Possible Room-Temperature Ferromagnetism in ... - ACS Publications

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Possible Room-Temperature Ferromagnetism in Self-Assembled Ensembles of Paramagnetic and Diamagnetic Molecular Semiconductors Barun Dhara,§ Kartick Tarafder,‡ Plawan K. Jha,§ Soumendra N. Panja,# Sunil Nair,# Peter M. Oppeneer,⊥ and Nirmalya Ballav*,§ §

Department of Chemistry, Indian Institute of Science Education and Research (IISER), Pune 411008, India Department of Physics, National Institute of Technology Karnataka (NITK), Mangalore 575 025, India # Department of Physics, Indian Institute of Science Education and Research (IISER), Pune 411008, India ⊥ Department of Physics and Astronomy, Uppsala University, Box 516, S-751 20 Uppsala, Sweden ‡

S Supporting Information *

ABSTRACT: Owing to long spin-relaxation time and chemically customizable physical properties, molecule-based semiconductor materials like metal-phthalocyanines offer promising alternatives to conventional dilute magnetic semiconductors/oxides (DMSs/ DMOs) to achieve room-temperature (RT) ferromagnetism. However, air-stable moleculebased materials exhibiting both semiconductivity and magnetic-order at RT have so far remained elusive. We present here the concept of supramolecular arrangement to accomplish possibly RT ferromagnetism. Specifically, we observe a clear hysteresis-loop (Hc ≈ 120 Oe) at 300 K in the magnetization versus field (M−H) plot of the self-assembled ensembles of diamagnetic Zn-phthalocyanine having peripheral F atoms (ZnFPc; S = 0) and paramagnetic Fe-phthalocyanine having peripehral H atoms (FePc; S = 1). Tauc plot of the self-assembled FePc···ZnFPc ensembles showed an optical band gap of ∼1.05 eV and temperature-dependent current−voltage (I−V) studies suggest semiconducting characteristics in the material. Using DFT+U quantum-chemical calculations, we reveal the origin of such unusual ferromagnetic exchange-interaction in the supramolecular FePc···ZnFPc system.

I

Among various organic/molecule-based semiconductor materials, porphyrins and phthalocyanines have recently emerged as suitable candidates for molecular-spintronic applications.8,9 Of particular interest is the onset of chemically tunable spinterface at room-temperature.10,11 Furthermore, an antiferromagnetic ordering in thin-films and nanostructures of cobalt(II) phthalocyanine (CoPc; S = 1/2);12 and noticeably long population-relaxation-time (T1) as well as phase-memorytime (T2) in thin-films of diluted copper(II) phthalocyanine (CuPc; S = 1/2) 13 were observed above the boiling temperature of liquid nitrogen (∼100 K). Here, we present a new approach of supramolecular arrangement in obtaining selfassembled binary-ensemble of paramagnetic iron(II) phthalocyanine (FePc; S = 1) and diamagnetic zinc(II) phthalocyanine (ZnFPc, S = 0), bearing hydrogen (H) and fluorine (F) as respective peripheral atoms (see Figure 1a for molecular schemes of FePc and ZnFPc), possibly exhibiting ferromagnetism at room-temperaturethe first material of its kind which can be named a supramolecular magnetic semiconductor (SMS). In the solid-state, phthalocyanines are known to exhibit

n quest of combining semiconductivity and ferromagnetism together in one platform for spintronic applications,1,2 two types of generic materials evolved: inorganic and organic/ molecule-based systems. In the former type, doping of magnetic impurities like spin-bearing transition metal ions in classical III−V and II−VI semiconductors was performed to induce ferromagnetic ordering at low-temperatures.3,4 Such impuritydoping was subsequently extended to high-band gap semiconductors as well as oxide insulators to achieve Curie temperature (Tc) close to room-temperature and beyond.5 These materials are conventionally referred as dilute magnetic semiconductors (DMSs) and dilute magnetic oxides (DMOs). However, the difficulty in synthetic methodology together with a limitation in doping concentration (∼10% doping concentration giving rise to magnetization in the order of ∼10−5 emu) and chemical sensitivity make room-temperature operation of inorganic-based systems rather onerous. In the case of molecule-based systems, room-temperature (RT) ferromagnetism along with semiconducting property has not yet been observed. Vanadium-tetracyanoethylene (V(TCNE)x) is a notable exception of ferromagnetic half-semiconductor-like molecule-based material.6,7 However, the stability and thinfilm growth of V(TCNE)x remained extremely delicate, primarily due to high air and solvent-sensitivity, and pyrophoric nature of the material. © 2016 American Chemical Society

Received: September 9, 2016 Accepted: November 18, 2016 Published: November 18, 2016 4988

