Publications by Klaus Kern | EPFL Graph Search

Charging and Electric Field Effects on Hydrogen Molecules Physisorbed on Graphene

Klaus Kern, Shai Mangel, Alessio Scavuzzo, Tobias Wollandt

Molecular hydrogen adsorbed on graphene was investigated by analyzing rotational excitation spectra obtained with a gate-tunable scanning tunneling microscope (STM). Through the shift of the rotational excitation energy, the tunability of physisorbed H2 on graphene was evaluated in response to electric fields and surface charging. Since pristine graphene and molecular hydrogen are both inert in nature, direct observation of physisorbed H2 on graphene remains elusive as H2 binds too weakly to flat graphene, even at low temperatures. Here, molecular hydrogen was exposed to wrinkled graphene at 5 K. Localized charges in the wrinkled graphene stabilize the H2-graphene bonding and thus enable observation using gate-tunable STM. Gate biasing modifies the charge carrier density in graphene and the electric field applied to the system, allowing for control of the physisorption of H2 molecules on graphene. The interaction between the molecule and surface is altered significantly as the gate voltage changes from -40 to +40 V, which results in a shift of the J = 0 -> 2 rotational excitation of H2 from 47 to 53 meV. Our theoretical results show that the rotational energy barrier follows a parabolic function of the electric field whose maximum shifts with surface charge. This trend of electric field and charge dependence was experimentally observed by STM. Modulating the electric field and the amount of surface charging offers unique opportunities to control physisorbed molecules, which is reversible and nondestructive.

AMER CHEMICAL SOC2023

Controlled Surface Modification to Revive Shallow NV- Centers

Klaus Kern, Dinesh Kevin Mathais Pinto, Aparajita Singha

Near-surface negatively charged nitrogen vacancy (NV) centers hold excellent promise for nanoscale magnetic imaging and quantum sensing. However, they often experience charge-state instabilities, leading to strongly reduced fluorescence and NV coherence time, which negatively impact magnetic imaging sensitivity. This occurs even more severely at 4 K and ultrahigh vacuum (UHV, p = 2 x 10-10 mbar). We demonstrate that in situ adsorption of H2O on the diamond surface allows the partial recovery of the shallow NV sensors. Combining these with band-bending calculations, we conclude that controlled surface treat-ments are essential for implementing NV-based quantum sensing protocols under cryogenic UHV conditions.

2023

In-situ atomic level observation of the strain response of graphene lattice

Jz -Yuan Juo, Klaus Kern

Strain is inevitable in two-dimensional (2D) materials, regardless of whether the film is suspended or supported. However, the direct measurement of strain response at the atomic scale is challenging due to the difficulties of maintaining both flexibility and mechanical stability at low temperature under UHV conditions. In this work, we have implemented a compact nanoindentation system with a size of similar to 160 mm(2) x 5.2 mm in a scanning tunneling microscope (STM) sample holder, which enables the reversible control of strain and gate electric field. A combination of gearbox and piezoelectric actuator allowed us to modulate the depth of the indentation continuously with nanometer precision. The 2D materials were transferred onto the polyimide film. Pd clamp was used to enhance the strain transfer from the polyimide from to the 2D layers. Using this unique technique, strain response of graphene lattice were observed at atomic precision. In the relaxed graphene, strain is induced mainly by local curvature. However, in the strained graphene with tented structure, the lattice parameters become more sensitive to the indentor height change and stretching strain is increased additionally. Moreover, the gate controllability is confirmed by measuring the dependence of the STM tip height on gate voltage.

NATURE PORTFOLIO2023

Seeding and Emergence of Composite Skyrmions in a van der Waals Magnet

Marko Burghard, Klaus Kern, Lukas Powalla

Topological charge plays a significant role in a range of physical systems. In particular, observations of real-space topological objects in magnetic materials have been largely limited to skyrmions - states with a unitary topological charge. Recently, more exotic states with varying topology, such as antiskyrmions, merons, or bimerons and 3D states such as skyrmion strings, chiral bobbers, and hopfions, have been experimentally reported. Along these lines, the realization of states with higher-order topology has the potential to open new avenues of research in topological magnetism and its spintronic applications. Here, real-space imaging of such spin textures, including skyrmion, skyrmionium, skyrmion bag, and skyrmion sack states, observed in exfoliated flakes of the van der Waals magnet Fe3-xGeTe2 (FGT) is reported. These composite skyrmions may emerge from seeded, loop-like states condensed into the stripe domain structure, demonstrating the possibility to realize spin textures with arbitrary integer topological charge within exfoliated flakes of 2D magnets. The general nature of the formation mechanism motivates the search for composite skyrmion states in both well-known and new magnetic materials, which may yet reveal an even richer spectrum of higher-order topological objects.

WILEY-V C H VERLAG GMBH2023

Anionic character of the conduction band of sodium chloride

Abhishek Grewal, Klaus Kern

The alkali halides are ionic compounds. Each alkali atom donates an electron to a halogen atom, leading to ions with full shells. The valence band is mainly located on halogen atoms, while, in a traditional picture, the conduction band is mainly located on alkali atoms. Scanning tunnelling microscopy of NaCl at 4 K actually shows that the conduction band is located on Cl- because the strong Madelung potential reverses the order of the Na+ 3s and Cl- 4s levels. We verify this reversal is true for both atomically thin and bulk NaCl, and discuss implications for II-VI and I-VII compounds. The current work finds that ordinary table salt behaves unexpectedly. The chloride ions alone dominate the electronic states at both edges of its band gap although sodium ions are also present. This is important when NaCl is used as an insulator.

