Models of Surface Morphology and Electronic Structure of IndiumOxide and Indium Tin Oxide for Several Surface Hydroxylation LevelsJaren Harrell,† Muhammed Acikgoz,† Hela Lieber Sasson,‡ Iris Visoly-Fisher,‡ Alessandro Genova,†

and Michele Pavanello*,†

†Department of Chemistry, Rutgers University, Newark, New Jersey 07102, United States‡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

*S Supporting Information

ABSTRACT: Indium oxide (IO) and indium tin oxide (ITO) are importantmetal oxide materials with a wide array of applications. Particularly, ITO isemployed as a transparent conductive electrode in photovoltaic systems. Whilebulk metal oxides are typically well characterized, their surfaces, especiallyin real-life applications, can be hydroxylated and intrinsically disorderedto a level that a structure−function prediction becomes a daunting task.We tackle this problem by carrying out simulations based on Density FunctionalTheory. We propose IO and ITO hydroxylated surfaces derived from the bccand rombohedral IO polymorphs (100%, 66%, 33%, and 0% hydroxylationcoverages were considered). By correlating computed quantities such as sur-face partial density of states, work functions, and surface dipole strength, aclear picture of the structure−function relationships in these model systemsemerges. In line with conclusions drawn from experiments, we find that thedensity of states of 100% hydroxylated surfaces and bulk models are unalteredby Sn doping, with the only difference being the position of the Fermi level. The partially hydroxylated surfaces, instead show arich array of behaviors, including appearance of surface states in the gap and appearance of interesting morphologies, such aschemisorbed molecular oxygen. We also find that the hydroxylation level affects surface dipoles in a systematic way, that is, thehigher the hydroxylation level, the higher the surface dipole (screening/reducing the work function). Furthermore, models withIn-atom vacancies show a relatively small decrease in surface dipole with hydroxyl coverage due to surface distortions.

1. INTRODUCTION

Transparent conducting oxides are polycrystalline materials oftenused in optical devices such as photovoltaics (PV). Indium oxide(IO) and indium tin oxide (ITO) feature appealing properties forthese applications. An important factor when designing efficientdevices that include IO/ITO is understanding and predicting thephysics occurring at their interface with othermaterials. Phenomenasuch as photoconductance still present open questions, as com-plications arise when one considers band bending and charging anddischarging of surface states.Commonplace methods to tune energy levels, such as adsorb-

ing molecular layers on the surface, gives the ability to controlphysical factors such as work functions, surface energies, andcharge injection/collection barriers.1,2 In ITO’s case, because itsmorphology is strongly affected by fabrication conditions andpretreatments, there remain challenges in isolating desirablefeatures of the surface for improved efficiency.3,4 Despite thesecomplications, various surface treatments (e.g., chemical, ozone,plasma, etc.) as well as adsorption of organic species2,5,6 areknown to induce variation on the properties of ITO, influencingits efficiency when part of a device. Most of these reducing/oxidative treatments have been well characterized experimen-tally. For example, it is known that treatment of ITO with water

or ozone leads to surface modifications. In ITO, film resistancechanges as H2O protonates surface sites creating hydroxylgroups,7 which in turn alter its electronic properties.8 From achemical prospective, the presence of hydroxyl groups on thesurface of ITO have been associated with the development ofdifferent oxygen species as well as an increase in the adsorptionrate of molecules.3,9 Ozone treatment has been reported toincrease the surface work function.7,10

These surface modifications lead to various degrees of surfacedipoles. It is known that work function principally can changedepending on (1) variation of the Fermi level, EF, by, for example,doping; (2) band bending in semiconductor materials; and(3) presence of a surface dipole. It was reported that the workfunction of ITO thin films increased from 4.34 to 4.47 eV afterchemical oxidation and to 4.69 eV after plasma oxidation.11

Lee et al. found that the work function of ITO increased by 0.8 eVafter inductively coupled plasma (ICP) treatment.12 Further-more, it was observed that the work function of ITO (and alsoof IO) surface increases from 4.47 (3.97) eV to 5.20 (4.58) eV by

Received: October 17, 2017Revised: December 1, 2017Published: December 6, 2017

Article

pubs.acs.org/JPCCCite This: J. Phys. Chem. C 2018, 122, 584−595

© 2017 American Chemical Society 584 DOI: 10.1021/acs.jpcc.7b10267J. Phys. Chem. C 2018, 122, 584−595

Dow

nloa

ded

via

RU

TGER

S U

NIV

on

Aug

ust 1

, 201

8 at

15:

57:1

4 (U

TC).

See

http

s://p

ubs.a

cs.o

rg/s

harin

ggui

delin

es fo

r opt

ions

on

how

to le

gitim

atel

y sh

are

publ

ishe

d ar

ticle

s.

changing the oxygen fraction in sputter gas from 0% to 6.5(10.0)%, respectively.1

IO’s most thermodynamically stable crystal structure is thebody-center cubic (bcc).13−17 has been reported that bcc is alsothe most prevalent phase when ITO is prepared through typicalthin film methods.7,18−22 However, when the tin doping levelreaches above 6%, a rhombohedral (rh) phase of ITO startsemerging as well.23,24 Because common doping levels of ITOused in photovoltaics applications are 5−15%, simulations of rhIO and ITO are of particular relevance. Moreover, ITO prepara-tion often requires deposition on other substrates (e.g., sapphire),causing it to accommodate a different lattice parameter, whichmay ultimately lead to the less thermodynamically favor-able rh phase.17,23,25−28 This suggests that models basedexclusively on the bcc structure are deemed to not fully representthe real ITO surface. To fill this gap, we embark on a thoroughsimulation of the [001] surface of IO and ITO in their rh crystalstructure. We choose this facet because its morphology isthe most similar to the commonly studied and more stable[111] surface of the bcc phase,15 and it also happens to be acommon cleavage facet of other hydroxylated trivalent metaloxides.29

Through ab initio density functional theory (DFT) simula-tions we aim at predicting morphologies and correspondingelectronic structures of several IO and ITO surfaces. We believethat understanding effects resulting from surface hydroxyl cov-erage is a necessary step to optimize surface properties andadsorption techniques. Thus, we explore the influence of thehydroxyl coverage, as well as In-atom vacancy defects on theelectronic density of states (DOS) of IO and ITO. Also, weevaluate the work function, ϕ, for all IO and ITO structuralmodels. Trends in the changes in work function with respect tovarious levels of surface modifications provides us with a clearrelation between the hydroxylation level, Sn doping, and thesurface dipole.This work is organized as follows. After describing the com-

putational details and techniques employed for the DFTsimulations, 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 withthe In-atom vacancy models. In the conclusions, we summarizeour findings and formulate a simple predictive model aiming atexplaining the basic physical interactions at play.

