Perspectives of Defects for Gas Detecting Performance of WSe₂ Nano Flakes Selenized from Tungsten Metal Thin Film

1. INTRODUCTION

With the continuous development of science, technology, and industry, air pollution has become an increasingly serious problem. The main cause of air pollution is the toxic gases generated by fossil fuel combustion. The most representative toxic gas is NO₂, which can cause terrible effects (headache, eye, or respiratory irritation) in humans at only 1 ppm [1]. Therefore, various materials have been developed for toxic gas detection [2-6], with the most representative being metal oxides and transition metal dichalcogenides (TMDCs) [7].

Metal oxides have high operating temperatures, high sensitivity, and low selectivity. Most importantly, they are very unstable in humid environments. Graphite-based detectors do not detect low concentrations of toxic gases and have slow recovery times [8]. In contrast, TMDC-based detectors have various advantages that can be utilized in gas detectors [9]. First, they have a very high specific surface area because of the basal plane and clear active sites on the edge. Additional advantages are observed in manufacturing TMDC materials for 3D nanostructures [10]. For example, they are optimal for active-site exposure. In addition, the voids formed between the nanoparticles constituting the 3D nanostructure are favorable for the diffusion of the target gas, which is advantageous for detector performance [11,12]. Although many studies have been conducted on optimizing the structure or maximizing the specific area, very little interest has been displayed in the defects or phases that may occur in the material when manufacturing these TMDC nanostructures employing various processes for use as gas detectors. This is because analysis and control are difficult to implement. However, interest in these factors is growing, making it important to expand the industry of commercial gas detectors using TMDCs.

In this study, to manufacture WSe₂ nano flakes in a vertical 3D structure, the simplest and most economical process was used, which is to manufacture a tungsten (W) metal thin film by having it react with selenium vapor. Compared to other high-technology processes, this method is the most widely used in industry. Various defects of the WSe₂ nano flakes were accurately manufactured using this method. The possible locations of the most important metal-phase residuals and their influence on gas detection performance are presented.

2. EXPERIMENTS

3. RESULTS AND DISCUSSIONS

In a high-temperature heat-treatment atmosphere, the W thin films gradually agglomerate, even though the temperature is much lower than the melting point. When selenium vapor is attached to the agglomerated tungsten, WSe₂ nucleates as tungsten is consumed, and it gradually grows along the basal plane into a nanoflake shape. The WSe₂ nano flakes grow with a thermodynamically stable hexagonal facet. Even after the reaction, a small amount of the tungsten metal phase may remain, which is a residue that does not fully react because of the high vapor pressure of selenium. This growth process is illustrated in Fig. 1 (a). XRD images of a 100 nm thick W thin film are displayed as a function of selenization temperature in Fig. 1 (b). Selenization hardly occurred at 600℃, as no peak related to WSe₂ are observable. At 700℃, WSe₂ grows by selenization, and a tungsten metal phase residue remains. At 800℃, only tungsten metal remains, and the WSe₂ phase does not appear. This is because selenium vapor is not adsorbed on the W thin film; thus, no reaction occurs. When heat-treated at 900℃, only the crystallinity of the tungsten metal increases. In the SEM image displayed as a function of selenization temperature in Fig. 1 (c), a slight sharp flake edge is observed when heat-treated at 600℃; however, the thermal energy is insufficient to achieve crystallinity. Another characteristic is that empty space exists between the flakes (pink dashed line circle), which is consumed as the tungsten thin film reacts with WSe₂, resulting in the appearance of a void. At 700℃, the flake shapes become more abundant. At 800℃, only tungsten metal remains, and at 900℃, only agglomeration in the form of nano dots occur. The reaction at 700℃ was determined to be the most suitable.

Fig. 1.

WSe2 nano-flake according to annealing temperature: (a) Schematic of WSe2 nano-flake growth from tungsten thin film. (b) XRD and (c) SEM images of WSe2 nano-flake as a function of selenization temperature. (W thin film thickness: 100 nm)

The amount of tungsten suitable for the growth of WSe₂ nano flakes was determined by controlling the thickness of the thin W film. As shown in the SEM image in Fig. 2 (a), when the thickness is reduced, the density of the WSe₂ nano flakes increases as the W thin film is sufficiently consumed. This means that as the amount of tungsten decreases, agglomeration and diffusion occur more easily due to thermal energy, and the selenization reaction becomes more favorable. At 50 nm, tungsten is sufficiently consumed, and the nano flakes are more abundant. This abundant structure formation is a positive factor for gas detection performance because it creates voids between the nanoparticles, which is advantageous for the diffusion of the gas to be detected. The XRD data in Fig. 2 (b) show a similar trend. At a thickness of 10 nm, the tungsten film is almost completely consumed and grows into WSe₂ nano flakes. However, at a thickness of 50 nm, although WSe₂ nano flakes are abundant, the tungsten metal phase remains. This tungsten residue plays a role in stabilizing the sensitivity and improving the performance of the detector with a nanostructured thin film. The detector performance of the WSe₂ nano flakes as a function of tungsten thickness is shown in Fig. S1. For the 50 nm thickness, the baseline of the current value is high, and the current noise is minimized during detection. This can be attributed to the effect of not only the increased density of the WSe₂ nano flakes, but also the metal phase residue. Fig. 2 (c) shows a cross-sectional SEM image and EDS results of the nano flakes for a 50 nm W thin film. Point 1 is the part that directly contacts the selenium vapor during annealing; therefore, selenium is completely sufficient, whereas the ratio of selenium decreases sharply at point 2. A closer look at the cross-sectional SEM image reveals only a WSe₂ flake-shaped particle at point 1, and point 2 reveals dot-shaped particles, not flakes. These dot particles are inferred to be metal residues. The location of these metal residues is also crucial. Since the gas-sensing region should be only provided by WSe₂, the top surface (point 1) should be free of metal residues. Conversely, the path through which gas attaches to the sensing region transmits the electrical signal to the bottom interface (point 2). In addition, among the target gases, in certain cases, strong oxidizing properties are detected, and metal residues (especially tungsten) can be highly oxidized by the target gas; however, oxidation can degrade the effectiveness of the metal residue. This can be avoided by positioning the metal residue at Point 2. Thus, the metal-phase residue is distributed at the boundary with the substrate and is involved in the amplification and stability of the electrical signal rather than directly participating in NO₂ gas detection.

