Unidirectional Enhanced Emission from 2D Monolayer Suspended by

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Unidirectional Enhanced Emission from 2D Monolayer Suspended by Dielectric Pillar Array Xianyu Ao,*,† Xinan Xu,† Jinwu Dong,† and Sailing He*,†,‡,§ †

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Centre for Optical and Electromagnetic Research, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510006, China ‡ National Engineering Research Center for Optical Instruments, Centre for Optical and Electromagnetic Research, JORCEP, Zhejiang University, Hangzhou 310058, China § School of Electrical Engineering, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden S Supporting Information *

ABSTRACT: Monolayers of transition metal dichalcogenides show great promise for optoelectronic devices as atomically thin semiconductors. Although dielectric or metal nanostructures have been extensively studied for tailoring and enhancing emission from monolayers, their applications are limited because of the mode concentrating inside the dielectric or the high optical losses in metals, together with the low quantum yield in monolayers. Here, we demonstrate that a metal-backed dielectric pillar array can suspend monolayers to increase the radiative recombination, and simultaneously, create strongly confined band-edge modes on surface directly accessible to monolayers. We observe unidirectional enhanced emission from WSe2 monolayers on polymer pillar array. KEYWORDS: transition metal dichalcogenide, two-dimensional materials, unidirectional emission, surface mode, hybrid metal-dielectric nanostructure, band edge

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In this work, we report the utilization of distributed, strongly confined modes on top of dielectric pillars for enhancing emission from monolayers of tungsten diselenide (WSe2). The two-dimensional dielectric pillar array is further backed by a noble metal film to form a hybrid metal-dielectric structure. The integration of monolayers on top of the pillar array thus allows the monolayers to be suspended to increase the radiative recombination, and furthermore, to be at the right position of the optical modes. The dispersion of the hybrid structure can enable the emission to be truly unidirectional. We observed over 30 times photoluminescence enhancement (relative to monolayers on a SiO2/Si substrate) and unidirectional emission with a divergence angle of 2° along the surface normal direction. In this approach, the period of the twodimensional array is on the order of the excitonic emission wavelength, and thus, low-cost techniques like soft lithography can be used for fabrication. WSe2 monolayers possess good luminescence at room temperature. As they can generate light emission via electrical carrier injection,24 here we focus on the manipulation of the emission of radiation. We couple WSe2 monolayers to the hybrid metal-dielectric structure as depicted in Figure 1a, where the hybrid metal-dielectric structure consists of a square

ransition metal dichalcogenide (TMDC) monolayers are direct bandgap semiconducting materials in the twodimensional limit, bearing unique properties such as a strong excitonic emission and large exciton binding energy, which make them impressive for a broad range of optoelectronic applications.1 Being atomically thin, TMDC monolayers have been integrated with various plasmonic2−6 and photonic structures7−15 to tailor and enhance their excitonic light emission. Although plasmonic nanostructures support localized electromagnetic modes on their surface which can enhance the light absorption and emission, their application is limited due to the high optical losses in metals together with the low quantum yield16 in TMDC monolayers, and consequently, high-Q dielectric cavities are preferred instead in the recent demonstration of monolayer excitonic lasing. 17−20 For dielectric structures, however, the photonic modes are usually concentrated inside the structure, and in order to enhance the optical mode overlap, the monolayer has to be embedded inside the cavity, or be placed in close proximity to the maximal field intensity by thinning the dielectric membrane, whereas the latter is susceptible to damage.21 In addition, the supporting substrate may suppress the luminescence of monolayers, and it is found that a suspended monolayer can show a significant increase in the radiative recombination of excitons.8,22 On the other hand, the directionality of emission is also important.12,23 Emission via strongly localized optical modes or directional Mie scattering of a single particle, in general, does not exhibit high directionality. © XXXX American Chemical Society

Received: July 26, 2018 Accepted: October 3, 2018 Published: October 3, 2018 A

DOI: 10.1021/acsami.8b12701 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Letter

ACS Applied Materials & Interfaces

Figure 1. Metal-backed dielectric pillar array and its band-edge mode. (a) Schematic of suspended WSe2 monolayer supported by a periodic array of dielectric pillars on a gold film. (b) SEM image showing the SU-8 pillar array on a gold-coated silicon substrate. The pillars (top diameter d = 390 nm, height h = 460 nm) are arranged in a square lattice (the center to center spacing a = 720 nm). (c) Calculated zeroth-order reflection spectra of the pillar array on gold to show the dispersion and band edge for TE and TM polarizations. The band-edge resonance is located at normal incidence (marked with a small red circle). (d) Electric field intensity distribution of the band-edge resonance in a unit cell. The blue to red color map illustrates the localized electric field intensity distribution around the top of the pillar in a unit cell. (e) Calculated reflection at normal incidence. The sharp deep dip, which matches the A exciton emission energy of CVD-grown WSe2 (∼1.63 eV), corresponds to the band-edge resonance. The presence of the WSe2 monolayer shifts the band-edge resonance to the longer-wavelength side.

