Imaging Proteins at the Single Molecule Level.
In this work we demonstrate the imaging of individual proteins and protein complexes by low-energy electron holography. This is made possible by producing ultrapure samples of ultraclean freestanding graphene onto which individual, chemically selected molecules were prepared by soft-landing electrospray ion beam deposition, a preparation method which is chemical- and conformational specific and allows for a gentle deposition. Low-energy electrons do not induce radiation damage, which enables acquiring subnanometer resolution images which are not the result of an averaging process.
Proc. Natl. Acad. Sci. U.S.A., DOI:10.1073/pnas.1614519114 (2017)
Molecular Nanostructurs through Two-Dimensional Folding of Polypeptides at Surfaces.
Folding is nature’s powerful tool for crafting highly functional, yet adaptable protein structures from an amino acid sequence. The final structure including high specific interaction sites as well as the folding pathway are already encoded in this sequence. Here, we demonstrate the fabrication of molecular nanostructures via two-dimensional folding of a nine amino acid peptide on a surface in vacuum. We observe the variability of the peptides conformation as well as a uniform folded state with subnanometer resolution using scanning tunneling microscopy. By an in depth theoretical analysis based on a unique combination of DFT and MD simulations we gain a detailed understanding of the mechanism, revealing similarities and differences to the folding in a native, aqueous environment.
ACS Nano, DOI: 10.1021/acsnano.6b06145 (2017)
Correlation Driven Transport Asymmetries Through Coupled Spins in a Tunnel Junction
Spin–spin correlations are fundamental to many material properties because they favour certain ground states and are key in numerous models that describe the behaviour of strongly correlated materials. While the sum of collective correlations usually lead to a macroscopically measurable change in properties, a direct quantification of correlations in atomic scale systems is difficult. Using a scanning tunnelling microscope we now determined the correlations between a localized spin and its electron bath by observing the appearance of differential conductance asymmetries during the coupling to a second spin in the junction.
Nature Communications 8, 14119 (2017)
Sensing the quantum limit in scanning tunnelling spectroscopy.
What would happen if an electric current no longer flowed, but trickled instead? This was the question we recently investigated. It involved cooling our scanning tunnelling microscope down to fifteen Millikelvin. At these extremely low temperatures, the electrons reveal their quantum nature. The electric current is therefore a granular medium, consisting of individual particles. The electrons trickle through a conductor like grains of sand in an hourglass, a phenomenon that can be explained with the aid of quantum electrodynamics.
Nature Communications 7, 13009 (2016)
Driving the Oxygen Evolution Reaction by Nonlinear Cooperativity in Bimetallic Coordination Catalysts
The oxygen evolution reaction is the limiting factor of the water splitting reaction. Developing efficient catalysts for the demanding four electron reaction is one of the prominent challenges in today’s energy conversion technologies. Here, we report porphyrin-based heterobimetallic-organic network inspired by metalloproteins and single atom electrocatalysts that show a dramatic enhancement in oxygen production by nearly two orders of magnitude compared to monometallic porphyrins. Non-linear increasing catalytic activity is facilitated through the electronic coupling of the catalytically active metal centers through the extended π-system of the molecules. This step forward towards efficient oxygen reduction catalysts using earth abundant materials is highlighted in Nature Energy
J. Am. Chem. Soc., 138 (11), pp 3623–3626, (2016)
Coherent spin tunneling in molecular magnet-graphene hybrids
Understanding the dynamics of spins on surfaces is of great importance for the design of devices that exploit novel means of spin manipulation. One attractive strategy involves the use of graphene for the electrical manipulation of close-by spins. To this end, we have studied the classical and quantum spin dynamics of molecular magnets on an underlying graphene substrate. While the presence of graphene leaves the static spin response of the molecules unaffected, their quantum spin dynamics and the associated selection rules are strongly altered. Theoretical modeling reveals that coupling of the molecular spins to the Dirac electrons of graphene enables a fully coherent, resonant spin tunneling process.
Nature Materials 15, 164 (2016)
Two-Dimensional Honeycomb Network through Sequence-Controlled Self-Assembly of Oligopeptides
In nature, the blueprint for function and shape of proteins and peptides is inherently comprised in the amino-acid sequence. Thus, manipulating the sequence changes the assembly of these biomolecules. In this work, we demonstrate that it is possible to use this sequence control to steer the surface confined assembly of peptides: we manipulate the bond motifs to yield a porous honeycomb network of peptides on the surface. Despite the flexibility of the building blocks, the network is long-ranged ordered and highly fault-tolerant. Our approach demonstrates the potential of the vast sequence space to build different 2D peptide structures and functional surfaces.
Nature Communications 7, 10335 (2016)
Quantum Engineering of Spin and Anisotropy in Magnetic Molecular Junctions
Single spins can be individually addressed when coupled to contacts forming an electrical junction. The chemical environment and local symmetry largely define the magnetic anisotropy. However, magnetic anisotropy also depends upon how the spin couples to the conduction electron bath via Kondo exchange. Here, we utilize a highly corrugated hexagonal boron nitride monolayer to mediate the coupling between a spin and the metal contact. Our experiments demonstrate how the Kondo exchange interaction mimics chemical tailoring and changes the magnetic anisotropy.
