Topological insulator Bi₂Se₃ for highly sensitive, selective and anti-humidity gas sensors

Summary

Chemiresistive gas sensors generally surfer from low selectivity, inferior anti-humidity, low response signal or signal-to-noise ratio, severely limiting the precise detection of chemical agents. Herein, we exploit high-performance gas sensors based on topological insulator Bi₂Se₃ that is distinguished from conventional materials by robust metallic surface states protected by time-reversal symmetry. In the presence of Se vacancies, Bi₂Se₃ nanosheets exhibit excellent gas sensing capability toward NO₂, with a high response of 93% for 50 ppm and an ultralow theoretical limit of detection concentration about 0.06 ppb at room temperature. Remarkably, Bi₂Se₃ demonstrates ultrahigh anti-humidity interference characteristics, as the response with standard deviation of only 3.63% can be achieved in relative humidity range of 0–80%. These findings are supported by first-principles calculations, with analyses on adsorption energy and charge transfer directly revealing the anti-humidity and selectivity. This work may pave the way for implementation of exotic quantum states for intelligent applications.

Graphical abstract

Highlights

• •Topological insulator Bi₂Se₃ based high-performance gas sensors are demonstrated
• •Se vacancies enhance the gas sensing capability of Bi₂Se₃ toward NO₂
• •The sensing device exhibits ultrahigh anti-humidity interference characteristics
• •First-principles analyses reveal the mechanism of anti-humidity and selectivity

Sensor; Condensed matter properties; Structural property of matter

Introduction

Gas monitoring has penetrated into various fields, such as environmental protection, medical health, military aerospace, life safety, industrial production, among many others.¹ Detection of factory exhaust gases, such as NO_x (X = 1, 2) and CO₂, is crucial for monitoring the emission of waste gases and supervising the industry production processes,² while sensing of gaseous biomarkers (volatile organic compounds, NO_x, CO etc.) in exhaled breath is vital for early diagnosis, prognosis prediction, and screening of diseases.³ Over the past decades, metal-oxide semiconductors (MOS) have been extensively investigated in the context of sensing materials and devices, and many strategies have been employed to improve the gas sensing performance, such as by introducing defects,⁴ band bending,⁵ nanostructuring,⁶ surface functionalization⁷ and chemical doping.⁸ The gas sensing performance of MOS is closely related to the surface oxygen vacancies, which can reduce the adsorption potential barrier and promote charge transfer at the molecule-oxide interface.^9,10 However, the high operating temperature of MOS based sensor demands high energy consumption, and may also induce safety issues when detecting explosive gases.¹¹ Moreover, MOS easily chemisorb H₂O with ambient moisture, which passivates active sites and degradates sensing capability.¹² The boom of two-dimensional (2D) materials such as graphene, black phosphorene, transition metal sulfides (MoS₂, WSe₂, etc.) and MXenes, opens up new opportunities for gas sensors due to the high surface area and room temperature sensing properties.^{13,14,15,16,17,18} Nevertheless, they usually suffer from low anti-humidity, low response signal or unsatisfactory signal-to-noise ratio, severely limiting their practical applications.¹⁹ High-performance gas sensors must simultaneously meet three key requirements, i.e., high selectivity with desired molecular adsorption, low noise with high electrical conductivity, and high anti-humidity interference characteristics.²⁰ In practice, however, these requirements are strongly coupled, leaving it difficult to improve the overall performance. For instance, graphene and black phosphorene exhibit high response signal but suffer from low stability and limited selectivity.^21,22 MXenes have high signal-to-noise ratio but inferior anti-humidity performance due to the covered functional groups (-OH, -O and –F, etc.).²³ Therefore, it is highly desirable to exploit new materials and new mechanisms for high-performance gas sensors.

As a novel quantum state of matter, topological insulators (TIs), as characterized by robust metallic surface or edge states residing inside an insulating bulk gap, have attracted tremendous interest in condensed matter and materials science communities.²⁴ As the topological states are protected by time-reversal symmetry, they have negligible elastic scattering and Anderson localization,²⁵ rendering significant implications in dissipationless transport and quantum computing devices.²⁶ In this regard, the metallic surface states contribute to conductivity, which may render great potential for gas sensing applications.^27,28 As a prototype example of three-dimensional (3D) TIs, Bi₂Se₃ has been extensively investigated from both experimental and theoretical sides.^29,30 Meanwhile, Se vacancies on the surface of Bi₂Se₃ can be controllably introduced during material growth or post-treatment processes,³¹ thus providing sufficient active sites for gas adsorption. More importantly, as compared to conventional materials, the metallic surface states of Bi₂Se₃ may serve as an effective electronic bath in enhancing the detection of gas molecules, and the topological robustness against adsorbed adatoms, local defects and modified terminations, holding significant advantages for sensing devices. It is thus tempting to explore TIs as potential gas sensing materials.

