Article pubs.acs.org/cm
Fundamental Semiconducting Properties of Perovskite Oxynitride SrNbO2N: Epitaxial Growth and Characterization Ryosuke Kikuchi,* Toru Nakamura, Satoru Tamura, Yasushi Kaneko, and Kazuhito Hato Advanced Research Division, Panasonic Corporation, Osaka 570-8501, Japan S Supporting Information *
ABSTRACT: We carried out theoretical calculations and demonstrated epitaxial growth of SrNbO2N by RF reactive sputtering. The SrNbO2N (001) epitaxial film on a SrTiO3 (001) substrate had a single orientation and was of high crystalline quality. The film’s band gap was experimentally determined as being 1.81 eV using a spectroscopic ellipsometer. Its transition type was indirect. In Hall effect measurements, the SrNbO2N film showed low Hall mobility despite our theoretical calculations showing the electron effective mass to be relatively low. Arrhenius plots of the Hall mobility and carrier concentration suggested the low mobility to result from conduction band bending due to the presence of grain boundaries.
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INTRODUCTION Photoelectrochemical water splitting by solar light is receiving much attention due to the increasing demand for hydrogen production methods that do not emit carbon dioxide. A number of studies have focused on semiconductors that have potential for use as solar absorbers for water splitting.1 Studies of numerous novel semiconductors, such as transition metal oxide materials, continue to explore earth-abundant, nontoxic, low-cost materials for use as solar absorbers.2,3 However, the solar-to-hydrogen (STH) efficiencies achieved by employing these materials have been lower than those employing typical high-purity semiconductors such as III−V group materials.1,4 To address higher STH efficiency using these novel materials, efforts to understand their basic properties such as carrier dynamics5−8 and to improve their performance therefore continue.9,10 Oxynitride materials such as baddeleyite-type oxynitrides (TaON, NbON) 11,12 and perovskite-type oxynitrides (AMO2N: A = La, Ca, Sr, Ba, M = Ti, Ta, Nb)13−17 are increasingly promising candidates as photoanodes for solar water splitting using visible light. Perovskite-type oxynitrides have especially suitable band gaps (1.7−2.3 eV)14,18,19 for use as the top cell of a tandem structure,20,21 and there are some reports on their photocatalytic performance as powder-based photoanodes.22−27 Furthermore, perovskite oxynitrides have attracted much attention due to their ferroelectricity by anionordering,28−30 prompting many fundamental studies that utilize first-principle calculations31−33 and epitaxial growth methods to obtain a high crystalline quality film with a single orientation29,30,34,35 and powder or bulk growth methods.13,15,18,28,36 First-principles calculations of perovskite-type oxynitrides have recently been carried out to address hydrogen © 2017 American Chemical Society
production and CO2 reduction, taking into account crystal structures, chemical compositions, and anion-ordering.16,33,35−41 However, there have been few relevant studies of epitaxial growth of SrNbO2N combined with first-principles calculations based on hybrid functionals, which focus on their optical and electrical properties. For efficient conversion of photons to current, and design of these solar absorbers’ thickness, an understanding is needed of these properties and the effects of lattice defects on carrier dynamics, especially carrier mobility and lifetime. In this study, the optical properties and electron and hole effective masses of SrNbO2N are calculated using a firstprinciples approach. We demonstrate the epitaxial growth of SrNbO2N thin films and the characterization of their semiconducting properties, and discuss the comparison between theoretical and experimental results. Finally, the electrical transport property is evaluated using Hall effect measurements, and the relationship between mobility and grain boundaries is discussed.
