Distributed Bragg Reflectors as Broadband and Large- Area Platforms

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Distributed Bragg Reflectors as Broadband and Large-Area Platforms for Light Coupling Enhancement in 2D Transition Metal Dichalcogenides Yen-Chun Chen, Han Yeh, Chien-Ju Lee, and Wen-Hao Chang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02845 • Publication Date (Web): 24 Apr 2018 Downloaded from http://pubs.acs.org on April 24, 2018

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ACS Applied Materials & Interfaces

Distributed Bragg Reflectors as Broadband and LargeArea Platforms for Light Coupling Enhancement in 2D Transition Metal Dichalcogenides Yen-Chun Chen1, Han Yeh1, Chien-Ju Lee1 and Wen-Hao Chang1,2* 1

Department of Electrophysics, National Chiao Tung University, Hsinchu 30010, Taiwan

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Center for Emergent Functional Matter Science, National Chiao Tung University, Hsinchu 30010, Taiwan. KEYWORDS: Transition metal dichalcogenide, two-dimensional layered materials, substrate interference, photoluminescence, Raman ABSTRACT: Two-dimensional (2D) semiconductors, particularly the direct-gap monolayer transition metal dichalcogenides (TMDs), are currently being developed for various atomicallythin optoelectronic devices. However, practical applications are hindered by their low quantum efficiencies in light emissions and absorptions. While photonic cavities and metallic plasmonic structures can significantly enhance the light-matter interactions in TMDs, the narrow spectral resonance and the local hot spots considerably limit the applications when broadband and large area are required. Here we demonstrate that a properly designed distributed Bragg reflector (DBR) can be an ideal platform for light-coupling enhancement in 2D TMDs. The main idea is

based on engineering the amplitude and phase of optical reflection from the DBR to produce optimal substrate-induced interference. We show that the photoluminescence, Raman and second harmonic generation signals of monolayer WSe2 can be enhanced by a factor of 26, 34 and 58, respectively. The proposed DBR substrates pave the way for developing a range of 2D optoelectronic devices for broadband and large-area applications.

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INTRODUCTION Two-dimensional (2D) monolayer transition metal dichalcogenides (TMDs) with direct band gap have been discovered as a new class of atomically-thin and optically-active materials for novel optoelectronic applications, such as ultrathin photodetectors and light emitters1-6. However, their low quantum efficiency7,8 in light emissions and the weak light absorptions9 considerably hinder the implementation of TMDs for practical photonic devices. In the past few years, considerable efforts have been focusing on enhancing the light-matter interactions by incorporating TMDs with photonic cavities (such as planar photonic crystal cavities10-12, microdisk resonators13 and Fabry-Pérot cavities14,15), plasmonic nanostructures16-19, and recently hyperbolic metamaterials20. Photonic cavities can provide resonant enhancement, however, only in a very narrow spectral range, limiting their applications when broadband response is required, such as light-emitting diodes and photodetections. While plasmonic nanostructures (such as metallic nanoparticles and patterned metallic nanostructures) or hyperbolic metamaterials enable enhancements in a broader bandwidth through local plasmonic fields, it is usually restricted to a very small area of hot spots and hence limiting their applications in large-area photonic devices. Conventional way to achieve broadband and large-area light-coupling enhancement is based on interference induced by multi-reflection between 2D materials and substrates with a dielectric spacer, such as the most commonly used 2D/SiO2/Si configurations. By optimizing the spacer thickness, the interference in the 2D/SiO2/Si structure not only increases the contrast and visibility of 2D materials for identifying the layer number, but also enhances the photoluminescence (PL) and Raman signals21-23. However, even with the optimized SiO2 thickness, the substrate induced interference can accommodate only a moderate (~4 times) increase in Raman and PL intensities of TMDs, as compared with those on glass substrates.


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Here we demonstrate that distributed Bragg reflectors (DBRs) can be a broadband and large-area platform for enhancing the light-coupling efficiency of 2D materials (Fig. 1a). By optimizing the center wavelength of DBRs, the PL and Raman signals of monolayer WSe2 can be enhanced by ~25 and ~34 times, respectively, comparing to flakes on glass substrates. The full PL emission band (~700-800 nm) can be enhanced by the DBR substrates, in contrast to a narrowband enhancement of photonic resonators. In addition, the observed enhancements are comparable with TMDs on plasmomic nanostructures16-18. We further apply a double-band DBR to enhance the optical second harmonic generation (SHG). We achieve a maximum enhancement factor for SHG up to ~58, which is much higher than the enhancement by the exciton resonance reported recently24,25.