DOI: 10.1021/acs.jpclett.6b02063 J. Phys. Chem. Lett. 2016, 7, 4988−4995

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The Journal of Physical Chemistry Letters

physical vapor transport technique, sequential deposition of αphase CuPc and CuFPc ended-up with the formation of uniaxially commensurate p−n-type charge-transfer junction interface.15 Co-deposition in molecular beam epitaxy technique can provide single-phase thin-films from a binary combination of α-phase H2Pc and CuPc, though spatial arrangement is not unprecedented. In fact, co-deposition of manganese(II) phthalocyanine (MnPc) and FePc did reveal random mixing in the monolayer coverage.16 A well-ordered chess-board like pattern could only be achieved after introducing additional noncovalent hydrogen-bonding (H···F) interaction (MnPc and FeFPc).16 We wanted to bring this molecular registry feature in a single-phase binary ensemble of FePc and ZnFPc via equilibrium-driven self-assembly in solutionso-called supramolecular arrangement. Solutions of equimolar concentration of commercially available FePc and ZnFPc were mixed together in ∼1:1 ratio (v/v) at ambient conditions. Self-assembly lead to the precipitation of supramolecular binary ensemble, namely, FePc···ZnFPc (1) (small amounts of self-precipitated FePc and ZnFPc could be easily washed away upon rinsing with solvent). Morphological patterns as revealed by field-emission scanning electron microscopy (FESEM) images showed small crystals of ZnFPc (Figure 2a), which could be due to shortrange structural order in the α/γ-phase phthalocyanine.14 On the other hand, plate/block like crystals of FePc (Figure 2b) represent the existence of long-range structural order in the βphase, additionally stabilized by herringbone arrangement.17 In general, morphological patterns of phthalocyanines are mainly

Figure 1. (a) Molecular schemes of FePc and ZnFPc. (b) π−π stacking arrangements in α and β polymorphs of phthalocyanine.

polymorphism and an elegant example is the isolation of α-, β-, γ-, δ-, ε-, ζ- and π-phases of copper(II) phthalocyanine (CuPc). 14 The key features behind polymorphism in phthalocyanines are the molecular stacking in a columnar structure and the intercolumnar packing; for example, distinctive molecular stacking in the α- and β-phase of phtahlocyanine are schematically shown in Figure 1b.14 Thus, obtaining a single-phase material in-bulk or thin-films from a binary combination is not trivial unless the parent phthalocyanines are isomorphic. In non-equilibrium processes like the

Figure 2. (a−c) Field-emission scanning electron microscopy (FESEM) images revealing crystallites of ZnFPc (a), FePc (b) and self-assembled FePc···ZnFPc (1) (c). (d) A zoomed-in FESEM image of self-assembled FePc···ZnFPc (1). The Red scale-bar in each of the image represents a length of 500 nm (a−d). (e) Transmission electron microscopy (TEM) image of self-assembled FePc···ZnFPc (1) crystallite. The scale-bar represents a length of 100 nm. (f) Energy dispersive X-ray spectroscopy (EDXS) analysis on self-assembled FePc···ZnFPc (1) showing elemental ratio of Fe:Zn ≈ 1:1 (an average value of 16-points measured; red-spots). (g) UV−vis absorption spectra of FePc (green), ZnFPc (orange), and selfassembled FePc···ZnFPc (1) (blue). (h) Powder X-ray diffraction (PXRD) patterns of FePc (green), ZnFPc (orange) and self-assembled FePc··· ZnFPc (1) (blue) recorded at room-temperature. (i) Matrix-assisted laser desorption-ionization time-of-flight (MALDI-TOF) patterns of FePc (green), ZnFPc (orange) and self-assembled FePc···ZnFPc (1) (blue). Additional moieties (Nim−FePc and Nim−FePc−Nim) characteristic of FePc are also detected in FePc···ZnFPc. 4989


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Figure 3. (a) M−H plots of FePc (green; open-triangles up), ZnFPc (orange; open-triangles down), self-assembled FePc···ZnFPc (1) (red; opencircles) and a mechanical mixture FePc+ZnFPc (2) (blue; open-squares) recorded in superconducting quantum interference device (SQUID) at 300 K. (b) M−H plot of self-assembled FePc···ZnFPc (1) at 300 K in lower fields clearly showing the ferromagnetic (FM) hysteresis loop. (c) M−H plots of FePc and ZnFPc at 300 K in lower fields showing the paramagnetic and diamagnetic characteristics, respectively, and also the absence of reasonable ferromagnetic (FM) hysteresis loops (Hc < 20 Oe could fall within the sensitivity of SQUID). (d) M−H plot of a mechanical mixture FePc+ZnFPc (2) at 300 K in lower fields clearly revealing the absence of a reasonable ferromagnetic (FM) hysteresis loop.

