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Ordinary and hot electroluminescene from single-molecule devices: controlling the emission color by chemical engineering Michael C. Chong, Lydia Sosa-Vargas, Herve Bulou, Alex Boeglin, Fabrice Scheurer, Fabrice Mathevet, and Guillaume Schull Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b02997 • Publication Date (Web): 21 Sep 2016 Downloaded from http://pubs.acs.org on September 25, 2016
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Ordinary and hot electroluminescene from single-molecule devices: controlling the emission color by chemical engineering Michael C. Chong,†,¶ Lydia Sosa-Vargas,‡,¶ Hervé Bulou,† Alex Boeglin,† Fabrice Scheurer,† Fabrice Mathevet,∗,‡ and Guillaume Schull∗,† Institut de Physique et Chimie des Matériaux de Strasbourg, UMR 7504 (CNRS – Université de Strasbourg), 67034 Strasbourg, France, and Institut Parisien de Chimie Moléculaire, Sorbonne Universités, UPMC Univ Paris 06, CNRS, 75005 Paris, France E-mail:
[email protected];
[email protected] Abstract Single-molecule junctions specifically designed for their optical properties are operated as light emitting devices using a cryogenic scanning tunneling microscope. They are composed of an emitting unit – a molecular chromophore – suspended between a Au(111) surface and the tip of the microscope by organic linkers. Tunneling electrons flowing through these junctions generate a narrow-line emission of light whose color is controlled by carefully selecting the chemical structure of the emitting unit. Besides the main emission line, red and blue-shifted vibronic features of low intensity are also detected. While the red-shifted provide a spectroscopic fingerprint of the emitting unit, the blue-shifted are interpreted in terms of hot-luminescence from a vibrationally excited state of the molecule. ∗ To
whom correspondence should be addressed de Physique et Chimie des Matériaux de Strasbourg, UMR 7504 (CNRS – Université de Strasbourg), 67034 Strasbourg, France ‡ Institut Parisien de Chimie Moléculaire, Sorbonne Universités, UPMC Univ Paris 06, CNRS, 75005 Paris, France ¶ These authors contributed equally to this work. † Institut
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Keywords: Scanning tunneling microscopy, single-molecule spectroscopy, STM-induced light emission, low-temperature fluorescence, vibronic microscopy, porphyrin-based emitters Single-molecule junctions are envisioned as components for miniaturized electronic circuits since the 1970’s. 1 As any electronic device, single-molecule components have a bandwith limited to the GHz range. 2 Combining such junctions with photonic or plasmonic elements may be the key to extend the bandwidth by several orders of magnitude, 3–5 a pre-requisite for ultrafast devices of nanometer scale. In this framework, the realization of molecular-scale devices capable of transforming an electrical stimulus into a well controlled optical signal are of great interest. To convey information in future electro-photonic or electro-plasmonic circuits, it may also be necessary to adapt the emission wavelength of the molecular device to the energy gap of a receiver. To date, a way to predict and control the color of the emitted light at the scale of a single-molecule junction is lacking. The scanning tunneling microscope (STM) is here a well adapted instrument which enables the study of nanoscale optoelectronic systems thanks to its sub-nanometric imaging resolution, its precise object manipulation capability 6,7 and to the possibility to locally stimulate surface plasmon polaritons (SPP) 8–10 or nanocavity plasmons (NCP). 11,12 STM-induced molecular fluorescence is usually only observed when the electronic levels of the molecule are decoupled from the metallic electrodes (substrate and tip). Usually it is obtained in tunnel regime on a molecule located on a thin insulating layer. 13–17 It is only recently that a fluorescence signal was observed for junctions where the molecule directly bridges two electrodes. 18–20 In this paper we demonstrate the possibility of selecting the emission wavelength of molecular light emitting devices in the range 750-1000 nm, without modifying other spectral properties. This is achieved by refining the chemical structure of the chromophore unit, a porphyrin derivative. The proposed strategy offers a clear path to extend the emission of the molecular-devices further to the blue or the near-infrared ranges. In addition to the ordinary luminescence, low intensity vibronic features are identified in the spectra which are traced back to the emission from non-thermalized molecular excitons. Known as hot-luminescence, 21,22 these so far unevidenced contributions to a single-molecule emission spectrum shed light on the luminescence mechanism of the molecular
