High-Resolution Vibronic Spectra of Molecules on Molybdenum

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High Resolution Vibronic Spectra of Molecules on Molybdenum Disulfide Allow for Rotamer Identification Nils Krane, Christian Lotze, Gaël Reecht, Lei Zhang, Alejandro L. Briseno, and Katharina J Franke ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b07414 • Publication Date (Web): 31 Oct 2018 Downloaded from http://pubs.acs.org on November 1, 2018

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High Resolution Vibronic Spectra of Molecules on Molybdenum Disulfide Allow for Rotamer Identification Nils Krane,† Christian Lotze,∗,† Gaël Reecht,† Lei Zhang,‡ Alejandro L. Briseno,‡ and Katharina J. Franke† †Fachbereich Physik, Freie Universität Berlin, Arnimallee 14, 14195 Berlin, Germany. ‡University of Massachusetts, Department of Polymer Science and Engineering, Amherst, Massachusetts, 01003, USA E-mail: [email protected]

Abstract Tunneling spectroscopy is an important tool for the chemical identification of single molecules on surfaces. Here, we show that oligothiophenebased large organic molecules which only differ by single bond orientations can be distinguished by their vibronic fingerprint. These molecules were deposited on a monolayer of the transition metal dichalcogenide molybdenum disulfide (MoS2 ) on top of a Au(111) substrate. MoS2 features an electronic band gap for efficient decoupling of the molecular states. Furthermore, it exhibits a small electron-phonon coupling strength. Both of these material properties allow for the resolution of vibronic states in the range of the limit set by temperature broadening in our scanning tunneling microscope at 4.6 K. Using DFT calculations of the molecule in gas phase provides all details for an accurate simulation of the vibronic spectra of both rotamers.

Keywords vibronic states, molecular vibrations, rotamer, BTTT, molybdenum disulfide, MoS2 , scanning tunneling microscopy Scanning tunneling microscopy (STM) is an

ideal tool for the determination of molecular structures on conducting surfaces. It enables the investigation of the orientation of single molecules or the assembly of large molecular islands. Furthermore, tunneling spectroscopy provides the energy level alignment of the molecular frontier orbitals with atomicscale precision. However, the electronic levels are not an unambiguous fingerprint of the chemical structure of the molecules. Additional information can be provided by molecular vibrations, which are excited in tunneling spectroscopy in the form of off-resonant excitations 1,2 or by resonant vibronic states. 3 The resolution of the latter is challenging, since the vibrational energy bands extend up to hundreds of milli electronvolt (meV) with differences of a few meV for the individual vibrations. The effective energy resolution, on the other hand, is limited by the hybridization of molecular states with the electronic bands of a metal substrate. 3,4 Typical hybridization energies are in the order of a few hundred meV, thus obscuring vibronic excitations. One approach to suppress hybridization broadening is to include thin layers with a band gap or a low density of states between molecule and metallic substrate. 3,5–11 Effective widths of the probed molecular orbitals thus have

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reached down to tens of meV. 9,12 However, this effective resolution is still not sufficient when compared to energy differences in vibrational modes of large organic molecules. Hence, a chemical identification of similar species or isomeric forms often is beyond reach. In particular, molecules which only differ by bond orientations, such as molecular rotamers, are typically indistinguishable. Here, we employ molybdenum disulfide (MoS2 ) as a thin decoupling layer. MoS2 is a three-atomic layer material, where molybdenum atoms are sandwiched between layers of sulfur atoms. A monolayer of MoS2 exhibits a direct electronic band gap of 2.8 eV. 13–15 Due to the saturation of its chemical bonds, MoS2 is considered as a van der Waals material. Hence, adsorbed molecules on these materials are expected to weakly interact with MoS2 . Nonetheless, the combination of transition metal dichalcogenides with organic molecules is promising for a variety of applications due to the versatile functionalization possibilities. 16 As a test molecule for the resolution limit of vibronic states, we chose 2,5-Bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene (BTTT) as depicted in Figure 1a. BTTT was previously synthesized and reported in literature by Zhang and coworkers. 17 The molecule exists in four rotamer forms as a consequence of a small rotation barrier of C–C single bonds between the thienothiophene and the thiophene units. Due to the high charge carrier mobility of BTTT and its polymers, 18–21 this molecule is of particular importance as a donor for applications in solar cell devices, mostly in combination with C60 -derived acceptor molecules. 17 Vibration mediated charge transfer has been discussed in similar devices as a highly efficient route of charge separation. 22 We show that BTTT molecules adsorbed on a single layer of molybdenum disulfide (MoS2 ) on Au(111) exhibit highly resolved vibronic tunneling spectra with peak widths getting close to the limit of the experimental temperature resolution at 4.6 K. Such narrow lineshapes allow for the resolution of fine details in the vibronic spectra, which reflect the fingerprint of different

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rotamer species of BTTT. Moreover, we show that these details can be simulated by simple gas phase DFT calculations of the molecules.

