Tuning the Band Bending and Controlling the Surface Reactivity at

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Tuning the band bending and controlling the surface reactivity at polar and non-polar surfaces of ZnO through phosphonic acid binding Alexandra R. McNeill, Adam R. Hyndman, Roger J. Reeves, Alison J. Downard, and Martin Ward Allen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10309 • Publication Date (Web): 21 Oct 2016 Downloaded from http://pubs.acs.org on October 24, 2016

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ACS Applied Materials & Interfaces

Tuning the band bending and controlling the surface reactivity at polar and non-polar surfaces of ZnO through phosphonic acid binding †,‡

Alexandra R. McNeill, , Adam R. Hyndman, ‡¶ Allen*, , † ‡ § ¶

‡,§

Roger J. Reeves

‡ ,§

Alison J. Downard,*,

†,‡

Martin W.

Department of Chemistry, University of Canterbury, Christchurch 8140, New Zealand MacDiarmid Institute for Advanced Materials and Nanotechnology, New Zealand Department of Physics and Astronomy, University of Canterbury, Christchurch 8140, New Zealand

Department of Electrical and Computer Engineering, University of Canterbury, Christchurch 8140, New Zealand

KEYWORDS: ZnO, band bending, surface reactivity, surface electron accumulation, wettability, photoconductivity.

ABSTRACT: ZnO is a prime candidate for future use in transparent electronics however development of practical materials requires attention to factors including control of its unusual surface band bending and surface reactivity. In this work, we have modified the O-polar0001, Zn-polar (0001), and m-plane 1010 surfaces of ZnO with phosphonic acid (PA) derivatives and measured the effect on the surface band bending and surface sensitivity to atmospheric oxygen. Core level and valence band synchrotron X-ray photoemission spectroscopy were used to measure the surface band bending introduced by PA modifiers with substituents of opposite polarity dipole moment: octadecylphosphonic acid (ODPA) and 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctylphosphonic acid (F13OPA). Both PAs act as surface electron donors, increasing the downward band bending and the strength of the 2dimensional surface electron accumulation layer on all the ZnO surfaces investigated. On the O-polar 0001, and m-plane 1010 surfaces, the ODPA modifier produced the largest increase in downward band bending relative to the hydroxyl-terminated unmodified surface of 0.55 and 0.35 eV, respectively. On the Zn-polar (0001) face, the F13OPA modifier gave the largest increase (by 0.50 eV) producing a total downward band bending of 1.00 eV, representing ~30 % of the ZnO band gap. Ultraviolet (UV) photoinduced surface wettability and photoconductivity measurements demonstrated that the PA modifiers are effective at decreasing the sensitivity of the surface towards atmospheric oxygen. Modification with PA derivatives produced a large increase in the persistence of UV-induced photoconductivity and a large reduction in UV-induced changes in surface wettability.

1. INTRODUCTION ZnO is an earth-abundant, biocompatible, wide band-gap (3.35 eV at 300 K) semiconductor that possesses the unusual combination of high transparency and high mobility n-type conduction.1,2 Consequently, it is attracting strong interest for use in transparent electronic devices such as ultraviolet (UV) photodiodes, thin film transistors, chemical- and biosensors, piezoelectric transducers, and as a transparent electrode for organic/inorganic solar cells.3-6 ZnO possesses an unusual surface chemistry in that its surfaces tend to be metallic in nature.7-12 This is due to the natural termination of the surface by hydroxyl groups that results in the structures shown in Figures 1a-c.7,13,14 As indicated in Figure 1a, the hydroxyl termination on the O-polar 0001 face is formed by hydrogen atoms attached to the outer plane of O atoms. On the Zn-polar (0001) face, the hydroxyl termination consists of OH groups bonded on top of the outer Zn atoms (Figure 1b), while on the non-polar m-plane 1010 face, that has an outer plane containing equal numbers of Zn and O atoms, both H and OH species are present (Figure 1c). Hydroxyl termination causes the conduction and valence bands to bend

downwards on all ZnO faces, as illustrated in Figure 1d, thereby creating a potential well in which a 2-dimensional surface electron accumulation layer (SEAL) is confined.11-14 We have recently shown a direct association between the extent of the hydroxyl coverage and the surface downward band bending,13,14 while Piper et al.11 and Ozawa et al.12 have found evidence of quantized electronic states inside the potential wells at ZnO surfaces. The surface metallicity produced by this SEAL may be advantageous for gas sensing, catalysis, and the formation of ohmic electrical contacts, but can also introduce unwanted environmental sensitivity, particularly towards atmospheric oxygen, in the conductivity of ZnO thin films and nanostructures. Oxygen readily adsorbs onto ZnO, capturing electrons from the SEAL and thus decreasing the surface electron density and conductivity of a particular sample.15 The extent of oxygen adsorption depends on the oxygen partial pressure, the amount of exposure to UV light, and the temperature of the sample.15,16 Such environment-dependent conductivity is clearly problematic for the use of ZnO in devices such as UV photodetectors, transparent thin film transistors, and solar cell electrodes.

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Figure 1. The (1 × 1) hydroxyl termination of the (a) O-polar , (b) Zn-polar (0001), and (c) m-plane 1010 ZnO sur0001 faces; and (d) energy band schematic of the surface potential well and the surface electron accumulation layer (SEAL) at ZnO surfaces (note: the downward band bending Vbb and related parameters ζ and ξ are defined in Section 2.2).

