Molecular conducting magnetic heterostructures

Feng Hu , Yong Hu , Yulong Huang , Changning Li , Ruizhe Yang and Shenqiang Ren *
Department of Mechanical and Aerospace Engineering, Chemistry, Research and Education in Energy, Environment & Water (RENEW), University at Buffalo, State University of New York, Buffalo, New York 14260, USA. E-mail: shenren@buffalo.edu

Received 5th October 2019 , Accepted 26th December 2019

First published on 30th December 2019


Building molecular conducting magnets has been of considerable interest due to their metallic transport and magnetic properties. Conjugated aniline anions and transition-metal cations are recognized to synthesize such π–d interaction frameworks. Here, an interfacial assembly of quasi-two-dimensional aniline heterojunction frameworks is developed, which consists of a proton-aniline conducting layer (conductivity of 9.2 × 102 S cm−1) and a transition metal–aniline magnetic layer. The two-dimensional aniline coordination framework with pronounced π–d interactions is indispensable for obtaining ordered spin states and metallic transport conductivity. We apply optical and Raman spectroscopy to study the interactions between the metal cations and nitrogen atoms in the aniline chains. The bilayer heterostructure maintains a high conductivity of 3.5 × 102 S cm−1 and increased magnetization, while the coupling of metallic conducting and magnetic layers leads to an enhanced magnetoconductance of 0.4% under a magnetic field of 70 kOe.


Multifunctional materials with charge and spin degrees of freedom, such as molecular conducting magnets, are receiving significant interest as potentially transformative components in quantum technologies.1 In this context, the molecular conducting magnet is particularly attractive due to the coupling of molecular conductor and magnet, in which the magnetic interactions are triggered between the π conduction electrons and localized d electrons. The molecular conducting magnets prepared thus far are mainly radical salts between low-dimensional conjugated donor molecules and magnetic acceptor anions.2 Such molecular magnets are generally characterized as strongly correlated materials with π electron-dominant transport due to the small transfer integral from the neighboring π-electron orbitals in donor molecules.3 Such unique packing structures enable the formation of a direct d–d spin interaction of magnetic anions and indirect π–d interaction between the π-conjugated cation and d electrons for electron transport. Magnetic ordering in the anion layer can be obtained by the combined effect of d–d and π–d interactions.4 However, the π–d interaction could reduce electron mobility in the conjugated layer. In addition, antiferromagnetic ordering is usually expected from the d–d interaction in such π–d system, while ferromagnetic ordering of d electrons can result from the spin-polarized π electrons.

Supramolecular assembly emerges as an effective approach to design and synthesize molecular conducting magnets from molecular synthons of alternately stacked π-conjugated donor molecules and transition-metal d-spin anion layers. Here, we present a facile strategy to prepare freestanding molecular conducting magnets by self-assembly of a quasi-two-dimensional aniline conjugated cation layer and a transition-metal anion layer. Polyaniline (PANi), selected here as one of the extensively investigated polymer materials, exhibits metallic transport, inducible magnetic property, and remarkable environmental stability.5 PANi is considered as both Brønsted base and Lewis base, and it can be doped to significantly improve metallic transport or magnetic properties through coordination with a Brønsted acid (HCl) or Lewis acid (transition-metal halides) for the formation of conducting and magnetic building unit layers, respectively. The self-assembly-driven chain ordering of quasi-two-dimensional nanosheets greatly increases the effects of magnetic alignment. The proton-doped PANi has a significantly increased conductivity, while the iron cation-incorporated PANi shows ferromagnetic-like behavior. More importantly, the conducting magnetic bilayer heterostructure can maintain a high conductivity and enhanced magnetization due to the induced localized spin-ordered states at the interface. When a magnetic field is applied to the molecular conductor, the aligned d electrons induce ferromagnetic ordering, together with the disrupted π–d interaction for enhanced electron mobility, leading to the magneto-conductance effect.

