Article Cite This: Inorg. Chem. XXXX, XXX, XXX-XXX
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Wurtzite-Derived Quaternary Oxide Semiconductor Cu2ZnGeO4: Its Structural Characteristics, Optical Properties, and Electronic Structure Masao Kita,*,† Issei Suzuki,‡,⊥ Naoki Ohashi,§ and Takahisa Omata∥ †
Department of Mechanical Engineering, National Institute of Technology, Toyama College, 13 Hongo-machi, Toyama, 939-8630, Japan ‡ Division of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, 565-0871, Japan § National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki, 305-0044, Japan ∥ Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, 980-8577, Japan S Supporting Information *
ABSTRACT: The quaternary I 2 −II−IV−O 4 semiconductor, Cu2ZnGeO4, with a wurtz-kesterite structure and 1.4 eV energy band gap has been synthesized for the first time via ion exchange of precursor Na2ZnGeO4. Its crystal structure was refined by Rietveld analysis, and the structural distortion was quantitatively evaluated based on the cation tetrahedral tilting and angle distortion indexes. The tetrahedral distortion in Cu2ZnGeO4 was smaller than in Ag2ZnGeO4 but larger than in β-CuGaO2, suggesting an indirect band gap of Cu2ZnGeO4. Density functional theory calculations using the functional of the local density approximation with corrections for on-site Coulomb interactions indicated that Cu2ZnGeO4 is an indirect semiconductor as expected from its structural feature. However, the energy difference between the direct and indirect band gaps is very small, suggesting that Cu2ZnGeO4 shows strong light absorption near the band edge. Among the I−III−O2 semiconductors, β-CuGaO2 and βAgGaO2 exhibit narrow band gaps, respectively, of 1.47 and 2.19 eV,16,17 which is an uncommon feature in oxide semiconductors. This feature enables expansion of the application area of oxide semiconductors into visible and infrared regions, such as visible light photocatalysts and solar cell absorbers. Band gap engineering of ternary I−III−O2 semiconductors has also been demonstrated: β-Cu(Ga,Al)O2 and β-Ag(Ga,Al)O2 alloys, respectively, cover wide energy regions of 1.47−2.09 eV18 and 2.19−2.83 eV.19 The ternary I− III−O2 semiconductors have undergone evolutions in the narrow band gap region. Contrary to the ternary I−III−O2 semiconductors, quaternary I2−II−IV−O4 has not received much attention yet. Na2MIIMIVO4,21,22 Li2ZnGeO4,23 and Ag2ZnMIVO424 (MII = Mg, Zn; MIV = Si, Ge) are quaternary wurtzite-derived oxides. While their crystal structures are studied well, i.e., three structures of monoclinic wurtz-kesterite (space group Pn), orthorhombic wurtz-stannite (space group Pnm21), and orthorhombic Li2CoSiO4 type (space group Pna21) as shown in Figure 1, their optical and electrical properties are relatively unknown. Further, quaternary wurtzite-derived oxides contain-
1. INTRODUCTION II−VI chalcogenide compound semiconductors possessing a cubic zincblende structure are practically used in a variety of optoelectronic devices, especially in photoelectric transducers in infrared and visible regions, such as (Hg,Cd)Te for infrared detectors,1 CdTe for thin-film solar cells,2 and CdSe quantum dot phosphors for liquid crystal displays.3 In response to the recent demands for environmentally conscious materials, replacing the binary II−VI zincblende compounds with the ternary I−III−VI2 chalcopyrite and quaternary I2−II−IV−VI4 kesterite and stannite compounds is examined in terms of avoiding the use of toxic cadmium: for example, Cu(In,Ga)Se2 and Cu2ZnSnS4 as alternatives to CdTe for solar cells4−7 and CuInS2 as an alternative to CdSe for quantum dot phosphors.8,9 Such an expansion of the material system from binary to ternary and quaternary systems in II−VI based chalcogenides is spreading to the III−V based pnictides; investigations into the application of ZnSnP2 in solar cell absorbers has recently begun.10,11 In oxide semiconductors, because ZnO is the only binary II− O compound semiconductor possessing a hexagonal wurtzite structure except for the carcinogenic BeO,12 ternary I−III−O2 semiconductors with a wurtzite-derived β-NaFeO2 structure and their alloys have recently been studied to expand the energy band gap region that oxide semiconductors cover.13−20 © XXXX American Chemical Society
