Nanoscale plasmonic phenomena in CVD-grown MoS2 monolayer revealed by ultra-broadband synchrotron radiation based nano-FTIR spectroscopy and near-field microscopy

Nanoscale plasmonic phenomena observed in single and bilayers of molybdenum disulfide (MoS2) on silicon dioxide (SiO2) are reported. A scattering type scanning near-field optical microscope (sSNOM) with a broadband synchrotron radiation (SR) infrared source was used. We also present complementary optical mapping using tunable CO2laser radiation. Specifically, there is a correlation of the topography of welldefined MoS2 islands grown by chemical vapor deposition, as determined by atomic force microscopy, with the infrared (IR) signature of MoS2. The influence of MoS2 islands on the SiO2 phonon resonance is discussed. The results reveal the plasmonic character of the MoS2 structures and their interaction with the SiO2 phonons leading to an enhancement of the hybridized surface plasmon-phonon mode. A theoretical analysis shows that, in the case of monolayer islands, the coupling of the MoS2 optical plasmon mode to the SiO2 surface phonons does not affect the infrared spectrum significantly. For two-layer MoS2, the coupling of the extra interplane acoustic plasmon mode with the SiO2 surface transverse phonon leads to a remarkable increase of the surface phonon peak at 794 cm. This is in agreement with the experimental data. These results show the capability of the s-SNOM technique to study local multiple excitations in complex nonhomogeneous structures. ©2016 Optical Society of America OCIS codes: (120.0120) Instrumentation, measurement, and metrology; (180.4243) Near-field microscopy; (240.0310) Thin films; (300.6340) Spectroscopy, infrared; (310.6188) Spectral properties. References and links 1. J. A. Wilson and A. D. Yoffe, “The transition metal dichalcogenides discussion and interpretation of the observed optical, electrical and structural properties,” Adv. Phys. 18(73), 193–335 (1969). 2. K. F. Mak, C. Lee, J. Hone, J. Shan, and T. F. Heinz, “Atomically thin MoS2: a new direct-gap semiconductor,” Phys. Rev. Lett. 105(13), 136805 (2010). 3. J. Mann, Q. Ma, P. M. Odenthal, M. Isarraraz, D. Le, E. Preciado, D. Barroso, K. Yamaguchi, G. von Son Palacio, A. Nguyen, T. Tran, M. Wurch, A. Nguyen, V. Klee, S. Bobek, D. Sun, T. F. Heinz, T. S. Rahman, R. #252198 Received 19 Oct 2015; revised 17 Dec 2015; accepted 19 Dec 2015; published 13 Jan 2016 © 2016 OSA 25 Jan 2016 | Vol. 24, No. 2 | DOI:10.1364/OE.24.001154 | OPTICS EXPRESS 1154 Corrected: 22 February 2016 Kawakami, and L. Bartels, “2-dimensional transition metal dichalcogenides with tunable direct band gaps: MoS2(1-x) Se2x monolayers,” Adv. Mater. 26(9), 1399–1404 (2014). 4. S. Y. Zhou, G.-H. Gweon, A. V. Fedorov, P. N. First, W. A. de Heer, D.-H. Lee, F. Guinea, A. H. Castro Neto, and A. Lanzara, “Substrate-induced bandgap opening in epitaxial graphene,” Nat. Mater. 6(10), 770–775 (2007). 5. R. Balog, B. Jørgensen, L. Nilsson, M. Andersen, E. Rienks, M. Bianchi, M. Fanetti, E. Laegsgaard, A. Baraldi, S. Lizzit, Z. Sljivancanin, F. Besenbacher, B. Hammer, T. G. Pedersen, P. Hofmann, and L. Hornekaer, “Bandgap opening in graphene induced by patterned hydrogen adsorption,” Nat. Mater. 9(4), 315–319 (2010). 6. K. Nakada, M. Fujita, G. Dresselhaus, and M. S. Dresselhaus, “Edge state in graphene ribbons: Nanometer size effect and edge shape dependence,” Phys. Rev. B Condens. Matter 54(24), 17954–17961 (1996). 7. Y.-W. Son, M. L. Cohen, and S. G. Louie, “Energy gaps in graphene nanoribbons,” Phys. Rev. Lett. 97(21), 216803 (2006). 8. M. Y. Han, B. Özyilmaz, Y. Zhang, and P. Kim, “Energy band-gap engineering of graphene nanoribbons,” Phys. Rev. Lett. 98(20), 206805 (2007). 9. M.-W. Lin, C. Ling, Y. Zhang, H. J. Yoon, M.-M. C. Cheng, L. A. Agapito, N. Kioussis, N. Widjaja, and Z. Zhou, “Room-temperature high on/off ratio in suspended graphene nanoribbon field-effect transistors,” Nanotechnology 22(26), 265201 (2011). 