Models of Surface Morphology and Electronic Structure of Indium

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Models of Surface Morphology and Electronic Structure of Indium Oxide and Indium Tin Oxide for Several Surface Hydroxylation Levels Jaren Harrell, Muhammed Acikgoz, Hela Lieber Sasson, Iris Visoly-Fisher, Alessandro Genova, and Michele Pavanello J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b10267 • Publication Date (Web): 06 Dec 2017 Downloaded from http://pubs.acs.org on December 7, 2017

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The Journal of Physical Chemistry

Models of Surface Morphology and Electronic Structure of Indium Oxide and Indium Tin Oxide for Several Surface Hydroxylation Levels Jaren Harrell1, Muhammed Acikgoz1, Hela Lieber Sasson2, Iris Visoly-Fisher2, Alessandro Genova1 and Michele Pavanello1* 1

Department of Chemistry, Rutgers University, Newark, NJ 07102

2

Department of Solar Energy and Environmental Physics, Swiss Institute for Dryland Environmental and Energy Research, Jacob Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, Sede Boqer 8499000, Israel * Corresponding author, e-mail: [email protected] Abstract Indium Oxide (IO) and Indium Tin Oxide (ITO) are important metal oxide materials with a wide array of applications. Particularly, ITO is employed as transparent conductive electrode in photovoltaic systems. While bulk metal oxides are typically well characterized, their surfaces, especially in real-life applications, can be hydroxylated and intrinsically disordered to a level that a structure-function prediction becomes a daunting task. We tackle this problem by carrying out simulations based on Density Functional Theory. We propose IO and ITO hydroxylated surfaces derived from the bcc and rombohedral IO polymorphs (100%, 66%, 33%, and 0% hydroxylation coverages were considered). By correlating computed quantities such as surface partial density of states, work functions and surface dipole strength, a clear picture of the structure-function relationships in these model systems emerges. In line with conclusions drawn from experiments, we find that the density of states of 100% hydroxylated surfaces and bulk models are unaltered by Sn doping with the only difference being the position of the Fermi level. The partially hydroxylated surfaces, instead show a rich array of behaviors, including appearance of surface states in the gap and appearance of interesting morphologies, such as chemisorbed molecular oxygen. We also find that the hydroxylation level affects surface dipoles in a systematic way. I.e., the higher the hydroxylation level, the higher the surface dipole (screening/reducing the work function). Furthermore, models with In-atom vacancies show a relatively small decrease in surface dipole with hydroxyl coverage due to surface distortions.

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1. Introduction Transparent conducting oxides are polycrystalline materials often used in optical devices such as photovoltaics (PV). Indium oxide (IO) and indium tin oxide (ITO) feature appealing properties for these applications. An important factor when designing efficient devices that include IO/ITO is understanding and predicting the physics occurring at their interface with other materials. Phenomena such as photoconductance still present open questions, as complications arise when one considers band bending and charging and discharging of surface states. Commonplace methods to tune energy levels, such as adsorbing molecular layers on the surface, gives the ability to control physical factors such as work functions, surface energies, charge injection/collection barriers1,2. In ITO’s case, because its morphology is strongly affected by fabrication conditions and pretreatments, there remain challenges in isolating desirable features of the surface for improved efficiency.3,4 Despite these complications, various surface treatments (e.g. chemical, ozone, plasma, etc.) as well as adsorption of organic species2,5,6 are known to induce variation on the properties of ITO, influence its efficiency when part of a device. Most of these reducing/oxidative treatments have been well characterized experimentally. For example, it is known that treatment of ITO with water or ozone leads to surface modifications. In ITO, film resistance changes as H2O protonates surface sites creating hydroxyl groups,7 which in turn alter its electronic properties.8 From a chemical prospective, the presence of hydroxyl groups on the surface of ITO have been associated with the development of different oxygen species as well as an increase in the adsorption rate of molecules.3,9 Ozone treatment has been reported to increase the surface work function.7,10 These surface modifications lead to various degrees of surface dipoles. It is known that work function principally can change depending on: (1) variation of the Fermi Level, EF, by e.g.


