Biopolymer Derived Thin Carbon Film As a Novel Sensing Material for Low-Cost Resistive and Fast-Response Humidity Sensors

Abstract

Introduction Recently, fast response humidity sensors are an active area of research for applications such as industrial, agricultural and point of care devices. Among various types of humidity sensor materials, thin film based resistive humidity sensors have been actively studied because those exhibit high surface area and the sensor configuration is simple. However, most of these sensors are relatively slow with long response time (over 60 seconds) and narrow detection limits (30-50 RH%), making them difficult to use in many applications [1-2]. Besides, the need of an external heater for fast response increases power consumption and complicates the manufacturing process of the sensor. Functionalized graphene, the most popular humidity sensor material, offers significant advantages due to its unique 2D structure and oxygen-containing functional groups, providing a large surface reaction site. However, it also has limitations such as poor stability in a moist environment due to its hydrophilic nature and weak adhesion with the substrate, complex synthesis procedure, makes it difficult to develop practical sensing devices.Here, we developed the low-cost and fast-response humidity sensors by integration of thermally decomposed carbon (TDC) thin film and carbon nano-sized interdigitated electrodes (IDEs). Carbon IDEs were used as a sensor platform due to its excellent thermal compatibility with TDC films and it provides low-cost wafer-level fabrication of three-dimensional nanoelectrodes [3], resulting in the miniaturized sensor configuration at a wafer level. Shellac, a low-cost biopolymer, was used as a precursor for synthesizing TDC films via single-step pyrolysis. The obtained film showed conformal coating with no grain boundaries or defects and formed high sp2 hybridized carbon network on the carbon nano-sized IDEs, thus providing excellent stability, rapid response/recovery and low power consumption as a room temperature humidity sensor. Our results demonstrate the potential application of TDC film as an alternative sensing material for environmental humidity, instant calibration for gas sensing measurements and human respiration in real time. Methods We integrated a nanometer-thick TDC film onto the carbon nano-IDEs, using two-step process, to facilitate a simple and cost-effective humidity sensor. Firstly, microscale interdigitated polymer patterns were fabricated using photolithography and then pyrolyzed into nanoscale carbon IDEs due to volume reduction in pyrolysis. The interdigitated carbon nanoelectrodes were annealed at 1000 °C using a rapid thermal annealing process to enhance the electrical conductivity. The next step is the integration of a TDC film on the carbon IDEs. For that, shellac solution (4 wt.%) was uniformly spray-coated on the carbon IDEs and air-dried (1 hour). This shellac film was pyrolyzed at 600 °C to obtain a uniform TDC thin film. Results and Conclusions Figure a shows a scanning electron microscopy (SEM) image of the TDC coated nano-sized carbon IDEs (width = 800 nm, height = 300 nm, gap = 2.5 μm). The film displays a conformal coating with no signature of defects or grain boundaries. The electrical properties of TDC film were evaluated using a two-probe I-V technique (Figure b), which indicated the excellent ohmic contact with carbon IDEs. Raman spectra of the TDC film (Figure c) indicated the presence of local intrinsic defects and disorder (intense D band) due to the presence of oxygen functional groups, especially at the edges and basal plane of the TDC film. X-ray photoelectron spectroscopy (XPS) for carbon (Figure d) also confirmed the presence of oxygen-containing functional groups (C-OH and C=O) and a high sp2 hybridized carbon network. Higher dynamic response (40 %) is attributed to the presence of oxygen functional groups (Figure e), whereas sp2 carbon network induce hydrophobicity, which lowers the condensation of water molecules without using any external heating source, resulted in fast response (37s) and recovery time (8s) (Figure f), compared to previously reported rGO-based sensors [4]. The obtained thin film humidity sensor also displayed the linear response over the wide humidity range (10-90 RH%), with high sensitivity > 0.7/RH%. The dynamic response of the sensor revealed that the protonic conduction mechanism is dominant in TDC film.