Enhanced Out-of-Plane Electrical Transport in n-Type SnSe

Subscriber access provided by UNIV OF NEW ENGLAND ARMIDALE

Enhanced out-of-plane electrical transport in n-type SnSe thermoelectrics induced by resonant states and charge delocalization Hulei Yu, Abdul Rehman Shaikh, Fen Xiong, and Yue Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b18871 • Publication Date (Web): 09 Mar 2018 Downloaded from http://pubs.acs.org on March 9, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 15 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Enhanced out-of-plane electrical transport in n-type SnSe thermoelectrics induced by resonant states and charge delocalization Hulei Yu,†,‡ Abdul Rehman Shaikh,† Fen Xiong,†,‡ and Yue Chen∗,†,‡ †Department of Mechanical Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong SAR, China ‡HKU Zhejiang Institute of Research and Innovation, 1623 Dayuan Road, Lin An 311305, China E-mail: [email protected]

Abstract Doping effects of various elements in the boron, carbon and pnictogen groups on the electronic structure and electrical transport properties of SnSe were studied from first principles. It is identified that Sb and Bi induce significant resonant states near the conduction band minimum, and increase the delocalization of charge density in the out-of-plane direction. Our Boltzmann transport calculations further demonstrate that these doping effects on the electronic structure are related to the simultaneously improved Seebeck coefficient and electrical conductivity. Using the band unfolding technique, we analyze the resonant states in detail based on the effective band structures.

1

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 15

Keywords thermoelectrics, defect formation energy, electronic structure, charge density, Boltzmann transport theory Around two-thirds of the energy generated around the world is evidently lost in the form of heat. 1 This, coupled with the ever growing concern for climate change, has heightened an immense interest in thermoelectric materials for their ability in harvesting waste heat and converting it into electricity. Their potential to generate low-cost green energy without the use of moving parts, has led to thermoelectric materials being intensively studied over the past few decades. While thermoelectric materials retain their popularity in the field of refrigeration and power generation, 2 various energy-intensive industries such as the automobile industy are also showing a growing interest towards them. The performance of thermoelectric materials is determined by the dimensionless figure of merit:

ZT =

σS 2 T κ

(1)

where κ, σ, S and T are the total thermal conductivity, electrical conductivity, the Seebeck coefficient and absolute temperature, respectively. The total thermal conductivity κ is equal to the sum of the lattice thermal conductivity (κlat ) and the electronic thermal conductivity (κele ). As evident from the definition of thermoelectric figure of merit, a high power factor σS 2 and a low thermal conductivity κ favor a high efficiency. Over the past decades, various strategies have been employed to enhance the performance of thermoelectric materials, which involve band engineering, 3 phonon engineering, 4 nano-composite engineering 5 and chemical doping. 6 While many of the methods have improved the ZT value through effectively reducing the thermal conductivity κ, band engineering via the induction of resonant states 3 has been among the popular strategies that have led to an improvement in ZT through power-factor enhancement. Despite the potential effects of resonant states on enhancing the thermoelectric efficiency, it worth noting that strong localization with poor hybridization

2


Page 3 of 15 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


with the host states may be detrimental to the overall thermoelectric performance. 7 Carrier mobility may also be reduced due to additional scattering induced by resonant states. 7 Thus, an enhanced Seebeck coefficient with a moderate decrease of carrier mobility is usually expected for resonant doping. Of the large amount of thermoelectric materials, SnSe has been a long-known semiconductor with favorable thermoelectric properties. A recent break-through research measured a record-high ZT value of approximately 2.6 at 923 K, 8–10 mainly attributed by the ultralow thermal conductivity caused by its distinctive anharmonic structure. Although controversy exists in the literature regarding the intrinsic thermal conductivity of SnSe single crystal, 11,12 its composition, comprising of relatively less toxic earth-abundant materials compared to tellurium or lead, 13 has made SnSe appear favorable for use in various industrial applications. Although both n-type and p-type materials are vital to construct a thermoelectric module, 14,15 studies have mainly focused on p-type SnSe. 8 It is yet interesting to observe that first-principle calculations 16 have predicted n-type SnSe to exhibit a better thermoelectric performance than p-type SnSe. SnSe has a layered crystal structure with an orthorhombic lattice, which gives rise to strong anisotropic characteristics. While SnSe has been shown to exhibit superior thermoelectric performance at high temperatures, 8,9 the ZT value remains low at the lower temperature range. Because SnSe has a very low intrinsic defect concentration, 17 many studies have focused on chemical substitution as a favorable way to improve the poor electrical transport properties. 18 Dopants such as Na and Ag have effectively enhanced the ZT value of SnSe. 19 The promising thermoelectric properties of SnSe have very well led us to study the effects of the boron group (Ga, In and Tl), the carbon group (Ge and Pb), and the pnictogen group (As, Sb and Bi) elements on its electronic structure and electrical transport properties. Density functional theory (DFT) and Boltzmann transport calculations were carried out for doped SnSe in this study. First-principles DFT calculations were performed using the Vienna ab-initio simulation package (VASP). 20 All crystal structures were relaxed into their

