Structural and Magnetic Depth Profiling and Their Correlation in Self

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Structural and Magnetic Depth Profiling and Their Correlation in Self-Assembled Co and Fe Based Phthalocyanine Thin Films Surendra Singh,†,* A. Singh,‡ M. R. Fitzsimmons,§ S. Samanta,‡ C. L. Prajapat,‡ S. Basu,† and D. K. Aswal‡ †

Solid State Physics Division, Bhabha Atomic Research Center, Mumbai 400085, India Technical Physics Division, Bhabha Atomic Research Center, Mumbai 400085, India § Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States ‡

ABSTRACT: The family of phthalocyanine (Pc) is suitable functional molecules in the field of molecular electronics because of their thermal stability and the possibility to tune their structure, chemical, magnetic, and transport properties by means of different metallic cations within the Pc molecular cage. Here we report the depth dependent chemical composition and magnetization of iron phthalocyanine (FePc), cobalt phthalocyanine (CoPc) and binuclear (Co−Fe)-phthalocyanine [(Co− Fe)Pc] thin films grown on sapphire substrates by molecular beam epitaxy. The binuclear (Co−Fe)Pc films grown by coevaporation of pure FePc and CoPc exhibited a new structure (binuclear) which show drastically different conducting and magnetic properties. Using X-ray reflectivity (XRR) and polarized neutron reflectivity (PNR), we demonstrated that the structural changes in binuclear (Co− Fe)Pc films as compared to pure film is responsible for about three to four order reduction in resistivity and presence of ferromagnetism in this film at low temperature. PNR data clearly suggest that the binuclear (Co−Fe)Pc film is ferromagnetic with a magnetization of 50 ± 15 kA/m at 10 K, indicating an increase in magnetic transition temperature. However, the pure FePc or CoPc films show negligible magnetization at 10 K. PNR data in combination of XRR also revealed detail magnetic and chemical structure across the molecule which is highly correlated along the normal and in the plane of the film.

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mobility of 110 cm2 V−1s−1 at room temperature.20 The high mobility of the binuclear (Co−Fe)Pc film compared to the mononuclear (pure) CoPc and FePc films was attributed to structural ordering of (Co−Fe)Pc dimers. In the (Co−Fe)Pc dimer, CoPc and FePc molecules are covalently joined through the corner benzene ring via two C−C bond formation, as shown in the inset (i) of Figure 1.21 These binuclear molecular thin films have also shown improved application for gas sensing as compared to pure films.21 TMPc films are also expected to bridge two novel disciplines of spintronics and molecular electronics because of their unique electronic and magnetic characteristics arising from the transition metal atoms in their framework.22−24 Utilizing spins localized in such molecule provides the possibility of molecular spintronics devices consisting of one or a few magnetic molecules. In order to realize such devices, it is crucial to unveil depth profiling of structure and magnetic properties and the coupling of molecular spin with environment. Here we report the correlation of depth dependent chemical composition and magnetism of pure CoPc, FePc, and binuclear (Co− Fe)Pc films with thickness ∼800 Å, using X-ray reflectivity (XRR) and polarized neutron reflectivity (PNR). These are complementary nondestructive techniques which provide

INTRODUCTION Transition-metal phthalocyanines (TMPcs) are archetypal organic semiconductors and their properties have been used in applications such as dyes, light emitting diodes, solar cells, field effect transistors, gas sensors, field emission applications, and single-molecule-devices.1−6 TMPc are planar molecules with the transition metal ion core surrounded by organic rings. The important feature of TMPc family is the possibility to replace the metal ion and to modify the metal ion’s spacing using different substrates and preparation methods in order to modify the electronic and magnetic properties of TMPc. Recently, there has been considerable interest in the magnetic properties of TMPc films, e.g., manganese phthalocyanine (MnPc),7−9 iron phthalocyanine (FePc),8,10−12 and cobalt phthalocyanine (CoPc).10,13−18 The magnetic properties of TMPc films are strongly influenced by absorption on magnetic and nonmagnetic metallic surfaces. Gredig et al.,19 observed ferromagnetism in a FePc film below 4.5 K due to inter-iron−chain interactions. The interactions were strongly influenced by the growth of the film on different substrates. Different scenarios have been observed regarding magnetic properties of the CoPc molecule, e.g., loss of magnetic moment of Co ion upon absorption on gold substrate,10,13 and quenching of the magnetic moment of Co upon absorption on magnetic substrates.18 Earlier we observed that binuclear (Co−Fe)-phthalocyanine [(Co−Fe)Pc] films of ∼200 Å thickness exhibited very high © 2014 American Chemical Society

