Bonnell, Dawn A

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2000 - 200918
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  • 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
    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
    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
    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
    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
    Probing Polarization and Dielectric Function of Molecules with Higher Order Harmonics in Scattering-near-field Scanning Optical Microscopy
    (2009-12-03) Nikiforov, Maxim P.; Kehr, Susanne C.; Park, Tae-Hong; Milde, Peter; Zerweck, Ulrich; Loppacher, Christian; Eng, Lukas M.; Engheta, Nader; Therien, Michael J.; Bonnell, Dawn A
    The idealized system of an atomically flat metallic surface [highly oriented pyrolytic graphite (HOPG)] and an organic monolayer (porphyrin) was used to determine whether the dielectric function and associated properties of thin films can be accessed with scanning–near-field scanning optical microscopy (s-NSOM). Here, we demonstrate the use of harmonics up to fourth order and the polarization dependence of incident light to probe dielectric properties on idealized samples of monolayers of organic molecules on atomically smooth substrates. An analytical treatment of light/ sample interaction using the s-NSOM tip was developed in order to quantify the dielectric properties. The theoretical analysis and numerical modeling, as well as experimental data, demonstrate that higher order harmonic scattering can be used to extract the dielectric properties of materials with tens of nanometer spatial resolution. To date, the third harmonic provides the best lateral resolution(~50 nm) and dielectric constant contrast for a porphyrin film on HOPG.
  • Publication
    Local Polarization, Charge Compensation, and Chemical Interactions on Ferroelectric Surfaces: a Route Toward New Nanostructures
    (2001-11-26) Bonnell, Dawn A; Kalinin, Sergei V
    The local potential at domains on ferroelectric surfaces results from the interplay between atomic polarization and screening charge. The presence of mobile charge affects surface domain configuration, switching behavior, and surface chemical reactions. By measuring the temperature and time dependence of surface potential and piezo response with scanning probe microscopies, thermodynamic parameters associated with charge screening are determined. This is illustrated for the case of BaTiO3 (100) in air, for which the charge compensation mechanism is surface adsorption and enthalpy and entropy of adsorption are determined. The local electrostatic fields in the vicinity of the domains have a dominant effect on chemical reactivity. Photoreduction of a large variety of metals can be localized to domains with the appropriate surface charge. It has been demonstrated that proximal probe tips can be used to switch polarization direction locally. Combining the ability to 'write' domains of local polarization with domain specific reactivity of metals, vapors of small molecules, and organic compounds leads to a new approach to fabricating complex nanostructures.
  • Publication
    Scanning Impedance Microscopy: From Impedance Spectra to Impedance Images
    (2001-11-26) Kalinin, Sergei V; Bonnell, Dawn A
    Impedance spectroscopy has long been recognized as one of the major techniques for the characterization of ac transport in materials. The primary limitation of this technique is the lack of spatial resolution that precludes the equivalent circuit elements from being unambiguously associated with individual microstructural features. Here we present a scanning probe microscopy technique for quantitative imaging of ac and dc transport properties of electrically inhomogeneous materials. This technique, referred to as Scanning Impedance Microscopy (SIM), maps the phase and amplitude of local potential with respect to an electric field applied across the sample. Amplitude and phase behavior of individual defects can be correlated with their transport properties. The frequency dependence of the voltage phase shift across an interface yields capacitance and resistance. SIM of single interfaces is demonstrated on a model metal-semiconductor junction. The local interface capacitance and resistance obtained from SIM measurements agrees quantitatively with macroscopic impedance spectroscopy. Superposition of a dc sample bias during SIM probes the C-V characteristics of the interface. When combined with Scanning Surface Potential Microscopy (SSPM), which can be used to determine interface I-V characteristic, local transport properties are completely determined. SIM and SSPM of polycrystalline materials are demonstrated on BiFeO3 and p-doped silicon. An excellent agreement between the properties of a single interface determined by SIM and traditional impedance spectra is demonstrated. Finally, the applicability of this technique for imaging transport behavior in nanoelectronic devices is illustrated with carbon nanotube circuit.
  • Publication
    Potential and Impedance Imaging of Polycrystalline BiFeO3 Ceramics
    (2002-12-01) Kalinin, Sergei V.; Davies, Peter K; Suchomel, Matthew R.; Bonnell, Dawn A
    Electrostatic-force-sensitive scanning probe microscopy (SPM) is used to investigate grain boundary behavior in polycrystalline BiFeO3 ceramics. Scanning surface potential microscopy (SSPM) of a laterally biased sample exhibits potential drops due to resistive barriers at the grain boundaries. In this technique, the tips acts as a moving voltage probe detecting local variations of potential associated with the ohmic losses within the grains and at the grain boundaries. An approach for the quantification of grain boundary, grain interior, and contact resistivity from SSPM data is developed. Scanning impedance microscopy (SIM) is used to visualize capacitive barriers at the grain boundaries. In SIM, a dc-biased tip detects the variations of local potential induced by the lateral ac voltage applied to the sample. Unlike the traditional dc and ac transport measurement, both of these techniques are sensitive to the variation of local potential (SSPM) or local voltage oscillation amplitude and phase (SIM), rather than to current. Therefore, special attention is paid to the relationship between SSPM and SIM images and data obtained from traditional impedance spectroscopy and dc transport measurements. For BiFeO3 ceramics excellent agreement between the local SIM measurements and impedance spectroscopy data are demonstrated.
  • 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.