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[Masanobu Iwanaga](https://orcid.org/0000-0002-8930-6940), [Xu Yang](https://orcid.org/0000-0001-8195-5850), [Vasilios Karanikolas](https://orcid.org/0000-0002-4829-8921), [Takashi Kuroda](https://orcid.org/0000-0001-6445-7673), [Yoshiki Sakuma](https://orcid.org/0000-0001-6804-7217)

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[2D‐Material‐Fused High‐Emittance Plasmo–Photonic Metasurfaces](https://mdr.nims.go.jp/datasets/9e2d65ce-ffef-4448-9361-390707af0163)

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2D‐Material‐Fused High‐Emittance Plasmo–Photonic MetasurfacesRESEARCH ARTICLEwww.advopticalmat.de2D-Material-Fused High-Emittance Plasmo–PhotonicMetasurfacesMasanobu Iwanaga,* Xu Yang, Vasilios Karanikolas, Takashi Kuroda, and Yoshiki SakumaTransition metal dichalcogenides (TMDC) are a family of atomic-layer 2Dmaterials (2dMs). Many researches have attempted to reinforce the uniqueproperties by incorporating artificially designed nanostructures. Here, a highlyphotoluminescence (PL) enhancing platform is reported that is formed bycombining high-emittance plasmo–photonic metasurfaces with a large-area2dM, which is a continuous monolayer of TMDC of cm2 dimension. It isexperimentally demonstrated that the 2dM-PL intensity at room temperatureis enhanced by more than 1030-fold in comparison with that observed on theflat gold film regions. The enhancement is associated with spectraltransformation indicating a resonant effect of the metasurfaces. It is moreoverrevealed that the confocal PL image originates from a coherent single-modeemission that forms a photon number distribution obeying the Poissonprobability distribution. Thus, the 2dM-fused metasurfaces function asefficient coherent quantum light sources. PL decay-time analysis also revealsthe coexistence of two components in the enhanced PL. The fast componentexhibits a lifetime of 18 ps whereas the slow component decays at 4.4 ns.Both components are most likely to undergo radiative processes, whichindicates that metal-induced PL quenching is suppressed on these 2dM-fusedmetasurfaces. The long lifetime component is ascribed to an excitonicintervalley transition.1. IntroductionQuantum 2D materials (2dM) such as transition metal dichalco-genides (TMDCs) are attracting great interest as a new seriesM. Iwanaga, X. Yang[+], V. Karanikolas[++], T. Kuroda, Y. SakumaNational Institute for Materials Science (NIMS)1-1 Namiki, Tsukuba 305-0044, JapanE-mail: iwanaga.masanobu@nims.go.jpThe ORCID identification number(s) for the author(s) of this articlecan be found under https://doi.org/10.1002/adom.202303309[+]Present address: Institute of Materials and Systems for Sustainability,Nagoya University, Nagoya 464-8601, Japan[++]Present address: Materials Modelling, Institute of Materials Science,Technical University of Darmstadt, 64287 Darmstadt, Germany© 2024 The Authors. Advanced Optical Materials published byWiley-VCH GmbH. This is an open access article under the terms of theCreative Commons Attribution-NonCommercial-NoDerivs License,which permits use and distribution in any medium, provided the originalwork is properly cited, the use is non-commercial and no modificationsor adaptations are made.DOI: 10.1002/adom.202303309of atomic-layer materials that exhibit re-markable optical[1–3] and electronic[4,5]properties owing to the peculiar bandstructures originating from the ultimatelythin 2D structures. Numerous studieshave been conducted to clarify the basicproperties of the TMDC atomic layers. Forfuture applications, large-scale growth isalso being studied to enable the productionof wafer-scale TMDC monolayers.[6–9]By combining the direct-bandgap lu-minescence of TMDC monolayers withsurface-enhancing effects that are at-tempted to be artificially realized onmetasurfaces and nanostructures, noveland highly efficient light-emitting systemsshowing spontaneous emissions[10–27] anda stimulated emission[28] have been exten-sively explored. The stimulated emissionwas reported using a photonic crystal cavitywhereas the contrast of the stimulatedemission to spontaneous emission was notso high, being ≈10;[28] it is thus unclearwhether the stimulated emission deservespractical use. Let us hereafter focus onspontaneous emissions from TMDCmonolayers. To date, most trials havereported a few tenfold spontaneously photoluminescence (PL) in-tensity enhancement. This tendency suggests that the interplaybetween the TMDC monolayer and the artificial nanostructureswas insufficient in the most cases, where local electric-field en-hancement (or so-called hot spot) was thought highly of at ei-ther of excitation or emission wavelength. To our knowledge, thelargest PL-intensity enhancement factor (EF) using plasmonicplatforms was approximately 200-fold regarding the WSe2 mono-layer transferred on Au nanogap structures under excitation at532 nm[15] and MoS2 monolayer placed between Ag nanocubeand Au film under excitation at 420 nm.[18] We point out thatartificial manipulations for PL-intensity EF were conventionallyconducted by dividing the observed EFs with geometrical factors,which were typically ratio of nanogap width to diameter of ex-citation laser spot and took small values in a range from 1/100to 1/10; consequently, far larger PL-intensity EFs than the ob-served EFs were claimed in the previous reports;[10,12,15,18,19] how-ever, these artificially manipulated EFs are not observable and arepractically meaningless because the geometrical factors mix upthe observed quantity with effects under the diffraction limit.Apart from the artificially manipulated EFs, optical resonancesin the artificial nanostructures were expected to selectively en-hance the exciton transitions in the TMDC monolayer. However,Adv. Optical Mater. 