Results/Discussion Structure of BTTT on MoS2 Monolayers islands of MoS2 were prepared in vacuo on a clean Au(111) surface (see Methods). 23,24 The islands exhibit the typical Moiré structure due to a lattice mismatch with the substrate. 24,25 Deposition of BTTT molecules at a sample temperature of 200 K leads to large molecular islands on the bare Au(111) surface and small islands on the MoS2 layer (Figure 1b). The unequal distribution of molecules suggests a low diffusion barrier and sticking coefficient of the molecules on MoS2 in contrast to Au(111). However, small assemblies of BTTT can be found on the MoS2 probably stabilized by intermolecular interactions. We first note that the BTTT may be found in four possible adsorption configurations (see Supporting Information). The most prominent difference is the different orientation of the alkyl legs on the same or opposite sides of the thiophene backbone, similar to the observations of similar alkyl-substituted oligothiophene molecules. 26,27 Not as easily detectable - but as we will show important for the vibrational modes - the thienothiophene may be oriented in two directions with respect to the alkyl legs. The reason for this flexibility is a negligible rotation barrier of the C–C single bonds between the thienothiophene and the thiophene units in gas phase. Hence, when the molecules land on the surface, they may end up in four different configurations, which can be classified as cis-cis, trans-trans, trans-cis, and cis-trans rotamers (the latter two being prochiral) when considering the positions of the S atoms with respect to the C–C bonds. Both the cis-cis and trans-trans configuration expose their alkyl legs at opposite sides of the thiophene backbone (Figure 1a). On MoS2 the trans-trans and cis-cis rotamer assemble in quasi one-dimensional rows, as


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shown in Figure 1c. Here, the BTTT molecules are aligned with their thiophene backbone as well as alkyl legs parallel to each other, thus maximizing hydrogen and van der Waals bonds. From their appearance in the STM images, it is difficult to identify the two rotamer configurations. Electronic Structure and Vibronics In order to characterize the electronic structure, we record differential conductance spectra. The spectrum on MoS2 reflects an electronic band gap between −1.4 eV and 0.5 eV (black line in Figure 2a), in agreement with earlier studies of MoS2 on Au(111). 24,28,29 On the BTTT, STS reveals a sharp onset of conductance at −1 eV. We assign the onset of tunneling to the removal of an electron from the highest occupied molecular orbital (HOMO). Though typically referred to probing the HOMO, we emphasize that the tunneling process leaves the molecule as a positive ion for a short time. The positive ion resonance (PIR) is followed by conductance peaks at even larger negative bias voltages. At positive bias voltage, we do not observe any states inside the band gap of MoS2 , but a small resonance at higher energies around 2 eV. We assign it to the negative ion resonance (or LUMO) of BTTT, resulting in a HOMO–LUMO gap of 3 eV of BTTT adsorbed on MoS2 /Au(111) which corresponds to the predicted gap of 3.3 eV from our DFT calculation in gas phase. Figure 2c shows the details of the PIR reflecting a fine structure with a large number of peaks as a higher-energy side band. We attribute these peaks to tunneling via vibronic states of the molecule. Upon hole attachment the molecule can not only be excited electronically but simultaneously left in a vibrationally excited states, a so-called vibronic state (see Figure 2d). If the vibronic level spacing is larger than the level broadening, the individual vibronic states are resolved as peaks in the differential conductance spectrum 4,5 In our case, the peaks are of ≈ 6 meV width. We note that the energy spacing between the vibronic resonances does not follow any periodicity. Hence, the vi-

bronic states cannot be associated to a simple set of higher harmonics of only a few vibrational modes. The absence of higher-harmonic excitations can be explained within the FranckCondon picture. The peak intensity Ikn of the nth excitation level of a mode k is given by a Poisson distribution: 30–32 Ikn = e−Sk

Skn , n!

