Electron Donor Molecule on the Oxide Surface: Influence of Surface

Sep 27, 2011 - and Kazuhiko Mase. §. †. Department of Chemistry and Materials Science, Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8550, Ja...
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Electron Donor Molecule on the Oxide Surface: Influence of Surface Termination of ZnO on Adsorption of Tetrathiafulvalene Kenichi Ozawa,*,† Shiori Munakata,‡ Kazuyuki Edamoto,‡ and Kazuhiko Mase§ †

Department of Chemistry and Materials Science, Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8550, Japan Department of Chemistry, Rikkyo University, Toshima-ku, Tokyo 171-8501, Japan § Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), Tsukuba 305-0801, Japan ‡

ABSTRACT:

Adsorption states of tetrathiafulvalene (TTF) on single-crystal zinc oxide (ZnO) surfaces are examined by photoelectron spectroscopy and X-ray absorption spectroscopy utilizing synchrotron radiation light. Comparison of TTF coverages, its adsorption states, and TTF-induced band bending is made among three ZnO surfaces with orientations of (0001), (1010), and (0001). TTF adsorbs both molecularly and dissociatively on all three ZnO surfaces at room temperature. The total amount of TTF at the saturation coverage is proportional to the surface Zn density. Contrastingly, there is a negative correlation between the amount of dissociated TTF and the surface Zn density. These indicate that TTF is bonded mainly to the surface Zn atoms, while the surface O atoms are necessary for TTF decomposition. The formation of the TTF overlayer induces downward band bending on ZnO(1010) and (0001), whereas the band is bent upward in energy on ZnO(0001). The origin of the surface dependence of band bending is discussed in terms of charge donor/acceptor behavior of the overlayer on the ZnO surfaces.

1. INTRODUCTION Organic-molecule-based optical and electronic devices such as organic thin-film transistors, organic light-emitting diodes, organic solar cells, etc. are considered to be the main stream of nextgeneration semiconductor devices.1 Performances of these devices are influenced by an energy-level alignment at the interface between organic layers and surfaces of metal electrodes or semiconductor substrates because the alignment is a crucial factor to determine electron-/hole-injection-barrier heights from the electrodes to the organic layers and vice versa.2 5 Thus, many studies have been devoted to elucidate structural and electronic properties of the interfaces between organic thin films and metal and semiconductor surfaces. Tetrathiafulvalene (TTF) and tetracyanoquinodimethane (TCNQ) are prototype building blocks of charge-transfer salts, in which they act as an electron donor and an acceptor, respectively. These molecules are good candidates to control the surface charge densities of metals and semiconductors by adsorption, a technique known as “surface transfer doping”.6 For example, densities of charge carriers of graphene can be tuned in r 2011 American Chemical Society

the range of 1012 cm 2 by adsorption of both an electron donor (TTF) and acceptors [TCNQ and tetrafluoro-TCNQ (F4TCNQ)].7 9 Regarding metal and semiconductor surfaces, adsorption states of TCNQ and F4TCNQ have been extensively investigated by several groups. On coinage metal surfaces (Cu, Ag, Au), these molecules actually act as electron acceptors, although they exhibit a rather complex charge transfer behavior;10 13 electrons are transferred from the metal surfaces to a π orbital of TCNQ (F4TCNQ), as expected from the charge acceptor character of these molecules, whereas partial charge donation from the molecules to the substrates is also induced through the cyano groups, which interact strongly with the surface metal atoms to form chemical bonds. F4TCNQ on Si surfaces also acts as a charge acceptor and induces upward bending of the Si band as a result of the formation of the charge depletion layer at the surface.14,15 Received: August 13, 2011 Revised: September 20, 2011 Published: September 27, 2011 21843

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Figure 1. Schematic models of (a) TTF, (b) a bulk structure of wurtzite ZnO, and (c) surface structures of ZnO(0001), (1010), and (0001). The surface unit cells are indicated by diamonds for ZnO(0001) and (0001) and by a rectangle for ZnO(1010). On ZnO(0001), a model of a straight step structure is exhibited. The S S distance and the overall size of TTF are taken from ref 43. The lattice constants of ZnO are from ref 44. The S S distance and the neighboring distance between two Zn atoms are nearly equal.

Contrary to the studies of TCNQ-adsorption systems, studies of electron donor TTF on metal and semiconductor surfaces are rare. Fernandez-Torrente et al. have shown in their study using scanning tunneling microscopy (STM) and density functional theory (DFT)16 that TTF on Au(111) forms a molecular array resembling a one-dimensional Wigner crystal, originating from the Coulomb repulsion between positively charged TTF. This infers a charge-donating character of TTF. Later, Hofmann et al. have systematically investigated the interaction of TTF on the (111) surfaces of Cu, Ag, and Au by DFT calculations and have obtained a strong indication that TTF is a good charge donor on these metal surfaces with the amount of transferred charge between 0.3 and 0.4 electrons per one molecule.17 However, unfortunately, such charge transfer between TTF and the substrate surfaces has not been experimentally scrutinized. Moreover, adsorption state, adsorption structure, as well as surface chemistry of TTF on not only these coinage metals but also any other metal and semiconductor surfaces have not been elucidated yet. In the present study, we investigate the interaction of TTF (Figure 1a) on zinc oxide (ZnO) surfaces by photoelectron spectroscopy (PES) and near-edge X-ray absorption fine structure spectroscopy (NEXAFS) utilizing synchrotron radiation. Our main interest is to estimate the influence of the surface termination of ZnO on the adsorption state and the adsorption structure of TTF. ZnO crystallizes into wurtzite structure (Figure 1b), and three surfaces with Millar’s indices of (0001), (1010), and (0001) (Figure 1c) are the object of intensive scrutiny. If the ideal bulk-truncated surfaces are realized, ZnO(0001) and (0001) surfaces are terminated by Zn and O atoms, respectively, while ZnO(1010) is composed of equal numbers of the Zn and O atoms. Because of the difference in the atomic composition and arrangement of these surfaces, chemical reactivities of ZnO should be dependent on which surface is exposed. For example, the ZnO(0001) surface is easily etched when the surface is being exposed to atomic H, whereas the (0001) surface is passivated after the surface O atoms are terminated by H.18 The termination dependence of the surface reactivity has also

