Synthesis and Characterization of Free-Stand Graphene / Silver Nanowire / Graphene Nano Composite as Transparent Conductive Film with Enhanced Sti ﬀ ness

Featured Application: In this work, free-stand Gr / AgNW / Gr nanocomposite has been synthesized and characterized for interfacial bonding study, ultrasensitive sensor, and actuator application. Abstract: As-grown graphene via chemical vapor deposition (CVD) has potential defects, cracks, and disordered grain boundaries induced by the synthesis and transfer process. Graphene / silver nanowire / graphene (Gr / AgNW / Gr) sandwich composite has been proposed to overcome these drawbacks signiﬁcantly as the AgNW network can provide extra connections on graphene layers to enhance the sti ﬀ ness and electrical conductivity. However, the existing substrate (polyethylene terephthalate (PET), glass, silicon, and so on) for composite production limits its application and mechanics behavior study. In this work, a vacuum annealing method is proposed and validated to synthesize the free-stand Gr / AgNW / Gr nanocomposite ﬁlm on transmission electron microscopy (TEM) grids. AgNW average spacing, optical transmittance, and electrical conductivity are characterized and correlated with di ﬀ erent AgNW concentrations. Atomic force microscope (AFM) indentation on the free-stand composite indicates that the AgNW network can increase the composite ﬁlm sti ﬀ ness by approximately 460% with the AgNW concentration higher than 0.6 mg / mL. Raman spectroscopy shows the existence of a graphene layer and the disturbance of the AgNW network. The proposed method provides a robust way to synthesize free-stand Gr / AgNW / Gr nanocomposite and the characterization results can be utilized to optimize the nanocomposite design for future applications.