Nature Communications 6, 8536 (2015)
Exciton dynamics of C60-based single-photon emitters explored by Hanbury Brown–Twiss scanning tunnelling microscopy
The creation of excitons and their annihilation are crucial processes in the energy conversion between electric circuits and light, e.g. in light sources and solar cells. We recently measured the dynamics of excitons by combining electroluminescence in the scanning tunneling microscope and Hanbury Brown-Twiss interferometry. The measured temporal correlation between photons created at structural defects of solid C60 reveals single photon emission and allows to determine an exciton lifetime of 0.75ns. For increasing current the lifetime reduces strongly proving that excitons are efficiently quenched by passing charge carriers. Our experiment introduces a unique way to study exciton dynamics and exciton-charge interaction employing current injection with ultimate molecular precision.
Nature Communications 6, 8461 (2015)
Tuning the isoelectric point of graphene by electrochemical functionalization
The surface charge behavior of the solid-liquid interface as governed by the isoelectric point plays a crucial role in the resulting physico-chemical properties. The pI is a key parameter which needs to be controlled for realizing applications in catalysis, sensing and energy conversion. For the first time, we present here a way to identify the isoelectric point of bare graphene and estimate it to be less than 3.3. Through controlled attachment of aromatic amino groups (by electrochemical modification of graphene) and supported by elaborate theory we unambiguously show that the pI of graphene can be continuously tuned from up to 7.5.
Scientific Reports 5, 11794 (2015)
Local vibrational fingerprinting of chemical functionalization on single nanotubes and graphene
Chemical functionalization of nanostructures allows for realizing materials and systems with novel physical and chemical properties. One important challenge is the need for obtaining chemical information about attached functional groups at a single object level. Here we present a new kind of tool called local vibrational fingerprinting which enables mesoscopic identification of chemical groups attached to individual carbon nanotubes or monolayer graphene. Moreover, we quantify the relative proportions of different functionalities with diffraction-limited spatial resolution. The method is generic and simple, can be applied to other kinds of nanostructures and can be carried out using just a confocal Raman microscope without the need for more specialized instruments.
ACS Nano 9, 3314 (2015)
Chemical Modification of Graphene via Hyperthermal Molecular Ion Beams
A wide range of device applications of graphene rely upon suitable methods for tailoring its electronic properties. Along these lines, band gap opening in graphene has been achieved via covalent chemical modification of its basal plane. We have developed a novel approach to chemically functionalize graphene in covalent manner, involving the use of hyperthermal molecular cation beams of 4,4’-azobis(pyridine) in electrospray ion beam deposition (ES-IBD). The one-step, room temperature ion-surface reaction process takes place in high vacuum (10-7 mbar) above a threshold kinetic energy of 165 eV of the molecular ions. Covalently linked azopyridine moieties are obtained with a functionalization degree of 3% of the carbon atoms of graphene after 3-5 hours of ion exposure of 2⋅1014 azopyridinium/cm 2 of which 50% bind covalently. This method is highly promising for the patterned functionalization of extended graphene areas under conditions compatible with state-of-the-art device fabrication technologies
J. Am. Chem. Soc. 136, 13482 (2014)
A fast light emitting transistor of molecular dimension
Optical communication requires the fast conversion of electronic signals to light pulses. We realized an electro-optical interface for this purpose on the ultimate scale of a few nanometers. The signal conversion takes place in a tunnel junction between two metal electrodes with a single decoupled molecule in the middle. The molecule operates as a sensitive and fast transistor. A millivolt modulation of the molecules’ potential is converted to plasmonic light emission with an on/off ratio of several orders of magnitude. The emitted light follows the voltage modulation in less than a nanosecond.
Nano Lett. 14, 5693 (2014)
First atomic-scale insights into dye-sensitized solar cell interfaces
Dye-sensitized solar cells constitute a promising approach to sustainable and low-cost solar energy conversion. Our work provides for the first time atomic-scale insights into the relevant N3 dye - TiO2 anatase (101) interface revealing multi-conformational adsorption and energy level alignment of the photosensitizers as well as supramolecular interaction. Most importantly, the findings show that optimization strategies based solely on the electrochemical properties of the dye molecules should be replaced by a more comprehensive approach considering kinetic aspects in the dye-substrate coupling, possible strain in the molecular chromophore, and the atomic-scale structure of the dye−semiconductor interface.
Nano Lett. 14, 5563 (2014)
Bio-inspired nanocatalysts for the oxygen reduction reaction
We show that bio-inspired catalytic metal centers can be effectively mimicked in two-dimensional metal-organic coordination networks self-assembled on electrode surfaces. Networks consisting of organic molecules coordinating to single iron and manganese atoms on Au(111) catalyze effectively the oxygen reduction and reveal distinctive catalytic activity in alkaline media. The results demonstrate the high potential of surface-engineered metal-organic networks for electrocatalytic conversions.
