Dysprosium Acetylacetonato Single-Molecule

materials Article

Dysprosium Acetylacetonato Single-Molecule Magnet Encapsulated in Carbon Nanotubes Ryo Nakanishi 1, *, Mudasir Ahmad Yatoo 1 , Keiichi Katoh 1 , Brian K. Breedlove 1 and Masahiro Yamashita 1,2, * 1

2

*

Department of Chemistry, Graduate School of Science, Tohoku University, 6-3 Aza-Aoba, Aoba-ku, Sendai, Miyagi 980-8578, Japan; [email protected] (M.A.Y.); [email protected] (K.K.); [email protected] (B.K.B.) WPI Research Center, Advanced Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan Correspondence: [email protected] (R.N.); [email protected] (M.Y.); Tel.: +81-22-795-6544 (M.Y.)

Academic Editor: Wolfgang Linert Received: 25 October 2016; Accepted: 20 December 2016; Published: 23 December 2016

Abstract: Dy single-molecule magnets (SMMs), which have several potential uses in a variety of applications, such as quantum computing, were encapsulated in multi-walled carbon nanotubes (MWCNTs) by using a capillary method. Encapsulation was confirmed by using transmission electron microscopy (TEM). In alternating current magnetic measurements, the magnetic susceptibilities of the Dy acetylacetonato complexes showed clear frequency dependence even inside the MWCNTs, meaning that this hybrid can be used as magnetic materials in devices. Keywords: single-molecule magnet; carbon nanotube

1. Introduction Single-molecule magnets (SMMs) [1–4], which are composed of isolated molecules, usually with large spin angular momenta (S) in the ground state and strong uniaxial magnetic anisotropies (D), exhibit an extensive range of functional properties, like magnetic bistability [1], quantum tunneling of magnetization [5–8], and quantum coherence [9]. Thus, they can be considered as not only molecular equivalents of classical bulk ferromagnets but also as next-generation quantum magnets. Therefore, SMMs are being developed for application in memory storage and in the processing of quantum information [10,11]. Moreover, novel applications of SMMs, including their use in molecular spintronics [12] and quantum computing [13], are being explored. To use SMMs, we must be able to exploit the functionality of individual SMM molecules and combine them with other functional materials. There have been a few reports on combining SMMs with materials. For example, SMMs have been combined with carbon nanotubes (CNTs) [14] and graphene [15]. From these examples, when lanthanoid SMMs interact with nanocarbon materials, their electronic properties are affected. Another example involves the encapsulation of SMMs into nanoscopic one-dimensional pores, such as the internal nano-space of CNTs [16] and metal-organic frameworks [17], in which SMMs become aligned and their magnetic properties are controlled. SMM-nanomaterial hybrids may have new structures and unique physical properties. If SMMs are encapsulated in one-dimensional pores, the stacking structure can be controlled, and the SMM properties should be enhanced. Furthermore, when SMMs are encapsulated in CNTs, they are protected from the surrounding environment, and thus, the hybrids are easier to use in real applications. However, little has been reported on lanthanoid SMMs encapsulated inside CNTs. In this work, we encapsulated Dy acetylacetonato SMMs [18] in multi-walled CNTs (MWCNTs) by using a capillary

Materials 2017, 10, 7; doi:10.3390/ma10010007

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method [19,20]. Encapsulation was verified by using transmission electron microscopy (TEM). It was shown that Dy complexes maintained their SMM-like properties in the MWCNTs. Materials 2017, 10, 7

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2. Results and Discussion using a capillary method [19,20]. Encapsulation was verified by using transmission electron microscopy

