In our work, Au@N-GQD is used to fabricate a high-speed photodetector without using a conductive or charge-transport layer in a device with better performance. Giri, "Quantitative Understanding of Ultra-Sensitive and Selective Detection of Dopamine Using Graphene Oxide/WS2 Quantum Dot Hybrid".
Introduction
Morphology and Structure of GQDs
GQDs are one of the emerging derivatives of graphene, classified as a zero-dimensional (0D) semiconductor material with a lateral size <10 nm.12 GQDs have a graphene core with sp2 hybridized carbon and a large number of oxygen functional groups at their edge sites as well as basal planes. Among them, there are epoxy (–COC–) groups mainly on the basal plane of GQD and hydroxyl (–OH), carbonyl (–HC=O), carboxyl (–COOH), etc. are attached at the edge sites of GQDs. 14, 16 Normally, GQDs are circular/elliptical in shape, but the shape change of GQDs from circular to polygonal has been reported experimentally. 10, 34-36 The size and shape of GQDs can be changed by controlling the synthesis conditions, such as the reaction time, temperature, reaction medium, etc.14, 37 To synthesize GQDs, the graphene sheet is mainly cut in two different crystallographic directions, leading to two types of GQD edge structures, namely armchair and zigzag. 10, 38 Figure 1.1 shows armchair and zigzag edges in GQD. Among them, the (002) planes correspond to the graphitic GQD planes with a lattice spacing of nm, and the other GQD planes are formed due to the attachment of functional groups.39-41 Peng et al.
Optical Properties of GQDs
Bandgap modulation and corresponding PL emission color in GQDs via size variation. Band structure of GQDs for intrinsic and extrinsic PL emission together with the center of the non-radiative trap.
Synthesis of GQDs
- Top-down Methods
- Bottom-up Methods
Based on the above, the present thesis focuses on a top-down method for the synthesis of GQDs. Schematic of the hydrothermal cutting mechanism of a graphene sheet using a top-down method for the synthesis of GQDs.
Chemical Doping in GQDs
Moreover, the origin of PL emission was not well addressed and there was no information on the PL QY of B-GQDs. Thus, doping is a very useful tool for tuning the optical and electronic properties of GQDs according to requirements.
Functionalization of GQDs
It was proposed that the coexistence of three independent luminescent centers, C=O, C=N and C=S, is the reason for the single excitation band-dependent PL emission in S,N co-doped GQDs. However, the identification of the doping sites and the understanding of the doping mechanism were not addressed in the literature80, 90, 91.
GQDs Based Heterostructures
Applications of Various Type of GQDs and their Heterostructures
- Applications as Sensor: Metal Ions, Bio-Molecules and
- Optoelectronic Applications: Photodetector, Light Emitting Diodes
- Photocatalytic Applications
- Energy Storage Applications: Supercapacitor and Battery
- Bio-Medical Applications: Bioimaging and Drug Delivery
Although numerous applications of GQDs and their HSs have been reported, there is great scope for improving the sensing performance for sophisticated applications. GQDs are widely used for the fabrication as well as development of more useful optoelectronic devices such as high-performance photodetectors (PDs), light-emitting diodes (LEDs), solar cells, etc.
Unresolved Issues and Challenges …
Understanding the mechanism of PL emission and pinpointing the origin of emission centers are the major challenges in GQDs research. Identifying the individual contribution of different factors in the SERS process is an important problem for the improvement of SERS-based sensors.
Focus of the Present Thesis
The use of bare GQDs or their hybrid structures in the optoelectronic devices without any commercial charge transporting layers can open a new direction in their applications. Detection of DA with ultrahigh sensitivity by a semiconductor-semiconductor-based hybrid structure with GO and tungsten disulfide QDs (GO/WS2).
Organization of the Thesis
Moreover, a comprehensive study of the DA sensitivity with the hybrid structure of GQDs and WS2 QDs have been addressed to reveal the reasons behind the ultra-high sensitivity of GO/WS2 to DA. The metal-semiconductor-based hybrid structure of N-GQDs, such as with Au plasmonic NPs (Au@N-GQD) have been investigated for multipurpose applications, such as DA, Fe3+ sensing.
Anomalous Photoluminescence Enhancement and Photoluminescence
Introduction
Chen and co-workers reported nuclear-targeted drug delivery with TAT conjugated to GQD peptide along with a drug release process.142 In the drug delivery process, GQDs acted as drug carriers, and TAT helped to attach the drug to the cell nucleus. In this chapter, the PL intensity tuning of U-GQD by variation of SWCNT concentration is studied along with their interaction mechanism.
