Ultrafast Bidirectional Charge Transport and Electron Decoherence

Letter pubs.acs.org/NanoLett

Ultrafast Bidirectional Charge Transport and Electron Decoherence at Molecule/Surface Interfaces: A Comparison of Gold, Graphene, and Graphene Nanoribbon Surfaces Olgun Adak,† Gregor Kladnik,‡,§ Gregor Bavdek,∥ Albano Cossaro,⊥ Alberto Morgante,*,§,⊥ Dean Cvetko,*,‡,⊥ and Latha Venkataraman*,†,# †

Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York 10027, United States Faculty of Mathematics and Physics, University of Ljubljana, Ljubljana, Slovenia § Department of Physics, University of Trieste, Trieste, Italy ∥ Faculty of Education, University of Ljubljana, Ljubljana, Slovenia ⊥ CNR-IOM Laboratorio Nazionale TASC, Basovizza SS-14, km 163.5, I-34012 Trieste, Italy # Department of Chemistry, Columbia University, New York, New York 10027, United States ‡

S Supporting Information *

ABSTRACT: We investigate bidirectional femtosecond charge transfer dynamics using the core−hole clock implementation of resonant photoemission spectroscopy from 4,4′-bipyridine molecular layers on three different surfaces: Au(111), epitaxial graphene on Ni(111), and graphene nanoribbons. We show that the lowest unoccupied molecular orbital (LUMO) of the molecule drops partially below the Fermi level upon core−hole creation in all systems, opening an additional decay channel for the core−hole, involving electron donation from substrate to the molecule. Furthermore, using the core−hole clock method, we find that the bidirectional charge transfer time between the substrate and the molecule is fastest on Au(111), with a 2 fs time, then around 4 fs for epitaxial graphene and slowest with graphene nanoribbon surface, taking around 10 fs. Finally, we provide evidence for fast phase decoherence of the core-excited LUMO* electron through an interaction with the substrate providing the first observation of such a fast bidirectional charge transfer across an organic/graphene interface. KEYWORDS: Charge transfer dynamics, photoemission spectroscopy, core−hole clock, molecule−metal interface, molecule−graphene interface, charge decoherence

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AFS), and resonant photoemission spectroscopy (RPES) with the core−hole clock method, as illustrated in Figure 1A, to probe charge transfer dynamics across heterogeneous interfaces.19−26 Core−hole clock spectroscopy has previously been used to measure femtosecond charge-transfer times across different molecule−metal interfaces.22−24,27−34 We apply this technique to investigate femtosecond charge transfer dynamics from 4,4′-bipyridine (BP) molecular layers to Au(111), epitaxial graphene on Ni(111) and graphene nanoribbon (GNR) surfaces (Figure 1B). We find that charge transfer from BP to the carbon surfaces is slow compared with the Au(111) surface. We attribute this difference to a reduced electronic interaction between the molecule and the carbonbased surfaces due to a reduced density of electronic states of

raphene and its derivatives have been subject to extensive research due to their extraordinary electronic, mechanical, and optical properties.1−10 Molecular electronics has also attracted attention from the physics and chemistry community for providing a fundamental understanding of charge transfer at the nanometer scale.11−14 Recently, efforts for incorporating carbon-based components into organic electronic devices have been fruitful; for example, researchers have demonstrated that hybrid devices can function as solar cells and light emitting diodes.8,15−18 Clearly, the performance and quality of such devices depends strongly on the electronic properties of their hybrid interfaces. Therefore, a complete understanding of charge transfer dynamics at such interfaces is fundamental for designing functional and efficient carbon-based organic electronic devices. Here, we use a combination of X-ray based spectroscopy techniques including X-ray photoemission spectroscopy (XPS), near-edge X-ray absorption fine-structure spectroscopy (NEX© XXXX American Chemical Society

Received: September 29, 2015 Revised: November 11, 2015

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DOI: 10.1021/acs.nanolett.5b03962 Nano Lett. XXXX, XXX, XXX−XXX