2024, 12, 2303309 2303309 (1 of 9) © 2024 The Authors. Advanced Optical Materials published by Wiley-VCH GmbHhttp://www.advopticalmat.demailto:iwanaga.masanobu@nims.go.jphttps://doi.org/10.1002/adom.202303309http://creativecommons.org/licenses/by-nc-nd/4.0/http://crossmark.crossref.org/dialog/?doi=10.1002%2Fadom.202303309&domain=pdf&date_stamp=2024-04-03www.advancedsciencenews.com www.advopticalmat.deFigure 1. Structural overview. a) Illustration of a 2D material (2dM) transferred onto a plasmo–photonic metasurface. b) Photo of a 2dM-fused meta-surface substrate. Three metasurfaces are located vertically at the center of the substrate. Four arrows indicate the corners of the 2dM monolayer. c)Optical microscopy image with black scale bar of 50 μm. d) Top-view scanning-electron-microscopy (SEM) image, taken near the edge of metasurface.The triangle indicates the edge. 2dM-fused flat Au region is located near the lower edge. The black scale bar indicates 10 μm. e) Magnified SEM image.Arrows indicate the edges of the 2dM film. The white scale bar indicates 500 nm.the resonances did not contribute to significant changes in theTMDC PL spectra, except for a plasmon-based study incorporat-ing MoS2[18] and dielectric nanophotonic structures coupled withWSe2;[25] in the former,[18] sub-10 nm gap between Ag nanocubeand Au file was used for the enhanced PL experiment, the PLEF for A exciton was 17-fold and that for B exciton was 211-fold,and the B exciton was particularly enhanced under excitationwavelength at 420 nm, which was matched with a wavelength ofnanogap cavity resonance; in the latter,[25] the PL was measuredonly at 5 K to study single-photon emissions, the temperature de-pendence of the PL spectra was not examined, and consequentlyresonant effect for the spectral shapes coming from the dielectricnanostructures is difficult to be evaluated here.As another drawback in the previous studies, the TMDCmonolayers were limited to micro-domains, which were typicallytriangles of several-micrometer sides and resulted in small light-emitting devices for most practical use. It is highly preferred thatmonolayers of larger scale are used for the light-emitting devices.A plasmo–photonic metasurfaces have eigen modes at 600–1400 nm, resulting from coupling of plasmonic and photonicguided modes, and exhibit a feature that they have several largelight absorption bands.[29] Absorptance is equivalent to emittancedue to reciprocity.[30] Emittance is a macroscopic property ofmetasurfaces and does not depend on local hot spots. The meta-surfaces exhibited extremely efficient fluorescence (FL) enhanc-ing capabilities that exceed 2000-fold.[31,32] In particular, the meta-surfaces coated with a self-assembled monolayer suppressedmetal-induced FL quenching[31] and selectively enhanced Ramanscattering with controlling the interface between the moleculesand the metasurfaces.[32] Recently, the metasurfaces have beenshown to serve as high-precision FL biosensors.[33] All-dielectricmetasurfaces consisting of periodic Si-nanopellet arrays exhib-ited a similar capability of enhancing FL[34,35] and have been ap-plied to biosensing.[36–41] These two types of metasurfaces haveopened new routes for developing extremely efficient biosensors.Here, we report a new platform fusing a continuous TMDCmonolayer with the plasmo–photonic metasurfaces, which havethe macroscopic feature of high emittance and do not rely onthe conventional hot-spot strategy. To date, the plasmo–photonicmetasurface have not been applied to any platform including2dM. Optical properties and resonances of the metasurface arenumerically clarified. Strong PL enhancement is presented onthe resonance and the enhanced PL dynamics is examined byanalyzing PL decay time. Furthermore, a coherent single-modeemission including single-photon emissions is revealed throughanalysis for photon number distributions of confocal PL images.Additionally, Raman-scattering enhancement is addressed.2. Results and Discussion2.1. 2dM-Fused MetasurfacesFigure 1a schematically illustrates a plasmo–photonic metasur-face fused with a TMDC monolayer, which was visualized usingVESTA software.[42] The monolayer was nearly WS2, described inthe Experimental Section; the gray and yellow spheres denote Wand S atoms, respectively. The monolayer was transferred ontoa metasurface substate conducting a procedure described in theExperimental Section.The appearance of the 2dM-fused metasurface substrate isshown in Figure 1b. The metasurface substrate was approxi-mately a square of 2 cm × 2 cm, based on a silicon-on-insulator(SOI) substrate (top Si layer 200 nm/middle buried oxide 375nm/base Si wafer 675 μm). The TMDC atomic layer with an areaof approximately 1 cm2, whose corners are indicated by arrows,was transferred onto three metasurface areas of 0.96 mm × 0.60mm, which were fabricated along the vertical center line of thesubstrate. The nanofabrication procedures of the metasurfacesare described in the Experimental Section.An optical microscopy image of the TMDC-transferred meta-surface is shown in Figure 1c, where the black scale bar indi-cates 50 μm. The optical image indicates that the transferredTMDC film was sectioned on the metasurface by forming bound-aries, mainly resulting from wrinkles and rifts that emerged dur-ing the transfer process. Magnified images were obtained usingfield-emission scanning electron microscopy (SEM), as shown inFigure 1d,e, with black and white scale bars of 10 μm and 500nm, respectively. The triangle in Figure 1d indicates the borderAdv. Optical Mater. 