# Configurational Effects on Strain and Doping at Graphene-Silver Nanowire Interfaces
## 2 Results and Discussion
As graphene is previously p-doped in the graphene/ITO heterostructure [19], the insertion of AgNW between graphene and ITO forms a p-n-p junction due to modulation in the E\({}_{\text{F}}\) of graphene. Figure 1: Stages of the preparation of graphene (Gr)/silver nanowire (AgNW) heterostructure. (**a**) Initially, AgNW is spray-coated on indium tin oxide (ITO) substrate. (**b**) Deposited AgNW is subsequently covered by Poly(methyl methacrylate) (PMMA)/Gr hybrid by wet transfer technique. (**c**) The PMMA layer is then removed in acetone bath and only the graphene monolayer remains on AgNW surface. The transparent hybrid layer of graphene-covered AgNW on ITO substrate shows the background of the logo of University of Sussex. There is reduction in the transmittance by 10% between 488 and 650 nm wavelength. The physical and electronic modification to graphene by AgNW is investigated by Raman spectroscopy, where the typical Raman spectra are illustrated in Figure 3a. Graphene Raman modes comprise G peak and 2D peak, which are located at 1585 cm\({}^{-1}\) and 2680 cm\({}^{-1}\), and respectively correspond to the first-order double-degenerated scattering process due to E\({}_{\text{2g}}\) symmetry and the second-order phonon mode between two iTO phonons [20]. D peak, situating at 1340 cm\({}^{-1}\), requires structural defects for its activation. The absence of prominent D peak in the Raman spectrum in Figure 3a indicates minimal density of defects. G and 2D modes are sensitive to strain and doping, while the effect of strain (doping) is more significant to the shift of 2D (G) peak [15]. An intensity enhancement of both peaks are observed at the graphene-covered AgNW region compared to graphene only, suggesting a surface-enhanced Raman scattering (SERS) effect [21]. SERS originates from the resonant response from localised surface plasmons, where the metal nanostructure serves as a hotspot to strengthen the surrounding electric field [22]. The intensity ratio I\({}_{\text{2D}}\)/I\({}_{\text{G}}\) being larger than 1 also confirms the enveloping graphene is monolayer [23]. Apart from the intensity enhancement, both G and 2D modes are observed at the graphene-covered AgNW region, and the effect of strain is observed at the graphene-covered AgNW region. The  
and 2D peaks also red shift at the AgNW region, revealing an influence on strain or doping by the curvature of the nanowire. A clear transition of peak intensity and peak position from the bare graphene to the AgNW-embedded graphene is reflected by a thorough investigation of a Raman line map across the AgNW along the axial direction (Figure 3b) and the transverse direction (Figure 3c). We observe an increase of G peak intensity and 2D peak intensity by 50%, and a redshift of G\(\omega\) (G peak position) and 2D\(\omega\) (2D peak position) of 4 cm\({}^{-1}\) and 11 cm\({}^{-1}\), respectively. The peak shifts can be correlated with the induced strain \(\varepsilon\) and the charge carrier density \(n\) of graphene by  
\[\left( \begin{array}{c}\omega_{G}\\ \omega_{2D}\end{array} \right)=T\left( \begin{array}{c}\varepsilon\\ n\end{array} \right) \tag{2}\]  
where  
\[T=\left( \begin{array}{cc}-2\gamma_{G}\omega_{G}^{0}&k_{G}\\ -2\gamma_{2D}\omega_{2D}^{0}&k_{2D}\end{array} \right) \tag{3}\]  
\(\gamma\) is the Gruneisen parameter, \(k\) is the doping shift constant, and \(\omega^{0}\) is the no-strain no-doping peak position. Literature reveals \(\gamma_{G}=1.95\), \(\gamma_{2D}=3.15\), \(k_{G}=-1.407\times 10^{-12}\)\(cm^{-1}\), and \(k_{2D}=-0.285\times 10^{-12}\)\(cm^{-1}\)[15; 24]. This linear transformation of \(\varepsilon\)-\(n\) space to the space of G\(\omega\) and 2D\(\omega\) realises a correlation plot of 2D\(\omega\) against G\(\omega\), with inclined isolines (grey dashed lines) representing equal strain and doping.