Introduction
Graphene, a two-dimensional material consisting of single layer carbon atoms in hexagonal lattice, has attracted much attention since Geim 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 Figure 1 shows the low preassure chemical vapor deposition (LPCVD) system for graphene synthesis. The mass flow controller (MFC) controls the quantity of gas that flows into the quartz tube for reaction. A 125 µm thick copper foil (99.9%, Alfa Aesar 13380, Alfa Aesar, Ward Hill, MA, USA) was pretreated in acetic acid (99.7%, Sigma Aldrich 695092, Sigma Aldrich, St Louis, MO, USA) for 48 h and then cleaned with DI water, IPA and acetone. Monolayer graphene was synthesized on the copper foil in the reaction chamber (quartz tube) with 5 sccm (standard cubic centimeter per minute) hydrogen flow for 30 min and then 5 sccm methane flow for 10 min under 1030 • C at~100 mTorr vacuum pressure. The as-grown graphene on copper was then cooled down to room temperature for next step usage. Appl. Sci. 2020, 10, x 3 of 12  Figure 2 shows the procedure of free-stand two-layer graphene and Gr/AgNW/Gr nanocomposite synthesis process. As-grown graphene on copper was spin coated with Poly (methyl methacrylate) (PMMA, Sigma Aldrich 182265, Mw = 996,000) in chlorobenzene (46mg/mL) solution at 4000 RPM for 30 s. The PMMA/Gr/copper was heated under 170 °C for 10 min to evaporate the chlorobenzene solvent and then floated on the copper etchant (Sigma Aldrich 667528) for 2 h to dissolve the copper substrate. The floated PMMA/Gr film was cleaned with deionized (DI) water twice and flipped over to make the graphene layer on the top. The Gr/PMMA film was transferred on a square meshed TEM grid (EMS 400-Ni, hole size 50 µm). A 3 µL solution of AgNW dispersed in isopropyl alcohol (IPA) (ACS Materials, Agnw-L100, diameter = 30 nm, length = 100~150 µm) with five different concentrations (0.2, 0.4, 0.6, 0.8, and 1.0 mg/mL) was applied on the Gr/PMMA surface through a micro syringe with needle. After the IPA dried at room temperature, another top layer PMMA/Gr film was transferred onto the AgNW/Gr/PMMA sample. The Gr/AgNW/Gr composite was free-standed on the TEM grid after a 30 min annealing process in vacuum (~50 mTorr) at 380 °C to evaporate the PMMA film. For comparison, a two-layer free-stand graphene film was also prepared by the similar procedures without applying the AgNW. The average thickness of the Gr/AgNW/Gr nanocomposite is ~31 nm based on the diameter of the AgNW (30 nm) and thickness of monolayer graphene (0.335 nm).   Figure 2 shows the procedure of free-stand two-layer graphene and Gr/AgNW/Gr nanocomposite synthesis process. As-grown graphene on copper was spin coated with Poly (methyl methacrylate) (PMMA, Sigma Aldrich 182265, M w = 996,000) in chlorobenzene (46mg/mL) solution at 4000 RPM for 30 s. The PMMA/Gr/copper was heated under 170 • C for 10 min to evaporate the chlorobenzene solvent and then floated on the copper etchant (Sigma Aldrich 667528) for 2 h to dissolve the copper substrate. The floated PMMA/Gr film was cleaned with deionized (DI) water twice and flipped over to make the graphene layer on the top. The Gr/PMMA film was transferred on a square meshed TEM grid (EMS 400-Ni, hole size 50 µm). A 3 µL solution of AgNW dispersed in isopropyl alcohol (IPA) (ACS Materials, Agnw-L100, diameter = 30 nm, length = 100~150 µm) with five different concentrations (0.2, 0.4, 0.6, 0.8, and 1.0 mg/mL) was applied on the Gr/PMMA surface through a micro syringe with needle. After the IPA dried at room temperature, another top layer PMMA/Gr film was transferred onto the AgNW/Gr/PMMA sample. The Gr/AgNW/Gr composite was free-standed on the TEM grid after a 30 min annealing process in vacuum (~50 mTorr) at 380 • C to evaporate the PMMA film. For comparison, a two-layer free-stand graphene film was also prepared by the similar procedures without applying the AgNW. The average thickness of the Gr/AgNW/Gr nanocomposite is~31 nm based on the diameter of the AgNW (30 nm) and thickness of monolayer graphene (0.335 nm).
Appl. Sci. 2020, 10, x 3 of 12  Figure 2 shows the procedure of free-stand two-layer graphene and Gr/AgNW/Gr nanocomposite synthesis process. As-grown graphene on copper was spin coated with Poly (methyl methacrylate) (PMMA, Sigma Aldrich 182265, Mw = 996,000) in chlorobenzene (46mg/mL) solution at 4000 RPM for 30 s. The PMMA/Gr/copper was heated under 170 °C for 10 min to evaporate the chlorobenzene solvent and then floated on the copper etchant (Sigma Aldrich 667528) for 2 h to dissolve the copper substrate. The floated PMMA/Gr film was cleaned with deionized (DI) water twice and flipped over to make the graphene layer on the top. The Gr/PMMA film was transferred on a square meshed TEM grid (EMS 400-Ni, hole size 50 µm). A 3 µL solution of AgNW dispersed in isopropyl alcohol (IPA) (ACS Materials, Agnw-L100, diameter = 30 nm, length = 100~150 µm) with five different concentrations (0.2, 0.4, 0.6, 0.8, and 1.0 mg/mL) was applied on the Gr/PMMA surface through a micro syringe with needle. After the IPA dried at room temperature, another top layer PMMA/Gr film was transferred onto the AgNW/Gr/PMMA sample. The Gr/AgNW/Gr composite was free-standed on the TEM grid after a 30 min annealing process in vacuum (~50 mTorr) at 380 °C to evaporate the PMMA film. For comparison, a two-layer free-stand graphene film was also prepared by the similar procedures without applying the AgNW. The average thickness of the Gr/AgNW/Gr nanocomposite is ~31 nm based on the diameter of the AgNW (30 nm) and thickness of monolayer graphene (0.335 nm).

