Symposium F.MT02: Multimodal, Functional and Smart Scanning Probe Microscopies for Characterization and Fabrication

Joshua Agar, Lehigh University

Practical Deep Learning in Scanning Probe Microscopy 

Written by Emma Perry 

Whilst open source machine learning algorithms promise speed and low mean square values, Joshua Agar offers a word of caution to scientists planning to use them without carefully studying the mathematics and applying a rigorous testing regime.

The analysis of the band excitation piezoresponse spectroscopy is a problem that could greatly benefit from machine learning. These experiments produce a complex 4 channel hyperspectral image.

In this talk a dictionary learning method is compared to a neural network deep autoencoder method. First the methods are applied to a toy data set and then a benchmark band excitation piezoresponse spectroscopy dataset. It is quickly seen that the dictionary learning method is the simplest and fastest. But this method fits based on the mean square error and the subtleties, that carry physical meaning, are lost. The neural network deep autoencoder method does manage to deliver the detail of the functions. In the live Q & A session Joshua Agar warns that there is no one solution for all. In your own work you must start with the simplest model, rigorously test it, and work from there.

Symposium F.SM03: Materials and Mechanics Challenges in Haptics for Human-Machine Interfaces

Han Yu, Columbia University

Mixed-Conducting Particulate Composites for High Resolution Human-Machine Interfaces

Written by Jessalyn Low Hui Ying

Human-machine interfaces should require the interfacing of soft, biocompatible probes with hard electronic devices for signal processing. However, this involves several considerations like mechanical mismatch between soft and hard materials. “Limitations of existing device-based approach for interfacing electronics suggest that a materials-based approach for making the connections is more effective,” says Han Yu. For that, she and her colleagues developed a mixed-conducting particulate (MCP) film. This MCP film consists of a controlled density of conducting polymer-based PEDOT:PSS particles, integrated within a nonconducting, biocompatible adhesive matrix. This results in a MCP film that is anisotropic conducting as well as organic and metal-free. Studies showed that the MCP was able to create an interface between soft probes and hard electronics to achieve high spatiotemporal biopotential sensing.

Moreover, as this MCP is biocompatible and with tunable electronic properties, it was also demonstrated that the MCP can directly interface with the human body and achieve high spatiotemporal resolution sensing. This was observed when MCP-coated electrodes were able to acquire spatially distinctive activity in response to fingers from the wrist, as well as localized neural spike signals from the skin surface. This highlights the application of the MCP film to achieve high spatiotemporal resolution and non-invasive acquisition of electrophysiological potential from the human skin.

Symposium F.EN08 : Scientific Basis for Nuclear Waste Management

Marc Weber, Washington State University

Assessment of Positrons for Defect Studies in UO2 and CeO2 Materials

Written by Emma Perry 

Positron annihilation spectroscopy offers an alternative perspective in the search for defects in spent nuclear fuel and its analogues. A particularly valuable perspective since unlike most mainstream techniques, it is sensitive to neutral and negatively charged, vacancy-like point defects and impurity-vacancy complexes. The detectable range is from 1 vacancy in 10⁶-10⁷ atoms to 1 vacancy in 10³ atoms. Penetration depths range from the nanometer to the micrometer. It is possible to understand the materials behavior at the surface and in the bulk.

Samples are placed into a beam of positrons. These positrons travel into the material before annihilating to produce two gamma photons each. The detector counts the energy range of these photons as a function of the depth at which they formed. S and W are shape parameters describing the photon energy function. Anion vacancy sites create potential wells to trap positrons, so most annihilation events occur at vacancy sites. The shape parameters of the gamma photon energy function depend on the number of vacancies and on the chemistry around the vacancy sites. 

By comparing plots of S against W for pure CeO₂, CeO₂-doped by 4% and 15% yttrium and those irradiated at 6 dpa and 33 dpa, it can be seen that yttrium doping causes samples to have a different annihilation site to irradiated samples. Irradiation defects are vacancies or clusters of vacancies but doping defects are yttrium clusters or Y-Ce vacancy complexes.

A further application of positron annihilation spectroscopy is to the leaching of vitrification glasses in which a third signal detects voids.

