Keynote Abstracts

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Prof. Sergei V. Kalinin: "Machine Learning for Scanning Probe Microscopy for Materials Discovery"

 

Department of Materials Science and Engineering, University of Tennessee, Knoxville

Prof. Sergei V. Kalinin: "Machine Learning for Scanning Probe Microscopy for Materials Discovery"

 

Department of Materials Science and Engineering, University of Tennessee, Knoxville

          Machine learning and artificial intelligence (ML/AI) are rapidly becoming an indispensable part of physics research, with domain applications ranging from theory and materials prediction to high-throughput data analysis. However, the constantly emerging question is how to match the correlative nature of classical ML with hypothesis-driven causal nature of physical sciences. In parallel, the recent successes in applying ML/AI methods for autonomous systems from robotics through self-driving cars to organic and inorganic synthesis are generating enthusiasm for the potential of these techniques to enable automated and autonomous experiment (AE) in imaging.

            In this presentation, I will discuss recent progress in automated experiment in scanning probe microscopy, ranging from feature to physics discovery via active learning. The applications of classical deep learning methods in streaming image analysis are strongly affected by the out of distribution drift effects, and the approaches to minimize though are discussed. We further present invariant variational autoencoders as a method to disentangle affine distortions and rotational degrees of freedom from other latent variables in imaging and spectral data. The analysis of the latent space of autoencoders further allows establishing physically relevant transformation mechanisms. I will further illustrate transition from post-experiment data analysis to active learning process, including learning structure-property relationships and materials discovery in composition spread libraries. Here, the strategies based on simple Gaussian Processes often tend to produce sub-optimal results due to the lack of prior knowledge and very simplified (via learned kernel function) representation of spatial complexity of the system. Comparatively, deep kernel learning (DKL) methods allow to realize both the exploration of complex systems towards the discovery of structure-property relationship, and enable automated experiment targeting physics (rather than simple spatial feature) discovery. The latter is illustrated via experimental discovery of microstructural origins of ferroelectric domain dynamics and ferroelectric non-linearities in PFM. For composition spread libraries, I will demonstrate the combination of the structured Gaussian process and reinforcement learning, the approach we refer to as hypothesis learning. The further applications of hypothesis learning towards physics discovery in SPM are discussed.

About Sergei V. Kalinin:

            Sergei Kalinin is a professor at the Department of Materials Science and Engineering at the University of Tenness, Knoxville, following 20-year career at Oak Ridge National Laboratory. He received his MS degree from Moscow State University in 1998 and Ph.D. from the University of Pennsylvania (with Dawn Bonnell) in 2002. His research presently focuses on the applications of big data and artificial intelligence methods in atomically resolved imaging by scanning transmission electron microscopy and scanning probes for applications including physics discovery and atomic fabrication, as well as mesoscopic studies of electrochemical, ferroelectric, and transport phenomena via scanning probe microscopy.

            Sergei has co-authored >650 publications, with a total citation of >42,000 and an h-index of ~100. He is a fellow of MRS, APS, IoP, IEEE, Foresight Institute, and AVS; a recipient of the Blavatnik Award for Physical Sciences (2018), RMS medal for Scanning Probe Microscopy (2015), Presidential Early Career Award for Scientists and Engineers (PECASE) (2009); Burton medal of Microscopy Society of America (2010); 4 R&D100 Awards (2008, 2010, 2016, and 2018); and a number of other distinctions.

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Dr. Bizan Balzer: "Stereographic force spectroscopy: a tool to study the directional anisotropy of molecular systems"

 

Institute of Physical Chemistry, Cluster of Excellence livMatS @ FIT and FMF

University of Freiburg, Germany

The atomic force microscope (AFM) is a versatile tool for imaging and force spectroscopy at the nanoscopic to mesoscopic scale. It is used to understand adhesion and friction properties of molecular systems and polymer mechanics in liquid environment.

