Chapter 6: Photoluminescence Modulation of Monolayer WS 2 by ZnO encapsulation and
6.3. Results and Discussion
6.3.2. Compositional and Structural Analysis
Fig. 6.2 displays the high-resolution XPS spectra of W 4f for the CVD grown 1L-WS2. The peaks at 32.0 eV and 34.1 eV are ascribed to the W4+ state of W 4f7/2 and W 4f5/2, respectively, which confirm the formation of 2H-WS2 (Fig. 6.2(a)). In addition, the presence of the metallic 1T- phase of WS2 is affirmed by the presence of the peaks at 31.1 eV and 33.1 eV5-6. The appearance of the small shoulder peak at 35.2 eV is attributed to the W6+ state of the unreacted WO3. The S 2p core-level spectrum of the 1L-WS2 is depicted in Fig. 6.2(b). The S envelope has been fitted following Gaussian deconvolution – the peaks at 162.2 eV and 163.4 eV correspond to the S2- states, S 2p3/2 and S 2p1/2, respectively5, 7. The as-grown 1L-WS2 film consists of S vacancies confirmed by the peaks at lower binding energies 161.2 eV and 162.7 eV8. Fig. 6.2(c) displays the W 4f core level spectrum corresponding to the HS ZnO/WS2/ZnO which shows the presence of all the characteristic peaks, mentioned earlier. However, there is a pronounced blueshift in the W 4f7/2
and W 4f5/2 peaks by 2.5 eV. This could be a clear indication of the transfer of electrons from ZnO film to the 1L-WS2, leading to a shift in the Fermi level of the WS2 towards the conduction band9-
10. Similarly, the S 2p spectrum of the ZnO/WS2/ZnO QW exhibits the signature of all the characteristic peaks (Fig. 6.2(d)). However, the spectral weight corresponding to S vacancies increases from 31.8% in the as-grown 1L-WS2 to 41.5% in ZnO/WS2/ZnO. This may be due to the transfer process of the 1L-WS2 film onto ZnO to fabricate the QW. No pronounced shift in peak position is observed. Fig. 6.2(e) shows the Zn 2p core-level spectrum for the RF sputtered ZnO film consisting of the characteristic peaks 1020.9 eV (Zn 2p3/2) and 1043.9 eV (Zn 2p1/2). The inset displays a redshifted spectrum of the ZnO/WS2/ZnO QW indicating electron transfer from the ZnO film to 1L-WS2. Fig. 6.2(f) presents the Gaussian deconvoluted spectrum corresponding to O 1s of the ZnO film. The peaks at binding energies 528.4 eV and 531.0 eV are ascribed to the lattice oxygen and oxygen vacancies in the as-grown sample11. The downshift of these peaks in
the HS (inset) further supports the assertion of the charge-transfer aided shift in the Fermi level away from the conduction band for ZnO.
Fig. 6.2. High-resolution XPS spectra corresponding to (a) W 4f and (b) S 2p of the CVD grown 1L-WS2. High- resolution XPS spectra corresponding to (c) W 4f and (d) S 2p of the ZnO/WS2 HS. High-resolution XPS spectra of (e) Zn 2p and (f) O 1s of the sputtered ZnO film. Insets show the respective spectra for Zn 2p and O 1s corresponding to the ZnO/WS2.
141 | P L M o d u l a t i o n o f 1 L - W S2 b y Z n O e n c a p s u l a t i o n a n d Q u a n t u m w e l l e f f e c t
6.3.2.2. XRD and Raman Analysis
Fig. 6.3.XRD pattern of the RF-sputtered ZnO thin film.
The XRD pattern of the ZnO thin film is displayed in Fig. 6.3. The diffraction peaks at 31.7°
and 34.3° correspond to the (100) and (002) lattice planes of the hexagonal wurtzite crystal phase of the as-synthesized ZnO thin film12.
Micro-Raman measurement was carried out on different 1L-WS2 samples (as-WS2-SiO2, as- WS2-Sap, and t-WS2) and the ZnO/1L-WS2/ZnO QW. The laser excitation of 488 nm was used and for comparison, the spectra are presented in a stacked plot in Fig. 6.4(a). The characteristic optical Raman modes E2g and A1g appear at 356.5 cm-1 and 418.3 cm-1, respectively, for as-WS2- SiO2. Sapphire presents itself as a strain-free system for CVD grown 1L-WS2, as discussed in Chapter 2, Section 2.4.4. Thus, the E2g mode in as-WS2-Sap exhibits a redshift to 357.7 cm-1, while the A1g mode almost remains unaltered. The release of strain in a transferred 1L-WS2 film, with respect to as-grown 1L-WS2, is further confirmed by the stiffening of the E2g mode for t-WS2. One might argue the doping effect that may be induced by PMMA that has been used as a transfer agent. However, utmost care has been taken to remove its traces from the transferred samples, in order to minimize any chances of electron transfer from PMMA to the WS2. Thus, no shift in the A1g mode is observed in t-WS2 with respect to as-WS2-SiO2. In the Raman spectrum corresponding to ZnO/WS2/ZnO, the A1g mode softens. For better understanding, we carry out Lorentzian deconvolution of the spectra of t-WS2 and ZnO/WS2/ZnO. The deconvoluted Raman spectra for t-
characteristic E2g and A1g modes appear at 357.2 cm-1 and 418.6 cm-1, respectively. The frequency difference Δk amounts to 61.4 cm-1, which is characteristic of 1L-WS2. However, for the ZnO/1L- WS2/ZnO QW, the in-plane E2g mode exhibits a slight redshift to 356.8 cm-1, indicative of an induced strain in the system.
Fig. 6.4. (a) Comparative micro-Raman spectra corresponding to CVD grown 1L-WS2 on SiO2, Sapphire, transferred 1L-WS2 film (on SiO2), and ZnO/1L-WS2/ZnO. (b) Deconvoluted Raman spectra for 1L-WS2 film (on SiO2) and ZnO/1L-WS2/ZnO QW structure. The excitation wavelength used is 488 nm.
Likewise, the A1g mode downshifts to 417.5 cm-1. Since the out-of-plane A1g mode is highly sensitive to doping, this redshift implies there is a transfer of electrons from ZnO to WS2, making it more n-type doped. This hypothesis is further backed by the increase in the FWHM of the A1g mode from 4.5 cm-1 in pristine t-WS2 to 5.3 cm-1 in the HS13. Along with these optical modes of WS2, another peak at ~353 cm-1 is assigned 2LA mode, i.e., the second-order longitudinal acoustic mode of WS214-15.