10. L. Ci, L. Song, C. Jin, D. Jariwala, D. Wu, Y. Li, A. Srivastava, Z. F. Wang, K. Storr, L. Balicas, F. Liu, and P. M. Ajayan, “Atomic layers of hybridized boron nitride and graphene domains,” Nat. Mater. 9(5), 430–435 (2010). 11. A. H. Castro Neto, F. Guinea, N. M. R. Peres, K. S. Novoselov, and A. K. Geim, “The electronic properties of graphene,” Rev. Mod. Phys. 81(1), 109–162 (2009). 12. P. Avouris, “Graphene: electronic and photonic properties and devices,” Nano Lett. 10(11), 4285–4294 (2010). 13. S. Das Sarma, S. Adam, E. H. Hwang, and E. Rossi, “Electronic transport in two-dimensional graphene,” Rev. Mod. Phys. 83(2), 407–470 (2011). 14. Q. Yue, J. Kang, Z. Shao, X. Zhang, S. Chang, G. Wang, S. Qin, and J. Li, “Mechanical and electronic properties of monolayer MoS2 under elastic strain,” Phys. Lett. A 376(12-13), 1166–1170 (2012). 15. A. Molina-Sánchez, D. Sangalli, K. Hummer, A. Marini, and L. Wirtz, “Effect of spin-orbit interaction on the optical spectra of single-layer, double-layer, and bulk MoS2,” Phys. Rev. B 88(4), 045412 (2013). 16. A. N. Grigorenko, M. Polini, and K. S. Novoselov, “Graphene plasmonics,” Nat. Photonics 6(11), 749–758 (2012). 17. T. Low and P. Avouris, “Graphene plasmonics for terahertz to mid-infrared applications,” ACS Nano 8(2), 1086–1101 (2014). 18. T. Stauber, “Plasmonics in Dirac systems: from graphene to topological insulators,” J. Phys. Condens. Matter 26(12), 123201 (2014). 19. M. Polini, R. Asgari, G. Borghi, Y. Barlas, T. Pereg-Barnea, and A. H. MacDonald, “Plasmons and the spectral function of graphene,” Phys. Rev. B 77(8), 081411 (2008). 20. A. Scholz, T. Stauber, and J. Schliemann, “Plasmons and screening in a monolayer of MoS2,” Phys. Rev. B 88(3), 035135 (2013). 21. P. Alonso-González, A. Y. Nikitin, F. Golmar, A. Centeno, A. Pesquera, S. Vélez, J. Chen, G. Navickaite, F. Koppens, A. Zurutuza, F. Casanova, L. E. Hueso, and R. Hillenbrand, “Controlling graphene plasmons with resonant metal antennas and spatial conductivity patterns,” Science 344(6190), 1369–1373 (2014). 22. K. M. Goodfellow, R. Beams, C. Chakraborty, L. Novotny, and A. N. Vamivakas, “Integrated nanophotonics based on nanowire plasmons and atomically thin material,” Optica 1(3), 149–152 (2014). 23. S. Butun, S. Tongay, and K. Aydin, “Enhanced light emission from large-area monolayer MoS2 using plasmonic nanodisc arrays,” Nano Lett. 15(4), 2700–2704 (2015). 24. Ž. Bonačić Lošić, “Coupling of plasmon and dipolar modes in a monolayer of MoS2,” Mod. Phys. Lett. B 28(12), 1450099 (2014). 25. G. Plechinger, J. Mann, E. Preciado, D. Barroso, A. Nguyen, J. Eroms, C. Schüller, L. Bartels, and T. Korn, “A direct comparison of CVD-grown and exfoliated MoS2 using optical spectroscopy,” Semicond. Sci. Technol. 29(6), 064008 (2014). 26. A. M. van der Zande, P. Y. Huang, D. A. Chenet, T. C. Berkelbach, Y. You, G.-H. Lee, T. F. Heinz, D. R. Reichman, D. A. Muller, and J. C. Hone, “Grains and grain boundaries in highly crystalline monolayer molybdenum disulphide,” Nat. Mater. 12(6), 554–561 (2013). 27. C. Lee, H. Yan, L. E. Brus, T. F. Heinz, J. Hone, and S. Ryu, “Anomalous lattice vibrations of singleand fewlayer MoS2.,” ACS Nano 4(5), 2695–2700 (2010). 28. H. Li, Q. Zhang, C. Yap, B. Tay, T. Edwin, A. Olivier, and D. Baillargeat, “From bulk to monolayer MoS2: evolution of Raman scattering,” Adv. Funct. Mater. 22(7), 1385–1390 (2012). 29. F. Bonaccorso, P.-H. Tan, and A. C. Ferrari, “Multiwall nanotubes, multilayers, and hybrid nanostructures: new frontiers for technology and Raman spectroscopy,” ACS Nano 7(3), 1838–1844 (2013). 30. X. Yin, Z. Ye, D. A. Chenet, Y. Ye, K. O’Brien, J. C. Hone, and X. Zhang, “Edge Nonlinear Optics on a MoS2 Atomic Monolayer,” Science 344(6183), 488–490 (2014). 