Sn doping induced n-type to p-type transition in Bi₂Se₃ nanosheets for flexible temperature sensing

Abstract

Flexible temperature sensors capable of simultaneously delivering high sensitivity, precision, and stability are essential to meet the increasing demands for monitoring temperature changes associated with infections and diseases. Herein, we fabricated a flexible temperature sensor using Bi₂Se₃-based thermosensitive materials. Through Sn-doping, an n-type to p-type transition was realized in Bi₂Se₃ nanosheets, leading to enhanced temperature sensing performance. The Bi_1.97Sn_0.03Se₃ nanosheets with optimal doping level exhibited a high sensitivity of –0.63%/°C. The fabricated temperature sensor could detect skin temperature with high precision and stability. Moreover, by taking advantage of the n–p transition, a flexible double-chain thermoelectric generator consisting of n-type Bi₂Se₃ and p-type Bi_1.97Sn_0.03Se₃ was also fabricated, demonstrating its potential for thermal energy harvesting and self-powered temperature sensing.

Graphical abstract

Keywords

Bi ₂Se ₃ / metal doping / n-p transition / temperature sensing

1 Introduction

Flexible temperature sensors have garnered tremendous interest and find wide-ranging applications in human health detection [1, 2], biomedical engineering [3], information wireless communication [4], robots [5], and spacecraft [6]. To meet the requirements of temperature sensors in various applications, especially for reflecting human body health, the characteristics of high precision, fast response/recovery rate and highly stability as well as the comfortable wearing should be fulfilled simultaneously. Temperature sensors are commonly divided into thermocouple-type [7], resistive-type [8], and thermistor-type [9]. On the basis of different working principles, various strategies, including the screening of sensing materials and substrates, the introduction of extra additives, and the optimization of fabrication strategies, etc., have been widely employed to improve the temperature sensing properties. As the core components of flexible temperature sensors, thermosensitive materials with suitable microstructure or defect structure should be rationally designed for the higher temperature sensitivity.

Bismuth selenide (Bi₂Se₃) is a three-dimensional topological insulator with a rhombohedral layer crystal structure [10], which is formed by five covalently bonded atomic sheets with the sequence of Se(1)-Bi-Se(2)-Bi-Se(1) [11]. Bi₂Se₃ possesses gapless surface states with a single Dirac cone and an insulator-like bulk band gap of 0.3 eV [12]. In the defect chemistry of Bi₂Se₃, the dominated selenium vacancy acted as electron donors that lead to n-type behavior [13, 14]. Additionally, p-type conducting behavior in Bi₂Se₃ with varying carrier concentrations can be achieved by introducing Ca or Cu dopant [15]. Due to its smaller energy gap and tunable electrical properties, Bi₂Se₃ has been regarded as a promising thermoelectric material. To further improve the thermoelectric performance, substantial effort has been dedicated to producing Bi₂Se₃ with different morphologies or defect structures [16−20]. For instance, flake-like Bi₂Se₃ prepared by solvothermal route exhibited a higher Seebeck coefficient and a low thermal conductivity compared with the bulk counterpart due to their nanoscale size [21]. In another study, the effect of various doping elements, such as Sn, Cu, Ag, and Pb, on the thermoelectric properties were investigated [22]. After systematic testing, it was concluded that Sn doping resulted in best performance, with an overall 60% improvement in the thermoelectric figure of merit of Bi₂Se₃. Building upon previous research, Ren et al. [23] also demonstrated the effect of Sn doping with different atomic ratios on the microstructure and thermoelectrical properties of Bi₂Se₃. With increasing Sn doping concentration, the carrier concentration increased accordingly, and the figure of merit of Sn_xBi₂Se₃ reached maximum when the Sn contents x was 0.3. The excellent thermoelectric property of Sn-doped Bi₂Se₃ means that it can be utilized in a variety of diverse application, such as temperature sensing. However, the effect of Sn doping on carrier type transformation in Bi₂Se₃, as well as its impact on enhanced temperature sensing properties, has been rarely revealed.

