N-type SnSe crystal with second phase SnSe2 introduced and preparation method thereof

Abstract

The application discloses an N-type SnSe crystal with a second phase SnSe2 and a preparation method thereof. The SnSe crystal growth method adopts a temperature gradient cycle chemical vapor transport method. Specifically, I2 is used as a transport agent, Sn powder and Se powder are mixed with the transport agent, and then the mixture is vacuum sealed in a quartz tube and placed in a two-temperature-zone tube furnace. Two different temperature gradient cycles are set in the growth temperature program, and natural cooling is performed to room temperature to obtain the N-type SnSe crystal with the second phase SnSe2. The size of the SnSe crystal grown by the method can reach millimeter level, the conductive carrier is electron, the carrier concentration can reach 10 18 cm ‑3 order at room temperature, and the mobility is 37 cm 2 ·V ‑1 ·S ‑1 order, which has important research and application value in the thermoelectric field.

Description

Technical Field

[0001] This invention belongs to the field of energy materials and crystal growth technology, and specifically relates to an N-type SnSe crystal with a second phase SnSe2 introduced therein and its preparation method. Background Technology

[0002] Thermoelectric materials are materials that can directly convert heat energy into electrical energy and vice versa. The energy conversion efficiency is generally characterized by the dimensionless thermoelectric figure of merit (ZT value), expressed as: Where S represents the Seebeck coefficient, σ represents electrical conductivity, κ represents thermal conductivity, and T represents absolute temperature. In recent years, SnSe has attracted widespread attention due to its superior thermoelectric properties. This is not only because SnSe's two-dimensional layered structure has very low thermal conductivity, but also because the compound is non-toxic and environmentally friendly. Furthermore, SnSe can achieve both P-type and N-type conductivity, which is crucial for constructing thermoelectric modules for practical applications. Therefore, there have been numerous research reports on SnSe in recent years.

[0003] SnSe crystal is a two-dimensional layered material. At room temperature, its crystal structure belongs to the orthorhombic crystal system with space group Pnma. A structural phase transition occurs at approximately 475–525 °C, transforming the crystal structure from orthorhombic to tetragonal and the space group from Pnma to Cmcm. SnSe crystals contain Sn vacancies, thus exhibiting intrinsic P-type conductivity. N-type SnSe crystals require experimental preparation. However, due to the presence of anti-site defects in Sn... Se The presence of [a specific element] makes the preparation of N-type SnSe crystals with excellent thermoelectric properties a persistent challenge.

[0004] Currently, the preparation of N-type SnSe crystals typically involves elemental doping, where high-valence elements such as Bi, Sb, Pr, and La partially replace Sn. 2+ Site, or by partially replacing Se with a low-valence element such as a halogen. 2- Site. For example, Sn using Bi element. 2+ N-type SnSe single crystals obtained by site doping can achieve a ZT value of 2.2 along the b-axis at 773 K (AT Duong, VQ Nguyen, G. Duvjir, VT Duong, S. Kwon, JY Song, JK Lee, JELee, S. Park, T. Min, J. Lee, J. Kim, and S. Cho, Nat. Commun. 7, 13713 (2016)). Se was obtained by Br element doping. 2-Through site doping, Zhao Lidong et al. achieved a ZT value as high as 2.8 in the out-of-plane direction of N-type SnSe crystals at 773 K, and found that its excellent thermoelectric performance mainly comes from the ultra-low out-of-plane thermal conductivity (C. Chang, MHWu, DSHe, YLPei, CFWu, XFWu, HLYu, FYZhu, KDWang, Y.Chen, L.Huang, JFLi, JQHe, and LDZhao, Science 360, 778 (2018)). Furthermore, by using Ce and Br co-doping, the carrier concentration of N-type SnSe single crystals can be increased from 10... 16 cm -3 Upgraded to 10 19 cm -3 The power factor at room temperature can reach 1.57 μW·cm. -1 ·K -2 (Xiao-Li Zhou, Yang-Yang Lv, Hang-Fei Zhang, Yong Zhang, Jinglei Zhang, Jian Zhou, Shu-Hua Yao, YBChen and Yan-Feng Chen, Appl. Phys. Lett. 120, 022102 (2022)).

[0005] However, preparing N-type SnSe crystals by element doping has many drawbacks, such as difficulty in controlling the doping level, and the introduction of other elements besides Sn and Se during the doping process, which increases defects in the crystal and affects scattering in the crystal, resulting in many problems. Summary of the Invention

[0006] Objective of the Invention: The present invention aims to provide an N-type SnSe crystal with the introduction of a second phase SnSe2. This crystal does not introduce other elements and does not generate additional crystal defects, thus ensuring the quality and mobility of the crystal. Another objective of the present invention is to provide a method for preparing the aforementioned N-type SnSe crystal. This method uses a temperature gradient cyclic chemical vapor transport method to introduce the second phase SnSe2 into the SnSe crystal, thereby changing the overall conductivity type of the crystal from P-type to N-type. This method is simple, easy to implement, low in cost, and makes it easier to prepare N-type SnSe crystals.