2. COMPUTATIONAL DETAILS

All simulations are carried out with the Quantum ESPRESSOsimulation package.30 The electronic structure is obtained atthe DFT level, using a plane wave basis set, periodic bound-ary 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 kineticenergy cut-offs for the expansion of Kohn−Sham (KS) orbitalsand electron densities are 50 and 500 Ry, respectively. To easethe SCF convergence, we apply a Gaussian smearing to theoccupations of the KS states, with a width (sigma) of 0.001 Ry.31

In geometry optimizations, we sample the FBZ at the Γ pointonly, while for single point calculations, computation of densityof states and work functions, we use a 4 × 4 × 1 k-point grid.Finally, to obtain smoother surface partial density of states(PDOS) plots, additional non-self-consistent (NSCF) calcu-lations are performed on a 8 × 8 × 1 k-point grid.

The bulk structure of IO in the rh and bcc crystal is retrievedfrom the crystal database at crystallography.net.32 To make oursurface models as realistic as possible, we include a large numberof independent atoms in the simulation cell. Specifically, we carryout simulations containing 120 atoms for the rh bulk and derivedsurface 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 an8% Sn-doped material. Bulk bcc ITO is obtained by replacingthree random In atoms with Sn atoms, which leads to a 9% Sndoping level. Input files with the given structures are accumulatedin the Supporting Information.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 imple-mented in such a way that at the surface either oxygen or metalIn/Sn atoms would be exposed. In the ITO surface slabs, two Snatoms are located on the surface neighboring each other (with asurface tin concentration of 0.016 Sn/Å2), while the other two Snatoms were randomly distributed in the layer underneath thesurface layer. Bcc IO/ITO surfaces are generated by cleaving thebulk along the 111 plane. In the ITO surface, two Sn atoms areonce again located on the surface neighboring each other, andthe remaining two are randomly distributed in the lower layer.The neighboring Sn atoms are placed in non-nearest neighborlocations. The models chosen are by no means comprehensive ofall possible doping configurations. Nevertheless, the chosenconfigurations are realistic as they reproduce the overall dopinglevel and feature dopant atoms both near the surface and deepertoward the bulk as it would be expected in realistic models.In future works, we will focus on more thorough investigations ofproperties’ fluctuations induced by local disorder in the dopingmicrostructures.To simulate the features of the 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 maintainoverall charge stoichiometry, when needed, the slabs were passi-vated at the bottom layer with iodine atoms. Modeling a stoi-chiometric slab should provide us with a more realistic picture ofthe 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 toidentify each surface/bulk. A glossary is reported in Table 1.

To investigate whether there is a correlation between theformation of gap states and the partial charges on the atoms, wecompute Bader charges of the surface atoms of IO100, ITO100,IO0, ITO0, IO0

m, and ITO0m models. Comparison of the atomic

charges of surface atoms helps us explain features seen in theDOS of IO and ITO models.Pictures of the surface morphologies have been created with

the VESTA program.33

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-bcc100% OH IO100 ITO100

66% OH IO66 ITO66

33% OH IO33 ITO33

O-terminated IO0 ITO0 IO0-bcc ITO0-bccO- and metal terminated IO0

m ITO0m

The Journal of Physical Chemistry C Article

DOI: 10.1021/acs.jpcc.7b10267J. Phys. Chem. C 2018, 122, 584−595

585

3. RESULTS3.1. Structure and Electronic Properties of Rhombohe-

dral and BCC Bulk IO and ITO. In this section, we compare IOand ITO bulk models, paying particular attention to the change ofgeometrical parameters from the original crystal structure of IO.In a nutshell, what we find is that geometry optimizations ofIO induce little change of the ionic positions compared to thecrystal structure. This validates our computational method forthese systems. The ITO bulk model is considered here only forcomparison purposes and to better understand the properties ofthe surface models presented afterward. We refer to ref 19 for amore in-depth analysis of bulk ITO models.3.1.A. Undoped Indium Oxide (IOb). Upon geometry

optimization of IOb and IOb-bcc models, we record only smalldeviations from the original crystallographic positions. Using theoptimized geometry, we compute the DOS, see Figure 1. As it isknown, semilocal exchange-correlation functionals underesti-mate material’s optical gaps. Unsurprisingly, our calculationsyield an optical gap for IOb of roughly 2.0 eV while the actualexperimental 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 opposedto experimental values ranging from 2.3 to 3.7 eV.25,29,34−38

Themajor features of the simulated DOS of these bulk systemsagree with previous investigations.15,25,34,39,40 Specifically, thevalence band displays a sharp peak at −1.0 eV.3.1.B. Indium Tin Oxide (ITOb). Upon substituting three In

atoms with Sn atoms, the geometry optimization results in amoderate geometry reorganization. Bond lengths between Snand O are shorter than the ones between In and O on average by5−10%. At same time, In−O bond lengths increase. This observa-tion is in agreement with previous investigations and in line withcommon knowledge.24 The calculated DOS for ITOb has a shapealmost identical to IOb but due to the extra electrons from the Snatoms, the Fermi level shifts toward the conduction band, seeFigure 1. Again, for ITOb our semilocal DFT calculation resultsin an underestimated band gap compared to the experimentaloptical gap for amorphous ITO which ranges between 3.58 and4.3 eV.40−42 Inspecting Figure 1, we observe almost identicaltrends in the DOS of ITOb-bcc compared to the rh phase ofITOb. For ITOb-bcc, the Fermi energy lies at the beginning of theconduction band, and the KS band gap is well within the range ofprevious theoretical predictions (1.0−2.7 eV).15,18,43 Finally, theDOS of both ITOb and ITOb-bcc have a peak arising at the top ofthe valence band at about −9.0 eV. Analysis of the PDOS showthat this peak is due to the additional Sn states in the crystal andis a known trait of the material.15,18,20,35 This peak hascontributions from Sn and In atoms. Its largest contribution isfrom s-type orbitals, however p and d also contribute.In conclusion, the simulated DOS of both phases of ITO are

consistent with the degenerate n-type semiconductor nature ofthe material, and no dramatic or apparent changes in the elec-tronic properties from the bcc phase to the rh crystal phase arepredicted by the bulk models.3.2. Simulated Surface Morphologies and Electronic

