Surface Modification of ZnO(0001) - American Chemical Society

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Surface Modification of ZnO(0001)−Zn with Phosphonate-Based SelfAssembled Monolayers: Binding Modes, Orientation, and Work Function Melanie Timpel,†,∇ Marco V. Nardi,†,∇ Stefan Krause,‡ Giovanni Ligorio,† Christos Christodoulou,† Luca Pasquali,⊥,∥,# Angelo Giglia,∥ Johannes Frisch,† Berthold Wegner,† Paolo Moras,§ and Norbert Koch†,‡,* †

Institut für Physik & IRIS Adlershof, Humboldt-Universität zu Berlin, Newtonstr. 15, 12489 Berlin, Germany Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Albert-Einstein-Str. 15, 12489 Berlin, Germany § Istituto di Struttura della Materia and ∥Istituto Officina dei Materiali, Consiglio Nazionale delle Ricerche, Area Science Park, SS. 14 Km 163.5, 34149 Basovizza, Trieste, Italy ⊥ Engineering Department “E. Ferrari”, University of Modena e Reggio Emilia, Via Vignolese 905, 41125 Modena, Italy # Department of Physics, University of Johannesburg, PO Box 524, Auckland Park 2006, South Africa ‡

S Supporting Information *

ABSTRACT: We used partially fluorinated alkyl and aromatic phosphonates as model systems with similar molecular dipole moments to form self-assembled monolayers (SAMs) on the Zn-terminated ZnO(0001) surface. The introduced surface dipole moment allows tailoring the ZnO work function to tune the energy levels at the inorganic−organic interface to organic semiconductors, which should improve the efficiency of charge injection/extraction or exciton dissociation in hybrid electronic devices. By employing a wide range of surface characterization techniques supported by theoretical calculations, we present a detailed picture of the phosphonates’ binding to ZnO, the molecular orientation in the SAM, their packing density, as well as the concomitant work function changes. We show that for the aromatic SAM the interaction between neighboring molecules is strong enough to drive the formation of a more densely packed monolayer with a higher fraction of bidentate binding to ZnO, whereas for the alkyl SAM a lower packing density was found with a higher fraction of tridentate binding.

1. INTRODUCTION Hybrid structures and materials that are composed of organic and inorganic semiconductors combine the favorable properties of their constituents and offer the possibility to overcome currently existing restrictions of all-organic and all-inorganic structures for electronic applications. Organic semiconductors, for instance, often exhibit efficient light-matter coupling, whereas inorganic semiconductors contribute high charge carrier mobility. During the past decade, the inorganic−organic semiconductor interface has received increasing attention, and means of tuning the energy level alignment between inorganic and organic semiconductors via a molecular interlayer have emerged.1−5 This tuning offers the optimization of this interface for charge injection as well as extraction, for example, with small molecules in sub- to monolayer coverage to adjust the substrate work function. However, the approach has one inherent problemit can create regions of different local work function and energy level alignment.6 Furthermore, advanced solar cell design concepts also include surface area maximized © 2014 American Chemical Society

and light harvesting substrate morphologies, such as nanorods, nanowhiskers, or nanoparticles. In these so-called hybrid devices, the exciton produced by light absorption in the organic layer must be dissociated at the inorganic−organic interface. Among the inorganic materials used for hybrid electronic devices, ZnO as wide band gap semiconductor and electron acceptor is a promising material, since it has many favorable properties (e.g., it is inexpensive, abundant, and nontoxic). It is (if doped) a good electron transport material, and its large band gap makes it transparent in the visible light regime. Furthermore, well-ordered ZnO nanostructures can be easily grown by both vacuum-based and wet-chemical methods in desired shape and density7−12 with excellent properties for hybrid electronic devices, such as nanowire field-effect transistors13 and nanowire solar cells.2,14 ZnO is thus an ideal Received: June 15, 2014 Revised: August 15, 2014 Published: August 17, 2014 5042

