Seebeck Coefficients of Layered BiCuSeO Phases: Analysis of Their Hole-Density Dependence and Quantum Confinement Effect

Abstract

Hole-doped layered BiCuSeO phases include substitutionally doped Bi1–x A x CuSeO (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–x A x CuSeO (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.

Significant Optimization of Electron–Phonon Transport of n-Type Bi2O2Se by Mechanical Manipulation of Se Vacancies via Shear Exfoliation

“…Very recently, a new versatile 2D material Bi 2 O 2 Se nanosheet composed of buckled Bi–O layers (which consist of edge-sharing OBi 4 tetrahedra) alternately stacked with planar Se layers, which has quite similar crystal structure to BiCuSeO, was found to exhibit ultrahigh electron mobility (>20 000 cm 2 V –1 s –1 ) in fabricated transistors. , Subsequent studies showed that the Bi 2 O 2 Se thin film exhibits good properties in highly sensitive infrared (IR) photodetectors and magnetoresistance devices, which were predicted to be promising candidates for realizing novel quantum phenomena, future logic devices, and flexible electronic, ferroelectric, and TE materials. ,− In particular, Bi 2 O 2 Se has been expected to be a perfect n-type counterpart of BiCuSeO (Figure S1). , The predicted promising TE properties of Bi 2 O 2 Se are attributed to its intrinsic high Seebeck coefficient and low lattice thermal conductivity, which originate from quantum confinement of carriers and interfacial scattering of phonons with weak atomic bonding between layers in its natural superlattice crystal structure. , However, high TE performance of bulk Bi 2 O 2 Se has not been realized experimentally so far because of its low electrical conductivity no matter what kind of methods adopted, including doping, solid solution, defects engineering or composites. ,− In the present work, a novel and facile method using a kitchen blender was developed for large-scale production of Bi 2 O 2 Se nanosheets (Figure ). Moreover, benefited from synergistical optimization of electron–phonon transport, the TE performance of the resultant bulk Bi 2 O 2 Se from spark plasma sintering (SPS) of Bi 2 O 2 Se nanosheets has been greatly improved ( ZT ≈ 0.5@793 K).…”

Electronic structure of a thermoelectric material: BiCuSO

“…5(c)], indicating the formation of a quasi-one-dimensional electronic structure in real space (Fig. 6), which is favorable for the TE performance [37][38][39][40]. We also notice some contribution from the Bi orbitals at the topmost band along -X and -Z (Fig.…”

Enhanced Thermoelectric Performance of c-Axis-Oriented Epitaxial Ba-Doped BiCuSeO Thin Films

“…The undoped film has the room temperature carrier concentration n of about 6.6 × 10 19  cm −3 , which is nearly an order of magnitude larger than those previously reported in the most bulk samples [5]. The higher n may originate from Cu or Bi vacancies in the films which can contribute holes [17–19]. As the Ba-doping content increases, the hole carrier concentration n of the films increases owing to the substitution of Bi 3+ by Ba 2+ .…”

“…This compound crystallizes in a tetragonal ZrCuSiAs structure with P4/nmm space group, which consists of the insulating (Bi 2 O 2 ) 2+ layers and the conductive (Cu 2 Se 2 ) 2− layers alternatively stacked along the c axis. Over the past several years, extensive works have been done in enhancing the TE performance of BiCuSeO bulks by optimization of its power factor and thermal conductivity via element doping [3–13], c -axis texturing [14], band gap tuning [15, 16], creating Bi or/and Cu vacancies [17–19], engineering grain boundaries [20, 21], adding nano-inclusions [22], introducing spin entropy by magnetic ion [23], and etc. For example, Zhao LD et al reported a high ZT of about 1.4 at 923 K in the c -axis-textured Ba-doped BiCuSeO bulks.…”