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Controlling n‑Type Doping in MoO3 H. Peelaers,* M. L. Chabinyc, and C. G. Van de Walle Materials Department, University of California, Santa Barbara, California 93106-5050, United States ABSTRACT: We study the electronic properties of native defects and intentional dopant impurities in MoO3, a widely used transparent conductor. Using first-principles hybrid functional calculations, we show that electron polarons can be self-trapped, but they can also bind to defects; thus, they play an important role in understanding the properties of doped MoO3. Our calculations show that oxygen vacancies can cause unintentional n-type doping in MoO3. Mo vacancies are unlikely to form. Tc and Re impurities on the Mo site and halogens (F, Cl, and Br) on the O site all act as shallow donors but trap electron polarons. Fe, Ru, and Os impurities are amphoteric and will compensate n-type MoO3. Mn dopants are also amphoteric, and they show interesting magnetic properties. These results support the design of doping approaches that optimally exploit functionality.
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INTRODUCTION
α-MoO3, which has a layered structure, is an attractive functional material. Due to its large band gap (3.2 eV1,2), it can be used as a transparent contact for organic photovoltaics or organic light-emitting diodes.3−6 Photo- and electrochromic behavior has also been reported,7,8 and MoO3 has shown great promise as a catalyst or sensor material9−11 and as an electrode material in lithium batteries.9,10,12 For all of these applications, a thorough understanding of the behavior of point defects and impurities is essential; we show how computations can enable the design of doping approaches that support the functionality. MoO3 is found to be unintentionally n-type doped,13 and oxygen vacancies, which are often observed,14 are thought to be responsible for the ntype conductivity.15 p-Type material is unlikely to form since the valence-band edge is very low compared to the vacuum level,6,13,15,16 making it improbable that shallow acceptors can be found. While studies on intercalated MoO3 have been performed,9,10,12,17,18 which consider interstitial impurities (in the van der Waals gap between the layers), no information is present in the literature on intentionally adding substitutional dopants to MoO3. In this work, we perform explicit calculations to elucidate the role of native defects and to investigate the possibility of controlled n-type doping by substitution of impurities. α-MoO3 has an orthorhombic structure (space group Pbnma, #62), that consists of van der Waals bonded sheets of distorted edge-sharing Mo−O6 octahedra (Figure 1). All Mo atoms are symmetry equivalent, but three symmetry-inequivalent O positions exist. We label these according to the number of Mo−O bonds: O(1) indicates the O atoms that are facing the interlayer spacing (in the b direction) and are bonded to only one Mo atom; O(2) connects two Mo atoms and is oriented along the a direction; O(3) connects three Mo atoms and forms chains in the c direction. © XXXX American Chemical Society
Figure 1. The crystal structure of MoO3. Large spheres indicate Mo atoms, while smaller spheres indicate O atoms. The different O sites are labeled and have different colors. The shape of a single unit cell is indicated.
Our computational results will show that, among substitutional impurities on the Mo site, Re is the most attractive n-type doping impurity. On the oxygen site, substitutional halogens (F, Cl, and Br) also act as shallow donors. All these donors, however, bind electron polarons; we report binding energies of Special Issue: Computational Design of Functional Materials Received: October 19, 2016 Revised: February 26, 2017 Published: February 27, 2017 A
DOI: 10.1021/acs.chemmater.6b04479 Chem. Mater. XXXX, XXX, XXX−XXX
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Chemistry of Materials the polarons to the donor to guide the choice of optimal impurities. Surprisingly, Fe, Ru, Os, and Mn on Mo sites show self-compensating behavior. We identified that Mn exhibits interesting magnetic behavior.
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METHODS
Our first-principles calculations are performed using density functional theory (DFT), with projector augmented wave potentials19 as implemented in the VASP20 code. We used the HSE06 hybrid functional21 and a 400 eV energy cutoff. To simulate defects, a 3 × 1 × 3 supercell was used, and the atomic structures were relaxed so that all residual forces were smaller than 0.005 eV/Å. The van der Waals interactions between the layers were explicitly included using the D2 method of Grimme.22 This approach of combining the HSE06 functional with the D2 method has proven to yield very good structural as well as electronic properties.23 We obtained an indirect band gap of 3.19 eV, in very good agreement with the experimental value of 3.2 eV.1,2 All structures were visualized using the VESTA24 program. Formation energies are key quantities to investigate the effects of defects or impurities on the electronic structure, as they provide information on defect concentrations, stability of defects, and thermodynamic transition levels.25 The location of the transition levels indicates the nature of the defect: shallow or deep, n- or p-type. The formation energy Ef of the defect is defined as defect bulk E f = Etot − Etot − nMoμMo − nOμO − nX μ X
+ q(E VBM + E F) + Δq
(1) Figure 2. Structure and isosurfaces corresponding to 10% of the maximum spin density for (a) a self-trapped electron polaron in bulk MoO3; (b) the 2+ charge state of an O(2) vacancy with a bound electron polaron; (c) the 1+ charge state of ReMo with a bound polaron (localized on the Re atom); (d) the 1+ charge state of BrO(3), with a bound polaron. For clarity, only part (a single layer) of the MoO3 structure is shown. Different colors are used to indicate the sign of the spin density.