Received: September 4, 2013 Revised: January 31, 2014 Published: February 3, 2014 4072

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EXPERIMENTAL SECTION

Films of CoPc, FePc, and binuclear (Co−Fe)Pc with thickness ∼800 Å, were deposited on (0001) sapphire substrate using molecular-beam epitaxy (MBE) (RIBER system, model EVA 32).21,28 Weight ratios (1:1) of FePc and CoPc were mixed and evaporated simultaneously to make the binuclear (Co−Fe)Pc films.21,28 All the films were grown at a substrate temperature of 200 °C using a deposition rate of 2 Å s−1 under a base vacuum of better than 10−8 Torr. In order to probe the depth dependent structure and magnetization of these films, we carried out XRR and PNR at Los Alamos Neutron Science Center (LANSCE). XRR provides a quantitative measure of the chemical depth profile. PNR provides chemical and magnetic depth profiles. Both techniques are nondestructive and provide depth profiles with nanometer resolution.25−27 We used a linear position sensitive detector (PSD) to simultaneously measure the specular (angle of incidence, θi = angle of reflection, θf) and off-specular (angle of incidence ≠ angle of reflection) XRR,27,29 over a large range of wave vector transfer parallel, Qx [= 2π/λ (cos(θi) − cos(θf)), where λ is wavelength of X-ray] and perpendicular, Qz [= 2π/λ (sin(θi) + sin(θf))], to the sample’s surface. The specular reflectivity is qualitatively related to the square of the Fourier transform of the scattering length density (SLD) depth profile ρ(z) (normal to the film surface or along the zdirection).25,26 For XRR, ρx (z) is proportional to electron density,25,26 whereas for PNR, ρ(z) consists of nuclear and magnetic SLDs such that ρ±(z) = ρn(z) ± CM(z), where C = 2.9109 × 10−9 Å−2 m/kA, and M(z) is the magnetization (kA/ m) depth profile.25 The sign +(−) is determined by the condition when the neutron beam polarization is parallel (opposite) to the applied field and corresponds to reflectivities, R±(Q). The difference between R+(Q) and R−(Q) divided by the sum, called the spin asymmetry, asym = (R+(Q) − R−(Q))/ (R+(Q) + R−(Q)), can be a very sensitive measure of small M.

Figure 1. Matrix-assisted laser desorption ionization (MALDI) timeof-flight mass spectrometry data for binuclear (Co−Fe)Pc Film. Inset (i) show the representation of binuclear (Co−Fe)-phthalocyanine molecule, which makes a dimer of Co-phthalocyanine (CoPc) and Fephthalocyanine (FePc). Inset (ii) show the enlarged version of dimer peak at 1143 amu.

deeper insight into the interfacial structure and magnetic properties of the thin films.25−27 The results indicate formation of dimer and contraction of binuclear (Co−Fe)Pc molecular plane compared to the pure films and a concomitant appearance of magnetization at 10 K, which is lacking in the pure films. The binuclear (Co−Fe)Pc film also show the reduction of resistivity by about three to four order at low temperature as compared to pure films.

Figure 2. Reciprocal space map (Qx − Qz map) of X-ray reflectivity from FePc (a), CoPc(b), and binuclear (Co−Fe)Pc (c) thin film samples. Specular X-ray reflectivity (XRR) from FePc (d), CoPc (e), and binuclear (Co−Fe)Pc (f) films. Electron scattering length density (ESLD) depth profile for FePc (g), CoPc (h), and binuclear (Co−Fe)Pc (i) samples which gave best fit to XRR data. 4073

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The specular reflectivity data shown here were normalized to the Fresnel reflectivity (RF = 16π2/Q4).25

