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ABSTRACT: Surface potentials and electrostatic interactions in biological systems are key elements of cellular regulation and interaction.
Examples include cardiac and muscular activity, voltage-gated ion channels, protein folding and assembly, and electroactive
cells and electrotransduction. The coupling between electrical, mechanical, and chemical signals and responses in cellular
systems necessitates the development of tools capable of measuring the distribution of charged species, surface potentials,
and mechanical responses to applied electrical stimuli and vice versa, ultimately under physiological conditions. In this
chapter, applications of voltage-modulated atomic force microscopy (AFM) methods including Kelvin probe force microscopy (KPFM)
and piezoresponse force microscopy (PFM) to biological systems are discussed. KPFM is a force-sensitive non-contact or intermittent-contact
mode AFM technique that allows electrostatic interactions and surface potentials to be addressed. Beyond long-range electrostatic
interactions, the application of bias can lead to a mechanical response, e.g., due to linear piezoelectric coupling in polar
biopolymers or via more complex electrotransduction and redox pathways in other biosystems. The use and development of PFM,
based on direct electromechanical detection, to biological systems will also be addressed. The similarities and limitations
of measuring surface potentials and electromechanical coupling in solution will be outlined.
12/2011: pages 243-287;
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ABSTRACT: Nonvolatile bias-controlled polarization states in ferroelectric materials offer unique opportunities for information technology
and data storage applications. The ability to probe ferroelectric properties at the nanoscale by piezoresponse force microscopy
(PFM) has enabled fundamental studies of polarization dynamics and the role of defects and disorder on domain nucleation and
wall motion and has led to advances in the design and implementation of such applications. This has resulted in the development
of fast spectroscopic modes to collect polarization switching data from every point in an image. The emergence of fast, configurable
data processing electronics has prompted the development of dynamic and nonsinusoidal data acquisition methods for PFM. Further,
the recent synergy of spectroscopic and dynamic modes has necessitated the development of multivariate data analysis and processing
in PFM. These recent advances in the applications of PFM for imaging and spectroscopy of the ferroelectric switching processes
will be discussed.
12/2010: pages 491-528;
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ABSTRACT: Probing the functionality of materials locally by means of scanning probe microscopy (SPM) requires a reliable framework for identifying the target signal and separating it from the effects of surface morphology and instrument non-idealities, e.g. instrumental and topographical cross-talk. Here we develop a linear resolution theory framework in order to describe the cross-talk effects, and apply it for elucidation of frequency-dependent cross-talk mechanisms in piezoresponse force microscopy. The use of a band excitation method allows electromechanical/electrical and mechanical/topographic signals to be unambiguously separated. The applicability of a functional fit approach and multivariate statistical analysis methods for identification of data in band excitation SPM is explored.
Nanotechnology 10/2010; 21(40):405703. · 3.98 Impact Factor
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ABSTRACT: Polarization switching in ergodic relaxor and ferroelectric phases in the PbMg <sub>1/3</sub> Nb <sub>2/3</sub> O <sub>3</sub>– PbTiO <sub>3</sub> (PMN-PT) system is studied using piezoresponse force microscopy, single point electromechanical relaxation measurements, and voltage spectroscopy mapping. The dependence of relaxation behavior on voltage pulse amplitude and time is found to follow a universal logarithmic behavior with a nearly constant slope. This behavior is indicative of the progressive population of slow relaxation states, as opposed to a linear relaxation in the presence of a broad relaxation time distribution. The role of relaxation behavior, ferroelectric nonlinearity, and the spatial inhomogeneity of the tip field on hysteresis loop behavior is analyzed in detail. The hysteresis loops for ergodic PMN-10%PT are shown to be kinetically limited, while in PMN with larger PT content, true ferroelectric hysteresis loops with low nucleation biases are observed.
Journal of Applied Physics 09/2010; · 2.17 Impact Factor
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ABSTRACT: The evolution of ferroelectric domain structure in Remeika-grown BaTiO3 crystals under an inhomogeneous electric field has been investigated using piezoresponse force microscopy. The ac imaging bias was found to affect the metastable polarization state and lead to the formation of a web-like domain structure. It is suggested that this behavior is due to the existence of subsurface domains arising in single crystals with a layered structure.
Applied Physics Letters 07/2010; 97(4):042902-042902-3. · 3.84 Impact Factor
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ABSTRACT: Harnessing electrical bias-induced mechanical motion on the nanometer and molecular scale is a critical step toward understanding the fundamental mechanisms of redox processes and implementation of molecular electromechanical machines. Probing these phenomena in biomolecular systems requires electromechanical measurements be performed in liquid environments. Here we demonstrate the use of band excitation piezoresponse force microscopy for probing electromechanical coupling in amyloid fibrils. The approaches for separating the elastic and electromechanical contributions based on functional fits and multivariate statistical analysis are presented. We demonstrate that in the bulk of the fibril the electromechanical response is dominated by double-layer effects (consistent with shear piezoelectricity of biomolecules), while a number of electromechanically active hot spots possibly related to structural defects are observed.
