Electronic Resonances and Gap Stabilization of Higher Acenes on a

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Electronic Resonances and Gap Stabilization of Higher Acenes on a Gold Surface Justus Krüger, Frank Eisenhut, Dmitry Skidin, Thomas Lehmann, Dmitry A. Ryndyk, Gianaurelio Cuniberti, Fatima Garcia, José M. Alonso, Enrique Guitian, Dolores Pérez, Diego Peña, Georges Trinquier, Jean-Paul Malrieu, Francesca Moresco, and Christian Joachim ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b04046 • Publication Date (Web): 30 Jul 2018 Downloaded from http://pubs.acs.org on July 31, 2018

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Electronic Resonances and Gap Stabilization of Higher Acenes on a Gold Surface Justus Krüger1,2, Frank Eisenhut1,2, Dmitry Skidin1,2, Thomas Lehmann1, Dmitry A. Ryndyk1,3, Gianaurelio Cuniberti1,2,4, Fátima García5, José M. Alonso5, Enrique Guitián5, Dolores Pérez5, Diego Peña5, Georges Trinquier6, Jean-Paul Malrieu6, Francesca Moresco*,2,1 and Christian Joachim7 1

Institute for Materials Science and Max Bergmann Center of Biomaterials, TU Dresden, 01062

Dresden, Germany 2

Center for Advancing Electronics Dresden, TU Dresden, 01062 Dresden, Germany

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Bremen Center for Computational Materials Science (BCCMS), Universität Bremen, 28359

Bremen, Germany 4

Dresden Center for Computational Materials Science (DCMS), TU Dresden, 01062 Dresden,

Germany 5

Centro de Investigación en Química Biolóxica e Materiais Moleculares (CIQUS) and

Departamento de Química Orgánica, Universidade de Santiago de Compostela, 15782-Santiago de Compostela, Spain 6

Laboratoire de Chimie et Physique Quantiques, IRSAMC-CNRS-UMR5626, Université Paul-

Sabatier (Toulouse III), 31062 Toulouse Cedex 4, France. 7

Centre d’élaboration de matériaux et d’études structurale (CEMES), UPR 8011 CNRS,

Nanosciences Group & MANA Satellite, 29 Rue J. Marvig, P.O. Box 94347, 31055 Toulouse, France

*Corresponding Author: [email protected]


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ABSTRACT

On-surface synthesis provides a powerful method for the generation of long acene molecules, making possible the detailed investigation of the electronic properties of single higher acenes on a surface. By means of scanning tunneling microscopy and spectroscopy combined with theoretical considerations, we discuss the polyradical character of the ground state of higher acenes as a function of the number of linearly fused benzene rings. We present energy and spatial mapping of the tunneling resonances of hexacene, heptacene, and decacene, and discuss the role of molecular orbitals in the observed tunneling conductance maps. We show that the energy gap between the first electronic tunneling resonances below and above the Fermi energy stabilizes to a finite value, determined by a first di-radical electronic perturbative contribution to the polyacene electronic ground state. Up to decacene, the main contributor to the ground state of acenes remains the lowest-energy closed-shell electronic configuration.

KEYWORDS: scanning tunneling spectroscopy, on-surface synthesis, deoxygenation, acenes, molecular resonances, energy gap, molecular orbitals.


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Acenes are a series of polycyclic aromatic hydrocarbons with an increasing number of linearly fused benzene rings.1, 2, 3, 4 The nature of their electronic structure is actively discussed4, 5, 6 and fundamental questions regarding the electronic configuration of the ground state as a function of the number of rings are still open, which can influence the application of acenes as semiconducting materials and are important to understand the electronic transport properties of molecules that can be considered as the narrowest graphene nanoribbons with zigzag edge. Acenes with more than five rings are chemically extremely reactive, light unstable and have a very low solubility.7 Spectroscopy of their low-energy excitations has been mainly restricted to optical absorption spectrum experiments up to nonacene from matrix isolation studies8,

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and

very recently undecacene.10 On-surface synthesis allows to generate long acenes,11, 12, 13, 14, 15, 16, 17

and to map their electronic structure molecule per molecule using scanning tunneling

microscopy (STM) and spectroscopy (STS). Here, we compare STS spectra on acenes of different length synthetized on the Au(111) surface, and we show that the energy gap between the first observed electronic tunneling resonances below and above the Fermi energy is not decreasing to zero for large N (up to N = 10) but is stabilized at about 0.95 eV. We provide a simple interpretation of this non-zero gap involving a di-radical electronic perturbative contribution to the polyacene electronic ground state.

