Simultaneous Surface-Enhanced Resonant Raman and Fluorescence

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Simultaneous Surface-Enhanced Resonant Raman and Fluorescence Spectroscopy of Monolayer MoSe2: Determination of Ultrafast Decay Rates in Nanometer Dimension Yexin Zhang, Wen Chen, Tong Fu, Jiawei Sun, Daxiao Zhang, Yang Li, Shunping Zhang, and Hongxing Xu Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.9b02425 • Publication Date (Web): 20 Aug 2019 Downloaded from pubs.acs.org on August 25, 2019

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Simultaneous Surface-Enhanced Resonant Raman and Fluorescence Spectroscopy of Monolayer MoSe2: Determination of Ultrafast Decay Rates in Nanometer Dimension

Yexin Zhang, † Wen Chen, † Tong Fu, † Jiawei Sun, ‡ Daxiao Zhang, † Yang Li, † Shunping Zhang, *, † Hongxing Xu, *, †, ‡ †School

of Physics and Technology, Center for Nanoscience and Nanotechnology, and Key

Laboratory of Artificial Micro- and Nano-structures of Ministry of Education, Wuhan University, Wuhan 430072, China. ‡The

Institute for Advanced Studies, Wuhan University, Wuhan 430072, China.

Corresponding Author *E-mail: [email protected]. Phone: +8627 68752219. *E-mail: [email protected]. Phone: +8627 68752253.

ACS Paragon Plus Environment

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ABSTRACT: The fact that metallic nanostructures are an excellent light receiver and transmitter connects the underlying principles of two widely applied optical processes: surface-enhanced Raman scattering (SERS) and surface-enhanced fluorescence (SEF). A comparative study of SERS and SEF can eliminate the typical unknown quantities of the system and reveal important parameters that cannot be accessed by conventional techniques. Here, we use this simultaneous SERS and SEF technique in a monolayer MoSe2 coupled plasmonic nanocavity. After optimizing the spatial and the spectral overlap between excitonic and plasmonic resonances, the SERS and SEF enhancement factors can exceed 107 and 6000, respectively, at the same time on the same nanocube. The comparison of the SERS and SEF enhancements allows the estimation of the ultrafast total decay rate of the bright exciton in monolayer MoSe2 in the nanocavity down to tens of femtoseconds, which is otherwise hard to realize using time-resolved techniques. KEYWORDS: surface-enhanced Raman scattering, surface-enhanced fluorescence, plasmonic nanocavity, light-matter interaction, transition metal dichalcogenides, total decay rate.


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Introduction By virtue of the localized surface plasmon resonances, metallic nanostructures behave as optical nanocavities/antennas capable of confining light to nanoscale1-3. This leads to orders of magnitude enhancement of the electromagnetic field, which usually happens in a narrow gap of coupled nanostructures4. Such light confinement and field enhancement features open the exciting possibility of boosting light-matter interaction at the nanoscale via coupling of the plasmonic modes with Raman-active, luminescent or nonlinear nanomaterials/emitters. This facilitates various remarkable phenomena, including single-molecule surface-enhanced Raman scattering (SERS)5-9, surface-enhanced

fluorescence

(SEF)10-14,

exciton-polariton

(strong

coupling)

at

room

temperature15-18, as well as the enhanced non-linearity,19-21 etc. Recently, transition metal dichalcogenides (TMDs) have received considerable attention due to their distinctive optical properties and flexible features for fabrication22-25. As a layered crystal spacer, TMDs flakes also facilitate the fabrication of robust plasmonic nanocavities with a gap-width down to sub-nm26,

27.

These nanocavities make the TMDs an ideal and promising

platform to couple emitters with nanocavities for various novel applications. For instance, the TMDs-nanocavity coupling enables a straightforward orientation matching between the lattice vibrations of the MoS2 and the plasmonic field components. This enables the quantum-limited plasmonic field enhancement to be probed via quantitative SERS study27. As luminescent emitters with a large transition dipole moment, TMDs can also be incorporated with nanocavities for plasmon-exciton interaction when the excitonic and plasmonic resonances are spectrally matched. This results in the formation of plexcitons (strong coupling) at room temperature28, 29, maximum brightness of fluorescence in the intermediate coupling regime30 or a tremendous fluorescence enhancement in the weak coupling regime via the double resonances of the plasmonic modes31, 32. However, previous research rarely pays attention to the comparative study of SERS and SEF processes in TMDs, since the Raman peaks are always overwhelmed by fluorescence33. Or alternatively, the luminescence of excitons is intentionally quenched for SERS studies27. Developing a reliable platform to realize comparable spectral intensities of SERS and SEF could


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gain an insight into novel applications due to the basic connection between SERS and SEF

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34, 35.

