Optical Analysis

Chapter 2: Controlled Growth of WS 2 Quantum dots by liquid exfoliation and Monolayer

2.1. Synthesis of WS 2 Quantum dots by Chemical Exfoliation

2.1.4. Results and Discussion

2.1.4.2. Optical Analysis

To further investigate the properties of the as-prepared nanostructured WS2 QDs and nanosheets, the samples were characterized by absorption spectroscopy. Fig. 2.4(a) shows the UV–vis absorption spectra of bulk WS2 as a reference. Fig. 2.4(b) depicts the UV-Vis absorption spectra for the sample obtained at Stage I of the synthesis process, before centrifugation. There are clear signatures of the QDs as well as NS in the sample. WS2 QDs exhibit strong UV absorbance, while the peak around ~620 nm corresponds to the layered nanosheets and is attributed to the optical transition at the K point of the Brillouin zone^{10, 20-21}. This feature is, however, absent in the absorption spectrum of the WS2 QDs. There is a strong UV absorption peak at ~297 nm (see Fig. 2.4(c)). It is to be noted that the sample used for this measurement is obtained after centrifugation at 12000 rpm. There is a broad absorption tail extending over the visible region. This property is typical of WS2 QDs²². To determine the

optical band gap of the as-prepared QDs, we use the Tauc plot for direct bandgap semiconductors. The inset of Fig. 2.4(c) shows the bandgap of the WS2 QDs as ~3.48 eV.

Fig. 2.4. UV-Vis absorption spectra for (a) bulk WS2 powder, (b) WS2 QDs and NS, and (c) WS2 QDs. Excitation wavelength-dependent PL spectra of WS2 QDs obtained from different centrifugation rates: (d) 4000 rpm, (e) 8000 rpm, and (f) 12000 rpm.

The excitonic Bohr radius of WS2 QDs is ~1.2 nm^23-24. Thus, the expected quantum confinement effect of the as-prepared WS2 QDs was studied and characterized with the excitation-dependent PL spectra. The excitation wavelength (λex) was varied from 300 nm to 440 nm using a monochromator and a Xenon lamp. Fig. 2.4(d, e) display the spectra for the

31 | C o n t r o l l e d G r o w t h o f W S2 Q D s b y l i q u i d e x f o l i a t i o n a n d 1 L - W S2 b y C V D

QDs corresponding to 4000 and 8000 rpm, respectively. In both cases, the PL intensity is maximum for the λex ~340-360 nm. The emission spectra are broad and exhibit redshift with the increase in the λex. Note that these QD samples exhibit broad-size distributions, as discussed in the previous section. However, the QDs obtained at 12000 rpm exhibit a narrower size distribution. Likewise, the corresponding λex dependent PL spectra differ from the former case.

The emission spectra are broad at lower λex and become gradually narrower and exhibit a systematic redshift with the increase in the λex (see Fig. 2.4(f)). Thus, it is clear that apart from the quantum size effect, the PL emission is essentially contributed by recombination at different selective states, depending on the wavelength of excitation. The contributing recombination pathways are discussed in detail in Chapter 3.

2.1.4.3. Compositional and Structural analysis

Fig. 2.5. (a) XPS survey scan of as-prepared WS2 QDs. XPS spectra of the WS2 QD showing (b) W 4f and (c) S 2p respectively. The symbols are the experimental data, and the solid curves are the Gaussian fittings. (d) Comparative Raman spectra of bulk WS2 and WS2 QDs films at excitation of 532 nm Ar laser.