Here we experimentally demonstrate TI Bi₂Se₃ for chemiresistive NO₂ sensors, which exhibit excellent gas sensing properties and ultrahigh anti-humidity interference performance. The Bi₂Se₃ nanosheets were synthesized by a facile solvothermal method, and abundant surface Se vacancies can be controllably introduced, which serve as active sites for a high response signal to NO₂. We show that Bi₂Se₃ sensors have a high signal-to-noise ratio (SNR) and a sub-ppb limit of detection (LOD) concentration, and can maintain a high performance in relative humidity (RH) range of 0–80%. Density functional theory (DFT) based first-principles calcualtions were performed to understand the underlying mechanism, especially the atomic adsorption structures, adsorption energies and electronic properties. We reveal the crucial role of the topological surface states in regulating the gas molecule adsorption, and Bi₂Se₃ tends to have stronger interaction toward NO₂ compared to other small molecules (H₂O, NH₃, etc.), thus leading to high selectivity and anti-humidity.

Results

Crystal structure and morphology

Bi₂Se₃ possesses a layered rhombohedral crystal structure and the adjacent quintuple layers are bonded by van der Waals forces, so that 2D layers of Bi₂Se₃ can be easily obtained.³² In our study, the Bi₂Se₃ nanosheets were synthesized through a solvothermal method. The crystal structure and morphology of Bi₂Se₃ were characterized, and the results are presented in Figure 1. X-ray diffraction (XRD) pattern reveals that the as-prepared Bi₂Se₃ can be exclusively indexed as the rhombohedral Bi₂Se₃ phase (ICDD-PDF # 0214) and no impurity phase can be identified (Figure 1A). There are two narrow peaks at 128 and 172 cm⁻¹ in the Raman spectrum, which can be assigned to $E_g^2$ (in-plane) and ${\mathrm{A}}_{1g}^2$ (out-of-plane) modes of Bi₂Se₃, respectively. No other peaks were observed, indicating the high-quality structure of Bi₂Se₃ (Figure 1B). Scanning electron microscopy (SEM) characterization on the morphology reveals that Bi₂Se₃ nanosheets have a large lateral size over 1 μm (Figure 1C) with an ultrathin thickness of ∼3 nm (Figure S1). Transmission electron microscopy (TEM) and X-ray energy-dispersive spectroscopy (EDS) mappings suggest that Biand Se are homogeneously distributed throughout the sample without segregation (Figures 1D–1F). Elemental analysis reveals the atomic ratio of Bi/Se to be 44.84/55.16, suggesting the abundance of Se vacancies in Bi₂Se₃ (Figure S2).³³ High-resolution TEM (HRTEM) clearly shows the (015) crystal plane of Bi₂Se₃ with a measured lattice spacing of 0.307 nm (Figure 1G), in accordance with the XRD data. Structural defects, such as vacancies and voids, can also be observed on the surface. The corresponding selected area electron diffraction (SAED) pattern along [100] zone axis indicates that the synthesized Bi₂Se₃ nanosheets are single crystals (Figure 1H).

Figure 1.

Morphology and structure of the as-synthesized Bi₂Se₃ nanosheets

(A) XRD pattern.

(B) Raman spectrum.

(C and D) SEM and (d) TEM images of the Bi₂Se₃ nanosheets.

(E and F) EDS mapping of the Biand Se elements.

(G and H) HRTEM image and (h) SAED pattern of the Bi₂Se₃ nanosheet.

Selectivity of Bi₂Se₃ gas sensor

To explore the gas sensing properties of Bi₂Se₃, the prepared nanosheets were coated on an alumina-based interdigital electrode. The structures of gas sensor and interdigital electrode are presented in Figure S3A, and the morphologies of sensing films are shown in Figures S3B and S3C. Obviously, Bi₂Se₃ nanosheets are stacked layer-by-layer on the interdigital electrode. XRD pattern shows that the sensing films are oriented in the (001) direction (Figure S4). Gas sensing performance of the device was evaluated by its resistance variation ΔR/R₀, where R₀ is the resistance in air and ΔR is the resistance change upon exposure to target gas.