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COMPUTATIONAL PROCEDURES
First-principles calculations were conducted using the projector augmented-wave (PAW) method42 and the Perdew−Burke−Ernzerhof generalized gradient approximation (PBE-GGA) functional43 or the Heyd−Scuseria−Ernzerhof (HSE06) hybrid functional44,45 as implemented in the Vienna Ab initio Simulation Package (VASP).46,47 The PAW data sets with radial cutoffs of 2.1, 1.5, 0.8, and 0.7 Å for Sr, Nb, O, and N, respectively, were used with a plane-wave cutoff energy of 400 eV. Five structures were prepared of completely anion-ordered, Received: March 31, 2017 Revised: September 1, 2017 Published: September 15, 2017 7697
DOI: 10.1021/acs.chemmater.7b01320 Chem. Mater. 2017, 29, 7697−7703
Article
Chemistry of Materials
Table 1. Calculated Band Gaps, Absorption Onsets, DOS Effective Masses, and Static Dielectric Constants of SrNbO2Na structure
space group
ΔH [kJ/mol]
transition type
Eg [eV]
Eabs [eV]
ΔE [eV]
me*/m0
mh*/m0
εr
trans-A trans-B trans-C cis-A cis-B
I4/mcm Fmmm P42/mcm Ima2 C2
+12.57 +18.10 +16.45 0 +1.20
indirect direct direct indirect indirect
1.35 1.23 1.34 1.79 1.48
1.60 1.39 1.52 2.30 1.53
0.24 0.16 0.18 0.51 0.06
0.54 0.40 0.58 0.20 0.85
0.30 0.34 0.75 2.25 1.07
580 184 29.8 51.0 49.3
The space group after geometry optimization, the relative formation enthalpy ΔH to the most stable structure, the transition type, the band gap Eg, the absorption onset Eabs (defined as energy at which absorption coefficient reaches 5 × 103 cm−1), the difference ΔE between Eg and Eabs, electron and hole DOS effective masses, and the static dielectric constants εr are given.
a
2000FI). The refractive index n and the extinction coefficient k were obtained by model fitting. The Gaussian model and the Tauc−Lorentz model were applied to Nb-doped SrTiO3 substrate and SrNbO2N film, respectively. The chemical compositions were determined by Rutherford backscattering spectrometry (RBS, National Electrostatics, Pelletron 3SDH). The irradiated ions were H+ and He2+. The O and N compositions were determined by RBS using H+, and the Sr and Nb compositions were determined by RBS using He2+. The carrier concentration and mobility of the SrNbO2N film were evaluated by Hall effect measurements (Toyo, ResiTest8400). The Hall effect measurements were carried out using the Van der Pauw method. The Miller index was determined by regarding the crystal structure of the SrNbO2N epitaxial film as a tetragonal-perovskite structure. For example, the lattice constants of the a-axis and c-axis refer to in-plane and out-of-plane, respectively. The SrNbO2N films on undoped SrTiO3 substrates were used for RBS, Hall effect measurements, TEM, and HAADF-STEM, and the SrNbO2N films on Nb-doped SrTiO3 substrates were used for XRD, AFM, and spectroscopic ellipsometry.
trans-type and cis-type, based on the crystal structures of ICSD no. 55395.13 The PBE + U calculations were used for geometry optimization and calculation of static dielectric constants. An on-site potential, U = 3 eV, was applied to Nb d-states.48 This U potential was tested by another Nb compound, suggesting that the PBE + U with U = 3 eV gives reasonable lattice constants (see the Supporting Information). The atomic positions were optimized until the residual force acting on all atomic coordinates fell below 0.01 eV Å−1. Electronic density of states (DOS) and complex dielectric functions neglecting excitonic effects were calculated using the HSE06 functional. Optical absorption spectra were obtained from the dielectric functions. The effective masses of electrons and holes were calculated by the calculated density of states using the semiconductor equation:49 −3/2 ⎛ m * ⎞3/2 1 ⎛ 2πm0kBT ⎞ ⎟ ⎜ i ⎟ = ⎜ 2 ⎠ 2⎝ h ⎝ m0 ⎠
∫ Z(E)f (E) dE
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with i = e, h. m0 is the free-electron mass, kB is the Boltzmann constant, T is temperature, h is the Planck constant, Z(E) is the total density of states, and f(E) is the Fermi distribution function. The band structures of five structures were calculated using PBE + U. The effective masses considering the direction were calculated from the band structures by parabolic curve fitting within a k-point range of ±0.05 Å−1. Band paths reported in ref 50 were used. These results were shown in the Supporting Information. Γ-Centered k-meshes were employed, and the k-points were sampled according to 12 × 12 × 12 except for transC, and a 10 × 10 × 8 k-mesh was employed for trans-C. Because spin− orbit coupling (SOC) has little noticeable effect on the band gap and the DOS effective masses in the present systems, our calculations were carried out excluding SOC. The comparison of cis-A’s band gaps and DOS effective masses either with or without SOC is shown in the Supporting Information.
