Room-temperature ultra-high-selectivity moisture-resistant gas sensor based on Bi2WO6 single crystal film

Abstract

The invention discloses a room-temperature ultra-high-selectivity moisture-resistant gas sensor based on a Bi2WO6 single-crystal thin film, Bi2WO6 thin films with different thicknesses are prepared by adopting a laser molecular beam epitaxy (LMBE) technology, and when the thickness is not greater than 130 nm, the thin film keeps an excellent single-crystal texture; by analyzing the thickness effect, the dependence of NO2 response barrier change and morphological parameters is evaluated. The gas sensor based on the 12 nm single crystal film realizes breakthrough performance at room temperature: ultrahigh NO2 selectivity, ultrafast response / recovery, ultralow detection limit, 86% RH response fluctuation of less than 5%, resettable in 8 months under extreme humidity, and small deviation between devices. According to the invention, the status of an Aurivillius phase film perovskite gas sensing platform is determined, and the achievement is the highest level of a room-temperature single-phase (quality) perovskite oxide film NO2 gas sensor.

Description

Technical Field

[0001] This invention belongs to the field of electronic thin film materials and devices, and relates to a room temperature ultra-high selectivity moisture-resistant gas sensor based on Bi2WO6 single crystal thin film. Background Technology

[0002] Metal-oxide-semiconductor (MOS) resistive gas sensors have long been a major research focus in the field of gas sensing due to their core advantages such as wide application range, strong gas detection capability, high sensitivity, and significant cost-effectiveness. However, in complex and multi-scenario operating environments, ambient humidity, as a common and strong interference factor in the natural environment, easily interacts strongly with the surface of sensor materials, severely damaging the performance stability of sensing devices that rely on electronic signal transmission, and greatly restricting their reliable application in real-world complex environments. Although existing technologies can mitigate humidity interference by adding moisture-proof isolation layers or moisture-absorbing sacrificial layers, these solutions often come with derivative problems such as increased structural complexity, incompatibility with semiconductor manufacturing processes, and unverified long-term stability, making it difficult to meet the demands of industrial mass production.

[0003] Furthermore, traditional MOS gas sensors typically require high operating temperatures of 200–500°C, which not only significantly increases device power consumption but also accelerates the chemical degradation of sensing materials, leading to key application bottlenecks such as decreased device stability and shortened lifespan (usually only 2–3 years). A more critical technical challenge lies in the fact that, to date, no single oxide thin-film system can simultaneously possess core performance indicators such as room-temperature sensing (low power consumption), resistance to humidity interference, high selectivity for a single gas, repeatable device fabrication, and ultra-long-term stability. Therefore, overcoming the limitations of traditional technologies and constructing solutions centered on high-performance materials through nanoscale materials engineering has become a crucial issue urgently needing to be addressed in the field of gas sensing.

[0004] Thin-film technology is a core support for the mass production of micro / nano devices, especially suitable for fabricating highly uniform devices with stringent requirements for dimensional accuracy, surface quality, and crystallinity. High-quality ultrathin films (<20 nm) possess unique advantages such as ultra-high specific surface area, fewer grain boundaries, and extremely low charge trap density. Their carrier mobility is at least two orders of magnitude higher than that of polycrystalline materials, significantly enhancing charge separation and transport efficiency, thereby endowing devices with superior sensitivity and response speed. Furthermore, ultra-high specific surface area, high-quality single-crystal structure, and grain size matching the depth of the space charge layer are core characteristics of high-performance gas sensing materials, ensuring significant and rapid electrical response modulation when the material comes into contact with the target gas. Therefore, the thin-film-based technology route provides a solid technical platform for high-performance gas sensing research and the development of cost-effective integrated sensors.

