Nowadays, we approach the time at which AFM imaging simultaneously quantifies and structurally maps the various physical and chemical properties of surfaces and interfaces. In all these efforts, the progress in instrumentation was essential and still is in recent developments today. Two prominent examples here are the qPlus-based AFM and liquid-phase AFM, which exhibit unprecedented resolution and sensitivity under different environments.
GMT 8:00 am – 9:00 am
Franz Giessibl
Professor at Institute of Experimental and Applied Physics, University of Regensburg, Germany
The Chemical Bond on the Test Bench – Revealing its Secrets by Atomic Force Microscopy
Abstract: CO terminated AFM tips have been shown to provide outstanding spatial resolution on organic molecules [1], metallic clusters [2] and other samples. Experimental evidence and calculations show that the CO tip is chemically inert and probes organic molecules mainly by Pauli repulsion [3].
Thus, images of organic molecules, graphene etc. observed with a CO tip can be interpreted as a map of the absolute charge density of the sample. The total charge density of a single adatom is approximately given by a Gaussian peak. While single silicon adatoms appear similar to a Gaussian peak when imaged by AFM with a CO terminated tip, copper and iron adatoms adsorbed on Cu(111) and Cu(110) appear as tori. Initially, we explained these images by spz hybridization of the valence electrons of the adatoms [2]. DFT calculations show that the total charge density of Cu and Fe adatoms is approximately Gaussian – in contrast to what surface induced hybridization would predict [4]. However, the presence of the CO tip induces a hybridization that is strongly dependent on the tip position and results in a subatomic contrast as shown in A-C. When observing a metallic cluster by AFM, we find that the bonding strength between the AFM tip and the atoms of the sample depends not only on the chemical identity but also on the coordination – corner atoms more reactive than center atoms [5].
Fig.1AFM images of various metallic adatoms using a CO terminated tip. A Copper adatom on Cu(111). B Copper adatom on Cu(110). C Iron adatom on Cu(111).
References: [1] L. Gross et al., Science 325, 1110 (2009) [2] M. Emmrich et al., Science 348, 308 (2015) [3] N. Moll et al., New Journal of Physics 12, 125020 (2010) [4] F. Huber et al., Science 366, 235 (2019) [5] J. Berwanger et al., Phys. Rev. Lett. 124, 096001 (2020)
GMT 9:00 am – 10:00 am
Takeshi Fukuma
Professor at Nano Life Science Institute (WPI-NanoLSI), Kanazawa University, Japan
Visualizing Inside of 3D Self-Organizing Systems by 3D-AFM
Abstract: Recently, three-dimensional atomic force microscopy (3D-AFM) has been proven to be a powerful tool for investigating various structures and phenomena at solid-liquid interfaces [1,2]. In the method, a tip is scanned in the XY and Z directions in a 3D interfacial space. During the tip scan, the variations in the force applied to the tip is recorded to produce a 3D force image.
At a solid-liquid interface, the tip interacts with surrounding solvent molecules during the tip scan. Thus, the obtained 3D image represents the distribution of solvent molecules. So far, the method has been used for visualizing 3D hydration structures on minerals, organic thin films, and biological systems with subnanometer-scale resolution. This emerging technology has attracted attention due to its potential applications in the research on interfacial control technologies for anti-fouling, lubrication, anti-freezing, colloidal dispersion, cosmetics and cleaning.
Meanwhile, here I would like to draw attention to another important implication of the success of the 3D hydration measurements. In the AFM community, it has been a common sense that we should fix atoms or molecules to a solid surface to visualize them with atomic or molecular resolution. However, 3D-AFM allows us to visualize subnanometer-scale distribution of mobile water molecules that are not fixed on a solid surface. This is a big surprise and may lead to the breakthrough for the aforementioned limitation of AFM. Then, the next question would be on the requirements for the 3D objects that can be imaged by 3D-AFM. We believe that the answer is capability of self-organization. For example, in the case of a 3D hydration structure, it is heavily disturbed during the vertical tip scan, yet it is quickly reconstructed before starting the next vertical scan. Such a self-organization capability is essential for visualizing inside of 3D structures. One may think this is too severe condition, yet we can find a large number of important 3D self-organizing systems in both natural and artificial systems. Examples ranges from interfacial phenomena and devices (hydration, lubrication, electric double layer devices and liquid crystal devices) to biological systems (cells, nucleus, chromosomes and proteins). 3D-AFM may allow us to directly visualize inside of these various 3D self-organizing systems.