dx.doi.org/10.1021/cm502171m | Chem. Mater. 2014, 26, 5042−5050

Chemistry of Materials

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phosphonate contains a partially fluorinated alkyl chain (F13OPA) that represents a typical SAM with an alkyl chain spacer and a dipolar group, which has been widely used for surface modification of metals20 and metal oxides.22,37−41 The second phosphonate studied has a partially fluorinated phenyl ring (pCF3PhPA); that is, it contains a conjugated moiety that is directly connected to the phosphonate anchoring group and provides better conductivity for electronic device applications. The first step was to produce reproducible SAMs of these two phosphonates that ideally show a full coverage and measure their influence on the ZnO work function. For this purpose, both unmodified and PA-modified ZnO surfaces were expansively characterized by scanning force microscopy (SFM), contact angle measurements, and ultraviolet photoemission spectroscopy (UPS). The latter was supported by density functional theory (DFT) calculations in order to unambiguously identify the contribution of the phosphonate to the valence band spectra of the PA-modified ZnO surface. The binding scheme of the phosphonates to ZnO was investigated with synchrotron-based photoelectron spectroscopy (PES), and a thorough analysis of the O 1s core level and its underlying chemical components was carried out. The orientation of the phosphonates was studied by X-ray adsorption spectroscopy (XAS) experiments in comparison with DFT calculations. Based on our results, the surface coverage (i.e., the packing density) of both partially fluorinated phosphonates on the ZnO surface was quantified. With the comprehensive picture obtained we rationalize the observed work function changes and the driving forces in the formation of the SAM on ZnO, which will aid the future design of improved molecules for SAM-modification of inorganic−organic interfaces.

candidate for the transfer of concepts elaborated under ultrahigh vacuum (UHV) conditions to more cost-efficient (e.g., solution-based fabrication methods). In addition, it is a candidate for replacing InGaN in optoelectronic applications,15 TiO2 as an anode material for dye-sensitized solar cells (DSSCs),16,17 as well as tin-doped indium oxide (ITO) as a transparent electrode in both light-emitting18 and photovoltaic19 devices. A solution-based and low-cost method to realize molecular interlayers that cover not only planar surfaces more homogeneously but also more complex substrate morphologies is given by the application of highly ordered and covalently surface-bound self-assembled monolayers (SAMs) with dipolar moieties.20 The SAM alters the chemical reactivity, surface free energy, and/or polarity of the inorganic surface, which can make it more compatible with organic materials, while the magnitude of the surface work function can be tuned by modifying the chemical structure of the SAM molecule.21−26 Recently, hybrid photovoltaic cells based on ZnO thin films2,4 and nanowires3 with molecular interlayers have been fabricated, however, with rather limited power conversion efficiency. This was most likely due to the choice of SAMs with small permanent dipoles, and thus poor adjustment of the interfacial energy levels, which ultimately determines whether efficient exciton dissociation (subsequent to light absorption) and energy or charge-carrier transfer across the interface occurs. To increase the performance of hybrid devices, it is therefore necessary to establish fundamental knowledge and reliable means to control the energy level alignment at interfaces between organic semiconductors and SAM-modified ZnO surfaces. There are several studies of SAM-modified ZnO substrates in the literature, ranging from planar polycrystalline ZnO thin films, 27,28 single crystal ZnO(000−1)−O, 29 ZnO(0001)−Zn,30 and ZnO(01−10)31 wafers. A few studies even include the complex morphology and higher surface area of ZnO nanorods.30,32 In those previous reports, the SAM molecules mainly comprised (nonfluorinated) alkyl chains with different anchoring groups, which were compared with respect to their thermal and corrosion stability,29 their surface morphology and their binding to ZnO.28,29,31 We chose the phosphonic acid (PA) as anchoring group for the SAM since it has been shown to readily form stable and strong bonds to ZnO and ITO surfaces.28,33−35 Each PA anchoring group has three potential binding sites (two hydroxyl groups and one phosphoryl group) that allow for covalent binding to the substrate in either monodentate, bidentate, and/or tridentate mode. The binding of phosphonates to ZnO was previously characterized via infrared (IR) spectroscopy and/or standard Xray photoemission spectroscopy (XPS), that is, using commercially available XPS systems with Al or Mg X-ray sources.28−32 Both techniques do not provide enough surfacesensitivity to clearly elucidate the true PA binding mechanisms.36 Furthermore, the ability to reliably control and tune the energy levels of ZnO using phosphonate-based interlayers requires a more detailed understanding not only of their binding to ZnO but also of their orientation and packing density in the molecular interlayer, since the strength of the interface dipole for effectively tuning the energy level alignment strongly depends on all of these factors. In the present study, a comprehensive experimental characterization of the surface modification of single crystal ZnO(0001)−Zn surfaces supported by DFT calculations was carried out using two phosphonates with archetypical organic moieties. One

