Topological Electronic States in Metal-Coordinated OrganicNetworks

Matthias Muntwiler (main proposer)Paul Scherrer Institut, 5232 Villigen PSI, matthias.muntwiler@psi.ch

Thomas A. Jung (faculty responsible)Universitat Basel/Paul Scherrer Institut, thomas.jung@psi.ch

1 Introduction

Topological insulators (TI) are materials which exhibit gapless electronic states at their boundary (surface ofa three-dimensional, or edge of a two-dimensional system) while the conduction and valence bands in thevolume are separated by a finite band gap induced by spin-orbit coupling [1]. The topological protection ofthe boundary states has implications which make the systems interesting for applications in spintronics orquantum computation. For instance, the topologically protected states are insensitive to scattering centerssuch as imperfections of the lattice structure. On the other hand, organic nanostructures such as metal-organic networks have been explored for a bottom-up route of building and connecting molecular electronicdevices at surfaces [2]. The fusion of the two concepts has lead to the prediction of organic topological insulators(OTI), i.e. organic materials that exhibit properties known from anorganic TIs [3]. In an OTI, a spin-orbitgap of several meV is induced by the coordination of a heavy-metal atom with organic molecules that havea large π-system. On the edge of the metal-organic network, symmetry-protected edge states may existanalogously to anorganic TIs. In this project, the topological properties of theoretically predicted OTIs shall beverified experimentally by means of scanning tunnelling microscopy (STM), tunnelling spectroscopy (STS),and angle-resolved photoelectron spectroscopy (ARPES).

2 Project Description

The starting point of the proposed project is the regular metal-organic network of 9,10-dicyano-anthracene(DCA) molecules on Cu(111). Recent first-principle calculations suggest that a free-standing metal-organiclayer consisting of DCA molecules and metal coordination centers of copper or gold atoms exhibits topo-logical properties [6]. These properties should be observable in the spectral function as Dirac cones with a

1 nm

(a) (b)

25 nm

(c)

1 nm1 nm

(d)

KK'

Figure 1: Self-assembled porous network of Cu-DCA on Cu(111). (a) Wide-area STM topography scan. The inset showsthe structural formula of a DCA molecule. (b) High resolution STM topography image recorded with a DCA moleculeattached to the STM tip An (approximate) model of the molecular structure with ad-atoms forming the coordination cen-ters is overlaid [4]. (c) STM topography scan of a DCA island at V = −0.5 V (unoccupied states). The increased apparentheight at the boundary may be due to enhanced LDOS induced by topological edge states [5]. (d) Calculated momentum-resolved local density of states of the semi-infinite DCA lattice. The broad features correspond to the projection of the 2Dband structure, the narrow bands are enhanced at the edge. They cross the spin-orbit gap at the K and K’ points [6].

2016-05-31 1/2

M. Muntwiler, T. A. Jung Topological Electronic States. . .

spin-orbit gap of the order of 10 meV at the Fermi level. Within the gap, one-dimensional electronic statesthat are localized at the border of the layer are expected.

In a previous project, the proposers have experimentally studied confined electronic states in the pores ofthe extended lattice, as well as electronic states derived from molecular orbitals [4]. The detailed molecularstructure of the regular Cu-DCA layer consisting of DCA molecules and Cu ad-atoms is shown in Fig. 1, pan-els a and b. Measurements of isolated Cu-DCA islands show increased topographic contrast at the borders, asin Fig. 1c [5]. This contrast is likely attributed to an enhanced local density of states (LDOS) at the border, andmay be an indication of a topological edge state. The topological origin of this contrast, however, needs to becorroborated by spectroscopic measurements. This will be done by scanning tunnelling spectroscopy (STS)and angle-resolved photoelectron spectroscopy (ARPES). STS can resolve occupied and unoccupied states ofthe electronic structure locally, specifically at the edge versus the interior of Cu-DCA patches. With ARPES,the presence of a spin-orbit gap and the one-dimensional dispersion of the edge states in reciprocal space, asshown in Fig. 1d, can be probed.

