Binding Modes of Fluorinated Benzylphosphonic Acids on the Polar

Article pubs.acs.org/JPCC

Binding Modes of Fluorinated Benzylphosphonic Acids on the Polar ZnO Surface and Impact on Work Function Christopher Wood,† Hong Li,† Paul Winget,† and Jean-Luc Brédas*,†,‡ †

School of Chemistry & Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, United States ABSTRACT: The interfaces formed between a zinc-terminated polar zinc oxide (0002) surface and a series of chemisorbed fluorinated benzylphosphonic acids have been studied at the density functional theory level. The results indicate that there occur substantial changes in the adsorption energy and surface work function whether the binding mode is bidentate or tridentate. Also, the trends and magnitude of the various factors that determine the total modifications in work function markedly vary between the two binding modes. We have also calculated the oxygen core-level binding energy shifts of the PO3 moiety with respect to the oxygen atoms in bulk ZnO; good agreement is obtained between the calculated values of the core-level binding energy shifts for the tridentate binding mode and available X-ray photoelectron spectroscopy data.

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The work by Perkins61 compared the use of hexylphosphonic acid and hexanethiol as surface modifiers for ZnO; it was found that the phosphonic acid derivative forms tridentate binding phosphonate monolayers that provide both better corrosion resistance against Brønsted acids and better thermal stability in comparison to monolayers formed from the thiol analogue. An experimental study based on XPS and IRRAS has also identified predominantly a tridentate binding mode for the adsorption of PA molecules on an oxygen-plasma-treated ZnO surface,31 while Fourier transform infrared spectroscopy (FT-IR) experiments on ZnO wafers and nanowires have pointed to a coexistence of both tridentate and bidentate configurations.35 It is useful to keep in mind that, due to a varying number of oxygen atoms bound to the surface and possible involvement of hydrogen bonding, variations in binding mode can lead to different adsorption energies and different orientations of the modifier with respect to the surface normal (in which case the component of the molecular dipole moment perpendicular to the surface will vary). These two factors can lead to substantial changes in surface work function. Therefore, to have the ability to reliably tune the surface properties of ZnO upon modification with PAs requires a detailed understanding of the nature of the PA binding to ZnO. Here, a series of fluorinated phosphonic acid self-assembled monolayers (SAMs) chemisorbed on the technologically relevant zinc-terminated polar ZnO(0002) surface have been modeled by taking into account the two major binding modes, bidentate and tridentate, proposed in earlier experimental studies.31,35,61 The adsorption energies for the two binding configurations have been calculated along with the 1s core-level binding energy shifts for the oxygen atoms in the PO3 moiety

INTRODUCTION Zinc oxide (ZnO) is a transparent conductive oxide that has attracted significant attention due to recent extensive applications as electron-transport material in dye-sensitized and hybrid solar cells (DSC and HSC),1−7 as electronselective/hole-blocking interlayer in organic solar cells (OSC) and light-emitting diodes (OLEDs) with inverted device structures,4,6,8−28 and in sensor applications.29 In all these instances, charge transfer across the interface between ZnO and an organic layer plays a key role in determining the device performance. Therefore, chemical modifications of the ZnO surface aiming at controlling the interfacial mechanical and electrical properties have been explored in a number of recent experimental works.30−35 It is well established that chemical modification of metal or metal oxide surfaces with small molecule adsorbates provides a method for tailoring the inorganic/organic interface to improve efficiency in organic electronics applications.36−38 The smallmolecule surface modifiers generally used in these applications include silanes, 30 amines, 39−41 thiols, 32,33,42 carboxylic acids,34,43−48 and phosphonic acids.31,35,49 Among these groups, phosphonic acids (PAs), due to their robust binding on metal oxide surfaces and easy processability, have recently been extensively used to successfully modify indium tin oxide (ITO) surfaces.50−57 X-ray photoelectron spectroscopy (XPS) and infrared reflection−absorption spectroscopy (IRRAS) experiments have shown that PAs can form stable monolayers on ITO, generally in a bidentate configuration, that lead to a more homogeneous surface with a lower surface energy and better interfacial compatibility with organic overlayers.58 Theoretical modeling based on DFT calculations has also demonstrated that PA monolayers can form on the ITO surfaces in both bidentate and tridentate configurations.59,60 The modification of ZnO surfaces with PA molecules has been characterized in several recent experimental studies.31,35,61 © 2012 American Chemical Society

