Method of improving thermoelectric performance of SnSe thermoelectric material

Abstract

Provided is a method of preparing an SnSe thermoelectric material including (a) heating a mixture including Sn.sup.2+ and Se.sup.2−, (b) cooling the mixture at a cooling rate greater than 0 and equal to or less than 3 K/h, and forming single crystal Sn.sub.1-xSe (where 0<x<1), and an SnSe thermoelectric material prepared thereby and including Sn vacancies.

Claims

1. A method of preparing an SnSe thermoelectric material, the method comprising: (a) heating a mixture comprising Sn.sup.2+ and Se.sup.2−; (b) cooling the mixture at a cooling rate greater than 0 and equal to or less than 3 K/h; and (c) forming single crystal Sn.sub.1-xSe (where 0<x<1).

2. The method of claim 1, wherein the single crystal Sn.sub.1-xSe has a power factor (PF) value of 3 to 6 μW/cm.Math.K.sup.2.

3. The method of claim 1, wherein the single crystal Sn.sub.1-xSe has an electrical conductivity of 2 to 20 S.Math.m.sup.−1.

4. The method of claim 1, wherein holes of Sn.sub.1-xSe are doped as the x value increases in step (c).

5. The method of claim 1, wherein a valence band maximum energy (E.sub.VBM) increases as the cooling rate decreases in step (b).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) These and/or other aspects will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings in which:

(2) FIG. 1 is a schematic diagram illustrating a crystal structure of tin (II) selenide (SnSe) according to an embodiment of the present disclosure;

(3) FIG. 2 is a schematic diagram illustrating a unit cell structure of SnSe according to an embodiment of the present disclosure;

(4) FIG. 3 is a graph illustrating a low energy electronic band structure of SnSe according to an embodiment of the present disclosure;

(5) FIG. 4A and FIG. 4B are, respectively, a graph illustrating an electronic band structure of SnSe having Sn vacancies according to an embodiment of the present disclosure calculated using a KKR-CPA method;

(6) FIG. 5 is a graph illustrating calculated doping dependance of valence band maximum (VBM) energy of SnSe according to an embodiment of the present disclosure; and

(7) FIG. 6 is a graph illustrating ΔE-E.sub.VBM and PF values according to a cooling rate of SnSe according to an embodiment.

DETAILED DESCRIPTION

(8) In the following detailed description, reference is made to the accompanying drawings that show, by way of illustration, specific embodiments in which the invention may be practiced. The invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete and will fully convey the concept of the invention to those skilled in the art. Throughout the specification, like reference numerals denote like elements. Furthermore, various components and regions are schematically illustrated in the drawings. Therefore, the technical conception of the present invention is not limited by relative sizes and intervals shown in the accompanying drawings.

(9) Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings for one of ordinary skill in the art to be able to perform the disclosure without difficulty.

(10) FIG. 1 is a schematic diagram illustrating a crystal structure of tin(II) selenide (SnSe) according to an embodiment of the present disclosure. Referring to a of FIG. 1, SnSe is a metal chalcogenide including Se.sup.2− and Sn.sup.2+ aligned in a layered structure and has a layered orthorhombic crystal structure at room temperature. B and c of FIG. 1 are a plan view and a side view of SnSe, respectively. SnSe belongs to a space group Pnma satisfying lattice constants a=4.15 Å, b=4.44 Å, and c=11.57 Å. Two layers of SnSe are stacked by week van der Waals interactions along the c-direction.

(11) According to an embodiment of the present disclosure, thermoelectric performance of SnSe depends on the low energy electronic band structure. Among thermoelectric properties that determine thermoelectric performance, dimensionless figure of merit (ZT) is an important indicator to determine a thermoelectric conversion energy efficiency and may be expressed by the following equation.
ZT=(S).sup.2σT/(k.sub.e+k.sub.l)

(12) Here, S, σ, k.sub.e, and k.sub.l indicate Seebeck coefficient, electrical conductivity, electron thermal conductivity, and lattice thermal conductivity, respectively. ZT determines efficiency of a thermoelectric material. In this regard, a power factor often used to measure thermoelectric properties is defined as follows.
PF=(S).sup.2σ

(13) The power factor is a value representing an output of unit length per unit area of a thermoelectric material, and a high ZT value may be obtained from a high power factor. That is, a material having a high Seebeck coefficient, a high electrical conductivity, and a low thermal conductivity may have excellent thermoelectric properties. By manufacturing such a thermoelectric material, cooling efficiency and power generation efficiency may be increased.

(14) In an embodiment of the present disclosure, synthesis of SnSe may be performed by reaction between tin and selenium at a temperature over 350° C. For example, after dissolving tin powder and selenide powder in a solvent, the solution is maintained at a temperature slightly higher than a saturation temperature, and then the temperature is slowly lowered to create SnSe crystals. This synthesis process may be performed in a crucible formed of platinum or alumina.