Actually, a unified treatment of these two processes has been developed in theory36, 37, and has further been exploited to experimentally estimate the modified decay rates of emitters38. However, so far this technique is limited to the study of dye molecules, with large uncertainty in the extracted decay rates because of the statistics over different nanocavities. Such uncertainty can be eliminated if one can measure accurately the enhancement factor of SERS and SEF in response to the plasmonic resonance in one nanocavity. Here, we show how to implement simultaneous SERS and SEF measurement of a monolayer (1L) of MoSe2 (a typical TMDs) in a single plasmonic nanocavity, to determine the ultrafast decay rate of excitons in response to the plasmonic resonance. This is accomplished by developing a robust MoSe2-spaced nanocube-over-mirror (MoSe2-NCOM) hybrid system and the plasmon-scanned technique27, 29, 30, which controls the redshift of plasmonic resonance by successively coating the MoSe2-NCOM surface with Al2O3 layers. As shown in Fig. 1a, the MoSe2-NCOM nanocavity consists of an Al2O3-sealed Ag nanocube on an ultra-smooth gold film separated by a 1L MoSe2 flake. It supports a cavity-plasmon mode (Fig. 1b-d) with a magnetic dipole emission pattern, which strongly amplifies the lattice vibrations and at the same time the spontaneous emission of excitons in the MoSe2. To tune quantitatively the plasmonic enhancement on an individual MoSe2-NCOM nanocavity, we redshift the cavity plasmon mode that eventually scans over the excitation laser at 785 nm, the A-exciton resonance at 786 nm and the Raman lines around ~790 nm. This is realized by successive depositing an Al2O3 layer onto the sample by atomic layer deposition. For the optimized spectral matching, the maximal SEF enhancement is as high as ~6000 times, and the SERS enhancement exceeds ~107 times. Moreover, we show how to use the ratio of the SERS to SEF enhancement factors (EF) to indirectly determine the nanocavity-modulated decay rate of the 1L MoSe2. The resulting lifetime of the total decay process varies from 22 to 280 fs, depending on the local density of states (LDOS) of the NCOM cavities, entering an ultrafast time regime that is intractable for conventional time-resolved methods. Our results expand the interest for developing a well-defined platform for further comparative studies of plasmon-enhanced processes and provide novel insights for their wider applications in other nanomaterials.


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Figure 1. MoSe2-NCOM nanocavity/antenna system for SERS and SEF. (a) 3D schematic of a MoSe2-NCOM system, the inset schematically shows its cross-section. (b) Simulated scattering spectrum of a MoSe2-NCOM without (dashed blue) and with (solid red) a 14-nm-thick Al2O3 surface coating. For comparison, a PL spectrum of a 1L of MoSe2 on quartz, excited by a 532 nm laser (yellow shaded). (c, d) Surface charge and electric field distributions at 785 nm, which is also the laser line.

Photoluminescence (PL) of 1L of MoSe2 on quartz (yellow shaded in Fig. 1b) excited by a 532-nm laser shows a single Lorentz peak at 786 nm with a bandwidth of 20 nm, resulting from radiative recombination of A-excitons39. Under incident excitation by a 785 nm continuous wave laser that spectrally matches with the A-exciton resonance, excitation/absorption enhancement of the PL and resonant Raman scattering can be accomplished simultaneously. Furthermore, we positioned the 1L MoSe2 into the NCOM nanocavities whose plasmonic gap modes were tuned to overlap spectrally with the A-exciton resonance (Fig. 1b). This gives rise to orders of magnitude amplification of both the laser excitation as well as the PL and Raman emission intensities via plasmonic enhancement, which is one of our goals in this article. To fabricate the MoSe2-NCOM nanocavity system, an ultra-smooth gold layer with a


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root-mean-square surface roughness of ~0.32 nm was first fabricated using the template method40. Then, 1L thick MoSe2 flakes were exfoliated onto the gold film using ultrasound in acetone after annealing the sample, forming a strong Au-Se interaction between the gold film and the bottom Se atoms of the MoSe2. Next, the sample surface was deposited with a ~2-nm-thick Al2O3 layer, followed by a drop-casting of a solution of Cetyltrimethylammonium chloride (CTAC)-capped colloidal Ag nanocubes. Then, most of the CTAC ligands on the nanocubes were removed by ethanol cleaning. This was followed by the growth of a 5-nm-thick Al2O3 layer on the nanocubes to finish the fabrication (see more details in Methods). As a result, the Ag nanocubes were totally sealed by the Al2O3 layer to prevent the oxidation or sulfuration effects from the air efficiently, and also to suppress its morphologic deformation under the laser-heating. The individual MoSe2-NCOM can be identified from the dark-field image on a color CCD camera and dark-field spectroscopy. After all the optical measurements, the MoSe2-NCOM was finally characterized by a top-view scanning electron microscope (SEM) to determine the size of the nanocube. As shown in Fig. 1b, the simulated scattering spectrum of a MoSe2-NCOM nanocavity (with or without the Al2O3 surface coating) shows a strong plasmonic resonance peak, labeled as the M mode. For the MoSe2-NCOM nanocavity without Al2O3 surface coating, the M mode is on the blue side of the resonance of the A-excitons in the 1L of MoSe2. After adding a ~14-nm-thick Al2O3, it can be redshifted to match with the A-exciton thanks to the dielectric screening effect. To identify the M mode, we simulated its surface charge distribution at its resonance peak (786 nm), as shown in Fig. 1c. It shows that the opposite charges are symmetrically distributed on the two sides of the bottom facet of the nanocube, with a null node in the center area. Correspondingly, an antipodal charge pattern is formed on the gold film with respect to the nanocube. The M mode can be recognized as the lowest frequency cavity plasmon mode excited in a nanocavity with a flat and narrow gap29,

41-43.

