Growth and Functionalization of Monolayer WS2 Quantum Dots and Films for Photoluminescence Modulation and Photodetector application

We investigate the charge transfer mechanism and tune the optical properties of 1L-WS2 by incorporating non-van der Waals Bi2O2Se QDs. Thus, the quenching of PL is largely traced to the charge transfer from Bi2O2Se QDs to 1L-WS2, resulting in the conversion of the neutral exciton to negative trion.

Introduction

Properties of WS 2

Crystal Structure
Electronic Band Structure
Optical properties

Inset shows the PL spectra of monolayer and bilayer WS2. b) The normalized PL spectra (with respect to peak A) of WS2. In addition to the VB cleavage of the strong spin-orbit coupling, monolayer WX2 shows a cleavage of the CB occurring at 11–40 meV. As such, two types of excitonic states are possible depending on the spin selection rule: 1) the spin allows bright exciton, formed by an electron in the highest conductance level and a hole in the highest valence level, at the K-point; and 2) the spin-forbidden dark state, which is formed by an electron in the lowest conductivity level and a hole in the highest valence level, at the K point.

Synthesis of 0D-2DWS 2

Mechanical Exfoliation
Intercalation and Chemical Exfoliation
Physical and Chemical vapor deposition

The exposure to air during ultrasonication aids in the expansion of the interlayer distance, and finally separation of layers leads to the synthesis of nanosheets as well as QDs. Therefore, when it comes to growing high-quality atomic-thickness thin films, chemical vapor deposition (CVD) is by far one of the most reliable methods.

WS 2 Based Heterostructures

Compared with the case of monolayer MoS2, large-area growth of monolayer WS2 is a challenging task, and several growth parameters affect the growth process.

Application of WS 2 and Its Heterostructures

Photoluminescence and Imaging
Sensors
Electrocatalytic and Photocatalytic Hydrogen Production …
Photodetectors & Solar Cells
Light-emitting diode (LED)

Both MoS2 and WS2 exhibited excellent catalytic activity, due to two main factors – (a) the QDs embedded in the respective nanosheets are rich in defects and provide abundant active edge sites for the hydrogen evolution reaction (HER ); (b) the random stacking of exfoliated nanosheets provided a larger surface-to-volume ratio aiding in improved electron transfer between the active edge regions and the underlying electrode 15, 59. Performance parameters of some of the WS2/QD films and of heterojunction photodetectors based on WS2 are summarized in Table 1.

Challenges in Fabrication and Applications of WS 2 Based Heterostructures …. 17

Controlled growth of large-area monolayer WS2 by CVD technique and study of the effect of growth conditions on the thickness and flake size of WS2. Tunability of the PL emission by the encapsulation of WS2 film with high bandgap ZnO and study of the quantum well effect on the excitation dynamics of monolayer WS2.

Organization of the Thesis

Detailed analysis of excitation wavelength-dependent photoluminescence spectrum of the WS2 QDs and investigation of interaction with single-walled carbon nanotubes. In Chapter 5, we have grown a WS2 film with monolayer thickness and investigated the modulation of PL and doping caused by decorating it with Bi2O2Se QDs.

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

Synthesis of WS 2 Quantum dots by Chemical Exfoliation

Introduction
Experimental details…
Characterization Techniques
Results and Discussion

Morphology Studies
Optical Analysis

Experimental details
Characterization techniques …

Morphology Study
Micro-Raman and Photoluminescence Analysis

CVD growth of monolayer WS 2 film

Introduction

Morphology & Raman study
Photoluminescence Analysis

4.4(c) shows the high-resolution S 2p XPS spectra for the WS2 QDs and Si/Au/WS2. We noticed that there is an improvement in the exciton binding energy in WS2. 5.10.: (a) Integrated PL intensities of the neutral exciton (IA0), trion (IAtr) and defect-bound exciton (IX) and their total (Itotal) as a function of the concentration of Bi2O2Se QDs.

However, at higher powers, there is a greater possibility for the formation of biexcitons in 1L-WS2.

Salt assisted large area CVD growth of monolayer WS 2

Introduction

Morphology Study
Raman Study
XPS Study
Photoluminescence Study

Growth mechanism

Conclusion

Kossacki, P.; Potemski, M., Orbital, Spin and Valley Contributions to Zeeman splitting of excitonic resonances in Mose ₂, Wse ₂ and Ws ₂ monolayers. Koçak, Y.; Akaltun, Y.; Gür, E., Magnetron Sputtered Ws₂; Optical and structural analysis. Lan, C.; Li, C.; Yin, Y.; Liu, Y., Large-Area Synthesis of Monolayer Ws2 and Its Environmentally Sensitive Photodetection Performance.

Wang, Z., et al., Nacl-assisted one-step growth of Mos₂–Ws₂ in-plane heterostructure.

