School of Physics

Development of glass-ceramic scintillators for gamma-ray astronomy

2017-08-14T15:34:46Z

Development of glass-ceramic scintillators for gamma-ray astronomy de Faoite, Daithí; Hanlon, Lorraine; Roberts, O.; Ulyanov, A.; McBreen, S.; Tobin, I.; Stanton, Kenneth T. Scintillators synthesised as glass-ceramics have several potential benefits compared to the currently-used halide scintillators, including non-hygroscopicity, mechanical ruggedness, ease of producing customisable shapes, and the potential for low-cost synthesis. The use of these scintillators is considered for a gamma-ray telescope operating in the 0.2 MeV¿50 MeV photon range. Inorganic scintillator compounds suitable for incorporation into glass-ceramics are assessed. In addition, several families of glass suitable for use as hosts for scintillating compounds are also reviewed. Applications of Novel Scintillators for Research and Industry (ANSRI 2015), Dublin, Ireland, 12–14 January 2015

Motion of Euglena gracilis: Active fluctuations and velocity distribution

2017-05-26T16:12:19Z

Motion of Euglena gracilis: Active fluctuations and velocity distribution Romanczuk, M.; Romensky, Maksym; Scholz, Dimitri; Lobaskin, Vladimir; Schimansky-Geier, L. We study the velocity distribution of unicellular swimming algae Euglena gracilis using optical microscopy and active Brownian particle theory. To characterize a peculiar feature of the experimentally observed distribution at small velocities we use the concept of active fluctuations, which was recently proposed for the description of stochastically self-propelled particles [Romanczuk, P. and Schimansky-Geier, L., Phys. Rev. Lett. 106, 230601 (2011)]. In this concept, the fluctuating forces arise due to internal random performance of the propulsive motor. The fluctuating forces are directed in parallel to the heading direction, in which the propulsion acts. In the theory, we introduce the active motion via the depot model [Schweitzer, et al., Phys. Rev. Lett. 80(23), 5044 (1998)]. We demonstrate that the theoretical predictions based on the depot model with active fluctuations are consistent with the experimentally observed velocity distributions. In addition to the model with additive active noise, we obtain theoretical results for a constant propulsion with multiplicative noise.

Thermal and aqueous stability improvement of graphene oxide enhanced diphenylalanine nanocomposites

2017-05-12T11:58:28Z

Thermal and aqueous stability improvement of graphene oxide enhanced diphenylalanine nanocomposites Ryan, Kate; Neumayer, Sabine M.; Maraka, Harsha Vardhan R.; Nicolae-Viorel, Buchete; Kholkin, Andrei L.; Rice, James H.; Rodriguez, Brian J. Nanocomposites of diphenylalanine (FF) and carbon based materials provide an opportunity to overcome drawbacks associated with using FF micro- and nanostructures in nanobiotechnology applications, in particular, their poor structural stability in liquid solutions. In this study, FF/graphene oxide (GO) composites were found to self-assemble into layered micro- and nanostructures, which exhibited improved thermal and aqueous stability. Dependent on the FF/GO ratio, the solubility of these structures was reduced to 35.65% after 30 min as compared to 92.4% for pure FF samples. Such functional nanocomposites may extend the use of FF structures to, e.g., biosensing, electrochemical, electromechanical or electronic applications.

Charge and topography patterned lithium niobate provides physical cues to fluidically isolated cortical axons

2017-05-12T11:32:35Z

Charge and topography patterned lithium niobate provides physical cues to fluidically isolated cortical axons Kilinc, Devrim; Blasiak, Agata; Baghban, M. A.; Carville, N. Craig; Al-Aladi, A.; Al-Shammari, R. M.; Rice, James H.; Lee, Gil U.; Rodriguez, Brian J.; et al. In vitro devices that combine chemotactic and physical cues are needed for understanding how cells integrate different stimuli. We explored the suitability of lithium niobate (LiNbO3), a transparent ferroelectric material that can be patterned with electrical charge domains and micro/nanotopography, as a neural substrate. On flat LiNbO3 z-surfaces with periodically alternating charge domains, cortical axons are partially aligned with domain boundaries. On submicron-deep etched trenches, neurites are aligned with the edges of the topographical features. Finally, we bonded a bicompartmental microfluidic chip to LiNbO3 surfaces patterned by etching, to create isolated axon microenvironments with predefined topographical cues. LiNbO3 is shown to be an emerging neuron culture substrate with tunable electrical and topographical properties that can be integrated with microfluidic devices, suitable for studying axon growth and guidance mechanisms under combined topographical/chemical stimuli.

Tricritical points in a Vicsek model of self-propelled particles with bounded confidence

2017-04-06T16:12:41Z

Tricritical points in a Vicsek model of self-propelled particles with bounded confidence Romensky, Maksym; Lobaskin, Vladimir; Ihle, Thomas We study the orientational ordering in systems of self-propelled particles with selective interactions. To introduce the selectivity we augment the standard Vicsek model with a bounded-confidence collision rule: a given particle only aligns to neighbors who have directions quite similar to its own. Neighbors whose directions deviate more than a fixed restriction angle α are ignored. The collective dynamics of this system is studied by agent-based simulations and kinetic mean-field theory. We demonstrate that the reduction of the restriction angle leads to a critical noise amplitude decreasing monotonically with that angle, turning into a power law with exponent 3/2 for small angles. Moreover, for small system sizes we show that upon decreasing the restriction angle, the kind of the transition to polar collective motion changes from continuous to discontinuous. Thus, an apparent tricritical point with different scaling laws is identified and calculated analytically. We investigate the shifting and vanishing of this point due to the formation of density bands as the system size is increased. Agent-based simulations in small systems with large particle velocities show excellent agreement with the kinetic theory predictions. We also find that at very small interaction angles, the polar ordered phase becomes unstable with respect to the apolar phase. We derive analytical expressions for the dependence of the threshold noise on the restriction angle. We show that the mean-field kinetic theory also permits stationary nematic states below a restriction angle of 0.681 π. We calculate the critical noise, at which the disordered state bifurcates to a nematic state, and find that it is always smaller than the threshold noise for the transition from disorder to polar order. The disordered-nematic transition features two tricritical points: At low and high restriction angle, the transition is discontinuous but continuous at intermediate α. We generalize our results to systems that show fragmentation into more than two groups and obtain scaling laws for the transition lines and the corresponding tricritical points. A numerical method to evaluate the nonlinear Fredholm integral equation for the stationary distribution function is also presented. This method is shown to give excellent agreement with agent-based simulations, even in strongly ordered systems at noise values close to zero.

Coarse-grained model of adsorption of blood plasma proteins onto nanoparticles

2017-04-06T16:02:47Z

Coarse-grained model of adsorption of blood plasma proteins onto nanoparticles Lopez, Hender; Lobaskin, Vladimir We present a coarse-grained model for evaluation of interactions of globular proteins with nanoparticles (NPs). The protein molecules are represented by one bead per aminoacid and the nanoparticle by a homogeneous sphere that interacts with the aminoacids via a central force that depends on the nanoparticle size. The proposed methodology is used to predict the adsorption energies for six common human blood plasma proteins on hydrophobic charged or neutral nanoparticles of different sizes as well as the preferred orientation of the molecules upon adsorption. Our approach allows one to rank the proteins by their binding affinity to the nanoparticle, which can be used for predicting the composition of the NP-protein corona. The predicted ranking is in good agreement with known experimental data for proteinadsorption on surfaces.