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doping; (2) band bending in semiconductor materials; (3) presence of a surface dipole. It was reported that the work function of ITO thin films increased from 4.34 eV to 4.47 eV after chemical oxidation and to 4.69 eV after plasma oxidation.11 Lee et al found that the work function of ITO increased by 0.8 eV after inductively coupled plasma (ICP) treatment.12 Furthermore, it was observed that the work function of ITO (and also of IO) surface increases from 4.47 (3.97) eV to 5.20 (4.58) eV by changing the oxygen fraction in sputter gas from 0% to 6.5 (10.0)%, respectively.1 IO's most thermodynamically stable crystal structure is the body-center cubic (bcc).13-17 It has been reported that bcc is also the most prevalent phase when ITO it is prepared through typical thin film methods. 7,18-22 However, when the tin doping level reaches above 6%, a rhombohedral (rh) phase of ITO starts emerging as well.23,24 Because common doping levels of ITO used in photovoltaics applications are 5-15%, simulations of rh IO and ITO are of particular relevance. Moreover, ITO preparation often requires deposition on other substrates (e.g. sapphire), causing it to accommodate a different lattice parameter, which may ultimately lead to the less thermodynamically favorable rh phase.17,23,25-28 This suggests that models based exclusively on the bcc structure are deemed to not fully represent the real ITO surface. To fill this gap, we embark on a thorough simulation of the [001] surface of IO and ITO in their rh crystal structure. We choose this facet because its morphology is the most similar to the commonly studied and more stable [111] surface of the bcc phase15, and it also happens to be a common cleavage facet of other hydroxylated trivalent metal oxides.29 Through ab initio density functional theory (DFT) simulations we aim at predicting morphologies and corresponding electronic structures of several IO and ITO surfaces. We believe that understanding effects resulting from surface hydroxyl coverage is a necessary step to 3 ACS Paragon Plus Environment


optimize surface properties and adsorption techniques. Thus, we explore the influence of the hydroxyl coverage, as well as In-atom vacancy defects on the electronic density of states (DOS) of IO and ITO. Also, we evaluate the work function, ϕ, for all IO and ITO structural models. Trends in the changes in in work function with respect to various levels of surface modifications provides us with a clear relation between the hydroxylation level, Sn doping and the surface dipole. This work is organized as follows. After describing the computational details and techniques employed for the DFT simulations, in section 3 we present the bulk of our results, starting from bulk IO and ITO, the oxygen terminated surfaces, the hydroxylated ones (33%, 66%, and 100%), and conclude with the In-atom vacancy models. In the conclusions, we summarize our findings and formulate a simple predictive model aiming at explaining the basic physical interactions at play. 2. Computational Details All simulations are carried out with the Quantum ESPRESSO simulation package.30 The electronic structure is obtained at the DFT level, using a plane wave basis set, periodic boundary conditions, and k-point sampling of the first Brillouin zone. The PBE functional31 is chosen to approximate the exchange-correlation energy functional and Projector Augmented Wave (PAW) pseudopotentials are employed. Plane wave kinetic energy cut-offs for the expansion of KohnSham (KS) orbitals and electron densities are 50 and 500 Ry, respectively. To ease the SCF convergence, we apply a Gaussian smearing to the occupations of the KS states, with a width (sigma) of 0.001Ry. 31 In geometry optimizations, we sample the FBZ at the Γ point only, while for single point calculations, computation of density of states and work functions, we use a 4 × 4 × 1 k-point grid. Finally, to obtain smoother surface partial density of states (PDOS)


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plots, additional non-self-consistent (NSCF) calculations are performed on a 8 × 8 × 1 k-point grid. The bulk structure of IO in the rh and bcc crystal is retrieved from the crystal database at crystallography.net.32 To make our surface models as realistic as possible, we include a large number of independent atoms in the simulation cell. Specifically, we carry out simulations containing 120 atoms for the rh bulk and derived surface slabs, and 80 atoms for the bcc models. Bulk rh ITO is constructed directly from the IO structure by swapping four random In atoms with Sn atoms. This provides an 8% Sn-doped material. Bulk bcc ITO is obtained by replacing three random In atoms with Sn atoms, which leads to a 9% Sn doping level. Input files with the given structures are accumulated in the Supplementary Information document. Regarding the slab models employed in this work, rh surfaces are constructed by cutting the along the 001 plane of bulk IO/ITO and adding a 20 Å vacuum layer. The cuts were implemented in such a way that at the surface either oxygen or metal In/Sn atoms would be exposed. In the ITO surface slabs, two Sn atoms are located on the surface neighboring each other (with a surface tin concentration of 0.016 Sn/Å2), while the other two Sn atoms were randomly distributed in the layer underneath the surface layer. Bcc IO/ITO surfaces are generated by cleaving the bulk along the 111 plane. In the ITO surface, two Sn atoms are once again located on the surface neighboring each other and the remaining two are randomly distributed in the lower layer. The neighboring Sn atoms are placed in non-nearest neighbor locations. The models chosen are by no means comprehensive of all possible doping configurations. Nevertheless, the chosen configurations are realistic as they reproduce the overall doping level and feature dopant atoms both near the surface and deeper toward the bulk as it