2. MATERIALS AND METHODS Single-crystal ZnO(0001)−Zn substrates were annealed in a tube furnace (Carbolite, TZF 12/38S) under ambient atmosphere at 1000 °C for 2 h (heating rate ∼44 K/min, cooling rate ∼3 K/min) to get well-defined crystal terraces with ultrasmooth surfaces.42 The partially fluorinated PA molecules used for surface modification, 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl phosphonic acid (F13OPA) and p-(trifluoromethyl)phenylphosphonic acid (pCF3PhPA), were purchased from Aculon, Inc. (U.S.A.). Adsorption of PA molecules on the ZnO surface was carried out by immersion of the substrates in a 1 mM tetrahydrofuran (THF) (anhydrous) solution of the corresponding PA for 6 h. All adsorption steps were done in Teflon (PTFE) vessels in order to prevent nonspecific PA adsorption on the preparation equipment. After immersion, the samples were annealed on a hot plate under ambient atmosphere at a temperature of 140 °C for 2 h, followed by sonication in THF for 15 min. Another two preparation cycles of immersion (3 h), annealing (0.5 h), and sonication in THF (15 min) were carried out to deposit a well-ordered and uniform molecular interlayer. An increased lateral homogeneity of the phosphonate-based interlayers by the additional preparation cycles was confirmed by XPS core level analysis (see Zn 3s + P 2p core levels in Figure S3, Supporting Information) taken after each preparation cycle at different positions across the sample surface. Before and after surface modification, the ZnO substrates were imaged by SFM in intermittent contact mode under ambient conditions using a NanoWizard 3 (JPK Instruments AG, Germany) instrument. Height and phase images were recorded using aluminumcoated silicon cantilevers (Olympus Corporation, Japan) with a typical resonant frequency of 70 kHz and a spring constant of about 2 N·m−1. To compensate for thermal drifts and sample inclination, first-order line subtraction and plane correction were applied to the images. Water contact angle measurements were carried out with a Theta Lite (L.O.T.-Oriel, Germany) instrument. The recorded images were analyzed with drop shape analysis software to determine the contact 5043



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angle by the Young Laplace fitting method. For each sample, the results of 5−8 measurements were averaged. Surface XPS/UPS studies were performed at the VUV beamline of the ELETTRA synchrotron facility in Trieste (Italy).43 To avoid possible radiation damage of the PA molecules during beam exposure, the C 1s core level line-shape of the molecular interlayers was carefully analyzed as a function of photon dose (see Figure S1, Supporting Information). Photon fluxes were minimized by detuning the undulator gap with respect to the grating. Under the adopted working conditions, no damaging of the phosphonates was observed (see Figure S2 and Table S2, Supporting Information). XPS/UPS data were collected using a hemispherical electron energy analyzer with an energy resolution of 50 meV in normal emission geometry. A photon energy of 35 eV was selected for the secondary electron cutoff (SECO) and the valence band (VB) analysis. The Zn 3s + P 2p, C 1s, and O 1s core level regions were measured with a photon energy of 244, 390, and 636 eV, respectively. In this way, photoelectrons with kinetic energies that correspond to the minimum inelastic mean free path (Λ ≈ 8 Å) were detected for all chemical species (∼100 eV) in order to probe similar sample depths and maximize the surface sensitivity. The Zn 3s peak is energetically very close to the P 2p peak (∼7 eV). This allowed us to check the lateral homogeneity of the molecular interlayers (see Figure S3, Supporting Information). Binding energies (BEs) of core levels and valence bands were referenced to the Au 4f7/2 level (84.00 eV) and the Au Fermi edge, respectively, which were obtained from a sputter-cleaned Au surface. Core level analyses were performed by Voigt line-shape deconvolution after background subtraction of a Shirley function. Typical uncertainties are ±0.05 eV for the energy position, less than ±5% for the full width at half-maximum (fwhm), and about ±2.5% for the area evaluation. The XAS experiments were performed at the BEAR endstation (BL8.1L), at the left exit of the 8.1 bending magnet of the ELETTRA synchrotron facility in Trieste (Italy).44,45 All XAS spectra were collected in total electron yield (TEY) mode (i.e., drain current mode) at the C K-edge and normalized to the incident photon flux and to the clean substrate signal. The spectral energy was calibrated by referring to CO2 C 1s−π* transitions. The incident light was horizontally polarized; the incidence angle of the light with respect to the sample surface plane was kept fixed at 10°, and the sample was then rotated around the beam axis to change the polarization from s to p. This leads to an effective rotation of the electric field plane at the surface while keeping the excitation volume constant. In order to keep both the illuminated area (i.e., the excited volume) and the incidence angle constant, we changed the direction of the electric field vector E⃗ from perpendicular to parallel with respect to the scattering plane. This was achieved by rotating the chamber angle ψC with respect to the beam axis from ψC = 0° (s-scattering) to ψC = 90° (p-scattering).46 The synchrotron beam was elliptically polarized with the dominating components in the horizontal plane (H), and the corresponding ⎯→ ⎯ ⎯→ ⎯ ellipticity defined as ε = | E V |2 /| E H|2 was 0.1 (where V is the vertical plane and ε = 1 (0) for circularly (linearly) polarized light). In order to correctly process the acquired data, each absorption spectrum collected at different angle ψC was first normalized to the drain current, which was measured on an optical element (refocusing mirror) placed along the beamline, and then normalized to the absorption spectrum acquired under the same experimental conditions and the energy range, on a Au(001) sputtered (i.e., carbon free) sample. The energy scale of each single spectrum was recalibrated taking into account the energy fluctuation of characteristic absorption features measured on the refocusing mirror. The DFT calculations of the free-molecules F13OPA and pCF3PhPA equilibrium geometry, density of states (DOS) and dipole moments μ were carried out by applying the StoBe code47 and using all-electron triple-valence plus polarization (TZVP) atomic Gaussian basis sets and gradient-corrected RPBE-PBE exchange-correlation functional. The calculated total and partial density of states (DOS and PDOS) were