[0007] Technical solution: The N-type SnSe crystal of the present invention has a size on the millimeter scale, and a second phase SnSe2 is introduced into the crystal during the growth process.

[0008] The method for preparing the N-type SnSe crystal employs a temperature gradient cyclic chemical vapor transport method, and the specific steps are as follows:

[0009] (1) Weigh Sn powder, Se powder and transport agent I2, mix them evenly and put them into a quartz tube, and seal them under vacuum.

[0010] (2) Place the sealed quartz tube in a two-temperature zone tube furnace, set two different temperature gradients in the growth temperature program, and circulate the temperature multiple times. Then, let it cool naturally to room temperature. After growth, crystals can be obtained.

[0011] Furthermore, in step (1), the quartz tube used to grow SnSe crystals is 100-200 mm long and 15-20 mm in diameter; the quartz tube is sealed by a gas flame, an acetylene flame, or a hydrogen flame.

[0012] Furthermore, in step (2), the two different temperature gradients set in the growth temperature program are: the first growth temperature gradient is: 470℃~480℃ at the growth end and 570℃~580℃ at the raw material end, and the temperature is maintained for 6-24 hours; the second growth temperature gradient is: 570℃~580℃ at the growth end and 670℃~680℃ at the raw material end, and the temperature is maintained for 6-24 hours.

[0013] Furthermore, the growth cycle described in step (2) is 5-10 days.

[0014] The crystalline material described in this invention has electrons as its conductive charge carriers, and the charge carrier concentration can reach 10⁻⁶ at room temperature. 18 cm -3 The order of magnitude, with a mobility of 37 cm⁻¹ 2 ·V -1 ·S -1 It has a high electrical conductivity and is of great research and application value in the field of thermoelectricity.

[0015] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages: (1) The N-type SnSe crystal does not introduce other elements and does not generate additional crystal defects, thus ensuring the quality and mobility of the crystal. The carrier concentration at room temperature can reach 10. 18 cm -3 The order of magnitude, with a mobility of 37 cm⁻¹ 2 ·V -1 ·S -1 (2) The method for preparing the N-type SnSe crystal uses a temperature gradient cyclic chemical vapor transport method. The N-type SnSe crystal can be prepared by controlling the temperature gradient of crystal growth. It is simple, easy and inexpensive. Attached Figure Description

[0016] Figure 1These are (a) an optical photograph of the N-type SnSe crystal grown in Example 1 of this invention at room temperature; (b) a Raman spectrum; and (c) a resistivity ρ. xy Curve showing how the magnetic field B changes;

[0017] Figure 2 These are (a) an optical photograph of the N-type SnSe crystal grown in Example 2 of this invention at room temperature; (b) a Raman spectrum; and (c) a resistivity ρ. xy Curve showing how the magnetic field B changes;

[0018] Figure 3 These are (a) an optical photograph of the N-type SnSe crystal grown in Example 3 of this invention at room temperature; (b) a Raman spectrum; and (c) a resistivity ρ. xy The curve showing how the magnetic field B changes. Detailed Implementation

[0019] The technical solution of the present invention will be further described below with reference to the embodiments and accompanying drawings.

[0020] High-purity Sn powder and Se powder were used as raw materials in the experiment. The transport agent I2 was also a high-purity reagent. The quartz tube for growing crystals was made of high-purity quartz, with a length of 100-200 mm and a diameter of 15-20 mm.

[0021] Example 1

[0022] When I2 is used as the transport agent, the two cyclic temperature gradients during crystal growth are: 480℃ (growth end) ~ 580℃ (raw material end), held for 24 hours; 580℃ (growth end) ~ 680℃ (raw material end), held for 24 hours. The preparation method is as follows:

[0023] Weigh 0.01 mol of Sn powder (1.1870 g) and 0.0122 mol of Se powder (0.9651 g), and weigh 50–100 mg of transport agent I2. Mix them thoroughly and load them into a pre-cleaned quartz tube. Seal the tube under vacuum using a mechanical pump and a molecular pump. Place the sealed quartz tube in a two-zone tube furnace. Cycle with two different temperature gradients in the growth temperature program. After approximately 8 days of growth, allow it to cool naturally to room temperature to obtain millimeter-sized crystals, with a maximum size of about 6 mm. Figure 1 As shown in (a).

[0024] like Figure 1 As shown in (b), Raman spectroscopy analysis revealed that the Raman spectrum of the prepared crystal contained both SnSe and other Raman spectroscopy elements. B 3g The pattern also includes SnSe2's A. 1gThe pattern indicates that the crystal prepared above contains both SnSe phase and SnSe2 phase.

[0025] The charge carrier information of the crystal was tested using a four-probe method on a Physical Properties Measurement System (PPMS). For example... Figure 1 As shown in (c), Hall effect testing of the grown crystal indicates that the crystal's conductivity type is electronic.

[0026] The N-type SnSe crystal has a carrier concentration of 1.93 × 10⁻⁶ at room temperature. 18 cm -3 The migration rate was 34.95 cm. 2 ·V -1 ·S -1 .

