# Directly Synthesized Graphene-Based Photonics and Optoelectronics Devices
###### Abstract  
In the past two decades, extensive research and studies have been performed on graphene because of its exceptional physical properties. Owing to its ultrahigh carrier mobility, quantum Hall effect and unique optical transmittance, graphene is considered to be a multi-functional component for realizing next-generation optoelectronic and photonic devices. Significant efforts have been made towards efficient synthesis, transfer, and integration of graphene for use in device scale. However, the critical hurdles lie in developing 3D and conformal graphene, which are ideal for integrated hybrid photonic systems. Here, we review different methods of synthesizing graphene, specifically recent advances in the synthesis of direct, conformal, 3D graphene. In addition, we comprehensively summarize the latest progress made towards directly grown, 3D, conformal graphene-based photonic and optoelectronic applications. Finally, several important challenges for large-sale implementation of directly grown graphene-based optoelectronic and photonic devices are discussed.  
direct synthesis of graphene; 3D graphene; ACS method; optical nonlinearity; graphene photonics; ultrafast laser; ultrafast optical switching +
Footnote †: journal: Applied Sciences  
**2021**, _11_, _2768_.  
characteristics forge the path for graphene to be a potential material for optoelectronic and photonic applications [15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30].  
Graphene synthesis was first accomplished by applying techniques such as micro- mechanical rift and epitaxial growth on SiC at elevated temperatures. For practical utilization, it is imperative to use synthesis methods that produce defect free, high quality graphene. Therefore, only limited approaches, such as mechanical exfoliation, carbon nanotube unzipping, epitaxial growth, self-assembly of surfactants, chemical vapor deposition (CVD), and chemical conversion, have been employed by researchers to synthesize graphene to meet the conditions of practical utilization [31; 32; 33; 34; 35; 36; 37; 38]. Synthesis of monolayer graphene on a larger scale was achieved by large improvements to the CVD technique [39; 40; 41; 42; 43; 44; 45]. However, these approaches cannot ensure direct use of graphene in devices for many notable reasons, such as the use of metal catalysts, chemical etching of catalytic materials, transfer or exfoliation, complex chemical reactions, and high operational temperatures.  
More importantly, it has been seen that optoelectronic and photonic devices are usually compact and not always planar; therefore, the transfer of 2D graphene synthesized by the prevailing methods onto practical non-planar, curvilinear, or customized substrates has critical limitations and disadvantages. Additionally, graphene is usually damaged under the mechanical and chemical stress of post-transfer treatment [46; 47; 48; 49]. Therefore, a transfer-free, direct synthesis of graphene on arbitrary surfaces is highly anticipated for the development of optoelectronic and photonic devices. Furthermore, in the case of optoelectronic and photonic devices, it is not possible to exploit the efficient light-graphene interaction because 2D graphene cannot cover 3D devices in a conformal manner. Accordingly, an advanced study of graphene synthesis that promises to be more direct, less complex, cost-efficient, and readily applicable to 3D structures for device manufacturing is highly demanded, confirming the utilization of the potentials that graphene offers.  
In this review, we present a brief summary of growth of graphene on metal catalysts and dielectric substrates through CVD. Additionally, transfer-free and direct synthesis of graphene realized in most recently adopted processes, in situ growth, and atomic carbon spraying (ACS) are discussed in this review. The demonstration of the excellent optical properties of the resultant graphene derived from the direct synthesis methods for use in optical devices is also discussed.