dominated by π−π interaction tending to form extended platelike structures along the stacking direction; due to specific orientation of phthalocyanine molecules, polarities of various facets are different and even could be influenced by peripheral halogen substitutions.14 Morphological patterns of the selfassembled FePc···ZnFPc (1) (Figure 2c) system are similar to those of FePc and can be assigned to a β/γ-phase material.18 A zoomed-in FESEM image of the self-assembled FePc···ZnFPc (1) ensemble indicated the presence of layered-structure originating from the π−π stacking interaction of the phthalocyanine rings (Figure 2d). Layered-structure of the self-assembled FePc···ZnFPc (1) ensemble was further confirmed by the TEM analysis (Figure 2e). Thus, our approach of ‘supramolecular arrangement’ indeed resulted in the formation of a single-phase material from a binary combination of phthalocyanines. The presence of FePc and ZnFPc in the binary ensemble of self-assembled FePc···ZnFPc (1) was confirmed by various complementary spectroscopic techniques and their composition was estimated to be ∼1:1 from a meticulous energy dispersive X-ray spectroscopy (EDXS) analysis (Figure 2f and Supporting Information (SI) Figures S1−S2, Tables S1−S2) as well as UV−vis absorption studies (Figure 2g). Interestingly, we did not observe any charge-transfer interaction between FePc and ZnFPc as was revealed by the UV−vis absorption spectra both in-solution (Figure 2g and Figure S3) and in solid-state (discussed later), which could result in the formation of blended structure (see SI).19,20 At first glance, room-temperature PXRD patterns of FePc, ZnFPc, and self-assembled FePc···ZnFPc (1) suggest the existence of β-phase, γ-/α-phase, and β/γ-phase, respectively (Figure 2h). To elucidate the structural aspects of supramolecular self-assembled FePc···ZnFPc (1) ensemble, we have explored a number of complementary techniques due to lack of single-crystal data. In particular, we have correlated our PXRD

patterns with the simulated PXRD patterns obtained from the single-crystal data of relevant metal-phthalocynines reported earlier and available from the Cambridge Crystallographic Data Centre (CCDC)18,21−26 (see Figure S4). In the case of the selfassembled FePc···ZnFPc (1) sample, an intensely sharp peak at 2θ ≈ 28.58° reflecting the F···π interaction within a distance of ∼3.12 Å, which in fact is also present in the ZnFPc sample (2θ ≈ 28.86, ∼ 3.09 Å) and perhaps enabled FePc (2θ ≈ 28.06, ∼ 3.18 Å) to commensurate in the course of their self-assembly. So as to say, an intermediate distance corroborated the F···π interaction pathways stabilizing molecular chains of FePc and ZnFPc in the self-assembled FePc···ZnFPc (1) ensemble. Our attribution of the characteristic PXRD peak as a result of the F···π interaction is based on the reported single crystal data on CuFPc system, where an average distance of ∼3.00 Å between F atoms (benzene ring) and C atoms (benzene ring) of adjacent phthalocyanine moieties in a π-stack was observed in a γ-phase structure and assigned to the F···π interaction.18 An extensive matrix-assisted laser desorption-ionization timeof-flight (MALDI-TOF) analysis revealed characteristic monomeric, dimeric, and even trimeric m/z signatures of FePc and ZnFPc along with the [FePc···ZnFPc] conjugate moieties in the self-assembled FePc···ZnFPc (1) ensemble (Figure 2i and Figure S5). It is not only the combination of FePc and ZnFPc, the combinations of MnPc and CoFPc, and MnPc and ZnFPc consistently showed similar MALDI-TOF patterns (see Figure S6−S7), which is rather unexpected from a blended structure originating from charge-transfer interaction19,20 (m/z values will be dominated by the monomers and conjugate). Furthermore, ionization pattern of the self-assembled FePc··· ZnFPc (1) ensemble is appreciably different from that of pure FePc and ZnFPc, apart from the fact that in both patterns characteristic monomeric and dimeric peaks at m/z ∼ 571 and m/z ∼ 1142 were observed. However, in the case of the FePc··· ZnFPc (1) sample, additional peaks of equal intensity at m/z ∼ 4990


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Figure 4. (a) χ−T plot of FePc at 100 Oe (green; open-triangles up). Inset: Curie−Weiss parameters. (b) χ−T plot of self-assembled FePc···ZnFPc (1) at 1000 Oe (red; open-circles). Inset: Curie−Weiss parameters. (c) χ−T plot of self-assembled FePc···ZnFPc (1) at 100 Oe (red; open-circles). Inset: Curie−Weiss parameters. (d) Inverse of magnetic susceptibility versus temperature plots of self-assembled FePc···ZnFPc (1) at various fields (1000 Oe, red, open-circles; 100 Oe, red, open-stars; 10 Oe, red, open-diamonds) and FePc (100 Oe, green, open-triangles up).