2.1. Synthesis (TEM). It was shown that Dy complexes maintained their SMM-like properties in the MWCNTs. MWCNTs an internal diameter of ~5 nm were purified by using centrifugation [21], and then 2. Resultswith and Discussion the end-caps were opened by heating in air. The impurities in the internal nano-space were removed 2.1. Synthesis by heating in a vacuum. Next, Dy(acac)3 (H2 O)2 was dissolved in 1,2-dichloroethane, and the solution was heatedMWCNTs at 65 ◦ C with for 2an h to obtain a saturated solution. Cap-opened MWCNTs were[21], added internal diameter of ~5 nm were purified by using centrifugation and to the thensolution the end-caps openedby byusing heatingultrasonication. in air. The impurities thesolution internal nano-space saturated and were dispersed Theninthe was left towere stand for removed by heating in a vacuum. Next, Dy(acac) 3 (H 2 O) 2 was dissolved in 1,2-dichloroethane, and 3 d in order to encapsulate Dy(acac)3 (H2 O)2 into the MWCNTs via a capillary phenomenon [19,20]. the solution heated at 65surfaces °C for 2 hwith to obtain a saturated solution. Cap-opened MWCNTs were After filtering andwas washing the 1,2-dichloroethane, Dy(acac) 3 (H2 O)2 encapsulated in added to the saturated solution and dispersed by using ultrasonication. Then the solution was left MWCNTs (Dy(acac)3 (H2 O)2 @MWCNTs) were obtained. to stand for 3 d in order to encapsulate Dy(acac)3(H2O)2 into the MWCNTs via a capillary phenomenon [19,20]. After filtering and washing the surfaces with 1,2-dichloroethane, 2.2. Transmission Electron Microscopy, Elemental Analysis and Thermogravimetry Dy(acac)3(H2O)2 encapsulated in MWCNTs (Dy(acac)3(H2O)2@MWCNTs) were obtained.

TEM was used to view the interior of the MWCNT hybrids; the structure images are illustrated 2.2. 1a. Transmission Electron Microscopy, Analysis and Thermogravimetry in Figure In the TEM images, onlyElemental Dy(acac) 3 (H2 O)2 @MWCNTs as free-standing entities were used to view interior of on the MWCNT hybrids; the structure images are illustrated observed, TEM and was there were no the complexes the external surfaces of the MWCNTs (Figure 1b). in Figure 1a. In athe TEMcontrast images, only Dy(acac) 2O)2@MWCNTs as free-standing entities were In enlarged images, stark between the3(H Dy(acac) (H O) @MWCNT (Figure 1c) and the 3 2 2 observed, and there were no complexes on the external surfaces of the MWCNTs (Figure 1b). In empty MWCNTs was observed, as shown in Supplementary Materials Figure S1, showing that enlarged images, a stark contrast between the Dy(acac)3(H2O)2@MWCNT (Figure 1c) and the empty Dy(acac)3 (H2 O)2 was encapsulated. In order to confirm the encapsulation and characterize the MWCNTs was observed, as shown in Supplementary Materials Figure S1, showing that material present inside the MWCNTs, energy-dispersive X-ray (EDX) spectroscopy was used to Dy(acac) 3(H2O)2 was encapsulated. In order to confirm the encapsulation and characterize the detect material the Dy present ions (Figure The results clearly indicate that Dy(acac) is encapsulated 3 (H2 O)2was inside 1d). the MWCNTs, energy-dispersive X-ray (EDX) spectroscopy used to in the detect MWCNTs. Thermogravimetric analysis (TGA) was performed MWCNTs the Dy ions (Figure 1d). The results clearly indicate that Dy(acac)3(Hon 2O)2pristine is encapsulated in and ◦ the MWCNTs. Thermogravimetric analysis (TGA) was performed on pristine MWCNTs and Dy(acac)3 (H2 O)2 @MWCNT (Figure 2). For the pristine MWCNTs, when T > 600 C, all of the carbon Dy(acac) 2O)2@MWCNT (Figure 2). For the pristine MWCNTs, when T > 600 °C, all of the carbon was lost as CO32(H . However, in the case of Dy(acac)3 (H2 O)2 @MWCNT, 22.3 wt % of a white compound was lost as CO2. However,◦in the case of Dy(acac)3(H2O)2@MWCNT, 22.3 wt % of a white compound remained even when T > 1000 C. This material is thought to be Dy2 O3 . From the TGA data, the amount remained even when T > 1000 °C. This material is thought to be Dy2O3. From the TGA data, the of Dy(acac)3 (H2 O)2 was estimated to be 1.2 mmol in 1 g of Dy(acac)3 (H2 O)2 @MWCNT. amount of Dy(acac)3(H2O)2 was estimated to be 1.2 mmol in 1 g of Dy(acac)3(H2O)2@MWCNT.

