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*,† †
Materials Science & Technology Division, Naval Research Laboratory, Washington, D.C. 20375, United States Sensors Directorate, Air Force Research Laboratory, Wright-Patterson AFB, Ohio 45433, United States § KBRwyle, Beavercreek, Ohio 45431, United States
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ABSTRACT: We present a paradigm for encoding strain into two-dimensional materials (2DMs) 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 60 K, 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 further investigations and future applications of strain engineering of 2DM and 2DM devices. KEYWORDS: two-dimensional materials, tungsten diselenide, strain engineering, single-photon emitter, atomic force microscopy
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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
ingle-photon emitters (SPEs), or quantum emitters, are key components in a wide range of nascent quantumbased 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 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 © XXXX American Chemical Society
Received: November 15, 2018 Accepted: January 3, 2019 Published: January 4, 2019 A
DOI: 10.1021/acsnano.8b08730 ACS Nano XXXX, XXX, XXX−XXX
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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 to 1640 nN. The sample was WSe2/PMMA (70 nm). (e) Indent depth as a function of applied load. Error 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 close up 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 to 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.
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 nanopillars. 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 nanoindents and exhibit single-photon emission up to 60 K, the highest temperature reported for SPEs in these materials. These emitters are bright, producing photon rates of 105/s at low laser pump powers (∼10 nw/μm2) 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 nanoimprinting approach will be effective in creating large arrays or patterns of quantum emitters for wafer-scale manufacturing of quantum photonic systems.
RESULTS AND 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 outB
DOI: 10.1021/acsnano.8b08730 ACS Nano XXXX, XXX, XXX−XXX
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deviation for the bare polymer indent depth in Figure 1e is 0.4 nm. The slight increase in standard deviation of indent depth for the WSe2/polymer composite could be due to small heterogeneities in the WSe2/polymer interface resulting from the transfer process. In addition to enabling excellent depth repeatability, AFM indenting can create nearly identical indent shapes, which is a significant improvement over existing strategies for strainengineering. Figure 1f shows two cross-sectional profiles of an indent produced with 1640 nN applied load. The red curve corresponds to the cross-section along the horizontal dashed red line shown in the inset. The blue curve corresponds to the cross-section along the vertical dashed blue line in the inset. The error bars on the curves correspond to the standard deviation of nine indents created with the same applied load. The average standard deviation within the indent (position = 50−90 nm) is 1.0 nm for the horizontal cross section (red curve) and 0.7 nm for the vertical cross section (blue curve). We can control the shape and size of the indents by using different AFM tips, providing another degree of freedom in strain engineering. The shape of the indents generally is the inverse of the tip shape. In Figure 1f, the shape of the indent is nominally a three-sided pyramid, which mimics the tip shape of the SSS-NCHR tip used for these indents. The use of a conical tip would result in a cone shaped indent, which may be advantageous for introducing smoother strain gradients. The use of blunt AFM tips (i.e., tips with a large tip radius) for the indenting procedure leads to larger indents, which are advantageous in some applications, such as enhancement of single-photon emission, as discussed in the following section. Figure 1g shows AFM topography of a WSe2 layer on 320 nm of PMMA after numerous indents with a blunt tip using a range of applied cantilever displacements from 1000 to 2000 nm. For blunt tips, the required force to achieve a given indent depth is larger due to the larger contact stiffness resulting from a larger contact radius. In our experiments, the required force to achieve the desired indent depth was often larger than the maximum range allowed by the AFM (limited by the reflected laser beam moving outside the four-quadrant photodiode). In these instances, indents of specified cantilever displacement (i.e., z-piezo displacement) still enable highly repeatable indents. Figure 1h shows the maximum indent depth as a function of cantilever displacement. This blunt tip produced indents ranging from 9 to 240 nm for cantilever displacements of 150 to 2000 nm. The standard deviation of maximum depth for indents produced with the same cantilever displacement is less than 3.1 nm for all cantilever displacements shown in Figure 1h. The blunt tip also produces highly repeatable indents as shown in Figure 1i, which shows cross sections of the indent labeled by the red box in Figure 1g. Similar to Figure 1f, the error bars represent the standard deviation of five indents produced with the same cantilever displacement. The average standard deviation within the indent (position = 300−600 nm) is 2.2 nm for the horizontal cross section (red curve) and 3.0 nm for the vertical cross section (blue curve). Importantly, the 2DM/polymer composite scheme and subsequent AFM indenting to produce strain can be extended to any choice of 2DM and also to van der Waals heterostructures (i.e., stacks of multiple 2DM). The ability to apply strain with nanometer-scale precision to 2DM and van der Waals heterostructures offers exciting possibilities for
of-plane displacements and high strain limits (up to 30%).13,22,23 Recently, strain engineering has been shown to significantly modify the optical properties10,24 and has been demonstrated at the wafer scale 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 nanopillars enabled deterministic placement and array formation of 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 Figure 1a. 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 Figure 1b. 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 Figure 1c. Parts d and e of Figure 1 demonstrate our ability to control the indent geometry by controlling the applied load. Figure 1d 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 to 1640 nN. The indents were performed with an approach and retract speed of 600 nm/s with a 1 s hold time at the maximum applied load. Each row of 15 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 1e 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