To fully exploit ZnO in electronic applications, strategies are required for tuning the surface band bending and controlling the surface reactivity. Several studies have demonstrated that physi- or chemisorption of donor and acceptor species on ZnO can modify the band bending. Ozawa et al.17 investigated the adsorption of tetrathiafulvalene (TTF) and found that it behaved as an electron donor on the O-polar and m-plane faces and an electron acceptor on the Zn-polar face. This was attributed to the degree of dissociation of the TTF molecules that was inversely correlated with the density of surface Znatoms. Undissociated TTF acted as an electron acceptor producing upward band bending on the Zn-polar face while dissociated TTF on the O-polar and m-plane faces resulted in downward band bending, due to the donor nature of the dissociated sulfur atoms. Schlesinger et al.18 investigated the energy-level alignment following the deposition of the molecular acceptor 2,3,4,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) on the O-polar and Zn-polar faces of ZnO and measured a significant upward shift in surface band bending (0.5 and 0.8 eV, respectively) on both polar faces. They also showed that only a relatively small electron transfer between the adsorbate and the semiconductor surface was required to significantly influence the surface band bending.18 Most recently, Ozawa et al.19 used X-ray spectroscopy to investigate the deposition of acridine orange base (AOB) on ZnO surfaces and found that AOB behaved as an electron donor inducing downward bending of the ZnO bands (up to 0.74 eV on the Opolar face) although without the formation of a potential well as the resulting bands were effectively flat. These studies demonstrate that the addition of chemical species to the ZnO surface can tune the band bending. However, in those studies, the modifying species were introduced to the ZnO surface from the vapor phase under ultra high vacuum (UHV) and the stability of the bonding interaction (if present)

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between the modifier and the surface is unknown. Furthermore, in each case the ZnO surface was prepared by repeated cycles of Ar-ion sputtering and annealing that removes nearly all of the native hydroxyl termination that is always present in air-exposed samples. In contrast, our interest is in the control of surface properties via modification strategies that can be undertaken in solution without the exclusion of air, and that result in layers bound to the surface through stable and welldefined bonding. The hydroxyl termination of ZnO surfaces provides a direct mechanism for chemical modification, through condensation reactions and hydrogen bonding with species such as silanes, carboxylic acids, and phosphonic acids (PAs).20-23 However, complexity is introduced by the varying chemical nature of the hydroxyl termination on the different polar and non-polar surfaces of ZnO (Figure 1a-c). PAs have been shown by a number of authors to form robust, covalently bonded self-assembled monolayers (SAMs) on ZnO surfaces,24-29 although systematic studies at different ZnO crystallographic surfaces are scarce. These studies have established that PAs bind to the ZnO surface through Zn−O−P linkages, replacing H at the surface sites shown in Figure 1. The available studies have also mainly focused on the use of SAMs to modify the work function and band-offset potential barriers of ZnO electrodes on organic semiconductors, particularly those used in solar cells. However, the unusual electronic activity and environmental sensitivity of ZnO surfaces are determined by their surface band bending, rather than their work function. Consequently, this work examines the ability of PA modifiers to influence the band bending and the surface electron accumulation layers at the common polar and nonpolar surfaces of ZnO. With our goal of finding practical, scalable methods for controlling the surface properties of ZnO, we have modified ZnO samples with two PA derivatives, using wet chemical methods. Octadecylphosphonic acid (ODPA) and 3,3,4,4,5,5,6,6, 7,7,8,8,8-tridecafluorooctylphosphonic acid (F13OPA) were selected for this study (Figure 2) because they have substituents with opposite polarity dipole moments (–2.3 D and 1.7 D for ODPA and F13OPA, respectively),30 thus allowing the influence of modifier polarity on band bending to be examined. Our methodology was to use synchrotron X-ray photoemission spectroscopy (XPS) in conjunction with UV photoconductivity and UV photo-induced surface wettability measurements, to examine the changes in surface chemistry and surface band bending introduced by these two PA modifiers on the Opolar 0001, Zn-polar (0001), and m-plane 1010 surfaces of hydrothermally grown bulk ZnO single crystals.

Figure 2. Octadecylphosphonic acid (ODPA) and 3,3,4,4,5,5, 6,6,7,7,8,8,8-tridecafluorooctylphosphonic acid (F13OPA) and their respective dipole moments.

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2. EXPERIMENTAL SECTION 2.1. Materials and Surface Modification. Acetone (HPLC grade, RCI Labscan), methanol (HPLC grade, Loba Chemie), isopropanol (IPA; HPLC grade, Sigma Aldrich), tetrahydrofuran (THF; HPLC grade, Sigma Aldrich), ODPA (Sigma Aldrich, 97%) and F13OPA (Sigma Aldrich, ≥ 95%) were used as supplied. Milli-Q water (resistivity > 18 MΩ cm) was used for rinsing. PA modification experiments were performed with hydrothermally-grown, double-sided polished, bulk single crystal ZnO 0001 and 1010 wafers (10×10×0.5 mm3) from Toyko Denpa Co. Ltd., cut to an offset of less than 0.5o.31 The resistivity, carrier concentration and electron mobility of these wafers were typically 0.18 Ωcm, 2 × 1017 cm-3, and 180 cm2 V-1 s-1, respectively. Two double-sided polished 0001 wafers and one 1010 wafer were each diced to provide three 10 × 3 × 0.5 mm3 samples per wafer. In this way, each of the two chemical modifiers (i.e. F13OPA and ODPA) could be applied to identical O-polar 0001, Znpolar (0001), and m-plane 1010 face samples from the same wafer, with the third sample from each wafer providing an unmodified reference. Each sample was ultrasonically cleaned using acetone, methanol, and IPA, and then dried with N2 gas. When required, unmodified ZnO samples were annealed in a UHV chamber at ~600 °C to reduce the surface hydroxyl coverage, as specified in the text. O-polar, Zn-polar, and m-plane samples were modified with ODPA and F13OPA following the method of Timpel et al.24 Specifically, each sample was immersed in 10 mL of 1 mM solutions of either F13OPA or ODPA in THF for 6 hours and then rinsed in Milli-Q water. The samples were then annealed at 190 oC for 3 hours followed by ultrasonic cleaning in THF for 5 minutes to remove any physisorbed material. The above immersion/annealing/sonication process was then repeated a further two times, but with the immersion and annealing steps reduced to 3 hours and 30 minutes, respectively. All immersion steps were performed with minimal exposure to light. 2.2. Core level and valence band XPS. Surface sensitive, variable photon energy (1486 – 150 eV) XPS was performed on F13OPA-modified, ODPA-modified, and unmodified reference samples for each of the O-polar 0001, Zn-polar (0001), and m-plane 1010 surfaces at the soft X-ray beamline of the Australian Synchrotron. XPS measurements were carried out at room temperature (RT) at a base pressure of < 2 × 10-10 mbar and without the use of an electron flood gun. Photoemission spectra were collected using a Specs Phoibos 150 hemispherical electron energy analyzer with the detector axis arranged in the direction normal to the sample surface. Care was taken to ensure that the samples were electricallygrounded to the spectrometer via a tightly-fitting tantalum foil sample holder, to avoid sample charging and to enable the zero of the binding energy (BE) scale to be directly referenced to the Fermi level of each sample. No BE shifts were observed on varying the incident photon flux, confirming the absence of any sample charging. The BE scale at each photon energy was calibrated using the Au 4 f core level doublet and Fermi edge of a sputter-cleaned Au reference foil. F 1s, O 1s, Zn 3s + P 2p core level and valence band (VB) spectra were measured at photon energies of 835 eV, 680 eV, 280 eV, and 150 eV, respectively, to maintain a similar electron photoemission kinetic energy of ~150 eV corresponding to an inelastic mean free path of ~6.2 Å, as

