Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
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Electronic and Thermoelectric Properties of Layered Oxychalcogenides (BiO)CuCh (Ch = S, Se, Te) Shibghatullah Muhammady,† Yudhi Kurniawan,† Seiya Ishiwata,‡ Awabaikeli Rousuli,§ Toshiki Nagasaki,§ Shogo Nakamura,∥ Hitoshi Sato,⊥ Atsushi Higashiya,# Atsushi Yamasaki,¶ Yoshiaki Hara,▲ Andrivo Rusydi,9,■ Kouichi Takase,‡ and Yudi Darma*,† †
Department of Physics, Faculty of Mathematics and Natural Science, Institut Teknologi Bandung, Bandung 40132, Indonesia College of Sciences and Technology, Nihon University, Chiyoda, Tokyo 101-0062, Japan § Graduate School of Science, Hiroshima University, Kagamiyama 1-3-1, Higashi-Hiroshima 739-8526, Japan ∥ Faculty of Science, Hiroshima University, Kagamiyama 1-3-1, Higashi-Hiroshima 739-8526, Japan ⊥ Hiroshima Synchrotron Radiation Center, Hiroshima University, Kagamiyama 2-313, Higashi-Hiroshima 739-0046, Japan # Faculty of Science and Engineering, Setsunan University, Neyagawa, Osaka 572-8508, Japan ¶ Faculty of Science and Engineering, Konan University, Kobe 658-8501, Japan ▲ National Institute of Technology, Ibaraki College, Ibaraki 312-8508, Japan 9 NUSNNI-NanoCore, Department of Physics, National University of Singapore, Singapore 117411 ■ Singapore Synchrotron Light Source, National University of Singapore, Singapore 117603
Inorg. Chem. Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 08/08/18. For personal use only.
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ABSTRACT: We study the new details of electronic and thermoelectric properties of polycrystalline layered oxychalcogenide systems of (BiO)CuCh (Ch = Se, Te) prepared by using a solid-state reaction. The systems were characterized by using photoemission (PE) spectroscopy and four-probe temperaturedependent electrical resistivity ρ(T). PE spectra are explained by calculating the electronic properties using the generalizedgradient approximation method. PE spectra and ρ(T) show that (BiO)CuSe system is a semiconductor, while (BiO)CuTe system exhibits the metallic behavior that induces the high thermoelectric performance. The calculation of electronic properties of (BiO)CuCh (Ch = S, Se, Te) confirms that the metallic behavior of (BiO)CuTe system is mainly induced by Te 5p states at Fermi energy level, while the indirect bandgaps of 0.68 and 0.40 eV are obtained for (BiO)CuS and (BiO)CuSe systems, respectively. It is also shown that the local symmetry distortion at Cu site strongly stimulates Cu 3d-t2g to be partially hybridized with Ch p orbitals. This study presents the essential properties of the inorganic systems for novel functional device applications.
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INTRODUCTION Layered oxychalcogenides (RO)TmCh (R = La, Ce, Nd, Pr, Bi; Tm = Cu, Ag; and Ch = S, Se, Te) have become attractive because of their transparent p-type semiconducting behavior.1 As the example, (LaO)CuCh systems have wide bandgaps (Eg) of 3.1,2 2.8,2d and 2.3 eV3 for (LaO)CuS, (LaO)CuSe, and (LaO)CuTe systems, respectively. It has also been shown that the Eg of (LaO)CuS1−xSex system can be tuned by adjusting x and the exciton emission can be emerged at room-temperature in the system.4 Regarding the crystal structure, the space group of these systems is P4/nmm (No. 129) based on the tetragonal (LaO)AgS system.5 (LaO)CuCh system is built by [LaO]+ and [CuCh]− layers stacked along the c-axis direction.6 The [CuCh]− layer plays the important role in resulting the ptype conductivity because of the presence of Cu 3d10 and Ch p6 states at the top of the valence band (VB). From these © XXXX American Chemical Society
properties, (LaO)CuCh systems are attractive for optoelectronic devices applications in blue or ultraviolet regions.7 Recently, we have presented the details of electronic and optical properties of (LaO)CuCh (Ch = S, Se, Te) systems by means of the first-principles calculation. The following important results have been shown. (i) (LaO)CuSe exhibits the strongest p-type conductivity. (ii) (LaO)CuTe has the strongest optical dichroism and the highest dielectric constant. (iii) From electron-energy-loss function, the energy levels of plasmonic states can be tuned by changing the Ch element.8 Concerning the layered two-dimensional structures, (LaO)CuCh can be assumed as the natural superlattice systems exhibiting two-dimensional electron confinement in the Received: May 23, 2018
A
