Abstracts of the 2018 Dutch SPM symposium

  • Topological quantum phases in graphene nanoribbons - Invited

    O. Gröning1, S. Wang1, X. Yao2, C. A. Pignedoli1, G. Borin Barin1, C. Daniels3, A. Cupo3, V. Meunier3, X. Feng4, K. Müllen2, A. Narita2, P. Ruffieux1 & R. Fasel1

    1 Empa, Swiss Federal Laboratories for Materials Science and Technology, 8600 Dübendorf (Switzerland)
    2 Max Planck Institute for Polymer Research, 55128 Mainz (Germany)
    3 Department of Physics, Rensselaer Polytechnic Institute, Troy, NY (USA)
    4 CAED and Department of Chemistry, TU Dresden, Dresden (Germany)

    Topological materials have attracted great interest in solid state physics due to their ability to support robust, yet exotic quantum states at their boundaries (or interfaces) such as spin-momentum locked transport channels or Majorana fermions. Very recently, it has been found theoretically that localized zero energy modes can be obtained at the junctions of topologically dissimilar graphene nanoribbons (GNR) [1]. We have experimentally realized GNRs exhibiting well defined periodic sequences of these topological electronic modes [2]. This leads to one-dimensional electronic bands which are described by the Su-Schrieffer-Heeger (SSH) Hamiltonian representing the dimerized atomic chain. By manipulating the intra- and inter-cell coupling strength we could create SSH analogs with different Chern number and therefore topological class. Experimentally, the topological class is determined by the presence or absence of zero energy end states at the termini of the corresponding GNR or its junctions to structurally dissimilar GNRs. The realization of 1D topological quantum phases in GNRs enables a novel route to bandgap (and effective mass) control in GNR structures, which can be readily integrated in CMOS type electronic devices [3]. Furthermore, the topological GNR structures might be extended to a size where magnetic ordering occurs and 1D spin chains can be realized. In the longer term the topological end states might be used to host qubits for quantum information applications.

    [1] T. Cao, F. Zhao, S.G. Louie, Phys. Rev. Lett. 119, 076401 (2017).
    [2] O. Gröning et al., Nature 560, 209–213 (2018).
    [3] J.P. Llinas et al., Nat. Commun. 8, 633 (2017).


    Figure: (a) Synthesis of edge-extended 7-AGNR hosting topological electronic bands. (b) Constant height nc-AFM image (with CO functionalized tip) of the ribbon structure on Au(111). (c) Series of constant height dI/dV maps of the GNR shown in b) at selected energies close to Ef. (d) Sequence of TB derived constant height charge density maps at the VB and CB extrema (at -1 eV, -0.2 eV, +0.2 and +1.0 eV from bottom VB to top CB). The 1 nm scale bar in b) applies to all maps.
  • Ion adsorption, hydration and specific ion effects at mineral electrolyte interfaces - Invited

    Frieder Mugele

    Physics of Complex Fluids University of Twente Department of Science and Technology

    Ions adsorbing to a solid surface immersed in an aqueous electrolyte have an important impact on the physical and chemical properties of the solid-liquid interface. Amongst others, they affect the hydration structure of the interface, the local surface charge and the structure of the adjacent electric double layer. These changes of the microscopic structure of the interface affect macroscopic properties of the system such as the wettability of the surface and its chemical reactivity.
    In this lecture, I will focus on the adsorption of mono- and divalent alkali and earth alkali cations to mica and gibbsite surfaces. I will discuss the relative importance of electrostatic as well as hydration forces for the adsorption process. Using dynamic force spectroscopy measurements in aqueous electrolytes of variable salt concentration, we observed oscillatory solvation forces on mica reflecting the periodicity of water molecules. For the lighter alkali cations, this structure prevailed all the way from sub-mM up to molar concentrations without major variations. For heavier alkalis, in contrast, a transition from oscillatory force profiles in pure water to purely attractive ones is observed for concentrations beyond approximately 1mM. This observation is consistent with the well-known ‘structure breaking’ and ‘hydrophobic’ character of $Rb^+$ and $Cs^+$ ions, which is also seen in molecular simulations. Divalent cations in contrast, are found to conserve the oscillatory hydration profile at the interface. I will discuss these observations in the context of recent molecular dynamics simulations, which confirm all qualitative aspects of the present measurements. I will demonstrate briefly that – as a consequence of the strong adsorption of the ions – the macroscopic wettability of the electrolyte on the mica surface in ambient oil changes from complete water wetting to partial wetting.
  • The complex magnetic structure of the Neodymium surface at 1.3 K