estimated using the TPP-2M formula.32 The PA modification was assessed using the relative peak areas and the BE separation of the surface and bulk O 1s core level components fitted using pseudo-Voigt functions [Gaussian:Lorentzian ratio = 60:40] on a Shirley background, with the full width at half maximum (FWHM) constrained to a FWHM of 1.0 ± 0.2 eV and 1.9 ± 0.2 eV for the bulk and surface O 1s components, respectively. The PA modifier coverages were compared via the relative peak area ratio of the P 2p and Zn 3s emission, which have closely lying BEs of 134.5 and 140.5 eV, respectively. The near surface band bending was determined from the VB spectra, using the approach of Chambers et al.,33,34 whereby the energetic separation ζ (= EV − EF) between the VB maximum (EV) and the Fermi level (EF) in the near-surface region was extracted from a linear fit of the low BE edge of the VB spectrum to a line fitted to the instrument background. This was then used to determine the near-surface band bending Vbb using Vbb = Eg − ζ − ξ, where ξ = (kT/q)ln(NC/n) is the energy difference between EF and the conduction band minimum (EC) in the bulk of the sample, n is the bulk carrier concentration and NC is the conduction band effective density of states (2.94 × 1018 cm-3 for ZnO) and Eg is the RT bandgap of ZnO = 3.35 eV. Negative values of Vbb indicate downward surface band bending and surface electron accumulation, while positive values of Vbb correspond to upward band bending and surface electron depletion.13,14 Survey spectra (using hv = 680 eV and hv = 1486 eV Xrays) were also collected from the unmodified and PAmodified samples to check for any sample contamination. Apart from a small P 2p signal and some adventitious C 1s emission detected on the unmodified reference samples (that will be discussed further in Section 3.1.1), no sample contamination was found. 2.3. UV-Induced Photoconductivity. Measurements were made on 5 mm × 5 × mm × 210 nm samples of a ZnO thin film grown by oxygen plasma assisted molecular beam epitaxy (MBE) on an epi-polished c-plane sapphire substrate. These films preferentially grow in the O-polar 0001 direction. The RT resistivity of unmodified and ODPA-modified samples was measured in air, before and after UV irradiation (365 nm, 1.5 mWcm-2). The unmodified sample was irradiated for 30 seconds and the ODPA-modified sample for at least 10 minutes to achieve the same photo-induced decrease in sample resistivity. After irradiation, samples were kept in the dark. Resistivity measurements were made using four Ti/Au (50 nm/ 50 nm) ohmic contacts (van der Pauw geometry), deposited before surface modification via electron beam evaporation. 2.4. UV Photo-Induced Wettability. Water contact angles were measured on unmodified and PA-modified ZnO samples before UV irradiation (300 nm, 0.22 mWcm-2, for 60 minutes) and up to seven days afterwards. Samples were kept in the dark after irradiation, and light exposure was minimized during the contact angle measurements. The contact angles of static water droplets (1 µL) were obtained using an Edmund Scientific camera and macro-lens to record the image, followed by analysis using ImageJ v1.48 software (NIH, Bethesda, MD) with a drop analysis plug-in. Multiple droplets were imaged per sample and the mean value reported.



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face of unmodified, 600 oC annealed, and F13OPA and ODPA modified, hydroFigure 3. XPS measurements from the O-polar 0001 thermal bulk ZnO: (a) O 1s core level XPS spectra using hv = 680 eV X-rays (dots represent measured data points, full lines represent fitted functions, ∆surf is the BE difference between the surface and bulk O 1s components, Asurf given as a percentage); (b) Zn 3s + P 2p core level spectra using hv = 280 eV X-rays; (c)-(d) valence band (VB) XPS spectra at hv = 150 eV showing the parameter ζ (= EV – EF) obtained by extrapolating the low BE edge to the instrument background and the calculated values of the surface band bending Vbb; (e) F 1s core level spectra taken using hv = 835 eV X-rays.

