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ZnO Functionalization: Metal-Dithiol Superstructures on ZnO(0001) by Self-Assembly Yongfeng Tong, Tingming Jiang, Shunli Qiu, Konstantin Koshmak, Angelo Giglia, Stefan Kubsky, Azzedine Bendounan, Lin Chen, Luca Pasquali, Vladimir A. Esaulov, and Hicham Hamoudi J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b12071 • Publication Date (Web): 22 Jan 2018 Downloaded from http://pubs.acs.org on January 28, 2018

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

ZnO Functionalization: Metal-Dithiol Superstructures on ZnO(0001) by Self-Assembly

Yongfeng Tong,1-3 Tingming Jiang,1,2,4 Shunli Qiu,5 Konstantin Koshmak,6 Angelo Giglia,6 Stefan Kubsky,3 Azzedine Bendounan,3 Lin Chen,5 Luca Pasquali,4,6,7 Vladimir A. Esaulov*1,2 and Hicham Hamoudi*8

1

Institut des Sciences Moléculaires d’Orsay, Université-Paris Sud, 91405 Orsay, France

2

CNRS, UMR 8214, Institut des Sciences Moléculaires d’Orsay, Orsay ISMO, Bâtiment 351,

Université Paris Sud, 91405 Orsay, France 3

Synchrotron SOLEIL, L’Orme des Merisiers, Saint-Aubin, BP 48, F-91192 Gif-sur-Yvette

Cedex, France 4

Dipartimento di Ingegneria “E. Ferrari,” Università di Modena e Reggio Emilia, Via Vivarelli

10, 41125 Modena, Italy 5

School of Nuclear Science and Technology, Lanzhou University, Lanzhou 730000, China

6

IOM-CNR, s.s. 14, Km. 163.5 in AREA Science Park, 34149 Basovizza, Trieste, Italy

7

Department of Physics, University of Johannesburg, P.O. Box 524, Auckland Park 2006,

South Africa 8

Qatar Energy and Environment Research Institute, Hamad Bin Khalifa University, Doha,

Qatar

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The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract: We present a study of the functionalization of a monocrystalline ZnO surface using aromatic dithiols. The aim is to obtain a sulfur terminated self-assembled monolayer (SAM), which is then used for creation of further molecular superstructures with intercalated metal atoms. These metal-molecule self-assembled structures are characterized by high resolution X-ray photoelectron spectroscopy (XPS) and near edge X ray adsorption fine structure (NEXAFS) measurements. Formation of a 5,5- bis(mercaptomethyl)-2,20- bipyridine (BPD) SAM on ZnO(0001) is demonstrated using a protocol developed by us earlier for dithiol assembly on gold, allowing production of a standing up SAMs with free SH groups. Thereafter the formation of a metal intercalated dithiol super lattice is achieved by first grafting metal atoms (Ag or Ni) and then attaching a second BPD molecular layer. Metal atoms bind to both sulfur and the pyridine nitrogens within the SAM. Clear changes in the valence band region near the Fermi level are observed and the highest occupied system orbital positions are determined along with work function evolution.


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1.INTRODUCTION ZnO has many interesting properties: high electron mobility, a wide band gap, strong room temperature luminescence and good transparency. It has been considered in a number of applications including transistors, light-emitting diodes, sensors, and solar cells.1-6 It is considered that ZnO should also be a better anode material for dye-sensitized solar cells than TiO2 because of much higher carrier mobility.4 Yet, ZnO solar cells achieve lower efficiency than the TiO2 ones.5 There are still many problems7 regarding finding proper binding groups and passivating molecules at the ZnO interface: a point essential towards improving the performance of these devices. A number of works have thus focused on functionalization of ZnO surfaces with various molecules7-26 such as oligothiophenes,15,16 pentacene,25 organic acids,24 pyridine, phosphonate based molecules,21 l-cysteine,19 and other thiols.8-13 Several theoretical investigations of ZnO functionalization with molecules have been performed.8, 10, 15, 27,28 A theoretical study8 of functionalization of ZnO with carboxyl, amine and thiol groups showed that while carboxyl and amino groups did not modify the transport and conductivity properties of ZnO, thiols lead to insertion of molecular states in the band gap, thus suggesting that the optical properties of ZnO nanomaterials could be customized using them. ZnO functionalization with alkanethiol SAMs was found to improve the performance in solar cells.7,12,13 Sulfur end groups allow to bind molecules with various functionalities to clusters and surfaces. Thus thiol functionalized dyes have been attached to ZnO particles.13,14 There is also much interest in attachment of e.g. gold nanoparticles to ZnO26-28 surfaces and in this context attachment of dithiol ((SH-R-SH), where R is some functional group) clad gold nano



