Rapid visualization of grain boundaries in monolayer MoS2 by multiphoton microscopy

Abstract

Grain boundaries have a major effect on the physical properties of two-dimensional layered materials. Therefore, it is important to develop simple, fast and sensitive characterization methods to visualize grain boundaries. Conventional Raman and photoluminescence methods have been used for detecting grain boundaries; however, these techniques are better suited for detection of grain boundaries with a large crystal axis rotation between neighbouring grains. Here we show rapid visualization of grain boundaries in chemical vapour deposited monolayer MoS₂ samples with multiphoton microscopy. In contrast to Raman and photoluminescence imaging, third-harmonic generation microscopy provides excellent sensitivity and high speed for grain boundary visualization regardless of the degree of crystal axis rotation. We find that the contrast associated with grain boundaries in the third-harmonic imaging is considerably enhanced by the solvents commonly used in the transfer process of two-dimensional materials. Our results demonstrate that multiphoton imaging can be used for fast and sensitive characterization of two-dimensional materials.

Figure 1. Multiphoton imaging of CVD-grown large-area MoS₂ flakes.

(a) SHG and (b) THG images of Sample 1. An area of ∼25 × 30 μm² is first exposed (marked by a red dashed border) by scanning over it with the same laser used for the multiphoton laser-scanning microscopy. The laser beam had a fluence of 21 mJ cm⁻² and scanning speed was 20 μs per pixel. After exposure, the SHG and THG images have been captured using a laser fluence of 11 mJ cm⁻². In a, the GBs with larger crystal orientation mismatch are visible in SHG images, which show no visible difference between the exposed and unexposed areas. In b, all the GBs are clearly visible in THG images on the exposed area. Similar THG images of (c) as-grown MoS₂ sample without any post treatments and (d,e) as-grown MoS₂ samples after ACE+IPA treatment. The GBs appear bright after ACE+IPA treatment. The laser fluence in c–e was ∼177 mJ cm⁻². Scale bars, 25 μm (a,b) and 50 μm (c–e).

Figure 2. Multiphoton characterization results.

(a) Generated multiphoton spectra with different input fluences, (b) optical image with marked grains (A1, A2, B1 and B2) and grain boundaries (GB1, GB2, GB3 and GB4), (c) experimental SHG image without analyser, (d) experimental SHG image with parallel polarized excitation and detection (as indicated by a double-headed arrow), (e) experimental THG image without analyser, (f) simulated SHG image without analyser, (g) simulated SHG image with parallel polarized excitation and detection (as indicated by a double-headed arrow) and (h) simulated THG image without analyser. Scale bars, 10 μm. Colour scale bars are the counts from photomultiplier tubes.

Figure 3. Raman and photoluminescence characterization.

(a) Raman spectrum, (b) intensity of peak, (c) centre position of peak, (d) intensity of A_1g peak and (e) centre position of A_1g peak, (f) PL spectra from the middle of the grain A1 (red curve, taken from the area marked with white line in g and from GB1 in the middle of the flake (black curve, taken from the area marked with a black dashed line in g. (g) PL intensity image of the 680 nm peak and (h) centre position of the 680 nm peak. Intensities and peak positions are acquired by Gaussian fitting. Scale bars, 10 μm. Colour scale bars are in CCD counts (Raman and PL intensities), in cm⁻¹ (Raman peak positions) and in nm (PL peak position).