Poly(ε-caprolactone)-Banded Spherulites and Interaction with MC3T3-E1 Cells

We report that protein adsorption, cell attachment, and cell proliferation were enhanced on spherulites-roughened polymer surfaces. Banded spherulites with concentric alternating succession of ridges and valleys were observed on spin-coated thin films of poly­(ε-caprolactone) (PCL) and two series of PCL binary homoblends composed of high- and low-molecular-weight components when they were isothermally crystallized at 25–52 °C. Their thermal properties, crystallization kinetics, and surface morphology were examined. The melting temperature (<i>T</i>_m), crystallinity (χ_c), crystallization rate, and spherulitic patterns showed strong dependence on the crystallization temperature (<i>T</i>_c) and the blend composition. The surface roughness of the spherulites was higher when <i>T</i>_c was higher; thus, the larger surface area formed in banded spherulites could adsorb more serum proteins from cell culture media. In vitro mouse preosteoblastic MC3T3-E1 cell attachment, proliferation, and nuclear localization were assessed on the hot-compressed flat disks and spherulites-roughened films of the high-molecular-weight PCL and one of its homoblends. The number of attached MC3T3-E1 cells and the proliferation rate were greater on the rougher surfaces than those on the flat ones. It is interesting to note that cell nuclei were preferentially, though not absolutely, located in or close to the valleys of the banded spherulites. The percentage of cell nuclei in the valleys was higher than 78% when the ridge height and adjacent ridge distance were ∼350 and ∼35 nm, respectively. This preference was weaker when the ridge height was lower or at a higher cell density. These results suggest that isothermal crystallization of semicrystalline polymers can be an effective thermal treatment method to achieve controllable surface roughness and pattern for regulating cell behaviors in tissue-engineering applications

Mapping Nanoscale Variations in Photochemical Damage of Polymer/Fullerene Solar Cells with Dissipation Imaging

We use frequency-modulated electrostatic force microscopy to track changes in cantilever quality factor (<i>Q</i>) as a function of photochemical damage in a model organic photovoltaic system poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-<i>b</i>:4,5-<i>b</i>′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-<i>b</i>]thiophenediyl]] (PTB7) and 3′<i>H</i>-cyclopropa[8,25][5,6]fullerene-C71-D5h(6)-3′-butanoic acid, 3′-phenyl-, methyl ester (PC₇₁BM). We correlate local <i>Q</i> factor imaging with macroscopic device performance and show that, for this system, changes in cantilever <i>Q</i> correlate well with changes in external quantum efficiency and can thus be used to monitor local photochemical damage over the entire functional lifetime of a PTB7:PC₇₁BM solar cell. We explore how <i>Q</i> imaging is affected by the choice of cantilever resonance frequency. Finally, we use <i>Q</i> imaging to elucidate the differences in the evolution of nanoscale structure in the photochemical damage occurring in PTB7:PC₇₁BM solar cells processed with and without the solvent additive 1,8-diiodooctane (DIO). We show that processing with DIO not only yields a preferable morphology for uniform performance across the surface of the device but also enhances the stability of PTB7:PC₇₁BM solar cellsan effect that can be predicted based on the local <i>Q</i> images

Breaking the Time Barrier in Kelvin Probe Force Microscopy: Fast Free Force Reconstruction Using the G‑Mode Platform

