOCN Thin Films
    (2001-12-01) Smith, Jackson; French, Roger H.; Duscher, Gerd; Bonnell, Dawn A
    Macroscopic properties of CrOCN thin films are related directly to composition and property variations on multiple length scales. Compositions resolved on a nanometer scale were measured in-depth in 120–150 nm thick CrOCN films by sputtered neutral mass spectroscopy. A statistical analysis of composition identifies the particular coordinations of the various anions with Cr that form preferentially under relevant processing conditions. Near-edge structure in electron energy loss from transmission electron microscopy and the Cr core level shift in X-ray photoemission spectroscopy further support this conclusion. A wide range of compositions are described in terms of mixtures of binary and ternary compounds, and optical absorption is found to be correlated with the presence of Cr4+ within this description. It appears that the presence of the unfilled t2g state is responsible for optical absorption in the range of 0.5–6 eV and that a critical concentration of Cr4+ in certain species within the system is required for the transition to occur. These results conflict with the suggestion that a percolated network of metallic clusters is responsible for the change in properties.
  • Publication
    Photo-induced Charge Dynamics on BaTIO3 (001) Surface Characterized by Surface Probe Microscopy
    (2006-09-11) Shao, Rui; Nikiforov, Maxim P; Bonnell, Dawn A
    The surface potential of a multidomain BaTiO3 (001) surface was imaged in the presence and absence of ultraviolet (UV) illumination. The UV radiation induces a decrease in the surface potential contrast between c+ and c− domains with a time constant of a few seconds due to redistribution of photocarriers and screening. A slower process of recovery was observed after illumination. In addition, scanning a conducting atomic force microscopy tip in contact under UV illumination destabilizes some ferroelectric domains.
  • Publication
    Atomic structure and charge-density waves of blue bronze K0.3MoO3 (20[overline 1]) by variable-temperature scanning tunneling microscopy
    (2007-07-23) Nikiforov, Maxim P; Isakovic, Abdel; Bonnell, Dawn A
    Blue bronze (K0.3MoO3) has been the focus of a number of scattering, transport, scanning tunneling microscopy (STM), and theoretical studies that have provided insight into the relation between atomic structure and charge-density wave (CDW) formation. However, the full extent of a relation of the CDWs to the atomic lattice and the microscopic origin of CDW pinning are still not completely resolved. In this study STM is used to distinguish the atomic structure and CDWs at the (201) surface. Within the STM's spatial resolution, the CDWs are incommensurate with the lattice at midrange temperatures and approach commensurability at low temperatures. Incommensurate CDWs are present on the surface and the degree of the incommensurability between blue bronze lattice and CDW lattice agree well with those determined from bulk scattering techniques
  • Publication
    Evolution of the Structure and Thermodynamic Stability of the BaTiO3(001) Surface
    (2008-07-18) Kolpak, Alexie M; Li, Dongbo; Rappe, Andrew M; Shao, Rui; Bonnell, Dawn A
    We report a series of new surface reconstructions on BaTiO3(001) as a function of environmental conditions, determined via scanning tunneling microscopy and low energy electron diffraction. Using density functional theory calculations and thermodynamic modeling, we construct a surface phase diagram and determine the atomic structures of the thermodynamically stable phases. Excellent agreement is found between the predicted phase diagram and experiment. The results enable prediction of surface structures and properties under the entire range of accessible environmental conditions.
  • Publication
    Polarization reorientation in ferroelectric lead zirconate titanate thin films with electron beams
    (2006-04-01) Li, D. B.; Strachan, Douglas R.; Ferris, J. H.; Bonnell, Dawn A
    Ferroelectric domain patterning with an electron beam is demonstrated. Polarization of lead zirconate titanate thin films is shown to be reoriented in both positive and negative directions using piezoresponse force and scanning surface potential microscopy. Reorientation of the ferroelectric domains is a response to the electric field generated by an imbalance of electron emission and trapping at the surface. A threshold of 500 µC/cm2 and a saturation of 1500 µC/cm2 were identified. Regardless of beam energy, the polarization is reoriented negatively for beam currents less than 50 pA and positively for beam currents greater than 1 nA.
  • Publication
    Nanoimpedance Microscopy and Spectroscopy
    (2002-12-02) Shao, Rui; Bonnell, Dawn A; Kalinin, Sergei V
    One of the key limiting factors in current-based scanning probe microscopies (SPM) is the quality of tip-sample contact and stray capacitance in the probe-surface junction. We conduct impedance spectroscopy over a broad frequency range (40Hz~110MHz) through an AFM tip to quantify local electrical properties. Equivalent circuit for the tip-surface contact is constructed based on the impedance data and is used to study the mechanisms of relaxation in the near-tip region. Relative contributions of tip-surface contact and materials properties to the signal are discussed. This technique, referred to as Nanoimpedance Microscopy/Spectroscopy, is demonstrated in the imaging of an electronic ceramic: a ZnO varistor. Analysis of impedance spectra allows separation of tip-surface interactions and grain boundary behavior.
  • Publication
    Low-Temperature Resistance Anomaly at SrTiO3 Grain Boundaries: Evidence for an Interface-Induced Phase Transition
    (2005-11-04) Shao, Rui; Chisholm, Matthew F; Duscher, Gerd; Bonnell, Dawn A
    Variable temperature transport between 1.4 and 300 K, structural imaging, and theoretical calculations were used to characterize the properties of electrically active 24º and 36.8º [001] tilt SrTiO3 grain boundaries with 0.1 at.% niobium doping. An anomaly in boundary resistance and capacitance characteristics typical of a positive temperature coefficient effect is observed. This behavior is indicative of interface-induced dipole ordering. The detailed atomic structures of these grain boundaries were determined from a comparison of ab initio calculations and Z-contrast TEM images. The number of excess electrons at the boundaries determined experimentally and theoretically agrees and is associated with the boundary structural units.