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Article 2

Enhanced Photoluminescence of Monolayer WS on Ag Films and Nanowire-WS-Film Composites 2

Fei Cheng, Alex D Johnson, Yutsung Tsai, Ping-Hsiang Su, Shen Hu, John G Ekerdt, and Chih-Kang Shih ACS Photonics, Just Accepted Manuscript • Publication Date (Web): 10 May 2017 Downloaded from http://pubs.acs.org on May 15, 2017

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Enhanced Photoluminescence of Monolayer WS2 on Ag Films and Nanowire−WS2−Film Composites Fei Cheng,† Alex D Johnson,† Yutsung Tsai,† Ping-Hsiang Su,† Shen Hu,‡ John G. Ekerdt,‡ and Chih-Kang Shih†,* †

Department of Physics, University of Texas at Austin, Austin, Texas 78712, United

States ‡

Department of Physics, Chemical Engineering, University of Texas at Austin, Austin,

Texas 78712, United States ABSTRACT Monolayer transition metal dichalcogenides (TMDCs), due to their structural similarity to graphene, emerge as a promising alternative material of integrated optoelectronic devices. Recently, intense research efforts have been devoted to the combination of atomically thin TMDCs with metallic nanostructures to enhance the light−matter interaction in TMDCs. One crucial parameter for semiconductor-metallic nanostructure hybrids is the spacer thickness between the gain media and the plasmonic resonator which needs to be optimized to balance radiation enhancement and radiation quenching. In current investigations of TMDCs−plamonic coupling, one often adopts a spacer thickness of ~5 nm or larger, a typical value for transitional gain media-plasmonic composites. However, it is unclear whether this typical spacer thickness represents the optimal value for TMDCs-plasmonic hybrids. Here we address this critical issue by studying the spacer thickness dependence of the luminescent efficiency in the monolayer tungsten-disulfide (WS2)−Ag film hybrids. Surprisingly, we discovered that the optimal thickness occurs at ~1 nm spacer much smaller than the typical value being used previously. In a WS2−Ag film system, at this 1

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optimal spacer thickness, the photoluminescence (PL) is increased by more than an order of magnitude due to exciton-coupled surface plasmon polaritons (SPPs), as compared to the as-grown WS2 on sapphire. We further explore a new composite system comprising of Ag nanowires on top of a WS2−Ag film and observe additional enhancement of the PL (by a factor of 3) contributed by SPPs that are reflected from the end of the wires. Interestingly, in such a composite system, the additional improvement of the PL signal is observed only when the underlying Ag film is an epitaxial film instead of a commonly available thermal film. This is attributed to the reduction of propagation loss of the SPPs on atomically smooth, epitaxial films. KEYWORDS: Single layer WS2, exciton, photoluminescence, surface plasmon polaritons, epitaxial Ag films In the past few years, two-dimensional (2D) transition metal dichalcogenide (TMDC) materials have been a focus of research1-2 because these layered van der Waals semiconductors undergo an indirect-to-direct bandgap transition when scaled down to single layer (SL) thickness and consequently provide efficient light emission even at room temperatures. Despite the direct band-gap character of SL−TMDCs with a PL efficiency orders of magnitude higher than their bulk counterparts,3 the absolute PL quantum yield remain relatively low due to weak light absorption by the sub-nm thickness.4 In order to compensate for their small absorption length and moderate PL quantum

yield,

extensive

approaches

including

chemical

doping,5

defect

engineering,6-7 multilayer stacking8 and so on are employed to enhance light emission of SL−TMDCs. Recently, efforts have been devoted to utilizing metallic nanostructures to enhance light emission efficiency of monolayer TMDCs.9-18 The basic notion is to enhance the absorption of the material and the radiative recombination rate of the excitons by taking advantage of the proximal enhancement effect of surface plasmon resonances (SPRs) supported by metallic nanostructures. Nevertheless, a close proximity can also lead to quenching of the luminescence19-20 by opening up the non-radiative energy transfer process. Such a trade-off between 2