‘supramolecular arrangement’ between FePc and ZnFPc molecules is in the heart of the observed real-phenomenon, and we anticipate that the concept can be explored to engineer electro-magnetic ordering in various other binary combinations of phthalocyanines as well as porphyrins. Note that porphyrins and phthalocyanines are cheaply abundant materials playing an inevitable role in photosynthesis, oxygen-transport in blood, and also serving as pigments and dyes in relevant industries.29−31 Temperature dependence of the direct current (DC) magnetic susceptibility (χ−T) of FePc and self-assembled FePc···ZnFPc (1) was investigated in the range 100−300 K, and the results are presented in Figure 4. In the case of the FePc (Figure 4a), data points in the χ−T plot recorded at 100 C Oe field can be well-fitted with Curie−Weiss law: χ = (T − θ)

570 + 14 and m/z ∼ 570 + 28 clearly indicated coordination of Nim ligands to the Fe(II) ion in the π−π stack of FePc. On the contrary, such coordination of Nim ligands to Zn(II) ions was noted to be much less pronounced for pure ZnFPc; specifically the peak corresponding to [Nim−MPc−Nim] moiety was almost negligible. Thus, FePc moieties in the self-assembled FePc··· ZnFPc (1) ensemble are clearly in the same arrangement as the β-/γ-phase and favoring the ferromagnetic interaction among the two nearest Fe ion via Fe···Nim···Fe 90° exchange path as per empirical GKA rules.27 Room-temperature magnetization versus field (M−H) plots recorded in SQUID confirmed that FePc and ZnFPc are paramagnetic and diamagnetic species, respectively (Figure 3a). Also, a mechanical mixture FePc+ZnFPc (2) was observed to be paramagnetic. The self-assembled FePc···ZnFPc (1) ensemble apparently appeared to be also paramagnetic in high-fields; however, at low-fields, a reproducibly observed hysteresis-loop suggests the presence of ferromagnetic ordered regions at room-temperature (Figure 3b). For clarity, roomtemperature M−H plots of FePc, ZnFPc and mechanical mixture FePc+ZnFPc (2) at low-field (likewise self-assembled FePc···ZnFPc (1)) are also presented (Figure 3c,d). The coercive magnetic field (Hc ∼ 120 Oe; ± 10 Oe across samples) and the saturation magnetization (Ms ∼ 0.002 emu/g) values of molecule-based magnetic materials like self-assembled FePc··· ZnFPc (1) are comparatively better than those observed earlier for inorganic-based DMSs. Notably, our observation on the hysteresis-loop for self-assembled FePc···ZnFPc (1) is not an impurity-driven phenomenon. Earlier, many reports showed loops in the M−H plots of inorganic oxides/semiconductors and were assigned due to undesirable magnetic impurity, for example, Fe.28 The absence of a reasonable hysteresis-loop in the M−H plot of mechanical mixture FePc+ZnFPc (2) strongly overruled such a possibility, i.e., even if we deliberately add high-amount of Fe-impurity as FePc molecules to ZnFPc at matching mole ratio, RT ferromagnetism was never achieved in the 1:1 mechanical mixture FePc+ZnFPc (2). Therefore,

where C is the Curie constant (= 1.44 emu·Oe−1·f.u.−1·K) and θ is Weiss constant (= −20 K). The large negative value of θ and the positive value of C clearly indicate an antiferromagnetic (AFM) correlation in the paramagnetic phase of FePc, and our data are in good agreement with the previously reported magnetic data on β-FePc.32,33 From the χ−T plot we get an effective magnetic moment of ∼3.5 μB at 300 K employing the equation: μeff = 2.828 χT BM. Since there are two molecules in the formula unit (f.u.) of β-FePc, an effective magnetic moment of ∼1.75 μB can be assigned to single FePc molecule and thereby suggesting the spin-state of S = 1.33 Similar to FePc, the magnetic susceptibility data of self-assembled FePc··· ZnFPc (1) (Figure 4b) also obeyed Curie−Weiss law (however, at higher-field of 1000 Oe) with C = 0.24 emu· Oe−1·f.u.−1·K and θ = −20 K − clearly an AFM interaction among FePc moieties (ZnFPc is diamagnetic). An effective magnetic moment of ∼1.4 μB was estimated for the FePc molecule in the self-assembled FePc···ZnFPc (1) ensembles (the formula unit consists of one FePc molecule and one ZnFPc molecule). 4991