[0027] Example 2

[0028] When I2 is used as the transport agent, the two cyclic temperature gradients during crystal growth are: 470℃ (growth end) ~ 570℃ (raw material end), held for 24 hours; 570℃ (growth end) ~ 670℃ (raw material end), held for 24 hours. The preparation method is as follows:

[0029] Weigh 0.01 mol of Sn powder (1.1870 g) and 0.0122 mol of Se powder (0.9651 g), and weigh 50–100 mg of transport agent I2. Mix them thoroughly and load them into a pre-cleaned quartz tube. Seal the tube under vacuum using a mechanical pump and a molecular pump. Place the sealed quartz tube in a two-zone tube furnace. Cycle with two different temperature gradients in the growth temperature program. After approximately 8 days of growth, allow it to cool naturally to room temperature to obtain millimeter-sized crystals, with a maximum size of about 4 mm. Figure 2 As shown in (a).

[0030] like Figure 2 As shown in (b), Raman spectroscopy analysis revealed that the Raman spectrum of the prepared crystal contained both SnSe and other Raman spectroscopy elements. B 3g The pattern also includes SnSe2's A. 1g The pattern indicates that the crystal prepared above contains both SnSe phase and SnSe2 phase.

[0031] The carrier information of the crystal was measured on a PPMS using a four-probe method. For example... Figure 2 As shown in (c), Hall effect testing of the grown crystal indicates that the crystal's conductivity type is electronic.

[0032] The N-type SnSe crystal has a carrier concentration of 2.42 × 10⁻⁶ at room temperature.18 cm -3 The migration rate was 31.87 cm. 2 ·V -1 ·S -1 .

[0033] Example 3

[0034] When I2 is used as the transport agent, the two cyclic temperature gradients during crystal growth are: 480℃ (growth end) ~ 580℃ (raw material end), held for 24 hours; 580℃ (growth end) ~ 680℃ (raw material end), held for 24 hours. The preparation method is as follows:

[0035] Weigh 0.01 mol of Sn powder (1.1870 g) and 0.015 mol of Se powder (1.1844 g), and weigh 50–100 mg of transport agent I2. Mix them thoroughly and load them into a pre-cleaned quartz tube. Seal the tube under vacuum using a mechanical pump and a molecular pump. Place the sealed quartz tube in a two-zone tube furnace. Cycle with two different temperature gradients in the growth temperature program. After approximately 8 days of growth, allow it to cool naturally to room temperature to obtain millimeter-sized crystals, with a maximum size of about 5 mm. Figure 3 As shown in (a).

[0036] like Figure 3 As shown in (b), Raman spectroscopy analysis revealed that the Raman spectrum of the prepared crystal contained both SnSe and other Raman spectroscopy elements. B 3g The pattern also includes SnSe2's A. 1g The pattern indicates that the crystal prepared above contains both SnSe phase and SnSe2 phase.

[0037] The carrier information of the crystal was measured on a PPMS using a four-probe method. For example... Figure 3 As shown in (c), Hall effect testing of the grown crystal indicates that the crystal's conductivity type is electronic.

[0038] The N-type SnSe crystal has a carrier concentration of 7.43 × 10⁻⁶ at room temperature. 18 cm -3 The migration rate was 37.07 cm. 2 ·V -1 ·S -1 .

Claims

1. A method for preparing an N-type SnSe crystal with a second phase SnSe2, characterized in that: This method employs a temperature gradient cycle chemical vapor transport method, and the steps are as follows: (1) Weigh Sn powder, Se powder and transport agent I2, mix them evenly and put them into a quartz tube, and seal them under vacuum; (2) Place the sealed quartz tube in a two-temperature zone tube furnace, set two different temperature gradients in the growth temperature program and cycle the heat multiple times, then let it cool naturally to room temperature, and the crystal can be obtained after growth. The two different temperature gradients set in the growth temperature program in step (2) are as follows: the first growth temperature gradient: 470 ℃~480 ℃ at the growth end and 570 ℃~580 ℃ at the raw material end, and kept at this temperature for 6-24 hours; the second growth temperature gradient: 570 ℃~580 ℃ at the growth end and 670 ℃~680 ℃ at the raw material end, and kept at this temperature for 6-24 hours. The conductive carriers of the crystal are electrons, and the charge carrier concentration can reach 10 at room temperature. 18 cm -3 The order of magnitude, with a mobility of 37 cm⁻¹ 2 ·V -1 ·S -1 It has a high electrical conductivity and is of a certain magnitude.

2. The method for preparing an N-type SnSe crystal with a second phase SnSe2 introduced according to claim 1, characterized in that: The quartz tube used to grow SnSe crystal in step (1) is 100~200 mm long and 15~20 mm in diameter.

3. The method for preparing an N-type SnSe crystal with a second phase SnSe2 introduced according to claim 1, characterized in that: The quartz tube sealing method described in step (1) is a gas flame, an acetylene flame, or a hydrogen flame.

4. The method for preparing an N-type SnSe crystal with a second phase SnSe2 introduced according to claim 1, characterized in that: The growth cycle of the crystal in step (2) is 5-10 days.

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