In vitro study of the interaction of heregulin-functionalized magnetic-optical nanorods with MCF7 and MDA-MB- 231 cells

2017-03-28T17:15:45Z

In vitro study of the interaction of heregulin-functionalized magnetic-optical nanorods with MCF7 and MDA-MB- 231 cells Lesniak, Anna; Kilinc, Devrim; Rashdan, Suad Ahmed; Kriegsheim, Alexander von; Ashall, B.; Zerulla, Dominic; Kolch, Walter; Lee, Gil U. Multifunctional nanoparticles that actively target specific cells are promising tools for cancer diagnosis and therapy. In this article we review the synthesis and surface chemistry of Fe–Au nanorods and their characterization using microscopy. The diameter of the rods used in this study was selected to be 150–200 nm so that they did not enter the cells. The 80 nm-long Au tips of the nanorods were functionalized with heregulin (HRG), and the micron-long Fe portion was coated with a poly(ethylene glycol) monolayer to minimize non-specific interactions. Nanorods functionalized with HRG were found to preferentially bind to MCF7 cells that express high levels of the receptor tyrosine-protein kinase ErbB2/3. Magnetic tweezers measurements were used to characterize the kinetic properties of the bond between the HRG on the rods and ErbB2/3 on the surface of the cells. The strong magnetization of Fe–Au nanorods makes them excellent candidates for in-vitro and in-vivo imaging, and magnetic therapeutic applications targeting cancer cells in circulation.

Quantifying nanoscale biochemical heterogeneity in human epithelial cancer cells using combined AFM and PTIR absorption nanoimaging

2017-02-16T12:56:40Z

Quantifying nanoscale biochemical heterogeneity in human epithelial cancer cells using combined AFM and PTIR absorption nanoimaging Kennedy, Eamonn; Al-Majmaie, Rasoul; Al-Rubeai, Mohamed; Zerulla, Dominic; Rice, James H. Subcellular chemical heterogeneity plays a key role in cell organization and function. However the biomechanics underlying the structure-function relationship is governed by cell substructures which are poorly resolved using conventional chemical imaging methods. To date, advances in sub-diffraction limited infrared (IR) nanoscopy have permitted intracellular chemical mapping. In this work we report how image analysis applied to a combination of IR absorption nanoimaging and topographic data permits quantification of chemical complexity at the nanoscale, enabling the analysis of biochemical heterogeneity in mammalian cancer cells on the scale of subcellular features.

Plasmon enhanced fluorescence studies from aligned gold nanorod arrays modified with SiO2 spacer layers

2017-02-06T13:12:40Z

Plasmon enhanced fluorescence studies from aligned gold nanorod arrays modified with SiO2 spacer layers Damm, Signe; Fedele, Stefano; Murphy, Antony; Barry, James N.; Dowling, Denis P.; Rice, James H.; et al. Here we demonstrate that quasi self-standing Au nanorod arrays prepared with plasma polymerisation deposited SiO2 dielectric spacers support surface enhanced fluorescence (SEF) while maintaining high signal reproducibility. We show that it is possible to find a balance between enhanced radiative and non-radiative decay rates at which the fluorescent intensity is maximized. The SEF signal optimised with a 30 nm spacer layer thickness, showed a 3.5-fold enhancement with a signal variance of <15% thereby keeping the integrity of the nanorod array. We also demonstrate the decreased importance of obtaining resonance conditions when LSPR is positioned within the spectral region of Au interband transitions. Procedures for further increasing the SEF enhancement factor are also discussed.

Application of AAO matrix in aligned gold nanorod array substrates for surface-Enhanced fluorescence and Raman scattering

2017-02-03T17:33:34Z

Application of AAO matrix in aligned gold nanorod array substrates for surface-Enhanced fluorescence and Raman scattering Damm, Signe; Lordan, Frances; Murphy, Antony; Rice, James H.; et al. In this paper, we probed surface-enhanced Raman scattering (SERS) and surface-enhanced fluorescence (SEF) from probe molecule Rhodamine 6G (R6G) on self-standing Au nanorod array substrates made using a combination of anodization and potentiostatic electrodeposition. The initial substrates were embedded within a porous alumina template (AAO). By controlling the thickness of the AAO matrix, SEF and SERS were observed exhibiting an inverse relationship. SERS and SEF showed a non-linear response to the removal of AAO matrix due to an inhomogeneous plasmon activity across the nanorod which was supported by FDTD calculations. We showed that by optimizing the level of AAO thickness, we could obtain either maximized SERS, SEF or simultaneously observe both SERS and SEF together.

Strong coupling in molecular exciton-plasmon Au nanorod array systems

2017-01-30T15:46:51Z

Strong coupling in molecular exciton-plasmon Au nanorod array systems Fedele, Stefano; Hakami, Manal; Murphy, Antony; Pollard, Robert; Rice, James H. We demonstrate here strong coupling between localized surface plasmon modes in self-standing nanorods with excitons in a molecular J-aggregate layer though angular tuning. The enhanced exciton−plasmon coupling creates a Fano like line shape in the differential reflection spectra associated with the formation of hybrid states, leading to anti-crossing of the upper and lower polaritons with a Rabi frequency of 125 meV. The recreation of a Fano like line shape was found in photoluminescence demonstrating changes in the emission spectral profile under strong coupling.

Interface modulated currents in periodically proton exchanged Mg doped lithium niobate

2017-03-01T02:00:34Z

Interface modulated currents in periodically proton exchanged Mg doped lithium niobate Neumayer, Sabine M.; Manzo, Michele; Kholkin, Andrei L.; Gallo, Katia; Rodriguez, Brian J. Conductivity in Mg doped lithium niobate (Mg:LN) plays a key role in the reduction of photorefraction and therefore doping is widely exploited in optical devices. However, charge transport through Mg:LN and across interfaces such as electrodes also yields potential electronic applications in devices with switchable conductivity states. Furthermore, the introduction of proton exchanged (PE) phases in Mg:LN enhances ionic conductivity thus providing tailorability of conduction mechanisms and functionality dependent on sample composition. To facilitate the construction and design of such multifunctional electronic devices based on periodically PE Mg:LN or similar ferroelectric semiconductors, fundamental understanding of charge transport in these materials, as well as the impact of internal and external interfaces is essential. In order to gain insight into polarization and interface dependent conductivity due to band bending, UV illumination, and chemical reactivity, multi composite wedge shaped samples consisting of polar orientated Mg:LN and PE phases, were investigated using conductive atomic force microscopy. In Mg:LN, three conductivity states (on/off/transient) were observed under UV illumination, controllable by the polarity of the sample and the externally applied electric field. Measurements of currents originating from electrochemical reactions at metal electrode - PE phase interfaces demonstrate a memresistive and rectifying capability of the PE phase. Furthermore, internal interfaces such as domain walls and Mg:LN - PE phase boundaries were found to play a major role in the accumulation of charge carriers due to the polarization gradients, which can lead to increased currents. The insight gained from these findings yield the potential for multifunctional applications such as switchable UV sensitive micro- and nanoelectronic devices and bistable memristors.