(1)

with Sk being the Huang-Rhys factor of the vibrational mode k. This factor depends on the relaxation energy k of a vibrational mode when charging the molecule (Figure 2d): 30,32 Sk =

k . ~ωk

(2)

Assuming a vibrational mode with a HuangRhys factor of, e.g., S < 0.5, the peak of the second excitation level would already be less then an eighth of the intensity of the elastic peak. Hence, the missing periodicity in the vibronic spectra of the BTTT molecules suggests a low Sk of all modes. Before turning to an identification of the vibronic modes, we note that we found two distinct sets of vibronic spectra on the BTTT molecules. For direct comparison, both are displayed in Figure 2c. The most pronounced difference is an energy shift of the PIR onset of 46 meV. Plotting the spectra to the same scale (Figure 3 bottom panel) allows for a close inspection of further differences. We realize that the overall lineshape is very similar. However, there are small differences in peak intensities, e.g., seen in the ratio of the three peaks found at 20 −35 meV above the onset, and slight energy shifts, e.g., from 165 meV to 169 meV. We will show that these characteristic differences provide an unequivocal fingerprint of the transtrans and cis-cis rotamer. Simulation and Vibrational Analysis For this, we first simulate the vibronic spectrum for both rotamers. We performed DFT calculations of both BTTT rotamers in their positive and neutral charge state. For the relaxed structure of both charge states we com-


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pared the position of each atom, in order to obtain their displacements from their equilibrium positions. Additionally we calculated the vibrational modes and their energies of the positively charged molecule. From the atomic displacements we determined the amplitude of each mode in mass weighted normal coordinates, yielding Sk for each mode. 33 A more detailed description of the calculation is provided in the Supporting Information. For simulating the spectra we extracted all vibrational eigenmodes with Sk ≥ 0.001 and included coupled vibronic states with up to three excitations (e.g., 3νk or 2νk +νk0 ). Each of these modes was weighted with the Huang-Rhys factor as a measure of the electron-phonon coupling strength, according to equation (1). A full width at half maximum (FWHM) of 6 meV for the Lorentzian lineshape was applied, corresponding to the experimental observation. The resulting lineshape of the vibronic spectra associated to the PIR in the cis-cis and trans-trans rotamer are shown in Figure 3 (top panel). A figure with the Sk of all modes is provided in the Supporting Information. These spectra can be directly compared to the experimental spectra, where the PIR onset has been set to zero energy. Furthermore, the energy scale of the measured spectra has been rescaled by a factor of 0.9. This value agrees with the expected voltage drop over the MoS2 layer. As mentioned above, distinct differences between two types of BTTT molecules were resolved. We recognize these differences are the same as those between the simulated transtrans and cis-cis spectra. Hence, we conclude that these differences signify the two rotamer species and suggest that the vibronic spectra of the individual molecules can be used as a fingerprint for rotamer identification. Additionally, the shift of the PIR, i.e., a different ionization energy, by 46 meV in experiment closely resembles the difference in the HOMO alignment from the vacuum level of 53 meV in DFT calculations of BTTT in gas phase. In the following we discuss, in which modes the rotamers differ in intensity or energy. The prominent low energy modes ν1−5 consist of

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bending and stretching modes of the whole backbones with longitudinal or transversal vibrations along the C12 H25 chains. The higher energy modes around ν18 correspond to various C-C stretching modes. For these modes, the Huang-Rhys factor varies between the two rotamers, but the energies are nearly identical. As a result the intensity distribution varies for the two rotamers. On the contrary, the two modes ν12 and ν15 are shifted in energy, when comparing transtrans and cis-cis configuration. They are displayed in mass weighted coordinates in Figure 4, together with the modes ν9 , ν14 and ν16 , which do not vary in energy between the rotamers. For equal-energy modes, the molecular vibrations are confined to either the thienothiophene group (ν9 ) or the thiophene group (ν14 ). In case of ν16 both sub groups show vibrational modes. However, the C-C bonds between the subgroups are only slightly distorted, thus they are not strongly coupled. The two split modes ν12 and ν15 , on the other hand, consist of vibrations involving the whole backbone. In particular, the C-C bond between the thiophene and thienothiophene group is distorted in a different way for the two rotamers. Therefore the modes are not fully analogous any more and have slightly different energies. MoS2 as Decoupling Layer In general, molecular rotamers only differ in minute details in their vibrational modes. Nonetheless, we could identify these differences in conductance spectra of BTTT thanks to highly resolved dI/dV spectra on MoS2 . We suggest that MoS2 is an ideal decoupling layer for molecules due to three important properties. First, compared to other single-layer materials frequently used as decoupling layers, such as graphene 10,11,34 or hexagonal boron nitride, 9,35,36 MoS2 provides a larger spatial tunneling barrier between molecule and substrate due to its three atomic layer thickness. Secondly, MoS2 provides a significant electronic band gap in contrast to single- and bilayer graphene. These characteristics enlarge the lifetime of the excited state, on the one hand due


