Electronic Structure and Spin Polarization of Metal (Mn, Fe, Cu

Aug 20, 2012 - First-principles calculations have been performed to study the electronic and magnetic properties of metal phthalocyanine molecules (MP...
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Electronic Structure and Spin Polarization of Metal (Mn, Fe, Cu) Phthalocyanines on an Fe(100) Surface by First-Principles Calculations X. Sun,*,†,‡ B. Wang,† and Y. Yamauchi‡ †

Hefei National Laboratory for Physical Sciences at the Microscale and Department of Physics, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China ‡ National Institute for Materials Science, 1-2-1 Sengen, Tsukuba 305-0047, Japan ABSTRACT: First-principles calculations have been performed to study the electronic and magnetic properties of metal phthalocyanine molecules (MPc, M = Mn, Fe, Cu) on an Fe(100) surface. All of the three MPc molecules prefer their central atom on the top site and orientation with the M−N1 bond along the ⟨110⟩ direction. Similar molecular distortions are observed among these molecules upon adsorption. The spin-resolved electronic structures reveal that the spinpolarized states of the central Mn and Fe atoms in MnPc and FePc are significantly changed due to their relatively strong interactions with the Fe atoms in the substrate, whereas the spin-polarized states of the central Cu atom in CuPc is only slightly varied. The ferromagnetic Fe substrate may induce a slight spin polarization of the Pc macrocycle. Negative spin polarization is obtained at around the Fermi level for all three of the adsorbed MPc molecules. Adsorption-induced charge transfers are analyzed according to the differential charge densities. The spin polarization is revealed to be dependent on the spatial symmetries of the unfilled d orbitals of each MPc molecule.

I. INTRODUCTION Metal phthalocyanine (MPc) molecules are promising for a wide range of applications, such as light-emitting diodes,1 organic field-effect transistors,2 single-molecule and spintronic devices,3 and organic photovoltaic cells.4 An understanding of the interactions of MPc molecules with metal substrates is crucial for practical applications, especially with respect to the magnetic properties. It has been revealed that the magnetic moment of the central metal atom can be induced,5 preserved,6,7 or even quenched.8 The Kondo effect has been experimentally observed with scanning tunneling microscopy (STM) in dehydrogenated CoPc molecules on a Au surface,8 MnPc molecules on a Pb surface,9 and FePc molecules on a Au surface,10 although the Kondo temperatures vary among these systems. The temperature difference suggests that the interactions between the central metal atoms and the substrates are different. Moreover, the spin polarization of the MPc molecule is very important in application to spintronic devices.11 High spin polarization may be more easily obtained by placing MPc molecules on 3d ferromagnetic substrates, such as Fe, Co, and Ni, rather than on nonmagnetic substrates, because the electronic states of these ferromagnetic substrates are highly spin-polarized around the Fermi level (EF). Spinpolarized STM has been used to detect the spin states of CoPc on ferromagnetic cobalt nanoislands12 and Fe(110) thin film.13 Cinchetti et al.14 used spin-resolved two-photon photoemission spectroscopy to determine that the energy level alignment of © 2012 American Chemical Society

the CuPc/Co interface can be modified and the efficiency of spin injection is enhanced by alkali-metal doping. Another useful technique is spin-polarized metastable-atom deexcitation spectroscopy (SPMDS), which is highly sensitive to the spin signals on surfaces and has been used to investigate the electronic and magnetic properties of adsorbates, such as crotyl alcohol on Pd(111),15 benzene on Fe(100),16 and atomic hydrogen on Fe3O4(100).17 Suzuki et al. reported that the magnitude of spin polarization near EF differs among the various MPc molecules adsorbed on an Fe(100) surface;18 however, the mechanism behind this phenomenon remains unclear. In this study, first-principles calculations are employed to investigate several 3d transition-metal MPc molecules (M = Mn, Fe, and Cu) on a ferromagnetic Fe(100) surface. The spinresolved electronic structures and charge transfer are calculated and analyzed. The spin polarization of the adsorbed MPc molecules near EF is discussed and compared with experimental results. Received: May 5, 2012 Revised: August 20, 2012 Published: August 20, 2012 18752