Biocompatible Gold Nanoparticle Arrays Photodeposited on Periodically Proton Exchanged Lithium Niobate

2016-08-19T11:19:03Z

Biocompatible Gold Nanoparticle Arrays Photodeposited on Periodically Proton Exchanged Lithium Niobate Carville, N. Craig; Neumayer, Sabine M.; Manzo, Michele; Gallo, Katia; Rodriguez, Brian J. Photodeposition of silver nanoparticles onto chemically patterned lithium niobate having alternating lithium niobate and proton exchanged regions has been previously investigated. Here, the spatially defined photodeposition of gold nanoparticles onto periodically proton exchanged lithium niobate is demonstrated. It is shown that the location where the gold nanoparticles form can be tailored by altering the concentration of HAuCl4. This enables the possibility to sequentially deposit gold and silver in different locations to create bimetallic arrays. The cytocompatibility of photodeposited gold, silver, and bimetallic ferroelectric templates to osteoblast-like cells is also investigated. Gold samples provide significantly greater cell biocompatibility than silver samples. These results highlight a potential route for using photodeposited gold on lithium niobate as a template for applications in cellular biosensing.

Wettability gradient-induced alignment of peptide nanotubes as templates for biosensing applications

2017-04-01T01:00:14Z

Wettability gradient-induced alignment of peptide nanotubes as templates for biosensing applications Almohammed, Sawsan; Oladapo, Sarah O.; Ryan, Kate; Kholkin, Andrei L.; Rice, James H.; Rodriguez, Brian J. Self-assembled diphenylalanine (FF) peptide nanotubes (PNTs) have attracted significant attention due to their well-ordered supramolecular structure and wide range of functional capabilities that may enable potential nanobiotechnology applications. However, self-assembled PNTs are generally inhomogeneous at the macroscale, which has limited their potential use. Reproducibly controlling the assembly and alignment of PNTs is therefore critical to enable the widespread use of PNTs, e.g., in sensing applications. In this study, a surface patterning technique based on UV/ozone exposure through a mask is used to align PNTs. Exposed regions become hydrophilic, leading to directed spreading of the FF solution and alignment of the PNTs that improves as the difference in wettability between adjacent regions increases. Alignment was further found to depend on the concentration- and temperature-dependent diameter of the PNTs formed and the size of the hydrophilic area. Finally, aligned PNTs decorated with silver nanoparticles are used to sense an analyte molecule using surface enhanced Raman spectroscopy.

Applications of piezoresponse force microscopy in materials research: from inorganic ferroelectrics to biopiezoelectrics and beyond

2017-02-25T02:00:14Z

Applications of piezoresponse force microscopy in materials research: from inorganic ferroelectrics to biopiezoelectrics and beyond Denning, Denise; Guyonnet, Jill; Rodriguez, Brian J. Piezoresponse force microscopy (PFM) probes the mechanical deformation of a sample in response to an electric field applied with the tip of an atomic force microscope. Originally developed more than two decades ago to study ferroelectric materials, this technique has since been used to probe electromechanical functionality in a wide range of piezoelectric materials including organic and biological systems. Piezoresponse force microscopy has also been demonstrated as a useful tool to detect mechanical strain originating from electrical phenomena in non-piezoelectric materials. Parallelling advances in analytical and numerical modelling, many technical improvements have been made in the last decade: switching spectroscopy PFM allows the polarisation switching properties of ferroelectrics to be resolved in real space with nanometric resolution, while dual ac resonance tracking and band excitation PFM have been used to improve the signal-to-noise ratio. In turn, these advances have led to increasingly large multidimensional data sets containing more complete information on the properties of the sample studied. In this review, PFM operation and calibration are described, and recent advances in the characterisation of electromechanical coupling using PFM are presented. The breadth of the systems covered highlights the versatility and wide applicability of PFM in fields as diverse as materials engineering and nanomedicine. In each of these fields, combining PFM with complementary techniques is key to develop future insight into the intrinsic properties of the materials as well as for device applications.

Influence of annealing on the photodeposition of silver on periodically poled lithium niobate

2016-04-14T16:43:16Z

Influence of annealing on the photodeposition of silver on periodically poled lithium niobate Carville, N. Craig; Neumayer, Sabine M.; Manzo, Michele; Rodriguez, Brian J.; et al. The preferential deposition of metal nanoparticles onto periodically poled lithium niobate surfaces, whereby photogenerated electrons accumulate in accordance with local electric fields and reduce metal ions from solution, is known to depend on the intensity and wavelength of the illumination and the concentration of the solution used. Here, it is shown that for identical deposition conditions (wavelength, intensity, concentration), post-poling annealing for 10 h at 200°C modifies the surface reactivity through the reorientation of internal defect fields. Whereas silver nanoparticles deposit preferentially on the +z domains on unannealed crystals, the deposition occurs preferentially along 180° domain walls for annealed crystals. In neither case is the deposition selective; limited deposition occurs also on the unannealed –z domain surface and on both annealed domain surfaces. The observed behavior is attributed to a relaxation of the poling-induced defect frustration mediated by Li+ ion mobility during annealing, which affects the accumulation of electrons, thereby changing the surface reactivity. The evolution of the defect field with temperature is corroborated using Raman spectroscopy.

Thickness, humidity, and polarization dependent ferroelectric switching and conductivity in Mg doped lithium niobate

2016-01-13T17:00:53Z

Thickness, humidity, and polarization dependent ferroelectric switching and conductivity in Mg doped lithium niobate Neumayer, Sabine M.; Strelcov, Evgheni; Manzo, Michele; Rodriguez, Brian J.; et al. Mg doped lithium niobate (Mg:LN) exhibits several advantages over undoped LN such as resistance to photorefraction, lower coercive fields, and p-type conductivity that is particularly pronounced at domain walls and opens up a range of applications, e.g., in domain wall electronics. Engineering of precise domain patterns necessitates well founded knowledge of switching kinetics, which can differ significantly from that of undoped LN. In this work, the role of humidity and sample composition in polarization reversal has been investigated under application of the same voltage waveform. Control over domain sizes has been achieved by varying the sample thickness and initial polarization as well as atmospheric conditions. In addition, local introduction of proton exchanged phases allows for inhibition of domain nucleation or destabilization, which can be utilized to modify domain patterns. Polarization dependent current flow, attributed to charged domain walls and band bending, demonstrates the rectifying ability of Mg:LN in combination with suitable metal electrodes that allow for further tailoring of conductivity.