Received: September 15, 2017
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DOI: 10.1021/acs.inorgchem.7b02379 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
occupancies of respective atoms were fixed to the value determined, based on the chemical composition determined by ICP-AES. Isotropic atomic displacement parameters, B, were applied to all ions, and the values of the same kind of element were constrained to be the same as each other. A split-type pseudo-Voigt profile function was used in the Rietveld refinements. The background was corrected using the standard background function in RIETAN-FP. The parameters optimized in the present structural refinements were the following: background, zero point, scale factor, pseudo-Voigt parameters of the peak shape, lattice parameters, and fractional coordinates and isotropic displacement parameters of respective atoms. The following agreement indices were calculated: profile, Rp =∑|yio − yic|/∑yio; weighted profile, Rwp = [∑wi(yio−yic)2/∑wi(yio)2]1/2; and the goodness of fit S = Rwp/Re, where Re = [(N − P)/∑wi(yio)2]1/2; yio and yic are the observed and calculated intensities, respectively, wi is the weighting factor, N is the total number of yio data, and P is the number of adjusted parameters. The diffuse reflectance spectra for the powdered sample were recorded by a double-beam spectrophotometer (U4000, Hitachi, Japan) equipped with an MgO-coated integrating sphere. MgO powder was used as the reference material. The diffuse reflectance, Rd, was transformed to F(Rd), which was proportional to the absorption, α, using the Kubelka−Munk formalism: α ∝ F(Rd) = (1 − Rd)2/(2Rd). The calculations of the electronic band structure of Cu2ZnGeO4 were performed using DFT with LDA+U26 as implemented in the CASTEP code.27 The norm-conserving pseudopotentials28 generated with OPIUM29 were used for the valence electrons of Cu 3d, 4s, 4p; Zn 3d, 4s, 4p; Ge 3d, 4s, 4p; and O 2s and 2p. The Hubbard correction, U, for Cu 3d electrons was set as 6 eV because the appropriate U value for the LDA+U calculation of β-CuGaO2, which similarly possesses the wurtzite-derived structure, was previously reported to be 5−7 eV.30,31 Brillouin-zone sampling was performed with a 5 × 4 × 5 k-point mesh. The plane-wave cutoff energy was set at 880 eV. The convergence conditions of the geometry optimization with imposed monoclinic symmetry with the space group of Pn were as follows: the energy convergence tolerance was 0.5 × 10−5 eV atom−1, the maximum ionic displacement tolerance was 5.0 × 10−4 Å, the maximum force tolerance was 1.0 × 10−2 eV Å−1, and the maximum stress tolerance was 2.0 × 10−2 GPa.
Figure 1. Schematic illustrations of cation ordering in (a) a β-NaFeO2type structure, (b) a wurtz-kesterite type structure (space group Pn), (c) a wurtz-stannite type structure (space group Pmn21), and (d) a Li2CoSiO4 type structure (space group Pna21). Thin solid lines indicate their unit cells.
ing monovalent Cu have yet to be reported. It is expected that an I2−II−IV−O4 semiconductor consisting of monovalent Cu exhibits narrow band gap and p-type conduction similar to the ternary β-CuGaO2. Here, a wurtzite-derived quaternary Cu2ZnGeO4 oxide semiconductor was synthesized for the first time, and we found that it has a narrow band gap of 1.4 eV. The crystal structure was refined by Rietveld analysis and discussed in terms of the deviation from the ideal wurtzite structure. We theoretically studied the electronic structure of Cu2ZnGeO4 based on density functional theory (DFT) calculations using the functional of the local density approximation with corrections for on-site Coulomb interactions (LDA+U). The calculation indicated that the energy band gap of Cu2ZnGeO4 is indirect, and the effective masses of electron and hole are respectively 0.24−0.45 and 2.1−10.0 in units of free electron mass, m*/m0.