where the first two terms are the total energy difference between the MoO3 supercell with and without the defect. The next terms account for the energy cost or gain of exchanging ni atoms of type i with a reservoir with energy μi (the chemical potential). For charged systems (q ≠ 0), the q(EVBM + EF) term accounts for electrons being exchanged with the Fermi level EF, which is referenced to the energy of the valence-band maximum EVBM. The last term (Δq) is used to remove the spurious interaction of charged defects due to periodic boundary conditions.26 The chemical potentials are referenced to the bulk or diatomic molecules; they are in principle variable, but their range is restricted by the enthalpy of formation ΔHf of limiting phases (MnO, MnO2, Tc2O7, ReO2, ReO3, Fe2O3, RuO2, OsO4, MoF6, MoCl2, and MoBr3) and by the formation of MoO3. We will report results for two limiting cases: Mo-rich (which implies O-poor) and O-rich (which implies Mopoor) conditions. For Mo-rich conditions, μMo is set to the energy of bcc Mo metal, and for O-rich conditions, μO is equal to half the energy of an O2 molecule. Details of the methodology to calculate formation energies can be found in ref 25. The self-trapping energy of a polaron is defined as the difference in energy between a delocalized electron and an electron polaron (localized electron). To calculate the binding energy of an electron polaron to a defect (or impurity), we calculate the energy difference, measured at the conduction-band minimum (CBM), between the defect state without the polaron and the state with the polaron. From this quantity, the self-trapping energy of the polaron is subtracted, yielding the polaron binding energy.
is 0.16 eV lower in energy than a delocalized electron. Recent calculations based on the GGA+U approach found a larger selftrapping energy of 0.63 eV.30 Such a large value for the selftrapping energy is highly unlikely; it would severely impede ntype conductivity in MoO3, since it would imply that at room temperature nearly all electrons would be self-trapped. The only conductivity that can take place is polaronic hopping, a slow process, which contradicts the use of the material as a contact and as an electrode. In contrast, our value of 0.16 eV indicates that some n-type band conductivity, in addition to polaronic hopping, is feasible, which will give rise to a larger overall conductivity. Next, we look at the native defects. We find that the Mo vacancy is always high in energy, independent of its charge state or chemical potential conditions, indicating that it is extremely unlikely to form. This high formation energy can be understood by looking at the MoO3 structure (Figure 1): forming a Mo vacancy results in the formation of an unbound (atomic) oxygen atom or in major reconstructions and the formation of an O2 molecule. Both are energetically very unfavorable. For the oxygen vacancy, we find that a vacancy is most likely to form on a 2-fold coordinated O(2) atom. Relaxation of vacancies initially located on O(1) positions leads to structural distortions such that the final position closely resembles the O(2) vacancy. The O(3) position is higher in energy compared to the O(2) position by 6.17 eV at the VBM. In the 2+ charge state, the presence of the vacancy causes one of the nearby Mo
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RESULTS AND DISCUSSION Experimental results suggest that, in n-type material, electron transport is often dominated by polaron hopping.27,28 This is corroborated by first-principles calculations that obtain similar barriers and polaron hopping asymmetry.29,30 Here, we focus on the binding of electron polarons to defects or impurities in MoO3. For self-trapped electron polarons in bulk MoO3 (Figure 2a), we find that an electron localizes on a Mo dxz orbital, consistent with the results of refs 29 and 30. The selftrapping of this polaron is barrierless, and the electron polaron B
DOI: 10.1021/acs.chemmater.6b04479 Chem. Mater. XXXX, XXX, XXX−XXX
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such as Fe, Ru, and Os. For the O site, the candidates are the halogens F, Cl, and Br. The formation energies of substitutional Mn, Tc, and Re on the Mo sites are shown in Figure 3b. Substitutional Mn does not lead to n-type doping; in fact, while it acts as a donor under p-type conditions (Fermi level low in the gap), it acts as an acceptor under n-type conditions. The thermodynamic transition levels occur at 0.57 eV above the VBM for (1+/0), 1.72 eV for (0/1−), and 2.57 eV for (1−/2−). Isolated Mn atoms have five unpaired d-electrons, and the Mn impurity prefers to be in a high spin state. Upon increasing the Fermi level, the preferred charge state changes incrementally from 1+ to 2−. At the same time, the total magnetic moment increases from 0 to 3 μB. Negative charge states beyond 2− [which would be stabilized only if the Fermi level would be above the conduction-band minimum (CBM)] would lead to a further increase in total magnetic moment (up to μB = 5). However, this total magnetic moment masks the magnetic moment of the individual Mn atom. This can be seen in Figure 4, which shows
atoms to move away from the vacancy, while the nearby O(1) atoms that point toward the interlayer spacing tilt toward the vacancy (see the structure in Figure 2b). The formation energy as a function of Fermi level is shown in Figure 3a. The slope of
Figure 3. (a) Formation energies of O vacancies on the O(2) site as a function of Fermi level for Mo-rich and O-rich conditions. The Fermi level is referenced to the VBM. The different charge states (1+ and 2+) are indicated. (b) Formation energies of substitutional Mn and Re on Mo sites as a function of Fermi level. For clarity, only the O-rich conditions are shown. In both panels, dashed lines indicate the binding of electron polarons.