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would be expected in realistic models. In future works, we will focus on more thorough investigations of properties’ fluctuations induced by local disorder in the doping microstructures. To simulate the features of underlying bulk, in the slab models the position of the bottom layers (4 for rh and 3 for bcc) is kept fixed throughout the geometry optimization. To maintain overall charge stoichiometry, when needed, the slabs were passivated at the bottom layer with iodine atoms. Modeling a stoichiometric slab should provide us with a more realistic picture of the electronic structure of this system. Hydroxylated surfaces where considered in our investigation including 4 levels of surface hydroxyl coverage: 100%, 66%, 33%, and 0%. Throughout the manuscript, we use abbreviations to identify each surface/bulk. A glossary is reported in Table 1.

Table 1: Glossary of the simulated IO and ITO models Termination

IO

ITO

BCC – IO

BCC – ITO

No termination (bulk)

IOb

ITOb

IOb-bcc

ITOb-bcc

100% OH

IO100

ITO100

66% OH

IO66

ITO66

33% OH

IO33

ITO33

IO0

ITO0

IO0-bcc

ITO0-bcc

IO

ITO

O-terminated O-& metal terminated

To investigate whether there is a correlation between the formation of gap states and the partial charges on the atoms, we compute Bader charges of the surface atoms of IO100, ITO100, IO0, ITO0, IO , and ITO models. Comparison of the atomic charges of surface atoms helps us

explain features seen in the DOS of IO and ITO models. Pictures of the surface morphologies have been created with the VESTA program.33


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3. Results 3.1. Structure and Electronic Properties of Rhombohedral and BCC Bulk IO and ITO In this section, we compare IO and ITO bulk models, paying particular attention to the change of geometrical parameters from the original crystal structure of IO. In a nutshell, what we find is that geometry optimizations of IO induce little change of the ionic positions compared to the crystal structure. This validates our computational method for these systems. The ITO bulk model is considered here only for comparison purposes and to better understand the properties of the surface models presented afterwards. We refer to Ref.19 for a more in-depth analysis of bulk ITO models.

A. Undoped Indium Oxide (IOb) Upon geometry optimization of IOb and IOb-bcc models, we record only small deviations from the original crystallographic positions. Using the optimized geometry, we compute the DOS, see Figure 1. As it is known, semilocal exchange-correlation functionals underestimate material’s optical gaps. Unsurprisingly, our calculations yield an optical gap for IOb of roughly 2.0 eV while the actual experimental value is 3.0 eV for a rh IO single crystal.29,34 Similarly, the IOb-bcc theoretical model has a band gap of 1.6 eV as opposed to experimental values ranging from 2.3-3.7 eV.25,29,34-38 The major features of the simulated DOS of these bulk systems agree with previous investigations.15,25,34,39,40 Specifically, the valence band displays a sharp peak at -1.0 eV.



B. Indium Tin Oxide (ITOb) Upon substituting three In atoms with Sn atoms, the geometry optimization results in a moderate geometry reorganization. Bond lengths between Sn and O are shorter than the ones between In and O on average by 5-10%. At same time, In-O bond lengths increase. This observation is in agreement with previous investigations and in line with common knowledge.24 The calculated DOS for ITOb has a shape almost identical to IOb but due to the extra electrons from the Sn atoms, the Fermi level shifts towards the conduction band , see Figure 1. Again, for ITOb our semilocal DFT calculation results in an underestimated band gap compared to the experimental optical gap for amorphous ITO which ranges between 3.58-4.3 eV.40-42 Inspecting Figure 1, we observe almost identical trends in the DOS of ITOb-bcc compared to the rh phase of ITOb. For ITOb-bcc, the Fermi energy lies at the beginning of the conduction band, and the KS band gap is well within the range of previous theoretical predictions (1.0–2.7 eV).15,18.43 Finally, the DOS of both ITOb and ITOb-bcc have a peak arising at the top of the valence band at about 9.0 eV. Analysis of the PDOS show that this peak is due to the additional Sn states in the crystal, and is a known trait of the material. 15,18,20,35 This peak is not fully contributed by the Sn but also the In atoms comprising of the s-orbitals with slight contribution form the p and d orbitals. In conclusion, the simulated DOS of both phases of ITO are consistent with the degenerate n-type semiconductor nature of the material, and no dramatic changes in the electronic properties from the bcc phase to the rh crystal phase are predicted by the bulk models.