DOI: 10.1021/acs.inorgchem.8b01396 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry conductive [CuCh]− layer. The confinement is shown by the deep levels of O 2p states. The confinement can be used for generating thermoelectric power.9 However, (LaO)CuCh systems are not suitable for thermoelectric power device application because of their high electrical resistivity (ρ) at room temperature (RT).7 Concerning the layered oxychalcogenides, (BiO)CuSe and (BiO)CuTe systems are more suitable for thermoelectric power devices. Thermoelectric performance is measured by the figure of merit (ZT) expressed as10 ZT =
S2σT (dimensionless) κ
powder X-ray diffraction (XRD) (Rigaku Ultima-IV, Cu Kα radiation, λ = 1.5418 Å) and supported by simulation. The simulated XRD data is obtained by using RIETAN-2000.22 The inputs of the simulation are space group, lattice parameters, and atomic coordinates of (BiO)CuSe and (BiO)CuTe systems from the previous report.17 Tdependence of electrical resistivity ρ(T) of the samples were measured through a four-probe method23 using physical properties measurement system (PPMS) (Quantum Design) with T range of 10 to 300 K. Seebeck coefficient S(T) was measured by a two-probe method with a temperature range of RT to 650 K under air circumstance. Thermocouples were attached at edges of the samples to evaluate T differences. Furthermore, PE spectroscopy measurements were performed at RT on the beamline BL7 at Hiroshima Synchrotron Radiation Center (HiSOR) equipped with the hemispherical photoelectron analyzer (GAMMA-DATA SIENTA R4000 analyzer) with an incident photon energy (hυ) of 50 eV.24 The total energy resolution is set to be ∼55 meV. A clean surface of each sample for the PE measurement was obtained by fracturing in situ in an ultrahigh vacuum of 6 × 10−9 Pa just before each measurement. The ratios of the photoionization cross sections of Se 4p, Te 5p, and O 2p states to Cu 3d state are 0.05 (Se/Cu), 0.04 (Te/Cu), and 0.58 (O/Cu), respectively.25 Here, Fermi energy level (EF) in each PE spectrum is determined by the Fermi edge of Au spectrum.26
(1)
where σ and κ are the electrical and thermal conductivities, respectively, S is the Seebeck coefficient, and T is the temperature. (BiO)CuTe system exhibits ZT of 0.42 at 373 K and 0.66 at 673 K, which is higher than that of (BiO)CuSe system (ZT = 0.50 at 923 K).9a,10 The higher ZT of (BiO)CuTe system is because of the lower κ and ρ rather than for (BiO)CuSe.11 Furthermore, ZT of (BiO)CuSe can be enhanced using substitution doping at Bi site, i.e., Sr2+ (ZT = 0.76 at 873 K)12 or Ca2+ (ZT = 0.8 at 773 K).13 The increase of ZT by Sr2+ doping is induced by the increase of σ.14 ZT can also be increased from 0.5 to 1.4 (923 K) by the texturation of optimally Ba-doped system,15 or by modulation Ba doping.16 The previous experimental and theoretical approaches have also been carried out to investigate the electronic properties of (BiO)CuCh (Ch = S, Se, Te) systems.17 For instance, the energy-dependence of the density of states (DOS) or D(E) shows that the conduction band minimum (CBM) mainly comes from Bi 6p states, while the VB mainly comes from the orbital states from Cu 3d and Ch p states. Besides, the optical properties of (BiO)CuCh are strongly dependent on Eg and the formation of Cu 3d and Ch p states in the electronic properties by the theoretical approach,18 as also presented for (LaO)CuCh systems in our previous study.8 Regarding the bondings between ions, the Cu−Se bond in (BiO)CuSe system exhibits the covalent bonding characteristics. The Bi−O bond exhibits both ionic and covalent bonding characteristics.14 However, the role of Cu 3d and another orbital states in forming the electronic and thermoelectric properties are yet to explore further. Especially, the details of electronic properties of Cu 3d suborbitals, that is, Cu 3d-t2g (dzx, dzy, dxy) and Cu 3d-eg (dz2,dx2−y2), are not clear yet. In this paper, we describe new details of the valence band structures and thermoelectric properties of (BiO)CuCh (Ch = Se, Te) systems prepared by using solid-state reaction. The systems are characterized using photoemission (PE) spectroscopy. Temperature (T)-dependent ρ of (BiO)CuCh (Ch = Se, Te) systems is used to study the electrical properties of our inorganic material systems. The PE spectra are explained by the calculation of the electronic properties using the planewave method within the generalized-gradient approximation (GGA). The electronic properties of (BiO)CuS are also calculated as a comparison with the other systems. Here, the details of electronic properties of Cu 3d suborbitals, crystalfield (CF) splitting,19 Jahn−Teller (JT) distortion20 at the Cu 3d site, and orbital states contributing to the electronic and thermoelectric properties are presented as the essential points.