Fig. 2.

WSe2 nano-flake according to metal film thickness: (a) SEM and (b) XRD images of WSe2 nano flake as a function of W thin film thickness. (c) Cross-section image and EDS data of WSe2 nano flake of 50 nm thickness W thin film.

The TEM image in Fig. 3 (a) shows that the WSe₂ nano flakes are uniformly thin and hexagonal, and the high-magnification image in Fig. 3 (b) shows that the crystal structure has hexagonal closed packing (HCP). Notably, the step terrace is on the basal plane of the flakes (Fig. 3 (c)). Because WSe₂ flakes (Fig. 3 (c)) and TMDC are thermodynamically more unstable at the edges than at the basal plane, they are active sites. The drawback of the WSe₂ nano flakes is that the density of these edges is very low compared to that of the basal plane. Therefore, the edge caused by the step terrace in the basal plane can amplify the sensor sensitivity of the WSe₂ nano-flake structure. A factor that can further amplify the sensitivity of the sensor is the 1T phase, which has a relatively high free energy. As shown in Fig. 3 (d), the two phases coexist in one nano flake. This provides more active sites, which can improve gas detection performance. To demonstrate the gas detection performance of these defects, the response of the WSe₂ nano flakes to NO₂ gas was measured using a simple method.

Fig. 3.

TEM images of WSe2 nano-flake: (a) low magnification, (b) high magnification and Fourier-filtered image of WSe2 nano-flake (Inset: corresponding Fourier-transformed diffraction pattern). (c) Image of step-terrace and (d) 2H, 1T phase of WSe2 nano-flake.

The NO₂ gas detection performance of WSe₂ nano flakes grown from 50 nm thick W thin films was evaluated at an operating temperature of 80℃. An electrode was manufactured to evaluate the performance of the gas detector (Fig. 4 (a)). The relative response shown in Fig. 4 (b) was obtained from the following equation;

Fig. 4.

Responsivity of the fabricated gas detector: (a) Schematic of gas detector electrode. (b) Real-time relative response curve of WSe2 nano-flake at a concentration of 3 ppm NO2. (c) Real-time resistance changes of the detector in response to 3 ppm NO2 at 80℃ demonstrating repeatability. (d) Real-time response of WSe2 nano-flake at 80℃ and different NO2 concentrations. (W thin film thickness: 50 nm)

where R_a is the baseline resistance of the sensor in the background gas, and R_g is the resistance after exposure to the target gas. When NO₂ gas was introduced, the resistance decreased because of the p-type semiconductor characteristics of WSe₂. These p-type characteristics become more pronounced at the nanoscale, and very thin nano flakes can be synthesized using this method. The relative response increased to ~9% within 574 s and required 3346 s for recovery. This response was maintained under repeated NO₂ gas on/off cycles (Fig. 4 (c)). Furthermore, the rate of resistance change varied with NO₂ concentration (Fig. 4 (d)). This high performance is attributed to the various defects (metallic residues, step terraces, and thermodynamically unstable phases) that are easily generated by the simple selenization process of tungsten metal. The ability to generate such defects, along with the accurate identification and analysis methods presented in this study, is expected to serve as an important strategic asset for the development of NO₂ gas sensors. Moreover, this approach can be extended to other TMDC materials beyond WSe₂.

4. CONCLUSIONS

In this study, WSe₂ nano flakes were synthesized by selenizing W thin films using a simple and economical method. The WSe₂ nano flakes exhibited various structural defects, including tungsten metal residues, step terraces, and mixed phases, which played a crucial role in enhancing NO₂ gas detection performance. The detector demonstrated a stable reproducible response to NO₂ at an operating temperature of 80℃.

These results highlight the significance of defect engineering in TMDC-based gas sensors. By accurately identifying and controlling the defects generated during synthesis, electronic properties and active site availability can be tailored, thereby improving detector performance. The approach demonstrated herein can be extended to other TMDC materials, providing a valuable strategy for the development of highly sensitive and reliable gas detection devices for environmental monitoring applications.

Acknowledgments

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean Government (MSIT) (No. RS-2023-00208311).

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