3



(a) 4

(b) 4 3

2

Ef (eV)

Ef (eV)

As-doped

0 Sn

q= -1 q= 0

2

q=+1 q=+3

1

Se

-2

0 Ga In Tl GePb As Sb Bi

(c) 4

0 (d) 4

Sb-doped

0.2 0.4 ∆EF (eV)

0.6

Bi-doped 3 Ef (eV)

3 Ef (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 15

2 1

2 1

0

0 0

0.2 0.4 ∆EF (eV)

0.6

0

0.2 0.4 ∆EF (eV)

0.6

Figure 1: (a) Formation energies of different doped SnSe systems with dopants in their neutral states. Formation energies of charged defects in (b) As-doped, (c) Sb-doped and (d) Bi-doped SnSe. The solid lines represent the occupations of the Sn site, and the dashed lines represent the occupations of the Se site. ground states, and the exchange-correlation energy functional was applied with the generalized gradient approximation (GGA) in the form of Perdew-Burke-Ernzerhof (PBE) parameterization. 21 A plane-wave energy cut-off value of 350 eV was applied in all DFT calculations. The Brillouin zone was meshed with a density of about 2π×0.03 Å−1 , adopting the Γ-centered Monkhorst-Pack scheme. 22 The electronic convergence criterion was set to 10−6 eV. To calculate the electrical transport properties, a denser k-mesh of about 2π×0.01 Å−1 was employed. To simulate doped SnSe, a 1 × 3 × 3 supercell containing 72 atoms was constructed with one Sn or Se atom substituted by the doping atom, corresponding to a doping concentration of ∼2.78%. Upon the calculations of electronic structures, the electrical transport properties were calculated using BoltzTraP. 23 In order to recover the primitive cell band structure of doped SnSe from its supercell lattice, an algorithm involving the calculation of spectral weight was used to unfold the electronic bands. This technique allows us to reconstruct an effective band structure and enables a direct comparison with the band dispersion of pristine SnSe. 24 The lattice constants of our relaxed SnSe are a=4.21 Å, b=4.54 Åand c=11.77 Å, which are in good agreement to the experimental values obtained by Zhao et al . 19

4


Page 5 of 15

9

(b) Pristine Sb-doped

DOS (states/eV)

DOS (states/eV)

(a)

6 3 0 0 3 2 1

0.1 0.2 E-ECBM (eV)

(d) Sn s Sn p Se s Se p Sb s Sb p

15

Pristine In-doped Tl-doped

10 5 0 -0.3

0.3

DOS (states/eV/atom)

(c) DOS (states/eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

-0.2 -0.1 E-EVBM (eV)

0

0.2 Sn px p 0.1 py z

Se

Sb

0 0

0.1 0.2 E-ECBM (eV)

0.3

0

0.3 0 0.3 0 E-ECBM (eV)

0.3

Figure 2: (a) Density of states of Sb-doped and pristine SnSe. The vertical dashed lines represent the positions corresponding to a carrier concentration of −1020 cm−3 . The conduction band minimum is shifted to zero. (b) DOS of In-doped, Tl-doped and pristine SnSe. The vertical dashed lines represent the positions corresponding to a carrier concentration of 1020 cm−3 . The valence band maximum is shifted to zero. (c-d) The projected DOS of Sb-doped SnSe. The preferable lattice site for substitution for each dopant, is identified by the calculation of the formation energy, in accordance to the following equation: 3

E f = Etot − Epristine − ER + EX + q(EV + ∆EF + ∆V )