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been found for adsorption of simple molecules such as CO and H2O as well as organic molecules.19 Thus, it is expected that the TTF ZnO interaction also depends on the surface orientation of ZnO, and this must affect surface transfer doping of ZnO by TTF. ZnO is a wide-band-gap n-type semiconductor with the Fermi level lying just below the conduction band minimum (CBM).20 Surface transfer doping, either electron doping or hole doping, of the ZnO surfaces is achieved by introducing O vacancies or adsorption of foreign species. In recent angle-resolved PES studies, we have found that H, H2O, and CH3OH act as electron donors and that adsorption of these species induces the insulatorto-metal transition on the surfaces of ZnO(1010) and (0001) by creating a two-dimensional electron gas with the charge density of 1012 1013 cm 2.21 23 Since TTF is a strong charge donor molecule, the metallic state with a high charge density is expected for the TTF/ZnO systems. Such metallization should be accompanied by large downward bending of the ZnO band. We, therefore, focus also on the TTF-adsorption-induced band bending to access the TTF-induced surface metallization.

2. EXPERIMENTAL METHODS The PES and NEXAFS experiments were performed at beamline (BL) 13A of the Photon Factory,24 High Energy Accelerator Research Organization (KEK), utilizing linearly polarized synchrotron radiation light. For the PES measurements, a hemispherical electron energy analyzer (VG Scienta SES200) was used with a typical overall energy resolution of 0.12 and 0.16 eV at photon energies (hν) of 350 and 600 eV, respectively. The sample specimens were placed so that the surface normal direction was parallel to the axis of the analyzer lens system. The incidence angle of the light (θi) was 65° from the sample normal direction. For NEXAFS measurements, the spectra were acquired by a total electron yield mode using a homemade electron-counting detector composed of a retarding mesh, a microchannel plate, and an anode plate.25 The detector was placed at about 40 mm away from the sample. The incidence light was polarized in the incidence plane, and θi was varied between 0° and 60°. All the measurements were done at the substrate temperature of 300 K. For the binding energy reference of the PES spectra, the Fermi cutoff position in the spectrum of the tantalum sample holder was used. The photon energy was calibrated using the N K-edge absorption peaks of gaseous nitrogen. The NEXAFS spectra were normalized by the photocurrent of a gold mesh, which was placed in a chamber between the analysis chamber and a focusing mirror chamber. Single-crystal ZnO samples with (0001), (1010), and (0001) orientations (SPC Goodwill, Russia) were used throughout the present study. The sample surfaces were cleaned in situ by cycles of Ar+ sputtering (2 kV, ∼5 μA) and annealing at about 1000 K in UHV. To restore possible oxygen vacancies, annealing at 700 K in an O2 atmosphere (1.3  10 4 Pa) was carried out after the sputtering annealing cycles. The clean surface of each ZnO sample exhibited a (1  1) low-energy electron diffraction (LEED) pattern with sharp spots. The sample cleaning and the LEED measurements were carried out in the preparation chamber, which was connected to the analysis chamber but was pumped separately to the ultrahigh vacuum. After surface cleaning, the ZnO samples were transferred to an evaporation chamber for TTF adsorption. TTF (Acros, >99%) was deposited on the ZnO surfaces at room temperature. 21844

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Figure 2. O 1s and Zn 3s core-level PES spectra for clean and TTFsaturated ZnO surfaces. The photon energy used was 600 eV. The spectra are shown after Shirley-type backgrounds are subtracted from the raw spectra. Peaks drawn by solid lines in the O 1s spectra indicate the components which make up the observed O 1s peaks.

A homemade effusion cell for organic solids with low sublimation temperature26 was used for TTF sublimation at the sublimation temperature of 370 K. The amounts of adsorbed TTF were estimated from intensities of the C 1s and S 2p core-level PES peaks.

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Figure 3. (a) C 1s and (b) S 2p core-level spectra for the TTF-saturated ZnO surfaces. Shirley-type background is subtracted from each observed spectrum. (c) Plots of the amounts of adsorbed TTF at saturation (FTTF) against the densities of the Zn atoms on the ZnO surfaces (FZn). In the upper panel, FZn corresponds to the ideally bulk-truncated surfaces. In the lower panel, the values of FZn on ZnO(0001) and (0001) are assumed to be 3/4 and 1/4, respectively, of the value of the ideal ZnO(0001) surfaces (10.9  1014 cm 2). A linear line in each panel is obtained by least-squares fitting of three points.