ACS Nano 02/2010; 4(2):689-98. · 10.77 Impact Factor
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ABSTRACT: Spatial variability of polarization relaxation kinetics in the relaxor ferroelectric 0.9Pb(Mg(1/3)Nb(2/3))O(3)-0.1PbTiO(3) is studied using time-resolved piezoresponse force microscopy at room temperature. Both the statistical principal component and correlation function analysis and the stretched exponent fits of relaxation curves illustrate the presence of mesoscopic "fast" and "slow" 100-200 nm regions. The spatial distribution of activation energies is reconstructed using a neural-network-based inversion of the relaxation data. The results directly prove the presence of mesoscopic heterogeneities associated with static and dynamic components of the order parameter on the surfaces of ferroelectric relaxors in the ergodic phase.
Physical Review B 02/2010; 81(6). · 3.69 Impact Factor
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ABSTRACT: Spatial homogeneity of polarization relaxation behavior on the surface of 0.9Pb(Mg1/3Nb2/3)O3–0.1PbTiO3 crystals in the ergodic relaxor phase is studied using three-dimensional time-resolved spectroscopic piezoresponse force microscopy. The number of statistically independent components in the spectroscopic image is determined using principal component analysis. In the studied measurement time interval, the spectra generally exhibit logarithmic behavior with spatially varying slope and offset, and the statistical distribution of these parameters are studied. The data illustrate the presence of mesoscopic heterogeneity in the dynamics of the relaxation behavior that can be interpreted as spatial variation in local Vogel–Fulcher temperatures.
Applied Physics Letters 10/2009; 95(14):142902-142902-3. · 3.84 Impact Factor
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ABSTRACT: The spatial variability of polarization reversal behavior in the relaxor 0.9 Pb ( Mg <sub>1/3</sub> Nb <sub>2/3</sub> O <sub>3</sub>)–0.1 PbTiO <sub>3</sub> crystal, is revealed on the ∼100 nm scale using switching spectroscopy piezoresponse force microscopy. Quenched fields conjugate to polarization are found, which show mesoscopic (∼100–200 nm ) spatial fluctuations around near-zero bias values. The mapping of the stability gap of the bias-induced phase and conjugate random fields is demonstrated. The origin of the observed nanoscale domains and the field-induced part of the polarization are discussed.
Applied Physics Letters 09/2009; · 3.84 Impact Factor
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ABSTRACT: Spatial variability of polarization relaxation kinetics in relaxor ferroelectric 0.9Pb(Mg1/3Nb2/3)O3-0.1PbTiO3 is studied using time-resolved Piezoresponse Force Microscopy. Local relaxation attributed to the reorientation of polar nanoregions is shown to follow stretched exponential dependence, exp(-(t/tau)^beta), with beta~~0.4, much larger than the macroscopic value determined from dielectric spectra (beta~~0.09). The spatial inhomogeneity of relaxation time distributions with the presence of 100-200 nm "fast" and "slow" regions is observed. The results are analyzed to map the Vogel-Fulcher temperatures on the nanoscale.
09/2008;
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ABSTRACT: The kinetics and thermodynamics of first order transitions are universally controlled by defects that act as nucleation sites and pinning centers. Here we demonstrate that defect-domain interactions during polarization reversal processes in ferroelectric materials result in a pronounced fine structure in electromechanical hysteresis loops. Spatially resolved imaging of a single defect center in multiferroic BiFeO3 thin film is achieved, and the defect size and built-in field are determined self-consistently from the single-point spectroscopic measurements and spatially resolved images. This methodology is universal and can be applied to other reversible bias-induced transitions including electrochemical reactions.
Physical Review Letters 04/2008; 100(15):155703. · 7.37 Impact Factor
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ABSTRACT: In order to better understand dopant incorporation in quantum dot infrared photodetectors, the application of cross-sectional scanning capacitance microscopy (SCM) has been used to investigate carrier occupation/distribution in a multilayer InAs/GaAs quantum dot (QD) heterostructure for different doping techniques. The doping schemes in the QD structure include direct doping (in InAs QD layers) and remote doping (in GaAs barrier layers), each with different doping concentrations. The SCM image suggests that large band bending occurs due to highly doped, remote-doping layers, thereby causing electron redistribution in direct-doping layers. The experimental result is supported by a band structure calculation using the Schrödinger–Poisson method by NEXTNANO3.