Received: May 24, 2012 Revised: July 24, 2012 Published: August 8, 2012 19125

dx.doi.org/10.1021/jp3050725 | J. Phys. Chem. C 2012, 116, 19125−19133

The Journal of Physical Chemistry C

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with respect to the oxygen atoms in the bulk zinc oxide; the latter results have also been compared to experimental data.31,35,61 The change in the surface work function upon application of the SAMs has been determined, and the factors contributing to the work-function modification have been calculated individually and compared among all the investigated systems. Our work provides a better understanding of the factors that determine the changes in the observed surface work functions, which can be used to guide the synthesis of new phosphonic acids for device applications.

Scheme 1. Possible Metal Oxide Binding Modes for Phosphonic Acids

COMPUTATIONAL DETAILS a. Surface and Choice of Compounds. The theoretical model of the zinc-terminated polar (0002) ZnO surface used in this investigation consists of an orthogonal 6.50 × 5.63 Å surface unit cell containing four zinc and four oxygen atoms within each Zn−O layer in a repeated slab configuration. The slabs are separated by a vacuum space larger than 20 Å.62 Each slab consists of six Zn−O layers with the lower three layers frozen at the optimized crystal structure; the top three layers, along with any surface adsorbate, are allowed to relax over the course of the geometry optimizations. To passivate the oxygen layer at the bottom of the slab, the dangling oxygen bonds on the bottom of the slab are saturated by a sheet of “virtual hydrogen atoms” with a nuclear charge of Z = 1/2|e| and an electron charge of 1/2e; this guarantees a formal charge of −2 for each oxygen atom at the bottom layer.62 To compensate for the possible dipole−dipole interactions between the periodic asymmetric slabs, a dipole sheet is introduced in the middle of the vacuum gap. The chosen surface model contains one Zn and one O vacancy within the top Zn−O layer and is passivated by two hydroxyl groups per unit cell, which corresponds to a surface density of 5.47 × 1014 molecules·cm−2; upon geometry optimization, one of the hydroxyls is located in a bridging position between two surface zinc atoms while the other fills the surface oxygen vacancy.62 Each surface unit cell contains one benzyl-PA molecule, which is equivalent to a packing density of 2.73 × 1014 molecules·cm−2. Here, four benzylphosphonic acids with varying degrees of fluorination have been considered; they include o-difluorobenzylphosphonic acid (o2FBPA), benzylphosphonic acid (BPA), p-fluorobenzylphosphonic acid (pFBPA), and pentafluorobenzylphosphonic acid (5FBPA). The molecular structures are shown in Figure 1. b. Binding Modes. Several different binding modes have been proposed for PA adsorption on oxide surfaces and are sketched in Scheme 1. The major adsorption modes are particularly sensitive to both the type of oxide surface and the

surface treatment. For example, modes a (monodentate) and b (bidentate + electrostatic) have been suggested for PA adsorption on TiO2,63 Al2O3,64,65 and BaTiO3,66 while tridentate mode d has been proposed to be prevalent on ZrO267 and SiO2.68 Our previous work on PA adsorption on an ITO surface has indicated in that instance that adsorption occurs via multiple configurations, with a predominance of bidentate and tridentate modes c and d that involve P−O−In or P−O−Sn bonds.59 Among all these possible binding configurations, only two, b and d, were found to lead to stationary points on the ZnO(0002) surface. Thus, they were selected for more detailed investigations. The surface, as well as a typical adsorbate is shown in Figure 2.

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Figure 2. Side view of the bare ZnO(0002) surface (a) and the PA− ZnO interfaces in bidentate (b) and tridentate (c) binding modes (shown for the difluorinated o2FBPA SAM).