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RESULTS AND DISCUSSION Secondary ion mass spectrometry (SIMS) and matrix-assisted laser ionization and desorption (MALDI) time-of-flight mass spectrometry studies on these Pc films were performed to study the molecular formation.21 Figure 1 shows MALDI results from binuclear (Co−Fe)Pc film. We observed a monomer peak at 570 amu. The additional peak at higher molecular mass (1136 amu) was observed only in case of (Co−Fe)Pc films suggests the formation of binuclear (Co−Fe)Pc molecules. Inset (ii) of Figure 1 show the enlarge version of dimer peak at ∼1136 amu. Figure 2(a)−(c) shows XRR (Qx − Qz scattering map at small angle) corresponding to the FePc, CoPc, and binuclear (Co−Fe)Pc films, respectively. The X-ray intensity as a function of Qz at Qx = 0 (vertical dash line in Figure 2 (a)) is the specular XRR. The X-ray intensity as a function of Qx at fixed Qz (horizontal dash line in Figure 2(a)) is off-specular (diffuse) XRR.25−27,29 Figure 2(d)−(f) shows the specular XRR from FePc, CoPc and binuclear (Co−Fe)Pc films. The electron SLD was inferred from the XRR data by fitting a model ρ (z) whose reflectivity best fits the XRR data (solid line in Figure 2(d)−(f)). A model consisted of layers representing regions with different electron SLDs. The parameters of the model included layer thickness, interface (or surface) roughness and electron SLD. The reflectivity was calculated using the dynamical formalism of Parratt,30 and parameters of the model were adjusted to minimize the value of weighted measure of goodness of fit, χ2.31 Figure 2(g)−(i) shows the electron SLD depth profile of FePc, CoPc, and binuclear (Co− Fe)Pc samples. We obtained the thicknesses for FePc, CoPc, and binuclear (Co−Fe)Pc samples of 760 ± 6, 775 ± 8, and 785 ± 8 Å, respectively. Bragg peaks at Qz ≈ 0.45, 0.47, and 0.49 Å−1, which correspond to a bilayer thickness (∼2π/Qz) of ∼13.9, 13.3, and 12.9 Å for FePc, CoPc and binuclear (Co−Fe)Pc films, respectively are seen in the XRR data (Figure 2(d)−(f)). XRR data depicts that within a film the molecules are self-assembled in a multilayer (repetition of bilayer along the thickness) structure. The bilayer (thickness ≈ 14 Å) extracted from the analysis of the XRR data from these films corresponds to a metal phthalocyanine molecule (Figure 3(a)), which represents the organic (small SLD) and metallic (large SLD) parts of the molecule (Figure 3(b)). The bilayer (larger lattice spacing ≈ 14 Å) structure for all the films is aligned normal to the surface indicates that all the films are grown such that the shorter (lattice spacing ≈ 3.5 Å)19 molecular b axis is oriented parallel to the substrate plane, i.e., edge-on stacking, as shown schematically in Figure 3(a). This was further confirmed by XRD measurements on these films (discussed later). In this model for all the films, the metallic ion (Co2+ or Fe2+) is at the center of the molecule that forms a quasi one-dimensional (1D) chain when several molecular planes are stacked face to face, i.e., along shorter molecular b axis.19 Thus the one-dimensional (1D) chains are parallel to the substrate plane in all the films (Figure 3). The reduction in bilayer thickness of binuclear (Co−Fe)Pc films as compared to the pure films may suggest that either these organic molecules are grown at an angle from the surface normal (i.e., tilted with respect to the surface normal) or the molecular planes perpendicular to surface normal are compressed. Figure 3(c) is a schematic diagram of a molecular arrangement with small tilt for binuclear (Co−Fe)Pc

Figure 3. Representation of phthalocyanine (Pc) film grown on sapphire. Center part (red) is metal ion (Fe or Co) and trapezium (blue) shape is the organic part of Pc molecule (∼14 Å). Dash line with arrow represents the crystalline b axis of molecule along the in plane direction. (a) Represents the structure of FePc molecule on substrate. (b) Represents the electron scattering density (ESLD) depth profile for binuclear (Co−Fe)Pc film (shown in Figure 2(i)) suggesting that the metallic ion has relatively higher ESLD as compared to organic part. (c) Tilted phthalocyanine molecule on substrate as shown by CoPc and binuclear (Co−Fe)Pc film.