2024, 12, 2303309 2303309 (2 of 9) © 2024 The Authors. Advanced Optical Materials published by Wiley-VCH GmbH 21951071, 2024, 17, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adom.202303309 by National Institute For, Wiley Online Library on [17/06/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advopticalmat.dewww.advancedsciencenews.com www.advopticalmat.deFigure 2. Optical resonances of the plasmo–photonic metasurface. a) Optical configuration and xyz coordinate: 3D view (left) and xz-section view (right).b) Simulated reflectance spectra at x and y polarizations, shown with black solid and red dotted lines, respectively. c,d) Resonant electric- and magnetic-field intensity distributions at 638.9 nm in an xz-section view, respectively. e,f) Electric-field intensity distributions at 532.0 nm in xz- and xy-section views,respectively. g) Magnetic-field intensity distribution at 532.0 nm in the xz-section view that is the same with (e). We note that incident intensity was setto unity in common to (c–g).between the metasurface area (upper) and flat Au region (lower).The boundaries and flat-less regions in the TMDC layer are ob-served as white parts. Other regions are flat. The TMDC layeris mostly flat in the Au region. In the magnified SEM image inFigure 1e, a rift of the TMDC layer is indicated by arrows andseen as a white part because the outermost surface is Au, whichis brighter than that of the TMDC layer. Figure 1e also showsthat the TMDC layer except for the rifts covers the metasurface,including the air holes.2.2. Electromagnetic Resonances of MetasurfaceFigure 2 shows the reflectance spectra and the resonant electro-magnetic (EM) field distributions of the plasmo–photonic meta-surface. The optical configuration and xyz coordinates are illus-trated in Figure 2a:3D (left) and a xz-section views (right), respec-tively. The metasurface includes a periodic hexagonal array of airholes; the periodic length was set to 410 nm and the diameterof the hole 260 nm, indicated by dotted lines in the xz section.Thickness of Au and Si layers was 30 and 200 nm, respectively.Incident wavevector kin and polarization Ein are shown in the3D view.The computed reflectance spectra at the normal incidence areshown in Figure 2b. The incident polarization (Ein) was set to x-or y-polarization, as shown with solid black and dotted red curves,respectively. More than ten resonances appear in the reflectancespectra as peaks and dips. The dependence on the incident polar-izations is not significant at the normal incidence.A set of resonant EM-field intensity distributions at 638.9 nm,which corresponds to a dip in the reflectance spectra, is shownin Figure 2c,d, where an xz-section view through the center ofthe circular air holes is presented. The electric-field intensity isdominantly localized and significantly enhanced at the sidewallof the Si nanohole, reaching at the maximum of 227.7 whenthe incident electric-field intensity is set to 1.0. Accordingly, themagnetic-field intensity is the most enhanced, inside the Si layerbetween the strong electric fields, at the maximum intensity of67.8. Both EM components are significantly enhanced in themetasurface compared with the incidence. Simultaneously, theEM fields are strongly localized in the metasurface, implying thatthe transmission component is small. This point is quantitativelyaddressed later (Figure 3).At the excitation wavelength of 532.0 nm for the PL spectra, theelectric-field intensity distribution is the most enhanced at the topof Au layer, as shown in Figure 2e, where the xz section throughthe center of the air holes is presented. Figure 2f shows an xysection representing the unit cell of the metasurface and locat-ing 0.5 nm above the top of the perforated Au layer. The xy sec-tion corresponds to the position of the transferred TMDC mono-layer. The maximum of the electric-field intensity is 17.3, whichis enhanced but not as strong as that at 638.9nm (Figure 2c).The magnetic field in the xz-section is shown in Figure 2g in asimilar manner to Figure 2e, and is the most enhanced in the Silayer, though the field intensity is substantially weaker than thatat 638.9 nm.To explicitly evaluate the emittance of the plasmo–photonicmetasurface, we studied the metasurface on a transparent SiO2substrate, as illustrated in Figure 3a. The experimental SOI con-figuration in Figure 2a includes light absorption by the base Sisubstrate. Consequently, it is difficult to identify light absorptionby the metasurface alone.Adv. Optical Mater. 