Symposium F.EN08: Scientific Basis for Nuclear Waste Management

Gabriel Murphy, Forschungszentrum Jülich GmbH1

Secondary Phase Derived Uranium Oxy-Hydroxides as Potential Host Matrices for Anionic Fission Daughters

Written by Emma Perry 

Gabriel Murphy passionately presents the preliminary results of the latest experiments at FZ Julich, unravelling the mystery of how negatively charged fission daughters become incorporated into the negatively charged matrix of secondary phases. This is a particularly important phenomena in the instance of a nuclear accident wherein it could impede radioactive release to the environment.

Work so far has focused on the layered uranium oxyhyroxide phases, shoepite and comprignacite, and the anionic fission daughter, Iodine-131. Powders of each phase were submerged in potassium iodide for 4 weeks before being studied by laser ablation mass spectroscopy. In shoepite, iodine is found uniformly distributed through the layers. In comprignicite no iodine was found, perhaps due to the potassium in this phase.

In another experiment, crystals were formed by increasing the experimental temperature and pressure and by submerging the powders in rubidium iodide. Using single crystal x-ray diffraction it was possible to infer the resulting structure and find that the intercalation mechanism is based on salt inclusion. The structures possess voids which function as positive wells that draw in negative species. Then hydroxyl groups draw in cations, in this case rubidium, to counterbalance. In shoepite there is more free space so the intercalation occurs more readily.

Ongoing work investigates other negatively charged fission daughter products, such as selenite, and many other promising secondary phases.

Symposium F.EL08: Frontiers of Halide Perovskites—Linking Fundamental Properties to Devices (Includes Symposium S.CT02—Halide Perovskites—From Lead-Free Materials to Advanced Characterization and Deposition Approaches)

Aron Walsh, Imperial College London

Beyond Dilute Imperfections—Extended Defects in Halide Perovskites

Written by Parul, Indian Institute of Technology Roorkee, India

Defects: boon or bane? Defects in traditional semiconductors were not as welcomed as in perovskite semiconductors. This lecture will give us insights into the chemistry of defects behind the photophysics of halide perovskites. This year power conversion efficiency of solar cells with semiconductors exceeded 25%. The comparative studies between the power conversion efficiency of GaAs and perovskite had given clues to the scientific community that reasons for less efficiency might lie in the concept of defects. Three defects are discussed by Aron Walsh, including point defects, grain boundaries, and phase intermixing. Furthermore, there are types of point defects according to interaction with charge carriers i.e. inert defects, active traps, and active recombination. Inert defects are the ones which means their presence does not lead to any interaction with charge carriers. Active traps can capture holes and active recombination can capture an electron and hole. The effective mass of perovskite is quite low, and the dielectric constant is quite large which limits the interaction of defects with charge carriers, therefore, empty defects are not so detrimental in perovskites.

Theoretical to experimental consideration is discussed extensively in this talk. Crystal defects cause rapid non-radiative recombination even in low concentrations. First principle modeling was used to show a band structure that gives rise to the radiative limit. The first photovoltaics application was applied successfully on Cu₂ZnSnS₄ (CZTS) with interesting results; ideally, efficiency should be around 32% but the population of point defects leads to efficiency down to 20.9%. A similar methodology was applied to perovskites. Point defects in perovskites are mainly discussed in terms of vacancies, interstitials, and antisites. Vacancies often dominate in the case of perovskite present in higher concentration.

Spatially resolved technique photoemission electron microscopy gives information about the mapping of the electronic density of states and their sub-band gaps. Spatial mapping of high-quality films shows photovoltaic performance is limited by exterior grain boundaries in perovskite. Electron microscopy proves traps are not distributed uniformly but selected near grain boundaries. In the later part of talk, phase intermixing is discussed, particularly composition between the yellow and black phase of perovskite material. In a nutshell, defect tolerance is used as a method to address critical issues.   