film which is consistent with the electron density depth profile inferred from the XRR data. The Pc molecules are highly correlated along the out-of plane direction which is evident from the presence of high Xray scattering intensity29 (Bragg sheet)32 as a function of Qx at fixed Qz (corresponds to Bragg peak) as shown in Figure 2 (a)−(c)(Qx − Qz scattering map at small angle). Formation of Bragg sheet clearly suggests high correlation of molecules (bilayer) along the thickness of the film over a length scale of the total thickness of the film (∼1000 Å).34 Figure 4(a) show the diffuse XRR data from binuclear (Co−Fe)Pc film as a

Figure 4. (a) Offspecular (diffuse) X-ray reflectivity (XRR) from binuclear (Co−Fe)Pc film near Bragg peak (Qz ≈ 0.48 Å−1). Inset shows the diffuse XRR from same sample at Qz ≈ 0.24 Å−1. Parts (b) and (c) show the GIXRD data from thick (∼800 Å) and thin (∼200 Å) binuclear (Co−Fe)Pc films, respectively. (d) Magnetization vs field at 5 K from binuclear (CO−Fe)Pc film measured by SQUID magnetometry. 4074

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Figure 5. (a) Polarized neutron reflectivity measurements from the samples. Spin asymmetry [(R+ − R−)/(R+ + R−)] data from FePc (b), CoPc (c), and binuclear (Co−Fe)Pc (d) samples. Parts (e)−(g) show the nuclear scattering length density (SLD) depth profile from the different samples. Part (h) shows the corresponding magnetization depth profile of the samples.

Figure 6. (a) Nuclear scattering length density (NSLD) profile across a molecule of different phthalocyanine (Pc) films. (b) Spin asymmetry asym = [(R+ − R−)/(R+ + R−)] data from binuclear (Co−Fe)Pc sample. (c) Magnetization (M) depth profile across the dimer of binuclear (Co−Fe)Pc film which gave the best fit to the spin asymmetry data in (b). Assuming average magnetization (□) across the dimer (or whole film) of binuclear (Co− Fe)Pc film did not fit (□) to spin asymmetry data in (b). (d) Normalized resistivity [ρ(T)/ρ (300 K)] data from three films.

function of Qx around Bragg peak (Qz ≈ 0.48 Å−1). Inset of the Figure 4(a) show the off-specular (diffuse) XRR data from binuclear (Co−Fe)Pc film at Qz ≈ 0.24 Å−1 (half of the Bragg peak). In order to study the correlation of atoms in the plane of the film, we analyzed the diffuse XRR data using a self-affine fractal surface model.27,29 We obtained ξ (in-plane correlation length) of 0.30 ± 0.05 μm, for the interfaces of binuclear (Co− Fe)Pc film.27 Similarly an in-plane correlation length of 0.20 and 0.25 μm for pure FePc and CoPc films, respectively, was obtained from the analysis of diffuse XRR data from these films. The in-plane correlation length is the lateral dimensions of the sample over which atomic-scale morphology is correlated (i.e., in-plane domain size),27,29 suggesting the molecules are also highly correlated in a length scale of micrometer in the plane of the films. Figure 4(b),(c) show the comparison of grazing incidence Xray diffraction (GIXRD) pattern from thick (∼800 Å) and thin (∼200 Å) binuclear (Co−Fe)Pc films. Similar GIXRD patterns are observed for pure films. The peak at 2θ ≈ 6.9° corresponds to the (200) peak of the α phase19 with lattice spacing of 12.7 Å indicating that the molecular b axis is oriented parallel to the substrate plane as observed from the analysis of XRR data. The GIXRD data (Figure 4(c)) from thinner films do not show the low-angle peak, but instead show a prominent peak at 27.6°. Thus, the thinner films (∼200 Å) exhibits face-on stacking (i.e.,

