Submolecular Electroluminescence Mapping of Organic Semiconductors

Submolecular Electroluminescence Mapping of Organic Semiconductors Christoph Große,*,†,# Pablo Merino,† Anna Rosławska,† Olle Gunnarsson,† Klaus Kuhnke,† and Klaus Kern†,‡ †

Max-Planck-Institut für Festkörperforschung, Heisenbergstraße 1, 70569 Stuttgart, Germany Institut de Physique, École Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland

‡

ABSTRACT: The electroluminescence of organic films is the central aspect in organic light emitting diodes (OLEDs) and widely used in current display technology. However, its spatial variation on the molecular scale is essentially unexplored. Here, we address this issue by using scanning tunneling microscopy (STM) and present an in-depth study of the electroluminescence from thin C60 films (2.45 eV.26 Comparing the emission energy and morphology of individual emission centers, we find that strong structural defects, such as dislocations or domain boundaries tend to exhibit lower emission energies than emission centers at flat C60 terraces. All these observations indicate that the emission centers arise from the radiative decay of perturbed Frenkel excitons20 trapped at

Figure 2. Emission characteristics of individual emission centers. (a−d) Perspective view on the STM topography of different structural defects, with the color representing the simultaneously acquired STM-induced luminescence intensity: (a) orientational disorder, (b) chemical impurity, (c) screw dislocation, (d) C60 domain boundary at substrate step edge. (e) Normalized emission spectra of various emission centers (yellow), shifted vertically for clarity (Vs = −3.0 V, Is = 20 pA to 1 nA) The line width is determined by the experimental resolution. The spectra of the emission centers in panels a−d are displayed with thicker lines and are labeled accordingly. The black curve at the bottom shows the absorption spectra below the fundamental absorption edge provided by Akimoto et al.19

structural defects, whose emission energy is determined by the energy of the local trap. A common criterion used to discriminate the emission from such traps, known as X traps, from those of chemical impurities is the existence of vibrational satellites that result from the coupling of electronic and vibrational transitions. When referencing the spectra to the main emission line, the vibrational features of all emission centers occur at similar energies (see Figure 3a). The spectral shapes resemble those observed by Rossel et al. for thin C60 nanocrystals on ultrathin NaCl layers27 (gray line in Figure 3a), albeit the authors did not report information on the local topography or possible structure defects. In addition, the spectral features in our spectra strongly resemble the vibrational satellites observed in photoluminescence spectra of C60 crystals reported by Akimoto et al.19,20 (black line in Figure 3a). These findings corroborate an emission from C60 molecules and rule out an emission from chemical impurities. Aside from this, the energies of the observed vibronic features provide crucial information on the emission process itself. Indeed, all prominent vibronic peaks can be ascribed to Raman-active intramolecular vibrational modes of C60 with gerade inversion symmetry.19 Coupling to these modes requires a Jahn−Teller mechanism and thus implies that the wave functions of the lowest excited singlet 1232

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Figure 3. Vibrational fine structure in the light emission of thin C60 films. (a) Yellow: normalized STM-induced luminescence spectra acquired on different emission centers (Vs = −3.0 V, Is = 20 pA to 1 nA), gray: emission spectrum of C60 nanocrystals on an ultrathin NaCl film at 50 K (reproduced from Rossel et al.27), black: photoluminescence spectrum of a C60 single crystal at 5 K (provided by Akimoto et al.19). Spectra are referenced to the main emission line and shifted vertically by multiples of 0.4. (b) Schematic energy diagram and assignment of the emission peaks in panel a to the vibrational modes of C60 denoted on the right-hand side.

