Molecular Orientation and Electronic States of Vanadyl

Article pubs.acs.org/JPCC

Molecular Orientation and Electronic States of Vanadyl Phthalocyanine on Si(111) and Ag(111) Surfaces Keitaro Eguchi,† Yasumasa Takagi,†,‡ Takeshi Nakagawa,†,‡ and Toshihiko Yokoyama*,†,‡ †

Department of Structural Molecular Science, The Graduate University for Advanced Studies (SOKENDAI), Myodaiji-cho, Okazaki 444-8585, Japan ‡ Department of Materials Molecular Science, Institute for Molecular Science, Myodaiji-cho, Okazaki 444-8585, Japan

ABSTRACT: We have investigated the molecular orientation and electronic structures of nonplanar vanadyl phthalocyanine (VOPc) on the Si(111)-(7 × 7) and Ag(111) surfaces by X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, and X-ray magnetic circular dichroism. The VOPc molecule adsorbs on Ag(111) in a parallel orientation to the surface with an oxygen-up configuration. A strong interaction between the N and C atoms of VOPc and surface Ag atoms is observed at the interface, although no marked change in the electronic state is observed for the V atom, similarly to the case for VOPc in a multilayer. On the other hand, the chemical interaction of the O atom of VOPc with the surface Si atoms favors the oxygen-down configuration. This chemical interaction causes the cleavage of the VO π bond and facilitates electron charge transfer to the V−N−C molecular orbitals. Such intermediation of the oxygen atom between the V atom and Si surface suppresses the direct interaction between them, and the spin magnetic moment of V remains the same as that of bulk VOPc molecules.

1. INTRODUCTION Controlling the wide range of functionalities of metal complexes on substrate surfaces for applications such as molecular spintronics,1,2 molecular devices,3 and molecular catalysis4 is of special importance. Metal phthalocyanines (MPcs) and metal porphyrins (MPs) consisting of a central metal atom and π-conjugated aromatic macrocycle have attracted attention as among the most promising materials for these applications owing to the wide variety of electronic and magnetic states that can be made available in these systems by changing the central metal atom and/or modifying the ligand. The electronic structures and magnetic properties of MPcs and MPs are influenced by surface interactions. Interfacial interactions such as charge transfer,5,6 loss of the spin magnetic moment of the central atom,7−10 ferromagnetic11,12 and/or antiferromagnetic13,14 couplings, and changes in the magnetic anisotropy15 are important examples. Understanding the surface interactions of MPcs and MPs is vital for tailoring their electronic and spin states on substrates for specific applications. Use of 3d transition-metal ions as the central metal atom imparts a spin magnetic moment to the resultant complex. In © 2013 American Chemical Society

general, two types of structures are formed. When a central metallic ion such as iron, cobalt, or nickel is present, a planar structure is formed. When an oxidized metal such as TiO or VO is present in a phthalocyanine, a nonplanar pyramidal structure is formed. In most cases, planar-structured MPcs are adsorbed on flat surfaces in a parallel orientation, whereas pyramidal-structured MPcs can exhibit two molecular configurations: upward and downward orientations with respect to the surface. Pyramidal-structured TMPcs, especially titanyl phthalocyanine (TiOPc) and vanadyl phthalocyanine (VOPc), have been studied from the viewpoint of their molecular orientation on the surface,16 magnitude and orientation of their electric dipole moments, and change in the work function of the substrate surface.17−19 Less attention, however, has been paid to their interactions with surfaces or the behavior of their electronic spin states, although interface interactions of TiOPc with semiconductors have been studied.20,21 Received: July 18, 2013 Revised: October 4, 2013 Published: October 4, 2013 22843

dx.doi.org/10.1021/jp406906k | J. Phys. Chem. C 2013, 117, 22843−22851

The Journal of Physical Chemistry C

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Figure 1. (a) Schematic molecular structures of (left) VOPc and (right) H2Pc. Four pyrrolic N (Np) and four imminic N (Ni) atoms are contained in VOPc, whereas two Np and six Ni atoms are contained in H2Pc. (b) Magnetization curves of purified VOPc at several temperatures measured using a superconducting quantum interference device (SQUID) magnetometer. (c) Low-energy electron diffraction (LEED) pattern of 1 ML of VOPc on Ag (E = 11 eV). A red arrow means artificial light from the filament. (d) Experimental configuration in XAS. The sample can be rotated. θ is the angle between the X-ray and surface normal directions. (e) N 1s XPS signal from VOPc (black squares) and background signal from the clean Ag surface (red circles).

