Article pubs.acs.org/cm
Seebeck Coefficients of Layered BiCuSeO Phases: Analysis of Their Hole-Density Dependence and Quantum Confinement Effect Changhoon Lee,†,‡,¶ Tae-Ho An,§,¶ Elijah E. Gordon,∥ Hyo Seok Ji,† Chan Park,§ Ji-Hoon Shim,*,†,‡ Young Soo Lim,*,⊥ and Myung-Hwan Whangbo*,∥ †
Department of Chemistry, Pohang University of Science and Technology, Pohang 790-784, Korea Division of Advanced Nuclear Engineering, Pohang University of Science and Technology, Pohang 790-784, Korea § Department of Materials Science and Engineering, Seoul National University, Seoul 151-744, Republic of Korea ∥ Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204, United States ⊥ Department of Materials System Engineering, Pukyong National University, Busan 48547, Republic of Korea ‡
S Supporting Information *
ABSTRACT: Hole-doped layered BiCuSeO phases include substitutionally doped Bi1−xAxCuSeO (A = alkali, alkaline earth) as well as vacancy-doped Bi1−δCu1‑γSeO and Bi1−δCuSeO. To probe how their Seebeck coefficients are related to their hole density p, we calculated the Seebeck coefficient for defect-free BiCuSeO as a function of the hole density, which is generated by lowering the Fermi level from the valence band maximum (VBM). In addition, we calculated the Seebeck coefficient for Bi1−δCuSeO (δ = 1/32, 1/16) with a large number of Bi vacancies. The Seebeck coefficients of the hole-doped BiCuSeO phases are governed by the electronic states lying within ∼0.5 eV from the VBM. These states are composed of largely Cu 3d xz/yz and Se 4p x/y states and possess the character of a uniform one-dimensional (1D) chain rather than a uniform two-dimensional (2D) lattice expected for a layered phase. The observed S-vs-p relationship for Bi1−xAxCuSeO (A = alkali, alkaline earth) as well as Bi1−δCu1‑γSeO (δ = 0, 0.025; γ = 0, 0.025) and Bi1−δCuSeO (δ = 0.0, 0.025) is very well reproduced by the calculated relationship for defect-free BiCuSeO within the rigid band approximation. The observed S-vs-p relationship reflects the quantum confinement effect of uniform 1D chains, despite that the hole-doped BiCuSeO phases consist of 2D layers, (Cu2Se2)2− and (Bi2O2)2+. The drastic decrease in the S values of Bi1−δCuSeO with large δ (= 0.05, 0.075, 0.10) arises from the loss of the quantum confinement effect in the (Cu2Se2)2− layers; that is, the uniform 1D chain character is lost because of their geometry distortion induced by a large number of Bi vacancies. where S is the Seebeck coefficient, σ the electrical conductivity, and κ the thermal conductivity.4 For metals and degenerate semiconductors described by a parabolic band, the Seebeck coefficient S is expressed as
1. INTRODUCTION Over the past several decades, thermoelectric (TE) materials have been extensively studied because of their ability to convert heat into electricity. They are expected to play an important role in generating primary power and recovering waste heat.1−3 Finding new efficient TE materials has been a challenging task. At a given temperature T, the efficiency of a TE material is measured by its figure of merit ZT, a dimensionless quantity, ZT =
S2 σ T κ
S=
(2)
Received: January 4, 2017 Revised: February 17, 2017 Published: February 21, 2017
(1) © 2017 American Chemical Society
⎛ π ⎞2/3 ⎜ ⎟ m * T ⎝ 3n ⎠ 3eh2
8π 2kB2
