Chapter 4: Highly suppressed dark current and fast photoresponse in Au nanoparticle
4.3. Results and Discussion
4.3.1. Morphology Studies
Fig. 2.13. Optical microscopy images of CVD-grown WS2 on SiO2 at (a) 800 °C, (b) 900 °C, and (c) 940 °C, respectively.
The optical microscopy images of the WS2 film grown on SiO2 substrates at different temperatures ranging from 800 to 940 °C are displayed in Fig. 2.13(a-c). It is observed that under different growth temperatures, WS2 flakes attain a triangular shape which was observed in earlier sections as well. The as-grown triangular flakes are seen to be interconnected to form a continuous film. The triangular or near triangular shape arises from the fact that the flake shape is determined by the growth rate of the WS2 edges38. The film grown at 800 °C consists of a combination of monolayer to few-layer WS2, as seen in Fig. 2.13(a). The triangles marked with dotted lines are monolayered and are of the order ~5 µm. At a higher growth temperature, i.e., 900 °C (Fig. 2.13(b)), a uniform monolayer WS2 film was successfully grown. However, the film appears to have grain boundaries and is not continuous over a large area. Lastly, in the case of the WS2 grown at 940 °C (Fig. 2.13(c)), the film quality is observed to be superior.
Smaller triangular flakes merge to form a sizably continuous and homogenous film over an area of the order of hundreds of micrometers with excellent uniformity.
Fig. 2.14(a-c) displays the OM images of the 1L-WS2 films successfully grown on different substrates (SiO2, quartz, and sapphire, respectively) at a growth temperature of 940
°C. The dotted lines mark the region of monolayer growth. To confirm the monolayer nature of the as-grown film, AFM is carried out for WS2 grown on the SiO2 substrate (Fig. 2.14(d)).
The inset displays the height profile along the white line, which reveals a step height of ~ 0.8 nm, corresponding to monolayer WS2. Thus, the salt-assisted CVD growth yields a large area of 1L-WS2 film on a variety of substrates.
43 | 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
Fig. 2.14. Optical microscopy images of CVD grown 1L-WS2 at 940 °C on (a) SiO2, (b) Quartz, and (c) Sapphire substrates, respectively. (d) AFM image of CVD grown 1L-WS2 on a SiO2 substrate. The inset shows the height profile along the white line.
2.4.3.2. Raman Study
Fig. 2.15(a) shows the comparative micro-Raman spectra of the CVD-grown WS2 at different growth temperatures in the range of 800-940 oC. Laser excitation of 488 nm was used for the Raman measurement. The Raman spectra show the presence of the two characteristic optical Raman modes (E2g and A1g), which correspond to the in-plane and the out-of-plane vibrations of the S atoms with respect to the W atoms, respectively, confirming good crystallinity of the as-grown WS2 layer. The frequency difference between the two peaks (∆k) can be used to identify the layer number of TMDs39. At 800 °C, the spectrum was acquired in one of the dotted triangles, and ∆k is found to be ~ 61 cm -1, which confirms their monolayered nature28. The FWHM of the E2g and A1g modes were ~2.9 cm-1 and ~3.5 cm-1, respectively, indicating good crystallinity. At higher temperatures, 900 and 940 °C, the Raman spectra analyses yielded ∆k of ~ 61.6 cm-1 and 61.3 cm-1, respectively. Note that there is an
Fig. 2.15. (a) Comparative micro-Raman spectra of 1L-WS2 grown on SiO2 substrates by CVD at growth temperatures 800 °C, 900 °C, and 940 °C. (b) Comparative substrate-dependent micro-Raman spectra of 1L-WS2
grown at 940 °C on SiO2, quartz, and sapphire substrates.
All other parameters in these experiments were fixed, to solely study the effect of the growth temperature. With different growth temperatures, there is a difference in the concentration of precursors on the growth substrate. We find that the WS2 film grown at 940 °C shows Raman peaks with higher intensities and narrower peak widths. Therefore, although the synthesis of monolayer WS2 is possible over the said temperature range, there is an overall improvement in the film quality and the crystallinity of the as-grown WS2 with an increase in the temperature.
Furthermore, we observed similar results for the films grown on other substrates (see Fig.
2.15(b)). The ∆k values are found to be ~61.0 ± 0.7 cm-1 for the WS2 film on the different substrates, as tabulated in Table 1.
Table 1: Summary of the E2g and A1g Raman modes for the different substrates. The peak separation (Δk ~61 cm-
1) signifies the growth of monolayer WS2.