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binding energies across these systems. These surfaces are prepared following published methods as detailed in the Supporting Information document and characterized by XPS and NEXAFS.35,36 We observe that in all systems, the N 1s peak is a single-component peak, as determined by fitting the data with the Doniach−Sunjic line shape.37 We find that the N 1s electron binding energy is larger than that in the multilayer. We also see that the N 1s binding energy varies with the substrate. It is smallest (398 eV) on Au(111) most likely due to the ability of the metal to effectively screen the core−hole.38 The N 1s binding energy is increased to 399 eV on the epitaxial graphene on Ni(111) surface, consistent with a decrease in the free electron density of this surface. Finally, we observe that the N 1s binding energy is closest to that of the multilayer for the GNR on Au(111) substrate. The first implication of this observation is that the GNR cover the underlying Au substrate completely as the spectrum is composed of a narrow single peak. Second, we can conclude that the density of electronic states for this surface is not sufficiently high to screen the core− hole as efficiently as Au, consistent with the fact that GNR presents a significant band gap;39 the effective image plane is likely closer to the underlying Au surface, resulting in a smaller shift in the N 1s binding energy. Finally, we note that the XPS peak is most asymmetric on the Au(111) surface, which is indicative of a coupling between the N and the surface. We determine the molecular orientation on these surfaces by exploiting the polarization dependence of the N 1s to LUMO transition, which is subject to dipole selection rules.40 The LUMO of BP has π character with its nodal plane coinciding with the molecular aromatic ring.41 The N 1s to LUMO transition is therefore forbidden for light polarized in the plane of aromatic rings. We determine an average angle for BP by comparing the N 1s to LUMO transition cross-section with incident electric field perpendicular (p-pol) and parallel (s-pol) to the surfrace.40 In Figure 2B, we show a NEXAFS spectrum where we observe that the N 1s to LUMO transition is strongly suppressed in s-pol on all surfaces. We find that the average angle is around 14° ± 3° on Au(111), 20° ± 3° on epitaxial graphene, and 26° ± 3° on GNR, while the N 1s to LUMO transition peak is broadest on Au(111) and narrowest on GNR. These two results together allow us to conclude that the

Figure 1. (A) Schematic representation of photoemission processes. (i) Direct photoemission. (ii) Core−hole/LUMO* electron-hole pair generation. (iii) Participator decay. (iv) Spectator decay. (v) Charge transfer from the LUMO* to the substrate and Auger decay. (B) Structure of 4,4′-bipyridine and the three surfaces studied in this work.

the surface. We further observe that creation of a core−hole on BP significantly alters the alignment of the BP molecular levels relative to the substrate Fermi level (EF), bringing the LUMO in the presence of a core−hole (LUMO*) close to EF. This leads to electron transfer between the LUMO* and the substrate continuum states within the core−hole lifetime. We can therefore probe the bidirectional charge transfer times and provide evidence for ultrafast phase decoherence of the photoexcited LUMO* electron through an interaction with the substrate. When the LUMO* remains above the Fermi level, the LUMO* electron can escape to the substrate as long as the LUMO is coupled to the substrate.28 However, in the systems studied here, the excited core electron remains partially on the LUMO* as a significant portion of the LUMO* drops below the Fermi level. The phase information on the core excited LUMO* electron is lost through an interaction with the substrate, and thus, subsequent decay of the LUMO* electron is not degenerate with direct photoemission. The angledependent RPES measurements presented here provide, for the first time, direct evidence for this phase decoherence occurring on the femtosecond time scale due to the LUMO* coupling to the substrate states. We first present XPS spectra for monolayer films of BP on all three surfaces and for a multilayer film on Au(111) in Figure 2A, which allow us to compare the nitrogen 1s (N 1s) electron

Figure 2. (A) N 1s binding energies of BP monolayer on Au(111), epitaxial graphene, and GNR and a multilayer BP film on Au(111). All measurements are carried out at a photon energy of 500 eV. The data is fit with the Doniach−Sunjic line shape (solid lines) to illustrate that the peaks have a single component. (B) Nitrogen K-edge NEXAFS spectra of a BP monolayer on Au(111), epitaxial graphene, and GNR measured with the incident electric field polarized perpendicular (p-pol) or parallel (s-pol) to the surfaces. Inset schematic illustrates the orientation of the field relative to the substrate. B


