materials science

Kinetic-controlled Crystallization of α-FAPbI3 Inducing Preferred Crystallographic Orientation Enhances Photovoltaic Performance

Physical properties of the polycrystalline materials are mostly determined by their microstructure. As the crystallization process can determine the microstructure, the nucleation, and growth can also control whether the materials will be resulted in single crystalline or polycrystalline. Along with the morphological changes, anisotropic properties of the materials can also be controlled. *

As a result, preferential orientation with advanced optoelectronic properties can enhance the photovoltaic devices’ performance. *

Although incorporation of additives is one of the most studied methods to stabilize the photoactive α-phase of formamidinium lead tri-iodide (α-FAPbI3), no studies focus on how the additives affect the crystallization kinetics. *

In the article “Kinetic-Controlled Crystallization of α-FAPbI3 Inducing Preferred Crystallographic Orientation Enhances Photovoltaic Performance” along with the role of methylammonium chloride (MACl) as a “stabilizer” in the formation of α-FAPbI3, Sooeun Shin, Seongrok Seo, Seonghwa Jeong, Anir S. Sharbirin, Jeongyong Kim, Hyungju Ahn, Nam-Gyu Park and Hyunjung Shin point out the additional role as a “controller” in the crystallization kinetics. *

With microscopic observations, for example, electron backscatter diffraction and selected area electron diffraction, it is examined that higher concentration of MACl induces slower crystallization kinetics, resulting in larger grain size and [100] preferred orientation. *

Optoelectronic properties of [100] preferentially oriented grains with less non-radiative recombination, a longer lifetime of charge carriers, and lower photocurrent deviations in between each grain induce higher short-circuit current density (Jsc) and fill factor. *

Resulting MACl40 mol% attains the highest power conversion efficiency (PCE) of 24.1%.

The results provide observations of a direct correlation between the crystallographic orientation and device performance as it highlights the importance of crystallization kinetics resulting in desirable microstructures for device engineering. *

The electrical characterizations with atomic force microscopy (AFM) and conductive atomic force microscopy ( C-AFM) were done to measure the local conductance of FAPbI3 films. All measurements were performed under illumination (green LED) with a 1.3 V bias using a Pt-coated C-AFM probe (NanoWorld PlatinumIridium coated Pointprobe® CONTPt ). FTO was used for the conductive substrates. *

Conductive atomic force microscopy (C-AFM) indicated much more homogeneous photocurrent generation along the surface of (100) preferentially oriented layers. *

Figure 4 from Sooeun Shin et al. 2023 “Kinetic-Controlled Crystallization of α-FAPbI3 Inducing Preferred Crystallographic Orientation Enhances Photovoltaic Performance”:
Electrical properties of (100)-oriented α-FAPbI3 thin films. a,b) Surface topography and photocurrent measurement by C-AFM of MACl10% and MACl40%. Measurements were taken under illumination (green LED) with a 1.3 V bias. Current images indicate that photocurrents were induced grain by grain, as each grain showed a distinct photocurrent. The photocurrent deviation for each grain is larger in MACl10% than in MACl40%, which shows a similar photocurrent grain by grain. c,d) Current line profile extracted from the current image (a and b). The standard deviation calculated from the magnitude of the photocurrent in each grain is displayed in the inset (0.019 and 0.014 nA for MACl10% and MACl40%, respectively). A low photocurrent was exhibited at the grain boundaries in both MACl10% and MACl40%. The dark areas measured within the current images are most likely distributed between grain edges. This may be caused by the formation of PbI2 or the loss of contact between the grain and the conducting substrate.

*Sooeun Shin, Seongrok Seo, Seonghwa Jeong, Anir S. Sharbirin, Jeongyong Kim, Hyungju Ahn, Nam-Gyu Park and Hyunjung Shin
Kinetic-Controlled Crystallization of α-FAPbI3 Inducing Preferred Crystallographic Orientation Enhances Photovoltaic Performance
Advanced Science, Volume 10, Issue 14, May 17, 2023, 2300798
DOI: https://doi.org/10.1002/advs.202300798

Open Access The article “Kinetic-Controlled Crystallization of α-FAPbI3 Inducing Preferred Crystallographic Orientation Enhances Photovoltaic Performance” Sooeun Shin, Seongrok Seo, Seonghwa Jeong, Anir S. Sharbirin, Jeongyong Kim, Hyungju Ahn, Nam-Gyu Park and Hyunjung Shin is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

An explicit model to extract viscoelastic properties of cells from AFM force-indentation curves

The viscoelastic behavior of soft materials, especially cells and tissues, has been extensively investigated due to its importance in many biological and physiological processes that take place during development and even disease.*
Many techniques are used to quantify the mechanical properties of cells, among them micropipette aspiration, optical stretching, deformability cytometry and atomic force microscopy (AFM).*