The oleylamine molecule is a known surfactant for controlled assembly due to the strong binding effect between the amino function with the surface atoms and the hydrophobic interaction of hydrocarbon chain.6 Here, we report for the first time the utilization of self-assembled oleylamine monolayer as the substrate to induce the ordering of aniline chains at the liquid/air interface. Fig. 1a depicts the assembly process of freestanding quasi-two-dimensional PANi nanosheets at the dimethyl sulfoxide (DMSO)/air interface. An increased amount of oleylamine accelerates the assembly process in PANi formation (Fig. S1 and S2, ESI). The as-formed freestanding PANi nanosheets can be transferred to a transition-metal salt or acid solution for the formation of magnetic or conductive PANi networks binding with imine and amine nitrogen atoms, with the color changing at completion from purple to green. Such coordinated PANi networks can be further assembled layer-by-layer to form molecular conducting magnetic heterostructures. The optical photographs in Fig. 1b and c show the PANi solution and self-assembled PANi nanosheet at the air/liquid interface. The scanning electron microscopy (SEM) image (Fig. 1d) shows the uniform and flexible surface morphology of PANi nanosheets, while transmission electron microscopy (TEM, Fig. 1e and Fig. S3, ESI) suggests its layer-like morphology.


image file: c9tc05450b-f1.tif
Fig. 1 (a) An illustration of the synthetic procedure for the 2D PANi nanosheets in DMSO solution and the doping process, as well as the assembly of multifunctional conducting magnetic heterostructures. (b and c) Photographs of DMSO solution before and after PANi formation. (d) SEM and (e) TEM images of the PANi nanosheet.

The freestanding PANi nanosheets provide a versatile platform for the incorporation of transition-metal cations and protons to control their physicochemical properties. Fig. 2a shows the absorption spectra of freestanding PANi, HCl-doped PANi (HPANi), and transition-metal-doped PANi (MPANi) nanosheets. The absorption band at 312 nm of the self-assembled PANi nanosheet is assigned to the π–π* transition of the conjugated benzenoid rings.7 The π–π* transition band shows a hypsochromic shift from 312 nm to 299 nm after Fe cation doping, suggesting a decrease in the extent of conjugation and an increase in the band gap. It is inferred that the adjacent phenyl rings of aniline have larger torsion angles with respect to the plane of the nitrogen atoms due to the possible steric repulsion between the FeCl3 and hydrogen atoms on the adjacent phenyl rings.8 For the proton doping and other transition-metal ions (NiCl2 and CoCl2), the π–π* transition band shows negligible shift, indicating the low steric repulsion, possibly due to the weak binding interaction. The absorption band at 620 nm of the self-assembled PANi nanosheet corresponds to the exciton transition.9 The exciton transition band in transition-metal-doped PANi shows a blue shift to 560 nm after Fe cation doping. It is inferred that Fe3+ ions strongly interact with the PANi chain, increasing the degree of freedom and therefore the loss of planarity of the chain.8 For Co2+ doping, there are two new absorption bands located at 430 nm and 900 nm, corresponding to the polaron and bipolaron transition in the doped form,9 respectively, which are not observed in the Ni2+ doping. For the proton doping, the exciton transition band disappears, while the bipolaron transition band exhibits a bathochromic shift compared to the Co2+-doped PANi, indicating a conductive structure with a more delocalized bipolaron transition band.9,10 Raman spectroscopy was further used to investigate the self-assembled structural conformation and interaction between dopants and aniline matrix chains. As shown in Fig. 2b, the resonance Raman spectra of PANi, HPANi, and MPANi nanosheets are recorded using 785 nm excitation. The resonance bands that appeared in the range of 1160 cm−1 to 1600 cm−1 correspond to the stretching modes of different bonds in PANi.11 The Raman peak at 1600 cm−1 in PANi is assigned to the C–C stretching vibration of benzene rings. The bands at 1220 cm−1 and 1164 cm−1 are assigned to C–N stretching in benzenoid units and C–H deformation vibration of a quinonoid ring.12 To evidence the effect of doping on the structure of PANi, on the one hand, the band at 1510 cm−1 in HPANi corresponds to N–H deformation vibrations, which are not observed in MPANi due to the doping of transition-metal ions. On the other hand, the band at 1460 cm−1 corresponds to the C[double bond, length as m-dash]N stretching vibrations in quinonoid units in MPNAi; it is absent in HPANi, indicating more benzenoid units in the complex coordinated networks. As shown in Fig. 2c–e, the as-formed PANi is blue, which can be converted to green and purple by proton doping and Fe cation doping, respectively. The SEM image in Fig. 2f of Fe cation-doped PANi shows similar morphology to PANi. The EDS mapping (C and Fe elements) of Fe cation-doped PANi indicates the uniform doping of Fe3+ throughout the PANi network, while the HPANi nanosheet only shows the C and N elements (Fig. S4 and S5, ESI). No visible iron clusters or particles are observed in the TEM image (inset of Fig. 2f) of Fe-doped PANi.