state S1 and the ground state S0 must have different inversion symmetries at the emission centers, in contrast to an isolated C60 molecule, in which both states have gerade symmetry. Such a local change in the symmetry of the corresponding wave functions can be rationalized only by the local mixing of higherlying unoccupied states or lower-lying occupied states of ungerade character, as a result of the broken translational symmetry at the structural defects. In the following we will discuss the observable size of the emission centers and investigate the spatial variations of their emission characteristics on the molecular scale (Figure 4). When moving away from the emission centers, the emission efficiency falls sharply over a distance of ∼2 nm (Figure 4c). The normalized emission spectra acquired at the positions marked by the arrows in Figure 4a show only a weak shift of 2 meV (Figure 4b), which is within the error limit of the experiment. Previous measurements of the photon statistics of the emission centers have further proven that they behave like single-photon sources and thus like single quantum systems.28 The dI/dV spectra in Figure 4d corroborate this observation by revealing the presence of intrabandgap states only in the midpoint of the emission center. These may act as charge carrier and exciton traps. Imaging these states by constant height dI/dV maps shows that they are localized to only one or very few C60 molecules and thus are much more localized than the area of enhanced electroluminescence.24 These observations suggest that the STM-induced luminescence in the periphery of the bright areas arises from the diffusion of charge carriers and excitons to the emission center, where they finally recombine radiatively. Considering the current (Figure 4e) and

Figure 4. Lateral dependence of the emission characteristics of an individual emission center. (a) Perspective view on the STM topography and simultaneously acquired STM-induced luminescence intensity of the emission center in Figure 2b. The colored arrows mark the lateral position of the measurements in panel b and d−f. (b) Normalized emission spectra (Vs = −3.0 V, Is = 100 pA) on the lateral positions in panel a and shifted vertically by multiples of 0.5. Circles display measured spectra; solid lines show a fit of these data by three Gaussians. The percentage on the righthand side denotes the relative intensity of the S2 peak with respect to the main emission line S1. (c) Radial dependence of the absolute photon yield. Symbols display the mean photon yield on surface molecules with a given distance to the position marked by the black cross in panel a. The solid curve shows an exponential fit as a guide to the eye. (d) Logarithmic plot of normalized dI/dV spectra acquired at the positions in panel a. The black arrow marks an electronic state within the bandgap. (e) Photon yield at the positions in panel a as a function of the tunnel current and (f) as a function of the applied sample voltage. The data in panel d−f were acquired simultaneously by sweeping the sample voltage at a constant tip height (tunneling set point: Vs = −3.0 V, Is = 100 pA); the photon yield is corrected by the experimental collection efficiency (see Experimental Section); the data in panel f are additionally binned by four points. The gray curves show an additional measurement at even larger distances from the emission center. 1233

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Figure 5. STM-induced emission of a C60 multilayer film in a defect-free region (top row) and at a structural defect acting as an emission center (bottom row). (a,b) Emission spectra for different sample voltages denoted at the top right in panel a. The gray curve in panel a displays the emission spectrum obtained with the same tip on a pristine Ag(111) surface (Vs = −3 V). The colored arrows at the bottom mark the high-energy cutoff of the spectra. The Ag(111) spectrum in panel a and the spectra in panel b are shifted vertically for clarity. The inset in panel b shows that the emission peak positions do not change with sample voltage. (c) Energy diagram of the processes leading to plasmon excitation and (d) to the radiative recombination of electron−hole pairs. (e,f) Constant height electroluminescence map and (g,h) simultaneously recorded current map (Vs = −3 V, images low-pass filtered).

intensity ratio from 18% (yellow curve) to 30% (red curve) when moving away from the emission center. With this observation we can also rule out that the S2 emission peak arises from hot electroluminescence,4,5,12 that is, from a vibrationally excited level of the electronic state leading to the S1 peak. If that were the case, moving the tip away from the emission center would both reduce the coupling to tip-induced plasmons and increase the time for the excitons to cool down to their vibrational ground state; therefore, the S2/S1 peak intensity ratio would decrease, which is contrary to our observation. By similar reasoning we can rule out an emission from two different excited states that are localized on the same molecule, as the diffusion of excitons or charge carriers should not affect the occupational probability of both states at the trap. However, the spatially varying S2/S1 peak ratio can be rationalized by a diffusion to two traps that are localized on different sites, because the diffusion to both sites might involve different paths. Considering the constant energy separation of the S1 and S2 emission peaks of 30 meV for different emission centers (Figure 3a), the most obvious origin is the diffusion to one surface trap and one subsurface trap. Indeed, very similar energy differences for the surface and subsurface excitons in anthracene and tetracene have been reported.36−38 The emission from the higher-lying S2 state might become observable due to a substantially enhanced radiative decay constant in the surface layer, resulting from the much stronger perturbation of the translation invariance.37 When the weak S2 peak is absent, as for the majority of emission centers, we assume the exciton trap to be fully localized in either the surface layer or a subsurface layer. At lateral distances several nanometers from the emission centers, the sharp emission line vanishes and we only observe a weak, homogeneous luminescence background with a photon yield of ∼10−5 photons per tunneling charge carrier. The corresponding emission spectra are substantially broader (full width at half-maximum ∼300 meV, Figure 5a), they exhibit a

