Excitonic States in Narrow Armchair Graphene Nanoribbons on Gold

Article pubs.acs.org/JPCC

Excitonic States in Narrow Armchair Graphene Nanoribbons on Gold Surfaces Christopher Bronner,† David Gerbert, Alexander Broska, and Petra Tegeder* Ruprecht-Karls-Universität Heidelberg, Physikalisch-Chemisches Institut, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany S Supporting Information *

ABSTRACT: Narrow graphene nanoribbons (GNRs) exhibit electronic and optical properties that are not present in extended graphene. Most importantly, they possess band gaps in the order of a few electron volts, which has been subject to numerous studies. Here we report on the experimental observation of exctionic states in the band gap of N = 7 armchair GNRs (7-GNR) on Au(111) and Au(788) using energy- and angle-resolved two-photon photoemission spectroscopy. Thereby, an exciton binding energy in the 7-GNR on Au(111) of 160 ± 60 meV has been determined. On the stepped Au(788) surface, the exciton binding energy is in the same range.

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INTRODUCTION Graphene nanoribbons (GNRs) are narrow quasi-one-dimensional structures which offer tunable electronic and magnetic properties depending on their width and edge symmetry.1−9 Particularly, they exhibit a band gap that depends inversely on the GNR width. Defect-free GNRs with nanometer scale widths and atomically precise edge structures can be synthesized using a bottom-up approach based on a thermally activated and surface-assisted two-step reaction involving a polymerization of suitable precursor molecules followed by a cyclodehydrogenation.4 Following this approach, various armchair-edged GNR (AGNR) structures,4,8,10 N-,3,11,12 B-,13,14 and S-doped15 AGNRs; AGNR heterostructures;3,5,6 and very recently a zigzag edge topology have been generated.16 For the experimental characterization of the electronic properties, i.e., mainly the determination of transport gaps, scanning tunneling spectroscopy (STS) has been the most widespread method. In some cases also photoemission,11,17−22 inverse photoemission,19 and high-resolution electron energy loss (HREELS)11,17 as well as optical spectroscopy23 have been utilized. Notably, in the latter case the optical response of an N = 7 AGNR (7AGNR) on Au(788) has been measured, identifying excitonic resonances.23 To date, this is the only experimental observation of excitonic states in GNRs, apart from theoretical studies.23−28 Here, we investigate an N = 7 AGNR on Au(111) and Au(788) using energy- and angle-resolved two-photon-photoemssion (2PPE) spectroscopy. Generally, 2PPE enables us to gain insight into energetic positions of occupied and unoccupied electronic states (bands) as well as excitonic states (bands) of adsorbed species, which has been demonstrated for several adsorbate−metal systems.29−35 Angle-resolved 2PPE allows to obtain information about the dispersion, viz., the electron localization−delocalization. For 7-AGNR, we report © XXXX American Chemical Society

the identification of an excitonic state which possesses on both the Au(111) and the Au(788) surface a binding energy in the order of 160 meV.

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EXPERIMENTAL SECTION The Au(111) and Au(788) single crystals were prepared by Ar+ sputtering and annealing at 800 K. 10,10′-Dibromo-9,9′bianthryl precusor molecules were deposited from an effusion cell held at a temperature of 470 K while the surface was kept at 300 K. In two-photon photoemission, a pump pulse hν1 excites an electron from below the Fermi level (EF) to intermediate unoccupied states at energies E − EF = Ekin + Φ − hν2 (with Φ the work function), while the probe pulse hν2 photoionizes the sample by lifting the excited electron above the vacuum level (Evac). Photoelectrons are detected with a time-of-flight (TOF) spectrometer and are analyzed with respect to their kinetic energy (Ekin). To determine the origin of peaks in the 2PPE spectrum, i.e., to identify whether they arise from occupied initial or unoccupied intermediate electronic molecular states in the 2PPE process, the dependence of Ekin on the photon energy was investigated. Variation of the detection angle provided insights into the dispersion parallel to the surface (k∥), i.e., the degree of electron localization−delocalization of occupied and unoccupied electronic states (for details, see ref 36). The 7-AGNRs are generated using an on-surface reaction. In this process, adsorption of several layers of 10,10′-dibromo9,9′-bianthryl (DBBA) on Au(111) or Au(788) and subsequent heating at 470 K leads to desorption of the second and higher layers as well as halogen dissociation and coupling of the resulting activated biradical monomers in the first layer, yielding Received: October 27, 2016 Published: October 28, 2016 A