The AFM, in particular, is still nowadays one of the most popular methods due to its conformity with various material types and geometries and the rather simple analysis process of the material properties.*

For a typical AFM indentation measurement, an AFM cantilever, with a distinct AFM tip shape, moves toward the sample with a predefined velocity and indents it until a prescribed force is reached. The AFM cantilever then moves upwards while detaching from the sample. The deflection and displacement signals of the AFM cantilever are processed further to extract the mechanical properties of the sample. Generally, a Hertzian model is fitted to the approach part of the force-indentation curves to quantify the apparent Young’s modulus.*

When applying the Hertzian model, few assumptions need to be considered, such as the material being homogeneous, isotropic, and linearly elastic. *

Cells and tissues, however, show not only elastic but also viscous behavior that is evident from the hysteresis between the approach and retraction segments of the force-indentation curve. Consequently, assessing this viscoelastic behavior is imperative for understanding the complex nature of biological matter.*

A number of studies utilized AFM to measure the viscoelastic properties of cells in both time and frequency domains.*

Ideally, to investigate the whole range of the viscoelastic behavior one needs to probe the material for a long time and observe its response or apply oscillatory signals and evaluate its phase lag. These approaches require the user to alter the probing method and add several steps to account for the time-dependent drift or the effect of the hydrodynamic drag of the surrounding medium. On top of that, in many of studies, the biological materials were probed with a linear approach followed by immediate retraction. The force-indentation curves from these studies were used to evaluate the apparent elastic modulus of the probed material using the standard Hertzian model. However, additional information concerning energy dissipation can still be extracted from the same curves to evaluate the viscoelasticity of the material.*

In the article “An explicit model to extract viscoelastic properties of cells from AFM force-indentation curves”, Shada Abuhattum, Dominic Mokbel, Paul Müller, Despina Soteriou, Jochen Guck and Sebastian Aland propose a new fitting model to extract the viscoelastic properties of soft materials from AFM force-indentation curves. *

To construct the explicit relation of force and indentation, the authors first use a generalization of Maxwell and Kelvin-Voigt models to describe soft materials, and numerically simulate the indentation of such material with a spherical indenter. *

Shada Abuhattum et al. show that the proposed Kelvin-Voigt-Maxwell (KVM) model adequately captures the force-indentation curves of materials having different mechanical characteristics. *

Based on the simulation results, Shada Abuhattum et al. further propose an explicit force-indentation relation to be fitted to the force-indentation curves. This explicit relation simplifies the association of the mechanical properties with physically meaningful components and processes.
Finally, the authors apply the fitting model to a number of samples, including poroelastic and viscoelastic hydrogels as well as HeLa cells in two different cell cycle phases, interphase and mitotic. *

Shada Abuhattum et al. demonstrate that the distinct nature of the hydrogels, arising from the different crosslinking mechanisms, can be described with the fitting model. For the HeLa cells, the mitotic cells had a higher apparent elasticity and a lower apparent viscosity, implying a stiffer actin cortex and a diluted cytoplasm protein concentration, when compared with interphase cells.*

Their findings demonstrate that the proposed model can reliably extract viscoelastic properties from conventional force-indentation curves. Moreover, the model is able to assess the contribution of the different elastic and viscous elements, and thus allows a direct comparison between the viscoelastic nature of different materials.*

AFM measurements were preformed using a commercially available Atomic Force Microscope. To indent the samples, NanoWorld Pyrex-Nitride tipless AFM cantilevers PNP-TR-TL with a nominal spring constant of 0.08 mN/m were modified by gluing 5 μm diameter polystyrene beads to the underside of the AFM cantilevers using two component glue.*

The AFM cantilevers were calibrated prior to each experiment using the thermal noise method and their accurate spring constant ranged between 0.047-0.059 mN/m. For PAAm and agarose hydrogels, the AFM cantilever was lowered with a constant velocity (5, 10, or 15 μm/s) toward the surface of the sample until a force of 2 nN for agarose and 4 nN for PAAm was reached. These force set points accounted for an indentation in the range of 0.5–1 μm. For HeLa cells, the AFM cantilever was lowered with a constant velocity of 2 μm/s and the cells were indented until a force of 2 nN was reached, which accounted for an indentation depth in the range of 0.5–1.5 μm.*