convolved with Gaussians (0.6 eV fwhm) before comparison with the experiment. The XAS simulations were done using the Slater transition state method.48,49 Dipole transitions and angle-dependent absorption at the C K-edge were calculated separately at each C atomic center and then summed together to obtain the total molecular absorption. To better describe relaxation effects, an IGLO-III basis set was used on each excitation center, while effective core potentials were used for the remaining C atoms. The dipole-excitation spectra were Gaussian convoluted with an energy-dependent broadening. For a correct energy scale alignment of the absorption spectra of different centers, a ΔKohn−Sham adjustment50,51 was applied to the lowest core-excited state for each center.

3. RESULTS AND DISCUSSION Work Function Modification and Valence Band Spectra. Changes in the surface electronic properties of the annealed ZnO surface before (i.e., unmodified) and after surface modification were characterized by UPS (see Figure 1).

Figure 1. (a) SECO and (b) valence band region UPS spectra of the unmodified, F13OPA- and pCF3PhPA-modified ZnO surface as indicated in the figure; (c) calculated total and partial density of states of the both PA molecules (background subtracted).

As can be seen in Figure 1a, the SECO shifts and yields an increase in the work function Φ for both PA-modified ZnO surfaces relative to the annealed (unmodified) ZnO surface (ΔΦ = +1.65 eV for F13OPA-, and ΔΦ = +1.35 eV for pCF3PhPA-modified ZnO surface). Figure 1b shows the experimental spectra from valence band states of the unmodified, F13OPA- and pCF3PhPA-modified ZnO surfaces, respectively. New photoemission bands are clearly detectable for both PA-modified ZnO surfaces, although these are overlapped with residual ionizations from the O 2p band of the ZnO. The DFT-calculated total DOS and PDOS of both PA (free) molecules are reported in Figure 1c. The highest occupied electronic states have been studied in the binding energy range of 0−11 eV. For better comparison between experimental and theoretical spectra, the most intense peak of each DOS in Figure 1c was aligned to the binding energy of the corresponding orbital in the experimental spectrum, that is, peak D in the case of F13OPA and peak C in the case of pCF3PhPA. 5044



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The lower BE region (4−8 eV) of the F13OPA-modified ZnO surface (upper panel in Figure 1b) is characterized by the presence of three bands, which are labeled as A, B, and C and located at 5.0, 6.5, and 7.5 eV, respectively. Another feature (labeled as D) is located at about 10.2 eV. By comparison with the DFT calculations in Figure 1c, the feature D is readily ascribed to the fluorinated carbon atoms delocalized along the entire alkyl chain, whereas the features B and C are ascribable to molecular orbitals (MOs) prevalently centered on the fluorinated part of the alkyl chain (C6F13) and the phosphonate group, respectively. The feature A is associated with the highest occupied molecular orbital (HOMO) of the F13OPA molecule and arises mainly from the phosphonate group, whereas very low (C2H4) and no contribution (C6F13) from the partially fluorinated alkyl chain are observed in the PDOS. For the pCF3PhPA-modified ZnO surface (lower panel in Figure 1b), three bands that are labeled as A, B, and C can be identified in the valence spectrum at 4.5, 6.0, and 7.6 eV, respectively. A broader feature (labeled as D) is centered at about 10.3 eV. Different from the F13OPA molecule, all frontier MOs of the pCF3PhPA molecule in the energy range 6.5−11 eV can be described by a superposition of the MOs stemming from the phenyl ring, the C−F3 component and the phosphonate group. From the PDOS analysis presented in the lower panel of Figure 1c, the features C and D are mainly formed from MOs located on the C−F3 component and the phenyl ring, whereas feature B is mainly stemming from the phosphonate group. The feature A is associated with the HOMO, and is formed by the superposition of MOs stemming from the phenyl ring and the phosphonate group. The orbital assignments as described above are summarized in Table S1, Supporting Information. In Figure 2, the experimentally measured change in work function is plotted vs the calculated molecular dipole moment μ

Figure 3. Zn 3s + P 2p core level spectra (background subtracted) of the (a) F13OPA- and (b) pCF3PhPA-modified ZnO surface, respectively. The core level components are described in the legend.

component of the F13OPA- (pCF3PhPA-) modified ZnO surface exhibits a binding energy of 133.20 eV (133.66 eV), which is in agreement with observed BE values of phosphonates covalently bound to different metal oxides.32,40,52 The F13OPAmodified ZnO surface shows an additional P 2p component at higher binding energy (134.9 eV) that can be either attributed to a slight multilayer contribution and/or charging of these molecules at the interface. To elucidate the binding modes of the phosphonate anchoring group to the ZnO surface, both shape and constituents of the O 1s peak before and after surface modification were analyzed in high-resolution; see Figure 4.