31. N. Kumar, A. Rae, and D. Roy, “Accurate measurement of enhancement factor in tip-enhanced Raman spectroscopy through elimination of far-field artefacts,” Appl. Phys. Lett. 104(12), 123106 (2014). 32. L. M. Malard, T. V. Alencar, A. P. M. Barboza, K. F. Mak, and A. M. de Paula, “Observation of intense second harmonic generation from MoS2 atomic crystals,” Phys. Rev. B 87(20), 201401 (2013). #252198 Received 19 Oct 2015; revised 17 Dec 2015; accepted 19 Dec 2015; published 13 Jan 2016 © 2016 OSA 25 Jan 2016 | Vol. 24, No. 2 | DOI:10.1364/OE.24.001154 | OPTICS EXPRESS 1155 33. N. Kumar, S. Najmaei, Q. Cui, F. Ceballos, P. M. Ajayan, J. Lou, and H. Zhao, “Second harmonic microscopy of monolayer MoS2,” Phys. Rev. B 87(16), 161403 (2013). 34. Y. Lin, X. Ling, L. Yu, S. Huang, A. L. Hsu, Y.-H. Lee, J. Kong, M. S. Dresselhaus, and T. Palacios, “Dielectric screening of excitons and trions in single-layer MoS2.,” Nano Lett. 14(10), 5569–5576 (2014). 35. K. Andersen, K. W. Jacobsen, and K. S. Thygesen, “Plasmons on the edge of MoS2 nanostructures,” Phys. Rev. B 90(16), 161410 (2014). 36. Z. Fei, G. O. Andreev, W. Bao, L. M. Zhang, A. S McLeod, C. Wang, M. K. Stewart, Z. Zhao, G. Dominguez, M. Thiemens, M. M. Fogler, M. J. Tauber, A. H. Castro-Neto, C. N. Lau, F. Keilmann, and D. N. Basov, “Infrared nanoscopy of Dirac plasmons at the graphene-SiO interface,” Nano Lett. 11(11), 4701–4705 (2011). 37. Z. Fei, A. S. Rodin, G. O. Andreev, W. Bao, A. S. McLeod, M. Wagner, L. M. Zhang, Z. Zhao, M. Thiemens, G. Dominguez, M. M. Fogler, A. H. Castro Neto, C. N. Lau, F. Keilmann, and D. N. Basov, “Gate-tuning of graphene plasmons revealed by infrared nano-imaging,” Nature 487(7405), 82–85 (2012). 38. J. Chen, M. Badioli, P. Alonso-González, S. Thongrattanasiri, F. Huth, J. Osmond, M. Spasenović, A. Centeno, A. Pesquera, P. Godignon, A. Z. Elorza, N. Camara, F. J. García de Abajo, R. Hillenbrand, and F. H. L. Koppens, “Optical nano-imaging of gate-tunable graphene plasmons,” Nature 487(7405), 77–81 (2012). 39. M. Wagner, Z. Fei, A. S. McLeod, A. S. Rodin, W. Bao, E. G. Iwinski, Z. Zhao, M. Goldflam, M. Liu, G. Dominguez, M. Thiemens, M. M. Fogler, A. H. Castro Neto, C. N. Lau, S. Amarie, F. Keilmann, and D. N. Basov, “Ultrafast and nanoscale plasmonic phenomena in exfoliated graphene revealed by infrared pump-probe nanoscopy,” Nano Lett. 14(2), 894–900 (2014). 40. S. Kawata and Y. Inouye, “Scanning probe optical microscopy using a metallic probe tip,” Ultramicroscopy 57(2-3), 313–317 (1995). 41. F. Zenhausern, Y. Martin, and H. K. Wickramasinghe, “Scanning interferometric apertureless microscopy: optical imaging at 10 angstrom resolution,” Science 269(5227), 1083–1085 (1995). 42. R. Bachelot, P. Gleyzes, and A. C. Boccara, “Near-field optical microscope based on local perturbation of a diffraction spot,” Opt. Lett. 20(18), 1924–1926 (1995). 43. B. Knoll and F. Keilmann, “Near-field probing of vibrational absorption for chemical microscopy,” Nature 399(6732), 134–137 (1999). 44. F. Huth, A. Govyadinov, S. Amarie, W. Nuansing, F. Keilmann, and R. Hillenbrand, “Nano-FTIR absorption spectroscopy of molecular fingerprints at 20 nm spatial resolution,” Nano Lett. 12(8), 3973–3978 (2012). 45. Y. Ikemoto, T. Moriwaki, T. Kinoshita, M. Ishikawa, S. Nakashima, and H. Okamura, “Near-field spectroscopy with infrared synchrotron radiation source,” J. Surf. Sci. Nanotech. 9, 63–66 (2011). 46. P. Hermann, A. Hoehl, P. Patoka, F. Huth, E. Rühl, and G. Ulm, “Near-field imaging and nano-Fouriertransform infrared spectroscopy using broadband synchrotron radiation,” Opt. Express 21(3), 2913–2919 (2013). 47. P. Hermann, A. Hoehl, G. Ulrich, C. Fleischmann, A. Hermelink, B. Kästner, P. Patoka, A. Hornemann, B. Beckhoff, E. Rühl, and G. Ulm, “Characterization of semiconductor materials using synchrotron radiation-based near-field infrared microscopy and nano-FTIR spectroscopy,” Opt. Express 22(15), 17948–17958 (2014). 