“Appl. Surf. Sci.”: Tunable p–n Characteristics of Ammonia Gas Sensors Based on Tungsten Disulfide Nanosheets Fabricated via Drop‑Coating and Aerosol Jet Printing
Product Introduction
Equipment Parameters
I. Abstract of the Paper’s Core Research
- Aerosol-based processing enables reversible p–n type modulation in WS₂.
The drop‑coated sample is a p‑type semiconductor, and its resistance increases upon exposure to ammonia. In contrast, the aerosol‑printed film, which incorporates O and N donor dopants via ultrasonic atomization, exhibits n‑type behavior with electrons as the majority carriers; its resistance decreases in the presence of ammonia. This one‑step printing process achieves in situ doping without requiring any additional post‑processing steps.
- The interlayer spacing has been significantly expanded, and the response recovery rate has increased by a factor of two.
The interlayer spacing of the drop‑coated WS₂ film is only 0.35 nm—close to the diameter of an NH₃ molecule (0.37 nm), facilitating molecular adsorption. In contrast, the AJP‑deposited film exhibits an expanded interlayer spacing of 0.51 nm, ensuring unobstructed gas adsorption and desorption pathways. Under a 100 ppm ammonia exposure: the drop‑coated sample responds in 65 s and recovers in 530 s, whereas the aerosol‑based device achieves response and recovery times of just 33 s and 80 s, respectively, demonstrating a substantial improvement in dynamic sensing performance.
- Full-range stability and high ammonia selectivity
With a wide concentration range of 5–500 ppm, it exhibits a stable baseline and a specific response to ammonia, remaining unaffected by common interfering gases, making it well-suited for monitoring in highly complex environments.
- Ultra-thin, uniform, and controllable sensing film
The AJP film has an average thickness of 0.9 μm, significantly thinner than the 2.5 μm achieved by spin coating. The film surface is uniform and free of agglomerates, enabling digital, precise control over both film thickness and sensor pattern dimensions.
- The integrated process offers distinct advantages.
The ultrasonic atomization–sheath‑gas delivery system simultaneously achieves elemental doping and lamellar delamination during the printing and deposition process, enabling one‑step film formation and modification while eliminating complex pre‑treatments such as hydrothermal processing and post‑doping.
II. Complete Aerosol-Spraying WS Gas-Sensing Membrane Process and Core Advantages
(1) Complete Preparation Procedure for Aerosol-Based Gas-Sensing Sensors
- WS₂ nanosheet powder, dispersed in a 1:1 ethanol/deionized water slurry and subjected to ultrasonic treatment for 2 hours;
- Base substrate pretreatment, with interdigitated metal electrodes pre-printed;
- The AJP device was operated at an ultrasonic voltage of 26.8 V, with an atomization flow rate of 25 SCCM, sheath gas flow of 60 SCCM, and a 150-μm nozzle; the WS₂ sensing layer was directly written at room temperature.
- It dries naturally at room temperature, eliminating the need for high-temperature annealing.
- Gas-sensing test platform with ammonia calibration across a 5–500 ppm gradient, coupled with SEM/EDS/TEM microstructural characterization.
(II) Five Core Process Advantages (Comparison of Dip Coating vs. Spin Coating)
- One-step synchronous modification and film formation, eliminating the need for post‑doping treatment.
Conventional methods require hydrothermal treatment and plasma‑assisted doping for modification, whereas the AJP atomization process automatically introduces oxygen‑ and nitrogen‑based donor impurities, enabling in‑situ control of the conductivity type and significantly streamlining the fabrication process.
- A controllable porous lamellar structure addresses the challenge of gas entrapment.
High‑velocity droplet deposition disrupts the tight stacking of WS₂, permanently widening the interlayer spacing and fundamentally addressing the industry’s longstanding pain point of excessively long recovery times for conventional thin films.
- Micrometer-scale pattern precision printing and array-based batch fabrication
Maskless digital fabrication enables the design of dot‑matrix and strip‑type sensor arrays, compatible with miniature wearable sensing chips.
- Ultra-thin, uniform film with repeatability far superior to spin coating.
Dip-coating results in thick edges and a thin center, with significant batch-to-batch variability; in contrast, aerosol deposition yields uniform thickness across the entire surface, and device‑to‑device sensing variability is markedly reduced within the same batch.
- Room-temperature, all‑process technology, compatible with flexible PI and PET substrates.
No high-temperature baking is required, and the flexible substrate remains undamaged, enabling the development of ammonia‑sensing patches for skin‑adhesive wearable applications.
III. Five Core Technological Innovations of This Study
Innovation 1: A Novel Mechanism for In-Situ Doping via Aerosol Ultrasonic Atomization
Innovative Technology for Preparing WS₂ Thin Films with Controllable Interlayer Spacing and Porosity
Innovation 3: A Single-Ink, Dual-Conductive Device Fabrication Paradigm
Innovation 4: Ultra-fast Room-Temperature Two-Dimensional Gas-Sensing Thin-Film Fabrication Process
Innovative 5-step green gas-sensing fabrication route without post-processing
IV. Existing Process Challenges and Corresponding Solutions for Overcoming Them
Challenge 1: The WS₂ flakes are densely packed upon drop-casting, with ammonia molecules trapped between the layers, resulting in extremely slow desorption.
Challenge 2: Conventional WS₂ can only yield p-type thin films, making it difficult to fabricate heterojunction-based sensors.
Challenge 3: Uneven coating thickness and poor batch-to-batch repeatability of the sensor.
Challenge 4: Doping of two-dimensional materials requires multiple steps, including hydrothermal treatment and plasma processing, resulting in a cumbersome procedure.
Challenge 5: High-temperature modification can degrade the flexible polymer substrate.
V. Future Diversified Industrial Application Scenarios
- Wearable non-invasive breath test patch for kidney disease
- Rapid Early Warning for Ammonia Gas Leaks in Industrial Workshops
- Intelligent Environmental Monitoring for Agricultural Livestock and Poultry
- Portable environmental gas detector
- Laboratory-Based Rapid Development Platform for Two-Dimensional Gas-Sensing Materials
Figure 1. Resistance–time curves of WS₂ ammonia sensors fabricated by drop‑coating (a) and aerosol jet printing (b); as well as the response time (c) and recovery time (d) of the sensor to ammonia at various concentrations (at room temperature, 25°C).
Figure 2. Scanning electron microscopy (SEM) images of WS₂ prepared by drop-casting (a) and by aerosol jet printing (b); insets show the corresponding transmission electron microscopy (TEM) images, as well as high-resolution transmission electron microscopy (HRTEM) images of WS₂ thin films prepared by drop-casting (c)(e) and by aerosol jet printing (d)(f).
Figure 3. EDS spectra (real-time mapping) of WS₂ sensing films prepared by drop-casting (a) and aerosol jet printing (b).
Figure 4. Schematic illustration of WS2 aerosol jet printing and the mechanism underlying its p-to-n type transition.
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