## 1 Introduction  
Graphene, a two-dimensional material consisting of single layer carbon atoms in hexagonal lattice, has attracted much attention since [PERSON] et al. [1, 2] first produced it from graphite in a lab through the mechanical exfoliation method. Thanks to its excellent optical, electrical, mechanical, and thermal properties, graphene is an ideal material for many applications such as flexible touchscreen, organic light emitting diode (OLED), chemical sensor, and biological devices [3, 4, 5, 6, 7]. In order to achieve industry-scale production, various graphene synthesis methods were developed by researchers [8, 9, 10, 11, 12]. Among them, the chemical vapor deposition (CVD) technique is most effective owing to its robust capability of large area monolayer graphene production [9, 13]. However, as-grown graphene film on metal catalyst (Cu, Ni) through the CVD process has intrinsic disorders at grain boundaries induced by the metal recrystallization under high temperature (\(>\)1000 \({}^{\circ}\)C) [14]. In addition, the transfer process via polymer thin film and etching will introduce defects, cracks, and wrinkles on the graphene layer [9; 15; 16; 17]. These drawbacks will reduce the electrical conductivity and mechanical strength of the as-grown graphene. Recently, a silver nanowire (AgNW) network has been embedded between two graphene layers to form a Gr/AgNW/Gr sandwich-structure composite [3; 6; 18; 19; 20]. The proposed composite has excellent electrical conductivity and optical transmittance. In particular, the electrical conductivity remains stable after more than 20,000 bending cycles [18], which indicates its robust mechanical strength and flexibility. Moreover, the two graphene layers encapsulation can prevent AgNW from oxidization, thus the composite has a long-term stability of performance.  
Although the Gr/AgNW/Gr composite shows excellent properties, there are three main technical challenges that limit its broader applications: (1) the bottom substrate (silicon, PET, glass, and so on) for composite synthesis reduces the optical transmittance, electrical conductivity, and mechanical flexibility. For applications such as battery, transparent electrode, and biological devices, free stand Gr/AgNW/Gr film is essential to achieve the required functions. (2) Interfacial bonding mechanism between AgNW and graphene layers is unknown, which is critical for the mechanics model and performance analysis. (3) Correlation between the composite properties and AgNW concentrations has not been studied, which is necessary for the optimization of the structure design.  
In this paper, a vacuum annealing method is proposed for the first time to synthesize free-stand two-layer graphene and Gr/AgNW/Gr composite on TEM grids. AgNW concentrations varying from 0.2 to 1.0 mg/mL are added for the nanocomposite to study the concentration effect. The average spacing of the AgNW network is measured first through scanning electron microscope (SEM) images and its correlation with concentration is then established. Optical transmittance at 550 nm and sheet resistance of the composite are characterized and compared with the two-layer graphene. Atomic force microscope (AFM) indentation is conducted and the stiffness of both the two-layer graphene and composites with different AgNW concentrations is obtained and compared from the force-displacement curve. Raman spectroscopy is conducted to prove the graphene existence and the disturbance of the AgNW network. The characterization results can be utilized for nanocomposite design and interfacial bonding study between the AgNW and graphene layer.