the formation energy indicates the charge state (eq 1), which is also labeled. Kinks in the curve indicate thermodynamic transition levels between different charge states. We find that oxygen vacancies are shallow donors: only the 2+ and 1+ charge states are stable for the entire range of Fermi levels in the band gap, meaning that the vacancy will always donate an electron to the conduction band. The formation energy of the vacancy is low, particularly under oxygen-poor conditions. Our calculations therefore confirm that oxygen vacancies are a likely cause of unintentional n-type conductivity in MoO3, as was suggested in ref 13 and recently also shown computationally.15,30,31 Closer inspection of the 1+ charge state reveals that this is actually a 2+ charge state with a bound electron polaron (see Figure 2b). The binding energy of a single electron polaron to the O vacancy is 1.54 eV. In principle, a second electron polaron could be bound to the 2+ O vacancy; however, we found that this is a higher energy configuration. Next, we will study what elements are able to intentionally ntype dope MoO3 and how likely these elements can be incorporated in the material. We are particularly interested in substitutional, as opposed to interstitial, doping since the latter is unlikely to result in stable properties. Candidates for n-type dopants on the Mo site are the elements to the right of Mo in the periodic table: Mn and Re. For completeness, we also added Tc, despite it having no stable isotopes, which excludes its use in practical applications. Our results for oxygen vacancies inspired us to also examine potential double donors. In the case of the oxygen vacancy, we observed that one electron is bound as a polaron but that binding a second electron is energetically unfavorable, effectively making the vacancy a shallow donor. It is interesting to examine whether the same behavior would be observed in the case of impurities that act as double donors, i.e., elements two columns to the right of Mo in the periodic table,
Figure 4. Isosurfaces corresponding to 10% of the maximum spin density for a Mn atom on a Mo site in different charge states: (a) 1+, (b) 0, (c) 1−, and (d) 2−. Different colors are used to indicate the sign of the spin density.
the spin density for the different stable charge states, and from Table 1, which lists the calculated Bader magnetic moments32 on the Mn atom and the nearby O(1) and O(2) atoms; note that these values should be taken only as an indication of the magnetic moment, not as exact values. For the 1+, 0, and 1− charge states, the Bader magnetic moments of the Mn atom is around 2.7 μB; it increases to 3.55 for the 2− charge state, Table 1. Charge State (q), Total Magnetic Moment (μB), and the Bader Magnetic Moments32 on the Mn, O(1), and O(2) Atoms for the Substitutional Mn on a Mo Site
C
q
μtot B
μMn B
μO(1) B
μO(2) B
1+ 0 1− 2−
0 1 2 3
2.77 2.78 2.69 3.55
−1.74 −0.89 −0.87 −0.83
−0.93 −0.91 0.11 0.04
DOI: 10.1021/acs.chemmater.6b04479 Chem. Mater. XXXX, XXX, XXX−XXX
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Chemistry of Materials indicating that the additional electron of the 2− charge state also localizes on the Mn atom. The Mn magnetic moment is compensated by a magnetic moment of opposite spin on the nearby O(1) and O(2) atoms. The spin density on the O atoms has a p-orbital shape, except in the case of the larger magnetic moment on the O(1) atom in the 1+ charge state (see Figure 4), which is the superposition of 2 p orbitals. In contrast to substitutional Mn, Tc and Re do exclusively act as donors. The (1+/0) transition level is located 1.34 eV below the CBM for Tc and 0.68 eV for Re (see Figure 3b). The spin density of the 0 charge state is mainly localized on the Tc or Re atom, but some is also localized on nearby O atoms: same spin on the nearby bonded O(2) atom and smaller opposite spin on the other bonded O atoms (see Figure 2c). Therefore, this charge state can be considered to be a 1+ charge state with a bound electron polaron, with a binding energy of 1.18 eV for Tc and 0.52 eV for Re. Figure 3b also shows that the position of the thermodynamic transition levels, when going from 3d (Mn) to 4d (Tc) to 5d (Re) orbitals, shifts toward higher Fermi levels, as can be seen, e.g., for the (1+/0) transition level. Fe, Os, and Ru show amphoteric behavior (see Figure 3b): for Fermi levels low in the gap (p-type conditions), they are donors, but for Fermi levels close to the CBM (n-type conditions), they act as acceptors, thereby compensating n-type conditions. Doping with these elements would therefore counteract n-type doping of MoO3. Our hope that double donors might lead to n-type conductivity was therefore not fulfilled. This