## 3 Results and Discussion
### Non-Linear Concentration Effects of Optical Transparency and Electrical Conductivity  
Initially we studied the influence of centrifugation and composition on suspension stability (Figure 5). Data suggest that centrifugation, although it leads to a significant decrease in suspension concentration (down to ca. 10 wt.% of the initial concentration), does not significantly affect a suspension's electrical conductivity. This was previously found for aqueous suspensions of pure FLG [70]. The fact that even after a serious decrease in concentration the electrical conductivity of suspensions remains almost intact may suggest two statements:  
* Percolation threshold in the system (suspensions) is achieved even at concentrations as low as ca. 0.5 mg/mL (see Section 3.2); therefore, centrifugation does not significantly affect the level of electrical conductivity;
* Individual nanoparticles are primarily responsible for charge carrier properties in the suspensions; the influence of agglomerates and larger particles is not very pronounced. Figure 4: Mean particle size vs. FLG content for hybrid FLG/AgNP suspensions. Figure 5: Specific electrical conductivity vs. FLG content for hybrid FLG/AgNPs suspensions. Although a low percolation threshold [71; 72] and network-like structuring of solid phase in suspensions [73] have been previously described for many, including graphene-based, systems, the latter statement requires more careful evaluation. Data from Figure 5 suggest that, whereas initial AgNPs and FLG suspensions had electrical conductivity around 150 \(\upmu\)S/cm and 100 \(\upmu\)S/cm, respectively, the hybrid suspension containing 65 wt.% of FLG showed conductivity of almost 400 \(\upmu\)S/cm independently of centrifugation. This non-linear concentration is obviously non-percolative in nature (overall concentration remains almost the same, and a sharp maximum with respect to both components concentration is observed); therefore, specific interactions between components in hybrid suspension should be responsible for the observed behavior. A high level of local contact resistances is a well-known problem limiting potential electrical applications and achieving theoretical values of charge carrier properties for most semimetals, which include graphite-based materials and graphene [74]. In this particular case, the effect may be due to electrical bridging of FLG platelets through AgNPs, thus drastically decreasing contact resistance. The non-linear concentration effect is also evident from optical transmission data (Figure 6). Optical spectra of centrifuged suspensions have two main features:  
* Non-linear dependence of average transmission on suspension composition;
* Absence of the surface plasmon resonance (SPR) peak at 350-450 nm characteristic of AgNPsPs [75; 76] (the peak is not very pronounced even at high AgNP loadings due to elevated overall optical absorption). Both of these feature specific mechanisms of carrier transport in the FLG/AgNP system that affect both direct current electrical conductivity and optics (i.e., interaction with a high-frequency electromagnetic field). Therefore, mixing and collectively ultrasonicing FLG and AgNP suspensions proves sufficient to provide interactions strong enough to significantly affect electrical and optical properties, as well as to reveal synergistic effects. Figure 7 represents optical transmission data more clearly by showing suspensions' transmission at the red edge (950 nm), commonly used 650 nm, and blue edge (350 nm) of the visual region of the optical spectrum. It can be readily seen that throughout the whole visual region the maximal transmission is observed for the hybrid suspension containing 65 wt.% of FLG. Figure 6: Optical transmission spectra for hybrid FLG/AgNPs suspensions at different component ratios (see legend).