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Figure 5. (a) Coercive field (Hc) versus temperature plot obtained from M−H plots recorded at different temperatures from 300 K down to 100 K. Inset: zoomed-in section of the M−H plots shown in Figure S9. (b) Zoomed-in M−H plots recorded at 250 K in field-cooled (5000 Oe, red solid circles; 10000 Oe, blue solid up-triangles; and 30000 Oe, green solid down-triangles) modes (zoomed-out M−H plots are provided in Figure S10) and for comparison zero-field cooled data (orange solid-squares) is provided. (c) Calculated magnetization density for the FePc in β-phase (sideview; stacking angle ϕ of ∼51.3°). Yellow hypersurfaces depict positive spin density (FM interaction), dominantly on the Fe ions, and cyan hypersurfaces negative spin density (AFM interaction), mainly on the N atoms. (d) Calculated magnetization density of neighboring FePc and ZnFPc stacks in supramolecular self-assembled FePc···ZnFPc (1) binary ensemble (top-view). The negative (cyan) spin density on the ZnFPc molecules forms an exchange path to stabilize FM coupling between neighboring FePc stacks, where each individual FePc stack orders ferromagnetically due to indirect Fe−Nim−Fe exchange. Estimated distance between two nearest neighbor (NN) atoms (Fe···Fe and Fe···Zn) and stacking angle (ϕ) in the FePc and ZnFPc chains are provided. Red spheres depict Fe, blue N atoms, light brown Zn, gray C, orange H, and pink F atoms.

Interestingly, the χ−T plot of self-assembled FePc···ZnFPc (1) in the temperature range of 100−300 K and recorded at 100 Oe field appeared to be complicated (Figure 4c) and we had to fit the data points by a modified Curie−Weiss law C χ = χ0 + (T − θ) where χ0 (= 0.0004 emu·Oe−1·f.u.−1) is a

Typically, in bulk (and thin-films), FePc was realized to be paramagnetic at RT, and long-range magnetic order was achieved only at very low-temperatures (below 10 K).33,35,36 Although, in a self-assembled FePc···ZnFPc (1) system, the structural stability given to FePc upon 1:1 supramolecular arrangement with ZnFPc, the long-range ferromagnetic order at room-temperature, i.e., finite hysteresis-loop in the M−H plot at 300 K is apparently counterintuitive with the Curie−Weiss behavior in the temperature dependence of DC susceptibility measurements, i.e., the χ−T plot. So, we have recorded M−H plots at various temperatures starting from 300 K down to 100 K and consistently, finite hysteresis-loops were observed. A plot of coercive field versus temperature (Hc−T) is provided in Figure 5a (see also Figure S9 for M−H plots). A gradual decrease in the Hc value upon decreasing the temperature is unusual and could possibly arise from magneto-crystalline anisotropy and/or magnetoelectric effect.37−39 Furthermore, we have noted the existence of an exchange-bias (EB) of ca. 10 Oe in the self-assembled FePc···ZnFPc (1) sample at 300 K (and consistently down to 150 K) which is a clear signature of the presence of ferromagnetic (FM)/antiferromagnetic (AFM) interface. The exchange strength of FM/AFM interface is of crucial importance at the EB phenomenon where Curie temperature of the FM part was usually observed to be much higher than Néel temperature of the AFM counterpart.40 Interestingly, in our supramolecular FePc···ZnFPc (1) system no EB was observed at ∼100 K, and it is worthwhile to mention that thin-films (and powders) of CoPc (S = 1/2) exhibited strong antiferromagnetic (AFM) coupling at ∼100 K.12 To strengthen our observation on the EB, we have applied different magnetic fields during cooling down the FePc···ZnFPc (1) sample from 300 to 250 K, switched-off the field, subsequently recorded the M−H plots at 250 K (so-called field-cooled

temperature independent offset.12 The positive values of θ (= 20 K) and C (= 0.15 emu·Oe−1·f.u.−1·K) clearly indicate the presence of ferromagnetic (FM) correlations. If we assume paramagnetic Fe(II) ions with S = 1 is contributing to the ferromagnetism (since Zn(II) is diamagnetic) and neglect the interchain interaction between FePc and ZnFPc, then the strength of ferromagnetic interaction along FePc chain can be evaluated using mean-field approximation of the Weiss constant as follows: θ = 2zJS (S+1)/3kB.34 In the temperature range of 100−300 K, one can propose the existence of weakly coupled FM chains of FePc (J/kB ∼ 7.5 K for z = 2 and S = 1) in the self-assembled FePc···ZnFPc (1) ensemble. To support the FM correlation in the self-assembled FePc···ZnFPc (1) system, we have further carried out χ−T measurement at even lower-field of 10 Oe. All the field-dependent inverse magnetization (χ−1) plots are summarized in Figure 4d. Overall, field-dependent magnetic susceptibility data in Figure 4d (deviating strongly from the Curie−Weiss law) are corroborating the observed FM hysteresis at room-temperature (Figure 3). Notably, the χ−T plot of the mechanical mixture FePc+ZnFPc (2) recorded at 100 Oe field could be well-fitted by Curie−Weiss law with C = 0.79 emu·Oe−1·f.u.−1·K and θ = −30 K (Figure S8a); thereby no signature of FM correlation was observed in the material. Also, the χ−T plot of ZnFPc recorded at 100 Oe confirmed diamagnetic characteristic (Figure S8b) and excluded the possibility of any paramagnetic and/or ferromagnetic impurity in the material. 4992