Based on this idea, we have recently started to explore inside of various 3D self-organizing systems. At polymer-water interfaces, gel-phase polymer chains with a thickness of a few nanometers were visualized. At an ionic liquid – Au electrode interface, ordered ionic liquid distributions with ~5 nm thickness were visualized. Furthermore, a carbon nanotube (CNT) tip was developed and used for visualizing inside of chromosomes with a thickness higher than 500 nm. Finally, a focused ion beam (FIB) milled Si needle probe was fabricated and used for visualizing inside of a live cell with a thickness of several microns. With these examples, here I would like to propose to apply 3D-AFM not only for visualizing hydration structures but also for imaging inside of various 3D self-organizing systems.
References: [1] T. Fukuma, Y. Ueda, S. Yoshioka, H. Asakawa, “Atomic-Scale Distribution of Water Molecules at the Mica-Water Interface Visualized by Three-Dimensional Scanning Force Microscopy”, Phys. Rev. Lett. 104 (2010) 016101. [2] T. Fukuma, R. Garcia, “Atomic- and Molecular-Resolution Mapping of Solid−Liquid Interfaces by 3D Atomic Force Microscopy”, ACS Nano 12 (2018) 11785−11797.
Session 2. Quantum Sensing
Imaging by SPM methods relies on various tip-surface interactions. However, the quantum coherence of the targets could be easily destroyed by those interactions such as the tunneling current, force, electron/phonon scattering and charge/spin noise. Therefore, most of the results obtained by SPM reflect only incoherent processes. SPM combined with quantum sensing technology may provide a solution to spatially follow the coherent evolution and achieve coherent control on various quantum processes.
GMT 11:00 am – 12:00 pm
Patrick Maletinsky
Professor at Department of Physics, University of Basel, Switzerland
Abstract: Quantum two-level systems offer attractive opportunities for sensing and imaging at the nanoscale. In the fifteen years since its inception, this idea [1] has advanced from proof of concept [2] to a mature quantum technology [3], which already finds applications in condensed matter physics, materials science and engineering. In this talk, I will present the key engineering challenges [4] we have addresses in this development and highlight particularly rewarding applications of single-spin, scanning probe microscopy.
Specifically, I will discuss how we employ single electronic spins in diamond for nanoscale probing of antiferromagnetic systems [5-9] and high-resolution imaging of atomically thin “van der Waals” magnets [10,11]. For both, the combination of sensitivity, spatial resolution and quantitative imaging enables unprecedented insights such as quantitative imaging of nanoscale domains [8] and domain-walls [9] in antiferromagnets and nanoscale imaging of spin textures in magnetic systems down to the atomic monolayer limit [11].
I will conclude with an outlook of future developments of single spin magnetometers for extreme conditions, such as high magnetic fields, millikelvin temperatures or for high-frequency sensors to probe the dynamics of nanomagnetic systems.
References: [1] B. Chernobrod and G. Berman, J. of Applied Physics 97, 014903 (2004) [2] G. Balasubmaranian et al., Nature 455, 644 (2008) [3] P. Appel et al., Review of Scientific Instruments 87, 063703 (2016) [4] N. Hedrich et al. arXiv:2003.01733 (2020) [5] T. Jungwirth et al., Nature Nanotechnology 11, 231 (2016) [6] T. Kosub et al., Nature Communications 8, 13985 (2017) [7] I. Gross et al., Nature 549, 252 (2017) [8] P. Appel et al., Nano Letters 19, 1682 (2019) [9] N. Hedrich et al., arXiv: 2009.08986 (2020) [10] M. Gibertini et al., Nature Nanotechnology 14, 408 (2019) [11] L. Thiel et al., Science 364, 973 (2019)
GMT 12:00 pm – 1:00 pm
Yujeong Bae
Group Leader at the IBS Center for Quantum Nanoscience, Ewha Womans University, Seoul, South Korea
Abstract: We combined the spin-polarized scanning tunneling microscopy (STM) with electron spin resonance (ESR) [1], which enabled us to study single atoms and inter-atomic coupling with unprecedented spatial and energy resolution.