[0005] Bi₂WO₆ (BWO), a typical two-dimensional ionic layered perovskite material, is the simplest aurivillius phase oxide. Its orthorhombic crystal structure consists of alternating [WO₄] atoms. 2- Layers and [Bi2O2] 2+ The layered structure exposes abundant free Bi. 3+ and Active sites. Crucially, its valence band apex is formed by Bi 6s-O 2p orbital hybridization, creating a narrow band gap of 2.5-2.8 eV. This band gap characteristic is insensitive to film thickness, inherently ensuring the long-term stability and reproducibility of the material. Density functional theory calculations show that BWO possesses significant ionic properties, with an effective charge distribution of Bi +2.83 valence, W +4.89 valence, and O -1.7 to -2.0 valence, far exceeding that of traditional binary oxides such as WO3. Combined with its low metal-oxygen bond energy, this unique structure significantly promotes interlayer ion migration, electron delocalization (charge transfer), and surface reactivity.

[0006] In theory, the above properties can significantly enhance the sensitivity, selectivity, and moisture resistance of BWO to polar gas molecules—this is mainly due to the highly active Bi exposed on its surface. 3+ (W) 6+ These sites can inhibit the adsorption of hydrated species while selectively promoting the adsorption of acidic (or basic) analytes. However, to date, systematic experimental verification of the gas sensing performance of BWO single-crystal thin films is still lacking. Their sensing characteristics and underlying mechanisms in complex environments (especially under high humidity conditions) remain unclear, greatly limiting their application exploration in the field of high-performance gas sensors. Summary of the Invention

[0007] To address the problems of existing MOS gas sensors, such as lack of room temperature sensing capability, poor tolerance to humidity interference, insufficient gas selectivity, low repeatability of device fabrication, and poor long-term stability, this invention provides a room temperature ultra-high selectivity and moisture-resistant gas sensor based on Bi2WO6 single-crystal thin film. The invention aims to use laser molecular beam epitaxy (LMBE) technology to fabricate a Bi2WO6 single-crystal thin film with controllable thickness and morphology, thereby achieving high selectivity and moisture-resistant room temperature NO2 sensing function based on the hydrophobic surface properties of Bi2WO6. This provides material and technical support for solving the problem of high-performance room temperature gas sensing of MOS under complex environments.

[0008] To achieve its objectives, the present invention employs the following technical solution: This invention first provides a method for preparing BWO single-crystal thin films, characterized by: using LMBE technology to prepare films with a thickness of 2~130 nm on a SrTiO3 (STO) substrate, with out-of-plane (00)... lBWO single-crystal thin films with preferred crystal plane orientation. The specific steps include: Step 1: STO substrate processing The STO substrate was immersed in deionized water at 45-55°C for 10-14 hours, and then ultrasonically cleaned in ethanol, acetone and deionized water for 10-20 minutes respectively to thoroughly remove oil and impurities from the substrate surface. Finally, the cleaned substrate was placed in air and annealed at 900-1000°C for 8-10 hours to achieve crystal plane regularization and residual stress release on the substrate surface.

[0009] Step 2: Preparation of Bi2WO6 single crystal thin film Thin films were grown on STO substrates using LMBE technology: the distance between the substrate and the target was controlled at 4–6 cm, and the substrate was heated to 650–750 °C and preheated for 5–15 min. Under an oxygen atmosphere of 15–22 Pa, a 248 nm KrF excimer laser with an effective energy of 100–130 mJ was used to ablate a high-purity Bi₂WO₆ target with a purity of not less than 99.99%, with the pulse sputtering frequency set to 1–3 Hz. After deposition, the sample was placed in an oxygen pressure environment of 25–35 Pa and annealed in situ at 720–780 °C for 20–40 min, followed by natural cooling to room temperature to obtain a BWO single-crystal thin film. Samples of different thicknesses could be grown by adjusting the number of laser ablation pulses. For ease of subsequent performance characterization and comparison, the sample naming rule was defined as "BWO - film thickness" (e.g., a sample with a thickness of 12 nm was named BWO-12).

[0010] The present invention further discloses a gas sensor based on the above-mentioned BWO single crystal thin film, the structure of which is designed as follows: using a pretreated STO substrate as a base, a BWO single crystal thin film is grown on the surface of the substrate as a core sensitive layer by the above-mentioned preparation method, and then an Au top electrode is prepared on the surface of the BWO single crystal thin film, and finally a layered gas sensor structure of "STO substrate - BWO sensitive layer - Au top electrode" is constructed.