Upon formation of a bidentate complex, one of the surface hydroxyl groups reacts with a PA hydrogen to release a water molecule: OH−ZnO + RPO3H 2 → RPO3H−ZnO + H 2O

Thus, in the bidentate mode, two of the PA oxygen atoms are bound to zinc surface atoms and one proton is still attached to the PA molecule. In the tridentate case, the proton migrates to the surface, resulting in each of the three PA oxygen atoms becoming bound to a different surface zinc atom. c. Methodology. The calculations were carried out at the density functional theory (DFT) level using the Vienna Ab Initio Package (VASP).69,70 As in our previous work,51,59,60 we make use of the generalized-gradient approximation (GGA) exchange-correlation functional of Perdew, Burke, and Ernzerhof (PBE)71,72 and the projector-augmented wave (PAW) method.73 A plane-wave energy cutoff of 400 eV is applied in all instances. The tetrahedral smearing with Blöchl corrections74 with σ = 0.1 eV is used for the Brillouin-zone integrations on a 2 × 2 × 1 k-point mesh. The total energy convergence for the self-consistent iterations is set at 10−6 eV, and the maximal residual force on each atom in the course of geometry optimizations is 0.02 eV/Å. To describe the strongly localized zinc 3d-orbitals, the GGA+U approximation75 with an effective Hubbard U-parameter (Ueff = 8.5 eV)76 is applied. Our earlier work showed that this parameter results in a simulated

Figure 1. The four benzylphosphonic acids considered in this study: odifluorobenzylphosphonic acid (o2FBPA), benzylphosphonic acid (BPA), p-fluorobenzylphosphonic acid (pFBPA), and pentafluorobenzylphosphonic acid (5FBPA). 19126



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Table 1. Comparison of the Adsorption Energies (AE) and Geometries for the Various Bidentate and Tridentate PA−SAMs angles (deg) Bidentate o2FBPA BPA pFBPA 5FBPA average Tridentate o2FBPA BPA pFBPA 5FBPA average

bond lengths (Å)

AE (eV)

Ben/ZnO

C−P/ZnO

P−O−Zn

C−P

P−O

PO−Zn

PO(H)−Zn

−1.53 −1.56 −1.54 −1.56 −1.55

46.0 43.4 44.1 47.2 45.2

81.2 82.4 82.5 81.3 81.9

120.4 120.3 120.2 120.3 120.3

1.836 1.829 1.828 1.837 1.833

1.553 1.554 1.554 1.552 1.553

1.983 1.987 1.988 1.986 1.986

2.332 2.332 2.331 2.362 2.339

−2.12 −2.14 −2.13 −2.16 −2.14

46.2 45.5 45.1 47.4 46.1

80.6 80.4 81.1 80.7 80.7

117.4 117.6 117.6 117.3 117.5

1.826 1.819 1.820 1.826 1.823

1.553 1.557 1.557 1.550 1.554

1.855 1.855 1.856 1.857 1.856

terms, ΔΦtot, can be compared to the DFT-derived value, ΔΦcalcd, to demonstrate the consistency and accuracy of this decomposition procedure. The potential energy step, ΔV int.dip. , that effectively corresponds to the formation of a dipole at the very interface, is calculated by solving Poisson’s equation for the planeaveraged electron density difference between the combined PA−ZnO system and the isolated components, along the direction normal to the surface. For bidentate systems, the electron density difference Δρ(z) is calculated as:

band gap of 1.8 eV for bulk ZnO62 (which is still much smaller than the experimental value of 3.3−3.4 eV).77 The O(1s) core-level binding energy shift for the three oxygen atoms in the PO3 moiety with respect to the oxygen in bulk ZnO is calculated using the method developed by Scheffler and Kresse.78,79 The calculated difference in energy required to remove a core electron from an atom can be compared to the binding energy shift determined by X-ray photoelectron spectroscopy (XPS). The shift in core-level binding energy between the PA oxygen atoms and the oxygen atoms in the bulk ZnO is determined as: ESCLS = [Esurface(nc − 1) − Esurface(nc)]

Δρ(z) = ρPA−ZnO (z) − [ρPA−H (z) − ρH (z)] − [ρZnO (z)]