Interface and thickness dependent domain switching and stability in Mg doped lithium niobate

2016-01-12T17:08:07Z

Interface and thickness dependent domain switching and stability in Mg doped lithium niobate Neumayer, Sabine M.; Ivanov, Ilia N.; Manzo, Michele; Rodriguez, Brian J.; et al. Controlling ferroelectric switching in Mg doped lithium niobate (Mg:LN) is of fundamental importance for optical device and domain wall electronics applications that require precise domain patterns. Stable ferroelectric switching has been previously observed in undoped LN layers above proton exchanged (PE) phases that exhibit reduced polarization, whereas PE layers have been found to inhibit lateral domain growth. Here, Mg doping, which is known to significantly alter ferroelectric switchingproperties including coercive field and switching currents, is shown to inhibit domain nucleation and stability in Mg:LN above buried PE phases that allow for precise ferroelectric patterning via domain growth control. Furthermore, piezoresponse force microscopy(PFM) and switching spectroscopy PFM reveal that the voltage at which polarization switches from the 'up' to the 'down' state increases with increasing thickness in pure Mg:LN, whereas the voltage required for stable back switching to the original 'up' state does not exhibit this thickness dependence. This behavior is consistent with the presence of an internal frozen defect field. The inhibition of domain nucleation above PE interfaces, observed in this study, is a phenomenon that occurs in Mg:LN but not in undoped samples and is mainly ascribed to a remaining frozen polarization in the PE phase that opposes polarization reversal. This reduced frozen depolarization field in the PE phase also influences the depolarization field of the Mg:LN layer above due to the presence of uncompensated polarization charge at the PE-Mg:LN boundary. These alterations in internal electric fields within the sample cause long-range lattice distortions in Mg:LN via electromechanical coupling, which were corroborated with complimentary Raman measurements.

Investigation of the mechanism of polarization switching in ferroelectric capacitors by three- dimensional piezoresponse force microscopy

2015-11-18T11:07:18Z

Investigation of the mechanism of polarization switching in ferroelectric capacitors by three- dimensional piezoresponse force microscopy Rodriguez, Brian J.; Gruverman, A.; Kingon, A. I.; et al. A mechanism for the switching behavior of (111)-oriented Pb(Zr,Ti)O3-based 1×1.5 μm2 capacitors has been investigated using three-dimensional piezoresponse force microscopy (3D-PFM). A combination of vertical and lateral piezoresponse force microscopy (VPFM and LPFM) has been used to map the out-of-plane and the in-plane components of the polarization. The three-dimensional polarization distribution was reconstructed by quantitative analysis of the PFM amplitude images of poled PZT capacitors while taking into account contrast variations in the PFM phase images. The switching behavior of the capacitors was determined by comparison of the static domain patterns in the same capacitors after both positive and negative poling. While 180° degree switching was observed, surprisingly, the switching process was dominated by 90° polarization vector rotation. Furthermore, central regions of the capacitors were characterized by the presence of charged domain boundaries, which could lead to imprint (preference of one polarization state over another.

Piezoresponse force microscopy for piezoelectric measurements of III-nitride materials

2015-11-18T10:57:54Z

Piezoresponse force microscopy for piezoelectric measurements of III-nitride materials Rodriguez, Brian J.; Gruverman, A.; Kingon, A. I.; Nemanich, R. J. Piezoelectric constants and polarity distributions of epitaxial AlN and GaN thin films are investigated by piezoresponse force microscopy (PFM). The magnitude of the effective longitudinal piezoelectric constant d(33) is determined to be 3+/-1 and 2+/-1 pm/V for wurtzite AlN and GaN/AlN layers grown by organo-metallic vapor phase epitaxy on SiC substrates, respectively. Simultaneous imaging of surface morphology as well as the phase and magnitude of the piezoelectric response is performed by PFM on a GaN film with patterned polarities on a c-Al2O3 substrate. We demonstrate that the polarity distribution of GaN based lateral polarity heterostructures can be deduced from the phase image of the piezoresponse with nanometer scale spatial resolution. We also present images of AlN/Si samples with regions of opposite piezoresponse phase, which indicate the presence of antiphase domains. We discuss the potential application of this technique for determination of the orientation of bulk crystals.

Local piezoresponse and polarization switching in nucleobase thymine microcrystals

2015-11-10T13:27:33Z

Local piezoresponse and polarization switching in nucleobase thymine microcrystals Bdikin, Igor; Heredia, Alejandro; Neumayer, Sabine M.; Rodriguez, Brian J.; et al. Thymine (2-oxy-4-oxy-5 methyl pyrimidine) is one of the four nucleobases of deoxyribonucleic acid (DNA). In the DNA molecule, thymine binds to adenine via two hydrogen bonds, thus stabilizing the nucleic acid structure and is involved in pairing and replication. Here, we show that synthetic thymine microcrystals grown from the solution exhibit local piezoelectricity and apparent ferroelectricity, as evidenced by nanoscale electromechanical measurements via Piezoresponse Force Microscopy. Our experimental results demonstrate significant electromechanical activity and polarization switchability of thymine, thus opening a pathway for piezoelectric and ferroelectric-based applications of thymine and, perhaps, of other DNA nucleobase materials. The results are supported by molecular modeling of polarization switching under an external electric field.

Tip-induced domain structures and polarization switching in ferroelectric amino acid glycine

2015-11-10T13:23:40Z

Tip-induced domain structures and polarization switching in ferroelectric amino acid glycine Seyedhosseini, E.; Bdikin, Igor; Ivanov, M.; Rodriguez, Brian J.; et al. Bioorganic ferroelectrics and piezoelectrics are becoming increasingly important in view of their intrinsic compatibility with biological environment and biofunctionality combined with strong piezoelectric effect and a switchable polarization at room temperature. Here, we study tip-induced domain structures and polarization switching in the smallest amino acid b-glycine, representing a broad class of non-centrosymmetric amino acids. We show that b-glycine is indeed a roomtemperature ferroelectric and polarization can be switched by applying a bias to non-polar cuts via a conducting tip of atomic force microscope (AFM). Dynamics of these in-plane domains is studied as a function of an applied voltage and pulse duration. The domain shape is dictated by polarization screening at the domain boundaries and mediated by growth defects. Thermodynamic theory is applied to explain the domain propagation induced by the AFM tip. Our findings suggest that the properties of b-glycine are controlled by the charged domain walls which in turn can be manipulated by an external bias.

Kelvin Probe Force Microscopy in liquid using Electrochemical Force Microscopy

2015-04-20T11:45:34Z

Kelvin Probe Force Microscopy in liquid using Electrochemical Force Microscopy Collins, Liam; Jesse, S.; Kilpatrick, J. I.; Rodriguez, Brian J.; et al. Conventional closed loop-Kelvin probe force microscopy (KPFM) has emerged as a powerful technique for probing electric and transport phenomena at the solid-gas interface. The extension of KPFM capabilities to probe electrostatic and electrochemical phenomena at the solid-liquid interface is of interest for a broad range of applications from energy storage to biological systems. However, the operation of KPFM implicitly relies on the presence of a linear lossless dielectric in the probe-sample gap, a condition which is violated for ionically-active liquids (e.g., when diffuse charge dynamics are present). Here, electrostatic and electrochemical measurements are demonstrated in ionically-active (polar isopropanol, milli-Q water and aqueous NaCl) and ionically-inactive (non-polar decane) liquids by electrochemical force microscopy (EcFM), a multidimensional (i.e., bias- and time-resolved) spectroscopy method. In the absence of mobile charges (ambient and non-polar liquids), KPFM and EcFM are both feasible, yielding comparable contact potential difference (CPD) values. In ionically-active liquids, KPFM is not possible and EcFM can be used to measure the dynamic CPD and a rich spectrum of information pertaining to charge screening, ion diffusion, and electrochemical processes (e.g., Faradaic reactions). EcFM measurements conducted in isopropanol and milli-Q water over Au and highly ordered pyrolytic graphite electrodes demonstrate both sample- and solvent-dependent features. Finally, the feasibility of using EcFM as a local force-based mapping technique of material-dependent electrostatic and electrochemical response is investigated. The resultant high dimensional dataset is visualized using a purely statistical approach that does notrequire a priori physical models, allowing for qualitative mapping of electrostatic and electrochemical material properties at the solid-liquid interface.

