Article Cite This: Chem. Mater. 2017, 29, 9378-9385
pubs.acs.org/cm
Systematic Band Gap Tuning of BaSnO3 via Chemical Substitutions: The Role of Clustering in Mixed-Valence Perovskites Seunghun Lee,†,‡ Haihang Wang,§ Priya Gopal,∥,⊥ Jongmoon Shin,‡ H. M. Iftekhar Jaim,†,‡ Xiaohang Zhang,†,‡ Se-Young Jeong,# Demet Usanmaz,⊥,@ Stefano Curtarolo,⊥,@ Marco Fornari,∥,⊥ Marco Buongiorno Nardelli,§,⊥ and Ichiro Takeuchi*,†,‡ †
Center for Nanophysics and Advanced Materials, Department of Physics, University of Maryland, College Park, Maryland 20742, United States ‡ Department of Materials Science and Engineering, University of Maryland, College Park, Maryland 20742, United States § Department of Physics, University of North Texas, Denton, Texas 76203, United States ∥ Department of Physics and Science of Advanced Materials Program, Central Michigan University, Mt. Pleasant, Michigan 48859, United States ⊥ Center for Materials Genomics, Duke University, Durham, North Carolina 27708, United States # Department of Cogno-Mechatronics Engineering, Department of Optics and Mechatronics Engineering, Pusan National University, Busan 46241, Republic of Korea @ Department of Mechanical Engineering and Materials Science, Duke University, Durham, North Carolina 27708, United States S Supporting Information *
ABSTRACT: By combining high-throughput experiments and firstprinciples calculations based on the DFT-ACBN0 approach, we have investigated the energy band gap of Sr-, Pb-, and Bi-substituted BaSnO3 over wide concentration ranges. We show that the band gap energy can be tuned from 3 to 4 eV by chemical substitution. Our work indicates the importance of considering the mixed-valence nature and clustering effects upon substitution of BaSnO3 with Pb and Bi. Starting from the band gap of ∼3.4 eV for pure BaSnO3, we find that Pb substitution changes the gap in a nonmonotonic fashion, reducing it by as much as 0.3 eV. Bi substitution provides a monotonic reduction but introduces electronic states into the energy gap due to Bi clustering. Our findings provide new insight into the ubiquitous phenomena of chemical substitutions in perovskite semiconductors with mixed-valence cations that underpin their physical properties.
1. INTRODUCTION Attention has been focused on BaSnO3 as a versatile functional perovskite material with potentials for a range of applications, including transparent conductors,1,2 dye-sensitized solar cells,3 infrared (IR) luminescent materials,4 and room-temperature magnetic semiconductors.5 The electronic structure, especially the energy band gap, governs most of the properties of this wide band gap semiconductor (EG = 3.1−3.5 eV).6−9 The physical properties of BaSnO3 can be tailored in various ways,2,7,10,11 especially by chemical substitutions that can introduce new charge carriers and magnetic effects and modify the general features of the band structure.12,13 The substitutions explored for BaSnO3 can be classified in terms of valence or crystallographic sites (replacing Ba, the A site, or replacing Sn, the B site). In earlier studies, Sb-substituted BaSnO3 has been investigated14,15 in analogy to perovskite superconductors such as BaPb1−xBixO316 and BaBi1−xKxO3 systems.17 Although superconductivity has not been found in the BaSnO3 system so far, these studies have spurred interest in the electronic structure of the BaSnO3 system, and it has led to a diverse © 2017 American Chemical Society
range of research. For instance, La replaces Ba on the A site (SnSn + BaBa → SnSn + LaBa• + e′), turning it into an n-doped transparent conductor.9 Sb is also a possible n-type dopant for BaSnO3 via substitution into the B site, but the presence of mixed-valence Sb (i.e., Sb3+ and Sb5+) has been suggested to cause poor electrical conductivity.18 Fe replaces Sn on the B site (SnSn + BaBa → FeSn′ + BaBa + h•), leading to magnetism and a p-type characteristic.19 Sr is an isovalent A-site substitution in BaSnO3 that has been shown to result in photocatalytic activities.20 Recently, stable p-type BaSnO3 via K doping (SnSn + BaBa → SnSn + KBa′ + h•) and pn junctions with n-type Ladoped BaSnO3 have been experimentally demonstrated,21 and the theoretical band structure of K-doped BaSnO3 has been discussed.22 Effects associated with intrinsic defects (including vacancies and dislocation), strain, and dimensionality reduction have also been investigated.2,10,23 Received: August 9, 2017 Revised: October 19, 2017 Published: October 23, 2017 9378
DOI: 10.1021/acs.chemmater.7b03381 Chem. Mater. 2017, 29, 9378−9385
Article
Chemistry of Materials