(15) According to measurement using angle-resolved photoemission spectroscopy (ARPES) and calculation using density functional theory (DFT), a multi-valley valence band maximum (VBM) binding energy of SnSe may be adjusted by Sn vacancies. In addition, the VBM value may vary according to a cooling rate while single crystals of SnSe are growing. ARPES intensity maps on SnSe crystals that have grown with different cooling rates were measured along a Y-Γ-Y direction. As a result, the entire electronic band structure moves to a higher binding energy as the overall cooling rate increases. This indicates that charge carrier density of SnSe is controlled by the cooling rate. According to an embodiment, the amount of generated nuclei may be increased by adjusting the cooling rate to 3 K/h or less during a process of growing single crystals of SnSe. This may lead to an increase in the concentration of a carrier, thereby contributing to an increase in the PF value.

(16) FIG. 4 is a graph illustrating an electronic band structure of SnSe having Sn vacancies according to an embodiment of the present disclosure calculated using a KKR-CPA method. In this regard, x represents Sn vacancies in Sample Sn.sub.1-x, and x values of (a) and (b) of FIG. 4 are 0.04 and 0.01, respectively. According to the electronic band structure calculated in the Γ-Y-Γ direction, it may be confirmed that the overall band structure moves to a lower binding energy, but the electronic band structure hardly changes as x increases. A non-dispersive purge state observed below the Fermi energy results from scattering of impurities.

(17) FIG. 5 is a graph illustrating doping dependance of valence band maximum (VBM) energy of SnSe according to an embodiment of the present disclosure calculated by KKR. A function of E.sub.VBM with respect to doping ratio x was observed. As x increases, ΔE.sub.VBM gradually increases leading to Sn vacancies to hole doping of SnSe. As the cooling rate increases to move E.sub.VBM, Sn vacancies are reduced. As the cooling rate decreases, hole doping of SnSe may increase.

(18) Hereinafter, the present disclosure will be described in more detail with reference to the following examples. However, the following examples are merely presented to exemplify the present disclosure, and the scope of the present disclosure is not limited thereto.

EXAMPLE

(19) Preparation of Single Crystal SnSe Sample

(20) After mixing tin powder (purity of 99.99%, product of Alfa Aesar) with selenium powder (purity of 99.98%, product of Alfa Aesar) in a molar ratio of 1:1, the mixture was heated while slowing increasing temperature from room temperature to 500° C. for 32 hours and then increasing the temperature to 950° C. for 45 hours. The resultant was immersed at 950° C. for 15 hours, cooled to 900° C. for 10 hours, and then cooled to 800° C. In this case, the cooling rate was adjusted to 0.5, 1, 2, 3, 4, and 5 K/h. Finally, the resultant was cooled over 100 hours at room temperature.

EXPERIMENTAL EXAMPLE

(21) FIG. 2 is a schematic diagram illustrating a unit cell structure of single crystals of SnSe prepared in the above-described example showing Brillouin zone of Pnma. Based on ARPES intensity maps obtained in a Y-Γ-Y direction shown in FIG. 2, 4 peaks and 3 valleys are observed in E.sub.VBM around site Y.

(22) FIG. 3 illustrates a low energy electronic band structure of SnSe prepared in the above-described example. As a result of calculating the electronic band structure of SnSe using a Perdew-Burke-Ernzerhof functional combined with modified Becke-Johnson potential (PBE+mBJ) method, distinguishable 4 peaks were observed around the site Y. This result is consistent with that of the observed ARPES intensity, indicating that the PBE+mBJ method accurately explains electronic interactions of SnSe.

(23) Thermoelectric performance of an SnSe thermoelectric material selected based on such characteristics as described above may be identified in FIG. 6. FIG. 6 shows dependence of ΔE-E.sub.VBM and PF extracted by ARPES data and the like on the cooling rate for growing crystals. In comparison with PF (red empty circle), cooling rate dependence of E.sub.VBM (circle filled with black) is shown. No significant change was visually observed in E.sub.VBM until the cooling rate reached 3 K/h. In the case where the cooling rate is further increased, E.sub.VBM decreased so that SnSe started to be gradually doped with electrons as Sn vacancies decreased. The ΔE.sub.VBM of the single crystal SnSe prepared in this example of the present disclosure showed a tendency consistent with cooling rate dependence of the power factor PF at 300 K. This indicates that a high power factor PF observed in hole-doped SnSe is closely related to Sn vacancies, i.e., charge carrier density (hole concentration) determined by the cooling rate while the single crystals of SnSe are growing.

(24) According to technical conception of the present disclosure, a high-efficiency thermoelectric material having improved thermoelectric performance may be prepared by increasing a carrier concentration by introducing Sn vacancies into single crystal SnSe.

(25) The above effects of the present disclosure are illustrative, and the scope of the present disclosure is not limited by these effects.

(26) According to the embodiments of the present disclosure as described above, flexible nanostructured film connected in three dimensions having various sizes may be formed on surfaces of stents formed of various materials by a bottom-up method using the ionic surfactant and the auxiliary spacer under chemically mild conditions.