3. RESULTS 3.1. Binding of Phosphonic Acids and Band Bending at ZnO surfaces. In order to study the effect of PA modification on pristine low-defect O-polar 0001, Zn-polar (0001), and m-plane 1010 ZnO surfaces, hydrothermally-grown bulk single crystal ZnO samples were used for all experiments, unless specified otherwise. O 1s, Zn 3s + P 2p, and F 1s core level spectra at X-ray photon energies hv = 680 eV, 280 eV, and 835 eV, respectively, and VB spectra at hv = 150 eV were measured on the O-polar 0001, m-plane 1010 and Znpolar (0001) faces of unmodified, and F13OPA- and ODPAmodified samples with the results shown in Figures 3-5. Data obtained from the spectra are tabulated in Table 1 which also lists the surface band bending Vbb for each sample, calculated from the valence band to Fermi-level separation parameter ζ that was extracted from the corresponding VB spectra, as described in Section 2.2. On each face, the VB spectra consisted of two main features–a lower binding energy (BE) peak attributed to predominately O 2p derived states and a higher BE peak with a hybridized O 2p, Zn 4s, and possible Zn 3d character.35,36 3.1.1. O-polar face. The O 1s core level emission, shown in Figure 3a, was dominated by two components - a low BE component at 530.9 eV due to bulk oxygen (i.e. O atoms tetrahedrally coordinated to four Zn atoms), and a higher BE surface-oxygen component at 532.6 eV attributed to surface O atoms coordinated to three Zn atoms and a surface-terminating species. In the case of the unmodified O-polar face, the surface-terminating species is known to be a hydrogen atom (Figure 1a) forming an outer surface hydroxyl (OH) group.13,14

The binding energy shift of the surface O 1s component relative to the bulk O 1s component (referred to as ∆surf) was 1.70 eV, similar to that previously reported for the fullyhydroxylated O-polar face of ZnO.14 The surface OH component could be significantly decreased from a relative peak area fraction (referred to as Asurf) of 38.2 to 19.6 % by heating the sample to 600 oC in a separate UHV preparation chamber for ~15 min. This treatment also increased Vbb from –0.15 to +0.17 eV (Figure 3c), indicating a transition from downward to upward surface band bending.13,14 The decrease in the surface OH component and the increase in Vbb could be fully reversed by briefly exposing the sample to the atmosphere, which resets both the unmodified O 1s spectra and the surface downward band bending. This reversible behavior establishes a direct link between the surface OH component of the O 1s spectra and the surface band bending Vbb for the unmodified sample. The changes in Vbb were reflected in the BE shifts of the bulk-oxygen O 1s (and Zn 3s) core level peaks that shifted towards lower BE on 600 oC annealing and towards higher BE on PA-modification. However, the absolute changes in Vbb were more accurately determined from the corresponding shifts in the VB spectra, as the Australian Synchrotron soft x-ray beam-line calibration was most stable and precise at photon energies of 150 eV. For the F13OPA modified sample, the surface O 1s component increased in intensity to an Asurf of 47.4 %, while ∆surf decreased from 1.70 to 1.30 eV, indicating a change in the chemical nature of the surface termination, consistent with the binding of PA to the surface. At the same time, Vbb decreased from –0.15 to –0.50 eV, indicating a significant increase in surface downward band bending, while a distinct P 2p peak


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0 face of unmodified, 600 oC annealed, and F13OPA and ODPA modified, hydroFigure 4. XPS measurements from the m-plane 101 thermal bulk ZnO: (a) O 1s core level XPS spectra using hv = 680 eV X-rays (∆surf values are the BE differences between the surface and bulk O 1s components, Asurf values given as a percentage); (b) Zn 3s + P 2p core level spectra using hv = 280 eV X-rays; (c)-(d) valence band (VB) XPS spectra at hv = 150 eV showing the parameter ζ (= EV – EF) and the calculated values of the surface band bending Vbb; (e) F 1s core level spectra taken using hv = 835 eV X-rays.

appeared in the Zn 3s + P 2p core level spectra, confirming the successful attachment of F13OPA molecules to the O-polar surface. Further evidence of the successful attachment of F13OPA molecules was provided by the appearance of F 1s emission that was absent in the unmodified sample. Surprisingly, this F 1s emission consisted of two components: a component at 687.6 eV due to F bound to C and a component at 685.1 eV, a BE consistent with F bound to Zn.37 The latter signal suggests there is some decomposition of the F13OPA modifier, possibly occurring at least partially during the XPS measurement. Supporting this suggestion, the 685.1 eV signal increased from 50 % to 58 % of the total F 1s signal, when spectra were collected after 1 and 15 scans from the same spot on the sample. Furthermore, changes in the C 1s XPS signal during repeat scans of F13OPA-modified ZnO (Zn-polar face) have also been reported,24 although the use of different measurement conditions means that the relevance to the present work is uncertain. The same effects (apart from the appearance of a F 1s peak) were observed and were even more pronounced in the ODPA modified sample, with a dominant surface O 1s component (Asurf = 88.7 %) at a ∆surf of 1.04 eV in the O 1s spectra, and a stronger decrease in Vbb to –0.70 eV. The larger attenuation of the bulk O 1s component and the higher P 2p : Zn 3s peak area ratio indicated that a denser surface coverage was achieved with the ODPA modifier compared to the F13OPA modifier. A strength of this study is that the XPS spectra of the PAmodified, 600 oC annealed, and the unmodified reference samples were collected at the same time on samples mounted in the same electrically-grounded tantalum foil holder, so that the relative BE shifts could be accurately compared. However, this approach led to a small amount of phosphorous contami-

nation on the unmodified reference sample, as shown in Figure 3b. Survey scans (at hv = 680 eV and 1486 eV) indicated no other contaminants apart from a small amount of adventitious carbon. The O 1s and VB spectra (and the extracted values of ζ and Vbb) of the unmodified reference were very similar to those of several other uncontaminated O-polar 0001 samples of the same hydrothermal bulk ZnO material, measured on the same instrument, including those reported in the literature.13,14 This shows that the low-level P-contamination had a very minor effect on the measured values of Asurf and Vbb in the reference sample. Note: similar checks were made on the m-plane 1010 and Zn-polar (0001) reference samples, whose O 1s and VB spectra were also very similar to those of several other completely uncontaminated samples. face. In the case of the unmodified 3.1.2. m-plane m-plane 1010 face, a single surface O 1s component (Asurf = 57.4 %) was observed at a ∆surf of 1.45 eV in the O 1s spectra, with a surface band bending Vbb (again determined from the corresponding VB spectra) of –0.20 eV. These findings are consistent with those that we have previously reported for fully hydroxyl-terminated m-plane ZnO surfaces (as shown in Figure 1c).13 The hydroxyl-related surface O 1s component could again be significantly reduced from Asurf = 57.4 % to 10.8 % by 600 oC heating for ~15 min under UHV conditions, accompanied by a transition from downwards to upwards band bending with Vbb increasing from –0.20 to +0.20 eV. MoraFonz et al.38 calculated an upward bending of the valence band of 0.32 eV on the clean 1010 ZnO surface, while the 600 oC heated surfaces here still had a small residual OH coverage, possibly explaining the difference. The surface band bending and hydroxyl coverage changes after 600 oC heating were fully reversible on brief exposure of the sample to the atmosphere.