Figure 4. O 1s core level spectra (background subtracted) of the (a) unmodified (lower panel) and F13OPA-modified ZnO surface (upper panel), and (b) unmodified (lower panel) and pCF3PhPA-modified ZnO surface (upper panel). (c) Schematic illustration of the surface− OH groups and possible PA binding modes on the ZnO surface, which are attributed to O 1s core level components in parts a and b.

Figure 2. Correlation between the experimental change in work function (ΔΦ) and the molecular dipole moment μ of the PA molecules for the (F13)OPA- and (pCF3)PhPA-modified ZnO surfaces.

for the above-described partially fluorinated phosphonates, and their corresponding nonfluorinated counterparts, namely octyl(OPA) and phenyl- (PhPA) phosphonic acid. The nonfluorinated phosphonates do not remarkably change the work function of ZnO. Binding of PAs to ZnO. XPS was used to probe the binding of the PAs to the ZnO surface. In Figure 3a and b, the Zn 3s + P 2p core level regions are shown for F13OPA- and pCF3PhPA-modified ZnO surface, respectively. The binding energy of the Zn 3s peak is ∼140 eV. The main P 2p

Both annealed (unmodified) ZnO surfaces (lower panel in Figure 4a and b) exhibit a main peak at ∼531 eV with a shoulder at 532.4 eV. The main peak is attributed to the bulk oxygen, whereas the shoulder is mainly associated with surface hydroxyl (−OH) groups,28,29,53 as schematically shown in the lower panel in Figure 4c. A similar binding energy shift (+1.6 eV) for surface −OH groups on polycrystalline ZnO films has been reported by Hotchkiss et al.28 5045



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Figure 5. (a) Experimental C K-edge XAS spectra of the F13OPA-modified ZnO surface; (b and c) DFT-calculated spectra of the F13OPA molecule.

Figure 6. (a) Experimental C K-edge XAS spectra of the pCF3PhPA-modified ZnO surface; (b and c) DFT-calculated spectra of the pCF3PhPA molecule.

After surface modification (upper panel in Figure 4a and b), both the bulk oxygen and the surface−OH component in the O 1s core level spectra are markedly attenuated for both PAmodified ZnO surfaces. The attenuation of the bulk oxygen component of the pCF3PhPA-modified ZnO surface is higher than that of the F13OPA-modified ZnO surface, which is in agreement with the P 2p/Zn 3s ratios in Figure 3. Globally, the O 1s core levels of both PA-modified ZnO surfaces can be fitted with three additional components. They are assigned to two possible PA binding modes, that is, bidentate and tridentate binding (oxygen atoms marked in red and blue, as schematically shown in the upper panel in Figure 4c), which

have been previously calculated to be stable configurations for PA adsorption on the ZnO(0001)−Zn surface by Wood et al.54 For the component assigned to tridentate binding mode of the pCF3PhPA molecule to the ZnO surface (blue component in Figure 4b), the O 1s binding energy shift with respect to the bulk oxygen was found to be +1.1 eV, which is in agreement with the calculated shift for core-level binding energies of (partially fluorinated) benzyl PA molecules with tridentate binding on ZnO (+1.07 eV).54 For the bidentate binding mode (red component in Figure 4b), the two bound PA oxygen atoms exhibit a shift of +2.1 eV (Wood et al.:54 +1.99 eV), whereas the unbound PA oxygen atom exhibits a shift of +3.2 5046