[0011] This invention explores the gas sensing application characteristics of dense single-crystal thin films, clarifying their unique advantages compared to traditional sensing materials. Particularly noteworthy is the gas sensor integrated based on a 12 nm thick BWO single-crystal thin film with precisely matched Debye lengths, achieving breakthrough comprehensive sensing performance at room temperature. This effectively solves the core technical pain points of existing MOS thin-film gas sensors, such as poor room temperature performance, weak tolerance to humidity interference, insufficient selectivity, poor stability, and poor fabrication repeatability. Specifically, the advantages of room temperature response are as follows: 1. Ultra-high NO2 selectivity: The response to common interfering gases such as H2S, SO2, ethanol, and NH3 is negligible, enabling accurate identification of NO2 in complex multi-component gas environments and significantly improving detection accuracy; 2. Ultrafast response / recovery kinetics: At a NO2 concentration of 10 ppm, the response time is only 24 s and the recovery time is only 71 s, meeting the practical application requirements of real-time monitoring and rapid response; 3. Low detection limit: It can accurately detect 27 ppb NO2, expanding the application range of the sensor in low-concentration pollutant monitoring scenarios; 4. Excellent moisture resistance and stability: In an environment with a relative humidity of up to 86%, the response value fluctuates by less than 5%, achieving stable sensing independent of humidity, and can be adapted to complex humid environments without the need for additional moisture-proof structures; 5. Long-term stability and resetability: Under extremely wide humidity conditions (5% RH~86% RH), the sensing performance can still be restored by simple reset after eight months of use, and the response value fluctuation is less than 4% and 3% respectively, which greatly extends the service life of the device. 6. Excellent device repeatability: The performance deviation between devices is only 7.5%, which provides a technical basis for large-scale mass production and reduces the cost of industrial applications.

[0012] This invention further clarifies the core position of Aurivillius phase perovskite oxides as a multifunctional biomimetic sensing platform. To date, the comprehensive performance achieved by this invention represents the highest level of existing single perovskite oxide-based gas sensors at room temperature, providing key technical support and unprecedented scientific insights for the material design and development of sensing systems. Attached Figure Description

[0013] Figure 1 The structure of BWO in Example 1 is characterized, where (a) is the XRD-2θ scan curve of BWO-130 and (b) is the XRD-2θ scan curve of BWO-195.

[0014] Figure 2 The structure of BWO-65 in Example 1 is characterized, wherein: (a) is the XRD-2θ scan curve of BWO-65, and the inset is the rocking curve of the main diffraction peak (006) in the 2θ scan curve. ω (a) scan); (b) for BWO-65 and STO φ Scan curves; (c) shows the exponential function relationship between grain size and film thickness, and the inset shows (00 l BWO lattice structure with orientation ).

[0015] Figure 3The structure of BWO-12 in Example 1 is characterized as follows: (ac) is the fine photoelectron spectrum of Bi 4f, W 4f and O 1s of BWO-12, and (d) is the Raman scattering spectrum of BWO-12.

[0016] Figure 4 The following are gas-sensing performance data of BWO in Example 1, where: (a~c) are curves showing the response of BWO-9 / 12 / 65 sensors of different thicknesses as a function of NO2 concentration, and the inset shows the dynamic gas sensing response performance of the corresponding samples under the corresponding NO2 concentration variation conditions; (d) is a radar chart comparing the response values ​​of BWO of different thicknesses to 10 ppm NO2; (e) shows the response / recovery time of BWO films of different thicknesses to 10 ppm NO2; (f) shows the response recovery time and selectivity of BWO-12 single crystal film, and the inset shows the response recovery time of BWO-18.

[0017] Figure 5 To test the humidity resistance of the BWO-12 gas sensor in Example 1 at different operating temperatures, the sensing characteristics of BWO-12 for 10 ppm NO2 gas were investigated at different operating temperatures and relative humidity levels. The test conditions were: at operating temperatures of (a) room temperature, (b) 40°C, (c) 60°C and (d) 100°C, the sensor was exposed to NO2 gas atmospheres containing 5% RH (high purity air), 11%, 33%, 57%, 75% and 86% RH, respectively.