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CALCULATION DETAILS The electronic properties of (BiO)CuCh (Ch = S, Se, Te) systems with P4/nmm space group (No. 129)5 were calculated using GGA method with Perdew−Burke−Ernzerhof (PBE) exchange-correlation energy.27 The method has been carried out for our previous reports to investigate the electronic properties of different systems.28 Plane-wave functions represented by Quantum-ESPRESSO code29 are employed. Norm-conserving pseudopotentials are employed to represent all-electron potentials.30 The Broyden mixing algorithm31 is employed to perform the self-consistent calculation. A kinetic energy cutoff of 1361 eV and threshold energy of 10−4 a.u. (∼2.7 meV) are applied. All atomic positions are optimized with a relaxation calculation using the Broyden−Fletcher− Goldfarb−Shanno (BFGS) algorithm32 with a threshold force of 0.01 Ry/Bohr (∼0.26 eV/Å). Initial lattice parameters of the systems are adopted from the previous experimental results,17 as shown in Table 1. Table 1. Lattice Parameters of (BiO)CuCh (Ch = S, Se, Te) Systems lattice parameters (Å)17 system
a
c
(BiO)CuS (BiO)CuSe (BiO)CuTe
3.8691 3.9287 4.0411
8.5602 8.9291 9.5237
A k-point mesh of N × N × P of each system in a tetragonal Brillouin zone is obtained by an optimization based on calculated total energy. Lengths of corresponding reciprocal vectors (1/a, 1/a, 1/c) should have equal densities where (1/ a)/N = (1/c)/P or N/P = c/a. From Table 1, we obtain c/a of 2.21, 2.27, and 2.36 for (BiO)CuS, (BiO)CuSe, and (BiO)CuTe systems, respectively. It means that N/P = c/a ≈ 2 for all the systems. Then, we performed a convergence test by setting P = 1, 2, 3, 4, and 5 where N = 2P, as shown in Figure 1. For all the systems, the convergence is reached at P = 4 within accuracies of 0.001, 0.008, and 0.027 eV for (BiO)CuS, (BiO)CuSe, and (BiO)CuTe systems, respectively. Thus, the k-point mesh of 8 × 8 × 4 is used.
EXPERIMENTAL METHODS
(BiO)CuSe and (BiO)CuTe samples were prepared by the solid-state reaction.21 Crystal structures of the samples were characterized using B
DOI: 10.1021/acs.inorgchem.8b01396 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
shift is due to the different lattice parameters between the measured and simulated XRD data. Through a Rietveld analysis33 by using RIETAN 2000,22 we obtain lattice parameters of a = 3.9284 Å and c = 8.9292 Å in (BiO)CuSe system, which are very close to that of the simulated XRD data (a = 3.9287 Å and c = 8.9291 Å). This result causes the measured XRD peaks to the simulated data. However, (BiO)CuTe system exhibits the lattice parameters of a = 4.0568 Å and c = 9.5951 Å, which are slightly higher with that of the simulated XRD data (a = 4.0411 Å and c = 9.5237 Å), indicating a higher unit cell volume. Additionally, a complete Rietveld analysis of the present XRD pattern will be reported elsewhere. For (BiO)CuSe system, several significant peaks are observed such as at 2θ of 30.27°, 52.54°, and 56.22° which are appropriate with (102), (114), and (212) planes, respectively. For (BiO)CuTe system, several significant peaks are also observed such as at 2θ of 28.55°, 44.49°, and 53.90° which are appropriate with (102), (200), and (212) planes, respectively. These XRD data confirm that our systems are polycrystalline and the samples are an almost single phase with the (LaO)AgS structure.5b Any peak originating from impurities, such as CuO crystalline34 is not observed within our experimental accuracy. Figure 3 presents ρ(T) in the range of T = 10 to 300 K for (BiO)CuSe and (BiO)CuTe systems. For (BiO)CuSe system,
Figure 1. Calculated total energy as a function of P for (a) BiOCuS, (b) (BiO)CuSe, and (c) (BiO)CuTe systems for testing the convergence of the k-point mesh.