RESULTS AND DISCUSSION On-Surface Synthesis of Acenes On-surface chemistry allows to produce molecular compounds that cannot be prepared by standard solution-based methods due to their low solubility and instability.18, 19, 20, 21 On a surface maintained in ultra-high vacuum, the high chemical reactivity of long acenes can be kept under


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control by inducing the final steps of the synthesis through deoxygenation,11,

12,

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dehydrogenation,14, 22 or decarbonylation15, 16 (see Scheme 1). Choosing the Au(111) surface as substrate has the advantage that the electronic coupling between acenes and surface is relatively weak, in contrast to Cu(111)23,

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or Ag(111).16 In the experiments described here, we have

deposited the respective epoxy-precursors for hexacene, heptacene, and decacene onto a clean Au(111) surface kept at room temperature. After the on-surface chemical reduction of those compounds by thermal annealing,11 fully deoxygenated acenes are obtained on the surface. By dosing the precursor molecules at a sub-monolayer coverage, the produced acenes are well separated from each other on the surface and can be studied by STM with high intramolecular spatial resolution. The on-surface synthesis of hexacene and decacene is described in ref.12,

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while heptacene is now also obtained following the same surface reaction. High-resolution STM images of the so-obtained hexacene, heptacene, and decacene are presented in Figure 1.

Electronic Resonances In Figure 2a, a STS dI/dV spectrum is presented for heptacene, showing three well defined resonances at -1.4 V, -0.55 V and 1.0 V, that we call for simplicity R-1, R0, and R1 respectively. In Figure 2b, the corresponding differential conductance maps recorded in constant-current mode are shown. The maps are very similar to the case of hexacene,12 showing however an additional lobe. The STS dI/dV spectrum and the constant-current dI/dV maps of decacene on Au(111) are presented in Figure 3. Apart from the R0 and R1 frontier resonances, which were reported earlier13, higher-order resonances are now accessible. Figure 3 shows the maps of the three resonances below and above EF, characterized by nodal planes along the molecule long axis


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separating 16, 18 and 20 lobes, respectively. The resonances above EF have pronounced conductance maxima at either end. To describe the electronic structure of decacene, we have used elastic scattering quantum chemistry (ESQC) to calculate the dI/dV maps (see Methods section for details). With this method, the mono-electronic Hamiltonian is built up using full valence Slater atomic orbitals per atom present in the STM junction. Counting the number of maxima and minima on the calculated differential conductance maps leads to a first identification of the tunneling resonances.12 We can therefore assign in a first approximation the tunneling resonances R-1, R0 and R1 in Figure 2 to the contribution of HOMO-1, HOMO and LUMO to the tunneling current of the free heptacene molecule, respectively.12, 25, 26, 27 For the case of decacene (Figure 3) R3 can be attributed to the LUMO+2 contribution, R2 to LUMO+1, R1 to LUMO, R0 to HOMO, R-1 to HOMO-1, and R-2 to HOMO-2. This does not mean that the observed electronic resonances can be in general assigned solely to those molecular orbitals (MOs), as discussed below (see section “Role of Molecular Orbitals”). The constant current dI/dV maps of hexacene have been reported in ref.12, while a comparison between differential conductance measurements of hexacene at constant height and constant current is presented in the SI.