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DOI: 10.1021/acs.chemmater.7b00028 Chem. Mater. 2017, 29, 2348−2354
Article
Chemistry of Materials under the energy-independent scattering approximation, where n is the carrier concentration, and m* the effective mass of the carrier.5 Optimizing the ZT is difficult, because enhancing one parameter comes typically at the expense of diminishing another one. For example, one might imagine increasing ZT by increasing σ, but it would decrease ZT by decreasing S2, because S2 ∝ (1/n)4/3. Over the years, a number of suggestions have been made to maximize the power factor S2σ and lower the thermal conductivity κ.6−16 One important suggestion by Hicks and Dresselhaus7 is based on the work of Cutler and Mott.17 The electronic structure of a material can be described in terms of the density of states (DOS), N(E), as a function of energy E. In general, the conductivity σ, the carrier density n, and the effective mass m* depend on the band energy E, and hence, they are written as σ(E), n(E), and m*(E), respectively. Cutler and Mott showed that the Seebeck coefficient S is related to σ(E) as1,17 S=
π 2kB2T ⎛ dln σ(E) ⎞ ⎜ ⎟ 3e ⎝ dE ⎠ E = E
F
Figure 1. (a) Stacking of the (Bi2O2)2+ and (Cu2Q2)2− layers along the c-axis in BiCuQO: blue circle = Cu, yellow circle = Q, white circle = Q, red circle = O. (b) A projection view of a (Cu2Q2)2− layer along the cdirection. (c) A projection view along the c-direction of how the Bi atoms are connected to the Q atoms of its adjacent (Cu2Q2)2− layer. (d) A perspective view of a Cu4Se2 ribbon chain made up of edgesharing Cu4Se square pyramids along the (a+b)-direction. (e) A perspective view of a Cu4Se2Bi2 ribbon chain made up of edge-sharing Cu4SeBi octahedra along the (a+b)-direction.
(3)
where EF is the Fermi level. For a metal or a degenerate semiconductor, σ(E) is written as σ (E ) =
e 2n(E)τ(E) m*(E)
(4)
where e is the free electron charge, and τ(E) the relaxation time. eqs 3 and 4 suggest that one way of enhancing S is to increase dσ(E)/dE. This can be achieved by increasing dn(E)/dE, namely, by increasing dN(E)/dE. A large value of dN(E)/dE occurs around the region of a sharp peak in the N(E) vs E plot (hereafter referred to as a sharp DOS peak). For a uniform 1D chain, the DOS peak occurs at the bottom and at the top of the band. For a uniform 2D lattice, the DOS peak occurs in the middle of the band. In general, low-dimensional materials have quantum confinement effect hence possessing DOS peaks.3 Thus, Hicks and Dresselhaus predicted15,16 that the figure of merit ZT can be enhanced by utilizing low-dimensional materials. The layered phases BiCuQO (Q = S, Se, Te) have attracted much attention as promising high-performance thermoelectric materials. They consist of (Bi2O2)2+ layers alternating with (Cu2Q2)2− layers along the c-axis (Figure 1a).18,19 The insulating (Bi2O2)2+ layers are made up of edge-sharing OBi4 tetrahedra and the semiconducting (Cu2Q2)2− layers edgesharing CuQ4 tetrahedra. In each (Cu2Q2)2− layer, the square net of Cu+ ions is sandwiched between two square nets of Q2− ions such that the Cu+ ions form Cu4Q square pyramid above and below the plane of the Cu net, as depicted in Figure 1b. The Bi3+ ions of each (Bi2O2)2+ layer form Se4Bi square pyramids with the adjacent (Cu2Q2)2− layers (Figure 1c). BiCuQO (Q = Se, Te) is a semiconductor with a small indirect band gap,20 and it has a high Seebeck coefficient21−23 and a very low lattice thermal conductivity ( 1 in Ba heavily doped BiCuSeO oxyselenides. Energy Environ. Sci. 2012, 5, 8543−6547. (25) Li, F.; Wei, T.