RESULTS AND DISCUSSION The main idea is based on engineering the amplitude and phase of the effective reflection coefficient at the 2D/substrate interface. Figure 1b shows the generic layer structure of 2D materials on a substrate for considering the substrate induced interference. Here we consider WSe2 monolayers as an example. The substrate can be glass, SiO2/Si or DBR, etc. In general, the effective reflection coefficient between the monolayer WSe2 and the substrate can be expressed as = . The emission intensity thus depends on the amplitude and phase due to the substrate induced interference (see Supporting Information). Figure 1c shows the twodimensional contour map of the emission intensity as a function of and at an emission wavelength of = 750 nm (i.e., near the typical PL peak wavelength of monolayer WSe2). For a comparison purpose, the intensities have been normalized to the intensity of a monolayer WSe2 on a homogeneous (semi-infinite) glass substrate. If we neglect the optical phase shift by


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light propagation in the monolayer-thick WSe2 as a first approximation, the optimal constructive interference for maximizing is expected to occur at = 1 and = 0 . Indeed, the calculation shows that the maximum (~6.5 times) occurs at = 1 and ≃ −0.16 rad, which has a small phase shift from = 0 due to the light propagation in the WSe2 monolayer. We have also analyzed and for a monolayer WSe2 on SiO2/Si with various SiO2 thickness ranging from 0 to 300 nm. As shown in Fig. 1d, and vary periodically with the SiO2 thickness due to the substrate induced interference. The calculated and for the WSe2/SiO2/Si structure shows a closed trajectory in the contour map (dashed line in Fig. 1c), yielding a moderate enhancement (~2.3 times) in the emission intensity when the SiO2 thickness equal to 127 nm with = 0.7 and = 0.1. The optimal enhancement can be fulfilled by using a high-reflectivity DBR as the substrate. Figure 1e shows the calculated and for emissions ( = 750 nm) from a monolayer WSe2 on a typical DBR with different center wavelengths . The optimal condition of = 1 and = −0.16 rad can be fulfilled using a DBR with ≃ 0.97 , providing an ideal platform for engineering the outcoupling of light emission from 2D materials. The same idea can be applied to analyze the effect of substrate induce interference on the excitation intensity . A similar contour map of as a function of and

is obtained at an excitation wavelength of = 532 nm (see Supporting Information). To verify the effect of DBR substrates on light coupling enhancement, we prepared two DBRs with center wavelengths at = 655 nm and 755 nm (see Methods) for enhancing PL and Raman intensities, respectively. CVD-grown monolayer WSe2 flakes with a typical side length of ~10-20 µm26 were transferred onto the DBR surface using a polydimethylsiloxane (PDMS) membrane27 (see Methods). Figure 2a-c show PL spectra of WSe2 flakes on DBRs ( = 755 nm), SiO2/Si (SiO2 thickness = 100 nm) and glass substrates using different excitation


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wavelengths at = 532, 633 and 690 nm, respectively. It is worth to mention that the PL intensity of monolayer TMDs would be affected by the substrate doping effect28-30. Since the topmost layer of all the DBR substrates used in this study is SiO2, we chose the PL intensity of monolayer TMDs on glass substrates as the reference in order to avoid different doping effects caused by different substrate materials. For monolayer WSe2 on the DBR substrates, the PL enhancement depends significantly on . Using = 690 nm, a PL enhancement factor up to ~26 can be achieved in comparison with those on glass substrates. It is noteworthy that the full PL emission band (~700-800 nm) can be enhanced by the DBR substrates, in contrast to the enhancement in a narrow band by photonic resonators. Besides, the PL spectral features are not influenced by the DBR substrates. Figure 2d shows the Raman spectra for monolayer WSe2 flakes on the DBR with = 655 nm. A Raman enhancement factor up to ~34 in is achieved by using an excitation wavelength at = 633 nm. In addition, E2g/A1g and 2LA Raman modes of WSe2 flakes on the DBR have the same enhancement in Fig. 2d. Comparing with SiO2/Si substrates, both and of DBRs are sensitive to the light incident angle. Due to the large numerical aperture (N.A. = 0.9) of the objective lens used in the experiments, oblique incidence should be considered in the calculations. We found that the center wavelengths for maximizing and shift to ≃ and ≃ 1.07 , respectively (see Supporting information). Figure 2e shows the excitation wavelength dependence of the PL enhancements for WSe2 flakes on the SiO2/Si substrate and the DBR with =755 nm. Solid curves in Fig. 2e are the calculated PL enhancement factor, Γ = × , where ≡ ,#$% / ,'()** is the excitation enhancement and ≡ ,#$% / ,'()** is the emission enhancement. For the DBR with = 755 nm, ≃ 5 is a constant factor for PL emissions at = 750 nm. However, for < 600 nm, no excitation enhancement is