Excitation wavelength-dependent spectral shift and large exciton binding

Experimental Details

Synthesis of WS 2 QDs
Details of SWCNTs for PL quenching

The excess solvent from the supernatant was evaporated and the WS2 QDs were redispersed in NMP at a concentration of 1 mg/mL for further use. The PL spectra of the WS2 QD solutions with the different concentrations of SWCNT1 were collected over the wavelength range of 425–750 nm under an excitation wavelength of 405 nm. The morphology, size and structural properties of the as-prepared WS2 QDs and WS2.

For low-temperature PL, a QD WS2 thin film inside a liquid nitrogen-cooled optical cryostat (Optistat DNV, Oxford Instruments) was used as a sample.

Results and Discussion

Structural and Compositional Analysis
Optical Analysis

UV-Vis Absorption Study
Photoluminescence Study
Time-Resolved Photoluminescence Study
Low-temperature Photoluminescence Study

Study of the interaction between WS 2 QDs and SWCNTs…

Raman Analysis of SWCNT/ WS 2 QD hybrid
Optical studies and PL quenching …

3.4(c) presents the comparative Raman spectra of bare WS2 QDs and the composite, showing the presence of characteristic WS2 modes. With the addition of SWCNT1, which acts as a PL quencher, a systematic decrease in the PL emission intensity of WS2. With further increase in SWCNT1 concentration, a systematic decrease in PL intensity is observed.

The scheme of SWCNT/WS2 QD complex formation is shown in the inset of Fig. 3b.

Conclusion

Yan, Y.; Zhang, C.; Gu, W.; Ding, C.; Li, X.; Xian, Y., Facile synthesis of water-soluble Ws2 quantum dots for switched-on fluorescent measurement of lipoic acid. Xu, S.; Li, D.; Wu, P., One-Pot, Facile and Versatile Synthesis of Monolayer Mos2/Ws2 Quantum Dots as Bioimaging Probes and Efficient Electrocatalysts for Hydrogen Evolution Reaction. Bai, X., et al., Ultrasmall Ws2 Quantum Dots with Visible Fluorescence for Protection of Cells and Animal Models from Radiation-Induced Damage.

Shi, J.; Liu, J.; Tian, F.; Yang, M., A fluorescence-triggered biosensor based on graphene quantum dots (Gqds) and molybdenum disulfide (Mos2) nanosheets for detection of epithelial cell adhesion molecule (Epcam).

Highly suppressed dark current and fast photoresponse in Au nanoparticle

Introduction

5.6(a) shows the multilayer F(R) curve of 1L-WS2 and the micro-PL spectrum obtained using 488nm laser excitation. 5.9(a) shows the evolution of PL spectra of 1L-WS2 with different QD concentrations. We performed Gaussian deconvolution of the PL spectra of 1L-WS2 and ZnO/WS2/ZnO QW.

In addition, the presence of trap states plays a critical role in the PL emission properties of the system.

Experimental Details

Synthesis of WS 2 QDs
Growth of Au Nanoparticles …
Fabrication of Si/WS 2 and Si/Au/WS 2 Heterojunction Photodetectors. 85

Results and Discussion

Structural and Compositional Analysis

Raman Analysis
XPS Analysis

Optical Studies on WS 2 QDs and Au/WS 2 Hybrid…
Performance as Photodetectors
Working Principle of the Photodetectors

Note that in the HS sample, the WS2 QDs are cast on top of the Au NPs grown on the Si substrate. In this section we discuss and compare the performance of the Si/WS2 and Si/Au/WS2. Next, we study the power-dependent photocurrent response of the Si/WS2 and Si/Au/WS2.

4.9(c, d) shows the photocurrent transient responses under different bias voltages for the Si/WS2 and Si/Au/WS2 PD heterojunction.

Conclusion

Liu, B., et al., A high-performance photodetector based on a graphene/Mos2/graphene lateral heterostructure with Schottky junctions. Xue, Y., et al., Scalable fabrication of a few-layer vertical Mos2/Ws2 heterojunction array and its application to photodetectors. Pak, Y., et al., Enhanced photoresponse of Ws2 photodetectors by interfacial defect engineering using a Tio2 interlayer.

Wang, K., et al., Interlayer coupling in twisted Wse2/Ws2 bilayer heterostructures revealed by optical spectroscopy.

Photoluminescence Modulation and Doping of CVD grown Monolayer WS 2 by

Introduction

At room temperature, these trions occupy an energy level lower than that of neutral excitons and serve as a relaxation pathway for the latter. The presence of these states can greatly modify the PL spectrum of single-layer TMDs. Experimental investigation and understanding of carrier recombination dynamics have been carried out in the past based on techniques such as time-resolved PL11-13 and ultrafast pump–probe spectroscopy 14-16 .

In this chapter, we present a thorough study on the photoinduced charge transfer and a theoretical model involving coupled rate equations to gain a better understanding of the carrier relaxation dynamics in the 1L-WS2/Bi2O2Se QD HS.