in a previous study that the Cu atom in CuPc loses its spin through direct interaction with specific Si atoms on Si(111).26 In the present study, we have investigated the molecular orientation and other structural and electronic properties of VOPc (Figure 1a) on the reactive Si(111)-(7 × 7) surface and the rather inert Ag(111) surface by X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS). For comparison, metal-free phthalocyanine (H2Pc, Figure 1a) was studied by XPS to examine the direct interaction of the phthalocyanine skeleton with the Si surface. In addition, the magnetic properties of VOPc on these surfaces were investigated by X-ray magnetic circular dichroism (XMCD).

To date, only the oxygen-up configuration of TiOPc and VOPc on the highly oriented pyrolytic graphite (HOPG)16,22 and Au(111)17,19 surfaces have been experimentally demonstrated. As examples of MPcs with the chlorine-down configuration, GaClPc18 and AlClPc23 on Cu(111) have been reported, where the Cl ligand bonds strongly with the surface Cu. Based on a theoretical study, Mattioli et al. reported that the oxo ligand in TiOPc and VOPc should form chemical bonds with Ga on a GaAs surface, leading to the energetically favored oxygen-down configuration.24,25 The oxygen-down configuration in TiOPc and VOPc is expected to be obtained using reactive surfaces that interact strongly with the oxo ligand. In addition, it is possible that this design for molecular adsorption will allow the metal ion to maintain its spin state by avoiding direct interactions between the central metal and substrate atoms. This might be important because it was shown

2. EXPERIMENTAL SECTION A commercial n-type (P-doped) Si(111) wafer (1−3 Ω·cm) was cut in a rectangle shape (14 mm × 6 mm, 0.5 mm thick) and cleaned with ethanol in a ultrasonic bath. The Si(111) 22844



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Furthermore, V L- and O K-edge XAS spectra were normalized to those for the clean surface to remove the influence of absorption dips at the O K edge, which were observed because of contamination in the beamline optics. μ+ (μ−) in the XAS and XMCD analyses represent the helicity of the X-rays parallel (antiparallel) to the majority spins. The X-ray energies were calibrated using the metallic V LIII and LII peaks at 512.1 and 519.8 eV, respectively. Although synchrotron radiation was typically irradiated onto the samples for more than 10 h, no significant beam damage was observed. The XPS experiments were performed with nonmonochromatized Mg Kα X-rays (hν = 1253.6 eV) at room temperature in an ultrahigh-vacuum (UHV) chamber (base pressure of ∼4 × 10−8 Pa). The X-ray beam irradiated the sample at an incidence angle of 45° with respect to the surface normal. The photoelectron signals were detected using a hemispherical electron analyzer (SPECS, Phoibos 100) that faced the sample surface (normal emission detection). The binding energies were calibrated using the Ag 3d5/2 and 3d3/2 lines at 368.2 and 374.2 eV, respectively.35 The full width at half-maximum (fwhm) of the Ag 3d5/2 line was found to be 1.0 eV, which is consistent with the expected fwhm of 1.0 eV estimated from the core-hole lifetime of Ag 3d5/2 (0.31 eV),36 the natural line width of Mg Kα1,2 (0.7 eV), the calculated resolution of analyzer (0.7 eV), and the measurement temperature (0.09 eV).37,38 The XPS binding energies for the Si surface were calibrated by assuming the energy of Si 2p to be 99.3 eV, which is consistent with the previous report,39 and no drift of the peak energy due to charging was observed during the experiments. The largest uncertainty in the binding energy was found to be ∼0.1 eV. No radiation damage to the samples was observed during the XPS measurements. To prevent a signal change in the O 1s region due to the oxidation of the Si surface, the O 1s spectra were recorded within 2 h after the preparation of the samples. Because the XPS spectrum in the N 1s region overlaps with the plasmon loss peaks from Ag(111),31,40 the N 1s XPS spectrum of VOPc was obtained by subtracting the spectrum of clean Ag from that of VOPc on Ag, as shown in Figure 1e. Peak-fitting analysis was performed using the Voigt functions and the Shirley background. Lorentzian widths were set to 0.295 eV for C 1s26 and 0.84 for N 1s, except for 1 ML of VOPc on Ag, for which the width was 0.50 eV for N 1s.41 The ratio of benzene carbon (Cb) to pyrrole carbon (Cp) (see Figure 1a) obtained from the peak fitting was in the range from 2.90 to 3.23 and that of iminic N (Ni) to pyrrolic N (Np) (see Figure 1a) was found in a range from 0.98 to 1.05. These results are close to the stoichiometric values of 3.00 (24:8) for C and 1.00 (4:4) for N in metal phthalocyanine.