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enhancement and quenching lead to an optimal value for the spacer thickness to achieve the maximum efficiency. Unfortunately, this optimal value cannot be predicted apriori and must be established experimentally. Work so far in the TMDCs−plasmonic systems have adopted an empirical value of 5 nm21 or larger22 typically used for traditional gain media such as III−V semiconductor23 or dye molecule24-26 nanostructures. However, it is unclear whether the above value should also be the optimal thickness used for TMDCs−plasmonic systems. This critical issue is being addressed here by investigating the WS2−Ag film hybrid with a systematic variation of the dielectric spacer thickness. In addition, we explore a new composite system comprising of Ag nanowires (NWs) on top of a WS2−Ag film to gain additional enhancement of the light emission. In our investigation of the optimal spacer thickness for PL enhancement in WS2−Ag film hybrid, we found that this occurs at a spacer thickness of ~1 nm, which is dramatically different from the typical value of 5 nm or larger being used in gain media−metallic nanostructures. The PL of WS2 is increased by more than an order of magnitude on the Ag films due to exciton-coupled SPPs, as compared to as-grown WS2 on sapphire. This general behavior of an optimal dielectric spacer as thin as ~1 nm is observed at all temperatures and on both polycrystalline and epitaxial, single-crystalline Ag films. In the case of the composite system comprising of Ag NWs on top of a WS2−Ag film, additional enhancement of the PL (by a factor of 3) is achieved. This is due to the re-excitation effect of SPPs that are reflected from the end of the wire. Interestingly this additional enhancement in the composite Ag NW−WS2−Ag film system is significantly reduced if the underlying Ag film is a polycrystalline film grown using traditional thermal evaporation instead of an epitaxially grown film. We attribute the difference to the suppressed propagation loss of SPPs on epitaxial, atomically smooth Ag films. The results of our investigations provide important insights in the current development of TMDC−plasmonic hybrid systems for nanophotonic and plasmonic applications.

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RESULTS AND DISCUSSION Figure 1a presents an optical image of triangular-shaped WS2 flakes on sapphire grown by chemical vapor deposition (CVD).27-31 A better color contrast can be obtained after the WS2 flakes are transferred to SiO2/Si substrates (consisting of 300 nm SiO2 film on Si) using the poly(methyl methacrylate) (PMMA) wet-transfer method, as shown in Figure 1b. The regular edge size of triangular shaped, single-crystal domains is about 30~50 µm. At optimal growth conditions, we can get single-crystal WS2 flakes with edge size up to 270 µm (Figure S1, Supporting Information). Atomic force microscope (AFM) scan reveals a uniform height of ~0.6 nm at the edge of one randomly selected WS2 flake (Figure 1c), indicating the uniform monolayer property of CVD obtained WS2. Figure 1d and 1e show the Raman spectra ଵ of SL−WS2 on sapphire and the corresponding in-plane vibrational (Eଶg ) mode

intensity map, respectively. A relatively stronger Raman intensity is observed at the edges while dimmer signals are observed at inner areas of the WS2 flakes16, 31-32, indicating the possible lattice strain or as-grown structural defects induced by thermal shrinking during the cooling process of growth.28,

31

This nonuniformity is also

observed in a PL mapping of monolayer WS2 (Figure S2, Supporting Information). Figure 1f shows a representative PL spectrum taken at 79 K for SL−WS2 on sapphire. It is worth noting that the PL spectrum has a weak asymmetric profile, indicating two possible components assigned to the emission from charged exciton at lower energy (blue, X- at 1.97 eV) and neutral free excitons at higher energy (red, X0 at 1.99 eV), respectively.28, 33 Similar to the quenched fluorescence of fluorophores located close to metal surfaces,34 quenching behavior of TMDCs are also expected with direct contact with metal films or nanostructures as a result of charge transfer from the TMDCs to the metals.19 One approach to avoid such quenching relies on inserting an insulating dielectric spacer or a functionalized molecular layer16 between the TMDCs and the metals. As discussed earlier, an optimal spacer thickness is an important parameter 4