Selectivity is one most important parameter for gas sensor, which determines the anti-interference ability to other gases and contributes to high detecting accuracy. To examine the selectivity of Bi₂Se₃, the device was dynamically exposed to various inorganic gases [ammonia (NH₃), nitric oxide (NO) and nitrogen dioxide (NO₂)] and gaseous volatile organic compounds [acetone (C₃H₆O), ethanol (C₂H₅O) and methane (CH₄)] at room temperature (25°C). The dynamic responses toward 50 ppm of NH₃, C₃H₆O, C₂H₅O, NO and NO₂ were evaluated to be 1.0%, 2.0%, 2.8%, 15% and 93%, respectively (Figures 2A–2E). It can be seen that Bi₂Se₃ exhibits low response toward reducing gases (1–3%) and high response toward oxidizing gases (>15%). Furthermore, the Bi₂Se₃ sensor shows excellent baseline recovery performance. Compared to other layered materials that can hardly return to the baseline resistance during gas desorption process,³⁴ here Bi₂Se₃ exhibits 100% recovery efficiency and stability response/recovery performance at room temperature (Table S1). This could be understood by the appropriate adsorption energy of NO₂ on defective Bi₂Se₃ (−1.9 eV).³⁵ On other 2D structures with defects (MoS₂, phosphorene and graphene, etc.), the adsorption energies of NO₂ are much larger (∼−4.0 eV),^36,37,38 leading to poor recovery performance.

Figure 2.

Selectivity of Bi₂Se₃ sensors at room temperature

(A–E) Resistance variation of the sensor exposure to 50 ppm of NH₃, acetone (C₃H₆O), ethanol (C₂H₆O), NO and NO₂ at room temperature (25°C).

(F) The corresponding maximal resistance change for NH₃, C₃H₆O, CH₄, C₂H₆O, NO and NO₂.

By comparing the resistance variation upon exposure to various target gases, we found that the resistance of Bi₂Se₃ sensor increases regardless of exposure to reducing gas molecules (NH₃, C₃H₆O, C₂H₅O and CH₄) or oxidizing gas molecules (NO and NO₂) (Figure 2). Such intriguing behaviors are quite different from graphene, black phosphorene or MoS₂, where the atmosphere-dependent conductivity variation is governed by the type of gas molecules.^39,40,41 This could be undestood by the fact that the adsorbed reducing and oxidizing gases reduce the charge carrier concentration and cause increased resistance associated with the metallic surface of Bi₂Se₃.⁴² In Figure 2F, we summarize the corresponding maximal response of each gas molecule. For inorganic gases, the resistance responses for 50 ppm of NO₂ and NO are 93% and 15% at room temperature respectively, while the 50 ppm of NH₃ shows a low response of 0.9%. For the organic vapors including C₃H₆O, C₂H₅O and CH₄, the resistance responses are generally low. The selectivity of Bi₂Se₃ nanosheets for NO and NO₂ are shown in Figure S5, which suggests that Bi₂Se₃ nanosheets are extremely sensitive to NO₂. Physically, Se vacancies form during growth process, which lead to a native n-type doping of Bi₂Se₃,³¹ while the adsorption of strong oxidizing gas molecules on the surface partially occupies these vacancies, which reduces the doping effects and thus increases the resistance.³⁵

Dynamic sensing performance toward NO_X

Due to the excellent selectivity toward NO₂, it is necessary to explore the dynamic sensing performance of the device. We recurrently exposed the Bi₂Se₃ sensor to NO₂ with concentrations ranging from 0.5 to 50 ppm, and found that the device presents excellent cyclic responses and recovery performance at room temperature (Figures 3A and S6). The sensor exhibits responses of 3.5% and 93% at 0.5 ppm and 50 ppm of NO₂, respectively. We also analyzed the response/recovery time and stability of Bi₂Se₃ sensor upon exposure to 20 ppm of NO₂ (Figures 3B and S7). The four-cycle dynamic response is 30.35 ± 0.28% and the response/recovery time is 2.9/18.8 min at room temperature.

Figure 3.

Dynamic response performance toward NO₂

(A) Dynamic response curve for a Bi₂Se₃ sensor upon exposure to NO₂ with concentrations ranging from 0.5 to 50 ppm at room temperature (25°C).

(B) Response at 20 ppm of NO₂.

(C) Response variation as a function of NO₂ concentration ranging from 0.5 to 50 ppm.

(D) SNR as a function of NO₂ concentration.