dx.doi.org/10.1021/jp304361n | J. Phys. Chem. C 2012, 116, 18752−18758

The Journal of Physical Chemistry C

Article

4.0 eV) functionals. Similar to the result for free NiPc,5 the GGA functional strongly underbinds the molecular orbitals that involve the central Mn atom of the MnPc molecule and disorders the molecular orbitals, as specified by blue arrows in Figure 1. Differently, both the energy levels and the ordering of molecular orbitals by the GGA+U are much closer to those obtained with HSE06 or PBE0 than the conventional GGA functional (Figure 1). The situation in free FePc and CuPc is similar with that in free MnPc, which suggests that the GGA+U method provides a better description than the conventional GGA method. The GGA+U technique has been widely used in theoretical calculations of transition-metal organic molecules, such as metal−porphines30−32 and metal−phthalocyanines.5 The physical idea behind the GGA+U approach comes from the Hubbard Hamiltonian, which is expressed in terms of the on-site Coulomb interaction parameter U and the exchange parameter J. The value of U strongly influences the calculated electronic structures. In the present calculation, U is determined by comparison of the energy levels and ordering of the molecular orbitals for each MPc (M = Mn, Fe, Cu) calculated at different U values (from 2 to 8 eV) referenced using the results from the HSE06 or PBE0 hybrid functionals, because these can provide better agreement with the experimental results.25,26 The U values are chosen to be 4, 4, and 6 eV for Mn, Fe, and Cu atoms, respectively. The exchange parameter J is set as 1 eV. The following discussions are based on calculations using the GGA+U method.

II. COMPUTATIONAL METHODS All calculations were performed within the framework of density functional theory (DFT) using a plane-wave basis set via the Vienna Ab-initio Simulation Package (VASP).19,20 The electron−ion interaction is described using the projectoraugmented wave (PAW) method, of which implementation is reported by Kresse and Joubert,21 and Blöchl.22 The spin interpolation reported by Vosko et al.23 was adopted for the spin-polarized calculations. The plane-wave energy cutoff was set to 400 eV for all calculations. The surface is modeled by a four-layer Fe slab through a (7 × 7) unit cell with a vacuum region of 15 Å, and an MPc (M = Mn, Fe, or Cu) molecule is adsorbed on one side of the Fe slab. During structural optimizations, the bottom three substrate layers are fixed. The MPc molecule and the topmost substrate layer are allowed to relax freely until all forces are less than 0.01 eV/Å. Only a single Γ point is used due to the numerical limitations. A test calculation is performed with a five-layer Fe(100) slab and relaxation of the top two substrate layers. Compared to the model of the fourth-layer slab, the variation of bond length between atoms in the adsorbed MPc is less than 0.006 Å, and the variation of the vertical distance between the molecule and the surface is approximately 0.02 Å. The negligible difference suggests that the four-layer slab is sufficient to model the MPc molecule adsorption on the Fe(100) surface. The conventional generalized gradient approximation (GGA) functional24 cannot describe the electronic structures of transition-metal organic molecules well; instead, hybrid functionals, such as B3LYP and PBEh, provide better agreement with experimental results.25,26 Our previous study shows that the GGA+U approach based on the Hubbard model27 is also better than GGA and is very close to the HSE0628 and PBE029 hybrid functionals in the description of the electronic structure of free NiPc.5 We calculated other free MPc molecules using the GGA+U method, for example, MnPc, and compared the result with those obtained using other calculation methods. Figure 1 shows the local density of states (DOS) of the Mn atom in free MnPc calculated using the respective GGA, HSE06, PBE0, and GGA+U (with a U value of