In this paper, Sn-doped Bi₂Se₃ rhombus nanosheets with a series of doping concentrations were successfully prepared by solvothermal method. To achieve a high-performance flexible temperature sensor, Sn-doped Bi₂Se₃ blended with a crosslink agent and hardener was printed on PET substrate for stable sensing. It is noteworthy that the as-fabricated thin film composed of extra additives exhibited a high resistance that was not suitable for resistive-type temperature sensing. Therefore, the commonly used thermoelectric materials, PEDOT:PSS, known for its high conductivity, good environmental stability, and ease of synthesis, was added for effective temperature sensing. The Bi_1.97Sn_0.03Se₃/PEDOT:PSS composite based flexible temperature sensor exhibited a maximum resistance response of 50.31% with a high sensitivity of −0.63%/°C, and real-time skin temperature detection was achieved with high precision. Moreover, a thermoelectric chip composed of p-type and n-type Sn-doped Bi₂Se₃ was also fabricated, demonstrating comparable output voltage.

2 Experiment

2.1 Materials

The selenium powders (Se, 99.0%), acetone (≥ 99.8%), and ethylene glycol (EG, ≥ 99.0%) were obtained from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). Sodium borohydride (NaBH₄, 96.0%), bismuth nitrate pentahydrate [Bi(NO₃)₃·5H₂O, > 99%], PVP (Mw ≈ 55 000), stannous chloride (SnCl₂·2H₂O, ≥ 99.0%), and Sodium Citrate Dihydrate (EG, ≥ 99.0%) were purchased from Sigma-Aldrich. All the chemicals were of analytical reagent grade and used without further purification. Ultrapure water (18.25 MΩ·cm, 25 °C) was used in the experiments.

2.2 Synthesis of Bi_2−xSn_xSe₃ nanosheets

Fig.1 shows the schematic synthesis process of Bi₂Se₃ and Sn-doped Bi₂Se₃. First, the NaHSe aqueous solution was prepared by reacting NaBH₄ with Se powders with a molar ratio of 2:1 in an ice-water bath. The obtained transparent solution was sealed for the following reaction. Second, 0.5 g of PVP and 20 mL EG were added to a 100 mL round-bottom flask under magnetic stirring. After dissolution, the solution of Bi(NO₃)₃·5H₂O (0.226 g in 12.5 mL of EG) was added with continuous magnetic stirring at room temperature. The mixture was then sealed and heated to 160 °C under N₂ atmosphere. As the temperature increased, the transparent mixture turned yellow and then turbid at 160 °C. After reaching the target temperature, the freshly synthesized oxygen-free NaHSe solution (0.667 mol·L⁻¹, 1.048 mL) was rapidly injected into the mixture with further reaction for 10 min. After the reaction, the flask was naturally cooled to room temperature, and the obtained sediment was washed three times using a mixture of acetone and ultra-pure water with a volume ratio of 5:1. To synthesize Sn-doped Bi₂Se₃, tin chloride acetone solutions with different concentrations (0.19 mg/mL, 0.38 mg/mL, 0.57 mg/mL, 0.95 mg/mL, and 1.33 mg/mL; 20 mL each) and prepared Bi₂Se₃ (0.65 g) were added to a round-bottomed flask, along with 0.02 g of citric acid. The mixture was refluxed at 52 °C for 10 min. After the reaction, the sediment was washed with hot ethanol and hot acetone (volume ratio 1:1) at 60 °C to obtain final Sn-doped Bi₂Se₃ nanosheets. The Sn-doped Bi₂Se₃ nanosheets with doping concentrations of 2%, 4%, 6%, 10%, and 14% were named as Bi_2−xSn_xSe₃ (x = 0.01, 0.02, 0.03, 0.05, 0.07), respectively.

2.3 Characterizations

The samples were characterized with a transmission electron microscope (TEM, JEOL 2100 Plus, Japan), a high-resolution transmission electron microscope (HRTEM, JEOL 2100 F, Japan) coupled with an energy dispersive X-ray (EDS) spectrometer, an X-ray diffractometer (XRD, Smart Lab Rigaku, Japan, with Cu Kα radiation at λ = 1.54 Å), and a Semiconductor Characterization System (SCS, Keithley 4200, America).

