@article{habelitz_rodriguez_marshall_marshall_kalinin_gruverman_2007, title={Peritubular dentin lacks piezoelectricity}, volume={86}, ISSN={["1544-0591"]}, DOI={10.1177/154405910708600920}, abstractNote={ 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. }, number={9}, journal={JOURNAL OF DENTAL RESEARCH}, author={Habelitz, S. and Rodriguez, B. J. and Marshall, S. J. and Marshall, G. W. and Kalinin, S. V. and Gruverman, A.}, year={2007}, month={Sep}, pages={908–911} }
 @article{watanabe_funakubo_osada_uchida_okada_rodriguez_gruverman_2007, title={Probing intrinsic polarization properties in bismuth-layered ferroelectric films}, volume={90}, number={11}, journal={Applied Physics Letters}, author={Watanabe, T. and Funakubo, H. and Osada, M. and Uchida, H. and Okada, I. and Rodriguez, B. J. and Gruverman, A.}, year={2007} }
 @article{kalinin_rodriguez_shin_jesse_grichko_thundat_baddorf_gruverman_2006, title={Bioelectromechanical imaging by scanning probe microscopy: Galvani's experiment at the nanoscale}, volume={106}, ISSN={["1879-2723"]}, DOI={10.1016/j.ultramic.2005.10.005}, abstractNote={Since the discovery in the late 18th century of electrically induced mechanical response in muscle tissue, coupling between electrical and mechanical phenomena has been shown to be a near-universal feature of biological systems. Here, we employ scanning probe microscopy (SPM) to measure the sub-Angstrom mechanical response of a biological system induced by an electric bias applied to a conductive SPM tip. Visualization of the spiral shape and orientation of protein fibrils with 5 nm spatial resolution in a human tooth and chitin molecular bundle orientation in a butterfly wing is demonstrated. In particular, the applicability of SPM-based techniques for the determination of molecular orientation is discussed.}, number={4-5}, journal={ULTRAMICROSCOPY}, author={Kalinin, SV and Rodriguez, BJ and Shin, J and Jesse, S and Grichko, V and Thundat, T and Baddorf, AP and Gruverman, A}, year={2006}, month={Mar}, pages={334–340} }
 @article{rodriguez_kalinin_shin_jesse_grichko_thundat_baddorf_gruverman_2006, title={Electromechanical imaging of biomaterials by scanning probe microscopy}, volume={153}, ISSN={["1095-8657"]}, DOI={10.1016/j.jsb.2005.10.008}, abstractNote={The majority of calcified and connective tissues possess complex hierarchical structure spanning the length scales from nanometers to millimeters. Understanding the biological functionality of these materials requires reliable methods for structural imaging on the nanoscale. Here, we demonstrate an approach for electromechanical imaging of the structure of biological samples on the length scales from tens of microns to nanometers using piezoresponse force microscopy (PFM), which utilizes the intrinsic piezoelectricity of biopolymers such as proteins and polysaccharides as the basis for high-resolution imaging. Nanostructural imaging of a variety of protein-based materials, including tooth, antler, and cartilage, is demonstrated. Visualization of protein fibrils with sub-10 nm spatial resolution in a human tooth is achieved. Given the near-ubiquitous presence of piezoelectricity in biological systems, PFM is suggested as a versatile tool for micro- and nanostructural imaging in both connective and calcified tissues.}, number={2}, journal={JOURNAL OF STRUCTURAL BIOLOGY}, author={Rodriguez, BJ and Kalinin, SV and Shin, J and Jesse, S and Grichko, V and Thundat, T and Baddorf, AP and Gruverman, A}, year={2006}, month={Feb}, pages={151–159} }
