Controlling the Magnetic Properties of a Single Phthalocyanine

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Controlling the Magnetic Properties of a Single Phthalocyanine Molecule through its Strong Coupling with the GaAs Surface G. Mattioli,†,‡ F. Filippone,† and A. Amore Bonapasta*,† †

Istituto di Struttura della Materia (ISM) del Consiglio Nazionale delle Ricerche, Via Salaria Km 29.5, CP 10, 00015 Monterotondo a degli Studi di Roma “La Sapienza”, P.le A. Moro 2, 00185 Roma, Italy Stazione, Italy, and ‡Department of Chemistry, Universit

ABSTRACT Results of an ab initio DFT (density functional theory) investigation of the paramagnetic vanadyl-phthalocyanine (VOPc) adsorbed on the GaAs (001) surface indicate that a Pc molecule equipped with a central metal-nonmetal pair gives rise to a strong molecule-semiconductor coupling that directly involves the central transition metal (TM) and permits to manipulate its magnetic properties. The nonmetal atom plays a key role in achieving such a remarkable effect by binding the molecule to the surface, permitting a substrate-to-molecule chargetransfer, and inducing a robust TM high spin configuration. The present model of organic-inorganic coupling can have a particular value because it involves semiconductor substrates, which generally give rise to a weak coupling with Pc molecules. It also indicates an alternative route for controlling the molecular magnetic properties, thus offering further degrees of freedom and opening wider perspectives for realizing single-molecule-based spintronics devices. SECTION Electron Transport, Optical and Electronic Devices, Hard Matter

semiconductors,7,8 likely because they are generally characterized by a quite weak coupling and van der Waals interactions, with minor effects on the molecule and substrate properties.16 A major goal of all of these investigations is to control and manipulate individual spins in a single molecule by exploiting their coupling to the environment. In this context, we propose here a third route for the TMsubstrate coupling where the role of the intermediate agent is played by the central nonmetal atom of the Pc in place of the atoms of the organic macrocycle. More specifically, we focus on the paramagnetic vanadyl-phthalocyanine (VOPc, Figure 1A) adsorbed on the Ga-rich GaAs (001) surface. (See Figure 2A.) The VOPc/GaAs system has been chosen on the ground of previous theoretical results,17,18 where it was suggested that the replacement of the Pc central metal with a metal-nonmetal group (here the V-O group) combined with a cation-rich semiconductor surface can favor the formation of an appreciable molecule-surface chemical bond (here an O-Ga bond). A major result of the present study is that in the VOPc/GaAs system the nonmetal O atom forms a V-O-Ga chemical bridge between the molecule and the surface, which deeply affects the magnetic properties of the V central atom. In fact, the ensuing molecule-surface strong coupling: (i) modifies the order of the molecular electronic levels by making a level localized on the V atom suitable for electron injection processes; (ii) permits

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n the discipline of spintronics, the spin degree of freedom of mobile charge carriers is exploited, in addition to their charge, to carry information as well as to realize different interactions with magnetic materials.1 Recent research efforts in this field have been directed toward organic materials, in particular, toward the properties of single organic molecules.2,3 In this regard, the architecture of metallo-organic molecules (MOMs), like phthalocyanines (Pcs), presents interesting features because some representatives of this wide family of molecules can be seen as single magnetic atoms dressed with an organic macrocycle. Magnetic Pc molecules can be considered indeed as the sum of two units (Figure 1A): (i) an organic macrocycle ligand and (ii) a central transition metal (TM) (e.g., Cu) or a TM-nonmetal group (e.g., VO), where the TM carries a local magnetic moment. Thus, when a Pc is adsorbed on a substrate, its architecture is suitable for a coupling of the magnetic atom to the atoms of the surface both direct and mediated by atoms of the macrocycle. Several studies have investigated electron and spin injection as well as magnetic coupling and switching in MOMs adsorbed on magnetic or nonmagnetic metals and semiconductors.4-14 Some of these investigations have been focused on the spin coupling of the central TM with metallic substrates,4-6,9,12,13 as described, for example, by Kondo resonances arising from the coupling between localized spins and conduction electrons.15 These studies confirm the potentialities of a Pc-like architecture by showing the occurrence of TM-substrate coupling both direct4,5,12,13 and mediated by the organic macrocycle.6,9 Less attention has been given to magnetic Pc's interacting with

r 2010 American Chemical Society

Received Date: June 22, 2010 Accepted Date: September 1, 2010 Published on Web Date: September 07, 2010