hope was based on a parallel with the oxygen vacancy, but evidently, the physics that governs doping behavior is very different in the case of the substitutional impurities and dominated by the precise position of the d states. Once again, the thermodynamic transition levels (e.g., the (1+/0) transition level) shift toward higher Fermi levels when the relevant d orbitals increase in electron shell. While we did not explore the magnetic properties of Fe, Os, and Ru in as much detail as those of Mn, they may also be interesting for applications in this area. For substitutional halogen doping, we study F, Cl, and Br. Since F atoms are similar in size to oxygen, they can replace oxygen on any of the three O sites without significant distortions of the crystal structure. This is no longer the case for Cl and Br. Because they have only one or two bonds with Mo, the O(1) and O(2) sites can accommodate the larger bond lengths of Cl and Br with minimal distortion, but this is not possible in case of the O(3) site, which has three bonds. In this case, the Cl or Br atom moves upward, almost to the plane formed by the Mo and O(2) atoms (see Figure 2d), where it is located close to the O(2) atoms (distance of 2.43 Å). This relaxation also causes the nearby Mo−O(3) bond length to increase from 2.37 to 2.47 Å in the case of Cl or to 2.79 Å for Br. Figure 5 shows that the substitutional halogens on the various O sites all exhibit the same behavior: they all have (0/ +) transition levels well below the CBM, but as confirmed by the spin density, the natural charge state of all these donors is actuallly the + charge state and they intrinsically behave as shallow donors. The “neutral” charge state reflects an electron polaron bound to the positively charged donor. The formation energy of F is lower than that for Cl and Br, because of the better size match. For Cl and Br, the O(1) position is the preferential position, likely because this position introduces the least amount of structural distortion. The thermodynamic transition levels (1+/0) for the energetically
Figure 5. Formation energies of substitutional halogen atoms: (a) F, (b) Cl, and (c) Br on the three different O sites. For clarity, only the Mo-rich conditions (which favor incorporation on the O site) are shown. Dashed lines indicate the presence of polarons.
preferred positions (FO(2), FO(1), ClO(1), and BrO(1)) are all at approximately 1.1 eV below the CBM. The comparison of this level to the self-trapping energy of a polaron (0.16 eV) yields a binding energy for an electron polaron of 0.94 eV. This binding energy is significantly larger than in the case of Re, indicating that Re will be a better candidate for donor doping than the halogens.
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CONCLUSIONS We performed a detailed first-principles study of native and intentional doping of MoO3 with the goal of designing doping schemes that support functionality. We found that the formation of Mo vacancies is unlikely, but that O vacancies can be readily formed and act as shallow donors, explaining the observed unintentional n-type character of MoO3. Substitutional Tc or Re on Mo sites and substitutional halogens (F, Cl, and Br) on O sites are all shallow donors that bind electron polarons; the reported binding energies will guide the choice of impurity. Surprisingly, Mn, Fe, Os, and Ru on Mo sites show compensating behavior, as they act as donors under p-type conditions and as acceptors under n-type conditions. Our hope that double donors such as Fe, Os, and Ru would lead to shallow donor behavior (in analogy with the oxygen vacancy) was therefore not fulfilled. Thermodynamic transition levels shift toward higher energies when the electron shell of the d orbitals is increased. While not suitable for n-type doping, Mn atoms exhibit interesting magnetic behavior and may enable new applications of this versatile material.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
H. Peelaers: 0000-0002-7141-8688 M. L. Chabinyc: 0000-0003-4641-3508 C. G. Van de Walle: 0000-0002-4212-5990 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS H.P. and M.L.C. were supported by the National Science foundation MRSEC program (DMR-1121053). C.G.V.d.W. D
DOI: 10.1021/acs.chemmater.6b04479 Chem. Mater. XXXX, XXX, XXX−XXX
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was supported by the Office of Science of the U.S. Department of Energy (Grant No. DE-FG02-07ER46434). Computing resources were provided by the Center for Scientific Computing at the CNSI and MRL (an NSF MRSEC, Grant No. DMR-1121053) (Grant No. NSF CNS-0960316) and by the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.
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