Optical Transmittance Characterization
The optical transmittance of a bare glass slide was first characterized through the optic transmittance meter (WTM-1100) at 550 nm as baseline. Monolayer and bilayer graphene and the Gr/AgNW/Gr with different AgNW concentrations were transferred on the slides independently and characterized by the meter. The measured values were divided by the baseline value to get the characterized transmittance accordingly.
As shown in Figure 3, a three-layer thin film model is introduced to characterize the optical transmittance of the composite. The wavelength of the light source is 550 nm, which is far greater than the AgNW diameter (~30 nm). Therefore, the diffraction effect on AgNW and interference between two graphene layers are neglected. The total optical transmittance T can be expressed as follows: where T G is the transmittance of graphene layer. For monolayer graphene, T G is 97%, while for two-layer graphene, the value drops to 93%. T A is the transmittance of the AgNW network, which varies with concentrations. T A values with five concentrations were measured first and then the total optical transmittance could be calculated using Equation (1).

Optical Transmittance Characterization
The optical transmittance of a bare glass slide was first characterized through the optic transmittance meter (WTM-1100) at 550 nm as baseline. Monolayer and bilayer graphene and the Gr/AgNW/Gr with different AgNW concentrations were transferred on the slides independently and characterized by the meter. The measured values were divided by the baseline value to get the characterized transmittance accordingly.
As shown in Figure 3, a three-layer thin film model is introduced to characterize the optical transmittance of the composite. The wavelength of the light source is 550 nm, which is far greater than the AgNW diameter (~30 nm). Therefore, the diffraction effect on AgNW and interference between two graphene layers are neglected. The total optical transmittance T can be expressed as follows: where TG is the transmittance of graphene layer. For monolayer graphene, TG is 97%, while for twolayer graphene, the value drops to 93%. TA is the transmittance of the AgNW network, which varies with concentrations. TA values with five concentrations were measured first and then the total optical transmittance could be calculated using Equation (1).

Sheet Resistance Characterization
The sheet resistance was characterized using the Van der Pauw (four-point method) measurement [9], as shown in Figure 4. Four copper foil (25 µm thick) electrodes were connected to each corner of the sample. A direct current (DC) was applied on two adjacent points and the voltage between the other two points was measured. The sheet resistance was then calculated as follows: Multiple measurements were conducted and averaged to obtain the sheet resistance value.

Sheet Resistance Characterization
The sheet resistance was characterized using the Van der Pauw (four-point method) measurement [9], as shown in Figure 4. Four copper foil (25 µm thick) electrodes were connected to each corner of the sample. A direct current (DC) was applied on two adjacent points and the voltage between the other two points was measured. The sheet resistance was then calculated as follows: Appl. Sci. 2020, 10, x 5 of 12 Sheet resistance of the composite RS is determined by three components: sheet resistance of the graphene layer RG and AgNW network RA, and contact resistance RC between the first two components. As shown in Figure 5, three types of contact conditions and corresponding circuit models are defined based on the contact condition. The fully contact model is an assumed ideal condition in which the AgNW-to-graphene and graphene-to-graphene are in perfect adhesion with no contact resistance. The no contact model is another assumed condition in which AgNW and graphene layers have no contact with each other. These two assumptions are set to capture the upper and lower bond of the actual sheet resistance of the nanocomposite, which is described by the actual contact model. Sheet resistances of these three conditions can be expressed as follows: For monolayer graphene synthesized by the LPCVD method, the measured sheet resistance RG is approximately 275 Ω/□. The AgNW sheet resistance RA was measured with different concentrations first using the four-point method. The sheet resistances of no contact and fully contact conditions can be calculated using Equations (3) and (5). The actual sheet resistances of the composite were then measured and compared with various AgNW concentrations.