Based on the development of functionalization protocols for covalent binding of single polymers to AFM cantilever tips [1] the adhesion and mechanics of single polymers has been studied on a great variety of solid substrates under liquid conditions.[2,3] Now we have extended this method towards stereographic force spectroscopy using angle-dependent pulling to monitor direction dependent properties of single polymers.[4,5]

Here, I will present stereographic force spectroscopy of single polymers to study the multidirectional and velocity dependent rupture of covalent bonds, coordination bonds and π-π stacking as well as receptor-ligand-protein complexes.

Our studies are fundamental to understand the role of force application direction for molecular adhesion and friction. This will help to discover the molecular origin of adhesion and friction and to design substrates with optimized interfacial properties.

References:

 

[1] Kolberg et al., J. Vis. Exp., 157, e60934 (2020).

[2] Horinek et al., PNAS, 105(8), 2842–2847 (2008).

[3] Kolberg, et al., J. Am. Chem. Soc., 141 (29), 11603 (2019).

[4] Balzer et al., Angew. Chem. Int. Ed. 52,25, 6541 (2013).

[5] Balzer et al., Soft Materials 12, S106-S114 (2014).

https://www.balzer-lab.com

Twitter: @BalzerBizan

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Dr. Nilanthy Balakrishnan: "Physics and Applications of Indium Selenide Based van der Waals Heterostructures"

 

Keele University, United Kingdom 

The pressing demand for device minaturizaton can be fulfilled by two-dimensional (2D) semiconducting materials. Among the 2D semiconducting materials, indium selenide (InSe) compounds are attracting great attention due to their desirable electronic and optical properties [1-2]. Indium selenide compounds can exist with different stoichiometries (e.g. InSe, In2Se3 and In4Se3) and polytype phases (α, β, γ, etc.), providing band gaps tunable from the near infrared to the visible range (1.2 - 2 eV) of the electromagnetic spectrum [2], a high electron mobility at room temperature (> 0.1 m²/Vs) [1], room temperature ferroelectricity [3] and strong carrier correlations in atomically thin layers due to an inverted “Mexican hat” valence band [4].

Here, we present our recent work on InSe based van der Waals heterostructures of interest for optoelectronics, thermoelectrics and nanoelectronics. Both InSe/GaSe and InSe/In2O3 heterojunctions exhibit room temperature electroluminescence and spectral response from the near-infrared to the visible and near-ultraviolet ranges [5-6]. On the other hand, the nanoscale thermal properties of InSe layers show an anomalous low and anisotropic thermal conductivity, which is smaller than that of low-κ dielectrics, such as silicon oxide [7]. The thermal response of free-standing InSe layers and layers supported by a substrate, reveals the role of interfacial thermal resistance, phonon scattering, and strain. These thermal properties are critical for future emerging technologies, such as field-effect transistors that require efficient heat dissipation or thermoelectric energy conversion with both low thermal conductance and high electron mobility 2D materials, such as InSe.

Furthermore, we report on the ferroelectric semiconductor α-In2Se3 embedded between two single-layer graphene electrodes. We show how the ferroelectric polarization of the In2Se3 layer can modulate the transmission of electrons across the graphene/In2Se3 interface, leading to memristive effects that can be controlled by wither applied voltages or by light [8].


References


1. Bandurin, D.A., et al., Nat. Nanotechnol.. 12, 223-227 (2017).
2. Huang, W., et al., Cryst. Eng. Comm.. 18, 3968-84 (2016).
3. Zheng, C., et al., Sci. Adv., 4, eaar7720 (2018).
4. Debbichi, L., et al., J Phys Chem Lett, 6, 3098-103 (2015).
5. Balakrishnan, N., et al., Adv. Opt. Mater. 2, 1064 – 9 (2015).
6. Balakrishnan, N., et al., 2D Mater., 4, 025043 (2017).
7. Buckley, D., et al., Adv. Funct. Mater., 31, 2008967 (2020).
8. Xie, S., et al., 2D Mater., 8, 045020 (2021).