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, m-plane and Zn-polar Table 1. Summary of the fitted synchrotron XPS parameters from the O-polar (0001) faces of hydrothermal bulk ZnO: peak area fraction (Asurf) of the surface O component and its binding energy shift (∆surf) relative to the bulk O component in the O 1s spectra (hv = 680 eV X-rays); the peak area ratio (Zn 3s : P 2p) of the P 2p emission compared to the Zn 3s emission (both using hv = 835 eV X-rays); and the surface band bending (Vbb) calculated from the measured value of ζ, using Vbb = Eg − ζ − ξ (hv = 150 eV X-rays) where Eg = 3.35 eV at 300 K. O-polar 0001 face

m-plane 1010 face

Zn-polar 0001 face

Modifier

Asurf (%)

∆surf (eV)

Zn 3s: P 2p

Vbb (eV)

Asurf (%)

∆surf (eV)

Zn 3s: P 2p

Vbb (eV)

Asurf (%)

∆surf (eV)

Zn 3s: P 2p

Vbb (eV)

Unmodified

38.2

1.70

1:0.2

–0.15

57.4

1.45

1:0.3

–0.20

52.2

1.16

0.0

–0.50

600 oC anneal

19.6

1.84

-

+0.17

10.8 7.6

1.32 2.40

-

+0.20

-

-

-

-

F13OPA

47.4

1.30

1:2.1

–0.50

69.1 3.9

1.07 2.73

1:2.1

–0.25

46.3 6.9

1.12 2.58

1:1.6

–1.00

ODPA

88.7

1.04

1:6.0

–0.70

88.3 5.1

1.18 2.81

1:7.8

–0.55

54.3 21.5

1.03 2.11

1:3.9

–0.65

Figure 5. XPS measurements from the Zn-polar (0001) face of unmodified, and F13OPA and ODPA modified, hydrothermal bulk ZnO: (a) O 1s core level XPS spectra using hv = 680 eV X-rays (∆surf values are the BE differences between the surface and bulk O 1s components, Asurf values given as a percentage); (b) Zn 3s + P 2p core level spectra using hv = 280 eV X-rays; (c)-(d) valence band (VB) XPS spectra at hv = 150 eV showing the parameter ζ (i.e. EV – EF) and the calculated values of the surface band bending Vbb; (e) F 1s core level spectra taken using hv = 835 eV X-rays.

For the F13OPA modified sample, the surface O 1s component increased in intensity to Asurf = 69.0 %, while ∆surf decreased from 1.45 to 1.07 eV, with the appearance of a second very small surface component Asurf = 3.9 % at a ∆surf of 2.73 eV. A strong P 2p peak was observed in the Zn 3s + P 2p spectrum indicating the attachment of F13OPA molecules to the mplane surface, although in this case Vbb decreased only very slightly. As noted for the O-polar face, more pronounced changes were observed for the ODPA modified sample with a dominant surface O 1s component Asurf = 88.3 % at a ∆surf of +1.18 eV (with an additional small component Asurf = 5.1 % at

∆surf of 2.81 eV) and a decrease in Vbb from –0.20 to –0.55 eV, corresponding to a significant increase in surface downward band bending. 3.1.3. Zn-polar (0001) face. For the unmodified Zn-polar (0001) face, a single surface O 1 s component Asurf = 52.2 % was observed at a ∆surf of 1.16 eV in the O 1s spectra, while the value of Vbb for the unmodified Zn-polar face was –0.50 eV. This represents a significantly larger downward band bending compared to the unmodified O-polar and m-plane faces. UHV heating measurements were not performed on the


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Zn-polar face sample, as we have previously established that the hydroxyl termination on the Zn-polar face has a much higher thermal stability compared to the other ZnO surfaces and does not show significant OH desorption under UHV heating.14 For the F13OPA modified sample, there was a small decrease in intensity for the surface O 1s component to Asurf = 46.3 % and a small decrease in ∆surf from 1.16 to 1.12 eV. A second small surface O 1s component with Asurf = 6.9 % appeared at a ∆surf of 2.58 eV. For the ODPA modified sample, more pronounced changes in the O 1s spectra were again observed with two significant surface components: the largest component Asurf = 54.3 % at a ∆surf of 1.03 eV and a second component Asurf = 21.5 % at a ∆surf of 2.11 eV. Timpel et al.24 have previously observed O 1s components of F13OPA- and p(trifluoromethyl) phenylphosphonic acid-modified Zn-polar single crystal ZnO surfaces at ∆surf values of +1.1 eV and +2.1 eV that they assigned to tridentate and bidentate PA binding modes. Surprisingly, the ODPA modifier only produced a small decrease in Vbb from –0.50 eV to –0.65 eV on the Zn-polar face, while the F13OPA modifier produced a much larger decrease in Vbb to –1.00 eV. This is the reverse of the trend observed on the O-polar and m-plane faces, where the ODPA modifier produced the largest decrease in Vbb. Another significant difference was observed in that the peak area intensity of the ZnF component (685.1 eV) relative to the C-F component (687.6 eV) in the F 1s spectra was significantly higher on the F13OPA modified Zn-polar face, suggesting a higher degree of modifier decomposition. 3.2 Effect of PA Modification on Oxygen Sensitivity of ZnO. The surface electronic properties of ZnO are known to be sensitive to environmental conditions, particularly the presence of atmospheric O2. Such behavior is a disadvantage for many proposed electronics-based applications of ZnO and hence we have investigated whether PA modification can mitigate the environmental sensitivity of ZnO. It is generally understood that sensitivity to air arises due to oxygen adsorption on the ZnO surface via the capture of a surface electron (from the SEAL) to become O , i.e. O e → O .39-41 On above band gap UV irradiation, photo-generated holes recombine with the captured surface electrons, oxidizing the O molecules which desorb from the surface as O2, ( O → O ). This leaves the photo-generated electrons to increase the surface electron density of the sample. After termination of the UV irradiation, atmospheric oxygen slowly readsorbs onto the ZnO surface, recapturing the SEAL electrons, thereby returning the surface towards its initial preillumination state. These processes make the conductivity of ZnO particularly sensitive to its recent UV illumination history producing an effect known as persistent photoconductivity (PPC).39-41 This refers to the phenomenon whereby an increase in conductivity produced by exposure to UV radiation persists after termination of the illumination, with the recovery to the initial dark conductivity occurring on a timescale of hours or even days. To investigate the effect of PA surface modification on the PPC of ZnO, the resistivity of unmodified and ODPAmodified samples from the same O-polar 0001 ZnO thin film grown by MBE on c-plane sapphire was measured before and after exposure to 365 nm UV radiation. Resistivity measurements are considerably less surface sensitive than XPS and