48. H. A. Bechtel, E. A. Muller, R. L. Olmon, M. C. Martin, and M. B. Raschke, “Ultrabroadband infrared nanospectroscopic imaging,” Proc. Natl. Acad. Sci. U.S.A. 111(20), 7191–7196 (2014). 49. F. Peragut, J.-B. Brubach, P. Roy, and Y. De Wilde, “Infrared near-field imaging and spectroscopy based on thermal or synchrotron radiation,” Appl. Phys. Lett. 104(25), 251118 (2014). 50. J. Feikes, M. von Hartrott, M. Ries, P. Schmid, G. Wüstefeld, A. Hoehl, R. Klein, R. Müller, and G. Ulm, “Metrology Light Source: The first electron storage ring optimized for generating coherent THz radiation,” Phys. Rev. ST Accel. Beams 14(3), 030705 (2011). 51. N. Ocelic, A. Huber, and R. Hillenbrand, “Pseudoheterodyne detection for background-free Near-field spectroscopy,” Appl. Phys. Lett. 89(10), 101124 (2006). 52. J. Mann, D. Sun, Q. Ma, J.-R. Chen, E. Preciado, T. Ohta, B. Diaconescu, K. Yamaguchi, T. Tran, M. Wurch, K. Magnone, T. F. Heinz, G. L. Kellogg, R. Kawakami, and L. Bartels, “Facile growth of monolayer MoS2 film areas on SiO2,” Eur. Phys. J. B 86(5), 226 (2013). 53. R. F. Frindt, “Single crystals of MoS2 several molecular layers thick,” J. Appl. Phys. 37(4), 1928–1929 (1966). 54. P. Nemes-Incze, Z. Osváth, K. Kamarás, and L. P. Biró, “Anomalies in thickness measurements of graphene and few layer graphite crystals by tapping mode atomic force microscopy,” Carbon 46(11), 1435–1442 (2008). 55. A. A. Govyadinov, S. Mastel, F. Golmar, A. Chuvilin, P. S. Carney, and R. Hillenbrand, “Recovery of permittivity and depth from near-field data as a step toward infrared nanotomography,” ACS Nano 8(7), 6911– 6921 (2014). 56. F. Maugé, J. Lamotte, N. S. Nesterenko, O. Manoilova, and A. A. Tsyganenko, “FT-IR study of surface properties of unsupported MoS2,” Catal. Today 70(1-3), 271–284 (2001). 57. A. Molina-Sánchez and L. Wirtz, “Phonons in single-layer and few-layer MoS2 and WS2,” Phys. Rev. B 84(15), 155413 (2011). 58. M. Ishikawa, M. Katsura, S. Nakashima, Y. Ikemoto, and H. Okamura, “Broadband near-field mid-infrared spectroscopy and application to phonon resonances in quartz,” Opt. Express 20(10), 11064–11072 (2012). 59. A. J. Huber, J. Wittborn, and R. Hillenbrand, “Infrared spectroscopic near-field mapping of single nanotransistors,” Nanotechnology 21(23), 235702 (2010). 60. S. Amarie and F. Keilmann, “Broadband-infrared assessment of phonon resonance in scattering-type near-field microscopy,” Phys. Rev. B 83(4), 045404 (2011). #252198 Received 19 Oct 2015; revised 17 Dec 2015; accepted 19 Dec 2015; published 13 Jan 2016 © 2016 OSA 25 Jan 2016 | Vol. 24, No. 2 | DOI:10.1364/OE.24.001154 | OPTICS EXPRESS 1156 61. A. S. McLeod, P. Kelly, M. D. Goldflam, Z. Gainsforth, A. J. Westphal, G. Dominguez, M. H. Thiemens, M. M. Fogler, and D. N. Basov, “Model for quantitative tip-enhcanced spectroscopy and the extraction of nanoscaleresolved optical constants,” Phys. Rev. B 90(8), 085136 (2014). 62. W. G. Spitzer and D. A. Kleinman, “Infrared lattice bands of quartz,” Phys. Rev. 121(5), 1324–1335 (1961). 63. E. H. Hwang, R. Sensarma, and S. Das Sarma, “Plasmon-phonon coupling in graphene,” Phys. Rev. B 82(19), 195406 (2010). 64. R. Hillenbrand and F. Keilmann, “Complex optical constants on a subwavelength scale,” Phys. Rev. Lett. 85(14), 3029–3032 (2000). 65. T. Stauber and G. Gómez-Santos, “Plasmons and near-field amplification in double-layer graphene,” Phys. Rev. B 85(7), 075410 (2012). 66. R. E. V. Profumo, R. Asgari, M. Polini, and A. H. MacDonald, “Double-layer graphene and topological insulator thin-film plasmons,” Phys. Rev. B 85(8), 085443 (2012). 67. N. Kumar, Q. Cui, F. Ceballos, D. He, Y. Wang, and H. Zhao, “Exciton diffusion in monolayer and bulk MoSe2.,” Nanoscale 6(9), 4915–4919 (2014). 68. A. C. Ferrari and D. M. Basko, “Raman spectroscopy as a versatile tool for studying the properties of graphene,” Nat. Nanotechnol. 8(4), 235–246 (2013).


