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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
