68 | Nature | Vol 577 | 2 January 2020
Article
S along a magnetic easy direction and the onset of superparamagnet-
ism. In support of this observation, variable-field magnetization data
collected for 1 (FeCl 2 ) 19 with a sweep rate of 9 mT s−1 revealed magnetic
hysteresis at 2 K with a coercive field of Hc = 70 mT (Fig. 4f).
To determine the magnitude of the barrier to spin reversal for the
confined sheets in 1 (FeCl 2 ) 19 , we collected temperature-dependent
a.c. magnetic susceptibility data under zero d.c. field and at discrete
frequency values ranging from 1 to 1,000 Hz (Extended Data Fig. 6).
A maximum was observed in both the in-phase (χ′) and out-of-phase
(χ′′) magnetic susceptibility data, with the peak shifting only 1.3 K
over the measured frequency range. The frequency dependence in
χ′′ precludes the existence of long-range magnetic ordering, and this
low-temperature behaviour can instead be attributed to either super-
paramagnetism or a glassy magnetic phase transition. The magnitude
of the frequency shift can be quantified using the Mydosh parameter
(γ), which adopts characteristic values for different magnetic behav-
iours^23. We find a Mydosh parameter of 0.14 for the FeCl 2 clusters, which
is most consistent with superparamagnetism. An Arrhenius fitting of
the a.c. susceptibility data affords physically meaningful values for the
spin reversal barrier of Ueff = 16 cm−1 and a relaxation attempt time of
τ 0 = 10−10 s. Notably, these values are competitive with iron(ii) cluster-
based single-molecule magnets^24.
As an additional probe of the magnetic behaviour of 1 (FeCl 2 ) 19 ,
Mössbauer spectra were collected on a microcrystalline sample at
temperatures ranging from 5 to 295 K (Extended Data Fig. 7 and Sup-
plementary Fig. 24). At all temperatures, the spectra exhibit iron(ii)
quadrupole doublets, consistent with the crystallographic environ-
ments in 1 (FeCl 2 ) 17 (Supplementary Fig. 25 and Supplementary Tables 4
and 5), and the spectral fits between 10 and 295 K indicate that 1 (FeCl 2 ) 19
behaves as a paramagnet at these temperatures. Upon cooling from
8 to 5 K, a substantial broadening is observed concomitant with the
gradual appearance of a superparamagnetic sextet with an average
iron(ii) hyperfine field of 9.8(2) T. In conjunction with the a.c. mag-
netic susceptibility data, these results support the observation that
the framework-confined iron(ii) chloride sheet fragments exhibit
superparamagnetism below 8 K.
As research into the electronic and magnetic properties of two-
dimensional materials intensifies^25 ,^26 , increased attention is being paid
towards lateral confinement of monolayers to yield two-dimensional
clusters or quantum dots^27 ,^28. Analogous to the confinement of three-
dimensional materials, confinement of two-dimensional materials is
anticipated to reveal distinct or enhanced physical and chemical prop-
erties, including those associated with edge states^29 ,^30. Understanding
and exploiting the structural influences on the properties of these
two-dimensional clusters will therefore require chemical syntheses
that yield monodisperse and well defined materials.
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- Bawendi, M. G., Steigerwald, M. L. & Brus, L. E. The quantum mechanics of larger
semiconductor clusters (“quantum dots”). Annu. Rev. Phys. Chem. 41 , 477–496
(1990). - Billas, I. M., Châtelain, A. & de Heer, W. A. Magnetism from the atom to the bulk in iron,
cobalt, and nickel clusters. Science 265 , 1682–1684 (1994). - Lee, S. C. & Holm, R. H. Nonmolecular metal chalcogenide/halide solids and their
molecular cluster analogues. Angew. Chem. 29 , 840–856 (1990). - Gatteschi, D., Caneschi, A., Pardi, L. & Sessoli, R. Large clusters of metal ions: the
transition from molecular to bulk magnets. Science 265 , 1054–1058 (1994). - Xia, Y., Xiong, Y., Lim, B. & Skrabalak, S. E. Shape-controlled synthesis of metal
nanocrystals: Simple chemistry meets complex physics? Angew. Chem. 48 , 60–103
(2009). - Papatriantafyllopoulou, C., Moushi, E. E., Christou, G. & Tasiopoulos, A. J. Filling the gap
between the quantum and classical worlds of nanoscale magnetism: giant molecular
aggregates based on paramagnetic 3d metal ions. Chem. Soc. Rev. 45 , 1597–1628 (2016). - Kim, C. R., Uemura, T. & Kitagawa, S. Inorganic nanoparticles in porous coordination
polymers. Chem. Soc. Rev. 45 , 3828–3845 (2016). - Meilikhov, M. et al. Metals@MOFs – Loading MOFs with metal nanoparticles for hybrid
functions. Eur. J. Inorg. Chem. 2010 , 3701–3714 (2010). - Lin, H. et al. Bulk assembly of organic metal halide nanotubes. Chem. Sci. 8 , 8400–8404
(2017). - Chen, K.-J., Perry, J. J., Scott, H. S., Yang, Q.-Y. & Zaworotko, M. J. Double-walled pyr
topology networks from a novel fluoride-bridged heptanuclear metal cluster. Chem. Sci.