Table 1. Amounts of Adsorbed TTF at Saturation (GTTF) and Densities of Surface Zn Atoms (GZn) on Ideally Bulk-Truncated ZnO and on Reconstructed ZnO (units: cm 2) FTTF FZn

ZnO(1010)

ZnO(0001)

1.7((0.6)  1014

1.3((0.4)  1014

0.6((0.4)  1014

10.9  1014

5.9  1014

0.0

8.2  1014

5.9  1014

2.7  1014

(bulk-truncated) FZn

3. RESULTS 3.1. Saturation Coverage. As the ZnO surfaces are exposed to the TTF vapor at room temperature, core-level peaks originating from the substrate surfaces (the O 1s and Zn 3s peaks) are attenuated to a certain extent, while the TTF-derived C 1s and S 2p peaks are emerged. Figures 2 and 3 show the core-level spectra of the substrate- and adsorbate-related elements, respectively, for the clean and 10 min TTF-dosed ZnO surfaces. On the clean ZnO surfaces, each O 1s spectrum is composed of an intense peak and a weak contribution at the higher-binding-energy side of the main peak. The main components are observed at 530.8, 530.4, and 529.7 eV on ZnO(0001), (1010), and (0001), respectively, and are associated with the bulk and surface O atoms of ZnO, while the subcomponents are located between 532.0 and 532.2 eV. Judging from their binding energies, the weak contribution is associated with the OH species.27,28 The Zn 3s spectra, on the other hand, have symmetric lineshapes, indicating that each spectrum consists of a single component, with the peak positions at 140.1, 139.7, and 139.2 eV on ZnO(0001), (1010), and (0001), respectively. These energy differences are partly due to different bulk doping concentrations (1013 1015 cm 2) of the crystals used in the present study22 but mostly due to the different direction and magnitude of band bending at the surfaces. We will come back to the band bending issue in Section 3.4. TTF adsorption on the ZnO surfaces induces the shift of the peak positions of both O 1s and Zn 3s core levels along with the

ZnO(0001)

(1/4-modified)

change of the spectral lineshapes. The peak shift is caused by adsorption-induced bending of the ZnO band (see Section 3.4). Adsorption of TTF seems to reach saturation at 10 min deposition irrespective of the termination of the ZnO surface because no changes are observed in each core-level spectrum as the duration of TTF exposure is prolonged. On the TTF-saturated surfaces, the spectral line shape of the O 1s, C 1s, and S 2p core levels shows a strong termination dependence of the ZnO surfaces, whereas the Zn 3s spectra have nearly identical line shapes among three TTF adsorption systems. Aside from the difference in the line shape, the peak intensities depend on the ZnO surfaces. Both C 1s and S 2p peaks are most intense on TTF/ZnO(0001), while they are minimal on TTF/ZnO(0001) (Figures 3a and 3b). The substrate-related peaks (Figure 2), on the other hand, show an opposite trend; the TTF/ZnO(0001) surface gives the O 1s and Zn 3s peaks with the lowest intensities, and the intensities are increased in the order of TTF/ZnO(1010) and (0001). These trends reflect the amounts of adsorbed TTF with the largest amount on ZnO(0001) and the least on ZnO(0001). From the intensities of the C 1s and S 2p peaks on the TTFcovered ZnO surfaces as well as the Zn 3s peak intensity on the clean ZnO(1010) surface, the amounts of adsorbed TTF (FTTF) are estimated, in the same manner as that employed in the literature,29 to be 1.7  1014, 1.3  1014, and 0.6  1014 cm 2 for 21845