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DOI: 10.1021/jz100852a |J. Phys. Chem. Lett. 2010, 1, 2757–2762

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Figure 2. VOPc molecule interacting with the Ga-rich GaAs (001) ζ(4 2)/c(8 2) surface. (A) Equilibrium geometry of the VOPc/ GaAs system and isosurface of the difference electron density (Fdiff). The Fdiff map shows the displacements of electronic charge at the molecule-surface interaction. Red surfaces cover areas where the difference is positive, and blue surfaces cover areas where it is negative. (B) Equilibrium geometry of the VOPc/(n-doped)GaAs system and |ψ| 2 plot of the LUMO(V 0 ). (See the text.) The |ψ|2 isosurfaces correspond to the value of 0.005e/(a.u.).3 The green and red full circles indicate a correspondence between the equilibrium configurations of the present Figure and the electronic levels of Figure 3.

above modifications of the molecular properties as well as the surface-to-molecule charge transfers can be achieved only by means of that strong molecule-surface coupling. As a matter of fact, molecules nonbonded to the same Ga-rich GaAs (001) surface, like a VOPc with an upside-down central group (with respect to that shown in Figure 2A) or a paramagnetic CuPc molecule carrying only a central TM atom, investigated here as well, show no such effect. (ii) Such a strong molecule-substrate coupling shows a route to affect the molecular magnetic properties even with semiconductor substrates. The electronic structure of the first component of the VOPc/ GaAs system, the isolated magnetic VOPc, is characterized by a HOMO(π) (highest occupied molecular orbital) and a doubly degenerate LUMO(π*) (lowest unoccupied molecular orbital) highly delocalized on the organic macrocycle. (See Figure 1B.) These orbitals represent a common feature of the Pc molecules and are responsible for their semiconducting and optical properties.19-21 Moreover, the VOPc presents two frontier orbitals involving the central atom and responsible for its magnetic properties (Figure 1B): a single-occupied MO composed of a V 3d atomic orbital and of orbitals of the N atoms, referred to as HOMO(V), and an unoccupied MO composed of a different V 3d atomic orbital and an O 2p atomic orbital, referred to as LUMO(V). These orbitals represent the global

Figure 1. (A) Equilibrium structures of the vanadyl-phthalocyanine (VOPc) and copper-phthalocyanine (CuPc) molecules, top and side views. (B) |ψ|2 plots of the HOMO (π) and LUMO (π*) belonging to the VOPc macrocyclic ligand and of the HOMO and LUMO of the central VO group, HOMO(V) and LUMO(V), respectively. |ψ|2 isosurfaces correspond to the value of 0.005e/(a.u.).3 (C) The green curve represents the spin density of states (SpinDOS) of the VOPc molecule. The orange curve represents the spin-resolved DOS projected over the V 3d atomic orbitals (SR-PDOS). Spin up (spin down) states are located at the upper (lower) part of the Figure.

substrate-to-molecule charge transfers; and (iii) most important, permits a change of the V spin configuration from a doublet configuration to a robust triplet configuration, which can be reversed by injecting holes in the semiconductor. All of these changes are expected to affect the molecular magnetic properties also by favoring the appearance of Kondo-like effects similar to those reported in refs 4, 5, 12, and 13. Two peculiar features of the present results deserve to be mentioned: (i) The