The sensitivity of device is defined by, S=In(ΔR/R₀)/In(ΔC), where ΔC is the variation concentration of target gas. The obtained results on the sensitivity are shown in Figure 3C, which can be divided into two parts with respect to the molecule concentration. The relative resistance increases linearly with NO₂ concentration in a logarithmic fashion. In a concentration range of 0.5–10 ppm, the value of sensitivity is 0.45%/ppb with a fitting quality R² of 0.995, and the value is 1.2%/ppb with a fitting quality R² of 0.993 in the concentration range of 10–50 ppm. As the value of sensitivity increases with the NO₂ concentration, we may understand this by the following two reasons. First, as a strong oxidizing gas, NO₂ captures electrons from the substrate, which decreases the carrier concentration of the sensing material. Adsorption of NO₂ also hinders the electron transport and reduces the charge carrier mobility. With reduced carrier concentration and mobility upon NO₂ adsorption, the resistance of Bi₂Se₃ will be increased, and with a higher NO₂ concentration, the sensitivity can be enhanced. Second, at low NO₂ concentration, monolayer adsorption dominates the gas adsorption process, which has a low saturated response; at high NO₂ concentration, multilayer adsorption dominates, which also contributes to a high response. As a matter of fact, similar results have also been observed in other sensing materials.^6,43

Moreover, Bi₂Se₃ sensors exhibit low electrical noise and high SNR. As shown in Figure S8, the electrical noise is 0.02% when exposed in the dry air. The SNR is calculated as the response divided by the electrical noise,²⁰ and a high SNR of 163.5 is achieved for NO₂ at a low concentration of 0.5 ppm (Figure S9). The LOD concentration in our testing apparatus is limited to sub ppm, and the theoretically achievable LOD (corresponding to an SNR of 3) can be derived by using a power-law equation.²⁰ The result of fitting SNR with NO₂ concentration is displayed in Figure 3D, which demonstrates a sub-ppb level LOD of 0.06 ppb.

We also considered the dynamic response of Bi₂Se₃ sensor toward NO with the molecular concentration ranging from 20 to 50 ppm at room temperature (Figure S10). It was found that the device has a varied output signal of 8%–15% when the NO concentration changes from 20 to 50 ppm. The response/recovery time of 20 ppm is 3.4/10.8 min. Compared to NO₂ molecule, NO shows slower response but faster recovery due to its lower electron affinity.

Se vacancy dependent sensing performance

To verify the importance of Se vacancy (V_se) on the sensing performance of Bi₂Se₃ nanosheets, we explored the influence of V_Se concentration on the sensing properties. In order to fabricate more V_Se on Bi₂Se₃ surface, we annealed the nanosheets in vacuum at 250°C for 0.5 h. Figure S11 presents the XRD pattern, which suggests that the annealed sample is still pure rhombohedral Bi₂Se₃ phase. We compared the sensing performance of as-prepared Bi₂Se₃ and the annealed nanosheets. Figure 4A shows the resistance variation upon exposure to NO₂ with a concentration of 25–500 ppb. Obviously, the annealed Bi₂Se₃ sensor exhibits much higher gas response than the as-prepared sensor, and the device also has faster response speed after annealing. This can be attributed to the increased number of V_Se in Bi₂Se₃ with annealing, which provides more active sites for NO₂ adsorption.

Figure 4.

Vacancy dependent sensing performance

(A) Response curves for the as-prepared and annealed Bi₂Se₃ sensors upon exposure to NO₂ with concentrations of 25–500 ppb.

(B) Current-voltage curves for the two sensors.

(C and D) Bi 4f and (d) Se 3 days XPS survey scan spectra of the as-prepared Bi₂Se₃ and annealed Bi₂Se₃.

(E) XPS valence band spectra of the two samples, with the energies of valence band and the annealing induced shift (Δ) indicated.

Figure 4B shows the current-voltage curves of as-prepared and annealed Bi₂Se₃ sensors in dry air. It can be seen that the annealed gas sensor has lower resistance due to the presence of more V_Se that enhances charge carrier concentration. Figures 4C and 4D present the X-ray photoelectron spectroscopy (XPS) survey scan spectra of as-prepared and annealed Bi₂Se₃ nanosheets, where the binding energy for Bi 4f and Se 3 days in the two samples are obviously shifted. For the as-prepared Bi₂Se₃, the binding energies at 162.81 and 157.48 eV can be indexed to Bi 4f_5/2 and 4f_7/2, and the energies at 52.87 and 53.65 eV are ascribed to Se 3d_5/2 and Se 3d_3/2 respectively.⁴⁴ For the annealed sample, the binding energies of Bi 4f and Se 3 days shift by ∼0.2 eV to higher energy positions. This energy upshift indicates more V_Se in the annealed Bi₂Se₃ than that in the as-prepared sample, consistent with previous reports.^45,46 We also analyzed the XPS valence band spectra, and the results are shown in Figure 4E. For the as-prepared Bi₂Se₃ nanosheets, the valence band is located at a binding energy of 0.24 eV, i.e., 0.24 eV below the Fermi level (set to 0 eV binding energy in XPS). For the annealed sample, the valence band sits at a binding energy of 0.30 eV, and this energy shift of 0.06 eV indicates an enhanced n-type doping in Bi₂Se₃ due to the increased number of V_Se after annealing.⁴⁷