particles to ZnO nanorods28,29 has been explored. Similarly, gold nanoparticles have been attached to amine groups of bifunctional SAMs on zinc nanorods.30 Our aim is to explore further the functionalization of ZnO surfaces using molecules and investigate strategies of grafting more complex structures involving metals. We focus here on use of dithiols as anchors since they allow attachment of e.g. metal particles and thus creation of metal organic structures.29-36 While this aspect is attractive, dithiol selfassembly on surfaces is prone to problems,37-51 since it is not so trivial to obtain the usually desired thiol terminated surface on a nano particle or a planar surface. In a number of cases there arise problems in the (frequently adopted) solution-based self-assembly: oxidation, multilayer formation, mixed standing up and lying down layers (with both sulfur atoms tethered to the surface). Various strategies to circumvent these problems have been proposed,31 including thiol end group protection,37,39,41 post assembly processing44,49 using disulfide reducing agents, and use of solvents other than the frequently used ethanol.40,43 Recently some of us developed a robust protocol for the preparation of standing up sulfur terminated dithiol SAMs 31, 34-36, 46-48, 50 on gold surfaces. This was then successfully used31,34 to construct by self-assembly metal (M) atom intercalated 5,5- bis (mercaptomethyl)-2,20bipyridine (BPD; HSCH2-(C5H3N)2-CH2-SH) dithiol multilayers on gold (BPD-M-BPD-MBPD...Au). The structures thus built showed interesting properties, such as changes in HOMO–LUMO splitting, confirmed by DFT calculations, which leads to changes in optical properties and also significant bias dependence of the I–V characteristics of related devices based on the number of the BPD layers. This bottom-up building strategy is extended here to a semiconductor surface: ZnO. Note that while it has been found that ordered alkanethiol SAMs form on ZnO monocrystalline surfaces,8,9 dithiol self-assembly has not, to our knowledge, been


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investigated in any detail, although dithiol clad Au nanoparticles have been employed28,29 to attach to ZnO nanostructures. Here we present results of a high resolution XPS and NEXAFS investigation of BPD self-assembly on a monocrystalline ZnO (0001) surface and demonstrate the construction of a BPD-Ag(/Ni)-BPD layer on it. We chose a monocrystalline surface since it allows a better analysis of self-assembly characteristics than nanostructures used in more applied nanoscience research. The valence band characteristics for the different systems are reported delineating their changes as a function of structure evolution.

2. Materials and Methods. Sample and SAM Preparation. The ZnO(0001) monocrystals were purchased from Mateck. The ZnO surface preparation can be performed either through sputtering and annealing in vacuum, as described by some authors,53 or else through UHV sputtering, followed by annealing in air in a furnace, following a protocol developed by Goëtzen and Witte.52 It was demonstrated52 that this procedure leads to better results than annealing in UHV.53 We have tested both methods but finally the latter was adopted, as in this work SAMs were prepared by immersion into BPD solutions and thus in all cases the samples were extracted from UHV. We sputtered the ZnO surface with 1 keV Ar ions in the UHV preparation chamber and then the samples were annealed in air in a furnace at a 1000 °C temperature, as suggested by Goëtzen and Witte.52 Thereafter the samples were allowed to cool down under N2 flow to minimise possible contamination from the ambient atmosphere. This procedure is in fact the one used extensively by us in thiol self assembly on gold surfaces. LEED imaging was then performed on the thus prepared pristine ZnO sample. We consistently observed clear LEED patterns (Figure S1 in the supporting information) as reported earlier,52 with no superstructures. This procedure was adopted for the selfassembly