Experimental Details

CVD growth of monolayer WS 2
Synthesis of Bi 2 O 2 Se QDs and decoration on the 1L-WS 2 …

Chemical exfoliation of the as-synthesized NS was performed in N-methyl-2-pyrrolidinone (NMP) using a tip sonicator with ultrasonic frequency for 1 hour. For the decoration and formation of the HS, the QDs of different concentrations are spin-coated on the 1L-WS2 and dried before performing the optical characterization. The morphology and structural properties of the CVD-grown 1L-WS2 were studied using a transmission electron microscope (TEM) (JEOL-JEM 2100 operated at 200 kV).

The details of the AFM and micro-Raman and PL measurements were discussed in detail in Chapter 2 (section 2.2.3).

Results and Discussion

Optical analysis of 1L-WS 2 and Bi 2 O 2 Se QDs
Photoluminescence modulation and charge transfer dynamics….….… 121

The core-level XPS survey scans of the pristine 1L-WS2 and HS are shown in Fig. Summary of the deconvolution parameters of the Raman spectra of the 1L-WS2 and 1L-WS2/Bi2O2Se QDs. The Gaussian components in the deconvolution are neutral exciton (A0), negative trion (Atr) and defect bound exciton (X).

From the PL fit, we obtain the integral PL intensities corresponding to the exciton (IA0), trion (IAtr), and defect exciton (IX) of 1L-WS2 HS at different concentrations of Bi2O2Se QDs.

Photoluminescence Modulation of Monolayer WS 2 by ZnO encapsulation and

Introduction

Although their occurrence is mostly investigated in cryogenic systems, room temperature bi-exciton channel emission is only achieved at high laser excitation powers2. Here we constructed a quantum well by encapsulating the 1L-WS2 by higher bandgap ZnO in a sandwich structure. Thus, while the signature of biexcitons is rarely observed at room temperature4, the lack of dielectric screening and quantum confinement in the QW system helps at high exciton density1.

As such, we managed to observe rich PL features in the QW system, which are discussed in detail below.

Experimental Details

Growth of ZnO Thin film
Growth of 1L-WS 2 and Formation of ZnO/WS 2 /ZnO heterostructure. 136

Then, another layer of ZnO film is directly deposited on 1L-WS2 via RF sputtering to create a ZnO/WS2/ZnO QW-like encapsulated heterostructure. Details of AFM, micro-Raman, micro-PL and XPS measurements are discussed in Chapter 2 (Section 2.4.3). The morphology and structural properties of CVD-grown 1L-WS2 were studied using a transmission electron microscope.

Kelvin Probe Force Microscopy (KPFM) measurements were performed for the 1L-WS2 and the HS to estimate the work function.

Results and Discussion

Morphology Study
Compositional and Structural Analysis

XPS Analysis
XRD and Raman Analysis

Optical Analysis of as-grown and transferred 1L-WS 2
Photoluminescence tuning and QW effect…
Temperature-dependent PL study
Mechanism of PL Modulation

This may be due to the transfer process of the 1L-WS2 film to ZnO to fabricate the QW. The inset shows a red-shifted spectrum of the ZnO/WS2/ZnO QW, indicating electron transfer from the ZnO film to 1L-WS2. 6.6(a) shows the comparative UV-Vis absorbance spectra of ZnO and ZnO/1L-WS2 on quartz substrates.

6.9(a, b) shows the temperature dependence of the PL emission spectra corresponding to as-1L-WS2 and ZnO/WS2/ZnO, respectively.

Conclusion

The marked enhancement is also mainly due to the essentially trion-dominated PL spectrum of 1L-WS23, at low powers. Su, L.; Yu, Y.; Cao, L.; Zhang, Y., Effects of substrate type and material-substrate bond on the high-temperature behavior of a Ws2 layer. Kajino, Y.; Arai, M.; Oto, K.; Yamada, Y., Effect of dielectric screening on exciton resonance energy in Ws2 monolayer on Sio2/Si substrate.

Mother, J.; Amsalem, P.; Schultz, T.; Scheen, D.; Xu, X.; Koch, N., Energy Level Alignment at the C60/Monolayer-Ws2 Interface on Insulating and Conductive Substrates.

Summary and Outlook

Highlights of the Thesis

By changing the Bi2O2Se QD concentration, a systematic attenuation of the PL 1L-WS2 intensity was observed. Kelvin probe force microscopy (KPFM) was used to assess the work function of cultured 1L-WS2. Thus, the controlled doping of 1L-WS2 was achieved for the first time simply by varying the concentration of Bi2O2Se QDs.

Interestingly, the PL spectra of transferred (unstrained) 1L-WS2 consist of a biexciton signature even at room temperature.

Scope for Future Work

At low excitation powers, the charge carrier influx from ZnO to WS aids in a ~10-fold PL enhancement due to the QW effect. Bi-excitons are less likely to be formed by the collision of excitons at low powers, and the formation rate of trions is also relatively low. However, at higher excitation powers, the ability of the neutral exciton to trion formation in ZnO/WS2/ZnO dominates, hindering the enhancement of PL intensity.

Therefore, by fabricating the ZnO/WS2/ZnO QW, we were able to successfully tune the optical properties of 1L-WS2 by manipulating the excitation and electron density of the system for the first time.