DySc2N@C80 Single-Molecule Magnetic Metallofullerene

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DyScN@C Single-Molecule Magnetic Metallofullerene Encapsulated in a Single-Walled Carbon Nanotube Ryo Nakanishi, Junya Satoh, Keiichi Katoh, Haitao Zhang, Brian K. Breedlove, Masahiko Nishijima, Yusuke Nakanishi, Haruka Omachi, Hisanori Shinohara, and Masahiro Yamashita J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b06983 • Publication Date (Web): 20 Aug 2018 Downloaded from http://pubs.acs.org on August 20, 2018

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Journal of the American Chemical Society

DySc2N@C80 Single-Molecule Magnetic Metallofullerene Encapsulated in a Single-Walled Carbon Nanotube Ryo Nakanishi,*,† Jyunya Satoh,‡ Keiichi Katoh,‡ Haitao Zhang,‡,○ Brian K. Breedlove,‡ Masahiko Nishijima,§ Yusuke Nakanishi,║ Haruka Omachi,║ Hisanori Shinohara,║ and Masahiro Yamashita*,†, ‡,┴,#

WPI Research Center, Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan § The Electron Microscopy Center, Tohoku University, Sendai 980-8577, Japan ║ Department of Chemistry & Institute for Advanced Research, Nagoya University, Nagoya 464-8601, Japan ┴ Center for Spintronics Research Network (CSRN), Tohoku University, Sendai 980-8577, Japan # School of Materials Science and Engineering, Nankai University, Tianjin 300350, China † ‡

Supporting Information Placeholder ABSTRACT: DySc2N@C80 is an endohedral metallofullerene

showing single-molecule magnet (SMM) behaviour. In this work, we encapsulated DySc2N@C80-SMMs into the internal onedimensional nano-space of single-walled carbon nanotubes (SWCNTs). Using transmission electron microscopy, “peapod” structures were clearly observed. From magnetic field dependent measurements, DySc2N@C80 showed stepwise hysteresis characteristic of SMMs even inside the SWCNTs, and the coercivity increased from 0.5 to 4 kOe. In addition, it showed slow relaxation of the magnetization without an applied external magnetic field. This system is the first example where SMMs have been encapsulated in SWCNTs, and this system could be used in future molecular-spintronics devices.

Single-molecule magnets (SMMs)1-3 are molecules which work as isolated magnets owing to large magnetic anisotropies and slow relaxation of the magnetization. Because of their functional properties, such as magnetic bistability,1 quantum tunnelling of magnetization (QTM),4-7 and quantum coherence,8 they are considered as next-generation quantum magnets. Thus, they are being developed for applications in memory storage and in the processing of quantum information.9,10 Moreover, new applications of SMMs, including their use in molecular spintronics11 and quantum computing,12 are being explored. To use SMMs as such devices, an information pathway is needed to allow read-and-write processes, and electrons would be the best messenger because they can deliver the information by using their spin orientation (up/down). Molecular-based materials are good candidates for a such pathway because they mainly consist of light elements and can maintain their spin currents longer than the metals due to the weak hyperfine coupling. Moreover, nanocarbon materials are of interest because 98.9% of naturally occurring carbon, 12C, has no nuclear spin. Therefore, SMMs have been combined with nano-carbon materials, such as carbon nanotubes (CNTs)13 and graphene,14 by attaching the SMMs onto their surfaces. When SMMs interact with nano-carbon materials, their

electronic properties are affected, and spintronics properties, such as giant magnetoresistance, are generated. Another example involves the encapsulation of SMMs into nanoscopic onedimensional (1D) pores of multi-walled CNTs (MWCNTs).15,16 Encapsulation of SMMs into CNTs can cause them to orient with respect to their easy axes, and their magnetic properties can be controlled, as in the case of [Mn12O12(O2CCH3)16(H2O)4] SMMs encapsulated in MWCNTs.15 However, little has been reported on SMMs encapsulated in CNTs. In particular, there have been no reports on SMMs encapsulated in “single-walled” CNTs (SWCNTs), which should be more interesting because they can show metallic or semiconducting properties depending on their structures.17 Furthermore, their narrower inner space can cause the SMMs to stack, forming a quasi 1D arrangement, wherein their properties are enhanced.18 In addition, the neighboring intermolecular dipole-dipole interactions can be reduced, which is a basic method for enhancing SMM properties.2,19-21 Here we focused on an endohedral metallofullerene, DySc2N@C80, having a cage with Ih symmetry (the Ih label is hereafter omitted for clarity), as an SMM. The SMM properties of DySc2N@C80 were first reported in 201222 as part of a pioneering work on metallofullerene-SMMs. It shows high SMM properties because of the large ligand-field splitting of 1500 cm−1 due to the Dy ion having 6H15/2 multiplet structures with Jz = ±15/2 states in the ground doublet,23,24 and its discovery has led to the development of other metallofullereneSMMs and further applied research.25-29 One important feature of the fullerene species is that they can be easily encapsulated in SWCNTs with high filling yields due to strong π-π interactions.3034 SWCNTs with encapsulated fullerenes are commonly called “peapods” since the spherical fullerenes are regularly stacked onedimensionally inside the SWCNTs. These peapods have exhibited interesting properties, such as the modulation of electrical properties35 in SWCNTs. In this work, we encapsulated DySc2N@C80 into SWCNTs as shown in Figure 1 to control and enhance the SMM properties.