In this talk, I will introduce the detailed mechanisms of driving and sensing single atom ESR in STM and how we use this technique to probe the quantum states of single atoms or magnetically coupled atoms on a surface. Making use of the high-energy resolution of this single atom ESR, we characterize the magnetic dipolar [2,3], exchange [3,4], and even hyperfine interactions [5] of atoms placed on a magnesium oxide film. Our work provides a powerful probe of the quantum states of electron spins for individual atoms and nanostructures.
References: [1] Susanne Baumann et al., Science 350, 417 (2015) [2] Taeyoung Choi et al., Nat. Nanotechnol. 12, 420 (2017) [3] Kai Yang et al., Phys. Rev. Lett. 119, 227206 (2017) [4] Yujeong Bae et al., Sci. Adv. 4, eaau4159 (2018) [5] Philip Willke et al., Science 362, 336 (2018)
November 10, 2020
Session 3. Photon-assisted Tunneling and Ultrafast Dynamics
In spite of the unprecedented spatial resolution down to atomic scale, the traditional temporal resolution of SPM is limited by the bandwidth of electronics and the resonance frequency of the scanner head. One way to defeat this limitation is through photon-assisted electron tunneling, by coupling the tip-surface junction with pulsed radio-frequency waves, THz, near-infrared and visible lasers. With those techniques, one can track and control the ultrafast dynamics of electrons, phonons, charges and spins on the atomic scale.
Time-Resolved Scanning Tunneling Microscopy and its Applications
Abstract: For further advances in nanoscale science and technology, the development of a method for exploring the transient dynamics of local quantum functions is essential. Since the invention of scanning tunneling microscopy (STM), the addition of high time-resolution to STM has been one of the most challenging issues [1-4].
Recently, new laser technologies have become applicable, where the carrier-envelope phase (CEP) is the same and locked in the subsequent monocycle pulses. Furthermore, the CEP can be controlled. On the basis of such CEP technologies, a new microscopy technique, THz-STM, has been developed [5-7]. The transient electronic state excited by an optical pulse can be probed using the THz-STM, and ultrafast carrier dynamics was reproducibly measured with visualization of enhanced THz near field [8]. Snapshots of ultrafast dynamics were successfully obtained. Details will be discussed at the symposium.
References: [1] Y. Terada et al., Nat. Photonics 2010, 4, 869–874. [2] S. Yoshida et al., Nat. Nanotechnol. 2014, 9, 588–593. [3] Zi-han Wang et al., Physical Chemistry Chemical Physics, 2019, 21, 7256-7260. [4] H. Mogi et al, Appl. Phys. Express, 2019, 12, 045002. [5] C. Guo et al., Phys. Rev. Lett., 2020, 124, 206801. [6] Cocker, T. L.et al., Nat. Photonics 2013, 7, 620–625. [7] Cocker, T. L.et al., Nature 2016, 539, 263–267. [8] Yoshida et al., ACS Photonics 2019, 6, 1356–1364.
Photon-assisted Tunneling at the Atomic Scale: Probing Resonant Andreev Reflections from Yu-Shiba-Rusinov States
Abstract: Exchange coupling of magnetic adsorbates to a superconducting substrate leads to Yu-Shiba-Rusinov (YSR) states within the superconducting energy gap. These can be probed by scanning tunneling spectroscopy as a pair of resonances at positive and negative bias voltage and over a wide range of tunnel conductances.
At low tunneling rates, the current is carried by single-electron processes, where each excitation is sufficiently quickly followed by a relaxation into the energetic continuum. Upon increasing the junction conductance, the relaxation rates suppress single-electron tunneling and resonant Andreev processes start to dominate the transport process. The cross-over of these processes is expressed in the variation of the ratio of YSR peak height at positive and negative bias voltage [1].