Stabilization of the Energy Gap In Figure 4 we have plotted, as a function of the number of linearly fused benzene rings, the experimental tunneling electronic gap between the frontier resonances R0 and R1 in STS experiments of acenes up to decacene on Au(111). The data is fitted by an exponential decay function, similar to the one used to describe the optical transition data of acenes9 considering the


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simple model of a π electron system in a rigid quantum box.28 As one can see in Figure 4a, the experimental gap does not close and stabilizes to a finite value of about 0.95 eV. A definitive assignment of each observed tunneling resonances to a single MO contribution would imply that the experimental tunneling gap from R0 to R1 converges towards zero for long acenes. For example and as recently shown,28 a convergence of the S0-S1 optical gap of acenes towards zero can be obtained by density functional theory (DFT) calculations. The same convergence is also obtained with the semi-empirical Hamiltonian used in the ESQC STM image calculations. This is not the case of the experimental results (Figure 4a) indicating that using a MO basis set, further MOs than the HOMO and LUMO mono-electronic states are contributing to each of the observed R0 and R1 tunneling resonances, stabilizing this gap. The recently published case of undecacene confirms our interpretation, showing a STM energy gap on Au(111) of 1.1 eV22 and an optical gap of about 1.2 eV.10 While increasing N, configuration interaction (CI) calculations4, 5 of the ground electronic state of a series of acenes up to N = 12 show a progressive mixing of the native singlet closed-shell determinant with di-radical contributions. For N = 10, Ref.

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indicates a contribution to the

ground state of 90% of the closed-shell configuration with two electrons on the HOMO. To model this effect from an effective Hamiltonian point of view,29 we have used the simple Hückel-Hubbard model with the resonance integral β and the on-site bi-electronic repulsion energy U together with the perturbation theory to evaluate the weight of this singlet state configuration in the ground state as a function of N (see ref.6 and Methods for details). Starting from a molecular orbital basis set, doubly excited Slater determinants take an increasing weight in the ground state wave function, giving an increasing di-radical character to this state.


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Using U/β = -2.5, the electronic gap is not oscillating anymore and converges towards 0.34 eV for | β |= 3.4 eV for infinite N as presented in Figure 4b. Notice here that we have taken into account only the ground state stabilization effect. The S1 state is also destabilized by the U term. However, it is delicate to take into account this effect in our perturbation model.30 To compensate, we have selected a U value slightly larger than usual.31, 32 Interestingly, with this simple model the weight of the closed-shell determinant for N = 10 is only 62% of the total ground state, reproducing the contamination of the ground state by at least doubly excited electronic configurations, and confirming that the observed dI/dV electronic resonances cannot be attributed simply to mono-electronic MO resonances.33

Role of Molecular Orbitals The spatial distribution of a tunneling electronic resonance along a molecule recorded by STM or STS is usually called a molecular orbitals mapping.25, 26, 27 A recent debate in literature questions however on how a molecular orbital MO can really be observed in STM.34,

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We agree with

those authors that experimental STM dI/dV conductance maps are not simply a direct image of |Ψ|2 of the molecule in the tunnel junction, where Ψ is its total electronic wave function. To describe the spatial mapping of a tunneling resonance, a detail model of the multi-electrons contributions the tunneling process is required. More precisely, at low electronic couplings between tip, molecule, and surface, a conductance map provides the spatial distribution over the molecule of the square of the secular Heisenberg-Rabi oscillation frequency describing the electron transfer processes occurring between the end atom of the STM tip apex and the metal surface through this molecule.36,

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However, among an a-priori infinite possible choice of

mono-electronic basis set functions to construct the Slater determinants34 used to describe those


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electron transfer processes, canonical molecular orbitals are very well adapted. Here, the atomic orbitals of the tip apex are electronically overlapping with only a few canonical MOs entering in the construction of the many-electrons states contributing to the electron-transfer processes. On the other hand, being a low-pass-filter-like quantum to classical transduction,37 this quantum measurement cannot capture the contribution coming from all the existing electron transfer channels through the molecule showing up after this decomposition. The atomic orbitals of the tip-apex-end-atom are the pointer states of this quantum measurement. High frequency processes are saturating this transduction and therefore do not contribute to the dI/dV map. Notice also that, due to the electronic asymmetry in an STM tunnel junction, this saturation is generally weak and is only appearing in the background. Therefore, a dI/dV map is giving access to a few canonical MO entering in the construction of Ψ based on the superposition of Slater determinants.34 Depending on the dI/dV bias voltage, this can be the HOMO or LUMO contribution26 or a superposition of HOMOs and/or LUMOs.. A complete reversal of the HOMO and HOMO-1 order was also recently observed after preparing the adsorbed molecule in an open-shell configuration.39 We can therefore conclude that the description of the ground state of a given acene using a quantum superposition of a single Slater determinant doubly filled up to HOMO together with an increasing di-radical contribution obtained by emptying the HOMOs and populating the LUMOs is the most convenient decomposition of this ground state relative to the STM quantum measurement process. For small N, when the contamination of the ground state by doubly excited configurations is not severe, this gives a good agreement between ESQC calculated mono-electronic dI/dV maps and the corresponding experimental images. For example, ESQC is using this HOMO for calculating the R0 resonance map. In other words, molecular orbitals are