image file: c9tc05450b-f2.tif
Fig. 2 (a) Absorption spectra of PANi, HCl-doped PANi (HPANi), and transition metal-doped PANi (MPANi). (b) Raman spectra of PANi, HPANi, and MPANi. (c–e) Photographs and the corresponding optical microscopy images (inset) of PANi, HPANi, and FePANi on glass substrates. (f) SEM and TEM (inset) images of FePANi.

Conducting HPANi and magnetic FePANi layers play an important role in building a molecular conducting magnet with π–d interactions. The acid treatment increases the electrical conductivity of PANi due to the proton-doping process.13 As shown in Fig. 3a, HPANi nanosheets show increased conductivity compared with the as-formed freestanding PANi nanosheet, which is transferred to the surface of 1 M HCl aqueous solution for 5 min, 30 min, and 120 min. The 5 min HPANi offers a high conductivity of 6.0 × 102 S cm−1, which can be further increased to 8.4 × 102 S cm−1 (30 min) and 9.2 × 102 S cm−1 (120 min), respectively (Fig. S7, ESI). It should be noted that PANi in the emeraldine salt form shows an electrical conductivity with a range of 10−3 to 102 S cm−1 and varying conjugation lengths, doping levels, and types of dopant.14 Notably, HPANi nanosheets prepared here through supramolecular assembly show a remarkably high conductivity benefiting from the compact ordering of the aniline chain. The resistivity–temperature (RT) curve of the HPANi layer in the inset of Fig. 3a shows a low resistivity of 0.02 Ω cm at 150 K. Fig. 3b shows the temperature-dependent magnetization hysteresis (MH) curves of the FePANi layer, which exhibits ferromagnetic characteristics with a saturation magnetization of ∼0.6 emu g−1. It is known that ferromagnetism requires not only magnetic moments (Fig. S6, ESI) but also that the moments remain mutually aligned,15 where the existence of spin alignment and d–d interaction between iron atoms is enhanced by the π–d interaction through the aniline chain. As shown in the left inset of Fig. 3b, no magnetization is observed in PANi, while a paramagnetic behavior is shown in the mixed Fe–PANi network. The coercivity shows an increased trend with the decrease of temperature, as shown in the right inset of Fig. 3b, indicating the enhanced ferromagnetism with ferromagnetic coupling interactions in FePANi magnet under low temperature.


image file: c9tc05450b-f3.tif
Fig. 3 (a) IV curves of HPANi following different HCl doping periods at room temperature. The inset figures are the RT curve (left) and the calculated conductivities of each HPANi sample (right). (b) MH loops of FePANi at different temperatures. The inset figures are the MH loops of PANi and FePANi powder at 300 K (left) and the coercivity of FePANi (right).

The coupling interactions between magnetic FePANi and conducting HPANi heterostructures (FePANi/HPANi) result from the interfacial charge-transfer interactions. The interfacial structure was characterized by atomic force microscopy (AFM), shown in Fig. 4a. The current–voltage curve of FePANi/HPANi bilayer indicates its comparable conductivity with HPANi (Fig. 4b). In comparison to HPANi with a conductivity of 6.0 × 102 S cm−1, the FePANi/HPANi conducting magnet shows an average conductivity of 3.5 × 102 S cm−1, while the FePANi/HPANi conducting magnetic heterostructure shows an enhanced saturation magnetization of 0.80 emu g−1, which is 25% higher than that of FePANi (Fig. 4c). The enhanced magnetization of FePANi/HPANi could be a result of the localized spin-ordered states at the charge-transfer interface between the metallic HPANi and FePANi layers,16 where the induced charge from the metallic layer of HPANi into the magnetic layer of FePANi results in an increase of localized spin-ordered states. The coupling between the magnetic and conducting FePANi/HPANi heterostructures is further evidenced by magnetoconductance behavior (Fig. 4d). The change of conductance (ΔG%) is calculated by: ΔG% = (GHG0)/G0 = (R0RH)/RH; GH and RH correspond to the conductance and resistance under the magnetic field (H), respectively, and G0 and R0 represent the conductance and resistance without the magnetic field. The conductance of the FePANi/HPANi conducting magnet can be tuned under magnetic field with a symmetric enhancement by switching the direction of the magnetic field. The conductance can be increased by 0.4% under the magnetic field of 70 kOe, at which the disrupted π–d interaction enhances electron mobility for the positive magneto-conductance of FePANi/HPANi.