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Figure 1. Image of DySc2N@C80 (left) and DySc2N@C80 encapsulated in SWCNT (right).

Figure 2. (a,b) TEM images of [DySc2N@C80]@SWCNT. (c) Structure model of (b)

DySc2N@C80 metallofullerenes were synthesized by using a modified Krätschmer–Huffman arc discharge method36 with a trimetallic nitride template process in a mixture of NH3 and He gases37 and separated by using high performance liquid chromatography (HPLC) with toluene as the eluent (Figure S1). The isolation of DySc2N@C80 was confirmed by using mass spectrometry (Figure S2) and UV-Vis-NIR absorption spectroscopy (Figure S3) in comparison with reported spectra,38,39 and the SMM properties were confirmed by magnetic measurements (Figure S4). Then the DySc2N@C80-SMMs were introduced into SWCNTs having a narrow diameter distribution of 1.4 ± 0.1 nm by using a sublimation method. It should be noted that usually SWCNTs contain metal nanoparticles, which were used as a catalyst for their growth, and they act as magnetic impurities, strongly affecting the magnetic measurements. Therefore, the magnetic properties of the SWCNTs and the hybrid materials have not been well explored, except for a few reports.40-43 For magnetic studies using SWCNTs, it is important to develop easier and scalable purification methods. For that purpose, a few purification methods for SWCNTs, such as acid treatment,44 have been studied. However, it has been too difficult to remove enough of the metal impurities so that their effects can be ignored. Therefore, we first tried to find a suitable method for purifying SWCNTs and found that we could purify the materials by combining acid treatment and sublimation methods. In our method, a cationic surfactant, hexadecyltrimethylammonium bromide, must be used in the acid treatment procedure to disperse the SWCNTs well45 and prevent their aggregation under acidic conditions. From field-dependent magnetic susceptibility measurements, ferromagnetic or superparamagnetic contributions from the metal impurities, which are observed for as-obtained SWCNTs, became negligible, and diamagnetic behaviour for the SWCNTs was observed, as shown in Fig. S5. Although it seems that a small amount of the residue still remained, the amount of impurities is similar to that obtained using a density gradient ultracentrifugation method,43 meaning that it can be neglected for the magnetic measurements. Our method can be easily scaled up, which is very important to be able to perform magnetic studies on bulk samples of CNTs and their hybrids.

Figure 3. (a) M vs. H/T plots for [DySc2N@C80]@SWCNT in applied H in the range of 10–70 kOe. (b) M vs. H plots for [DySc2N@C80]@SWCNT at T of 1.8, 5, 7 and 10 K. (c) M vs. H plots for DySc2N@C80 (filled circles) and [DySc2N@C80]@SWCNT (open circles) at 1.8 K. Arrows indicate the direction of the measurements. (d) Magnification of (c) in the H range of –10 to 10 kOe.

The obtained hybrids were observed by transmission electron microscopy (TEM). Spherical structures were confirmed in the SWCNTs, as shown in Figure 2. In addition, signals for the Dy and Sc ions were observed in the same region in energy dispersive X-ray (EDX) spectra, as shown in Figure S6. Moreover, from high-angle annular dark field (HAADF) scanning TEM (STEM), white dots due to the heavy Dy ions were clearly observed at the same positions with those of the fullerenes in the TEM images (Figure S7). These results indicated that DySc2N@C80 were encapsulated in the SWCNTs to afford [DySc2N@C80]@SWCNT. Thermogravimetric (TG) analyses were performed on the purified SWCNTs and [DySc2N@C80]@SWCNT in the presence of air (Figure S8). For the purified SWCNTs, the weight became almost zero when T > 600 °C. On the other hand, in the case of

