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C: Surfaces, Interfaces, Porous Materials, and Catalysis
Synchrotron X-ray Photoelectron Spectroscopy Study of Electronic Changes at the ZnO Surface Following Aryldiazonium Ion Grafting: A Metal-to-Insulator Transition Alexandra R. McNeill, Kalib J Bell, Adam R. Hyndman, Rodrigo M Gazoni, Roger J. Reeves, Alison J. Downard, and Martin Ward Allen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00758 • Publication Date (Web): 01 May 2018 Downloaded from http://pubs.acs.org on May 1, 2018
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The Journal of Physical Chemistry
Synchrotron X-ray Photoelectron Spectroscopy Study of Electronic Changes at the ZnO Surface following Aryldiazonium Ion Grafting: a Metal-to-Insulator Transition †,‡
†,‡
Alexandra R. McNeill, Kalib J. Bell, Adam R. Hyndman, †,‡ †,‡ ‡§ Reeves Alison J. Downard,*, and Martin W. Allen*, , † ‡ §
†,‡
Rodrigo M. Gazoni,
†,‡
Roger J.
School of Physical and Chemical Sciences, University of Canterbury, Christchurch 8140, New Zealand MacDiarmid Institute for Advanced Materials and Nanotechnology, Wellington 6012, New Zealand Department of Electrical and Computer Engineering, University of Canterbury, Christchurch 8140, New Zealand
ABSTRACT: ZnO is a member of a small class of semiconductors that includes In2O3, SnO2, CdO, and InN, whose surfaces are highly unusual due to the fact that their electronic bands bend downwards to form a quantized potential well in which a 2-dimensional electron gas (2DEG) is confined. At the O-polar ZnO0001 surface, this effect arises from the adsorption of H adatoms which produces a hydroxyl-terminated surface. In this work, we investigate the effect of covalently anchored organic layers on the band bending at ZnO0001 surfaces. We use aryldiazonium salt electrochemistry to deposit 4–5 nm thick layers of 4-nitrophenyl (NP) and 4-(trifluoromethyl)phenyl (TFMP) groups. Synchrotron X-ray photoelectron spectroscopy (XPS) showed that both NPand TFMP-modification permanently removed the downward band bending at the ZnO surface. This behavior can be explained by the direct covalent bonding of the aryl groups to the surface, also revealed by XPS analysis, and the electron withdrawing character of both modifiers. Surprisingly, the in-situ irradiation-induced reduction of the grafted NP groups to aminophenyl-like moieties resulted in a further band bending shift in the upwards direction, producing a combined change of more than 1.0 eV, corresponding to strong near-surface electron depletion. This phenomenon can be explained by the participation of electrons from the ZnO surface and possibly hydrogen from subsurface donors in the reduction process. Our study shows that electrochemically-grafted aryl layers can alter the fundamental nature of ZnO surfaces by producing a ‘metal-to-insulator transition’ in its unusual surface conductivity.
1. INTRODUCTION ZnO is a technologically-important wide band gap semiconductor1 that is finding increasing use in gas and biological sensing,2 organic solar cell electrodes,3 and in a range of transparent electronic devices.4 From a fundamental perspective, it is a member of a small class of intrinsically ntype semiconductors, that includes In2O3,5 SnO2,6 CdO,7 and InN,8 whose electronic bands bend downwards at the surface, thereby creating a potential well in which a two-dimensional electron gas (2DEG), also referred to in the literature as a surface electron accumulation layer, is confined. Remarkably, the surface potential well is sufficiently localized to create quantized electronic states that have been observed using angle-resolved photoelectron spectroscopy (ARPES).5,7,8 This behavior is in direct contrast to the depleted nature of most semiconductor surfaces and contributes to the well-known gas sensing properties of these materials, as the 2DEG can be
modulated by the presence of polar or electrophilic adsorbates, such as H2O, NO2, and O2, leading to measurable changes in the conductivity of thin films and nano-structures.9 However, this surface metallicity and environmental sensitivity is undesirable for many other applications, such as electronic biosensors, ultraviolet photodetectors, and transparent thin film transistors used in electronic displays. Consequently, robust surface modification strategies are required that can remove the unusual downward band bending and 2DEG at the surface of these materials. In the case of ZnO, the link between the downward bending of the surface bands and the presence of the 2DEG is well established: Piper et al.10 reported direct evidence for a quasi2DEG at O-polar ZnO0001 surfaces and suggested that H adatoms provide the charged surface states necessary to produce the required downward band bending; Ozawa and Mase11 showed that H adsorption onto clean ZnO0001 and surfaces, prepared in ultra-high-vacuum (UHV), ZnO1010