image file: c9tc05450b-f4.tif
Fig. 4 (a) An AFM image of the bilayer structure. The inset figure is the thickness measurement. (b) The IV curve of the FePANi/HPANi bilayer structure at room temperature. The inset figures are the corresponding optical microscopy image (left) and conductivities (right). (c) The MH loop of the FeCl3/HCl bilayer structure and FePANi at 300 K. (d) Magnetic conductance measurements of the FePANi/HPANi bilayer at 180 K.

Conclusions

In conclusion, a molecular conducting magnetic heterostructure is obtained in a quasi-two-dimensional π–d aniline framework through supramolecular assembly in the presence of aniline and transition-metal cations. HPANi shows metallic conductivity of up to 9.2 × 102 S cm−1, while FePANi shows ferromagnetism-like behavior, with a saturation magnetization value of ∼0.6 emu g−1 at 300 K. The enhanced magnetization of the FePANi/HPANi heterostructure is achieved through coupling the metallic and magnetic layers to induce localized spin-ordered states at the charge-transfer interface and π–d interactions between the metallic transport layer and magnetic layer, while a conductivity of 3.5 × 102 S cm−1 is observed in this molecular conducting magnetic heterostructure. In addition, when a magnetic field is applied to the molecular conductor, the aligned d electrons induce ferromagnetic ordering, together with disrupted π–d interactions for enhanced electron mobility, leading to a magneto-conductance effect. Such materials-by-design and the supramolecular assembly of hybrid materials with strong π–d interactions provide a general method for the development of organic conducting magnets.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering supports S. R. under Award DE-SC0018631 (Organic conductors). Financial support was provided by the U. S. Army Research Office for S. R. under Award W911NF-18-2-0202 (Materials-by-Design and Molecular Assembly).