6 , 4784–4789 (2015). - Gallington, L. C. et al. Regioselective atomic layer deposition in metal–organic
frameworks directed by dispersion interactions. J. Am. Chem. Soc. 138 , 13513–13516
(2016). - Braglia, L. et al. Tuning Pt and Cu sites population inside functionalized UiO-67 MOF by
controlling activation conditions. Faraday Discuss. 201 , 265–286 (2017). - An, B. et al. Confinement of ultrasmall Cu/ZnOx nanoparticles in metal-organic
frameworks for selective methanol synthesis from catalytic hydrogenation of CO 2. J. Am.
Chem. Soc. 139 , 3834–3840 (2017). - Chen, L., Chen, H., Luque, R. & Li, Y. Metal−organic framework encapsulated Pd
nanoparticles: towards advanced heterogeneous catalysts. Chem. Sci. 5 , 3708–3714
(2014). - Wang, X.-N. et al. Crystallographic visualization of postsynthetic nickel clusters into
metal–organic framework. J. Am. Chem. Soc. 141 , 13654–13663 (2019). - Gonzalez, M. I., Bloch, E. D., Mason, J. A., Teat, S. J. & Long, J. R. Single-crystal-to-single-
crystal metalation of a metal-organic framework: a route toward structurally well-defined
catalysts. Inorg. Chem. 54 , 2995–3005 (2015). - Gonzalez, M. I., Oktawiec, J. & Long, J. R. Ethylene oligomerization in metal-organic
frameworks bearing nickel(II) 2,2′-bipyridine complexes. Faraday Discuss. 201 , 351–367
(2017). - Ketelaar, J. A. A. Die Kristallstruktur des Nickelbromids und -jodids. Z. Kristallogr. Cryst.
Mater. 88 , 26–34 (1934). - Ferrari, A., Braibanti, A. & Bigliardi, G. Refinement of the crystal structure of NiCl 2 and of
unit-cell parameters of some anhydrous chlorides of divalent metals. Acta Crystallogr. 16 ,
846–847 (1963). - George, P. & McClure, D. S. The effect of inner orbital splitting on the thermodynamic
properties of transition metal compounds and coordination complexes. In Progress in
Inorganic Chemistry Vol. 1 (ed. Cotton, F. A.) 381–463 (Interscience, 1959). - Wilkinson, M. K., Cable, J. W., Wollan, E. O. & Koehler, W. C. Neutron diffraction
investigations of the magnetic ordering in FeBr 2 , CoBr 2 , FeCl 2 , and CoCl 2. Phys. Rev. 113 ,
497–507 (1959). - Tsubokawa, I. The magnetic properties of NiBr 2. J. Phys. Soc. Jpn. 15 , 2109 (1960).
- Mydosh, J. A. Spin Glasses (Taylor & Francis, 1993).
- Milios, C. J. & Winpenny, R. E. P. in Molecular Nanomagnets and Related Phenomena (ed.
Gao, S.) 1–109 (Springer, 2014). - Huang, B. et al. Layer-dependent ferromagnetism in a van der Waals crystal down to the
monolayer limit. Nature 546 , 270–273 (2017). - Gong, C. et al. Discovery of intrinsic ferromagnetism in two-dimensional van der Waals
crystals. Nature 546 , 265–269 (2017). - Wang, Y. et al. Cryo-mediated exfoliation and fracturing of layered materials into 2D
quantum dots. Sci. Adv. 3 , e1701500 (2017). - Lin, L. et al. Fabrication of luminescent monolayered tungsten dichalcogenides quantum
dots with giant spin-valley coupling. ACS Nano 7 , 8214–8223 (2013). - Ritter, K. A. & Lyding, J. W. The influence of edge structure on the electronic properties of
graphene quantum dots and nanoribbons. Nat. Mater. 8 , 235–242 (2009). - Wilcoxon, J. P. & Samara, G. A. Strong quantum-size effects in a layered semiconductor:
MoS 2 nanoclusters. Phys. Rev. B 51 , 7299–7302 (1995).
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