A gold nano-urchin-decorated quasi-freestanding graphene-based humidity sensor with enhanced responsivity and a wide relative humidity detection range for real-time applications

Excellent humidity response and detection threshold are essential attributes of a humidity sensor. In this study, quasi-freestanding graphene (QFSG) was grown epitaxially on vicinal silicon carbide via thermal annealing, followed by decoration with hydrophilic gold nano-urchins (AuNUs). The resulting humidity sensor consistently achieved highly effective non-contact humidity-sensing performance, which can be used to monitor breathing and skin moisture. The proposed humidity sensor demonstrated a non-linear response of ∼119.98 % over a wide relative humidity detection range of 4–96 % at a frequency of 1 kHz, outperforming a QFSG-only humidity-sensing device (∼11.92 % and 22–84 % relative humidity, respectively). This 10-fold increase in humidity response and wider detection range is attributed to the hydrophilicity and plasmonic behavior of the AuNUs. It also demonstrated transient impedance response/recovery time of 4.5 and 3.0 s, respectively; low hysteresis of 3.6 %; and stability over 25 days, which are key factors for effective humidity sensing. The fast response and sensitivity of the AuNUs/QFSG humidity sensor made it possible to test for various real-time monitoring applications such as human breathing and non-contact proximity testing.