[0018] Figure 6 The activation energy and stability of the BWO-12 film in Example 1 in a 10 ppm NO2 environment are evaluated, including: (a) the relationship between activation energy and relative humidity level, with the inset showing the contact angle of the BWO-12 film; (b) the periodic sensing performance and durability stability in a high-purity air environment and (c) an 86% relative humidity environment.

[0019] Figure 7 The devices based on BWO-12 in Example 1 can reproducibly respond to 10 ppm NO2 (for three consecutive cycles). Detailed Implementation

[0020] The technical solution of the present invention will be described in detail below through specific embodiments. The following embodiments are implemented under the premise of the technical solution of the present invention, and detailed implementation methods and specific operation processes are given. However, the protection scope of the present invention is not limited to the following embodiments.

[0021] Example 1 In this embodiment, a series of BWO single-crystal thin films were first grown. After material characterization and testing, a gas sensor was further fabricated, and NO2 gas sensing, selectivity, moisture resistance, and stability tests were completed. The detailed steps are as follows: Step 1: STO substrate processing The STO substrate was immersed in deionized water at approximately 50°C for 12 h, followed by ultrasonic cleaning in ethanol, acetone, and deionized water for 15 min each. Finally, the substrate was placed in air and annealed at 950°C for 9 h to complete the substrate pretreatment.

[0022] Step 2: Growth of BWO single-crystal thin films Thin films were prepared on STO substrates using LMBE technology: the distance between the substrate and the target was controlled to be 5 cm, the substrate was heated to 700℃ and preheated for 10 min; under an oxygen pressure of 18 Pa, a 248 nm KrF excimer laser with an effective energy of 115 mJ was used to ablate a high-purity Bi2WO6 target with a purity of 99.99%, and the pulse sputtering frequency was set to 2 Hz; after deposition, the sample was placed in an oxygen pressure of 30 Pa and annealed in situ at 750℃ for 30 min, and then naturally cooled to room temperature to obtain a BWO single crystal thin film.

[0023] By adjusting the number of laser ablation pulses, samples of different thicknesses were grown. When the number of pulses was 1100, 1500, 2200, 3200, 8000, 16000, and 24000, thin film samples with thicknesses of 9 nm, 12 nm, 18 nm, 26 nm, 65 nm, 130 nm, and 195 nm were obtained, respectively.

[0024] Step 3: Material Characterization Figure 1 (a) demonstrates the high-quality single-crystal texture of BWO-130 when the film thickness is increased to 195 nm, such as Figure 1 As shown in (b), the BWO-195 sample exhibits a distinct polycrystalline structure.

[0025] like Figure 2 As shown, XRD analysis results confirm the high crystallinity of the BWO-65 single-crystal film. Combined with XRD analyses of the BWO-130 single-crystal film and the BWO-195 polycrystalline sample, it can be concluded that when the sample thickness is ≤130 nm, (00 l The preferred orientation of the single-crystal diffraction peaks indicates that BWO achieved high-quality single-crystal epitaxy on an STO substrate. Taking BWO-65 as an example, Figure 2(a) The full width at half maximum (FWHM) of the (006) diffraction peak in the illustration is as low as 0.15°, demonstrating a large grain size and high-quality single-crystal texture. Figure 2 (b) φ Scanning revealed that the (113) crystal plane of BWO and the corresponding crystal plane of STO both exhibit quadruple symmetry, and the BWO diffraction peak is shifted by 45° relative to STO, verifying the cubic stacked epitaxial relationship that follows BWO

[100] ||STO

[110] . Figure 2 (c) illustrates the exponential function relationship between grain size and thickness of the BWO film. As the film thickness increases, the grain size grows exponentially and gradually approaches saturation. The inset visually demonstrates (00 l The BWO layered lattice structure with 0.5 orientation further clarifies its single-crystal epitaxial structural characteristics.