Symposium F.MT02 : Multimodal, Functional and Smart Scanning Probe Microscopies for Characterization and Fabrication

Krislon Voitchovsky

Tracking the Dynamics of Liquids and Solutes at Solid Nano-Interfaces 

Written by Emma Perry

The use of atomic force microscopy (AFM) to make a three-dimensional (3D) map of a hydration landscape at a solid liquid interface is well established. Now Krislon Voitchovsky and his colleagues have developed a “vortex scanner” to understand interfacial liquid flow. The small centimeter square device may look quotidian but when placed on top of a normal AFM scanner it can add a lateral vibration of at least 36 kHz. By tuning this vibration to the normal vertical vibration of the tip it is possible to understand the direction that fluid prefers to flow.

Initial experiments have studied the flow of potassium through graphene oxide flakes and the sieving effect has been observed for magnesium. From these experiments it was possible to calibrate for small asymmetries in the apparatus. The technique also correctly identified the no slip boundary condition characteristic of muscovite. For softer samples such as aquaporin a gliding effect has been observed. Once better understood this could prove to be advantageous in regards to sample preservation.

Symposium F.SM02: Hydrogel Technology for Humans and Machines

Abdon Pena-Francesch, University of Michigan

Bioinspired Self-Healing Protein Hydrogels for Soft Machines

Written by Jessalyn Low Hui Ying

Soft robotics is an emerging field, involving materials that are flexible and compliant to overcome challenges faced by traditional hard robotics. These properties, however, result in soft robots being vulnerable to mechanical damage, which hinders their performance. In this talk, Abdon Pena-Francesch presents his research work of how he and his research team take inspiration from nature, in particular the squid sucker ring teeth, of which proteins are semicrystalline, to design protein-based functional materials with self-healing properties.

First, the researchers sequenced the protein complexes from the squid ring teeth to design a squid-inspired master sequence. This sequence consisted of two blocks—an amorphous and a crystalline block, repeated in tandem. Interestingly, it was discovered that this segmented amino acid structure drives the self-assembly of the protein, stabilized by β-sheet nanocrystals networks formed from the crystalline segment. Furthermore, the formation of these β-sheet nanocrystals, which act as physical crosslinks, is fast and reversible. This means that when the protein network is damaged, it can self-heal quickly, driven by the β-sheet nanocrystals and diffusion from heat. “This is very important because most post-healing materials have the limitations of strength or kinetics,” says Pena-Francesch. “With this protein materials, by playing and optimizing the sequence and this β-sheet nanostructure, we can achieve the best of both worlds. We can achieve fast but also strong healing.”

The researchers validated how this designed bioinspired protein hydrogels performed under different types of damage, and it was demonstrated to exhibit excellent self-healing properties under scratch damage, puncture damage, and cut damage. To demonstrate its use in soft robotics, the researchers also built a soft actuator using this protein and subsequently fabricated this into soft grippers and artificial muscles, which exhibited good performance. This demonstrates the potential of these bioinspired protein hydrogels as self-healing materials for use in soft robotics.  

Symposium F.EL08: Frontiers of Halide Perovskites—Linking Fundamental Properties to Devices (Includes Symposium S.CT02—Halide Perovskites—From Lead-Free Materials to Advanced Characterization and Deposition Approaches)

Felix Deschler, Technical University Munich

Novel hybrid perovskites for spin control and magneto-optics

Written by Parul, Indian Institute of Technology Roorkee, India

Do you know trendy Ruddlesden-Popper layered structure of perovskite? If not, then this talk will help you in better understanding of such structures along with their properties. Results on spin-dynamics and magneto-optics were discussed. In the two-dimensional (2D) layered structure, lead iodide is separated by large organic molecules which will slice curt the molecule into lower dimensions. Strong excitonic formation can be easily observed on shifting from absorption spectra of 3D to 2D structures. Spin-orbit coupling, and non-centric structure makes the electronic states complex. Compositional and structural variations are tuned to get structure with favorable spin dynamics. Simple chemical modifications on the precursor molecule can generate a non-centric layered structure.

There is strong contribution from exciton scattering to spin relaxation. Also, spin polarized excitons leads to circularly polarized emission which is mandatory before understanding of the mechanism. Lifetime of materials also depends on excitonic formation.