Properties of rh IO and ITO. We simulated four degrees ofsurface hydroxylation of IO and ITO to gain insight on its effectson surface morphology and properties. For both IO and ITO, thesupercell slabmodels have a surface area of 124 Å2. This is as largeas seen in previous theoretical models15 and should provide areasonable description of a realistic surface. Geometry optimiza-tion of these surfaces results in surface distortions of variousmagnitudes depending on the degree of hydroxyl coverage.The ITO surface simulations agree with experiments in that,

similarly to previously presented bulk calculations, doping thesurface with Sn decreases the metal−oxygen bond lengths byabout 5−10%.20,22,24,39 The largest distortion is localized in thevicinity of the Sn atom. We also pay attention to the surfacehydrogen bond structure that forms upon hydroxylation, as wellas the formation of oxygen dimers on the surface. Below, wefocus on each of the models considered.

3.2.A. Oxygen Terminated Rhombohedral IO and ITOSurfaces (IO0 and ITO0).As IO and ITO surfaces are often treatedwith ozone and oxygen plasma, we also considered models fea-turing oxygen-only terminated surfaces. The IO0 surface oxygenlayer moves downward toward the In atom layer, however,still noticeably above the metal layer. Each surface O atom iscoordinated to two In atoms. In ITO0, Sn has substantial effects

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 isset to zero.

The Journal of Physical Chemistry C Article

DOI: 10.1021/acs.jpcc.7b10267J. Phys. Chem. C 2018, 122, 584−595

586

on the surface morphology, inducing the migration of one Inatom to the surface from the subsurface layer. This migrationleads to a subsurface vacancy and dimerization two subsur-face oxygen atoms. On the surface, we notice the formation ofan oxygen trimer atop the surface In atoms, see Figure 2. Presenceof oxygen dimers at the surface of ITO is an accepted consequenceof the treatment with ozone3,44 especially when polycrystallinesubstrates are studied. We believe that such dimers were notfound in previous theoretical models because the oxygen ter-minated 111 facet of the bcc IO (i.e., the commonplace modelsurface for this system) only exposes tricoordinate O atoms (videinfra, the topmost oxygen layer in IO0-bcc collapses onto themetal layer). These atoms are unlikely to dimerize due to the highcoordination. Clearly, this is a bias introduced by the bcc modelsystems. In fact, it is known from experiments that for surfaceterminations other than 111, oxygen dimers occur.45

In Figure 3 we show the PDOS of the IO0 and ITO0 surfaces.Both surfaces show some key changes from the previouslypresented bulk DOS. Specifically, due to the formation of themolecular oxygen species as well as undercoordinated surfaceoxygen atoms, the Fermi energy of ITO now also lies at the onsetof the valence. We also notice an additional peak at about −8 eVin the DOS of ITO0. PDOS shows that unlike the similar peakfound in the DOS of ITO100 this one is not associated with thesurface Sn atoms, but rather to the surface O trimers (vide infra inthe section devoted to ITO100). This is important because itshows how the electronic structure of the Sn−O bond is radicallydifferent in the ITO0 model compared to the bulk (ITOb) andthe ITO100 surface models.The PDOS plots present metallic character induced by the

doubly coordinated surface oxygen atoms in both IO0 and ITO0models, as well as that of molecular oxygen species near theFermi level in ITO0 with the presence of partially occupied states.This is elucidated by the difference of Bader charges of theoxygens of surface dimer and trimer (about−0.81) from those ofbulk oxygens (about −1.2).3.2.B. Mixed Metal Terminated Rhombohedral IO and ITO

Surfaces. The IO0m and ITO0

m models are obtained from cutt-ing the rh IO bulk along the 001 facet in such a way that bothmetal and oxygen atoms are exposed on the surface. Experiments

have reported that IO and ITO can have this detailed surfacemorphology.8,19,46 The optimized IO0

m surface has oxygen atomstricoordinated to In atoms on the same plane (see Figure 4a).On the other hand, ITO0

m shows extensive distortions among thesurface Sn atoms, with Sn atoms slightly raised above the surfacetoward the vacuum (see Figure 4b). However, surface oxygenatoms in ITO0

m remain 3 coordinated, and due to the fact that theSn atoms move slightly above the surface, the surface In/Sn−Obond lengths slightly increase with respect to IO0

m.On one hand, the PDOS of IO0

m in Figure 5a has the Fermilevel lying within the valence band (similarly to the IO0and ITO0 surfaces) and a band gap of about 2.0 eV. On theother hand, the PDOS of ITO0

m in Figure 5b is significantlydifferent: new surface states emerge in the gap due to Sn

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

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

The Journal of Physical Chemistry C Article

DOI: 10.1021/acs.jpcc.7b10267J. Phys. Chem. C 2018, 122, 584−595

587

hybridizing with O. In conclusion, orbital hybridization betweenthe s orbitals of Sn and the p orbitals of O arises due to the additionof Sn to the surface and is enabled by the associated surfacedistortions.

In IO0m 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, In atoms arecharged +1.73, while the second layer their charge is between+1.86 and +1.89. Finally, in ITO0

m, the charges of In and Sn atomsare surprisingly similar (+1.70), while in the other ITO modelsSn was up to +1.0 more positive than In.