Here, we investigate these transport processes by photon-assisted tunneling. While applying high-frequency radiation to the tunneling junction, we record the differential conductance spectra in the low and high-conductance regime. At low conductance, the YSR states exhibit symmetrically spaced sidebands with their spacing directly evidencing single-electron tunneling. Surprisingly, at large junction conductance, the spacing remains the same while the patterns become asymmetric. We show that this asymmetry is direct evidence of a resonant Andreev reflection with tunneling threshold conditions imposed on its electron and hole component [2]. We suggest that photon-assisted tunneling can be a powerful tool for the determination of the nature of the charge carriers in a single tunneling event.
References: [1] M. Ruby et al., Phys. Rev. Lett., 115 087001 (2015) [2] O. Peters et al., Nature Phys. (2020)
Session 4. Chemical and Bio-Imaging/Spectroscopy
The birth of SPM has opened up a fascinating opportunity of probing and controlling various chemical and biological phenomenon at single molecule level. Discrimination of the (bio-)chemical identities at single molecule level has been a longstanding issue in the SPM field. Two possible and effective methods include tip-enhanced vibrational spectroscopy and single molecule force spectroscopy, which can be also combined with scanning mode to allow spectromicroscopic imaging.
GMT 11:00 am – 12:00 pm
Zhenchao Dong
Professor at Hefei National Laboratory for Physical Sciences at Microscale University of Science and Technology of China
Abstract: Aspirations for reaching atomic resolution with light have been a major force in shaping nano-optics, whereby a central challenge is to achieve highly localized optical fields. The nanocavity defined by the coinage-metal tip and substrate in a scanning tunneling microscope (STM) can provide highly localized and dramatically enhanced electromagnetic fields upon proper plasmonic resonant tuning, which can modify the excitation and emission of a single molecule inside the nanocavity and produce intriguing new optoelectronic phenomena.
In this talk, I shall demonstrate two STM-based phenomena related to single-molecule optical spectroscopy. The first is single-molecule Raman scattering [1,2]. The spatial resolution of tip enhanced Raman spectroscopy (TERS) has been further driven down to a single chemical-bond scale. Such simultaneous chemically and spatially resolved capability for vibrational-mode mapping opens up a new possibility to determine molecular chemical structure by optical imaging at only a single molecule.
The second phenomenon is single-molecule photoluminescence. Unlike the Raman scattering process, molecular fluorescence is generally believed to be strongly quenched in the vicinity of metallic structures, which often imposes a harsh limit on the attainable spatial resolution. Recently, through precise junction control over both the tip apex with an atomistic protrusion and the molecular electronic states decoupled from a metal substrate [3,4], we beat fluorescence quenching and demonstrate nanocavity-plasmon enhanced photoluminescence imaging from a single molecule with sub-nanometer resolution. The fluorescence intensity is found to keep increasing, rather than quenched, as the tip gradually approaches the molecule until making a contact. By exploiting sub-nanometer resolved spectroscopic imaging, we can locally map the spatial distribution of molecular exciton energies and linewidths, probing subtle plasmonmolecule interactions with sub-molecular resolution. Our results provide new routes to optical imaging, spectroscopy and engineering of light–matter interactions at the sub-nanometer scale.
