Two-dimensional (2D) semiconducting materials have been studied extensively for their interesting excitonic and optoelectronic properties arising from strong many-body interactions and quantum confinement at 2D limit. Most of these materials have been inorganic, such as transition metal dichalcogenides (TMDCs), phosphorene, etc. Organic semiconductor materials, on the other hand been investigated for their excellent electrical conductivity and low dielectric coefficients for similar applications in the thin film or bulk material phase. The lack of crystallinity in the thin film and bulk phases has led to ambiguity over the excitonic and electronic/optical band gap characteristics. The recent emergence of 2D organic materials has opened a new domain of high crystallinity and controlled morphology, allowing for the study of low-lying excitonic states and optoelectronic properties. They have been demonstrated to have different excitonic properties compared with the Wannier-Mott excitons in inorganic 2D materials. Here we present our recent experimental observations and analysis of 2D organic semiconductor materials. We discuss the role of high-crystalline and morphology-controlled growth of single-crystalline materials and their optoelectronic properties. The report explains the Frenkel (FR) and charge-transfer (CT) excitons and subsequent light emission and absorption properties in organic materials. The true nature of low-lying excitonic states, which arises from the interaction between CT and FR excitons, is experimentally studied and discussed to reveal the electronic band structure. We then discuss the pure FR behaviour we observed in J–type aggregated organic materials leading to coherent superradiant (SR) excitonic emissions. The supertransport (ST) of excitons within the organic materials, facilitated by their pure FR nature, and the delocalization of excitons over a large number of molecules are also demonstrated. Finally, we discuss the applications and our vision for these organic 2D materials in fast OLEDs, high-speed excitonic circuits, quantum computing devices, and other optoelectronic devices.

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Controlling the polarization state is an efficient way to enhance the functionalities in sensing, imaging, and communication systems in the microwave and terahertz (THz) spectrum. This study proposed an anisotropic metasurface and numerically explained it as a highly efficient polarization conversion using far-infrared frequency. Structural design, optimization, and results examination of the metasurface are performed using the CST microwave studio electromagnetic simulator. The metasurface was developed surrounding the two individual arrow-shaped metal resonators with two bar resonators on the opposite angular side of the arrow. Aluminum (Al) was used as a metallic resonator, while gallium arsenide (GaAs) was as the substrate. The interference theory was used to describe the co-polarized and cross-polarized reflectance coefficients, where two different mediums and interference layers were considered along with the reflected and transmitted wave. The polarization conversion efficiency yielded over 90% from 282.9 to 302.3 µm (0.987–1.062 THz) and 558.78 to 676.7 µm (0.442–0.537 THz) indicating multiple resonances at 286.7, 298.25, 586.1, and 689.55 µm. In conclusion, the performances and diverse characteristics of the designed metasurface demonstrated potential applications in the far-infrared spectrum as an efficient polarization converter application.

As a rising star of two-dimensional (2D) materials, 2D organic materials have inspired massive interest due to their remarkable merits such as a large materials library, intrinsic flexibility, diverse synthesis techniques, etc., which provide them with great prospects for flexible optoelectronics applications. Moreover, highly ordered 2D organic materials exhibit ultrathin features, low symmetry and unique anisotropy, and all these open a new avenue to achieve high-performance 2D organic materials devices and benefit their integration into optoelectronics. Herein, we first review several important growth techniques of 2D organic materials. Second, we summarize the recent progress in anisotropy characterizations of 2D organic materials and their applications in the optoelectronics field. Finally, we presented an outlook of anisotropic 2D organic materials in terms of challenges and opportunities. We believe this chapter will be an important reference for designing and developing novel 2D organic materials and integrating them into next-generation optoelectronics, provoking more researchers to come to this field.