(2)

where Etot and Epristine represent the total energies of doped and pristine SnSe, respectively; EX and ER are the total energies of the substituted atom (Sn or Se) and the doping atom in their bulk phases, respectively; q represents the charge state of doping atom; EV denotes the energy at the top of the valence band of pristine SnSe, and ∆EF is the Fermi level relative to EV ; ∆V is a correction term to align the reference potentials of pristine and doped SnSe. The formation energies of dopants in their neutral states were firstly calculated, as shown in Figure 1(a). It is observed that all the neutral-state doping elements tend to occupy the Sn site, except for As, which shows an equal tendency to occupy either of the sublattice sites. To investigate the effects of different charge states on the preferable substitutional

5



lattice site, we have calculated the formation energies for the pnictogens in different charged states, as shown in Figure 1(b, c, and d). Because the valence states of Sn and Se atoms in SnSe compound are +2 and -2, respectively, and the common valence states of pnictogens (As, Sb and Bi) are +3, +5 and -3, we have considered +1 and +3 charged states when Sn is substituted and -1 charged state when Se is substituted. It is observed that the preferred substitutional site of As changes from Sn to Se depending on the Fermi energy. This is attributed to the extremely small difference in formation energies for neutral As, as identified from Figure 1(a). It is also found that the different charge states of Sb and Bi have no significant effect on the preferable substitutional site; both Sb and Bi always tend to substitute the Sn sublattice site. On the other hand, due to the larger differences in formation energies for the remaining dopants in their neutral states, it is expected that different charge states do not affect their preferred substitutional lattice site. (a)

Pristine

E-EF (eV)

2

High

1 0 -1

(b)

-2 Γ

Y

T

(b) 2 E-EF (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 15

Z Sb-doped

Γ

X

S

Low High

1 0 -1 -2 Γ

Y

T

Z

Γ

X

S

Low

Figure 3: Effective band structures of (a) pristine and (b) Sb-doped SnSe. Fermi level is located at 0 eV. In order to study the effects of dopants on the electronic structure of SnSe, the density of states (DOS) of doped SnSe is calculated. Results for selected systems are shown in Figure 2, and the results for the remaining dopants are included in the Supplementary Information. For the preferable substitutional site, i.e., the Sn site, of Sb, In and Tl, Sb is an n-type 6


Page 7 of 15 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


dopant while In and Tl are p-type dopants. Therefore, the conduction band DOS of n-type dopants and the valence band DOS of p-type dopants are of our interest. Because typical thermoelectric materials have a carrier concentration in the range of 1019 to 1021 cm−3 , we have indicated the Fermi level corresponding to a carrier concentration of 1020 cm−3 in Figure 2. It is observed from Figure 2(a) that Sb tends to enhance the DOS near the conduction band minimum (CBM), while Figure 2(b) illustrates no significant changes in the DOS for In and Tl doped SnSe at the focused carrier concentration. Although In induces a sharp rise in DOS near the top of the valence band, the carrier concentration needed to reach that sharp rise is beyond the practical range of carrier concentration for thermoelectrics, and is thus of lesser interest to us. To further analyze the increase in DOS of Sb-doped SnSe, projected density of states has been calculated. Through Figure 2(c) and Fig. 2(d), we notice that the DOS near the Fermi level is mainly dominated by the Sn 5p states. More interestingly, it is observed that Sb tends to exhibit a highly anisotropic increment in DOS, and resonant states at the CBM arise mainly from the px orbitals of Sb, which point to the out-of-plane direction of the layered crystal structure. The induction of resonant states near the Fermi level may point towards a potential improvement in the Seebeck coefficient, as suggested by the Mott’s formula: 25

S=

n 1 dn(E) 1 dµ(E) o π 2 kB kB T + 3e n dE µ dE E=EF 1 dn(E) g(E) = n dE n(E)

(3)

(4)

where n, µ, E, EF and g(E) represent the carrier concentration, mobility, energy of carriers, Fermi level, and DOS, respectively. The energy derivative of carrier concentration ( dn(E) ) dE is related to the band structure and is equivalent to the DOS at Fermi level. 26 Thus, an increase in DOS near Fermi level brought about by the resonant states in turn enhances the Seebeck coefficient. At the same time, the second term

dµ(E) dE

is associated with the

scattering parameter. An increased scattering factor will also result in an improved Seebeck 7



coefficient. This enhancement in the carrier scattering parameter may also be brought about by the resonant states, due to the interchange of electrons between the normal and impurity states. 27 As a result, an improved Seebeck coefficient is expected due to the effects of resonant states. 28 (a)

Pristine

(b)

Sb-doped

a c

Figure 4: Decomposed charge densities at the bottom of the conduction bands of (a) pristine and (b) Sb-doped SnSe for a carrier concentration of −4.3 × 1020 cm−3 . The iso-surfaces correspond to a charge density of 1.3 × 10−4 e/Å3 . Sn, Se and Sb atoms are colored by blue, green and red, respectively.