an applied H in the range of 10–70 kOe. The saturated M values depended on the applied H, which is the same for DySc2N@C80, indicating that the Dy ion in DySc2N@C80 still has uniaxial magnetic anisotropy and/or a low-lying excited state. In M vs. H plots, shown in Figure 3b, clear M loops were detected when T ≤ 5 K, which is the same for DySc2N@C80. This result strongly suggests that DySc2N@C80 maintains its SMM properties even inside the SWCNTs. Furthermore, to compare the M loops for DySc2N@C80 before and after the encapsulation into SWCNTs, the coercivity was increased from 0.5 to 4 kOe (Figure 3c,d). When DySc2N@C80 was diluted with C60, the coercivity increased (Figure S10), which is due to the longer Dy-Dy distances and the weakening of the intermolecular dipole-dipole interactions. Therefore, the enhancement of the SMM properties of

[DySc2N@C80]@SWCNT, 15.0 wt% of a white compound remained even when T > 1000 °C. This material is thought to be a mixture of Dy and Sc oxides. These results also indicate the success of the purification of SWCNTs and encapsulation of DySc2N@C80 into them. To determine the effects of encapsulation of the SMMs in SWCNTs on the magnetic properties, static magnetic measurements on [DySc2N@C80]@SWCNT were performed, and the results were compared with those for free DySc2N@C80 SMMs. Direct current (DC) measurements were used to obtain the magnetization (M), which depended on T and the magnetic field (H). Figure 3a shows M vs. H/T plots for [DySc2N@C80]@SWCNT in


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Journal of the American Chemical Society [DySc2N@C80]@SWCNT is due to the suppression of QTM by dilution upon encapsulation of DySc2N@C80 into the SWCNTs. At the same time, strong π-π interactions between DySc2N@C80 and SWCNTs could split the magnetic ground states of the time reversed symmetric doublet of DySc2N@C80, resulting in QTM and a large remanence, as in the case of DySc2N@C80 in MOF177.29,46 For SMMs, slow relaxation of the magnetization is an important parameter. For DySc2N@C80, long relaxation times of ca. 40 min for an undiluted sample and more than 5 h for a diluted sample (with C60) have been reported.22 In order to compare the relaxation times, time-dependent relaxation of the magnetization for [DySc2N@C80]@SWCNT was measured at 2, 2.4, 2.9, 3.2, 3.7, 4 and 5 K. The relaxation curve at 2 K in a zero field is shown in Figure 4 (those of the other temperatures are shown in

Figure 4. Relaxation of the magnetization for [DySc2N@C80] @SWCNT at T = 2 K, with ∆m(t) = m(t) − m(t→∞). msat is the saturation magnetization. Solid line corresponds to the best fit using triple exponential eqn 1. Inset shows magnetic relaxation times τA as a function of T–1, and the solid line corresponds to the best fit using eqn 2. Figure S11). In the case of [DySc2N@C80]@SWCNT, the relaxation occurred via a triple-exponential decay, although it was double for DySc2N@C80. The relaxation data in this T region were fitted with the following equation: ∆ → ∞ exp⁄ exp⁄ exp⁄ (1) where α, β and γ are the magnetization of relaxation processes A, B and C at t = 0, and τA, τB and τC are the corresponding relaxation times (τA > τB > τC). The values obtained from the best fit using eqn 1 for each T are shown in Table S1. τA at 2 K was determined to be 5650 s (94 min), which is longer than that of the undiluted DySc2N@C80 in a zero field. This elongation has been observed for samples diluted with C6022 and other non-magnetic matrices,29 showing that encapsulation in SWCNTs magnetically dilutes DySc2N@C80. The effective barrier for the thermal relaxation process (∆eff) and the temperature-independent decay times τc were estimated by using the following equation: exp∆ ⁄ exp∆ ⁄