2024, 12, 2303309 2303309 (3 of 9) © 2024 The Authors. Advanced Optical Materials published by Wiley-VCH GmbH 21951071, 2024, 17, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adom.202303309 by National Institute For, Wiley Online Library on [17/06/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advopticalmat.dewww.advancedsciencenews.com www.advopticalmat.deFigure 3. Emittance of plasmo–photonic metasurface. a) 3D-view illustra-tion of the metasurface on SiO2 substrate. Directions of light absorptanceand emittance are indicated by arrows. b) Computed emittance spectraat x and y polarizations, evaluated by using Equation (1) and shown withsolid black and dotted red curves, respectively.The emittance is equivalent to the absorptance because ofreciprocity,[30] therefore, the two quantities are equal. Both quan-tities are macroscopic parameters of the metasurface. We evalu-ated the absorptance (Abs) in % using Equation (1):Abs = 100 −∑m,n(Rm,n + Tm,n) (1)where m and n denote the diffraction orders open in the wave-length range of interest. R0, 0 and T0, 0 are the ordinary reflectanceand transmittance, respectively. The nonzero diffraction orderswere R0, ±1, T±1, 0, T0, ±1, T0, ±2, and T±1, ±1 in the present wave-length range. Figure 3b shows polarization-dependent Abs spec-tra (x polarization:solid black curve, y polarization:dotted redcurve) and several prominent emittance peaks, some of which ex-ceed 80%. The emittance peaks originate from the resonances ofthe plasmo–photonic metasurface because the other surround-ing materials, namely air and SiO2, are resonance-free in thiswavelength range. Figure 3b also shows that 56% of the incidentlight at 532 nm is resonantly absorbed; in other words, the emit-tance is 56% at this wavelength. The emittance peak overlaps withthe PL wavelengths of the TMDC monolayer, as shown later. Theresonant electric fields are significantly enhanced, as visualizedin Figure 2c,e,f.2.3. Photoluminescence Enhancement in the 2dM-Fused SystemThe enhanced PL (red curve) and the reference spectra measuredon flat Au (thin yellow lines) are shown in Figure 4a, and the in-set magnifies the reference PL spectrum. The PL EF is plottedfor the right axis with a dotted black curve where the EF wasdefined as the ratio of the enhanced PL and reference spectra.The maximum PL EF was 1033-fold and located at 634.2 nm.Thus, prominent PL enhancement was quantitatively evaluated.The enhanced PL peaks at 636.7 and 670 nm are ascribed to theA exciton and trion, respectively, in the TMDC monolayer.[43] Asmall peak at 563 nm is considered to originate from the B ex-citon. The largest PL peak at 636.7 nm agrees with a measuredreflectance dip (or an emittance peak) of the 2dM-fused metasur-face, as shown in the Supporting Information (Figure S1, Sup-porting Information). Thus, the prominent PL enhancement is aresonant effect incorporating the metasurface.The PL peak of the reference spectrum was 644 nm anddiffered from the peak of the enhanced PL at 636.7 nm. ThePL spectra of the TMDC monolayers have been reported fre-quently. It is known that the spectra can change due to circum-stances, including the substrate,[43–47] atomic components,[48,49]and oxidation.[50] In this case, the as-grown TMDC layer wastransferred onto the metasurface using an organic sorbent; there-fore, the chemical treatment and the change of substrate wereconsidered to change the PL peak to some extent. However, as wenoted above, the most PL-enhanced wavelength agrees with theresonant wavelength of this 2dM-fused system and therefore isprimarily ascribed to the resonant effect of metasurface. Besides,oxidation could explicitly change the PL peak after exposure to airfor more than 7 d;[50] however, we handled the TMDC sample inair within 12 h or less, and preserved it under an N2 gas atmo-sphere except for experiment; therefore, the oxidation is unlikelyin this case. We briefly note that the PL-peak wavelength is withinthe range reported so far.[44,48,49]The PL EF in the experimental configuration is expressedas[31,51]EF =NmN0×𝜂m𝜂0×𝛾m(k)𝛾0(k)(2)where the subscripts m and 0 denote on and off the metasurface,respectively. N is the photoexcited populations, 𝜂 is the quantumyield, and 𝛾(k) is the radiative decay rate dependent on outgoingwavevector k. In particular, 𝜂 = 𝛾/(𝛾 + 𝛾NR) where 𝛾NR is the non-radiative decay rate. The 𝛾m/𝛾0 ratio is often referred to as thePurcell factor.[52] From the electric-field intensity in Figure 2e,the TMDC monolayer was excited efficiently by ≈tenfold, givingthe Nm/N0 ratio. The other factors at the right-hand side of Equa-tion (2) are considered after examining the experimental and the-oretical results.An upright confocal PL image of the 2dM-fused metasur-face is shown in Figure 4b. Red denotes the PL from the 2dMmonolayer. The rectangular red area corresponds to the metasur-face area of 0.96 mm × 0.60 mm. The white scale bar indicates0.25 mm. The off-metasurface area is dark, exhibiting the zero-signal level in the confocal measurement. We also mention thatthe brightest spot on the bottom edge of the metasurface origi-nated from the aggregated 2dM that was not a simple monolayer.The confocal image was acquired through the photon-countingmeasurement (see the Experimental Section for details). Eachpixel has a detected photon number. Therefore, the photon num-ber distribution can be extracted as shown in Figure 4c. TheAdv. Optical Mater. 