Manifold Coupling Mechanisms of Transition Metal Dichalcogenides

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C: Physical Processes in Nanomaterials and Nanostructures

Manifold Coupling Mechanisms of Transition Metal Dichalcogenides to Plasmonic Gold Nanoparticle Arrays Sandra Diefenbach, Eric Parzinger, Jonas Kiemle, Jakob Wierzbowski, Sebastian Funke, Bastian Miller, Reka Csiki, Peter Thiesen, Anna Cattani-Scholz, Ursula Wurstbauer, and Alexander W. Holleitner J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01154 • Publication Date (Web): 13 Apr 2018 Downloaded from http://pubs.acs.org on April 19, 2018

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The Journal of Physical Chemistry

Manifold Coupling Mechanisms of Transition Metal Dichalcogenides to Plasmonic Gold Nanoparticle Arrays Sandra Diefenbach1, 2, Eric Parzinger1, 2, Jonas Kiemle1, Jakob Wierzbowski1, 2, Sebastian Funke3, Bastian Miller1, 2, Réka Csiki1, Peter Thiesen3, Anna Cattani-Scholz2, Ursula Wurstbauer1 2, and Alexander W. Holleitner1, 2,* 1) Walter Schottky Institute and Physics Department, Technical University of Munich, Am Coulombwall 4a, 85748 Garching, Germany. 2) Nanosystems Initiative Munich (NIM), Schellingstr. 4, 80799 Munich, Germany. 3) Accurion GmbH, Stresemannstr. 30, 87079 Göttingen, Germany

Abstract We reveal the manifold interaction mechanisms between monolayers of MoS2 and single layers of plasmon-active gold nanoparticles. The MoS2 shows a ten- to twenty-fold enhanced photoluminescence when covered with the gold nanoparticles. Surprisingly, we detect this enhancement also for excitation energies that are not resonant to the surface plasmon polaritons of the nanoparticles. Complementary Kelvin probe force measurements indicate a lowering of the work function, when the MoS2 is decorated with the gold nanoparticles. This is in agreement with a reduced band gap for the decorated MoS2 as determined from absorbance measurements. We furthermore demonstrate a dielectric coupling between the two layers by spectroscopic imaging ellipsometry as well as Raman spectroscopy. Combining the various results, we discuss the enhanced photoluminescence in terms of a modified emission pattern of the radiative dipole in the MoS2 monolayers at the presence of the gold nanoparticles. In particular, the studied systems elucidate the underlying physical mechanisms of the enhanced photoluminescence for decorated MoS2 that stems predominantly from incoherent contributions including the far-field emission pattern, the dielectric coupling, and the electronic interaction mechanisms.

Keywords Monolayer transition metal dichalcogenides, gold nanoparticle arrays, plasmonic enhancement, dielectric coupling, far-field spectroscopy