substrate was mounted on a Mo holder and quickly inserted into a load lock chamber. It was subsequently transferred to a preparation chamber (∼3 × 10−8 Pa) after the pressure in the load lock chamber reached 1 × 10−4 Pa. A Si(111)-(7 × 7) surface was obtained by flashing at 1200 K using the directcurrent method after the sample had been degassed at 800 K for 4−12 h. A clean Ag(111) surface was prepared from a commercial Ag(111) single crystal by repeated cycles of Ar+ sputtering (∼2 μA, 1 kV) and annealing at 850 K. In the XAS and XMCD measurements, a Ag/Cu(111) substrate was used to reduce the background signals from Ag. The clean and ordered Cu(111) surface was obtained by repeated cycles of Ar sputtering (∼2 μA, 1 kV) and annealing at 800 K, followed by the deposition of three atomic layers of Ag by electron bombardment at a substrate temperature of 300 K. The amount of Ag was estimated from the low-energy electron diffraction (LEED) patterns and intensity of the Auger electron spectra according to the literature.27,28 The cleanness of the surfaces was verified by Auger electron spectroscopy. Contaminations of C and O on the Si surface were estimated to be 2% and 0.6%, respectively, and no contamination was detectable on the Ag surface. Commercial VOPc (purity > 95%) and H2Pc (purity > 98%) were purified by train sublimation.29 The magnetization curves of purified VOPc were obtained using a superconducting quantum interference device (SQUID) magnetometer. The magnetic moment of VOPc was confirmed to be consistent with that reported in a previous study30 (Figure 1b). The purified VOPc was deposited onto the substrates at room temperature using a homemade Knudsen cell evaporator fitted with an alumel−chromel (K-type) thermocouple. During the deposition, the pressure was kept below 4 × 10−8 Pa. The deposition rates of VOPc and H2Pc were controlled by adjusting the cell temperature as monitored by the thermocouple and were ∼0.1 ML/min at 530 K for VOPc and 490 K for H2Pc. The molecular density of the monolayer was defined as 0.5 molecules/nm2 and was estimated from the LEED pattern of H2Pc on Ag(111).31,32 The amount of VOPc was estimated using the quartz crystal oscillator and comparing its C 1s XPS intensity with that of H2Pc. A clear LEED pattern originating from VOPc was observed for 1 ML of VOPc on the Ag surface (Figure 1c), whereas no LEED spot from VOPc was seen for that on the Si surface. XAS and XMCD measurements were conducted using an XMCD system (base pressure of ∼1 × 10−8 Pa) equipped with a 7 T superconducting magnet and a liquid He cryostat (T = 3.8 K) at the end station of the bending magnet Beamline 4B of UVSOR-II of the Institute for Molecular Science (IMS), Okazaki, Japan. Details of the XMCD measurement system have been reported elsewhere.33,34 N K-edge angle-dependent XAS measurements were performed using linearly polarized Xrays. The X-ray incidence angle θ was 0°, 30°, or 55° with respect to the surface normal (Figure 1d). Circularly polarized XAS and XMCD measurements at the V LIII,II and O K edges were performed in an external magnetic field of ±5 T, at a temperature of 5 K and X-ray incidence angles of 0° and 55°. The magnetic field was applied to the sample in both parallel and antiparallel geometries with respect to the X-ray beam (Figure 1d). All XAS and XMCD spectra were recorded in total-electron-yield (TEY) mode, and several spectral scans were averaged to enhance the signal-to-noise ratio. The TEY from the sample was divided by that of a Au mesh attached upstream of the sample to scale the incident X-ray intensity.