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HOMO(-1) and LUMO(þ1) of VOPc, as shown by the spinresolved DOS in Figure 1C. In the VOPc molecule, the Vatom has an electronic configuration 3d1 and an oxidation state of þ4, thus giving rise to a doublet spin configuration and a total magnetic moment of 1 μB. The ζ(4 2)/c(8 2) Ga-rich GaAs (001) surface is characterized by two parallel lines of three-fold coordinated sp2 Ga atoms.22,23 The interaction of the VOPc molecule with this surface leads to the formation of a moleculesurface O-Ga bond (Figure 2A), as indicated by an O-Ga distance of 1.77 Å, an adsorption energy E[ads] of 2.41 eV,24 and by the difference density map Fdiff[VOPc/GaAs], shown in the same Figure. At the onset of the molecule-surface interactions, this difference density25 indicates indeed a charge displacement toward the O atom and the O-Ga bond region, both from the V and surface Ga atoms, thus settling a V-O-Ga bridging bond. This pictorial view is confirmed by the L€ owdin charges estimated for the involved V, O, and (surface) Ga atoms, whose charges change from 12.68, 6.20, and 2.80 for the VOPc not interacting with the surface to 12.60, 6.35, and 2.58 for the moleculesemiconductor system, respectively. Therefore, charge is piled up on the O atom and lost mainly from the surface Ga atom and, in a small part, from the V atom. To clarify the effects of the V-O-Ga bond on the electronic and magnetic properties of the VOPc molecule, let us consider the electronic levels of the isolated molecule, of the molecule adsorbed on the GaAs surface with the O atom upward,26 and of the molecule bonded to the surface (shown in Figure 2A), schematically represented in Figure 3A-C, respectively, together with the valence band (VB) and conduction band (CB) edges of GaAs. The comparison of Figure 3A with Figure 3 B shows that without a bridging bond with the substrate, the VOPc electronic structure is unchanged. On the contrary, Figure 3C shows that the formation of the V-O-Ga bond has impressive effects on the LUMO(V), which decreases about 2 eV in energy, locates in the GaAs energy gap and, most interestingly, reverses its position with respect to the LUMO(π*). The strong effects of the formation of the V-O-Ga bond on the LUMO(V)are closely related to the role played by the O atom. In fact, the O 2p orbital, which significantly contributes to the LUMO(V) in the isolated VOPc (see the corresponding |ψ|2 in Figure 1B) also participates in both the LUMO(V) and the formation of the O-Ga bond in the bonded molecule (as indicated by an analysis of the contributions of the O 2p orbitals to the occupied and unoccupied eigenstates of the VOPc/GaAs system, not reported here). Therefore, in the latter case, the O 2p orbital acquires some bonding character that can account for the lowering of the LUMO(V) energy. Such changes of the VOPc electronic structure can have important effects on the magnetic behavior of this molecule. First, because carrier injection involves the frontier orbitals, let us note that in isolated molecules such orbitals are the macrocycle spin-insensitive HOMO (π) and LUMO (π*), whereas in the bonded molecule the new LUMO(V) has a majority spin character (Figure 1C) and is strongly localized on the VO central group, which carries the VOPc magnetic moment, as shown in Figure 1B. Then, we probed this new LUMO(V) by investigating a charge transfer from a GaAs donor impurity to this orbital. We have simulated an n-doped GaAs (n-GaAs) by substituting a Ga(III) with a Si(IV) atom, which induces a donor level (represented by a red line in


Figure 3. Total (spin up þ spin down) DOS and electronic levels of the VOPc and CuPc molecules, isolated or interacting with the GaAs (001) surface. All of the levels are aligned to a common reference. The HOMO(π) and LUMO(π*), related to the macrocyclic ligand, are represented by full and dashed black lines, respectively. The HOMO(V) and LUMO(V), (HOMO(Cu) and LUMO(Cu)) related to the central V (Cu) atom, are represented by full and dashed orange (green) lines, respectively. The blue color background delimits the valence band (VB) and conduction band (CB) regions relative to the GaAs band structure. A dopant donor level is indicated by a red line. (A) Isolated VOPc molecule; (B) VOPc molecule interacting with the GaAs surface (the O atom points upward); (C) VOPc molecule bonded to the GaAs surface (the O atom points downward); (D) VOPc molecule bonded to an n-type, Sidoped GaAs surface (i.e., the case of surface to molecule charge transfer inducing a high-spin configuration). (E) Isolated CuPc molecule; (F) CuPc molecule interacting with the GaAs surface; (G) CuPc molecule interacting with an n-doped GaAs surface. The green and red full circles indicate a correspondence between electronic levels of the present Figure and the equilibrium configurations of Figure 2.