experiment described here. On the other hand in order to obtain the valence band spectra of clean ZnO and its workfunction we used the vacuum preparation procedure.” The self-assembled BPD layers were prepared using the procedure elaborated previously by us46-48 for dithiol SAMs from solution, which avoids problems of photooxidation, multilayer and mixed (standing/lying down) layer formation. The BPD SAMs were prepared ex-situ before the spectroscopy study by immersing the just annealed ZnO sample, cooled under N2 flow, into a freshly prepared 1 mM solution of BPD in n-hexane for about 40 min at 60 °C in a dark room. Use of hexane rather than ethanol in this procedure has been shown by us46-48 to lead systematically to films of better quality, as it reduces oxidation related problems. The hexane solution was purged with N2 (or Ar) prior to and during assembly. The samples were rinsed in hexane and dried under N2 flow before analysis. Metal grafting was performed from solution. In using this solution deposition procedure, we extend our previous works on BPD SAMs on gold34-36 and works of other authors32 who developed similar approaches. This provides a facile way to graft metals without having to use more elaborate vacuum equipment and in particular, as opposed to metal evaporation can be used for throughout coating of ZnO nanoparticles. Ag was grafted onto the BPD SAM by immersing the sample for about 20 minutes into a 60 °C aquous AgNO3 solution as described previously.32,34 The Ag-BPD-ZnO sample was carefully rinsed in water to eliminate spurious metal salt residues (chlorides, nitrates etc) as this was shown previously 32,34-36 (checked here by photoemission) and then dried under N2 (or Ar) flow. Finally, the BPD-Ag-BPD SAM was prepared by immersing the Ag-BPD SAM again for 30 minutes into the purged 60 °C BPD/hexane solution. The samples were analyzed by photoemission at different steps in this procedure. A similar procedure was followed for


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Ni grafting onto BPD.35,36 The BPD-ZnO sample was immersed into a 20 millimolar aqueous NiCl2 solution held at 60°C. Thereafter the Ni-BPD-ZnO sample was rinsed and then incubated again in the BPD hexane solution. Photoemission Measurements. Photoemission was performed on the BEAR beamline at the Elettra synchrotron54, 55 (Trieste, Italy). This involved high resolution XPS performed at different photon energies in the 60 to 1150 eV energy range. A hemispherical electron analyzer was used with a constant pass energy. The overall energy resolution for a photon energy of 260 eV was better than 200 meV (due to analyzer and beamline settings). XPS spectra were acquired at normal emission, with the light incident at 45° to the surface normal. Generally, the photon energies were chosen in order to measure the photoelectron peaks of the different core levels approximately at the same final kinetic energies of about 100eV, in order to maximize the surface sensitivity. The energy calibration was performed on a sputter cleaned Au sample with respect to the Au 4f7/2 level taken as an energy reference and set at 84.0 eV. For this we used a polycrystalline Au foil mounted on the sample holder along with the ZnO samples. However, some charging was noted on the ZnO samples resulting in shifts of XPS peaks in general of the order of 0.4 +/- 0.2eV. The valence band spectra were acquired at 260eV and 60 eV photon energies. Care was taken to minimize X ray damage to the films and was cross checked by making more than one measurement on the sample and changing the irradiation spot. Near Edge X-ray Absorption. The NEXAFS spectra were measured at the C 1s and N1s edge keeping fixed the incidence angle with respect to the surface plane and varying the direction of the electric field vector from perpendicular to the scattering plane (s-incidence geometry) to parallel to it (p-incidence geometry). On BEAR this is achieved by rotating the experimental chamber55 around the beam axis by an angle ΨC, from ΨC = 0° (s-scattering) to



90° (p-scattering). The photon beam was linearly polarized in the horizontal plane (90% linear polarization). The incidence angle of the light on the sample was set to 20°. Measurements were carried out by acquiring the drain current from the sample (total yield mode) and also counting secondary electrons using a channel electron multiplier. The photon energy resolution was of 0.2 eV. A normalization was performed to a reference absorption spectrum taken under the same experimental conditions on a carbon- and nitrogen-free sample. All the measurements were taken at room temperature. Computational Method. As a complement to the study of valence band spectra reported here, in order to obtain a better insight into the molecular features, density functional theory (DFT) calculations for BPD and Ag-BPD adsorbed on a ZnO were performed. All simulations were conducted using the CASTEP package56. Our model system consists of a BPD molecule adsorbed on ZnO(0001) with one deprotonated S end attached to the surface. Additionally, calculations were performed for the case of BPD with an Ag atom attached to the free sulfur end of the chemisorbed molecule. The ZnO surface was simulated using a 3-layer bulk terminated slab with static ZnO atoms. Geometry was optimized using the gradient corrected Perdew-Burke-Ernzerhof (PBE)57 exchange-correlation functional. A (4×4) unit cell was considered, with a vacuum thickness of 40 Å. The density of states (DOS) was defined with a cutoﬀ energy of 300 eV, using ultrasoft pseudopotentials58. The integration over the surface Brillouin zone was done on a 2×2×1 k-point sampling grid. The molecule was allowed to relax.