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attainable because is far away from (Fig. 2f). Therefore, total PL enhancement is below 5 due to the influence of reflectivity fringes out of the DBR stop band. As approaching , the excitation enhancement increases gradually (Fig. 2f), leading to a total enhancement up to Γ ∼ 26 (at = 690 nm), which is comparable with TMDs on plasmonic nanostructures16-18. In contrast, for WSe2 flakes on SiO2/Si substrates with 100-nm SiO2, only a moderate PL enhancement Γ ∼ 7 is attainable (Fig. 2e). To illustrate how the center wavelength of the DBR affects the PL and Raman enhancements, we plotted contour maps of the total enhancement factor as a function of both and with = 655, 755 and 800 nm, as shown in Fig. 3a-c. Here, the solid and dash line indicate PL emission at = 750 nm and the Raman -.' mode (~250 cm-1) as a function of excitation wavelengths. While the DBR with = 800 nm exhibits a maximum near = 750 nm, but a moderate PL enhancement (~17 times) can be achieved by using an excitation wavelength at = 690 nm. The tradeoff between the excitation and emission wavelengths thus yields a maximum PL enhancement for the DBR with = 755 nm (Fig. 3d). For Raman measurements, two excitation wavelengths at 532 and 633 nm are used. Since the Raman E2g mode is close to the excitation wavelength, both and can be strongly enhanced by a DBR with a suitable . Using the 633-nm excitation, a Raman enhancement factor up to 34 is achieved for WSe2 flakes (-.' ~642 nm) on the DBR with = 655 nm. However, no apparent Raman enhancement can be observed on these DBRs when using the 532nm excitation due to the large detuning from the spectral ranges of excitation and emission enhancements (Fig. 3e) We further extend the DBR substrates to enhance the SHG from monolayer WSe2. Since monolayer TMDs exhibit non-zero second order nonlinearity due to their noncentrosymmetric


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crystal structure31-34, SHG is a powerful tool to characterize the crystal orientation, grain boundary or even twist angle of stacked TMDs35-37. However, the separation between the fundamental and the SH wavelengths ( = 2 ) exceeds the stop bandwidth of a typical DBR. To achieve both excitation and emission enhancements for SHG, we prepared a modified DBR (thickness: SiO2 = 103 nm and TiO2 = 142 nm, 12 pairs) consisting of two stop bands with center wavelengths at 470 nm and 842 nm (Fig. 4a). Figure 4b shows the SHG intensities from WSe2 flakes on the modified DBR and a glass substrate as a function of the square of excitation power using = 904 nm. The SHG intensity increases quadratically with the excitation power, indicative of a nonlinear optical process. From the slope of the power dependence, we estimated that the SHG efficiency is enhanced by a factor of ~ 60 for WSe2 flakes on the twostop-band DBR. Figure 4c shows the calculated spectral dependences of and for WSe2 flakes on the two-stop-band DBR. The maximum emission enhancement factor is 5.9, which is similar to the one of PL and Raman signals discussed above. By contrast, the maximum excitation enhancement factor can be up to 37, since the SHG intensity depends quadratically . |. on the excitation intensity (i.e. 234 ∝ |- ). Figure 4d shows the measured SHG

enhancement as a function of . Combining and for SHG, the two-stop-band DBR can provide a maximum enhancement factor ~58 at = 904 nm, in good agreement with experimental results. This measured SHG enhancement is much higher than the enhancement by two-photon resonance with the exciton energy24,25. We have also examined the distributions of SH intensities from single WSe2 flakes on the glass substrate (Fig.4e) and the DBR (Fig.4f) under a constant excitation power. Enhanced SHG can be observed on the entire WSe2 flake on the DBR substrate. Unlike using plasmonic nanostructures or hyperbolic metamaterials, the