− [E bulk (nc − 1) − E bulk (nc)]

where subscript PA−ZnO represents the optimized bidentate system, PA−H is the isolated PA monolayer with the same molecular structure as in the bound geometry and the hydrogen atom lost upon surface binding reattached, and ZnO is the bare surface taking the SAM-modified geometry. The tridentate case is a bit more complex but follows a similar concept:

where Esurface(nc) is the total energy of the PA−ZnO system in the ground state; Esurface(nc − 1) is the total energy of the system with a 1s-core electron of the oxygen atom belonging to the PO3 moiety removed and added to the conduction band; the two Ebulk terms represent the same values for the bulklike ZnO, taken here from an oxygen atom located in the third ZnO layer of the unmodified surface in the same unit cell. The methodology to evaluate the change in work function for a surface modified by a physisorbed or chemisorbed molecular layer has been discussed in detail elsewhere.60,80 Because ZnO is an n-type semiconductor due to unintentional n-type doping, which is generally attributed to interstitial zinc atoms, oxygen vacancies, and/or impurity hydrogen atoms,81−101 the Fermi level is found near the bottom of the conduction band (usually ∼0.1−0.2 eV away).27,101 Here, because no electron donor states have been considered, we simply use the conduction band minimum as the effective Fermi level for calculating the work function. As demonstrated in our earlier work,60 the work-function change of the modified surface can be decomposed into its contributing components:

Δρ(z) = ρPA−ZnO−H3 (z) − [ρPA−H1−H2 (z) − ρH1(z) − ρH2 (z)] − [ρZnO−OH (z) − ρH4−OH (z) + ρH4 (z)] − [ρH5−H3 (z) − ρH5 (z)]

Here, subscript PA−ZnO−H3 represents the optimized tridentate system with one hydrogen atom from the molecule (labeled H3) attached to the ZnO surface, PA−H1−H2 denotes the isolated PA monolayer with the same molecular structure as in the bound geometry and the two hydrogen atoms H1 and H2 lost upon surface binding reattached, and ZnO−OH corresponds to the bare surface but with H3 replaced by the hydroxyl group originally lost by the chemisorption of the PA−SAM.

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RESULTS AND DISCUSSION a. Binding Geometries and Adsorption Energies of the Bidentate and Tridendate Configurations. The adsorption energies and geometries of the bidentate and tridentate adsorbed configurations of the PA molecules are shown in Table 1. Both adsorption reactions are energetically favorable, with the total energy of the complex less than the sum of their unbound components. It appears that the adsorption energies for both binding motifs are not affected by the degree of fluorination of the PA molecules, with the

ΔΦcalcd ≈ ΔVSAM + ΔVint.dip. + ΔVgeom.rec. = ΔΦtot

where ΔVSAM is the potential energy step of an electron crossing the isolated molecular SAM in the same geometry as the one adopted when adsorbed on the ZnO surface, ΔVint.dip. is the potential energy step occurring at the interface due to the charge redistribution caused by adsorption of the PA−SAM on the ZnO surface, and ΔVgeom.rec. is the change in work function with respect to the bare ZnO surface due to geometry relaxation upon adsorption of the SAM. The sum of these three 19127



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Table 2. Calculated O(1s) Core-Level Energy Shifts in eV, for the Four PA−SAMs in Bidentate and Tridentate Configurations molecule

bidentate (P−O−H)

bidentate (P−O−Zn)

tridentate (P−O−Zn)

