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Quantum Calligraphy: Writing Single Photon Emitters in a Two-Dimensional Materials Platform Matthew R. Rosenberger, Chandriker Kavir Dass, Hsun-Jen Chuang, Saujan V. Sivaram, Kathleen M. McCreary, Joshua R. Hendrickson, and Berend T. Jonker ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b08730 • Publication Date (Web): 04 Jan 2019 Downloaded from http://pubs.acs.org on January 7, 2019
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Quantum Calligraphy: Writing Single Photon Emitters in a TwoDimensional Materials Platform Matthew R. Rosenberger,1a* Chandriker Kavir Dass,2,3 Hsun-Jen Chuang, 1b Saujan V. Sivaram,1a Kathleen M. McCreary,1 Joshua R. Hendrickson,2 and Berend T. Jonker1* 1
Materials Science & Technology Division, Naval Research Laboratory, Washington DC 20375, USA 2 Sensors Directorate, Air Force Research Laboratory, Wright-Patterson AFB, OH 45433, USA 3 KBRwyle, Beavercreek, OH, 45431 a Postdoctoral associate at the Naval Research Laboratory through the National Research Council b Postdoctoral associate at the Naval Research Laboratory through the American Society for Engineering Education *Correspondence and requests for materials should be addressed to M.R.R. (email:
[email protected]) or B.T.J. (email:
[email protected])
Abstract We present a paradigm for encoding strain into two dimensional materials (2DM) to create and deterministically place single photon emitters (SPEs) in arbitrary locations with nanometer-scale precision. Our material platform consists of a 2DM placed on top of a deformable polymer film. Upon application of sufficient mechanical stress using an atomic force microscope tip, the 2DM/polymer composite deforms, resulting in formation of highly localized strain fields with excellent control and repeatability. We show that SPEs are created and localized at these nanoindents, and exhibit single photon emission up to 60K, the highest temperature reported in these materials. This quantum calligraphy allows deterministic placement and real time design of arbitrary patterns of SPEs for facile coupling with photonic waveguides, cavities and plasmonic structures. In addition to enabling versatile placement of SPEs, these results present a general methodology for imparting strain into 2DM with nanometer-scale precision, providing an invaluable tool for
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further investigations and future applications of strain engineering of 2DM and 2DM devices.
Keywords: two-dimensional materials, tungsten disulfide, strain engineering, single photon emitter, atomic force microscopy
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Single photon emitters (SPEs), or quantum emitters, are key components in a wide range of nascent quantum-based technologies, including computing, communications, sensing and metrology.1 An ideal SPE generates one photon on demand at a high rate, with each photon indistinguishable from another, and is realized in a material platform which enables deterministic placement of SPEs in a fully scalable fashion. A solid state host offers many advantages for realization of a functional system, but single photon emission often originates from defects such as vacancy complexes whose existence and position are difficult to control with the reliability and nanoscale precision requisite for technological implementation.1
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Recent work has identified SPE behavior from seemingly random sites in single monolayer transition metal dichalcogenides (TMDs) such as WSe2.2–6 These monolayer materials are particularly attractive as an SPE host because they are readily coupled to photonic waveguides, cavities and plasmonic structures, and the emitter is not embedded in a high dielectric environment which would otherwise make extraction of the light difficult. In addition, SPEs in the TMDs can be electrically driven.6,7 Although the detailed origin of quantum emission in the TMDs is unclear, the physical position of the SPE sites are often correlated with areas of high strain.8,9 Subsequent work has demonstrated scalable array formation using a prefabricated pillar template10 over which a WSe2 monolayer is mechanically draped, inducing a strain field in the TMD at the peak of each pillar to localize the SPE with a positioning accuracy of 120 ± 32 nm in the best case.11,12 One potential limitation of this approach is the uncontrolled formation of wrinkles in the TMD around the nano-pillars. These wrinkles form in random orientations that may result in unpredictable and unrepeatable strain profiles. Here we report the direct writing of quantum emitters in two dimensional semiconductors using a materials platform consisting of a TMD layer on a deformable substrate. We use an atomic force microscope (AFM) to form nanoindents in monolayer WSe2 on a poly(methyl methacrylate) (PMMA) / SiO2 / Si substrate with positioning accuracy limited by the AFM and the width of the nanoindent. We demonstrate the ability to control the depth of indentation by controlling the applied load and achieve good process repeatability. We show that quantum emitters are created and localized at these nano-indents, and exhibit single photon emission up to 60K, the highest temperature reported for SPEs in these materials. These emitters are bright, producing photon rates
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of 105/sec at low laser pump powers (~ 10 nw / um2) with low spectral wandering. This quantum calligraphy allows deterministic placement and real time design of arbitrary patterns of SPEs for facile coupling with photonic waveguides, cavities and plasmonic structures. Our results also indicate that a nano-imprinting approach will be effective in creating large arrays or patterns of quantum emitters for wafer scale manufacturing of quantum photonic systems.