It can be understood as a standing wave pattern,

resulting from the interference of cavity plasmons when they travel along the metal-dielectric-metal interface and are reflected from the nanocube edges. This means that the resonance wavelength of the M mode is proportional to the lateral size of the nanocavity, namely the size of the nanocube. Besides, the resonance wavelength and field enhancement of the M mode also relies critically on


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the vertical size of the cavity (the gap distance). Here the lateral and vertical sizes of the cavity are chosen to be 65 nm (the bottom size of the nanocube equals its top size, as determined by SEM) and ~3 nm (~0.7-nm-thick MoSe2 and ~2-nm-thick Al2O3), to control the resonance wavelength of the M mode on the blue side of the A-exciton resonance. The additional Al2O3 layer within the cavity increases the gap, which decreases the SERS intensity due to the weaker plasmonic enhancement27, but increases the SEF intensity owing to the reduced non-radiative rate occurring in a very narrow gap14. The charge distribution of M mode can also be considered as an effective current loop, forming a strong magnetic field localized in the nanogap region with a direction normal to the cross-section plane shown in Fig.1d. The M mode can be regarded as a magnetic dipole mode with the dipole-axis parallel to the gold film, showing a high directional out-of-plane light emission that facilitates the signal collection efficiency44. Moreover, it can be conveniently excited through the magnetic field component of the incident light with in-plane polarization. The back-scattering microscope configuration used in our experiment (see more details in Methods) is enough to fulfill the polarization and emission matching of such a nanoantenna, resulting in a high-efficiency excitation and collection. As shown in Fig. 1d, the M mode shows a strong electric field confined within the nanogap region that contains the MoSe2. This enables optical processes of the MoSe2 including Raman and PL to be largely enhanced. The bottom panel of Fig. 2 shows a Raman spectrum of a 1L of MoSe2 on quartz before (red line) and after (black line) a reactive ion etching (RIE, see Methods). This RIE treatment quenches the PL intensity to one-fortieth, but it is still observable. This makes the flooded Raman peaks of A1g at 238 cm-1 and E12g at 288 cm-1 visible in the spectrum. Similarly, the Raman spectrum of a 1L of MoSe2 tightly contacted to the gold film shows a weak Raman peak along with a strong PL background (the middle panel of Fig. 2). Compared with 1L of MoSe2 on quartz without any treatment (the red line in Fig. 2), the tight contact between 1L of MoSe2 and the gold film leads to a 30-fold reduction of the PL intensity due to the largely enhanced non-radiative decay rate, enabling us to simultaneously investigate the SERS and SEF. The top panel of Fig. 2 shows a Raman spectrum of a MoSe2-NCOM with the M peak at 778 nm (8-nm Al2O3 surface coating), which


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overlaps spectrally with the A-exciton peak. The results show that both the PL background and the Raman intensity are enhanced by orders of magnitude through the M mode, demonstrating a comparable spectral intensity between the PL and Raman. As shown in the Raman imaging at 288 cm-1 (the inset of Fig. 2), a bright diffraction-limited spot feature at the position of the nanocube further demonstrates that such amplified spectral intensity originates from the resonant enhancement of the MoSe2-NCOM nanocavity. By virtue of the plasmonic resonance enhancement, the profile of the Raman lines for the excitonic resonance condition39, including longitudinal acoustic phonon modes LA (M) ~ 4 LA (M), can be clearly identified. Additionally, a new phonon mode (labeled as the star) emerges at the blue side of the 2LA (M), which may originate from the breakdown of the Raman selection rules under the high gradient plasmonic field. This is of interest for further investigation but beyond the scope of this article.

Figure 2. Resonant Raman and fluorescence spectra of a 1L of MoSe2 coupled with different system. Raman spectra of a 1L MoSe2 in a NCOM nanocavity with an 8-nm-thick Al2O3 surface coating (blue line, top panel), on an ultra-smooth gold film (orange line, middle panel) and on quartz before (red line, bottom


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panel) and after RIE treatment (black line, bottom panel). All the spectra share the same scale bar. The inset shows the Raman imaging at 290 cm-1 of a MoSe2-NCOM, the scale bar is 2 μm. The integration of the E12g mode (red peak) and the shaded area beneath it are used to represent the Raman fluorescence intensities used for the calculation of the SERS and SEF enhancement factors.

To obtain the optimized SERS and SEF signals, the plasmonic resonance should spectrally match well with the A-exciton resonance of the 1L of MoSe2, as well as with the excitation and outgoing Raman wavelengths, reaching doubly resonant conditions. In the case of using single plasmonic resonance, the optimized enhancement also relies on the resonance bandwidth with respect to the wavelength difference between the incoming laser and the outgoing signal45. The smaller the laser-signal spectral distance is, the stronger the maximum signal enhancement will be. In our MoSe2-NCOM system, both the PL and Raman peak positions are close to the laser line (spectral separations ~20 nm), ensuring that both the excitation and emission are enhanced by the M mode (~50 nm bandwidth). To achieve the spectral matching, we gradually control the wavelength detuning of the M mode with respect to the A-exciton mode and also the laser and Raman lines. This enables a quantitative study of the SERS and SEF processes in response to the change of the plasmonic enhancement. Similar to our previous works27,