Figure 3D) near the minimum of the GaAs CB. The donor level, higher in energy than the LUMO(V) (now the proper lowest unoccupied energy level of the VOPc/GaAs system), induces a charge-transfer process from the semiconductor to the molecule, as indicated by the red arrow in Figure 3D, which fills the LUMO(V) with one electron. The filled LUMO(V), hereafter referred to as LUMO(V0 ), maintains its composition, as clearly shown by a comparison of the corresponding |ψ|2 reported in Figure 2B with that of the LUMO(V) of Figure 1B, and, because of the above stabilizing effect, its location in the GaAs energy gap. These results indicate that the new LUMO(V) is potentially available for electron injection in the bonded VOPc molecule. A further remarkable consequence of the VOPc-GaAs bonding is that the filled LUMO(V0 ) is highly spin selective; the triplet configuration (total magnetic moment 2 μB), involving the LUMO(V0 ), is 1.0 eV lower in total energy than the singlet one. The contribution of the O 2p orbital to the LUMO(V0 ) is crucial for stabilizing the triplet configuration; the V-O-Ga bond indeed acts only on the triplet state resulting in the occupation of the HOMO(V)-LUMO(V0 ) pair, the singlet state being given instead by two paired electrons accommodated on a different MO localized on the Vatom only. Finally, it has to be taken into account that in the bonded VOPc the LUMO(V) orbital is partially overlapped with the LUMO(π*) orbital, as shown by the existence of overlap regions between

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transistors where, in addition, the Kondo resonances could be tuned reversibly using the gate voltage to change the spin configuration of the V atom. Such kind of experiment should evidentiate the key property of the proposed VOPc/GaAs system; that is, the injection of holes in the GaAs VB can neutralize the effects of the n-type doping by removing the electron transferred to the LUMO(V0 ), thus changing the VOPc triplet state to a doublet one. In other words, the VOPc triplet state can be switched on/off by injecting carriers in the semiconductor. In conclusion, present results show that the equipment of a paramagnetic Pc with a central metal-nonmetal pair permits us to realize a strong molecule-semiconductor coupling that directly involves the central metal and can be exploited to manipulate its magnetic properties. They also elucidate some remarkable, basic features of such a phenomenon, like the key role played by the nonmetal atom, which binds the molecule to the surface (the bond directly affecting the central-metal electronic properties), permits us to realize a substrate-to-molecule charge-transfer, and induces a robust TM high spin configuration.The original model of strong organic-inorganic coupling proposed here has a particular value because it regards semiconductor substrates, which generally give rise to a weak coupling with Pc molecules. It also indicates an alternative route for controlling the molecular magnetic properties, thus offering further degrees of freedom and opening wider perspectives for realizing singlemolecule-based spintronics devices.

Figure 4. Top views of the |ψ|2 plots of the LUMO(π*) and LUMO(V) orbitals relative to a VOPc molecule bonded to a Ga-rich GaAs (001) surface are shown in (A) and (B), respectively. A side view of the |ψ|2 plot corresponding to the same LUMO(V) orbital is reported in Figure 1B. Identical |ψ|2 plots are given by LUMO(V) and LUMO(V0 ) orbitals. (See the text.) The |ψ|2 isosurfaces correspond to the value of 0.001 e/(a.u.).3.