NATURE PORTFOLIO2022

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Experimental control of local spin-charge interconversion is of primary interest for spintronics. Van der Waals (vdW) heterostructures combining graphene with a strongly spin-orbit coupled two-dimensional (2D) material enable such functionality by design. Electric spin valve experiments have thus far provided global information on such devices, while leaving the local interplay between symmetry breaking, charge flow across the heterointerface and aspects of topology unexplored. Here, we probe the gate-tunable local spin polarisation in current-driven graphene/WTe2 heterostructures through magneto-optical Kerr microscopy. Even for a nominal in-plane transport, substantial out-of-plane spin accumulation is induced by a corresponding out-of-plane current flow. We present a theoretical model which fully explains the gate- and bias-dependent onset and spatial distribution of the intense Kerr signal as a result of a non-linear anomalous Hall effect in the heterostructure, which is enabled by its reduced point group symmetry. Our findings unravel the potential of 2D heterostructure engineering for harnessing topological phenomena for spintronics, and constitute an important step toward nanoscale, electrical spin control. Spin-based electronics offers significantly improved efficiency, but a major challenge is the electric manipulation of spin. Here, Powalla et al find a large gate induced spinpolarization in graphene/WTe2 heterostructures, illustrating the potential of such heterostructures for spintronics.

NATURE PORTFOLIO2022

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Transport through quantum coherent conductors, such as atomic junctions, is described by conduction channels. Information about the number of channels and their transmissions can be extracted from various sources, such as multiple Andreev reflections, dynamical Coulomb blockade, or shot noise. We complement this set of methods by introducing the superconducting excess current as a new tool to continuously extract the transport channel transmissions of an atomic scale junction in a scanning tunneling microscope. In conjunction with ab initio simulations, we employ this technique in atomic aluminum junctions to determine the influence of the structure adjacent to the contact atoms on the transport properties.

AMER PHYSICAL SOC2022

Landing Proteins on Graphene Trampoline Preserves Their Gas-Phase Folding on the Surface

Klaus Kern, Hannah Julia Ochner, Stephan Rauschenbach, Sven Alexander Szilagyi

Molecule-surface collisions are known to initiate dynamics that lead to products inaccessible by thermal chemistry. These collision dynamics, however, have mostly been examined on bulk surfaces, leaving vast opportunities unexplored for molecular collisions on nanostructures, especially on those that exhibit mechanical properties radically different from those of their bulk counterparts. Probing energy-dependent dynamics on nanostructures, particularly for large molecules, has been challenging due to their fast time scales and high structural complexity. Here, by examining the dynamics of a protein impinging on a freestanding, single-atom-thick membrane, we discover molecule-on-trampoline dynamics that disperse the collision impact away from the incident protein within a few picoseconds. As a result, our experiments and ab initio calculations show that cytochrome c retains its gas-phase folded structure when it collides onto freestanding single-layer graphene at low energies (similar to 20 meV/atom). The molecule-on-trampoline dynamics, expected to be operative on many freestanding atomic membranes, enable reliable means to transfer gas-phase macromolecular structures onto freestanding surfaces for their single molecule imaging, complementing many bioanalytical techniques.

AMER CHEMICAL SOC2022

Phase Reconstruction of Low-Energy Electron Holograms of Individual Proteins

Klaus Kern, Hannah Julia Ochner, Stephan Rauschenbach, Sven Alexander Szilagyi

Low-energy electron holography (LEEH) is one of the few techniques capable of imaging large and complex three-dimensional molecules, such as proteins, on the single molecule level at subnanometer resolution. During the imaging process, the structural information about the object is recorded both in the amplitude and in the phase of the hologram. In low energy electron holography imaging of proteins, the object's amplitude distribution, which directly reveals molecular size and shape on the single-molecule level, can be retrieved via a onestep reconstruction process. However, such a one-step reconstruction routine cannot directly recover the phase information encoded in the hologram. In order to extract the full information about the imaged molecules, we thus implemented an iterative phase retrieval algorithm and applied it to experimentally acquired low-energy electron holograms, reconstructing the phase shift induced by the protein along with the amplitude data. We show that phase imaging can map the projected atomic density of the molecule given by the number of atoms in the electron path. This directly implies a correlation between reconstructed phase shift and projected mean inner potential of the molecule, and thus a sensitivity to local changes in potential, an interpretation that is further substantiated by the strong phase signatures induced by localized charges.

AMER CHEMICAL SOC2022

Single Skyrmion Generation via a Vertical Nanocontact in a 2D Magnet-Based Heterostructure

Marko Burghard, Klaus Kern, Lukas Powalla

Skyrmions have been well studied in chiral magnets and magnetic thin films due to their potential application in practical devices. Recently, monochiral skyrmions have been observed in twodimensional van der Waals magnets. Their atomically flat surfaces and capability to be stacked into heterostructures offer new prospects for skyrmion applications. However, the controlled local nucleation of skyrmions within these materials has yet to be realized. Here, we composed of the 2D ferromagnet Fe3GeTe2 (FGT), an insulating hexagonal boron nitride layer, and a graphite top electrode. Upon a stepwise increase of the voltage applied between the graphite and FGT, a vertically conducting pathway can be formed. This nanocontact allows the tunable creation of individual skyrmions via single nanosecond pulses of low current density. Furthermore, timeresolved magnetic imaging highlights the stability of the nanocontact, while our micromagnetic simulations reproduce the observed skyrmion nucleation process.

AMER CHEMICAL SOC2022