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Figure 3. (A) Nitrogen K-edge RPES map of BP monolayer on Au(111) showing the LUMO* resonance (black dashed line), a line scan above the ionization edge (blue dashed line) and the superparticipator decay seen as a vertical feature starting at the LUMO* energy as indicated by yellow arrow. Nitrogen K-edge RPES line scans at the LUMO* (shaded trace), above the ionization edge (solid line), and multilayer LUMO* (black, upper panel) for a BP monolayer on (B) Au, (C) epitaxial graphene, and (D) GNR. Black arrows indicate the participator peaks and colored arrows indicate the superparticipator peaks.

electron is initially photoexcited to an orbital higher than the LUMO (LUMO + 1, etc.). Thus, the characteristic spectral lines due to the decay of the LUMO* electron are observed regardless of the incident light energy; we denote this process as superparticipator decay. As we will show below, we find evidence for this superparticipator decay channel demonstrating that when we create a N 1s core−hole, the LUMO* goes partly below the Fermi level, allowing an electron from the substrate to occupy the LUMO*. As detailed in the SI, by comparing the intensities of the superparticipator and participator decay channels in the molecule on surface system and the participator intensity in the multilayer system, we obtain the charge transfer time from the surface to the molecular LUMO* and estimate the fraction of the LUMO* that falls below the Fermi level upon creation of a core−hole. We measure the RPES spectra, which comprise of XPS measurements taken at a series of incident photon energies across the nitrogen K absorption edge, of a BP multilayer on Au(111) and BP monolayer on the three surfaces considered here. The multilayer film serves as a reference for the uncoupled or the isolated system and is used to obtain the charge transfer times for all monolayer films. Figure 3A shows a two-dimensional photon energy versus electron kinetic energy RPES map for a BP monolayer film on Au(111). We note first that the resonance intensity due to an excitation to the LUMO* at a photon energy of 399 eV is visible in the map. More importantly, we see a clear spectroscopic feature near an electron kinetic energy of 394 eV, which corresponds to the

molecule/surface interaction is strongest on Au(111) and weakest on the GNR surface. We now turn to the measurements of charge transfer dynamics using the RPES technique. In this technique, a core electron is excited to an unoccupied molecular orbital, or even to the free electron continuum, leaving a core−hole on the molecule. This excited state decays via multiple processes, involving emission of photons or electrons. The former, photoluminescence, is not relevant to the RPES technique; while the latter, photoemission, is fundamental to RPES as illustrated in Figure 1A. The first process that involves electron emission proceeds with the filling of the core−hole and the subsequent emission of an electron, leaving the LUMO empty; this decay process is called participator decay. The second decay process results in the filling of the core−hole and the subsequent electron emission, leaving the system with two holes and an additional (spectator) electron in the LUMO; this decay process is called spectator decay. When the molecule is electronically coupled to the substrate, the excited electron can escape to the substrate, quenching the participator and spectator decay channels. By comparing the participator decay intensity in the isolated and coupled molecular systems and knowing the core−hole lifetime (5 fs for N 1s),42 we can determine the charge transfer times across the molecule− surface interfaces.21 However, another core−hole decay scenario arises if the LUMO* drops below the Fermi level upon a core−hole creation.27 In this case, the LUMO* may get filled via electron transfer from the substrate even if the core C


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Figure 4. Nitrogen K-edge RPES line scans at the LUMO* energy showing the participator intensity for (A) BP on Au(111) at polarization angles of 90°, 75°, 60°, 45°, and 35° and (B) BP on epitaxial graphene at polarization angles of 90° and 35°. Inset: Nitrogen K-edge RPES line scans taken at 416 eV showing the superparticipator peaks.

