Session B23: Advances in Scanned Probe Microscopy I: Novel Approaches and Ultrasensitive Detection

Nanoscale magnetometry using scanning single spin quantum sensors

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Three-dimensional AFM imaging of hydration and flexible surface structures at solid-liquid interfaces

At a solid-liquid interface, water interacts with a surface to present a non-uniform distribution referred to as hydration structure. In addition, if the surface consists of soft organic or biological molecules, the flexible surface structures often exhibit thermal fluctuations to present a three-dimensional (3D) distribution. These 3D hydration and flexible surface structures play critical roles in various interfacial phenomena such as protein folding, crystal growth, self-assembly, molecular recognition, metal corrosion, friction and anti-fouling. However, these 3D structures are hard to visualize by conventional imaging techniques. Here, we aim to solve this problem by 3D-AFM [1]. In 3D-AFM, a tip is scanned in vertical and lateral directions to cover the 3D interfacial space. During the scan, the tip interacts with the surrounding molecules so that the measured 3D force distribution shows molecular-scale contrasts reflecting the molecular distributions. So far, several groups have shown that the method can visualize 3D hydration structures on minerals (e.g. mica, calcite and fluorite) and biological molecules (e.g. bR, GroEL and DNA). In addition, it was shown that 3D-AFM can visualize 3D distribution of fluctuating molecules such as lipid headgroups at a membrane surface [2] and surfactants on an HOPG substrate. More recently, the possibility of visualizing 3D hydrations above heterogeneous structures such as point defects and complex minerals have been explored. Furthermore, there has been an attempt to visualize dynamic changes of the 3D hydration structures by improving the operation speed of 3D-AFM. In this study, we present an overview of these advanced 3D-AFM techniques and their applications.

[1] T. Fukuma, Y. Ueda, S. Yoshioka, H. Asakawa, Phys. Rev. Lett. 104 (2010) 016101.
[2] H. Asakawa, S. Yoshioka, K. Nishimura, T. Fukuma, ACS Nano 6 (2012) 9013.   

Single-atom nuclear magnetic resonance (NMR) using scanning tunneling microscopy

Nuclear spins are sensitive probes in chemistry and materials science as well as promising candidates for quantum information processing. Manipulating nuclear spins in condensed matter systems is difficult due to the small nuclear magnetic moment, leading to low polarizations, and addressing them individually is particularly challenging. Here, we demonstrate NMR of individual atoms on a surface in a scanning tunneling microscope (STM) [1]. To achieve NMR, we first polarize nuclear spins using spin-polarized tunneling current. By employing the flip-flop hyperfine interaction, the spin angular momentum of tunneling electrons is transferred to the nucleus. The nuclear polarization is controlled by the current, and read out by electron spin resonance [2, 3]. We further use NMR to sense the local magnetic environment of the electron spin of a single atom. The electrical polarization and driving of the nuclear spin states enable the local spin manipulation for nuclear spintronics and detection of the atomic-scale magnetic environment in nanomagnets.
[1] Yang et al., Nat. Nanotechnol. doi:10.1038/s41565-018-0296-7 (2018).
[2] Yang et al., Phys. Rev. Lett. 119, 227206 (2017).
[3] Willke et al., Science 362, 336 (2018).   

Downscaling of multiprobe STM experiments: towards atomic level understanding of transport phenomena

Multiprobe scanning tunneling microscopy (STM) based techniques allow determination of electronic and spin transport in variety of systems supported on surfaces of solid materials. Classical 2- and 4-probe methods are currently considered as universal tools for in-situ transport measurements on mesoscopic scales (typically hundreds of nanometers). Alternatively, application of scanning tunneling potentiometry (STP) visualizes potential change during such mesoscopic charge current transport with a nominal nanometer resolution. Here, on chosen examples we would like to discuss our efforts towards changing of this mesoscopic experimental paradigm by downscaling of 2-probe STM and STP experiments towards the atomic level. In this case the charge and spin current supplying probes are positioned in atomically defined locations with respect to the characterized nanosystem. In such configuration multiprobe techniques could directly visualize quantum nature of coherent carriers.   

Array atomic force microscopy for real-time multi-parametric analysis

Nanoscale multipoint structure-function analysis is essential for deciphering complexity of multiscale biological and physical system. Here, we describe a prototype of dispersive optics-based array AFM capable of simultaneously monitoring multiple probe-sample interactions in air and in liquid. A single supercontinuum laser beam is utilized to spatially and spectrally map multiple cantilevers, so that the beam deflection from individual cantilever can be isolated and recorded by distinct wavelength selection. This new design provides a remarkably simplified yet effective solution to overcome optical crosstalk, while maintaining high sensitivity and compatibility with probe-based sensors. We demonstrate the versatility and robustness of our system on parallel multi-parametric imaging, as well as monitoring live hearts cells intercellular activity. This approach provides new opportunities for studying emergent properties of atomic-scale mechanical and physicochemical interactions in a wide range of biological and physical networks.   