stacking of molecules on the substrate such that the shorter molecular b axis is oriented perpendicular to the substrate plane),19,20 whereas thicker films (thickness ≈ 800 Å) exhibit an edge-on stacking (i.e., shorter molecular b axis oriented along the plane of the film (Figure 3)). This is attributed to the fact that the influence of the substrate is less pronounced for thick films. Different stacking of FePc molecules on different substrates was also observed previously.19 Further to study the magnetic properties of the films we performed macroscopic magnetic measurement using SQUID magnetometry at 5 K. Figure 4(d) shows the magnetization curve as a function of field at 5 K from binuclear (Co−Fe)Pc film. It is evident that binuclear (Co−Fe)Pc film show ferromagnetism at 5 K. However, we did not observed any magnetization hysteresis curve for pure FePc and CoPc sample at 5 K, because of large background contribution from substrate. The magnetization of such organic films with 1dimensional chain is very small19 and it is sometime difficult to measure using standard macroscopic magnetization techniques such as SQUID, because of higher diamagnetic/paramagnetic contribution of substrate and other sources of experimental errors.33 It is however noted that the advantage of determining the small values of magnetization for films using PNR over macroscopic magnetometry is that (1) PNR can discriminate the magnetization of the film from magnetic impurities that 4075

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for pure film) for this system. The reduction of bilayer thickness (due to either tilting or compression) may be responsible for strengthening of interion-chain-interaction (i.e., exchange). In addition stronger Fe−Co exchange interaction due to formation of the dimer structure in binuclear (Co− Fe)Pc film might stabilize the magnetization. Stronger Fe−Co exchange interaction as compared to Co−Co and Fe−Fe exchange interaction was also observed earlier in other systems.34,35

may be in the substrate or absorbed on to it, and (2) PNR provides a magnetization depth profile (averaged over the lateral dimensions of the film). To systematically investigate the depth profiling of magnetic properties of these metallic-organic films, PNR measurements were carried out at a sample temperature of 10 K in a magnetic field of 6 kOe after cooling the sample in a field of 6 kOe (FC) from 300 K. The magnetic field was applied parallel to the sample plane. Figure 5(a) shows the PNR measurements from these samples. Spin asymmetry [(R+ − R−)/(R+ + R−)] data from FePc, CoPc, and binuclear (Co−Fe)Pc samples are shown in Figure 5 parts (b), (c), and (d) respectively. PNR data were analyzed by constraining layer thicknesses and interface roughness to be within the 95% confidence limit, i.e., 2-σ error, established from the analysis of the XRR data.31 Figure 5(e)−(g) show the nuclear SLD depth profile for FePc, CoPc, and binuclear (Co−Fe)Pc samples, respectively, which gave best fit to PNR data [Figure 5(a−d)]. The magnetization depth profiles (solid line) of FePc, CoPc, and binuclear (Co−Fe)Pc films inferred from the PNR data [Figure 5(a)−(d)] are shown in Figure 5(h). It is evident from magnetization depth profile (Figure 5(h)) that magnetization is mainly concentrated along the in-plane Fe−Co chains (planes) with magnetization (averaged over lateral dimension of film) of 50 ± 15 kA/m for binuclear (Co−Fe)Pc film and negligible magnetization for pure films. Figure 6(a) shows the nuclear SLD depth profile across a Pc molecule of different films. It is evident that nuclear SLD near metallic ion (central part) of binuclear (Co−Fe)Pc molecule as compared to pure FePc and CoPc molecules is higher. Similar behavior is also seen in electron SLD profile [Figure 2(e)−(g)] obtained from specular XRR. Such an increase in scattering length density suggest a modification in structure of binuclear (Co−Fe)Pc film, which is consistent with formation of the FePc and CoPc dimer in the binuclear (Co−Fe)Pc film.20 Another important aspect of this study which is inherent advantage of PNR technique is depicted in Figure 6(b),(c), which show the variation of magnetization across a dimer in the binuclear (Co−Fe)Pc film along the thickness of the film. Assuming a uniform magnetization (□) curve, in Figure 6(c) across the dimer along the thickness of the film did not fit the asym (or PNR) data at 10 K as shown in Figure 6(b). Best fit to asym data (solid line in Figure 6(b)) reveal the actual magnetization depth profile (solid line Figure 6(c)) across dimer (along the thickness of the film), suggesting highest magnetization (∼50 kA/m) at transition metal and falls minimum in the organic part of Pc molecule. Figure 6(d) show the comparison of temperature dependent normalized resistivity (ρ(T)/ρ(300 K)) from three Pc films. It is interesting to note that compared to pure films, the binuclear (Co−Fe)Pc films show a reduction in resistivity by about three to four order in magnitude at low temperature, which may be due to ferromagnetism behavior of binuclear (Co−Fe)Pc film at low temperature. Our results indicate negligible magnetization in the FePc and CoPc pure films at 10 K after field cooling from 300 K in a magnetic field of 6 kOe. Absence of magnetization in FePc film at 10 K is consistent with the earlier results,19 which suggested nontraditional paramagnetic behavior of FePc film in the temperature range 5−25 K. In contrast, the binuclear (Co− Fe)Pc film, shows significant magnetization at 10 K. The significant magnetization at 10 K for binuclear (Co−Fe)Pc also indicate a considerable increase in transition temperature (∼5K