The chemical composition of the as-prepared WS2 QDs was determined using XPS analysis. Fig. 2.5(a) depicts the full range XPS survey scan spectrum for the WS2 QDs. In addition to W 4f and S 2p peaks, there is the presence of C 1s (~284 eV), N 1s (~399 eV), and

O 1s (~531 eV) peaks. The carbon contamination is well known in XPS analysis, and the presence of N may be due to the residual trace of NMP, which was used for the sample preparation^{10, 25}. In Fig. 2.5(b), the broad W 4f envelope is fitted with peaks at 32.5 eV and 34.8 eV, which are known to be from W 4f7/2 and W 4f5/2, respectively. These peaks correspond to the 4⁺ oxidation state of W, which concur with previous reports for 2H-WS222. The S 2p XPS of the WS2 QDs is displayed in Fig. 2.5(c). The peaks at ~161.8 eV and ~162.9 eV correspond to the S 2p3/2 and S 2p1/2, respectively, and are attributed to the divalent sulfide ions (S^2-). Additionally, there are extra peaks at 161.4 eV (with 10.3 % spectral weight) and 162.1 eV (with 14.4 % spectral weight) in the S-2p spectra of the WS2 QDs, which are due to the presence of surface defects (S vacancies) in the WS2 QDs. These defects are created during the synthesis by the liquid phase chemical exfoliation^{22, 26}.

Micro-Raman measurements were carried out on WS2 QD and bulk powder using Ar laser excitation at 532 nm. Fig. 2.5(d) shows that the bulk WS2 powder has two characteristic Raman modes E2g (356.6 cm⁻¹) and A1g (421.4 cm^-1), which correspond to the in-plane vibration and the out-of-plane vibration of the W-S bond of 2H WS2. In addition to these characteristic modes, there is another mode at 351.5 cm^-1 that is ascribed to the second-order longitudinal acoustic mode (2LA)^27-28. Additionally, the low-intensity peak at 325.6 cm^-1 is assigned to a combination of acoustic processes, LA, from longitudinal acoustic mode and ZA, attributed to out-of-plane acoustic phonons. WS2 QDs also exhibit a similar Raman spectrum with a marginal blue shift in the E2g peak by 0.4 cm^-1 and a redshift in the A1g peak by 0.8 cm^-

1 due to the decrease in the number of layers in the as-prepared WS2 QDs, as compared to the bulk counterpart. The presence of these Raman modes confirms good crystallinity of the as- prepared WS2 QDs¹⁰. Note that there is an increase in the relative intensity of the 2LA mode in the QDs that is attributed to the reduction of layer number and lateral size of WS2 due to the confinement effect on the Raman acoustic phonon mode²⁸.

More exhaustive analyses of the as-prepared WS2 QDs are carried out using various spectroscopic tools in the subsequent chapters. Their optical and photoconduction properties have been utilized for photoluminescence-based applications and in developing optoelectronic devices.

2.2. Lower temperature growth of monolayer WS

2

flakes via chemical vapor deposition

2.2.1 Introduction

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As discussed in the previous section, owing to their several novel properties, 2D WS2

structures have proven to be ideal for applications in various areas. However, despite their advantages, QDs display poor carrier transport in comparison to their 1D, 2D, and 3D counterparts. For practical applications, especially in optoelectronics, 2D monolayer-bilayer WS2 acts as a more robust platform. Hence, the synthesis of large uniform wafer-scale WS2

single crystal becomes crucial. Consequently, among different methods, chemical vapor deposition (CVD) is extensively employed for the growth of large-area, homogeneous, and high-quality monolayer 2D TMD films. Moreover, monolayer WS2 (1L-WS2) exhibits properties like excellent visible photoluminescence due to quantum confinement, large spin- orbit splitting, and high exciton binding energy. CVD has proved to be reliable for producing high-quality 2D WS2 films with high crystallinity favorable for investigating fundamental properties and device fabrication. Although this area is being extensively studied, the CVD growth of monolayer single-crystal WS2 with a large domain size is still a challenge without the incorporation of a seed layer²⁹ or H2 gas³⁰. The process primarily depends on the precursors, WO3 and S. To optimize controlled growth, besides varying the typical growth parameters, the proper design of a growth system plays a significant role. As such, given the melting point of WO3 is as high as 1300 °C, the importance of a space-confined growth system is explored^31-32. This is done to enhance the partial pressure of the WO3 vapor³³, to obtain uniform 1L-WS2 of large domain sizes.