III. RESULTS AND DISCUSSION A. Adsorption Energy and Geometric Structure. The geometry of the MPc molecules examined is similar. As with our previous study of NiPc on the Fe(100) surface,5 three adsorption sites (hollow, bridge, and top) are considered for the central metal atom and each with two orientations (I and II) for MPc (M = Mn, Fe, Cu) on the Fe(100) surface. The M−N1 bonds along the ⟨100⟩ and ⟨110⟩ directions are referred to as the I and II orientations, respectively (Figure 2). As indicated by the adsorption energies in Table 1, the most favorable adsorption geometry for all three MPc (M = Mn, Fe, Cu) molecules is that where MPc locates at the top site with the II orientation (Figure 2b), similar to the result for NiPc,5 which indicates that the adsorption configuration is independent of the central metal atom when the ferromagnetic Fe(100) surface is used. However, the situation on the Au(111) surface is quite different, such that MnPc prefers the top site, whereas FePc and CuPc prefer the hcp hollow site.7 The adsorption energies for MnPc, FePc, and CuPc are 6.31, 6.64, and 6.04 eV, respectively. The large adsorption energy indicates that the interactions of MPc molecules with the Fe(100) substrate are much stronger than those with the Au(111), Co(001), and Cu(001) substrates, where the adsorption energies typically range from 0.5 to 3.7 eV.6,7 The energy difference between the most favorable and the second most favorable configurations is less than 0.06 eV for MPc (M = Mn, Fe, and Cu) on the Au(111) surface,7 whereas it is as large as 0.54, 1.30, and 1.01 eV for MnPc, FePc, and CuPc on the Fe(100) surface, respectively. Thus, the top site with the II orientation is the most stable configuration for MPc on the Fe(100) surface. Figure 2c shows that the adsorbed MPc molecules are almost parallel to the surface, but with considerable distortions, which causes enlargement of the molecular rings, elongation of the bond lengths, tilting of the hydrogen atoms, and nonplanarity of the ligands. The distortion characteristics are approximately

Figure 1. Local DOS of the Mn atom in a free MnPc molecule calculated using different functionals with a smearing factor of 0.1. 18753

dx.doi.org/10.1021/jp304361n | J. Phys. Chem. C 2012, 116, 18752−18758

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Figure 2. One MPc (M = Mn, Fe, Cu) molecule adsorbs on the top site of the Fe(100) surface (top view) with (a) the I orientation (the M−N1 bond is along the ⟨100⟩ direction) and (b) the II orientation (the M−N1 bond is along the ⟨110⟩ direction). (c) Side view of adsorbed MPc molecules with the II orientation on the Fe(100) surface. The surface is represented by a (7 × 7) unit cell. The C, H, N, M (Mn, Fe, Cu), and substrate Fe atoms are represented as gray, pink, cyan, yellow, and green spheres, respectively. Only atoms at the topmost layer of the substrate are shown.

Table 1. Adsorption Energies (eV) of MPc (M = Mn, Fe, and Cu) on the Fe(100) Surface at Different Sites with I and II Orientations MnPc FePc CuPc

hollow I

hollow II

bridge I

bridge II

top I

top II

−5.38 −5.34 −5.03

−4.17 −4.50 −3.94

−3.25 −3.54 −3.07

−5.77 −4.01 −3.59

−3.77 −4.21 −3.97

−6.31 −6.64 −6.04

the same among the three MPc (M = Mn, Fe, Cu) molecules, and quite similar to that for NiPc on the same Fe(100) surface.5 The central metal atom bonds directly to the underlying Fe atom with vertical distances (bond lengths) of 2.59, 2.57, and 2.65 Å for MnPc, FePc, and CuPc, respectively. These molecule−substrate distances are slightly longer than that for NiPc on the same Fe(100) surface, but much shorter than those for MPc on other substrates, such as Au(111) (2.81, 2.73, and 3.04 Å, respectively). The bond lengths between C atoms in MPc and the substrate Fe atoms vary from 2.07 to 2.48 Å (Table 2), which are close to those for C atoms in a benzene

Figure 3. Spin-up and spin-down DOS of the adsorbed MPc (M = Mn, Fe, and Cu) molecules, the central metal atom, and the Pc macrocycle.