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CONCLUSIONS We observed magnetization at low temperature for a binuclear (Co−Fe)Pc film as compared to pure phthalocyanine films. Xray reflectivity measurements suggest the metal ion chains are parallel to the substrate and are highly correlated perpendicular to the sample plane. However, the binuclear (Co−Fe)Pc bilayers are compressed or tilted with respect to the surface normal. Our polarized neutron reflectivity analysis measurements indicate the absence of magnetization (≤5 ± 6 kA/m) in pure FePc and CoPc films at 10 K. Whereas the binuclear (Co−Fe)Pc film shows significant magnetization (along the inplane direction) of 50 ± 15 kA/m at 10 K after field cooling at 6 kOe. Magnetism of the binuclear (Co−Fe)Pc film may be associated with a reduced bilayer thickness and enhanced Fe− Co exchange interaction due to the formation of dimers as compared to the pure films. The enhanced Fe−Co exchange interaction may also resulted into an increase in transition temperature for binuclear (Co−Fe)Pc film as compared to pure film. The experiments revealed the improved transport and magnetic properties of the binuclear (Co−Fe)Pc film as compared to pure FePc and CoPc films, which are correlated to the possible changes in structures of these films. The importance of the detailed atomic and magnetic structures across the pure and binuclear (Co−Fe)Pc molecules should enable better theoretical modeling and manipulating the magnetic properties of self-assembled TMPc films in molecular electronics.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Work supported by the Office of Basic Energy Science, U.S. Department of Energy, BES-DMS funded by the Department of Energy’s Office of Basic Energy Science, DMR under Grant DE FG03-87ER-45332. Los Alamos National Laboratory is operated by Los Alamos National Security LLC under DOE Contract DE-AC52-06NA25396. This work was also supported by “DAE-SRC Outstanding Research Investigator Award” (2008/21/05-BRNS) granted to D.K.A.

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REFERENCES

(1) Tang, C. W.; VanSlyke, S. A. Organic Electroluminescent Diodes. Appl. Phys. Lett. 1987, 51, 913−915. (2) Garnier, F.; Hajlaoui, R.; Yassar, A.; Srivastava, P. All-Polymer Field-Effect Transistor Realized by Printing Techniques. Science 1994, 265, 1684−1686. (3) Scott, J. C. Is There an Immortal Memory? Science 2004, 304, 62−63.