3.2.C. 100% Hydroxylated Rhombohedral IO and ITOSurfaces.The 100% hydroxylated IO surface (IO100) features thesmallest ionic distortions compared to the bulk. The large surfacearea afforded by the model, allows us to inspect the morphologyand hydrogen-bond network arising from the many independentsurface hydroxyl groups. These sample multiple conformationsduring the geometry optimization. In Figure 6a we present thefinal configuration of the surface with a focus on the two-dimensional H-bond network.We observe the formation of threeH-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 surfaceOH 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-bondpattern compared to IO100. Figure 6b shows one of the OH ispointing away from the Sn atom. A simple way to explain this issuggested by comparing the Bader charges computed for the Inatoms (+1.8) and Sn atoms (+2.3). The larger net positive chargeon 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)

Figure 4. Surface structure resulting after optimization of (a) IO0m and (b) ITO0

m models.

Figure 5.Different states occur in (a) IO0m and (b) ITO0

m surface. PDOSin arbitrary units, energy in eV. The Fermi energy is set to 0.

The Journal of Physical Chemistry C Article

DOI: 10.1021/acs.jpcc.7b10267J. Phys. Chem. C 2018, 122, 584−595

588

with a slightly larger band gap (2.0 eV/2.5 eV for IO100 andITO100, respectively) and the Fermi energy lying within the gap.

3.2.D. 66% Hydroxylated Rhombohedral IO and ITOSurfaces. We have generated IO66 and ITO66 models visible inFigure 8a and b, respectively. We generated these structuresadding 8 hydroxyl groups in random positions on the previouslyconsidered IO0 and ITO0 surfaces. After optimization, we noticethe formation of an oxygen dimer on the surface of IO66.Whereasno oxygen dimers are observed in ITO66, see Figure 8b. We notean interesting detail in the PDOS of IO66 in Figure 9a. There aretwo new prominent peaks from gap states, and the peak at theFermi level is due to the oxygen dimer. The ITO66 PDOS inFigure 9b did not have any surface states within the band gap,which is consistent with the fact that no oxygen dimers wereformed on the surface of this particular model. However, wenotice a shoulder in the valence band of ITO66 facing the gap dueto doubly coordinated oxygens on the surface. Thus, to betterunderstand the role of these doubly coordinated oxygens, we alsoindicated their PDOS in Figure 9, which clearly demonstrate thatthese oxygens are responsible for the metallic shape of the PDOSnear the Fermi level in a way that resembles the IO0/ITO0models.The orientation of the hydroxyl groups differs in IO66 and

ITO66. Specifically, in both IO66 and ITO66, three out of eightOH groups lie essentially flat on the surface pointing toward ahollow site. The five remaining OH groups point upward with atilt angle. However, the ITO66 OH groups are almost perpen-dicular to the surface compared to the IO66, which instead has asignificant tilt angle. From these considerations, we expect thatthe contribution from the OH groups to the surface dipole inIO and ITO to be different (vide infra in the section devoted towork functions).

3.2.E. 33% Hydroxylated Rhombohedral IO and ITOSurfaces. Our models for the IO33 and ITO33 surfaces containfour hydroxyl groups each in random positions. The optimizedsurface of IO33 contains adjacent oxygen dimers. Once again,ITO33 features no surface oxygen dimers (Figure 10). An impor-tant difference between these IO and ITO surfaces is theorientation of the hydroxyl groups. Specifically, in IO33 one out offour OH groups lies essentially flat on the surface pointingtoward a hollow site. The three remaining OH groups pointupward (although with a slight tilt angle). In ITO33, two out offour surface OH groups lie flat pointing away from the Sn atom inthe same fashion as in the ITO100 surface. Once again, we expectthat the surface dipole of these two structures be different. Videinfra in the section devoted to work functions for a more detailedcharacterization of this effect.Shifting our attention to the PDOS of IO33 in Figure 11, we

notice that an unoccupied gap state at about 0.5 eV above theFermi level arises due to the surface oxygen dimers (as indicatedin the figure by the PDOS of the surface O2 fragment). On thecontrary, there is no evidence of any band gap surface states inthe DOS of ITO33. The Fermi level once again lies in the valenceband, as seen earlier in previous surfaces doped with Sn. More-over, as in the models IO66 and ITO66, similar role of the doublycoordinated surface oxygens is seen in Figure 10.

3.3. Simulated Surface Morphologies and ElectronicProperties of BCC IO and ITO. For sake of completeness, wealso carried out a comparison to the most commonly studied bccsurfaces of IO and ITO. We generate the thermodynamicallymore stable, [111] surfaces of the bcc phase. Upon relaxing theIO0-bcc surface we find the O and In atoms are on the surfaceplane, with O being tricoordinated to In atoms. The In-surface

Figure 7. Partial density of Kohn−Sham states (PDOS) for surfacesderived 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.

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

The Journal of Physical Chemistry C Article

DOI: 10.1021/acs.jpcc.7b10267J. Phys. Chem. C 2018, 122, 584−595

589

atoms are penta- and hexacoordinated to the surrounding surfaceand subsurface O atoms. The bond lengths between surfaceIn and O atoms are between 2.12 and 2.44 Å.The ITO0-bcc surface also has a similar structure with penta- and

hexacoordinated In/Sn surface atoms and O atoms that are onlyslightly above the surface plane while still being tricoordinated.The bond lengths of surface In atoms and O atoms are between2.13 and 2.36Å,while Sn−Obond lengths range from2.08 to 2.23Å.As we can see in Figure 12, we do not record extensive surface

distortions in either surfaces.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 modelthe Fermi level lies at the edge of the valence band, in agreementwith previous experimental and theoretical results.15,40 The changein the DOS of ITO compared to IO is due to the addition of Sninto the model, rather than the marginal geometrical distortions.Comparison of the morphology and DOSs obtained for rh

IO0 and ITO0 models with IO0-bcc and ITO0-bcc highlightsdifferences between these two phases. The band gap of IO0-bcc issmaller than those calculated for the rh IO slabs. As pointedout before, the formation of oxygen cluster and the presence ofundercoordinated oxygens at the surface of rh ITO (see Figure 2where a surface oxygen dimer and a trimer are highlighted) is the

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

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

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

The Journal of Physical Chemistry C Article

DOI: 10.1021/acs.jpcc.7b10267J. Phys. Chem. C 2018, 122, 584−595

590

most profound difference compared to bcc models. Contrary tothe rh models, in bcc models it is also seen that the surfaceoxygens in IO0-bcc/ITO0-bcc are mostly on the same plane asthe metal atoms.