3. RESULTS AND DISCUSSION The molecular orientation of the phthalocyanine framework was studied using N K-edge angle-dependent linearly polarized XAS of VOPc. The results are shown in Figure 2. The XAS spectrum of 10 ML of VOPc exhibits at least seven peaks (A− G), which resemble those of VOPc,42 TiOPc,43 and CuPc.44 Peaks A−C are mainly attributed to the transition from the N 1s orbital to the π* molecular orbitals, and peaks D−G are due to the transition to σ* molecular orbitals.45,46 Adopting the Fermi golden rule for these π-conjugated molecular systems, the dipole transition probability is most intense when an electric field vector of linearly polarized X-rays points in the direction of the unoccupied orbital, and no transition occurs 22845



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Figure 2. Angle-dependent linearly polarized XAS spectra at the N K edge of 10 ML of VOPc on Si(111) (top), 0.6 ML of VOPc on Si(111) (middle), and 1 ML of VOPc on Ag/Cu(111) (bottom). The energy resolution (E/ΔE) is 2000 for 10 ML of VOPc on Si and 1333 for the other two samples.

Figure 3. V LIII,II- and O K-edge XAS spectra of 10 ML of VOPc on Si(111) (top), 0.6 ML of VOPc on Si(111) (middle), and 1 ML of VOPc on Ag/Cu(111) (bottom) at 5 K. Each spectrum was averaged for the circularly polarized XAS at a magnetic field of ±5 T.

when the vector is perpendicular to the orbital.47 The π* absorption intensity of 10 ML of VOPc increases with the X-ray incidence angle. It should be noted that the π* peak can still be seen at θ = 0°, indicating that the phthalocyanine framework is slightly tilted but nearly parallel with respect to the sample surface. Similarly, in the case of 0.6 ML of VOPc on Si(111), the π* intensity in the XAS spectrum at θ = 55° is greater than that at θ = 0°, and the π* peak can still be seen at θ = 0°. This means that the phthalocyanine framework is nearly parallel but slightly tilted with respect to the surface, similarly to the 10 ML of VOPc case. This result is inconsistent with CuPc on Si(111), which prefers a flat orientation.26 Therefore, it is expected that no direct interaction of the molecular frameworks with the Si surface occurs. In addition, it should be noted that the intensity of peak A for 0.6 ML of VOPc on Si(111) decreases dramatically in comparison to that of 10 ML of VOPc, and peak A in this case broadens and overlaps with peak B. Broadening did not result from a difference in the energy resolution, implying that other peaks exist between peaks A and B. Similar features can be observed for 1 ML of VOPc adsorbed on the Ag surface in a parallel orientation, where a separated peak is included within peak B. Such an additional peak can be observed in NiPc.45,48 Theoretical calculations revealed that peak A and the separated peak are attributable to transitions from the two nonequivalent N atoms, namely, pyrrolic N (inside) and iminic N (outside), in the macrocycle.46 The main peak A for 10 ML of VOPc represents the transitions from two nonequivalent N atoms, as seen in the case of CuPc.46,49 The electronic states of nitrogen in the VOPc molecules adsorbed on the Si and Ag surfaces are thus expected to be different from those of VOPc in the multilayer. Next, we discuss the orientation of the oxygen atom (whether the oxygen atoms are pointing upward or downward on the Si and Ag surfaces). The V L- and O K-edge XAS spectra for VOPc at θ = 0° and 55° are shown in Figure 3. The XAS spectrum for 10 ML of VOPc is very similar to that for the VOPc film reported in the literature.42 First, we focus on the O K-edge region around 530 eV. The doublet peaks at 529.1 and 530.6 eV are observed in 10 ML of VOPc and are attributed to the transition from the O 1s orbital to VO π* and σ* orbitals, respectively. The π* peak clearly shows an angular dependence, and the π* intensity at θ = 0° is stronger than that at θ = 55°. Although it is difficult to estimate quantitatively the