Figure 1. Summary of key experimental results of chemically substituted BaSnO3. (a) Lattice constant and (b) band gap of chemically modified BaSnO3 for different substitution elements and their concentrations. Each set of data for the same element is taken from one composition spread. symmetry (c/a = 1 within 1%), and strains associated with the lattice mismatch with the substrate (approximately 7−8%) are fully relaxed, as revealed by reciprocal space mapping (not shown). The lattice parameters are calculated from the position of the BaSnO3 (002) peak in the θ−2θ scan of X-ray diffraction (XRD) patterns. More detailed information can be found in the Supporting Information. The lattice parameter changes linearly with substitution concentration (see Figure 1a), and its variation can be roughly explained by the difference in the ionic radius of each substituting ion (Sr2+, Pb4+, Bi5+, and Bi3+) and the substituted ion (i.e., Ba2+ and Sn4+) based on the sample fabrication process. On the basis of effective ionic radii determined by Shannon,31 the ionic radius of XIISr2+ is 144 pm, which is smaller than that of XIIBa2+ (161 pm), consistent with the decreased lattice parameter caused by substitution. The fact that the ionic radius of VIPb4+ (77.5 pm) is larger than that of VISn4+ (69 pm) is likely to explain the increase in the lattice parameter caused by Pb substitution. However, we will show later that band gap alteration and theoretical picture evidence the cosubstitution of Pb in both A and B sites of BaSnO3, which is indicative of the possibility for a mixed-valence composition (type I, according to Robin and Day32). The fact that the ionic radius of XII Pb2+ (149 pm) is smaller than that of XIIBa2+ cannot explain the increase in the lattice expansion. For B-site Bi-substituted BaSnO3, the possible stable oxidation states are 3+ and 5+.33,34 Both Bi3+ and Bi5+ have ionic radii (the ionic radii of VIBi3+ and VIBi5+ are 103 and 76 pm, respectively) larger than that of VISn4+, and thus, the increase in the lattice constant with an increase in Bi concentration appears to be consistent. Bi-substituted BaSnO3 can be classified as a type II mixed-valence system similar to its parent compound BaBiO3.32 Figure 1b shows the measured band gap of chemically substituted BaSnO3 for the different elements and their concentrations. The optical band gap was determined by Tauc fitting of the absorption spectra measured by an ultraviolet−visible (UV−vis) spectrometer.35 The optical band gap of Sr-substituted BaSnO3 has been previously reported, and we use their values as a reference for checking the validity of our study.8 To the best of our knowledge, the optical properties of Pb- and Bi-substituted BaSnO3 have not been reported to date. As shown in Figure 1b, the band gap of Pb-substituted BaSnO3 decreases at first, and then at x = 20 mol %, it starts to increase in with an increase in Pb concentration. The band gap of Bi-substituted BaSnO3 monotonically decreases with an increase in Bi concentration to ≤40 mol %. In simple terms, the change in band gap can be understood by interpolating the ratio of the bulk systems used to form the alloys. For example, the optical band gap of BaSnO3 gradually increases with an increase in Sr concentration, which is consistent with the fact that the band gap of SrSnO3 is 3.93 eV, 15% larger than that of the observed value of BaSnO3.8 The band gap of Sr-substituted BaSnO3 is also consistent with a previous report. BaPbO3 in the cubic phase was found to be metallic in our calculations and others,36 and BaBiO3 has a
The electronic structure of BaSnO3 is characterized by a charge transfer energy gap between an O-p occupied manifold and Sn-s at the bottom of the unoccupied manifold. The Ba empty states also contribute to the conduction bands, albeit at higher energies. Understanding its band structure is critical for pursuing transparent conductivity because it exhibits a very dispersive isolated s band that, after n-doping, becomes active in transport. Indeed, (Ba,La)SnO3 displays excellent conductivity and optical transparency, and it is a great candidate for transparent conducting electrodes.9,23−25 Thus, it is of great interest to pursue engineering of the electronic structure of BaSnO3 and to explore novel functionalities and opportunities for applications in other devices. Few factors are relevant when investigating the effects of chemical substitutions. A range of concentration must be taken into account to explore a sufficiently large configuration space, and the structural distortions as well as the site occupancy must be carefully analyzed. Additionally, theoretical methods that can capture local effects must be employed to accommodate the possibility of symmetry breaking associated with different ionization states for the same chemical species. Here, we demonstrate systematic tuning of the optical band gap in chemically modified BaSnO3 by combining highthroughput (HT) experiments and HT informed theoretical calculations based on the ACBN0 (Agapito, Curtarolo, and Buongiorno Nardelli) approach.26,27 We focus on Sr, Pb, and Bi substitution with the goals of clarifying the role of different ionization states that are possible for Pb (+2 and +4) and Bi (+3 and +5) and identifying the effects of the mixed valence on clustering of impurities. This work illustrates design principles that can be applied to other materials systems.
