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Possible Room-Temperature Ferromagnetism in Self-Assembled Ensembles of Paramagnetic and Diamagnetic Molecular Semiconductors Barun Dhara, Kartick Tarafder, Plawan Kumar Jha, Soumendra Nath Panja, Sunil Nair, Peter M. Oppeneer, and Nirmalya Ballav J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b02063 • Publication Date (Web): 18 Nov 2016 Downloaded from http://pubs.acs.org on November 19, 2016

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The Journal of Physical Chemistry Letters

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 AUTHOR INFORMATION Corresponding Author *[email protected]

ACS Paragon Plus Environment

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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 molecule-based 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 Znphthalocyanine 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 quantumchemical calculations we reveal the origin of such unusual ferromagnetic exchange-interaction in the supramolecular FePc---ZnFPc system.

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In quest of combining semiconductivity and ferromagnetism together in one platform for spintronic applications1, 2, two types of generic materials evolved, inorganic and organic/molecule-based systems. In the former type, doping of magnetic impurities like spinbearing transition metal ions in classical III-V and II-VI semiconductors was performed to induce ferromagnetic ordering at low-temperatures3, 4. Such impurity-doping was subsequently extended to high-band gap semiconductors as well as oxide insulators to achieve Curie temperature (Tc) close to room-temperature and beyond5. 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 material6, 7. However, the stability and thin-film growth of V(TCNE)x remained extremely delicate, primarily due to high air and solvent-sensitivity, and pyrophoric nature of the material. Among various organic/molecule-based semiconductor materials, porphyrins and phthalocyanines have recently emerged as suitable candidates for molecular-spintronic applications8, 9. Of particular interest is the onset of chemically-tunable spinterface at roomtemperature10, 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-memory-time (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


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present a new approach of supramolecular arrangement in obtaining self-assembled binaryensemble 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 as supramolecular magnetic semiconductor (SMS). In the solid-state, phthalocyanines are known to exhibit 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 inter-columnar packing; for example, distinctive molecular stacking in the α- and β-phase of phtahlocyanine are schematically shown in Figure 1b14. 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 process like 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 interface15. 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 coverage16. A well-ordered chessboard like pattern could only be achieved after introducing additional non-covalent hydrogenbonding (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 selfassembly in solution – so called supramolecular arrangement.


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

Solutions of equimolar concentration of commercially available FePc and ZnFPc were mixed together in ~1:1 ratio (vol/vol) 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 short-range structural order in the α/γphase phthalocyanine14. On the other hand, plate/block like crystals of FePc (Figure 2b) represent existence of long-range structural order in the β-phase, additionally stabilized by herringbone arrangement17. In general, morphological patterns of phthalocyanines are mainly dominated by π-π interaction tending to form extended plate-like structures along the stacking


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direction; and due to specific orientation of phthalocyanine molecules, polarities of various facets are different and even could be influenced by peripheral halogen substitutions14. Morphological patterns of the self-assembled FePc---ZnFPc (1) (Figure 2c) system are similar to those of FePc and can be assigned to a β/γ-phase material18. 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). Layeredstructure 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 see Supporting Information (SI) Figure S1-S2, Table 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 below) 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


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the Cambridge Crystallographic Data Centre (CCDC)18, 21-26 (see Figure S4). In case of the selfassembled FePc---ZnFPc (1) sample, an intensely sharp peak at 2θ ≈ 28.58o reflecting the F---π interaction within a distance of ~3.12 Å which in fact 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 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 results 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 interaction18.


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Figure 2. (a-c) Field-emission scanning electron microscopy (FESEM) images revealing crystallites of FePc (a), ZnFPc (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 selfassembled 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 self-assembled FePc---ZnFPc (1) (blue). (h) Powder X-ray diffraction (PXRD) patterns of FePc (green), ZnFPc (orange) and selfassembled 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 (N-FePc and N-FePc-N) characteristic of FePc are also detected in FePc---ZnFPc.

An extensive matrix-assisted laser desorption-ionization time-of-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 MALDITOF 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


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characteristic monomeric and dimeric peaks at m/z~571 and m/z~1142 were observed. However, in case of the FePc---ZnFPc (1) sample additional peaks of equal intensity at m/z~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 [N-MPc-N] moiety was almost negligible. Thus, FePc moieties in the self-assembled FePc---ZnFPc (1) ensemble are clearly in the arrangement likewise in the β-/γ-phase and favoring the ferromagnetic interaction among two nearest Fe ion via Fe---Nim---Fe 90o exchange path as per empirical GKA rules27.

Figure 3. (a) M-H plots of FePc (green; open-triangles up), ZnFPc (orange; open-triangles down), self-assembled FePc---ZnFPc (1) (red; open-circles) and a mechanical mixture FePc+ZnFPc (2) (blue; open-squares) recorded in superconducting quantum interference device


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(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 absence of reasonable ferromagnetic (FM) hysteresis loops (Hc

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