Atomic Force Microscopy
The stiffness of the nanocomposite was characterized by the atomic force microscope (AFM) indentation. As shown in Figure 6a, the AFM consists of a cantilever probe, photo diode, and laser. The laser and diode are used to measure the displacement of the probe. The cantilever probe is designed to measure the force. Figure 6b,c shows the probe tip after indentation, which has the residual graphene and nanocomposite on it. Multiple measurements were conducted and averaged to obtain the sheet resistance value. Sheet resistance of the composite R S is determined by three components: sheet resistance of the graphene layer R G and AgNW network R A , and contact resistance R C between the first two components. As shown in Figure 5, three types of contact conditions and corresponding circuit models are defined based on the contact condition. The fully contact model is an assumed ideal condition in which the AgNW-to-graphene and graphene-to-graphene are in perfect adhesion with no contact resistance. The no contact model is another assumed condition in which AgNW and graphene layers have no contact with each other. These two assumptions are set to capture the upper and lower bond of the actual sheet resistance of the nanocomposite, which is described by the actual contact model. Sheet resistances of these three conditions can be expressed as follows: Appl. Sci. 2020, 10, x 5 of 12 Sheet resistance of the composite RS is determined by three components: sheet resistance of the graphene layer RG and AgNW network RA, and contact resistance RC between the first two components. As shown in Figure 5, three types of contact conditions and corresponding circuit models are defined based on the contact condition. The fully contact model is an assumed ideal condition in which the AgNW-to-graphene and graphene-to-graphene are in perfect adhesion with no contact resistance. The no contact model is another assumed condition in which AgNW and graphene layers have no contact with each other. These two assumptions are set to capture the upper and lower bond of the actual sheet resistance of the nanocomposite, which is described by the actual contact model. Sheet resistances of these three conditions can be expressed as follows: For monolayer graphene synthesized by the LPCVD method, the measured sheet resistance RG is approximately 275 Ω/□. The AgNW sheet resistance RA was measured with different concentrations first using the four-point method. The sheet resistances of no contact and fully contact conditions can be calculated using Equations (3) and (5). The actual sheet resistances of the composite were then measured and compared with various AgNW concentrations.

Atomic Force Microscopy
The stiffness of the nanocomposite was characterized by the atomic force microscope (AFM) indentation. As shown in Figure 6a, the AFM consists of a cantilever probe, photo diode, and laser. The laser and diode are used to measure the displacement of the probe. The cantilever probe is designed to measure the force. Figure 6b,c shows the probe tip after indentation, which has the residual graphene and nanocomposite on it. For monolayer graphene synthesized by the LPCVD method, the measured sheet resistance R G is approximately 275 Ω/ . The AgNW sheet resistance R A was measured with different concentrations first using the four-point method. The sheet resistances of no contact and fully contact conditions can be calculated using Equations (3) and (5). The actual sheet resistances of the composite were then measured and compared with various AgNW concentrations.

Atomic Force Microscopy
The stiffness of the nanocomposite was characterized by the atomic force microscope (AFM) indentation. As shown in Figure 6a, the AFM consists of a cantilever probe, photo diode, and laser. The laser and diode are used to measure the displacement of the probe. The cantilever probe is designed to measure the force. Figure 6b,c shows the probe tip after indentation, which has the residual graphene and nanocomposite on it. Appl. Sci. 2020, 10, x 6 of 12  Figure 7 shows the optical microscope images of the two-layer graphene and the Gr/AgNW/Gr nanocomposite (higher illumination to emphasize the AgNW). Some ruptured holes can be observed on the grid owing to the synthesis process. Figure 8 shows the SEM images of two-layer graphene and Gr/AgNW/Gr composites with different AgNW concentrations. As the nanowire is randomly distributed between two graphene layers, average AgNW spacing is first characterized. Multiple measurements of the AgNW spacing were conducted and averaged to obtain the average spacing value. As shown in Figure 9, the average spacing decreases linearly from 4.2 µm to 1.5 µm with the increased AgNW concentrations.  Figure 7 shows the optical microscope images of the two-layer graphene and the Gr/AgNW/Gr nanocomposite (higher illumination to emphasize the AgNW). Some ruptured holes can be observed on the grid owing to the synthesis process. Figure 8 shows the SEM images of two-layer graphene and Gr/AgNW/Gr composites with different AgNW concentrations. As the nanowire is randomly distributed between two graphene layers, average AgNW spacing is first characterized. Multiple measurements of the AgNW spacing were conducted and averaged to obtain the average spacing value. As shown in Figure 9, the average spacing decreases linearly from 4.2 µm to 1.5 µm with the increased AgNW concentrations. on the grid owing to the synthesis process. Figure 8 shows the SEM images of two-layer graphene and Gr/AgNW/Gr composites with different AgNW concentrations. As the nanowire is randomly distributed between two graphene layers, average AgNW spacing is first characterized. Multiple measurements of the AgNW spacing were conducted and averaged to obtain the average spacing value. As shown in Figure 9, the average spacing decreases linearly from 4.2 µm to 1.5 µm with the increased AgNW concentrations.    Table 1 lists the transmittance of the AgNW network, as well as calculated and measured values of the composite with five different AgNW concentrations. All the values decrease with the increasing AgNW concentrations because a larger quantity of AgNW will block more light. It can also be seen that the proposed model fits the measured value well with a maximum error of 6%.    Table 1 lists the transmittance of the AgNW network, as well as calculated and measured values of the composite with five different AgNW concentrations. All the values decrease with the increasing AgNW concentrations because a larger quantity of AgNW will block more light. It can also be seen that the proposed model fits the measured value well with a maximum error of 6%.   AgNW concentrations because a larger quantity of AgNW will block more light. It can also be seen that the proposed model fits the measured value well with a maximum error of 6%.  Figure 10a shows the measured actual sheet resistance of the composite together with the calculated no contact, fully contact resistances. It is obvious that the measured sheet resistance of the composite locates between the upper and lower values calculated from the no contact and fully contact models. Therefore, the actual contact model is assumed to represent the average contact condition of the Gr/AgNW/Gr composite well.