Substrate E2g (cm-1) A1g (cm-1) ∆k (cm-1)
SiO2 357.9 419.2 61.3
Quartz 357.2 419 61.7
Sapphire 358.6 419.2 60.4
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The positions of the E2g peak are seen to vary from substrate to substrate. This is primarily due to the built-in strain induced during the growth due to the difference in the thermal expansion coefficients (TEC) between the substrate and WS2. The TEC of WS2 is about one order higher than that of SiO2, while it is almost comparable to that of sapphire40-41. The E2g mode is believed to be more sensitive to uniaxial strain since it corresponds to in-plane vibrations41-42. Thus, the SiO2 substrate introduces uniaxial strain in the WS2 film, while for sapphire, it is nearly a strain-free film, as evident from the redshift in the E2g mode by ~1.3 cm-
1 with respect to SiO2. On the other hand, the A1g band is impacted by the doping level of the system41. Thus, the slight variation in the A1g peak positions might be due to the presence of charged impurities at the interface between the WS2 film and the substrate. Nevertheless, in all cases, the Δk values imply the growth of monolayer WS2 film.
2.4.3.3. XPS Study
The elemental composition of the CVD grown 1L-WS2 on an SiO2 substrate is ascertained by the XPS study. Fig. 2.16(a) presents the survey scan spectrum of the sample over the range of 0-600 eV. In addition to W 4f (~34 eV) and S 2p (~162 eV) peaks, there are peaks corresponding to S 2s (~244 eV), W 4p (~425 eV), Si (~102 eV and ~153 eV) and O 1s (~531 eV). The O 1s peak may be due to the presence of WO3 residue as well as due to the SiO2 substrate.
Fig. 2.16(b) displays the high-resolution XPS spectra of W 4f for the 1L-WS2. The peaks attributed to W 4f7/2 (~32.5 eV) and W 4f5/2 (~34.7 eV), respectively, verify the formation of highly crystalline 2H-WS2. The additional peaks at 31.7 eV and 33.5 eV match with the 4th oxidation state of W of the metallic 1T-phase of WS2, which concurs with the literature15, 22. The shoulder peak at 35.9 eV, which corresponds to the W6+ state of the oxide confirms the presence of unreacted WO3. Additionally, a small peak corresponding to the W 5p state is evident in the spectrum. The high-resolution core-level spectrum corresponding to S 2p of the 1L-WS2 is shown in Fig. 2.16(c). The presence of the S2- states, S 2p3/2 and S 2p1/2 is confirmed by the peaks at 162.2 eV and 163.4 eV, respectively41. The as-grown samples of 1L-WS2
contain sulfur vacancies as evident from the peaks at 161.2 eV and 162.7 eV26, which collectively constitute ~16% of the spectrum.
Fig. 2.16. (a) XPS survey scan spectrum of as-prepared 1L-WS2. High-resolution XPS spectra of the 1L-WS2
showing (b) W 4f and (c) S 2p respectively. The symbols are the experimental data, and the solid curves are the Gaussian fittings, with Shirley baseline.
47 | 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
2.4.3.4. Photoluminescence Study
Fig. 2.17. Comparative micro-PL spectra of 1L-WS2 grown on SiO2 substrates by CVD at growth temperatures 800 °C, 900 °C, and 940 °C.
Fig. 2.17 shows the growth temperature-dependent PL spectra of 1L-WS2 grown on SiO2 substrates, taken at the same spots as the corresponding Raman spectra in Fig. 2.15(a).
The emission characteristics are similar for each case, however, there is an overall enhancement in the intensity with higher growth temperature. Fig. 2.18(a) displays the comparative PL spectra on different substrates on a stacked plot. As discussed in Section 2.4.3.2., the effect of the built-in strain determines the PL peak position, which differs from substrate to substrate.
As expected, sapphire being the substrate with nearly no strain exhibits a blue shift with respect to the as-grown film on the SiO2 substrate. Additionally, although the PL peak positions for SiO2 and quartz substrate is nearly the same (~630 nm), the PL emission spectrum of the 1L- WS2 on quartz is broader than that of the SiO2 case. To understand the origin of the PL emission band, we deconvoluted each spectrum of the WS2 film grown on different substrates by three Gaussian peaks (see Fig. 2.18(b-d)) – the neutral exciton or A0 peak, derived from the direct bandgap of WS2, the negative trion or Atr peak, that arises from a three-body quasi excitonic state, and a defect bound X exciton. The negative trion emission is caused by light n-type
on the experimental conditions. We observed the presence of peak X at ~650 nm in all the samples, which is attributed to the radiative recombination of defect-bound excitons. The 1L- WS2 film grown on an SiO2 substrate shows the highest PL intensity, while the film grown on quartz shows the lowest intensity, measured under identical conditions. The deconvolution is carried out noting that the trion binding energy is of the order ~35 meV34. The spectral weight of A0 is as high as ~74.5% for the SiO2 substrate, while it is the lowest (~56.0 %) for the quartz case. The corresponding spectral weights of the Atr are observed to be ~16.4% and ~ 24%, while the defect-bound X amounted to 9% and 20%, respectively. The PL spectrum of the 1L- WS2 grown on sapphire is similar to that of SiO2, however, the peak is substantially blue- shifted. The neutral excition A0, trion Atr, and defect X exciton, respectively, constitute 70.5%, 21.5%, and 8% of the overall spectrum. Thus, the spectral weights of the trion Atr and the defect peak X are found to be highest in the case of WS2 grown on the quartz substrate. The trion spectral weight is the lowest for the WS2 grown on the SiO2 substrate, explaining the highest PL intensity. Different weights of the constituents give rise to change in the overall peak positions for different substrates.