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only one of many mono-electronic basic set able to describe tunneling electronic resonances. Among the possible decompositions however, molecular orbitals are a very convenient monoelectronic basis set, well adapted also to describe the atomic orbitals of the tip apex. Considering the R0 dI/dV resonance, the corresponding hole transfer channel through the molecule can be described in first approximation by a virtual quantum state with one electron instantaneously missing in the HOMO of the corresponding ground state Slater determinant. This is the channel mainly captured here by the quantum transduction process.33 The R0 and R1 resonances can be therefore interpreted using a description based on HOMO and LUMO single hole or electron transfer virtual channels. In this case, all the other MO contributions to the electronic ground state via their respective Slater determinants are not measured in the dI/dV map, indicating that the main contributor to the ground state tunneling resonance (R0) is a closed-shell state. For N = 10, the contamination by the doubly excited configurations (38% in our simple model) is preventing the R0-R1 gap to reach zero. However, it is not large enough to be observed in the dI/dV map. For N > 10, such contamination will certainly become more severe and the conductance maps will deviate from a simple mono-electronic MO interpretation.

CONCLUSIONS We have presented the electronic resonance maps of hexacene, heptacene, and decacene synthetized on the Au(111) surface by deoxygenation. Among the many possible decompositions, molecular orbitals are a very convenient mono-electronic basis set to describe tunneling electronic resonances, because they are well adapted to describe the atomic orbitals of the tip apex. For long acenes, the energy gap between the first observed electronic tunneling resonances below and above the Fermi energy is stabilized to a value of about 0.95 eV due to a


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first di-radical electronic perturbative contribution to the polyacene electronic ground state. Up to decacene, the ground state quantum structure captured by the STM dI/dV maps is a singlet state with all the molecular orbitals doubly filled up to HOMO. The other Slater determinants contributing to this ground state are stabilizing the electronic gap, but do not contaminate enough the ground multi-electronic state to be observed in the STM conductance maps.

METHODS Experimental Details. STM experiments were performed using a Createc instrument operating at a low temperature of T ≈ 5 K and in ultra-high vacuum (p ≈ 1 × 10−10 mbar). A Au(111) surface single monocrystal was used as substrate and prepared by repeated cycles of sputtering (Ne+) and annealing (720 K). Suitable epoxy precursors of hexacene, heptacene and decacene were deposited in separate experiments onto the clean sample kept at room temperature. Temperature-induced deoxygenation was triggered by transferring the sample out of the STM to anneal it up to temperatures in the range between 423 K and 493 K. Subsequently, the sample was cooled again and transferred back inside the STM without breaking the ultra-high vacuum at any time.

Computational Details. The differential conductance maps of the electronic resonances of various acenes were calculated within the mono-electronic elastic scattering quantum chemistry (ESQC) approach40. It is based on a single-electron Hamiltonian describing the electronic structure of the STM tunneling junction within the extended Hückel theory and scattering matrix approach. It allows to obtain constant- height and constant-current STM images and differential conductance maps including local tip apex interaction with the electronic cloud of the molecule.