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directly led to the formation of a 2DEG; Allen et al.12 determined the carrier concentration profile within the 2DEG for different ZnO surfaces and used magnetotransport measurements to separate the influence of the surface and bulk electron populations; Finally, Heinhold et al.13,14 quantified the relationship between the hydroxyl termination and the downward band bending at polar and non-polar ZnO surfaces, and further showed that the downward band bending could be reversed by decreasing the surface OH coverage by heattreatment under UHV. However, they also showed that the surface immediately rehydroxylates on exposure to atmosphere, recreating the potential well and associated surface metallicity.13,14 Other studies have confirmed that, under most conditions, the polar and non-polar surfaces of ZnO are naturally terminated by OH groups.15,16 The grafting of covalently-bonded groups offers a potentially simple and robust method for modifying the chemical and electronic nature of semiconductor surfaces. Attempts at chemically modifying the electronic properties of ZnO surfaces have mainly focused on either (i) the in-situ application of modifiers to the ‘clean’ hydroxyl-free ZnO surface, prepared by repeated cycles of Ar-ion bombardment and O2 annealing under UHV, or (ii) chemical substitution reactions involving the surface-terminating OH groups under non-vacuum laboratory conditions. The ‘clean-surface’ approach has the advantage of creating a highly reactive surface, rich in dangling bonds, prior to chemical modification, but relies on UHV gas-phase reactions and is not readily-scalable for industrial use. Using this approach, Ozawa et al.17 showed that tetrathiafulvalene (TTF) acts as an electron acceptor on the Zn-polar 0001 face, and faces, as a donor on the O-polar 0001 and m-plane1010 due to different levels of decomposition of the TTF molecule. In contrast, acridine orange base acted as an electron donor on all of these ZnO surfaces.18 Schlesinger et al.19 showed that 2,3,4,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane acts as surfaces of ZnO an acceptor on the 0001 and 1010 producing a significant upwards shift in band bending of 0.5 and 0.8 eV, respectively. While these experiments have convincingly established the electronic tunability of the ZnO surface, the robustness of these adsorbed molecules and their stability under atmospheric conditions were not investigated. In contrast, the ‘surface-hydroxyl substitution’ method has been widely used to covalently bind a variety of chemical modifiers, including silanes,20 thiols,21 alkenes,22 carboxylic acids,23 and phosphonic acids,24 to the ZnO surface. Much of this work has focused on the ability of these modifiers to tune the work function and band alignment of ZnO organic solar cell electrodes or to act as immobilization platforms for biosensing, while their ability to tune the surface conductivity of ZnO has been largely unexplored. However, McNeill et al.25 have recently shown that octadecylphosphonic acid and 3,3,4,4,5,5,6,6,7,7,8,8,8-(tridecafluoro)octylphosphonic acid, covalently-attached via a condensation reaction with surfaceterminating OH groups, act as surface donors on ZnO significantly increasing the downward band bending. In another approach, the reduction of aryldiazonium salts has proven to be a robust and flexible surface modification strategy that has been extensively used in modifying carbonbased materials,26,27 and also to deposit organic layers on the surfaces of silicon, metals and some metal oxides.28,29 The popularity of this method is due to its simplicity and speed, its
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applicability to a range of aryl derivatives and the possibility of forming covalent aryl-surface bonds. Wang et al.30 have shown that 4-nitrophenyl (C6H5-NO2) layers can be successfully deposited onto nanocrystalline Aldoped ZnO films. These groups were then deliberately reduced to 4-aminophenyl (C6H5-NH2) groups in order to couple DNA sequences for subsequent fluorescence imaging. As the focus of their work concerned the selective-immobilization of biomolecules, neither the nature (or even presence) of bonds between aryl groups and the surface, nor the effect of the NP and aminophenyl layers on the surface conductivity of ZnO were investigated. However, Cottineau et al.31 have shown that the downward band bending at p-type Si(111) electrodes can be significantly decreased by replacement of the Si-H termination with arylamine groups, although a surprisingly small decrease was observed following 4-nitrobenezene diazonium (NBD) grafting. In contrast, Hunger et al.32 observed a much larger reduction in the downward band bending of H-terminated p-Si(111) wafers from 0.47 to 0.09 eV after grafting with NBD, indicating significant electronic passivation of surface states. In this work, we investigate the ability of aryldiazonium salt electrochemistry to modify the electronic nature of ZnO0001 surfaces and remove the native downward band bending and 2DEG that are undesirable for many ZnO-based electronic applications. Two aryldiazonium salts with strong electron-withdrawing substituents: 4-nitrobenzenediazonium tetrafluoroborate and 4-(trifluoromethyl)benzenediazonium tetrafluoroborate were used. These derivatives are well-known to yield layers of nitrophenyl (NP) and (trifluoromethyl)phenyl (TFMP) groups respectively.33 It is anticipated that these surface modifiers will withdraw electron density from the ZnO surface via the aromatic ring, with the transfer of negative charge from the surface to the modifier causing the surface bands to bend upwards.19 Since 4nitrobenzene has a dipole moment of 4.22 ± 0.08 D compared to 2.86 ± 0.08 D for 4-(trifluoromethyl)benzene,34 the former is expected to produce a larger electron withdrawing effect. Synchrotron X-ray photoelectron spectroscopy (XPS) was used to simultaneously correlate the chemical and electronic changes to the ZnO surface introduced by these different modifiers, while atomic force microscopy was used to investigate the multilayered nature of the electrografted films.