    A. Eich, U. Kamber, A.A. Khajetoorians and D. Wegner

    Institute for Molecules and Materials, Radboud University, Nijmegen, the Netherlands

    Chiral magnetism has gained great attention due to its potential for magnetic storage devices or computing [1,2]. Following this development lanthanide metals shift into focus as they exhibit complex magnetic structures, e.g. helical/conical spin spirals or linear spin waves [3]. However, most investigations of lanthanides so far are based on magnetic neutron scattering experiments, lacking spatial resolution. Therefore, the nano-magnetic properties of lanthanides are poorly understood.
    Neodymium shows the most complicated magnetic phase diagram among the lanthanides. It exhibits several magnetic phase transitions below its Néel temperature of 19.9 K, resulting in incommensurate multi-q magnetic order [4,5].
    Here, we present first results and analysis of SP-STM measurements of Neodymium (Nd) bulk-like films on W(110) taken at 1.3 K. While we can retrieve some of the q-vectors found by previous neutron scattering studies, we discover additional vectors present at the Nd surface and study their evolution in an out-of-plane magnetic field.

    [1] A. Fert et al., Nat. Nanotechnol. 8, 152 (2013). [2] S. Krause & R. Wiesendanger, Nat. Mater. 15, 493 (2016). [3] W.C. Koehler, J. Appl. Phys. 36, 1078 (1965). [4] R. M. Moon & R.M. Nicklow, J. Magn. Magn. Mater. 100, 139 (1991). [5] E.M. Forgan et al., PRL 62, 470 (1989).
  • Examining crystal growth and breakdown at the nanoscale

    Helen E. King

    Department of Earth Sciences, Utrecht University

    The reactivity of the inorganic materials that make up our planet is governed by processes occurring at the interface between crystals and the local environment. At the Earths surface, interfaces between crystals and aqueous solutions are responsible for many natural processes, including element mobilisation to form ore deposits. These interfaces are also critical in engineering problems such as the remediation of environmental contamination by pollutants and pipe scale formation. To be able to probe crystal interfaces we need a technique that can monitor nanoscale changes of the surface topography in situ during the interaction of crystals with aqueous solutions. Thus, atomic force microscopy (AFM) is the optimal tool for this task. In this talk, I will discuss how AFM can be used to monitor inorganic crystal dissolution and growth. In addition, I will examine how AFM can act as an in situ probe of the interfacial feedbacks between dissolution and the nucleation and growth of new materials that remove contaminants such as heavy metals from aqueous solutions.
  • Scanning probe microscopy at ultra-high magnetic fields

    L. Rossi1,2, J. Gerritsen2, L. Nelemans1, A. Khajetoorians2, B. Bryant1,2

    1 High Field Magnet Laboratory (HFML-EMFL), Radboud University, Nijmegen, Netherlands
    2 Institute for Molecules and Materials, Radboud University, Nijmegen, Netherlands