Measuring the spectral properties of OTIs, however, poses several challenges. The solutions to thesechallenges can be considered milestones of the project:

• ARPES requires a dense, ordered array of domain boundaries so that the edge states can be identifiedat a high signal-to-noise ratio. One way to increase the areal density of edges on the sample is to growribbons of Cu-DCA on a stepped surface.

• It is possible that the presence of a metallic substrate below the metal-organic layer modifies the elec-tronic structure of the OTI by doping, hybridization, or screening. To reduce the influence of the sub-strate, it may be necessary to substitute copper for a less conductive surface, or by introducing a thindecoupling layer between the substrate and the DCA network.

• The predicted energy gap in Cu-DCA is rather small due to the low spin-orbit coupling in the Cu-organic bond. Substitution of the coordinating metal atom by a heavier element may increase the gapdue to intrinsically higher spin-orbit coupling within the atom, or by enhanced chemical interactionbetween the metal atom and the organic molecule, as observed in other metal-organic networks [7].

3 Resources

The suggested milestones emphasize that the development of suitable samples will be a major task of theproject, and that an iterative approach combining real and reciprocal space characterization methods is re-quired. The project will profit from the joint expertise of the two proposers and their groups at PSI and atthe University of Basel in scanning probe microscopy and angle-resolved photoelectron spectroscopy, in thepreparation of metal-organic networks and the characterization of their ligands.

By this proposal, we ask for the funding of the salary of a PhD student for up to 4 years. The PhD candidatewill be embedded in the Microsopy & Magnetism group at the Paul Scherrer Institut (PSI). At the PSI, he(she) will have access to the PEARL beamline of the Swiss Light Source where the ARPES measurementswill be done. Access to other beamlines will be applied for if high energy resolution or spin resolutionis needed. Samples will be developed and characterized at the Nanolab of the University of Basel. Thenecessary instrumentation for this project is available in the two labs.

References1M. Z. Hasan, and C. L. Kane, Rev. Mod. Phys. 82, 3045 (2010).2J. V. Barth, G. Costantini, and K. Kern, Nature 437, 671 (2005).3Z. Wang, Z. Liu, and F. Liu, Nat. Commun. 4, 1471 (2013).4J. Zhang, et al., Chem. Commun. 50, 12289 (2014).5J. Zhang, T. A. Jung, and M. Muntwiler, unpublished.6L. Z. Zhang, et al., Nano Lett. 16, 2072 (2016).7A. Shchyrba, et al., J. Am. Chem. Soc. 136, 9355 (2014).

2016-05-31 2/2

Matthias Muntwiler

Curriculum Vitae

Name Matthias Kurt MuntwilerBirth 17.10.1973, Olten, Switzerland

E-Mail matthias.muntwiler@psi.chHomepage www.psi.ch/lsc/matthias-muntwiler

ORCID 0000-0002-6628-3977

Academic Degrees2004 Dr. sc. nat. in Experimental Physics, University of Zurich, Nanostructured mag-

netic interfaces: case studies and new experiment control software.Thesis supervisors: Dr. T. Greber, Prof. Dr. J. Osterwalder

2000 Diploma in Experimental Physics, University of Zurich, Structural analysis ofhexagonal boron nitride on nickel (111) and additional cobalt clusters by angle-scannedphotoelectron diffraction.

Employment2009–present Beamline scientist at PEARL, Paul Scherrer Institut, Villigen PSI.

2007–2009 Research associate, Department of Chemistry, University of Minnesota, Min-neapolis, Minnesota, United States, group of Prof. Xiaoyang Zhu.

2004–2007 Postdoctoral associate, Department of Chemistry, University of Minnesota, Min-neapolis, Minnesota, United States, group of Prof. Xiaoyang Zhu.

2000–2004 Research and teaching assistant, Institute of Physics, University of Zurich.