therefore thin films were used since they have a significantly higher surface-to-volume ratio than bulk wafers. The inset of Figure 6 shows the typical UV photo-response of unmodified thin film ZnO. After UV irradiation (30 seconds), one sample was first held in vacuum followed by nitrogen and then oxygen atmospheres (at 1 atmosphere) for consecutive 10 hour periods. The second sample was continually exposed to air after termination of the UV irradiation. Compared to the response in air, a very slow increase in resistivity is observed for the sample maintained under vacuum, with little change on exposure to nitrogen. However when the sample is exposed to oxygen, the rate of increase in resistivity is greater than in air, confirming that the PPC effect is directly linked to the re-adsorption of oxygen onto the ZnO surface. The behavior of the UV-irradiated ODPA-modified sample on exposure to air is shown in the main part of Figure 6. In contrast to the unmodified sample (red) that returned to its initial resistivity in 40 hours, the ODPA-modified sample (blue) showed only a small recovery in resistivity after 120 hours, with the film taking many weeks to return to its initial resistivity. Clearly the PPC effect is greatly enhanced in the ODPA modified sample. Interestingly, although separate UV transmission measurements showed that the ODPA modifier did not affect the transmission of UV radiation into the ZnO film, at least 10 minutes of 365 nm UV irradiation (i.e. 20 times longer) was required to decrease the resistivity of the ODPAmodified film to a similar level to the unmodified film.

Figure 6. Resistivity of the O-polar 0001 face of unmodified and ODPA modified ZnO thin film samples grown by MBE, before and after illumination with 365 nm UV radiation in air (note: respective resistivity scales indicated via circles/arrows). Inset shows results of a separate experiment in which a UV-irradiated face) was maintained in unmodified ZnO sample (O-polar 0001 vacuum, then exposed to nitrogen followed by oxygen atmospheres (both at 1 atm).

ZnO surface hydrophilicity is also influenced by surface adsorbed oxygen as demonstrated by Sun et al.42 who reported a significant increase in the surface wettability of ZnO films upon exposure to 365 nm UV radiation and a slow reversal in dark conditions over the course of 7 days, as O2 slowly readsorbed onto the ZnO surface and it regained its initial wettability. Hence we also investigated the effect of PA modifiers on UV-induced changes in surface wettability.



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Figure 7 presents plots of the water contact angles of unmodified and PA modified bulk single crystal ZnO samples, before and after exposure to 300 nm UV light. Note that these samples had undergone the prior XPS analysis, described earlier. Figure 7a shows that all three faces of the unmodified samples undergo a significant (~30 °) decrease in water contact angle (i.e. an increase in hydrophilicity) after UV exposure, followed by an increase in water contact angle over the course of several days, to close to their initial values. As expected, before UV irradiation, all modified samples exhibited higher contact angles than the unmodified samples (Figure 7b), consistent with the presence of the hydrophobic PA layers. However the contact angles measured on the ODPA-modified Zn-polar face, and the F13OPA-modified O-polar face were unexpectedly low, possibly indicating damage to the surface during prior handling or the XPS measurements. Nevertheless, after exposure to UV light under the same conditions as for the unmodified samples, Figure 7b shows that the UV-induced decrease in water contact angle was ≤ 10°. Evidently PA modification has significantly decreased the sensitivity of the ZnO surfaces to changes associated with surface oxygen content, as monitored by both PPC and surface hydrophilicity.

Figure 7. (a) Water contact angles on the unmodified Zn-polar (0001), O-polar0001, and m-plane 1010 faces of hydrothermal bulk ZnO samples before and after illumination with 300 nm radiation, and (b) water contact angles of the F13OPA and ODPA modified Zn-polar, O-polar, and m-plane samples before and after UV irradiation using the same UV exposure conditions.