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(a) IOb

(b) ITOb

(c) IOb-bcc

(d) ITOb-bcc

Figure 1: DOS of (a) IOb, (b) ITOb (8% doped), (c) IOb-bcc and (d) ITOb-bcc (9.4% doped). DOS is arbitrary units. The Fermi energy is set to zero.



3.2. Simulated Surface Morphologies and Electronic Properties of rh IO and ITO We simulated four degrees of surface hydroxylation of IO and ITO to gain insight on its effects on surface morphology and properties. For both IO and ITO, the supercell slab models have a surface area of 124 Å2. This is as large as seen in previous theoretical models15 and should provide a reasonable description of a realistic surface. Geometry optimization of these surfaces results in surface distortions of various magnitudes depending on the degree of hydroxyl coverage. The ITO surface simulations agree with experiments in that, similarly to previously presented bulk calculations, doping the surface with Sn decreases the metal-oxygen bond lengths by about 5-10%.20,22,24,39 The largest distortion is localized in the vicinity of the Sn atom. We also pay attention to the surface hydrogen bond structure that forms upon hydroxylation, as well as the formation of oxygen dimers on the surface. Below, we focus on each of the models considered.

A. Oxygen Terminated Rhombohedral IO and ITO Surfaces (IO0 and ITO0) As IO and ITO surfaces are often treated with ozone and oxygen plasma, we also considered models featuring oxygen-only terminated surfaces. The IO0 surface oxygen layer moves downward towards the In atom layer, however, still noticeably above the metal layer. Each surface O atom is coordinated to two In atoms. In ITO0, Sn has substantial effects on the surface morphology, inducing the migration of one In atom to the surface from the subsurface layer. This migration leads to a subsurface vacancy and dimerization two sub-surface oxygen atoms. On the surface, we notice the formation of an oxygen trimer atop the surface In atoms, see Figure 2. Presence of oxygen dimers at the surface of ITO is an accepted consequence of the treatment with ozone3,44 especially when polycrystalline substrates are studied. We believe that


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such dimers were not found in previous theoretical models because the oxygen terminated 111 facet of the bcc IO (i.e., the commonplace model surface for this system) only exposes tricoordinate O-atoms (vide infra, the topmost oxygen layer in IO0-bcc collapses onto the metal layer). These atoms are unlikely to dimerize due to the high coordination. Clearly, this is a bias introduced by the bcc model systems. In fact, it is known from experiments that for surface terminations other than 111, oxygen dimers occur.45 (a)

(b)

Figure 2: Surface structure resulting after optimization of (a) IO0 and (b) ITO0 the colored inset shows the formation of surface oxygen dimer and trimer.

In Figure 3 we show the PDOS of the IO0 and ITO0 surfaces. Both surfaces show some key changes from the previously presented bulk DOS. Specifically, due to the formation of the molecular oxygen species as well as undercoordinated surface oxygen atoms, the Fermi energy of ITO now also lies at the onset of the valence. We also notice an additional peak at about -8 eV in the DOS of ITO0. PDOS shows that unlike the similar peak found in the DOS of ITO100 this one is not associated to the surface Sn atoms, but rather to the surface O trimers (vide infra in the section devoted to ITO100). This is important because it shows how the electronic structure of the



Sn-O bond is radically different in the ITO0 model compared to the bulk (ITOb) and the ITO100 surface models. (a) IO0

(b) ITO0

Figure 3: The PDOS of (a) IO0 and (b) ITO0 of the oxygen terminated surface. PDOS is arbitrary units. The Fermi energy is set to zero.

The PDOS plots present metallic character induced by the doubly coordinated surface oxygen atoms in both IO0 and ITO0 models, as well as that of molecular oxygen species near the Fermi level in ITO0 with the presence of partially occupied states. This is elucidated by the difference of Bader charges of the oxygens of surface dimer and trimer (about -0.81) from those of bulk oxygens (about -1.2).


