Localized optical-quality doping of graphene on silicon waveguides through a TFSA-containing polymer matrix

The use of graphene in optical and photonic applications has gained much attention in recent years. To maximize the exploitation of graphene's extraordinary optical properties, precise control over its Fermi level (e.g. by means of chemical doping) will be of vital importance. In this work, we show the usage of a versatile p-doping strategy based on the incorporation of bis(trifluoromethanesulfonyl)amide (TFSA), functioning as an active p-dopant molecule, into a poly(2,2,3,3,4,4,5,5-octafluoropentyl methacrylate) (POFPMA) polymer matrix. The TFSA/POFPMA dopant can be utilized both onto large size graphene regions via spin coating and on small predefined spatial zones of micrometer dimension by localized inkjet printing. Whereas pure TFSA suffers from a clustered layer deposition combined with environmental instability, the application of the POFPMA polymer matrix yields doping layers revealing superior properties counteracting the existing shortcomings of pure TFSA. A first key finding relates to the optical quality of the dopant layer. We obtain a layer with an extremely low surface roughness (0.4–0.8 nm/25 μm2) while exhibiting very high transparency (absorbance <0.05%) over the 500–1900 nm wavelength range, with strongly enhanced doping stability as a function of time up to several weeks (for inkjet-printed deposition) and months (for spin coated deposition). Finally, the doping efficiency is very high, reaching a carrier density around +4 × 1013 cm−2 whereas the optical transmission of a graphene-covered Si waveguide revealed a strong improvement (4.22 dB transmission increase per 100 μm graphene length at the wavelength of 1550 nm) after deposition of the dopant via inkjet printing.