The carrier concentration of Bi₂Se₃ can be estimated by, $n\left(E_f\right)={\int }_0^{\infty }D\left(E\right)f\left(E,E_f\right)dE$, where $D\left(E\right)$ is the density of states and $f\left(E,E_f\right)=(1+\exp \left(\frac{E-E_f}{kT}\right))$ is the Fermi-Dirac distribution. Since Se vacancies are n-type dopants, in the absence of detectable concentrations of other dopants in the BiSe system, the Fermi level position can be related to the number of Se vacancies.³⁰ Figure 4E characterizes the Fermi level position of the as-prepared and annealed Bi₂Se₃ samples, for which an energy difference of 0.06 and −0.01 eV can be obtained between the conduction band minimum and Fermi level $E_f$. Thus, the number of Se vacancies in the as-prepared and annealed Bi₂Se₃ can be estimated as 2×10¹² and 3×10¹³ respectively. These analyses suggest that a higher concentration of Se vacancies contributes to a higher response of the gas sensor.

High anti-humidity performance

As many sensing materials can easily chemisorb H₂O, it is difficult to achieve high sensing response and high anti-humidity simultaneously. In order to increase the anti-humidity performance of gas sensors, strategies of hydrophobic coating and moisture absorbent have been widely applied to construct the sensing films.⁴⁸ However, these strategies improve the anti-humidity, but sacrifice the gas sensing performance at the same time.⁴⁹ It is therefore of great significance to develop sensing materials with intrinsic anti-humidity capability. Here, we demonstrate that the proposed Bi₂Se₃ is an excellent candidate. We have conducted contact angle measurements to compare the surface wettability of various sensing materials. The As shown in Figures 5A–5C, the typical MOS exhibit low contact angles, about 16° and 34° for In₂O₃ and graphene oxide films respectively. However, Bi₂Se₃ films have a high contact angle of 78°, indicating high anti-humidity characteristics.

Figure 5.

Anti-humidity performance of Bi₂Se₃ gas sensor

(A–C) Contact angle measurements of (a) MOS In₂O₃, (b) graphene oxide (GO) and (c) Bi₂Se₃.

(D) Dynamic response of the Bi₂Se₃ sensor in different RH for 10 ppm of NO₂.

(E) Response variation of the Bi₂Se₃ sensor with different RH for 10 ppm of NO₂.

(F) Baseline resistance variation with different RH.

We then investigated the anti-humidity performance of the fabricated Bi₂Se₃ sensors. Figure 5D shows the dynamic response of the device in different RH. We can see that with increased RH, the baseline resistance increases slightly (∼7%), and in contrast with the response in dry air, the response remains 91% with RH increased to 80% (Figures 5E and 5F). The coefficient of variation (CV) is used to estimate the anti-humidity performance as defined by, CV = S_SD/S_AVG×100%, with S_SD and S_AVG the standard deviation and the average of response values in different RH, respectively.⁵⁰ As such, lower CV values mean weaker dependence to humidity, and larger CV values suggest stronger dependence. We found that the CV of Bi₂Se₃ sensor varies by only ∼3.6% when the RH changes from 0% to 80%. For comparison, in Table S2 we list the obtained CV of various sensing materials with state-of-art strategies to remove the humidity interference. It can be seen that gas sensors based on MoS₂, graphene, black phosphorene, MXenes and In₂O₃ generally exhibit low anti-humidity properties with large CV values of 11.3%, 8%, 41%, 43% and 54.3%, respectively. Here, the low CV value of Bi₂Se₃ sensor (3.6%) demonstrate excellent anti-humidity performance, even superior to the materials engineered by sophiscated strategies such as doping, surface decoration, making heterostructures and hydrophobic coating (Table S2).