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B. Mixed Metal Terminated Rhombohedral IO and ITO Surfaces The IO and ITO models are obtained from cutting the rh IO bulk along the 001 facet in

such a way that both metal and oxygen atoms are exposed on the surface. Experiments have reported IO and ITO can have this detailed surface morphology. 8,19,46 The optimized IO surface has oxygen atoms tri-coordinated to In atoms on the same plane (see Figure 4a). On the other hand, ITO shows extensive distortions among the surface Sn atoms, with Sn atoms slightly raised above the surface towards the vacuum (see Figure 4b). However, surface oxygen atoms in ITO remain 3 coordinated, and due to the fact that the Sn atoms move slightly above the surface, the surface In/Sn – O bond lengths slightly increase with respect to IO . On one hand, the PDOS of IO in Figure 5a has the Fermi level lying within the valence band (similarly to the IO0 and ITO0 surfaces) and a band gap of about 2.0 eV. On the other hand, the PDOS of ITO in Figure 5b is significantly different: new surface states emerge in the gap due to Sn hybridizing with O. In conclusion, orbital hybridization between the s orbitals of Sn and the p orbitals of O arises due to the addition of Sn to the surface and is enabled by the associated surface distortions.



(a) IO

(b) ITO

Figure 4: Surface structure resulting after optimization of (a) IO and (b) ITO models.


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(a) IO

(b) ITO

Figure 5: Different states occur in (a) IO , and (b) ITO surface. PDOS in arbitrary units, energy in eV. The Fermi energy is set to 0.

In IO O atoms in the first layer have a -1.18 charge, while in the second layer the charge is -1.26. In the top layer of In atoms are charged +1.73, while the second layer their charge is between +1.86 and +1.89. Finally, in ITO the charges of In and Sn atoms are surprisingly similar (+1.70), while in the other ITO models Sn was up to +1.0 more positive than In.



C. 100% Hydroxylated Rhombohedral IO and ITO Surfaces The 100% hydroxylated IO surface (IO100) features the smallest ionic distortions compared to the bulk. The large surface area afforded by the model, allows us to inspect the morphology and hydrogen-bond network arising from the many independent surface hydroxyl groups. These sample multiple conformations during the geometry optimization. In Figure 6a we present the final configuration of the surface with a focus on the two-dimensional H-bond network. We observe the formation of three H-bonded hydroxyls surrounding the In atoms on the surface. However, the H-bond arrangement lacks a network structure. This could be an artifact of the large-yet-finite number of surface OH groups and should be investigated further in the future. Incorporating Sn atoms to form the 100% hydroxylated ITO surface (ITO100) causes an interesting deviation of the H-bond pattern compared to IO100. Figure 6b shows one of the OH is pointing away from the Sn atom. A simple way to explain this is suggested by comparing the Bader charges computed for the In atoms (+1.8) and Sn atoms (+2.3). The larger net positive charge on Sn causes the observed orientation of the OH dipole. Figure 7 collects the PDOS of IO100 and ITO100. In both cases, the PDOS profiles are similar to the bulk (i.e., IOb, and ITOb) with a slightly larger band gap (2.0 eV/2.5 eV for IO100 and ITO100, respectively) and the Fermi energy lying within the gap.


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(a) IO100

(b) ITO100

Figure 6: The optimized surface morphology of the top layer of metal atoms and hydroxyls on (a) IO100 and (b) ITO100. Indium atoms are gray and tin are black. Sticks are hydroxyl groups (red: oxygen, white: hydrogen).



(a) IO100

(b) ITO100

Figure 7: Partial density of Kohn-Sham states (PDOS) for surfaces derived from the rh crystals (a) IO100, and (b) ITO100 (8%-doped). PDOS in arbitrary units, energy in eV. The Fermi energy is set to 0.