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The Journal of Physical Chemistry C ZnO(0001), (1010), and (0001), respectively. These values are summarized in Table 1. In Table 1, the Zn concentration (FZn) of the bulk-truncated ZnO surfaces is also displayed. Interestingly, there is a strong correlation between FTTF and FZn. The upper panel in Figure 3c shows the plot of FTTF against FZn on the bulktruncated ideal ZnO surfaces. A linear positive correlation is depicted, suggesting that the surface Zn atoms are active sites for TTF adsorption. At this point, one should be aware of the fact that ZnO is an ionic crystal so that the bulk truncated (1  1) surfaces of ZnO(0001) and (0001) are unstable because of the nonvanishing dipole moment perpendicular to these surfaces. Simple electrostatic considerations30,31 show, however, that the counterfield is created to cancel the dipole moment if 1/4 of surface charge is compensated. X-ray diffraction (XRD) measurements by Jedrecy et al.32 have revealed that the occupancy in the topmost plane on Zn-terminated ZnO(0001) is reduced to 0.75; i.e., a quarter of the surface Zn atoms is missing. On the basis of the STM observations, it is proposed that the lower occupancy of the surface Zn atoms is achieved by the formation of the step structures whose edges are terminated by the O atoms and that the density of the steps is defined to reduce the surface Zn/O atomic ratio to 3/4 from the value on the bulk-terminated surface (Zn/O = 1).33,34 Although the charge compensation mechanism has not been clarified on O-terminated ZnO(0001), if surface charge is compensated by a similar manner to ZnO(0001), the Zn/O atomic ratio should be increased from 0 to 1/4. The bottom line in Table 1 indicates the surface Zn concentrations having the Zn/O atomic ratios of 3/4 on ZnO(0001) and 1/4 on ZnO(0001). The above-mentioned modification of the Zn/O atomic ratios of the clean ZnO(0001) and (0001) surfaces is inferred from the intensity analysis of the O 1s and Zn 3s core-level peaks. In the analysis, we assume that the observed photoemission intensity is the sum of the emission intensities from the atomic planes with different depths in ZnO and that the atomic defects are introduced only in the topmost layer. Furthermore, the Zn/O ratio of 1.0 is assumed to be preserved on the real ZnO(1010) surface. Under these assumptions, it is estimated that about 10% of the surface Zn atoms and 20% of the surface O atoms (the error of (10% is expected for both cases) are missing on ZnO(0001) and (0001), respectively. Although the PES measurements are not ideal to determine the surface defect densities, the formation of the atomic defects to compensate surface charge is also supported. In the lower panel in Figure 3c, we replot FTTF against FZn assuming the modified Zn/O ratios on ZnO(0001) and (0001). The result of the least-squares fitting of the three points is shown by a linear line intersecting the point near the origin. This is a strong indication that TTF should be bonded exclusively to the surface Zn atoms, and the saturation amount of TTF on each ZnO surface is determined only by the density of the surface Zn atoms. The slope of the linear line is 0.2, meaning that every five Zn atoms can host one TTF molecule. Although there is no direct evidence in the present study, TTF is probably bonded to Zn through its S atoms. This Zn S bonding is reasonable because Zn has empty Zn 4sp dangling bonds20 and because of the substantial contribution of the S 3p atomic orbital to the highest occupied molecular orbital of TTF.35 The ZnO TTF interaction can thus be viewed as a Lewis acid base interaction. 3.2. Adsorption State. The linear dependence of FTTF on FZn (Figure 3c) is indicative of the TTF Zn bond formation.

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Figure 4. Results of the least-squares fitting of the C 1s and S 2p corelevel spectra. The C 1s spectra are reproduced from Figure 3a. The S 2p spectra shown here are measured by the light at hν = 350 eV. Solid lines indicate deconvoluted components. For each component in the C 1s spectra, Gaussian Lorentzian sum functions are used, whereas each S 2p spin orbit doublet is reproduced by two Gaussian functions, whose intensity ratio is 2:1, and the energy separation of 1.2 eV. In the insets, the integrated intensities of the components from molecular TTF (circles) and dissociated TTF (squares) are plotted against FZn.

However, no state associated with this bonding has emerged in the Zn 3s core-level spectra (Figure 2b). Instead, subcomponents are developed in the O 1s spectra upon TTF adsorption. As mentioned in the previous section, each O 1s peak from the TTFfree ZnO surface is composed of a main component, associated with the bulk and surface O atoms, and an OH component (Figure 2a). The OH species are due to adsorption of residual water in the ultrahigh vacuum chamber, and the coverages of OH, estimated from the peak intensities, are 0.05 on ZnO(1010) and 0.1 on ZnO(0001) and (0001) (the coverage of 1.0 is defined here to be equal to the density of the surface atoms). The OH coverages are small enough that the influence of the OH species on the adsorption behavior of TTF should be limited. As the ZnO surfaces are being covered with TTF, two additional components are formed at both sides of the main O 1s component, shifted by 0.9 and 1.8 eV, on ZnO(1010) and (0001). This means that, although TTF is most likely bonded to Zn, adsorbed TTF also affects the chemical environment of the surface O atoms on ZnO(1010) and (0001). Interestingly, no such component has emerged in the O 1s peak from the TTF/ ZnO(0001) surface, although the TTF coverage is the highest among three adsorption systems. The TTF-induced O 1s components on ZnO(1010) and (0001) are observed at 529.5 and 532.2 eV. The binding energy of the latter component is in good agreement with the O 1s peak from the O atoms bonded to C27 as well as H.27,28 This suggests that C-containing species other than TTF should be present on the surface to form O C bonding along with the O H bonding. The origin of the component at 529.5 eV, on the other hand, is unknown at present because the binding energy of this feature is too low to associate it to any kind of O-related species. If the chemical shift reflects valence charge of the O atoms, highly negatively charged O species should be present on the surfaces. The formation of O C bonding is suggestive of TTF decomposition. Dissociative adsorption of TTF is directly proved by analyzing the C 1s peak. The C 1s peaks of all TTF/ZnO systems are composed of main peaks at 285.2 285.3 eV with several 21846