Nat. Commun. 4, 2904 (2013)
Strong evidence for weak coupling Kondo Physics
While many aspects in physics can be treated with single-particle models, many-body correlations are of great interest because only by taking them into account intriguing effects like superconductivity, heavy Fermion systems, and the Kondo effect can be explained. The Kondo effect arises due to the interaction between a localized spin and the electrons of a surrounding host and manifests itself as a characteristic zero bias anomaly in tunneling experiments. Studies of magnetic impurities by scanning tunneling spectroscopy have renewed interest in Kondo physics, however a quantitative comparison with theoretical predictions remained challenging. Here we show that both temperature and magnetic field dependence of the zero-bias anomaly detected on an organic radical molecule weakly coupled to a metal surface can be described with astonishing agreement by perturbation theory as originally developed by Jun Kondo 50 years ago. These results are not only a benchmark for predictions of theories for the Kondo effect itself but also for correlated electron materials in general.
Record energy resolution in scanning tunneling spectroscopy
Progress in research is intimately connected with progress in developing novel and improved experimental methods. We have constructed a scanning tunneling microscope (STM) operating at a base temperature of 10 mK and in high magnetic fields up to 14 Tesla. It is combined with an ultra high vacuum preparation chamber allowing to study a large variety of samples and tips. Our mK-STM offers ultimate energy resolution combined with the atomic scale spatial resolution. The energy resolution of the setup, determined from an aluminum tip on a copper sample (see figure), is 11.4±0.3 µeV (Teff = 38±1 mK).
Rev. Sci. Instr. 84, 033903 (2013)
Nature's richness manifested in topological insulators
Nature is a rich source of technologically relevant materials, such as diamond, one of the hardest known materials, or graphite as a suitable precursor of graphene. This list has now gained a new member belonging to the new class of topological insulators (TIs). The surface states of TIs display a linear dispersion combined with spin polarization due to spin-momentum locking, which opens intriguing application perspectives in spintronics. We have found that the mineral Kawazulite with the general chemical composition Bi2(Te,Se)2(Se,S) is a natural TI whose electronic properties compete well with its synthetic counterparts. Based on this discovery, natural TIs may exist in other minerals, which due to their minimized defect densities display even better electronic characteristics.
Nano Letters 13, 1179 (2013)
High mobility Bi2Te2Se Platelets on h-Boron Nitride
Topological insulators (TIs) are attracting increasing attention owing to their peculiar helical states at their boundaries. However, electrical detection of these states is complicated by the interference of bulk conduction that arises due to pronounced doping originating from lattice defects. We have demonstrated the growth of high quality nanoplatelets of the topological insulator Bi2Te2Se through van der Waals epitaxy on thin hexagonal boron nitride (hBN) substrates. In this manner, a significant increase of the surface state carrier mobility is achieved, thus opening the possibility to observe well-developed Shubnikov-de Haas oscillations in the platelets. Furthermore, the platelets' small thickness enables effective tuning of the Fermi level position with the aid of a back gate.
Nano Letters 12, 5137 (2012)
Imaging of individual protein molecules at the single amino acid level
The folding of a polypeptide chain into a specific three-dimensional protein is probably the most fascinating example for the self-assembly of functional nanostructures. Using electrospray ion beam deposition (ES-IBD) of selectively folded and unfolded Cytochrome c protein ions on atomically defined solid surfaces in ultrahigh vacuum we demonstrated that it is possible to employ folding processes at the same level of complexity within a technological environment like the vacuum processing of surfaces. In this environment the unprecedented resolution of the scanning tunneling microscope allowed for the imaging of individual protein molecules at the single amino acid level. On the surface folded proteins are found to retain their three dimensional structure. Unfolded proteins are observed as extended polymer strands displaying submolecular features with resolution at the amino acid level. On weakly interacting surfaces unfolded proteins refold into flat, irregular patches composed of an individual molecule. This suggests the possibility of two-dimensionally confined folding of peptides of an appropriate sequence into regular two dimensional structures as new approach towards functional molecular surface coatings
Nano Letters 12, 2452 (2012)
The Quantum Magnetism of Individual Manganese-12-Acetate Molecular Magnets Anchored at Surfaces
Single molecular magnets (SMM) inspire the research in information storage, spintronics, and quantum computation due to their magnetic properties which lie at the interface between classical and quantum mechanical description. Manganese-12-Acetate (Mn12) is the archetypical SMM due to its high spin and long spin relaxation time. While these properties have been observed on bulk samples, the deposition on surfaces has not yet been possible while retaining these characteristics. In our experiment we demonstrate the surface deposition of intact Mn12 molecules by electrospray ion beam deposition. Using a scanning tunneling microscope we detect directly the preserved magnetism as low-energy fingerprint in inelastic spin-flip spectroscopy. These results are supported by DFT based calculations and establish that individual Mn12 molecules retain their intrinsic quantum magnetism.
Nano Letters 12, 518 (2012)