Results/Discussion AFM Indentation for Nanometer-scale Strain Engineering Two-dimensional materials (2DM) such as graphene and the TMDs exhibit many intriguing mechanical, electronic, and optoelectronic properties that make them promising for a wide range of applications, including flexible and transparent electronics, conformal optoelectronics, and sensing.13,14 Their intrinsic high surface-to-volume ratio renders them highly sensitive to the environment, including the adsorption of gas molecules,15,16 changes in the surrounding material’s dielectric constant,17 and mechanical strain induced by topographical features or pressure.10,18–21 Strain engineering is a particularly exciting possibility for 2DM due to their small stiffness for out-of-plane displacements and high strain limits (up to 30%).13,22,23 Recently, strain engineering has been shown to significantly modify the optical properties,10,24 and has been demonstrated at the waferscale by modifying substrate and 2DM relative thermal expansion during growth.25 In particular, introduction of strain fields by draping a TMD layer over a prefabricated template of nano-pillars enabled deterministic placement and array formation of
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SPEs.11,12 There has also been some work on using tip-based approaches to scratch2 or indent26 TMDs in order to modify their light emission properties. We have developed an AFM-based approach to generate local strain fields and write single photon emitter patterns into a 2DM with nanometer-scale precision. The technique uses a simple material platform consisting of a 2DM on top of a polymer layer. After indentation with an AFM tip, the polymer layer serves as a deformable layer which holds the 2DM in place, forcing it to follow a deformation contour and resulting in a highly localized strain field. Figure 1 shows the concept for local strain engineering of 2DM using an AFM tip. The 2DM is transferred onto a polymer layer that serves as a deformable substrate, as shown in Fig. 1(a). Bringing the cantilever into contact with a small applied load leads only to elastic deformation of the surface, producing no permanent indent. At a critical load, the polymer begins to plastically deform, resulting in a permanent indent in the material. The 2D layer is deformed with the polymer while the AFM tip is in contact, resulting in tensile strain buildup in the 2D layer, as shown in Fig. 1(b). When the AFM tip is removed, the adhesive interaction between the polymer and the 2D layer prevents the 2D layer from relaxing back to its original, strain-free geometry, which results in a permanent strain applied to the 2D layer, as shown in Fig. 1(c). Figure 1(d-e) demonstrate our ability to control the indent geometry by controlling the applied load. Figure 1(d) shows the sample topography at the edge of a WSe2 flake on a 70 nm thick PMMA layer after numerous indents have been formed with applied loads varying from 30 nN to 1640 nN. The indents were performed with an approach and retract speed of 600 nm/sec with a 1 second hold time at the maximum applied load. Each
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Figure 1. (a) Schematic of the 2DM/polymer structure used in the experiments. (b) The AFM tip applies sufficient load to plastically deform the polymer. (c) The adhesive interaction between the 2DM and polymer is strong enough to hold the 2DM in place, thus imparting strain to the 2DM. (d) AFM image after a grid of AFM indents made with sharp tip and a range of applied force from 30 – 1640 nN. The sample was WSe2/PMMA(70 nm). (e) Indent depth as a function of applied load. Bars represent standard deviation of nominally identical indents. (f) Line profiles averaged from nine nominally identical indents along the lines labeled in the inset image. The inset image is a closeup of the indent labeled with the red box in (d). The error bars represent standard deviation. (g) AFM image after a grid of AFM indents made with a blunt tip and a range of cantilever displacement from 1000 – 2000 nm. The sample was WSe2/PMMA(320 nm). (h) Indent depth as a function of cantilever displacement for a blunt AFM cantilever. Error bars representing standard deviation of five nominally identical indents are smaller than the size of the data points. (i) Line profiles averaged from five nominally identical indents along the lines labeled in the inset image. The inset image is a closeup of the indent labeled with the red box in (g). The error bars represent standard deviation.
row of fifteen indents correspond to indents with the same applied load. We performed these indents near the edge of the WSe2 flake in order to compare the bare polymer film to the WSe2/polymer composite. Figure 1(e) shows the maximum indent depth as a function of applied load for both the bare polymer layer and the WSe2/polymer composite. As expected, increased load leads to increased indent depth. At loads