voltage dependence (Figure 4f) of the photon yield, the diffusion of excitons might be the dominant contribution. In the center of the bright spot (yellow and orange curves), the photon yield decreases with increasingly negative sample voltage and current due to the charge-induced quenching of singlet electron−hole pairs.28 However, at larger lateral distances, the photon yield is constant and thus independent of the applied voltage and the electric field in the STM junction. If the size of the bright spot were determined by the diffusion of unbound charge carriers, we would expect a strong reduction in the photon yield also in the periphery of the bright spots, because the electric field should reduce the number of charge carriers moving laterally to the trap. The observed intensity decay lengths agree well with the nonradiative energy transfer reported for C60/C70 nanocrystal domain boundaries on ultrathin NaCl films of 1.5−2 nm.29 At room temperature, exciton diffusion lengths of 5 nm,30−32 8 nm33 and 34 nm34 have been reported for solid C60. The smaller intensity decay lengths of ∼2 nm in our experiment may arise from the much smaller film thickness. In contrast to the bulk or thicker films, the close proximity of the metal substrate and the possible diffusion of excitons to the substrate provides additional nonradiative decay channels, which reduce the effective exciton lifetime. Aside from this, the exciton diffusion length might be reduced by a lower exciton diffusivity due to the lower temperature in our experiment. Temperature-dependent measurements might clarify this point. For a small number of emission centers, the electroluminescence spectrum exhibits an additional weak peak S2 at an energy 30 meV above the main emission line S1. The energy separation between the S1 and S2 peaks of 30 meV is significantly smaller than the lowest-energy Herzberg−Teller active gu(1) mode of C60 (43 meV35). Consequently, we can rule out that the more intense S1 peak results from an intensity borrowing from the S2 transition via Herzberg−Teller coupling. Furthermore, Figure 4b shows an increase in the S2/S1 peak 1234

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excitation of tip-induced plasmons. In our specific case, however, the excitation of tip-induced plasmons is more than just an undesired side effect. In fact, their detection is a probe for the typically inaccessible electron injection by the substrate. At the same time, the observation of the HOMO-derived states in current maps at −3 V (Figure 1b) confirms the dominant injection of holes by the tip. Accordingly, both electrons and holes are injected into the organic film and might form electron−hole pairs. The absence of any C60 emission in highly crystalline regions suggests that in these regions the lowest singlet transitions are still parity-forbidden like in an isolated C60 molecule; thus, most of the singlet excitons may undergo an intersystem crossing to triplet excitons or may be quenched by diffusion to the metal substrate, instead of recombining radiatively at the emission center. However, if the generated singlet excitons are able to diffuse within their lifetime (∼0.7 ns28,40) to a structural defect of sufficient ungerade character of the exciton wave function, their radiative recombination is substantially enhanced. Additionally, the trapping of excitons might hamper their diffusion to the substrate and reduce their nonradiative decay channels.

