Increasing the light extraction and longevity of TMDC monolayers using liquid formed micro-lenses

The epoxy SILs used in this article were produced and aligned to the monolayers using the following procedure, as illustrated in figures 2(b)–(d). The samples were first placed into a bath of glycerol, which acts as the filler solution that enables high contact angle droplets to form on the sample. The UV-curable epoxy (Norland adhesive 81) was dispensed onto the wafer through a 32-gauge needle, using an air dispensing system to accurately control the droplet size (figure 2(c)). The needle was positioned above previously studied monolayers using a custom built stage with an endoscope camera, allowing relocation accuracy at the micron scale. A bias between the needle and sample could then be applied to tune the contact angle of the SIL, if required [11]. Finally, once the size and shape of the SIL was optimised, the needle was retracted and the epoxy was cured using an external UV light source (figure 2(d)).

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Figure 3 shows multiple optical microscope images of a flake of WSe₂, with both monolayer and bilayer thick regions, before and after mounting a SIL on top of it. The mounted epoxy SIL had a radius of 550 ± 5 μm, and a height from apex to base of 700 ± 50 μm. The optical magnification through the SIL was measured to be 1.80, this helps to resolve fine features of the flake, as shown by the cracks in the flake which are now clearly visible. Optical artefacts seen in figure 3(d) are a result of using a narrower aperture in the microscope to take the image. It can also be observed that a few small bubbles have become encapsulated, but providing the bubbles are not directly over the monolayer, these defects will have no effect on the light collection efficiency. Additional SILs with very similar dimensions were mounted onto different flakes of WSe₂ and MoS₂, giving similar enhancements in magnification.

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To fully assess the performance of the SIL, PL intensity maps of the WSe₂ monolayer (from figure 3) were taken both before and after mounting the SIL, the results are shown in figure 4. It is clear from figure 4 that there is a significant increase in magnification of the PL map between (a) and (b), due to the mounting of the SIL. The extra resolution this provides enables more detail to be captured in PL maps, making edges and intensity differences easier to see. It should be noted that the horizontal artefacts are due to the PL maps being interpolated, to enable a better comparison with the optical images. The observed increase in resolution arises not only due to the magnification of the flake, but also from a slight reduction of the laser's spot size. Theoretically, the increase in resolution from a h-SIL in μPL arises from a decrease in laser spot diameter, which is proportional to $1/n$ (where n is the refractive index of the SIL) [19]. The laser acting as a plane wave will approximately project an airy pattern with a half-width at half-maximum of:

$$\begin{eqnarray}&&{\rm{HWHM}}=\,\tfrac{0.26\lambda }{n{\rm{N}}{{\rm{A}}}_{{\rm{o}}{\rm{b}}{\rm{j}}}},\end{eqnarray} \tag{ 2 }$$

where n = refractive index of the volume above the TMDC, NA_obj = numerical aperture of the μPL system and λ = excitation wavelength [13]. Calculating (2) with the experimental parameters here gives an expected resolution increase of 1.6 times. The value measured from figure 4 is slightly larger than this, at 1.8 times, this discrepancy is attributed to the SIL having a contact angle to the substrate greater than 90°. The overall improvement in magnification is thus somewhere between that of an h-SIL (linear dependence with n) and an s-SIL (quadratic dependence with n) [19]. This result shows that SILs in between the h and s-SIL geometries can give optical properties that are a combination of the two, with the studied SIL showing a greater magnification than an h-SIL without introducing strong chromatic aberrations. This suggests that gradually increasing the contact angle to the substrate from 90° will change the magnifications dependence on refractive index from a reciprocal to an inverse square relationship. This means that epoxy SILs have the potential to be formed into geometries that can provide optimised trade-offs between resolution and image quality.

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Figure 4(c) shows a map taken at cryogenic temperatures (20 K); it can be seen that the temperature change has had no detrimental effect on the optical properties of the SIL, or flake. This result shows that the SILs are highly resistant to temperature changes, and can thus be used for PL studies at cryogenic temperatures. Some example spectra from (c) are shown in figure 4(d), these correspond to areas where there was some overlap between monolayer and bilayer, as observed by the two main peaks. Sharp exciton-like lines can be observed in each of these spectra, this is unusual as these sharp peaks do not appear anywhere else on this flake, nor do they appear at room temperature. Since these peaks all occur at the same wavelength, it is unlikely that they are due to quantum dots, but could be due to defects/impurities that are more likely to occur at the monolayer-bilayer boundary.