Sheet Resistance
Appl. Sci. 2020, 10, x 8 of 12 Figure 10a shows the measured actual sheet resistance of the composite together with the calculated no contact, fully contact resistances. It is obvious that the measured sheet resistance of the composite locates between the upper and lower values calculated from the no contact and fully contact models. Therefore, the actual contact model is assumed to represent the average contact condition of the Gr/AgNW/Gr composite well.

Sheet Resistance
On the basis of Equation (3) The measured sheet resistance of the AgNW network, Gr/AgNW/Gr nanocomposite, and calculated contact resistance value with five AgNW concentrations is listed in Table 2. It can be seen that all the values decrease with the increased AgNW concentrations. This is because the AgNW network with a higher concentration has more connection joints within the network and a greater connection area with the two graphene layers that can increase the conductivity.  On the basis of Equation (3) and the measured sheet resistance of the composite, contact resistances with different AgNW concentrations can be derived and are shown in Figure 10b. Nonlinear regression of the sheet resistance R S and contact resistance R C is conducted and the correlation functions with various AgNW concentrations φ are obtained as follows: The measured sheet resistance of the AgNW network, Gr/AgNW/Gr nanocomposite, and calculated contact resistance value with five AgNW concentrations is listed in Table 2. It can be seen that all the values decrease with the increased AgNW concentrations. This is because the AgNW network with a higher concentration has more connection joints within the network and a greater connection area with the two graphene layers that can increase the conductivity.  Table 3 and Figure 11a show the stiffness values and the force-displacement curves of the AFM indentation on the free-stand two-layer graphene and Gr/AgNW/Gr composites. Figure 11b,c shows the SEM images of the two-layer graphene and the Gr/AgNW/Gr nanocomposite after the AFM indentation. The cut-wire phenomenon observed on the nanocomposite indicates the strengthening effect of the AgNW on increasing the mechanical property of the nanocomposite. Compared with two-layer graphene, the stiffness of the composite increases with higher AgNW concentrations, while it becomes stable after 0.6 mg/mL with the value of around 2150 N/m, which indicates the maximum stiffness enhancement by the AgNW network reaches 0.6 mg/mL. This is owing to the limited interfacial bonding strength between the graphene layer and the AgNW network. When the AgNW concentration is higher than 0.6 mg/mL, the interfacial bonding between the Gr and AgNW dominates the stiffness of the nanocomposite. Therefore, the continuously increased AgNW concentration after 0.6 mg/mL cannot enhance the mechanical strength of the nanocomposite. it becomes stable after 0.6 mg/mL with the value of around 2150 N/m, which indicates the maximum stiffness enhancement by the AgNW network reaches 0.6 mg/mL. This is owing to the limited interfacial bonding strength between the graphene layer and the AgNW network. When the AgNW concentration is higher than 0.6 mg/mL, the interfacial bonding between the Gr and AgNW dominates the stiffness of the nanocomposite. Therefore, the continuously increased AgNW concentration after 0.6 mg/mL cannot enhance the mechanical strength of the nanocomposite.   Figure 12 shows the Raman spectra of two-layer graphene and the Gr/AgNW/Gr composite. Raman shift at D, G, D', and 2D peaks and intensity ratio of IG/I2D are listed in Table 4. The higher intensity at D peak in the composite spectra indicates that the AgNW between two graphene layers increases the disorder level of the sp 2 hybridization. Besides, the localized vibration mode of AgNW splits the D' peak from the original G peak. The IG/I2D ratio is commonly used to determine the number of graphene layers [9,16,[21][22][23]. As both samples have two layers of graphene, the ratio values are close to each other. The blue shift of the Gr/AgNW/Gr sample is generated by the decreased Fermi velocity [24], which may correspond to the change in twist angle and layer separation between the two graphene layers induced by the AgNW network.  Figure 12 shows the Raman spectra of two-layer graphene and the Gr/AgNW/Gr composite. Raman shift at D, G, D', and 2D peaks and intensity ratio of I G /I 2D are listed in Table 4. The higher intensity at D peak in the composite spectra indicates that the AgNW between two graphene layers increases the disorder level of the sp 2 hybridization. Besides, the localized vibration mode of AgNW splits the D' peak from the original G peak. The I G /I 2D ratio is commonly used to determine the number of graphene layers [9,16,[21][22][23]. As both samples have two layers of graphene, the ratio values are close to each other. The blue shift of the Gr/AgNW/Gr sample is generated by the decreased Fermi velocity [24], which may correspond to the change in twist angle and layer separation between the two graphene layers induced by the AgNW network.