high-energy quantum cutoff that depends on the applied bias voltage (marked by colored arrows), and closely resemble spectra of tip-induced plasmons measured with the same tip on the pristine Ag(111) surface (gray curve). These observations indicate that the homogeneous luminescence background arises from the excitation of tip-induced plasmons, instead of the electron−hole pair recombination within the C60 film. In contrast to the latter, tip-induced plasmons are excited by inelastic tunneling through the vacuum barrier between tip and sample (Figure 5c). In other words, their excitation requires an energy loss of the tunneling electrons that corresponds to the energy of the emitted photons. Figure 1d shows that the HOMO-derived states appear at an energy of ∼3 eV below EF,s; therefore, these states cannot provide the necessary energy loss of Ephot > 1.5 eV visible in Figure 5a. Furthermore, an excitation by tunneling from occupied states of the metal substrate to the STM tip without a resonant tunneling through C60 states can be ruled out, based on the large distance between both electrodes (at least 4 nm for 5 ML of C60). Only electrons that are injected by the substrate into the LUMO-derived states and then subsequently tunnel inelastically to the tip (Figure 5c, red arrows) can provide the necessary energy loss required to excite light in the visible range. Finally, we investigate the electroluminescence characteristics of the C60 films on the submolecular scale. Specifically, we analyze the shape of the molecular orbitals visible in simultaneously recorded current and electroluminescence maps.39 For C60 molecules with a hexagonal face pointing toward the STM tip, the interpretation of the orbital shape is particularly straightforward because the HOMO is mostly localized on the bonds connecting two hexagonal faces (blue ovals in Figure 1b and Figure 5c,d), while the LUMO is mostly localized on bonds connecting a hexagonal face and its neighboring pentagonal faces (red ovals in Figure 1c and Figure 5c,d). At sites away from the emission centers, the electroluminescence map (Figure 5e) and the current map (Figure 5g) show different orbital shapes (see encircled molecule). This observation arises from the fact that light can be generated only by electrons inelastically tunneling from the LUMO-derived states, while the measurable tunnel current is dominated by the elastic tunneling of holes into the HOMOderived states. In contrast, at the emission centers both the electroluminescence map (Figure 5f) and the current map (Figure 5h) show the same orbital shapes. At these sites, light is generated by the radiative recombination of electron−hole pairs, which is maximal at tip positions providing the most efficient hole injection, that is, at positions where the tip is localized on top of the HOMO-derived states. With the same arguments we can also rule out a pumping of electron−hole pairs by tip-induced plasmons4,5,12−14 at the emission centers. If the radiatively decaying electron−hole pairs were generated by an excitation transfer from tip-induced plasmons, the photon emission would be maximal at tip positions of most efficient inelastic tunneling, i.e., on top of the LUMO-derived states. Likewise, if excitons created in regions off the emission centers transferred their energy to tip-induced plasmons, the photon emission would be maximal at tip positions of most efficient elastic tunneling, i.e., on top of the HOMO-derived states. These results demonstrate that submolecularly resolved photon maps provide valuable information on the primary light excitation process, that is, whether light is primarily generated by the intrinsic recombination of electron−hole pairs within the investigated organic film or by the STM-related

CONCLUSION In conclusion, the presented study provides detailed insight into the molecular scale electroluminescence of thin C60 films as a model system for other organic semiconductors. We have shown that their local electroluminescence characteristics may vary substantially on the molecular scale due to the local mixing of electronic states by local disorder. Electroluminescence maps at and away from the observed emission centers further demonstrate that the primary excitation process leading to the STM-induced luminescence of organic multilayers can be derived from the visible orbital pattern. In the present case, the pronounced differences in the shape of the C60 HOMO and LUMO enable an unambiguous discrimination between the recombination of injected charge carriers and the excitation of tip-induced plasmons. Here, a strong radiational coupling of both processes, as suggested for other systems,4,5,12−14 can be excluded. Instead, both processes occur along two different tunneling pathways. The presented concepts are widely applicable to other organic films and will help to separate their intrinsic emission characteristics from the STM-specific excitation of tip-induced plasmons or an energy transfer between both processes. Moreover, mapping of their local electroluminescence on the molecular scale will enable studying charge carrier and exciton dynamics in real-space. EXPERIMENTAL SECTION The experiments are performed in an in-house built scanning tunneling microscope with optical access, working at low-temperatures (4 K) and under ultrahigh vacuum (UHV, < 10−11 mbar).41 Outside the UHV chamber, two of the three optical ports are equipped with a spectrally integrating avalanche single photo diode (APD, PerkinElmer SPCM-AQRH-15) and an optical spectrograph (Acton Research Spectra Pro 300i, 150 lines/inch blazed grating, combined with a Peltier-cooled, intensified charge-coupled device, CCD). Spectrally integrated electroluminescence maps are acquired simultaneously to constant current or constant height images by the APD. For the determination of the total photon yield, that is, the number of photons per tunneling charge carrier, an overall photon collection efficiency of the instrument on the order of 10%41 is assumed. Emission spectra are recorded by a two-dimensional CCD chip without correction of the spectral response of the detector and the setup. The observed line width is limited by the experimental resolution and depends on the 1235