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eV (Wood et al.:54 +3.75 eV). Therefore, we provide direct experimental evidence that PA adsorption on ZnO occurs via multiple configuration of bidentate and tridentate modes, as theoretically calculated for ZnO54 and ITO55 substrates. With the fitting results shown in Figure 4a and b, it is obvious that the pCF3PhPA molecule exhibits a higher fraction of bidentate binding to the ZnO surface than the F13OPA molecule. Orientation of PAs on ZnO. XAS spectra were collected from C K-shell electrons excited to unoccupied MOs, and are shown in Figure 5 for the F13OPA- and in Figure 6 for the pCF3PhPA-modified ZnO surface, respectively. Figure 5a shows the XAS spectra for the F13OPA-modified ZnO surface collected at five different ψC angles spanning from 0−90°. The principal structures identified in the spectra in Figure 5a are numbered I−V. On the basis of our DFT simulated absorption spectra shown in Figure 5b, we ascribe the main peak at 292.9 eV (labeled as III in Figure 5a) to transitions from the C 1s to the C−F σ*-antibonding orbital involving mainly the carbon atoms from C3−C7 along the fluorinated alkyl chain, as depicted in Figure 5b, where contributions from the different centers have been separated. The transition dipole moment associated with this peak lies in the y−z plane of the molecule. This is reconciled considering the calculated angular resolved spectra reported in Figure 5c, obtained with the electric field vector aligned along x, y, and z axis, respectively. This transition dipole is therefore perpendicular to the main axis of the alkyl chain (x axis; see Figure S4, Supporting Information). The structures at higher energies from 295−305 eV (labeled as IV and V) are assigned to C−C σ*-antibonding transitions involving once again the carbon atoms C3−C7. A significant contribution to the peak IV stems from the transition originated on the carbon atom C8, where carbon is coordinating three fluorine atoms. The peaks at 289.1 and 290.3 eV (labeled as I and II, respectively) are both associated with C−H transitions involving the carbon atoms C1 and C2. No contribution from fluorinated carbon atoms are present in this region. Observing the experimental angular dependence evolution of the intensity of peak III, it is possible to calculate the average orientation of the molecules with respect to the substrate normal. In particular, on the basis of the angular resolved spectra of Figure 5c, the progressive reduction of the intensity of peak III with the change of polarization from s to p suggests that the F13OPA molecules adopt an upward orientation on the ZnO surface. Figure 6a shows the XAS spectra collected at different angles for the pCF3PhPA-modified ZnO surface. Mainly four different features (labeled as I, II, III, and IV) are detectable. The most intense transition I located at 285.2 eV exhibits a strong angular dependence and is ascribable to the CC π* transitions located on the phenyl ring. This is also confirmed by siteresolved calculated spectra in Figure 6b and angular-resolved calculated spectra in Figure 6c. The transition dipole moment associated with feature I lies in a plane perpendicular to the phenyl ring (z axis; see Figure S4, Supporting Information). Other notable features are located at 289.1 eV (II) and 290.5 eV (III). Comparing with the calculations, they are attributable to transitions involving the C−H groups and C−C σ* states, respectively (carbon atoms C1−C6). A π*-type transition due to atoms C2−C5 also contributes to feature II. The structure at higher photon energies, at 292−300 eV (i.e., feature IV), is attributed to transitions involving mainly C−F3 σ* states from carbon atom C7, with the dipole moment mostly in the x−y plane. An

overview about the above-described assignments to transitions and atom contributions is given in Table S3, Supporting Information. As in the case of the F13OPA-modified ZnO surface, a strong anisotropy is also observable for the pCF3PhPA-modified ZnO surface. The angular behavior of the strong π* resonance at 285.2 eV supports the idea that also the pCF3PhPA molecules tend to adopt a vertical standing configuration at the ZnO surface. To quantify the average orientation of the molecules with respect to the surface normal, we monitored the intensity of the features III and I (see orientation of the transition dipole moments in Figure S4, Supporting Information) for both the F13OPA- and pCF3PhPA-modified ZnO surface, respectively, collected at five different angles. Assuming that the surface presents an isotropic molecular distribution in the azimuthal plane, the intensity I of a resonance peak corresponding to excitation into a vector orbital can be described by eq 1 reported in ref 46. The best fits of this equation, assuming θ as a free parameter, are reported in Figure 7 for the F13OPA- and

Figure 7. Plots of the relative σ*- and π*-orbital intensities as a function of the photon incidence angle ΨC. The solid curve corresponds to the best fit of the intensity evolution for (a) F13OPA-, and (b) pCF3PhPA-modified ZnO surface with molecule tilt angle of 26° and 28°, respectively, referred to the surface normal.

pCF3PhPA-modified ZnO surface, respectively. The fit results indicate an average tilt angle of the fluorinated alkylchain (F13OPA) and the phenyl ring (pCF3PhPA) of 26° and 28°, respectively, with respect to the substrate normal. Similar average orientation angles have been previously found for (partially fluorinated) alkyl and aromatic phosphonates on ITO.36,56,57 It should be noted that the angles determined by XAS data represent an average orientation of the phosphonates and do not allow for estimation of the statistical distribution of orientations, which can be different for the two phosphonates. Surface Coverage of PAs on ZnO. The quality and efficiency of the SAM preparation procedure adopted was investigated by SFM measurements to check for uniform interlayer coverage. Figure 8 shows SFM data of the annealed ZnO surface before (i.e., unmodified, Figure 8a) and after (Figure 8c−f) surface modification. The intermittent contact mode SFM height image (2 × 2 μm2) of the annealed, unmodified ZnO surface in Figure 8a reveals the presence of extended, atomically flat terraces (widths of up to 1.5 μm) that are separated by distinct steps. The corresponding cross-section analysis in Figure 8b yields steps of multiple Zn−O layer height (2−4× , with a step height of neighboring Zn−O layers to be c/ 2 = 2.6 Å), thus indicating a step bunching of ZnO terraces as previously observed.42 The terraces exhibit a remarkably 5047