2.4 Fabrication of flexible temperature sensors

The flexible temperature sensors were fabricated using the screen-printing method. The printing paste was prepared via a two-step process. Firstly, 68.50 wt% liquid epoxy resin, 12.6 wt% methylhexahydrophthalic anhydride (MHHPA) and 18.9 wt% 2-ethyl-4-methyl-1H-imidazole-1-propanitrile (EMIP) were mixed together to form a well-distributed paste. It is important to note that MHHPA acts as a hardener, while the purpose of EMIP catalyst is to reduce the reaction temperature and increase the reaction speed simultaneously. Subsequently, the Bi_2−xSn_xSe₃ powder and PEDOT:PSS were added to the original slurry in a weight ratio of 10:1 for screen printing.

Fig.2 illustrates the fabrication process of a flexible temperature sensor using screen printing technology. The sensor’s overall dimensions are designed to be 5 mm × 5 mm. Initially, the prepared Bi_2−xSn_xSe₃ mixed ink is applied to the printing plate, and a conductive pattern is printed on the surface of the polyethylene terephthalate (PET) film. Subsequently, the printed plate is removed, and the printed serpentine-shaped electrode pattern and the PET substrate are baked in a vacuum drying oven at 80 °C for 30 min until the pattern is firmly attached to the substrate surface. Next, electrode coating is applied to both ends of the conductive silver pulp, and thin copper foil is cut and applied on the conductive silver pulp to form the electrical signal output electrode. Finally, the device undergoes another baking process at 80 °C for 50 min to dry the silver pulp, enabling the achievement of lower contact resistance between the electrode and sensing materials.

2.5 Temperature sensing test and thermoelectric performance test

The resistive-type temperature sensors and dual-chain thermoelectric devices were measured using a data acquisition system (Agilent 34972A, USA). During testing, the device was positioned on a copper plate equipped with a temperature controller (PE150-30-P4, China). The initial temperature was set at 20 °C, and the heating rate is 10 °C/min. Temperature readings were calibrated using a thermal infrared imager (G165-X, FLIR, USA). The ambient temperature, humidity, and atmospheric pressure were 25 °C, 43% RH and 1026 kPa respectively. All sensing experiments were conducted in a stable environment.

3 Results and discussion

3.1 Characterization of Bi_1.97Sn_0.03Se₃ nanosheets

The phase structure of bare Bi₂Se₃ and Bi_1.97Sn_0.03Se₃ was first examined by XRD analysis. All the diffraction peaks [Fig.3(a)] can be indexed as the hexagonal morphology structure (JCPDS No. 33-0214). No other diffraction peaks were detected, indicating the high purity of as-prepared samples. However, compared to bare Bi₂Se₃, the (015) peak shifted to a higher diffraction angle [Fig.3(b)], possible attributing to the Sn doping. The morphological structure of Bi₂Se₃ and Bi_1.97Sn_0.03Se₃ was also characterized by TEM and HR-TEM. Uniform nanosheets with an average transverse size of 100 nm can be clearly seen in both bare Bi₂Se₃ and Bi_1.97Sn_0.03Se₃ [Fig. S1(a) of the Electronic Supplementary Materials and Fig.3(c)]. According to the HR-TEM image in Fig.3(d) and Fig. S1(b), the synthesized nanosheets exhibited good crystallinity, with crystal lattice fringes clearly visible, belonging to the (110) plane of Bi₂Se₃ with an interplanar spacing of 0.21 nm [24], which are consistent with the electron diffraction rings of bare Bi₂Se₃ [insert in Fig. S1(b)] and the pattern [110] obtained from FFT (Fast Fourier Transform) analysis of Bi_1.97Sn_0.03Se₃ [insert in Fig.3(d)] [25]. Especially, a laterally oriented structure with a thin-layered structure of less than 1 nm thickness and a gap of about 0.48 nm width between the layers were also confirmed, as shown in Fig.3(e) [23]. Additionally, their chemical compositions were determined by EDS mapping. For Bi₂Se₃ and Bi_1.97Sn_0.03Se₃, Bi, Se, and Sn are uniformly distributed on the rhomboid nanostructure [Fig. S1(c) and Fig.3(f)]. The sparse Sn distribution indicates the relatively low doping content. The quantitative elemental composition of Bi_1.97Sn_0.03Se₃ was analyzed using EDX mapping. As shown in Fig. S2, the molar ratio of Bi, Sn and Se elements was approximately 1.97:0.03:3, which is in good agreement with the stoichiometric ratio of Bi_1.97Sn_0.03Se₃ nanosheets.