information. Therefore, any subsequent decay would be of an Auger type with almost no angular dependence on the light polarization.44,45 The intensity of the emitted electrons will therefore show only a weak or no dependence on the light polarization as long as the charge transfer time is significantly shorter than the core−hole lifetime. To probe the phase decoherence of the LUMO* electron through an interaction with the substrate, we measure the nitrogen K-edge RPES spectra for the BP monolayer on Au(111) at five different polarization angles relative to the surface, ranging from 35° to 90°. In Figure 4A, we show the participator intensity at the LUMO* excitation for all polarization angles measured, where we see a weak dependence on the polarization angle. This demonstrates that most of the participator decay occurs after the electron loses its phase information through an interaction with the substrate, consistent with the very fast charge transfer observed in this system. In Figure 4B, we show similar measurements at two angles for the BP monolayer film on epitaxial graphene. The intensity of the emitted electron shows a 30% decrease when the angle between the light polarization and the surface goes from 90° to 35°. This points to a significant resonant photoemission component in the total photoemission signal, consistent with the charge transfer time comparable to the core−hole lifetime determined above for this system. As a comparison, we also report the angular dependence of the superparticipator decay intensity measured at a photon energy of 416 eV, which is entirely due to electron donation from the substrate to the LUMO* and as such bears no phase correlation with the core−electron excitation. In both the Au(111) and the graphene system, we see no angular dependence, consistent with an Auger type decay of the core−hole.46 Therefore, the measurements presented here provide evidence for ultrafast phase decoherence of hot electrons on Au(111) and epitaxial graphene surfaces. In conclusion, we have probed interfacial charge transfer between a BP monolayer on Au(111), epitaxial graphene on Ni(111), and GNR on Au(111) surfaces. We find that the charge transfer is fastest on Au(111), which indicates a strong electronic interaction between BP and the surface. Charge transfer is slower on the epitaxial graphene surface where the BP/graphene interaction is primarily through a van der Waals coupling. Finally, we find an even slower charge transfer on the semiconducting GNR surface, which could stem from the

decay of the LUMO* electron independent of the photon energy above 405 eV. This is the signature for the superparticipator decay process. In Figure 3B−D, we present RPES line scans at the LUMO* and above the ionization edge for all three monolayers (at energies indicated by the dashed lines in Figure 3A) and compare these to the line scan at the LUMO* of the multilayer system. First, we see that in all systems the intensity of the participator decay is reduced relative to the multilayer indicating a charge transfer from the LUMO* to the substrate within the core−hole lifetime. Second, we observe that BP monolayer on Au(111) and epitaxial graphene exhibits a clear HOMO−LUMO* superparticipator Auger lines once a core− hole is created regardless of the photon energy. This indicates that the LUMO* drops at least partially below the Fermi level once the N 1s core−hole is created, leading to a charge transfer from the substrate to the LUMO* and followed by the superparticipator and spectator decay processes.27 Next, we obtain the charge transfer times from the surface to the BP LUMO* and estimate the fraction of the LUMO* that falls below the Fermi, as described in the SI. We obtain a charge transfer time of 2.0 ± 0.5 fs for the BP/Au(111) system, 4 ± 1 fs for the BP/epitaxial graphene system, and 10 ± 2 fs for the BP/GNR/Au(111) system, while the fractions of the LUMO* that fall below the Fermi level are around 1/5 for both Au(111) and epitaxial graphene. For the GNR surface, where Fermi is likely to fall within the band gap,39 we do not observe a measurable backfilling of the LUMO*, similar to what has been observed on semiconducting surfaces.43 We note that the observed charge transfer times are consistent with the trends in the spectator shifts in the Auger spectra on the LUMO* as seen in Figure 3B−3D.32 The strong modification to the level alignment induced by the core−hole not only lets us observe bidirectional charge transport in the RPES measurements but it also opens up the possibility of observing electron decoherence via an interaction with the substrate. The participator decay process is a resonant photoemission process, degenerate with the direct photoemission with an identical final state having a single hole in the valence band. This process is subject to dipole selection rules.40 Specifically, the angle between the light polarization and the normal of the HOMO nodal plane dictates the photoemission intensity. However, if the LUMO* electron interacts with the substrate before filling the core−hole, it loses the phase D


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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.5b03962. Measurements, calculations, and data analysis (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Laerte Patera and Cristina Africh for help preparing the graphene on Ni(111) surface. O.A. acknowledges support from the support from the NSF DMR-1122594. G.K. and D.C. acknowledge support from the Slovenian Research Agency (proj. Z1-6726 and P1-0112). L.V. thanks the Packard Foundation for Support. A.C. acknowledges support from the ANCHOR project of the MIUR FIRB 2010. Support from MIUR (PRIN 20105ZZTSE) and MAE (US14GR12) is acknowledged. Part of this research was also funded through “Progetto Premiale 2012″ project ABNANOTECH.

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Ultrafast Bidirectional Charge Transport and Electron Decoherence

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