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to reduction of the electron tunneling rate from the molecule into the metal substrate, and on the other hand due to the suppression of the vibrational relaxation via electron-hole-pair creation at Fermi energy. Thirdly, MoS2 is not an ionic material, in contrast to Al2 O3 or NaCl, which are also often used as decoupling layers. 5,7,37–40 The ionic character of those materials causes a strong electron-phonon coupling, leading to Gaussian broadening of molecular ion resonances and, thus, obscures low-energy details in the spectra. 32 A recent study showed that the relaxation energy of a charged molecule on a NaCl is mainly due to lattice distortion of the decoupling layer, 41 which could possibly cause a superposition of molecular vibrations and substrate phonons in STS. The spectra on MoS2 presented above were well described by peak widths of 6 meV. However, a close inspection of the onset of the PIR shows that its low-energy flank is actually even sharper (Figure 5). We ascribe the broader appearance of the high-energy side to vibrational modes with energies below 5 meV. Our DFT calculations reveal the presence of lowenergy vibrations, such as flapping modes of the backbone (4.2 meV) or the whole molecule (0.4 meV). In gas phase, their Huang-Rhys factor is zero. In contrast, the presence of a surface breaks the symmetry, allowing these vibrations to be excited by electrons. Additionally, vibrations of the molecule against the surface are expected with similar energies. 42 The sum of these vibrations leads to the asymmetry of the overall lineshape at the onset of the PIR. For an estimation of the excited state lifetime we then need to consider the broadening associated with the low-energy flank in the spectrum. 9 Fitting a Voigt function to the lowenergy flank of the elastic peak of the transtrans and cis-cis configuration (grey areas in Figure 5) yields a FWHM of 4.0 meV and 4.3 meV, respectively. The Voigt function is a convolution of a Gaussian and a Lorentzian profile, accounting for different broadening mechanisms. In order to estimate a lower boundary for the lifetime, we assume the Lorentzian part of the Voigt profile

to be caused solely by lifetime broadening. The FWHM of the Lorentzian is 1.7 meV (transtrans) and 2.4 meV (cis-cis), indicating a lifetime of the excited state of more than 300 fs. This brings the lifetime broadening into the range of the thermal broadening of 1.4 meV at 4.6 K.

Conclusions In conclusion, we suggest that MoS2 is an interesting substrate for studying low-energy phenomena in single molecules. We ascribe this promising potential to a sizable electronic band gap in combination with a low electron-phonon coupling strength. These properties yielded an effective energy resolution that enabled us to use small differences in energy and intensity of vibronic resonances to unambiguously identify the two rotamers. By simple DFT calculations of the BTTT molecules in gas phase we extracted the vibrational energies and HuangRhys factors. Those ingredients were sufficient to simulate the experimental spectra to a surprising degree. The excellent match between experiment and DFT allowed us to assign the peaks in the spectra to specific vibrational modes.

Methods/Experiment The Au(111) single crystal was cleaned under ultra-high vacuum conditions using the typical sputtering–annealing cycles. MoS2 was subsequently grown by deposition of Mo and heating in H2 S atmosphere, adapting a recipe by Grønborg et al. 43 Subsequently, a submonolayer amount of BTTT was evaporated from a Knudsen cell evaporator at 365 K onto the surface, held at 200 K. The sample was then transferred into the STM operated at a base temperature of 4.6 K. For scanning tunneling spectroscopy (STS) a lock-in amplifier was used with modulation frequency f = 921 Hz. The STM tip was coated with gold by controlled indentation into the Au(111) surface and its quality assured by reference spectra on Au(111) and MoS2 .


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DFT calculations were performed for single molecules in gas phase with the Gaussian 09 package, 44 using the B3PW91 functional with the 6-31g(d,p) basis set.

6. Wang, S.; Wang, W.; Hong, Y.; Zhong, B.; Lin, N. Vibronic State Assisted Resonant Transport in Molecules Strongly Anchored at an Electrode. Phys. Rev. B. 2011, 83, 115431.

Supporting Information

7. Repp, J.; Meyer, G.; Stojković, S. M.; Gourdon, A.; Joachim, C. Molecules on Insulating Films: Scanning-Tunneling Microscopy Imaging of Individual Molecular Orbitals. Phys. Rev. Lett. 2005, 94, 026803.