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where θ is the measured contact angle of the phosphonatebased interlayer, f is the fractional coverage of the interlayer on the substrate, θref is the contact angle for a complete reference layer, and (1 − f) and θ′ are the fractional coverage and contact angle of the unmodified ZnO substrate, respectively. For the F13OPA-modified ZnO surface, we used the static contact angle (i.e., the mean value of the advancing and receding water contact angle) of a related thiol CF3(CF2)5(CH2)2SH on a Au film,59 yielding a coverage of f = 0.76. With the values for a well-packed monolayer coverage of a reference layer (e.g., 3.4 molecules/nm2 for CF3(CF2)7(CH2)2SH on Au(111)),60 we can estimate the surface coverage of F13OPA on ZnO to be 2.6 molecules/nm2. This value is comparable to previously reported values for F13OPA on ZnO thin films28 and ITO22 (using contact angle data and the same thiol-on-Au reference layer). Since no contact angle data for a reference layer of pCF3PhPA are available in the literature, the coverage of pCF3PhPA relative to F13OPA was derived from XPS data (see Figure 3) to be 60% higher than the F13OPA coverage. With the comprehensive experimental and theoretical results presented above, the surface coverage N/A (i.e., molecules/ nm2) of the partially fluorinated phosphonates on ZnO (as roughly determined using contact angle data) can be estimated on the basis of the Helmholtz equation: Figure 8. (a) SFM height image of the unmodified (annealed) ZnO surface. (b) Cross-section along the dotted line in part a. SFM (c, e) height and (d, f) phase images of the F13OPA- and pCF3PhPAmodified ZnO surface, respectively; water contact angles of the corresponding surface are shown in the inset of a, c, and e.

ΔΦ = eN

ε0εr A

(2)

where e is the elementary charge, N is the number of dipoles (i.e., molecules), μz is the molecular dipole moment along the surface normal, ε0 is the vacuum permittivity, εr is the effective dielectric constant of the molecular interlayer, and A is the area of the surface. Taking into account the angle θ of the molecules perpendicular to the surface normal μz = μ cos θ, the Helmholtz equation can be modified and written as

smooth and featureless surface morphology with a roughness (root-mean-square, RMS) of ∼1 Å. After surface modification, the terrace structure of the annealed ZnO are preserved as clearly visible from the SFM height images (10 × 10 μm2) of the F13OPA-modified (Figure 8c) and the pCF3PhPA-modified (Figure 8e) ZnO surface. This indicates that surface modification is achieved without severe etching of the ZnO surface, which can be attributed to the use of high-quality (single crystal) ZnO surfaces with low surface defect density that usually accompanies other thin film ZnO samples. The lateral homogeneity of PA molecules across the ZnO terraces was greatly increased by applying three preparation cycles as described in the experimental section (see also Figure S3, Supporting Information). The corresponding SFM phase images (Figure 8d and f) indicate a uniform surface coverage for both PA molecules. On the same ZnO surfaces, we also investigated the wettability using water contact angle measurements before and after surface modification. As shown in the insets in Figure 8a, c, and e, the water contact angle increased from (65.6 ± 2.1)° for the annealed, unmodified ZnO surface to (101.3 ± 0.5)° for the F13OPA-, and (92.6 ± 2.6)° for the pCF3PhPAmodified ZnO surface. The contact angle data can be used to roughly estimate the surface coverage based on Cassie’s law58 for a chemically heterogeneous surface, in combination with available reference data for complete well-packed monolayers, for example, corresponding thiols on Au substrates. The fractional surface coverage of a molecular interlayer is given by cos θ = f cos θref + (1 − f )cos θ′

μz

dϕ N e cos θ = dμ A ε0εr

(3) 61

where dϕ/dμ can be deduced from Figure 2 for both the aromatic and the alkyl phosphonate, assuming similar binding modes for the partially fluorinated SAMs and their nonfluorinated counterparts, and thus a linear relationship54 between work function change and molecular dipole moment μ. The angle θ is derived from our XAS data; see Figure 7. Higher surface coverage increases the number of dipoles but also enhances the effect of depolarization of neighboring dipoles,62,63 which is accounted for by the effective dielectric constant εr. In the present study, εr was assumed to be in the order of εr ∼ 1.4−1.5 for high coverage densities (i.e., ≥ 1.1 molecules/nm2), in agreement with theoretical calculations of trifluoro-methylthiol on Au(111)64 and 3,4,5-trifluorophenyl PA on ITO.63 Hence, the surface coverage of F13OPA and pCF3PhPA on ZnO is calculated to be N/A = 2.0 and 4.4 molecules/nm2, respectively. The higher surface coverage of pCF3PhPA compared to F13OPA is in good agreement with the above-described estimation based on XPS attenuation. Furthermore, our coverage values for the F13OPA-modified ZnO surface are comparable with those previously reported on ZnO thin films28 and ITO22 (using contact angle data and a thiol-on-Au reference layer). Based on XPS attenuation, Koh et al.65

