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Observation of room temperature photoluminescence blinking in armchair-edge graphene nanoribbons Markus Pfeiffer, Boris V. Senkovskiy, Danny Haberer, Felix R. Fischer, Fan Yang, Klaus Meerholz, Yoichi Ando, Alexander Grüneis, and Klas Lindfors Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b03006 • Publication Date (Web): 18 Oct 2018 Downloaded from http://pubs.acs.org on October 19, 2018
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Nano Letters
Observation of room temperature photoluminescence blinking in armchair-edge graphene nanoribbons Markus Pfeiffer,∗,† Boris V. Senkovskiy,‡ Danny Haberer,¶ Felix R. Fischer,¶ Fan Yang,‡ Klaus Meerholz,† Yoichi Ando,‡ Alexander Gr¨uneis,‡ and Klas Lindfors∗,† †Department f¨ ur Chemie, Universit¨at zu K¨oln, Luxemburger Strasse 116, 50939 K¨oln, Germany ‡II. Physikalisches Institut, Universit¨ at zu K¨oln, Z¨ ulpicher Strasse 77, 50937 K¨oln, Germany ¶Department of Chemistry, University of California at Berkeley, Tan Hall 680, Berkeley, CA 94720, USA E-mail:
[email protected];
[email protected] Abstract By enhancing the photoluminescence from aligned 7-atom wide armchair-edge graphene nanoribbons using plasmonic nanoantennas, we are able to observe blinking of the emission. The on- and off-times of the blinking follow power law statistics. In time-resolved spectra, we observe spectral diffusion. These findings together is a strong indication of the emission originating from a single quantum emitter. The room temperature photoluminescence displays a narrow spectral width of less than 50 meV, which is significantly smaller than the previously observed ensemble linewidth of 0.8 eV. From
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spectral time traces, we identify three optical transitions, which are energetically situated below the lowest bulk excitonic state E11 of the nanoribbons. We attribute the emission to transitions involving Tamm states localized at the end of the nanoribbon. The photoluminescence from a single ribbon is strongly enhanced when its end is in the antenna hot spot resulting in the observed single molecule characteristics of the emission. Our findings illustrate the essential role of the end termination of graphene nanoribbons in light emission and allow us to construct a model for photoluminescence from nanoribbons.
Keywords Graphene nanoribbons, plasmonic nanoantennas, fluorescence blinking, fluorescence enhancement, single emitters In the last few years, there has been a large interest in the properties of graphene nanoribbons (GNRs). For example, their electronic properties, 1–3 energy level structure and excitonic properties, 4–8 as well as their optical characteristics 6,9,10 have been recently studied both theoretically and experimentally. A particularly exciting aspect of graphene nanoribbons is that they can be grown with atomic precision and with a high degree of alignment using bottom-up fabrication. 2,11–16 The synthesis of atomically precise graphene nanoribbons has even enabled selective doping to tailor their electronic properties. 17,18 Recently the first successful implementation of GNRs in electronic devices has been demonstrated 19,20 . Finally, theoretical work has predicted strong non-linear optical response 21 and high magnetoresistance 22 making GNRs interesting candidates for a plethora of future applications in electronics and optoelectronics. 23 The most studied GNRs until now are seven-atom wide armchair-edge graphene nanoribbons (7-AGNRs). Recent optical studies using Raman, 12,18,24 and Raman excitation, 9 differential reflection, 25 absorption, 16 photoluminescence, 9,26 and electroluminescence 10 spectroscopy have unveiled already several of their characteristic properties such as the energetic 2
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positions of the excitonic transitions and absorption and emission spectra. So far the optical studies on GNRs have focused on the bulk and ensemble properties of the ribbons. In the first optical spectroscopy experiment on single ribbons, Chong et al. observed spectrally narrow electroluminescence for dehydrogenated GNRs with a linewidth down to 40 meV at cryogenic temperatures and in ultra-high vacuum (UHV) using a scanning tunneling microscopy (STM) tip. 10 This is significantly narrower than the approximately 0.8 eV linewidth measured on the ensemble level. 9 Based on density functional theory calculations, Ref. 10 provided a theoretical model attributing the emission to transitions between states, so called Tamm states, 3,27–30 localized at the zigzag termini of 7-AGNRs, and bulk states, which are delocalized along the AGNRs. Experimental studies on the significance of a localized state in the emission process and its role in photoluminescence is however still scarce. To address these issues, we use plasmonic enhancement to increase the photoluminescence of pristine GNRs from regions smaller in size than the length of the ribbon and thus only probe a short section along the GNRs. In this study we use plasmonic nanoantennas to enhance the photoluminescence from a layer of aligned 7-AGNRs. We have recently shown on the ensemble level that the nanoantennas significantly increase the photoluminescence and Raman signals. 24 The antennas localize the electromagnetic field and enhance the light-matter interactions to allow us to study the photoluminescence properties of only a few AGNRs within the dense layer of ribbons. We observe spectrally narrow room temperature photoluminescence bursts from (down to) single 7-AGNRs in the hot spots of the antennas. The emission has the typical characteristics of single emitter photoluminescence. 31 We fabricate an array of resonant plasmonic nanoantennas on a glass substrate. The optical antennas are round gold disks with a diameter of 140 nm and a thickness of 30 nm. The pitch of the square lattice of antennas is 650 nm. Aligned 7-AGNRs are transferred from a Au 788 single crystalline surface onto the array as described in our previous publication. 9 Figure 1a shows a sketch of one antenna covered with GNRs. The orientation of the AGNRs
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