VO direction with respect to the surface, it is expected from the results of N K-edge XAS that the molecules are slightly tilted. The oxygen-up or -down direction cannot be clearly determined because both configurations give the same π* transition intensity. However, both oxygen-up and -down orientations are expected to be formed in the multilayer by stacking one after another, because the electric dipole−dipole interaction is dominant compared to the van der Waals interaction for molecular stacking. This structure has already been proposed for the multilayer on HOPG19,50 and Si(100)42 when VOPc was deposited at room temperature under UHV conditions. In the V LIII-edge region, at least four peaks denoted A−D are seen, as shown in Figure 3. Stöhr and König reported the dependence of the intensity of the transition from the 2p orbitals to the 3d orbitals on the X-ray electric field vector.51,52 The contribution of the 3d orbitals in a spectrum can be determined from an angle dependence measurement. The intensities of peaks A−D at θ = 0° were found to be stronger than those at θ = 55°. The 3d electronic structure of the V atom in VOPc has an unpaired electron occupying the 3dxy orbital,24,53 whereas the unoccupied 3dx2−y2 is the highestenergy orbital.54 Hence, a smaller peak A appearing at the lower-energy side is attributed to 3dxy, and the larger peak D at a higher energy is assigned mainly to 3dx2−y2. In addition, the transition to the 3dπ orbital is also included in peak D, as shown by DFT calculations.42 In contrast, the intensity of peak C at θ = 55° is stronger than that at θ = 0°, indicating that peak C arises mainly from the transition to dz2. Because peak B shows a small angle dependence, the 3dπ character is considered to be dominant. In the XAS spectrum for 0.6 ML of VOPc on Si, no oxygen π* peak is observed at around 529.1 eV, whereas a peak at 530.1 eV and a broad peak at 535 eV are seen in the O K-edge region, which is different from those observed for 10 ML of VOPc. The disappearance of the π* peak implies that the π bond between the V and O atoms diminishes upon the formation of a new Si−O chemical bond. This indicates the oxygen-down configuration. The formation of the Si−O bond is also suggested by the observation of a broad peak above ∼532 eV in the O K-edge XAS spectrum, which is likely caused by the mixing of the O 2p band with a Si sp band.55 On the other hand, the features of the V L-edge XAS spectrum for 0.6 ML of VOPc on Si are similar to those for 10 22846



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Figure 4. XPS spectra of VOPc and H2Pc in the (a) V 2p and O 1s, (b) N 1s, and (c) C 1s regions at room temperature. In the V 2p region, mainly two peaks, namely, V 2p3/2 and V 2p1/2, are observed because of the spin−orbit splitting. Kα3,4 satellite peaks are shown by the gray lines. The red (blue) line in panel b represents Ni (Np) in VOPc or Np (Ni) in H2Pc. The stoichiometric ratios of Np to Ni in VOPc and H2Pc are 1 (4:4) and 3 (6:2), respectively. The red (blue) line in panel c represents Cb (Cp), and a satellite peak in the green line is located almost at the same energy as that of SiC. The Si−C interaction is indicated.

dependent. The V 2p and O 1s XPS spectra of VOPc, except for those of 0.6 ML of VOPc on Si(111) (Figure 4a), show almost the same features and resemble the spectrum of VOPc in a previous report.42 The binding energies of V 2p3/2, V 2p1/2, and O 1s in 10 ML of VOPc on Ag are 516.4, 523.9, and 531.0 eV, respectively, and the V 2p3/2 binding energy is close to that of VO2.61,62 The binding energies for 1 ML of VOPc on Ag(111) are almost the same as those for 10 ML of VOPc within the statistical errors, although the peak shift (>0.2 eV) caused by charge transfer from Ag to the central metal ions was observed in the planar-structured cobalt phthalocyanine10 and cobalt tetraarylporphyrins.63,64 This implies that the interaction of the central V atom with the Ag surface is not significant, and this finding can be explained by the surface trans effect demonstrated by Gottfried et al.65,66 The strong VO bond in VOPc is believed to suppress the interaction between the V and Ag atoms. Although the interaction between V and Ag is not very strong, a rather strong interaction occurs between N and Ag (Figure 4b). The N 1s XPS spectrum for 10 ML of VOPc exhibits two peaks, namely, the main peak at 398.9 eV and a shakeup peak at ∼401 eV, which are similar to the spectrum of phthalocyanines.42,43,67 The main peak is composed of two overlapping peaks originating from Np and Ni. Although the peaks are not well resolved because of the small energy difference (0.3 eV),67 we performed peak-fitting analysis by assuming the two chemical states of N. Both peaks for 1 ML of VOPc on Ag(111) shift by 0.3 eV toward lower energy with respect to those for 10 ML of VOPc. This indicates that the N atoms of VOPc interact strongly with the Ag atoms. This result is in good agreement with that for ZnPc on Au(100)68 and in disagreement with the case of CoPc on Ag(111), where the N 1s binding energy of submonolayer (∼0.9 ML) CoPc is almost the same as that of multilayer CoPc.10 Such a peak shift is also seen in the C 1s XPS spectrum (Figure 4c). The C 1s XPS spectrum consists of benzene carbon (Cb), pyrrole carbon (Cp), and these shakeup satellite peaks,67,69 and the shifts of the Cb and Cp binding energies for 1 ML of VOPc with respect to the multilayer are 0.2 and 0.3 eV, respectively, toward lower energy. In addition, it was found from the peak fittings that the energy difference between Cb and Cp for 1 ML of VOPc on Ag is