Figure 1. (a) Drawings of Dy(acac)3(H2O)2 complex and the complexes encapsulated in multi-walled

Figure 1. (a) Drawings of Dy(acac)3 (H2 O)2 complex and the complexes encapsulated in multi-walled carbon nanotubes (MWCNT); (b) Low magnification and (c) high magnification transmission carbonelectron nanotubes (MWCNT); (b)images Low magnification (c) high magnification transmission electron 2O)2@MWCNTs; (d) energy dispersive X-ray microscopy (TEM) of Dy(acac)3(Hand microscopy (TEM)(EDX) images of Dy(acac) 3 (Hfor 2 O) 2 @MWCNTs; spectroscopy spectrum acquired the sample in (c). (d) energy dispersive X-ray spectroscopy (EDX) spectrum acquired for the sample in (c).

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Figure2. 2. Thermogravimetric Thermogravimetricanalyses analyses of ofempty emptyMWCNTs MWCNTs(black) (black)and andDy(acac) Dy(acac)333(H (H222O) @MWCNTs (red). Figure Thermogravimetric analyses of empty MWCNTs O)222@MWCNTs (red). Figure 2. (black) and Dy(acac) (H

2.3.Magnetic MagneticProperties Properties 2.3. 2.3. Properties Todetermine determinethe theeffects effectsof ofencapsulation encapsulationof ofthe theSMMs SMMsin inMWCNTs MWCNTson onthe themagnetic magneticproperties, properties, To To determine the effects of encapsulation of the SMMs in MWCNTs on the magnetic properties, bothstatic staticand anddynamic dynamicmagnetic magneticmeasurements measurementson onDy(acac) Dy(acac) 3(H (H222O) O)222@MWCNTs @MWCNTs were were performed, performed, both both static and dynamic magnetic measurements on Dy(acac) O) 33(H and the results were compared with those for free Dy complexes. Direct current (DC) and the results were compared with those for free Dy complexes. Direct current (DC) measurements and the results were compared with those for free Dy complexes. Direct current (DC) measurements were used to obtain molar magnetic susceptibilities (χ m ), which depended on T and were used to obtain susceptibilities (χm ), which depended on T and the magnetic measurements were molar used tomagnetic obtain molar magnetic susceptibilities (χm), which depended on T and the magnetic magnetic field (H).for T-T plots plots3 (H for2 O) Dy(acac) (H22O) O)22@MWCNTs @MWCNTs and pure pure (H O)22 are are field (H). χm T-T plots Dy(acac) and pure Dy(acac) O)2 are33(H shown in the field (H). χχmmT-T for Dy(acac) 33(H and 22O) 2 @MWCNTs 3 (HDy(acac) 2Dy(acac) shown in Figure 3a. After correcting the diamagnetism of the MWCNTs (see Supplementary Figure After 3a. correcting the diamagnetism of the MWCNTs (see Supplementary Materials shown 3a. in Figure After correcting the diamagnetism of the MWCNTs (see Supplementary Materials Figure S2), we we determined determined the m values values3 (H for2 O) Dy(acac) (H22O) O)by @MWCNTs by using using the Figure S2),Figure we determined the χm valuesthe forχχDy(acac) using the ratio obtained Materials S2), m for Dy(acac) 33(H 22@MWCNTs by the 2 @MWCNTs ratio obtained obtained from TGA, and the the resulting χmmTT value at 300 300 K Kwith agrees with that for an isolated isolated Dy(III) from TGA, and the TGA, resulting χmresulting T value χ at 300 K agrees that forthat an for isolated Dy(III) ion ratio from and value at agrees with an Dy(III) 3·K·mol −1), which suggests that the estimated amount of Dy(acac)3(H2O)2 is reliable. χmT 3 − 1 3 −1 ion (14.2 cm (14.2 cm cm ·K··K·mol mol ), ),which the estimated estimatedamount amountof of Dy(acac) is reliable. ion (14.2 whichsuggests suggests that the Dy(acac) 3(H32(H O)22 O) is 2reliable. χmT values for Dy(acac) Dy(acac) (H223O) O) @MWCNTs decreased with a decrease in T, whereas those for pure χvalues for Dy(acac) (H22@MWCNTs O) @MWCNTs decreased with a decrease in T, whereas those for pure for 33(H decreased with a decrease in T, whereas those for pure m T values 2 2 Dy(acac)333(H (H222O) O)222 did did not. not. This Thisdifference differencewas wasascribed ascribedtoto todepopulation depopulation of high energy m states due Dy(acac) ofof high energy mJ m states duedue to Dy(acac) (H difference was ascribed depopulation high energy JJ states to configurational and orientational changes in the ligands upon encapsulation [22,23]. configurational andand orientational changes in the ligands upon encapsulation [22,23]. to configurational orientational changes in the ligands upon encapsulation [22,23].