Graphical abstract for the article “An explicit model to extract viscoelastic properties of cells from AFM force-indentation curves” by Shada Abuhattum, Dominic Mokbel, Paul Müller, Despina Soteriou, Jochen Guck and Sebastian Aland consisting of 4 squares. showing a symbol for numerical simulations in the top left square, an arrow points to the bottom left square showing a graph and a formula as symbols for fitting algorithm a further arrow points to the bottom right square symbolizing the extraction of viscoelastic properties. The pictures in this square show on the left a drawing of the end of a tipless AFM cantilever on which a sphere is glued pressing on a cell, on the right of this picture there is another picture showing the end of a tipless AFM cantilever on which a sphere is glued pressing on a sphere or bead, underneath a graph symbolizing the mechanical properties of hydrogels is shown. Above this square on the top right a graph with a symbol for the mechanical behavior of the indented material is shown.NanoWorld Pyrex-Nitride tipless AFM cantilevers PNP-TR-TL with a nominal spring constant of 0.08 mN/m were modified by gluing 5 μm diameter polystyrene beads to the underside of the AFM cantilevers using two component glue were used for the atomic force microscopy indentation measurements described in the cited article. — Graphical abstract for the article “An explicit model to extract viscoelastic properties of cells from AFM force-indentation curves” by Shada Abuhattum at al. 2022. NanoWorld Pyrex-Nitride tipless AFM cantilevers PNP-TR-TL with a nominal spring constant of 0.08 mN/m were modified by gluing 5 μm diameter polystyrene beads to the underside of the AFM cantilevers using two component glue were used for the atomic force microscopy indentation measurements described in the cited article.

*Shada Abuhattum, Dominic Mokbel, Paul Müller, Despina Soteriou, Jochen Guck and Sebastian Aland
An explicit model to extract viscoelastic properties of cells from AFM force-indentation curves
iScience, Volume 25, ISSUE 4, 104016, April 15, 2022
DOI: https://doi.org/10.1016/j.isci.2022.104016

The article “An explicit model to extract viscoelastic properties of cells from AFM force-indentation curves” by Shada Abuhattum, Dominic Mokbel, Paul Müller, Despina Soteriou, Jochen Guck and Sebastian Aland is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third-party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit https://creativecommons.org/licenses/by/4.0/.

Electrically and mechanically driven rotation of polar spirals in a relaxor ferroelectric polymer

Flux-closure structures, vortices/antivortices, skyrmions, and merons in oxides, metals and polymers represent non-trivial topologies in which a local polar/magnetic order undergoes quasi-continuous spatial variations in a host crystal lattice. These structures are now extensively studied due to emergent functionalities, but the application of electrical/mechanical fields has so far only served to destroy the polar topologies of interest. *

Topology created by quasi-continuous spatial variations of a local polarization direction represents an exotic state of matter, but field-driven manipulation has been hitherto limited to creation and destruction. *In the article “Electrically and mechanically driven rotation of polar spirals in a relaxor ferroelectric polymer” Mengfan Guo, Erxiang Xu, Houbing Huang, Changqing Guo, Hetian Chen, Shulin Chen, Shan He, Le Zhou, Jing Ma, Zhonghui Shen, Ben Xu, Di Yi, Peng Gao, Ce-Wen Nan, Neil. D. Mathur and Yang Shen report that relatively small electric or mechanical fields can drive the non-volatile rotation of polar spirals in discretized microregions of the relaxor ferroelectric polymer poly(vinylidene fluoride-ran-trifluoroethylene).*

These polar spirals arise from the asymmetric Coulomb interaction between vertically aligned helical polymer chains, and can be rotated in-plane through various angles with robust retention. *

Given also that their manipulation of topological order can be detected via infrared absorption, Mengfan Guo et al.’s work suggests a new direction for the application of complex materials. *

Mengfan Guo et al. produced a 100-nm-thick monolayer of face-on lamellae with vertically aligned polymer chains by melt-recrystallizing spin-coated thin films of P(VDF-TrFE).

The resulting melt-recrystallized thin film of the relaxor ferroelectric polymer was characterized by the authors using a commercial atomic force microscope for in-plane piezo-response force microscopy (IP-PFM).

NanoWorld Platinum Iridium coated Arrow-CONTPt AFM probes (typical resonant frequency: 14 kHz, typical force constant: 0.2 N/m, typical AFM tip radius 25 nm) were used for the in-plane (IP) PFM tests and the PFM lithography tests.

For piezo-response force microscopy (PFM) imaging, Vector mode was used where AFM tips were modulated at around 240 kHz for IP imaging, with the AC voltage set at 2 V. The images obtained by Vector Mode were double checked by using dual AC resonance tracking (DART) mode and the patterns could be reproduced. *

For angle-resolved IP-PFM tests, the rotation of sample was controlled by a protractor. To ensure identical position was imaged after rotating the sample, the authors made cross-scratches as a mark on the sample surface in advance. This method was applied to locate the scanning position in other situations if Mengfan Guo et al. had to move the sample in between the scanning probe microscopic studies.

For electric-field-induced manipulations using PFM lithography, the DC voltage on AFM tip was previously edited in the software. The scan speed was set at 1.95 Hz and no AC voltage was applied during the scanning. The DC voltage was divided by film thickness (100 nm) to obtain the electric field value. And an electric field with downward direction is defined with a positive sign.