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Figure 1: On-surface synthesis of molecular copolymers. (a) Sketch of the chemical reactions used to produce the molecular structures. STM topographies (5 × 9.5 nm2 , I = 0.1 nA, V = -0.1 V) of copolymers composed of oligothiophene and (b) fused-MPP, (c) fused-DPP and (d) fused-DNP molecules. device. Three different molecular architectures were fabricated by in situ copolymerization via dehalogenative Ullmann-type coupling reactions 23,24 between 5,5”-dibromo-2,2’:5’,2”-terthiophene (DBrTT) and different free-base porphyrin derivatives: 5-(phenyl)-10,20-(dibromo)-porphyrin (Br2 MPPH2 ), 5,15-(diphenyl)-10,20-(dibromo)-porphyrin (Br2 -DPPH2 ) and 5,15-(dinaphthalene)-10,20(dibromo)-porphyrin (Br2 -DNPH2 ) (Fig. 1a and Supporting Information S1). The molecules were co-deposited under vacuum by sublimation on a clean Au(111) single-crystal kept at room temperature. Then, the samples were annealed at a temperature of 475 K to activate an on-surface copolymerization of the molecules. 23–25 The samples were then transferred into the STM. STM images obtained after the on-surface polymerization reactions show copolymers made of covalently bonded porphyrin and oligothiophene units (Fig. 1b,c,d). As previously reported for 3 ACS Paragon Plus Environment
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DPPH2 , 20 the annealing procedure results also in a intramolecular cyclodehydrogenation transformation 20,26 of the porphyrin derivatives where the peripheral units (phenyl rings for MPPH2 and DPPH2 or naphthalene units for DNPH2 ) are fused to the macrocycle. This leads to the formation of new molecular species: fused-MPP, fused-DPP and fused-DNP (see Fig. 1a). As will be demonstrated below, these modifications of the chemical structure of the porphyrin core play a crucial role for the control of the emitted color. The next step is to lift the copolymers in the STM junction 7,19,20 so as to decouple a fusedporphyrin unit from the Au(111) surface. The procedure consists in: (i) positioning the STM tip above the extremity of a molecular wire, (ii) approaching the tip to the molecule up to the formation of a contact, and (iii) retracting the tip with the attached wire sufficiently high that a single emitting unit is suspended in the junction. An idealized representation of the final configuration of the molecular devices is provided in Fig. 2a. Tunneling electrons flowing through these junctions at elevated voltages generate an emission of light. 20 Typical electroluminescence spectra for the three different molecular devices are displayed in Fig. 2b. All the spectra are characterized by an intense and remarquably narrow (FWHM ≤ 10 meV) emission line (0-0 line in Fig. 2b) and a series of low intensity peaks at higher and/or lower energies. Whereas the overall shape of the spectra is similar for the three cases, the energy of the main peak is shifted depending on the chemical structure of the emitting unit. Time-dependent density functional theory (TD-DFT) calculations of the first optical transition (S1 → S0 transition) of each emitting unit are reported in Fig. 2c and compared to the experimental energies of the main emission lines (see details in Supporting Information S2). The good agreement between these values confirms that we are exciting the fluorescence of the fused-porphyrins. While this seems an obvious conclusion, it is interesting to note that the origin of the emission spectra from single-molecules excited by STM could rarely be unambiguously determined in previous experiments (e.g., Refs 13,14). These results also indicate that the oligothiophene chains efficiently decouple the porphyrin emitters from the electrodes while simultaneously ensuring the transport of charge from one electrode to the other. The difference in emission energy for the three
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Figure 2: Controlling the light emission color of single-molecule junctions (a) Scheme of the experimental configuration: the emitting unit (red) is suspended between the tip and the sample of the STM by oligothiophene chains. (b) Typical light emission spectra for the different molecular devices (fused-MPP: V = 1.75 V, I = 1.85 nA, z = +2.38 nm; fused-DPP: V = 2.0 V, I = 0.26 nA, z = +2.7 nm; fused-DNP: V = 1.4 V, I = 1.27 nA, z = +2.6 nm, where z is the tip-sample distance). The green boxes represent the experimental scattering of the main emission line energies for each of the emitters. (c) Experimental and TD-DFT calculated values for, respectively, the main emission lines and the S1 → S0 transitions. 27 We attribute the scattering of the experimental emission energies to small changes of the emitter-thiophene and thiophene-electrode couplings from junction to junction. The experimental energies in (c) correspond to the average values recorded for different molecular junctions. The uncertainties are the standard deviation.