Introduction
Among the 2D electronic materials that have received increased attention recently are the transition metal dichalcogenides. They consist of hexagonally structured layers of metal atoms, typically Mo, or W-embedded between two layers of chalcogenes (typically S, Se, or Te) [1], although the overall point group symmetry is C 3v . Depending on the combination of the metal and the chalcogen, different material properties are obtained. In this work we focus on MoS 2 . In contrast to graphene, MoS 2 is another intensely studied 2D electronic material, but has a direct band gap (E g = 1.87 eV) in the monolayer limit [2,3] and therefore does not require influence of the substrate [4], substrate hydrogen passivation [5], or dimensional reduction to break metallicity [6][7][8][9]. Likewise doping with nitrogen or boron [10] is not possible for MoS 2 . There is extensive literature for both MoS 2 and graphene describing their electronic [1,2,[11][12][13][14][15] as well as plasmonic properties [16][17][18][19][20], partially because of their potential in plasmonic circuitry [21][22][23].
There are important, characteristic differences between MoS 2 and graphene: for instance, in graphene the low-energy collective oscillations originate from intraband electron resonances [20], whereas in single layer MoS 2 (1L-MoS 2 ) there are not only a low-energy oscillations [18,20] but also a coupled system with an interband dipolar collective mode, which lowers the plasmon dispersion [24]. Specific emphasis was put on the electronic and optical properties of 1L-MoS 2 , as probed by photoluminescence (PL) spectroscopy [2,3,25,26], Raman spectroscopy [27][28][29], and second harmonic generation [30][31][32][33]. These methods make use of primary excitation in the optical regime. The use of photons with energies exceeding the 1L-MoS 2 direct band gap leads to exciton modes that are present in photoluminescence (PL) measurements [34], as well as strongly damped plasmons, in which energy losses are connected to the interband transition processes. The latter ones reduce the transition energy and the lifetime of the plasmons [35]. Direct excitation of the intraband plasmons in such systems requires, however, infrared (IR) radiation. Since the wavevector of the plasmons in extended monolayer graphene or 1L-MoS 2 is much larger than the wavevector of freely propagating photons of the same energy, one needs to find a way to overcome this mismatch for the successful excitation of plasmon polaritons.
Recent experiments by Fourier-transform infrared (FTIR) nanoscopy and nanoimaging on exfoliated graphene demonstrated that confining IR radiation to a nanosized scattering object results in an increased coupling to the in-plane momentum component and thus enables the optical excitation of plasmons [36][37][38][39]. These experiments used the near-field based technique namely scattering-type scanning near-field optical microscopy (s-SNOM) [40][41][42][43]. This method has gained broad interest because it not only enables imaging with a spatial resolution below 20 nm [44] but in combination with a broadband IR radiation source i.e., difference frequency generation sources [44] or ultra-broadband synchrotron radiation [45][46][47][48][49], it allows for probing the spectroscopic response of nanoscale objects as well as thin films.
Here, we present high sensitivity imaging and nano-FTIR spectroscopy on 1L-MoS 2 samples using the electron storage ring Metrology Light Source (MLS) [46,47,50] in the mid-IR regime. We demonstrate that plasmonic enhancement of the SiO 2 phonon due to coupling with the plasmon from a 2D material does not require the presence of an adlayer that exhibits a "Dirac cone plasmon", as surmised from previous experiments on exfoliated graphene [39]. At fixed carrier concentration, the optical plasmon frequency in 1L-MoS 2 is significantly lower as compared to the graphene plasmon frequency. Therefore, in order to observe the effect of the exciton-plasmon coupling in 1L-MoS 2 on SiO 2 , one needs an order of magnitude higher carrier concentrations in MoS 2 compared to graphene. However, one can already observe an enhancement of the SiO 2 phonons by the MoS 2 plasmon in the case of the double layer (2L-MoS 2 ), i.e. at rather low charge carrier concentration levels. Specifically, as is demonstrated below, we observe in the case of 2L-MoS 2 an additional inter-layer (acoustic) plasmon mode which is coupled to the corresponding SiO 2 transverse phonon mode. This leads to a significant increase in intensity in the s-SNOM spectrum, as shown here.