    Up to now, low temperature scanning probe microscopy (SPM) has been limited to a magnetic field strength of around 18 T, as the majority of designs have been based on superconductor magnets. For some experimental applications, e.g. the study of fractal spectra in graphene superlattices, the room temperature quantum Hall effect and some metamagnetic transitions, higher fields are required. Static fields of more than 30 T can be generated in dedicated high-field facilities by water-cooled, resistive Bitter magnets or hybrid resistive-superconducting magnets. However, implementing SPM in a Bitter magnet is a major challenge, due to the high level of vibrational noise produced by the turbulent cooling water, in addition to the space constraints resulting from the small magnet bore.
    We present a novel cryogenic Scanning Tunnelling Microscope (STM) designed to operate inside a water-cooled Bitter magnet, which can reach a magnetic field of 38 T. The performance of the STM is demonstrated through Landau level tunnelling spectroscopy of graphite, at 4.2 K in magnetic fields up to 34 T (1). Additionally we show the design of a highly compact Atomic Force Microscope (AFM) for operation at cryogenic temperatures in high magnetic fields (2). We show preliminary imaging data on the frustrated spinel CdCr2O4 at up to 30 T.

    1. Tao, W. et al. Rev. Sci. Instrum. 88, 093706 (2017).
    2. Rossi, L., Gerritsen, J. W., Nelemans, L., Khajetoorians, A. A. & Bryant, B. http://arxiv.org/abs/1805.01302
    • Membrane remodelling: Live action of the ESCRT machinery at the molecular level

      Sourav Maity

      Moleculair Biofysica, Zernike Instituut, Rijksuniversiteit Groningen, Nijenborgh 4, 9747 AG, Groningen, The Netherlands

      Endosomal sorting complexes required for transport (ESCRT) are involved in many cellular membrane remodelling processes including membrane deformation and fission. Examples of such processes are the formation of endosomal vesicles, virus budding, cytokinesis and nuclear envelope closure. In membrane fission, first a budded neck is formed assisted by ESCRT III filaments that stabilize the highly curved membrane neck. After remodelling of these filaments by vacuolar protein sorting-associated protein 4 (VPS4), the actual fission can occur. It was previously shown that a continuous VPS4 activity removes ESCRT III subunits leading to complete filament disassembly. While the molecular constituents in this process are well described, the actual mechanism of action remains poorly understood. Using High Speed - Atomic Force Microscopy (HS-AFM), we investigate how VPS4 dynamically remodels ESCRT III filaments, which might ultimately lead to membrane fission once such polymers are assembled within a membrane neck structure. Our real-time movies show that prior to disassembly, the filaments go through local constriction, which we propose is essential to lead the budded neck to trigger fission. Combining this data with experiments on the VPS4 concentration dependency of constriction, we are able to propose a novel model of the molecular pathway involved in the final stage of cellular membrane budding.
    • Exploring magnetic frustration in atomically engineered closed chains

      J. Gobeil, D. Coffey, S. J. Wang, A. F. Otte

      Delft University of Technology, Delft, The Netherlands

      Modelling quantum systems with a large number of degrees of freedom can be a daunting task from a computational standpoint. Scanning Tunneling Microscopy (STM) offers an alternative path, by enabling atom-by-atom engineering and probing of such systems[1]. Spin-Polarized STM (SP-STM) can provide direct insight into a system's spin configuration[2], while at the same time providing a tunable interaction parameter[3]. This enables the study of frustrated spin systems, which pose a particular modelling challenge as they are governed by a delicate balance of competing interactions.
      Here we present the study of such a frustrated spin system, consisting in D-shaped chains of single iron atoms assembled on a single nitride layer grown on Cu3Au(100)[4]. As in the similar Cu2N system[5], the nitride layer provides a uniaxial framework with different ferromagnetic and antiferromagnetic interatomic couplings depending on the relative position on the lattice. This allows us to assemble closed loop chains with an odd number of antiferromagnetic couplings, leading to frustration. We explore the role of an external magnetic field, interatomic exchange, as well as the exchange interaction with the spin-polarized tip in the stabilization of the resulting spin configuration.