2024, 12, 2303309 2303309 (4 of 9) © 2024 The Authors. Advanced Optical Materials published by Wiley-VCH GmbH 21951071, 2024, 17, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adom.202303309 by National Institute For, Wiley Online Library on [17/06/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advopticalmat.dewww.advancedsciencenews.com www.advopticalmat.deFigure 4. Enhanced photoluminescence (PL) from the 2dM-fused metasurface (MSF). a) PL spectra on and off the MSF, shown with a solid red curveand thin yellow lines, respectively. Inset shows a magnified PL spectrum off the MSF. The PL enhancement factor (EF) is represented with a dotted blackcurve, plotted for the right axis. b) Confocal PL image represented using a pseudo red color. Rectangular red area at the center corresponds to the MSF.White scale bar indicates 250 μm. c) Photon number distribution evaluated in the confocal image in (b). d,e) Temporal profiles of the enhanced PL inps and ns ranges, respectively. Measured data are shown with closed red circles. Fitted exponential curves are shown with black curves.analysis region for the photon numbers was set inside the meta-surface area by choosing a nearly uniform area, which is shownin the Supporting Information (Figure S2, Supporting Informa-tion). Figure 4c shows that the photon numbers were predomi-nantly less than ten in each pixel and that single-photon detectionfrom the TMDC monolayer occupied 20.3% of the photon num-ber distribution. Conducting fitting using the Poisson distribu-tion, we reproduced the photon number distribution fairly well(Figure S3, Supporting Information) and determined the meanphoton number to be 2.41 for Figure 4c. The zero-photon eventsoccupying 11.6% are primarily attributable to the Poisson dis-tribution that governs the probability of small-number events.Small deviation from the ideal Poisson distribution probably re-sulted from broken holes in the monolayer and a small portionof bilayers. Overall, the photon number distribution obeying thePoisson distribution is ascribed to a coherent single-mode pho-ton emission;[53] in other words, this 2dM-fused system is a quan-tum coherent light source.We note that, as a corollary of the coherent single-mode emis-sion obeying the Poisson distribution, the second-order coher-ence g(2) is unity, meaning perfect coherence, irrespective of themean photon number;[53] in other word, the photon number dis-tribution obeying the Poisson distribution is direct evidence forthe quantum coherent state. To the best of our knowledge, the co-herent state has not been observed on any plasmonic platform.Regarding the appearance of Figure 4b, we note that dark spotsexist on the metasurface area and result from the quantum na-ture of light emission, which means that the zero-photon spotsappear inevitably under low-power excitation.The PL decay profile in a ps range is shown on the semi-logscale in Figure 4d; closed red circles denote the measured PLdata and the solid black curve is a fitted curve using a single-exponential function such as y = y0 + A0exp [− (t − t0)/𝜏S] where𝜏S denotes the decay time, A0 is the proportional constant, t0 isthe time offset, and y0 is the background constant. The measureddata were fitted 15 ps after from the peak because the temporalwidth of the excitation laser pulses was determined by the in-strumental response function (IRF) in the setup and was approx-imately 10 ps at the full width at half maximum.[54] The IRF isshown in green in Figure 4d. Thus, the PL decay time in the psrange was determined to be 𝜏S = 18.0 ± 1.2 ps.The PL lifetime was also studied in a ns range. The tempo-ral PL growth is shown with red dots in Figure 4e, where thephoton numbers were accumulated using a photon-counting de-tector. The time-integrated PL intensity I for the gate time T isexpressed asI(T) = ∫T0[C exp(−t∕𝜏S) + D exp(−t∕𝜏L)]dt≈ C𝜏S + D𝜏L[1 − exp(−T∕𝜏L)] (3)where 𝜏S and 𝜏L denote the short and long decay times, respec-tively, and C and D are the proportional constants. A relation ofAdv. Optical Mater. 2024, 12, 2303309 2303309 (5 of 9) © 2024 The Authors. Advanced Optical Materials published by Wiley-VCH GmbH 21951071, 2024, 17, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adom.202303309 by National Institute For, Wiley Online Library on [17/06/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advopticalmat.dewww.advancedsciencenews.com www.advopticalmat.deFigure 5. Energy diagrams of the PL dynamics. a) Enhanced PL processeson the plasmo–photonic metasurface. b) Excitation–relaxation dynamicsoff the metasurface (i.e., on the flat Au film), where the nonradiative pro-cess(es) is predominant.𝜏S ≪ T is reasonably assumed to derive Equation (3), because of𝜏S = 18.0 ps (Figure 4c) and T in the order of ns. Fitting by Equa-tion (3) worked well, as indicated by the black curve in Figure 4e,and determined the 𝜏L to be 4.4± 0.5 ns. We note that decay curveoff the metasurface was not successfully measured because of theextremely low-signal level.The parameters C and D in Equation (3) are proportional tothe initial populations contributing to the fast and slow exciton-PL components, respectively. It was determined from the fittingresult in Figure 4e that C:D was 151.7:1, implying that the dom-inant populations were consumed as the fast component. Thisanalysis suggests that there are two possible excitonic transitionsin the TMDC monolayer. A theoretical study on the exciton bandof MoS2 monolayer showed that the A excitons have K–K di-rect and K–Γ intervalley transitions at approximately the sameenergy,[55] where Γ and K denote the high-symmetry points inthe Brillouin zone. Because WS2 is reasonably assumed to haveexciton bands similar to those of MoS2, we can ascribe the two ex-citonic transitions to the two observed PL components. The twoPL components have not been frequently reported; however, twotime-resolved studies showed the fast (ps) and slow (ns or longer)PL components in a WS monolayer.