o2FBPA BPA pFBPA 5FBPA Average

3.75 3.77 3.77 3.71 3.75

1.98 2.01 2.02 1.94 1.99

1.07 1.07 1.07 1.07 1.07

the unbound PA oxygen atom has a shift of 3.75 (±0.03) eV. The 1.99 eV shift turns out to be nearly identical to the 1.98 eV shift calculated in our earlier work for a hydroxyl group bound to an oxygen vacancy site on the ZnO surface.62 It can be expected from these results that the XPS spectra of the ZnO surface modified by such PA−SAMs would generate a strong O(1s) peak shifted by +1.07 eV from the dominant ZnO peak, with two weaker peaks corresponding to the bidentate motif around +1.99 eV and +3.75 eV, with the 1.99 eV peak likely coinciding with the peak corresponding to hydroxyl groups remaining on the surface. The computational results can be compared to prior experimental work on polycrystalline ZnO, ZnO nanowires, and sputter-deposited ZnO films.31,35,61 In ref 61, the XPS spectra of 1-hexanephosphonic acid on polycrystalline ZnO show the main O (1s) peak at 531.2 eV with two additional peaks, one appearing at +1.0 eV vs the main peak, attributed to the P−O−Zn bonding, and the second at +1.9 eV, attributed to surface hydroxyl groups.61 Such assignments are in very good agreement with our computed binding energies for the O(1s) core level for P−O−Zn bonding in a tridentate configuration (+1.07 eV), and our earlier computational result62 for the O(1s) core level corresponding to the surface hydroxyl groups bound to an oxygen vacancy site (+1.98 eV). The binding mode of PAs on sputter-deposited ZnO31 follows the same pattern as in ref 61. A solvent-clean ZnO surface presents an XPS O (1s) peak at 530.0 eV, attributed to the bulk oxygen species, and a shoulder at 531.6 eV associated with surface hydroxyl groups. Upon the binding of PA molecules, an additional peak at 531.0 eV (+1.0 eV) appears, accompanied by a decrease in the intensity of the peak associated with surface hydroxyl groups, consistent with the formation of P−O−Zn bonds. In ref 35, the O (1s) spectra of a carboxyalkylphosphonic acid on ZnO nanowires was fitted to five separate oxygen types. The bulk peak was found at 530.3 eV, with additional components at +1.1 eV (assigned to both P−O−Zn and P O) and +1.8 eV (assigned to unbound P−O−H) from the phosphonic acid and two components at +2.6 and +3.5 eV from the unbound carboxylic acid. Note that the PO bonding in a bidentate configuration, c in Scheme 1, reported in ref 35 is actually not discussed in refs 31 and 61 and is not found in our optimized binding geometries. Optimizations starting from such an initial structure directly led to the tridentate mode. The combination of these experimental observations with the calculated adsorption energies and O (1s) core level binding energy shifts strongly suggests that the thermodynamically favored tridentate configurations are the dominant components. However, with bidentate configurations being energetically favored over an unbound state, it might be possible to find surface regions presenting this binding mode, though with a lower probability.

bidentate and tridentate PA molecules having a adsorption energy around −1.5 eV and −2.1 eV, respectively. Thus, the adsorption energy is essentially determined by the local effects due to the formation of the bonds between the phosphonic acid group and the ZnO surface. That the tridentate configuration is calculated to be more stable by 0.6 eV per molecule than its bidentate counterpart is reasonable in view of the additional O−Zn bond formed. The orientation of each PA molecule on the surface can be characterized by two tilt angles (see Table 1): the angle of the benzyl ring relative to the oxide surface and the angle formed between the C−P bond and the surface. Our results indicate that the molecular orientations for the two binding motifs are similar. Taking the benzyl-PA molecule as an example, in the tridentate configuration, the angle of the benzyl ring relative to the oxide surface is 45.5°; the angle is 43.4° for the bidentate geometry. Similar differences in the angles between the C−P bond and the surface are also found for the benzyl-PA molecule in the two binding configurations. Fluorination of the aromatic ring leads to minimal changes to this structure as the benzene ring in the other fluorinated PAs forms an angle ranging from 45.1° to 47.4° for the tridentate configuration and 43.4° to 47.2° for the bidentate configuration. The angle between the C−P bond and the surface for the fluorinated PAs also varies within a very limited range, 80.4° to 81.1° for the tridentate motif and 81.3° to 82.4° for the bidentate motif. The bond lengths between the C−P, P−O, and PO−Zn or PO(H)−Zn atoms are used to characterize the molecular binding geometry illustrated in Table 1. The C−P and P−O bond lengths for each PA molecule are nearly identical for the two binding configurations: 1.82 and 1.55 Å for the tridentate PAs and 1.83 and 1.55 Å and for the bidentate PAs. The average (PO−Zn) bond lengths for the three oxygen atoms of the tridentate PAs are nearly identical, around 1.86 Å for all four molecules. By comparison, the two PO−Zn bond lengths for the bidentate PAs are about 0.13 Å longer and average 1.99 Å; a much longer O−Zn distance is obtained between the third oxygen atom of the bidentate PAs (belonging to the OH group) and the closest surface Zn atom, as it averages 2.34 Å, i.e., 0.48 Å longer than the average PO−Zn bond length of the tridentate systems. These results are consistent with the 0.6 eV adsorption energy difference between the two binding configurations. b. O(1s) Core-Level Binding Energy Shifts. Because the 1s core-level electrons of the oxygen atoms in the phosphonic acid group are expected to be sensitive to their chemical environment, the 1s binding energy for the three PA oxygen atoms in each molecular SAM are calculated relative to that of the O(1s) core electrons in bulk ZnO. The results are collected in Table 2. For the energetically favored tridentate binding mode, the calculated O(1s) binding energy shift is +1.07 eV for all four systems. For the bidentate binding mode, the bound PA oxygen atoms have a calculated shift averaging +1.99 (±0.03) eV and 19128