Experimental part
The experimental setup has been described previously [46,47]. It consists of a commercial s-SNOM system (NeaSNOM, Neaspec GmbH, Germany), which combines an atomic force microscope (AFM) operating in tapping mode with a typical oscillation amplitude of about 50 nm and an asymmetric Michelson interferometer. Au-coated silicon AFM cantilevers with a metal layer thickness of about 20 nm and resonance frequencies in the range from 75 to 265 kHz were used for the experiments described below. The average tip diameter of near-field probes did not exceed 50 nm. As an IR radiation source we either used a continuous wave, grating-tuned CO 2 gas laser (PL5, Edinburgh Instruments, UK) working in sealed-off mode or the IR radiation from the MLS storage ring (Berlin, Germany). The former source is characterized by high intensity and frequency stability at discrete transition lines enhancing suitability for nano-imaging experiments. The latter source is predominantly designed as an ultra-broadband source for spectroscopy by delivering high brilliance radiation in the IR and THz regimes.
In our experiments the emitted synchrotron radiation (SR) was directed to the experimental setup from the storage ring by using several parabolic and planar mirrors. At the end of the beamline a diamond window is mounted. From there the radiation penetrates ambient conditions under which the s-SNOM experiments were carried out. At this point the IR beam has a rhomboidal shape of approximately 10 mm x 25 mm in vertical and horizontal directions, respectively. The integrated power was typically about 2 mW in the wavelength range from 1 µm to 20 µm at a ring current of 100 mA. The radiation is then guided through a periscope-like mirror configuration [P in Fig. 1], in order to convert the initial horizontal polarization of the SR into vertically polarized radiation. The SR is focused on the AFM tip apex by a parabolic mirror [PM in Fig. 1].
The change in polarization direction is essential for s-SNOM operation, since a larger component of the electric field vector parallel to the tip axis increases the electric field enhancement at the tip apex. These fields interact locally with the surface of the sample by exciting a spectroscopic response. This is collected by the same near-field probe which acts as one arm of a Michelson interferometer. The second arm consists of a planar mirror which is mounted on a piezo stage and has been translated over a distance of 800 µm [I in Fig. 1]. The collected signal is recorded as an interferogram as a function of the optical beam path in the interferometer, which is subsequently Fourier transformed in order to derive an IR-spectrum. The spectral resolution of about 6 cm −1 is solely determined by the translation distance of the reference arm. A liquid-nitrogen cooled Mercury-Cadmium-Telluride (MCT) detector (J15D14, Teledyne Judson Technologies) [D in Fig. 1] with a sensitivity range from 2 µm to 13.5 µm was used for these experiments. For near-field imaging using a CO 2 gas laser we have used a pseudoheterodyne interferometric detection technique [51]. It is based on the phase modulation of the reference wave and enables unambiguous imaging of the near-field optical amplitude and phase contrasts at a single laser line.
The investigated 1L-MoS 2 samples were grown in a tube furnace by chemical vapor deposition (CVD) onto a 300 nm thin SiO 2 -layer on a Si substrate. Elemental sulfur and MoO 3 powder are used as S-and Mo-sources, respectively. The reactants were placed into alumina crucibles and inserted into a quartz tube, so that ultra-high purity nitrogen, used as a carrier gas, transports the sulfur vapor to the MoO 3 powder and onto the substrate, which is placed on top of the crucible containing MoO 3 . The growth temperature was adjusted between 650 °C and 700°C [25, 52].   The height profile measured across the flake reveals a thickness of (0.8 ± 0.1) nm, representing a monolayer [cf. Figure 2(b)]. This result slightly differs from the expected theoretical thickness of 0.615 nm [53], which is commonly ascribed to a combination of different hygroscopicity/water accumulation on the SiO 2 substrate and the MoS 2 film as well as the AFM tapping mode, which has been reported to produce lower accuracy than in contact mode measurements, which has also been derived from the characterization of graphene samples [54]. The CVD MoS 2 islands contain occasionally bilayer growth seeds near their center. Compared to exfoliated layers CVD-grown samples exhibit stronger adhesion to the substrate [25]. Simultaneously to the topographic information, near-field intensity maps were acquired using a CO 2 laser source at 959.39 cm −1 (P2 line) [see Figs. 3(a)-3(e)]. An area of 7 µm x 7 µm in size was scanned at a resolution of 256 x 256 pixels and at an integration time of 20 ms per pixel. The first to the fifth harmonic signals were recorded (S 1 -S 5 ), respectively. They denote the harmonics of the cantilever's oscillation frequency (Ω) at which the signal was demodulated. This is of crucial importance due to the relatively large size of the focused SR spot [47], which illuminates both tip and sample simultaneously. With the demodulation of the signal, and in combination with a lock-in amplifier, the strong far-field background can be separated from the weak near-field signal, where the latter is the signal of interest. Note that the demodulation works better when using higher harmonics (S n , n>1). Higher harmonics signals correspond to decreasing the probing volume around the tip allowing for surface sensitive measurements [see Fig. 3(f)], as derived from ref [55].). The intensity maps of S 1 -S 3 do not show a clear optical response of the 1L-MoS 2 . The presence and definition of the 1L-MoS 2 can only be resolved by the increased contrast resulting from the side products of the MoS 2 growth process, such as MoO 3 , which surround the triangular flakes and have an IRabsorption in the same spectral regime [56]. The signals recorded at higher harmonics (S n , n > 3) reveal a difference in the near-field response between 1L-MoS 2 and SiO 2 . Fig. 3. The near-field intensity maps recorded at higher harmonics (S n , 1 ≤ n ≤ 5) of the tip's oscillation frequency using a CO 2 laser at 959.39 cm −1 : (a)-(e) illustrates the signals detected at S 1 -S 5 respectively; (f) schematic presentation of the probed volume for different harmonics (reproduced according to [55]).