[54,56] We note that the fullyintegrated PL intensities at T = ∞ are given by the ratio of C𝜏S:D𝜏L because the intensities are described using the double expo-nential functions (see the definition of I(T), Equation (3)). Theanalysis of Figure 4e resulted in C𝜏S:D𝜏L = 1:1.6. This ratio in-dicates that the slow component substantially contributed to theobserved confocal image.On the basis of the experimental results shown in Figure 4,the PL dynamics on and off the metasurfaces are depicted inFigure 5a,b, respectively. Excitation is indicated by the green ar-row in Figure 4a. Subsequently, the photoexcited states are gen-erated in the continuum and relax into the excitonic states or tononradiative process(es). The excitonic states are mainly A exci-tons or trions owing to the emission wavelengths.[43,44] Figure 5also shows that the A exciton energy is in matched with a peakenergy of emittance on the metasurface, while the exciton offthe metasurface is not enhanced by the emittance. The height ofemittance is comparable in Figure 5. In addition, some excitonicstates can also relax into the nonradiative process, due to exciton–exciton collisions that depend on the excitation density.[57] As isreferred to in the Introduction, many attempts aiming at sig-nificant enhancement effect for the PL dynamics in 2dM havebeen conducted;[10–27] however, there were hardly studies to ex-amine the whole PL dynamics described in Figure 5. Because thePL EF is a multiple quantity (Equation (2)), the three factors re-garding excitation, excited-state transfer, and emission should belarge simultaneously. This point of view has been neglected inthe most of previous studies, which optimized only one or two ofthe three factors.The PL dynamics off the metasurfaces are simpler than thoseon the metasurfaces because nonradiative processes are predom-inant, and nearly all the photoexcited states relax through thenonradiative process in an ultrafast manner. Such metal-inducedquenching was studied in the 1970s using a damping oscillatormodel,[58] and it was determined that a fluorescent dipole locatedwithin 1 nm from an Au-film surface decayed nonradiatively ata rate exceeding 1 × 104 in comparison with the original decayrate.[31] Although this model does not address excitons in 2D ma-terials, it can account for the heavy reduction of the PL off themetasurface. A very small portion of the photoexcited states re-laxes into the excitonic state and results in the excitonic PL, asshown in the inset of Figure 4a.In the theoretical Purcell factor evaluation, we determined thatPurcell factors of approximately four can be observed in the exper-iment (Figure S4, Supporting Information) and that large Purcellfactors are unlikely in this 2dM-fused system. The Purcell factoris equal to 𝛾m/𝛾0 in Equation (2). From the resonant electric-fieldintensity, it was noted that the Nm/N0 ratio is approximately ten.Therefore, the observed large EF implies that the 𝜂m/𝜂0 ratio wasapproximately 25.8, reflecting a drastic change in the PL dynam-ics illustrated in Figure 5. This large ratio 𝜂m/𝜂0 suggests sup-pression of metal-induced quenching.2.4. Enhanced Raman ScatteringFigure 6a shows a typical Raman scattering spectra measured onand off the metasurface, which are shown as purple and orangelines, respectively. Raman signals coming from WS2 appearedat 320.2, 349.7 (E12g mode), and 415.0 cm−1 (A1g mode), whereasthose from MoS2 are seen at 373.4 and 395.2 cm−1 (arrows). Al-though these are slightly shifted from the Raman peaks of theas-grown monolayer (Figure S5, Supporting Information), theyare almost consistent with previous studies.[44,49] Interestingly,Raman scattering at 518 cm−1 from Si also became prominent.This is nontrivial because the top layer was Au and Si was lo-cated under the Au film. Nevertheless, the Si line was clearly ob-served. The Raman scattering signals of the TMDC layer off themetasurface were 11.2-fold decreased compared to those on theAdv. Optical Mater. 2024, 12, 2303309 2303309 (6 of 9) © 2024 The Authors. Advanced Optical Materials published by Wiley-VCH GmbH 21951071, 2024, 17, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adom.202303309 by National Institute For, Wiley Online Library on [17/06/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advopticalmat.dewww.advancedsciencenews.com www.advopticalmat.deFigure 6. Enhanced Raman scattering by the metasurface. a) EnhancedRaman spectra measured on the metasurface (purple) and off the meta-surface (orange), i.e., on the flat Au film. The former is much more promi-nent than the latter. b) Raman scattering of a Si wafer.metasurface. The Raman signals of the TMDC layer off the meta-surface were hardly detected under the same measurement con-ditions as those on the metasurface.As a reference to the enhanced Raman scattering of Si,Figure 6b shows an ordinary Raman scattering spectrum of a Siwafer, displayed in a comparable scale with Figure 6a. The Ra-man signal of Si comprising the metasurface was 6.7-fold largerthan that of the Si wafer. In the perforated structure of the meta-surface, the volume of Si is reduced by 36.5%; therefore, the netincrease in the Raman signals is estimated to be 