3.4. Surface Defects. Surface defects, such as an indiumatom vacancies, have the potential of creating band gap surfacestates. It is known that electronic properties of a material can begreatly affected by the presence of defects in the bulk.47 For thisreason, we have simulated the bulk of ITO with an In vacancy(ITObv), as well as a series of derived surfaces characterizedby various degrees of hydroxylation (ITO100−0v). The vacancieswere introduced at the surface. Although in principle thereare several nonequivalent possible vacancy sites, due to therelatively small size of the employed slab model, only two siteswere available and for our simulations we chose the onesnearest to the Sn atom. Surface morphologies and PDOSsfor the various vacancy models are presented in SupportingInformation (Figures S1−S5). The surface morphologiesof all ITO models with an In atom vacancy show significantsurface distortions. However, only the DOS of ITO66v andITO33v show surface states in the gap. These states are due tothe formation of oxygen dimers at the surface. For the remainingsurface models, no gaps states emerge.

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

Figure 12. Optimized surface morphology of (a) IO0-bcc and (b) ITO0-bcc.

The Journal of Physical Chemistry C Article

DOI: 10.1021/acs.jpcc.7b10267J. Phys. Chem. C 2018, 122, 584−595

591

Furthermore, in order to see the effect of levels of surfacehydroxyl coverage: 100%, 66%, 33%, and 0% on the local dis-tortions, we determine the ligand length distances (Sn−O andIn−O) as well as the separation between In and Sn ions forboth rf and bcc phases. These values are tabulated in Table 2where we see that the distance between In−Sn decreases in thesurface models with respect to bulk model. As expected that the

substitution of Sn4+ for In3+ ion decreases the O ligand distances(Sn−O < In−O) due to both leading more electrostatic attrac-tion and having smaller ionic size, that is, rSn4+ = 0.71 Å/rIn3+ =0.80 Å. It is important to state that there is no unique change onthe distances with the levels of surface hydroxyl coverage. Whenwe compare rf ITO0v model with bcc ITO0v model we see thatthe difference between max and min values of each length ishigher in ITO0v-bcc. Moreover, having various lengths for eitherSn−O or In−O and In−Sn reveals that the symmetry around theIn and Sn vacancy sites was broken after the lattice relaxation.

3.5. Work Functions (ϕ) and Surface Dipole. In thissection, we present simulations of work functions, ϕ, of all IOand ITO structural models. The calculated work functions aredepicted in Figures 14 and 15. In the Supporting Information(Table S1) we collect all work function numerical values. Uponinspection of the figure, we notice that ϕ (IO) > ϕ (ITO) foralmost all models. We also see that the introduction of vacanciesleads to an increase of work functions for almost all casesconsidered.The change of work function of IO and ITO models with

respect to OH coverage are shown in Figure 14. We notice thatϕsignificantly decreases with increasing OH coverage.The change of work function (Δϕ) between ITO and ITO

with vacancy models as a function of surface OH coverage isshown in Figure 15.We notice a trend, for example,Δϕ generallyincreases with increasing OH coverage. This trend is explainedby a simple solvation effect. The OH groups stabilize the vacancysites by pointing toward them. This effect is noticeable in allstructures but particularly in the 100% hydroxylated model(ITO100v).In the literature, we find experimental values of ϕ for dif-

ferently prepared IO and ITO thin films (different substratetemperature (Tsub), oxygen fraction in sputter gas (%O2).

1 ForITO, at Tsub = 400 °C, 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 at400 °C and 0% O2, 3.97 eV at 200 °C and 0% O2, at 400 °C and0%O2, and 4.58 eV at 200 °C and 10%O2 fraction.

1 The value ofaround 5.0 eV calculated by us for IO100, ITO0-bcc, ITO33v, andITO0

m is in fairly good agreement with most of these values.In addition, the experimental trend of increasing ϕ with increas-ing oxygen on the surface is nicely reproduced by our simulations.We collect plane-averaged electrostatic potential plots for all

IO and ITOmodels in Supporting Information (Figures S6−S17).To better understand the computed trends, we decided to also

Table 2. Various Atom−Atom Lengths (Å) for the Effect ofLevels of Surface Hydroxyl Coverage in Vacancy IncludedITO Modelsa

model In−Sn Sn−O In−O

ITObv 3.286/4.121 2.058/2.199 2.141/2.317ITO100v 3.144/3.659 2.055/2.237 2.151/2.297ITO66v 3.173/3.293 2.020/2.267 2.117/2.300ITO33v 3.276/3.385 2.169/2.246 2.040/2.257ITO0v 3.139/3.289 2.051/2.240 2.141/2.319ITO0

m 3.115/3.378 2.060/2.206 2.135/2.282ITObv-bcc 3.327/3.825 2.077/2.183 2.117/2.288ITO0v-bcc 3.253/4.066 1.990/2.327 2.103/2.352

aThe values are given as min/max.

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.

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

The Journal of Physical Chemistry C Article

DOI: 10.1021/acs.jpcc.7b10267J. Phys. Chem. C 2018, 122, 584−595

592

investigate the size of the surface dipole contribution toϕ (whichwe indicate by ϕd). The dipole was estimated by computing thedifference between the left and right vacuum levels.48 The resultsare given in Table 3 for the all models under study. The mostprofound contribution was found for the IO33/ ITO33 models,which we relate to the orientation of OH groups on the opti-mized surface of IO33/ ITO33 (see Figure 10), that is, when theOH groups are vertical, we witness a strong reduction of the workfunction (e.g., for 100% OH coverage), and when they are tilted(e.g., for 33 and 66% OH coverage), the reduction of workfunction is smaller.Turning to the mixed oxygen−metal terminated surfaces when

we compare the value of ϕd and surface structure of IO0m with

those of ITO0m (Figure 4), we see that the introduction of Sn ions

leads to a considerable reduction in ϕd. We correlate this withthe slightly raised surface Sn atom in ITO0

m compared to IO0m.