References: [1] R. Zhang, Y. Zhang, Z. C. Dong, S. Jiang, C. Zhang, L. G. Chen, L. Zhang, Y. Liao, J. Aizpurua, Y. Luo, J. L. Yang, J. G. Hou, “Chemical mapping of a single molecule by plasmon enhanced Raman scattering”, Nature 498, 82-85 (2013) [2] Y. Zhang, B. Yang, A. Ghafoor, Y. Zhang, Y. F. Zhang, R. P. Wang, J. L. Yang, Y. Luo, Z. C. Dong, J. G. Hou,”Visually constructing the Chemical Structure of a Single Molecule by Scanning Raman Picoscopy”, Natl. Sci. Rev. 6, 11691175 (2019) [3] Y. Zhang, Y. Luo, Y. Zhang, Y. J. Yu, Y. M. Kuang, L. Zhang, Q. S. Meng, Y. Luo, J. L. Yang, Z. C. Dong, J. G. Hou, “Visualizing coherent intermolecular dipole-dipole coupling in real space”, Nature 531, 623-627 (2016) [4] B. Yang, G. Chen, A. Ghafoor, Y. F. Zhang, Y. Zhang, Y. Zhang, Y. Luo, J. L. Yang, V. Sandoghdar, J. Aizpurua, Z. C. Dong, J. G. Hou, “Sub-nanometre resolution in single-molecule photoluminescence imaging”, Nat. Photonics (2020)
GMT 12:00 pm – 1:00 pm
Daniel Müller
Professor at ETH Zurich, Biosystems Science and Engineering, Basel, Switzerland
Mechanically Quantifying and Directing Biological Systems
Abstract: Mechanobiology emerges at the crossroads of medicine, biology, biophysics and engineering and describes how the response of proteins, cells, tissues and organs to mechanical cues contribute to development, differentiation, physiology and disease. The grand challenge in mechanobiology is to quantify how biological systems sense, transduce, respond and apply mechanical signals.
Over three decades, atomic force microscopy (AFM) has emerged as a key platform enabling the simultaneous morphological and mechanical characterization of living biological systems. Here, I will introduce the use of AFM-based nanoscopic assays to characterize the mechanical process guiding the drastic shape change of animal cells progressing through mitosis. We apply our assay in a massive screen to study the contribution of > 1’000 individual human genes in mitotic cell shape change. After having found the major genes responsible for regulating cell shape changes in mitosis, we apply our assay to control cancer cells progressing through mitosis.
After this, we introduce high-resolution AFM-based assays to characterize individual cellular machines (proteins) playing commanding roles in animal cells.
First, we developed AFM-based imaging to observe cellular machines at sub-nanometer resolution at work.
Second, we extended these imaging possibilities of AFM to image native membrane receptors and at the same time detect their interactions and binding steps to ligands and determine the free-energy landscape of receptor-ligand bonds.
Third, we apply AFM-based single-molecule force spectroscopy to image and structurally map, at amino acid accuracy, the interactions that functionally modulate a membrane receptor.
Fourth, we will exemplify how to use AFM-based assays to characterize viruses binding to mammalian cells and demonstrate how to use these insights to direct virus infection in vitro and in vivo for controlling cellular function and to restore vision.
References: [1] Atomic force microscopy imaging modalities in molecular and cell biology. Y.F. Dufrêne et al., Nature Nanotechnology (2017) 3, 295. [2] Atomic force microscopy-based characterization and design of biointerfaces. D. Alsteens et al., Nature Review Materials (2017) 2, 17008. [3] Combined activities of hydrostatic pressure and the actomyosin cortex drive mitotic cell rounding. M.P. Stewart et al., Nature (2011) 469, 226. [4] Cdk1 dependent mitotic enrichment of cortical myosin II promotes cell rounding against confinement. S.P. Ramanathan et al., Nature Cell Biology (2015) 17, 148. [5] Mechanical control of mitotic progression in single animal cells. C.J. Cattin et al., Proc. Natl. Acad. Sci. USA (2015) 112, 11258. [6] Multi-parametric force mapping of biological systems to molecular resolution. Y.F. Dufrene et al., Nature Methods (2013) 10, 847. [7] Imaging G protein-coupled receptors while quantifying their ligand-binding free-energy landscape. D. Alsteens et al., Nature Methods (2015) 12, 845. [8] Nanomechanical mapping of first binding steps of a virus to animal cells. D. Alsteens et al., Nature Nanotechnology (2017) 12, 177. [9] Genome-scale single-cell mechanical phenotyping reveals disease-related genes involved in mitotic rounding. Y. Toyoda et al., Nature Communications (2017) 8, 1266. [10] Inertial picobalance reveals fast mass fluctuations of mammalian cells. D. Martínez-Martín et al., Nature (2017) 550, 500. [11] Virus stamping for targeted single cell infection in vitro and in vivo. R. Schubert et al., Nature Biotechnology (2018) 36, 85. [12] AFM-based Mechanobiology. M. Krieg et al., Nature Reviews Physics (2019).