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Prof. Brian Rodriguez: "Tailoring Collagen Piezoelectricity"

 

University College Dublin, Ireland 

Piezoelectric biomaterials are being considered for numerous sensing, nanoelectronics, and tissue engineering applications. At the same time, biomedical applications of inorganic piezo- and ferroelectric materials continue to grow. Progress in investigating and exploiting the piezoelectric properties and surface charge of these functional materials will be presented, with a focus on understanding the origin and role of piezoelectricity in type I collagen using piezoresponse force microscopy (PFM). Specifically, the role moisture content and crosslinking on electromechanical coupling in type I collagen will be presented.

Type I collagen piezoelectricity, as determined by PFM, was found to persist at high (> 60%) relative humidity (Figure 1). The piezoresponse was highest at intermediate values of relative humidity, suggestive of a role of water on the measured signal. It was also found that non-enzymatic crosslinking, which may have implications for understanding the functional changes of collagen, e.g., during aging, led to reduced piezoelectricity; crosslinked samples showed improved stability in collagenase, supporting that crosslinking took place.

Additionally, aligned collagen films prepared with different fibril sizes have been investigated to explore whether the characteristic ‘piezoelectric domain’ size of the films can be controlled in a deterministic way. Engineered collagen films with aligned fibrils and uniform polarization might be useful in, e.g., energy harvesting and tissue engineering applications. The interaction of such piezoelectric domains (and ferroelectric domains in the case of lithium niobate surfaces) with cell migration and the influence of topographic cues are explored.

Figure 1: Topography (left), PFM amplitude (middle), and PFM phase (right) images of tendon collagen recorded under different relative humidity values.

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Dr. Tobias Cramer: "Taking multichannel AFM images of electronic materials while they break"

 

Department of Physics and Astronomy, University of Bologna, Viale Berti Pichat 6/2, 40127 Bologna, Italy 

Stretchable conductors are of crucial relevance for emerging technologies such as wearable electronics, low-invasive bioelectronic implants or soft actuators for robotics.¹ Understanding of defect formation and deterioration of electronic properties of such materials during tensile strain cycles is important to optimize stretchable electronic materials.

Here we present an atomic force microscopy (AFM) method that provides multichannel images of surface morphology, conductivity, and elastic modulus during sample deformation. To develop the method, we investigate in detail the mechanical interactions between the AFM tip and a stretched, free-standing thin film sample. Our findings reveal the conditions to avoid artifacts related to sample bending modes or resonant excitations. As an example, we analyze strain effects in thin gold films deposited on a soft silicone substrate.² Our technique allows to observe the details of microcrack opening during tensile strain and their impact on local current transport and surface mechanics. We find that although the film fractures into separate fragments, at higher strain a current transport is sustained by a tunneling mechanism. The microscopic observation of local defect formation and their correlation to local conductivity will provide novel insight to design more robust and fatigue resistant stretchable conductors.³

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AFM investigations on a thin film deposited on an elastic substrate during tensile strain. Three imaging modes are explored: height, conductivity and surface stiffness.

 

1.          Decataldo, F. et al. Stretchable Low Impedance Electrodes for Bioelectronic Recording from Small Peripheral Nerves. Sci. Rep. 9, 1–9 (2019).

2.          Cortelli, G. et al. Atomic Force Microscopy Nanomechanics of Hard Nanometer-Thick Films on Soft Substrates: Insights into Stretchable Conductors. ACS Appl. Nano Mater. 4, 8376–8382 (2021).