The presence of In vacancy (considered in the ITO0vm model)

results in a significant increase of ϕd due to the fact that the Snatom fills in the vacancy site. A similar trend is also seen in bccsymmetry models (IO0-bcc, ITO0-bcc, and ITO0v-bcc). How-ever, the value ofϕd for bcc models is quite smaller than that of rhmodels. This is consistent with the observation that the oxygenlayer in bcc models lies at the same level of the metal layer, thus,resulting in an overall small surface dipole.

4. CONCLUSION

To conclude, we have computationally investigated metal oxidesurfaces based on indium oxide (IO) and its Sn-doped analog,indium tin oxide (ITO). Our investigations aim at drawing a “bigpicture” of the factors influencing surface electronic properties.Obtaining such a big picture is of pivotal importance because itultimately allows us to control the electronic properties ofthe surface. Controlling such properties is at the foundation oftoday’s rational design of energy materials and the involvedinterfaces. Specifically, we analyzed the surface density of statesand the work function in correlation to the surface morphology.The models considered are by all means nonstandard. Theyinclude four surface hydroxylation levels ranging from 0% to100%, two bulk IO polymorphs (bcc and rhombohedral), as wellas In vacancy models.Our simulations reveal a trend in both the presence of sur-

face states, as well as the work functions. The 100% and 0% OHcovered surfaces constitute the two extrema of behaviors, withthe oxygen terminated surfaces featuring the highest workfunctions, strongest surface dipole, and least coordinated sur-face oxygens leading to a partially filled valence band. The 100%OH covered surface mostly behaves like the bulk, and featuresreduced work functions and surface dipoles compared to theother models. By correlating these findings with the surfacemorphologies, we draw a simple conclusion that we summarize inFigure 16.We also find that with low OH coverage, there is a tendency to

form molecular oxygen species on the surface. We find thesespecies to be partially negatively charged and to present levelseither in the gap or at the edge of the valence band.Furthermore, from the plotted density of states we evince a

general trend, that is, Sn doping moves the Fermi level upwardin energy. Conversely, the more oxidized the surface is (e.g.,the lower the OH coverage), the more the Fermi level moves

Figure 15. Change of work function difference (Δϕ) between ITO andITO with vacancy models with respect to the OH coverage.

Table 3. Calculated Surface-Averaged Electrostatic PotentialChange (in eV) Due to Dipole Correction (ϕd) for All IO andITO Models

IO IO0 IO0m IO100 IO66 IO33 IO0-bcc

3.76 2.06 1.52 2.97 3.40 0.21ITO ITO0 ITO0

m ITO100 ITO66 ITO33 ITO0-bcc2.81 1.15 0.00 2.63 4.41 −0.32

ITO ITO0v ITO0vm ITO100v ITO66v ITO33v ITO0v-bcc

2.88 1.82 1.20 2.70 2.37 0.49

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

The Journal of Physical Chemistry C Article

DOI: 10.1021/acs.jpcc.7b10267J. Phys. Chem. C 2018, 122, 584−595

593

toward the valence band. In the limiting case of a purely oxygen-terminated surface, the Fermi level enters the valence band givingrise to a metallic phase on the surface which is dominated by low-coordination oxygen atoms. Because we employ a slab model, itis very difficult for our simulations to capture band bendingeffects. However, in the oxidized surface cases likely the valenceband would bend upward (due to e− accumulation filling theempty oxygen states). This picture is consistent with experi-mental findings reporting two-dimensional e− gas accumulationat the surface of oxidized ITO.1

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.jpcc.7b10267.

Collection of input structures of all models considered,Figures S1−S17, and Table S1 (PDF).

■ AUTHOR INFORMATIONCorresponding Author*E-mail: m.pavanello@rutgers.edu.ORCIDIris Visoly-Fisher: 0000-0001-6058-4712Michele Pavanello: 0000-0001-8294-7481NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis material is based on work supported by the National ScienceFoundation under Grant Nos. DMR-1507812, DMR-1742807,and OIIA-1404739. H.L.S. is grateful to the High-tech, Biotechand ChemoTech Scholarship for outstanding doctoral candidates.

■ REFERENCES(1) Gassenbauer, Y.; Schafranek, R.; Klein, A.; Zafeiratos, S.; Havecker,M.; Knop-Gericke, A.; Schlogl, R. Surface states, surface potentials, andsegregation at surfaces of tin-doped In2O3. Phys. Rev. B: Condens. MatterMater. Phys. 2006, 73 (24), 245312.(2) Paramonov, P. B.; Paniagua, S. A.; Hotchkiss, P. J.; Jones, S. C.;Armstrong, N. R.; Marder, S. R.; Bredas, J. L. Theoretical character-ization of the indium tin oxide surface and of its binding sites foradsorption of phosphonic acid monolayers. Chem. Mater. 2008, 20,5131−5133.(3) Morales, E. H.; Diebold, U. The structure of the polar Sn-dopedindium oxide (001) surface. Appl. Phys. Lett. 2009, 95 (1), 6−9.(4) Cheng, W.; He, L.; Fan, X.; Ou, Q.; Liang, R. Surface modificationof indium tin oxide by oxygen plasma immersion ion implantation. Sci.China: Technol. Sci. 2013, 56 (4), 925−929.(5) Zhou, Y.; Fuentes-Hernandez, C.; Shim, J.; Meyer, J.; Giordano, A.J.; Li, H.; Winget, P.; Papadopoulos, T.; Cheun, H.; Kim, J.; et al. Auniversal method to produce low-work function electrodes for organicelectronics. Science 2012, 336, 327−332.(6) Li, H.; Paramonov, P.; Bredas, J. Theoretical study of the surfacemodification of indium tin oxide with trifluorophenyl phosphonic acidmolecules: impact of coverage density and binding geometry. J. Mater.Chem. 2010, 20, 2630−2637.(7) Swint, A. L.; Bohn, P. W. Effect of the interfacial chemicalenvironment on in-plane electronic conduction of indium tin oxide: roleof surface charge, dipole magnitude, and carrier injection. Langmuir2004, 20 (28), 4076−4084.(8) Donley, C.; Dunphy, D.; Paine, D.; Carter, C.; Nebesny, K.; Lee, P.;Alloway, D.; Armstrong, N. R. Characterization of indium−tin oxideinterfaces using x-ray photoelectron spectroscopy and redox processes

