In recent advancements in gas sensor technology, a groundbreaking study has presented a selective gas sensor utilizing a graphene-silicon Schottky junction. This innovative sensor operates under varying voltage conditions, which enables it to toggle between distinct regions of gas sensitivity depending on ambient lighting conditions. The research highlights significant detection capabilities, achieving a low detection limit of 36 parts per billion (ppb) for nitrogen dioxide (NO2), 238 ppb for ammonia (NH3), and 640 ppb for acetone (C3H6O) when subjected to ultraviolet (UV) irradiation at a voltage of -0.4 V. However, the operation of this sensor requires a light power supply to maintain effective sensitivity.
The sensor's performance hinges on the unique characteristics of the Schottky junction formed when graphene is combined with n-doped silicon (Si). The study discovered that the height of the Schottky barrier becomes significantly sensitive to gas adsorption, providing a measurable response. Moreover, when functioning in the absence of light, the sensor demonstrates a gating effect, illustrated by a bend in the current-voltage (I-V) curve around 0.7 V. This bias shift occurs owing to the charge interaction of the gas molecules with the graphene surface, enhancing the device's sensitivity.
Graphene has captivated researchers for more than a decade, primarily due to its unique structural, optical, electrical, thermal, and mechanical properties. The high reactivity of its two-dimensional surface and high carrier mobility have facilitated its use in sensitive probe applications across varied sensor types, particularly gas detection systems. Despite these advantages, traditional graphene-based sensors have grappled with issues like low selectivity and rapid aging when exposed to varying relative humidity levels in the air.
To counteract these limitations, various strategies have been employed to improve the sensitivity, selectivity, and durability of graphene-based sensors. Approaches include decorating the graphene surface with catalytic materials, creating hybrid structures, and implementing UV irradiation techniques. The Schottky junction device discussed in this article emphasizes a notable advantage over conventional field-effect transistors (FET) by demonstrating similar sensitivity at significantly lower voltage biases, thereby enhancing the efficacy of gas detection in humid conditions.
To fabricate the Schottky diodes, researchers began by cleaning an n-type silicon wafer, followed by the deposition of a 90 nm layer of thermal oxide (SiO2) on its surface. They then selected specific areas for selective etching before transferring a layer of graphene, obtained through chemical vapor deposition (CVD) on copper foil, onto the silicon substrate. Metal contacts (Ni/Au) were deposited onto the graphene to enable electrical measurements. The size of the active area of the sensors varied, with a base of 25,000 μm² up to 50,000 μm², enhancing their practical application in gas sensing experiments.
Results reveal that the G-Si diode exhibits prominent responsiveness towards the tested gases, with current changes being showcased as the relative change in the current flowing through the sensor (IIS in the presence of the gas versus I0 in baseline conditions). The detected limit under dark conditions for NO2 was initially observed at 47 ppb at a bias of 0.7 V, later reducing to 36 ppb with the aid of UV light. Furthermore, results noted a current increase of around 69% when exposed to 3 ppm of NO2 in darkness, which jumped to 181% under UV conditions.
Ammonia, while consistently producing responses, demonstrated a higher detection limit in dark conditions at 629 ppb, tapering down to 238 ppb with UV light, indicative of the efficiency of enhanced energy input methods. The sensor was also tested with acetone, revealing less pronounced changes in current, yet still achieved a detection limit of 640 ppb under UV illumination — a significant improvement from dark conditions where responses were notably subdued.
Moreover, the performance of the sensor in humid environments was closely examined, highlighting how the presence of moisture can obstruct the sensor's sensitivity. It was shown that interactions with H2O molecules may compete with target gas adsorption, thereby adversely affecting detection thresholds. Yet what stands out is this sensor’s resilience against humidity effects, making it a superior candidate compared to other graphene-based systems.
While the efficacy of the G-Si diode was reaffirmed through various trials, the need for further exploration into its real-world applicability is evident, particularly concerning repeatability across different production batches. The study emphasizes a potentially transformative approach to low-power, cost-effective gas sensing applications while ensuring the device remains sensitive even under variable humidity conditions. Such innovations in sensor technology present vast potential for addressing critical environmental and health monitoring challenges.