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RESULTS AND DISCUSSION Experimental Results. Figure 2 shows the XRD pattern of (BiO)CuCh (Ch = Se and Te) from the XRD measurement and the simulation. Almost all XRD peaks are close to the simulated XRD data from the previous report.17 We find a slight shift in 2θ of some peaks of (BiO)CuTe system. The
Figure 3. Four-probe temperature-dependence of electrical resistivity of (BiO)CuCh (Ch = Se, Te) systems with the temperature range of 10 to 300 K.
it is observed that ρ is decreased by the increase of T. However, for (BiO)CuTe system, ρ is slightly increased by the increase of T. Hence, (BiO)CuSe system exhibits semiconductor behavior because of the negative gradient of ρ(T). On the other hand, (BiO)CuTe system exhibits metallic behavior because of the positive gradient. (BiO)CuTe shows lower ρ(T) rather than that for (BiO)CuSe for the whole range of T. Especially at RT, ρ in (BiO)CuTe system is 1.05 × 10−7 Ωm (= σ −1) which is lower than that of (BiO)CuSe, that is, 6.65 × 10−5 Ωm. These results are in line with that of the previous report.11 The metallic behavior of (BiO)CuTe is in contrast with the semiconducting behavior in the previous experimental reports.11,17 This contrasting result might be due to the higher lattice parameters in the present work. In a crystal, as the lattice parameters are increased, the interatomic distances are also enlengthened. Therefore, binding forces between valence electrons and their parent atoms are reduced. This possible mechanism might let the valence electrons to be
Figure 2. Measured and simulated X-ray diffraction pattern of (a) (BiO)CuSe and (b) (BiO)CuTe samples. C
DOI: 10.1021/acs.inorgchem.8b01396 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry easier to move freely, leading to a metallic behavior and lower ρ(T). This mechanism also explains the higher ρ(T) of (BiO)CuSe due to the lower lattice parameters than that of (BiO)CuTe. Figure 4a,b present wide-range valence-band PE spectra as functions of binding energy of (BiO)CuSe and (BiO)CuTe
Table 2. Gaussian Peak Levels Generating Fit Curves of Photoemission Spectra of (BiO)CuCh (Ch = Se, Te) Systems Gaussian photoemission peak level (eV) system
E1
E2
E3
E4
E5
E6
E7
(BiO)CuSe (BiO)CuTe
12.07 12.13
9.99 9.38
5.48 6.02
3.82 4.91
3.07 3.62
2.79 2.99
1.60 1.55
It is also suggested that VB of (BiO)CuCh systems are divided into three primary levels (i.e., bonding, nonbonding, and antibonding levels), as also proposed in our previous studies on (LaO)CuCh systems.8,35 The peaks at E3 represent stable states (i.e., bonding-level states). The peaks at E1 and E2 represent more stable states (i.e., additional bonding-level states). On the other hand, the peaks at E7 represent less stable states near EF (i.e., antibonding-level states). The highest peak level (i.e., E5) followed by the peaks at E4 and E6 represent almost localized states (i.e., nonbonding-level states). The nonbonding level of (BiO)CuTe is located at deeper binding energy level than that of (BiO)CuSe systems. These results will be further explained by the calculated electronic properties. Figure 4c presents short-range PE spectra near EF. For (BiO)CuSe system, it is observed that the VBM is located at a binding energy level at 0.13 eV. However, VBM of (BiO)CuTe system overlaps EF. Hence, (BiO)CuSe and (BiO)CuTe systems have semiconductor and metallic behavior, respectively, which is consistent with ρ(T) in Figure 3. Furthermore, the terms of (dD(E)/dE)Se and (dD(E)/dE)Te are DOS slope at VBM for (BiO)CuSe and (BiO)CuTe, respectively. It is observed that (dD(E)/dE)Se is larger than that of (dD(E)/ dE)Te. Using the terms of (dD(E)/dE)Se and (dD(E)/dE)Te, one can obtain S shown in Equation 1 using Mott formula36 Ä ÉÑ π 2kB2T ÅÅÅÅ 1 dD(E) 1 dμ(E) ÑÑÑ ÅÅ ÑÑ S= (in V/K) + 3q ÅÅÅÇ D(E) dE μ(E) dE ÑÑÑÖE = E F (2)
where kB is the Boltzmann constant (kB = 1.381 × 10−23 m2 kg s−2 K−1), q is the elementary electric charge, D(E) and μ(E) are DOS and mobility as a function of energy (E), respectively. Hence, using the term of (dD(E)/dE) in Equation 2, (BiO)CuSe exhibits a higher S than that of (BiO)CuTe systems. This result is consistent with measured S(T) in the inset of Figure 4c. The inset shows a larger S(T) of (BiO)CuSe system which is in a good agreement with the previously reported results.11 It is also shown that S(T) of (BiO)CuTe system is almost linearly increased by the increase of T in the whole T range, which is consistent with Equation 2. On the other hand, S(T) of (BiO)CuSe system is maximum at T ∼ 575 K and decreased at T > 575 K. It is suggested that the BiO(CuSe) sample might experience a chemical decomposition at T ∼ 475 K. The chemical decomposition can occur by a chemical reaction between the sample and oxygen (O2) in air. However, the higher κ from the previous report37 and the lower ρ(T) in (BiO)CuTe induces the higher ZT rather than that for (BiO)CuSe systems. Hence, (BiO)CuTe system exhibits the best thermoelectric performance. It is confirmed in calculation result section that Ch element also plays an important role in thermoelectric properties. Calculation Results. In this section, the PE spectra are explained by the calculated electronic properties of (BiO)CuCh (Ch = Se, Te) systems. The electronic properties of
Figure 4. Wide-range photoemission (PE) spectra of (a) (BiO)CuSe and (b) (BiO)CuTe systems. Circles represent experimental data, while the solid blue lines represent the fit curves. The fit curves are obtained of the sum of seven Gaussians (solid red lines). (c) Shortrange PE spectra near Fermi energy level is also shown. All the PE spectra were normalized by the integrated spectral intensity between the binding energy of 15 eV and the Fermi energy level after subtracting the Shirley-type background.26 Inset shows temperaturedependence of Seebeck coefficient (S) with the temperature range from room temperature to 650 K.