29, 30,

we used the plasmon-scanned

technique (Fig. 3a) to redshift the M mode. This is due to the increased dielectric screening effect by the high refractive index Al2O3. Fig. 3a shows typical dark-field spectra of a MoSe2-NCOM with Al2O3 surface coating thicknesses varying from 4 nm to 29 nm. It corresponds to a peak shift from 750 nm to 810 nm, a wavelength regime that almost covers the spectral profile of A-exciton resonance, laser and Raman wavelengths. In principle, we can also do the measurement on the nanocavities without Al2O3 surface coating. However, this would result in the risk of oxidation of the Ag nanocubes, so we tend to start the measurements from 4-nm-thick Al2O3 coating. An SEM image of the structure is shown in the inset of Fig.3a. The corresponding Raman spectra of the MoSe2-NCOM as a function of the Al2O3 surface thickness are shown in Fig. 3b. It shows an increased (decreased) intensity of both phonon modes and the PL background as the plasmonic resonance gradually approaches (redshifts away from) the A-exciton resonance.


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To quantitatively study the enhancement of the SERS and SEF processes, we determine the SERS and SEF enhancement factors (EFs) averaged over the NCOM cavity region EFSERS and EFSEF by:

EFSERS

I Rc A0  0 c IR A

(1)

c I PL A0 0 I PL Ac

(2)

EFSEF 

c where I Rc and I PL are the Raman and PL intensities of the 1L of MoSe2 in the NCOM,

respectively, which are taken by the integral area of peaks fitted by a Lorentzian function and the 0 shaded areas beneath the Raman peaks (yellow and red curves and rectangles shown in Fig. 2). I PL

is the reference PL intensity for EFSEF, which was taken from a pristine 1L MoSe2 on quartz (without RIE) instead of the MoSe2 on the Au film. This is because the Raman and PL intensity of the former are more uniform within one flake and are more repeatable among different samples, c compared to the latter. I PL is taken from an integration area that just below the E12g peak (shaded

pink area in the top panel of Fig.2), which is ~94% compared to the intensity at the PL peak position. Note that the PL peaks of the MoSe2 in the NCOM and on the Au film show a redshift 0 compared to that of the MoSe2 on quartz. Thus for a fairer calculation of the EFSEF, I PL is

obtained from the shaded pink areas (~100 cm-1) in the bottom panel of Fig. 2, whose intensity is also ~94% with respect to that at PL peak position (~30 cm-1), instead of the shaded blue area (Fig. 2) below the E12g peak. I R0 is the reference Raman intensity for EFSERS which should also be taken from the same pristine 1L MoSe2 on quartz but the Raman peaks are overwhelmed by the PL. To address this issue, we performed a mild RIE treatment on the 1L MoSe2 on quartz, which results in the reduced PL and hence the visible Raman peaks (black line in Fig. 2). Besides, to obtain EFSERS and EFSEF, the intensity enhancements calculated by the spectra should be normalized by the area where the signals emitted. For the reference spectra, the effective emission areas are equal to the collection area A0 ≈ 3.5 μm2 (due to the large-area excitation configuration, see Methods). On the


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other hand, the signal of a MoSe2-NCOM is predominately emitted from a volume of the plasmonic M mode that approximately equals the area of the nanocavity Ac, the square of the edge size of the nanocube (65 × 65 nm2).

Figure 3. Plasmon-scanned SERS and SEF measurements on a MoSe2-NCOM nanocavity system. (a) Dark-field scattering spectra of a MoSe2-NCOM system with an Al2O3 surface coating thickness varying from 4 nm to 29 nm. The inset shows a top view SEM image of the nanocube (with edge size about 65 nm) taken after all the optical measurements. The scale bar is 100 nm. (b) Raman map of the MoSe2-NCOM in response to the Al2O3 surface thickness. (c) SERS EF of E12g and A1g phonon modes and SEF EF as a function of Al2O3 surface thickness.

Fig. 3c shows the Al2O3 thickness dependences of the EFSERS of A1g and E12g phonon modes as well as EFSEF . Compared with EFSEF, EFSERS is increasing faster before reaching the maximum but then decreasing slowly. The maximum EFSERS of both phonon modes and EFSEF can reach ~107 and 6000. This corresponds to a plasmonic resonance at 786 nm. The linear power dependence of SEF (Fig. S1) suggests that the SEF can still be viewed as a weak excitation without saturation. The