Table 2. Bond Lengths (Å) between Atoms in the Adsorbed MPc with Substrate Fe Atoms MnPc FePc CuPc

dM−Fe

dN1−Fe

dN2−Fe

dC1−Fe

dC2−Fe

dC3−Fe

dC4−Fe

2.59 2.57 2.65

3.18 3.15 3.22

2.11 2.10 2.10

2.45 2.44 2.48

2.20 2.19 2.19

2.07 2.08 2.07

2.29 2.29 2.29

molecules (black lines in Figure 3) involve a dominant contribution from their corresponding Pc macrocycles (blue lines in Figure 3), due to the overwhelming number of atoms in the Pc macrocycle, in contrast to the single central metal atom. Therefore, the overall electronic structures mainly reflect the characteristics of the Pc macrocycles, especially at the deep energy levels (far away from EF). This is in good agreement with the previous experimental results, where similar behavior was observed among different MPc molecules using SPMDS.18 The electronic states of the Pc macrocycle are slightly spinpolarized (Figure 3), which is induced by the hybridization with the ferromagnetic Fe(100) substrate. In addition to the electronic structure, the spin polarization of the Pc macrocycle is also very similar for the different MPc molecules, due to the similarity in their atomic position and interaction with the substrate. However, the magnetic properties and spin polarization of electronic states for the central metal atom differ very much among the three MPc molecules. Figure 4 shows the local DOS (LDOS) and decomposed DOS for the metal atoms before and after adsorption. For direct comparison, the DOS of the MPc before adsorption is calculated for a quasi-free molecule by adopting a distorted geometry from that upon adsorption to exclude the effect of molecular distortion. The Mn atom has the highest total magnetic moment, and Cu has

molecule on an Fe(100) surface (from 2.02 to 2.82 Å) through chemisorption.16 The strong interaction between NiPc and the Fe(100) surface induces spin-splitting in NiPc upon adsorption, although the NiPc molecule is nonmagnetic before adsorption.5 The Fe(100) surface may significantly modify the electronic and magnetic properties of MPc upon adsorption, due to these strong interactions. B. Electronic Properties and Spin Polarization. The densities of states (DOS) that originated from Pc (macrocycle excluding a metal atom) are almost identical for the adsorbed MPc (M = Mn, Fe, Cu) molecules (blue lines in Figure 3). This can be attributed to the similar interaction strength of the Pc macrocycle with the Fe(100) surface, because Pc has the same atomic composition and similar geometric structure upon adsorption, which is indicated by the negligible difference in the bond lengths of C and N atoms with the substrate Fe atoms (Table 2). The total electronic structures of the adsorbed MPc 18754

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Figure 4. Total and projected DOS (PDOS) of the Mn, Fe, and Cu atoms in MPc molecules before (top and bottom panels) and after (middle panels) adsorption on the Fe(100) surface. To exclude the effect of molecular distortion, the DOSs of quasi-free MPc molecules were calculated with distorted geometries, similar to those when adsorbed on the substrate.

middle panel of Figure 4a), which can be assigned to the unfilled state originally located at 1.25 eV above EF of the quasifree MnPc molecule (bottom panel of Figure 4a). Thus, the spin polarization is changed from positive to negative in the energy range from −2.0 eV to EF. For Fe in FePc, the spin polarization in the range from −2.0 eV to EF is negative before the molecule adsorption. Upon adsorption, the two spin-down states from the dx2−y2 orbitals located at −2.31 and −1.30 eV combine into one spin-down state at −1.68 eV. A spin-down dz2 orbital simultaneously appears at −1.52 eV, which is assigned to the dz2 electronic state originally at 1.22 eV. Meanwhile, the state at EF composed of spin-down dxz(yz) orbitals (lower middle panel of Figure 4b) shifts above EF. Therefore, the spin polarization of Fe is also highly negative in the energy range from −2.0 eV to EF for FePc upon adsorption. For CuPc, the variation of electronic states of Cu upon adsorption of CuPc is much smaller, and the spin polarization of Cu remains positive, although the unoccupied spin-down dxy orbital becomes partially filled. This is consistent with the smallest adsorption energy for CuPc among the three MPc molecules on the Fe(100) surface. The calculated results can be compared with some experimental results. SPMDS measurements of electrons ejected from the surface by He(23S) beam irradiation were performed with parallel and antiparallel electron spin orientations of He(23S) in the beam with respect to the majority of electrons in the ferromagnetic substrate.18 The spin asymmetry, A, is defined as