Atomic force microscopy (AFM) offers unparalleled insight into structure and material functionality across nanometer length scales. However, the spatial resolution afforded by the AFM tip is counterpoised by slow detection speeds compared to other common microscopy techniques (<i>e.g.</i>, optical, scanning electron microscopy, <i>etc.</i>). In this work, we develop an ultrafast AFM imaging approach allowing direct reconstruction of the tip-sample forces with ∼3 order of magnitude higher time resolution than is achievable using standard AFM detection methods. Fast free force recovery (F³R) overcomes the widely viewed temporal bottleneck in AFM, that is, the mechanical bandwidth of the cantilever, enabling time-resolved imaging at sub-bandwidth speeds. We demonstrate quantitative recovery of electrostatic forces with ∼10 μs temporal resolution, free from influences of the cantilever ring-down. We further apply the F³R method to Kelvin probe force microscopy (KPFM) measurements. F³R-KPFM is an open loop imaging approach (<i>i.e.</i>, no bias feedback), allowing ultrafast surface potential measurements (<i>e.g.</i>, <20 μs) to be performed at regular KPFM scan speeds. F³R-KPFM is demonstrated for exploration of ion migration in organometallic halide perovskite materials and shown to allow spatiotemporal imaging of positively charged ion migration under applied electric field, as well as subsequent formation of accumulated charges at the perovskite/electrode interface. In this work, we demonstrate quantitative F³R-KPFM measurementshowever, we fully expect the F³R approach to be valid for all modes of noncontact AFM operation, including noninvasive probing of ultrafast electrical and magnetic dynamics

Decoding Apparent Ferroelectricity in Perovskite Nanofibers

Ferroelectric perovskites are an important group of materials underpinning a wide variety of devices ranging from sensors and transducers to nonvolatile memories and photovoltaic cells. Despite the progress in material synthesis, ferroelectric characterization of nanoscale perovskites is still a challenge. Piezoresponse force microscopy (PFM) is one of the most popular tools for probing and manipulating nanostructures to study the ferroelectric properties. However, the interpretation of hysteresis data and alternate signal origins are critical. Here, we use a family of scanning probe microscopy (SPM) techniques to systematically investigate the ferroelectric behavior of electrospun potassium niobate (KNbO₃) nanofibers. Band Excitation (BE) SPM scans reveal that PFM signals are dominated by changes in resonant frequency due to rough nanofiber surfaces, rather than the actual local piezoelectric strength. We investigate the bias-induced charge injection properties and electrostatic interactions on the PFM response of the nanofiber using contact mode Kelvin probe force microscopy (cKPFM). Furthermore, the impact of relative humidity on the KNbO₃ nanofiber’s piezoresponse, switching behavior, and tip-induced charges are explored. The resultant data from BE scans were utilized to estimate the piezoelectric constants of the KNO nanofiber. These observations will provide clarity in studying newly developed ferroelectric nanostructures and unambiguously interpreting the PFM data

Photoinduced Bulk Polarization and Its Effects on Photovoltaic Actions in Perovskite Solar Cells

This article reports an experimental demonstration of photoinduced bulk polarization in hysteresis-free methylammonium (MA) lead-halide perovskite solar cells [ITO/PEDOT:PSS/perovskite/PCBM/PEI/Ag]. An anomalous capacitance–voltage (CV) signal is observed as a broad “shoulder” in the depletion region from −0.5 to +0.5 V under photoexcitation based on CV measurements where a dc bias is gradually scanned to continuously drift mobile ions in order to detect local polarization under a low alternating bias (50 mV, 5 kHz). Essentially, gradually scanning the dc bias and applying a low alternating bias can separately generate continuously drifting ions and a bulk CV signal from local polarization under photoexcitation. Particularly, when the device efficiency is improved from 12.41% to 18.19% upon chlorine incorporation, this anomalous CV signal can be enhanced by a factor of 3. This anomalous CV signal can be assigned as the signature of photoinduced bulk polarization by distinguishing from surface polarization associated with interfacial charge accumulation. Meanwhile, replacing easy-rotational MA⁺ with difficult-rotational formamidinium (FA⁺) cations largely minimizes such anomalous CV signal, suggesting that photoinduced bulk polarization relies on the orientational freedom of dipolar organic cations. Furthermore, a Kelvin probe force microscopy study shows that chlorine incorporation can suppress the density of charged defects and thus enhances photoinduced bulk polarization due to the reduced screening effect from charged defects. A bias-dependent photoluminescence study indicates that increasing bulk polarization can suppress carrier recombination by decreasing charge capture probability through the Coulombic screening effect. Clearly, our studies provide an insightful understanding of photoinduced bulk polarization and its effects on photovoltaic actions in perovskite solar cells

