Enhanced Optical Nonlinearity in Noncovalently Functionalized Amphiphilic Graphene Composites

In the past decades, there has been great progress in development of organic and inorganic optical limiters with large nonlinear optical (NLO) response. Carbon-based materials, including carbon black suspension (CBS), singleand multi-walled carbon nanotubes (CNTs), 3] and some small p-electron systems, such as fullerenes, porphyrins, and phthalocyanines have been widely explored as advanced optical limiters. For example, the thermally induced nonlinear scattering of CBS and CNTs is generally accepted as the principal mechanism for the optical limiting (OL). As known, the OL effect of graphitic systems covers a broad wavelength, ranging from the visible to near infrared. However, good OL behavior of graphitic systems only performs in solution, owing to the dominance of scattering mechanism. The solution-assisted performance is a serious obstacle for practical applications, as the graphitic system tends to aggregate into large bundles because of its relatively high surface energy. Therefore, more research interests have been directed towards the development of graphitic materials which have good dispensability and can be processed in liquid dispersion. It was reported that the small p-electron systems can be homogenously dispersed in solution or solid phases, showing good OL properties in the sub-nanosecond timescale. Unfortunately, these systems only have a narrow band OL behavior as the ratio of excited state to ground state cross section strongly depends on the excitation wavelength. For example, C60 has very poor OL properties beyond 700 nm, which cannot meet the requirement of OL devices for sensor protection, because band protection for the entire operating wavelength of the sensor system is required. Graphene, as the newly explored single-layer carbon material, is found to be a promising broadband optical limiter owing to the strong nonlinear scattering mechanism. Moreover, the enhanced OL behaviors have been reported in graphene hybrid materials covalently functionalized with porphyrin, organic dye ionic complex, oligothiophene, fullerene, phthalocyanine, upconversion rare-earth nanoparticles, and poly(N-vinylcarbazole). However, the poor solubility of these graphene hybrid materials is one of the big problems limiting their practical application. Although covalent functionalization was used to improve the solubility of graphene hybrid materials, it is a destructive method that may alter the chemical structures of graphene and its derivatives, such as graphene oxide (GO) and reduced graphene oxide (rGO). To maintain the intrinsic property of graphene, an alternative method that can protect the basic plane of graphene is preferred. Moreover, on one hand, the good thermal conductivity of graphene is beneficial to the excellent OL response. On the other hand, graphene is an innate energy acceptor, and its OL behavior can be enhanced by functionalization with nonlinear optical chromophores (donors), which can promote the energy transfer between them. Therefore, the nonconvalent functionalization of graphene with amphiphilic conjugated molecules,