2. EXPERIMENTAL METHODS Sr-, Pb-, and Bi-substituted BaSnO3 thin films were deposited on LaAlO3 (LAO) (100) substrates using a combinatorial pulsed laser deposition system equipped with automated shadow masks and a multitarget carousel that enables fabrication of pseudobinary linear gradient composition films with epitaxial quality.28−30 The designs of the spreads were such that one end of the concentration range is always pure BaSnO3 and the other end is pure SrSnO3, BaPbO3, or BaBiO3. Each spread film is typically 8 mm long in the composition gradient direction, and the film thickness across the spread is kept constant at 100 nm. Details of sample fabrication procedures are described in the Supporting Information. The composition mapping of the spread films (i.e., substitution element concentration) is performed using wavelength dispersive X-ray spectroscopy (WDS) with a JEOL 8900 electron microprobe. All films were found to have pseudocubic 9379
DOI: 10.1021/acs.chemmater.7b03381 Chem. Mater. 2017, 29, 9378−9385
Article
Chemistry of Materials
Figure 2. Experimental and theoretical values of (a) the lattice constant and (b) the band gap of Sr-substituted BaSnO3. (c) Theoretical absorption spectra of Sr-substituted BaSnO3 as a function of Sr concentration. (d) Experimental (αhν)2 plots of Sr-substituted BaSnO3 as a function of photon energy (E), which were used to determine the band gap (Tauc plot) (units of mole percent). very small gap;37−40 it can thus be expected that the band gap of BaSnO3 decreases with Pb or Bi substitution. However, the observed degree of reduction in the band gap caused by Bi substitution is rather small, and the nonmonotonic response of the band gap to Pb concentration cannot be understood with such a simple picture. Thus, we turn to computational work to look for explanations of the observed experimental results.
3. COMPUTATIONAL METHODS
While norm-conserving (NC) pseudopotentials were used initially to determine the effective Hubbard U for the relevant elements using ACBN0, all other calculations using the AFLOWπ43 package were performed with ultrasoft pseudopotentials from the GBRV library.44 A kinetic energy cutoff of 60 Ry and a k-point mesh of 3 × 3 × 3 were used for the supercell calculations. We computed the imaginary part and the real part of the dielectric function following the procedure described by D’Amico et al.45 and derived the absorption coefficient, α(ω).
First-principles calculations based on density functional theory (DFT) were used to obtain an atomistic insight into the nature of the observed changes in the band gap of BaSnO3. We performed generalized gradient approximation (GGA) within the ACBN0 approach that corrects the electronic structure with a local Hubbard U computed self-consistently for all the inequivalent atoms in the unit cell.27 ACBN0 has previously been shown to improve the description of the structural and electronic properties in a range of complex materials from Zn/Cd-based chalcogenides to transition metal oxides.41,42 In all cases, a 3 × 3 × 3 (135-atom) supercell was used to optimize the geometry and compute the electronic structure and the optical absorption within an independent particle approximation. The choice of a 3 × 3 × 3 supercell forbids octahedral rotations and tiltings. Calculations with a 2 × 2 × 2 supercell were used to confirm that such distortions are not relevant when the Ba concentration remains above 50% for A-site substitution using either Sr2+ or Pb2+. For the alloyed systems, 1, 4, 8, and 12 Ba or Sn atoms were substituted to model the 3.7, 14.8, 29.6, and 40% alloy concentrations within the 135-atom supercell, respectively. Several chemical orderings were investigated, which led to both clustered and uniform alloy models (see Figure 4 for a conceptual illustration of the different orderings).