3. RESULTS AND DISCUSSION


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XPS spectra were acquired in the Zn2p, O1s, S2p, C1s, N1s binding energy (BE) regions, on the valence band and in correspondence of the Zn LMM Auger lines. Measurements were also made in the Ag 3d, Ag MVV Auger and Ni 2p, 3p regions after deposition of the metal and then the growth of the second BPD layer. In the following we first focus on the important point of self-assembly characteristics of BPD on ZnO by XPS and NEXAFS. We then discuss metal attachment and the growth of the second BPD overlayer and how these changes affect the characteristics in the valence band region. BPD-ZnO SAM. A very strong attenuation of the substrate (Zn, O) peaks occurs due to BPD adsorption. Therefore, we focus mainly on the molecular layer features. Characteristic S 2p3/2,1/2, C1s and N1s spectra are shown in Figure 1a-c. As in earlier works on ZnO, charging was noted, which resulted in shifts of the spectra from known positions for e.g. the BPD SAM on Au.31,34-36 Such variable shifts for a ZnO surface have been corrected in previous works by placing either Zn 2p at 1021.7eV,59 adjusting to Zn 3p positions18 at about 88.7eV, or also in case of alkanethiols, by placing the carbon peak at 284.8 eV.7 We chose to correct for charging by placing the peaks at the same positions as for BPD on Au (calibrated with respect to Au 4f set at 84eV) ,34-36 i.e. the S2p thiolate peak corresponding to the sulfur bound to the surface at 162 eV, the N1s peak at 399 eV, the C1s peak at 285.2 eV and Zn3p and Zn3d at 88.85 and 10.25 eV, respectively. Experiments performed at a high photon energy allowing simultaneous measurement of these main transitions, confirm this relative positioning of peaks. As will be seen later, peaks corresponding to the deposited Ag appear at the position of 368.1 eV. The S(2p) spectra in Figure 1a are plotted after a Shirley background subtraction. For alkane thiol adsorption on ZnO a single doublet is reported, with the 2p3/2 contribution at about 162eV.7,23 In our dithiol case the spectra can be decomposed into two doublets with a



spin-orbit splitting of 1.18 eV and a branching ratio of 1:2 for the 2p1/2-2p3/2 components, corresponding to thiolate S atoms bound to ZnO ( ST at 162.0 eV) and “free” SH atoms on top of the BPD SAM (SH at 163.7 eV). The ST component is very small at 260 eV photon energy, because of attenuation of the intensity of these electrons passing through the organic layer.48 A measurement was also performed at a higher photon energy of 630 eV (Figure S2), for which the ST component appears as a much more intense shoulder. If one assumes that the BPD layer is homogeneous, the layer thickness can be estimated by measuring the intensity attenuation of a photoelectron peak originated from the substrate (or from the buried interface, as in the present case for ST),43,45-47 considering the inelastic mean free path determined for alkanethiols: λ =0.6 nm60 for circa 100 eV kinetic energy photoelectrons (260eV photon energy) passing through the organic film. For the 630 eV photon data we used λ =1.4 nm.60 In a previous study of BDMT assembly,45-47 where a well-defined standing up monolayer was formed, the ratio of intensities of the thiolate ST to free SH (on top of the SAM) gave a thickness in agreement with estimates by spectroscopic ellipsometry.


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Figure 1. XPS spectra in the S2p, C1s and N1s regions for BPD-ZnO, Ag-BPD-ZnO and BPDAg-BPD-ZnO. Lines are fits mentioned in the text.