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While increasing N, the oscillations towards zero of the optical gap can also be reproduced using a simple Hückel Hamiltonian model with a Hubbard term in the 2pz Hamiltonian6 and by introducing a small transverse through-space electronic coupling β’ between each 1,4 carbon atoms along the polyacene skeleton41. Here, β is the hopping electronic integral between 2pz adjacent carbons along this polyacene skeleton. Its value ranges from | β |= 2.8 eV to 3.4 eV31, 32. Accordingly, the above small transverse β’= 0.06 β used in our calculation was optimized to get the first HOMO-LUMO crossing for N = 10 following Korytar et al.28 The adsorption configurations of the molecules were first obtained by ab initio density functional theory (DFT) calculations using a mixed Gaussian and plane wave approach implemented in the Quickstep code of CP2K42. Goedecker-Teter-Hutter pseudo potentials43 with the Perdew-BurkeErnzerhof exchange-correlation functional44 and a valence double-zeta basis set were chosen. Dispersion correction was included by using the nonlocal revised Vydrov-van Voorhies (rVV10) functional45. The Au(111) surface was modelled by a periodic slab of six layers, with three bottom layers fixed at bulk positions during relaxation. Surface reconstruction effects were neglected.

ASSOCIATED CONTENT Supporting Information. Additional STM and STS data for anthracene, tetracene hexacene, heptacene, and decacene synthetized on Au(111) are shown in the Supporting Information file.

AUTHOR INFORMATION


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Corresponding Author * Correspondence should be addressed to Francesca Moresco, E-mail: [email protected].

ACKNOWLEDGMENT This work was funded by the ICT-FET European Union Integrated Project PAMS (agreement no. 610446). Support by the German Excellence Initiative via the Cluster of Excellence EXC1056 ‘‘Center for Advancing Electronics Dresden’’ (cfaed), the International Helmholtz Research School ‘‘NANONET’’and the Agencia Estatal de Investigación (MAT2016-78293-C63-R and CTQ2016-78157-R), the Xunta de Galicia (Centro singular de investigación de Galicia accreditation 2016–2019, ED431G/09), and the European Regional Development Fund (ERDF) is gratefully acknowledged.


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FIGURES

Scheme 1. (a) The on-surface generation of large acenes starting from three different types of precursors: epoxyacenes (a left), hydroacenes (a center) and alpha-diketone (a right) precursors. (b) Large acenes obtained and studied on Au(111) through on-surface deoxygenation.


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Figure 1. STM images of hexacene, heptacene and decacene (top to bottom). All three images were acquired with a functionalized tip and while scanning at constant height. Used bias: 20 mV (top, down images) and -5 mV (middle image).


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Figure 2. Spectroscopic data of a single heptacene on Au(111). (a): Differential conductance spectra of a heptacene molecule (black curve) and of the bare Au(111) surface (gray curve). (b): Constant-current differential conductance measurements at I = 0.3 nA and the indicated bias values (1.0 V, -0.55 V, and -1.4, respectively).


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Figure 3. Electronic resonances of decacene on Au(111). (a) Differential conductance spectra of a decacene molecule recorded at constant-height in a broad bias range from -2.5 V up to 2.5 V. Left: Filled-state data of decacene (black) and of the bare surface (grey). Right: Empty-state data. (b-c): Filled-state resonances. Constant-current maps of the differential conductance (b) and ESQC-calculated constant-current maps (c) for R-2 (HOMO-2), R-1 (HOMO-1) and R0 (HOMO). (d-e): Unfilled-state resonances, i.e. R1 (LUMO), R2 (LUMO+1) and R3 (LUMO+2).


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Figure 4. The energy gap of acenes as function of number of linearly fused benzene rings. (a) Tunneling energy gap between the R0 and R1 frontier resonances in STS experiments on Au(111). The experimental value for pentacene is taken from W.-H. Soe et al.27 The data was fitted by the exponential decay function E = E0 + A*exp[-n/t], with n being the number of rings and E0, A, t as parameters. In particular, the fitting results in E0 = (0.95 +- 0.05) eV. (b) Black curve: Calculated energy gap in unit of | β | for our simple Hückel model considering a transverse electronic coupling β’ per benzene unit. The oscillation of the gap obtained in DFT calculation is well reproduced. Blue curve: the same model plus a perturbative contribution of a simple double excited HOMO→LUMO di-radical state. Here, the gap is not going to zero (see Methods section for details).


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REFERENCES

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Electronic Resonances and Gap Stabilization of Higher Acenes on a

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