lattice of SU-8 pillars backed by a gold film. SU-8 (Microchem) is epoxy-based, UV-curable polymer, with high optical transparency in visible and near IR regions. Soft imprinting method25 was used to fabricate the SU-8 pillar array (see the Supporting Information). Briefly, a thin SU-8 film on a gold-coated silicon substrate was imprinted by a poly(dimethylsiloxane) (PDMS) stamp which contained the negative surface structure of the desired pillar array. The finished pillar shows a tapered shape at the bottom (Figure 1b), resulting from the silicon template used for casting PDMS stamps. The pillar has a top diameter d = 390 nm, height h = 460 nm, and the center to center spacing a = 720 nm. These structure parameters of the pillar array were designed through a numerical optimization. The gold film beneath acts as a mirror to redirect the incident field and the pillar array induces grazing diffraction. With an optimized pillar height one can achieve a sharp band-edge resonance associated with strong field enhancement on top of the pillars, while the resonance peak can be tuned to the exciton emission wavelength λ0 by adjusting the pillar diameter (for a given spacing a slightly shorter than λ0). To map out the band structure of the SU-8 pillar array on gold, angle-dependent zeroth-order reflection spectra (Figure 1c) were calculated by using the scattering matrix method.26 The dispersion bands are manifested by the reflection minima (dark color), resulting from the incident light being coupled into bound modes by Bragg scattering. The bare hybrid metal-

dielectric structure possesses a band-edge mode at normal incidence (marked with a small red circle), for which the electric field intensity distribution shows localized enhancement around the top surface of the dielectric pillar (Figure 1d). The electric field is predominantly polarized in the plane to allow efficient coupling of the in-plane excitons of monolayers with the band-edge resonance. This surface mode mimics dipolar surface plasmon resonances in terms of spatial localization while staying away from metal. Due to the spatial periodicity of the pillar array, such localized enhancements are distributed over a large area on the structure surface, and the field enhancement |E|2/|E0|2 can be over 2000 (Figure S1). When the incident angle deviates from 0° (near the band edge), the slope of the dispersion band (the group velocity) increases significantly, leading to a high local density of states (LDOS) only in a small angle range around 0°, as LDOS is inversely proportional to the group velocity. This band-edge feature will be responsible for the unidirectional enhanced emission along the surface normal, since LDOS determines the intensity of radiation. The simulated reflection spectra of the bare hybrid metaldielectric structure at normal incidence are presented in Figure 1e as the red curve. The dip at 720 nm is attributed to Rayleigh-Wood anomaly, and the dip at 950 nm is a surface plasmon mode localized at the pillar/gold interface. The sharp deep dip at 753 nm corresponds to the target band-edge resonance, and only this resonance spectrally overlaps with the B

DOI: 10.1021/acsami.8b12701 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Letter

ACS Applied Materials & Interfaces WSe2 A exciton emission energy (∼1.63 eV). We then modeled the WSe2 monolayer as a 0.7 nm thick layer with its complex refractive index from spectroscopic ellipsometry measurement.27 The presence of the WSe2 monolayer on the pillar array causes a red shift of the band-edge resonance to 763 nm, and also broadening of the resonance line width. This broadening is primarily due to the added losses from the monolayer. The dip at 950 nm is almost stable against the monolayer since its mode field distribution is far away from the monolayer. We transferred chemical vapor deposition (CVD)-grown WSe2 monolayer from the growth substrate (sapphire) onto the pillar array by using water-soluble polymer film28 as a sacrificial mediator. The wet transfer process introduced some air bubbles that might result in broken monolayers. The suspension of monolayers is clearly visible by the color contrast near the cracked gaps (Figure 2a). The rough surface between pillars is attributed to polymer residual after imprinting. The unshifted Raman peak of WSe2 over the pillar array suggests that no strain was induced by the transfer process (Figure S2). Figure 2b shows the measured reflection spectra at normal incidence. The band-edge resonance of the bare hybrid metaldielectric structure was at 752 nm (with a resonance line width ∼4 nm), and it was shifted to 760 nm resulting from the presence of WSe2 monolayers, in good agreement with simulation. The kink in the blue curve (marked by an arrow) is attributed to the noncomplete coverage of WSe2 monolayers. Photoluminescence (PL) measurements were carried out with a 532 nm pump laser at an incident angle of 45°. Figure 2c shows the emission spectra along the surface-normal direction. As a reference, WSe2 monolayers from the same growth substrate were also transferred onto silicon with 300 nm thick thermally grown oxide and onto unimprinted SU-8 film on gold. The emission spectra from WSe2 on SiO2/Si and on SU-8 film/Au exhibit similar peak position and spectral width, indicating that they have similar substrate doping effect. The slightly weaker emission intensity from WSe2 on SU-8 film/Au than on SiO2/Si could be attributed to the interference effect which causes different local field enhancements. The spectral shape, peak position, and intensity of WSe2 excitonic emission were strongly modified by the metal-backed pillar array. On the SiO2/Si substrate, the emission peak is at 754 nm. On the metal-backed pillar array, the emission peak is at 760 nm, which coincides with the reflection dip at the band edge in Figure 2b. The modified emission peak has a full width at half-maximum (in wavelength) of about 7.5 nm, in contrast to 25 nm for emission from WSe2 on the SiO2/Si substrate. The modified spectral shape and peak position suggest that most of the emitted photons were coupled into the band-edge mode instead of emitting directly into free space. We observed a 32-fold enhancement in the peak intensity as compared with the emission from WSe2 on the SiO2/Si substrate, higher than the previous reported all-dielectric structures.9,11 To be more specific, the enhancement at pump absorption (relative to the SiO2/Si substrate) is about 1.7−3 times depending on the polarization, evaluated by integrating the simulated electric field intensity. Therefore, the main contribution to the observed enhancement occurs at the emission wavelength. As LDOS is high at the band edge, a large Purcell enhancement would be expected,29 and the