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The Journal of Physical Chemistry C subcomponents. Figure 4a shows the results of peak fitting using Gaussian Lorentzian sum functions. Aside from the main component, there are two components at 286.8 and 288.5 eV for ZnO(0001) and three components at 283.6, 286.6, and 288.5 289.0 eV for ZnO(1010) and (0001). The main feature can be associated with molecular TTF. The subcomponents, on the other hand, are related to the dissociated species of carbonaceous C (283.6 eV), and the C atoms bonded one substrate O atom (286.6 eV) and two O atoms (288.5 289.0 eV).36 In the inset of Figure 4a, the intensities of dissociated TTF and molecular TTF are plotted against FZn. A negative correlation between the intensity from dissociated TTF and FZn is depicted, whereas the intensity from molecular TTF is positively correlated with FZn. The S 2p spectra of TTF have characteristic profiles depending on the surface termination (Figure 3b). The S 2p spectrum from the TTF/ZnO(0001) surface is characterized by a two-peak structure. On the TTF/ZnO(1010) surface, the S 2p spectrum has a single peak with a shoulder at the lower-binding-energy side. The spectrum on TTF/ZnO(0001) exhibits a triangular shape with several humps. Since the difference in the S 2p line shape is so obvious and well-reproducible, the S 2p peak of TTF can be used as a fingerprint for determination of the ZnO surface termination. Although the S 2p spectra have complex lineshapes, they can be reproduced by only three components. Figure 4b shows the results of least-squares fitting using three spin orbit splitting doublets, each of which is constructed by two Gaussian functions with the energy separation of 1.2 eV and the branching ratio of 2:1 for the S 2p3/2 and 2p1/2 core levels.37 The S 2p3/2 peak positions of the three components are 161.6, 162.5, and 164.1 eV. The two components with the higher binding energies are intense on ZnO(0001) to form the characteristic double-peak structure. On ZnO(1010), the intensity of the component at 162.5 eV is largely suppressed in comparison with the component at 164.1 eV. On ZnO(0001), the lowest-binding-energy component becomes dominant. Plots of the relative intensities of these three components against FZn show that the 161.6 eV component is negatively dependent on FZn (see the plot by squares in the inset of Figure 4b), whereas the other two components are positively correlated (the sums of the intensities of these two components are shown by circles). The negative correlation of the 161.6 eV feature is well compared with that of the dissociated-TTF-derived subcomponents in the C 1s peak. Furthermore, it is known that the S adatoms bonded to Zn on ZnO give the S 2p3/2 peak at around 161.5 eV.38 These imply that the S atoms formed by TTF decomposition contribute to the lowest-binding-energy feature. On the other hand, the other two components at 162.5 and 164.1 eV, having a positive correlation with FZn as in the case of the main component of the C 1s peak, should originate from molecularly adsorbed TTF. There are four equivalent S atoms in TTF. Thus, two S 2p components for molecularly adsorbed TTF may be interpreted either as inequivalency of the S atoms in single TTF as a result of tilted adsorption or as two differently charged TTFs on ZnO. The annealing experiments support the latter view: as the TTFsaturated ZnO surfaces are annealed to around 500 K, the intensity of the 164.1 eV component is decreased faster than that of the 162.5 eV component (results not shown). This means that these S 2p components arise from different TTF species with different thermal stability. Ikemoto et al. have measured the S 2p spectra of a thick TTF layer deposited onto Al plates by X-ray

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Figure 5. C K-edge NEXAFS spectra for the TTF-saturated ZnO surfaces. The intensity of each spectrum is normalized by the height of the continuum step structure. The incidence angle of the light θi is measured from the surface normal, i.e., the normal incidence corresponds to θi = 0°. The results of the least-squares fitting to the spectra at θi = 0° are shown at the bottom curves in each panel. TTF-derived molecular orbitals of π*(b3g, au, b2g), σ*(b2u), and σ*(b3u, b2u) are resolved along with the fourth broad peak, which is attributed to absorption to the higher molecular orbital levels. The insets show changes of the integrated intensities of the first three absorption peaks as a function of θi.

PES (hν = 1253.6 eV) and have found that neutral TTF (TTF0) gives the S 2p3/2 peak at about 164 eV.39 In the TTF TCNQ charge-transfer salt, both TTF0 and positively charged TTF (TTF+) are formed with the ratio of about 6:4, and the corresponding S 2p3/2 peaks are observed at 163.8 and 164.8 eV, respectively.39,40 On the basis of these literature values, the 164.1 eV feature can be associated with TTF0. If this assignment is true, the other molecular contribution at 162.5 eV, which is too low for TTF0, should originate from negatively charged TTF (TTF ). We, thus, temporarily assign two molecular TTF species to TTF0 and TTF . The possible formation of TTF will be again discussed in Section 3.4. 3.3. NEXAFS Measurements. As indicated in Section 3.2, molecular and dissociated TTF species are coexistent on three ZnO surfaces. The ratio of the molecular species to the dissociated ones is proportional to the Zn concentrations of the ZnO surfaces. The same trend is also found in the C K-edge NEXAFS spectra. Figure 5 shows the C K-edge NEXAFS spectra for the TTF-saturated ZnO surfaces measured at various incidence angles of the light (θi). At normal incidence (θi = 0°), where the electric vector of the light is parallel to the surface, the spectrum for TTF/ZnO(1010) bears four absorption peaks between 285 and 289 eV (Figure 5b). The spectral line shape resembles that of crystal TTF,41 and four peaks are assigned to the transition from the C 1s core level to the molecular orbitals of π*(b3g, au, b2g), σ*(b2u), σ*(b3u, b2u), and higher levels. These peaks are resolved by the line shape analysis as shown in the bottom spectrum in Figure 5b. In this analysis, the first three absorption peaks are reproduced by Gaussian functions, and an asymmetric Gaussian function42 is used for the fourth broad peak. The error function, whose edge is fixed at 285.2 eV (the C 1s core-level peak position), represents the step continuum. On TTF/ZnO(0001), the four absorption peaks are also resolved (Figure 5a). Contrastingly, the absorption structures are less obvious on TTF/ZnO(0001) (Figure 5c), reflecting the small 21847