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that still remains to be explored. We transferred SL−WS2 flakes from sapphire substrates to Ag films capped by thin layers of Al2O3 and investigated the PL response of this hybrid structures (Figure 2a). Two representative PL spectra are presented in Figure 2b (~2 nm Al2O3 spacer), which shows a substantial enhancement factor (more than an order of magnitude) as compared to that taken on as-grown SL−WS2 on sapphire. We note that using epitaxial Ag film only leads to a marginal improvement (by a factor of ~1.2-1.3) over the usage of polycrystalline Ag film, indicating that the strong reduction of SPP propagation loss in epi-Ag plays only a small beneficial role in luminescent enhancement, a point to be discussed further below. Thus the spacer thickness dependence is systematically investigated using the polycrystalline Ag films. The thickness of the Ag films is kept at 25 nm while that of the Al2O3 layers varies from 1 to 5 nm which can be controlled precisely by atomic layer deposition (ALD). As shown in Figure 2c, the temperature-dependent PL study is carried out on the WS2−Ag film hybrids with different spacer thicknesses. The maximum PL intensities decreases gradually when the temperature of sample is increased from 79 to 300 K. At each temperature point, a thinner Al2O3 spacer presents a higher PL intensity, e.g., the maximum value for 1 nm spacer is about twice of that for 5 nm spacer at both 79 and 300 K (inset of Figure 2c). A quenching behavior is observed for WS2−Ag film hybrid without Al2O3 spacer, indicating a non–radiative energy transfers between WS2 and Ag film without the spacer. Note that this quenching at zero spacer thickness is in comparison to the case with non-zero spacer. As compared to the as-grown WS2 on sapphire, the PL intensity for the WS2 directly on Ag film is still larger by about a factor of 2 at 79K. At 300 K, the PL evolved from high intensity to almost complete quenching. The higher PL intensity for thinner spacers is mainly attributed to the larger spatial overlap of the plasmonic near-field and the WS2, which will be discussed below. We suspect that the surprisingly small thickness (1~2 nm) of the optimal dielectric spacer is due to the extreme 2D geometry and the small exciton diameters (1~1.5 nm) of monolayer TMDC,35 which has a different distance dependent behavior than previously studied emitter-metal systems that do not have 5

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this 2D constraint. We would like to mention that the low temperature measurement condition plays an important role in the observed PL enhancement because the PL quantum efficiency is much higher at low temperatures due to the decreased non-radiative decay. Furthermore, the effect of SPPs on PL enhancement is more prominent at low temperatures than at room temperature, as can be seen in the inset of Figure 2c. Our result is quite surprising because the value found here is much smaller than the empirically optimized thickness (~5 nm) of dielectric layers found in other systems. Fluorescence or luminescence quenching were found to happen for fluorophores or quantum dots if the distance between them and metal nanostructures is <5 nm,24, 36 while efficient fluorescence or luminescence enhancement could be achieved at larger distances (7-20 nm).25-26, 37 Furthermore, the typical value of ~5 nm or larger are indeed adopted in recent researches combining TMDCs and plasmonic nanostructures.17, 21-22, 38 In spite of the fact that the optimal thickness of the dielectric layer may differ from system to system, our results provide a quantitative determination of the optimal spacer thickness for the TMDCs/plasmonic hybrid structures, which is critical in the development of TMDCs/plasmonic hybrids for novel photonics and biological/biochemical sensors.2, 39 We anticipate that the observed PL enhancement from WS2−Ag film hybrids is associated with the combined contribution from the enhanced light absorption by WS2 in the hybrid structure, the exciton–SPP–photon conversion by surface roughness and the SPP enhanced exciton generation/recombination process. On the one hand, the light absorption of a thin layer, highly absorbing semiconductor can be enhanced when it is placed in intimate planer contact on a reflecting metal film, due to the strong interference effects in the highly absorbing media.40-41 We find that a strongly enhanced absorption of a thin layer WS2 is indeed obtained for the WS2/Al2O3/Ag stack with respect to the WS2/Al2O3 stack (by a factor of ~2.5, Figure S3a, Supporting Information). A similar scenario was also reported recently for a thin layer of WSe2 or 6