Research Interest and Expertise� Surfaces and interfaces of organic semiconductors� Self-assembled organic monolayers, metal-organic networks� Charge transfer, interfacial electronic states� Single-molecule electronics, single-ion molecular magnets� Photoelectron Spectroscopy (angle-resolved, time-resolved, spin-resolved)� Photoelectron Diffraction (angle-scanned, energy-scanned)

Selected Publications2014 J. Zhang, A. Shchyrba, S. Nowakowska, E. Meyer, T. A. Jung, M. Muntwiler,

“Probing the spatial and momentum distribution of confined surface states in ametal coordination network”, Chem. Commun. 50, 12289 (2014).

2009 M. Muntwiler, Q. Yang, X.-Y. Zhu, “Exciton dynamics at interfaces of organicsemiconductors”, J. Electron Spectrosc. Relat. Phenom. 174, 116 (2009).

2008 M. Muntwiler, Q. Yang, W. A. Tisdale, X.-Y. Zhu, “Coulomb barrier for chargeseparation at an organic semiconductor interface”, Phys. Rev. Lett. 101, 196403(2008).

2016-05-31 1/1

CV: Thomas A. Jung 1

Curriculum Vitae: Thomas A. Jung

Academic Achievements

2009 Professor in Nanoscale Science, University of Basel, Switzerland1992 PhD in Physics, University of Basel1987 Diplom in Experimentalphysik, ETH Zurich, Switzerland1986 Diploma Thesis, Prof. Dr. H.-C. Siegmann, ETH Zurich

Employment

1997− Molecular Nanoscience group at Paul Scherrer Institute (PSI),Switzerland, since 1999 group leader

1998− Group Leader, Nanolab at the Physics Department, University of Basel1994 − 1996 Postdoc IBM Zurich Research Laboratory, Switzerland1992 − 1994 Postdoc IBM T.J. Watson Research Center, Yorktown Heights NY, USA1992 PSI Zurich (formerly RCA, now CSEM), Switzerland1987 − 1992 Research and teaching assistant, University of Basel

Research Interests and Expertise

Molecular devices, quantum wells and metamaterials; Molecular Electronics and Molecular Spin-tronics; Combining surface / interface science and supramolecular self assembly; On-surfaceChemistry;Spectro-microscopy correlation at high resolution e.g. by Scanning Probe Microscopy and Pho-toemission / Photoabsorption Spectroscopy; Industry collaboration: e.g. Nanonis (today Specs),Sony, IBM, Straumann and Roche

Honours and Awards

1999 Visiting Professor at Kyoto University, Kyoto Japan1998 Visiting Scientist at University of Madison, Wisconsin1997 IBM Outstanding Innovation Award, together with J. K. Gimzewski1987 Research fellowships by IBM Switzerland, Ciba Geigy and Swiss NSF

Publication Record

Coauthored ∼ 120 articles and two textbook chapters, Science (2009, 1996), Nature (1997), Na-ture Communications (2016, 2015, 2010), Angewandte Chemie (2011, twice in 2008, 2007, 2005,2004, 2013), Applied Physics Letters (2012, 2009, 2007, 2001, 1993), Journal of the AmericanChemical Society (2014, 2013, 2010), Advanced Materials (2013, 2006), Chemical Communi-cations (2015, three times in 2014, 2013, 2011, 2007), Journal of Physical Chemistry Letters(twice in 2010), Physical Review Letters, Chemical Physics Letters, Chemical Science; About4200 total citations; 35 average citations per article; citation rate of ∼300 citations per year;h-index 35; coauthored more than 5 patents

Outreach

Achievements, events covered ∼ 60 times by the general press (e.g. NZZ, Basler Zeitung, Aar-gauer Zeitung, 20-minuten, New Scientist), ∼ 20 feature pages in newspapers and journals andradio; tv-broadcasts. Book contribution Highlights aus der Nanowelt , Herder Verlag Germany,2006

Websites

http://www.psi.ch/lmn/molecular-nanoscience

http://www.physik.unibas.ch/dept/pages/en/personnel/jung.htm

http://nanolab.unibas.ch/

1