The resonant states in DOS near the CBM in Sb-doped SnSe indicate that Sb is a favorable candidate for enhancing the n-type electrical transport properties of SnSe. In order to better understand the origins of the resonant states, the effective band structure of Sb-doped SnSe has been calculated, as shown in Figure 3. It is noticed that the band dispersions of pristine SnSe are well preserved in Sb-doped SnSe. More interestingly, relatively non-dispersive impurity bands are introduced in the Γ-Y, Y-T and Z-Γ directions near the CBM after Sb is doped. The resonant states in DOS near the CBM is thus believed to be related to these relatively non-dispersive bands. For a more complete understanding of the effects of Sb on the electronic structures, we have also calculated the decomposed charge density associated with the bottom of conduction bands (see Figure 4). Comparing to pristine SnSe, a more delocalized charge density is clearly observed in Sb-doped SnSe, with most of the charges being delocalized in the out-of-plane direction. We realize that this change in decomposed charge density near the CBM causes the formation of additional interlayer electrical conductive pathways. Thus, an anisotropic enhancement in the electrical conductivity is expected for Sb-doped SnSe. 8


Page 8 of 15

Page 9 of 15

300K

S (µV/K)

-600

-600

-450

-450

-300

-300

-150

-150

PF/τ σ/τ (1014µW/(cm*K2*s)) (1016S/(cm*s))

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0 -1018 16 12 8

-1019

0 -1020 -1018 16

a b c

-1019

-1020

-1019

-1020

12 8

4 0 -1018 36

600K

4 -1019

0 -1020 -1018 80

27

60

18

40

9

20

0 0 -1018 -1019 -1020 -1018 -1019 -1020 -3 Carrier density (cm ) Carrier density (cm-3)

Figure 5: Seebeck coefficients (top), electrical conductivities normalized by the relaxation times (middle), and normalized power factors (bottom) of pristine (solid curves) and Sbdoped SnSe (dashed curves) at different temperatures. To provide further evidence for the above analysis based on the electronic structures, Boltzmann transport calculations have been carried out. It is seen from Figure 5 that Sb induces an obvious improvement in Seebeck coefficient at 300 K. Consistent with previous discussion, this improvement is mainly attributed to the resonant states near the CBM. Nonetheless, the improvement of Seebeck coefficient becomes less significant at higher temperatures. The decrease of Seebeck coefficients along the b and c directions at 600 K when the carrier concentration is low is due to the underestimated band gap in our DFT calculations. 10 Moreover, it is also found that the electrical conductivity normalized by the relaxation time is favorably improved along the out-of-plane direction (a direction); this may be attributed to the more delocalized charge density near the CBM and the additional interlayer electrical conductive pathways. In contrast to the doping effects on Seebeck coefficient, the improvement of normalized electrical conductivity can be retained at 600 K. The improvements of both Seebeck coefficient and normalized electrical conductivity at 300 K consequently result in a significant enhancement of the normalized power factor along the out-of-plane direction. However, the absolute value of power factor is affected by the relaxation time, and a definite conclusion can only be drawn after experiments have been carried out. The results 9



obtained from our electrical transport calculations are in good agreement with the analysis based on the electronic structures. It is also worth noting that SnSe possesses the lowest thermal conductivity along the out-of-plane direction due to the strong inter-layer phonon scattering. 16,29,30 Therefore, the enhancement of out-of-plane power factor together with the ultralow thermal conductivity could very well result in a remarkable improvement of the thermoelectric figure of merit. In summary, pnictogen group elements (Sb and Bi) are found to result in a significant improvement in the normalized power factor along the out-of-plane direction of n-type SnSe, especially in the lower temperature range. The improvement in power factor is due to a combination of simultaneous advantages brought by the dopants; the coexistence of resonant states, which lead to an improved Seebeck coefficient, together with a more delocalized charge density at the CBM, which provides additional interlayer electrical conductive pathways, contributes towards an enhancement of power factor. Because SnSe exhibits a superior thermoelectric efficiency at high temperatures, an improvement of its efficiency at lower temperatures is beneficial for the overall performance.

Acknowledgement This work is supported by the Research Grants Council of Hong Kong under project numbers 27202516 and 17200017, and the National Natural Science Foundation of China under project number 51706192. The authors are grateful for Lidong Zhao for helpful discussion, and the research computing facilities offered by ITS, HKU.