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c. Electronic Density of States. As previously mentioned, the conduction-band minimum (CBmin) is used as the effective Fermi-level for the calculation of the work function Φ to provide a meaningful comparison with the experimental measurements on unintentionally n-type doped ZnO. A decomposition of the charge density associated with the CBmin and valence-band maximum (VBmax) was performed for each system and shown in Figure 3. For both bidentate and

Figure 4. Density of states for bidentate (a) and tridentate (b) configurations of o2FBPA/ZnO.

Table 3. ΔΦ and μz(SAM) Values for the Bidentate and Tridentate PA−SAMs in Figure 5

Figure 3. Charge distribution corresponding to VBmax and CBmin for the bidentate (a) and tridentate (b) configurations of the o2FBPA− ZnO system.

bidentate o2FBPA BPA pFBPA 5FPBA

tridentate systems, the CBmin level is mainly located within the bulk of ZnO, which indicates that the CBmin of the ZnO surface is not affected by surface modification with PA−SAMs. This also confirms that the use of CBmin as the effective Fermi-level is valid for the PA−ZnO system. However, a significant difference is obtained regarding the VBmax for the two binding geometries. In the bidentate systems, VBmax essentially corresponds to O(2p) orbitals within the top two Zn−O layers; for the tridentate systems, it corresponds to contributions from the molecular SAM. The projected density of states (PDOS) onto the various atom types is presented in Figure 4. The PDOS for the F and C atoms in the PA molecule shows that the energy level of the highest occupied molecular orbital (HOMO) of the tridentate PA−SAM is right above the top of the ZnO valence band, which is consistent with the charge distribution shown in Figure 3b. For o2FBPA−SAM in the bidentate configuration, the HOMO level is 1.8 eV below the VBmax of the ZnO surface. Thus, our results underline that the binding mode has a significant effect on the energy-level alignment of the molecular frontier orbitals with respect to the band edges of the metal oxide. d. Change in Work Function. The work function of the bare ZnO surface model adopted in this work is calculated to be 4.68 eV. This value is slightly higher than the experimental values that vary between 3.5 and 4.3 eV.101,102 The calculated change in work function for each PA-modified ZnO surface is given in Table 3. As a function of the fluorination pattern, the change can reach over 1.5 eV for the bidentate case and 1.3 eV in the tridentate case. This is the same range as in previous studies using semifluorinated alkylthiols on gold surfaces103 and fluorinated benzylphosphonic acids on the ITO surface.51,60

μz(SAM) (D)

ΔΦ (eV)

+1.12 +0.72 −0.04 −0.29

−1.48 −1.09 −0.28 +0.03

tridentate o2FBPA BPA pFBPA 5FPBA

μz(SAM) (D)

ΔΦ (eV)