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Introduction Optically active, atomically thin semiconductors are an emergent class of two-dimensional materials. Particularly, monolayers of semiconducting transition metal dichalcogenides (SCTMDs), such as MoS2, excel in their outstanding optical response 1–6. In the monolayer limit, an increased confinement turns these two-dimensional crystals from an indirect to a direct band-gap semiconductor with optical band gaps in the visible to near infrared 4,5. Due to a reduced screening in two dimensions, the light-matter interaction in such monolayers is dominated by exciton phenomena 7–12, resulting in a layer- and energy-dependent extraordinarily high absorbance of more than 15% for a less than 1nm thick crystal 13–15. The outstanding optical properties together with intriguing spin- and valley-properties 16, field-effect transistor (FET) performance comparable to silicon 17,18, catalytic activity 19–21, and photocatalytic stability 22 renders possible manifold applications based on SC-TMD monolayers. Among these are applications particularly in the area of optoelectronics, e.g. for solar energy conversion in solar and photocatalytic cells, photo-detectors, photo-transistors and light-emitting diodes to name just a few 1,2,20,23,24. Key to the integration of SC-TDM monolayers into optoelectronic circuitries is the possibility to tune and engineer the optical properties on demand and on-chip. In this respect, the optical properties of SC-TMD monolayers can be adjusted by strain 25–28, dielectric engineering 29,30, doping 31–34, and defect engineering 35–38. Strain results in a modified electronic band structure, whereas the binding energy of the localized excitons can be altered by changing the screening media nearby the SC-TMD monolayers in a so-called dielectric engineering approach 30. Also the quasi-particle band gap is sizably affected by the dielectric environment 39,40. Increasing the electron doping results in a band gap renormalization effectively reducing the band gap, and simultaneously, in the reduction of the exciton binding energy 31,34,39,41. Both effects partially cancel out each other, such that the energy of the lowest excitons only moderately changes, while the light-matter interaction strength is reduced for an increased doping density. The light-matter interaction can be increased by integrating the SC-TMD monolayers into external cavities 42–45 or into plasmonic nanostructures acting as nano-antennas 46–59. In this respect, the emission of a SCTMD based emitter can couple to localized surface plasmons. In the weak coupling regime, the plasmonic coupling can result in an enhanced light-matter interaction with reported ten-fold 47,48,51,52,57,60 up to ~20.000 fold enhancement factors 50. Moreover, hot electron injection has been reported for such plasmonic SC-TMD structures to effectively dope the two-dimensional crystals 53–56. Furthermore, excitons in monolayers of MoS2 can be regarded as pure in-plane dipoles with a far-field emission pattern that strongly depends on the refractive index of the surrounding materials 61,62,63. Due to non-trivial emission pattern, an optical measurements can be significantly affected by the numerical aperture (NA) of the optical set-up 63 and by substrate dependent interference effects 64,65 without changing the intrinsic optical properties of twodimensional materials. In this report, we investigate SC-TMD monolayers with their A- and B-exciton transitions resonant to surface plasmon polaritons (SPP) of a two-dimensional array of gold nanoparticles (Au-NPs) on top (Figure 1a). We investigate monolayers of MoS2 as prototypical system for SCTMDs, and expect similar behavior for all SC-TMDs. We observe a ten- to twenty-fold increase in the photoluminescence intensity of the MoS2 monolayers, when they are decorated with the Au-NPs. However, we do not observe evidence for a direct near-field coupling of the SPPs to the excitons in the MoS2. Most significantly, we reveal that the decoration with Au-NPs gives rise to red-shifted A- and B-exciton transitions and similarly, to a reduced work function of the MoS2, ACS Paragon Plus Environment

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both of which we interpret to stem from a modified dielectric environment that affects the quasiparticle band gap 40. The magnitude of the optical absorbance is almost identical for decorated and pristine flakes, which indicates that the magnitude of the light matter interaction strength for MoS2 with and without Au-NPs is very similar. Instead, our study suggests that the enhanced photoluminescence signal for Au-NP decorated MoS2 originates from a modified far-field emission pattern which is dominated by the dielectric response function of the Au-NPs 62,63,65. Our insights are based on a complementary approach of Kelvin probe measurements, photoluminescence spectroscopy, spectroscopic imaging ellipsometry, and Raman spectroscopy studies together with finite-difference time- domain (FDTD) simulations numerical of the plasmonic behavior of the Au-NPs and the modified far-field emission pattern of the in-plane dipole of the excitons in MoS2 61. Our results may prove essential for the development of SCTMD-based plasmonic hybrid structures for optoelectronic applications and the interpretation of optical experiments on such SC-TMDS-plasmonic hybrid structures.