Models and STM images of the four different rotamer species for adsorbed BTTT molecules, details on the DFT calculations and extraction of Huang-Rhys factors, as well as a plot including the calculated Huang-Rhys factors of all vibronic modes can be found in the Supporting Information, which is available free of charge on the ACS Publications website. Acknowledgement We thank K. Weinel for the participation in the first experiments on BTTT on MoS2 . Also we thank R. Bittl, R. Steyrleuthner and J. Behrends for the fruitful discussions. We are grateful to the German research foundation for funding within the framework of the SFB 658 and TRR 227.

References 1. Stipe, B. C.; Rezaei, M. A.; Ho, W. SingleMolecule Vibrational Spectroscopy and Microscopy. Science 1998, 12, 1732–1735. 2. Heinrich, A. J.; Lutz, C. P.; Gupta, J. A.; Eigler, D. M. Molecule Cascades. Science 2002, 298, 1381–1387. 3. Pradhan, N. A.; Liu, N.; Ho, W. Vibronic Spectroscopy of Single C60 Molecules and Monolayers with the STM. J. Phys. Chem. B 2005, 109, 8513. 4. Franke, K. J.; Pascual, J. I. Effects of Electron-Vibration Coupling in Transport through Single Molecules. J. Phys.: Condens. Matter 2012, 24, 394002. 5. Qiu, X. H.; Nazin, G. V.; Ho, W. Vibronic States in Single Molecule Electron Transport. Phys. Rev. Lett. 2004, 92, 206102.

8. Pavliˇcek, N.; Swart, I.; Niedenf¨ uhr, J.; Meyer, G.; Repp, J. Symmetry Dependence of Vibration-Assisted Tunneling. Phys. Rev. Lett. 2013, 110, 136101. 9. Schulz, F.; Drost, R.; Hämäläinen, S.; Liljeroth, P. Templated Self-Assembly and Local Doping of Molecules on Epitaxial Hexagonal Boron Nitride. ACS Nano 2013, 7, 11121–11128. 10. Riss, A.; Wickenburg, S.; Tan, L. Z.; Tsai, H.-Z.; Kim, Y.; Lu, J.; Bradley, A. J.; Ugeda, M. M.; Meaker, K. L.; Watanabe, K.; Taniguchi, T.; Zettl, A.; Fischer, F. R.; Louie, S. G.; Crommie, M. F. Imaging and Tuning Molecular Levels at the Surface of a Gated Graphene Device. ACS Nano 2014, 8, 5395–5401. 11. Wickenburg, S.; Lu, J.; Lischner, J.; Tsai, H.-Z.; Omrani, A. A.; Riss, A.; Karrasch, C.; Bradley, A.; Jung, H. S.; Khajeh, R.; Wong, D.; Watanabe, K.; Taniguchi, T.; Zettl, A.; Neto, A. H. C.; Louie, S. G.; Crommie, M. F. Tuning Charge and Correlation Effects for a Single Molecule on a Graphene Device. Nat. Commun. 2016, 7, 1. 12. Frederiksen, T.; Franke, K. J.; Arnau, A.; Schulze, G.; Pascual, J. I.; Lorente, N. Dynamic Jahn-Teller Effect in Electronic Transport through Single C60 Molecules. Phys. Rev. B 2008, 78, 233401.


Page 7 of 13 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

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13. Cheiwchanchamnangij, T.; Lambrecht, W. R. L. Quasiparticle Band Structure Calculation of Monolayer, Bilayer, and Bulk MoS2 . Phys. Rev. B 2012, 85, 205302. 14. Ramasubramaniam, A. Large Excitonic Effects in Monolayers of Molybdenum and Tungsten Dichalcogenides. Phys. Rev. B 2012, 86, 115409. 15. Qiu, D. Y.; da Jornada, F. H.; Louie, S. G. Optical Spectrum of MoS2 : Many-Body Effects and Diversity of Exciton States. Phys. Rev. Lett. 2013, 111, 119901. 16. Huang, Y. L.; Zheng, Y. J.; Chi, D.; Wee, A. T. S.; Quek, Organic-2D Transition Metal genide Heterointerface. Chem. 2018, 47, 3241–3264.

Song, Z.; S. Y. The DichalcoSoc. Rev.