D. 66% Hydroxylated Rhombohedral IO and ITO Surfaces We have generated IO66 and ITO66 models visible in Figure 8a and Figure 8b, respectively. We generated these structures adding 8 hydroxyl groups in random positions on the previously considered IO0 and ITO0 surfaces. After optimization, we notice the formation of an oxygen dimer on the surface of IO66. Whereas no oxygen dimers are observed in ITO66, see Figure 8b. We note an interesting detail in the PDOS of IO66 in Figure 9a. There are two new prominent peaks from gap states, and the peak at the Fermi level is due to the oxygen dimer. The 18 ACS Paragon Plus Environment

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ITO66 PDOS in Figure 9b did not have any surface states within the band gap, which is consistent with the fact that no oxygen dimers were formed on the surface of this particular model. However, we notice a shoulder in the valence band of ITO66 facing the gap due to doubly coordinated oxygens on the surface. Thus, to better understand the role of these doubly coordinated oxygens we also indicated their PDOS in Figure 9, which clearly demonstrate that these oxygens are responsible for the metallic shape of the PDOS near the Fermi level in a way that resembles the IO0/ITO0 models. The orientation of the hydroxyl groups differs in IO66 and ITO66. Specifically, in both IO66 and ITO66 three out of eight OH groups lie essentially flat on the surface pointing towards a hollow site. The five remaining OH groups point upward with a tilt angle. However, the ITO66 OH groups are almost perpendicular to the surface compared to the IO66 which instead has a significant tilt angle. From these considerations, we expect that the contribution from the OH groups to the surface dipole in IO and ITO to be different (vide infra in the section devoted to work functions). (a)

(b)

Figure 8: Surface structure resulting after optimization of (a) IO66 and (b) ITO66.



(a) IO66

(b) ITO66

Figure 9: The PDOS of (a) IO66 and (b) ITO66. PDOS in arbitrary units, energy in eV. The Fermi energy is set to 0.

E. 33% Hydroxylated Rhombohedral IO and ITO Surfaces Our models for the IO33 and ITO33 surfaces contain four hydroxyl groups each in random positions. The optimized surface of IO33 contains adjacent oxygen dimers. Once again, ITO33 features no surface oxygen dimers (Figure 10). An important difference between these IO and ITO surfaces is the orientation of the hydroxyl groups. Specifically, in IO33 one out of four OH groups lies essentially flat on the surface pointing towards a hollow site. The three remaining OH groups point upward (although with a slight tilt angle). In ITO33, two out of four surface OH


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groups lie flat pointing away from the Sn atom in the same fashion as in the ITO100 surface. Once again, we expect that the surface dipole of these two structures be different. Vide infra in the section devoted to work functions for a more detailed characterization of this effect. (a)

(b)

Figure 10: Surface structure resulting after optimization of (a) IO33 and (b) ITO33.

Shifting our attention to the PDOS of IO33 in Figure 11, we notice that an unoccupied gap state at about 0.5 eV above the Fermi level arises due to the surface oxygen dimers (as indicated in the figure by the PDOS of the surface O2 fragment). On the contrary, there is no evidence of any band gap surface states in the DOS of ITO33. The Fermi level once again lies in the valence band, as seen earlier in previous surfaces doped with Sn. Moreover, as in the models IO66 and ITO66, similar role of the doubly coordinated surface oxygens is seen in Figure 10.



(a) IO33

(b) ITO33

Figure 11: PDOS of (a) IO33 and (b) ITO33. PDOS in arbitrary units, energy in eV. The Fermi energy is set to 0.

3.3. Simulated Surface Morphologies and Electronic Properties of BCC IO and ITO For sake of completeness, we also carried out a comparison to the most commonly studied bcc surfaces of IO and ITO. We generate the thermodynamically more stable, [111] surfaces of the bcc phase. Upon relaxing the IO0-bcc surface we find the O and In atoms are on the surface plane, with O being tricoordinated to In atoms. The In-surface atoms are penta- and hexacoordinated to the surrounding surface and sub-surface O atoms. The bond lengths between surface In and O atoms are between 2.12 and 2.44 Å.


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The ITO0-bcc surface also has a similar structure with penta- and hexacoordinated In/Sn surface atoms and O atoms that are only slightly above the surface plane while still being tricoordinated. The bond lengths of surface In atoms and O atoms are between 2.13 and 2.36 Å, while Sn-O bond lengths range from 2.08 to 2.23 Å. As we can see in Figure 12, we do not record extensive surface distortions in either surfaces.



(a) IO0-bcc

(b) ITO0-bcc

Figure 12: The optimized surface morphology of (a) IO0-bcc and (b) ITO0-bcc.