Methods Preparation of the SC-TMDS monolayers. Monolayers (MLs) of MoS2-flakes are micromechanically exfoliated from naturally occurring bulk crystals (SPI Supplies) and then transferred by an all-dry viscoelastic stamping technique 66,67 . Substrates are (110)-oriented p-type doped silicon (Si) wafers with 285 nm silicon dioxide (SiO2) with a refractive index of nSiO2 (1.8 eV) ≈ 1.45668. In total, we fabricated 13 samples. Seven samples are MoS2 monolayers, and six further samples are Au-NP decorated MoS2 monolayers. Preparation of the Au-NP arrays. The Au-NP arrays are fabricated via a combination of a self-assembly process69–72 and microcontact printing73. Ethanole (analytical standard), chloroform (anhydrous, > 99%), gold(III) chloride trihydrate (HAuCl4 · 3H2O) ,tannic acid, sodium citrate, and 1- octanethiole (CH3(CH2)7SH, > 98.5 %) are purchased from Sigma- Aldrich. To this end, 80 ml of a 1% wt. solution of HAuCl4 in DI-water is heated up to 80°C under constant stirring at 500 rpm with a glass paddle agitator 70. A reduction solution containing 4 ml of 1% tri-sodium citrate in DIwater and 80 µl of 1% tannic acid in DI-water is added to 14 ml DI-water and heated up to 60°C 71 . The reduction solution is poured into the colloidal solution under constant stirring and heating. Constant stirring and heating is continued for 10 minutes until the end of the nanoparticle growth process 70. For the functionalization of the Au-NPs with 1-octanethiol (C8) 74 , the Au-NP solution in DI-water is transferred to ethanol via centrifugation at 14.5 krpm for 1 hour 72. The Au-NP solution in ethanol is mixed with a 0.5 M solution of 1-octanethiol inside a glovebox and stored under exclusion of light for 1 day 74. The excess ethanol molecules are removed, and the remaining solution is rinsed with fresh ethanol to further remove the unbound 1-octanethiols. In a next step, the dried Au-NPs are dissolved in 3.5 ml chloroform. Dropping 350 µl of the Au-NP solution onto a convex air-water interface, a hexagonally close-packed array of gold nanoparticles is formed by fast evaporation of the solvent and subsequent convective flow 74. We transfer the Au-NP arrays on top of MoS2 MLs via micro contact printing with a PDMS stamp (Sylgard 184, cf. Figure 1b)73,75. Scanning electron microscope (SEM) images are taken on Au-NP arrays transferred onto the SiO2/Si substrates to confirm the formation of single layers with domains with hexagonal ordering up to a micrometer (cf. Figure 1c and Supporting Figure S2). The nanoparticles have a diameter of ~10 nm and they are ACS Paragon Plus Environment

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encapsulated by 1-octanethiols (C8) with a length of ~1.3 nm refractive index of about nTh(1.8 eV) ≈ 1.45 76.

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69,73

. The 1-octanethiols have a

Results µ-Raman and photoluminescence characterization. For the optical characterization of the samples, a helium-neon gas laser is narrowed with a monochromator to an emission line of 632.8 nm corresponding to a photon energy of Ephoton = 1.96 eV. In addition, an emission line of 487.6 nm (Ephoton = 2.54 eV) of a tunable Krypton/Argon gas laser is used. In both cases, the linearly polarized laser light is focused by an objective an NA of 0.75 (efl = 2.61 mm) onto the samples with a spot size of ~1 µm, and it is scanned across the sample via an x-y-piezo stage. The emitted luminescence and/or the scattered Raman signal are collected and focused onto the grating of a spectrometer with 300 lines/mm in the case of the photoluminescence (PL), and with 1800 lines/ mm in case of the Raman measurements. The corresponding spectra are read-out via a charge coupled device (CCD) camera at nitrogen temperatures. All Raman and PL spectra are taken on samples at ambient conditions and room temperature. Figure 1d shows typical non-resonantly excited Raman spectra (Ephoton = 2.54 eV) for a MoS2 ML with and without Au-NP arrays on top (red and black lines). We detect both first order Raman modes E’ and A’1 with mode energies of 384.4 cm-1 and 404.6 cm-1 for MoS2 MLs without Au-NPs (dashed lines in Figure 1d) and 384.5 cm-1 and 404.8 cm-1 with Au-NPs, respectively. The corresponding energy difference between the A’1- and the E’-modes of 20.2 cm-1 and 20.3 cm-1, respectively, indicates that the MoS2 flakes are indeed MoS2 monolayers, in accordance with the literature. 77. By comparing different samples, the variation in the A’1-mode energy is statistically independent from the coverage with or without Au-NP arrays indicating the absence of significant charge transfer between MoS2 and Au-NPs. For the photoluminescence, we observe remarkable differences between the MoS2 with and without Au-NPs on top (cf. red and black lines Figure 1e). The intensity is enhanced by a factor of up to twenty when the MoS2 MLs are decorated with Au-NPs. Surprisingly, this enhancement is very similar for an excitation energy being resonant and non-resonant to the surface-plasmon polariton of the Au-NP arrays. Spectroscopic imaging ellipsometry. To explore the dielectric coupling between the SC-TMD MLs and the Au-NP arrays, spectroscopic imaging ellipsometry (SIE) measurements are performed with a spectroscopic imaging nulling ellipsometer EP4 (Accurion Gmbh, Göttingen) in ambient conditions at room temperature, as described in detail in 14. Spectroscopic data are obtained under an angle of incidence of 50° in the spectral range between 1.4 eV (886 nm) and 3.0 eV (413 nm) with constant wavelength steps of 5 nm. We calculate the absorbance of each layer of our multilayer sample separately as a function of the photon energy for the investigated samples. Figure 2 shows such absorbance spectra of a pristine MoS2 ML (black line), a MoS2 ML from the Au-NP hybrid (blue line), a pristine Au-NP arrays (orange line), and an Au-NP array from the hybrid with a MoS2 ML (red line). For the MoS2 ML (black line in Figure 2), one can clearly see the A-, B-, and C-absorbance lines of the excitonic transitions at ~1.97 eV, ~2.03 eV, and 2.87 eV, consistent with literature 13,14,78,79. For the hybrid structure, the A- and B-exciton resonances are ACS Paragon Plus Environment