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WS2 on top of metal films (Ag or Au) with a near-unity absorption when the thickness of WSe2 or WS2 is ~12−15 nm,42 whereas the absorption of WSe2 or WS2 on SiO2 is limited to a maximum value of 0.5−0.643-44 On the other hand, like the case of InGaN quantum well─Ag film45 and CdSe quantum dot─Au film46 hybrid, here the excitons excited in WS2 can couple to SPPs on Ag film when their energy are overlapped (Figure S3b, Supporting Information). The exciton-coupled SPPs will undergo the plasmon-photon conversion during propagation as a result of scattering at surface roughnesses and imperfections on metal films, leading to the enhancement of light emission. Not only that, given the fact that a SPP propagating along a nanowire can excite an emitter in its propagation path,47 the strong near field of exciton-coupled SPPs can also re-excite surrounding excitons since the exciton diameter (~1 nm)35 is much smaller than the SPP propagation distance, which not only increases exciton generation in WS2 but also enhances the spontaneous emission rate of generated excitons. The observed spacer-thickness dependent PL enhancement shown above complies with process of exciton–SPP–photon conversion and SPP enhanced exciton generation/recombination: the closer excitons located within the near-field of SPPs, the more energy transfer between excitons and SPPs is enabled and thus higher PL enhancement is expected. With the key role played by the formation of SPPs in luminescent enhancement, an interesting question arises: since epitaxial Ag films grown by molecular beam epitaxy (MBE, see Methods for growth detail) have been proven to have smaller intrinsic ohmic loss and much longer propagation length of SPPs with respect to regularly obtained polycrystalline films,48 will one get even more enhancement if we employ atomically smooth, epitaxial Ag films for TMDCs−metal nanostructure hybrids? It has been recently discussed that single-crystalline Ag NWs are used to control the monolayer molybdenum disulfide (MoS2) exciton emission.17, 22 This has motivated us to compare the PL performance of WS2−Ag film hybrids based on two kinds of Ag films, a commonly used thermal film and an epitaxial film (Figure 3).

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In order to investigate systematically how a flat metal film influences the PL performance of SL−WS2, WS2−Ag film hybrids based on atomically smooth Ag films grown by MBE and films prepared by regular thermal evaporation are compared. The surface quality of an AlOx capped expitaxial Ag film (25 nm) and one thermally deposited Ag film (25 nm) are tested by atomic force microscopy (AFM) shown in Figure 3a and 3b. The epitaxial film has a root mean square (RMS) roughness (~0.32 nm) an order of magnitude smaller than that of the poly-film (~3.4 nm) and is thus expected to provide a much longer propagation distance of SPPs by suppressing the propagation loss caused by surface roughness and grain boundaries. As also shown in Figure 3c, the imaginary optical constants (ε2) of epitaxial Ag film around 2 eV is ~2 times smaller than that of the JC values.49 Furthermore, the crystallinity of the epitaxial Ag film was confirmed by X-ray diffraction (XRD) analysis (Figure S5, Supporting Information). Temperature-dependent PL spectra of SL−WS2 transferred to the thermal and epitaxial Ag films are shown in Figure 3d and 3e, respectively. At 79 K, the maximum PL intensity of epitaxial and thermal WS2−Ag film hybrids is ~16 and ~13 times larger than that of as-grown WS2 on sapphire; in the case of 200 K, the enhancement factor is ~13 and ~10 for epitaxial and thermal platforms, respectively. Although our experiment shows that the epitaxial film indeed provides an addition enhancement, it is only a minor improvement by a factor of ~1.25. The improvement is not as dramatic as one might expect in light of the fact that the propagation length of SPPs is much longer on epitaxial films, indicating that the enhancement occurs at a local length scale and thus, the much longer propagation length of SPPs on epitaxial Ag films does not contribute much. According to recent work on MoS2−nanowire hybrids,17 another key factor is the reflection of exciton-coupled SPPs from the end of the nanowire, yielding cyclic reexcitation of excitons and enhanced spontaneous emission. For our case of WS2 on flat metal films, the film geometry would not allow for such a reflection and thus cyclic reexcitation of excitons would not occur. It is therefore not surprising that only a minor improvement by the epitaxial film is observed. To explore this matter further and to develop the full 8

