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ZnO thin films. We studied the tuning of the PL emission of 1L-WS2 when encapsulated by a higher bandgap ZnO and investigated the change in the exciton, biexciton, and trion dynamics using power-dependent and temperature-dependent PL measurements (Chapter 6).
The major contributions of the present thesis are summarized as follows:
A. Controlled Growth of Monolayer WS2 quantum dots and film by Chemical Exfoliation and Chemical Vapor Deposition Method
Firstly, we have devised a synthesis protocol for highly luminescent monolayer and bilayer WS2 quantum dots with controlled sizes using a straightforward chemical exfoliation technique followed by centrifugation. Spectroscopic tools such as UV-Visible-NIR absorption, Raman, and XPS analysis confirmed the growth of 2D QDs. Next, we have demonstrated the growth of triangular 1L-WS2 flakes using the CVD technique at a relatively lower temperature ~750°C in a semi-confined growth system. The size of these 1L-WS2 flakes was ~20 µm. By tuning the growth parameters such as carrier gas pressure, temperature, and duration, we were able to grow a larger area monolayer to bilayer WS2.Finally, we have grown a large area of continuous 1L-WS2 by using a NaCl-assisted CVD growth approach. The role of growth temperature, pressure, duration, precursors, and substrates is significant in the growth of highly crystalline 1L-WS2. Moreover, controlling the vapor pressure locally using a one-end sealed quartz tube inside the system and the Na+ ions from the NaCl ensure the growth of large-area and high-quality monolayer WS2 on a variety of substrates. We have carried out systematic studies of the substrate-dependent Raman and PL spectra of the as-grown WS2. The growth substrates have a significant impact on the PL line shape, peak position, and emission intensities. The sapphire substrates yield better quality 1L- WS2 film in all aspects. Advancing the ability to grow continuous and defect-free monolayer WS2
on at least 100 mm wafers is crucial to realize any applications for WS2 as well as any other 2D materials. It can be realized in more high end sophisticated experimental setups such as the MOCVD.
B. Detailed Spectral Analysis of WS2 Quantum dots and Study of Photoluminescence Quenching using Single-walled Carbon Nanotubes
We have demonstrated the optimized synthesis of ultra-small WS2 QDs that exhibit a high PL quantum yield of ~15%, without any functionalization. The QDs have a high exciton binding
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energy and rich PL emission features. A detailed analysis reveals that the excitation wavelength- dependent broad PL emission results from different contributions from the excitonic transitions from the spin-orbit split valence band at the K, K´ point, and also from the defect states. The PL emission of WS2 QDs is systematically quenched in the presence of single-walled carbon nanotubes (SWCNT). The quenching is pronounced in SWCNT with higher defect densities. XPS and Raman analyses support the formation of WS2 QD/SWCNT complexes that lead to the overall improvement in the crystallinity of the SWCNTs, shown for the first time. The PL quenching is attributed to the formation of non-fluorescent ground state complexes, which leads to lesser availability of the highly fluorescent QDs in the system. The strong bonding between the WS2 QDs and the SWCNTs allows charge transfer from the QDs to the SWCNTs, which is also partially responsible for the quenching. The results are insightful in understanding the interaction between WS2 QD and SWCNTs and regulating the fluorescence intensity of WS2 QDs, which is important for various applications in biomedical areas. This work has been published in “J. Colloids & Interf.
Sci., 2020, 561, 519.”
C. A High-Performance Si/Au/WS2 QD-based Schottky Photodetector with Highly Suppressed Dark Current and Fast Photoresponse
We have fabricated a high-performance p-n junction photodetector (PD) by combining n- type WS2 QDs with p-type Si platforms, for the first time. The I–V characteristics of PD confirm the formation of rectifying p-n heterojunction at the interface between the n-type WS2 QDs and p- type Si. The hybrid Si/WS2 QD PD acts as a self-powered photodetector and shows good photodetection performance. Furthermore, a Schottky Si/Au/WS2 PD was fabricated using a p-Si substrate with a uniform array of Au NPs (average size ~6.7 nm) and the n-WS2 QDs. The incorporation of Au NPs greatly improved Ion/Ioff ratio (~1.3×105), by the suppression of the dark current by several orders of magnitude accompanied by an increase in the photocurrent. The Schottky PD exhibited a fast response with photocurrent rise/fall time of~4.4/43.5 s. The device responsivity and detectivity were ~276.2 A/W and ~4.3 × 1013 Jones, respectively. The Au NPs serve as a carrier tunneling pathway and lead to the generation and transport of hot electrons for ultra-fast transport of photo-generated charge carriers. This PD can be operated without any external bias. Our findings pave way for designing 2D TMD and plasmonic NP-based
heterojunction devices with superior performances for state-of-the-art optoelectronic applications.