systems, respectively. The PE spectra are modeled by fit curves obtained by the sum of seven Gaussian peaks. The binding energy levels of those Gaussian peaks are shown in Table 2. Almost all the peaks level of (BiO)CuSe are closer to EF rather than that of (BiO)CuTe systems except E2 and E7 peak levels. It means that Ch element significantly contributes to VB structures of (BiO)CuCh systems. D
DOI: 10.1021/acs.inorgchem.8b01396 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry Table 3. Atomic Coordinate of Bi and Ch along c-Axis in (BiO)CuCh (Ch = S, Se, Te) Systems (BiO)CuS
(BiO)CuSe
(BiO)CuTe
present work
other experiment17
present work
other experiment17
present work
other experiment17
0.15307 0.66441 3.223 −0.982
0.14829(5) 0.6710(2)
0.14458 0.67420 3.124 −0.237
0.14020(4) 0.6758(1)
0.13083 0.68054 2.749 −0.068
0.12733(9) 0.6810(1)
zBi zCh ΔzBi (%) ΔzCh (%)
(BiO)CuS system were also calculated as the comparison with the other systems. The structural properties of the systems are first calculated. Based on the previous reports,8,17 the lattice parameters of all the systems are increased as the increase of ionic radii of Ch as shown in Table 1. The effective ionic radii of S2−, Se2−, and Te2− are 1.84 Å, 1.98 Å, and 2.21 Å, respectively.38 Thus, the lattice parameter of (BiO)CuTe system is the largest among the systems. Table 3 summarizes atomic coordinates of Bi and Ch along c-axis in (BiO)CuCh (Ch = S, Se, Te) systems, compared with those of the previous experimental results.17 In Table 3, zBi and zCh are Wyckoff positions of Bi and Ch ions, respectively, along with the c-axis in (BiO)CuCh unit cell, which are obtained by GGA method. The terms of ΔzBi and ΔzCh are the differences between the Wyckoff positions from the calculation with those of the previous experimental report.17 Also, ΔzCh is smaller than ΔzBi indicating that the distance between [BiO]+ layer and Ch site is close to those of the experimental results. Table 4 presents the calculated bond lengths (dcalc), compared with the estimated (dc + da) and measured bond Table 4. Bond Lengths and Angles in (BiO)CuCh (Ch = S, Se, Te) Systemsa
Figure 5. (a) Schematic illustration of the position of bond angles in the crystal structure of (BiO)CuCh (Ch = S, Se, Te) systems. The bottom panes show bond angles of the systems for (b) Ch−Cu− Ch(α, β), (c) O−Bi−O, and Ch−Bi−Ch bonds.