 @article{hanson_rodriguez_nemanich_gruverman_2006, title={Fabrication of metallic nanowires on a ferroelectric template via photochemical reaction}, volume={17}, ISSN={["0957-4484"]}, DOI={10.1088/0957-4484/17/19/028}, abstractNote={Fabrication of silver nanowires on a domain-patterned lithium niobate template by inducing a photochemical reaction in an aqueous solution is reported. Silver deposition occurs preferentially along the domain boundaries which separate antiparallel domains. The nanowires can reach lengths of hundreds of micrometres, and their location can be controlled by generating domain patterns of a desired configuration while their width depends on deposition conditions, such as temperature, solution concentration and ultraviolet (UV) light exposure time. The selective deposition process is explained by a combination of the inhomogeneous distribution of the electric field in the vicinity of the domain wall and the polarization screening mechanism of the template material. Controllable and selective deposition of metal species onto nanoscale domain-patterned ferroelectric templates may provide an alternative bottom-up route to lithographic fabrication methods.}, number={19}, journal={NANOTECHNOLOGY}, author={Hanson, J. N. and Rodriguez, B. J. and Nemanich, R. J. and Gruverman, A.}, year={2006}, month={Oct}, pages={4946–4949} }
 @article{kalinin_rodriguez_jesse_shin_baddorf_gupta_jain_williams_gruverman_2006, title={Vector piezoresponse force microscopy}, volume={12}, ISSN={["1431-9276"]}, DOI={10.1017/S1431927606060156}, abstractNote={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 (3D) 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.}, number={3}, journal={MICROSCOPY AND MICROANALYSIS}, author={Kalinin, Sergei V. and Rodriguez, Brian J. and Jesse, Stephen and Shin, Junsoo and Baddorf, Arthur P. and Gupta, Pradyumna and Jain, Himanshu and Williams, David B. and Gruverman, Alexei}, year={2006}, month={Jun}, pages={206–220} }
 @article{dalmau_schlesser_rodriguez_nemanich_sitar_2005, title={AlN bulk crystals grown on SiC seeds}, volume={281}, ISSN={["1873-5002"]}, DOI={10.1016/j.jcrysgro.2005.03.012}, abstractNote={AlN layers with thickness between 0.1 and 3 mm were grown on on-axis and off-axis (0 0 0 1), Si-face SiC seeds by physical vapor transport (PVT) from an AlN powder source. A two-step deposition process was developed for the growth of thick layers. Cracks formed in the AlN layers due to the thermal expansion mismatch between AlN and SiC were observed to decrease with increase in AlN thickness. AlN grown on on-axis SiC was primarily Al-polar, but contained N-polar inversion domains (IDs) revealed by wet etching in hot, aqueous phosphoric acid or potassium hydroxide solutions. Regions of opposite polarity on basal plane surfaces were imaged by piezoresponse force microscopy (PFM). IDs were not observed in crystals grown on off-axis seeds. The influence of SiC seed orientation and stability on the polarity of the AlN layers is discussed.}, number={1}, journal={JOURNAL OF CRYSTAL GROWTH}, author={Dalmau, R and Schlesser, R and Rodriguez, BJ and Nemanich, RJ and Sitar, Z}, year={2005}, month={Jul}, pages={68–74} }
 @article{dehoff_rodriguez_kingon_nemanich_gruverman_cross_2005, title={Atomic force microscopy-based experimental setup for studying domain switching dynamics in ferroelectric capacitors}, volume={76}, ISSN={["1089-7623"]}, DOI={10.1063/1.1850652}, abstractNote={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 1μm2 with switching times resolved down to 10ns. The incorporation of switching current measurement capability into piezoresponse AFM makes detailed analysis of switching behavior in ferroelectric memory devices possible.}, number={2}, journal={REVIEW OF SCIENTIFIC INSTRUMENTS}, author={Dehoff, C and Rodriguez, BJ and Kingon, AI and Nemanich, RJ and Gruverman, A and Cross, JS}, year={2005}, month={Feb} }
 @article{gruverman_rodriguez_dehoff_waldrep_kingon_nemanich_cross_2005, title={Direct studies of domain switching dynamics in thin film ferroelectric capacitors}, volume={87}, ISSN={["1077-3118"]}, DOI={10.1063/1.2010605}, abstractNote={An experimental approach for direct studies of the polarization reversal mechanism in thin film ferroelectric capacitors based on piezoresponse force microscopy (PFM) in conjunction with pulse switching capabilities is presented. Instant domain configurations developing in a 3×3μm2 capacitor at different stages of the polarization reversal process have been registered using step-by-step switching and subsequent PFM imaging. The developed approach allows direct