Bonnell, Dawn A
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Publication High-resolution characterization of defects in oxide thin films(2008-11-03) Nikiforov, Maxim N; Brukman, Matthew J; Bonnell, Dawn ANanometer sized defects in thin HfOx films are detected by atomic force microscopy facilitated leakage current measurements. Differences in the electrical properties of individual defects were distinguished. The effects of two mechanisms that localize the tip-sample interaction and increase spatial resolution were calculated. The expected increase in tip-sample current due to stress induced phase transformations and band gap narrowing has been calculated, and a behavior diagram is presented that shows the pressure necessary to generate a detectable current increase as a function of tip radius.Publication Determining the Electronic Properties of Individual Nanointerfaces by Combining Intermittent AFM Imaging and Contact Spectroscopy(2010-11-01) Kraya, Ramsey A.; Bonnell, Dawn AA method to determine the electronic properties at nanointerfaces or of nanostructures by utilizing intermittent contact atomic force microscopy and contact spectroscopy in one system is developed. By combining these two methods, the integrity of the interface or structure is maintained during imaging, while the extraction of the electronic information is obtained with contact spectroscopy. This method is especially vital for understanding interfaces between metal nanoparticles and substrates, where the nanoparticles are not tethered to the surface and can be combined with new and evolving techniques of thermal drift compensation to allow for a larger range of experiments on nanointerfaces and nanostructures in ambient environments. An experimental probe for quantifying the properties of individual interfaces with diameters in the range of 20 to 100 nm is developed, which is based on scanning probe microscopy.Publication Direct Determination of the Effect of Strain on Domain Morphology in Ferroelectric Superlattices with Scanning Probe Microscopy(2012-09-04) Kathan-Galipeau, K.; Wu, P. P; Li, Y. L; Chen, L. Q; Soukiassian, A; Zhu, Y.; Muller, D. A; Xi, X. X.; Schlom, D. G; Bonnell, Dawn AA variant of piezo force microscopy was used to characterize the effect of strain on polarization in [(BaTiO3)n/(SrTiO3)m]p superlattices. The measurements were compared to theoretical predictions based on phase-field calculations. When polarization is constrained to be perpendicular to the substrate, the measured polarization and domain morphology agree quantitatively with the predictions. This case allows the presence of an internal electric field in the thin film to be identified. The measured trend in piezoelectric response with strain state was in qualitative agreement with predictions, and the differences were consistent with the presence of internal electrical fields. Clear differences in domain morphology with strain were observed; and in some cases, the lateral anisotropic strain appeared to influence the domain morphology. The differences in magnitude and morphology were attributed to the internal electric fields and anisotropic strains.Publication Atomic and Electronic Structure of the BaTiO3(001) (√5×√5)R26.6° Surface Reconstruction(2012-12-20) Martirez, John Mark P; Morales, Erie H; Bonnell, Dawn A; Saidi, Wissam A; Rappe, A MThis contribution presents a study of the atomic and electronic structure of the (√5×√5)R26.6° surface reconstruction on BaTiO3 (001) formed by annealing in ultrahigh vacuum at 1300 K. Through density functional theory calculations in concert with thermodynamic analysis, we assess the stability of several BaTiO3 surface reconstructions and construct a phase diagram as a function of the chemical potential of the constituent elements. Using both experimental scanning tunneling microscopy (STM) and scanning tunneling spectroscopy measurements, we were able to further narrow down the candidate structures, and conclude that the surface is either TiO2-Ti3/5, TiO2-Ti4/5, or some combination, where Ti adatoms occupy hollow sites of the TiO2 surface. Density functional theory indicates that the defect states close to the valence band are from Ti adatom 3d orbitals (≈1.4 eV below the conduction band edge) in agreement with scanning tunneling spectroscopy measurements showing defect states 1.56±0.11 eV below the conduction band minimum (1.03±0.09 eV below the Fermi level). STM measurements show electronic contrast between empty and filled states’ images. The calculated local density of states at the surface shows that Ti 3d states below and above the Fermi level explain the difference in electronic contrast in the experimental STM images by the presence of electronically distinctive arrangements of Ti adatoms. This work provides an interesting contrast with the related oxide SrTiO3, for which the (001) surface (√5×√5)R26.6° reconstruction is reported to be the TiO2 surface with Sr adatoms.Publication Defect-Mediated Adsorption of Methanol and Carbon Dioxide on BaTiO3(001)(2009-07-14) Vohs, John M; Garra, J.; Bonnell, Dawn AThe surface chemistry of single crystal barium