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ACS Nano emission wavelength41 as well on the focus alignment, which may vary slightly for different measurements. Owing to the weak emission intensity in the experiment (∼103 photons/s), we use the abovementioned blazed grating, which enables a spectral resolution of ∼10 meV. Gratings with smaller line spacings enable a higher energy resolution but result in substantially lower detection efficiencies. For a more convenient comparison, emission spectra are plotted as a function of photon energy, with the intensity being corrected by the wavelength-dependent bin size. Differential conductance (dI/dV) spectra are recorded via standard lock-in technique by modulating the bias voltage (5 mV, 524 Hz). All indicated bias voltages refer to the sample voltage with respect to the grounded STM tip. The samples are prepared in situ at room temperature by evaporating a thin C60 film with a nominal thickness of ∼4 ML on either a Ag(111) or a Au(111) single crystal, which was cleaned before by repeated sputtering and annealing cycles. Locally, the coverage reaches up to 9 ML due to the pyramid-like growth of C60 at room temperature. Subsequently, the sample is transferred into the microscope, where it cools down to 4 K. All measurements were performed with an electrochemically etched Au tip that was prepared on the pristine substrate crystal prior to the experiment. The STM images in Figure 2a−d and Figure 4a were visualized by the freeware program WSxM.42

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AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Christoph Große: 0000-0003-2474-0997 Present Address #

NanoPhotonics Centre, Cavendish Laboratory, University of Cambridge, Cambridge CB3 0HE, United Kingdom. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS We are grateful to Ikuko Akimoto for providing the absorption and photoluminescence data displayed in Figures 2,, and 3, respectively. Furthermore, we thank Jens Pflaum and Hagen Klauk for helpful comments on the manuscript. C.G. and P.M. acknowledge the support by the A. v. Humboldt Foundation. REFERENCES (1) Organic and Printed Electronics SummaryOE-A Roadmap, 6th ed; 2015. (2) Qiu, X. H.; Nazin, G. V.; Ho, W. Vibrationally Resolved Fluorescence Excited with Submolecular Precision. Science 2003, 299, 542−546. (3) Reecht, G.; Scheurer, F.; Speisser, V.; Dappe, Y. J.; Mathevet, F.; Schull, G. Electroluminescence of a Polythiophene Molecular Wire Suspended between a Metallic Surface and the Tip of a Scanning Tunneling Microscope. Phys. Rev. Lett. 2014, 112, 47403. (4) Chong, M. C.; Reecht, G.; Bulou, H.; Boeglin, A.; Scheurer, F.; Mathevet, F.; Schull, G. Narrow-Line Single-Molecule Transducer between Electronic Circuits and Surface Plasmons. Phys. Rev. Lett. 2016, 116, 36802. (5) Chong, M. C.; Sosa-Vargas, L.; Bulou, H.; Boeglin, A.; Scheurer, F.; Mathevet, F.; Schull, G. Ordinary and Hot Electroluminescence from Single-Molecule Devices: Controlling the Emission Color by Chemical Engineering. Nano Lett. 2016, 16, 6480−6484. (6) Chen, C.; Chu, P.; Bobisch, C. A.; Mills, D. L.; Ho, W. Viewing the Interior of a Single Molecule: Vibronically Resolved Photon Imaging at Submolecular Resolution. Phys. Rev. Lett. 2010, 105, 217402. (7) Lee, J.; Perdue, S. M.; Perez, A. R.; Apkarian, V. A. Vibronic Motion with Joint Angstrom−Femtosecond Resolution Observed 1236

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