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Al2O3 interlayer in the NCOM brings an additional gap distance and hence a smaller plasmonic enhancement. This gives rise to (i) lower EFSERS compared to our previous results of 108 obtained in MoS2-coupled atom-thick nanocavity system; (ii) stronger EFSEF due to a reduction of the dominant non-radiative processes occurring in such a narrow gap distance. (iii) a blue-shifted M mode in the blue side of the A-exciton resonance adapting to the plasmon-scanned techniques. Interestingly, the A1g phonon mode of the MoSe2 has an out-of-plane component in the Raman polarizability to match with the much stronger vertical plasmonic field, whose EFSERS is comparable with that of the E12g mode. This result is in line with the plasmon-enhanced resonant Raman spectroscopy of a MoS2 coupled with a nanocavity in the previous work26, but in contrast with our previous results from a similar system that almost avoids the excitation of the excitonic resonance27. Therefore, this effect may originate from the excitonic transition resonance, which affects the A1g phonon though exciton-phonon coupling, but it requires further investigation. Next, based on treating the SERS and SEF processes in the same context of modified spontaneous emission38, we show how the ratio of EFSERS to EFSEF can be used to determine the ultrafast decay rate of 1L of MoSe2 modulated by the plasmonic field. Fig. 4a and 4b show the Raman and fluorescence processes of 1L of MoSe2 with and without the presence of a plasmonic nanocavity. For a 1L of MoSe2 on quartz, its PL intensity is determined by a spontaneous emission 0 0 0 0 0 rate of  em .  em is associated with the excitation rate  exc and quantum yield Q0 =  r /  =  r

0

0 0 0 0 /(  r +  int ), where  r ,  int and

 0 represent the radiative, intrinsic non-radiative and total

decay rates of the 1L of MoSe2 on quartz, respectively46 (Fig. 4a). The coupling of the MoSe2 c 0 within an NCOM nanocavity leads to both the Purcell enhancement with a factor of  em /  em , with c  em being the modified spontaneous emission rate in the cavity, as well as the change of the c angular distribution of the light emission14, 44. In this case, the excitation rate  exc of the MoSe2 is

strongly modulated by local field E of the plasmonic M mode induced from the incident field E0 at 785 nm. This gives rise to the amplification of the absorption cross-section of the PL with a factor c 0 of Mexc = |E/E0|2 =  exc /  exc (Fig. 4b). As the spectral profile of the PL overlaps well with that of


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c c the M mode, the radiative decay rate  r and non-radiative decay rate  nr of the MoSe2 are also c enhanced by the local field. Note that  nr is also affected by the charge transfer between the

monolayer MoSe2 and the ultrasmooth gold film. This results in a variation of the quantum yield Qc c = r /

c

c c c 0 =  r /(  r +  nr +  int ) in the cavity with respect to the Q0 (Fig. 4b), with

 c being the

total decay rate of the MoSe2 in the presence of an NCOM nanocavity. Here the intrinsic 0 non-radiative decay rate  int is considered unaffected by the modulation of the nanocavity.

Therefore, EFSEF in our MoSe2-NCOM system can be expressed as:

EFSEF 

c c c M  em M  exc  rc  0 Q c M  exc   0 0 0 Q 0 0  exc 0  em 0  exc  r0  c

(3)

where ηM and η0 are collection efficiencies of a 1L of MoSe2 in an NCOM nanocavity and on quartz using a 0.8-NA objective in a backscattering configuration. ηM and η0 are assumed to be 75% and c 0 15%31. Here we define a radiative rate enhancement of Mrad =  r /  r

and an excitation

c 0 enhancement of Mexc =  exc /  exc . Then, Eq. 3 can be redefined as:

EFSEF 

M 0 M exc M rad c 0 

(4)

Similarly, the SERS process in the MoSe2 coupled with the NCOM nanocavities can also be treated in the framework of modified spontaneous emission as a two-step process47. First, the Raman excitation of the MoSe2 by the local field of the M mode shows an enhancement of Mexc at 785 nm compared with that by the incident field. Second, the oscillating Raman dipole moment (Raman polarizability) of the 1L of MoSe2 within the nanocavity is substantially modified by the local field at the Raman frequency, resulting in an enhancement of the Raman emission. Note that the emission enhancement induced by the Purcell effect behaves as a modification of the quantum yield, which is related to the competition between radiative and non-radiative processes. However, the Raman scattering process does not have such competition (Fig. 4a and 4b). Therefore, the emission enhancement of SERS is directly defined as Mrad, proportional to the square of the local


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field at the Raman frequency38. As a result, the SERS enhancement of the MoSe2 coupled with the nanocavity is determined by:

EFSERS 

M M M 0 exc rad

(5)

Based on the optical reciprocity theorem, MRad is comparable with MLoc4, 47, namely Mrad ≈ Mexc = |E/E0|2. We then have EFSERS ∝ |E/E0|4, which is the well-accepted E4 model4, 47. By defining the ratio REF = EFSERS/EFSEF, in combination with the Eq. 4 and 5, we can find an indirect way to estimate the total decay rate of a 1L of MoSe2 in the presence of a nanocavity:

  REF  REF c

0

 r0

(6)

Q0

It should be noted that Mrad and Mexc in Eq. 4 and 5 can only be canceled out to obtain Eq. 6 if the SEF and SERS processes are excited by the same plasmonic local field component. The PL of the MoSe2 results from the radiative recombination of A-excitons, in which the transition dipole moment of A-excitons is almost oriented towards the in-plane directions48 and thereby mainly responds to the in-plane local field of the nanocavities. Therefore, the EFSERS should be taken from the in-plane phonon mode E12g.27 Based on Eq. 6, the total decay rate

 c of A-excitons in a 1L of MoSe2 coupled with the

NCOM cavity as a function of the Al2O3 surface coating thickness are shown in Fig. 4c. In this case, REF in Eq. 5 was obtained from the EFSERS and EFSEF at the frequency of the E12g mode (Fig. 3c). The quantum yield Q0 of 1L of MoSe2 on quartz at room temperature was measured to be ~3.6% (see Supporting Information, Fig. S2), comparable to the previous measurement49. The corresponding effective radiative decay lifetime 1/  r

0

previously result50,

51.

was ~0.9 ns which was taken from the

The results show that the total decay lifetime of the 1L of MoSe2 1/ 

c

largely reduces from 280 fs to 22 fs as the SEF process gradually reaches its optimized spectral matching between the plasmon and the exciton.