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(4) Xue, J. G.; Rand, B. P.; Ushida, S.; Forrest, S. R. A Hybrid Planar−Mixed Molecular Heterojunction Photovoltaic Cell. Adv. Mater. 2005, 17, 66−71. (5) Service, R. F. Organic LEDs Look Forward to a Bright, White Future. Science 2005, 310, 1762−1763. (6) Chen, L.; Hu, Z. P.; Zhao, A. D.; Wang, B.; Luo, Y.; Yang, J. L.; Hou, J. G. Mechanism for Negative Differential Resistance in Molecular Electronic Devices: Local Orbital Symmetry Matching. Phys. Rev. Lett. 2007, 99, 146803−146806. (7) Fu, Y.-S.; Ji, S.-H.; Chen, X.; Ma, X.-C.; Wu, R.; Wang, C.-C.; Duan, W.-H.; Qiu, X.-H.; Sun, B.; Zhang, P.; Jia, J.-F.; Xue, Q.-K. Manipulating the Kondo Resonance through Quantum Size Effects. Phys. Rev. Lett. 2007, 99, 256601−256604. (8) Hu, Z.; Li, B.; Zhao, A.; Yang, J.; Hou, J. G. Electronic and Magnetic Properties of Metal Phthalocyanines on Au(111) Surface: A First-Principles Study. J. Phys. Chem. C 2008, 112, 13650−13655. (9) Wu, W.; Kerridge, A.; Harker, A. H.; Fisher, A. J. StructureDependent Exchange in the Organic Magnets Cu(II)Pc and Mn(II)Pc. Phys. Rev. B 2008, 77, 184403−184414. (10) Stepanow, S.; Miedema, P. S.; Mugarza, A.; Ceballos, G.; Moras, P.; Cezar, J. C.; Carbone, C.; de Groot, F. M. F.; Gambardella, P. Mixed-Valence Behavior and Strong Correlation Effects of Metal Phthalocyanines Adsorbed on Metals. Phys. Rev. B 2011, 83, 220401− 220404. (11) Gao, L.; Ji, W.; Hu, Y. B.; Cheng, Z. H.; Deng, Z. T.; Liu, Q.; Jiang, N.; Lin, X.; Guo, W.; Du, S. X.; Hofer, W. A.; Xie, X. C.; Gao, H.-J. Site-Specific Kondo Effect at Ambient Temperatures in IronBased Molecules. Phys. Rev. Lett. 2007, 99, 106402−106405. (12) Tsukahara, N.; Noto, K. ichi; Ohara, M.; Shiraki, S.; Takagi, N.; Takata, Y.; Miyawaki, J.; Taguchi, M.; Chainani, A.; Shin, S.; Kawai, M. Adsorption-Induced Switching of Magnetic Anisotropy in a Single Iron(II) Phthalocyanine Molecule on an Oxidized Cu(110) Surface. Phys. Rev. Lett. 2009, 102, 167203−167206. (13) Zhao, A.; Li, Q.; Chen, L.; Xiang, H.; Wang, W.; Pan, S.; Wang, B.; Xiao, X.; Yang, J.; Hou, J. G.; Zhu, Q. Controlling the Kondo Effect of an Adsorbed Magnetic Ion Through Its Chemical Bonding. Science 2005, 309, 1542−1544. (14) Li, Z.; Li, B.; Yang, J.; Hou, J. G. Single-Molecule Chemistry of Metal Phthalocyanine on Noble Metal Surfaces. Acc. Chem. Res. 2010, 43, 954−962. (15) Iacovita, C.; Rastei, M. V.; Heinrich, B. W.; Brumme, T.; Kortus, J.; Limot, L.; Bucher, J. P. Visualizing the Spin of Individual CobaltPhthalocyanine Molecules. Phys. Rev. Lett. 2008, 101, 116602−116605. (16) Brede, J.; Atodiresei, N.; Kuck, S.; Lazíc, P.; Caciuc, V.; Morikawa, Y.; Hoffmann, G.; Blugel, S.; Wiesendanger, R. Spin- and Energy-Dependent Tunneling through a Single Molecule with Intramolecular Spatial Resolution. Phys. Rev. Lett. 2010, 105, 047204−047207. (17) Chen, X.; Alouani, M. Effect of Metallic Surfaces on the Electronic Structure, Magnetism, and Transport Properties of Cophthalocyanine Molecules. Phys. Rev. B 2010, 82, 094443−094453. (18) Baran, J. D.; Larsson, J. A.; Woolley, R. A. J.; Cong, Y.; Moriarty, P. J.; Cafolla, A. A.; Schulte, K.; Dhanak, V. R. Theoretical and Experimental Comparison of SnPc, PbPc, and CoPc Adsorption on Ag(111). Phys. Rev. B 2010, 81, 075413−075424. (19) Gredig, T.; Colesniuc, C. N.; Crooker, S. A.; Schuller, I. K. Substrate-Controlled Ferromagnetism in Iron Phthalocyanine Films Due to One-Dimensional Iron Chains. Phys. Rev. B 2012, 86, 014409− 014414. (20) Kumar, A.; Singh, A.; Samanta, S.; Debnath, A. K.; Aswal, D. K.; Gupta, S. K. Metallic-like Conduction in Co-phthalocyanine/Fephthalocyanine Composite Films Grown on Sapphire Substrates. Appl. Phys. Lett. 2011, 99, 112102−3. (21) Kumar, A.; Debnath, A. K.; Samanta, S.; Singh, A.; Prasad, R.; Veerender, P.; Singh, S.; Basu, S.; Aswal, D. K.; Gupta, S. K. Enhanced Cl2 Response of Ultrathin Bi-nuclear (Cobalt−Iron) Phthalocyanine Films. Sens. Actuators B 2012, 171−172, 423−430. (22) Bogani, L.; Wernsorfer, W. Molecular Spintronics Using SingleMolecule Magnets. Nat. Mater. 2008, 7, 179−186.