Nanofabricated tips as a platform for double-tip and device based scanning tunneling microscopy

We introduce a new kind of tip for scanning tunneling microscopy (STM) and report on its fabrication and performance [1]. By fully incorporating a metallic tip on a silicon chip using modern micromachining and nanofabrication techniques, we realize so-called smart tips and show the possibility of device-based STM tips. Contrary to traditional etched/grinded wire tips, these can be integrated in lithographically defined electrical circuits, photonic circuits and mechanical systems. We experimentally demonstrate the high performance of the smart tips, both in stability and resolution. In situ tip preparation methods are possible and we verify that they can resolve the herringbone reconstruction and Friedel oscillations on Au(111) surfaces. Smart tips can allow to considerably extend the range of STM, for example by enabling high-frequency tips to study noise on majorana zero modes and spin resonances, local gating using two tips or spin sensitive devices. As an example we present in detail two isolated tips with sub-50 nm apex-to-apex distance and calculations of how this can be used to measure electron correlations at the nanoscale.

[1] M. Leeuwenhoek et al., arXiv:1712.08620 (2017).   

FEBID-grown iron and cobalt nanowires as magnetic force sensors

Nanowires (NW) fabricated by focused electron beam induced deposition (FEBID) of magnetic materials such as iron and cobalt [1] are ideal candidates as magnetic force transducers [2]. The ability to produce nanometer-scale structures with extremely high aspect ratios should allow for magnetic probes with both high force sensitivity and fine spatial resolution. Here, we characterize the mechanical properties of magnetic FEBID-grown NWs using optical interferometry. Furthermore, we study their magnetic behavior through measurements of dynamic torque magnetometry [2]. Due to the large shape anisotropy, the equilibrium magnetization points along the NW, giving rise to a tiny magnetic monopole-like tip for magnetic force sensing. We confirm such behavior by scanning the NW over a micron-sized, current-carrying wire and recording its mechanical response driven by the Biot-Savart magnetic field. Our results, combined with ongoing progress in FEBID manufacturing of nanowires, hold great promise for new types of mechanical sensors for magnetic field imaging at the nanometer-scale.

[1] J. Pablo-Navarro et al., J. Phys. D: Appl. Phys. 50, 18LT01 (2017)
[2] N. Rossi et al., arXiv:1810.10865   

Imaging and Controlling Pseudospin and Berry Phase of Dirac Fermions by Symmetry-Assisted Coherent Scattering

In lattice materials, the solution of the Schrödinger equation is a set of Bloch waves. The dispersion of these components defines the band structure and is crucial to electronic transport and thermodynamic properties. Scanning tunneling microscopy (STM) is a powerful tool to study electronic properties with exceptional spatial resolution. Because coherent scattering induced by impurities strongly depends on the band structure of a material, by imaging the pattern of coherent scattering, the band structure can be probed in momentum space as well. Using atom manipulation with STM, we show that the coherent scattering term can be separated from the non-scattering term. As a result, the contrast of the coherent scattering pattern is amplified and the details of the band structure of Dirac fermions in graphene, including complete Dirac cones and their trigonal warping, are revealed. Furthermore, by tuning the symmetry of the scattering center using atom manipulation, we show that pseudospin can be resolved and that a signature of the underlying Berry phase can be observed. The results indicate that the interplay of the symmetry of wave functions and that of a single scattering center provides a new way to image and control the internal degrees of freedom of quantum matter.   

Using real space pseudopotentials to simulate non-contact atomic force microscopy images of organic molecules

Non-contact atomic force microscopy (nc-AFM) is a popular instrument to visualize the nano world with unprecedented resolution. nc-AFM, with a CO functionalized probe tip, can be used to distinguish different organic molecules, and serves as a very powerful analytical tool in organic chemistry research to identify the molecular structures of reactants, intermediates and products. However, a one-to-one mapping between the functional groups and nc-AFM images is often lacking. We employ a real-space pseudopotential method to simulate nc-AFM images and to provide a “database” for various functional groups, such as -C≡C-, -C=C-, and -C=O. We will also assess new functionalized tips by performing calculations to simulate nc-AFM images.   

Subtleties in the use of a quadrant cell photodiode in an optical lever

Quadrant cell photodiodes are often used in an optical lever to deliver a digital “zero-crossing” signal (either for determining the period or for a static position signal). However, one can also continuously monitor the analog output of a quad-cell photodiode to determine time dependent positioning information (albeit over a narrow range). Unfortunately, the response of a quadrant cell photodiode to the passage of a gaussian laser beam is a non-trivial function of the beam radius. We have created a detailed (python based) computational model of a quadrant cell photodiode, and compared the model to experimental measurements. We remark the accuracy of the model as well as some important frequency spectrum implications of the detector response which are very relevant when analyzing the power spectrum of a quadrant cell signal.