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Figure 4. Determination of the decay rate of the 1L of MoSe2 in the NCOM nanocavity by unified treatment of the SERS and SEF processes. (a, b) Energy diagrams of fluorescence and Raman (SEF and SERS) processes of a 1L of MoSe2 (a) on quartz (b) in the NCOM nanocavity. The SEF and SERS are treated in the framework of modified spontaneous emission. They show the same excitation enhancement. The emission enhancement of the SEF results from the competition between the radiative and non-radiative processes, whereas the enhancement of the SERS process is purely determined by the radiative process. (c) The measured total decay rate of the MoSe2 (orange) and the in-plane LDOS (blue) in the NCOM cavity as a function of the Al2O3 surface thickness, showing a qualitative agreement. This suggests that the variation of the LDOS accounts for the variation of the total decay rate. The former is obtained by Eq. 6 based on the comparison of the SERS and SEF enhancement factors, and the latter is extracted from the center position (also the strongest point) of (d). (d) Map of in-plane LDOS within the NCOM nanocavity cross-section at the peak of the A-exciton (786 nm).


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Based on Fermi’s golden rule, the total decay rate of the MoSe2 depends critically on its surrounding electromagnetic environment. We simulated an in-plane LDOS map in the gap region of an NCOM nanocavity with a 14-nm-thick Al2O3 surface coating, where its M mode matches best with the A-exciton mode spectrally (Fig. 4d, Methods). It shows a pattern similar to its in-plane local field distribution (see Fig. S3), where the maximum LDOS enhancement of 3900 is distributed at the center region of the cavity. This LDOS pattern is in contrast with our previous result in the similar NCOM system30, where the strongest LDOS is at the corner region. This is due to the MoSe2 here is located at the bottom region of the gap of the NCOM (contacts with the Au film) instead of the center position between the nanocube and the Au film (see details in Fig. S3). To further clarify the relationship between the LDOS and

 c , we calculated the in-plane LDOS of the

nanocavity (obtained at the center position in Fig. 4d) as a function of Al2O3 coating thickness for comparison (orange curve in Fig. 4c). The result shows a ‘’ shape curve that follows the curve of

 c in response to the M mode, which suggests that the LDOS can account for the majority of the variation of

 c . This result also indicates that the quenching effect52 of exciton resonance

(including charge transfer) contributes a minor part to the total decay rate in the NCOM. Although the maximum in-plane LDOS enhancement of 3900 is comparable with the 6000-folds SEF enhancement obtained in the experiment, our simulations in Fig. 4c and 4d only treat the MoSe2 as a pure dielectric medium without considering the oscillator strength f of the A-excitons (f = 0, see details in Methods). As we have not found any dip or splitting in the dark-field spectra, the Purcell effect (SEF) observed in our experiments still falls in the weak coupling regime. In the SEF process the oscillator strengths f is weak but should not be equal to zero. To clarify the role of the oscillator strength f, we simulated its dependence on the maximum in-plane LDOS of the nanocavity at the optimized resonance enhancement (Supporting Information, Fig. S4). The results show that introducing only a very small f of 0.01 can make the in-plane LDOS increase dramatically to ~7400, which is enough to explain the measured PL enhancement. This suggests that EFSEF is not only determined by the plasmonic enhancement but also the probable contribution by a strong excitonic effect in the MoSe230.


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By substituting the maximum EFSERS of the in-plane phonon mode E12g into Eq. 5 based on the E4 approximation (Mexc ≈ Mrad), we can experimentally obtain the maximum plasmonic enhancement g = (η0(ηM)-1EFSERS)1/4= 36.6, which is larger than to the corresponding simulated results of 17.1 based on the in-plane local field distribution (Fig. S3 and Fig. S5). This difference may be due to the tiny wrinkle of MoSe2 that makes the in-plane phonon mode E12g to be excited by the much stronger out-of-plane plasmonic field. The roughness of the Au film may also create additional local ‘hotspots’ compared to the simulation which assumes a perfect flat film. Another possibility might be the charge transfer between the MoSe2 and the Au, which is sensitive to the in-plane electric field and can account for increasement in the measured SERS enhancement factor53.