ML of VOPc, although the spectra are broadened because of a lower energy resolution. It is noticeable that the main peak D shifts by 0.7 eV toward the lower-photon-energy side with respect to that for 10 ML of VOPc. This indicates that the V oxidation state is reduced by electron charge transfer from O (and/or Si) atoms to V atoms, because the peak positions in XAS spectra are strongly related to V oxidation states.56 The XAS spectrum of 1 ML of VOPc on Ag(111) resembles that of 10 ML of VOPc in both the V L- and O K-edge regions. The observation of π* and σ* peaks at the O K edge indicates that the VO π bond remains unbroken at the interface. As a result, it was confirmed that VOPc adsorbed on the Ag surface should have the oxygen-up orientation, which is in agreement with the results for VOPc on Au(111).16,22 Here, we can exclude the possible formation of bilayer staking as observed in VOPc57 and TiOPc58 on HOPG, because no LEED spot was observed after annealing of 0.6 ML of VOPc on Ag whereas a clear LEED pattern can be seen in 1 ML of VOPc on Ag (Figure 1c). Contrary to the results for 0.6 ML of VOPc on the Si surface, the main peak position for 1 ML of VOPc at the V L edge is very close to that for 10 ML of VOPc. This result indicates that the V oxidation state in VOPc at the interface is equivalent to that of VOPc in the multilayer. On the other hand, the XAS spectra for planar-structured MPcs on metal surfaces are dramatically modified because of a strong interaction between the central metal atoms and the surfaces.59,60 For instance, a shift of the peak energy and a disappearance of the peak in the Co L-edge XAS spectrum were observed for CoPc on Ag(111) as a result of charge transfer at the interface.59 In this study, the interaction between V and Ag was found to be effectively suppressed. In summary, the oxygen-up orientation is dominant on Ag(111) owing to the lack of effective surface interaction between O and Ag. In contrast, the VOPc molecule adsorbs on the Si surface in the oxygen-down configuration because VOPc is stabilized by the formation of a Si−O chemical bond. To ensure the robustness of our conclusions, XPS measurements were performed in the V 2p, O 1s, N 1s, and C 1s regions, as shown in Figure 4. All XPS spectra were recorded at room temperature. Although the molecular vibration is more enhanced than at the XAS measurement temperature of 5 K, the core binding energies might not be strongly temperature 22847



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Figure 5. Circularly polarized XAS and XMCD spectra at the V LIII,II edges of (a) 10 ML of VOPc on Si(111), (b) 0.6 ML of VOPc on Si(111), and (c) 1 ML of VOPc on Ag(111) at a temperature of 5 K, an external magnetic field of ±5 T, and X-ray incidence angles of 0° and 55°. The green dashed line represents the integral of μ+ − μ− from 507 to 527 eV. In the inset of panel a, the magnified XAS spectrum at θ = 0° and XMCD spectra at θ = 0° and 55° at the O K edge are shown.