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red-shifted by ~0.1 eV compared to the pristine layer with an almost identical absorbance-value exceeding 10%. The red-shift of the A-exciton resonance for the decorated MoS2 MLs indicates either a reduction of the single-particle band gap, an increase of the exciton binding energy, or a combination of a modification of both. The C-exciton resonance from the hybrid device seems to be slightly red-shifted and increased in absorbance. For the pristine Au-NP array (orange line), the maximum absorbance is centred at ~1.87 eV with a maximum value of ~30%. The absorbance of the Au-NP array in the hybrid structure is slightly blue-shifted with a similar value at the maximum compared to the pristine array. Both values are consistent with a collective SPPmode of Au-NP arrays on SiO2/Si substrates 80,81. Atomic force and Kelvin probe force microscopy. We perform atomic force microscopy (AFM) and Kelvin probe force microscopy (KPFM) on a Bruker MultiMode 8 microscope to investigate the contact potential difference (CPD) and respective work functions of the TDMs with and without Au NP arrays 82. For the measurements of the MoS2-based systems, we utilize Bruker PFQNE-Al cantilevers with a resonance frequency of 300 kHz and a spring constant 0.8 N/m. The AFM and KPFM images are simultaneously taken in ambient conditions at room temperature. Figure 3a shows such an AFM image of a MoS2 ML on SiO2/Si-substrate which is largely covered with Au-NPs. However, small parts are free from Au-NPs (arrow). These sections allow to measure the contact potential difference between the MoS2 ML without Au-NPs and the area with Au-NP coverage. In particular, Figure 3b clearly demonstrates that MoS2 ML without Au-NPs (arrow) exhibits a more positive contact potential difference compared to the sections with coverage (∆Ewf ~ 0.13 eV) . The Au pad on the right side of the images has a height of ~20 nm, and it is connected to ground, such that it gives the reference work function of the measurement (set to be -4.85 eV). Figure 3b demonstrates that the area of the Au-decorated MoS2 has a more positive work function than the Au-NPs on the SiO2/Si-substrate. All values are summarized in Figure 3c with the error from the actual measurements. Raman spectra vs laser power for resonant and non-resonant excitation. To further explore the coupling between the SC-TDM MLs and the Au-NP arrays, we compare power-dependent Raman spectra for a resonant and non-resonant excitation of the SPP in the AuNP arrays. Figures 4a and 4b show the corresponding Raman response of an Au-NP decorated MoS2 ML for non-resonant excitation (Ephoton = 2.54 eV and open triangle in Figure 2) in the energy range from 360 cm-1 to 440 cm-1 (Figure 4a) and from 500 cm-1 to 540 cm-1 (Figure 4b). We find, that the mode energies of E’ and A’1 are independent of the laser power in the investigated range of powers (dashed lines in Figure 4a) indicating the absence of changes in the lattice temperature or the doping level 3. At ~520 cm-1 (Figure 4b), we observe the 1TO Raman mode of the silicon substrate. The Raman intensity of the modes E’ and A’1 as well as the one of the Si 1TO mode increase linearly with increasing laser power, as it is consistent for first order Raman processes. For a resonant excitation (Ephoton = 1.96 eV and black triangle in Figure 2), we additionally detect a Raman mode at 379.0 cm-1 (cf. arrow in Figure 4c), which we assign to be the E1u phonon mode near the Γ point 59,83. The energy of all modes (E1u, E’, A’1, and Si 1TO) is power independent also for the resonant excitation (dashed lines). However, the intensity of the Si 1TO-mode is strongly suppressed for all laser intensities (Figure 4d), as will be discussed below. Numerical simulation of the surface plasmon polaritons.