the |ψ|2 functions relative to the two orbitals; see the plots given in Figures 4A,B, respectively. The same holds for the filled LUMO(V0 ), whose |ψ|2 density is indistinguishable from that of LUMO(V). This suggests the possibility of coupling of electrons injected in the LUMO(π*) orbital with the electron occupying the LUMO(V0 ). It is worth stressing that none of the above effects have been found with the nonbonded VOPc (O in the upward direction) because no substrate-to-molecule charge transfer is induced by a Si donor in the GaAs substrate. In this case, the LUMO(V) and the LUMO(π*) are higher in energy than the Si donor level, which rests occupied by its exceeding electron. A similar result is found for the CuPc/GaAs system. The electronic levels of the CuPc molecule (where the Cu atom has an electronic configuration 3d9 and an oxidation state of þ2), when isolated, adsorbed on the surface, and adsorbed on the n-type surface, are shown in Figure 3E-G, respectively. These panels show that such levels are almost unperturbed by the interaction with the surface and that no surface-to-molecule charge transfer occurs in presence of a Si donor. As for the nonbonded VOPc, the exceeding donor electron occupies its shallow level close to the CB and located below both the LUMO(Cu) and the LUMO(π*). It may be noted that the systems considered in the panels A-F of Figure 3 are all characterized by a total magnetic moment of 1 μB, except for the system of panel D, which presents a total magnetic moment of 2 μB. This theoretical picture suggests that the effects of a VOPcGaAs strong coupling on the magnetic behavior of the VOPc molecule could be observed in scanning tunneling microscopy (STM) experiments involving a single molecule adsorbed on an n-GaAs surface or in electron transport experiments performed in single-molecule transistors (refs 27 and 28), where an n-GaAs substrate is used as a back gate. In the former case, electron injected in the LUMO(π*) orbital is expected to give rise to an appreciable spin coupling to the LUMO(V0 ) electron because of the LUMO(π*)-LUMO(V0 ) interactions, the high spin localization of the latter orbital, and the high stability of the triplet spin configuration discussed above. Altogether, these conditions should favor the appearance of Kondo-like resonances. Similarly, Kondo effects should be observed in single-molecule


COMPUTATIONAL METHODS We have investigated the interactions of the VOPc and CuPc molecules with the Ga-rich GaAs (001) surface by using first-principles DFT methods in the U-corrected local spin density generalized gradient approximation (LSD-GGAþU) and a supercell approach.29 Hubbard U corrections of 4.3 and 8.9 eV for the d electrons of the V and Cu atoms in the corresponding Pc molecules have been calculated as described in refs 30 and 31. Such corrections have been applied to circumvent partially the too poor LSD-GGA electron correlation description,32,33 which can affect the position of metallic d states with respect to the ligand MOs and to the semiconductor bands as well as the degree of localization of the corresponding electronic charge. Total energies have been calculated by using nonrelativistic ultrasoft pseudopotentials,34 plane-wave basis sets, and the PBE gradient corrected exchange-correlation functional.35 We have investigated the electronic properties of the VOPc/GaAs and CuPc/GaAs systems by analyzing the electronic eigenvalues calculated at the Γ point.36 The convergence of results has been carefully checked. We have achieved satisfactorily converged results by using cutoffs of 30 Ry on the plane waves and of 180 Ry on the electronic density. The Garich GaAs (001) surface cell has been modeled by the addition of ∼15 Å of empty space to a 4 4 crystal slab, formed by eight atomic layers of bulk GaAs (128 atoms) cut along the (001) crystal axis. The unrelaxed lower surface of the GaAs slab has been saturated by pseudo H atoms.22 Geometry optimization procedures have been performed by fully relaxing the positions of all of the atoms in a supercell, except for the atoms of the bottom layer of the semiconductor. The effects of van der Waals

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forces on the molecule-surface interactions have been taken into account with a semiempirical model.37,38 (See also the Supporting Information.)

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SUPPORTING INFORMATION AVAILABLE This material is available free of charge via the Internet at http://pubs.acs.org. (14)

AUTHOR INFORMATION Corresponding Author: *To whom correspondence should be addressed. (15)

ACKNOWLEDGMENT We would like to thank G. Erbacci, CNR(16)

INFM, and the CINECA consortium for computational support and G. Rossi, A. Paoletti, and G. Pennesi for several useful discussions.

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