Supporting Information Available DOS of pristine and doped SnSe (Figure S1). DOS and PDOS of As-doped (Figure S2) and Bi-doped (Figure S3) SnSe. Electrical transport properties of As-doped (Figure S4) and Bi-doped (Figure S5) SnSe. 10


Page 10 of 15

Page 11 of 15 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


This material is available free of charge via the Internet at http://pubs.acs.org/.

References (1) Zhou, Y.; Zhao, L.-D. Promising Thermoelectric Bulk Materials with 2D Structures. Adv. Mater. 2017, 29 . (2) Chen, G. Nanoscale heat transfer and nanostructured thermoelectrics. IEEE Trans. Compon. Packag. Technol. 2006, 29, 238–246. (3) Shen, J.; Chen, Y. Silicon As an Unexpected n-Type Dopant in BiCuSeO Thermoelectrics. ACS Appl. Mater. Interfaces 2017, 9, 27372–27376. (4) Wang, X. W.; Lee, H.; Lan, Y. C.; Zhu, G. H.; Joshi, G.; Wang, D. Z.; Yang, J.; Muto, A. J.; Tang, M. Y.; Klatsky, J.; Song, S.; Dresselhaus, M. S.; Chen, G.; Ren, Z. F. Enhanced thermoelectric figure of merit in nanostructured n-type silicon germanium bulk alloy. Appl. Phys. Lett. 2008, 93, 193121. (5) Tan, G.; Zhao, L.-D.; Kanatzidis, M. G. Rationally designing high-performance bulk thermoelectric materials. Chem. Rev. 2016, 116, 12123–12149. (6) Wei, W.; Chang, C.; Yang, T.; Liu, J.; Tang, H.; Zhang, J.; Li, Y.; Xu, F.; Zhang, Z.; Li, J.-F.; Tang, G. Achieving High Thermoelectric Figure of Merit in Polycrystalline SnSe via Introducing Sn Vacancies. J. Am. Chem. Soc. 2018, 140, 499–505. (7) Ren, Z.; Lan, Y.; Zhang, Q. Advanced Thermoelectrics: Materials, Contacts, Devices, and Systems; CRC press, 2017; Vol. 11. (8) Zhao, L.-D.; Lo, S.-H.; Zhang, Y.; Sun, H.; Tan, G.; Uher, C.; Wolverton, C.; Dravid, V. P.; Kanatzidis, M. G. Ultralow thermal conductivity and high thermoelectric figure of merit in SnSe crystals. Nature 2014, 508, 373–377.

11



(9) Zhao, L.-D.; Tan, G.; Hao, S.; He, J.; Pei, Y.; Chi, H.; Wang, H.; Gong, S.; Xu, H.; Dravid, V. P.; Uher, C.; Snyder, G. J.; Wolverton, C.; Kanatzidis, M. G. Ultrahigh power factor and thermoelectric performance in hole-doped single-crystal SnSe. Science 2015, aad3749. (10) Yu, H.; Dai, S.; Chen, Y. Enhanced power factor via the control of structural phase transition in SnSe. Sci. Rep. 2016, 6, 26193. (11) Wei, P.-C.; Bhattacharya, S.; He, J.; Neeleshwar, S.; Podila, R.; Chen, Y.; Rao, A. The intrinsic thermal conductivity of SnSe. Nature 2016, 539, E1–E2. (12) Ibrahim, D.; Vaney, J.-B.; Sassi, S.; Candolfi, C.; Ohorodniichuk, V.; Levinsky, P.; Semprimoschnig, C.; Dauscher, A.; Lenoir, B. Reinvestigation of the thermal properties of single-crystalline SnSe. Appl. Phys. Lett. 2017, 110, 032103. (13) Chen, Z.; Jian, Z.; Li, W.; Chang, Y.; Ge, B.; Hanus, R.; Yang, J.; Chen, Y.; Huang, M.; Snyder, G. J.; Pei, Y. Lattice Dislocations Enhancing Thermoelectric PbTe in Addition to Band Convergence. Adv. Mater. 2017, 29, 1606768, 1606768. (14) Wang, X.; Xu, J.; Liu, G.; Fu, Y.; Liu, Z.; Tan, X.; Shao, H.; Jiang, H.; Tan, T.; Jiang, J. Optimization of thermoelectric properties in n-type SnSe doped with BiCl3. Appl. Phys. Lett. 2016, 108, 083902. (15) Tang, G.; Wen, Q.; Yang, T.; Cao, Y.; Wei, W.; Wang, Z.; Zhang, Z.; Li, Y. Rocksalt-type nanoprecipitates lead to high thermoelectric performance in undoped polycrystalline SnSe. RSC Adv. 2017, 7, 8258–8263. (16) Yang, J.; Zhang, G.; Yang, G.; Wang, C.; Wang, Y. X. Outstanding thermoelectric performances for both p-and n-type SnSe from first-principles study. J. Alloys Compd. 2015, 644, 615–620.