bond length (in Å) Ch
bond
dcalc
dexp17
dc + da17
S
Bi−O Bi−Ch Cu−Ch Ch−Cu−Ch
2.34 3.15 2.39
2.31 3.14 2.43
2.57 3.01 2.44
Se
Te
Ch−Bi−Ch, O− Bi−O Bi−O Bi−Ch Cu−Ch Ch−Cu−Ch Ch−Bi−Ch, O−Bi−O Bi−O Bi−Ch Cu−Ch Ch−Cu−Ch Ch-Bi-Ch, O−Bi−O
bond angle
other hand, the calculated Cu−Ch bond is lower than that of the experiment for (BiO)CuS system, provoked by the larger |ΔzCh|. For all the systems, the shorter calculated Cu−Ch bonds rather than dc + da indicate the strong covalent bonds. Bi−O bonds are longer than those of the experiment because of the large |ΔzBi|. It is also suggested that the shorter dexp and dcalc for Bi−O bond rather than dc + da indicates the strong covalent bonds between Bi and O atoms. Furthermore, Bi−Ch bonds are shorter than those of the experiment for (BiO)CuSe and (BiO)CuTe systems. On the other hand, the Bi−Ch bond of (BiO)CuS system is longer than that of the experiment because of the largest value of (|ΔzBi| + |ΔzCh|) among all Ch elements. Moreover, Bi−Ch bonds in (BiO)CuS and (BiO)CuSe systems are longer than their dc + da, indicating weak bonds. However, both dcalc and dexp are close to dc + da of (BiO)CuTe system, indicating a fair agreement of bond length between Bi and Ch ions. In an ideal tetrahedral-type local symmetry, the bond angle of Ch−Cu−Ch should be 109.47°.39 The calculation, however, reveals two bond angles, that is, Ch−Cu−Ch(α) and Ch−Cu− Ch(β), which indicates distortion of local symmetry at Cu site. The change of Ch from S to Se and Te decreases Ch−Cu− Ch(α) and increases Ch−Cu−Ch(β) bond angles, as shown in Figure 5b. Furthermore, distortion is also found for the polyhedral BiO4Ch4, indicated by the increase of O−Bi−O and
α = 107.93°, β = 110.25° 75.76°, 71.67° 2.35 3.21 2.51
2.33 3.23 2.51
2.57 3.15 2.58 α = 103.93°, β = 112.67° 75.32°, 72.45°
2.37 3.38 2.65
2.36 3.39 2.66
2.57 3.38 2.81 α = 99.21°, β = 114.84° 73.54°, 74.01°
a
The calculated values are calculated by using VESTA.40
lengths (dexp), and bond angles between ion sites. The position of each bond angle is shown in Figure 5(a). Here, the term of (dc + da) represents the summation of cation ionic radii (dc) and anion ionic radii (da).17 The calculated Cu−Ch bonds are close to those of the experiment for (BiO)CuSe and (BiO)CuTe systems because of the small |ΔzCh|. On the E
DOI: 10.1021/acs.inorgchem.8b01396 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
contrast with that of the previous report showing the Eg of 0.4 eV estimated by the Kubelka−Munk42 equation from optical absorption spectra17 and 0.05 eV calculated using GGA + U + spin−orbit coupling (SOC) method.43 The underestimation of Eg of (BiO)CuS and (BiO)CuSe systems might be because of the absence of U. However, U does not entirely describe exchange-correlation of electrons at Cu 3d orbital43,44 and not suitable for closed-shell systems, in which all valence orbitals are empty or fully occupied.8,45 Here, (BiO)CuCh is a closedshell system, as shown in Table 5. The use of U to correct the
the decrease of Ch−Bi−Ch bond angles, as shown in Figure 5c. These kinds of distortions have also been found in (LaO)CuCh systems.8 Figure 6 shows the calculated band structures of (BiO)CuCh systems. A semiconductor behavior is found for (BiO)CuSe
Table 5. Electronic Configuration of Ions in (BiO)CuCh (Ch = S, Se, Te) Systems ion 3+
Bi O2− Cu+ S2− Se2− Te2−
electronic configuration49 [Xe]4f145d106s2 [He]2s22p6 [Ar]3d10 [Ne]3s23p6 [Ar]3d104s24p6 [Kr]4d105s25p6
electronic properties of other closed-shells systems, such as ZnO46 and TiO2 systems,47 also produces lower Eg compared to the results from absorption spectra measured using UV−vis spectrophotometer.48 Thus, GGA without the additional corrections is suitable for the properties of the systems and the metallic behavior of (BiO)CuTe system from the calculation is reliable to explain the PE spectra. Figure 7 shows total DOS curves of (BiO)CuCh systems as well as the projected density of states (PDOS) of Bi 6p and O 2p states. For all the systems, PDOS centroids of O 2p states in VB lie at around −4.26, −4.18, and −4.22 eV, respectively. Despite being located at the deep levels, O 2p states spread near EF up to around the energy level of −1.00 eV. This level is higher with that of (LaO)CuCh systems, in which the highest
Figure 6. Band structures of (BiO)CuS (top), (BiO)CuSe (center), and (BiO)CuTe systems (bottom).