In the present case, the experimental intensity ratios derived from areas of the fitted peaks yield thickness values of about 1.7nm +/- 0.2nm for the cases of 260 and 630 eV photons. This is compatible with standing up molecules with an inclination to the surface normal of less than 40°, assuming a molecular length of about 2 nm. In earlier experiments of phenylpyridine63 thiol and biphenyl cyanide64 thiol on Au, XPS and NEXAFS data analysis showed inclination of these molecules to the normal of about 30°, although some of these authors63 also deduce a 18° inclination from their NEXAFS data. We would conclude that our BPD-ZnO data is compatible with these results on Au. The C1s spectrum after a Shirley background subtraction appears slightly asymmetric and can be decomposed into two components. We use Voigt contours in all the fits, with the same line profiles for a given system and given photon energy. The main C peak at about 284.8 eV is assigned to C–C and C=C bonds in the ring unit, whereas the peak 285.6 eV peak is assigned to C–N. This is consistent with works on BPD34-36 and pyridine-terminated thiol61 monolayer on Au. The N1s peak at 399eV is fitted with one Voigt contour. As in previous works on BPD on Au, the peak position is also close to that reported for a 2,2’ bipyridine salt 399.15 eV. K-edge NEXAFS spectra of carbon and nitrogen are presented in Figure 2 for s and p polarization. Regarding the C K-edge, in Figure 2a, one observes absorption resonances due to excitations into π* and σ* orbitals of the aromatic rings as well as into Rydberg molecular orbitals. A sharp double resonance structure appears with components at 285.0 eV and 285.6 eV close to the positions reported for pyridine62,64-66 and pyridine substituted phenyl



and alkane thiol61,63 SAMs. It corresponds to the (C1s - π*(3b1)) transition and the split results from interaction of C atoms with the N atom in the ring, giving rise to the higher energy component. Regarding the nitrogen K-edge (Figure 2b), a strong (N1s - π*(b1)) resonance appears at 397.3 eV. It has been reported to be 397.6 eV for a 2-2’ bipyridine salt.66 A clear dichroism is not observed when comparing spectra for s and p polarization for BPD. In previous experiments on phenyl pyridine SAMs58 a rather weak dichroism was reported. In our case this observation would be compatible with the large inclination of the molecular axis with respect to the normal deduced above and furthermore to molecular internal degrees of freedom, with possible variations in the relative twist of the two aromatic rings of BPD and tilt with respect to the surface normal.63 Summarizing, these results show that we indeed obtain a SAM of standing up BPD molecules presenting free SH groups at the outer SAM surface to which metal particles can be attached.


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Figure 2. NEXAFS spectra at the (a) C1s and (b) N1s edges for s and p light polarization. The cartoons show the experimental configuration. The small bars correspond to positions of π* and σ* transitions for pyridine and bipyridine.59,61,62

M-BPD-ZnO SAM and the BPD-M-BPD-ZnO Multilayer. Let us now turn to the bottom up assembly of the metal dithiol superstructures illustrated schematically in Figure 3 (drawn neglecting the other molecules in the SAM). The assembly was followed by XPS and the results for Ag and Ni are summarized in Figures 1 and S4 and Figure 4, respectively. The characteristics of metal attachment and growth of the second BPD layer bear close similarities.



Figure 3. Cartoon of the fabrication steps of metal intercalated BPD dithiol molecular layer formation on ZnO.

Upon immersion of the ZnO sample with the BPD SAM, into the AgNO3 (NiCl2) solution, the thiolate ST component of the S2p peak increases substantially as shown in Figure 1a and 4a. We ascribe this to the binding of Ag and Ni to the top sulfur SH atoms, resulting in a shift to lower core level binding energies. Indeed, this core level position corresponds to that of thiolate sulfur on e.g. Ag.50 Concurrently the SH peak decreases in intensity. In these experiments, part of the BPD SH groups remained unreacted with the metal atoms, as we observe that a part of the 163.7 eV SH component remained. In this work we sought to demonstrate the build-up of the BPD-M-BPD layers and did not go into the lengthy optimization procedures regarding solute concentration and incubation times, followed by XPS and NEXAFS measurements, that would be required to determine the conditions for attaining fully reacted SH groups.


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Figure 4. XPS spectra for BPD-ZnO, Ni-BPD-ZnO and BPD-Ni-BPD-ZnO in the S2p, N1s, C1s. Lines are fits of the spectra discussed in the text.