17. Zhang, L.; Liu, F.; Diao, Y.; Marsh, H. S.; Colella, N. S.; Jayaraman, A.; Russell, T. P.; Mannsfeld, S. C. B.; Briseno, A. L. The Good Host: Formation of Discrete One-Dimensional Fullerene “Channels” in Well-Ordered Poly(2,5bis(3-alkylthiophen-2-yl)thieno[3,2b]thiophene) Oligomers. J. Am. Chem. Soc. 2014, 136, 18120–18130. 18. McCulloch, I.; Heeney, M.; Bailey, C.; Genevicius, K.; MacDonald, I.; Shkunov, M.; Sparrowe, D.; Tierney, S.; Wagner, R.; Zhang, W.; Chabinyc, M. L.; Kline, R. J.; McGehee, M. D.; Toney, M. F. Liquid-Crystalline Semiconducting Polymers with high Charge-Carrier Mobility. Nat. Mater. 2006, 5, 328–333. 19. Hamadani, B. H.; Gundlach, D. J.; McCulloch, I.; Heeney, M. Undoped Polythiophene Field-Effect Transistors with Mobility of 1 cm V−2 s−1 . Appl. Phys. Lett. 2007, 91, 243512. 20. Zhang, Y.; Steyrleuthner, R.; Bredas, J.L. Charge Delocalization in Oligomers of Poly(2,5-bis(3-alkylthiophene-2yl)thieno[3,2-b]thiophene) (PBTTT). J. Phys. Chem. C 2016, 120, 9671–9677.

21. Steyrleuthner, R.; Zhang, Y.; Zhang, L.; Kraffert, F.; Cherniawski, B. P.; Bittl, R.; Briseno, A. L.; Bredas, J.-L.; Behrends, J. Impact of Morphology on Polaron Delocalization in a Semicrystalline Conjugated Polymer. Phys. Chem. Chem. Phys. 2017, 19, 3627–3639. 22. Falke, S. M.; Rozzi, C. A.; Brida, D.; Maiuri, M.; Amato, M.; Sommer, E.; De Sio, A.; Rubio, A.; Cerullo, G.; Molinari, E.; Lienau, C. Coherent Ultrafast Charge Transfer in an Organic Photovoltaic Blend. Science 2014, 344, 1001–1005. 23. Krane, N.; Lotze, C.; Läger, J.; Reecht, G.; Franke, K. J. Electronic Structure and Luminescence of Quasi-Freestanding MoS2 Nanopatches on Au(111). ACS Nano 2016, 16, 5163. 24. Krane, N.; Lotze, C.; Franke, K. J. Moiré Structure of MoS2 on Au(111): Local Structural and Electronic Properties. Surf. Sci. 2018, In press, DOI: 10.1016/j.susc.2018.03.015. 25. Sørensen, S. G.; F¨ uchtbauer, H. G.; Tuxen, A. K.; Walton, A. S.; Lauritsen, J. V. Structure and Electronic Properties of In Situ Synthesized Single-Layer MoS2 on a Gold Surface. ACS Nano 2014, 8, 6788. 26. Taber, B. N.; Kislitsyn, D. A.; Gervasi, C. F.; Mannsfeld, S. C. B.; Zhang, L.; Briseno, A. L.; Nazin, G. V. AdsorptionInduced Conformational Isomerization of Alkyl-Substituted Thiophene Oligomers on Au(111): Impact on the Interfacial Electronic Structure. ACS Appl. Mater. Interfaces 2015, 7, 15138–15142. 27. Kislitsyn, D. A.; Taber, B. N.; Gervasi, C. F.; Mannsfeld, S. C. B.; Zhang, L.; Briseno, A. L.; Nazin, G. V. CoverageDependent Self-Assembly Regimes of Alkyl-Substituted Thiophene Oligomers on Au(111): Scanning Tunneling Microscopy


ACS Nano 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

and Spectroscopy. J. Phys. Chem. C 2015, 119, 26959–26967. 28. Miwa, J.; Ulstrup, S.; Sørensen, S. G.; ˇ Dendzik, M.; Grubiˇsić Cabo, A.; Bianchi, M.; Lauritsen, J. V.; Hofmann, P. Electronic Structure of Epitaxial Single-Layer MoS2 . Phys. Rev. Lett. 2014, 114, 046802. ˇ 29. Grubiˇsić Cabo, A.; Miwa, J. A.; Grønborg, S. S.; Riley, J. M.; Johannsen, J. C.; Cacho, C.; Alexander, O.; Chapman, R. T.; Springate, E.; Grioni, M.; Lauritsen, J. V.; King, P. D. C.; Hofmann, P.; Ulstrup, S. Observation of Ultrafast Free Carrier Dynamics in Single Layer MoS2 . ACS Nano 2015, 15, 5883. 30. Kerner, E. H. Note on the Forced and Damped Oscillator in Quantum Mechanics. Can. J. Phys. 1958, 36, 371–377. 31. Gadzuk, J. W. Inelastic Resonance Scattering, Tunneling, and Desorption. Phys. Rev. B. 1991, 44, 13466. 32. Repp, J.; Meyer, G.; Paavilainen, S.; Olsson, F. E.; Persson, M. Scanning Tunneling Spectroscopy of Cl Vacancies in NaCl Films: Strong Electron-Phonon Coupling in Double-Barrier Tunneling Junctions. Phys. Rev. Lett. 2005, 95, 225503. 33. Wilson, E. B.; Decius, J. C.; Cross, P. C. Molecular Vibrations; London: McGrawHill, 1955.