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potential of epitaxial Ag films, we design a new composite system by combining the previously mentioned WS2−Ag film hybrids with Ag NWs, i.e., NW−WS2−film composite. Figure 4a illustrates a sectional view of the NW−WS2−film composite. According to Figure 2, a 1~2 nm Al2O3 spacer can effectively prevent PL quenching of WS2, we thus isolate the SL−WS2 from both bottom film and top wires by 2 nm Al2O3 or AlOx with comparable thickness in the following composite. Ag NWs (ACS Material Corp., nominal diameter ~120 nm) are spin casted across the whole sample surface and then a layer of PMMA (~150 nm) was spin coated on top to protect Ag NW from degradation (oxidation and sulfuration). Figure 4b presents the optical image (top view) indicating the relative position of the Ag NWs with respect to the WS2 flake and several specific locations where we collected PL. Two different configurations are investigated: NWs that crossed the flake edge (Location A, B, C and D) and a NW that fully overlapped with the flake (Location E). The PL signal of both configurations are compared with that taken near the bare flake edge (Location F), as shown in Figure 4c. Different from SPPs that propagate on Ag films alone, the exciton-coupled SPPs guiding along Ag NWs can now bounce back from the wire end and return to the laser input position,22 participating in the reexcitation process of excitons and enhancing their spontaneous emission rate (nonresonant Purcell enhancement effect).50-51 As a result, the NW effective length, defined as the non-overlapping portion of the NW that extends from the edge of the flake to the end of the wire (Figure 4b), combined with the underneath Ag film, determines how much of the residual SPPs return after wire’s end scattering, surface scattering and ohmic loss during propagation. Notably, the peak intensity taken at location A and B are significantly enhanced (~2.4 times for Location A and ~2 times for location B) compared with that (dashed line) taken at Location F. For Location C and D, the enhancement factors are relatively smaller due to longer effective NW lengths. The PL signal obtained at Location E is comparable to that taken at location F, indicating an absence of (or weak) re-excitation process. This result is similar to what was 9

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observed by Hyun et al. using only Ag nanowires on MoS2.17 It should be noted that by focusing the laser spot at the midsection of the NWs, the contribution of directly−launched SPPs on NWs can be eliminated because the launch of SPPs and subsequent emission of photons is selective and occurs only at the ends or discontinuities of the NWs.52 Since the propagation loss of the exciton−coupled SPPs determines the amount of reflected SPPs that contribute to enhanced spontaneous emission of excitons, it is beneficial to compare quantitatively the additional enhancement factor introduced by NW sitting on both epitaxial and polycrystalline Ag films. The temperature-dependent PL spectra of two NW−WS2−film composites are evaluated in Figure 5. The NW effective lengths are almost the same (~3 µm) for both configurations and the data taking positions are indicated in optical images (inset of Figure 5a: composite on epitaxial film; inset of Figure 5b: composite on polycrystalline film). Similar to the case shown in Figure 4c, an enhancement factor ~2.1 is achieved for the single-crystalline platform at all temperatures from 79 to 300 K (Figure 5a). In comparison, an average enhancement factor ~1.3 is obtained for the polycrystalline platform at different temperatures (Figure 5b). Figure 5c summarized the additional enhancement factor for both platforms as a function of temperature. We attribute the distinct difference of the additional enhancement factor to the fact that polycrystalline Ag films introduce intrinsic surface roughnesses and grain boundaries which cause additional scattering and thereby higher propagation loss of the exciton-coupled SPPs.48, 53 We would like to mention that the actual enhancement provided by the composite should be larger than the observed value in Figure 5c due to the fact that the effective area (with a width of double SPP decay length starting from NW−WS2 overlapping point, ~100 nm according to simulations) is much smaller than the laser spot size (~1.5 µm). A smaller laser spot size is expected to produce a larger enhancement factor.

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According to the above analysis, the epitaxial platform provides an advantage of less propagation loss of exciton-coupled SPPs that contribute to enhancing the spontaneous emission of WS2. Note that the above enhancement factor (~2.4) observed in Figure 4 is the additional boost of PL signal when Ag NWs are included in the WS2−film hybrids. Taking into account the enhancement factor (~16) on epitaxial Ag film only and the additional one (~2.4) introduced by the Ag NW, a total enhancement factor of ~38 with respect to as-grown WS2 on sapphire is obtained experimentally with our NW−WS2−film composite structure. Also, we would like to mention that one advantage of the above composite prototype is its non-resonant character. It is found that although the plasmonic resonance of a hybrid MoS2−Au nanostructure can enhance the absorption and emission of excitons, the resonant near field may heat MoS2 flake, leading to PL intensity reduction, line shape broadening and emission peak red-shift.10 In the above NW−WS2−film composite structure, we found that PL peak positions and their full-width-at-half-maxima (FWHM) have no power dependent behavior (Figure S6, Supporting Information). Moreover, the PL peak intensity increases linearly with respect to the laser power (Figure 5d), showing no observable local heating effects in our systems. In order to gain insight into the exciton−plasmon coupling mechanism, we calculate numerically by finite element method the near-field distribution in WS2−Ag film hybrids where the exciton is mimicked by an electric dipole placed near the Ag film. Since it has been shown that the excitons in TMDCs have only in-plane dipole moments54 due to their 2D geometry, the orientation of the electric dipole is set to be parallel to the surface of Ag film in the simulation. As shown in Figure 6a, the radiation energy from the dipole couples to SPP mode propagating along the Ag film surface. To examine how the electromagnetic field enhancement around the dipole changes at different spacer thicknesses, we extract the magnetic field magnitude at the height of dipole from Ag film in Figure 6a and plot them in Figure 6b: the magnetic field at the height of dipole increases strongly as the distance between them is decreased from 15 to 2 nm, demonstrating the stronger interaction of dipole and the 11