that for 10 ML of VOPc, whereas that of Ni 1s (lower energy peak) shifts by 0.3 eV toward lower energy. This result may imply that the electron originating from the cleavage of the V O π bond selectively occupies the Ni 2p orbital rather than the Np 2p orbital. For the C 1s XPS spectrum of 0.6 ML of VOPc, the binding energy of Cb 1s is the same with that for 10 ML of VOPc, but that of Cp 1s shifts by 0.2 eV toward lower energy compared to that in the multilayer. This indicates that the chemical state of Cb in 0.6 ML of VOPc on Si is similar to that in multilayer VOPc, which is in clear contrast to Cp in 0.6 ML of VOPc on Si or Cp and Cb in 1 ML of VOPc on Ag. This can understood from the orientation of VOPc and the cleavage of the VO π-bond. The interaction scenario might be as follows: VOPc adsorbs on the Si surface with oxygen down, and the cleavage of the VO π bond occurs because of the strong interaction of the O atom with the Si surface. Subsequently, the electron transfers to the V, Ni, and Cp molecular orbitals, but the electron does not occupy the Cb orbital, and no direct interaction of Cb with the Si surface can exist. The chemical state of Cb is similar to that in the multilayer. The results of the XAS and XMCD measurements are shown in Figure 5. The XMCD signal is observed in 10 ML of VOPc, and the appearance of the XMCD signal is consistent with the magnetization curve measured by SQUID (Figure 1e) and the electron spin resonance (ESR) measurements.53,74 The intensity of XMCD at an external magnetic field of ±5 T estimated from the magnetization curve (Figure 1b) is ∼60% of the saturation magnetization. The V L-edge XAS and XMCD spectra resemble those of vanadyl-bis-enaminoketone reported by Gallani et al.75 Although it is possible to estimate the spin and orbital magnetic moments of Fe and Co from XMCD signals by using sum rules related to XMCD,33,76 we cannot evaluate the V L-edge XMCD because of the overlap between the V LIII and LII edges. However, the sign of the orbital magnetic moment can be obtained from XMCD by integrating the XMCD signal in the V LIII,II region (507−527 eV). The sign of the integral of XMCD in 10 ML of VOPc is negative, implying that the orbital magnetic moment is positive. This system shows the coupling of J = L + S, whereas the total angular momentum J of vanadium is usually expressed as J = L − S on the basis of Hund’s third rule. The theoretical calculation for a VAu4 cluster suggested that an unusual LS coupling is ascribed to the ligand field effect surrounding the V atom.77 Similar results obtained by XMCD have been reported

smaller than that for 10 ML of VOPc because of a site-specific screening effect.68,69 In the oxygen-down configuration of VOPc, no direct interaction between the phthalocyanine framework and the Si surface was expected. To investigate the possibility of a direct interaction, a comparative XPS measurement was performed for H2Pc deposited on a Si(111) surface. Two distinct peaks at 398.5 and 400.1 eV were observed in the N 1s spectrum of the multilayer and attributed to the iminic nitrogen (N, Ni) and pyrrolic nitrogen (NH, Np), respectively. These results are consistent with those of previous studies.70,71 On the other hand, a large deviation between the experimental and fitted spectra was found in the peak-fitting analysis of the N 1s XPS spectrum of 0.6 ML of H2Pc, if only the two contributions from the iminic and pyrrolic nitrogen were similarly taken into consideration. In Figure 4b, the result obtained by using the same parameters as for 10 ML of H2Pc is depicted, although the deviation is not acceptable. Such a discrepancy indicates that H2Pc adsorbs on the surface in a parallel orientation and that the N atoms interact strongly with Si. Such a flat orientation and a strong interaction of phthalocyanine on Si(111) have been reported in the literature.26,72 In addition, a peak at ∼283.0 eV is observed in the C 1s XPS spectrum of 0.6 ML of H2Pc on the Si surface, which corresponds to silicon carbide.73 It is apparent that a part of the phthalocyanine framework interacts chemically with the Si surface. Such a peak was not observed in the spectra of 0.6 ML of VOPc on Si(111), indicating that the framework of VOPc is chemically separated from the Si surface because of the intermediation of the O atoms. The V 2p3/2 binding energy for 0.6 ML of VOPc on Si(111) shifts by 0.7 eV toward lower energy, and the O 1s binding energy shifts by 0.5 eV toward higher energy with respect to the corresponding energies for 10 ML of VOPc on Si. This indicates that the effective electronic density increases in the V 3d levels and decreases in the O 2p levels. The V 2p3/2 chemical shift is in agreement with the result of XAS, and the result for O 1s XPS is reasonable because the electronegativity of V is lower than that of Si. The main peak for 0.6 ML of VOPc in the N 1s XPS spectrum shifts by 0.2 eV toward the lower-energy side. The main peak can be fitted by assuming two components, namely, Np and Ni, and the peak-fitting analysis provides a different peak shift for each component. The binding energy of Np 1s (higher-energy peak) for 0.6 ML of VOPc is very close to 22848