Figure 3. 3. (a) χχmmTT vs. vs. T plots for for Dy(acac)33(H (H2O)22@MWCNTs @MWCNTs (red (red filled filled circles) circles) and and pure pure Figure Figure 3. (a) (a) χm T vs. TT plots plots for Dy(acac) Dy(acac)3 (H2O) 2 O)2 @MWCNTs (red filled circles) and pure (H22O) O)22 (blue (blue open open circles); circles); (b) (b) M M vs. vs. H H plots plots for for Dy(acac) Dy(acac)33(H (H22O) O)22@MWCNTs @MWCNTs at at 1.85 1.85 K. K. The The Dy(acac)33(H Dy(acac) Dy(acac)3 (H2 O)2 (blue open circles); (b) M vs. H plots for Dy(acac)3 (H2 O)2 @MWCNTs at 1.85 K. inset shows magnified curve in the range of −4–4 kOe. insetinset shows magnified curve in the of −4–4 The shows magnified curve in range the range of −kOe. 4–4 kOe.

In magnetization magnetization (M) (M) vs. vs. H H plots, plots, shown shown in in Figure Figure 3b, 3b, magnetic magnetic hysteresis hysteresis was was not not observed. observed. In In In In magnetization (M) vs. H plots, shown in Figure 3b, magnetic hysteresis was not observed. the case case of of Dy(acac) Dy(acac)33(H (H22O) O)22 diluted diluted with with 20 20 equivalents equivalents of of Y(acac) Y(acac)33(H (H22O) O)22,, slight slight hysteresis hysteresis has has been been the In the caseatof2Dy(acac) (H O)distance dilutedbetween with 20each equivalents of Y(acac) (H2 O)and , slight hysteresis has 3 2 2 3 2 observed K because the Dy(acac) 3 (H 2 O) 2 is large quantum tunneling observed at 2 K because the distance between each Dy(acac)3(H2O)2 is large and quantum tunneling been observed at 2 K because distance between each Dy(acac) O)not large and quantum 3 (His 2is 2 is suppressed of the the magnetization (QTM) the is suppressed suppressed [18]. Therefore, Therefore, QTM suppressed for the the of magnetization (QTM) is [18]. QTM not for tunneling of the magnetization (QTM) is suppressed [18]. Therefore, QTM is not suppressed Dy(acac)33(H (H22O) O)22@MWCNTs. @MWCNTs. In In addition, addition, itit is is possible possible that that the the coordination coordination environment environmentfor of Dy(acac) of the Dy(acac) (H22changed O)2 @MWCNTs. In addition, in it is possible that which the coordination environment of 3 Dy(acac) 3 (H 2 O) upon encapsulation the MWCNTs, promotes the QTM process Dy(acac)3(H2O)2 changed upon encapsulation in the MWCNTs, which promotes the QTM process and shortens shortens the the relaxation relaxation time. time. Similar Similar behavior behavior for for Mn Mn1212-acetate -acetate SMMs SMMs encapsulated encapsulated in in and MWCNTs has been reported [16]. In other words, no hysteresis was observed for the Dy hybrids. MWCNTs has been reported [16]. In other words, no hysteresis was observed for the Dy hybrids.