3.2 Temperature sensing properties of bare Bi₂Se₃ and Sn-doped Bi₂Se₃

The temperature sensing performances of both bare Bi₂Se₃ and Sn-doped Bi₂Se₃ were systematically evaluated. As depicted in Fig.4(a), Sn-doped Bi₂Se₃ demonstrated a higher response compared to bare Bi₂Se₃. Furthermore, with increasing Sn doping concentration, the temperature sensing response increased accordingly, reaching a maximum at a doping molar ratio of 6% (Bi_1.97Sn_0.03Se₃). However, further increase in Sn doping content led to a decrease in the response. The dynamic response-recovery curves of bare Bi₂Se₃ and Bi_2−xSn_xSe₃ to different temperatures were also presented in Fig.4(b) and Fig. S3, demonstrating rapid response and recovery speeds with stable initial resistance. These results indicate the stability and reliability of the fabricated temperature sensors. Considering that the temperature coefficient of resistance (TCR) is a crucial parameter for temperature sensors, it can be calculated using the formula: ΔR/R₀ = TCR (T − T₀) [26], where T₀ is the initial temperature, T represents the real-time temperature, R₀ is the initial resistance value, and ΔR is the change in resistance. Based on the fitting curves derived from the relative resistance changes as a function of temperature [Fig.4(c)], the Bi_1.97Sn_0.03Se₃-based temperature sensor exhibited a relatively higher sensitivity of 0.63% and high linearity (R² = 0.98) within the temperature range of 30 to 90 °C. Furthermore, the resolution, stability, and durability of the Bi_1.97Sn_0.03Se₃-based flexible temperature sensor were assessed. As depicted in Fig.4(d) and Fig. S4, the temperature resolution of the fabricated sensor was determined to be 0.2 °C, enabling real-time tracking of temperature changes. During continuous measurement over 5 minutes [Fig.4(e)], the nearly unchanged response values of the temperature sensors at different temperatures indicated the excellent stability of the fabricated sensor. Additionally, as shown in Fig.4(f), the temperature sensor maintained stable sensing characteristics even after 5 consecutive cycles at different temperatures, underscoring the excellent cycle stability of the fabricated sensors. The temperature sensing performances of the sensor remained unchanged after being bent 300 times, stretched, and compressed with a 5 N stress, demonstrating its excellent flexibility (Fig. S5). As shown in Table S1, compared with some reported temperature sensors, the sensor in this work exhibits higher temperature sensitivity over a wide sensing range.

3.3 Temperature sensing mechanism

As described previously, the fabricated flexible temperature sensors are all based on the mixing of Bi_2−xSn_xSe₃ and PEDOT:PSS, which exhibited decreased resistance with increasing temperature. To detail the different temperature sensitivities of bare Bi₂Se₃ and Sn-doped Bi₂Se₃ with various doping concentrations, the semiconducting behavior of single or composite materials was first confirmed. As shown in Fig.5(a), the increased drain-source current under positive gate voltage and the decreased drain-source current under negative gate voltage indicate the n-type conducting behavior of bare Bi₂Se₃, consistent with previous reports [27]. In contrast, the conducting behavior of Bi₂Se₃ was transformed from n-type to p-type after Sn doping, as seen in the decreased drain-source current under positive gate voltage [Fig.5(b)]. After blending with p-type PEDOT:PSS, the n-type conducting behavior of Bi₂Se₃ are maintained, as shown in Fig.5(c), possibly because the dominant Se vacancies in Bi₂Se₃ could neutralize the holes ionized from PEDOT:PSS. The p-type semiconducting behavior of Bi_2−xSn_xSe₃/PEDOT:PSS was forceable because of the p-type characteristics of both components, as confirmed by the I−V curves under different gate voltages in Fig.5(d) and Fig. S6. The electrical conductivity σ of a semiconductor depends on both electrons and holes, expressed as