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4. DISCUSSION Prior to PA modification, significant crystallographic polarity-related differences were observed in the hydroxyl termination and the downward band bending of the unmodified polar and non-polar surfaces of ZnO single crystal wafers. In particular, the binding energy shift (∆surf) between the hydroxylrelated surface O 1s component and the bulk O 1s component in the O 1s spectra was 1.70, 1.45, and 1.16 eV, while the surface band bending Vbb was –0.15, –0.20, and –0.50 eV on the O-polar0001, m-plane 1010, and Zn-polar (0001) surfaces, respectively. We have previously observed similar trends in surface band bending13,14 and other polarity-related differences in the photoluminescence, reflectivity, and barrier heights of Schottky contacts on different ZnO surfaces, that have been associated with the large internal spontaneous polarization field along the c-axis of ZnO.43,44 Furthermore, the bulk-terminated Zn-polar and O-polar surfaces are theoretically unstable due to a diverging electrostatic dipole.45-47 First principles density functional theory (DFT) calculations indicate that stabilization occurs via a competition between surface OH termination and triangular (hexagonal) reconstructions on the Zn-polar (O-polar) faces depending on the hydrogen and oxygen chemical potentials.45-47 The OH coverage of the unmodified Zn-polar (O-polar) surfaces investigated here were ~2.0 (~1.3) monolayers, estimated from the corresponding O 1s spectra in Figures 5a and 3a using a Lambert-Beer adsorption law approximation, as described elsewhere.13,14 These coverages indicate that OH termination and subsequent surface modification was likely to be the dominant stabilization mechanism. Following PA modification, the surface coverage of the ODPA layer was higher than that of the F13OPA layer on all ZnO surfaces investigated, as indicated by a significantly higher P 2p : Zn 3s peak area ratio and a significantly larger surface O 1s component (Asurf) in the corresponding O 1s spectra (Table 1). The same relative surface coverages were found by Hotchkiss et al.28 for ODPA and F13OPA on ZnO films deposited by RF magnetron sputtering; denser packing for ODPA was attributed to its longer tail increasing the stabilizing van der Waals interactions. However, we have also demonstrated that after reaction of ZnO with F13OPA, there is significant Zn-F coordination, which, if present at the surface (especially on the Zn-polar face), would decrease the binding sites available to F13OPA and therefore its surface coverage. On all ZnO surfaces, the binding energy of the surface O 1s component shifted towards lower energies after both F13OPA and ODPA modification, indicating an increase in the electron density in the vicinity of the surface O atoms. In most cases, the binding energy difference (∆surf) between the surface O 1s component and the bulk O 1s component was close to the value of 1.07 eV calculated by Wood et al.48 using DFT for the tridentate bonding configuration of different benzylphosphonic acids on the Zn-polar (0001) surface of ZnO. In nearly all cases, this ∆surf ≈ 1.07 eV tridentate component was the dominant surface O-component (accounting for more than 90% of the surface related O 1s emission), the exception being the ODPA modified Zn-polar (0001) face that also had a significant surface O 1s component at ∆surf = 2.11 eV, close to the value of 2.00 eV calculated by Wood et al.48 for bidentate PA coordination. Figure 8 shows energy level diagrams of the band bending at the unmodified and PA-modified ZnO surfaces. These dia-


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grams assume that the surface band bending at the conduction band minimum (EC) and the valence band maximum (EV) are the same. However, Mora-Fonz et al.38 used hybrid DFT to predict a smaller band gap at the clean ZnO m-plane 1010 surface compared to the bulk, due to an upward bending of EV. A similar effect was also predicted for other oxide materials.49 King et al.50 showed that the presence of electron accumulation layers at semiconductor surfaces can reduce the surface band gap, due to many-body effects (i.e. interactions between free carriers in the SEAL) that increase the downward band bending of EC relative to EV. If operative, both effects would mean that the downward band bending of EC shown in Figs. 8(a)-(c) would be underestimated, as it would be greater than the experimentally-determined value of Vbb for the downward band bending of EV. However, the relative changes in the band bending of both EC and EV on surface modification would be largely unchanged. Both F13OPA and ODPA modifiers increased the downward band bending on all the ZnO surfaces investigated. This increase in downward band bending indicates that the coordinated PA groups are stronger electron donors than the hydroxyl groups that naturally terminate unmodified ZnO surfaces. As illustrated in Figure 8, the ODPA modifier produced the largest increase in downward band bending (by ∆Vbb = –0.55 and –0.35 eV, respectively) on the O-polar 0001 and m-plane 1010 faces, resulting in modified Vbb values of –0.70, and – 0.55 eV, respectively. This finding is consistent with a higher surface coverage for ODPA than F13OPA, and also with the negative dipole of the ODPA headgroup. In contrast, the F13OPA modifier produced the largest increase in downward band bending on the Zn-polar (0001) face (by ∆Vbb = –0.50 eV) resulting in a modified Vbb of –1.00 eV that represents ~30 % of the band gap of ZnO. While the origin of this effect is unclear, the significantly larger Zn-F component in the F 1s emission from the Zn-polar face compared to the O-polar and m-plane faces, may be correlated with the larger observed increase in downward band bending. The unexpectedly small increase in downward band bending after modification of the Zn-polar (0001) face with ODPA also correlates with the relatively low surface coverage as evidenced by the Zn 3s: P 2p ratio in Table 1, and the anonymously low water contact angle (before UV illumination) in Figure 7b. Whether these measurements indicate an inherent difference between PA binding at the Zn-polar face, and the O-polar and m-plane faces needs to be further explored. The finding that both F13OPA and ODPA modifiers acted as surface donors increasing the surface downward band bending despite their opposite dipole moments (i.e. 1.7 D and –2.3 D, respectively) suggests that charge transfer to the ZnO surface is mainly determined by the interface (or bonding) dipole formed by the P–O–Zn bonds that anchor the PA modifiers to the ZnO surface, rather than the electronic nature of the headgroup of the modifier. For F13OPA, Zn-F bonding on the surface would be expected to withdraw charge, due to the strong electronegativity of F. However, this effect, if present, must be outweighed by the effect of the interface dipole of the P–O–Zn bonding. In their computational study involving the Zn-polar (0001) face, Wood et al.48 found that the adsorption energies and O 1s core-level shifts for both the tridentate and bidentate binding modes of different benzylphosphonic acids were not affected by their degree of fluorination. Although the change in work

Figure 8. Energy-level diagrams showing the surface band bending on the unmodified (black), F13OPA-modified (blue) and , (b) m-plane ODPA-modified (red) faces of (a) O-polar 0001 1010, and (c) Zn-polar (0001) ZnO.