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RESULTS AND DISCUSSION First-Principles Calculations. Calculated band gaps Eg and electron and hole effective masses are shown in Table 1. Absorption onset energy Eabs (defined as the energy at which the absorption coefficient reaches 5 × 103 cm−1) and the difference between Eg and Eabs, ΔE, are also described. The crystal structures, the lattice constants, and the atomic parameters after geometry optimization are shown in Figure S1 and Tables S1 and S2, respectively. The calculated absorption coefficient spectra are shown in Figure S3. The most stable structure is cis-A, whose band gap is close to the reported experimental value.19 The band gaps of other structures are smaller than cis-A’s band gap. Kubo et al. have reported the band gaps of various structures in CaTaO2N,33 noting that the structures with a wider band gap are stabler than those with a smaller one. Our results suggest that the various structures of SrNbO2N show the same tendency. The electron DOS effective masses are relatively low, at less than 1, as well as silicon.51 However, the hole DOS effective masses depend on the structures: cis-A’s hole mass is almost 7-fold that of trans-A’s hole mass. Figure 1 shows the partial density of states of the Nb d, O p, and N p states. The conduction band and valence band consist of the Nb d-orbital, and O and N p-orbitals, respectively. In trans-A, the slope of the N p-orbital that dominated near the valence band maximum is gentle. On the other hand, in cis-A, the slope of the N porbital is extremely abrupt. As a result, cis-A’s hole mass was larger than those of the other structures. Although all dNb−N and dNb−O of three trans-type structures are about 2 Å, the two cis-type structures have notably shorter dNb−N and larger asymmetry than those of the trans-type as shown in Table 2. The conduction band and valence band of SrNbO2N consist of the Nb−N antibonding orbital and the Nb−N bonding orbital, respectively. In the case of CaTaO2N,
EXPERIMENTAL METHODS
We grew SrNbO2N films on undoped SrTiO3 (001) and 0.5 wt % Nbdoped SrTiO3 (001) substrates (Shinkosha) by RF reactive sputtering in an ambient mixture of Ar, O2, and N2. Ar and N2 gas flows were constant, at 5 and 10 sccm, respectively. O2 gas flows were varied between 0 and 0.07 sccm for each sample to investigate the relationship between the carrier concentration and Hall mobility. The total pressure was 0.5 Pa during the growth. An RF power source was used, with the input power maintained at 20 W. Film deposition was carried out at 650 °C. A 50.8-mm diameter Sr2Nb2O7 target (Toshima Manufacturing) was used at 100 mm from the substrate. The crystalline quality of the SrNbO2N film was investigated by Xray diffraction (XRD) using a five-axis diffractometer (Rigaku, SmartLab). Its surface morphology was observed by atomic force microscopy (AFM) in tapping mode (Bruker, MultiMode 8). Transmission electron microscopy (TEM) was used to observe defects such as grain boundaries (Hitachi, H-9000UHR III), and highangle annular dark-field scanning transmission electron microscopy (HAADF-STEM) was used to obtain images of ordered atoms in SrNbO2N films (JEOL, JEM-ARM200F). The absorption spectrum of the SrNbO2N films was calculated from extinction coefficients evaluated by a spectroscopic ellipsometer (J. A. Woollam, M7698
DOI: 10.1021/acs.chemmater.7b01320 Chem. Mater. 2017, 29, 7697−7703
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Chemistry of Materials
Figure 1. Calculated partial density of states of trans-A, trans-B, transC, cis-A, and cis-B.
Figure 2. XRD results of a SrNbO2N film. (a) 2θ/ω scan profile under O2 gas flow of 0, 0.05, 0.10, and 0.20 sccm, and (b) φ scan profile under O2 gas flow of 0.05 sccm while the 2θ and ω values were fixed to a 103 reflection of SrNbO2N.