The fact that metal atoms are indeed attached to the BPD SAM is demonstrated by the appearance of characteristic metal atom related peaks: Ag 3d and Ni 2p are shown in Figure 5a,b. The Ni 2p3/2 peak appearing at 856eV (Figure 5b) is clearly shifted from the elemental Ni position at 852.4eV.67 This is consistent with presence of Ni2+ and agrees with results of Ni-BPD SAM on Au35,36 and with earlier reports of thiol adsorption68 on Ni. The shake-up satellites69 between the 2p3/2 and 2p1/2 peaks at 863eV and higher energy are also consistent with this. This effect must be due to interaction with sulfur or nitrogen. While the Ag 3d peak is close to that of the clean metal, the shape of the Ag Auger spectra (Figure S3) resembles that reported for silver sulfide rather than Ag metal and suggests a sulfidic nature of the Ag deposit as for Ni. One cannot rule out a degree of oxidation of the attached metals in the aqueous solution. Some oxidation of the sulfur atoms also occurs, as shown by the appearance of the 168 eV peak in Figure1a and 4a. Figure S4 in the supporting information shows absence of any nitrate peak in the N1s spectra. The exact structure of the metal layer on top of the SAM is not known from these measurements. Concerning the amount of



metal, for silver, from the intensities of N 1s and Ag 3d peaks in the XPS spectra (Figure S4) and taking into account the photoionization cross sections, one can estimate that the amount of Ag present is about 0.8 Ag atoms/molecule (see supporting information). Some of the metal is present also within the layer, as discussed below.

Figure 5. XPS spectra in the (a) Ag 3d and (b) Ni 2p regions. The pristine metallic, Ni 2p3/2 energy position is indicated by the vertical bar labelled NiM. Metal attachment also induces substantial changes in the N1s peak (Figure 1c and 4c), which broadens and, as can be seen in Figure 4c, shifts. This effect is very strong for Ni35,36 but exists also, though to a lesser extent, for the Ag case. The N 1s peak, can now be fitted with two components: one using the position and the shape of the BPD spectrum and a higher binding energy one. The latter is assigned to pyridine nitrogen interacting with metal ions, which penetrate into the SAM (cartoon in Figure 3). Earlier studies indeed


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indicate appearance of such a higher energy component near 400eV in the pyridine N 1s XPS spectrum due to interaction with e.g. Ru.70,71 This attachment of metals is not surprising, since bipyridine is a known chelating ligand. Also porphyrins and phtalocyanines with metal atoms bonded to nitrogens are well known to exist for Ni and other metals, including Ag. Complexation of Ag ions with various heterocyclic amines including bipyridine is known to occur.72 We will not go further into the exact nature of this metal-molecule bonding aspect here, and of the intermolecular interactions that it may involve possibly accompanied by some restructuring of the SAM, as our measurements do not provide more direct evidence on which such arguments could be based. When the M-BPD-ZnO sample is re-immersed into the n-hexane BPD solution we observe the growth of a BPD overlayer. The fingerprint of this is the increase of the “free” SH component in the XPS spectrum (Figure 1a and 4a), an effect ascribed to binding of BPD molecules on top of the M–BPD SAM. The layer has therefore again free sulfur on top. A concurrent increase in the C1s and N 1s intensity occurs (Figure 1 and 4) and the N1s peak becomes narrower as the metal related higher energy feature is attenuated. The BPD overlayer also induces a decrease in the Ag 3d peak intensity (Figure 5a). NEXAFS spectra were recorded for Ag and further BPD deposition. Rather small variations appear in the C1s spectra with appearance of feeble dichroism on addition of the second BPD layer compatible with more inclined molecules. The N1s data did not show any clear changes and is not shown here. The first π resonance positions were found to be 397.5 eV and 397.4 eV for the Ag-BPD and BPD-Ag-BPD cases respectively. Valence Band Characteristics. We now turn to the characteristics of the electronic structure of the functionalized ZnO. Characteristics of the VB region for pristine vacuum prepared ZnO(0001) have been reported previously by Jacobi et al.73 We performed similar



measurements on vacuum prepared ZnO which are in agreement with these73 reports and are summarised briefly in the supporting information (Figure S6). Measurements on the annealed ZnO prior to immersion into BPD were not performed, since, as could be expected, exposure to the ambient atmosphere would lead to erratic results.74 Spectra in the valence band (VB) region for BPD on ZnO are shown in Figure 6 for 60eV and 260eV nominal photon energies. These two energies were chosen since the photoionization cross sections of the different atomic levels are quite different (see the table in supporting information) and they allow to delineate better the origin of specific features in the spectra. The zero of the binding energy scale is referred to the polycrystalline Au Fermi level (work function ∼ 5.1eV). For the 260 eV photon energy the spectrum in Figure 6a is dominated by a strong peak due to the Zn3d electrons73 and a smaller lower energy peak. To avoid problems related to charging, the valence band spectra were in all cases adjusted in energy so that the Zn 3d peak lies at 10.25 eV. This procedure was followed in the photoelectron spectroscopy work function measurement studies on ZnO by Jacobi et al.73 This is, in our view, a reasonably reliable way to address the problems of charging in the valence band spectra region and in particular to determine the highest occupied system orbital (HOSO) positions and work function, which are the objectives of a number of studies. Jacobi’s et. al.’s work appears to be one of the very rare ones that use this approach for ZnO. At 60 eV one observes three large structures close to 4.5 eV, 7.5eV and 10.25 eV and smaller high energy features at 14.5 eV, and around 18 eV. The 10.25 eV feature is related mainly to Zn3d electrons.73 The 4.5 eV feature is related to both the molecule and O2p. The latter gives a broad low energy contribution (Figure S6) around 4eV for pristine ZnO as reported previously.73 This point becomes also more apparent if one compares the spectra


