Paper presentation: By: Tripti Ahuja Date: 11/06/2016 Background: - PowerPoint PPT Presentation

Paper presentation: By: Tripti Ahuja Date: 11/06/2016

Background:  MoS 2 sheets, a direct band gap semiconducting material has rapid advancement in opto- electronic devices and other processes like photoluminescence (PL) imaging.  Using PL property of MoS 2 , scanning near field optical microscopy (SNOM) was integrated with PL technique to identify nanosized line defects and GBs on ML-MoS 2 sheet with enhanced resolution.  Near field optical microscopy provides a non traditional route to explore material properties below the diffraction limit in 2D systems that confines optical excitation and collection without hindering spectral analysis.  Scanning tunneling and transmission electron microscopies are able to probe atomic scale electronic properties and structural defects in ML-TMDCs but exhibit limitations for optical investigations of the electronic properties and bound excited states (excitons and trions) of the 2D materials.

In this paper…  They have used SNOM PL imaging technique to resolve nanoscale structural defects as small as 20 nm, such as grain boundaries (GBs) or adlayers of CVD grown ML-MoS 2 , which were not discernible by conventional PL imaging.  SNOM PL imaging was correlated with scanning electron microscopy (SEM) to show that PL quenching at the GB sites on ML-MoS 2 resulted mainly from actual physical damage at the GBs rather than from the presence of localized states. From-Witec Manual

Method for sample preparation: 780 °C http://www.slideshare.net/rahulraghvendra5/mos2-46232395  CVD grown ML-MoS 2 sheet.  Substrate here is ITO coated glass or sapphire.

Results and Discussions: Fig. 1) SNOM and confocal PL images of butterfly-shaped monolayered MoS 2 prepared on an ITO-coated glass substrate. (a) Confocal PL image and (b) SNOM PL image of monolayered MoS 2 on the ITO-coated glass substrate. The gray arrows indicate the expected GB positions. (c) SEM image of the same sample area. The width of a narrow line defect was 20 nm. The scale bar indicates 3 µm. ( a’, b’, and c’) indicate the magnified views of the center area (red box) of each image shown in (a, b, and c). The scale bar in (a’) indicates 1 µm.

Fig. 2) SNOM PL images of polycrystalline monolayered MoS 2 flakes. (a) SNOM PL image of a 3-grain merged MoS 2 flake. (b, c) Re-imaged SNOM PL images of the two GB areas (red boxed regions in a). The expected GB position is indicated in each inset. (d) SNOM PL image of a 4-grain merged MoS 2 flake. The inset shows a high-resolution SNOM PL image of the boxed GB area. The white and orange arrows indicate the GB sites. The scale bars in (a, d) indicate 5 µm, and the scale bars in (b, c) and the inset in (d) indicate 1 µm.

Fig. 3) SNOM PL images of monolayered MoS 2 flakes with adlayer defects. (a) SNOM PL image of a polycrystalline monolayered MoS 2 flake. Dark triangular adlayer defects were observed on the MoS 2 surface. Comparison of (b) confocal PL and (c) SNOM PL images of the same area of the monolayered MoS 2 . (d) SNOM PL image showing the orientations of the adlayer defects. (e) Distribution of the relative orientations of the adlayer defects with respect to the underlying monolayered MoS 2 grain. The scale bars in (a, b, and c) indicate 2 µm, and the scale bar in (d) indicates 1 µm.

(b) Fig. 4) (a) PL-SNOM image to show that freshly cut edge of ML-MoS 2 do not show enhanced PL intensity. (b) Plot of the full width at half maximum (FWHM), as measured in the SNOM PL image vs. the estimated widths measured in the SEM image, of selected line defects. The solid dots represent the actual measured values and the red squares indicate the predicted FWHM values of the line profile obtained by convolution of a Gaussian profile having a 110 nm FWHM and the step function of the corresponding widths measured from the SEM images. The red line provides a guide for the eyes. The insets show the actual SEM and SNOM PL images used to estimate the FWHM.

