Synthesis and Device Applications of Graphene Derivatives and Quantum Dots

The world currently relies on carbon sources to meet its energy needs; main sources include oil, coal, and natural gas.

This chapter provides a comprehensive review of graphene-based materials, including graphene, graphene oxide (GO), and graphene quantum dots (GQDs), with a focus on their synthesis methods, chemical and optical properties, and applications, particularly in solar cell-related devices. We begin with the background of graphene in its structure and electronic properties. This chapter then delves into the synthesis of graphene oxide, detailing the various chemical and mechanical methods employed, along with characterization of graphene. The section on graphene quantum dots emphasizes their synthesis, which is pivotal due to their size-dependent properties and discusses the optical properties that make GQDs suitable for optoelectronic applications. This includes an analysis of the incorporation of graphene and its derivatives can enhance the efficiency and stability of solar cells, along with a discussion of the latest advancements and challenges in this field. The chapter aims to provide a thorough understanding of the state-of-the-art synthesis techniques, the unique properties of these materials, and their applications in solar cell technology.

Chapter 3 provides a comprehensive exploration of the synthesis of graphene oxide (GO), a critical precursor for graphene-based materials in solar cell applications. The chapter presents synthesis methods of GO, especially Hummer’s method, analyzing its efficiency and adaptability for producing high-quality GO. The introduction of oxygen groups, lattice defects, and surface functional groups disrupts the conjugated structure of graphene, effectively insulating GO. The modified electronic structure of GO provides strong scattering centers that reduce carrier transport. The produced GO with high quality is prepared to synthesize RGO, described in the next chapter. The chapter summarizes the critical importance of GO synthesis in advancing the performance and development of next-generation solar cell technologies.

Developing a novel two-step reduction method to synthesize reduced graphene oxide (RGO) can improve the electrical conductivity and tune the work function of RGO for optoelectronic applications. In GO, the sp³ C–O bonds disrupt charge transfer in the C=C clusters of its honeycomb structure. Restoring the graphene structure increases the conductivity by several orders of magnitude [117]. By removing oxygenated groups via chemical reduction, GO is converted to RGO, which is an ideal TCE to replace conventional TCEs like expensive, brittle ITO. However, as discussed in Chap. 2, the main challenge with RGO is achieving conductivity comparable to ITO. This chapter focuses on the synthesis of RGO via a two-step reduction technique, aimed at both augmenting its electrical properties and fine-tuning its work function. It explores the fabrication of transparent RGO films through spin coating on glass substrates and discusses the application of free-standing films, characterized by their high electrical conductivity,flexibility, and durability, in the realm of flexible electronics.

Chapter 5 presents an in-depth study on the utilization of reduced graphene oxide (RGO) as a gate metal in metal-oxide-semiconductor capacitor (MOSCAP) devices, highlighting its potential as an alternative to traditional metals. The chapter focuses on the synthesis of RGO through thermal annealing to enhance electrical conductivity and uniformity. The electrical properties of RGO, including its conductivity and work function, are measured by the four-point probe method and ultraviolet photoelectron spectroscopy (UPS). The calculated work function differences between the RGO and the semiconductor illustrates how RGO offers a better match compared to aluminum. The high performance of MOSCAP devices with RGO gate metals is demonstrated through capacitance-voltage (C-V) measurements, which are employed to assess the quality and reliability of the oxide layer in these devices. This chapter effectively shows the advancements in MOSCAP technology facilitated by the integration of RGO, marking a significant step forward in semiconductor device engineering.

Chapter 6 introduces a novel stepwise method for synthesizing graphene quantum dots (GQDs) using a combination of strong acids and hydrogen peroxide (H₂O₂) to oxidize graphite. The produced GQDs have quantum confinement effects and excellent stability. The addition of H₂O₂ enhances the oxidation process, facilitating the cleavage of graphite into smaller fragments, thereby achieving a photoluminescence (PL) peak centered between 480–520 nm. The chapter explores benefits of using graphite as a precursor, emphasizing how it helps maintain the graphitic structure, and how the functional groups and the conjugated structure influence the PL intensity. Additionally, the chapter explores the post-treatment processes, including pH modification of GQDs to neutralize the acidic solution. The resulting GQD film exhibits high PL intensity, showcasing the potential of this method in optoelectronic applications. The primary focus of this chapter is the exploration of the impact of H₂O₂ on the PL properties of GQDs and to investigate the underlying mechanisms contributing to their enhanced PL behavior.

Chapter 7 explores the significant potential of graphene quantum dots (GQDs) in the fabrication of high-performance optoelectronic devices, low toxicity, tunable photoluminescence, and cost-effectiveness. The chapter addresses the challenges in implementing GQDs in devices, particularly the issues of poor adhesion on substrates and interface contact resistance. To deeply understanding the physical mechanisms of GQDs in device contexts, the chapter explores surface functionalization and interface heterojunctions. It details various designs of GQD-based photodetectors utilizing top-down synthesized GQDs as the active layer. The GQDs are passivated by polyethylene glycol (PEG) to form the PEG-passivated GQDs to modify the surface to be hydrophilic. Furthermore, Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT: PSS) is functionalized as the hole transport layer, forming a heterojunction at the interface of PEG-GQDs/PEDOT: PSS in planar devices. The device is fabricated by physical vapor deposition (PVD) and solution processing method. The performance of the device is characterized by the behavior under the illumination of UV and LED light. Optical and electrical characterization methods are employed to study charge separation and carrier transport mechanisms. Conclusively, the GQD-based photodetectors as simple yet effective solutions, showcasing their promising performance in optoelectronic applications.

This work explores the synthesis of alternative graphene based materials and their applications in optoelectronics, focusing on photovoltaic and photodetector devices. It introduces innovative chemical synthesis methods for graphene products, emphasizing environmentally friendly processes that yield materials with exceptional electrical, optical, and chemical properties. Notably, reduced graphene oxide (RGO) films exhibited transparency and tunability in work function, comparable to traditional materials like ITO. The study also developed graphene quantum dots (GQDs) with high photoluminescence, optimized for use in photodetectors. Subsequent chapters detail the application of these materials in MOSCAP devices and photodetectors, highlighting their significant potential to revolutionize optoelectronic device technology and pave the way for future innovations.