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chromophores can be intuitively understood in terms of different conjugation lengths: the greater (smaller) the conjugation length, the smaller (larger) the optical gap. By gently tuning the chemical architecture of the chromophore we see that it is possible to adapt the emission wavelength of the device while conserving the almost monochromatic 28 character of the emission. We now focus on the description of low intensity features in the spectra. A careful inspection reveals a series of peaks at the low energy side of the 0-0 transition for fused-MPP and fused-DPP (labelled R-band in Fig. 2b). These features are not observed in the fused-DNP spectrum because of the low detection efficiency of the optical setup at low energy. In the case of fused-DPP, these features were previously associated to vibrational modes of the molecular emitter based on a comparison with Raman intensity calculations. 20 The modes detected in our spectra are expected to be symmetric (i.e., Raman active) with respect to the C2 axes of the porphyrin macrocycle and should only involve planar distortions of the molecular structure since the lowest electronic excitations implicate pi-electrons,. Moreover, the exact frequencies are also likely affected by the covalently linked thiophene units and the metallic electrodes. The R-band can therefore be viewed as a spectroscopic fingerprint of the molecular emitter. In Fig. 3a we represent the spectra of the three devices as a function of the energy shift (in cm−1 ) with respect to the 0-0 line. Unsurprisingly, the vibrational series follow a similar trend for fused-DPP and fused-MPP as the two molecules have quasi-identical chemical structures. The main differences lie in the peak at -1180 cm−1 which is attenuated in the spectra of the fused-MPP and in the splitting of the -495 cm−1 line. Understanding the origin of these small differences would require a detailed theoretical modelization which is beyond the scope of the present work. In the spectra of fused-MPP and fused-DNP, low intensity vibronic peaks are also detected on the high energy side of the 0-0 line (labelled as B-band in Fig. 2b and appearing at positive shifts in Fig. 3a). In the case of the fused-MPP, we can see that the energy dispersion of these peaks is strikingly similar to the one observed for the R-band, excepted for a small shift of the B-band to lower energy. This nearly ”mirror-symmetry” between the two bands can be better visualized by superimposing the spectrum of a fused-MPP with its mirror image with respect to the 0-0 line (see figure S2). This suggests that the blue and red shifted
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Figure 3: Vibronic features and emission mechanism (a) Light emission spectra of fused-DPP, fused-MPP and fused-DNP plotted as a function of the energy shift from the 0-0 lines. The raw (smoothed) data appear in grey (red). (b) Schematic representation of the emission mechanism. The energy lost by an inelastic tunneling electron (1) is transfered to the molecular emitter that is excited from S0 to S1 (2). Eventually, the emitter relaxes to its electronic ground state by emitting a photon (3). (c) Details of the excitation and de-excitation process. Three possible paths are described: the green arrows characterize the most efficient path leading to the 0-0 emission line. It corresponds to a molecule excited to the vibrational ground level of S1 relaxing to the vibrational ground level of S0. The red and blue arrows characterize much less efficient paths. In path II the molecule is also excited to the vibrational ground level of S1, but decays to vibrational excited levels of S0 (R-band). Path III corresponds to a molecule excited to vibrational excited levels of S1 and relaxes to the vibrational ground level of S0 (B-band).
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peaks can be associated to the same vibrational modes of the molecules. The resulting picture is reminiscent of what is traditionally observed in Raman spectra where stokes (red-shifted) and anti-stokes (blue-shifted) peaks are symetrically shifted with respect to the excitation line of the laser. We will see that, if the analogy with the stokes emission is partially relevant, the blue-shifted peaks have a rather different origin than the anti-stokes emission. The suggested mechanisms responsible for the different components of the spectra are detailed in Fig 3b and c. The energy lost by a tunneling electron crossing the junction (1) is transfered to the molecular emitter either directly or via the excitation/de-excitation of a localized plasmon (2). Eventually, the excited emitter relaxes by emitting a photon (3). This general picture has been established in a previous publication 20 and is similar to the mechanism proposed recently to explain the luminescence of molecular aggregates excited by STM. 21,29 Figure 3c presents a detailed view of the different excitation/de-excitation paths of the molecule. Because of the experimental conditions, thermal excitation of the emitter is negligeable, and the molecule is originaly in its ground state. When the molecule is excited to the vibrational ground level of S1 it can either deexcite to the vibrational ground level of S0 (path I, the most likely, yielding the 0-0 line) or excite a vibrational mode of S0 (path II, yielding the red-shifted lines). For higher excitation energies (i.e., higher bias voltages) it is possible to drive the molecular emitter directly in a vibrational excited level of S1. In this configuration, Kasha’s rule 30 states that the molecule should relax