Experimental Details
- Sample Preparation …
- Synthesis of Graphene Oxide
- Synthesis of Undoped GQDs
- Characterization Techniques
Initially, different concentrations of SWCNT solution were prepared separately in MQ water through appropriate sonication. Different concentrations of SWCNTs (10–60 g/ml indicated as 10–60, respectively, from right to left) (b) just after sonication and (c) 20 minutes after sonication.
Results and Discussion
- Morphology Studies
- Structural Analysis: XRD and Raman Analysis
- Optical Analysis
- UV-vis Absorption Study
- Photoluminescence Study
- Time-Resolved Photoluminescence Study
- Confocal Imaging
S and M indicate the semiconducting and metallic nature of the SWCNTs. the diameter of SWCNTs using eqn. In the high concentration region of SWCNTs (>10 g/mL), the systematic quenching of the PL intensity of U-GQDs and the corresponding change of the relative PL intensity with SWCNTs are shown in Fig.
Summary and Conclusions
The dominant metallic nature of SWCNTs at low concentration increases the incident local field on the U-GQD fluorophores through a plasmonic effect that results in PL enhancement in the U-GQD. At higher SWCNT concentrations, PL U-GQD damping occurs with contributions from both static and dynamic damping.
Origin of High Photoluminescence Yield and High SERS Sensitivity of Nitrogen-
Introduction
It is essential to understand the contribution of each process to engineer the SERS performance of GQDs for subsequent applications. By studying the structural and optical properties of different types of GQDs, the origin of the high PL QY and high SERS sensitivity of N-GQDs is investigated here.
Experimental Details
- Sample Preparation …
- Synthesis of Graphene Oxide
- Synthesis of Undoped GQDs
- Synthesis of Nitrogen-doped GQDs
- Synthesis of Sulfur-doped GQDs
- SERS Detection
- Fabrication of Liquid Phase White LED
- Characterization Techniques
A scheme of the synthesis process of different types of GQDs in different media and the characteristics of the resulting products is shown in Fig. Reaction scheme of different types of GQDs with different solvents and the chemical characteristics of the resulting products.
Results and Discussion
- Morphology Studies
- Structural Analysis
- XRD and XPS Analysis
- Raman and FTIR Spectral Analysis
- Optical Analysis
- UV-vis Absorption Study
- Photoluminescence Excitation Spectra Study
- Room Temperature Photoluminescence Study…
- Low-Temperature Photoluminescence Study
- Time-Resolved Photoluminescence Study
- Application of N-GQDs
- N-GQDs as SERS Substrate
- N-GQDs for Liquid Phase White LED Fabrication
The peak position of the deconvoluted Raman spectra in the range 1060–1650 cm-1 for the different types of GQDs. A digital photograph of N-GQDs with illumination of ~360 nm light is presented in the inset of Fig.
Summary and Conclusions
The EL spectra of the as-prepared UV LEDs/N-GQDs were recorded at different bias voltages in the V region as shown in the figure. the broad EL spectrum of N-GQD with a peak at ~503 nm both have a non-linear nature (see Fig.
Quantitative Understanding of the Ultra-Sensitive and Selective Detection of
Introduction
Based on the above, we study the interaction between GO and WS2 QDs and exploit the GO/WS2 hybrid for the detection of DA with ultra-high sensitivity and selectivity with PL quenching of the hybrid. Furthermore, the sensing of spiked DA in Brahmaputra River water and human blood serum is successfully achieved with the newly proposed GO/WS2 hybrid sensor.
Experimental Details
- Sample Preparation …
- Synthesis of Graphene Oxide
- Synthesis of WS 2 QDs
- Sensing of Dopamine with GO/WS 2 Hybrid
- Characterization Techniques
Therefore, the unusual nature of the PL quenching of GO/WS2 hybrid is explained for a wide range of DA concentrations (1 nM–10 M) by the combined process of non-fluorescence complex formation and charge transfer dynamics. Next, for the sensing of DA, the quenching efficiency for GO/WS2 hybrid was optimized by varying the GO concentration with 8.4 g/mL WS2 QDs solution and a fixed concentration of DA.