confirming the PE spectra and (BiO)CuS systems. Calculated Eg of 0.68 and 0.40 eV are found in (BiO)CuS and (BiO)CuSe systems, respectively, with an indirect type between CBM at Γ and VBM to 0.4Σ. The term of 0.4Σ is the path connecting from 0.6Γ to M points in the corresponding Brillouin zone. The technique in determining the position along Σ path has been used in the previous report.41 The calculated Eg values are lower than that of the previous experimental Eg = 1.1 and 0.8 eV in (BiO)CuS and (BiO)CuSe systems, respectively.17 On the other hand, the metallic behavior is found in (BiO)CuTe system confirming ρ(T) in Figure 3 and the PE spectra in Figure 4c. However, the metallic behavior of (BiO)CuTe is in
Figure 7. Total density of states of (a) (BiO)CuS, (b) (BiO)CuSe, and (c) (BiO)CuTe systems. Blue solid line and red dashed line denote projected density of states of O 2p and Bi 6p, respectively. F
DOI: 10.1021/acs.inorgchem.8b01396 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Figure 8. Projected density of states (PDOS) curves of (a) (BiO)CuS, (b) (BiO)CuSe, and (c) (BiO)CuTe systems. The left panel shows PDOS curves of Cu 3d and Ch p states, while the right panel shows PDOS curves of Cu 4s and sub-Ch p states.
energy level of O 2p states is around the energy level of −3.0 eV.8 By considering the deep levels of O 2p states in VB structure, two-dimensional electron confinement can arise. Hence, (BiO)CuCh systems can be assumed as natural superlattice systems which are potential to generate thermoelectric power.9a,b In the scheme of ionic bonding, O 2p states should be fully occupied, while Bi 6p states should be empty, as shown in Table 5. However, the calculated PDOS curves show that both O 2p and Bi 6p states lie in VB and conduction band (CB) indicating a strong hybridization between both orbital states. Looking back the PE spectra in Figure 4a,b, VB width for each system from the PE spectra is about 2 times wider than that of the calculated VB structures. Despite these results, the shape of calculated DOS is comparable with the PE spectra. The calculated DOS of (BiO)CuSe and (BiO)CuTe systems can be divided into four regions (i.e., antibonding, nonbonding, bonding, and additional bonding levels states at around −1, −3, −4, and −6 eV, respectively). Here, it is also suggested that E4 peak from the PE spectra is contributed by O 2p states. Left side of Figure 8 shows PDOS curves of Cu 3d decomposed into Cu 3d-t2g (dzx, dzy, dxy), Cu 3d-eg (dz2,dx2−y2), and Ch p (S 3p, Se 4p, and Te 5p) states in the VB. The separated Cu 3d-t2g and Cu 3d-eg states are the results of CF splitting.19 These results confirm our statement that Ch element significantly contributes to VB. Furthermore, VBM of each system mainly comes from Ch p states, which determine the Eg and the electrical properties. Regarding the divided VB, the bonding level is dominated by Ch p, while the antibonding level is dominated by Cu 3d-t2g states. Here, Cu 3d-t2g are strongly hybridized with Ch p. In both levels, S 3p, Se 4p, and Te 5p peaks for (BiO)CuS, (BiO)CuSe, and (BiO)CuTe systems are located at around −4.04 and −0.94 eV, −3.60 and −1.05 eV, −3.45 and −1.20 eV, respectively. From the PE spectra in Figure 4a,b, it is suggested that E3 and E7 peaks mainly come from Se 4p and Te 5p for (BiO)CuSe and (BiO)CuTe systems, respectively. The nonbonding level between the antibonding and bonding levels comes from localized Cu 3d-eg states with the highest dz2 peaks at −2.29, −2.30, and −2.55 eV for (BiO)CuS, (BiO)CuSe, and
(BiO)CuTe systems, respectively, followed by dx2−y2 states. Interestingly, few Cu 3d-t2g states also contribute the nonbonding level, as found in (LaO)CuCh systems.8 The mechanism behind this contribution will be discussed later. Hence, it is suggested that the peak levels at E5 and E6 peaks in the PE spectra (see Figure 4a,b) mainly come from dz2 and dx2−y2, respectively. Besides O 2p, dxy states also contribute E4 peak in the PE spectra. Moreover, the additional bonding level is found for the highest peaks of −5.89, −5.55, and −5.25 eV for (BiO)CuS, (BiO)CuSe, and (BiO)CuTe systems, respectively. Furthermore, the additional bonding level represented by E1 and E2 peaks from the PE spectra is the result of the hybridization of Ch px/py and Cu 4s states, as shown in the right side of Figure 8. However, the contribution of Cu 4s on the additional bonding level in (BiO)CuTe system is lower than that of (LaO)CuTe,8 indicating the weaker hybridization. The previous works have shown the energy level diagrams of molecular bonding structures of VB in (LaO)CuCh systems.8,17 The diagrams also summarize the VB structure of (BiO)CuCh systems in this work. Looking back