Kubo et al. have reported that the structures that have the three-dimensional ordering of nitrogen ions have lower VBM positions because the larger Ta−N d−pπ interactions induce the stabilization.33 In this study, cis-A and cis-B have the threedimensional ordering of nitrogen ions. We assume that the three-dimensional ordering of nitrogen ions makes the VBM positions of the cis-type structures lower than those of transtype structures because of the larger Nb−N d−pπ interactions. Thus, the cis-type structures’ band gaps are wider than those of the trans-type structures. The static dielectric constants of trans-A and trans-B are larger than for the other structures. The N−Nb−N ordering of trans-A and trans-B lies in same direction, although the other structures have Nb−N bonds in the distributed directions. The direction of the Nb−N bond affects the static dielectric constants. Crystallographic Characterization. Figure 2a shows a 2θ/ω scan profile of a SrNbO2N film on a SrTiO3 (001) substrate with O2 gas flow of 0, 0.05, 0.10, and 0.20 sccm during the growth. In all samples, both SrTiO3 and SrNbO2N peaks at 001, 002, and 003 reflections were clearly observed. The SrNbO2N films were epitaxially grown on SrTiO3 (001) substrates. Figure 2b shows a ϕ scan profile in the sample with O2 gas flow of 0.05 sccm, while the 2θ and ω values were fixed to a 103 reflection of SrNbO2N. Four peaks can also be clearly
observed at 360° around the ϕ axis. These results show that the SrNbO2N epitaxial film had a single orientation. The four peaks in the ϕ scan had almost identical intensities, indicating that the epitaxial film has a tetragonal structure. We also comfirmed that all of the samples with O2 gas flow of 0−0.20 sccm have a single orientation. Lattice constants were calculated from 2θ values of 002 and 103 reflections. Figure 3 shows the calculated lattice constants and volumes. The lattice constants in the sample with 0.05 sccm of O2 gas flow were a = 4.02 Å, c = 4.09 Å, and c/a = 1.02. They are consistently close to the values previous reported by Kim et al. (a = 4.04 Å, c = 4.05 Å, and c/a = 1.00).13 The c/a value in our study is greater than that in the previous study. The calculated lattice constants of cis-A are shown as dotted lines in Figure 3. The calculated values are almost the same as the previously reported values. The experimental value of the volume with 0.05 sccm of O2 gas flow corresponds with the volume of cis-A. We assumed the sample with O2 gas flow of 0.05 sccm to be stoichiometric. On increasing the O2 gas flow, the lattice constant of the a-axis decreased and that of the c-axis increased. The lattice constants of SrNbO3 (001) epitaxial film coherently grown on KTaO3 (001) were reported to be a = 3.99 Å, c = 4.07 Å, and c/a = 1.02.52 We anticipated that the epitaxial film would be
Table 2. Bond Lengths of Nb−N and Nb−O, and Bond Angle of N−Nb−N in Calculated Structures trans-A
trans-B
trans-C
cis-A
cis-B
dNb−N [Å]
2.060 (×2)
2.042 (×2)
2.039 (×2)
dNb−O [Å]
2.047 (×4)
2.043 (×2) 2.046 (×2)
2.040 (×2) 2.047 (×2)
1.906 2.074 2.037 2.039 (×2) 2.223
180
180
180
1.982 1.990 2.019 2.072 2.116 2.128 92.94
∠N−Nb−N [deg]
7699
93.34
DOI: 10.1021/acs.chemmater.7b01320 Chem. Mater. 2017, 29, 7697−7703
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Chemistry of Materials
indicating that the SrNbO2N growth by sputtering was a three-dimensional mode. The roughness had a root-meansquare value of 1.7 nm, indicating that the epitaxial film has a relatively flat surface. Figure 4b shows a plan-view TEM image of SrNbO2N film, and the inset shows the electron diffraction pattern obtained from the area including several grains in the SrNbO2N film. The electron diffraction indicates that the SrNbO2N film was epitaxially grown on a (001) plane, and highly oriented to the in-plane direction, corresponding to the XRD results previously discussed. The grain boundaries were also observed in the plan-view TEM image. We assumed that the grain boundaries were generated when the 3D islands coalesced with wrong bonding during the growth. They extended along the ⟨100⟩ direction according to the electron diffraction. Consequently, square domains were formed by ⟨100⟩ grain boundaries. It is possible that the grain boundaries were generated by a large lattice mismatch (3.4−3.8%) or poor wettability between SrNbO2N and SrTiO3. Figure 5a shows a cross-sectional HAADF-STEM image of the SrNbO2N film on SrTiO3 substrate in the [110] zone-axis.