3. RESULTS AND DISCUSSION 3.1. Characterization. The composition of the sample after i o n ex c h a n g e f o r 4 8 h w a s d e t e r m i ne d t o b e (Cu0.91Na0.09)2ZnGeO4. The concentration of residual Na+ ions after the ion exchange did not decrease, even when the ion exchange was prolonged to 120 h or the demonstrated ionexchange temperature was elevated. This indicates that the ion exchange of all Na+ ions in precursor Na2ZnGeO4 with Cu+ ions cannot be completed, although it was reported that the complete ion exchange of all Na+ ions in Na2ZnGeO4 with Ag+ ions is possible.24 The mechanism that determines attainable composition after ion exchange is an open question that should be answered in the future. For ease of representation, the sample after ion exchange is noted as Cu2ZnGeO4, whereas the actual composition is (Cu0.91Na0.09)2ZnGeO4. Figures 2a and 2b show the XRD patterns of the precursor Na2ZnGeO4 and Cu2ZnGeO4, respectively. For the precursor Na2ZnGeO4, all the diffractions were indexed as those of a wurtz-kesterite type structure, as reported previously. 21 Although the XRD pattern of Cu2ZnGeO4 was completely different from that of the precursor, all the diffractions could be indexed as those of a wurtz-kesterite type structure with a = 6.57219 Å, b = 5.51080 Å, c = 5.29790 Å, α = 90°, β = 90.370°, and γ = 90°, as indicated in Figure 2b. However, there are three structural types of wurtz-kesterite, wurtz-stannite, and Li2CoSiO4 type, among which cation ordering is different, as shown in Figure 1, in the quaternary hexagonal diamond-
2. EXPERIMENTAL SECTION Cu2ZnGeO4 was synthesized by ion exchange of Na+ ions in the precursor Na2ZnGeO4 with Cu+ ions supplied from a mixed molten salt of CuCl (99.9%; Wako Pure Chemical Industries, Ltd.; Japan) and KCl (99.9%; Wako Pure Chemical Industries, Ltd.) similar to the synthesis of ternary β-CuGaO2.16 The precursor Na2ZnGeO4 was prepared by solid-state reaction. Na2CO3 (99.8%; Wako Pure Chemical Industries, Ltd.), ZnO (99.99%; Kojundo Chemical Laboratory CO., LTD; Japan), and GeO2 (99.997%; Kojundo Chemical Laboratory CO., LTD) were weighed, mixed, and then pressed into disks (17.2 mm diameter) at 256 MPa. The disks were then fired at 1273 K for 10 h in air. The obtained Na2ZnGeO4 was crushed and mixed with CuCl−KCl powder to achieve a molar ratio of Na2ZnGeO4:CuCl:KCl = 1:1.30:0.671. The mixed powder was heated at 428 K for 48 h under vacuum (1.4 eV is observed in the spectrum in addition to weak absorption in the range 1.4 eV is attributable to direct band gap absorption. The weak absorption is possibly related to the electronic defects or indirect band gap absorption. Because the dispersion of the valence band is small in β-CuGaO2, β-CuAlO2, and γ-CuAlO2, in which tetrahedrally coordinated Cu+ ions are contained, the energies of direct and indirect band gaps in these materials are very close to each other.30 In Cu2ZnGeO4, because the valence band in Cu2ZnGeO4 should consist mainly of Cu 3d states similar to β-CuGaO2, β-CuAlO2, and γ-CuAlO2, the small valence band dispersion is highly expected in Cu2ZnGeO4. This implies that the energy of direct and indirect band gaps should be very close each other, even when Cu2ZnGeO4 is an indirect band gap semiconductor. Consequently, the weak absorption observed in the range