From the surface PDOS of IO0-bcc in Figure 13a, and ITO0-bcc’s in Figure 13b, we see that once again that for the undoped model the Fermi level lies at the edge of the valence band, in agreement with previous experimental and theoretical results.15,40 The change in the DOS of ITO


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compared to IO is due to the addition of Sn into the model, rather than the marginal geometrical distortions. Comparison of the morphology and DOSs obtained for rh IO0 and ITO0 models with IO0bcc and ITO0-bcc highlights differences between these two phases. The band gap of IO0-bcc is smaller than those calculated for the rh IO slabs. As pointed out before, the formation of oxygen cluster and the presence of undercoordinated oxygens at the surface of rh ITO (see Figure 2 where a surface oxygen dimer and a trimer are highlighted) is the most profound difference compared to bcc models. Contrary to the rh models, in bcc models it is also seen that the surface oxygens in IO0-bcc/ITO0-bcc are mostly on the same plane as the metal atoms.



(a) IO0-bcc

(b) ITO0-bcc

Figure 13: PDOS of [111] bcc surface for (a) IO0-bcc and (b) ITO0-bcc. PDOS in arbitrary units, energy in eV. The Fermi energy is set to 0.

3.4. Surface Defects Surface defects, such as an indium atom vacancies, have the potential of creating band gap surface states. It is known that electronic properties of a material can be greatly affected by the presence of defects in the bulk.47 For this reason, we have simulated the bulk of ITO with an In vacancy (ITObv), as well as a series of derived surfaces characterized by various degrees of hydroxylation (ITO100-0v). The vacancies were introduced at the surface. Although in principle there are several nonequivalent possible vacancy sites, due to the relatively small size of the employed slab model, only two sites were available and for our simulations we chose the ones 26 ACS Paragon Plus Environment

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nearest to the Sn atom. Surface morphologies and PDOSs for the various vacancy models are presented in Supplementary Information document (Figure S1-S5). The surface morphologies of all ITO models with an In atom vacancy show significant surface distortions. However, only the DOS of ITO66v and ITO33v show surface states in the gap. These states are due to the formation of oxygen dimers at the surface. For the remaining surface models, no gaps states emerge. Furthermore, in order to see the effect of levels of surface hydroxyl coverage: 100%, 66%, 33%, and 0% on the local distortions, we determine the ligand length distances (Sn-O and In-O) as well as the separation between In and Sn ions for both rf and bcc phases. These values are tabulated in Table 2 where we see that the distance between In-Sn decreases in the surface models with respect to bulk model. As expected that the substitution of Sn4+ for In3+ ion decreases the O ligand distances (Sn-O ϕ (ITO) for almost all models. We also see that the introduction of vacancies leads to an increase of work functions for almost all cases considered. The change of work function of IO and ITO models with respect to OH coverage are shown in Figure 14. We notice that ϕ significantly decreases with increasing OH coverage.


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(a) IO

(b) ITO

Figure 14: The change of work function (ϕ) of (a) IO and (b) ITO models with respect to OH coverage.

The change of work function (∆ϕ) between ITO and ITO with vacancy models as a function of surface OH coverage is shown in Figure 15. We notice a trend, e.g., ∆ϕ generally increases with increasing OH coverage. This trend is explained by a simple solvation effect. The OH groups stabilize the vacancy sites by pointing towards them. This effect is noticeable in all structures but particularly in the 100% hydroxylated model (ITO100v ).



Figure 15: The change of work function difference (∆ϕ) between normal ITO and ITO with vacancy models with respect to the OH coverage.

In the literature, we find experimental values of ϕ for differently prepared IO and ITO thin films (different substrate temperature (Tsub), oxygen fraction in sputter gas (%O2).1 For ITO, at Tsub = 400 0C, ϕ values were measured at various O2 fraction as follows: 4.47 eV [0%], 4.83 eV [1%], 5.04 eV [2%], 5.16 eV [3%], 5.11 eV [4%], 5.20 eV [6.5%], and 5.05 eV [10%]. On the other hand, for IO three values were reported: 3.98 eV at 400 0C and 0% O2, 3.97 eV at 200 0

C and 0% O2, at 400 0C and 0% O2, and 4.58 eV at 200 0C and 10% O2 fraction.1 The value of

around 5.0 eV calculated by us for IO100, ITO0-bcc, ITO33v, and ITO is in fairly good agreement with most of these values. In addition, the experimental trend of increasing ϕ with increasing oxygen on the surface is nicely reproduced by our simulations. We collect plane-averaged electrostatic potential plots for all IO and ITO models in Supplementary Information document (Figure S6-S17). To better understand the computed