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spatial distribution of light enhancement is usually not uniform due to the highly localized plasmonic fields in hot-spot areas16-20. In the discussion above, we used monolayer WSe2 as an example to demonstrate the lightcoupling enhancements by the DBR substrates. We emphasize that this method can be applied to other 2D materials as well. To demonstrate the general applicability of the DBR platform, we have also examined the PL enhancements for monolayer MoS2 and MoSe2 on DBR substrates. Figure 5a-c shows the PL spectra of monolayer MoS2, WSe2 and MoSe2 on the same DBR ( = 800 nm) and on a glass substrate using an excitation wavelength of = 532 nm. All the PL intensities are enhanced by the DBR substrate, but with different enhancement factors depending on the emission wavelength. Since the excitation wavelength = 532 nm is beyond the stop band of the DBR with = 800 nm (see Fig. S3 in Supporting information), the PL enhancements is contributed mostly from the emission enhancement (Γ ~ ) without significant excitation enhancement ( ~1). Figure 5d-f shows the calculated PL enhancement factors, Γ = × , as a function of emission wavelength for MoS2, WSe2 and MoSe2 monolayers on the DBR substrate, respectively. The calculations are based on the same model (see Supporting information), except that different refractive indices are used for different monolayer TMDs. For the DBR with = 800 nm, the maximum emission enhancement occurs near ~750 nm, which is closed to the typical PL peak wavelength of WSe2 monolayers. The measured PL enhancement factors (symbols in Figs. 5d-f) agree very well with the calculated enhancements. The slight difference among these enhancement curves is caused by the different refractive indices of TMD materials. The good agreement between in calculations and experiments thus indicates the general applicability of this methods to other 2D materials.


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CONCLUSION In conclusion, we demonstrated that a properly designed DBR can be an ideal platform for broadband and large-area light-coupling enhancements in 2D materials. By engineering the amplitude and phase of the reflection from the DBR substrate to achieve optimal constructive interference, both in-coupling and out-coupling efficiencies of light in 2D materials can be enhanced significantly. We show that the PL, Raman and SHG can be enhanced up to 26, 34 and 58, respectively. The calculated enhancement factors considering interference induced by the DBR substrates agree very well with the experimental findings. As a rule of thumb, a DBR substrate with a center wavelength close to the emission (excitation) wavelength can achieve optimal out-coupling (in-coupling) of light in 2D materials. The proposed DBR substrates pave the way for developing a range of 2D optoelectronic devices for broadband and large-area applications.


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METHODS Material Growth WSe2 monolayers were synthesized on sapphire substrates in a hot-wall chemical vapor deposition (CVD) chamber using WO3 and Se powders as source precursors. The WO3 powders were placed in a ceramic boat located in the heating zone center (925 ℃) of the furnace. The Se powders were prepared at the upper stream side at 270 ℃. The WSe2 layers were grown on sapphire substrates at 850 °C in Ar/H2 flowing gas at atmospheric pressure.

WSe2 flake transfer The CVD-grown monolayer WSe2 flakes were transferred onto different substrates by using polydimethylsiloxane (PDMS) membranes. The PDMS membrane was first attached to the asgrown WSe2/sapphire. To detach the PDMS/WSe2 film from the sapphire substrate, the PDMS/WSe2/sapphire was heated to 160 ℃ on a hot plate for 30 minutes. Then the PDMS/WSe2 film was peeled off from the sapphire substrate in deionized water. Finally, by attaching the PDMS/WSe2 layer to the target substrate and slowly peeling off the PDMS membrane, the WSe2 flakes can be transferred onto the target substrate.

DBR fabrications The DBR substrates were fabricated by depositing 12 periods of TiO2/SiO2 layers on clean glass substrates using an electron beam evaporator equipped with an in-situ optical monitor. The nominal thicknesses of the SiO2/TiO2 layers are 121/78 nm, 130/91 nm and 138/ 96 nm for DBRs with center wavelengths at 655 nm, 755 nm and 800 nm, respectively. The topmost layer


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is SiO2 for all the DBR substrates used in this study. The reflectivity spectra of bare DBRs were measured by a UV-Visible-NIR spectrophotometer (Hitachi U-4100).