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36. Joshi, S.; Bischoff, F.; Koitz, R.; Ecija, D.; Seufert, K.; Seitsonen, A. P.; Hutter, J.; Diller, K.; Urgel, J. I.; Sachdev, H.; Barth, J. V.; Auwärter, W. Control of Molecular Organization and Energy Level Alignment by an Electronically Nanopatterned Boron Nitride Template. ACS Nano 2013, 8, 430–442. 37. Gross, L.; Moll, N.; Mohn, F.; Curioni, A.; Meyer, G.; Hanke, F.; Persson, M. HighResolution Molecular Orbital Imaging Using a p-Wave STM Tip. Phys. Rev. Lett. 2011, 107, 086101. 38. Villagomez, C. J.; Zambelli, T.; Gauthier, S.; Gourdon, A.; Stojkovic, S.; Joachim, C. STM Images of a Large Organic Molecule Adsorbed on a Bare Metal Substrate or on a Thin Insulating Layer: Visualization of HOMO and LUMO. Surf. Sci. 2009, 603, 1526–1532. 39. Li, S.; Yuan, D.; Yu, A.; Czap, G.; Wu, R.; Ho, W. Rotational Spectromicroscopy: Imaging the Orbital Interaction between Molecular Hydrogen and an Adsorbed Molecule. Phys. Rev. Lett. 2015, 114, 206101. 40. Bombis, C.; Ample, F.; Lafferentz, L.; Yu, H.; Hecht, S.; Joachim, C.; Grill, L. Single Molecular Wires Connecting Metallic and Insulating Surface Areas. Angew. Chem., Int. Ed. 2009, 48, 9966–9970.

¨ 34. Cho, J.; Smerdon, J.; Gao, L.; S¨ uzer, O.; Guest, J. R.; Guisinger, N. P. Structural and Electronic Decoupling of C60 from Epitaxial Graphene on SiC. Nano Lett. 2012, 12, 3018–3024.

41. Fatayer, S.; Schuler, B.; Steurer, W.; Scivetti, I.; Repp, J.; Gross, L.; Persson, M.; Meyer, G. Reorganization Energy upon Charging a Single Molecule on an Insulator Measured by Atomic Force Microscopy. Nat. Nanotechnol. 2018, 13, 376– 380.

35. Järvinen, P.; Hämäläinen, S.; Banerjee, K.; Häkkinen, P.; Ijäs, M.; Harju, A.; Liljeroth, P. Molecular Self-Assembly on Graphene on SiO2 and h-BN Substrates. Nano Lett. 2013, 13, 3199–3204.

42. Cocker, T. L.; Peller, D.; Yu, P.; Repp, J.; Huber, R. Tracking the Ultrafast Motion of a Single Molecule by Femtosecond Orbital Imaging. Nature 2016, 539, 263–267.


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43. Grønborg, S. S.; Ulstrup, S.; Bianchi, M.; Dendzik, M.; Sanders, C. E.; Lauritsen, J. V.; Hofmann, P.; Miwa, J. A. Synthesis of Epitaxial Single-Layer MoS2 on Au(111). Langmuir 2015, 31, 9700. 44. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Petersson, G. A.; Nakatsuji, H.; Li, X.; Caricato, M.; Marenich, A.; Bloino, J.; Janesko, B. G.; Gomperts, R.; Mennucci, B.; Hratchian, H. P.; Ortiz, J. V.; Izmaylov, A. F. et al. Gaussian 09, Revision D.01. 2009; Gaussian Inc. Wallingford CT.

a)

trans-trans-rotamer

cis-cis-rotamer

b)

5 nm c)

1 nm

Figure 1: BTTT on MoS2 /Au(111) (a) Structure model of trans-trans- and cis-cisconfiguration of BTTT. While the alkyl legs point always in opposite direction with respect to the molecular backbones axis, the difference between trans-trans- and cis-cis-rotamer is the rotated central thienothiophene unit. (b) Most molecules stick to the Au(111) surface (left side of the image). A small fraction of the molecules sticks to MoS2 islands (recognizable by the hexagonal Moiré pattern) in unordered or ordered (red box) islands. (1 V/10 pA) (c) Zoom into the ordered BTTT island from (b) with superimposed molecular structure models. (1 V/10 pA)