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We numerically determine the electric field distribution of the Au-NP array by a finite difference time domain (FDTD) simulation for a resonant excitation of its SPP and a vertical polarization of the incident photon (‘vertical’ wrt. Figure 4e). Consistent with literature 80,81, the simulated SPPmode of the Au-NP array is polarized according to the hexagonal ordering of the array. In other words, the plasmons are predominantly excited when the linear polarization of the exciting photons are aligned along one of the 60°-orientations of the hexagonal lattice. Moreover, the SPPs are strongly localized within the gap between two adjacent NPs (Figure 4f). The maximum field-amplitude and -intensity of this so-called gap-mode has a distance of about 7 nm to the MoS2 crystal. PL measurements vs laser power for resonant and non-resonant excitation. In a next step, we present the power-dependent PL for a resonant and non-resonant excitation of the SPP in the Au-NP arrays. As highlighted by the black triangle in Figure 2, such an excitation energy is also resonant to the low-lying excitons of the MoS2 ML. Figure 5a shows the PL spectra for an Au-NP decorated MoS2 ML vs laser power for a non-resonant excitation and Figure 5b for resonant excitation. The PL signal for both, non-resonant and resonant excitation saturate for the highest powers, which can be explained e.g. by a phase-space filling argument. The contribution from the neutral A-exciton and the charged A--trion to the overall emission signal can be extracted from Gaussian fits to the data (data not shown). We observe that the ratio of the spectral weight from neutral A-exciton and charged A--trion varies from sample to sample with and without decorating Au-NPs, and it constitutes between 1.5 and 4. This variation can be explained by moderate fluctuations in the intrinsic doping level 84, which are in agreement with the sample to sample variation of the charge carrier sensitive A’1-phonon mode, as detected by us in Raman measurements (data not shown).

Discussion We detect a ten- to twenty-fold enhancement of the photoluminescence of MoS2 MLs, when they are decorated with Au nanoparticles as depicted in Figure 1e. In literature, this increase is occasionally explained in terms of a plasmonic enhancement of the excitonic luminescence of the transition metal dichalcogenides 47,49,51,57,60. Our set of data suggests further effects to be highly relevant, and as will be laid out in the following, these effects even dominate the optical and dielectric response of the investigated hybrid nanosystems. It is intriguing that the photoluminescence enhancement occurs for resonant as well as non-resonant excitations of the Au-NP arrays with a very similar magnitude within the statistical error of various samples. It is even more surprising that the light-matter interaction, which is proportional to the experimental absorbance strength, is almost unchanged for decorated MoS2 MLs compared to the pristine flakes (cf. Figure 2). In first order, the magnitude of the A- and B-exciton absorbance resonances are identical for pristine and decorated MoS2 MLs. Only the respective energies are red-shifted by about 0.1 eV as will be further discussed below. Generally, the ellipsometry data in Figure 2 confirm the presence of surface plasmon polaritons (SPPs) in the Au-NPs. The plasmonic response function of the Au-NP arrays is similar for the pristine arrays on SiO2/Si and for the hybrid structure with MoS2 in between. The center energy of the SPP is resonant to the A-exciton of the MoS2 (black triangle in Figure 2). It is important to note that the 1TO-phonon mode of Si is suppressed in the Raman experiments for an excitation in resonance with the plasmon energy of the Au-NPs (cf. Figure 4d). The excitation with linearly polarized light imprints the oscillatory direction of the plasmons such that the Au-NP array act like a grating. This grating effect can be seen in the numerically simulated distribution of the ACS Paragon Plus Environment