Band excitation Kelvin probe force microscopy utilizing photothermal excitation

2015-09-25T14:29:44Z

Band excitation Kelvin probe force microscopy utilizing photothermal excitation Collins, Liam; Jesse, S.; Balke, Nina; Rodriguez, Brian J.; et al. A multifrequency open loop Kelvin probe force microscopy (KPFM) approach utilizingphotothermal as opposed to electrical excitation is developed. Photothermal band excitation (PthBE)-KPFM is implemented here in a grid mode on a model test sample comprising a metal-insulator junction with local charge-patterned regions. Unlike the previously described open loop BE-KPFM, which relies on capacitive actuation of the cantilever, photothermal actuation is shown to be highly sensitive to the electrostatic force gradient even at biases close to the contact potential difference (CPD). PthBE-KPFM is further shown to provide a more localized measurement of true CPD in comparison to the gold standard ambient KPFM approach, amplitude modulated KPFM. Finally, PthBE-KPFM data contain information relating to local dielectric properties and electronic dissipation between tip and sample unattainable using conventional single frequency KPFM approaches

Macromolecularly crowded in vitro microenvironments accelerate the production of extracellular matrix-rich supramolecular assemblies

2015-04-10T10:22:53Z

Macromolecularly crowded in vitro microenvironments accelerate the production of extracellular matrix-rich supramolecular assemblies Kumar, Pramod; Satyam, Abhigyan; Fan, Xingliang; Rodriguez, Brian J.; et al. Therapeutic strategies based on the principles of tissue engineering by self-assembly put forward the notion that functional regeneration can be achieved by utilising the inherent capacity of cells to create highly sophisticated supramolecular assemblies. However, in dilute ex vivo microenvironments, prolonged culture time is required to develop an extracellular matrix-rich implantable device. Herein, we assessed the influence of macromolecular crowding, a biophysical phenomenon that regulates intra- and extra-cellular activities in multicellular organisms, in human corneal fibroblast culture. In the presence of macromolecules, abundant extracellular matrix deposition was evidenced as fast as 48 h in culture, even at low serum concentration. Temperature responsive copolymers allowed the detachment of dense and cohesive supramolecularly assembled living substitutes within 6 days in culture. Morphological, histological, gene and protein analysis assays demonstrated maintenance of tissue-specific function. Macromolecular crowding opens new avenues for a more rational design in engineering of clinically relevant tissue modules in vitro

Breaking the limits of structural and mechanical imaging of the heterogeneous structure of coal macerals

2015-10-09T03:00:12Z

Breaking the limits of structural and mechanical imaging of the heterogeneous structure of coal macerals Collins, Liam; Tselev, A.; Jesse, S.; Rodriguez, Brian J.; et al. The correlation between local mechanical (elasto-plastic) and structural (composition) properties of coal presents significant fundamental and practical interest for coal processing and for the development of rheological models of coal to coke transformations. Here, we explore the relationship between the local structural, chemical composition, and mechanical properties of coal using a combination of confocal micro-Raman imaging and band excitation atomic force acoustic microscopy for a bituminous coal. This allows high resolution imaging (10s of nm) of mechanical properties of the heterogeneous (banded) architecture of coal and correlating them to the optical gap, average crystallite size, the bond-bending disorder of sp2 aromatic double bonds, and the defect density. This methodology allows the structural and mechanical properties of coal components (lithotypes, microlithotypes, and macerals) to be understood, and related to local chemical structure, potentially allowing for knowledge-based modeling and optimization of coal utilization processes.

High viscosity environments: an unexpected route to obtain true atomic resolution with atomic force microscopy

2015-04-09T03:00:12Z

High viscosity environments: an unexpected route to obtain true atomic resolution with atomic force microscopy Weber, Stefan A. L.; Kilpatrick, J. I.; Brosnan, Timothy M.; Jarvis, Suzi; Rodriguez, Brian J. Atomic force microscopy (AFM) is widely used in liquid environments, where true atomic resolution at the solid–liquid interface can now be routinely achieved. It is generally expected that AFM operation in more viscous environments results in an increased noise contribution from the thermal motion of the cantilever, thereby reducing the signal-to-noise ratio (SNR). Thus, viscous fluids such as ionic and organic liquids have been generally avoided for high-resolution AFM studies despite their relevance to, e.g. energy applications. Here, we investigate the thermal noise limitations of dynamic AFM operation in both low and high viscosity environments theoretically, deriving expressions for the amplitude, phase and frequency noise resulting from the thermal motion of the cantilever, thereby defining the performance limits of amplitude modulation, phase modulation and frequency modulation AFM. We show that the assumption of a reduced SNR in viscous environments is not inherent to the technique and demonstrate that SNR values comparable to ultra-high vacuum systems can be obtained in high viscosity environments under certain conditions. Finally, we have obtained true atomic resolution images of highly ordered pyrolytic graphite and mica surfaces, thus revealing the potential of high-resolution imaging in high viscosity environments.

Piezoelectricity in collagen type II fibrils measured by scanning probe microscopy

2014-09-26T09:08:25Z

Piezoelectricity in collagen type II fibrils measured by scanning probe microscopy Denning, Denise; Kilpatrick, J. I.; Hsu, T.; Rodriguez, Brian J.; et al. The converse piezoelectric effect in collagen type II fibrils, the main collagen constituent in cartilage, was investigated using piezoresponse force microscopy. The fibrils exhibited shear piezoelectric behavior similar to that previously reported in collagen type I fibrils and followed the same cantilever-fibril angle dependence present for type I. A uniform polarization directed from the amine to carboxyl termini, as seen for collagen type I, was observed in all type II fibrils studied. The shear piezoelectric coefficient, d 15, however, for type II was roughly 28–32% of the value measured for type I fibrils. Possible explanations for the reduced piezoelectric coefficient of type II collagen are provided.

Nanoscale characterization of β-phase HxLi1−xNbO3 layers by piezoresponse force microscopy

2014-09-24T08:27:50Z

Nanoscale characterization of β-phase HxLi1−xNbO3 layers by piezoresponse force microscopy Manzo, Michele; Denning, Denise; Rodriguez, Brian J.; Gallo, Katia We investigate a non-destructive approach for the characterization of proton exchanged layers in LiNbO3 with sub-micrometric resolution by means of piezoresponse force microscopy (PFM). Through systematic analyses, we identify a clear correlation between optical measurements on the extraordinary refractive index and PFM measurements on the piezoelectric d 33 coefficient. Furthermore, we quantify the reduction of the latter induced by proton exchange as 83 ± 2% and 68 ± 3% of the LiNbO3 value, for undoped and 5 mol. % MgO-doped substrates, respectively.