comparison of experimentally measured microscopic switching behavior with parameters used by phenomenological switching models. It has been found that in the low field regime (just above the threshold value) used in the present study, the mechanism of polarization reversal changes during the switching cycle from the initial nucleation-dominated process to the lateral domain expansion at the later stages. The classical nucleation model of Kolmogorov–Avrami–Ishibashi (KAI) provides reasonable approximation for the nucleation-dominated stage of switching but is inapplicable to the slow switching stage. It has been suggested that the switching dynamics can be approximated by averaging the KAI model over a broad distribution of switching times.}, number={8}, journal={APPLIED PHYSICS LETTERS}, author={Gruverman, A and Rodriguez, BJ and Dehoff, C and Waldrep, JD and Kingon, AI and Nemanich, RJ and Cross, JS}, year={2005}, month={Aug} }
 @article{rodriguez_nemanich_kingon_gruverman_kalinin_terabe_liu_kitamura_2005, title={Domain growth kinetics in lithium niobate single crystals studied by piezoresponse force microscopy}, volume={86}, ISSN={["1077-3118"]}, DOI={10.1063/1.1845594}, abstractNote={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.}, number={1}, journal={APPLIED PHYSICS LETTERS}, author={Rodriguez, BJ and Nemanich, RJ and Kingon, A and Gruverman, A and Kalinin, SV and Terabe, K and Liu, XY and Kitamura, K}, year={2005}, month={Jan} }
 @article{kalinin_rodriguez_jesse_thundat_gruverman_2005, title={Electromechanical imaging of biological systems with sub-10 nm resolution}, volume={87}, number={5}, journal={Applied Physics Letters}, author={Kalinin, S. V. and Rodriguez, B. J. and Jesse, S. and Thundat, T. and Gruverman, A.}, year={2005} }
 @article{rodriguez_gruverman_kingon_nemanich_cross_2005, title={Investigation of the mechanism of polarization switching in ferroelectric capacitors by three- dimensional piezoresponse force microscopy}, volume={80}, ISSN={["1432-0630"]}, DOI={10.1007/s00339-004-2925-2}, abstractNote={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).}, number={1}, journal={APPLIED PHYSICS A-MATERIALS SCIENCE & PROCESSING}, author={Rodriguez, BJ and Gruverman, A and Kingon, AI and Nemanich, RJ and Cross, JS}, year={2005}, month={Jan}, pages={99–103} }
 @article{kalinin_gruverman_rodriguez_shin_baddorf_karapetian_kachanov_2005, title={Nanoelectromechanics of polarization switching in piezoresponse force microscopy}, volume={97}, number={7}, journal={Journal of Applied Physics}, author={Kalinin, S. V. and Gruverman, A. and Rodriguez, B. J. and Shin, J. and Baddorf, A. P. and Karapetian, E. and Kachanov, M.}, year={2005} }
 @article{yang_rodriguez_gruverman_nemanich_2005, title={Photo electron emission microscopy of polarity-patterned materials}, volume={17}, ISSN={["1361-648X"]}, DOI={10.1088/0953-8984/17/16/012}, abstractNote={This study presents variable photon energy photo electron emission microscopy (PEEM) of polarity-patterned epitaxial GaN films, and ferroelectric LiNbO3 (LNO) single crystals and PbZrTiO3 (PZT) thin films. The photo electrons were excited with spontaneous emission from the tunable UV free electron laser (FEL) at Duke University. We report PEEM observation of polarity contrast and measurement of the photothreshold of each polar region of the materials. For a cleaned GaN film with laterally patterned Ga- and N-face polarities, we found a higher photoelectric yield from the N-face regions compared with the Ga-face regions. Through the photon energy dependent contrast in the PEEM images of the surfaces, we can deduce that the threshold of the N-face region is less than ∼4.9 eV while that of the Ga-face regions is greater than 6.3 eV. In both LNO and PZT, bright emission was detected from the negatively poled domains, indicating that the emission threshold of the negative domain is lower than that of the positive domain. For LNO, the measured photothreshold was ∼4.6 eV at the negative domain and ∼6.2 eV at the positive domain, while for PZT, the threshold of the negative domain was less than 4.3 eV. Moreover, PEEM observation of the PZT surface at elevated temperatures displayed that the domain contrast disappeared near the Curie temperature of ∼300 °C. The PEEM polarity contrast of the polar materials is discussed in terms of internal screening from free carriers and defects and the external screening due to adsorbed ions.}, number={16}, journal={JOURNAL OF PHYSICS-CONDENSED MATTER}, author={Yang, WC and Rodriguez, BJ and Gruverman, A and Nemanich, RJ}, year={2005}, month={Apr}, pages={S1415–S1426} }