where τ0 is the pre-exponential factor for the temperaturedependent region of the relaxation process. For relaxation process A, because it should be dominated by direct process, the τA values were plotted as a function of T–1, as shown in the inset of Figure 4, and the values were fitted by using eqn. 2. ∆eff/kB and τ0 were determined to be 34.1 K and 2.0 s, respectively. These values are similar to or slightly higher than those of the original DySc2N@C80 (24 K for ∆eff/kB and 1 s for τ0 in H of 3 kOe).22 τc 3 for relaxation process A (τ ) was determined to be 5.0 × 10 , and 2 that for B (τ ) was 4.9 × 10 s (Figure S12a). These values are similar to those for DySc2N@C80. However, τc for relaxation process C could not be obtained due to fitting problems (Figure S12b). The difference between A and B for DySc2N@C80 is due to hyperfine interactions between the Jz level and the corresponding nuclear spin of different Dy isotopes.22 For [DySc2N@C80]@SWCNT, electron transfer from the SWCNTs to DySc2N@C80 occurred, causing a slight up-shift in the tangential C-C stretching mode of the SWCNTs (1570.7 cm–1 → 1580.3 cm– 1 ) in Raman spectra (Figures S13).47 Charge transfer would cause oxidation state changes in the Dy ions in DySc2N@C80, as in the case of the Sc ions in [Sc3N@C80]@SWCNT.48 As a result, the Kramers doublet would split, and therefore, relaxation process C may involve weak QTM. In summary, we encapsulated DySc2N@C80 (symmetry: Ih) SMMs in the internal nanospace of well-purified SWCNTs. From field dependent magnetic susceptibility measurements, DySc2N@C80 exhibited SMM properties even inside the SWCNTs, and the coercivity increased from 0.5 to 4 kOe. Moreover, the relaxation time of the magnetization of [DySc2N@C80]@SWCNT became about 94 min. These enhancements of the SMM properties are due to the suppression of QTM by dilution upon encapsulation into SWCNTs. To the best of our knowledge, this is the first report of SMMs encapsulated in “single-walled” CNTs without the loss of their magnetic properties. This hybrid system allows for easy one-dimensional alignment of SMMs while protecting the SMMs from the surrounding environment, which is important for future applications in memory storage devices. Moreover, it is possible to combine the molecule-based magnetic properties of SMMs and strong electronic properties of SWCNTs, which should lead to new device applications by exploiting the high magnetoresistivity.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xxxxxxxxxxxxxx. Experimental details; HPLC chromatograms; mass, UVVis-NIR, EDX, Raman spectra; TEM and STEM-HAADF images; Thermogravimetric analyses; M vs. T and M vs. H plots; and relaxation of the magnetization.

AUTHOR INFORMATION Corresponding Authors [email protected] (R.N.), [email protected] (M.Y.).

Present Addresses ○

(2)

Institute for Inorganic and Applied Chemistry, University of Hamburg, Hamburg 20146, Germany.



Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by CREST, JST, a Grant-in-Aid for Scientific Research (S) (grant No. 20225003), Grant-in-Aid for Scientific Research (C) (grant No. 15K05467), Grant-in-Aid for Young Scientists (B) (grant No. 24750119) from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT), Japan. A part of this study was supported by Tohoku University Microstructural Characterization Platform in Nanotechnology Platform project sponsored by the MEXT, Japan. Ryo Nakanishi thanks Shorai Foundation for Science and Technology. Masahiro Yamashita thanks the support by the 111 project (B18030) from China. We thank Dr. Takamichi Miyazaki (Technical Division, Department of Engineering, Tohoku University) for the support in the TEM and EDX analyses.

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Figure 1. Image of DySc2N@C80 (left) and DySc2N@C80 encapsulated in SWCNT (right). 125x42mm (300 x 300 DPI)


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Figure 2. (a,b) TEM images of [DySc2N@C80]@SWCNT. (c) Structure model of (b) 100x47mm (300 x 300 DPI)



Figure 3. (a) M vs. H/T plots for [DySc2N@C80]@SWCNT in applied H in the range of 10–70 kOe. (b) M vs. H plots for [DySc2N@C80]@SWCNT at T of 1.8, 5, 7 and 10 K. (c) M vs. H plots for DySc2N@C80 (filled circles) and [DySc2N@C80]@SWCNT (open circles) at 1.8 K. Arrows indicate the direction of the measurements. (d) Magnification of (c) in the H range of –10 to 10 kOe. 126x92mm (300 x 300 DPI)


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Figure 4. Relaxation of the magnetization for [DySc2N@C80]@SWCNT at T = 2 K, with ∆m(t) = m(t) − m(t→∞). msat is the saturation magnetization. Solid line corresponds to the best fit using triple exponential eqn 1. Inset shows magnetic relaxation times τA as a function of T–1, and the solid line corresponds to the best fit using eqn 2. 109x80mm (300 x 300 DPI)


DySc2N@C80 Single-Molecule Magnetic Metallofullerene

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