-R.; Kang, F.; Li, J.-F. Enhanced thermoelectric performance of Ca-doped BiCuSeO in a wide temperature range. J. Mater. Chem. A 2013, 1, 11942−11949. (26) Li, J.; Sui, J.; Barreteau, C.; Berardan, D.; Dragoe, N.; Cai, W.; Pei, Y.; Zhao, L.-D. Thermoelectric properties of Mg doped p-type BiCuSeO oxyselenides. J. Alloys Compd. 2013, 551, 649−653. (27) Inoue, S.; Ueda, K.; Hosono, H.; Hamada, N. Electronic structure of the transparent p-type semiconductor (LaO)CuS. Phys. Rev. B: Condens. Matter Mater. Phys. 2001, 64, 245211. (28) Sato, H.; Negishi, H.; Wada, A.; Ino, A.; Negishi, S.; Hirai, C.; Namatame, H.; Taniguchi, M.; Takase, K.; Takahashi, Y.; Shimizu, T.; Takano, Y.; Sekizawa, K. Electronic structure of oxysulfide (LaO)CuS and (La1−xCaxO)Cu1−xNixS (x < ∼ 0.10) studied by photoemission and inverse-photoemission spectroscopies. Phys. Rev. B: Condens. Matter Mater. Phys. 2003, 68, 035112. (29) Ueda, K.; Hiramatsu, H.; Ohta, H.; Hirano, M.; Kamiya, T.; Hosono, H. Single-atomic-layered quantum wells built in wide-gap semiconductors LnCuOCh (Ln = lanthanide, Ch = chalcogen). Phys. Rev. B: Condens. Matter Mater. Phys. 2004, 69, 155305. (30) Ueda, K.; Hosono, H.; Hamada, N. Valence-band structures of layered oxychalcogenides, LaCuOCh (Ch = S, Se, and Te), studied by ultraviolet photoemission spectroscopy and energy-band calculations. J. Appl. Phys. 2005, 98, 043506. (31) Hiramatsu, H.; Yanagi, H.; Kamiya, T.; Ueda, K.; Hirano, M.; Hosono, H. Crystal structures, optoelectronic properties, and electronic structures of layered Oxychalcogenides MCuOCh (M = Bi, La; Ch = S, Se, Te): Effects of electronic configurations of M3+ Ions. Chem. Mater. 2008, 20, 326−334. (32) Lan, J. L.; Zhan, B.; Liu, Y. C.; Zheng, B.; Liu, Y.; Lin, Y. H.; Nan, C. W. Doping for higher thermoelectric properties in p-type BiCuSeO oxyselenide. Appl. Phys. Lett. 2013, 102, 123905. (33) Pei, Y. L.; He, J.; Li, J. F.; Li, F.; Liu, Q.; Pan, W.; Barreteau, C.; Berardan, D.; Dragoe, N.; Zhao, L. D. High thermoelectric performance of oxyselenides: intrinsically low thermal conductivity of Ca-doped BiCuSeO. NPG Asia Mater. 2013, 5, e47. (34) Zhao, L. D.; Berardan, D.; Pei, Y. L.; Byl, C.; Pinsard-Gaudart, L.; Dragoe, N. Bi1‑xSrxCuSeO oxyselenides as promising thermoelectric materials. Appl. Phys. Lett. 2010, 97, 092118. (35) Sui, J.; Li, J.; He, J.; Pei, Y. L.; Berardan, D.; Wu, H.; Dragoe, N.; Cai, W.; Zhao, L. D. Texturation boosts the thermoelectric performance of BiCuSeO oxyselenides. Energy Environ. Sci. 2013, 6, 2916−2920. (36) Pan, L.; Berardan, D.; Zhao, L.; Barreteau, C.; Dragoe, N. Influence of Pb doping on the electrical transport properties of BiCuSeO. Appl. Phys. Lett. 2013, 102, 023902. (37) Luu, S. D. N.; Vaqueiro, P. Synthesis, structural characterisation and thermoelectric properties of Bi1−xPbxOCuSe. J. Mater. Chem. A 2013, 1, 12270−12275. (38) Lan, J.-L.; Liu, Y.-C.; Zhan, B.; Lin, Y.-H.; Zhang, B.; Yuan, X.; Zhang, W.; Xu, W.; Nan, C.-W. Enhanced thermoelectric properties of Pb-doped BiCuSeO ceramics. Adv. Mater. 2013, 25, 5086−5090.
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DOI: 10.1021/acs.chemmater.7b00028 Chem. Mater. 2017, 29, 2348−2354