Gas sensing mechanism

In order to understand the unique gas sensing properties of Bi₂Se₃, especially the superior anti-humidity and selectivity toward NO₂ as observed in experiments, we performed first-principles calculations to investigate adsorption behaviors of various small gas molecules on pristine Bi₂Se₃ surface and surface with V_Se. The calculated electronic band structures of perfect and defective Bi₂Se₃ surface are shown in Figure S12. As spin-orbit coupling (SOC) is crucial to describe the topological phase of Bi₂Se₃, we compared the band structures with and without SOC. A Bi₂Se₃ surface with 3 quintuple layers (QLs), with each of the QL comprising 5 alternating Bi and Se atomic layers in a sequence Se-Bi-Se-Bi-Se, behaves as a direct semiconductor with a band gap of 0.55 eV at Γ point without SOC (Figure S12A). With the inclusion of SOC, topological surface states emerge and there is an energy gap of ∼0.2 eV (Figure S12B), in agreement with previous reports.³⁰ The existence of this gap is due to the non-negligible coupling between the two surfaces of a very thin Bi₂Se₃ film of 3 QLs.⁵¹ To eliminate this gap, we considered a thicker Bi₂Se₃ surface with 9 QLs. As shown in Figures S12C and S12D, A distinctive feature of linearly dispersed topological surface states arises, leading to the formation of a Dirac cone. When a V_Se is introduced on the Bi₂Se₃, an electronic defect state appears below the Fermi level and can be split by SOC (Figures S12E and S12F). This results in n-type doping and contributes to the electrical conductivity as observed in the experiments.

Next, we explored the adsorption of small gas molecules, including NO₂, NO, NH₃, and H₂O on both pristine Bi₂Se₃ surface and surface with V_Se. Various adsorption configurations have been considered and the ground-state structures can be identified with the lowest total energy. Figures 6A and 6B show the adsorption structures of the four molecules on pristine and defective Bi₂Se₃ surface. The detailed adsorption distance, adsorption energy and charge transfer are listed in Table S3. We found that the adsorption distances (measured as the vertical length between molecules and the top Se layer of Bi₂Se₃) of NO₂, NO, NH₃, and H₂O on Bi₂Se₃ with V_Se are 0.58, 0.49, 1.54 and 0.89 Å respectively, which are much smaller than the case of pristine surface. Accordingly, the calculated adsorption energies greatly increase from −0.39, −0.36, −0.19 and −0.11 eV on pristine surface to −1.95, −0.81, −0.36 and −0.14 eV on defective Bi₂Se₃ surface for O₂, NO, NH₃ and H₂O, respectively (Figure 6C). Obviously, NO₂ has the strongest adsorption on Bi₂Se₃ surface, and with a V_Se, the molecule is even “pulled” to the vacancy site with a large adsorption energy (−1.95 eV). This confirms the experimentally observed high sensing selectivity of Bi₂Se₃ toward NO₂ and the annealing enhanced sensing performance. Interestingly, the adsorption of H₂O is weak on both pristine and defective Bi₂Se₃ surface, with the adsorption energy (∼0.1 eV) much smaller than that of other molecules. This distinctively reveals the intrinsically high anti-humidity of Bi₂Se₃, as well as the the measured contact angle that is much larger than other materials (Figures 5A–5C).

Figure 6.

First-principles explorations on the sensing mechanism

(A) Optimized configurations of NO₂, NO, NH₃ and H₂O molecules adsorbed on pristine Bi₂Se₃ surface.

(B) Adsorption configurations on the surface with V_Se.

(C) Calculated adsorption energies of NO₂, NO, NH₃ and H₂O on pristine Bi₂Se₃ surface and surface with V_Se.

(D) Plots of differential charge density for NO₂ adsorption on pristine Bi₂Se₃ surface (top) and surface with V_Se (bottom). The differential charge density (Δρ) is defined as, $\Delta \rho ={\rho }_{\text{total}}-\left({\rho }_{\text{sur}}+{\rho }_{\text{mol}}\right)$, with ${\rho }_{\text{total}}$, ${\rho }_{\text{sur}}$ and ${\rho }_{\text{mol}}$ representing the charge density of the total adsorbed system, Bi₂Se₃ surface and NO₂ molecule, respectively. Yellow color denotes electron accumulation and blue indicates electron depletion (isovalue = 0.002 e/ų).