Dual harmonic Kelvin probe force microscopy at the graphene-liquid interface

2014-04-28T14:00:21Z

Dual harmonic Kelvin probe force microscopy at the graphene-liquid interface Collins, Liam; Kilpatrick, J. I.; Vlassiouk, Ivan V.; Weber, Stefan A. L.; Rodriguez, Brian J.; et al. Kelvin probe force microscopy (KPFM) is a powerful technique for the determination of the contact potential difference (CPD) between an atomic force microscope tip and a sample under ambient and vacuum conditions. However, for many energy storage and conversion systems, including graphene-based electrochemical capacitors, understanding electrochemical phenomena at the solid¿liquid interface is paramount. Despite the vast potential to provide fundamental insight for energy storage materials at the nanoscale, KPFM has found limited applicability in liquid environments to date. Here, using dual harmonic (DH)-KPFM, we demonstrate CPD imaging of graphene in liquid. We find good agreement with measurements performed in air, highlighting the potential of DH-KPFM to probe electrochemistry at the graphene¿liquid interface.

Vector piezoresponse force microscopy

2014-04-04T10:44:39Z

Vector piezoresponse force microscopy Kalinin, S. V.; Rodriguez, Brian J.; Jesse, S.; et al. A novel approach for nanoscale imaging and characterization of the orientation dependence of electromechanical properties-vector piezoresponse force microscopy (Vector PFM)-is described. The relationship between local electromechanical response, polarization, piezoelectric constants, and crystallographic orientation is analyzed in detail. The image formation mechanism in vector PFM is discussed. Conditions for complete three-dimensional (313) reconstruction of the electromechanical response vector and evaluation of the piezoelectric constants from PFM data are set forth. The developed approach can be applied to crystallographic orientation imaging in piezoelectric materials with a spatial resolution below 10 nm. Several approaches for data representation in 2D-PFM and 3D-PFM are presented. The potential of vector PFM for molecular orientation imaging in macroscopically disordered piezoelectric polymers and biological systems is discussed.

Quantitative determination of tip parameters in piezoresponse force microscopy

2014-03-20T16:00:25Z

Quantitative determination of tip parameters in piezoresponse force microscopy Kalinin, S. V.; Jesse, S.; Rodriguez, Brian J.; et al. One of the key limiting factors in the quantitative interpretation of piezoresponse force microscopy (PFM) is the lack of knowledge on the effective tip geometry. Here the authors derive analytical expressions for a 180 degrees domain wall profile in PFM for the point charge, sphere plane, and disk electrode models of the tip. An approach for the determination of the effective tip parameters from the wall profile is suggested and illustrated for several ferroelectric materials. The calculated tip parameters can be used self-consistently for the interpretation of PFM resolution and spectroscopy data, i.e., linear imaging processes.

Piezoresponse force microscopy for polarity imaging of GaN

2014-02-06T11:57:09Z

Piezoresponse force microscopy for polarity imaging of GaN Rodriguez, Brian J.; Gruverman, A.; Kingon, A. I.; et al. The polarity distribution of GaN based lateral polarity heterostructures is investigated by piezoresponse force microscopy (PFM). Simultaneous imaging of surface morphology, as well as the phase and magnitude of the piezoelectric response, is performed by PFM on a GaN film with patterned polarities on a c-Al2O3 substrate. We demonstrate that the polarity distribution of GaN based lateral polarity heterostructures can be deduced from the phase image of the piezoresponse with nanometer scale spatial resolution.

Wavelength-dependent Raman scattering of hydrogenated amorphous silicon carbon with red, green, and blue light excitation

2014-02-06T11:45:22Z

Wavelength-dependent Raman scattering of hydrogenated amorphous silicon carbon with red, green, and blue light excitation Park, M.; Sakhrani, V.; Maria, J-P.; Rodriguez, Brian J.; et al. This study presents results of wavelength-dependent Raman scattering from amorphous silicon carbon (a-Si:C:H). The a-Si:C:H films were produced by radio-frequency plasma-enhanced chemical vapor deposition. Prior results with amorphous carbon indicate that laser excitation selectively probes clusters with differing sizes. Our measurements with a-Si:C:H indicate that when using red (632.8 nm), green (514.5 nm), and blue (488.0 nm) excitation, the Raman D and G peaks shift to higher wave numbers as the excitation energy increases. The higher frequency is associated with smaller clusters that are preferentially excited with higher photon energy. It appears that photoluminescence occurs due to radiative recombination from intracluster transitions in Si-alloyed sp(2)-bonded carbon clusters.

Spatial inhomogeneity of imprint and switching behavior in ferroelectric capacitors

2014-02-06T11:43:23Z

Spatial inhomogeneity of imprint and switching behavior in ferroelectric capacitors Gruverman, A.; Rodriguez, Brian J.; Kingon, A. I.; et al. Piezoresponse force microscopy has been used to perform nanoscale characterization of the spatial variations in the imprint and switching behavior of (111)-oriented Pb(Zr,Ti)O-3-based capacitors on Pt electrodes. Mapping of polarization distribution in the poled capacitors as well as local d(33)-V loop measurements revealed a significant difference in imprint and switching behavior between the peripheral and inner parts of the capacitors. It has been found that the inner regions of the capacitors are negatively imprinted (with the preferential direction of the normal component of polarization upward) and tend to switch back after application of the positive poling voltage. On the other hand, switchable regions at the edge of the integrated capacitors generally exhibit more symmetric hysteresis behavior. Application of an ac switching voltage, contrary to what was expected, resulted in an increase of the negatively imprinted regions. The observed effect has been explained by incomplete or asymmetric switching due to the mechanical stress conditions existing in the central parts of the capacitors. (C) 2003 American Institute of Physics.

Micro-Raman study of electronic properties of inversion domains in GaN-based lateral polarity heterostructures

2014-02-06T11:42:10Z

Micro-Raman study of electronic properties of inversion domains in GaN-based lateral polarity heterostructures Park, M.; Cuomo, J. J.; Rodriguez, Brian J.; et al. The electronic properties of inversion domains in a GaN-based lateral polarity heterostructure were investigated using micro-Raman spectroscopy. The piezoelectric polarization of each domain was calculated from strain determined via Raman scattering. The free carrier concentration and electron mobility were deduced from the longitudinal optical phonon-plasmon coupled mode. The electron concentration in the N-face domain was slightly higher than that in the Ga-face domain. It appears that during growth, a larger number of donor impurities may have been incorporated into the N-face domain than into the Ga-face domain.

Mechanical stress effect on imprint behavior of integrated ferroelectric capacitors

2014-02-06T11:40:20Z

Mechanical stress effect on imprint behavior of integrated ferroelectric capacitors Gruverman, A.; Rodriguez, Brian J.; Kingon, A. I.; et al. Stress-induced changes in the imprint and switching behavior of (111)-oriented Pb(Zr,Ti)O-3 (PZT)-based capacitors have been studied using piezoresponse force microscopy. Visualization of polarization distribution and d(33)-loop measurements in individual 1x1.5-mum(2) capacitors before and after stress application, generated by substrate bending, provided direct experimental evidence of stress-induced switching. Mechanical stress caused elastic switching in capacitors with the direction of the resulting polarization determined by the sign of the applied stress. In addition, stress application turned capacitors into a heavily imprinted state characterized by strongly shifted hysteresis loops and almost complete backswitching after application of the poling voltage. It is suggested that substrate bending generated a strain gradient in the PZT layer, which produced asymmetric lattice distortion with preferential polarization direction and triggered polarization switching due to the flexoelectric effect.