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Dy(acac)3 (H2 O)2 changed upon encapsulation in the MWCNTs, which promotes the QTM process Materials 2017, 10, 7 4 of 8 and shortens the relaxation time. Similar behavior for Mn12 -acetate SMMs encapsulated in MWCNTs has been [16]. other words, no hysteresis observed for Dy the hybrids. Thus,time by Thus, by reported controlling theIncoordination environment viawas encapsulation in the CNTs, relaxation controlling the coordination environment via encapsulation in CNTs, the relaxation time of the SMMs of the SMMs can be tuned. can beNext, tuned. the dynamic magnetic properties were studied, and the results are shown in Figure 4. Next, the dynamic magnetic properties were studied, and the results are shown in Figure 4. For Dy(acac)3(H 2O)2@MWCNTs, an out-of-phase (χ″) signal, which is indicative of slow relaxation For Dy(acac) (H O) @MWCNTs, anboth out-of-phase (χ”)(χ′) signal, is indicative of slowdependent. relaxation 3 2 2 Furthermore, of M, was observed. the in-phase and which χ″ signals were frequency 0 of M, was observed.isFurthermore, the in-phase (χ ) and χ” signals were frequency dependent. This dependence due to the both Dy(acac) 3(H2O)2 complexes because the susceptibilities of the This dependence is due to the Dy(acac) (H O) complexes because the susceptibilities of the S3). MWCNTs 3 2 2 MWCNTs themselves are not frequency dependent (Supplementary Materials Figure These themselves are not frequency dependent (Supplementary Materials Figure S3). These results results indicate that the observed slow relaxation is due to SMM behavior, that is, there is anindicate energy that the for observed slowofrelaxation is due to SMM behavior, thatMWCNTs. is, there isHowever, an energy barrier barrier relaxation the magnetic moment even inside the there wasfor no relaxation of the magnetic moment even inside the MWCNTs. However, there was no peak top for peak top for the Dy(acac)3(H2O)2@MWCNTs in the frequency range of 1–1000 Hz, whereas a clear the Dy(acac) the frequency range of 1–1000 Hz, whereas a clear peak was 3 (Hobserved 2 O)2 @MWCNTs peak top was for theinpure complex (Supplementary Materials Figure S4). Astop seen in observed for the pure complex (Supplementary Materials Figure S4). As seen in Figure 4b, peak top Figure 4b, peak top values of χ″ shifted towards higher frequencies. This indicates that the values of χ”times shifted higher frequencies. indicates relaxation times the hybrids relaxation fortowards the hybrids are faster thanThis those for the that purethe complex. In the χ″for versus T plots are fasterinthan those pure the χ” versus shown in Figure 5a, a peak top shown Figure 5a,fora the peak topcomplex. was stillInobserved in theT plots T region below 2 K, indicating thatwas the still observed in the T region below 2 K, indicating that the magnetic moment was not frozen and that magnetic moment was not frozen and that a different relaxation process, like QTM process, was adominant different relaxation process, like process, dominant in the low-T region. estimated the in the low-T region. WeQTM estimated thewas pre-exponential factor τ0 and the We activation energy 0 − 1 pre-exponential factor τT0−1and theK)activation energy ∆E from T of (6–10 K) plots, shown ΔE from χ″/χ′ versus (6–10 plots, shown in Figure 5b,χ”/χ in theversus ν range 240–1103 Hz by using in Figure 5b, in the ν range of 240–1103 Hz by using the Kramers-Kronig equation [23–27]: the Kramers-Kronig equation [23–27]:

′′⁄ 0 ′= ωτ χ00 /χ

(1) (1)

′′⁄0 = ′ ωτ0 + exp exp(∆E/k ∆ ⁄ B T) χ00 /χ 0 lnlnχ00 /χ ∆ ⁄ BT ′′⁄ ′ = ln ln(ωτ0 ) + ∆E/k