4. RESULTS AND DISCUSSION Panels a and b of Figure 2 show the lattice constant and the band gap of Sr-substituted BaSnO3, respectively, each comparing the experimental and theoretical results. As discussed in section 2, the lattice constant of Sr-substituted BaSnO3 gradually decreases with an increase in Sr concentration. Deviations between theory and experiment are naturally more relevant at high concentrations because of the possible existence of ordering that we did not include. For Sr, which is an isovalent A-site substitution (SnSn + BaBa → SnSn + SrBaX), the theoretical band gap values are within 10% of experimental measurements, and the overall trend is well-reproduced by our calculations. Panels c and d of Figure 2 show the theoretical absorption spectra of Sr-substituted BaSnO3 and the plots of experimentally obtained (αhν)2 (i.e., Tauc plot) as a function of photon energy, respectively. The absorption edge monotonically shifts to a higher energy with an increase in Sr concentration, indicating that there is a continuous increase in the band gap. This effect is associated with the volume 9380
DOI: 10.1021/acs.chemmater.7b03381 Chem. Mater. 2017, 29, 9378−9385
Article
Chemistry of Materials
Figure 3. Experimental and theoretical values of (a) the lattice constant and (b) the band gap of Pb-substituted BaSnO3. (c and d) Theoretical absorption spectra of Pb-substituted BaSnO3 as a function of Pb concentration for A-site and B-site substitution, respectively. (e) Experimental (αhν)2 plots of Pb-substituted BaSnO3 as a function of photon energy (E) (units of mole percent).
hybridization, resulting in the observed behavior. Alternatively, Pb4+ can replace Sn4+, and the lattice parameter can substantially increase; however, the band gap dramatically decreases to ∼50% for a Pb concentration of 40 mol %. Hypothetical cubic BaPbO3 is metallic because of the increased degree of overlap between the wave functions of anions and cations, and structural relaxations reduce such an overlap and lead to a band gap of 1.8 eV. This provides a reasonable justification for the trend observed in Figure 3b. We devise a scenario in which Pb enters both the A site and the B site, leading to a mixed-valence system with both Pb2+ and Pb4+ oxidation states. In this scenario, the major contribution to the lattice variation will be associated with the B-site substitution because the consequence of A-site replacement on the lattice parameter is expected to be minimal. The band gap, however, is affected by both B-site and A-site substitutions (up to the respective solubility limits). Figure 3b indicates a transition point at a Pb concentration of 20 mol %: up to 20 mol %, the band gap decreases, and above 20 mol %, it increases. As shown in Figure 3c, the theoretical absorption band edge shifts to a lower energy and then moves to a higher energy in the case of A-site substitution. On the other hand, the theoretical absorption band edge for B-site substitution shifts to the lower energy continuously (Figure 3d). The trend shown in the case of A-site substitution is consistent with the experimental observation shown in Figure 3e. The experimental evidence provided by the lattice parameter and band gap trends supports the idea that the mixed-valence state of Pb exists in Pb-substituted BaSnO3. Following the work of Robin and Day,32 we classify BaSnO3:Pb as a type I mixed-valence system. We have attempted to directly observe the presence of the mixed-valence state of Pb by employing X-ray photoelectron
change and the local disorder due to the reduction of the size of the A-site ion, which in turn affects the size of the octahedra46 and, consequently, increases the level of hybridization. Nonetheless, the bottom of the conduction manifold minimum in Sr-substituted BaSnO3 is likely to conserve the Sn-s dispersive character in the calculated projected density of states plots (Figure S5). Pb-substituted BaSnO3 has a much richer phenomenology because of the possibility of both A-site and B-site substitutions.47 As mentioned above, Figure 3a seems to indicate a better agreement with the experimental XRD data when Pb replaces Sn. However, the trend in the band gap (Figure 3b) tends to support the opposite effect, namely A-site substitution. Theoretical calculations of the energetics for A-site or B-site occupancy provide very similar results and do not discriminate between the two cases. It is worthwhile to note that the Ba−Pb−O system has many possible stable compounds, including Ba2PbO4 and Ba2PbO6, and Pb ions may also have a stable valence state of 2+.48,49 It has been reported that the oxygen vacancies can enable the partial reduction of Pb4+ to Pb2+ and the change in the Pb−O bonding angle, which is responsible for the semiconducting behavior of BaPbO3.50,51 In one possible theoretical scenario, Pb2+ enters the A site, which leads to a situation similar to what is observed with Sr: a small decrease in the lattice constant (