Fig. 2.18. (a) Comparative substrate-dependent micro-PL spectra of 1L-WS2 grown at 940 °C on SiO2, Quartz, and Sapphire substrates. Deconvoluted PL spectra of 1L-WS2 grown on (b) SiO2, (c) Quartz, and (d) Sapphire.
49 | 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
optimum growth. Finally, we have shown a simple salt-assisted approach to growing high- quality, large-area continuous monolayer WS2 film on different substrates by CVD. The influence of the change in the growth temperature (800-940 °C) was investigated to understand the effect on WS2 growth. We have systematically studied the Raman and PL properties of the as-grown WS2 on different substrates. We have briefly addressed the substrate-dependent built- in strain and doping effects. Thus, it is found that the growth substrates significantly affect the PL emission characteristics of the 1L-WS2. Our results indicate that the large-area monolayer WS2 grown on sapphire substrate contains fewer defects than that of the other substrates. The controlled synthesis procedures of these WS2 QDs and monolayer films could further open avenues for investigations into the fundamentals of light-matter interactions in these 2D systems and thus, pave way for the development of various optoelectronic devices.
51 | 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
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55 | S p e c t r a l a n a l y s i s o f W S2 Q D s a n d i n t e r a c t i o n w i t h S W C N T s
Chapter 3
1.4.), owing to their many unique properties. Excitation wavelength-dependent emission profiles have been reported for WS2 QDs, though the origin of the same has not been addressed properly.
The excitation-wavelength dependent change in emission color is very interesting, as it can serve as a multicolor biolabeling reagent8. Based on their high room temperature fluorescence, biosensors based on turn on- turn off fluorescence of these 2D semiconducting quantum dots, including graphene QDs9 and MoS2 QDs10-11, etc., have been developed. Yan et al. utilized the fluorescence tunability of WS2 QDs to fabricate a sensor for the determination of lipoic acid using Fe3+ ions as a quencher through photo-induced charge transfer5. The PL quantum yield of WS2
QDs is usually higher than that of 2D sheets, which may be associated with the confinement effect and higher exciton binding energy. However, there is no report on the exciton binding energy of WS2 QDs, though the same is known for the monolayer WS2.
Single-walled carbon nanotubes (SWCNTs) are rolled over graphene sheets that are quasi- one-dimensional tubules having sharp densities of electronic states12. As universal fluorescence quenchers, single-walled and multi-walled CNTs effectively quench several fluorophores by Förster resonance energy transfer (FRET) from a donor (fluorophores) to an acceptor (CNT) or by the formation of ground state non-fluorescent complexes12-15. Das et al. reported a detailed analysis on the anomalous behavior of the PL intensity of the graphene QDs with varying concentrations of the SWCNTs14. However, there have been no reports on the effect of SWCNT on the emission properties of highly fluorescent TMD QDs, such as MoS2, WS2, etc. Thus, the fluorescence quenching mechanism of TMD QDs in the presence of SWCNTs has not been explored to date. In this chapter, the optical properties of the as-prepared WS2 QDs are studied in detail. An in-depth study of the effect of SWCNTs on the fluorescence of the WS2 QDs is carried out. The mechanism of PL quenching of WS2 QDs is explored using various spectroscopic tools. There is an associated improvement in the structural quality and change in the electronic structure of the SWCNT with the attachment of the WS2 QD on the SWCNT walls, which is observed for the first time.
3.2. Experimental details
3.2.1. Synthesis of WS2 QDs by liquid exfoliation
WS2 bulk powder was dispersed in NMP to carry out liquid exfoliation for a duration of 15 hours. The obtained suspension was centrifuged at a low temperature to separate the supernatant and the centrifugate. The synthesis procedure was described in detail in Chapter 2