Quantitative Description of Crystal Nucleation and Growth from <i>in Situ</i> Liquid Scanning Transmission Electron Microscopy

Recent advances in liquid cell (scanning) transmission electron microscopy (S)TEM has enabled <i>in situ</i> nanoscale investigations of controlled nanocrystal growth mechanisms. Here, we experimentally and quantitatively investigated the nucleation and growth mechanisms of Pt nanostructures from an aqueous solution of K₂PtCl₆. Averaged statistical, network, and local approaches have been used for the data analysis and the description of both collective particles dynamics and local growth features. In particular, interaction between neighboring particles has been revealed and attributed to reduction of the platinum concentration in the vicinity of the particle boundary. The local approach for solving the inverse problem showed that particles dynamics can be simulated by a stationary diffusional model. The obtained results are important for understanding nanocrystal formation and growth processes and for optimization of synthesis conditions

MOESM1 of Feature extraction via similarity search: application to atom finding and denoising in electron and scanning probe microscopy imaging

Additional file 1. Additional figures

Deep Data Analysis of Conductive Phenomena on Complex Oxide Interfaces: Physics from Data Mining

Spatial variability of electronic transport in BiFeO₃–CoFe₂O₄ (BFO–CFO) self-assembled heterostructures is explored using spatially resolved first-order reversal curve (FORC) current voltage (IV) mapping. Multivariate statistical analysis of FORC-IV data classifies statistically significant behaviors and maps characteristic responses spatially. In particular, regions of grain, matrix, and grain boundary responses are clearly identified. <i>k</i>-Means and Bayesian demixing analysis suggest the characteristic response be separated into four components, with hysteretic-type behavior localized at the BFO–CFO tubular interfaces. The conditions under which Bayesian components allow direct physical interpretation are explored, and transport mechanisms at the grain boundaries and individual phases are analyzed. This approach conjoins multivariate statistical analysis with physics-based interpretation, actualizing a robust, universal, data-driven approach to problem solving, which can be applied to exploration of local transport and other functional phenomena in other spatially inhomogeneous systems

Co-registered Topographical, Band Excitation Nanomechanical, and Mass Spectral Imaging Using a Combined Atomic Force Microscopy/Mass Spectrometry Platform

The advancement of a hybrid atomic force microscopy/mass spectrometry imaging platform demonstrating the co-registered topographical, band excitation nanomechanical, and mass spectral imaging of a surface using a single instrument is reported. The mass spectrometry-based chemical imaging component of the system utilized nanothermal analysis probes for pyrolytic surface sampling followed by atmospheric pressure chemical ionization of the gas-phase species produced with subsequent mass analysis. The basic instrumental setup and operation are discussed, and the multimodal imaging capability and utility are demonstrated using a phase-separated polystyrene/poly(2-vinylpyridine) polymer blend thin film. The topography and band excitation images showed that the valley and plateau regions of the thin film surface were comprised primarily of one of the two polymers in the blend with the mass spectral chemical image used to definitively identify the polymers at the different locations. Data point pixel size for the topography (390 nm × 390 nm), band excitation (781 nm × 781 nm), and mass spectrometry (690 nm × 500 nm) images was comparable and submicrometer in all three cases, but the data voxel size for each of the three images was dramatically different. The topography image was uniquely a surface measurement, whereas the band excitation image included information from an estimated 20 nm deep into the sample and the mass spectral image from 110 to 140 nm in depth. Because of this dramatic sampling depth variance, some differences in the band excitation and mass spectrometry chemical images were observed and were interpreted to indicate the presence of a buried interface in the sample. The spatial resolution of the chemical image was estimated to be between 1.5 and 2.6 μm, based on the ability to distinguish surface features in that image that were also observed in the other images