3.          Cortelli, G. et al. In-situ force microscopy to investigate fracture in stretchable electronics: insights on local surface mechanics and conductivity, https://doi.org/10.48550/arXiv.2202.10119

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Dr. James Kerfoot: "Exploring the electrical and mechanical properties of layered materials using atomic force microscopy"

Max Planck Institute of Colloids and Interfaces, Potsdam Science Park, Potsdam-Golm, Germany

J. Kerfoot1* and V. V. Korolkov¹  *pse@parksystems.com

¹ Park Systems UK Ltd, MediCity Nottingham, D6 Thane Rd, Nottingham, UK, NG90 6BH

Among all state-of-the-art characterisation techniques, atomic force microscopy (AFM) is particularly powerful tool in the study of layered materials since it can be used to measure both atomic registry of such layered materials heterostructures and their associated functional properties at sub-nanometre length scales. In this talk, we provide a brief overview of the state-of-the-art in AFM-based characterisation of layered materials before proceeding to discuss two examples of interest both for fundamental studies and applications.

In the case of our first example, controlling the twist angle between flakes of layered materials leads to interfacial ferroelectricity in the case of parallel stacked hexagonal boron nitride (hBN) [1-3]. We provide an account of the fabrication of such parallel stacked hBN samples using a home-built transfer setup [4] and the mapping of ferroelectric domains in these samples using both electrostatic force microscopy (EFM) and Kelvin probe force microscopy (KPFM). We discuss different ways in which such domains may be mapped and manipulated using different AFM modalities. In particular, we address the measurement of these domains with different derivatives of KPFM (both amplitude modulated and sideband KPFM inside and outside of vacuum) with higher contrast extracted for sideband KPFM. We conclude by studying the manipulation of domains both electrically by applying bias via the tip and mechanically using contact mode AFM [5].

 

Fig. 1. Parallel stacked hBN may be formed by breaking and restacking exfoliated layers mechanically (a). Once formed, these structures may be characterised using AFM (b) to map the morphology of ferroelectric domains (c).

In our second example, we use high voltage conductive AFM to study the dielectric breakdown of hBN and mica in order to extract the dielectric strength of these materials and study the morphology and current voltage characteristics of layers before and after dielectric breakdown. We go on to apply this technique to large area thin films and demonstrate the ability of this technique to measure both the ‘intrinsic’ dielectric properties of thin films and at defects addressed individually via the probe. In summary, we exemplify the exceptional utility of AFM to provide insightful information for layered systems of both fundamental and applied interest.

 

[1] C. R. Woods et al. Nat Commun. 12, 347 (2021).

[2] K. Yasuda et al. Science 372, 6549 1458 (2021)

[3] M. Vizner Stern et al. Science 372, 6549, 1462, (2021).

[4] Q. Zhao et al. J. Phys. Mater. 3 016001 (2020)

[5] R. Ribeiro-Palau et al. Science 361, 6403, 690 (2018)

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Stanislas Rohart: "Current induced dynamics of skyrmions in ultrathin ferromagnetic films"

Laboratoire de Physique des Solides, Université Paris-Saclay, CNRS, Orsay, France

Skyrmions, a magnetic texture characterized by its unique topological charge, have attracted recently a concerted effort to study how they could be controlled and moved in ferromagnetic films, and open a new route for spintronics and information storage technologies [1]. Stabilization, nucleation and motion are the three challenges toward such an achievement [2]. In this talk, we show how skyrmions can be stabilized in ultrathin magnetic films. The films are in contact with high spin-orbit interaction, which induce a chiral magnetic exchange, the Dzyaloshinskii-Moriya interaction, that promotes magnetic non collinearity and the particular topology required for skyrmions. The magnetic textures are observed with magnetic force microscopy. In samples patterned in nanotracks, electrical current is injected and moves the skyrmions with velocities of few tens of meter per seconds for current densities of few 10¹¹A/m² [3,4]. Additionally, the skyrmions display an intrinsic deflection, related to the topology. In recent results, we show that, in synthetic antiferromagnetic films (two ferromagnetic layers coupled antiferromagnetically through a spacer), the deflection can be compensated thanks to the formation of a bound state of two skyrmions with opposite topological numbers [5].