Photoelectron emission microscopy observation of inversion domain boundaries of GaN-based lateral polarity heterostructures

2014-02-06T11:22:10Z

Photoelectron emission microscopy observation of inversion domain boundaries of GaN-based lateral polarity heterostructures Yang, W. -C.; Rodriguez, Brian J.; Park, M.; et al. An intentionally grown GaN film with laterally patterned Ga- and N-face polarities is studied using in situ UV-photoelectron emission microscopy (PEEM). Before chemical vapor cleaning of the surface, the emission contrast between the Ga- and N-face polarities regions was not significant. However, after cleaning the emission contrast between the different polarity regions was enhanced such that the N-face regions exhibited increased emission over the Ga-face regions. The results indicate that the emission threshold of the N-face region is lower than that of the Ga face. Moreover, bright emission was detected from regions around the inversion domain boundaries of the lateral polarity heterostructure. The PEEM polarity contrast and intense emission from the inversion domain boundary regions are discussed in terms of the built-in lateral field and the surface band bending induced by the polarization bound surface charges.

Direct measurement of periodic electric forces in liquids

2014-02-06T10:06:12Z

Direct measurement of periodic electric forces in liquids Rodriguez, Brian J.; Jesse, S.; Seal, K.; et al. 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.

Structural, magnetic, and electric properties of La0.7Sr0.3MnO3/PbZrxTi1-xO3 heterostructures

2014-02-06T10:04:21Z

Structural, magnetic, and electric properties of La0.7Sr0.3MnO3/PbZrxTi1-xO3 heterostructures Ziese, M.; Setzer, A.; Vrejoiu, I.; Rodriguez, Brian J.; et al. Epitaxial La0.7Sr0.3MnO3/PbZrxTi1-xO3 multilayers were fabricated by pulsed-laser deposition and studied by structural, magnetic, and electric characterization techniques. Transmission electron microscopy and x-ray diffractometry proved the excellent structural quality of the samples. A high ferroelectric polarization and stable piezoelectric switching were found for the lead zirconate titanate layers, whereas the manganite layers showed bulklike resistivity and magnetoresistance, both attesting to the high quality of the layers. In a detailed study of the magnetic response of the multilayers multiple magnetization switching was observed that was related to the complex strain state.

Three-dimensional high-resolution reconstruction of polarization in ferroelectric capacitors by piezoresponse force microscopy

2014-02-04T11:41:21Z

Three-dimensional high-resolution reconstruction of polarization in ferroelectric capacitors by piezoresponse force microscopy Rodriguez, Brian J.; Gruverman, A.; Kingon, A. I.; et al. A combination of vertical and lateral piezoresponse force microscopy (VPFM and LPFM, respectively) has been used to map the out-of-plane and in-plane polarization distribution, respectively, of (111)-oriented Pb(Zr,Ti)O-3-based (PZT) ferroelectric patterned and reactively-ion-etched capacitors. While VPFM and LPFM have previously been used to determine the orientation of the polarization vector in ferroelectric crystals and thin films, this is the first time the technique has been applied to determine the three-dimensional polarization distribution in thin-film capacitors and, as such, is of importance to the implementation of nonvolatile ferroelectric random access memory. Sequential VPFM and LPFM imaging have been performed in poled 1x1.5 mum(2) PZT capacitors. Subsequent quantitative analysis of the obtained piezoresponse images allowed the three-dimensional reconstruction of the domain arrangement in the PZT layers of the capacitors. It has been found that the poled capacitors, which appear as uniformly polarized in VPFM, are in fact in a polydomain state as is detected by LPFM and contain 90degrees domain walls. Despite the polycrystallinity of the PZT layer, regions larger than the average PZT grain size are found to have the same polarization orientation. This technique has potential for clarifying the switching behavior and imprint mechanism in micro- and nanoscale ferroelectric capacitors.

In situ cleaning and characterization of oxygen- and zinc-terminated, n-type, ZnO{0001} surfaces

2014-02-04T11:32:45Z

In situ cleaning and characterization of oxygen- and zinc-terminated, n-type, ZnO{0001} surfaces Coppa, B. J.; Fulton, C. C.; Hartlieb, P. J.; Rodriguez, Brian J.; et al. A layer containing an average of 1.0 monolayer (ML) of adventitious carbon and averages of 1.5 ML and 1.9 ML of hydroxide was determined to be present on the respective O-terminated (000 (1) over bar) and Zn-terminated (0001) surfaces of ZnO. A diffuse low-energy electron diffraction pattern was obtained from both surfaces. In situ cleaning procedures were developed and their efficacy evaluated in terms of the concentrations of residual hydrocarbons and hydroxide and the crystallography, microstructure, and electronic structure of these surfaces. Annealing ZnO(000 (1) over bar) in pure oxygen at 600-650 degreesC+/-20 degreesC reduced but did not eliminate all of the detectable hydrocarbon contamination. Annealing for 15 min in pure O-2 at 700 degreesC and 0.100+/-0.001 Torr caused desorption of both the hydrocarbons and the hydroxide constituents to concentrations below the detection limits (similar to0.03 ML=similar to0.3 at. \%) of our x-ray photoelectron spectroscopy instrument. However, thermal decomposition degraded the surface microstructure. Exposure of the ZnO(000 (1) over bar) surface to a remote plasma having an optimized 20\% O-2/80\% He mixture for the optimized time, temperature, and pressure of 30 min, 525 degreesC, and 0.050 Torr, respectively, resulted in the desorption of all detectable hydrocarbon species. Approximately 0.4 ML of hydroxide remained. The plasma-cleaned surface possessed an ordered crystallography and a step-and-terrace microstructure and was stoichiometric with nearly flat electronic bands. A 0.5 eV change in band bending was attributed to the significant reduction in the thickness of an accumulation layer associated with the hydroxide. The hydroxide was more tightly bound to the ZnO(0001) surface; this effect increased the optimal temperature and time of the plasma cleaning process for this surface to 550 degreesC and 60 min, respectively, at 0.050 Torr. Similar changes were achieved in the structural, chemical, and electronic properties of this surface; however, the microstructure only increased slightly in roughness and was without distinctive features.

Polarization-dependent electron affinity of LiNbO3 surfaces

2014-02-04T11:23:04Z

Polarization-dependent electron affinity of LiNbO3 surfaces Yang, W. -C.; Rodriguez, Brian J.; Gruverman, A.; Nemanich, R. J. Polar surfaces of a ferroelectric LiNbO3 crystal with periodically poled domains are explored using UV-photoelectron emission microscopy (PEEM). Compared with the positive domains (domains with positive surface polarization charges), a higher photoelectric yield is found from the negative domains (domains with negative surface polarization charges), indicating a lower photothreshold and a corresponding lower electron affinity. The photon-energy-dependent contrast in the PEEM images of the surfaces indicates that the photothreshold of the negative domains is similar to4.6 eV while that of the positive domains is greater than similar to6.2 eV. We propose that the threshold difference between the opposite domains can be attributed to a variation of the electron affinity due to opposite surface dipoles induced by surface adsorbates.