to the vibrational ground level of S1 before a radiative transition to S0 can occur. Nonetheless, it has been shown in the case of multilayers of molecules 21 that the presence of resonant plasmons can favor a direct radiative de-excitation to S0. This emission of light by a non-thermalized molecular exciton is known as hot-luminescence. This mechanism (path III in Fig. 3c) provides an explanation for the blue-shifted peaks of our spectra, which are therefore characteristic of the vibrational modes of the excited emitter. For this reason, they have a rather different origin than anti-stokes resonances which are characteristic of the vibrational modes of the ground state of a molecule. The nearly mirror-symmetry between the energy dispersion of the blue- and red-shifted peaks with respect to the 0-0 line and the slightly smaller energy shift
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of the B-band compared to the R-band are expected behaviors for large and rigid molecules of the porphyrin family. 31 These observations support the model of Fig. 3c. To our knowledge, this is the first observation of hot-luminescence from a single-molecule excited by STM. As stated above, this process has previously been reported for multilayers of molecules 21 and a theoretical description has been proposed 32,33 where the plasmons localized at the STM junction behave as a coherent excitation source. Such an interpretation may likely apply here as well, but would require a thorough theoretical treatment to be ascertained. In this paper we report on the STM-induced electroluminescence of single-molecule junctions. We first demonstrate the possibility to control the energy of the emitted light by a proper design of the chromophore that preserves the high monochromaticity of the emission. We show that the emission energy can be predicted in a reliable way by TD-DFT calculations for different chromophores. One can therefore envisage targeted molecular design yielding specific light emission properties. This extends the versatility of such molecular-scale optoelectronic devices, thus improving their potential use in future hybrid plasmonic-electronic circuits. Furthermore, we investigated the nature of low intensity features of the optical spectra. This resulted in the first observation of hot-luminescence from a single-molecule excited by STM, opening new questions regarding the nature of the interaction between the molecular emitters and the localized surface plasmons.
Methods The experiments were performed using a low temperature (4.5 K) Omicron STM, operating under ultra high vacuum conditions (base pressure < 1−10 mbar). The light collection setup (described in Ref. 19) is composed of a grating spectrograph (Princeton Instruments Acton Series SP2300i) connected to a liquid nitrogen cooled CCD camera (Princeton instruments PyLoN-100BReXcelon). The system allows for the use of two different gratings with high and low resolution, as described in Ref 20. Au(111) samples were systematically cleaned by cycles of argon sputtering and thermal annealing. 9 ACS Paragon Plus Environment
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Author information Notes The authors declare no competing financial interests.
Acknowledgement The authors thank Stephane Berciaud and Laurent Limot for stimulating discussions, and Virginie Speisser, Jean-Georges Faullumel, Michelangelo Romeo and Olivier Cregut for technical support. The Agence National de la Recherche (project SMALL’LED No. ANR-14-CE26-0016-01), the Labex NIE (Contract No. ANR-11-LABX-0058_NIE), the Equipex UNION (Contract No. ANR-10-EQPX-52-01), GENCI-IDRIS (Grant no. i2015097459) and the International Center for Frontier Research in Chemistry (FRC) are acknowledged for financial support.
Supporting Information Available The following files are available free of charge. • Supporting information : Synthesis of materials, TD-DFT calculations and mirror symmetry between R- and B- bands. This material is available free of charge via the Internet at http://pubs.acs.org/.
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(5) Wang, T.; Nijhuis, C. A. Appl. Mater. Today 2016, 3, 73. (6) Bartels, L.; Meyer, G.; Rieder, K.-H. Phys. Rev. Lett. 1997, 79, 697. (7) Lafferentz, L.; Ample, F.; Yu, H.; Hecht, S.; Joachim, C.; Grill, L. Science 2009, 323, 1193. (8) Wang, T.; Boer-Duchemin, E.; Zhang, Y.; Comtet, G.; Dujardin, G. Nanotechnology 2011, 22, 175201. (9) Bharadwaj, P.; Bouhelier, A.; Novotny, L. Phys. Rev. Lett. 2011, 106, 226802. (10) Dong, Z.; Chu, H. S.; Zhu, D.; Du, W.; Akimov, Y. a.; Goh, W. P.; Wang, T.; Goh, K. E. J.; Troadec, C.; Nijhuis, C. a.; Yang, J. K. W. ACS Photonics 2015, 2, 385. (11) Johansson, P.; Monreal, R.; Apell, P. Phys. Rev. B 42, 9210. (12) Berndt, R.; Gimzewski, J. K.; Johansson, P. Phys. Rev. Lett. 1991, 67, 3796–3800. (13) Qiu, X. H.; Nazin, G. V.; Ho, W. Science 2003, 299, 542. (14) Chen, C.; Chu, P.; Bobisch, C. A.; Mills, D. L.; Ho, W. Phys. Rev. Lett. 2010, 105, 217402. (15) Zhu, S.-E.; Kuang, Y.-M.; Geng, F.; Zhu, J.-Z.; Wang, C.-Z.; Yu, Y.-J.; Luo, Y.; Xiao, Y.; Liu, K.-Q.; et. al, J. Am. Chem. Soc. 2013, 135, 15794. (16) Lee, J.; Perdue, S. M.; Rodriguez Perez, A.; Apkarian, V. A. ACS Nano 2014, 8, 54. (17) Zhang, Y.; Luo, Y.; Zhang, Y.; Yu, Y.; Kuang, Y.; Zhang, L.; Meng, Q.; Luo, Y.; Yang, J.; Dong, Z.; Hou, J. G. Nature 2016, 531, 624. (18) Marquardt, C. W.; Grunder, S.; Blaszczyk, A.; Dehm, S.; Hennrich, F.; Lohneysen, H. V.; Mayor, M.; Krupke, R. Nat. Nanotechnol. 2010, 5, 863. (19) Reecht, G.; Scheurer, F.; Speisser, V.; Dappe, Y. J.; Mathevet, F.; Schull, G. Phys. Rev. Lett. 2014, 112, 047403.