Figure 3. Obtained lattice constants from XRD results under O2 gas flow of 0, 0.05, 0.10, and 0.20 sccm. Lattice constants of cis-A are also shown.
compressively stressed by the substrate with increasing oxygen composition. The full widths at half-maximum (FWHMs) of the X-ray rocking curves at the 002 and 103 reflections are 0.15° and 0.49° in the sample with 0.05 sccm of O2 gas flow. The other samples have also small FWHM values, and these small FWHM values reveal that the epitaxial film has high crystalline quality. We observed the surface morphology of the SrNbO2N film to evaluate the growth mode. Figure 4a shows an AFM image of the SrNbO2N film with O2 gas flow of 0.05 sccm during growth. Closely arranged square islands were observed,
Figure 5. (a) Cross-sectional HAADF-STEM image and (b) highresolution STEM image of a SrNbO2N film with O2 gas flow of 0.05 sccm.
The arrows indicate the grain boundaries. The grain boundaries thread from the interface between the film and substrate to the surface. The sputtering method used in this study leads to columnar growth of the SrNbO2N film. The cation-ordering was observed using the high-resolution HAADF-STEM, as shown in Figure 5b. Regularly ordered bright spots were observed, indicating Sr and Nb atoms. As shown in the enlarged figure (the inset of Figure 5b), there are stacks of brighter spots and darker spots. The HAADF-STEM image represents the Z-contrast. Thus, the brighter and darker spots are, respectively, Nb and Sr atoms. At the interface between the film and substrate, the STEM image was blurred and distorted, indicating that lattice defects were caused by a lattice mismatch. We confirmed that the cations are regularly ordered in a perovskite structure, and that the SrNbO2N film has a single orientation.
Figure 4. (a) Surface morphology observed by AFM and (b) plan-view TEM image on the [001] zone axis (an electron diffraction image is shown in the inset) of a SrNbO2N film with O2 gas flow of 0.05 sccm. 7700
DOI: 10.1021/acs.chemmater.7b01320 Chem. Mater. 2017, 29, 7697−7703
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Chemistry of Materials Optical Properties. The absorption coefficient spectrum of the SrNbO2N epitaxial film with O2 gas flow of 0.05 sccm during growth is shown in Figure 6 (measured Ψ, Δ, and fitting
Figure 6. Absorption coefficient spectrum of a SrNbO2N film with O2 gas flow of 0.05 sccm. The inset shows a Tauc plot of (αhν)1/2 versus hν. Figure 7. (a) Atomic ratios, Nb/Sr, O/Sr, and N/Sr by RBS in samples grown under various O2 gas flows (0−0.07 sccm). (b) Relationships between O2 gas flow during growth and carrier concentration, or Hall mobility in 100 nm-thick SrNbO2N films. (c) Relationship between carrier concentration and Hall mobility in samples grown under various O2 gas flows (0−0.07 sccm) at room temperature. (d) Arrhenius plots of the carrier concentration and Hall mobility in samples grown under O2 gas flow of 0.05 sccm.
spectra, and obtained refractive index and extinction coefficient in the Suppoting Information). A Tauc plot was drawn to determine the band gap of SrNbO2N as shown in the inset of Figure 6. Plotting (αhν)1/2 versus hν gave a straight-line segment, indicating that SrNbO2N film has an indirect transition characteristic. The band gap was determined to be 1.81 eV, which is consistent with that obtained in previous studies.19 Our study is the first to determine the transition type experimentally. The absorption coefficient reaches 5 × 103 cm−1 at 2.25 eV, revealing that ΔE = 0.44 eV. The absorption rises as fast as that of gallium phosphide (GaP),53,54 which is a typical indirect-band gap semiconductor. The Eg and ΔE values obtained in this experiment correspond to the calculated values of the most stable structure, cis-A. Carrier Transport Properties. Figure 7a shows the atomic ratios, Nb/Sr, O/Sr, and N/Sr, measured by RBS. Under O2 gas flows of 0−0.07 sccm, the SrNbO 2 N films had stoichiometric compositions. We evaluated the electrical transport properties of the SrNbO2N film using Hall effect measurements at room temperature. Under an O2 gas flow of 0 sccm, the SrNbO2N film had the highest carrier concentration of over 1017 cm−3, as shown in Figure 7b. Between 0.03 and 0.07 sccm, the films had small measured carrier concentrations. These results indicate that O vacancies or N occupied at O sites act as electron sources. Figure 7c shows the relationship between carrier concentration and Hall mobility in samples grown under various O2 gas flows (0−0.07 sccm). The films show n-type conductivity. As the carrier concentration increased, the Hall mobility also increased. Typically, at room temperature, mobility decreases due to a scattering caused by defects related to donors or acceptors when the carrier concentration increases. Moreover, the films show low motilities of less than 0.1 cm2 v−1 s−1, despite the small electron mass calculated in this study. We assumed the relationship between carrier concentration and Hall mobility in this study to be due to the grain boundaries threading the epitaxial film. Other studies in polycrystalline film have shown that depletion layers widen from the grain boundaries due to the charge trapped in the boundaries.55,56 As the carrier concentration decreases, the depletion layers widen. Thus,