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potentially promising system for organic field effect transistors and all the organic based electronic devices where the conductivity of the organic interlayer plays an important role.

estimated the surface coverage of (nonfluorinated) PhPA to be 76% of a nonfluorinated octadecyl-PA (ODPA) on ITO. In the present study, we observe an opposite trend, that is, the surface coverage of the aromatic phosphonate pCF3PhPA is approximately 60% higher than that of F13OPA. A priori this is an interesting finding since one would expect less steric hindering by an alkyl chain than a phenyl ring, if the packing density of molecules would only be defined by the steric radius of the molecules in the SAM. For a complete picture of the SAM formation mechanism, the interaction between the phosphonate anchoring group and ZnO has to be taken into consideration. Käfer et al.66 have shown for aromatic SAMs with different anchoring groups that in the case of a weaker anchoring group the interaction of the aromatic part of the SAMs is strong enough to define the arrangement of the molecules on the surface. Careful analysis of the substructure of the O 1s high-resolution core level spectra in Figure 4 shows that the fluorinated alkyl chain in the F13OPA prefers tridentate bonding to the ZnO surface, whereas pCF3PhPA has a significantly higher fraction of bidentate bonds. Assuming an orthogonal ZnO surface unit cell of 6.50 × 5.63 Å2 (comprised of 4× Zn and 4× O atoms), each surface unit cell can theoretically contain one (two) PA molecules bound via tridentate (bidentate) binding mode. This corresponds to a maximal surface coverage of 2.73 and 5.47 molecules/nm2 for complete tridentate and bidendate binding, respectively. Therefore, the higher fraction of bidentate binding of pCF3PhPA to the ZnO surface, as evident from the detailed O 1s core level analysis in Figure 4, can qualitatively explain the higher surface coverage, that is, more densely packed pCF3PhPA interlayer on ZnO with respect to the F13OPA. Following that finding, the energetic difference between the bidentate and tridentate arrangement seems small enough that the π−π interaction of the phenyl rings in the pCF3PhPA interlayer is strong enough to dominate the arrangement of the molecules, their overall surface coverage, and therefore the resulting work function change.

■

ASSOCIATED CONTENT

S Supporting Information *

Assignment of the features in the experimental valence band spectra to the contributions of the corresponding phosphonate (Table S1). Evolution of radiation damage of the F13OPAmodified ZnO surface as identified in the C 1s core level spectrum (Figure S1). C 1s core level analysis of the PAmodified ZnO surfaces (Figure S2). XPS peak area ratios as calculated from C 1s core level analysis of the PA-modified ZnO surfaces (Table S2). Lateral homogeneity vs number of preparation cycles for the pCF3PhPA-modified ZnO surface (Figure S3). Orientation of the transition dipole moments of both phosphonates (Figure S4). Assignment of the features in the XAS spectra to the atom contributions of the corresponding phosphonate (Table S3). This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Email: [email protected]. Author Contributions ∇

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. M.T. and M.V.N. contributed equally.

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We thank Jürgen P. Rabe for granting access to SFM. This work was financially supported by the SFB951 (DFG), the European Commission FP7 Project HYMEC (Grant No. 263073), and the Helmholtz-Energie-Allianz “Hybrid-Photovoltaik”.

■

4. CONCLUSIONS Two partially fluorinated phosphonates, namely an alkyl and an aromatic phosphonate, were used to modify the surface work function of ZnO(0001)−Zn single crystals. The unmodified and PA-modified ZnO surfaces were comprehensively studied by contact angle, SFM, high-resolution XPS, UPS, and XAS measurements. Both UPS and XAS data were supported by additional DFT calculations to unambiguously identify the contributions of the PA molecules to the experimental spectra. By combining all these methods, we could not only iteratively optimize the quality of molecular interlayers with uniform surface coverage but also derive a complete picture of bonding and arrangement of the phosphonates with respect to the ZnO surface. Therefore, this work provides a detailed understanding of the factors impacting the work function modification of the technologically relevant Zn-terminated polar ZnO surface via phosphonate-based SAMs. Such an understanding is critical to address the changes in work function observed experimentally since the details of the preparation 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 charge collection process between the ZnO surface and an organic semiconductor in electronic and optoelectronic applications. For instance, the aligned π-systems and the densely packed arrangement of aromatic SAMs offer a

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