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The Journal of Physical Chemistry Letters mode), and compared the data with the zero field-cooled data taken at 250 K (Figure 5b for zoomed-in M−H plots and see also M−H plots in Figure S10). At the 30000 Oe field-cooled M-H data, the EB was found to be lifted-off at 250 K. Thus, overall magnetic measurements (i) do support that at the origin of hysteresis there is RT ferromagnetism in the self-assembled FePc···ZnFPc (1) system, and (ii) stimulate further investigations of such supramolecular magnetic systems as “multiferroic”/“magnetoelectric” materials.41 Finally, to understand the origin of the possible FM order, we performed electronic structure calculations for the self-assembled FePc···ZnFPc (1) system on the basis of the density functional theory with additional on-site Coulomb U interactions added (DFT+U). The latter are required to capture the strong d−d correlations in the open 3d shell of FePc (for details, see SI). The structure of the self-assembled FePc···ZnFPc (1) system has been fully optimized through self-consistent relaxation of all forces in the simulation cell. We started from an initially assumed β-structure after which all atomic positions were allowed to relax. The ab initio calculations predict ferromagnetic order (at T = 0 K) as the ground state, with an on-site spin moment of ∼2.0 μB on the Fe ions (i.e., S = 1) and 0.00 μB on the Zn ions (i.e., S = 0), respectively. In addition, small magnetic moments (ca. −0.02 μB) are computed for the nitrogen and carbon atoms of the phthalocyanine macrocycle of FePc. The computed spin density is shown in Figure 5c,d. Note that for β-FePc, the total magnetic moment on single FePc is ∼1.8 μB; however, on the Fe atom the magnetic moment is ∼2.0 μB. Also, for the β-FePc···ZnPc system, the total magnetic moments on the FePc and ZnFPc molecules are ∼1.8 μB and 0.0 μB, respectively, while the magnetic moment on the Fe atom is ∼2.1 μB. Such values on the magnetic moments realized from the DFT+U calculations are in excellent agreement with the experimentally measured values (Figure 4). There are two exchange couplings that contribute to the overall FM order. First, the Fe−Fe distance of two nearest-neighbor FePc molecules is ∼5.41 Å in vertical stacking, with a stacking angle of ∼51.3° (consistent with, e.g., reported values for βCoPc12). A strong exchange coupling between the FePc molecules along their stacking axis is mediated by the Nim ligands, i.e., a FM coupling of two Fe ions is induced by a double Fe−Nim−Fe indirect exchange interaction (see the positive and negative spin densities on the Fe ions and nearest N atoms, respectively, in Figure 5c). Second, there is a FM exchange coupling between two FePc molecules in neighboring stacks that is mediated by the ZnFPc molecules separating the two stacks. Figure 5d shows a magnified view of the calculated spin density for the lateral arrangement of the molecules. Although the central Zn ion carries no spin density and the Zn−Fe ion separation is ∼10.46 Å, there exists nonetheless a spin density on the phthalocyanine ring of ZnFPc, which creates a lateral exchange path between two magnetic FePc centers, where the exchange between neighboring FePc stacks is conveyed by the π electrons of ZnFPc. Hence, the emerging overall exchange coupling between Fe(II) centers is FM, and supramolecular arrangement of FePc and ZnFPc molecules is perhaps the key for bringing the long-range magnetic order in the selfassembled FePc···ZnFPc (1) ensembles. Current−voltage (I−V) characteristics of both FePc and ZnFPc suggested Ohmic-type electrical conduction with conductivity value in the order of ∼10−9 S/cm and ∼10−10 S/cm (I−V plots not shown), similar to other metal-

phthalocyanines (MPcs) and undoped organic conducting polymers like polyaniline.24,42 In the case of self-assembled FePc···ZnFPc (1), the conductivity value was increased by a factor of ∼10 when compared with that of a mechanical mixture FePc+ZnFPc (2). From the temperature-dependent I−V plots (Figure 6a), we confirmed the semiconductivity characteristic