[1]  A. Fert, V. Cros and J. Sampaio Nature Nano. 8 (2013) 152

[2]  J. Sampaio et al. Nature Nano 8 (2013) 184422

[3] A. Hrabec et al. Nature Com. 8 (2017) 15765

[4] S. Mallick et al. under submission

[5] S. Panigrahy et al. in preparation

 

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Prof. Dr. Lukas Eng: "Infrared and THz spectroscopy down to the 1-nm length scale"

Lukas M. Eng ¹,²

¹ Institute of Applied Physics, TU Dresden, Nöthnitzerstr. 61, 01187 Dresden, Germany
² ct.qmat: Würzburg-Dresden Cluster of Excellence - EXC 2147, TU Dresden, Germany

Contact Email lukas.eng@tu-dresden.de

Scattering-type near-field optical microscopy (s-SNOM) relies on tapping or non-contact scanning force microscopy (SFM) where illuminating the cantilever tip with appropriate laser light induces a tip dipole. That dipole in turn, induces an image dipole into the sample under test, with the overall interaction then being scattered to the detector. Both elastic and inelastic interactions thus can be read and quantified hereafter. We have extended this concept to very low energies of ~1 meV only, needing special THz laser light sources, i.e. the using the Free-electron laser at the Helmholtz Center Dresden-Rossendorf (HZDR).

The reliable probing of nanoscale optical properties by s-SNOM, however, is often obscured by a manifold of local artefacts, with local electronic potential variations playing the major role into that game. In this contribution, we illustrate how to compensate for electrostatic artefacts in-situ, by elegantly combining s-SNOM with the capabilities of Kelvin-Probe Force Microscopy (KPFM) operated in frequency-modulation (FM) mode [1]. Not only are we then able to monitor nearly electronic-artefact-free near-field signals at any of the different higher harmonics demodulated in s-SNOM, but furthermore, also to gather quantitative local information on the sample surface electrostatic conditions quasi for free [2,3]. We will introduce into this technical merger [2] by demonstrating its necessity with a manifold of different s-SNOM examples, i.e. s-SNOM data recorded on pure metals (Au), semiconducting (Si) and dielectric (SiO2) samples, on different ferroelectric surfaces [2,3,4] and multiferroics [5] both at ambient and liquid-helium temperatures [4], but equally when investigating phase-change materials [6]. Notably, we show both resonant and non-resonant optical sample excitations in these experiments, hence demonstrating the great benefits of our s-SNOM/KPFM combinations at FELBE and TELBE when performing investigations over the broad wavelength range from VIS down to < 1 THz [7].

[1]   U. Zerweck et al., Phys. Rev. B 71, 125424 (2005); https://doi.org/10.1103/PhysRevB.71.125424.

[2]   T. Nörenberg et al., APL Photon. 6, 036102 (2021); https://doi.org/10.1063/5.0031395.

[3]   J. Döring et al., Nanoscale 10, 18074 (2018); https://doi.org/10.1039/C8NR04081H.

[4]   L. Wehmeier et al., Phys. Rev. B 100, 035444 (2019); https://doi.org/10.1103/PhysRevB.100.035444.

[5]   D. Lang et al., Rev. Sci. Instrum. 89, 033702 (2018); https://doi.org/10.1063/1.5016281.

[6]   J. Barnett et al, Nano Lett. 21, 9012 (2021); https://doi.org/10.1021/acs.nanolett.1c02353.

[7]   S.C. Kehr et al., Synch. Rad. News 30, 31 (2017); https://doi.org/10.1080/08940886.2017.1338421.