of a chemisorbed probe molecule: effect of surface pretreatmentconditions. Langmuir 2002, 18 (16), 450−457.(9) Purvis, K. K. L.; Lu, G.; Schwartz, J.; Bernasek, S. L. Surfacecharacterization and modification of indium tin oxide in ultrahighvacuum. J. Am. Chem. Soc. 2000, 122 (30), 1808−1809.(10) Hanson, E. L.; Guo, J.; Koch, N.; Schwartz, J.; Bernasek, S. L.Advanced surface modification of indium tin oxide for improved chargeinjection in organic devices. J. Am. Chem. Soc. 2005, 127 (17), 10058−10062.(11) Christou, V.; Etchells, M.; Renault, O.; Dobson, P. J.; Salata, O. V.;Beamson, G.; Egdell, R. G. High resolution x-ray photoemission study ofplasma oxidation of indium-tin-oxide thin film. J. Appl. Phys. 2000, 88,5180.(12) Lee, K. H.; Jang, H. W.; Kim, K.-B.; Tak, Y.-H.; Lee, J.-L.Mechanism for the increase of indium-tin-oxide work function by O2

inductively coupled plasma treatment. J. Appl. Phys. 2004, 95, 586.(13) Agoston, P.; Erhart, P.; Klein, A.; Albe, K. Geometry, electronicstructure and thermodynamic stability of intrinsic point defects inindium oxide. J. Phys.: Condens. Matter 2009, 21, 455801.(14) Hotovy, I.; Kups, T.; Hotovy, J.; Liday, J.; Buc, D.; Caplovicova,M.; Rehacek, V.; Sitter, H.; Simbrunner, C.; Bonnani, A.; et al. Structuralevolution of sputtered indium oxide thin films. J. Electr. Eng. 2010, 61(6), 382−385.(15) Walsh, A.; Catlow, C. Structure, stability and work functions ofthe low index surfaces of pure indium oxide and Sn-doped indium oxide(ITO) from density functional theory. J. Mater. Chem. 2010, 20, 10438−10444.(16) Wang, C. Y.; Dai, Y.; Pezoldt, J.; Lu, B.; Kups, T.; Cimalla, V.;Ambacher, O. Phase stabilization and phonon properties of singlecrystalline rhombohedral indium oxide. Cryst. Growth Des. 2008, 8 (4),1257.(17) Garcia-Domene, B.; Ortiz, H. M.; Gomis, O.; Sans, J.; Manjon, F.J.; Munoz, A.; Rodríguez-Hernandez, P.; Achary, S. N.; Errandonea, D.;Martínez-García, D.; Romero, A. H.; et al. High-pressure latticedynamical study of bulk and nanocrystalline In2O3. J. Appl. Phys.2012, 112, 123511.(18) Brewer, S. H.; Franzen, S. Calculation of the electronic and opticalproperties of indium tin oxide by density functional theory. Chem. Phys.2004, 300, 285−293.(19) Kim, H.; Gilmore, C. M.; Pique, a.; Horwitz, J. S.; Mattoussi, H.;Murata, H.; Kafafi, Z. H.; Chrisey, D. B. Electrical, optical, and structuralproperties of indium-tin-oxide thin films for organic light-emittingdevices. J. Appl. Phys. 1999, 86 (1999), 6451.(20) Korber, C.; Krishnakumar, V.; Klein, A.; Panaccione, G.; Torelli,P.; Walsh, A.; Da Silva, J.; Wei, S.; Egdell, R.; Payne, D. Electronicstructure of In2O3 and Sn-doped In2O3 by hard x-ray photoemissionspectroscopy. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 81,165207.(21) Meng, L.; dos Santos, M. Properties of indium tin oxide filmsprepared by rf reactive magnetron sputtering at different substratetemperature. Thin Solid Films 1998, 322, 56−62.(22) Parent, P.; Dexpert, H.; Tourillon, G.; Grimal, J. Structural studyof tin-doped indium oxide thin films using x-ray absorption spectros-copy and x-ray diffraction. J. Electrochem. Soc. 1992, 139 (1), 282.(23) Cha, G.-Y.; Baek, W.-W.; Lee, S.-T.; Lim, J.-O.; Huh, J.-S. Effectsof crystal structure and particle size on ethanol gas sensing character-istics of nanocrystalline ITO thick film. J. Ceram. Soc. Japan 2004, 112(1305), 252−258.(24) Nadaud, N.; Lequeux, N.; Nanot, M.; Jove, J.; Roisnel, T.Structural studies of tin-doped indium oxide (ITO) and In4Sn3O12. J.Solid State Chem. 1998, 135, 140−148.(25) Fuchs, F.; Bechstedt, F. Indium-oxide polymorphs from firstprinciples: Quasiparticle electronic states. Phys. Rev. B: Condens. MatterMater. Phys. 2008, 77 (15), 155107.(26) Sorescu, M.; Diamandescu, L.; Tarabasanu-Mihaila, D.;Teodorescu, S. Nanocrystalline rhombohedral In2O3 synthesized byhydrothermal and postannealing pathways. J. Mater. Sci. 2004, 39, 675−677.