(2) (2) (3) (3)

where By fitting fitting the the data, the ττ0 and where ωω (=2πν) (=2πν) isis the the angular angular frequency. frequency. By data, the and ∆E ΔE for for −6 –10−7 s and 4–5 cm−1 , Dy(acac) (H O) @MWCNTs were estimated to be in the range of 10 −6 −7 3 2 2 Dy(acac)3(H2O)2@MWCNTs were estimated to be in the range of 10 –10 s and 4–5 cm−1, respectively O) , τ 0 and respectively (Supplementary (Supplementary Materials Materials Table TableS1). S1). For For pure pure Dy(acac) Dy(acac)3 (H 3(H22O)22, τ0 and ∆E ΔE were were − 7 − 1 determined to be 8.0 × 10 s and 45.9 cm , respectively [18]. We think that ∆E for the hybrids −7 −1 determined to be 8.0 × 10 s and 45.9 cm , respectively [18]. We think that ΔE for the hybrids isis lower 22O) lowerbecause becauseof ofaaconformational conformationalchange changeininDy(acac) Dy(acac)3 (H 3(H O)22 inside inside the the MWCNTs. MWCNTs. The The values valuesare are consistent consistentwith withthe thedecrease decreasein inthe theχχmmTT value value and and magnetic magnetic hysteresis hysteresis behavior. behavior.

0 ) and Figure 4.4. Frequency Frequency dependence dependence of of the the (a) (a) in-phase in-phase (χ (χ′) and (b) (b) out-of-phase out-of-phase (χ”) (χ″) AC AC magnetic magnetic Figure Dy(acac) 3 (H 2 O) 2 @MWCNTs . The measurements were performed in an susceptibilities of susceptibilities of Dy(acac)3 (H2 O)2 @MWCNTs. The measurements were performed in an HDCHofDC0 of Oe0 Oe and H AC of 3 Oe in the T range of 1.85–10 K. The solid lines are guides for eyes. and HAC of 3 Oe in the T range of 1.85–10 K. The solid lines are guides for eyes.

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Figure 5. 5. (a) 3(H(H 2O)O) 2@MWCNTs. The solid lines are guides for eyes; (b) Figure (a) χ″ χ”vs. vs.TTplots plotsfor forDy(acac) Dy(acac) 3 2 2 @MWCNTs. The solid lines are guides for eyes; −1 (6–10 − 1 0 χ″/χ′ versus T K) plot in the ν range of 240–1103 Hz. The Hz. solidThe linessolid werelines fitted as described (b) χ”/χ versus T (6–10 K) plot in the ν range of 240–1103 were fitted as in Supplementary Materials Table S1. Table S1. described in Supplementary Materials