 @article{mecouch_wagner_reitmeier_davis_pandarinath_rodriguez_nemanich_2005, title={Preparation and characterization of atomically clean, stoichlometric surfaces of AIN(0001)}, volume={23}, ISSN={["0734-2101"]}, DOI={10.1116/1.1830497}, abstractNote={In situ exposure of the (0001) surface of AlN thin films to flowing ammonia at 1120 °C and 10−4Torr 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 Al2p and the N1s core level peaks acquired from the cleaned surfaces were 75.0±0.1eV and 398.2±0.1eV, respectively, which were similar to the values determined for the as-loaded samples. The cleaning process left unchanged the (1×1) 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 10−7Torr at 1175 °C for 15 min removed all detectable hydrocarbons; however, it did not remove the oxygen∕hydroxide species.}, number={1}, journal={JOURNAL OF VACUUM SCIENCE & TECHNOLOGY A}, author={Mecouch, WJ and Wagner, BP and Reitmeier, ZJ and Davis, RF and Pandarinath, C and Rodriguez, BJ and Nemanich, RJ}, year={2005}, pages={72–77} }
 @article{rodriguez_yang_nemanich_gruverman_2005, title={Scanning probe investigation of surface charge and surface potential of GaN-based heterostructures}, volume={86}, ISSN={["1077-3118"]}, DOI={10.1063/1.1869535}, abstractNote={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±0.5×1010cm−2), and a reduction of 0.3±0.1V in the surface potential difference between the N- and Ga-face surfaces.}, number={11}, journal={APPLIED PHYSICS LETTERS}, author={Rodriguez, BJ and Yang, WC and Nemanich, RJ and Gruverman, A}, year={2005}, month={Mar} }
 @article{shin_rodriguez_baddorf_thundat_karapetian_kachanov_gruverman_kalinin_2005, title={Simultaneous elastic and electromechanical imaging by scanning probe microscopy: Theory and applications to ferroelectric and biological materials}, volume={23}, number={5}, journal={Journal of Vacuum Science & Technology. B, Microelectronics and Nanometer Structures}, author={Shin, J. and Rodriguez, B. J. and Baddorf, A. P. and Thundat, T. and Karapetian, E. and Kachanov, M. and Gruverman, A. and Kalinin, S. V.}, year={2005}, pages={2102–2108} }
 @article{coppa_fulton_hartlieb_davis_rodriguez_shields_nemanich_2004, title={In situ cleaning and characterization of oxygen- and zinc-terminated, n-type, ZnO{0001} surfaces}, volume={95}, ISSN={["1089-7550"]}, DOI={10.1063/1.1695596}, abstractNote={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 (0001̄) 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(0001̄) in pure oxygen at 600–650 °C±20 °C reduced but did not eliminate all of the detectable hydrocarbon contamination. Annealing for 15 min in pure O2 at 700 °C and 0.100±0.001 Torr caused desorption of both the hydrocarbons and the hydroxide constituents to concentrations below the detection limits (∼0.03 ML=∼0.3 at. %) of our x-ray photoelectron spectroscopy instrument. However, thermal decomposition degraded the surface microstructure. Exposure of the ZnO(0001̄) surface to a remote plasma having an optimized 20% O2/80% He mixture for the optimized time, temperature, and pressure of 30 min, 525 °C, 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 °C 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.}, number={10}, journal={JOURNAL OF APPLIED PHYSICS}, author={Coppa, BJ and Fulton, CC and Hartlieb, PJ and Davis, RF and Rodriguez, BJ and Shields, BJ and Nemanich, RJ}, year={2004}, month={May}, pages={5856–5864} }