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experimentally for vanadyl-bis-enaminoketone, VOSO4,75 and V(TCNE)x.78 The g values for VOPc given by ESR, however, are 1.965−1.993,53 1.968 (g∥) and 1.987 (g⊥),74 indicating that the 3d electron is coupled according to J = L − S. Future theoretical calculations will be necessary to settle this discrepancy. The magnified O K-edge XAS and XMCD spectra are shown in the inset of Figure 5a. A very smal but positive XMCD signal is observed at the O K edge, in accordance with the results for some VO complexes.75 Analyzing the O K-edge XMCD spectrum is difficult because of the low intensity and small LS coupling, although XMCD in adsorbed CO on ferromagnetic surfaces has been well studied.79−81 The XMCD spectrum of 1 ML of VOPc on the Ag surface is shown in Figure 5c, resembling that of 10 ML of VOPc, although the spectrum is broader than that of 10 ML of VOPc because of the lower energy resolution. It is clear, however, that the V spin in VOPc on Ag(111) is maintained, even though the spins of the central metals present in FePc and CoPc are substantially lost on the Ag and Au surfaces.9,10 The XAS and XPS results are consistent and indicate that the interaction of the V atom with the Ag surface is not very strong. In the spectrum of 0.6 ML of VOPc (Figure 5b), the XMCD signals are similar to those observed in the spectrum of 10 ML of VOPc on Si(111) and 1 ML of VOPc on Ag(111). The intensity of the XMCD spectrum in 0.6 ML of VOPc is close to that of 1 ML of VOPc. This indicates that the V spin in VOPc is retained and does not contribute to the formation of an electron pair, and the spin is not enhanced as a result of the cleavage of the VO π-bond. The spin state (S = 1/2) of V is preserved, regardless of the thickness of VOPc and type of the surface, that is, metal or semiconductor, even though the orientation of VOPc and the interactions with the surfaces are different at these interfaces.

Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +81 (564)55 7341. Fax: +81 (564)55 7337. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge Mr. Motoyasu Fujiwara of IMS for his help in the SQUID measurements in the Instrument Center of IMS. We also thank Dr. Toshihiko Kaji and Prof. Masahiro Hiramoto for their technical support in purifying VOPc and H2Pc; Dr. Masaharu Matsunami and Dr. Hiroyuki Yamane for fruitful discussions; Takayuki Yano for his technical support in preparing the connectors; and Enago, Crimson Interactive Pvt. Ltd., for the English language review. This study was supported by a Grant-in-Aid for Scientific Research (A) (Grant 22241029) of the Japan Society for the Promotion of Science (JSPS).

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REFERENCES

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4. CONCLUSIONS We have investigated the orientation and electronic structure of VOPc on the Si(111)-(7 × 7) and Ag(111) surfaces by XPS, XAS, and XMCD. The VOPc molecule adsorbs on Ag(111) in a parallel orientation with respect to the surface in an oxygenup configuration, similar to the adsorption of VOPc on HOPG and Au surfaces.16,19 No obvious interaction between the central V and substrate Ag atoms is observed at the top layer on the Ag(111) substrate because of the oxygen atoms at the trans position bonding with the V atom, and accordingly, the V spin state in VOPc is preserved and remains the same as that of V in the VOPc multilayer. The electronic structure of the N and C, however, apparently differs from that of N and C in 10 ML of VOPc because of the screening effect at the interface. On the other hand, the oxygen-down configuration is dominant for VOPc adsorbed on Si(111) because of the formation of the Si−O chemical bond. This interaction results in the cleavage of the VO π bond and changes the electronic states of the central V and ligand Ni and Cp atoms, causing electron transfer to their molecular orbitals. This charge transfer, however, does not influence the magnetic properties of the central V atom. In either case, the O ligand plays a key role in the protection of the spin magnetic moment at the interface. 22849



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Molecular Orientation and Electronic States of Vanadyl

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