3. Materials Materials and and Methods Methods 3. 3.1. General Distilled water from a EYELA STILL STILL ACE SA-2100E deionizerdeionizer (Tokyo Rikakikai waterwas wasobtained obtained from a EYELA ACE SA-2100E (Tokyo Co., Ltd.,Co., Tokyo, Dy(acac)Dy(acac) 3(H2O)2 (STREM Chemicals, Inc., Newburyport, MA, USA), Rikakikai Ltd., Japan). Tokyo, Japan). Chemicals, Inc., Newburyport, MA, 3 (H2 O)2 (STREM 1,2-dichloroethane and methanol (Wako (Wako Pure Chemical Industries, Ltd., Osaka, Japan) were used as USA), 1,2-dichloroethane and methanol Pure Chemical Industries, Ltd., Osaka, Japan) were received. MWCNTs synthesized by using the CoMoCAT™ catalytic chemical vapor deposition used as received. MWCNTs synthesized by using the CoMoCAT™ catalytic chemical vapor with outer outer diameters diameters of of10 10± ± 0.1 nm, inner diameters of of 4.5 4.5 ± ± 0.5 method with 0.5 nm, nm, and lengths of 3–6 µm (Sigma-Aldrich Co. Co.LLC., LLC., Louis, purchased andafter used after removing the (Sigma-Aldrich St.St. Louis, MO,MO, USA)USA) were were purchased and used removing the magnetic magnetic impurities by using a centrifugation method [21]. The MWCNTs (30 mg) were dispersed impurities by using a centrifugation method [21]. The MWCNTs (30 mg) were dispersed with 60 mL of mL of cholate 1 wt %insodium cholate water by using ultrasonication with a(UP200S, tip-type Hielscher sonicator 1with wt %60sodium water by usingin ultrasonication with a tip-type sonicator (UP200S, Hielscher Ultrasonics GmbH, Teltow, Germany). The obtained black suspension was Ultrasonics GmbH, Teltow, Germany). The obtained black suspension was centrifuged at 18,500 rpm centrifuged at 18500 rpm for ~1 h using a tabletop centrifuge (AS185, AS ONE Japan), for ~1 h using a tabletop centrifuge (AS185, AS ONE Co., Osaka, Japan), andCo., the Osaka, upper 80% of andsupernatant the upper 80% the supernatant was collected.MWCNTs The well-dispersed MWCNTs were aggregated the wasofcollected. The well-dispersed were aggregated by adding methanol by adding and filtered over a Kiriyama glass having Co, Tokyo, Japan) a and filteredmethanol over a Kiriyama filter (Kiriyama glassfilter Co.,(Kiriyama Tokyo, Japan) a pore size having of 1 µm. ◦ poreaggregates size of 1 µm. The aggregates wereexcess then washed with excess at 200 °C in a The were then washed with methanol and driedmethanol at 200 Cand in a dried vacuum overnight, vacuum overnight, affording 15 mg ofbuckypaper. purified MWCNT buckypaper. affording 15 mg of purified MWCNT 3.2. Synthesis Purified decapped by heating at 550 C for°C 5 min heating by in Purified MWCNTs MWCNTswere were decapped by heating at◦ 550 for in 5 air minand in degassed air and by degassed aheating vacuuminjust before using. To a saturated solution of Dy(acac) (H O) in 10 mL of 1,2-dichloroethane, a vacuum just before using. To a saturated solution 3 2 of 2 Dy(acac)3(H2O)2 in 10 mL of which was heated atwhich 65 ◦ C for 2 hatto65 ensure Dy(acac) (H2 O)2that dissolved as 3much possible, 1,2-dichloroethane, wasabout heated °C forthat about 2 h to 3ensure Dy(acac) (H2O)as 2 dissolved 10 of decapped added. After 5 min of added. ultrasonication using bath-type sonicator as mg much as possible,MWCNTs 10 mg ofwere decapped MWCNTs were After 5 min of aultrasonication using and letting stand for 3 d, MWCNTs were collected by filtration and collected washed with 1,2-dichloroethane to a bath-type sonicator and letting stand for 3 d, MWCNTs were by filtration and washed completely remove the Dy(acac) the Dy(acac) surfaces 3of with 1,2-dichloroethane to completely remove (Hthe 2O)2MWCNTs. from the surfaces of the MWCNTs. 3 (H2 O) 2 from the 3.3. 3.3. TEM TEM Observation Observation High-resolution High-resolution transmission transmission electron electron microscopy microscopy (TEM) (TEM) and and energy energy dispersive dispersive X-ray X-ray spectroscopy (EDX) were carried out using a JEM2100F (acceleration voltage; 200 kV, JEOL Ltd., spectroscopy (EDX) were carried out using a JEM2100F (acceleration voltage; 200 kV, JEOL Ltd., Tokyo, Japan) with dry SD30GV detector (JEOL Ltd., Tokyo, Japan). The sample was dispersed Tokyo, Japan) with dry SD30GV detector (JEOL Ltd., Tokyo, Japan). The sample was dispersed in in ◦C methanol which was dried byby heating overnight at 100 methanol and and deposited depositedon onaacarbon-coated carbon-coatedCu Cugrid, grid, which was dried heating overnight at 100 −4 −4 vacuum before TEM was performed. in °Cain10a 10Pa Pa vacuum before TEM was performed.