Raman Spectroscopy
images of (b) two-layer graphene and (c) Gr/AgNW/Gr nanocomposites after AFM indentation. Figure 12 shows the Raman spectra of two-layer graphene and the Gr/AgNW/Gr composite. Raman shift at D, G, D', and 2D peaks and intensity ratio of IG/I2D are listed in Table 4. The higher intensity at D peak in the composite spectra indicates that the AgNW between two graphene layers increases the disorder level of the sp 2 hybridization. Besides, the localized vibration mode of AgNW splits the D' peak from the original G peak. The IG/I2D ratio is commonly used to determine the number of graphene layers [9,16,[21][22][23]. As both samples have two layers of graphene, the ratio values are close to each other. The blue shift of the Gr/AgNW/Gr sample is generated by the decreased Fermi velocity [24], which may correspond to the change in twist angle and layer separation between the two graphene layers induced by the AgNW network.

Normalized Results
To summarize the results more clearly, the optical transmittance, sheet resistance, and stiffness values are normalized with the value of two-layer graphene set to 1. Figure 13 and Table 5 show the normalized values of the optical transmittance, sheet resistance, and stiffness, respectively. From 0.2 to 1.0 mg/mL of AgNW concentration increasing, the optical transmittance and sheet resistance reduce to 79% and 16% of the original two-layer graphene value, respectively. The stiffness of the composite increases at first and then becomes stable after 0.6 mg/mL with a value of approximately 4.6 times higher than two-layer graphene.

Normalized Results
To summarize the results more clearly, the optical transmittance, sheet resistance, and stiffness values are normalized with the value of two-layer graphene set to 1. Figure 13 and Table 5 show the normalized values of the optical transmittance, sheet resistance, and stiffness, respectively. From 0.2 to 1.0 mg/mL of AgNW concentration increasing, the optical transmittance and sheet resistance reduce to 79% and 16% of the original two-layer graphene value, respectively. The stiffness of the composite increases at first and then becomes stable after 0.6 mg/mL with a value of approximately 4.6 times higher than two-layer graphene.   Figure 13. Normalized results of the stiffness, optical transmittance, and sheet resistance.

Conclusions
In summary, the free-stand Gr/AgNW/Gr nanocomposite was synthesized through the LPCVD and vacuum annealing process for the first time in this work. Average spacing of the AgNW network, optical transmittance, sheet resistance, and stiffness of the nanocomposite were characterized under various AgNW concentrations. The characterization results can be used for transparent conductive film design based on specific parameter requirements. The stiffness results can be utilized for the interfacial bonding study between the AgNW and the Gr layer in the future.