      [1] R. Toskovic, R. van der Berg et al., Nature Physics 12, 656-660 (2016).
      [2] A. A. Khajetoorians et al., Nature Physics 8, 497-503 (2012).
      [3] S. Yan et al., Nature Nanotechnology 10, 40-45 (2015).
      [4] J. Gobeil et al.,
    • Latest Advancements and Applications in Nanoscale IR Spectroscopy

      Panchal Vishal, Miriam Unger, Anirban Roy and Kevin Kjoller

      Bruker Nano Surfaces 112 Robin Hill Road Santa Barbara, CA 93117, USA

      Nanoscale infrared spectroscopy has been successfully demonstrated in an expanding range of applications in recent years due to significant increases in capability. One method of nanoscale infrared spectroscopy, atomic force microscope based infrared spectroscopy (AFM-IR) directly detects IR radiation absorbed by the sample using the AFM probe tip to sense thermal expansion. This thermal expansion depends primarily on the absorption coefficient of the sample and is largely independent of other optical properties of the AFM tip and the sample.
      One of the initial major improvements in the AFM-IR technique was the development of the resonance enhanced version of AFM-IR. In the resonance enhanced technique, an IR source with a tunable pulsed repetition rate is tuned such that the repetition rate is matched to a contact resonance of the AFM cantilever providing much larger oscillations of the cantilever. This has been employed to investigate samples as thin as an individual monolayer due to an improved sensitivity which is orders of magnitude higher than the non-resonant AFM-IR.
      Additionally, the resonance enhanced AFM-IR technology has demonstrated 100x faster spectral and 10x faster imaging acquisition times with better SNR. This development is instrumental to augment the reliability of nanoscale characterization by reducing the overall data acquisition time and enabling users to perform repeated measurements for statistical analysis.
      The resonance enhanced AFM-IR technique has previously been limited to the mid IR range which is accessible with QCL sources. The recent development and integration of a broad range Optical Parametric Oscillator (OPO) source which can be pulsed at rates compatible with resonance enhancement provides an extended spectral range. This has allowed measurements in the C-H, N-H and O-H vibrational stretching range (3600-2700 cm-1) with significantly improved sensitivity. More recently, building on the resonance enhanced AFM-IR technique we have introduced the Tapping AFM-IR mode. This mode allows chemical imaging while in the tapping mode such that soft polymer materials and loose particles can be analyzed. Due to the heterodyne detection method employed in this technique, the spatial resolution of the chemical measurements has improved to better than 10 nm.
      This presentation will describe the underlying technology including their recent advances and will also highlight numerous applications of nanoscale spectroscopy and chemical imaging in polymer, material and life sciences.

      References
      1. Dazzi, A.; Prazeres, R.; Glotin, F.; Ortega, J. M. Opt. Lett. 2005, 30, 2388-2390.
      2. Lu, F.; Jin, M.; Belkin, M. A. Opt. Express 2011, 29, 19942-19947.
      3. Dazzi, A.; Prater, C. Chem. Rev. 2016, DOI: 10.1021/acs.chemrev.6b00448.
    • Scanning noise spectroscopy on a cuprate high temperature superconductor

      Doohee Cho, Koen M. Bastiaans, Tjerk Benschop, Damianos Chatzopoulos, Irene Battisti, Maarten Leeuwenhoek, Jan Zaanen, Milan P. Allan

      Leiden University

      Valuable information about dynamics in electric charge transport cannot be accessed by conventional time-averaged spectroscopy techniques. An example is the granularity of charge that leads to current fluctuations; so called shot noise. Correlations can lead to deviations from Poissonian noise which are smeared out in the averaged current value. In mesoscopic systems, noise-spectroscopy measurements have been widely used to investigate the dynamics of strongly correlated phenomena. Here, we present a newly developed noise spectroscopy technique, for which we combined a Scanning Tunneling Microscope (STM) with a novel MHz amplifier to bring noise-spectroscopy measurements to the atomic scale. We demonstrate the Poissonian tunneling process on Au(111) surface. In addition, we observe unexpected non-Poissonian tunneling process on a cuprate high temperature superconductor with atomic resolution. This provides us a new way to unveil electronic properties hidden in the time-averaged transport measurements on exotic quantum materials.
    • Additive electrochemical nanofabrication using scanning probes in dilute electrolytes