ACS Nano

0.6 0.4 0.2 0.0 -2

c)

b)

MoS2 BTTT

-1 0 1 Bias Voltage (V)

2

d) Energy

10 5 0 -1.2

-1.1 -1.0 -0.9 Bias Voltage (V)

M*

ħωk

Abs. Bias

dI/dV (nS)

a)

dI/dV (nS)

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

εk M0 dI/dV

Normal Coord.

Figure 2: Electronic and vibronic structure of BTTT. (a) STS taken on pristine MoS2 (black) and a BTTT molecule (red). The molecular positive ion resonance (corresponding to tunneling through the HOMO) at −1 eV lies within the band gap of MoS2 . Feedback opened at 2.2 V/100 pA (black) and 2.2 V/75 pA (red), Vmod = 5 mV (b) Closeup STM image of BTTT molecules on MoS2 . (−0.85 V/10 pA) (c) High-resolution STS on the different BTTT-rotamers, taken at the positions indicated by the colored crosses in (b). A set of vibronic resonances is found to follow the first PIR resonance. Feedback opened at −0.85 V/10 pA, Vmod = 0.5 mV (d) Scheme of vibronic excitation of a molecule in a tunneling experiment in the Franck-Condon picture. The state M 0 corresponds to the neutral molecular ground state, while M ∗ is the charged, excited molecule. The horizontal lines correspond to higher harmonics of one vibrational mode ~ωk of the excited molecule. The scheme on the right sketches the Franck-Condon derived transition probabilities for the different vibronic modes translating into an energy-dependent change of the measured tunneling conductance.


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ν7 & 2ν4

50

0 1

ν1

2ν9 ν14

ν14+ ν3

ν15

ν16

trans-trans cis-cis

ν12 ν9+ ν4 ν9+ ν2,3 ν 9

150

ν18

ν18+ ν3

ν18+ ν4

sim. dI/dV (a.u.)

200 1

Vibronic Energy (meV) 100

0 1 dI/dV (a.u.)

0

Huang-Rhys Factor

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

ACS Nano

2ν3 ν6 & ν3+ ν4 ν5 ν4 ν3

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0

-200

-150 -100 Bias Voltage Difference (mV)

-50

0

Figure 3: Simulated and measured vibronic structure of BTTT. Top: Vibronic structure of the trans-trans (blue) and cis-cis (green) rotamers of BTTT obtained from ab-initio DFT simulations. The colored circles indicate the energy and Huang-Rhys factor of the relevant vibrational modes. A figure containing all modes is displayed in the Supporting Information. For the simulated spectra, all vibrations with a Huang-Rhys factor of Sk ≥ 0.001 were considered. Bottom: Experimental spectra of the two rotamers of BTTT. Feedback opened at −0.85 V/10 pA, Vmod = 0.5 mV. The energy of the elastic peaks were set to 0 eV and the bottom x-Axis scaled by a factor of 0.9 in order to account for the voltage drop over the MoS2 layer. The intensity of the elastic peak was normalized for all spectra.


ν12

ν14

ν15

ν16

cis-cis

ν9

Figure 4: Visualization of selected vibrational modes (compare to energies in Figure 3) of BTTT trans-trans and cis-cis rotamer. Dark gray, light grey and yellow colored circles indicate the positions of C, H and S atoms, respectively, for the positively charged molecules. Red circles are the atom displacements – massweighted for better visibility – induced by the vibrational mode. The arrows indicate the vibrational modes that are different for transtrans and cis-cis rotamer and, hence, lead to clear differences in their vibronic spectra.

10

trans-trans cis-cis Voigt

dI/dV (nS)

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

trans-trans

ACS Nano

0

-0.98

-0.94 Bias Voltage (V)

-0.90

Figure 5: Asymmetric onsets of the positive ion resonance for both BTTT rotamers. The two dotted lines display a Voigt-function fitted to the low-energy flank of the onsets (fitting range marked by grey areas). Feedback opened at −0.85 V/10 pA, Vmod = 0.5 mV.


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Experiment Simulation

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rotamer B

ν15

Au-tip

BTTT

ACS Paragon Plus Environment MoS2

Energy

Au(111)

High-Resolution Vibronic Spectra of Molecules on Molybdenum

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