Figure 6. (a) Temperature dependent current−voltage (I−V) characteristics of self-assembled FePc···ZnFPc (1) with estimated activation energy of ∼0.4 eV (inset). Different symbols represent various temperatures. (b) Tauc plots of FePc (green; open-triangles up), ZnFPc (orange; open-triangles down), and self-assembled FePc··· ZnFPc (1) (red; open-circles). The optical band gap values were obtained from the intercept of the green, orange, and red dotted lines with the X-axis.

of the self-assembled FePc···ZnFPc (1) material (with increasing temperature conductivity value was increasing). The conductivity value of the material increased up to 100 times at 90 °C and an activation energy (Ea) value of ∼0.4 eV was estimated. We have further measured solid-state UV−vis absorption spectra of FePc, ZnFPc and self-assembled FePc··· ZnFPc (1) and presented the data as Tauc plots where socalled Soret-band and Q-band maxima are clearly visible at ∼3.4 eV and ∼1.8 eV, respectively (Figure 6b).43 In all three materials, a similar band gap was realized (FePc ∼ 1.2 eV; ZnFPc ∼ 1.3 eV and self-assembled FePc···ZnFPc (1) ∼ 1.05 eV) which is in good agreement with the reported theoretical values of various metal-phthalocyaninesthe energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbitals (LUMO).44 4993


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(4) Dietl, T. A Ten-Year Perspective on Dilute Magnetic Semiconductors and Oxides. Nat. Mater. 2010, 9, 965−974. (5) Matsumoto, Y.; Murakami, M.; Shono, T.; Hasegawa, T.; Fukumura, T.; Kawasaki, M.; Ahmet, P.; Chikyow, T.; Koshihara, S.-y.; Koinuma, H. Room-Temperature Ferromagnetism in Transparent Transition Metal-Doped Titanium Dioxide. Science 2001, 291, 854− 856. (6) Manriquez, J. M.; Yee, G. T.; Mclean, S. R.; Epstein, A. J.; Miller, J. S. A Room-Temperature Molecular/Organic-Based Magnet. Science 1991, 252, 1415−1417. (7) Prigodin, V. N.; Raju, N. P.; Pokhodnya, K. I.; Miller, J. S.; Epstein, A. J. Spin-Driven Resistance in Organic-Based Magnetic Semiconductor V[TCNE]X. Adv. Mater. 2002, 14, 1230−1233. (8) Dediu, V. A.; Hueso, L. E.; Bergenti, I.; Taliani, C. Spin Routes in Organic Semiconductors. Nat. Mater. 2009, 8, 707−716. (9) Ballav, N.; Wäckerlin, C.; Siewert, D.; Oppeneer, P. M.; Jung, T. A. Emergence of on-Surface Magnetochemistry. J. Phys. Chem. Lett. 2013, 4, 2303−2311. (10) Sanvito, S. Molecular Spintronics: The Rise of Spinterface Science. Nat. Phys. 2010, 6, 562−564. (11) Wäckerlin, C.; Chylarecka, D.; Kleibert, A.; Müller, K.; Iacovita, C.; Nolting, F.; Jung, T. A.; Ballav, N. Controlling Spins in Adsorbed Molecules by a Chemical Switch. Nat. Commun. 2010, 1, 61. (12) Serri, M.; Wu, W.; Fleet, L. R.; Harrison, N. M.; Hirjibehedin, C. F.; Kay, C. W. M.; Fisher, A. J.; Aeppli, G.; Heutz, S. HighTemperature Antiferromagnetism in Molecular Semiconductor Thin Films and Nanostructures. Nat. Commun. 2014, 5, 3079. (13) Warner, M.; Din, S.; Tupitsyn, I. S.; Morley, G. W.; Stoneham, A. M.; Gardener, J. A.; Wu, Z.; Fisher, A. J.; Heutz, S.; Kay, C. W. M.; Aeppli, G. Potential for Spin-Based Information Processing in a ThinFilm Molecular Semiconductor. Nature 2013, 503, 504−508. (14) Erk, P.; Hengelsberg, H. The Porphyrin Handbook; Academic Press: Amsterdam, 2003. (15) Zhang, Y.; Dong, H.; Tang, Q.; Ferdous, S.; Liu, F.; Mannsfeld, S. C. B.; Hu, W.; Briseno, A. L. Organic Single-Crystalline P−N Junction Nanoribbons. J. Am. Chem. Soc. 2010, 132, 11580−11584. (16) Wäckerlin, C.; Nowakowski, J.; Liu, S.-X.; Jaggi, M.; Siewert, D.; Girovsky, J.; Shchyrba, A.; Hählen, T.; Kleibert, A.; Oppeneer, P. M.; Nolting, F.; Decurtins, S.; Jung, T. A.; Ballav, N. Two-Dimensional Supramolecular Electron Spin Arrays. Adv. Mater. 2013, 25, 2403− 2403. (17) Anthopoulos, T. D.; Shafai, T. S. Alternating Current Conduction Properties of Thermally Evaporated A-Nickel Phthalocyanine Thin Films: Effects of Oxygen Doping and Thermal Annealing. J. Appl. Phys. 2003, 94, 2426−2433. (18) Yoon, S. M.; Song, H. J.; Hwang, I.-C.; Kim, K. S.; Choi, H. C. Single Crystal Structure of Copper Hexadecafluorophthalocyanine (F16CuPc) Ribbon. Chem. Commun. 2010, 46, 231−233. (19) Lindner, S.; Knupfer, M.; Friedrich, R.; Hahn, T.; Kortus, J. Hybrid States and Charge Transfer at a Phthalocyanine Heterojunction: MnPcδ+ /F16CoPcδ‑. Phys. Rev. Lett. 2012, 109, 027601. (20) Lindner, S.; Mahns, B.; Konig, A.; Roth, F.; Knupfer, M.; Friedrich, R.; Hahn, T.; Kortus, J. Phthalocyanine Dimers in a Blend: Spectroscopic and Theoretical Studies of MnPcδ+/F16CoPcδ‑. J. Chem. Phys. 2013, 138, 024707. (21) Wang, C.; Fang, Y.; Wen, L.; Zhou, M.; Xu, Y.; Zhao, H.; De Cola, L.; Hu, W.; Lei, Y. Vectorial Diffusion for Facile SolutionProcessed Self-Assembly of Insoluble Semiconductors: A Case Study on Metal Phthalocyanines. Chem. - Eur. J. 2014, 20, 10990−5. (22) Robertson, J. M.; Woodward, I. An X-Ray Study of the Phthalocyanines. Part III. Quantitative Structure Determination of Nickel Phthalocyanine. J. Chem. Soc. 1937, 219−230. (23) Brown, C. J. Crystal Structure of β-Copper Phthalocyanine. J. Chem. Soc. A 1968, 2488−2493. (24) Schramm, C. J.; Scaringe, R. P.; Stojakovic, D. R.; Hoffman, B. M.; Ibers, J. A.; Marks, T. J. Chemical, Spectral, Structural, and Charge Transport Properties of the ″Molecular Metals″ Produced by Iodination of Nickel Phthalocyanine. J. Am. Chem. Soc. 1980, 102, 6702−6713.