When comparing figures 4(a) and (b) there is a significant increase in the intensity of light collected across the sample, with the integrated intensity increasing on average by 4.0 times for the same excitation power. Figure 5(a) shows the measured PL spectra of the flake from a single point in maps 4(a) and (b) (before and after the application of the SIL), the fourfold enhancement in the integrated intensity is easily observed as a large increase in the peak intensity. It was noticed that the relative enhancement varied with excitation power, this is shown in figure 5(b) where the ratio between SIL to no SIL is plotted versus excitation power for different monolayer positions. The measured PL enhancement was twice as high as predictions from classical optics, using equation (1) and extrapolating into solid angles. This increased enhancement could arise from constructive interference or confinement effects at a microscopic level, which cannot couple into the collection optics without a SIL. Simulating and quantifying these interactions is challenging due to the monolayer and the silicon dioxide substrate having dimensions that place the system into a semi-classical regime, making it computationally difficult. However, despite not being fully able to model the interactions, other emission increasing mechanisms such as doping of the sheet due to the epoxy can be discounted, as similar WSe₂ monolayers show no enhancement in their PL when they are coated with a film of cured epoxy, rather than a SIL (figure 6).

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The decrease of the intensity ratio with power is likely explained by the reduction in PL laser spot size with the presence of the SIL, and subsequent increase in power density exciting the monolayer. The increase in power density from the smaller spot size can reduce the relative PL intensity through charge screening effects, leading to a reduction of relative intensity with power. A decreased spot size could also cause an increase in localised heating of both the epoxy and the flake, which may affect both the emission of the monolayer and the optical properties of the SIL due to thermal effects.

SILs were also mounted on top of MoS₂ monolayers to see if they could be used to enhance the optical properties of the device in a similar fashion as for WSe₂. Upon mounting a SIL, the emission of the monolayers was found to be heavily suppressed, and to investigate why, a 1.5 μm thick layer of epoxy was spin-coated onto the samples containing both types of monolayer. PL spectra from these samples are compared in figure 6, where it can be observed that there is a significant reduction in PL intensity for the epoxy on MoS₂, with an accompanied red shift in the central wavelength. On the WSe₂ no significant change in the peak intensity was observed, however the central wavelength blue-shifts, which is accompanied by a narrowing of the peak. The shift of the WSe₂ peak and its sharpening are possibly attributed to a change in the proportion of emission from charged excitons, causing a reduction in the monolayer peak intensity at longer wavelengths. This would indicate that the monolayer WSe₂ can be doped by the epoxy. The effects of doping also allow us to explain the results observed for MoS₂. The mechanically exfoliated MoS₂ is known to be naturally n-type [22], and it has been previously reported by Li et al in [23] that additional n-type doping results in a quenching of the PL. Thus, the epoxy bonded to the flake may be introducing dopants that result in the observed reduction in PL. MoS₂ monolayers are highly sensitive to doping, with similar effects being reported to occur between MoS₂ monolayers and the substrate that they are placed on [24].

The UV curing process of the epoxy may also induce compressive strain, influencing the optical properties of TMDCs. It has been shown that compressive strain can change the bandgap of MoS₂ from direct to indirect [25], which would decrease the emission intensity. The compressive strain values needed to induce a shift from direct to indirect bandgaps for MoS₂ and WSe₂ are 0.5% and 1.5% respectively [26]. Since the threshold for WSe₂ is higher than MoS₂, the epoxy may have induced enough compressive strain to change the bandgap in MoS₂, but not in WSe₂.

It was hypothesised that, in addition to the improved optical properties, the SIL may also increase the longevity of TMDC monolayers in ambient conditions. Gao et al in [27] showed that under ambient conditions monolayer TMDCs degrade due to oxidation and the introduction of organic contaminants, but they themselves demonstrated encapsulation in transparent polymers can help prevent this. In our experiment over a time period of six months, there was no significant change observed in any of the encapsulated monolayers, despite being left in ambient conditions. Furthermore, the PL spectra's peak intensity remained almost identical for the same excitation powers, demonstrating that no degradation occurred during the time period. This can be observed in figure 6(b) (inset) where, within the uncertainty of the PL system, the intensity remained constant.

Monolayers were mechanically exfoliated [29–31] from natural/synthetic bulk crystals of MoS₂/WSe₂ respectively, and transferred to the surface of silicon wafers with a 290 ± 5 nm SiO₂ coating (this process is illustrated in figure 2(a)). An optical microscope was then used to identify individual monolayer flakes [32], which was additionally confirmed by observing the unique spectral signatures in the μPL spectra [33].

To analyse the optical properties of individual monolayers, the samples were placed in an evacuated cryostat and excited with a 532 nm CW laser through a 50× IR microscope objective lens with a NA of 0.65. PL was collected from the sample at 300 and 20 K through the same objective lens and passed through a single-stage spectrometer to a silicon EMCCD detector for wavelength-resolved measurements. The microscope objective was mounted on a piezo-driven XYZ stage with 20 nm step resolution, to produce PL maps as a function of position on individual monolayers.