Figure 2. Enhanced photoluminescence by coupling with band-edge mode. (a) 30°-tilted SEM image showing the WSe2 monolayers suspended by SU-8 pillars. (b) Measured reflection spectra at normal incidence. The coverage of WSe2 monolayers was not complete, and therefore the blue curve is a mixture of reflection spectra from both covered and not-covered areas, resulting in a kink at the position marked by the arrow. (c) Emission collected in the surface-normal direction. The emission spectra of WSe2 are dramatically modified by the metal-backed pillar array, in comparison with WSe2 monolayers on a flat substrate (with 300 nm SiO2 on silicon, or 220 nm SU-8 on gold).

radiative recombination rate would get enhanced because the band-edge mode is confined away from the lossy metal. To confirm that the emission enhancement takes place at the band edge, we performed angle-resolved photoluminescence and reflection measurements near the normal direction of the sample surface (Figure S3). The symmetry about 0° and the emission peak at 0° for the angle-resolved spectra in Figure 3a indicate that the highest emission enhancement takes place at the band edge located at 0°. The emission is nonpolarized since the in-plane excitons of monolayers are randomly oriented, and therefore the emission into TE and TM polarizations shows roughly equal peak intensity. The efficient C

DOI: 10.1021/acsami.8b12701 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 3. Directional emission from suspended WSe2 monolayers. (a) Angle-resolved photoluminescence spectra measured without polarizer and with polarizer for TM and TE polarizations. (b) Angle-resolved reflection spectra under illumination of 45°-, TM-, and TE-polarized light. One lattice vector of the square lattice was aligned parallel to the plane of incidence, and the collection probe was scanning angularly in the plane of incidence. The reflectance data are not available for incident angles within ±1° because the incident beam was blocked by the collection probe.

a pillar of elliptical shape or a pillar-dimer in the unit cell, the band edges of the two polarizations can be spectrally separated, and the one with strong field confinement can be tuned to match the target emission wavelength so that only one polarization gets enhanced (Figure S5). To summarize, we have introduced a hybrid metal-dielectric platform that can create band-edge resonances with strongly confined optical modes on top of the dielectric pillars and thus directly accessible to monolayers integrated on top. On the basis of polymer pillars fabricated by soft imprinting on a gold film, we demonstrated unidirectional enhanced emission from suspended monolayers of WSe2. As the band-edge surface resonance arises from the periodicity of the dielectric nanostructures, it can be tuned over a wide wavelength range to match the exciton emission energy of various TMDC monolayers, by simply changing the spacing and the size of dielectric pillars, and combing with appropriate highreflectivity metal films. We expect this hybrid platform will benefit applications in two-dimensional light-emitting devices.

coupling of emission from WSe2 monolayers into the bound modes of the hybrid structure is verified by the one-to-one correspondence between the emission bands (Figure 3a) and the reflection minima (Figure 3b), and also the weak background emission intensity (especially for the TE polarization). Also note that the measured dispersion behavior in Figure 3b (see Figure S4 for the case without WSe2) agrees well with simulations. At the peak wavelength, the photons are emitted primarily within ±1° around the surface normal (Figure 4), accomplishing the truly unidirectional emission (there is no emission into the supporting substrate due to the gold film beneath). Such a platform with unidirectional enhanced emission property can be useful for many applications.30 Additional control over the polarization can be achieved by unit cell structure design. For example, by using

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b12701. Device fabrication, calculated electric field intensity distribution, PL and Raman mapping of WSe 2 monolayers on pillar array, setup for angle-resolved optical measurements, angle-resolved reflection spectra without WSe2, calculated zeroth-order reflection spectra of a pillar-dimer array on gold to show polarization control (PDF)

Figure 4. Angle distribution of photoluminescence at the emission peak, showing a divergence angle of 2° around the surface normal. D

DOI: 10.1021/acsami.8b12701 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (X.A.). *E-mail: [email protected] (S.H.). ORCID

Xianyu Ao: 0000-0003-1987-849X Sailing He: 0000-0002-3401-1125 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Science Foundation of China (61204074 and 91233208) and National Key Research and Development Program of China (2017YFA0205700). X.A. thanks Y.Z. Jiang, X.X. Chen, Prof. H.J. Chen, and Prof. Y.Y. Meng for help with PL/Raman mapping.

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DOI: 10.1021/acsami.8b12701 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX