Supporting Information CVD grown 2D MoS2 layers: A photoluminescence and fluorescence lifetime imaging study *,1
2
4
1
3
2
Ayberk Özden , Hüseyin Şar , Aydan Yeltik , Büşra Madenoğlu , Cem Sevik , Feridun Ay , *,2 and Nihan Kosku Perkgöz 1
Department of Materials Science and Engineering, Faculty of Engineering, Anadolu University, 26555 Eskisehir, Turkey Department of Electrical and Electronics Engineering, Faculty of Engineering, Anadolu University, 26555 Eskisehir, Turkey 3 Department of Mechanical Engineering, Faculty of Engineering, Anadolu University, 26555 Eskisehir, Turkey 4 Department of Physics, UNAM Institute of Materials Science and Nanotechnology, Bilkent University, 06800 Ankara, Turkey 2
Received 30 June 2016, revised 24 August 2016, accepted 30 August 2016 Published online September 2016 Keywords MoS2, two-dimensional materials, fluorescence lifetime imaging microscopy, photoluminescence, Raman spectroscopy, chemical vapor deposition
_____________ *Corresponding authors: e-mail
[email protected], Phone: +99 535 4172654;
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This supplementary information contains following information: 1. AFM measurements on the flakes 2. Detailed analysis of Raman and PL spectra 3. Decay curves and fitting procedures 1 AFM measurement AFM measurements are performed to demonstrate the few layer nature of the MoS2 as indicated by the Raman Spectra in the manuscript. AFM image (Fig. S1a) and height profiles (Fig. S1b) are shown as below. The height difference of the two points are taken as an indication of the thickness. These are the crossing points of the vertical lines with the profile lines. Height difference indicates a thickness of 1 nm for Profile 1, 1.2 nm for Profile 2, 1.2 nm for Profile 3, and 1.7 nm for Profile 4. The thickness of CVD grown monolayer MoS2 is reported as ~0.8 nm [1], bilayer MoS2 as 1.7 nm and three layer MoS2 as 2.14 nm [2]. Our values range from 1 nm to 1.7 nm, smaller than three layers but between monolayer to bilayer. This result indicates the few layer nature of the CVD grown MoS2 in this work.
a)
b)
Figure S1 AFM measurement (a) AFM image and (b) profile heights. Color of the lines on AFM image correspond to color of the profile lines.
2 Raman spectra and PL spectra Raman spectra can be seen in Fig. 2S. Analysis of the Raman spectra and corresponding PL points can be seen in Table 1S. FWHM of the Raman peaks are around 2-3 cm-1 broader than the reported CVD grown MoS2 layers [3]. But the peak centers (1.82 eV to 1.86 eV) and FWHM of the PL peaks (59 to 68 meV) are perfectly within the range of CVD grown MoS2 layers (60-80 meV) [4]. Therefore, instead of the crystallinity, this broadening could be attributed to the spectral resolution of the Raman instrument [5, 6], as evident from the not sharp tip of the Raman peaks.
Figure S2 Raman data of the film area. Color code of the points matches with color code of the crosses on the film area.
Table S1 Raman points and FWHM of the corresponding peaks E2g and A1g.
Points
E2g -1
FWHM(cm )
A1g FWHM(cm-1)
Definition based on
PL Center
FWHM
Δ
(eV)
PL (meV)
1.82
61
Bilayer
to
Three
1
8.44
9.33
2
8.88
9.30
Mono to Bilayer
1.85
59
3
8.71
9.38
Bilayer
1.85
59
4
8.36
9.19
Three Layer
1.85
62
5
7.70
9.36
1.86
60
6
9.11
9.50
1.86
60
7
9.53
9.79
Monolayer
1.86
68
8
7.50
9.36
Monolayer
1.86
60
Layer
Bilayer to Three Layer Bilayer to Three Layer
3 Decay curves and fitting procedure In a Witec Alpha 300 R System, a Zeiss 50X objective is used with a numerical aperture (N.A.) of 0.8 for both lifetime and PL mapping measurements. A 75μm x 75μm area using 375 x 375 pixels (140625 spectra) with a 200 nm pixel size is used. Diffraction limited spot size for 532 nm and 485 nm lasers are about 400 nm and 360 nm, respectively. 532 nm CW laser with 1 mW laser power is used for PL and Raman maps. 485 nm pulsed laser with 80 MHz repetition frequency is used for the lifetime mapping. Fluorescence lifetime measurements are performed using a 485 nm pulsed laser with 80 MHz repetition frequency and a resulting pulse width (FWHM) of 90 ps. We used 0.9 mW pulsed laser power (maximum output of our pulsed laser) considering to have a better signal to noise ratio. IRF FWHM of the photon counting detector (MPD brand) is 49 ps at the wavelength of 670 nm. Normalized decay curves of the Points 1 to Point 8 was given in Fig. S3 below.
Figure S3 Normalized decay curves and corresponding lifetimes of the points taken from the film region. Color code refers to color of the cross marks on the film.
Fitting is performed with the Control FOUR program of the WITEC company by using exponential decay function:
n: Exponential decay order y0: Offset x0: Center Ampi: Amplitude ti: Decay constant Fitting procedure is explained by using two different points on the film region (Fig. 4) as an example. Decay curve ranges between from 0 to 12.5 ns as can be observed from Fig. 4. Shaded areas in the figure indicate selected fitting region ranging from 0.6 ns to 3 ns. We applied exponential decay fitting function having single decay component (n = 1) and neglected the data smaller than 0.6 ns to prevent the effect of fast transitions, which is out of scope of this work. Therefore, we specifically map the excitonic transitions in this study. IRF data present in Fig. 4c show that the FLIM decays are not in the limit of the detector’s response.
Figure S4 Lifetime decay data (red curves) and exponential fits (blue curves) within the range of 0.35 ns to 4 ns (shaded area). (a) Point 5: bilayer to three layer and (b) point 8: monolayer.
References [1] Liu, Z. et al., Strain and structure heterogeneity in MoS2 atomic layers grown by chemical vapour deposition. Nature Commun. 5, 5246 (2014). [2] Feng, J. et al.,“Strain-engineered artificial atom as a broad-spectrum solar energy funnel. Nature Photon. 6, 865 (2012). [3] Tongay, S. et al. Broad-range modulation of light emission in two-dimensional semiconductors by molecular physisorption gating. Nano Lett. 13, 2831 (2013). [4] Bao, W. et al., Visualizing nanoscale excitonic relaxation properties of disordered edges and grain boundaries in monolayer molybdenum disulfide. Nature Commun. 6, 7993 (2015). [5] Van Der Zande, A. M. et al., Grains and grain boundaries in highly crystalline monolayer molybdenum disulphide. Nature Mater. 12, 554 (2013). [6] Mak, K. F. et al., Tightly bound trions in monolayer MoS2. Nature Mater. 12, 207 (2013). [7] Dhakal, K. P. et al., Confocal absorption spectral imaging of MoS2: Optical transitions depending on the atomic thickness of intrinsic and chemically doped MoS2. Nanoscale 6, 13028 (2014).