10.6-fold, beingone order of the signals. This magnification of the Si line is as-cribed to the metasurface emittance at this wavelength, which isestimated to be approximately 50% in Figure 3b, and to the lo-cal electric fields at 532 nm (Figure 2e). When comparing the Silines off the metasurface and those of Si wafer, the latter was 1.66-fold larger; this is reasonable because Au film of 30 nm thicknesscovered the Si layer off the metasurface and reduced the incidentlaser power in the Si layer.3. ConclusionWe experimentally demonstrated a highly PL-enhancing com-pound system comprising the plasmo–photonic metasurfacefused with the atomic-layer 2dM, which was nearly WS2 mono-layer. The maximum PL intensity was as large as 1033-fold atroom temperature, compared with that of the 2dM on the ref-erence flat Au film. This result evidences that this platform re-alizes efficient PL emissions from the 2dM with suppressing PLquenching due to the contacting Au layer. We moreover deter-mined that the confocal image of the enhanced PL visualizesa quantum-mechanical coherent state through the analysis forphoton number distribution using the Poisson distribution, andrevealed that this 2dM–metasurface compound system can effi-ciently function as a quantum light source. The dynamics of theenhanced PL was studied by analyzing the decay time. Conse-quently, the two radiative components were determined to decayat 18 ps and 4.4 ns, respectively. These two components were at-tributed to the energy degeneracy of the A exciton in the TMDCmonolayer. The ns component manifests itself the intervalleytransition. Owing to the high emittance, which is a macroscopicproperty of the metasurface, the enhanced PL was primarily inde-pendent of the local nanostructures. This feature is distinct fromthat in the related studies focusing on local hot spots. Thanks tothe high emittance, the Raman scattering of the TMDC mono-layer was also found to be enhanced.4. Experimental SectionNanofabrication of Plasmo–Photonic Metasurfaces: Nanofabricationwas performed using electron-beam lithography. The nanopatterns weredrawn using a high-resolution electron-beam-drawing machine (JBX-6300FS, JEOL, Japan), and dry etching of the SOI layer was conductedusing a BOSCH instrument (MUC-21 ASE-SRE Sumitomo Precision Prod-uct, Japan).After the nanofabrication, metal deposition was conducted as follows.An adhesion Ti layer with thickness of 0.8 nm was normally deposited.Subsequently, an Au layer with thickness of 30 nm was formed via normaldeposition. The stacked complementary structure of Au was confirmedin a section-view SEM image.[33] A wide-view SEM image is provided inFigure S6 (Supporting Information) to confirm structural uniformity ofthe metasurface.Growth of the TMDC Atomic Layer: For the 2dM, monolayer WS2 filmswere grown on c-plane sapphire substrates via cold-wall chemical vapordeposition at 800 °C and 50 Torr for 60 min. H2S and WOCl4 were usedas sulfur and tungsten precursors, respectively. The flow rate of H2S was 1sccm, the feed rate of the N2 carrier gas through the WOCl4 canister was300 sccm, and the temperature and pressure of the canister were main-tained at 45 °C and 760 Torr, respectively. The total N2 flow rate in thereactor was 2500 sccm. In addition, 0.1 sccm of O2 was injected duringthe deposition of WS2. Dragontrail glass, serving as a catalyst reservoir,was placed upstream of the sapphire substrate during growth to provideNa and K catalysts and promote the lateral growth of WS2 with enlargedgrain size. Because of the resident Mo in the growth chamber, Mo wasdetected in the Raman scattering (Figure S5, Supporting Information); Al-though the monolayer is WxMo1 − xS2, it was estimated that x ⩾ 0.9,[49] ofwhich the most was WS2; accordingly, the TMDC was simply refferred toas WS2.Transfer Process of the TMDC Atomic Layer: The as-grown monolayerWS2 films were transferred from the sapphire substrate onto the metasur-face. Before delamination, the WS2/sapphire sample was coated with poly-methyl methacrylate (PMMA) to support and protect the ultrathin WS2.Subsequently, the PMMA/WS2 stack was peeled off the sapphire surfaceby allowing deionized water to penetrate the interface between the sap-phire and WS2. Finally, the sample of PMMA/WS2 on the metasurfacewas immersed in acetone at 50 °C for 60 min to remove the PMMA andcleaned in isopropanol, followed by drying with a N2 blow. This procedurewas etchant-free and conducted in a similar manner to those in previousreports.[6,45]After the transfer, rapid temperature annealing was coducted. It startedat room temperature (25 °C); the temperature was increased to 300 °C andmaintained for 10 min under N2 gas flow at 120 sccm; finally, the samplewas cooled to the room temperature.Adv. Optical Mater. 2024, 12, 2303309 2303309 (7 of 9) © 2024 The Authors. Advanced Optical Materials published by Wiley-VCH GmbH 21951071, 2024, 17, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adom.202303309 by National Institute For, Wiley Online Library on [17/06/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advopticalmat.dewww.advancedsciencenews.com www.advopticalmat.deNumerical Implementations: Numerical calculations based on rigor-ous coupled-wave analysis[59] and scattering-matrix algorithm[60] wereconducted. The two algorithms were combined and employed to evalu-ate the optical spectra and resonant EM field distributions of the plasmo–photonic metasurfaces.