Conclusions In conclusion, we have developed a 1L MoSe2 spaced NCOM nanocavity platform which allows simultaneous study of SERS and SEF processes of the MoSe2, and thereby enabling the extraction of the ultrafast total decay rate at the nanoscale. Based on a plasmon-scanned technique, the cavity-plasmon mode resonance at ~750 nm has been quantitatively redshifted to match best with the resonance of the A-excitons of the 1L of MoSe2 at ~786 nm. By exciting these overlapped resonances using a 785 nm laser, doubly resonant enhancement of both the SERS and SEF processes have been reached, with maximum signal enhancements approaching 107 and ~6000, respectively. Furthermore, by modeling SERS and SEF processes in the context of modified spontaneous emission based on the same in-plane plasmonic enhancement, we show how the enhancement-factor ratio of the SERS to SEF can be used to estimate the ultrafast total decay rate of the MoSe2 in the nanocavity. The resulting total decay lifetime ranges from 22 fs to 280 fs, which depends on the in-plane LDOS enhancement of the nanocavity. The direct measurement of such femtosecond process in a nanometer dimension is a difficult task for conventional time-resolved techniques. Our results gain a novel insight in the understanding and indirect measuring ultrafast processes of TMDs in the intense electromagnetic environment. Owing to the flexible and


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well-controlled resonance features of the nanocavity plasmons, our platform can be easily adapted to other nanomaterials and emitters to realize simultaneous investigation of different plasmon-enhanced optical processes.

Materials and Methods Fabrication of MoSe2-NCOMs. The gold film was prepared as previously reported. First, a 200 nm-thick gold film was deposited on a standard (100) silicon wafer after cleaning. Because of the poor adhesion between gold and the oxidized Si surface, the gold film can easily be peeled off using an optical adhesive (NOA 61, Norland Products) as a backing layer to reveal the ultra-smooth gold surface replicated from the silicon surface. Next, monolayer MoSe2 films were exfoliated from bulk MoSe2 crystals (SPI) using annealing and ultrasonic method. Briefly, the bulk MoSe2 on Au film was annealed at 200 °C with 8 h. Then the samples were immersed in the acetone solution with ultrasonic bath for 5 min. This removed the bulk flake of MoSe2, leaving the 1L MoSe2 that tightly contacts with the Au film. Then an Al film was deposited on the sample using electron beam evaporation, where evaporation on the sample started when the speed is stable at 0.5 Ǻ/s with 40 s duration to form ~ 2-nm-thick Al film. The Al film would be oxidized to a ~ 2-nm-thick Al2O3 film on the sample surface after exposing to the air several hours. Next, 60-nm CTAC-stabilized Ag nanocubes in purified water were drop-coated onto the areas containing single-layer MoSe2 films. Then, the samples were rinsed with ethanol and deionized water to largely remove the CTAC ligand. Finally, Al2O3 layers were grown on the sample surface with thicknesses varying from 4 nm to 29 nm using atomic layer deposition at 120 °C. To fabricate 1L MoSe2 on quartz with visible Raman peaks, a pristine 1L MoSe2 on quartz sample was put into the RIE chamber with Ar ion for 20 seconds. The power of RIE treatment is ~10 W, and the bias voltage is ~5 V.


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Spectroscopy. The SERS and SEF spectra were performed under ambient conditions using a Renishaw micro-Raman system with a backscattering configuration. A 785-nm continuous-wave laser was used for excitation. The size of the laser spot at the sample was ∼16 m2, focused by a 100× objective (Olympus, NA = 0.8). For the SERS and SEF measurements, a 1200 lines/mm grating was used with a spectral resolution of 1.2 cm-1. For the SERS and SEF measurements of the MoSe2 in the NCOM and on the Au film, the laser power was ∼0.36 μW. For the Raman measurements of 1L MoSe2 on quartz with and without RIE treatment, the laser powers were ~1.26 and 0.03 mW. For all the measurements, the integration time was 60 s, and the collection area was set to ∼3.5 m2 (1.8 m × 1.95 m). The Raman peak of Si (111) was used as the reference to calibrate the Raman shift and the laser power of each Raman measurement. All the Raman scattering and PL signals were collected without polarization selection. The dark-field scattering spectroscopy was performed using the same spectrometer, microscope, and configuration while removing the long-pass filter module. Through a commercial illuminator (Olympus BX 51), white light from the lamp was directed to a dark-field 100× objective with 0.8 NA to form uniform dark field illumination. The scattered signal of the NCOM (labeled as S) was collected by the same objective, which is then directed to the spectrometer with collection area set to ∼1 m2. The integration time was 20 s. The reference spectrum B was taken at the nearby bare MoSe2 region with the same setup. Then the dark-field scattering spectrum can be obtained by (S-B)/L, where L is the spectrum of the light source.

Simulation. COMSOL Multiphysics 5.2a was used to perform the full-wave electromagnetic simulations. Dispersive dielectric constants of silver and gold were determined from the experimental data by Johnson and Christy54, the refractive index of Al2O3 and CTAC were set to 1.5. The anisotropic in-plane relative permittivity of a monolayer of MoSe2 followed the Lorentz model


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 (E)     f

E0 2 E 2  E0 2  iE  0

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(7)

where ε∞ = 22.5 is the high-frequency contribution to the permittivity and f is the reduced oscillator strength of the exciton. E0 and Γ0 were taken from the absorption spectra of the MoSe2 layer on a silica substrate. The out-of-plane permittivity was set to a constant εout = 2.9. The silver nanocube was modeled with an edge length of 65 nm and the corners of the nanocube were rounded with a radius of 17.3 nm, taken from the SEM images. The thickness of a monolayer of MoSe2 was taken to be 0.7 nm, and the thickness of the CTAC shell was taken to be 1 nm.

