Introducing PbSe quantum dots and manipulating lattice strain contributing to high thermoelectric performance in polycrystalline SnSe
Graphical abstract
Introduction
The thermoelectric energy conversion is a promising energy conversion technology which can convert heat to electricity directly through Seebeck effect [[1], [2], [3]]. The energy conversion efficiency of thermoelectric materials depends on the figure of merit (ZT), which is defined as [4]:where S is the Seebeck coefficient, σ is the electrical conductivity, T is the absolute temperature, κT is the total thermal conductivity. In addition, κT is composed of the electronic thermal conductivity (κe) and the lattice thermal conductivity (κL) [5], and S2σ is also known as power factor (PF). A high ZT requires both a high PF and a low κT. In the past few years, several approaches have been proposed to improve thermoelectrical properties, such as band engineering approaches [[6], [7], [8], [9]], energy filtering effect [10,11], entropy engineering [[12], [13], [14]], defects design [[15], [16], [17], [18]] and nanoengineering [[19], [20], [21]].
Tin Selenide (SnSe), with its low toxicity, high cost-effectiveness and outstanding thermoelectric performance, has become one of the most promising thermoelectric materials [22,23]. SnSe is a layered semiconductor with an orthorhombic structure, which is in Pnma space group at room temperature and translates to Cmcm space group above 800 K. High ZT values for both p-type and n-type single crystal SnSe have been reported [24,25]. Nevertheless, SnSe single crystals are not suitable for large-scale thermoelectric applications due to its expensive and complex crystal growth process and poor mechanical properties [25,26]. Hence, polycrystalline SnSe with facile synthesis processing and machinability has become a promising alternative candidate. The nanoporous design was introduced in polycrystalline SnSe to serve as phonon scattering centers, contributing to a ZT of 1.7 at 823 K [27]. The Ag0.004Na0.016Sn0.98Se0.99Te0.01 sample realized ZT of 1.47 at 790 K and an average ZT of 0.67 between 300 and 790 K owing to the increased carrier concentration and the band effective mass [28]. By increasing the carrier concentration and introducing the valence band convergence, Sn0.99Na0.01Se-Ag8SnSe6 realized ZT of 1.33 at 773 K [29]. Although promising strategies have been proposed for polycrystalline SnSe, it is still challenging to simultaneously achieve a high peak ZT and a high average ZT within a wide temperature range.
The anharmonic bonding between Sn and Se atoms is helpful to obtain a low κL for SnSe [30], but the existence of Sn oxides worse the κL [31]. To further reduce the κL, static lattice strains are expected to be introduced. In one dimension single-atom harmonic model, the phonon dispersion is given by [32]:where F, M, k, and kc are the force constant, atomic mass, wave vector, and cut-off wave vector. Either change of M or F can affect the phonon transport process. To achieve a low κL, a large M and/or a small F are expected. A large fluctuation in M can be achieved by point defects, while the force constant F is determined by the “dynamic’’ lattice strain and the “static” lattice strain. For a given material, the “dynamic’’ lattice strain is caused by atomic thermal vibrations at nonzero Kelvin temperatures and determined by the temperature, which is equivalent to the effect of Umklapp-process scattering [33]. “Static” lattice strains can be created by defects and phase transitions [34,35]. Lattice strains can contribute to the broadening of the phonon dispersion (ω) and shorter the phonon relaxation time, leading to reduced κL.
Magnetic field is a noncontact physical tool used in biopharmaceuticals [36], chemical synthesis [37], materials engineering [38], etc. The influence of high magnetic field on the nucleation, crystallization, microstructure and crystal orientation has been reported [39,40]. Our previous study also proved that quantum dots can be obtained under a high magnetic field [19]. The 0D quantum dots show a dramatical change in the density of states compared with 3D crystalline solids [41]. It was found that quantum dots can increase Seebeck coefficients all in nanowire [42], monolayer [43] and bulk thermoelectric materials [44]. The high magnetic field can retard recrystallization and hinder the disappearance of dislocations, which may lead to high-density dislocations [45,46]. These encourage us to apply a high magnetic field during hydrothermal synthesis process.
Here, we fabricated the Pb and Cd dual doped polycrystalline SnSe by using an in situ high magnetic field-assisted hydrothermal route. The static lattice strain induced by dislocations and stacking faults shortens phonon relaxation time, resulting in ultralow lattice thermal conductivity. The PbSe quantum dots (PbSe QD) give rise to the increased density of states near the Fermi level due to the quantum confinement effect, contributes to the significantly enhanced Seebeck coefficients and power factor. As a result, besides a high ZT of 1.9, a high average ZT of 0.71 between 300 and 873 K is achieved by introducing PbSe quantum dots functional units and manipulating the lattice strain.
Section snippets
Sample fabrication
Reagents materials are SnCl2⋅2H2O, (99.99%, Aladdin), PbCl2 (99.9%, Aladdin), CdCl2 (99.95%, Aladdin), Se powder (99.99%, Aladdin) and NaOH (AR, Aladdin). PbSe QD/Sn0.965Pb0.01Cd0.025Se sample was synthesized using an ordinary hydrothermal method and an in-situ high magnetic field-assisted hydrothermal method under 5 T magnetic field intensity. SnCl2⋅2H2O (1.0808 g), PbCl2 (0.0068 g) and CdCl2 (0.0112 g) were weighted into a Teflon-lined stainless-steel autoclave (25 mL) with 20 mL deionized
Results and discussion
Fig. 1 shows XRD patterns of Cd and Pb codoped samples prepared without magnetic field and under 5 T. Since a high performance ZT = 1.7 can be achieved in polycrystalline SnSe doped with 1% Pb, the Pb doping level is fixed at 1% [48]. The identified peaks indicate the SnSe phase of Pnma structure (PDF#48–1224) and no impurity peak can be observed. Fig. 1b shows that (111) and (400) diffraction peaks of Cd and Pb dual doped SnSe shift toward a higher angle, indicating the lattice contraction
Conclusion
In summary, by applying a high magnetic field during hydrothermal synthesis, polycrystalline SnSe embedded with PbSe quantum dots was fabricated. The PbSe quantum dots were induced by the reduced critical nucleation free energy and increased nucleation rate under a high magnetic field. PbSe quantum dots functional units give rise to marked enhanced density of states near the Fermi level, contributing to a significant enhancement in the Seebeck coefficients and PF. The maximum PF for PbSe QD/Sn
Data availability
The datasets generated and/or analyzed during the current study are available from the corresponding author on reasonable request.
Credit author statement
S. Li prepared the samples, analyzed the data, and wrote the paper. X.N. Lou, B. Zou, Y.X. Hou, J. Zhang and D. Li measured the properties. J. Fang assisted with the High Magnetic Field Facilities experiments. T. Feng and Y.S. Liu assisted with the UPS measurement. D.W. Zhang assisted with the SPS experiment. J.Z. Liu carried out HAADF-STEM experiments and performed the microstructure analysis. G.D. Tang initialed the idea, conceived and designed the experiments, analyzed the data, and revised
Declaration of competing interest
There is no conflict to declare.
Acknowledgements
The work was supported by the National Natural Science Foundation of China (Nos. 52071182, U1732153 and 11802276), “Qinglan Project” of the Young and Middle-aged Academic Leader of Jiangsu Province, the Fundamental Research Funds for the Central Universities (No. 30921011107). This work was partially performed on the Steady High Magnetic Field Facilities (SM1 superconducting magnet), High Magnetic Field Laboratory, CAS.
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