 

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Thomas Modes: "Piezoelectric force microscopy on magnetron-sputtered (Al,Sc)N layers"

T. Modes, O. Zywitzki, S. Barth, H. Bartzsch

Fraunhofer Institute for Organic Electronics, Electron Beam and Plasma Technology FEP, Dresden, Germany

Aluminum scandium nitride layers with hexagonal wurtzite structure and a polar c axis were deposited by reactive pulse magnetron sputtering on silicon wafers. By incorporation of scandium into wurtzite structure the piezoelectric charge constant and electromechanical coupling are improved, which are important for application in radio frequency filters.The piezoelectric properties were determined by a Berlincourt piezometer (PM300, Piezotest) and by piezoelectric force microscopy PFM in off-resonance mode (AFM NX20, Park System). Piezometer measurements revealed for pure AlN layers a piezoelectric charge coefficient d33 of 8.4 pC/N. With increasing scandium content, the piezoelectric charge coefficient increased up to 27 pC/N for a scandium concentration of 43 %. The results of PFM measurements show that mean PFM amplitude is directly proportional to the piezoelectric charge coefficient determined by the piezometer.

In contrast to the piezometer, the PFM measurement allows a high lateral resolution of the piezoelectric properties. For non-optimized samples the PFM amplitude shows areas with lower piezoelectric response than the surrounding matrix. This can be attributed to grains with abnormal grain growth and different crystallographic orientation (Fig. 1). After optimizing the pulse parameters of magnetron sputtering, the growth of these abnormal oriented grains could be suppressed and the PFM amplitude exhibits a homogenous signal.

Furthermore, the PFM measurement allows to assess local changes of the polarity. It can be shown that in dependence of the process parameters the polarity of the (Al,Sc)N layer is changed from N polar to bipolar and further to Al polar.

The results show that the high-resolution PFM measurement enables a further optimization of piezoelectric properties of (Al,Sc)N layer by detection of local changes in amplitude and phase of piezoelectric signal.

Figure 1: (Al,Sc)N layer with some crystallites of abnormal grain growth which show a lower piezoelectric amplitude (left AFM topography, right PFM amplitude)
 

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Dr. Ilka Hermes: "Visualizing the electronic properties of topographical anomalies on wafer-scale graphene"

Ilka M. Hermes,¹,² Simonas Krotkus,³ George Sarau,⁴ Abdul Rauf,² Sergej Pasko³, Silke Christiansen,⁴ Michael Heuken,³ Ben Conran,⁵ Clifford McAleese,⁵ Xiaochen Wang,⁵ Oliver Whear⁵

¹ Leibniz Institut für Polymerforschung Dresden e.V., Dresden, Germany

² Park Systems Europe GmbH, Mannheim, Germany

³ AIXTRON SE, Herzogenrath, Germany

⁴ Fraunhofer-Institut für Keramische Technologien und Systeme IKTS, Forchheim, Germany

⁵ AIXTRON Ltd, Cambridge, United Kingdom

Wafer-scale fabrication of graphene on insulating substrates without transfer is a prerequisite for the materials’ industrial application in high-performance nanoelectronics [1]. Chemical vapor deposition (CVD) enables direct monolayer graphene growth on sapphire. The use of high temperature H2-annealed sapphire substrate instead of pristine sapphire improves the resulting graphene film by reducing the overall density of strain-induced wrinkles and increasing the crystal quality [2]. However, using Kelvin probe force microscopy (KPFM) imaging we observed that the annealing induced underlaying sapphire terraces introduce a heterogeneous surface potential distribution on the graphene monolayers. Moreover, we correlated the surface potential distribution to the nanomechanical behavior of graphene film, including its stiffness and adhesion, which was supplemented by micro-Raman measurements. Particularly, the correlation of the surface potential and the sample’s nanomechanical response indicates that the heterogeneous electronic properties are likely to originate from a film straining at underlying step edges.

[1] S. J. Kim, K. Choi, B. Lee, Y. Kim and B. H. Hong, Annu. Rev. Mater. Res., 45 (2015), 63–84

[2] N. Mishra, S. Forti, F. Fabbri, L. Martini, C. McAleese, B. R. Conran, P. R. Whelan, A. Shivayogimath, B. S. Jessen and L. Buß, Small, 15 (2019), 1904906