to Figure 4c, it is revealed that (dD(E)/dE)Se is larger than (dD(E)/dE)Se, leading to the larger S(T) of (BiO)CuSe system. From the calculated results, it is found that the calculated |dD(E)/dE| near EF is 10.1 eV−2 for (BiO)CuSe, which is larger than that of (BiO)CuTe (i.e., 6.52 eV−2). Hence, the calculation consistently reproduces the experimental results. Here, the term of |dD(E)/dE| is induced by Se 4p and Te 5p states for (BiO)CuSe and (BiO)CuTe systems, respectively. Besides, (BiO)CuS has |dD(E)/dE| near EF of 8.91 eV−2, which is induced by S 3p followed by Cu 3d states. Hence, the higher calculated |dD(E)/dE| of (BiO)CuSe rather than that of (BiO)CuTe is significantly affected by Ch element. In the tetrahedral-type crystal, like (BiO)CuCh and (LaO)CuCh,8,17 d orbital splits into 2-fold-degenerate eg states at lower levels and 3-fold-degenerate t2g states at higher levels. Figure 8 indicates that eg and t2g states are separated into the different levels. It means that the degeneracy levels of both states are decreased. Despite, dzx and dzy states are still at the same level, indicating that ab-plane local symmetry at Cu site is not distorted. Therefore, each (BiO)CuCh system have a distorted-tetrahedral-type local symmetry at Cu site along cG
DOI: 10.1021/acs.inorgchem.8b01396 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry axis induced by JT distortion.20 Based on the previous work,17 it is suggested that Cu ion should be bound with Ch ions in either zx, zy, or xy planes. Here, by the presence of local symmetry distortion at Cu site, Cu−Ch bonds are rearranged as indicated in Table 4 and Cu 3d-t2g is partially bound with Ch p orbitals. As one of the important points in this study, JT distortion induces few Cu 3d-t2g states to lie at the nonbonding level and to contribute E4, E5, and E6 peaks in the PE spectra. It is also found that the bonding level at around −4 eV is induced by dzx and dzy at the lower level, which are strongly hybridized with Ch p, and dxy at the higher level, which is weakly hybridized with Ch p states. On the other hand, the antibonding level at around −1 eV comes from dzx and dzyat the higher level and dxy states at the lower level. The hybridization of dzx (dzy) and Ch p is stronger rather than that of dxy and Ch p states. Interestingly, a small portion of dx2−y2, followed by dz2, are also located at the bonding and antibonding levels. The nonbonding level at around −2.5 eV mainly comes from dz2, followed by dx2−y2 states at the higher energy levels, which are in line with those of (LaO)CuCh systems.8
Physics, Institut Teknologi Bandung, Indonesia for providing technical support and calculation facilities.
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CONCLUSIONS In summary, the electronic and thermoelectric properties of (BiO)CuCh (Ch = S, Se, Te) systems have been studied by experimental and theoretical approaches. The PE spectra and ρ(T) show the semiconducting and metallic behaviors for (BiO)CuSe and (BiO)CuTe systems, respectively. These behaviors are confirmed by the calculated electronic properties which also show the semiconductor behavior in (BiO)CuS system. The metallic behavior and the lowest ρ(T) of (BiO)CuTe system arising from Te 5p hybridized with few Cu 3d states at EF induce the highest thermoelectric performance. From the fit curves of the PE spectra, each Gaussian fitting peak originates from the particular orbital states from the calculation. As the important result, the local symmetry distortion at Cu site plays the important role in E4, E5, and E6 peaks where Cu 3d-t2g are partially hybridized with Ch p orbitals. This study presents the new details and important properties of the inorganic layered systems for novel functional device applications.
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REFERENCES
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AUTHOR INFORMATION
Corresponding Author
*E-mail: yudi@fi.itb.ac.id. ORCID
Shibghatullah Muhammady: 0000-0002-2939-7887 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is supported by Ministry of Research, Technology, and Higher Education of Republic of Indonesia through Penelitian Dasar Unggulan Perguruan Tinggi (PDUPT) program 2018 (532w/l1.C01/PL/2018), Hibah Kompetensi Kemenristekdikti 2018, P3MI research program 2018 (1275G/ l1.C01/PL/2018), and Riset KK ITB 2018 (324f/l1.C01/PL/ 2018). S.M. thanks to WCU 2018 Postdoctoral program at Institut Teknologi Bandung. The PE spectroscopy with synchrotron X-ray was performed at HiSOR (Hiroshima University) under proposal No. 2016BU006. The authors also acknowledge Advanced Computing Laboratory, Department of H
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