This work has been published in “ACS Appl. Electron. Mater. 2021, 3, 11, 4891–4904.”
D. Photoluminescence Modulation and Charge Transfer Dynamics in Monolayer WS2
Decorated with Bi2O2Se Quantum Dots
We have demonstrated the modulation in the light emission of the CVD grown 1L-WS2 by decoration with non-van der Waals 2D semiconducting Bi2O2Se QDs. A systematic quenching of the PL intensity of 1L-WS2 was observed by varying the concentration of the Bi2O2Se QDs. Kelvin Probe Force Microscopy (KPFM) was employed to estimate the work function of the as-grown 1L-WS2. On being decorated with the Bi2O2Se QDs, the work function of 1L-WS2 decreased, which provided direct evidence of charge transfer from the QD to the 1L-WS2. A thorough analysis using a four-energy level model was carried out to explain the decrease in the PL intensity of the 1L-WS2 and the accompanied redshift with an increase in the concentration of the Bi2O2Se QDs.
The role of defects was also established in the doping density-dependent recombination dynamics of excitons in the system through quantitative analysis. The quenching of the PL emission is due to the conversion of neutral excitons to trions owing to the surplus electrons in the system. The doped electron density is as high as Δne ~6.6 × 1013 cm-2, which indicates high n-type doping in the 1L-WS2. Thus, controlled doping of 1L-WS2 was achieved simply by varying the concentration of the Bi2O2Se QDs, for the first time. Our results potentially facilitate further research in understanding the fundamentals of light-matter interactions in 2D heterostructure systems and further, pave way for various electronic and optoelectronic applications. This work has been published in “J. Phys. Chem. C 2022, 126, 30, 12623–12634.”
E. Photoluminescence Modulation in a Quantum Well of Monolayer WS2 sandwiched between ZnO layers
Herein, we have grown high-quality 1L-WS2 films on a variety of substrates by the CVD method. We studied the room temperature Raman and PL emission spectra of as-grown and transferred 1L-WS2, which are largely affected by factors such as strain and doping. As grown 1L- WS2 shows emission from biexcitons, at low temperatures. Interestingly, the PL spectra of transferred (unstrained) 1L-WS2 consist of the signature of biexcitons even at room temperature.
Subsequently, to tune the PL emission of 1L-WS2, we constructed a quantum well (QW) structure using ZnO layers, which is a higher band gap semiconductor. The power-dependent and
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temperature-dependent PL emission characteristics of the ZnO/WS2/ZnO QW and unstrained 1L- WS2 film were carefully examined. The biexciton emission becomes dominant at low temperatures and high laser powers for 1L-WS2. At low excitation powers, the influx of charge carriers from ZnO to WS2 aids in a ~10 folds PL enhancement due to the QW effect. Biexcitons are less likely to be formed through the collision of excitons at low powers, and also the trion formation rate is comparatively low. At higher excitation powers, however, the possibility of the neutral exciton to trion formation dominates in ZnO/WS2/ZnO, which hinders the enhancement of PL intensity.
Therefore, through the fabrication of the ZnO/WS2/ZnO QW, we were able to successfully tune the optical properties of 1L-WS2 by manipulating the exciton and electron density of the system, for the first time. The temperature-dependent studies enabled interesting insights into the QW system. The results offer a detailed insight into the carrier dynamics in 1L-TMDs on encapsulation with higher band gap semiconductors, which will be beneficial in the development of future optoelectronic and photonic devices.
7.2. Scope for Future Work
In the present thesis, we have achieved a controlled synthesis of ultra-small and highly fluorescent WS2 QDs by chemical exfoliation and highly crystalline large-area monolayer WS2 by CVD technique. Additionally, we have presented thorough investigative studies on the fabrication of heterostructures of WS2 QDs with SWCNTs and plasmonic Au NPs and of 1L- WS2 with non- van der Waals Bi2O2Se QDs and ZnO thin film for applications in photoluminescence and photodetection. There is enormous scope to extend the present work to understand the fundamentals of light-matter interactions in 2D TMD systems and in turn, utilize them for a broad range of applications:
1. WS2 QDs and monolayer WS2 can be combined to form heterostructures with several other 2D materials, e.g., MoSe2, WSe2, h-BN, Bi2O2Se, PdSe2, MXenes, and etc. to enable a wide range of device applications.
2. The monolayer and bilayer WS2 nanosheets (a byproduct of chemical exfoliation), given their excellent absorption in the UV-visible region and large surface-to-volume ratio serve as a promising system to form hybrid structures with new-age 2D semiconductors for photoelectrochemical hydrogen production and photodetection.