Results and discussion
Here, the intensity of the former (the 1L-MoS 2 ) is higher. We attribute this difference in response to an enhancement of the SiO 2 layer near-field excitation in the presence of the local carrier concentration originating from the interaction of the 1L-MoS 2 with the tip. Mid IR radiation can excite collective charge oscillations in 1L-MoS 2 over a broad energy range [20] suggesting the presence of near-field enhancement over a wider frequency range than that of the SiO 2 phonon. The most significant advantage of using SR as a broadband IR radiation source is the ability to perform nano-FTIR spectroscopy over an energy range that is limited by the sensitivity range of the MCT detector only. Figure 4 presents near-field spectra derived from the 4th harmonic (S 4 ) recorded at the positions marked A and B on the 1L-MoS 2 structure and a third one labeled C on the SiO 2 substrate [inset of Fig. 4]. The reference mirror was translated over a distance of 800 µm at an integration time per data point of 100 ms. Each response is an average of two interferograms gathered successively at the same spatial position. The scattering amplitudes S 4 were normalized to (i) the near-field response of the Si reference-substrate using the same acquisition parameters and (ii) to the current of the storage ring. The 1L-MoS 2 double bond vibration around 469 cm −1 [57] could not be observed since the sensitivity range of the used MCT detector is limited to the spectral range from 740 to 5000 cm −1 . All spatially resolved spectra are dominated by an intense feature at ω SP1 = 1132 cm −1 . This is due to the surface phonon-polariton originating from the resonant near-field interaction between the SiO 2 and the probing tip. This finding is in agreement with results reported by other groups [39,47,[58][59][60][61]. Additionally, the near-field signal gives rise to a less intense, lower-wavenumber response, ω SP2 , occurring around 794 cm −1 , which is due to a second surface phonon [39]. It occurs close to the transverse optical phonon frequency of SiO 2 (ω TO = 797 cm −1 ) [62].
The 1L-MoS 2 adlayer influences the near-field response of the SiO 2 substrate over a broad spectral range, as can be seen from the blue and red curves in Fig. 4(a) taken on the MoS 2 triangles as compared to the green curve on the bare substrate. Figure 4  The blue and red curves were acquired at positions A and B, respectively. They originate from 1L-MoS 2 that is deposited on 300 nm thick SiO 2 . Position C corresponds to the bare SiO 2 substrate (green curve). Each spectrum represents the average of two interferograms gathered successively at the same positions; they were normalized to the Si response S 4 (1L-MoS 2 or SiO 2 )/S 4 (Si). ω SP1 and ω SP2 denote the frequencies of the SiO 2 phonon modes; (b) Theoretical results for the magnitude of the 4th s-SNOM scattering harmonics of SiO 2 , 1L-MoS 2 -SiO 2 , and 2L-MoS 2 -SiO 2 systems. It was assumed that the system's carrier concentration is 0.01 electron per MoS 2 unit cell (see text for further details). contributes to the s-SNOM IR spectrum. In Eq. (4), ω 0 = (4e 2 ε F /(2ε 0 )) 1/2 is the frequency, which is proportional to the bulk optical plasmon frequency. q TF = me 2 /ε 0 is the Thomas-Fermi momentum. Furthermore, e and m are the electron charge and mass, respectively. The Fermi energy ε F = πn/m is defined by the electron carrier density n and m (see, e.g., Ref [20]. for details). The plasmon spectrum of 2L-MoS 2 [spot A in Fig. 4] is modeled by π ω ε = is the 1L plasmon dispersion. Note that there is one extra mode compared to the 1L system [63,65,66]. The spectrum contains two modes: (i) the "standard" optical ( + ) mode, which is proportional to q at low momenta and (ii) a new acoustic (-) mode (~q ), which corresponds to inter-plane oscillations. As we show below, the transverse low-energy mode plays a crucial role for the increase of the intensity of the 794 cm −1 peak in the experimental spectrum shown in Fig. 4(a). Importantly, phonons in MoS 2 islands do not play an important role in the energy range presented in Fig. 4. In particular, our calculations on 1L-MoS 2 yield a phonon spectrum with energies below 500 cm −1 , which is in agreement with earlier experimental results [27,67]. Furthermore, the phonon spectra of the 1L-and 2L-MoS 2 are rather similar in shape [57], as compared to the optical phonon energies. They are much lower than the energies of the SiO 2 modes that give rise to the intense spectral features shown in Fig. 4. Because of their different energies, one cannot expect strong hybridization between the MoS 2 and SiO 2 surface phonon modes and, hence, no significant shift of the s-SNOM spectra is anticipated. This resembles the case of graphene, where the relevant phonon modes have much higher energy than the SiO 2 modes (see, e.g., ref [68].) resulting in the absence of an observable spectral shift. For details of the derivation of the excitation spectra, dielectric functions, and r p (q,ω), we refer to Ref [36]. The modeled spectra for the 4th harmonic of SiO 2 , 1L-MoS 2 -SiO 2 , and 2L-MoS 2 -SiO 2 systems are presented in Fig. 4(b).