Supporting Information SERS and SEF intensities of a MoSe2-NCOM as a function of excitation power; measurement of quantum yield of 1L MoSe2 on quartz; in-plane local field distributions of the M mode at different gap position; in-plane LDOS of an NCOM nanocavity as a function of the oscillator strength f of the A-excitons; detaild description of calculation of "hotspot" area. Acknowledgments We thank Huatian Hu for providing the simulation model for the calculation of the LDOS. This work was supported by the National Key R&D Program of China (Grant 2017YFA0303504), the National Natural Science Foundation of China (Grants 91850207, 11674255 and 11674256).

Conflict of interests The authors declare no competing financial interest.

Contributions S.P.Z. and H.X.X. conceived the idea. Y.X.Z. prepared the samples and performed the experiments under the guidance of W.C. and S.P.Z.. T.F. performed the theoretical simulations. J.W.S., D.X.Z.,


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and Y.L. helped prepare the samples. W.C., Y.X.Z., S.P.Z. and H.X.X. analyzed the data and wrote the paper.

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TOC-graphic:

In this letter, we realize the simultaneous study of SERS and SEF processes of a monolayer MoSe2 coupled into a plasmonic nanocavity. After optimizing the spatial and the spectral overlap between excitonic and plasmonic resonances, the SERS and SEF enhancement factors can exceed 107 and 6000, respectively, at the same time on the same nanocube. The comparison of the SERS and SEF enhancements allows the estimation of the ultrafast total decay rate of the bright exciton in monolayer MoSe2 in the nanocavity down to tens of femtoseconds, which is otherwise hard to realize using time-resolved techniques.


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In this letter, we realize the simultaneous study of SERS and SEF processes of a monolayer MoSe2 coupled into a plasmonic nanocavity. After optimizing the spatial and the spectral overlap between excitonic and plasmonic resonances, the SERS and SEF enhancement factors can exceed 107 and 6000, respectively, at the same time on the same nanocube. The comparison of the SERS and SEF enhancements allows the estimation of the ultrafast total decay rate of the bright exciton in monolayer MoSe2 in the nanocavity down to tens of femtoseconds, which is otherwise hard to realize using time-resolved techniques.


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MoSe2-NCOM nanocavity/antenna system for SERS and SEF. (a) 3D schematic of a MoSe2-NCOM system, the inset shows its cross-section schematic. (b) Simulated scattering spectrum of a MoSe2-NCOM without (dashed blue) and with (solid red) a 14-nm-thick Al2O3 surface coating. For comparison, a PL spectrum of a 1L of MoSe2 on quartz, excited by a 532 nm laser (yellow shaded). (c, d) Surface charge and electric field distributions at 785 nm, which is also the laser line. 147x86mm (300 x 300 DPI)


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Figure 2. Resonant Raman and fluorescence spectra of a 1L of MoSe2 coupled with different system. Raman spectra of a 1L MoSe2 in a NCOM nanocavity with an 8-nm Al2O3 surface coating (blue line, top panel), on an ultra-smooth gold film (orange line, middle panel) and on quartz before (red line, bottom panel) and after RIE treatment (black line, bottom panel). All the spectra share the same scale bar. The inset shows the Raman imaging at 290 cm-1 of a MoSe2-NCOM, the scale bar is 2 μm. The integration of the E_2g^1 mode (red peak) and the shaded area beneath it are used to represent the Raman fluorescence intensities used for the calculation of the SERS and SEF enhancement factors.


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Plasmon-scanned SERS and SEF measurements on a MoSe2-NCOM nanocavity system. (a) Dark-field scattering spectra of a MoSe2-NCOM system with an Al2O3 surface coating thickness varying from 4 nm to 29 nm. The inset shows a top view SEM image of the nanocube (with edge size about 65 nm) taken after all the optical measurements. The scale bar is 100 nm. (b) Raman map of the MoSe2-NCOM in response to the Al2O3 surface thickness. (c) SERS EF of E1 2g and A1g phonon modes and SEF EF as a function of Al2O3 surface thickness. 162x80mm (300 x 300 DPI)


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Figure 4. Determination of the decay rate of the 1L of MoSe2 in the NCOM nanocavity by unified treatment of the SERS and SEF processes. (a, b) Energy diagrams of fluorescence and Raman (SEF and SERS) processes of a 1L of MoSe2 (a) on quartz (b) in the NCOM nanocavity. The SEF and SERS are treated in the framework of modified spontaneous emission. They show the same excitation enhancement. The emission enhancement of the SEF results from the competition between the radiative and non-radiative processes, whereas the enhancement of the SERS process is purely determined by the radiative process. (c) The measured total decay rate of the MoSe2 (orange) and the in-plane LDOS (blue) in the NCOM cavity as a function of the Al2O3 surface thickness, showing a qualitative agreement. This suggests that the variation of the LDOS accounts for the variation of the total decay rate. The former is obtained by Eq. 6 based on the comparison of the SERS and SEF enhancement factors, and the latter is extracted from the center position (also the strongest point) of (d). (d) Map of in-plane LDOS within the NCOM nanocavity cross-section at the peak of the A-exciton (786 nm)


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Simultaneous Surface-Enhanced Resonant Raman and Fluorescence

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