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Figure 6. Valence band spectra for clean and TTF-saturated surfaces of (a) ZnO(0001), (b) ZnO(1010), and (c) ZnO(0001). Spectra obtained from the TTF-saturated surfaces subjected to annealing at 500 and 495 K are also displayed in (a) and (c). The photon energy of 350 eV was used for the measurements. The positions of the VBM on the clean surfaces are indicated. Schematics of the energy band structures of the TTF-saturated ZnO surfaces are shown in the inset. The energy of the CBM at the surface (Esurf CBM) for each adsorption system is estimated from the VBM position on the clean surface, the TTF-induced Zn 3d shift, and the band gap energy of 3.37 eV.

amount of molecularly adsorbed TTF. In this adsorption system, the NEXAFS spectrum gives an additional feature at 288.5 eV. Although the exact origin of this peak is not clear, it may be related to C-containing species formed by TTF dissociation. For all adsorption systems, an absorption peak relating to the lowest unoccupied molecular orbital state [σ*(ag)] is not observed, in agreement with the literature.41 This is probably because of the low contribution of the C atomic orbitals to this state.41 The line shape of the NEXAFS spectra varies depending on θi. The four-peak structure becomes less obvious with increasing θi for the TTF/ZnO(1010) system. For the spectrum of TTF/ ZnO(0001), the peak at 286 eV becomes intense and is broadened at larger θi. These changes are due to the intensity variation of the absorption peaks. Inset figures show the θi dependence of the absorption intensities of the first three states [the first two states for TTF/ZnO(0001)]. If the TTF molecules adsorb flat on the surface, the σ* (π*) state is observed as an intense peak (a weak peak) at θi = 0°, and the peak intensity is gradually decreased (increased) with increasing θi.42 In the case of a standing-up orientation, a reverse trend is expected. In any cases, the symmetry arguments of the molecular orbitals allow us to speculate the opposite behavior of the σ* and π* peak intensities as a function of θi. However, we observe that the π* and σ* states show the same θi dependence. Therefore, TTF adsorbs on the ZnO surfaces without preferential orientation of its molecular plane. 3.4. TTF-Induced Band Bending. Adsorption of TTF on the ZnO surfaces induces the shift of both O 1s and Zn 3s core-level

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peaks (Figure 2). A similar shift is also found in a much shallower Zn 3d level. Figure 6 shows valence band spectra for three TTF adsorption systems. The valence band of ZnO is composed of the Zn 3d band and the O 2p Zn 4sp hybrid bands.20 The former is seen as an intense peak at 10 11 eV, while the latter contribution is barely observable between 3 and 5 eV. The valence band maximum (VBM) positions on the TTF-free clean ZnO surfaces, determined by extrapolating the leading edge of the spectrum to the baseline, are 3.10, 3.15, and 2.52 eV on ZnO(0001), (1010), and (0001), respectively. Since the bulk VBM of ZnO, calculated from the bulk carrier concentrations of the ZnO crystals,22 is at 3.1 3.2 eV below the Fermi level, the ZnO band is bent upward at the surface on ZnO(0001), while the bands are almost flat on ZnO(1010) and (0001). TTF adsorption induces the shift of the valence band as well as the Zn 3d band. On ZnO(0001), TTF adsorption results in the shift of these bands to the lower-binding-energy side by 0.27 eV. Contrastingly, the shift to the higher-binding energy side by 0.06 and 0.26 eV is induced on ZnO(1010) and (0001). A similar adsorption-induced shift is also found in the O 1s and Zn 3s corelevel regions (Figure 2). Thus, the shift is caused by bending of the ZnO band. Namely, adsorbed TTF pulls down the ZnO bands at surfaces of ZnO(1010) and (0001), whereas upward bending is induced on ZnO(0001). The direction of band bending is indicative of the direction of charge transfer between the TTF overlayer and the ZnO substrate surface. Judging from the direction of band bending, the TTF layers act as charge donors on ZnO(1010) and (0001), while the overlayer withdraws charge from ZnO(0001). One should be careful that there exist both molecular and dissociated TTF on ZnO(1010) and (0001). Thus, it is not straightforward to conclude whether molecular TTF acts as a charge donor or an acceptor. A clue to reveal the role of adsorbed TTF resides in the spectra obtained from annealed surfaces. The valence spectrum of TTF/ZnO(0001) after annealing at 495 K is shown in Figure 6c. On the annealed TTF/ZnO(0001) surface, almost all TTF molecules are decomposed into atomic S. However, the Zn 3d peak position remains the same before and after annealing. This suggests that the S adatoms on ZnO(0001) should behave as charge donors. Therefore, TTFinduced downward band bending on ZnO(0001) is largely owing to the surface S atoms rather than the TTF molecules. The situation on TTF/ZnO(0001) is much simpler than TTF/ZnO(0001) because more than 80% of adsorbed TTF remains molecularly (Figure 4b). TTF-induced upward band bending on ZnO(0001), thus, reflects the charge transfer between molecular TTF and the substrate, and upward bending means that charge is transferred from the substrate to TTF. Thus, molecularly adsorbed TTF should behave as a charge acceptor on ZnO(0001). This is in accordance with the conclusion drawn from the S 2p line shape analysis, which reveals the existence of two differently charged TTFs, i.e., TTF0 and TTF . As the TTF/ ZnO(0001) surface is annealed to 500 K and the ratio of dissociated TTF to molecular TTF is increased (molecular TTF is still persistent after annealing on this surface), the magnitude of the upward band bending is diminished from 0.27 to 0.05 eV (Figure 6a). This also supports the acceptor behavior of molecular TTF on ZnO(0001). Because the S 2p component at 162.5 eV is resolved not only on TTF/ZnO(0001) but also on the other two surfaces (Figure 4b), the negatively charged TTF molecules should be present on all three ZnO surfaces. 21848