(1)\sigma =\text{1}/\phantom{\text{1}\rho }\rho =en{\mu }_{\text{e}}+ep{\mu }_h,

where ρ is the electrical resistivity, n and p are the electron and hole concentration, e is the electrical charge, μ_e and μ_h are the electron and hole mobility, respectively. According to mass action law (np = n_i²), the conductivity of n-type semiconductor is dominated by electrons [28], formulated as

(2)\sigma =\text{1}/\phantom{\text{1}\rho }\rho = e{N}_d{\mu }_e+e\left(n_i^2/\phantom{n_i^2{N}_d}{N}_d\right){\mu }_h\approx e{N}_d{\mu }_e.

Similarly, the conductivity of a p-type semiconductor depends mainly on the hole concentration and hole mobility. For the n-type Bi₂Se₃/PEDOT:PSS composite-based temperature sensors, the electron concentration increases with increasing temperature due to the dominated ionization of Se vacancies, lead to decreased resistance. However, a portion of the electron concentration ionized by Se vacancy under elevated temperature is partially neutralized by the ionized acceptors in p-type PEDOT:PSS. It is just because of compensate doping effect, the bare PEDOT:PSS-based temperature sensor exhibit the higher sensitivity than that of Bi₂Se₃/PEDOT:PSS composite (Fig. S7) [29]. After Sn doping, the hole concentration remarkably increased due to both the ionization of p-type Bi_2−xSn_xSe₃ and p-type PEDOT:PSS, leading to increased sensitivity of p-type Bi_2−xSn_xSe₃/PEDOT:PSS compared to n-type Bi₂Se₃/PEDOT:PSS composite. With further increasing Sn doping concentrations, the thermal scattering effect [30] would be enhanced due to the existing excess defect sites, leading to decreased hole mobility under elevated temperature. That is why the temperature sensitivity of Bi_2−xSn_xSe₃/PEDOT:PSS composites reaches a maximum when the doping concentration is 6%, and the composite with further increased Sn doping concentration exhibits decreased temperature sensitivity.

3.4 Application of Bi_1.97Sn_0.03Se₃/PEDOT:PSS composite flexible temperature sensor in monitoring human skin

To demonstrate the feasibility of our fabricated sensor for practical application, the Bi_1.97Sn_0.03Se₃/PEDOT: PSS composite with the maximum sensitivity was employed for real-time human skin monitoring (Fig.6, Fig. S8). It is noted that the skin temperature was controlled using hot water or ice packs. According to the definition of temperature sensitivity (S = ΔR/R₀), the temperature sensitivity of sensor after attaching to arm at room temperature was calculated as −9.92% based on the initial resistance of sensors at 20 °C. Based on the linear fitting equation (ΔR/R₀ = –0.63 T + 5.27) in Fig.4(c), the current arm temperature value is 24.1 °C, which is equal to the temperature value obtained using infrared thermal imager shown in the top right row of Fig.6. With the hot water pack near the skin-temperature sensors, the temperature sensitivity of sensor reached –15.59% and –19.97%, corresponding to 33.1 °C and 40.0 °C, respectively, which are also consistent with the temperature values obtained from the infrared thermal imager. The accurate temperature sensing ability of the fabricated sensors indicates its potential application in human body temperature sensing.