12


Page 12 of 15

Page 13 of 15 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(17) Huang, Y.; Wang, C.; Chen, X.; Zhou, D.; Du, J.; Wang, S.; Ning, L. First-principles study on intrinsic defects of SnSe. RSC Adv. 2017, 7, 27612–27618. (18) Chere, E. K.; Zhang, Q.; Dahal, K.; Cao, F.; Mao, J.; Ren, Z. Studies on thermoelectric figure of merit of Na-doped p-type polycrystalline SnSe. J. Mater. Chem. A 2016, 4, 1848–1854. (19) Leng, H.-Q.; Zhou, M.; Zhao, J.; Han, Y.-M.; Li, L.-F. The thermoelectric performance of anisotropic SnSe doped with Na. RSC Adv. 2016, 6, 9112–9116. (20) Kresse, G.; Furthmüller, J. Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set. Comput. Mater. Sci. 1996, 6, 15–50. (21) Blöchl, P. E. Projector augmented-wave method. Phys. Rev. B 1994, 50, 17953–17979. (22) Monkhorst, H. J.; Pack, J. D. Special points for Brillouin-zone integrations. Phys. Rev. B 1976, 13, 5188–5192. (23) Madsen, G. K.; Singh, D. J. BoltzTraP. A code for calculating band-structure dependent quantities. Comput. Phys. Commun. 2006, 175, 67–71. (24) Medeiros, P. V. C.; Stafström, S.; Björk, J. Effects of extrinsic and intrinsic perturbations on the electronic structure of graphene: Retaining an effective primitive cell band structure by band unfolding. Phys. Rev. B 2014, 89, 041407. (25) Rowe, D. M. CRC Handbook of Thermoelectrics; CRC press, 1995. (26) Heremans, J. P.; Wiendlocha, B.; Chamoire, A. M. Resonant levels in bulk thermoelectric semiconductors. Energy Environ. Sci. 2012, 5, 5510–5530. (27) Zhang, Q.; Wang, H.; Liu, W.; Wang, H.; Yu, B.; Zhang, Q.; Tian, Z.; Ni, G.; Lee, S.; Esfarjani, K.; Chen, G.; Ren, Z. Enhancement of thermoelectric figure-of-merit by resonant states of aluminium doping in lead selenide. Energy Environ. Sci. 2012, 5, 5246–5251. 13



(28) Zhang, Q.; Cao, F.; Liu, W.; Lukas, K.; Yu, B.; Chen, S.; Opeil, C.; Broido, D.; Chen, G.; Ren, Z. Heavy doping and band engineering by potassium to improve the thermoelectric figure of merit in p-type pbte, pbse, and pbte1–y se y. J. Am. Chem. Soc. 2012, 134, 10031–10038. (29) Duong, A. T.; Nguyen, V. Q.; Duvjir, G.; Kwon, S.; Song, J. Y.; Lee, J. K.; Lee, J. E.; Park, S.; Min, T.; Lee, J.; Cho, S. Achieving ZT= 2.2 with Bi-doped n-type SnSe single crystals. Nat. Commun. 2016, 7, 13713. (30) Kutorasinski, K.; Wiendlocha, B.; Kaprzyk, S.; Tobola, J. Electronic structure and thermoelectric properties of n- and p-type SnSe from first-principles calculations. Phys. Rev. B 2015, 91, 205201.

14


Page 14 of 15

Page 15 of 15

Graphical TOC Entry DOS (states/eV)

9

Charge Density of Sb-doped

Pristine Sb-doped

6 3 0

DOS (states/eV/atom)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0 0.2

Sn px 0.1 py pz

0.1 Se

0.2

0.3

Sb

0 0

0.3 0 0.3 0 E-ECBM (eV)

0.3

15

Sb


Enhanced Out-of-Plane Electrical Transport in n-Type SnSe

Recommend Documents