      M. Aarts, E. Alarcon-Llado

      AMOLF institute, Amsterdam

      Additive manufacturing is an attractive way of fabrication as it reduces production costs and offers more design flexibility. The former is especially true when done with single-step, direct-write processes. At the nanoscale, Electrochemical Atomic Force Microscopy (EC-AFM) has the potential for deterministic placement of high-quality structures through electrocrystallization. It combines the power of electrochemical fabrication with the spatial freedom of the probe and in-situ imaging. In this work, we demonstrate highly localized electro-deposition of metal nanostructures with dimensions down to tens of nanometers grown in highly dilute metal-salt aqueous solutions (1 µM) in DC operation, by using a nano-electrode scanning probe.
      As the electrode-nanoprobe separation is now smaller than the electrostatic screening length (~150 nm), the resulting electric field profile and the ion movement within this nanometric gap are very complex. The EC-AFM gives us a handle to investigate fundamental questions regarding the electrochemical processes at the solid-liquid interface, and the charge carrier flow within the electric double layer structure, while enabling the growth of functional nano-architectures.
    • Self-assembly of para-hexaphenyl-dicarbonitrile on HOPG and graphene on Cu(111)

      Nico Schmidt1, Ida Delac Marion1, Mihaela Enache1, Stefano Gottardi1, Jun Li1, Juan-Carlos Moreno-Lopez1, Leticia Monjas Gomez2, Anna Hirsch2, Meike Stöhr1

      1: Zernike Institute for Advanced Materials, University of Groningen, Groningen, the Netherlands;
      2: Stratingh Institute for Chemistry, University of Groningen, Groningen, the Netherlands

      Herein, we report on the self-assembly of sexiphenyl-dicarbonitrile (NC-Ph6-CN) on highly oriented pyrolytic graphite (HOPG) and single-layer graphene on Cu(111). We studied the structural and electronic properties of the molecules using STM, STS, and LEED. For NC-Ph6-CN on HOPG, we found a close-packed structure where parallel molecules align in rows with a peculiar shift every fourth molecule. For NC-Ph6-CN on graphene on Cu(111), we found two related close-packed structures. This indicates that: (i) the observed shift is per se a unique feature of NC-Ph6-CN on graphitic substrates and (ii) one layer of graphene already suffices to induce it. Furthermore, we identified small but yet distinct differences in the NC-Ph6-CN structures on HOPG compared to graphene on Cu(111) demonstrating that even for weakly corrugated graphene substrates the role of the underlying metal substrate is not negligible.
    • Atomic-scale identification of the electrochemical roughening of platinum

      Leon Jacobse1, Marcel J. Rost2, Marc T.M. Koper1

      1 Leiden Institute of Chemistry
      2 Huygens-Kamerlingh Onnes Laboratory

      Platinum is arguably the most stable, highly active electrocatalyst under oxidizing conditions in acidic media. Nonetheless, the poorly understood electrode degradation process strongly limits the economic feasibility of large scale applications. Previously, we have shown by combination of simultaneous cyclic voltammetry and in-situ EC-STM that the total electrochemical signal of Pt(111) is directly correlated to its surface roughness [1, 2]. This analysis, however, did not yet provide a rationale for this correlation.
      Here we show that it is possible to derive the average, atomic-scale structure of the formed Pt nano-islands and their evolution via a detailed analysis of the EC-STM images. Correlating the resulting density of specific surface sites to the different hydrogen adsorption features in the voltammetry, allows us to identify the electrochemical reactivity of particular formed step, kink, and terrace sites on the roughened surface. This finally disentangles the rather complex evolution of the cyclic voltammograms: we can pinpoint each individual peak in the hydrogen adsorption region to a specific atomic structure on the roughened surface. Our detailed analysis delivers new insights into how to describe the electrochemical reactivity not only of the observed nano-islands but also of Pt nanoparticles in general.