Maybe such lowering of the band gap in the self-assembled FePc···ZnFPc (1) ensembles was responsible for manifesting relatively higher electrical conductivity than FePc and ZnFPc parents. Notably, MPcs are in general known to possess excellent semiconducting properties with mobility values in the range of 10−5−10−2 cm2 V−1 s−1 depending on the metal ion, peripheral substitution as well as the experimental conditions, and are suitable for fabricating air-stable thin-film transistors.15,45−48 Specifically, mobility values of FePc and ZnFPc could be tuned from 10−5 to 10−3 in cm2 V−1 s−1 in thin-film configurations upon simply varying the support-substrate temperature.46,47 In summary, we have presented a new approach of supramolecular arrangement to reach room-temperature ferromagnetism in molecular semiconductor. Specifically, selfassembly of FePc and ZnFPc molecules lead to the formation of air-stable binary-ensemble via noncovalent interactions. While respective molecular constituents FePc and ZnFPc are paramagnetic and diamagnetic, their supramolecular ∼1:1 combination was found to exhibit ferromagnetic interaction at T = 300 K. The origin of the ferromagnetic exchange can be understood with the help of DFT+U calculations. We anticipate our concept to be useful in the development of chemically customizable molecule-based magnetic semiconductor materials which are promising for molecular spintronic applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.6b02063. Experimental and computational details and data on EDXS, Raman, FTIR, UV−vis, MALDI-TOF, PXRD, M−H and χ−T plots (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Nirmalya Ballav: 0000-0002-7916-7334 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from IISER Pune, DST NanoMission (India; project number SR/NM/NS-42), DAE-BRNS (India; project number 2011/20/37C/17/BRNS), SERB (SB/FTP/PS-032/ 2014 and SB/S2/CMP-048/2013), the Swedish Research Council (VR), and Swedish National Infrastructure for Computing (SNIC) is thankfully acknowledged. B.D. and P.K.J. thank CSIR (India) and IISER Pune for Senior Research Fellowships. We thank Dr. Pramod Pillai (IISER Pune) for solid-state UV−vis data.

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REFERENCES

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