2. EXPERIMENTAL METHODS 2.1. Materials. HPLC-grade acetone (VWR International), methanol (Loba Chemie), isopropanol (IPA, Sigma-Aldrich), and acetonitrile (ACN, Merck) were used as supplied. Tetrabutylammonium tetrafluoroborate ([Bu4N]BF4) was prepared by standard methods and dried under vacuum at 80 o C for 2 days. 4-Nitrobenzenediazonium tetrafluoroborate (NBD) and 4-(trifluoromethyl)benzenediazonium tetrafluoroborate (TFMBD) were synthesized from their respective anilines using standard procedures.26 2.2. Plasma Assisted Molecular Beam Epitaxy. Sb-doped ZnO 0001 substrates were grown by plasma assisted molecular beam epitaxy (MBE) on double-sided polished cplane sapphire. Standard effusion cells (E-science Inc., USA) were used to produce metallic beams of Zn (99.9999 %, Osaka Asahi Metal Ltd.) and Sb (99.999 %, Kamis Inc.) at 320 and 255 °C, respectively. A 250 W RF plasma generator (Oxford Applied Research Ltd.) was used to produce an oxygen
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(99.999%) plasma beam at a partial pressure of 9 × 10-5 mbar. A 2 nm thick buffer layer was grown at a substrate temperature of 450 °C and annealed at 730 °C for 10 min in the oxygen plasma beam to accommodate the lattice mismatch between the film and substrate. The remaining growth was carried out at a substrate temperature of 730 °C at a rate of 1 nm/min to a thickness of 180 nm. Reflection high-energy electron diffraction patterns acquired during and after growth exhibited sharp streaks that confirmed the excellent crystalline quality of the film, its epitaxial growth, and the 2-dimensional nature of its 0001 surface (see Supporting Information). The O-polar surface termination was confirmed from the relative intensities of the lowest binding energy valence band states measured using XPS at 1486 and 680 eV, following the method of Allen et al.15 UV-visible transmission spectroscopy at room temperature (RT) using an Agilent Cary 6000i spectrophotometer recorded a transmission of more than 90% in the visible spectrum and a band gap of (3.285 ± 0.004) eV was extracted from standard Tauc plots (see Supporting Information). Hall Effect measurements using the van der Pauw contact geometry and a 0.51 T magnetic field confirmed the n-type conductivity of the Sb:ZnO films and yielded a resistivity, carrier concentration and mobility of 1.26 ± 0.01 Ω.cm, (9.1 ± 0.5) × 1018 cm-3, and (55 ± 3) cm2/Vs, respectively. This confirmed that the Sbdoping was successful in producing high-quality conducting ZnO 0001 substrates suitable for electrochemistry. 2.3. Electrochemical Methods. Electrografting from aryldiazonium salt solutions was carried out using an Autolab potentiostat. The ZnO substrate, acting as the working electrode, was placed on an insulating plate and a glass solution cell containing a bottom aperture was sealed above the sample to define a working electrode area of 50 mm2. Copper foil was taped to a titanium/gold strip, deposited along one side of the substrate by e-beam evaporation, to ensure a good electrical contact. A platinum wire counter electrode and a saturated calomel reference electrode (SCE) completed the cell. The electrolyte solution contained 10 mM NBD or TFMBD in 0.1 M [Bu4N]BF4-ACN. Prior to voltammetry, the cell solution was purged with N2 for 15 min. Nitrophenyl (NP) and trifluoromethylphenyl (TFMP) films were grafted to the surface of the ZnO substrate using two cyclic scans at 200 mV/s between 0.6 and -0.3 V. After modification, the samples were sonicated for 2 min in acetone, then rinsed in methanol and IPA, and dried using high-purity N2 gas. Control samples were prepared following the same procedures as above, but with the cyclic voltammetry performed in the absence of the aryldiazonium salt, to assess any modifying effect of the [Bu4N]BF4-ACN solution on the ZnO surface. 2.4. Core level and valence band XPS. XPS measurements were performed at the soft X-ray beamline of the Australian Synchrotron, Melbourne, Victoria, under UHV (< 2 x 10-10 mbar) and at RT on as-grown, control, NP-modified, and TFMP-modified ZnO 0001 substrates, sourced from the same MBE-grown wafer. All spectra were collected using a Specs Phoibos 150 hemispherical electron energy analyzer with the detector axis positioned normal to the sample surface. C 1s, N 1s, O 1s, F 1s, Zn 2p core level and valence band (VB) spectra were collected using photon energies (hν) of 435, 548, 680, 835, 1180, and 150 eV, respectively. This ensured a similar surface sensitivity for each spectrum, as in each case the collected photoelectrons had the same kinetic energy of ~150 eV, corresponding to an inelastic mean free path of ~6.2
Å (using the TPP-2M formula).35 Core-level spectra were fitted using pseudo-Voigt functions (Gaussian : Lorentzian ratio of 30:70) on Shirley or linear backgrounds, as appropriate. Full-width half maxima (FWHM) were constrained to
