Efficient cathodoluminescence of monolayer transitional metal dichalcogenides in a van der Waals heterostructure

Efficient cathodoluminescence of monolayer transitional metal dichalcogenides in a van der Waals heterostructure Shoujun Zheng1, Jinkyu So2, Fucai Liu3, Zheng Liu3, Nikolay Zheludev1,, and Hong Jin Fan1,2,

1Centre for Disruptive Photonic Technologies, Nanyang Technological University, 637371, Singapore 2Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, 637371, Singapore 3School of Materials Science and Engineering, Nanyang Technological University, 639798, Singapore S.Zheng and J. So contributed equally to this work. *Corresponding authors. Emails: NZheludev@ntu.edu.sg (N.Z.); fanhj@ntu.edu.sg (H.J.F.)

Abstract: Monolayer two-dimensional transitional metal dichalcogenides, such as MoS2, WS2 and WSe2, are direct band gap semiconductors with large exciton binding energy. They attract growing attentions for opto-electronic applications including solar cells, photo-detectors, light-emitting diodes and photo-transistors, capacitive energy storage, photodynamic cancer therapy and sensing on flexible platforms. While light-induced luminescence has been widely studied, luminescence induced by injection of free electrons could promise another important applications of these new materials. However, cathodoluminescence is inefficient due to the low cross-section of the electron-hole creating process in the monolayers. Here for the first time we show that cathodoluminescence of monolayer chalcogenide semiconductors can be evidently observed in a van der Waals heterostructure when the monolayer semiconductor is sandwiched between layers of hexagonal boron nitride (hBN) with higher energy gap. The emission intensity shows a strong dependence on the thicknesses of surrounding layers and the enhancement factor is more than 1000 folds. Strain-induced exciton peak shift in the suspended heterostructure is also investigated by the cathodoluminescence spectroscopy. Our results demonstrate that MoS2, WS2 and WSe2 could be promising cathodoluminescent materials for applications in single- photon emitters, high-energy particle detectors, transmission electron microscope displays, surface- conduction electron-emitter and field emission display technologies.

Keywords: transitional metal dichalcogenides; van der Waals heterostructures; cathodoluminescence; strain effect; 2D heterostructures

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Introduction Two-dimensional (2D) layered semiconductors, due to their weak interlayer van der Waals bonds, can be easily thinned down to atomic thickness by mechanical1 and chemical2 exfoliation methods, for example, graphene3 and hexagonal boron nitride (hBN)4. Monolayer transitional metal dichalcogenides with the formula of MX2 (M=Mo, W; X=S, Se, Te) are a type of unique semiconductors with narrow direct band gaps, large exciton binding energies, high optoconductivity, and high photoelectrochemical activity. Moreover, due to the inversion symmetry breaking, monolayer MX2 are widely employed for the study of valley polarization and spin-valley coupling5, 6. Recently, van der Waals heterostructures, composed of different 2D materials with unique band alignment and interlayer coupling, attract growing attentions not only for fundamental new physics but also for many potential applications such as tunneling transistors7, 8, 9 and light-emitting diodes10.
Cathodoluminescence, photon emission excited by a high-energy electron beam, is widely applied in the analysis of mineral compositions11, light emitting diodes12, 13, surface plasmon mapping14. Compared to photoluminescence excited by light, cathodoluminescence offers a much higher excitation energy allowing the study of wide band gap materials including diamond15 and hexagonal boron nitride (hBN)16, 17. Due to a small excitation hotspot cathodoluminescence has been extensively used to study nanostructures including hyper-spectral imaging of plasmonic gratings18, nanoparticles19, nano- antenna20, quantum well21, 22, three-dimensional nanoscale visualization of metal-dielectric nanoresonators23 and nanoscale light sources24, 25.
In atomic layers of MX2, it is challenging to detect the interband cathodoluminescence signal as the electron-hole creation cross section is extremely small. Moreover, the spatial distribution of electron- hole pairs at the interface, which is near the point of free-electron injection, is close to a 3D spherical shape of a few microns in diameter. Only a small fraction of recombination takes place in the top 2D material and most of them happen in the supporting slab. Indeed, so far only a few reports are available on cathodoluminescence study of 2D materials, including six atomic layer thick flakes of boron nitride.26, 27, 28 However, cathodoluminescence from monolayer MX2 has not reported. In this report we show that cathodoluminescence emissions from monolayer MX2 (MoS2, WS2 and WSe2) can be enhanced and efficiently detected in

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