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Ag film. The above simulation supports the experimental result that thinner spacer produces stronger exciton-SPP coupling and leads to stronger PL intensity. For the configuration of NW−WS2−Ag film composite, an eigen mode analysis is carried out to retrieve the effective mode indexes of a NW−Al2O3−Ag film waveguide (Figure 6c). The corresponding electric field magnitude distributions are shown in Figure 6d and 6e. We note that the electric field is mainly confined to the region underneath the NW and within the Al2O3 spacer (5 nm) for both modes. Figure 6f shows the lateral distribution of the electric field in the middle of the Al2O3 spacer for the lowest two eigen modes. Interestingly, a maximum magnitude is found under the NW for the eigen mode with an effective mode index εeff = 6.36 (red line) while an almost null intensity accompanied by contiguous field maxima on both sides is observed for another eigen mode with εeff = 2.33 (black line). Though both modes show a tightly confined electric field in the Al2O3 layer, only the latter mode with εeff = 2.33 propagates along the NW (i.e., the exciton-coupled SPPs) in the plasmonic waveguide and contributes to the exciton-SPP coupling. CONCLUSON In conclusion, we have studied systematically the PL emission of SL−WS2 on Ag film as a function of spacer thickness and demonstrated enhanced light-matter interaction in SL−WS2 by utilizing hybrid TMDC−plasmonic nanostructures. PL quenching of WS2 excitons can be effectively suppressed by a dielectric layer as thin as 1 nm. Due to direct plasmon-exciton coupling, the PL emission of WS2−Ag film hybrid is boosted more than an order of magnitude with respect to that of as-grown WS2 on sapphire. Furthermore, an additional enhancement of the PL signal is obtained in new NW−WS2−Ag film composite, benefitting from the contribution of remotely collected SPPs and suppressed propagation loss on atomically smooth, epitaxial Ag films. Our observation on the hybrid plasmon−exciton system unambiguously shows the advantage of epitaxial metal platforms in combination with

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2D materials, which may serve as important building blocks for the design of future TMDCs-based optoelectronic integrated circuits. EXPERIMENTAL METHODS Molecular beam epitaxial film growth, , in-situ oxidation and ex-situ atomic layer deposition. The epitaxial Ag film on silicon used in this work was grown using a 6N (99.9999%) high-purity silver source. A commercial Knudsen cell was used as the evaporator to ensure a highly stable deposition rate (calibrated through a quartz crystal monitor) and precise thickness control. Highly doped Si (111) wafers with a low miscut angle (~0.2º) are used as substrates and the surface is cleaned by passing current through the substrate for outgassing (0−2.5 A) and 7 × 7 reconstruction (10−17 A). The total thickness of the Ag film is 106ML (25 nm) and was completed after four growth cycles following a refined two-step growth process that guarantees single-crystalline growth of the film. For each cycle, 26 ML of silver was evaporated onto a liquid nitrogen cooled, surface-reconstructed Si (111) substrate (~90 K) with a low deposition rate of ~0.5 Å/min. After that, the film was naturally annealed to and kept for a while at room temperature in the UHV chamber. In order to avoid de-wetting, oxidation and sulfuration of Ag in an ambient environment, 5ML Al is deposited on top of the Ag using the same two-step growth method. A pure, densified native oxide (AlOx) layer with uniform thickness (~1.5 nm) will be formed on the Ag surface by passing high purity oxygen gas in the UHV chamber (under low pressure ~1×10-6 Torr) for 10 minutes. For protection of polycrystalline Ag films deposited by thermal evaporation and isolation of SL−WS2 from Ag NWs, ex-situ capping by atomic layer deposition 13

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(ALD) of Al2O3 is employed. ALD growth was performed at 500 K and with ~1 torr of ultrahigh purity Ar as the carrier gas using procedures presented elsewhere.55 Commercially-available trimethylaluminum (TMA) was used as the Al precursor and purified water as the oxidizing co-reactant. The Al2O3 thickness is controlled by number of ALD cycles; each cycle includes 1s TMA dosing followed with 20s Ar purge and 1s water dosing with 20s Ar purge. Ten ALD cycles lead to 1 nm of Al2O3.

CVD growth and flake transfer. The growth method of the SL−WS2 on sapphire is based on the chemical vapor phase reaction between WO3 and sulfur powder following previously reported recipes.56-59 After the AFM confirmation of monolayer property, the WS2 flakes are removed from sapphire to Si or Ag films following a refined, PMMA assisted wet transfer procedure that was also used by other works to transfer SL−MoS2.10, 22 The only difference for WS2 transfer operation resides in the etching liquid we use, which is diluted hydrofluoric acid (HF) solution instead of potassium hydroxide (KOH) solution that was used for MoS2 transfer. Optical Characterization. The sample’s temperature was varied from 79 to 300 K using an Oxford Instruments continuous flow cryostat and temperature controller. The sample was excited by normal incident 532 nm laser, and the PL emission from WS2 excitons was collected with the same optical microscope. The PL spectra were taken and

analyzed

using

an

ARC

Spectra

Pro−500i

spectrometer

and

a

liquid-nitrogen-cooled Si CCD detector. Numerical Simulations. Numerical simulations were performed with a finite element method (COMSOL Multiphysics, COMSOL Inc.) for a source dipole on Ag film and the NW−Al2O3−SF structure. The optical constants of polycrystalline Ag is 14

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taken from papers by Johnson49 and that of single-crystalline Ag from the SE measurement on our epitaxial Ag film grown by the two-step method.48 In Figure 6a, we plot the magnetic field instead of the electric field because the induced electric field of the SPP is much weaker than the radiation field of the source electric dipole and the magnetic component looks more prominent. The visible stripes are due to the residual intensity not being completely absorbed by perfectly matched layers (PMLs) and boundary reflection from geometry truncation. In Figure 6c−6f, we use a Ag NW with a radius of 60 nm in the eigen mode analysis. ASSOCIATED CONTENT

Supporting Information. Optical image of the large SL−WS2 flakes grown by CVD on sapphire, PL intensity mapping of an as-grown monolayer WS2 on sapphire, Absorption spectra of ultrathin WS2 on different substrates, Dispersion curve of SPP generated on Al2O3/Ag surface, AFM scans of Ag films before and after Al2O3 deposition, XRD 2θ scan pattern of a 25 nm epitaxial Ag film grown by the two-step method and power dependent PL peak energies and FWHM of SL−WS2 in NW−WS2−Ag film composites. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected]

Notes The authors declare no competing financial interest. 15

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ACKNOWLEDGMENT

This work was partially supported by the Welch Foundation (F-1672) and by the National Science Foundation (DMR-1306878, ECCS-1408302, CMMI-1437050, and EFMA-1542747). REFERENCES 1.

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Figure 1. (a) Optical image of SL–WS2 flakes grown by CVD on sapphire. (b) Optical image of one SL–WS2 flake transferred to SiO2/Si substrate. (c) AFM image of one SL–WS2 flake on sapphire and height profile along the indicated white line. (d) Raman spectra measured for one SL–WS2 flake grown on sapphire. (e) E2g1 Raman mode intensity map. The in-plane vibrational (E2g1) mode at 356 cm-1 and out-of-plane vibrational (A1g) mode at 420 cm-1 are indicated by dashed lines in (d). (f) PL spectrum of as-grown SL–WS2 on a sapphire substrate taken at 79K. The two Lorentzian peaks can be assigned in the fitting to the neutral exciton (red, X0) and the lower energy charged exciton (blue, X-). 239x156mm (300 x 300 DPI)