The theoretical results, summarized in Fig. 4(b), are in agreement with the experimental data [see Fig. 4(a)]. It is important to note that the experimental results reveal that 1L-MoS 2 -SiO 2 , and 2L-MoS 2 -SiO 2 systems do differ from the case of graphene [36]. Only for 2L-MoS 2 is a small spectral shift of ~5 cm −1 relative to the surface phonon peak of bare SiO 2 at 794 cm −1 seen. This redshift is due to the coupling of the silica phonon with the transverse 2L-MoS 2 plasmon mode. The fact that the corresponding coupling does not lead to a shift of the 1132 cm −1 peak in both 1L and 2L MoS 2 cases can be rationalized by the almost vertical orientation of the tip-sample electric field that leads to a preference for coupling predominantly to vertically oscillating modes.
Finally, we turn to the difference between the s-SNOM spectrum of MoS 2 /SiO 2 and the corresponding spectrum for graphene. Contrary to our results, one observes a strong shift of the 1132 cm −1 peak of SiO 2 when it is coupled to graphene. Besides different momentum dependence, in graphene the plasmon dispersion is given by important is the difference of the dependence of the plasmon frequencies on the Fermi energy in these two systems. For graphene, the Fermi energy is proportional to the plasmon frequency and also to the square root of the carrier density n, ε F = (2π) 1/2 ν F √n, whereas for MoS 2 it is proportional to the density of electrons, ε F = πn/m, a much smaller number for a semiconductor with ~2 eV band gap. This leads to significantly lower plasmon frequencies in MoS 2 , about 0.001 to 0.01 meV (∼n 1/2 ), as compared to graphene (∼n 1/4 ), where typical values of ~0.06 eV were found [36]. This is in agreement with the present experimental results. It is important to point out, that despite significantly lower plasmon frequencies as compared to graphene, one can still observe a notable shift of the ~800 cm −1 phonon mode in the case of 2L-MoS 2 . Evidently, such shift is a resonance mode of the out-of-plane SiO 2 phonon and 2L-MoS 2 plasmon excitations, which is absent in graphene and 1L-MoS 2 . Thus, we have demonstrated the capability of the s-SNOM technique to assign low-energy excitations in layered MoS 2 deposited on SiO 2 including substrate-MoS 2 interactions. Importantly, it is demonstrated that it is not required to have a Dirac-like spectrum to obtain a significant coupling between the substrate phonon-and 2D-material plasmon-modes. It cannot be fully excluded that some changes in the excitation spectra of 1L-and 2L-MoS 2 prepared by different techniques might occur, such as e.g. exfoliated MoS 2 samples, which can be rationalized due to possible defect states. However, we assume that such modifications of the system will not affect significantly the collective plasmon excitations, so that the results for the s-SNOM spectra shown in Fig. 4 will remain essentially unchanged.

Conclusion
We have demonstrated that SR-based near-field microscopy is a valuable tool not only for imaging monolayer thick MoS 2 structures deposited onto SiO 2 substrates, but also for investigating differences in the excitations in single and double layer islands of CVD-grown MoS 2 . Our results revealed evidence of a strong interaction between the optical phonon modes of the SiO 2 and the plasmon modes of MoS 2 giving rise to hybrid plasmon-phonon modes. Depending on the number of layers, these modes exhibit an intensity enhancement as well as a spectral redshift compared to the SiO 2 phonon modes. The coupling strength between the surface modes strongly depends on the geometry of the system, i.e. the electric field distribution around the tip apex, which allows for an efficient excitation of the vertically oscillating modes. Our results are in agreement with theoretical calculations based on the point dipole model. The model calculations confirm that s-SNOM can be used to detect distinctly local excitations in layered systems, such as thickness variations, with high spatial resolution and surface sensitivity, providing the experimental and theoretical foundation to investigate other 2D-electronic materials.