      [1] L. Jacobse, Y.-F. Huang, M.T.M. Koper, and M.J. Rost, Nature Materials, 17 (3), 277, (2018). [2] Movie: https://youtu.be/xSpRcsgxq3Q
    • Space- and energy- dependent periodic electron localization from intrinsic surface modified epitaxial graphene on Cu(111)

      Umamahesh Thupakula, Priya Laha, Ivan Madarevic, Aleksandr Seliverstov, Asteriona Maria Netsou, Ken Verguts, Steven Brems, Stefan De Gendt, Herman Terryn, Margriet Van Bael, and Chris Van Haesendonck

      (1) Solid-State Physics and Magnetism Section, KU Leuven, BE-3001 Leuven, Belgium.
      (2) Department of Materials and Chemistry, Vrije Universiteit Brussel, BE-1050 Brussels, Belgium.
      (3) Department of Chemistry, KU Leuven, BE-3001 Leuven, Belgium.
      (4) Interuniversitair Micro-Electronica Centrum (imec) vzw, BE-3001 Leuven, Belgium.

      The ability to modify the electronic properties of a true two-dimensional (2D) material, graphene, by internal spontaneous surface modification offers an additional degree of freedom for the fabrication of graphene nanoelectronic devices. Here, we present atomic-scale investigations of periodically localized electronic states, which appear upon intriguing interaction between graphene and Cu(111) substrate, using UHV scanning tunneling microscopy and spectroscopy (UHV STM-STS) at low temperature (4.5 K). Annealing the graphene/Cu(111) sample at 400 °C under UHV conditions results in the appearance of atomically flat phase separated regions, with triangular and irregular surface reconstructions appearing on the surface. The locally flat regions formed on the graphene regions with irregular surface reconstruction exhibit tunneling voltage dependent emergence of a moiré pattern at higher negative voltages. Our findings illustrate that these localized electronic states depend on both space and energy, leading to periodic variations of the LDOS with respect to both space and energy. We find that the formation of metal oxide (Cu2O) islands beneath the graphene, which are phase separated from pure Cu(111) regions, is responsible for the localized intriguing electronic states in the filled states. Our findings provide opportunities to overcome the complex processes involving graphene transfer from metallic to insulator surfaces by electronically decoupling it from the Cu(111).
    • Structural characterization of cobalt sulfide supported on Au(111)

      M.K.Prabhu, D. Boden, M. Stam, M. Rost, J. Meyer, I.M.N. Groot

      Leiden university

      Cobalt sulfide is a 2D material which is very important as a catalyst promoter for hydrodesulfurization (HDS) besides its use in fuel cell electrodes and super capacitors. In this study, we perform structural characterization of the various reconstructions of cobalt sulfide sheets supported on Au(111) using scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED) and density functional theory calculations. The results of this study shows that the reconstructions of the cobalt sulfide sheets are a strong function of the sulfur coverage on the surface. The possibility to tune the coordinatively unsaturated sites in the cobalt sulfide sheets provides the key to engineering smart catalysts and electronic devices.
    • Lowering the force noise in Magnetic Resonance Force Microscopy

      G. Welker, M. de Wit, F.G. Hoekstra, G. van de Stolpe, T.H. Oosterkamp

      Leiden Institute of Physics

      Magnetic Resonance Force Microscopy is a technique aimed at measuring forces exerted by small spin ensembles. In combines scanning an ultrasoft, magnetically tipped cantilever with magnetic resonance techniques. When reaching for higher sensitivities and the ability to measure smaller and smaller spin ensembles, the force noise of the measurement must be lowered. In our talk, we present an approach to lower the force noise that combines three measures: Lowering the operation temperature into the milliKelvin regime to decrease thermal noise, shielding from external vibrations and increasing the intrinsic force sensitivity of the ultrasoft cantilever. We present a specifically developed vibration isolation system that allows for high cooling powers, 113 micro-W/mK at 100mK. Furthermore, we elaborate on the use of so-called nanoladder cantilevers with spring constants as low as 5 micro-N/m.
    • Scanning probe microscopy for future semiconductor devices