 @article{yang_rodriguez_gruverman_nemanich_2004, title={Polarization-dependent electron affinity of LiNbO3 surfaces}, volume={85}, ISSN={["1077-3118"]}, DOI={10.1063/1.1790604}, abstractNote={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 ∼4.6eV while that of the positive domains is greater than ∼6.2eV. 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.}, number={12}, journal={APPLIED PHYSICS LETTERS}, author={Yang, WC and Rodriguez, BJ and Gruverman, A and Nemanich, RJ}, year={2004}, month={Sep}, pages={2316–2318} }
 @article{rodriguez_gruverman_kingon_nemanich_cross_2004, title={Three-dimensional high-resolution reconstruction of polarization in ferroelectric capacitors by piezoresponse force microscopy}, volume={95}, ISSN={["1089-7550"]}, DOI={10.1063/1.1638889}, abstractNote={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)O3-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 1×1.5 μm2 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 90° 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.}, number={4}, journal={JOURNAL OF APPLIED PHYSICS}, author={Rodriguez, BJ and Gruverman, A and Kingon, AI and Nemanich, RJ and Cross, JS}, year={2004}, month={Feb}, pages={1958–1962} }
 @article{gruverman_rodriguez_kingon_nemanich_tagantsev_cross_tsukada_2003, title={Mechanical stress effect on imprint behavior of integrated ferroelectric capacitors}, volume={83}, ISSN={["0003-6951"]}, DOI={10.1063/1.1593830}, abstractNote={Stress-induced changes in the imprint and switching behavior of (111)-oriented Pb(Zr,Ti)O3 (PZT)-based capacitors have been studied using piezoresponse force microscopy. Visualization of polarization distribution and d33-loop measurements in individual 1×1.5-μm2 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.}, number={4}, journal={APPLIED PHYSICS LETTERS}, author={Gruverman, A and Rodriguez, BJ and Kingon, AI and Nemanich, RJ and Tagantsev, AK and Cross, JS and Tsukada, M}, year={2003}, month={Jul}, pages={728–730} }
 @article{park_cuomo_rodriguez_yang_nemanich_ambacher_2003, title={Micro-Raman study of electronic properties of inversion domains in GaN-based lateral polarity heterostructures}, volume={93}, ISSN={["1089-7550"]}, DOI={10.1063/1.1570507}, abstractNote={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.}, number={12}, journal={JOURNAL OF APPLIED PHYSICS}, author={Park, M and Cuomo, JJ and Rodriguez, BJ and Yang, WC and Nemanich, RJ and Ambacher, O}, year={2003}, month={Jun}, pages={9542–9547} }
 @article{yang_rodriguez_park_nemanich_ambacher_cimalla_2003, title={Photoelectron emission microscopy observation of inversion domain boundaries of GaN-based lateral polarity heterostructures}, volume={94}, DOI={10.1063/1.1618355}, abstractNote={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.}, number={9}, journal={Journal of Applied Physics}, author={Yang, W. C. and Rodriguez, B. J. and Park, M. and Nemanich, R. J. and Ambacher, O. and Cimalla, V.}, year={2003}, pages={5720–5725} }
 @article{gruverman_rodriguez_kingon_nemanich_cross_tsukada_2003, title={Spatial inhomogeneity of imprint and switching behavior in ferroelectric capacitors}, volume={82}, ISSN={["0003-6951"]}, DOI={10.1063/1.1570942}, abstractNote={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)O3-based capacitors on Pt electrodes. Mapping of polarization distribution in the poled capacitors as well as local d33–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.}, number={18}, journal={APPLIED PHYSICS LETTERS}, author={Gruverman, A and Rodriguez, BJ and Kingon, AI and Nemanich, RJ and Cross, JS and Tsukada, M}, year={2003}, month={May}, pages={3071–3073} }
 @article{park_sakhrani_maria_cuomo_teng_muth_ware_rodriguez_nemanich_2003, title={Wavelength-dependent Raman scattering of hydrogenated amorphous silicon carbon with red, green, and blue light excitation}, volume={18}, ISSN={["2044-5326"]}, DOI={10.1557/JMR.2003.0106}, abstractNote={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 sp2-bonded carbon clusters}, number={4}, journal={JOURNAL OF MATERIALS RESEARCH}, author={Park, M and Sakhrani, V and Maria, JP and Cuomo, JJ and Teng, CW and Muth, JF and Ware, ME and Rodriguez, BJ and Nemanich, RJ}, year={2003}, month={Apr}, pages={768–771} }