+0.89 +0.45 −0.36 −0.53

+0.82 +1.16 +1.74 +2.14

As in our previous work on the PA-modified ITO surface,51,60 a clear correlation between the component of the PA molecular dipole moment perpendicular to the surface and the modification of the surface work function is obtained. Indeed, the SAM-related modifications correlate with the relative increase in electron density on the outer surface of the SAM: 5FBPA provides for the largest increase in Φ, ΔΦ = +2.14 eV, and o2FBPA has the smallest, ΔΦ = +0.82 eV. For each PA modifier, we have considered an isolated monolayer with the same geometry as the one it adopts when adsorbed on the ZnO surface, so as to be able to evaluate the component of the molecular dipole moment (μz) perpendicular to the surface (μz(SAM)), see Table 3. Interestingly, the tridentate PA−SAMs present a smaller z-dipole moment than their bidentate counterparts, which leads to substantially different work-function changes for the two binding modes. The difference in the z-dipole moment between the bidentate and tridentate configurations can be related to the changes in binding geometry of the PA anchoring groups because the relative orientations of the phenyl groups are nearly constant, as can be deduced from Table 1. Plotting μz(SAM) vs the calculated change in work function (ΔΦ) provides similar linear trends for the two binding geometries (see Figure 5). However, the entire data set for the bidentate systems is shifted 2.1−2.3 eV downward with respect to the corresponding tridentate systems. The intercepts on the y-axis of the lines drawn in 19129



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these three terms are referred to as ΔΦtot. The ΔΦtot values are found to agree completely with the ΔΦ calculated values using the DFT-derived potential for the PA−ZnO systems (given in Table 3). For both binding modes, a difference of at most 0.1 eV is obtained between ΔΦ and ΔΦtot. This is consistent with the trends observed in our earlier work.60 The ΔVSAM values are related to μz(SAM) through the Helmholtz equation and are found to provide the major component in the variations of ΔΦ (see Figure 6). The values for ΔVgeom.rec. are very similar for all SAMs in a given bonding configuration, indicating again that variations in the fluorination patterns of the aromatic rings lead to negligible changes in surface geometry relaxation. The bidentate configurations result in average ΔVgeom.rec. contributions on the order of −1.45 eV, which is some 0.6 eV larger than the −0.87 average shift in ΔVgeom.rec. for the tridentate configurations. The overall similarity within each binding mode is expected from the very similar binding geometries of each system; as a result, although the contribution to ΔΦ from the geometric relaxations is substantial, it has little impact on any attempt to tune the overall surface work-function change by chemically modifying the headgroup (substituent) of the PA molecules. The third factor, ΔVint.dip., has a much more significant level of variance among the tridentate SAMs (0.35 eV) when compared to the bidentate SAMs (0.06 eV) and provides the remaining contribution to the variance in ΔΦ found in the tridentate molecular SAMs. The level of variation in ΔVint.dip. is significantly larger than in the case of similar benzylphosphonic acids deposited on ITO (0.03 eV).51 On the basis of the ITO results, it was expected that the lack of significant differences among the optimized geometries for each system would result in the absence of any significant variations for the interface dipoles. However, the present calculations demonstrate that the interface dipoles for the tridentate configurations are sensitive to variations in the aryl electronic structure, pointing to a stronger interaction between the molecular headgroup and docking group in the tridentate configurations. This stronger interaction can be clearly seen by comparing the amount of charge transfer from the ZnO surface to the molecules for the two binding modes (Figure 7). In the tridentate case (Figure 7b), a total charge of about 0.5e is transferred from the ZnO surface to the PO3 group of the molecule; in the bidentate case, only about 0.2e is transferred from the ZnO surface to the molecular docking group (Figure 7a). This significant difference in charge transfer also rationalizes the large difference in ΔVint.dip. between the two binding motifs.

Figure 5. Correlation between the calculated work-function change (ΔΦ) and the component of the molecular dipole moment along the surface normal direction (μz(SAM)).