function due to the interface dipole of tridentate-bonded benzylphosphonic acids showed a small variation with their degree of fluorination, the work function change was always positive irrespective of the direction of the molecular dipole moment. Cornil et al.27 reported similar theoretical results for SAMs of various benzoic acid modifiers with different headgroups on the ZnO m-plane 1010 surface, showing that the work function change due to the interface dipole was also always positive, irrespective of the sign of the molecular dipole moment. These theoretical calculations suggest that the charge redistribution at the interface between different PA modifiers and the ZnO surface will only be slightly affected by the dipole moment of the modifier, which is consistent with the surface band bending results reported here. Surprisingly, Wood et al.48 calculated a charge transfer of ~0.2e from the ZnO surface to the benzylphosphonic acid molecule, that is, charge transfer in the opposite direction to that observed in this work. On the other hand, Cornil et al.27 reported a transfer of ~0.3e from the benzoic acid molecule to the ZnO surface. In their calculation of charge transfer, Wood et al.48 assumed a model involving two hydroxyl groups per unit cell with one in a bridging position between two surface Zn atoms and the other filling an oxygen vacancy. However, an experimental study of single crystal ZnO surfaces (with samples prepared in the same way as reported here) using surface X-ray diffraction showed that the hydroxyl termination on the Zn-polar (0001) face consists of a fully occupied (1 × 1) overlayer located at the on-top position above each surface Zn atom.51 Hence differences between the model and the surfaces used experimentally in this study may underlie the discrepancy between the calculated and experimental charge transfer direction. The presence of Zn-F coordination in the F13OPA modified samples is very intriguing with respect to both the mechanism of decomposition of the PA and the location of F in the ZnO structure. Although F should withdraw charge if coordinated at the surface (especially on the Zn-polar face if replacing



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surface terminating OH groups, see Figure 1b), when introduced onto the lattice oxygen-site, F is known to act as a shallow donor.52,53 Hence, the correlation of the relatively large downward band bending with the large Zn-F component of the F 1s emission for the Zn-polar face might suggest that more F is present as a sub-surface dopant, rather than coordinated at the surface. Polydorou et al.54 reported that F species introduced into ZnO films via a SF6 plasma produced a downward shift in the surface bands as measured using ultra-violet photoelectron spectroscopy. They also used DFT calculations to propose a mechanism whereby F can substitute oxygen atoms and/or oxygen vacancy sites on the O-polar face and eliminate near-surface electron-traps. Substitutional FO could contribute to the downward band bending on F13OPA-modified ZnO surfaces, however, the mechanism responsible for the decomposition of the F13OPA molecule to produce F species is unknown. As well as increasing the surface downward band bending, the F13OPA and ODPA modifiers were effective at decreasing the environmental sensitivity of the O-polar 0001, m-plane 1010, and Zn-polar (0001) surfaces. A very large increase in the UV-induced PPC of ODPA modified O-polar ZnO thin films in air was observed, and both PA modifiers almost completely prevented UV-induced hydrophilicity changes at all ZnO faces. The latter observation can be explained by noting that at the modified surfaces, the water contact angle should be determined by the wettability of the PA layer, rather than the underlying ZnO surface. Therefore, any changes that occur at the ZnO surface on UV irradiation should not affect the measured surface wettability, providing the PA layers are continuous over the surface and stable to UV exposure. On the other hand, the origin of the increase in UV-induced PPC, and the slow rate of decrease in resistivity during UV exposure, for ODPA-modified ZnO is currently unknown. Modification with PA might be expected to prevent the adsorption of oxygen at ZnO surfaces, in which case the decrease in resistivity during UV irradiation cannot be attributed to desorption of surface O2 with a concomitant increase in surface electron density. In the absence of surface oxygen, a different mechanism must be operative. One possibility is capture of the photo-generated holes by the ODPA itself, a state that would not be expected to be reversible on termination of the UV illumination. Alternatively, if the modified surface also incorporates adsorbed oxygen, a slower rate of desorption and readsorption of oxygen might be expected, simply due to the barrier properties of the ODPA layer, thereby accounting for the relatively slow rates of resistivity decrease on UV illumination, and resistivity increase post-illumination. Regardless of the origin of the extended PPC, the results demonstrate that the PA modifying layers are stable to UV illumination, and over an extended time period. 5. CONCLUSIONS Two PA modifiers, F13OPA and ODPA, of opposite dipole moment, were both found to increase the native downward band bending of the O-polar0001, m-plane 1010, and Znpolar (0001) ZnO surfaces. On the O-polar and m-plane surfaces, the ODPA modifier produced the largest increase in downward band bending (by 0.55 and 0.35 eV respectively) while on the Zn-polar face, the F13OPA modifier produced the largest increase (by 0.50 eV) and the largest total downward band bending of 1.00 eV (~30 % of the band gap). At the same

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time both modifiers were found to significantly decrease the sensitivity of these surfaces towards atmospheric oxygen, as evident from a significant decrease in UV photo-induced changes in H2O wettability and a significant increase in UVinduced persistent photoconductivity. Although the origin of the latter finding is currently unclear, these results indicate that modification with PAs is a promising approach to eliminate the undesirable reactivity of ZnO surfaces. In future work we will examine the origin and role of Zn-F coordination in the band bending at F13OPA modified surfaces, we will establish the basis of the increase in PPC at PA-modified ZnO and further characterize the stability of the PA SAMs. We will also expand our investigations to identify other modifiers that form stable attachments to ZnO surfaces under solution conditions, and are useful for tuning the surface band bending and eliminating unwanted surface reactivity.

AUTHOR INFORMATION Corresponding Authors * Fax: +64-3-364-2761. Tel: +64-3-369-3499. E-mail: [email protected]. * Fax: +64-3-364-2110. Tel: +64-3-364-2100. E-mail: [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.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We gratefully acknowledge beam-time and the expert assistance of B. Cowie, L. Thomsen, and A. Tadich at the Australian Synchrotron, Victoria, Australia. This work was financially supported by the MacDiarmid Institute for Advanced Materials and Nanotechnology and the Royal Society of New Zealand Rutherford Discovery Scheme.

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