(23) Rocha, A. R.; García-Suárez, V. M.; Bailey, S. W.; Lambert, C. J.; Ferrer, J.; Sanvito, S. Towards Molecular Spintronics. Nat. Mater. 2005, 4, 335−339. (24) Pramanik, S.; Stefanita, C.-G.; Patibandla, S.; Bandyopadhyay, S.; Garre, K.; Harth, N.; Cahay, M. Observation of Extremely Long Spin Relaxation Times in an Organic Nanowire Spin Valve. Nat. Nanotechnol. 2007, 2, 216−219. (25) Fitzsimmons, M. R.; Majkrzak, C. Modern Techniques for Characterizing Magnetic Materials; Springer: New York, 2005; Chap. 3; pp 107−155. (26) Singh, S.; Basu, S.; Gupta, M.; Majkrzak, C. F.; Kienzle, P. A. Growth Kinetics of Intermetallic Alloy Phase at the Interfaces of a Ni/ Al Multilayer Using Polarized Neutron and X-ray Reflectometry. Phys. Rev. B 2010, 81, 235413−235419. (27) Sinha, S. K.; Sirota, E. B.; Garoff, S.; Stanley, H. B. X-ray and Neutron Scattering From Rough Surfaces. Phys. Rev. B 1988, 38, 2297−2311. (28) Debnath, A. K.; Samanta, S.; Singh, A.; Aswal, D. K.; Gupta, S. K.; Yakhmi, J. V. Chem. Phys. Lett. 2009, 480, 185. (29) Holý, V.; Kuběna, J.; Ohlidal, I.; Lischka, K.; Plotz, W. X-ray Reflection From Rough Layered Systems. Phys. Rev. B 1993, 47, 15896−15903. (30) Parratt, L. G. Surface Studies of Solids by Total Reflection of Xrays. Phys. Rev. 1954, 95, 359−369. (31) Press, W. H.; Flannery, B. P.; Teukolsky, S. A.; Vetterling, W. T. Numerical Recipes in Fortran: The Art of Scientific Computation, 2nd ed.; Cambridge University Press: Cambridge, 1992. (32) Kaganer, V. M.; Stepanov, S. A.; Kohler, R. Bragg Diffraction Peaks in X-ray Diffuse Scattering From Multilayers with Rough Interfaces. Phys. Rev. B 1995, 52, 16369−16372. (33) Garcia, M. A.; Pinel, E. F.; Venta, J.; de la Quesada, A.; Bouzas, V.; Fernández, J. F.; Romero, J. J.; Martín González, M. S.; CostaKrämer, J. L. Sources of Experimental Errors in the Observation of Nanoscale Magnetism. J. Appl. Phys. 2009, 105, 013925−7. (34) Liu, X. B.; Altounian, Z.; Yue, M. Effect of Fe Partial Substitution for Co on the Magnetic Properties of From FirstPrinciples. J. Appl. Phys. 2010, 107, 09A718−3. (35) Liu, X. B.; Altounian, Z. The Mixing of Fe/Co and its Effect on the Exchange Interaction in Nanocomposites: A First-Principles Study. J. Appl. Phys. 2012, 111, 07B526−3.

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dx.doi.org/10.1021/jp408847z | J. Phys. Chem. C 2014, 118, 4072−4077