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At present, humidity sensors have promising prospects in disease monitoring, family life, environmental protection, and so on. Flexible humidity sensor is more and more popular because of its flexibility and portability. In our work, a flexible humidity sensor based on a tin disulfide (SnS) nanoflower and a zinc stannate (ZnSnO) hollow sphere film was fabricated though layer-by-layer self-assembly technique. The humidity performance showed that the SnS/ZnSnO hybrid film sensor was ultrasensitive to humidity at room temperature. The test results demonstrated superior response, fast response/recovery behavior, and excellent repeatability. Moreover, compared to the single SnS and single ZnSnO nanomaterials, the SnS/ZnSnO hybrid film sensor exhibited great improvement in humidity sensing. In addition, complex impedance spectroscopy was adopted to further explore the sensing mechanisms of the SnS/ZnSnO hybrid to various humidities. Human respiration, palm sweat, urine, and water droplets were delicately detected by the SnS/ZnSnO humidity sensor, indicating its great potential in multifarious application fields.

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The extraordinary properties of black phosphorus (BP) make it a promising candidate for next-generation transistor chemical sensors. However, BP films reported so far are supported on substrate, and substrate scattering drastically deteriorates its electrical properties. Consequentially, the potential sensing capability of intrinsic BP is highly underestimated and its sensing mechanism is masked. Additionally, the optimum sensing regime of BP remains unexplored. This article is the first demonstration of suspended BP sensor operated in subthreshold regime. BP exhibited significant enhancement of sensitivity for ultra-low-concentration mercury detection in the absence of substrate, and the sensitivity reached maximum in subthreshold regime. Without substrate scattering, the suspended BP device demonstrated 10 times lower 1/f noise which contributed to better signal-to-noise ratio. Therefore, rapid label-free trace detection of Hg was achieved with detection limit of 0.01 ppb, lower than the world health organization (WHO) tolerance level (1 ppb). The time constant for ion detection extracted was 3s. Additionally, experimental results revealed that good stability, repeatability, and selectivity were achieved. BP sensors also demonstrated the ability of detecting mercury ions in environment water samples. The underling sensing mechanism of intrinsic BP was ascribed to the carrier density variation resulted from surface charge gating effect, so suspended BP in subthreshold regime with optimum gating effect demonstrated the best sensitivity. Our results show the prominent advantages of intrinsic BP as a sensing material.