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electric field within the Au-NPs (cf. Figure 4e). Only light that is parallel polarized to the plasmons can pass, while perpendicularly polarized light is blocked. For the utilized (110) Si wafers the 1TO phonon is cross-polarized, meaning the polarization of the scattered light is rotated by 90° compared to the incoming light85. In turn, the Raman signal of this 1TO-phonon is blocked for an excitation energy being resonant to the Au-NP plasmon energy (cf. Figure 4b vs Figure 4d). We note that this blocking cannot be explained by the absorbance values as in Fig. 2 for a resonant and non-resonant excitation (cf. Supporting Information). From the fact that we do not see a plasmonic enhancement in the absorbance spectra together with the observation that the PL is enhanced for the decorated MLs for resonant and nonresonant excitation, we do not see evidence for a direct near-field interaction between the SPPs of the Au-NP array and the optical interband transitions in MoS2. In other words, the optical matrix element seems to be comparable for the pristine and the decorated MoS2 MLs. A possible explanation can be found in the small gap between the Au-NPs of less than 2 nm given by the length of the stabilizing octanethiols. Such a gap-plasmon has a maximum electric field intensity about 7 nm above the 2D crystal (radius of the Au-NPs plus the length of octanethiols), as can be seen in the numerical simulations of Figures 4e and 4f. The calculations are done for a linearly polarized, resonant excitation. The plasmon field is highly localized within this gap with almost no extent into the sheet of the MoS2. Therefore, the gap-plasmons do not couple with the dipole oscillator localized in the plane of the MoS2. We would like to note that laser-induced heating 86, photo-doping effects 87, and also a transfer of hot electrons generated from the Au-NPs 53–56,88,89 can be excluded to be of importance in the presented experiments, since the energies of the E’and the A1’-phonon modes for pristine and decorated MoS2 MLs are independent from the incoming laser power (Plaser ≤ 100 µW) for resonant and non-resonant excitation. Intriguingly, the KPFM measurements indicate that the work function of MoS2 decreases by a value of about 0.13 eV, when the MLs are decorated with Au-NPs (Figure 3). The determined work function of the decorated MoS2 MLs is even lower than the work function of the Au contact pad on Si/SiO2 suggesting that the work function of the decorated MOS2 is not determined by the one of the Au-NPs. Considering a constant intrinsic doping level and hence Fermi energy, the KPFM measurements point towards a reduction of quasi-particle band gap of ~0.13 eV. Taking into account typical sample-to-sample variations in the charge carrier density and exciton binding energy, the reduction of the single particle band gap is in good agreement with the reduction of the A- and B-exciton energies of ~0.1 eV found in ellipsometry measurements (cf. Figure 2). Covering MoS2 with Au-NPs and octanethiols alters sizable the dielectric environment compared to air as top interface (air: ε = 1; Au-NP: ε (1.85 eV) ≈ 4.98 + i ‧ 8.14, as determined from ellispometry measurements). The dielectric environment is expected to impact the single particle band gap as well as the exciton binding energy 39. The reduction of the work function for decorated MoS2 together with the red-shifted A- and B-exciton resonances indicate consequently a reduction of the band-gap due to the modified dielectric environment on the top interface of the MoS2 MLs by the octanethioles and Au-NP arrays with a higher permeability. We now turn to the discussion for a possible origin of the ten- to twenty-fold PL enhancement observed for resonant and non-resonant excitation for the decorated MoS2 flakes, while the strength of the light-matter interaction determined from ellipsometry is unchanged. The dielectric environment in close vicinity of a radiative dipole determines the far-field emission pattern. In particular, the density of optical states is given by the dielectric interface at a distance much smaller than the emission wavelength (d

Manifold Coupling Mechanisms of Transition Metal Dichalcogenides

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