This is the first report in which 5% of lead is replaced by magnetically active dopants, that is, manganese-doped perovskite films. EPR of manganese-doped perovskite prove that it is magnetically active. Also, it is evident that low temperatures are favorable for such materials. This talk shows how optical excitations interact with elemental spins in these materials. Circular polarization of 15% is achieved in excitonic photoluminescence at low temperatures. This gives strong magnetic field dependence.

In short, researchers are trying to get interactions including magnetic moments and electronic states to get tailored interactions.

Symposium F.GI01: Special Symposium on Materials Approaches for Tackling COVID-19

Roberto de la Rica, Health Research Institute of the Balearic Islands

Screening and Management of COVID-19 with Paper-Based Nanoparticle Tests

Written by Jessalyn Low Hui Ying

Current conventional methods for the diagnosis of COVID-19 require a nasopharyngeal swab test. However, this must be done by trained professionals, therefore limiting the number of tests done per day. Furthermore, it is important to detect biomarkers like cytokines that can prognosticate outcomes of COVID-19, so as to achieve better distribution of healthcare resources. In this talk, Roberto de la Rica reports the development of paper-based biosensors that can be used for non-invasive detection of COVID-19 antigens, as well as detection of prognostic biomarkers in blood and respiratory samples from patients.

In this paper-based biosensor, a piece of filter paper is first modified with polystyrene sulfonate (PSS), and then a drop of antibody-decorated gold nanoparticles. The PSS prevents the irreversible interaction of nanoparticles with the paper, which was validated using scanning electron microscopy. The gold nanoparticles from the paper reservoir can then be transferred to another substrate just by pressing against it. For the diagnosis of COVID-19, this receiving substrate can be a face mask. As the nanoparticles are decorated with anti-COVID-19 antibodies, antibody-antigen interactions will occur resulting in a color change should the target antigens be present. The level of antigens is reflected in the intensity of the color which can then be detected through a mobile application. When tested, the biosensor achieved a high specificity of over 0.97 and sensitivity over 0.7 for COVID-19 antigen detection.

This biosensor can also be used to detect the interleukin 6 (IL-6) protein, which is a cytokine that has been shown to be part of the cytokine storm in COVID-19 patients. The biosensor was able to achieve a very low limit of detection of 10^-3 pg/mL, and could detect IL-6 levels higher than 17 pg/mL in blood and higher than10 pg/mL in respiratory samples. “The ability of detecting this cytokine in blood and respiratory samples is important in order to evaluate both systemic and local inflammation in COVID-19 which are related but do not have to be necessarily the same,” says de la Rica.

Symposium F.NM03: Nanotubes, Graphene and Related Nanostructures

Yoke Khin Yap, Michigan Technological University

Ultra-High Brightness Fluorophores Based on Nanotubes

Written by Jessalyn Low Hui Ying

Flow cytometry is a laser-induced fluorescence technique commonly used for cell analysis. However, this technique has two major issues, especially in multicolor flow cytometry— autofluorescence, which contributes to background fluorescence noise, and spillover, which refers to signal overlapping. These issues can be resolved by using high-brightness fluorophores (HBFs). In this talk, Yoke Khin Yap presented on a novel nanotube technology that is able to produce these HBFs.

The principle behind this is that fluorescence intensity can be increased by enhancing the molar extinction coefficient. To achieve this, the researchers conjugated large amounts of dye molecules on nanotubes to form the fluorophore. Here, boron nitride nanotubes (BNNTs) are used as they are electrically insulating and optically transparent, thus preventing quenching and enhancing performance, unlike in carbon nanotubes (CNTs). The cylindrical shape also allows for high quantity loading of dye molecules. To conjugate the dye molecules onto the BNNTs, DSPE-PEG-amine linkers are first labeled onto the BNNT surface via non-covalent functionalization. Various dye molecules can be conjugated, such as FITC and Cy5. Additionally, antibodies can be conjugated for specific antigen detection.

The intensity of the fluorophore can be tuned by changing the concentration of dye molecules on the BNNT. Results showed that these produced HBFs offer a brightness of up to a thousand times higher than commercial dyes. This highlights the potential of these HBFs for use in enhancing capabilities of flow cytometry.