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3.4. Thermogravimetric Analysis Thermogravimetric analysis (TGA) was performed on a SHIMADZU DTG-60 (Shimadzu Corporation, Kyoto, Japan) using aluminum oxide powder as a standard material. Several milligrams of the sample were put in an aluminum cell, and the cell was heated to 1000 ◦ C with a heating rate of 2 ◦ C/min. 3.5. Magnetic Susceptibility Measurement Magnetic susceptibility measurements were performed on a SQUID magnetometer (model MPMS-XL SQUID magnetometer, Quantum Design, Inc., San Diego, CA, USA). Samples were put into gelatin capsules, and eicosane was added to fix the samples during the measurement. DC measurements for Dy(acac)3 (H2 O)2 were performed in an HDC of 500 Oe, and those for the purified MWCNTs and Dy(acac)3 (H2 O)2 @MWCNTs were recorded in HDC of 1000 Oe. T was changed from 300 K to 1.85 K with a sweep rate of 1 K/min. Field dependent DC measurements were performed at 1.85 K while changing the magnetic field as follows: 0 Oe → 70 kOe → −70 kOe → 70 kOe. AC measurements were recorded in an HAC of 3 Oe in the frequency range of 1–1500 Hz and T range of 1.85–10 K. Diamagnetic contributions from the eicosane and Dy(acac)3 (H2 O)2 were corrected by using Pascal’s constants, and then the magnetic susceptibility for the purified MWCNTs was subtracted from that for Dy(acac)3 (H2 O)2 @MWCNTs. Magnetic moments χCNT , χCNT 0 and χCNT ” (Supplementary Materials Figures S2 and S3) were obtained by normalizing the obtained magnetic moments with the mass of CNT after applying the diamagnetic corrections. 4. Conclusions In this work, we encapsulated Dy(acac)3 (H2 O)2 SMMs in the internal nanospace of MWCNTs by using a capillary method. Encapsulation was confirmed by using TEM. From AC magnetic susceptibility measurements, both the in-phase and out-of-phase signals were clearly frequency dependent, indicating that Dy(acac)3 (H2 O)2 complexes still exhibited SMM-like properties. To the best of our knowledge, this is the first example of a lanthanoid SMM encapsulated in CNTs. Although the encapsulation of Dy(acac)3 (H2 O)2 into MWCNTs did not enhance the SMM properties, this work shows that it is possible to control the coordination environment and tune the magnetic properties of SMMs via encapsulation. In addition, we believe that the magnetic and electronic properties of lanthanoid SMM-CNT hybrids can be combined to bring about new applications in devices, like spintronic devices. Supplementary Materials: The following are available online at www.mdpi.com/1996-1944/10/1/7/s1. Figure S1: TEM image and EDX spectrum of empty MWCNT, Figure S2: χCNT and χCNT T vs. T plots for MWCNT and Dy(acac)3 (H2 O)2 @MWCNTs without correction for the diamagnetism of the MWCNTs, Figure S3: Temperature-dependence of the in-phase (χ0 ) and out-of-phase (χ”) AC magnetic susceptibilities of MWCNT and Dy(acac)3 (H2 O)2 @MWCNTs, Figure S4: Frequency-dependence of χ0 and χ” AC magnetic susceptibilities of Dy(acac)3 (H2 O)2 , Table S1: Selected values of ∆E and τ0 for Dy(acac)3 (H2 O)2 @MWCNTs. Acknowledgments: This work was supported by CREST, JST, a Grant-in-Aid for Scientific Research (S) (grant No. 20225003, Masahiro Yamashita), Grant-in-Aid for Scientific Research (C) (grant No. 15K05467, Keiichi Katoh), Grant-in-Aid for Young Scientists (B) (grant No. 24750119, Keiichi Katoh) from the Ministry of Education, Culture, Sports, Science, and Technology, Japan (MEXT). Ryo Nakanishi thanks Shorai Foundation for Science and Technology. We thank Takamichi Miyazaki (Technical Division, Department of Engineering, Tohoku University) for the support in the TEM and EDX analyses. Author Contributions: Ryo Nakanishi, Keiichi Katoh and Masahiro Yamashita conceived and designed the experiments; Mudasir Ahmad Yatoo performed the experiments; Ryo Nakanishi, Mudasir Ahmad Yatoo and Keiichi Katoh analyzed the data; Ryo Nakanishi, Brian K. Breedlove and Masahiro Yamashita wrote the paper. Conflicts of Interest: The authors declare no conflict of interest. The funding sponsors had no role in the design of the study; in the collection, analyses, and interpretation of data, in the writing of the manuscript, and in the decision to publish the results.


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