Optical Measurements PL of monolayer WSe2 were excited by a supercontinuum laser with tunable wavelengths from 500 nm to 700 nm focused through a 100× objective lens (N.A.= 0.9). Raman measurements were performed using either a 532-nm solid state laser or a He-Ne laser (633 nm) as the excitation source. SHG was excited by a Ti:sapphire laser with tunable wavelengths from 800 nm to 920 nm focused through a 50× objective lens (N.A.= 0.42). The PL, Raman and SHG signals were collected by the focusing objective and analyzed by a 0.75-m monochromator equipped with a liquid-nitrogen-cooled CCD camera.


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ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. Details of excitation and emission enhancements; Angle depended enhancements. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Author Contributions W.H.C. and Y.C.C. conceived the idea and designed the experiment. Y.C.C. performed the spectroscopy measurements. The TMD materials were grown by H.Y. Modeling and numerical calculated by Y.C.C. and assisted by C.J.L. W.H.C. and Y.C.C. wrote the paper. All authors discussed the results and commented on the manuscript. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was supported by the Ministry of Science and Technology of Taiwan (MOST-1042628-M-009-002-MY3, MOST-105-2119-M-009-014-MY3), AOARD (FA2386-16-1-4035). W.H.C. acknowledges the supports from the Center for Interdisciplinary Science of NCTU.


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28. Ghatak, S.; Pal, A. N.; Ghosh, A. Nature of Electronic States in Atomically Thin MoS2 Field-Effect Transistors. ACS Nano 2011, 5, 7707-7712. 29. Sercombe, D.; Schwarz, S.; Del Pozo-Zamudio, O.; Liu, F.; Robinson, B. J.; Chekhovich, E.; Tartakovskii, I. I.; Kolosov, O.; Tartakovskii, A. I. Optical Investigation of the Natural Electron Doping in Thin MoS2 Films Deposited on Dielectric Substrates. Sci. Rep. 2013, 3, 3489. 30. Buscema, M.; Steele, G. A.; van der Zant, H. S. J.; Castellanos-Gomez, A. The Effect of the Substrate on the Raman and Photoluminescence Emission of Single-Layer MoS2. Nano Res. 2014, 7, 561-571. 31. Zeng, H.; Liu, G.-B.; Dai, J.; Yan, Y.; Zhu, B.; He, R.; Xie, L.; Xu, S.; Chen, X.; Yao, W.; Cui, X. Optical Signature of Symmetry Variations and Spin-Valley Coupling in Atomically Thin Tungsten Dichalcogenides. Sci. Rep. 2013, 3, 1608. 32. Malard, L. M.; Alencar, T. V.; Barboza, A. P. M.; Mak, K. F.; de Paula, A. M. Observation of Intense Second Harmonic Generation from MoS2 Atomic Crystals. Phys. Rev. B 2013, 87, 201401. 33. Kumar, N.; Najmaei, S.; Cui, Q.; Ceballos, F.; Ajayan, P. M.; Lou, J.; Zhao, H. Second Harmonic Microscopy of Monolayer MoS2. Phys. Rev. B 2013, 87, 161403. 34. Ribeiro-Soares, J.; Janisch, C.; Liu, Z.; Elías, A. L.; Dresselhaus, M. S.; Terrones, M.; Cançado, L. G.; Jorio, A. Second Harmonic Generation in WSe2. 2D Mater. 2015, 2, 045015. 35. Li, Y.; Rao, Y.; Mak, K. F.; You, Y.; Wang, S.; Dean, C. R.; Heinz, T. F. Probing Symmetry Properties of Few-Layer MoS2 and h-BN by Optical Second-Harmonic Generation. Nano Lett. 2013, 13, 3329-3333. 36. Kim, C.-J.; Brown, L.; Graham, M. W.; Hovden, R.; Havener, R. W.; McEuen, P. L.; Muller, D. A.; Park, J. Stacking Order Dependent Second Harmonic Generation and Topological Defects in h-BN Bilayers. Nano Lett. 2013, 13, 5660-5665. 37. Hsu, W.-T.; Zhao, Z.-A.; Li, L.-J.; Chen, C.-H.; Chiu, M.-H.; Chang, P.-S.; Chou, Y.-C.; Chang, W.-H. Second Harmonic Generation from Artificially Stacked Transition Metal Dichalcogenide Twisted Bilayers. ACS Nano 2014, 8, 2951-2958.