3.5 Fabrication and output voltage performance test of double-chain thermal device

Benefiting from the n−p transition characteristics, a flexible double-chain thermoelectric generator composed of n-type Bi₂Se₃ and p-type Bi_1.97Sn_0.03Se₃ was also fabricated to achieve the potential applications in thermal energy harvesting. The fabrication process consisted of two main parts, as illustrated in Fig.7(a). First, the n-type Bi₂Se₃-based ink was printed on the PET substrate using screen-printing technology. To prevent damage during the subsequent screen-printing procedure, the printed n-type thermoelectric legs were cured at 80 °C for 30 min. Secondly, the p-type Bi_1.97Sn_0.03Se₃ thermoelectric legs with patterns complementary to those of the previous screen were printed. It is noted that a partially overlapping area at the junction of two types of legs in the long axis direction was designed for a more efficient connection between the n-type Bi₂Se₃ and the p-type Bi_1.97Sn_0.03Se₃. Fig.7(b) illustrates the schematic diagram of the working mechanism in n-type Bi₂Se₃/p-type Bi_1.97Sn_0.03Se₃ thermoelectric generator based on the Seebeck effect. When a temperature difference (\mathrm{\Delta }T) is applied to the device, the electrons in the n-type Bi₂Se₃ or the holes in p-type Bi_1.97Sn_0.03Se₃ will diffuse from the hot side to the cold side, generating a potential in the circuit. The potential difference (\mathrm{\Delta }V) between the p–n junctions can be expressed as [31]

(3)\mathrm{\Delta }V=S_{ab}\mathrm{\Delta }T,

where S_ab is the differential Seebeck coefficient, which is determined by the properties of the thermoelectric material itself, and \mathrm{\Delta }T is the temperature difference at the junctions. As shown in Fig.7(c), the open-circuit output voltage of the fabricated thermoelectric generator was obtained. With increasing temperature differences, the output voltage increased accordingly. The output voltage of the double-chain thermoelectric generator reached 76.06 mV when the temperature difference was 70 K, indicating its potential application in thermal energy harvesting or self-power sensing.

4 Conclusion

In conclusion, Sn-doped Bi₂Se₃ with varying doping concentrations were successfully synthesized using the hot-injection method. The incorporation of extra crosslinkers, hardener, and highly conductive PEDOT:PSS enabled the stable temperature sensing properties in Bi_2−xSn_xSe₃-based thermosensitive materials. Our temperature sensing results demonstrated that the Bi_1.97Sn_0.03Se₃-based flexible sensors exhibited a maximum resistance response of 50.31% and a high sensitivity of −0.63 %/°C, with a temperature resolution of 0.2 °C. The enhanced temperature sensitivity of Sn-doped Bi₂Se₃ was attributed to the formation of more acceptors induced by p-doping effect of Sn incorporation, leading to the generation of additional free charge carriers under elevated temperature. As a proof of concept, the fabricated flexible temperature sensors exhibited high stable and precision in real-time body temperature monitoring. Additionally, the fabricated double-chain thermoelectric generator consisting of n-type Bi₂Se₃ and p-type Bi_1.97Sn_0.03Se₃ produced an output voltage of 76.06 mV under a 70 K temperature difference, indicating its potential application in thermal energy harvesting or self-powered temperature sensing.

References

[1] B. A. Kuzubasoglu, E. Sayar, C. Cochrane, V. Koncar, and S. K. Bahadir, Wearable temperature sensor for human body temperature detection, J. Mater. Sci. Mater. Electron. 32(4), 4784 (2021)

[2] M. Zhu, M. U. Ali, C. Zou, W. Xie, S. Li, and H. Meng, Tactile and temperature sensors based on organic transistors: Towards e-skin fabrication, Front. Phys. (Beijing) 16(1), 13302 (2021)

[3] J. Lee, N. Hong, W. Hong, D. Kim, Y. Hwang, J. Jang, and H. Kang, Semi‐transparent, micrometer resolution p-NiO/n-ZnO heterojunction diode temperature sensors with ultrathin metal anode, Adv. Mater. Technol. 7(6), 2100923 (2022)