Domain growth kinetics in lithium niobate single crystals studied by piezoresponse force microscopy

2014-02-04T10:16:51Z

Domain growth kinetics in lithium niobate single crystals studied by piezoresponse force microscopy Rodriguez, Brian J.; Nemanich, R. J.; Kingon, A. I.; et al. The kinetics of sidewise domain growth in an inhomogeneous electric field has been investigated in stoichiometric LiNbO3 single crystals by measuring the lateral domain size as a function of the voltage pulse magnitude and duration using piezoresponse force microscopy. The domain size increases linearly with the voltage magnitude suggesting that the domain size is kinetically limited in a wide range of pulse magnitudes and durations. In spite of that, the written domains exhibit strong retention behavior. It is suggested that the switching behavior can be described by the universal scaling curve. Domain kinetics can be described as an activation process by calculating the field distribution using the charged sphere model under the assumption of an exponential field dependence of the wall velocity. The activation energy is found to be a function of the external field.

Preparation and characterization of atomically clean, stoichlometric surfaces of AIN(0001)

2014-02-04T10:12:30Z

Preparation and characterization of atomically clean, stoichlometric surfaces of AIN(0001) Mecouch, W. J.; Wagner, B. P.; Reitmeier, Z. J.; Rodriguez, Brian J.; et al. In situ exposure of the. (0001) surface of AlN thin films to flowing ammonia at 1120 degreesC and 10(-4) Torr removes oxygen/hydroxide and hydrocarbon species below the detectable limits of x-ray photoelectron spectroscopy and decreases the Al/N ratio from 1.3 to 1.0. The positions of the Al 2p and the N 1s core level peaks acquired from the cleaned surfaces were 75.0 +/- 0.1 eV and 398.2 +/- 0.1 eV, respectively, which were similar to the values determined for the as-loaded samples. The cleaning process left unchanged the (1 X 1) low energy electron diffraction pattern, the step-and-terrace microstructure, and the root mean square roughness values observed for the surfaces of the as-loaded samples; i.e., the surface structure and microstructure were not changed by the high-temperature exposure to ammonia at low pressures. Vacuum annealing under 10(-7) Torr at 1175 degreesC for 15 min removed all detectable hydrocarbons; however, it did not remove the oxygen/hydroxide species. (C) 2005 American Vacuum Society.

Atomic force microscopy-based experimental setup for studying domain switching dynamics in ferroelectric capacitors

2014-02-04T10:10:03Z

Atomic force microscopy-based experimental setup for studying domain switching dynamics in ferroelectric capacitors Dehoff, C.; Rodriguez, Brian J.; Kingon, A. I.; et al. This article describes an experimental setup for combined measurements of domain switching dynamics and switching currents in micrometer scale ferroelectric capacitors. The setup is based on a commercial atomic force microscope (AFM) that is equipped with a piezoresponse mode for domain imaging and with a wide bandwidth current amplifier for switching current recording. The setup allows combined domain/current measurements in capacitors as small as 1 mum(2) with switching times resolved down to 10 ns. The incorporation of switching current measurement capability into piezoresponse AFM makes detailed analysis of switching behavior in ferroelectric memory devices possible.

Scanning probe investigation of surface charge and surface potential of GaN-based heterostructures

2014-02-04T09:58:02Z

Scanning probe investigation of surface charge and surface potential of GaN-based heterostructures Rodriguez, Brian J.; Yang, W. -C.; Nemanich, R. J.; Gruverman, A. Scanning Kelvin probe microscopy (SKPM) and electrostatic force microscopy (EFM) have been employed to measure the surface potentials and the surface charge densities of the Ga- and the N-face of a GaN lateral polarity heterostructure (LPH). The surface was subjected to an HCl surface treatment to address the role of adsorbed charge on polarization screening. It has been found that while the Ga-face surface appears to be unaffected by the surface treatment, the N-face surface exhibited an increase in adsorbed screening charge density (1.6 +/- 0.5x10(10) cm(-2)), and a reduction of 0.3 +/- 0.1 V in the surface potential difference between the N- and Ga-face surfaces.

Scanning frequency mixing microscopy of high-frequency transport behavior at electroactive interfaces

2014-01-31T10:06:54Z

Scanning frequency mixing microscopy of high-frequency transport behavior at electroactive interfaces Rodriguez, Brian J.; Jesse, S.; Meunier, Vincent; Kalinin, S. V. An approach for high-frequency transport imaging, referred to as scanning frequency mixing microscopy (SFMM), is developed. Application of two high-frequency bias signals across an electroactive interface results in a low-frequency component due to interface nonlinearity. The frequency of a mixed signal is chosen within the bandwidth of the optical detector and can be tuned to the cantilever resonances. The SFMM signal is comprised of an intrinsic device contribution and a capacitive mixing contribution, and an approach to distinguish the two is suggested. This technique is illustrated on a model metal-semiconductor interface. The imaging mechanism and surface-tip contrast transfer are discussed. SFMM allows scanning probe microscopy based transport measurements to be extended to higher, ultimately gigahertz, frequency regimes, providing information on voltage derivatives of interface resistance and capacitance, from which device characteristics such as Schottky barrier height, etc., can be estimated. (c) 2006 American Institute of Physics.

High Resolution Electromechanical Imaging of Ferroelectric Materials in a Liquid Environment by Piezoresponse Force Microscopy

2014-01-31T09:55:04Z

High Resolution Electromechanical Imaging of Ferroelectric Materials in a Liquid Environment by Piezoresponse Force Microscopy Rodriguez, Brian J.; Jesse, S.; Baddorf, A. P.; Kalinin, S. V. High-resolution imaging of ferroelectric materials using piezoresponse force microscopy (PFM) is demonstrated in an aqueous environment. The elimination of both long-range electrostatic forces and capillary interactions results in a localization of the ac field to the tip-surface junction and allows the tip-surface contact area to be controlled. This approach results in spatial resolutions approaching the limit of the intrinsic domain-wall width. Imaging at frequencies corresponding to high-order cantilever resonances minimizes the viscous damping and added mass effects on cantilever dynamics and allows sensitivities comparable to ambient conditions. PFM in liquids will provide novel opportunities for high-resolution studies of ferroelectric materials, imaging of soft polymer materials, and imaging of biological systems in physiological environments on, ultimately, the molecular level.

Nanoelectromechanics of polarization switching in piezoresponse force microscopy

2014-01-31T09:00:36Z

Nanoelectromechanics of polarization switching in piezoresponse force microscopy Kalinin, S. V.; Gruverman, A.; Rodriguez, Brian J.; et al. Nanoscale polarization switching in ferroelectric materials by piezoresponse force microscopy in weak and strong indentation limits is analyzed using exact solutions for coupled electroelastic fields under the tip. Tip-induced domain switching is mapped on the Landau theory of phase transitions, with domain size as an order parameter. For a point charge interacting with a ferroelectric surface, switching by both first and the second order processes is possible, depending on the charge-surface separation. For a realistic tip, the domain nucleation process is first order in charge magnitude and polarization switching occurs only above a certain critical tip bias. In pure ferroelectric or ferroelastic switching, the late stages of the switching process can be described using a point charge model and arbitrarily large domains can be created. However, description of domain nucleation and the early stages of growth process when the domain size is comparable with the tip curvature radius (weak indentation) or the contact radius (strong indentation) requires the exact field structure. For higher order ferroic switching (e.g., ferroelectroelastic), the domain size is limited by the tip-sample contact area, thus allowing precise control of domain size. (C) 2005 American Institute of Physics.