DOI: 10.1021/acs.jpcc.6b10834 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C a sterically crowded and hence twisted polyanthrylene (see Figure 1). In a second heating step at 670 K, this polymer

Figure 1. Synthesis of the N = 7 AGNR on gold surfaces. In the first step, the adsorbed 10,10′-dibromo-9,9′-bianthryl (DBBA) is dehalogenated. In the subsequent polymerization step, covalent coupling of the DBBA radicals leads to the formation of a polyanthrylene chain. Upon heating the polymer, a cyclohehydrogenation reaction yields the fully aromatic AGNR.

undergoes a subsequent cyclodehydrogenation reaction providing access to the desired 7-AGNR. By adsorption of xenon onto the GNR-covered surface we can roughly deduce a GNR coverage of approximately 65% of a monolayer (see the Supporting Information). The formation process can nicely be followed by angle-resolved HREELS.17 Note that recent scanning tunneling microscopy (STM) studies have shown that the 7-AGNR can be produced in the same fashion on stepped Au(788) surfaces and that the ribbons are aligned parallel to the steps.18,19

Figure 2. Angle-resolved 2PPE data of 7-AGNR (a) on Au(111) and (b) on Au(788). The spectra have been normalized to the d-band peaks. 2PPE measurements on 7-AGNR/Au(788) have been measured along the step edges. The spectra are displayed as a function of the final state energy of the photoelectrons, EFinal, relative to the Fermi level, EF. (Panel a is adapted with permission from ref 17. Copyright 2012 American Physical Society.)

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RESULTS AND DISCUSSION Figure 2 shows angle-resolved one-color (pump and probe pulses have the same energy) 2PPE data of 7-AGNR adsorbed on Au(111) and Au(788) recorded with a photon energy of hν = 4.69 eV and hν = 4.55 eV, respectively. Apart from photoemission features originating from the gold d-bands, three further peaks are observed. On the basis of detailed photonenergy-dependent measurements, they can be related to photoemission from unoccupied intermediate states (see the Supporting Information). On the Au(111) surface and for normal emission (0°, k∥ = 0), the peak labeled with A is located at 1.44 ± 0.06 eV with respect to the Fermi level (EF). The peaks labeled B and C possess an energetic position of 3.92 ± 0.06 eV and 4.30 ± 0.06 eV, respectively, with respect to EF. On Au(788), the peak labeled with A exhibits an energetic position of 1.40 ± 0.06 eV. The peaks labeled as B and C adopt energetic positions of 3.95 ± 0.06 eV for B and for C of 4.19 ± 0.06, which are similar to positions found on Au(111). Figure 3 depicts the binding energy of the states labeled as A, B, and C as a function of k∥. Of the three unoccupied states only the one labeled with B shows a dispersion, viz., states A and C are localized. The effective mass of B on Au(111) is m* = 1.2 ± 0.2 me and on Au(788) it is similar, namely, m* = 1.0 ± 0.2 me. On Au(111), the 7-AGNRs are oriented isotropically. Thus, one must consider that the measured momentum parallel to the surface, k∥, of a photoemitted electron has two components, namely, a component along the ribbon axis (which is the quantity of interest) and one perpendicular to it (and parallel to the surface). Therefore, the momentum