[4] A. Nag, R. B. V. B. Simorangkir, D. R. Gawade, S. Nuthalapati, J. L. Buckley, B. O’Flynn, M. E. Altinsoy, and S. C. Mukhopadhyay, Graphene-based wearable temperature sensors: A review, Mater. Des. 13, 2339 (2022)

[5] J. Shin, B. Jeong, J. Kim, V. B. Nam, Y. Yoon, J. Jung, S. Hong, H. Lee, H. Eom, J. Yeo, J. Choi, D. Lee, and S. H. Ko, Sensitive Wearable Temperature Sensor with Seamless Monolithic Integration, Adv. Mater. 32(2), 1905527 (2020)

[6] S. S. Courts, A standardized diode cryogenic temperature sensor for aerospace applications, Cryogenics 74, 172 (2016)

[7] Z. Liu, B. Tian, Z. Jiang, S. Li, J. Lei, Z. Zhang, J. Liu, P. Shi, and Q. Lin, Flexible temperature sensor with high sensitivity ranging from liquid nitrogen temperature to 1200 °C, Int. J. Extreme Manuf. 5(1), 5601 (2022)

[8] J. J. Wang, T. Wang, C. G. Wu, W. B. Luo, Y. Shuai, and W. L. Zhang, Highly precise Ti/Pt/Cr/Au thin-film temperature sensor embedded in a microfluidic device, Rare Met. 40(1), 195 (2021)

[9] R. Liu, L. He, M. Cao, Z. Sun, R. Zhu, and Y. Li, Flexible temperature sensors, Front Chem. 9, 539678 (2021)

[10] S. D. Yang, L. Yang, Y. X. Zheng, W. J. Zhou, M. Y. Gao, S. Y. Wang, R. J. Zhang, and L. Y. Chen, Structure-dependent optical properties of self-organized Bi₂Se₃ nanostructures: From nanocrystals to nanoflakes, ACS Appl. Mater. Interfaces 9(34), 29295 (2017)

[11] L. Liu, M. Zhang, Q. Liu, and The raman spectra of Co-, Fe-, and Mn-doped Bi₂Se₃ single crystals, Crystals (Basel) 13(3), 456 (2023)

[12] K. Mazumder, A. Sharma, Y. Kumar, P. Bankar, M. A. More, R. Devan, and P. M. Shirage, Enhancement of field electron emission in topological insulator Bi₂Se₃ by Ni doping, Phys. Chem. Chem. Phys. 20(27), 18429 (2018)

[13] J. Dai, D. West, X. Wang, Y. Wang, D. Kwok, S. W. Cheong, S. B. Zhang, and W. Wu, Toward the intrinsic limit of the topological insulator Bi₂Se₃, Phys. Rev. Lett. 117(10), 106401 (2016)

[14] X. Liu, D. Wang, H. Wu, J. Wang, Y. Zhang, G. Wang, S. J. Pennycook, and L. D. Zhao, Intrinsically low thermal conductivity in BiSbSe₃: A promising thermoelectric material with multiple conduction bands, Adv. Funct. Mater. 29(3), 1806558 (2019)

[15] K. Mazumder,P. M. Shirage, A brief review of Bi₂Se₃ based topological insulator: From fundamentals to applications, J. Alloys Compd. 888, 161492 (2021)

[16] G. Sun, X. Qin, D. Li, J. Zhang, B. Ren, T. Zou, H. Xin, S. B. Paschen, and X. Yan, Enhanced thermoelectric performance of n-type Bi₂Se₃ doped with Cu, J. Alloys Compd. 639, 9 (2015)

[17] S. Gautam, A. K. Verma, A. Balapure, B. Singh, R. Ganesan, M. S. Kumar, V. N. Singh, B. Gahtori, and S. S. Kushvaha, Structural, electronic and thermoelectric properties of Bi₂Se₃ thin films deposited by RF magnetron sputtering, J. Electron. Mater. 51(5), 2500 (2022)