We further analyzed the charge transfer between gas molecules and the underlying Bi₂Se₃ surface. By using Bader charge analysis,⁵² we are able to determine the direction and magnitude of charge transfer. As shown in Table S3, for H₂O, there is little charge transfer on both pristine Bi₂Se₃ surface and surface with V_Se, indicative of weak interaction that leads to high anti-humidity. The magnitude of charge transfer between NH₃ and Bi₂Se₃ is also small, reflecting weak adsorption. However, for the oxidizing gases, there are net electrons transferred from Bi₂Se₃ to NO, about 0.03 e for pristine surface and 0.44 e for the surface with V_Se. Strikingly, for NO₂, the transferred electrons increase significantly from 0.18 e on pristine Bi₂Se₃ to 0.7 e on defective surface, confirming the strong adsorption of NO₂. The defect regulated charge transfer can also be visualized by the charge density difference plots in real space, which are shown in Figures 6D and S13. The pronounced differential charge distributed at the NO₂-Bi₂Se₃ interface highlights the increased charge transfer with stronger interaction induced by V_Se (Figure 6D). Therefore, our theoretical analyses clearly reveal the fundamental origins of the high sensitivity, selectivity and anti-humidity of Bi₂Se₃ sensors that are tunable by introducing defects, a great benefit the future design of TI based electronic devices.

Discussion

We have successfully demonstrated the synthesis, characterization and performance meaurements of a novel chemiresistive gas sensor based on TI Bi₂Se₃ nanosheets. The Bi₂Se₃ sensor exhibits superior sensing performance toward NO₂ as compared to other 2D materials. Especially, a high response of 93% for 50 ppm and an ultralow theoretical limit of detection concentration about 0.06 ppb can be achieved at room temperature, with a response with standard deviation of only 3.63% in relative humidity range of 0–80%. Our detailed first-principles calculations reveal the fundamental mechamisms of the high sensitivity, selectivity and anti-humidity of Bi₂Se₃, providing insights into the vacancy enhanced sensing performance that can be facilely introduced by annealing. This work demonstrates the superior gas sensing capability of Bi₂Se₃ toward NO₂, and we highlight the utilization of topological surface states for molecular detection. In future, it will be highly attractive to fabricate high-quailty sensing devices based on single TI films, so that the molecular adsorption effects on the quantum transport of the topological surface states can be explicitly resolved. Our results show promise for using exotic topological materials with defect engineering to further our ability to design and fabricate optoelectronic devices for practical applications, such as in intelligent systems.

Limitations of the study

Our study showed the importance of defects, especially Se vacancies in enhancing the gas sensing performance of TI Bi₂Se₃ toward NO₂. However, it is still unclear how “topology” could affect the sensing properties. This calls for high-quality single Bi₂Se₃ films, so that quantum transport measurements could be conducted. Moreover, in view of controlling the physicochemical properties of nanomaterials for functional purposes, futher research is needed. At last, this study focused on the detecting of NO_X, while a lot of other toxic gases should be systematcially investigated before TI Bi₂Se₃ can be applied for practical applications.

STAR★Methods

Key resources table

REAGENT or RESOURCE	SOURCE	IDENTIFIER
Chemicals, peptides, and recombinant proteins
Polyvinylpyrrolidone	Alfa aesar	CAS:9003-39-8
Bismuth nitrate pentahydrate	Alfa aesar	CAS:10035-06-0
Selenium dioxide	Alfa aesar	CAS:7446-08-4
Oleic acid	Alfa aesar	CAS:112-80-1
N,N-Dimethylformamide	Alfa aesar	CAS:68-12-2
Ethylene glycol	Alfa aesar	CAS:107-21-1
Software and algorithms
Vienna Ab-initio Simulation Package	VASP Software GmbH	https://www.vasp.at
Other
X-rays diffraction	PANalytical B.V. l	PANalytical X’Pert Powder
Electron microscopy	Thermo Fisher	FEI Inspect; F50Tecnai G2F20
X-ray photoelectron spectroscopy	Thermo Fisher	Thermo Scientific Escalab Xi+
Contact angle measuring instrument	Biolin Scientific	https://www.biolinscientific.com/
Gas sensor measurement	Keithley	Keithley 2400 digital multimeter

Resource availability

Lead contact

Further information and other requests should be directed to and will be fulfilled by the lead contact, Miao Zhou (mzhou@cqu.edu.cn).

Materials availability

This work did not generate new unique reagents. All the materials and methods used for the generation of data and analysis are mentioned in the manuscript.

