Donor–acceptor type black phosphorus nanosheets covalently functionalized with a conjugated polymer for laser protection†
Covalent functionalization of two-dimensional black phosphorus (BP) with conjugated polymers would be favorable to improve its environmental stability and solubility in organic solvents, and thereby to open a way to explore more applications of BP in the fields of optoelectronics, electronics and photonics. As a proof-of-concept application, soluble donor–acceptor type BP nanosheets covalently functionalized with poly[(1,4-diethynyl-2,5-bis(hexyl-oxy)benzene)-alt-benzo[c]thiadiazole] (hereafter called PDBT-BP) were synthesized through Sonogashira–Hagihara coupling polymerization, and embedded into an optically nonactive transparent poly(methylmethacrylate) (PMMA) matrix to produce the PMMA-based PDBT-BP film for nonlinear optics. In contrast to BP/PMMA, PDBT-BP/PMMA and PDBT/BP blends/PMMA films, the annealed PDBT-BP/PMMA film, which was heated at 150 °C under dry argon for 1 h, shows more excellent reverse saturable absorption (RSA) behavior, with a larger nonlinear absorption coefficient (βeff) of 208.16 cm GW−1 and an imaginary third-order susceptibility (Im χ(3)) of 8.89 × 10−11 esu at a pulse energy of 400 μJ. The achieved optical limiting (OL) threshold is 4.44 J cm−2 (0.74 GW cm−2), smaller than that of the non-annealed PDBT-BP/PMMA film (5.64 J cm−2). The SA (saturable absorption) to RSA transition observed at a low incident laser intensity could be used to realize all-optical intensity-dependent logic gates in the near future.