[18] P. Singha, S. Das, N. Rana, S. Mukherjee, S. Chatterjee, S. Bandyopadhyay, and A. Banerjee, Investigation of thermoelectric and magnetotransport properties of single crystalline Bi₂Se₃ topological insulator, J. Appl. Phys. 135(2), 025001 (2024)

[19] Y. Xiong, G. Zhou, N. C. Lai, X. Wang, Y. C. Lu, O. V. Prezhdo, and D. Xu, Chemically switchable n-type and p-type conduction in bismuth selenide nanoribbons for thermoelectric energy harvesting, ACS Nano 15(2), 2791 (2021)

[20] M. A. Tumelero, M. B. Martins, P. B. Souza, R. D. Della Pace, and A. A. Pasa, Effect of electrolyte on the growth of thermoelectric Bi₂Se₃ thin films, Electrochim. Acta 300, 357 (2019)

[21] M. Hong, Z. G. Chen, L. Yang, G. Han, and J. Zou, Enhanced thermoelectric performance of ultrathin Bi₂Se₃ nanosheets through thickness control, Adv. Electron. Mater. 1(6), 1500025 (2015)

[22] J. Bao, S. Zeng, J. Dai, X. Wang, Q. Liu, H. Li, X. Huang, and W. Huang, Heterostructures between a tin-based intermetallic compound and a layered semiconductor for gas sensing, Chem. Commun. (Camb.) 57(45), 5590 (2021)

[23] R. Ren, P. Qin, J. Song, Y. Qin, W. Li, Y. Wang, X. Yang, J. Li, W. Löser, and C. Cao, Crystal growth and thermoelectric properties of Sn-doped Bi₂Se₃, J. Cryst. Growth 627, 127510 (2024)

[24] C. C. Wang, Y. S. Chang, P. T. Lin, F. S. Shieu, and H. C. Shih, Fabrication, characterization and optical properties of Au-decorated Bi₂Se₃ nanoplatelets, Sci. Rep. 12(1), 17761 (2022)

[25] A. Zhuang, J. J. Li, Y. C. Wang, X. Wen, Y. Lin, B. Xiang, X. Wang, and J. Zeng, Screw‐dislocation‐driven bidirectional spiral growth of Bi₂Se₃ nanoplates, Angew. Chem. Int. Ed. 53(25), 6425 (2014)

[26] H. Liu, K. Sun, X. L. Guo, Z. L. Liu, Y. H. Wang, Y. Yang, D. Yu, Y. T. Li, and T. L. Ren, An ultrahigh linear sensitive temperature sensor based on PANI: graphene and PDMS hybrid with negative temperature compensation, ACS Nano 16(12), 21527 (2022)

[27] Y. Min, J. W. Roh, H. Yang, M. Park, S. I. Kim, S. Hwang, S. M. Lee, K. H. Lee, and U. Jeong, Surfactant‐free scalable synthesis of Bi₂Te₃ and Bi₂Se₃ nanoflakes and enhanced thermoelectric properties of their nanocomposites, Adv. Mater. 25(10), 1425 (2013)

[28] L. Tian, G. Sun, D. Jing, C. Pan, Z. Ran, W. Shi, and C. Zhang, Temperature characteristic of carrier scattering and dark resistivity of semi-insulating GaAs, J. Appl. Phys. 130, 195107 (2021)

[29] L. C. Hao, Z. A. Chen, D. Y. Liu, W. K. Zhao, M. Zhang, K. Tang, S. M. Zhu, J. D. Ye, R. Zhang, and Y. D. Zheng, Suppression and compensation effect of oxygen on the behavior of heavily boron-doped diamond films, Chin. Phys. B 32, 038101 (2023)

[30] C. Yan, J. Wang, and P. S. Lee, Stretchable graphene thermistor with tunable thermal index, ACS Nano 9(2), 2130 (2015)

[31] D. L. Wen, H. T. Deng, X. Liu, G. K. Li, X. R. Zhang, and X. S. Zhang, Wearable multi-sensing double-chain thermoelectric generator, Microsyst. Nanoeng. 6(1), 68 (2020)

RIGHTS & PERMISSIONS

Higher Education Press