Method details

Synthesis of Bi₂Se₃ nanosheets

Bi₂Se₃ nanosheets were synthesized by a solvothermal method. The typical synthesis process includes the following steps. 4.5 g Polyvinylpyrrolidone (K30, Alfa aesar), 1 mmol of Bi(NO₃)₃·5H₂0 (99.99%, Alfa aesar) and 1.5 mmol SeO₂ (99.99%, Alfa aesar) were added into a mix solution of 12 mL Oleic acid (99%, Alfa aesar) and 24 mL N,N-Dimethylformamide (99%, Alfa aesar). The resulting suspension was transformed to an autoclave (50 mL) and holding at 200°C for 24 h. The product was collected by centrifugation, washed several times with absolute ethanol, and dried in vacuum oven at 60°C for 24 h.

Characterization

XRD was characterized by the PANalytical X’Pert Powder using Cu-Kα₁ radiation (λ = 1.5406 Å). The morphologies of Bi₂Se₃ nanosheets were measured by using field-emission SEM (FEI Inspect F50). HR-TEM was performed by the Tecnai G2F20, where EDS is equipped. The binding energy and valence band spectra were characterized using XPS (Thermo Scientific Escalab Xi+). The contact angle was measured using theta flow optical contact angle measuring instrument. The surface morphologies are presented in Figure S14.

Sensor fabrication and measurements

Bi₂Se₃ sensors were fabricated by dropping the mix solution of Bi₂Se₃/ethylene glycol onto a commercially interdigitated Ag/Pd-alloy electrodes, and then annealed at 300°C for 2h in N₂ atmosphere. The measurements of gas sensing properties were conducted by a home-made gas sensing analysis system. The gas sensor was placed in a sealed intelligent temperature-controlled test chamber with a volume of 150 mL. The concentration of testing gases was controlled by diluting the standard gas with dry air, where NO₂, NO, C₂H₅O, CH₄, C₃H₆O, NH₃, N₂ and dry air gases were purchased from NIMTT Co., Ltd., China. The diluting gas for NO is N₂ and for other gases is dry air. The artificial background air with different relative humidity was prepared by mixing dry air with different proportion of saturated air. The operating temperature for gas sensing was set at 25°C, and the total constant flow rate was set as 100 sccm. The current-voltage curve and resistance of the gas sensors were measured by the Keithley 2400 digital multimeter. In this work, the gas response is defined as, S= (R_g-R_a)/R_a×100%, where R_a and R_g are the resistance in air and upon exposure to target gases, respectively.

First-principles calculations

DFT-based first-principles calculations were performed by using the project augmented wave (PAW) approach^53,54 as implemented in the Vienna Ab-initio Simulation Package (VASP). The Perdew–Burke–Ernzerhof (PBE) parameterization of the generalized gradient approximation (GGA)^55,56 was employed to model the exchange-correlation energy. To properly treat the dispersion interaction, we included the van der Waals contributions (optPBE-vdw).⁵⁷ An energy cutoff of 400 eV was adopted for the plane-wave basis. The convergence criteria of energy and force were set to 10⁻⁵ eV and 0.02 eV Å⁻¹, respectively. A 2 × 2 supercell of Bi₂Se₃ surface with 3 QLs was constructed to model the adsorption of various small gas molecules on the surface with or without V_Se, and the Brillouin zone integration was sampled by a 6 × 6×1 Monkhorst-Pack grid for structural optimization and electronic structure calculations. We have also checked the results with larger supercells to ensure minimized interaction between adjacent defects or molecules. A vacuum region of 20 Å along the Z-direction was included to avoid the interaction between neighboring slabs. The inclusion of SOC was achieved by a second variational procedure on a fully self-consistent basis. The adsorption energy ($\Delta {\mathrm{E}}_{\mathrm{a}\mathrm{d}}$) of gas molecules on Bi₂Se₃ surface is defined as, $\Delta {\mathrm{E}}_{\mathrm{a}\mathrm{d}}={\mathrm{E}}_{\mathrm{t}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}}-\left({\mathrm{E}}_{\mathrm{s}\mathrm{u}\mathrm{r}}+{\mathrm{E}}_{\mathrm{g}\mathrm{a}\mathrm{s}}\right)$,where ${\mathrm{E}}_{\mathrm{t}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}}$, ${\mathrm{E}}_{\mathrm{s}\mathrm{u}\mathrm{r}}$ and $E_{\mathrm{g}\mathrm{a}\mathrm{s}}$ are the enegies of the total adsorbed system, Bi₂Se₃ surface and molecules in the gas phase, respectively. By this definition, larger negative value of $\Delta {\mathrm{E}}_{\mathrm{a}\mathrm{d}}$ suggests stronger adsorption.

Published: March 11, 2023

Data and code availability

All data reported in this paper will be shared by the lead contact upon request. This paper does not report original code. Any additional information required to reanalyze the data reported in this paper is available from the lead contact upon request.