Conclusions:  Near-field PL images of CVD grown ML-MoS 2 were presented to characterize the nanosized line defects and adlayer defects of MoS 2 .  Superb capability of optical PL-SNOM to identify the structural defects of ML-MoS 2 over confocal PL imaging.  PL signal reduction at the GBs across the centered GB line is not always necessarily true.

Visualizing nanoscale excitonic relaxation properties of disordered edges and grain boundaries in monolayer molybdenum disulfide Wei Bao, Nicholas J. Borys, Changhyun Ko, Joonki Suh, Wen Fan, Andrew Thron, Yingjie Zhang, Alexander Buyanin, Jie Zhang, Stefano Cabrini, Paul D. Ashby, Alexander Weber-Bargioni, Sefaattin Tongay, Shaul Aloni, D. Frank Ogletree, Junqiao Wu, Miquel B. Salmeron & P. James Schuck Molecular Foundry, Lawrence Berkeley National Laboratory, USA Published: 13 Aug 2015 DOI: 10.1038/ncomms8993

In this paper…  They have utilized their previously established sub-diffraction hyperspectral imaging capability of their campanile probe to spectroscopically map nanoscale excited-state relaxation processes in chemical vapor deposition (CVD) grown ML- MoS 2 sheet.  The enhanced resolution of ML-MoS 2 images revealed the significant optoelectronic heterogeneity.  These images have also enabled to quantify the exciton-quenching phenomena at GBs.

Fig 1: SEM images of the typical campanile probe used for the measurement analysis Fig 2: Schematic drawing of the optical beam path used for the near-field hyperspectral optical imaging of ML-MoS 2 using campanile probe, (b) A comparison of the spatially averaged emission spectra collected from the Campanile near field probe and traditional confocal microscopy (100X 0.7 NA objective) for the ML-MoS 2 flake.

Results and Discussions: Figure 3: Nano-optical imaging of PL in ML-MoS 2 . (a) Illustration of near-field excitation and collection of the PL from ML-MoS 2 using the Campanile near-field probe where the optical laser excitation (2.33 eV) and collection of sample emission are confined to the apex of the tip to produce high resolution optical maps. At each pixel a full PL spectrum is acquired. (b) Map of the PL emission intensity of a triangular ML-MoS 2 flake using the Campanile probe. The white dashed line indicates the flake boundary as determined from the shear-force topography. Scale bar, 1mm. (c) An image of the same flake acquired with scanning confocal microscopy using a 100-X, 0.7-NA air objective. Scale bar, 1mm. (d) Near-field nano- PL spectrum averaged over the spatial extent of the ML-MoS 2 flake. The emission contains two peaks arising from the radiative recombination from exciton (A) and trion (A -)states . The dashed vertical line shows the position of the spectral median (SM) that splits the spectrum into equal amounts of high and low energy countsand is used to quantify spectral variations in the lower signal-to-noise spectra of individual spatial positions acquired during fast scans. All data were acquired under ambient conditions. NA, numerical aperture

Comparative study of two papers: Difgerent Nanoscale paper Nature aspects Communications paper Basic study Characterization of Visualization of nanosized GBs and excitonic relaxation adlayer defects on phenomena at edges ML-MoS 2 sheet and GBs at nanoscale level Sample CVD grown ML- CVD grown ML-MoS 2 preparation MoS 2 method T echnique SNOM-PL but SNOM utilizing PL aperture cantilever property of ML-MoS 2 probe is used but hyperspectral imaging campanile probe is used Imaging and Only SNOM-PL SNOM imaging with spectral analysis imaging of ML- spectral analysis is MoS 2 is done done.

Future plan:  To study the optical and electronic effects of one layered material on the other layered material in Van der Waals layered heterostructures.  Since edge defects and the GBs are important from sense of catalytic activity, one can monitor the catalytic activity of biological molecules like proteins, enzymes etc with the different nanoparticle systems.