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trends, we decided to also investigate the size of the surface dipole contribution to ϕ (which we indicate by ϕd). The dipole was estimated by computing the difference between the left and right vacuum levels.48 The results are given in Table 3 for the all models under study. The most profound contribution was found for the IO33/ ITO33 models, which we relate to the orientation of OH groups on the optimized surface of IO33/ ITO33 (see Figure 10). I.e., when the OH groups are vertical, we witness a strong reduction of the work function (e.g., for 100% OH coverage), when they are tilted (e.g., for 33 and 66% OH coverage) the reduction of work function is smaller. Turning to the mixed oxygen-metal terminated surfaces when we compare the value of ϕd and surface structure of IO with those of ITO (Figure 4) we see that the introduction of Sn

ions leads to a considerable reduction in ϕd. We correlate this with the slightly raised surface Sn atom in ITO compared to IO . The presence of In vacancy (considered in the ITO model)

results in a significant increase of ϕd due to the fact that the Sn atom fills in the vacancy site. A similar trend is also seen in bcc symmetry models (IO0-bcc, ITO0-bcc, and ITO0v-bcc). However, the value of ϕd for bcc models is quite smaller than that of rh models. This is consistent with the observation that the oxygen layer in bcc models lies at the same level of the metal layer, thus resulting in a overall small surface dipole.



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Table 3. The calculated surface-averaged electrostatic potential change (in eV) due to dipole correction (ϕd) for all IO and ITO models. IO

ITO

ITO

IO0

IO

IO100

IO66

IO33

IO0-bcc

1.52

2.97

3.40

0.21

3.76

2.06

ITO0

ITO100 ITO

ITO66

ITO33

ITO0-bcc

2.81

1.15

2.63

4.41

-0.32

0.00

ITO0v ITO ITO100v ITO66v ITO33v ITO0v-bcc 2.88

1.82

1.20

2.70

2.37

0.49

4. Conclusion To conclude, we have computationally investigated metal oxide surfaces based on Indium Oxide (IO) and its Sn-doped analog, Indium Tin Oxide (ITO). Our investigations aim at drawing a “big picture” of the factors influencing surface electronic properties. Obtaining such a big picture is of pivotal importance because it ultimately allows us to control the electronic properties of the surface. Controlling such properties is at the foundation of today’s rational design of energy materials and the involved interfaces. Specifically, we analyzed the surface density of states and the work function in correlation to the surface morphology. The models considered are by all means nonstandard. They include four surface hydroxylation levels ranging from 0% to 100%, two bulk IO polymorphs (bcc and rhombohedral), as well as In vacancy models. Our simulations reveal a trend in both the presence of surface states, as well as the work functions. The 100% and 0% OH covered surfaces constitute the two extrema of behaviors, with


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the oxygen terminated surfaces featuring the highest work functions, strongest surface dipole, and least coordinated surface oxygens leading to a partially filled valence band. The 100% OH covered surface mostly behaves like the bulk, and features reduced work functions and surface dipoles compared to the other models. By correlating these findings with the surface morphologies, we draw a simple conclusion which we summarize in Figure 16. We also find that with low OH coverage, there is a tendency to form molecular oxygen species on the surface. We find these species to be partially negatively charged and to present levels either in the gap or at the edge of the valence band.

Figure 16: Structure-property relation. Above: the tilt angle of the surface OH groups influences the surface dipole. Below: the polymorph chosen to generate the surface models yields different surface dipoles which affect the surface electronic properties and its response to the dipole originating from the OH groups.

Furthermore, from the plotted density of states we evince a general trend. I.e., Sn doping moves the Fermi level upward in energy. Conversely, the more oxidized the surface is (e.g., the



lower the OH coverage), the more the Fermi level moves towards the valence band. In the limiting case of a purely oxygen-terminated surface, the Fermi level enters the valence band giving rise to a metallic phase on the surface which is dominated by low-coordination oxygen atoms. Because we employ a slab model, it is very difficult for our simulations to capture band bending effects. However, in the oxidized surface cases likely the valence band would bend upward (due to e- accumulation filling the empty oxygen states). This picture is consistent with experimental findings reporting 2-dimensional e- gas accumulation at the surface of oxidized ITO1. 5. Supporting Information Collection of input structures of all models considered, figures S1 to S17, and table S1. 6. Acknowledgements This material is based upon work supported by the National Science Foundation under Grants No. DMR-1507812, DMR-1742807 and OIIA-1404739.

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Models of Surface Morphology and Electronic Structure of Indium

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