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(20) Chong, M. C.; Reecht, G.; Bulou, H.; Boeglin, A.; Scheurer, F.; Mathevet, F.; Schull, G. Phys. Rev. Lett. 2016, 116, 036802. (21) Dong, Z.; Zhang, X. L.; Gao, H. Y.; Luo, Y.; Zhang, C.; Chen, L. G.; Zhang, R.; Tao, X.; Zhang, Y.; Yang, J. L.; Hou, J. G. Nat. Photonics 2010, 4, 50. (22) Rebane, K.; Saari, P. J. Lumin. 1978, 16, 223–243. (23) Grill, L.; Dyer, M.; Lafferentz, L.; Persson, M.; Peters, M. V.; Hecht, S. Nat. Nanotechnol. 2007, 2, 687. (24) Lafferentz, L.; Eberhardt, V.; Dri, C.; Africh, C.; Comelli, G.; Esch, F.; Hecht, S.; Grill, L. Nat. Chem. 2012, 4, 215. (25) Reecht, G.; Bulou, H.; Scheurer, F.; Speisser, V.; Carriere, B.; Mathevet, F.; Schull, G. Phys. Rev. Lett. 2013, 110, 056802. (26) Wiengarten, A.; Lloyd, J. a.; Seufert, K.; Reichert, J.; Auwärter, W.; Han, R.; Duncan, D. a.; Allegretti, F.; Fischer, S.; Oh, S. C.; Saglam, O.; et al., Chem. - A Eur. J. 2015, 21, 12285. (27) Supporting Information. (28) Born, M.; Wolf, E. Principles of optics: Electromagnetic Theory of Propoagation, Interference and Diffraction of Light, 7th ed.; Cambridge University Press, 1999; p 20. (29) Schneider, N. L.; Berndt, R. Phys. Rev. B 2012, 86, 035445. (30) Kasha, B. Y. M. Discuss. Faraday Soc. 1950, 9, 14. (31) Even, U.; Magen, J.; Jortner, J.; Friedman, J.; Levanon, H. The Journal of Chemical Physics 1982, 77, 4374–4383. (32) Tian, G.; Liu, J.-C.; Luo, Y. Phys. Rev. Lett. 2011, 106, 177401. (33) Tian, G.; Luo, Y. Phys. Rev. B 2011, 84, 205419. 12 ACS Paragon Plus Environment
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R-band
fused-DPP
0 0-0
8 B-band
4
fused-DNP
0 1.2
c
Molecule fused MPP fused DPP fused DNP
1.3 1.4 1.5 1.6 1.7 Photon Energy (eV)
Experimental (eV) 1.68 ± 0.06 1.51 ± 0.04 ACS Paragon Plus Environment
1.28 ± 0.06
TD-DFT (eV) 1.83 1.51 1.21
Nano Letters
0-0 B-band Energy shift from 0-0 (meV)
R-band
-124
-62
62
124 fused - DPP
2.0
0
-495
-1180
-186
1.0
800
fused - MPP
0.6
-370
1.2
530
350
0
-840
0 1.0
fused - DNP
Light Intensity (arb. units)
a
-570
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 16 of 17
0.5 0 -1500
-1000
-500
0
500
1000
Energy shift from 0-0 (cm-1)
b
c S1 1 S1
3 S0
Tip
Sample
2
ACS Paragon Plus Environment
S0 I
II
III
Page 17Nano of 17Letters
1 2 Paragon Plus Environmen ACS 3 4