[51] The permittivities of Au and Si were taken fromliterature,[61,62] and those of air and SiO2 were set to the representativevalues of 1.00054 and 2.1316, respectively.PL and Raman-Scattering Measurement: The PL of WS2 on the meta-surfaces were measured using a micro-PL setup for spectrum acquisitionand a confocal microscopy for spectrally resolved PL images. In the micro-PL setup, a single-mode continuous-wave laser light at a wavelength of532.0 nm was focused on the WS2 monolayer on the metasurface using a50× objective lens with numerical aperture (NA) of 0.55 (M Plan Apo, Mit-sutoyo, Japan) and a working distance of 13.0 mm. The laser power wasset to 0.5 mW at the entrance of the objective lens. The PL from the WS2film was collected using the objective lens, passed through a monochro-mator (ISO-160, Teledyne Princeton Instruments, Trenton, NJ, USA) andacquired using a cooling charge-coupled device camera (ProEM1024HS,Teledyne Princeton Instruments).The Raman scattering was measured using the same setup as that usedfor the PL measurements. The input laser power was set to 15 mW. Theincident laser light was filtered using line-sharped filters passing wave-lengths of 532 ± 2 nm. The scattered light was filtered using a Raman filterwith optical density of six for the incident light at 532 nm. In this setup,Raman shift larger than 250 cm−1 was observed, as shown in Figure 6.For the confocal PL imaging, a laser-scanning upright confocal micro-scope (Stellaris 5, Leica Microsystems, Wetzlar, Germany) was employed.A 5× objective lens of NA of 0.15 was used to acquire the wide confocal PLimage in Figure 4b. The wavelength of excitation laser pulses of ps widthand several tens of MHz repetition was set to 514.0 nm. Then, the lat-eral resolution was 1.748 μm. The confocal images were acquired usinga photon-counting detector, which recorded photon numbers in a point-to-point manner; in total, the confocal images were formed by scanningthe laser spot. By changing the gate time of the detector, the temporalgrowth of the PL was measured in the ns range of 0.5–11 ns, as shown inFigure 4e.For the ps range, the PL decay time measurements were conductedusing a pulsed ps laser (MIRA-OPO-X ps, Coherent, Santa Clara, CA,USA) with 76-MHz repetition, an emission wavelength of 540 nm, andthe pulse width of 2 ps in the laser system. The pulsed laser light was fo-cused using a 50 × objective lens of NA of 0.8 and a working distance of1.0 mm (MPFLN50X, Olympys, Tokyo, Japan). The PL was collected us-ing the same objective lens. The PL in ps ranges was acquired using asynchronously scanning streak camera (C5680, Hamamatsu Photonics,Hamamatsu, Japan) attached to a monochromator (250is, Chromex, Al-buquerque, NM, USA).Theoretical Evaluation of the Purcell Effect: When evaluating the Purcellfactors shown in Figure S4 (Supporting Information), the excitons in theWS2 layer were approximated as two-level energy quantum systems. TheirPurcell factors were calculated using the macroscopic quantum electro-dynamic theory. The excitation created by the electric dipole source wasrelated to the quantum mechanical terms with the relaxation of the exci-ton. In real terms, the relaxation of the exciton was obtained by solvingthe Maxwell equations for a point dipole excitation in a complicated en-vironment, which included the EM response of the different materials. Acommercial finite-difference time-domain software (Ansys, Canonsburg,PA, USA) was used to solve the Maxwell equations. The optical responsesof the different materials were determined from their experimentally mea-sured dielectric permittivity.[62]Physically, the Purcell factor value, Γ(r, 𝜔), represents the enhancementor inhibition of the relaxation rate of the exciton when the WS2 mono-layer is placed in a nanostructured environment and compared with thereference relaxation value of the WS2 monolayer. A reference value wasevaluated for the case where the WS2 layer was placed in a homogeneousmedia. In the reference media, the WS2 excitons follow an exponential re-laxation exp (− t/𝜏ref) from the excited state to the ground state, where 𝜏refis the reference lifetime. The presence of the nanostructured environmentaccelerates the relaxation by the Purcell factor value to exp (− Γt/𝜏ref). ThePurcell factor is given by Γ = 𝜏ref/𝜏NS, where 𝜏NS is the lifetime of theexciton in the nanostructured environment.Supporting InformationSupporting Information is available from the Wiley Online Library or fromthe author.AcknowledgementsM.I. acknowledges a partial support by JSPS KAKENHI Grant numberJP20K21134 and the support system for curiosity-driven research in NIMS.T.K. thanks a support of Innovative Science and Technology Initiative forSecurity, Grant Number JPJ004596, ATLA, Japan. Nanofabrication was con-ducted in part at the Namiki Foundry and Nanofabrication Platform inNIMS. 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See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advopticalmat.de 2D-Material-Fused High-Emittance Plasmo9040�Photonic Metasurfaces 1. Introduction 2. Results and Discussion 2.1. 2dM-Fused Metasurfaces 2.2. Electromagnetic Resonances of Metasurface 2.3. Photoluminescence Enhancement in the 2dM-Fused System 2.4. Enhanced Raman Scattering 3. Conclusion 4. Experimental Section Supporting Information Acknowledgements Conflict of Interest Data Availability Statement Keywords