electron transport is prevented by band bending caused by the presence of grain boundaries. To confirm the effect of grain boundaries, we measured the temperature dependences of Hall mobility and carrier concentration using a sample with O2 gas flow of 0 and 0.05 sccm between 300 and 460 K. Figure 7d shows Arrhenius plots of the carrier concentration and Hall mobility. All Arrhenius plots show a straight line segment. The slopes of the Hall mobilities were calculated at 0.06 eV (O2: 0 sccm) and 0.10 eV (O2: 0.05 sccm). The slopes of the carrier concentrations were also calculated at 0.24 eV (O2: 0 sccm) and 0.33 eV (O2: 0.05 sccm). With the widening of depletion layers and the bending of conduction band inside the grains, the slope of the mobility represents the barrier height of the band bending, and the slope of the measured carrier concentration represents the difference between the trap level on the grain boundary and the level of the equilibrium conduction band minimum.56 We therefore assumed that grain boundaries form a trap level that is 0.24− 0.33 eV below the conduction band minimum. Electrons enter under the trap level, and the grain boundaries take on negative charge. Finally, a depletion layer spreads and the conduction band bends, with a barrier height of 0.06−0.10 eV, from the grain boundary. Although Hall effect measurements cannot reveal the valence band state in n-type semiconductors, there is a possibility that grain boundaries represent a serious problem for photoexcited holes when SrNbO2N is used as a photoelectrode. If the valence band bends along the conduction band near the grain boundaries, photoexcited holes will drift toward the grain boundaries in the depletion layers during irradiation with solar light. To achieve a high incident photon to current conversion efficiency using a SrNbO2N photoelectrode, decreasing the 7701
DOI: 10.1021/acs.chemmater.7b01320 Chem. Mater. 2017, 29, 7697−7703
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density of the grain boundaries, in other words, increasing the grain size, is likely to be needed.
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CONCLUSION We demonstrated epitaxial growth of SrNbO2N and carried out theoretical calculations on it. The SrNbO2N (001) epitaxial film on a SrTiO3 (001) substrate had a single orientation and a high crystalline quality. Measuring the epitaxial film showed the band gap to be 1.81 eV and the transition type to have an indirect characteristic. This is the first report to experimentally determine indirect transition. The band gap and absorption onset correspond to the theoretical calculation of the most stable structure. We also conclude, after applying Hall effect measurements, that the grain boundaries prevent electron transport. The Arrhenius plots of the mobility and measured carrier concentration clearly show that the grain boundaries form a trap level 0.24−0.33 eV below the conduction band minimum, resulting in bending of the conduction band and the spread of depletion layers from the grain boundaries.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b01320. Calculated crystal structures (Figure S1), calculated lattice parameters (Table S1), calculated atomic parameters (Table S2), calculated lattice parameters after geometry optimization using PBE + U with U = 0, 1, 3, and 5 eV and experimental values of SnNb2O6 (Table S3), calculated band structures using PBE + U (Figure S2), calculated effective masses along each directions using PBE + U (Table S4), comparison with or without spin−orbit coupling in cis-A using PBE + U (Table S5), calculated optical absorption spectra (Figure S3), and measured Ψ, Δ, and fitting spectra by a spectroscopic ellipsometer and obtained refractive index and extinction coefficient of a SrNbO2N film (Figure S4) (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Ryosuke Kikuchi: 0000-0002-6058-2917 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Advanced Research Program for Energy and Environmental Technologies of the New Energy and Industrial Technology Development Organization (NEDO) of Japan.
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REFERENCES
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DOI: 10.1021/acs.chemmater.7b01320 Chem. Mater. 2017, 29, 7697−7703
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DOI: 10.1021/acs.chemmater.7b01320 Chem. Mater. 2017, 29, 7697−7703
