Fluorinated Graphene Films with High Temperature Sensitivity for Uncooled Bolometer Applications

Graphene possesses weak electron–phonon interactions, small electron heat capacity, and broad-band absorption, which make it an excellent candidate as a fast and sensitive bolometer. This work studies the bolometric performance of fluorinated graphene (FG) films, which were synthesized using a vacuum filtration-assisted method with FG and chitosan. Thermal annealing (200–1000 °C) under an Ar atmosphere was used to modulate the fluorine density. The electrical resistivity and thermal conductivity were measured from 320 to 10 K. As the ratio of F/C reduced from 4.06 to 0.003, the electrical resistivity decreased from 0.37 to 9.6 × 10 –5 Ω·m at RT, and the thermal conductivity increased from 0.5 to 2.7 W·m –1 ·K –1 at RT. In addition, the higher fluorine density resulted in a higher temperature coefficient of resistance (TCR). A mildly reduced FG film with a F/C ratio of 4.06 exhibited the highest |TCR| of 1%·K –1 at 10 K and 0.48%·K –1 at 320 K. The temperature-dependent resistivity revealed that the electrical transport mechanisms are mainly due to a combination conduction processes of thermal activation and variable range hopping. The thermal activation energy reduced from 21 to 1 meV as the concentration of fluorine decreased, which confirms that the conductivity of FG is controlled by fluorinated structures and can be adjusted by thermal annealing treatment. Last, the bolometric performance of FG films was demonstrated across a wide range of wavelengths from ultraviolet to near-IR. The FG films can detect a human hand 25 cm away with good sensitivity and repeatability. For a sample with a suspended length of 1.2 mm, the response time was measured to be 436–449 ms. These results demonstrated that the FG films show great potential as flexible and sensitive bolometric IR sensors.