For stress-induced manipulations, the deflection value of the PFM cantilever, which is a signal from photodetector, was preset to control the stress/force applied onto the sample. The difference in deflection value between a pressed AFM cantilever and a free AFM cantilever reflects how hard the AFM tip and sample surface are pressed to each other.*

To obtain the force value F, Mengfan Guo et al. first calibrated the AFM tips by the thermal noise method, and obtain the inverse optical lever sensitivity (InvOLS) and the spring constant k of the AFM tips.

The authors also conducted a polarization analysis based on their PFM measurements. *

To obtain the nominal toroidal order evaluated by the local curvature, the obtained IP-PFM amplitude image was firstly divided into 33 × 33 arrays, and each region was then subjected to a recognition of potential domain walls and measurement of an averaged curvature radius. *

To obtain polarization maps, angle-resolved IP-PFM images were first aligned to correct spatial distortion in nanoscale measurement. Positions with specific morphological characteristics were selected as reference points to determine the coordinate. After the correction, improved angle-resolved IP-PFM phase images would be divided into 64 × 64 arrays for deriving polarization maps. *

Fig. 1 from Mengfan Guo et al. (2024) “Electrically and mechanically driven rotation of polar spirals in a relaxor ferroelectric polymer”:Observation of a microregion containing an in-plane polar spiral. a Morphology of a melt-recrystallized thin film of the relaxor ferroelectric polymer. The scale bar is 2 μm. IP-PFM phase (b) and amplitude (c) images of the same area in a exhibiting concentric ring-shaped domains in curly stripe domains. d Distribution of domain wall curvatures in the same area in a–c evidencing nominal toroidal order. It is assumed that the local polarization is parallel to the nearest domain wall so that larger curvature (denoted red) reflects stronger toroidal order. IP-PFM phase images of identical concentric ring-shaped domains with the axis along vertical (e) and horizontal (f) measurement directions. The scale bar is 0.3 μm. The curl (g) and the divergence (h) of local polarization in the same area as e and f, revealing the polar spiral topology. i Schematic stereoscopic view of a CCW polar spiral, arrows represent regions of polarization. The red/blue arrows denote the polar source/sink that spirals in/out. The white arrows represent Néel rotation along the radial direction, as shown in more detail via the inset. NanoWorld Platinum Iridium coated Arrow-CONTPt AFM probes (typical resonant frequency: 14 kHz, typical force constant: 0.2 N/m, typical AFM tip radius 25 nm) were used for the in-plane (IP) PFM tests and the PFM lithography tests. — Fig. 1 from Mengfan Guo et al. (2024) “Electrically and mechanically driven rotation of polar spirals in a relaxor ferroelectric polymer”:
Observation of a microregion containing an in-plane polar spiral.
a Morphology of a melt-recrystallized thin film of the relaxor ferroelectric polymer. The scale bar is 2 μm. IP-PFM phase (b) and amplitude (c) images of the same area in a exhibiting concentric ring-shaped domains in curly stripe domains. d Distribution of domain wall curvatures in the same area in a–c evidencing nominal toroidal order. It is assumed that the local polarization is parallel to the nearest domain wall so that larger curvature (denoted red) reflects stronger toroidal order. IP-PFM phase images of identical concentric ring-shaped domains with the axis along vertical (e) and horizontal (f) measurement directions. The scale bar is 0.3 μm. The curl (g) and the divergence (h) of local polarization in the same area as e and f, revealing the polar spiral topology. i Schematic stereoscopic view of a CCW polar spiral, arrows represent regions of polarization. The red/blue arrows denote the polar source/sink that spirals in/out. The white arrows represent Néel rotation along the radial direction, as shown in more detail via the inset.

*Mengfan Guo, Erxiang Xu, Houbing Huang, Changqing Guo, Hetian Chen, Shulin Chen, Shan He, Le Zhou, Jing Ma, Zhonghui Shen, Ben Xu, Di Yi, Peng Gao, Ce-Wen Nan, Neil. D. Mathur and Yang Shen
Electrically and mechanically driven rotation of polar spirals in a relaxor ferroelectric polymer
Nature Communications volume 15, Article number: 348 (2024)
DOI: https://doi.org/10.1038/s41467-023-44395-5

The article “Electrically and mechanically driven rotation of polar spirals in a relaxor ferroelectric polymer” by Mengfan Guo, Erxiang Xu, Houbing Huang, Changqing Guo, Hetian Chen, Shulin Chen, Shan He, Le Zhou, Jing Ma, Zhonghui Shen, Ben Xu, Di Yi, Peng Gao, Ce-Wen Nan, Neil. D. Mathur and Yang Shen is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third-party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit https://creativecommons.org/licenses/by/4.0/.