The Journal of Physical Chemistry C Article

DOI: 10.1021/acs.jpcc.7b10267J. Phys. Chem. C 2018, 122, 584−595

594

(27) King, P. D. C.; Veal, T. D.; Payne, D. J.; Bourlange, A.; Egdell, R.G.; McConville, C. F. Surface electron accumulation and the chargeneutrality level in In2O3. Phys. Rev. Lett. 2008, 101 (11), 116808.(28) Kachouane, A.; Addou, M.; Bougrine, A.; El Idrissi, B.; Messoussi,R.; Regragui, M.; Bernede, J. C. Preparation and characterisation of tin-doped indium oxide films. Mater. Chem. Phys. 2001, 70, 285−289.(29) King, P. D. C.; Veal, T. D.; Fuchs, F.; Wang, C. Y.; Payne, D. J.;Bourlange, A.; Zhang, H.; Bell, G. R.; Cimalla, V.; Ambacher, O.; Egdell,R. G.; et al. Band gap, electronic structure, and surface electronaccumulation of cubic and rhombohedral In2O3. Phys. Rev. B: Condens.Matter Mater. Phys. 2009, 79 (20), 205211.(30) Giannozzi, P.; Baroni, S.; Bonini, N.; Calandra, M.; Car, R.;Cavazzoni, C.; Ceresoli, D.; Chiarotti, G. L.; Cococcioni, M.; Dabo, I.;et al. Quantum ESPRESSO: a modular and open-source softwareproject for quantum simulations of materials. J. Phys.: Condens. Matter2009, 21, 395502.(31) Perdew, J. P.; Burke, K.; Ernzerhof, M. of Physics, D.; Quantumtheory group tulane university, N. O. L. 70118 J. generalized gradientapproximation made simple. Phys. Rev. Lett. 1996, 77 (18), 3865−3868.(32) Karazhanov, S. Z.; Ravindran, P.; Vajeeston, P.; Ulyashin, A.;Finstad, T. G.; Fjellvag, H. Phase stability, electronic structure, andoptical properties of indium oxide polytypes. Phys. Rev. B: Condens.Matter Mater. Phys. 2007, 76 (7), 75129.(33) http://scripts.iucr.org/cgi-bin/paper?S0021889811038970.(34) Wang, C. Y.; Cimalla, V.; Romanus, H.; Kups, T.; Ecke, G.;Stauden, T.; Ali, M.; Lebedev, V.; Pezoldt, J.; Ambacher, O. Phaseselective growth and properties of rhombohedral and cubic indiumoxide. Appl. Phys. Lett. 2006, 89 (1), 77−80.(35) Erhart, P.; Klein, A.; Egdell, R. G.; Albe, K. Band structure ofindium oxide: Indirect versus direct band gap. Phys. Rev. B: Condens.Matter Mater. Phys. 2007, 75 (15), 153205.(36) Bender, M.; Katsarakis, N.; Gagaoudakis, E.; Hourdakis, E.;Douloufakis, E.; Cimalla, V.; Kiriakidis, G. Dependence of thephotoreduction and oxidation behavior of indium oxide films onsubstrate temperature and film thickness. J. Appl. Phys. 2001, 90 (10),5382−5387.(37) Weiher, R. L. Electrical properties of single crystals of indiumoxide. J. Appl. Phys. 1962, 33 (9), 2834−2839.(38) Weiher, R. L.; Ley, R. P. Optical properties of indium oxide. J.Appl. Phys. 1966, 37 (1), 299−302.(39) Tripathi, M. N.; Shida, K.; Sahara, R.; Mizuseki, H.; Kawazoe, Y.First-principles analysis of structural and opto-electronic properties ofindium tin oxide. J. Appl. Phys. 2012, 111, 103110.(40)Walsh, A.; Da Silva, J. L. F.;Wei, S. H.; Korber, C.; Klein, A.; Piper,L. F. J.; Demasi, A.; Smith, K. E.; Panaccione, G.; Torelli, P.; et al. Natureof the band gap of In2O3 revealed by first-principles calculations and x-ray spectroscopy. Phys. Rev. Lett. 2008, 100 (16), 167402−1.(41) Liu, X. DRIFTS study of surface of γ-alumina and itsdehydroxylation. J. Phys. Chem. C 2008, 112 (13), 5066−5073.(42) Malathy, V.; Sivaranjani, S.; Vidhya, V. S.; Prince, J. J.;Balasubramanian, T.; Sanjeeviraja, C.; Jayachandran, M. Amorphousto crystalline transition and optoelectronic properties of nanocrystallineindium tin oxide (ITO) films sputtered with high rf power at roomtemperature. J. Non-Cryst. Solids 2009, 355 (28−30), 1508−1516.(43) Bai, L. N.; Wei, Y. P.; Lian, J. S.; Jiang, Q. Stability of indium-tin-oxide and its optical properties: A first-principles study. J. Phys. Chem.Solids 2013, 74 (3), 446−451.(44) Zhou, C.; Li, J.; Chen, S.; Wu, J.; Heier, K. R.; Cheng, H. First-principles study on water and oxygen adsorption on surfaces of indiumoxide and indium tin oxide nanoparticles. J. Phys. Chem. C 2008, 112,14015−14020.(45) Hohmann, M. V.; Agoston, P.; Wachau, A.; Bayer, T. J. M.; Brotz,J.; Albe, K.; Klein, A. Orientation dependent ionization potential ofIn2O3: a natural source for inhomogeneous barrier formation atelectrode interfaces in organic electronics. J. Phys.: Condens. Matter2011, 23, 334203.(46) Carvalho, C. N. De; Botelho, a M.; Amaral, a; Brogueira, P.;Lavareda, G. Effect of substrate temperature on the surface structure,

composition and morphology of indium-tin oxide films. Surf. Coat.Technol. 2000, 124, 70−75.(47) Harvey, S. P.; Mason, T. O.; Gassenbauer, Y.; Schafranek, R.;Klein, A. Surface versus bulk electronic/defect structures of transparentconducting oxides: I. Indium oxide and ITO. J. Phys. D: Appl. Phys. 2006,39 (18), 3959−3968.(48) Cornil, D.; Li, H.; Wood, C.; Pourtois, G.; Bredas, J.; Cornil, J.Work-function modification of Au and Ag surfaces upon deposition ofself-assembled monolayers: influence of the choice of the theoreticalapproach and the thiol decomposition scheme. ChemPhysChem 2013,14, 2939−2946.

The Journal of Physical Chemistry C Article

DOI: 10.1021/acs.jpcc.7b10267J. Phys. Chem. C 2018, 122, 584−595

595