[0026] The XPS spectrum of BWO-12 is as follows: Figure 3 As shown in (ac): Figure 3 In the Bi 4f spectrum of (a), the characteristic peaks at 159.2 eV and 163.2 eV correspond to the 4f values ​​of Bi, respectively. 7 / 2 and 4f 5 / 2 Energy levels, this is Bi in BWO thin films 3+ Characteristic spectral lines. Figure 3 (b) The W4f spectrum is decomposed to 35.4 eV (4f 7 / 2 ) and 37.5 eV (4f 5 / 2 The double peaks of [WO4] correspond to [WO4]. 2- W in the layer 6+ . Figure 3 (c) The O 1s spectrum can be decomposed into adsorbed oxygen, oxygen vacancies and lattice oxygen components. Among them, the characteristic peak at 531.7 eV, which belongs to oxygen vacancies, accounts for 18%, which can form abundant active sites and provide favorable conditions for surface redox reactions (the core of the gas adsorption / desorption process). Figure 3 The Raman spectrum of (d) also exhibits characteristic peaks with significant intensity, mainly concentrated at 260, 283, 306, 417, 703, 722, 792, and 827 cm⁻¹. -1 At this location, in the low wavenumber region (260-306 cm⁻¹) -1 ) and 417 cm -1 The characteristic peak at 703–827 cm⁻¹ is attributed to the translational, bending, and oscillating vibrational modes of the [Bi₂O₂] chain and [WO₆] octahedron. -1 The peak at that point corresponds to the stretching vibration of the WO bond, further confirming the typical layered perovskite structure of BWO.

[0027] Step 4: Fabrication of Gas Sensor Devices A gas sensor was fabricated by using a thermal evaporation deposition process to prepare an Au top electrode pattern with a thickness of approximately 50 nm on the surface of a BWO single-crystal thin film.

[0028] Step 5: Device Testing Gas sensing performance testing was conducted at ambient temperature (25 ± 1℃) using a computer-controlled gas sensing workstation. A multi-channel mass flow controller system was used to generate precisely calibrated NO2 concentrations, with a total gas flow rate of 300 sccm. A 5 V DC bias voltage was applied using a Keithley 2612B digital source meter to achieve real-time monitoring of the device's resistance signal. The gas sensing performance of the BWO device is as follows: Figure 4 As shown.

[0029] Figure 4 (a~c) show that BWO-based devices of different thicknesses exhibit stable and reproducible gas sensing characteristics for NO2. The response trend shows a linear response from a thickness of 9 nm to a superlinear power-law response (significantly enhanced response at the same concentration) at a thickness of 12 nm. When the film thickness exceeds 12 nm, the response value at the same concentration gradually decreases with increasing thickness. Specifically, as the BWO thickness increases from 12 nm to 65 nm, the detection limit changes from 27 ppb (BWO-12) to 48 ppb (BWO-65), and the sensitivity increases from 2.2 %ppm. -1 (BWO-12) decreased to 1.24% ppm -1 (BWO-65). Therefore, in Figure 4 In the radar diagram (d), the 12 nm thick BWO device exhibits the strongest responsivity, and the response value shows a significant decreasing trend as the film thickness increases. Figure 4 (e~f) shows the response and recovery times of samples with different thicknesses. Among them, BWO-12 and BWO-18 films showed the fastest response speed, which reflects the dependence of adsorption / desorption kinetics on crystal size and also highlights the great potential of single-crystal oxide films in selective sensing systems.

[0030] The moisture resistance test results of the BWO-12 device are as follows: Figure 5 As shown, under constant temperature conditions, the reference resistance decreases slightly with increasing relative humidity, indicating that a humid environment slightly enhances the device's conductivity. When exposed to 10 ppm NO2 at different relative humidity (RH) levels, the BWO-12 exhibits minimal variation in its gas response value, with a response fluctuation coefficient of 4.75%. The response amplitude increases significantly with operating temperature, peaking at 40°C, and then slightly decreases upon further temperature increases. Crucially, when operating at temperatures below 100°C, the BWO-12 maintains excellent NO2 sensing humidity stability under all tested relative humidity conditions.