Applied Physics Letters 03/2008; 92(9):092101-092101-3. · 3.84 Impact Factor
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Advanced Materials 01/2008; 20(1):109 - 114. · 13.88 Impact Factor
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ABSTRACT: Ferroelectric domain nucleation and growth in multiferroic BiFeO(3) is studied on a single-domain level by using piezoresponse force spectroscopy. Variation of local electromechanical response with dc tip bias is used to determine the size of the domain formed below the conductive scanning probe tip. The domain parameters are calculated self-consistently from the decoupled Green function theory by using tip geometry determined from the domain wall profile. The critical parameters of the nucleating domain and the activation energy for nucleation are determined. The switching mechanism is modeled by using the phase-field method, and comparison with experimental results shows that the nucleation biases are within a factor of approximately 2 of the intrinsic thermodynamic limit. The role of atomic-scale defects and long-range elastic fields on nucleation bias lowering is discussed. These measurements open a pathway for quantitative studies of the role of a single defect on kinetics and thermodynamics of first order bias-induced phase transitions and electrochemical reactions.
Proceedings of the National Academy of Sciences 01/2008; 104(51):20204-9. · 9.68 Impact Factor
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ABSTRACT: Cross-section of multi-layer InAs/GaAs quantum dot heterostructure has been characterized using scanning capacitance microscopy to investigate dopant incorporation into quantum dots. Simulation of the corresponding band structure is used to better understand the experimental results.
Lasers and Electro-Optics Society, 2007. LEOS 2007. The 20th Annual Meeting of the IEEE; 11/2007
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ABSTRACT: Dentin is a mesenchymal tissue, and, as such, is based on a collagenous matrix that is reinforced by apatite mineral. Collagen fibrils show piezoelectricity, a phenomenon that is used by piezoresponse force microscopy (PFM) to obtain high-resolution images. We applied PFM to image human dentin with 10-nm resolution, and to test the hypothesis that zones of piezoactivity, indicating the presence of collagen fibrils, can be distinguished in dentin. Piezoelectricity was observed by PFM in the dentin intertubular matrix, while the peritubular dentin remained without response. High-resolution imaging of chemically treated intertubular dentin attributed the piezoelectric effect to individual collagen fibrils that differed in the signal strength, depending on the fibril orientation. This study supports the hypothesis that peritubular dentin is a non-collagenous tissue and is thus an exception among mineralized tissues that derive from the mesenchyme.
Journal of Dental Research 10/2007; 86(9):908-11. · 3.49 Impact Factor
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ABSTRACT: A dual-excitation method for resonant-frequency tracking in scanning probe microscopy based on amplitude detection is developed. This method allows the cantilever to be operated at or near resonance for techniques where standard phase locked loops are not possible. This includes techniques with non-acoustic driving where the phase of the driving force is frequency and/or position dependent. An example of the later is Piezoresponse Force Microscopy (PFM), where the resonant frequency of the cantilever is strongly dependent on the contact stiffness of the tip-surface junction and the local mechanical properties, but the spatial variability of the drive phase rules out the use of a phase locked loop. Combined with high-voltage switching and imaging, dual-frequency, resonance-tracking PFM allows reliable studies of electromechanical and elastic properties and polarization dynamics in a broad range of inorganic and biological systems, and is illustrated using lead zirconate-titanate, rat tail collagen, and native and switched ferroelectric domains in lithium niobate.
09/2007;
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ABSTRACT: The electric forces acting on an atomic force microscope tip in solution have been measured using a microelectrochemical cell formed by two periodically biased electrodes. The forces were measured as a function of lift height and bias amplitude and frequency, providing insight into electrostatic interactions in liquids. Real-space mapping of the vertical and lateral components of electrostatic forces acting on the tip from the deflection and torsion of the cantilever is demonstrated. This method enables direct probing of electrostatic and convective forces involved in electrophoretic and dielectroforetic self-assembly and electrical tweezer operation in liquid environments.
08/2007;
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ABSTRACT: Mapping energy transformation pathways and dissipation on the nanoscale and understanding the role of local structure on dissipative behavior is a challenge for imaging in areas ranging from electronics and information technologies to efficient energy production. Here we develop a novel Scanning Probe Microscopy (SPM) technique in which the cantilever is excited and the response is recorded over a band of frequencies simultaneously rather than at a single frequency as in conventional SPMs. This band excitation (BE) SPM allows very rapid acquisition of the full frequency response at each point (i.e. transfer function) in an image and in particular enables the direct measurement of energy dissipation through the determination of the Q-factor of the cantilever-sample system. The BE method is demonstrated for force-distance and voltage spectroscopies and for magnetic dissipation imaging with sensitivity close to the thermomechanical limit. The applicability of BE for various SPMs is analyzed, and the method is expected to be universally applicable to all ambient and liquid SPMs. Comment: 32 pages, 9 figures, accepted for publication in Nanotechnology
08/2007;
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ABSTRACT: Insulated cantilever probes with a high aspect ratio conducting apex have been fabricated and their dynamic and electrical properties analyzed. The cantilevers were coated with silicon dioxide and a via was fabricated through the oxide at the tip apex and backfilled with tungsten to create an insulated probe with a conducting tip. The stiffness and Q-factor of the cantilevers increased after the modifications and their resonances shifted to higher frequencies. The coupling strength between the cantilever and the coating are determined. The applications to conductive and electromechanical imaging of ferroelectric domains are illustrated, and a probe apex repair process is demonstrated.
08/2007;