measured in the experiment is larger than the momentum along the ribbon axis (see ref 17 for further details). The isotropy leads to a smearing of the measured dispersion toward higher k∥ and thus an overestimation of the effective mass; however, the isotropy does not affect the measured binding energies. Because the isotropic alignment of the 7-AGNR on the Au(111) surface leads to an overestimation of the effective mass, we cannot identify states A and C as localized, i.e., having a very high effective mass, with absolute certainty. Furthermore, the isotropy has been suspected frequently as a source of an experimental artifact which dissembles a high degree of electron localization. In order to clarify the dispersion of the seemingly localized states, we now turn to angle-resolved measurements on the stepped Au(788) single-crystal surface with its globally aligned 7-AGNRs. The missing dispersion of the states labeled as A and C on Au(788) (see Figure 3a,c) are consistent with the measurements on the Au(111) surface. Thus, we can rule out the isotropic orientation of the 7-AGNR on this crystal face to cause a false dispersion measurement and conclude that the states labeled as A and C do not exhibit a dispersion. The question we have to answer is what kind of electronic states can be attributed to the three observed photoemission features. Therefore, we must revisit our original assignment (see ref 17) because new insights on the electronic structure of B


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The Journal of Physical Chemistry C

Figure 3. (a) Dispersion of the 2PPE feature labeled as A on Au(111) (black diamonds) and Au(788) (red triangles) based on the measurements shown in Figure 2. (b) Dispersion of the peaks labeled as B and C on Au(111) and (c) on Au(788) based on the measurements shown in Figure 2. A parabolic fit yielding the effective mass, m*, is shown. (Panel b is adapted with permission from ref 17. Copyright 2012 American Physical Society.)

(bands) are ionized, creating a positive ion. Thus, in this case we obtain the ionization potential or the electron affinity of the respective molecular state (band). Hence, both STS and 2PPE enable the determination of transport levels and consequently transport gaps. Additionally, in 2PPE an intramolecular excitation, e.g., a direct highest occupied molecular orbital (HOMO) (or VB) to lowest unoccupied molecular orbital (LUMO) (or CB) transition is also possible (the molecule remains neutral), which is needed for the formation of excitons. The energy required for this process is Eopt, the optical gap. Eopt is lower than the difference between the ionization potential and electron affinity of the respective HOMO (VB) and LUMO (CB) levels (transport gap).42 For the exciton, the ionization potential is measured in 2PPE. Figure 4 displays an energy diagram for 7-AGNR on Au(111). The energetic position of the VB and CB have been determined using STS by Ruffieux et al.;18 thus, a transport gap of 2.3 eV has been obtained. Correlating the state at 1.44 eV to an exciton, we get an exciton binding energy of

7-AGNR on Au from STS and angle-resolved photoelectron spectroscopy (ARPES) studies published since ref 17 appeared must be considered.18,19,37,38 The peak labeled as B can be easily assigned to an image potential state (IPS), because it shows a dispersion with an effective mass typical for IPS and is pinned to the vacuum level, as can be seen in xenon coadsorption experiments (see the Supporting Information). The nondispersing state labeled as C located at 4.30 ± 0.06 eV can be associated with a higher-lying 7-AGNR-derived unoccupied state. However, most surprisingly, the lowest unoccupied state located at 1.44 eV on Au(111) and at 1.40 eV at the Au(788) surface (peak labeled A) exhibits no dispersion, in contrast to what would be expected for the 7AGNR conduction band (CB). Note that none of the observed states exhibit a considerable lifetime which can be detected with our femtosecond time-resolved 2PPE setup (see the Supporting Information), due to the strong electronic coupling to metal states. Excited-state dynamics of image potential states at the 7AGNR/Au(788) interface have been investigated previously, showing also no significant lifetimes of the states.39 Merging our results with measurements on the electronic structure of 7-AGNR on Au(111) and Au(788) known from literature, we may elucidate the origin of the state found around 1.4 eV. The electronic structure of the 7-AGNRs on Au(111) have already been studied using STS, resulting in a transport gap of 2.3 eV.18,38 The valence band (VB) is located −0.7 eV below EF of the Au(111) surface, while the energetic position of the conduction band (transport level) is 1.6 eV.18,37 ARPES measurements of aligned 7-AGNR on Au(788) indicate a clear dispersion of the VB with an effective mass of m* = 0.21me.18,19,38 If the here observed state A would originate from the CB, then it should possess a dispersion similar to the VB as shown in ARPES.18 Instead, we observe a localized feature and propose that the peak is caused by photoemission from an excitonic state.40 This is consistent with the fact that photoemission from an exciton does not show a dispersion because the photoionization process destroys the exciton (quasiparticle).41 One has to consider that electronic states determined by STS are transport states because electron tunneling into unoccupied molecular states (bands) results in the formation of negative ion resonances. On the other hand, tunneling out of occupied molecular states (bands) leads to the creation of positive ion resonances. The same is true for 2PPE measurements, that is, when unoccupied states (bands) are populated via an electron transfer from the metal to the molecule (creating a negative ion resonance) or when occupied molecular electronic states