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in Figure 6b to a surface condition with less BPD molecules (Figure S5), where the afore mentioned two features are more intense than the central one. BPD can thus be deduced to contribute to the central and also the lower energy peak as well as the higher energy features. This is in agreement with other observations on pyridine and mercapto-phenyl pyridine75 SAMs on metals and existing studies of the photoelectron spectra of the gas phase pyridine molecule alone. The latter show76 states indicated by vertical bars in Figure 6b, in which the energy scale of the data is shifted to take into account the different referencing: Fermi level vs vacuum level.

Figure 6. Valence band spectra for BPD-ZnO (black line), Ag-BPD-ZnO (red line) and BPD-AgBPD-ZnO (blue line) taken at 60eV and 260eV photon energies. The low energy secondary electron cutoff and the region near the Fermi level is shown expanded in the small panels for the 60eV photon energy. The small vertical bars in panel (b) are states of pyridine in gas phase spectra (see text.)76

Metal attachment leads to significant changes related to appearance of Ag and Ni related features as shown in Figures 6 and 7 for Ag and Ni respectively. The Zn3d peak is



attenuated. In the Ag case, there is a prominent contribution of the Ag d electrons centered in the 5-6 eV region. A small structure appears close to 2eV (marked by an arrow in Figure 6a), that is usually noted in chalcogen atom interaction with Ag.50, 77,78 In the Ni case, the intensity of the lower BE peak in the 260eV photon energy spectrum increases and the spectrum now extends to almost the Fermi level. This low energy evolution is also clearly observable in the 60eV photon energy spectrum of Figure 7b. One can furthermore see the existence of a structure in the 6eV region that “fills up” the clear minimum in the BPD-ZnO spectrum (arrows in Figure 7b). For metallic Ni one would expect a strong peak at the Fermi level, however this is not the case here. We noted that Ni appears sulfurized. For a number of Ni sulfides it is found theoretically that the Ni d band shifts down in the density of states, appearing between -1 and -2.5eV, depending upon the specific sulfide, whereas the sulfur p electrons appear between 4 and 7eV.79 This shift in the d band is also seen in nickel interaction with nitrogen.80 This can thus explain the origin of these features in the spectrum.


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Figure 7. Valence band spectra for BPD-ZnO (black line), Ni-BPD-ZnO(red line) and BPD-NiBPD-(blue line) on ZnO for the indicated photon energies.

The general conclusions concerning the different contributions to the VB spectrum are confirmed in the DFT calculations shown in Figure 8a,b for the case of the BPD-ZnO and Ag-BPD-ZnO systems. The figures show calculated projected DOS for the molecular components: C, N, and S as well as Ag and the overall substrate DOS (shaded region). For AgBPD case the calculation was only performed for Ag attached to the top S atom. One can see that the calculated molecular DOS has groups of structures A,B,C,D that, after appropriate alignment, correspond to the experimental ones. It confirms that the high binding energy structures (C/D) correspond to molecular states and that BPD contributes to the VB in the A and B regions as surmised above. Also, the intense structure in the VB region for Ag-BPD corresponds to the calculated Ag contribution. Sulfur contributes as mentioned in the region closer to the Fermi level and this contribution appears more prominent in the case of Ag linking to sulfur.