 @article{gruverman_rodriguez_nemanich_kingon_2002, title={Nanoscale observation of photoinduced domain pinning and investigation of imprint behavior in ferroelectric thin films}, volume={92}, ISSN={["1089-7550"]}, DOI={10.1063/1.1497698}, abstractNote={Piezoresponse force microscopy has been used to investigate the nanoscale mechanism of imprint behavior of ferroelectric PbTiO3 thin films by studying the photoinduced changes in the hysteresis loops of individual grains. Illumination of the film with UV light resulted in a voltage shift opposite to that observed in ferroelectric thin film capacitors. This effect is attributed to the generation of an electric field within the surface dielectric layer as a result of the interaction between photoinduced charges and polarization charges. Application of a small nonswitching bias to the film with simultaneous UV illumination resulted in domain pinning in the grains where the polarization direction coincided with the direction of the applied field, in agreement with the proposed model. Domain pinning was also observed in grains with polydomain structure suggesting that charge entrapment at the existing domain boundaries in the bulk of the film contributes to the suppression of switchable polarization. However, a symmetric character of hysteresis loops observed in such grains implies that charge entrapment in the bulk of the film does not cause the voltage shift. It has been suggested that a thin high-dielectric interfacial layer can improve the imprint behavior of ferroelectric capacitors.}, number={5}, journal={JOURNAL OF APPLIED PHYSICS}, author={Gruverman, A and Rodriguez, BJ and Nemanich, RJ and Kingon, AI}, year={2002}, month={Sep}, pages={2734–2739} }
 @article{rodriguez_gruverman_kingon_nemanich_2002, title={Piezoresponse force microscopy for piezoelectric measurements of III-nitride materials}, volume={246}, ISSN={["0022-0248"]}, DOI={10.1016/S0022-0248(02)01749-9}, abstractNote={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 d33 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.}, number={3-4}, journal={JOURNAL OF CRYSTAL GROWTH}, author={Rodriguez, BJ and Gruverman, A and Kingon, AI and Nemanich, RJ}, year={2002}, month={Dec}, pages={252–258} }
 @article{rodriguez_gruverman_kingon_nemanich_ambacher_2002, title={Piezoresponse force microscopy for polarity imaging of GaN}, volume={80}, ISSN={["1077-3118"]}, DOI={10.1063/1.1483117}, abstractNote={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.}, number={22}, journal={APPLIED PHYSICS LETTERS}, author={Rodriguez, BJ and Gruverman, A and Kingon, AI and Nemanich, RJ and Ambacher, O}, year={2002}, month={Jun}, pages={4166–4168} }
 @article{christman_kim_maiwa_maria_rodriguez_kingon_nemanich_2000, title={Spatial variation of ferroelectric properties in Pb(Zr-0.3, Ti-0.7)O-3 thin films studied by atomic force microscopy}, volume={87}, ISSN={["0021-8979"]}, DOI={10.1063/1.373492}, abstractNote={Imaging of the phase and magnitude of the piezoelectric strain in Pb(Zr0.3, Ti0.7)O3 (PZT) capacitors is performed with an atomic force microscope. The imaging reveals a significant spatial dependence of the ferroelectric properties of both fatigued and unfatigued PZT films. We propose that the variation is related to the domain structure of the PZT. Through the measurement of local piezoelectric hysteresis loops and imaging of the piezoelectric strain, areas are observed in fatigued PZT that exhibit hysteresis loops shifted along the polarization axis. In some regions of fatigued samples, the hysteresis loops are shifted such that both remanent points of the hysteresis curve have the same polarization direction. These results have important implications for the scalability of nonvolatile ferroelectric random access memory to higher device densities.}, number={11}, journal={JOURNAL OF APPLIED PHYSICS}, author={Christman, JA and Kim, SH and Maiwa, H and Maria, JP and Rodriguez, BJ and Kingon, AI and Nemanich, RJ}, year={2000}, month={Jun}, pages={8031–8034} }