Figure 4. Energy level diagram of 7-AGNR/Au(111). The blue levels are the ionization potentials (left axis) and the red ones are the electron affinities (right axis); EF denotes the Au(111) Fermi level. The values for the VB and CB (indicated by *) are mesured by Ruffieux et al.18 using STS. The exciton (Exc.) exhibts a binding energy of 160 meV. C



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160 ± 60 meV and accordingly an optical gap of 2.14 eV. This value matches with the optical gap measured with HREELS on 7-AGNR/Au(111).17 Because the VB of the 7-AGNR on both the Au(111) and the Au(788) possesses the same binding energy (−0.7 eV18), we assume a very similar electronic coupling between the 7-AGNR and the respective gold substrate, and accordingly, the energetic position of the CB is most likely the same on Au(788). Hence, on Au(788) the value for the exciton binding energy would be 200 ± 60 meV and the optical gap is 2.1 eV. This is in perfect agreement with reflectance difference spectroscopic measurement on 7-AGNR/ Au(788) in which an optical gap of 2.1 eV has been found as well,23 and this further supports our assignment of the photoemission peaks to arise from excitons. Considering the impreciseness in the measured energetic position of the photoemission feature resulting from the excitonic state (error of ±60 meV), we propose a similar exciton binding energy in the 7-AGNR on both the Au(111) and the Au(788) substrate. Note that the calculated exciton binding energy for the gas-phase 7-AGNR possesses a value of around 1.8 eV,23,28 which is considerably higher than the binding energy measured here. This difference can be attributed to not included substrate polarization effects (image charge contributions) which strongly affect the size of the transport gap (i.e., energetic position of the affinity level). Including these effects in the calculations would result in a reduction of the exciton binding energy of 1.0−1.4 eV,23 thus resulting in a value close to the here experimentally determined energy.

CONCLUSION We have identified excitonic states in N = 7 armchair edged graphene nanoribbons (7-AGNR) on gold surfaces using energy- and angle-resolved two-photon photoemission (2PPE) spectroscopy. On Au(111), the exciton possesses a binding energy of 160 ± 60 meV. A similar value is found for the 7-AGNR adsorbed on the stepped Au(788) surface. Our study shows that 2PPE is a powerful tool to determine important electronic properties such as exciton binding energies and optical gaps of graphene-based nanostructures. ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b10834. Additional information about the 2PPE experiments (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +49 (0) 6221 54 8475. Present Address †

C.B.: Department of Physics, University of California at Berkeley, Berkeley, CA 94720. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Funding by the Deutsche Forschungsgemeinschaft (DFG) through Project TE479/3-1 is gratefully acknowledged. D


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Excitonic States in Narrow Armchair Graphene Nanoribbons on Gold

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