[0031] To elucidate the moisture resistance mechanism of the device, this embodiment employs activation energy analysis, treating NO2 at different relative humidities as six independent analytes: NO2 (5% RH), NO2 (11% RH), NO2 (33% RH), NO2 (57% RH), NO2 (75% RH), and NO2 (86% RH). The differences in the thin film's selectivity for these six gas systems stem from differences in activation energy. Figure 6 (a) shows the activation energy of the NO2 response in the range of 5%–86% relative humidity. E a The voltage level remained stable around 0.086 ± 0.01 eV, with negligible changes due to humidity. This value is consistent with the significant change in relative barrier height during 10 ppm NO2 detection, confirming that when RH ≤ 86%, humidity does not significantly alter the device's conductivity mechanism or surface band structure, and NO2 detection maintains high surface activity with an extremely low activation energy (< 0.1 eV). Furthermore, the BWO-12 surface exhibits some hydrophobicity (contact angle > 90°, see...). Figure 6 (a) Illustration), which can further suppress the interference of water molecules on oxygen exchange and gas diffusion, ultimately enabling the BWO-12 single crystal thin film to achieve humidity-independent gas response stability (response fluctuation coefficient of 4.75%).

[0032] Long-term cyclic test results are as follows Figure 6 As shown in (b) and (c), the sensor maintained continuous response stability for 47 days in both dry and 86% RH humid environments, highlighting the superior practicality of the BWO-12 sensor. Notably, after 8 months of environmental exposure and subsequent aging treatment (200℃, 2h), the repeatability and sensing stability of the BWO-12 sensor remained comparable to newly fabricated devices. This superior stability stems from the dual synergistic characteristics of single-crystal orientation: (i) the inherent structural robustness of the material; and (ii) the ion- and vacancy-rich surface significantly reduces the NO2 response activation energy, enhancing both NO2 adsorption energy and charge transfer, jointly ensuring the long-term functional integrity of the device.

[0033] Furthermore, the BWO-12 epitaxial film grown using LMBE technology exhibits excellent uniformity, and combined with mature semiconductor integration processes, enables reproducible device fabrication. Parallel test results of four sensors ( Figure 7 The device exhibits a small response deviation (response fluctuation coefficient of 7.5%), confirming that single-crystal thin films are suitable for scalable mass production of micro sensors.

[0034] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a Bi₂WO₆ single crystal thin film, characterized in that: Bi2WO6 single-crystal thin films with a thickness of 2-130 nm were deposited on SrTiO3 substrates using laser molecular beam epitaxy deposition.

2. The method for preparing Bi2WO6 single crystal thin films according to claim 1, characterized in that, The Bi2WO6 single crystal thin film has an outward (00) plane. l Preferred orientation of crystal planes.

3. The method for preparing Bi2WO6 single-crystal thin films according to claim 1 or 2, characterized in that, Includes the following steps: Step 1: SrTiO3 substrate treatment The SrTiO3 substrate was immersed in deionized water at 45-55℃ for 10-14 h, and then ultrasonically cleaned in ethanol, acetone and deionized water for 10-20 min respectively. Finally, the cleaned substrate was placed in air and annealed at 900-1000℃ for 8-10 h. Step 2: Preparation of Bi2WO6 single crystal thin film Thin films were grown on SrTiO3 substrates using laser molecular beam epitaxy (LBE). The distance between the substrate and the target was controlled to be 4–6 cm. The substrate was heated to 650–750 °C and preheated for 5–15 min. Under an oxygen atmosphere of 15–22 Pa, a 248 nm KrF excimer laser with an effective energy of 100–130 mJ was used to ablate a high-purity Bi2WO6 target with a purity of not less than 99.99%. The pulsed sputtering frequency was set to 1–3 Hz. After deposition, the sample was placed in an oxygen pressure environment of 25–35 Pa and annealed in situ at 720–780 °C for 20–40 min. Then, it was naturally cooled to room temperature to obtain a Bi2WO6 single crystal thin film.

4. A Bi2WO6 single crystal thin film prepared by the preparation method according to any one of claims 1 to 3.

5. A room-temperature ultra-high selectivity moisture-resistant gas sensor based on Bi2WO6 single-crystal thin film, characterized in that: The Bi2WO6 single crystal thin film as described in claim 4 is used as a gas-sensitive material.

6. The gas sensor according to claim 5, characterized in that: The gas sensor is used to detect NO2.

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