Notes and references

  1. (a) M. C. Beeler, R. A. Williams, K. Jimenez-Garcia, L. J. LeBlanc, A. R. Perry and I. B. Spielman, Nature, 2013, 498, 201 CrossRef PubMed ; (b) W. Eerenstein, N. D. Mathur and J. F. Scott, Nature, 2006, 442, 759 CrossRef PubMed ; (c) A. K. Powell, Nat. Chem., 2010, 2, 351 CrossRef ; (d) Y. N. Geng, H. Das, A. L. Wysocki, X. Y. Wang, S. W. Cheong, M. Mostovoy, C. J. Fennie and W. D. Wu, Nat. Mater., 2014, 13, 163 CrossRef ; (e) M. Warner, S. Din, I. S. Tupitsyn, G. W. Morley, A. M. Stoneham, J. A. Gardener, Z. L. Wu, A. J. Fisher, S. Heutz, C. W. M. Kay and G. Aeppli, Nature, 2013, 503, 504 CrossRef .
  2. (a) E. Coronado and P. Day, Chem. Rev., 2004, 104, 5419 CrossRef PubMed ; (b) T. Enoki and A. Miyazaki, Chem. Rev., 2004, 104, 5449 CrossRef .
  3. (a) E. Coronado, J. R. Galan-mascaros, C. Gimenez-saiz, C. J. Gomez-garcia, S. Triki and P. Delhaes, Mol. Cryst. Liq. Cryst., 1995, 274, 89 CrossRef ; (b) E. Coronado, J. R. Galán-Mascarós, C. Giménez-Saiz, C. J. Gómez-García and S. Triki, J. Am. Chem. Soc., 1998, 120, 4671 CrossRef ; (c) J. R. Galán-Mascarós, C. Giménez-Saiz, S. Triki, C. J. Gómez-García, E. Coronado and L. Ouahab, Angew. Chem., Int. Ed. Engl., 1995, 34, 1460 CrossRef .
  4. (a) S. A. Crooker, D. A. Tulchinsky, J. Levy, D. D. Awschalom, R. Garcia and N. Samarth, Phys. Rev. Lett., 1995, 75, 505 CrossRef ; (b) H. Akutsu, E. Arai, H. Kobayashi, H. Tanaka, A. Kobayashi and P. Cassoux, J. Am. Chem. Soc., 1997, 119, 12681 CrossRef .
  5. (a) J. Huang, S. Virji, B. H. Weiller and R. B. Kaner, J. Am. Chem. Soc., 2003, 125, 314 CrossRef ; (b) X. S. Du, C. F. Zhou and Y. W. Mai, J. Phys. Chem. C, 2008, 112, 19836 CrossRef ; (c) H. Zhang, X. Wang, J. Li and F. Wang, Synth. Met., 2009, 159, 1508 CrossRef .
  6. (a) S. Mourdikoudis and L. M. Liz-Marzán, Chem. Mater., 2013, 25, 1465 CrossRef ; (b) C. J. Lockhart de la Rosa, R. Phillipson, J. Teyssandier, J. Adisoejoso, Y. Balaji, C. Huyghebaert, I. Radu, M. Heyns, S. De Feyter and S. De Gendt, Appl. Phys. Lett., 2016, 109, 253112 CrossRef ; (c) C. Jiang, Z. Wang, H. Lin, Y. Wang, C. Luo, B. Li, R. Qi, R. Huang, X. Tang and H. Peng, Colloids Surf., A, 2017, 529, 403 CrossRef ; (d) S. Soman, A. S. Chacko, V. S. Prasad, P. Anju, B. S. Surya and K. Vandana, Carbohydr. Polym., 2018, 182, 69 CrossRef .
  7. (a) S. Li, C. Zhu, L. Tang and J. Kan, Mater. Chem. Phys., 2010, 124, 168 CrossRef ; (b) Y. Zhang, C. Zhu and J. Kan, J. Appl. Polym. Sci., 2008, 109, 3024 CrossRef .
  8. J. Yue, Z. H. Wang, K. R. Cromack, A. J. Epstein and A. G. Macdiarmid, J. Am. Chem. Soc., 1991, 113, 2665 CrossRef .
  9. B. C. Roy, M. D. Gupta, L. Bhoumik and J. K. Ray, Synth. Met., 2002, 130, 27 CrossRef .
  10. (a) S. M. Roy, N. N. Rao, A. Herissan and C. Colbeau-Justin, Polymer, 2017, 112, 351 CrossRef CAS ; (b) W. S. Yin and E. Ruckenstein, Synth. Met., 2000, 108, 39 CrossRef CAS .
  11. A. B. Rohom, P. U. Londhe, S. K. Mahapatra, S. K. Kulkarni and N. B. Chaure, High Perform. Polym., 2014, 26, 641 CrossRef .
  12. Z. Moravkova and P. Bober, Int. J. Polym. Sci., 2018, 1797216, 1 Search PubMed .
  13. (a) A. Varela-Alvarez, J. A. Sordo and G. E. Scuseria, J. Am. Chem. Soc., 2005, 127, 11318 CrossRef CAS ; (b) D. W. Hatchett, M. Josowicz and J. Janata, J. Phys. Chem. B, 1999, 103, 10992 CrossRef CAS ; (c) G. M. do Nascimento and M. L. A. Temperini, J. Raman Spectrosc., 2008, 39, 772 CrossRef CAS .
  14. (a) Y. Yang, S. Chen and L. Xu, Macromol. Rapid Commun., 2011, 32, 593 CrossRef ; (b) J. Jin, Q. Wang and M. A. Haque, J. Phys. D: Appl. Phys., 2010, 43, 205302 CrossRef ; (c) J. Stejskal, D. Hlavatá, P. Holler, M. Trchová, J. Prokeš and I. Sapurina, Polym. Int., 2004, 53, 294 CrossRef CAS .
  15. I. M. L. Billas, A. Chatelain and W. A. Deheer, Science, 1994, 265, 1682 CrossRef CAS .
  16. F. A. Ma'Mari, T. Moorsom, G. Teobaldi, W. Deacon, T. Prokscha, H. Luetkens, S. Lee, G. E. Sterbinsky, D. A. Arena, D. A. MacLaren, M. Flokstra, M. Ali, M. C. Wheeler, G. Burnell, B. J. Hickey and O. Cespedes, Nature, 2015, 524, 69 CrossRef .

Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c9tc05450b

This journal is © The Royal Society of Chemistry 2020