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4. DISCUSSION 4.1. Surface Zn Concentration. The adsorption states of TTF on three single-crystal ZnO surfaces with orientations of (0001), (1010), and (0001) are examined by PES and NEXAFS. It is found that chemical reactivity of the ZnO surfaces toward TTF depends strongly on the surface termination of ZnO. The amount of adsorbed TTF (FTTF), determined from the intensities of the C 1s and S 2p core-level peaks, is the highest on ZnO(0001), and it decreases with decreasing surface Zn density (FZn). As shown in the lower panel in Figure 3c, the plotted points of FTTF against FZn, assuming the Zn/O ratios modified by a factor of 1/4 on ZnO(0001) and (0001), are fitted by a linear line which passes through the point near the origin with the slope of 0.2. This indicates that adsorbed TTF is bonded selectively to the surface Zn atoms and that five Zn atoms are required to host one TTF molecule irrespective of the atomic arrangement of the ZnO surfaces. Comparing the size of TTF (Figure 1a)43 with the lattice constant of ZnO (Figure 1b),44 it is not unreasonable for single TTF to block five Zn sites on both ZnO(0001) and (1010) as shown in Figure 1c. On ZnO(0001), whose surface morphology is characterized by hexagonal terraces with double steps,45 TTF should adsorb at the step edges where the Zn atoms are exposed to the surface. TTF at the step edge can directly affect four Zn atoms (see Figure 1c). Thus, again, it is not unreasonable that a single TTF molecule blocks five Zn atoms on ZnO(0001). A more important implication deduced from Figure 3c is the stabilization mechanism of the Zn-terminated ZnO(0001) and O-terminated (0001) surfaces. On the basis of the electrostatic model, surface positive and negative charge on bulk-terminated ZnO(0001) and (0001) must be reduced by a factor of 1/4.30,31 The present study indicates that this charge compensation should be achieved by changing the surface Zn/O atomic ratios to have 3/4 and 1/4 on ZnO(0001) and (0001), respectively. From the recent STM studies,33,34 it is proposed that the stabilization of the ZnO(0001) surface is achieved by surface reconstructions to have triangular-shaped islands and pits. In the triangular reconstruction model, the density of the islands and pits, whose edges are terminated by the O atoms, should be determined to have less positive surface charge by a factor of 1/4.33 In addition to the triangular reconstruction, local reconstruc√ √ tions with (1  3) and ( 3  3)R30° periodicities are found to be formed on the terraces of ZnO(0001).46,47 Interestingly, similar reconstructed structures are also observed on ZnO(0001),28,48 although triangular islands and pits are not formed on this surface. If these reconstructed structures are formed by missing the surface Zn or O atoms, surface charge on ZnO(0001) or (0001) should be reduced by a factor of 1/3. If we plot FTTF against FZn assuming the Zn/O ratios of 2/3 and 1/3 on ZnO(0001) and (0001), respectively, the plotted points are on a linear line intersecting the FTTF axis at 0.5  1014 cm 2. This suggests that the surface Zn concentration is underestimated Thus, the surface (overestimated) on ZnO(0001) [ZnO(0001)]. √ √ with the (1  3) and/or ( 3  3)R30° reconstructed regions should not be dominant but limited to a certain extent on the surfaces, as has already been pointed out by the STM studies.46 The stabilization mechanism of ZnO(0001) is far less understood than that of ZnO(0001). However, the present study provides evidence that the surface should undergo reconstruction to have the surface Zn/O atomic ratio of 1/4. In future works, therefore, we suggest that the stabilization of ZnO(0001) should be studied under this restriction.

