Hydrogen, a zero-carbon energy source with high heat of combustion, is crucial for various applications including rocket fuels, vehicles, and industrial processes. However, its flammability and explosive nature necessitate highly sensitive and rapid leak detection. Existing optical sensing techniques are complex and expensive, while electrical techniques often operate at high temperatures and require bias voltage. This research addresses this challenge by developing a novel on-chip hydrogen sensing platform. The platform integrates plasmonic sensing, photoelectric detection, and photocatalysis, aiming to achieve high sensitivity, rapid response, low cost, high portability, and flexibility – addressing the 3A criteria (Accessible, Affordable, Applicable) for hydrogen sensors. Current electrical sensing methods, such as those based on thermal conductivity, resistance, electrochemistry, and semiconductor junctions, often require high temperatures and biasing conditions, increasing complexity, explosion risk, and power consumption. Optical methods, like optical fiber sensing and tunable diode laser absorption spectroscopy (TDLAS), need complex and expensive instruments, skilled operators, and complicated data processing. Plasmonic sensing, with its highly localized electromagnetic field at the metal surface, offers the potential to meet the 3A criteria, but existing plasmonic sensors often still rely on complex and expensive instruments due to a lack of in situ photoelectric conversion. This work leverages a novel approach using a plasmonic metal-semiconductor nanojunction to achieve direct electric readout and enhanced photodetection, addressing the limitations of previous methods. The researchers propose a platform utilizing platinum-silicon nanojunctions with three functions: plasmonic sensing, photoelectric detection, and photocatalysis to achieve a strong interaction between plasmon-induced hot electrons and hydrogen molecules. This multi-functional combination is anticipated to produce significant improvements in sensitivity and response speed compared to non-plasmonic devices.

The existing literature extensively covers hydrogen sensing techniques, with a focus on catalytic reactivity and solubility of hydrogen with noble metals such as palladium and platinum. These methods typically measure changes in resistance, work function, volume, temperature, or refractive index to detect hydrogen. However, these methods often suffer from limitations such as high operating temperatures and the need for bias voltages, leading to increased complexity and safety risks. Optical sensing techniques provide alternatives but often rely on complex and expensive instrumentation such as spectrometers and tunable lasers, making them unsuitable for many applications. Plasmonic sensing is an emerging technology showing promise for high-sensitivity gas sensing, but frequently lacks integrated photoelectric conversion for direct, on-chip readout. Previous studies have explored plasmonic metal-semiconductor nanojunctions for sensing, but these have lacked the sensitivity needed for hydrogen detection. This paper builds upon this existing knowledge by combining the advantages of plasmonics with photoelectric detection and photocatalysis to create a high-performance hydrogen sensor.

The researchers designed a plasmonic-catalytic metal-insulator-semiconductor (MIS) nanojunction composed of a 50nm platinum layer on an n-type silicon substrate with a one-dimensional periodic surface relief structure. The platinum layer acts as both the plasmonic absorber and catalyst, while the silicon substrate serves as the photodetector. The MIS structure, with an atomically thin oxide layer, suppresses leakage current and enhances sensitivity. The periodic surface relief structure, fabricated using techniques like nanoimprint or nanosphere lithography, enhances the plasmonic resonance. The device's optical resonance was characterized by measuring the zero-order reflectance under normal incidence, which matched well with numerical simulations. The angle-dependent photocurrent response was measured at a fixed resonance wavelength of 1064nm with a slightly tilted incident angle (3 degrees). Time-dependent photoelectric responses were measured in air and a hydrogen/air mixture (3% H2 in air) to assess the sensitivity. The light and dark current-voltage (I-V) characteristics were also measured under both conditions to observe the effects of hydrogen exposure. A quantum tunneling model was developed to simulate the effects of a hydrogen-induced dipole layer on the photoelectric properties of the MIS junction. The model incorporated factors such as energy band diagrams, electric field distributions, and potential distributions across the junction. This model was used to explain the observed S-shaped I-V curves and the enhanced sensitivity to hydrogen.

The study demonstrated a highly sensitive and rapid hydrogen sensor based on a plasmonic-catalytic MIS nanojunction. The device achieved a detection limit of 1 ppm hydrogen at room temperature and zero bias. The plasmonic resonance significantly enhanced the photoelectric response, improving sensitivity by three orders of magnitude and response speed by one order of magnitude compared to non-plasmonic MIS sensors. The observed S-shaped I-V curve in the presence of hydrogen is attributed to a hydrogen-induced interfacial dipole layer, which alters the energy band alignment and the built-in potential of the MIS junction. The quantum tunneling model accurately predicted the experimental results, confirming the mechanism of hot electron transfer modulated by the hydrogen-induced dipole layer. The high photocurrent responsivity of 9.4 mA/W (corresponding to an external quantum efficiency of 1.1%) and the fast response time highlight the advantages of the proposed approach. The device's performance surpasses that of existing hydrogen sensors, offering a promising platform for practical applications.

The findings demonstrate the effectiveness of integrating plasmonics, photocatalysis, and photoelectric detection in a single on-chip device for sensitive and rapid hydrogen sensing. The superior performance compared to conventional methods is attributed to the unique plasmon-induced hot electron-hydrogen molecule interaction, facilitated by the hydrogen-induced dipole layer. The agreement between experimental and theoretical results validates the proposed mechanism of hot electron transfer modulated by the dipole layer. This research provides a significant advancement in hydrogen sensing technology, offering a pathway towards low-cost, portable, and highly sensitive hydrogen leak detection systems. The results contribute to the development of safer and more efficient hydrogen energy technologies.

This work successfully demonstrated a highly sensitive and fast on-chip hydrogen sensor based on a plasmonic-catalytic MIS nanojunction. The device's exceptional performance stems from the synergistic combination of plasmonic enhancement, photoelectric conversion, and the catalytic effect of platinum. The findings pave the way for developing portable, low-cost, and highly sensitive hydrogen sensors for various applications, enhancing safety and efficiency in hydrogen-related technologies. Future research could explore different plasmonic nanostructures and catalytic materials to further improve sensitivity and selectivity, as well as investigate the integration of this sensor into practical sensing systems.

While the developed sensor shows excellent performance, some limitations should be acknowledged. The current design is optimized for hydrogen detection and might require modifications for detecting other gases. The long-term stability and reliability of the device under diverse environmental conditions still need further evaluation. Further research could focus on improving the sensor's selectivity and reducing the influence of other gases present in the environment.