      Kristof Paredis

      IMEC

      In this talk, the focus will be on scanning probe microscopy developments and results targeting future semiconductor devices and materials. A first example will cover Fast Fourier transform Scanning spreading resistance microscopy, a newly developed extension of SSRM for improved carrier profiling in devices. It allows to significantly increase the sensitivity and eliminate the impact of parasitic resistances. Secondly, the application of SPM for 2D materials in the context of imec is shown. For instance, with the help of conductive AFM and lateral force microscopy, we show that the electrical impact of grain boundaries is not confined to a single layer but extends across multiple layers. Thirdly, we will go in detail on theory and application of scalpel-SPM for obtaining 3D information with SPM, examples on Si, oxides and magnetic materials will be shown.
    • Engineered electronic states in atomic and molecular lattices - Invited

      Peter Liljeroth

      Constructing designer materials where the atomic geometry and interactions can be precisely controlled is becoming reality. I will discuss our research towards this aim using atomic manipulation by the tip of a scanning tunnelling microscope (STM) and molecular self-assembly to reach the desired structures. Using atomic manipulation, it is possible to construct nanostructures and lattices where every atom is in a well-defined, predetermined position. This opens possibilities for creating artificial materials with engineered electronic structure via detailed control of lattice components, symmetries and interactions. I will illustrate this concept by showing how chlorine vacancies on Cu(100) [1] can be used to implement various one- and two-dimensional artificial lattices. In particular, I will focus on engineered topological domain wall states in one-dimensional dimer and trimer chains [2,3].
      In the second part of my talk, I will discuss similar concepts using metal-organic frameworks (MOFs) as a tuneable platform for realizing materials with engineered electronic structure. MOFs are an important class of materials that present intriguing opportunities in, e.g., the fields of sensing, gas storage, catalysis, and optoelectronics. While there is a tremendous number of examples of three-dimensional, bulk, MOFs, synthesis strategies for two-dimensional (2D), monolayer thick MOFs are more limited [4]. These systems are drawing growing interest as a promising platform for realizing designer materials with engineered electronic structures. The synthesis of 2D-MOFs is usually carried out on metal surfaces (e.g. Au, Ag, Cu), where it is difficult to access their intrinsic electronic properties. We have carried out synthesis of cobalt-dicyanobiphenyl and cobalt-dicyanoanthracene MOFs on epitaxial graphene and characterized their atomic geometriy and electronic structure using atomic force microscopy (AFM), STM and scanning tunneling spectroscopy (STS). We have observed the formation of a strongly coupled 2D electronic system in a MOF synthesized on a weakly interacting substrate.
      In summary, our work forms an important step towards 'designer quantum materials' with ultimate control over their electronic structure through atomic assemblies and molecular self-assembly.

      References
      [1] F.E. Kalff, M.P. Rebergen, E. Fahrenfort, J. Girovsky, R. Toskovic, J.L. Lado, J. Fernández-Rossier, A.F. Otte, A kilobyte rewritable atomic memory. Nat. Nanotech. 11, 926 (2016).
      [2] R. Drost, T. Ojanen, A. Harju, P. Liljeroth, Topological states in engineered atomic lattices, Nat. Phys. 13, 668 (2017).
      [3] M.N. Huda, S. Kezilebieke, T. Ojanen, R. Drost, P. Liljeroth, Tuneable topological domain wall states in engineered atomic chains, submitted.
      [4] L. Dong, Z. A. Gao, N. Lin, Self-assembly of metal-organic coordination structures on surfaces, Prog. Surf. Sci. 91, 101 (2016).
      [5] A. Kumar, K. Banerjee, A.S. Foster, P. Liljeroth, Two-dimensional band structure in honeycomb metal-organic frameworks, submitted (arXiv:1711.01128).