the smallest total magnetic moment in the quasi-free MPc molecules. The spin-up LDOS shifts toward a shallow level upon adsorption, which results in a decrease of the magnetic moment of Mn from 3.52 to 3.03 μB. Similar shifts in the energy levels are observed for FePc and CuPc upon adsorption, although the shifts are much less. The decreases in magnetic moment are from 2.15 to 1.90 μB for Fe and from 0.61 to 0.44 μB for Cu. The small magnetic moment of the latter can be attributed to the almost-filled 3d orbital in Cu. It is clear that the spin polarization of the central metal atom is dependent on the type of MPc molecule and varies with the shift in the energy level upon adsorption. For Mn and Fe in MPc, the majority electronic 3d states (spin-up) dominantly range from −6 to −2 eV with negligible minority ones (spindown) in this energy range (middle panels of Figure 4a,b), which leads to positive spin (spin-up) polarization. For Cu in CuPc, the spin polarization in this energy range is much smaller because the amounts of spin-up and spin-down electronic states (middle panels of Figure 4c) are almost equivalent. For the shallow levels (quite close to EF), the electronic structures of Mn and Fe in MnPc and FePc are more significantly changed upon adsorption, whereas the electronic structure of Cu changes only slightly for CuPc upon adsorption. Considering that the overall interactions of the MPc molecules with the Fe substrate are in the similar level, because of their similar adsorption energies and bond lengths (Tables 1 and 2), we suggest that the different behavior for Cu from those for Mn and Fe could be attributed to the different spatial symmetries of the related orbitals. For Mn and Fe, the dz2 and dxz(yz) orbitals for the shallow levels are significantly shifted, although a reasonably large shift is also observed for the dxy orbital in MnPc. However, the main contributions to the shallow levels are from the dxy orbitals in CuPc. Therefore, better symmetry matching is possible for the central Mn (Fe) dz2 and dxz(yz) orbitals with the related Fe substrate orbitals than that for the Cu dxy orbitals, which results in a relatively weak interaction between the central Cu and the Fe substrate. As an extended discussion, we further consider the contributions of different orbitals to the spin polarization in the energy ranges of interest. For Mn in MnPc, the spin polarization in the range from −2.0 eV to EF is positive before MnPc adsorption, because of the spin-up dxy orbital (top panel of Figure 4a). This orbital is shifted to 1.07 eV and becomes unfilled upon adsorption (upper middle panel of Figure 4a). Meanwhile, a spin-down dz2 orbital appears at −0.76 eV (lower

A=

Ip − Ia Ip + Ia

(1)

where Ip and Ia denote the intensities of ejected electrons for the parallel and antiparallel orientations, respectively. The spin asymmetry is of more concern near EF, because it is more important for spintronics than that at the deep energy level. Positive spin asymmetry was obtained for all three MPc molecules on the Fe(100) surface at the energy near EF. The inverse relation between the spin directions of surface electrons detected by the He(23S)-1s holes and the occupying 1s and 2s electrons in He(23S) mean that a positive spin asymmetry A indicates a dominance of minority electrons, that is, negative spin polarization.33 A good example is the clean Fe(100) surface, where positive spin asymmetry was obtained by SPMDS, due to the negative spin-polarized electronic states 18755

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at around EF. The magnitude of the detected spin asymmetry differs among the MPc molecules investigated. The value is the largest for FePc (>15%), medium for MnPc (ca. 7%), and smallest for CuPc (