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Figure 2. (a) Structural model illustrating a SL–WS2 flake transferred to Ag film capped by a thin layer of Al2O3 spacer. The thickness of Al2O3 spacer is controlled precisely from 1 nm to 5 nm. (b) Selected PL spectra of WS2 transferred to an epitaxial Ag film (red solid, with spacer thickness ~1.5 nm (5ML oxidized Al)) and a polycrystalline film (green solid, with 2 nm spacer thickness) taken at 79 K. The PL spectrum of as-grown SL–WS2 on sapphire substrate plotted for comparison (blue dashed). (c) Temperature-dependent PL intensity of WS2 flake transferred to poly-Ag films for different thicknesses of Al2O3 spacer. Every point is an averaged value with error bar from 5 measurements taken on 5 different flakes on each sample. 239x146mm (300 x 300 DPI)

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Figure 3. AFM images of (a) single- and (b) polycrystalline Ag films. The thickness of both films is 25 nm. The RMS surface roughness of the single- and polycrystalline films are 0.32 and 3.4 nm, respectively. According to AFM scans of polycrystalline Ag films before and after Al2O3 deposition by ALD, the surface morphology is well retained after ALD with a thin layer of Al2O3 capping (Figure S4, Supporting Information). (c) Energy dependence of ε2 extracted from spectroscopic ellipsometry (SE) measurements on an epitaxial, single-crystallline film (blue) and a thermally deposited polycrystalline film (black), which are plotted against data taken by Johnson and Christy (red).49 (d) Temperature-dependent (79 to 300 K) PL spectra of a SL– WS2 transferred to the epitaxial (black solid) and polycrystalline (black dashed) Ag films with 2 nm Al2O3 spacer by ALD. The PL spectrum in each panel is plotted against data (blue dashed) taken from as-grown SL–WS2 on sapphire. Note that the scale changes for 200 K and above. 199x136mm (300 x 300 DPI)

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Figure 4. (a) Schematics illustrating a SL–WS2 flake sandwiched between a Ag film and a Ag NW (d = 120 nm). 5ML oxidized Al (AlOx) and 2 nm Al2O3 are inserted below and above the WS2 flake. (b) Optical image showing the locations where the PL was collected (dashed circles). The effective NW length (from the flake’s edge to the wire’s end) is about 3 µm, 4 µm, 7 µm and 6 µm for location A, B, C and D, respectively. (c) PL spectra taken at different locations (A-E for on the NW and F for off the NW). Dashed lines indicate the maximum value of the PL spectra taken at location A and F. The former value is about 2.4 times of the latter one. All spectra are taken at the same lasing power. 239x83mm (300 x 300 DPI)

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Figure 5. (a) Temperature-dependent PL spectra of a NW–WS2–film composite structure (epitaxial) taken on the NW (black, on NW) and at edge of the flake (red, off NW). An optical image indicating the locations is shown on the right. (b) Temperature-dependent PL spectra of a NW–WS2–film composite structure (poly) taken on the NW (black, on NW) and taken at the edge of the flake (red, off NW). The NW effective length is about 3 µm for both cases. (c) Enhancement factor as a function of temperature. (d) PL peak intensity of SL–WS2 as a function of excitation laser power for on the NW and off the NW. 239x127mm (300 x 300 DPI)

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Figure 6. (a) Magnetic field distribution of an electric dipole (parallel to the film, radiation wavelength set to 610 nm) near the Ag film. The distance of the dipole from the Ag film is changed from 2 to 15 nm above the Ag film as marked. (b) Magnetic field magnitude along a cut line taken at the height of the dipole (log scale). The inset is a zoom-in view at the location of dipole. The sharp peaks centered at the original point are divergent data points due to geometry discontinuity and should be ignored. (c) Effective mode index calculated from the NW–Al2O3–film structures. The radius of the Ag NW is 60 nm. Electric field magnitude map of the NW–Al2O3–film structure cross sections for the effective mode index of 6.36 (d) and 2.33 (e). Note that only the field map in (d) represents a possible exciton-coupled SPP field in the structure because the electric field is mainly parallel to the axis of the NW. The electric field corresponding to εeff = 6.36 is mainly perpendicular to the metal surface. (f) Electric field profile within the spacer (taken at 2.5 nm above the Ag film) for both effective mode indexes which are extracted from (d) and (e). 199x250mm (300 x 300 DPI)

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