titanate (BaTiO3) has been studied using temperature programmed desorption (TPD). TPD measurements were performed with several probe molecules, including methanol and carbon dioxide. The role of oxygen vacancies in the adsorption and reaction of these molecules was examined by annealing the crystal under oxidizing or reducing conditions prior to performing TPD. It is shown that the adsorption and reaction of methanol and carbon dioxide are enhanced on BaTiO3(001) by annealing the crystal under reducing conditions.Publication Ferroelectric Polarization Dependent Interactions at Pd-LiNbO3(0001) Interfaces(2009-10-22) Bonnell, Dawn A; Zhao, Mosha H.; Vohs, John M.A combination of Auger electron spectroscopy and temperature-programed desorption was used to characterize the growth and interaction of Pd films with positively and negatively terminated ferroelectric LiNbO3(0001) surfaces. The growth mode of vapor-deposited Pd layers at 300 K was found to be dependent on the direction of the ferroelectric polarization with layer-by-layer growth occurring on the negative (c−) surface and particle formation occurring on the positive (c+) surface. The Pd metal layers were also found to be more thermally stable on the c− surface relative to the c+ surface. These results provide another example of how the polarization orientation in ferroelectric materials affects adsorption and reaction on their exposed surfaces.Publication Piezoresponse Force Microscopy: A Window into Electromechanical Behavior at the Nanoscale(2009-09-01) Bonnell, Dawn A; Kalinin, S V; Kholkin, A L; Gruverman, APiezoresponse force microscopy (PFM) is a powerful method widely used for nanoscale studies of the electromechanical coupling effect in various materials systems. Here, we review recent progress in this field that demonstrates great potential of PFM for the investigation of static and dynamic properties of ferroelectric domains, nanofabrication and lithography, local functional control, and structural imaging in a variety of inorganic and organic materials, including piezoelectrics, semiconductors, polymers, biomolecules, and biological systems. Future pathways for PFM application in high-density data storage, nanofabrication, and spectroscopy are discussed.Publication Real-Time TEM Imaging of the Formation of Crystalline Nanoscale Gaps(2008-02-08) Strachan, Douglas R; Davies, Peter K; Johnston, Danvers E; Bonnell, Dawn A; Guitton, Beth S; Johnson, Alan T; Datta, Sujit SWe present real-time transmission electron microscopy of nanogap formation by feedback controlled electromigration that reveals a remarkable degree of crystalline order. Crystal facets appear during feedback controlled electromigration indicating a layer-by-layer, highly reproducible electromigration process avoiding thermal runaway and melting. These measurements provide insight into the electromigration induced failure mechanism in sub-20 nm size interconnects, indicating that the current density at failure increases as the width decreases to approximately 1 nm.Publication Controlling Interface Properties for Advanced Energy Applications(2007-03-09) Shao, Rui; Li, D.; Kraya, Ramsay A; Bonnell, Dawn AInternal interfaces in materials play an important role in the performance of many devices used in energy applications including solar cells, LEDs, passive electronics, and fuel cells. Efficiencies in energy and power consumption may be realized by optimizing and often miniaturizing these devices. Our studies show that internal boundaries and biomaterial interfaces cause local property variations. These effects will dominate device performance as the systems become smaller. A fundamental understanding of the effect of atomic structure on local properties is a prerequisite to device optimization. Developing this understanding requires new probes that access local properties, controlled interface structure, atomic resolution electron microscopy and first principles calculations of geometric and electronic structure.Publication Probing Spin-charge Relation by Magnetoconductance in One-dimensional Polymer Nanofibers(2012-10-12) Choi, A.; Kim, K. H; Hong, S. J; Goh, M.; Akagi, K.; Kaner, R. B; Johnson, A.T. Charlie; Kirova, N. N; Bonnell, Dawn A; Brazovskii, S. A; Mele, Eugene J.; Park, Y. WPolymer nanofibers are one-dimensional organic hydrocarbon systems containing conducting polymers where the nonlinear local excitations such as solitons, polarons, and bipolarons formed by the electron-phonon interaction were predicted. Magnetoconductance (MC) can simultaneously probe both the spin and charge of these mobile species and identify the effects of electron-electron interactions on these nonlinear excitations. Here, we report our observations of a qualitatively differentMC in polyacetylene (PA) and in polyaniline (PANI) and polythiophene (PT) nanofibers. In PA, the MC is essentially zero, but it is present in PANI and PT. The universal scaling behavior and the zero (finite)MC in PA (PANI and PT) nanofibers provide evidence of Coulomb interactions between spinless charged solitons (interacting polarons which carry both spin and charge).