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4.2. Adsorption vs Decomposition Reactivities. If we take the amount of adsorbed TTF as a measure of the surface activity, the order of the activity for TTF adsorption is ZnO(0001) > (1010) > (0001) (Figure 3). Interestingly, however, the activity for TTF decomposition is opposite (Figure 4). On the basis of these findings, we conclude that TTF adsorbs on the surface Zn atoms, while decomposition of adsorbed TTF proceeds through the interaction with the surface O atoms. The involvement of the O atoms in TTF decomposition is understood as follows. On the ZnO(0001) surface, TTF must adsorb at surface defect sites such as step edges and O vacancies on the terrace. On these adsorption sites, several O atoms are available within the reach of adsorbed TTF so that the O H or O C interaction may be possible. Dissociation of TTF could be initiated by deprotonation of TTF by the surface O atoms or weakening of the C S or C C bonds in TTF as a result of the O C bond formation. On ZnO(1010), there are a fewer number of O atoms within the reach of adsorbed TTF (Figure 1c). The situation is more severe for TTF on ZnO(0001) because no O atoms are available on the terrace site where the majority of TTF adsorbs. Thus, the decomposition reactivity should be proportional to the number of available O atoms by adsorbed TTF. 4.3. Surface Transfer Doping. Finally, we discuss surface transfer doping of ZnO by TTF. TTF is a typical charge donor molecule in charge-transfer salts, and as an adsorbate, it acts as an electron donor on graphene8,9 and coinage metals.17 As for the TTF/ZnO systems, two differently charged TTF molecules are formed on the surfaces. We propose from the core-level peak analysis and the TTF-induced band bending that one is neutral TTF and the other is negatively charged TTF. Thus, contrary to the expectation, molecular TTF behaves as a charge acceptor on ZnO. Although the charge transfer mechanism from the ZnO surface to molecularly adsorbed TTF is a subject of future studies, the surface-state-mediated transfer49 is a possible candidate. At the interface between TTF and ZnO, several S Zn bonding states are formed, and some of them are expected within the ZnO band gap. One of such in-gap states could originate from the lowest unoccupied σ*(ag) molecular orbital, which must be located in the energy region near the CBM (Figure 5). This energy position is suitable to mediate charge transfer from the conduction band of ZnO to unoccupied states of TTF. The electron acceptor character is obvious on ZnO(0001) (Figure 6a). On ZnO(1010) and (0001), on the other hand, although molecularly adsorbed TTF shows an acceptor character, the deposited TTF overlayers can be viewed as a whole as electron donors because the ZnO bands at the surface are pulled down (Figures 6b and 6c). Dissociated TTF is found to be responsible for charge donation. Recent angle-resolved PES studies have proved that the ZnO(1010) surface undergoes insulator-to-metal transition when the surface is exposed to hydrogen, methanol, and water.21,22 Hydrogen and methanol can also metallize the ZnO(0000) surface.23 The adsorption layers of these species induce downward band bending so that the CBM is shifted from above to below the Fermi level on the ZnO surfaces. As a result, charge accumulation layers, within which two-dimensional electron gases are developed, are formed on the adsorbate-covered ZnO surfaces. Since net charge is donated from the overlayer to the substrate for TTF/ZnO(1010) and TTF/ZnO(0001), it is interesting to assess whether these ZnO surfaces are metallized under the TTF overlayers. In the inset of each panel in Figure 6, the schematic band structure of the TTF-saturated ZnO surface is shown. Here, the 21849

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The Journal of Physical Chemistry C energy position of the VBM on each TTF-saturated surface is deduced from the VBM position on the clean surface and the TTF-induced Zn 3d peak shift, which is equivalent to the magnitude of adsorptioninduced band bending. The bulk VBM of ZnO, calculated from the bulk carrier concentrations of the ZnO crystals,22 is at 3.1 3.2 eV below the Fermi level. The position of the CBM is assumed to be located at 3.37 eV (the band gap of ZnO20), above the VBM. For both TTF/ZnO(0001) and TTF/ZnO(0001) systems, the ZnO band bends upward at the surface with the CBM positions at 0.5 0.6 eV above the Fermi level; i.e., the charge depletion layers are formed on these surfaces. On the other hand, the ZnO band bends slightly downward with respect to the bulk band position on TTF/ZnO(1010), but the CBM still lies above the Fermi level. Thus, all TTF-saturated ZnO surfaces remain semiconductors. Although the TTF overlayer donates some charge to ZnO(1010) and (0001), the amount of donated charge is not enough to bring the surfaces to be metallic. From these observations, TTF is not an efficient charge dopant for the ZnO surface.

5. CONCLUSIONS PES and NEXAFS utilizing synchrotron radiation light are employed to examine the adsorption state of TTF on three singlecrystal ZnO surfaces, i.e., the (0001), (1010), and (0001) surfaces, with special focus on the influence of the surface atomic composition on the adsorption and decomposition activities as well as on surface transfer doping by TTF. TTF adsorbs both molecularly and dissociatively on all three ZnO surfaces at room temperature. The total amount of TTF at the saturation coverage is proportional to the density of the surface Zn atoms. Contrastingly, there is a negative correlation between the amount of dissociated TTF and the surface Zn density. These results indicate that TTF adsorbs selectively on the surface Zn atoms, and a part of adsorbed TTF undergoes dissociation with the help of neighboring O atoms. For molecularly adsorbed TTF, there are two differently charged states: one is in the neutral state, and the other is in the negatively charged state. Therefore, molecular TTF on the ZnO surfaces behaves as a charge acceptor rather than as a donor. This property leads to charge withdrawal from the substrate surface in the TTF/ZnO(0001) system. However, the TTF overlayer as a whole can be viewed as an electron donor on ZnO(1010) and (0001); i.e., charge is transferred from the overlayer to the substrate surface because a large amount of dissociated S, which has a donor character, is formed on ZnO(1010) and (0001). ’ AUTHOR INFORMATION Corresponding Author

*Fax: +81 3 5734 2655. E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by a Grant-in-Aid for Scientific Research from Ministry of Education, Culture, Sports, Science, and Technology of Japan (Grant No. 21560695) and Asahi Glass Foundation. The PES and NEXAFS measurements were performed under the approval of the Photon Factory Advisory Committee (Proposal No. 2009S2-007, 2010G044). ’ REFERENCES (1) See, for example: Koch, N. ChemPhysChem 2007, 8, 1438–1455. (2) Koch, N.; A. Kahn, A.; Ghijsen, J.; Pireaux, J.-J.; Schwartz, J.; Johnson, R. L.; Elschner, A. Appl. Phys. Lett. 2003, 82, 70–72.

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