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Figure 1. Light coupling enhancement by substrate induced interference. (a) Schematic of monolayer WSe2 flakes on a DBR substrate. Inset: The optical image of a monolayer WSe2 flake transferred onto a DBR substrate. The scale bar is 10 µm. (b) Schematic of the multiple reflections and optical paths for excitation and emission processes in the WSe2 layer. Here d is the thickness of WSe2 monolayers with typical value of 0.65 nm. (c) The contour map of the calculated emission intensity from the WSe2 layer ( = 750 nm) as a function of the amplitude and phase of the effective reflection coefficient = between the WSe2 and the substrate. The emission intensities have been normalized to that on a semi-infinite glass substrate (red symbol). The open symbol indicates the maximum emission intensity at = 1 and ≃ −0.16 rad. (d) The calculated (blue) and (red) for a monolayer WSe2 on a SiO2/Si substrate as a function of the SiO2 thickness. (e) The calculated (blue) and (red) for a monolayer WSe2 on a DBR substrate as a function of the center wavelengths . The solid line and dashed line in (c) are the calculated : , ; shown in (d) and (e), respectively.


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a

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Normalized Raman intensity (a.u.)

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Figure 2. Excitation wavelength dependence of PL and Raman enhancements. (a, b, c) PL spectra of WSe2 flakes on a glass substrate (gray), a SiO2/Si substrate (green, SiO2 thickness =100 nm) and a DBR substrate (blue, = 755 nm) using different excitation wavelengths : (a) 532 nm, (b) 632 nm and (c) 690 nm. The spectra have been normalized to the PL peak intensity of WSe2/glass. (d) Raman spectra of WSe2 flakes on a glass substrate (gray), a SiO2/Si substrate (green) and a DBR substrate with = 655 nm (blue) using = 633 nm. The nearly degenerated E2g/A1g modes and the 2LA second-order Raman mode of monolayer WSe2 can be observed. (e) The calculated (solid curves) and measured (symbols) PL enhancement factors of monolayer WSe2 on SiO2/Si (green) and on DBR (blue, = 755 nm) as a function of excitation wavelength . (f) The calculated excitation and emission enhancement of the DBR ( = 755 nm) as a function of excitation (red) and emission (blue) wavelengths, respectively.


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Figure 3. Effect of center wavelength on the PL and Raman enhancements. (a, b, c) Contour map of the calculated total enhancement factor as function of excitation and emission wavelengths for WSe2 on DBR substrates with different center wavelengths : (a) 655 nm, (b) 755 nm and (c) 800 nm. The dotted and dashed lines indicate the PL and Raman emission wavelengths as a function of excitation wavelengths . (d) The calculated (solid curve) and measured (symbols) PL enhancements Γ as a function of DBR center wavelengths . The excitation wavelength is = 690 nm. (e) The calculated (solid curves) and measured (symbols) Raman enhancements as a function of DBR center wavelengths using different excitation wavelengths at 532 nm (green) and 633 nm (red).


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Figure 4. SHG enhancements using a two-stop-band DBR. (a) Amplitude and phase shift of the effect reflection coefficient for WSe2 monolayers on the two-stop-band DBR. (b) SHG intensities as a function of the square of excitation power for WSe2 on the DBR (blue) and on a glass substrate (gray). Inset: SHG spectra using an excitation wavelength of 904 nm, corresponding SHG at 452 nm. (c) Calculated excitation (fundamental fields, red) and emission (SH fields, blue) enhancement factors as a function of the fundamental and SH wavelengths, respectively. (d) Experimental (symbols) and calculated (solid curve) SHG enhancement factors as a function of fundamental wavelengths. (e) SH image of monolayer WSe2 on glass. (f) SH image of monolayer WSe2 on the DBR. Inset: the corresponding optical image of (e) and (f). The scale bar is 5 µm.


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Figure 5. PL enhancements for different monolayer TMD materials on the same DBR ( = 800 nm) substrate. (a, b, c) PL spectra of (a) MoS2 ( = 654 nm), (b) WSe2 : = 745 nm) and (c) MoSe2 ( = 784 nm) monolayers on the DBR substrate (blue) and on a glass substrate (gray). The excitation wavelength is = 532 nm. (d, e, f) The calculated PL enhancement as a function of emission wavelength for (d) MoS2, (e) WSe2 and (f) MoSe2 monolayers on the DBR substrate (solid curve). The curves in (d-f) are calculated by the same model except that different refractive indices were used for different TMD materials.


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Distributed Bragg Reflectors as Broadband and Large- Area Platforms

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