Results and Discussion
- Morphology Studies
- Structural Analysis
- XRD and Raman Analysis
- XPS Analysis
- Optical Analysis
- UV-vis Absorption Study
- Photoluminescence Study
- Dopamine Sensing with GO/WS 2 Hybrid
- Mechanism of Superior Dopamine Sensing with GO/WS 2 Hybrid….… 100
- Analysis of Real-life Samples
The XRD pattern of GO/WS2 hybrid confirms the presence of the graphitic component together with WS2 in the hybrid. A comparative study of the PL quenching of WS2 QDs hybrid with U-GQDs and S-GQDs in the presence of DA is shown in Fig.
Summary and Conclusions
Mechanistic Insights into Highly Sensitive and Selective Label-free Detection of
Introduction
Based on the phenomenon of the compound formation, the newly developed sensor is used for the efficient detection of DA by measuring UV-vis absorption as well as the fluorescence quenching. Moreover, this sensor is applied for the detection of spiked DA in the real samples, such as Brahmaputra river water, and human serum, with high selectivity as well as recovery.
Experimental Details
- Sample Preparation …
- Synthesis of Au@N-GQDs…
- Detection of Dopamine…
- Characterization Techniques
For the effective sensing of DA, different concentrations of stock solution of Au@N-GQDs were used to find out the optimal concentration and repeatability of the sensing. For comparison, UV-vis absorption and PL spectra of bare N-GQDs were also recorded.
Results and Discussion
- Morphology Studies
- Structural Analysis
- XRD Analysis
- Raman and FTIR Spectral Analysis
- Optical Analysis
- Colorimetric Sensing of Dopamine.…
- Fluorescence Sensing of Dopamine
- Time-Resolved Photoluminescence Study
- Selectivity of Dopamine Sensing
- Analysis of Real Samples
The size distribution of N-GQD in the Au@N-GQD/DA composite is shown in the inset. The change in PL intensity is presented in figure a) Comparison of PL spectra of N-GQD and Au@N-GQD.
Summary and Conclusions…
In the photometric detection strategy, the relative change of PL intensity of Au@N-GQDs follows a linear Stern–Volmer equation due to the combined effect of ground-state complexation and efficient charge transfer from N-GQDs to the core GQDs. scale structure.
Quantitative Understanding of Charge Transfer Mediated Fe 3+ Sensing and Fast
Introduction
Herein, we demonstrate a major improvement in the performance of the N-GQDs-based photodetector through the incorporation of Au NPs through an in-situ growth along with the fast photoresponse, and we thoroughly investigate the underlying mechanism. Note that we do not use any charge transport layer in the N-GQDs-based photodetector device reported here.
Experimental Details
- Sample Preparation
- Synthesis of Graphene Oxide
- Synthesis of Nitrogen-doped GQDs
- Synthesis of Au@N-GQDs…
- Detection of Fe 3+ Ions…
- Device Fabrication for Photocurrent Measurements
- Characterization Techniques
When comparing the sensitivity, the effect of Fe3+ with N-GQD and bare N-GQD mixed with C-Au NPs (synthesized by the standard citrate reduction method) (N-GQD/c-Au NPs) was also considered ). For photocurrent measurement, 405 nm laser and 250 W xenon lamp (Newport, USA) along with a monochromator (Oriel Instruments, USA) were aligned for illumination.
Results and Discussion
- Morphology Studies
- Chemical and Structural Analysis…
- XPS Analysis
- XRD Analysis
- Raman Spectral Analysis
- Optical Analysis
- UV-vis Absorption Study
- Photoluminescence Excitation Spectra Study…
- Photoluminescence Spectra Study
- Time-Resolved Photoluminescence Study
- Applications of Au@N-GQDs
- Au@N-GQDs as Metal Ion Sensor…
- Au@N-GQDs as High-Speed Schottky Junction
A comparison of the (a) XRD pattern, (b) Raman spectra, and (c) FTIR spectra of N-GQDs and Au@N-GQDs. Evolution of the PL spectra of Au@N-GQDs with the increased Fe3+ concentration in (c) human serum, (d) Brahmaputra river water, (e) tap water and (f) drinking water.
Summary and Conclusions
Summary and Outlooks
Summary and Highlights of the Thesis Contribution
In the presence of DA, the enhancement of the UV-Vis absorption intensity and the quenching of the PL intensity of Au@N-GQDs have been used for the colorimetric and fluorometric detection of DA in the region M, respectively. The quenching behavior does. does not follow the known laws of quenching and the unusual quenching of the PL intensity of Au@N-GQDs in the presence of Fe3+ ions is modeled by solving the analytical rate equations, incorporating Langmuir's adsorption law and non-radiative charge transfer to the acceptor ions, for the first time.
Scope of Future Work