Figure 8. Calculated projected densities of states for the BPD-ZnO and Ag-BPD-ZnO cases, showing contributions from the substrate (shaded areas) and from different molecular ACS Paragon Plus Environment


constituents. The experimental data are shown by points and the binding energy reference EF is indicated by the dashed line. The growth of the second BPD layer leads to attenuation of the Ag and Ni features. In the Ni case, the valence band region measured at 260eV photon energy, is characterized by mainly two clear peaks corresponding to the Zn3d level and a lower binding energy peak presumably related mainly to Ni d electrons. Indeed, at this energy the photoionization cross section is by more than an order of magnitude greater for these two levels than for the other ones. An expanded view of the Fermi level region is shown in Figure 6d for the Ag case. Linear extrapolation gives the HOSO edge at 2.4, 1.4 and 1 eV below the Fermi level for the BPD, Ag-BPD and BPD-Ag-BPD layers. In the Ni case in the low photon energy spectrum the HOSO edge lies at about 0.5eV binding energy. For the vacuum prepared ZnO the HOSO edge was deduced to lie at 2.6 eV.

Finally, it was interesting to see how the work function changes for these systems. To address this point we measured the low energy secondary electron cutoff (SECO) for the SAM covered surfaces. The SECO region was measured by polarizing the sample and accelerating electrons by 30eV. The work functions of the pristine ZnO (0001) and (0001̅) surfaces prepared entirely in vacuum by sputtering and annealing have been reported to be 3.7eV and 3.4eV respectively by Jacobi et al73 using in situ He I ultraviolet photoelectron spectroscopy. The values mentioned here correspond to a stable situation reached after annealing in that work. Our measurements on the vacuum prepared ZnO in the VB region (Figure S6) gave a work function value of 3.8 eV.


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The SECO region is shown in Figures 6b, c and 7b. As mentioned earlier, the spectra in the VB region are aligned to best adjust the structures near 10 eV and 14 eV to avoid shifts due to charging. Small variations in the cutoff energies occur, indicating only minor relative variations in the work functions, which in these cases lie between 4.5eV (BPD and Ni-BPD) and 4.2eV (Ag-BPD), as referred to the 5.1eV work function of polycrystalline Au. For comparison for methanthiol dosing of ZnO, a work function of 4.1eV has been reported18.

4. CONCLUSIONS

We reported a study of the self-assembly of 5,5- bis(mercaptomethyl)-2,20- bipyridine monolayers on zinc oxide formed according to a protocol developed earlier for dithiol selfassembly on gold. High resolution XPS and X-ray adsorption measurements show the formation of a single molecular layer with BPD molecules presenting a free SH group at the SAM/ambient interface.

This SAM was then used to successfully attach Ag and Ni to the outer SH groups. Attachment of metal atoms to the nitrogen also occurs. Furthermore, it was demonstrated that a metal (M) atom intercalated 5,5- bis(mercaptomethyl)-2,20- bipyridine dithiol multilayer (BPD-M-BPD/ZnO) could be built by successive self-assembly. These modifications lead to clear changes in the valence band region near the Fermi level that can be of interest in molecular electronics and photovoltaic applications. An interesting question, that needs further investigation, concerns the structure of the self-assembled BPD monolayer and in particular how metal complexation affects it.



The procedure we outline can be extended to further deposition of metal dithiol layers and also the spacing between layers may be altered by choosing different length dithiol chains. This opens further perspectives in functionalization of zinc oxide surfaces.

Acknowledgments Y. T. and T. J. acknowledge support from the Chinese Government Scholarship Council for their PhD work. L. C. and S. L. Q. acknowledge the National Natural Science Foundation of China (Grant No.11474140 and No.11405078). Experiments at Elettra were carried out in the framework of proposal n. 20160134. We are grateful to Christiano Pedersini & Barbara Sartori for help in their Elettra chemistry laboratories. V.A.E. acknowledges a Conicyt (Chile) grant N° 80150073, which made his stay at the Universidad Tecnica Federico Santa Maria possible during part of this work.

Associated Content. SUPPORTING INFORMATION: LEED image of ZnO. BPD-ZnO S2p sp ectra at 630eV. Zn and Ag Auger spectra. Ag 3d and N1s spectrum. S2p and Valence band spectra for different samples. Photoionization cross sections.

Corresponding Authors. Vladimir Esaulov: [email protected] Hicham Hamoudi: [email protected]

ORCID. Vladimir A.Esaulov : 0000-0002-7263-9685


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Luca Pasquali: 0000-0003-0399-7240 Angelo Giglia: 0000-0002-1672-9029 Konstantin Koshmak: 0000-0002-6479-7923 Hicham Hamoudi: 0000-0003-2036-7072 Yongfeng Tong: 0000-0002-0029-5289

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