Figure 5 indicate that, when the z-components of the molecular dipole moments are vanishing, the intrinsic shifts in effective work function are −0.29 eV for the bidentate configuration and +1.56 eV for the tridentate configuration. We note that the correlation for the bidentate configuration is slightly stronger than in the tridentate case, which suggests that additional factors are involved in the latter case. e. Decomposition of the Work Function. As discussed above, the change in work function can be decomposed into three components: (i) the interface dipole electrostatic potential energy step due to the charge redistribution at the interface between the SAM and the ZnO surface (ΔVint.dip.); (ii) the change in electrostatic potential energy across the molecular SAM (ΔVSAM); (iii) the change in work function of the bare ZnO surface when taking account of the surface geometry relaxation upon adsorption of the SAM (ΔVgeom.rec.). Each of these terms has been evaluated separately and is collected in Table 4 and illustrated in Figure 6. The sums of Table 4. Decomposition of the Work-Function Modification into Its Contributing Factors: The Electrostatic Potential Step across the Molecular Layer, the Work-Function Change Due To Surface Geometry Relaxation, and the Interface Dipole (all values in eV)

Bidentate o2FBPA BPA pFBPA 5FBPA Tridentate o2FBPA BPA pFBPA 5FBPA

ΔVSAM

ΔVgeom.rec.

ΔVint.dip.

ΔΦtot

−1.15 −0.75 0.04 0.30

−1.48 −1.45 −1.46 −1.42

1.23 1.20 1.17 1.21

−1.38 −1.00 −0.24 0.08

−0.91 −0.46 0.36 0.54

−0.87 −0.88 −0.86 −0.82

2.47 2.40 2.12 2.27

0.69 1.05 1.62 2.00

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CONCLUSIONS We have reported the molecular adsorption geometries and energies, calculated at the DFT level, for a series of four benzylphosphonic acid SAMs with varying degrees of fluorination, chemisorbed on the polar (0002) ZnO surface. Two major binding modes have been identified, with a tridentate configuration some 0.6 eV more energetically favorable than a bidentate configuration. The calculated 1s core level binding energy shifts of the oxygen atoms in the PO3 moiety with respect to the oxygen atoms in the ZnO bulk have been compared to available XPS data. Very good agreement with the XPS experimental assignments is obtained for the tridentate configuration; on the other hand, the calculated shift for the 1s oxygen level in the (P−O−H) moiety of the bidentate configuration is not 19130



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Figure 6. Variations of the total work-function change (ΔΦtot) and of the contributing components (ΔVSAM, ΔVgeom.rec., and ΔVint.dip.) vs μz(SAM) for bidentate (a) and tridentate (b) systems.

Figure 7. Comparison of the change in electron density (Δρ) and accumulation of Δρ (Q) upon formation of the SAM on the ZnO surface for the bidentate (a) and tridentate (b) configurations of the o2FBPA−SAMs.

This work provides a thorough understanding of the factors impacting the work-function modification of the technologically relevant zinc-terminated polar ZnO surface via phosphonic acid SAMs. Such an understanding is critical to address the changes in work function observed experimentally since the details of the deposition process and the resulting surface coverage can lead to variations in the binding modes, which ultimately contributes to the efficiency of the charge-injection or chargecollection process between the ZnO surface and the active organic layer in optoelectronic applications.

observed in the XPS experiments, which points to a low probability of formation of bidentate binding of the SAMs. The binding configuration presents a significant impact on the molecular energy level alignment with respect to the ZnO band edges. For instance, in the tridentate systems, the molecular HOMO appears within the ZnO bandgap, whereas it is located 1.8 eV lower than the valence band maximum of ZnO in the bidentate systems. The deposition of the phosphonic acid SAMs results in substantial modifications of the ZnO work function. For the tridentate configurations, there occurs an overall increase in work function for all four benzylphosphonic acids; on the other hand, the more weakly bound bidentate systems would lead to reductions in work function, except in the case of the pentafluorobenzylphosphonic acid SAM. Interestingly, the various tridentate SAMs display a larger variation of their respective interface dipoles, which is related to a larger effect of the fluorination pattern of the benzyl groups on the interfacial charge redistributions and charge transfers for the tridentate systems.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. ‡ Also affiliated with Department of Chemistry, King Abdulaziz University, Jeddah 21589, Saudi Arabia. 19131



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Article

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ACKNOWLEDGMENTS This work was supported as part of the “Center for Interface Science: Solar Electric Materials (CISSEM)”, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award Number DE-SC0001084 and by Solvay. The computations reported here were performed mainly at Georgia Tech’s “Center for Computational Molecular Science and Technology”, funded through an NSF CRIF award (grant no. CHE-0946869) and by the Georgia Institute of Technology.

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