Anomalous Diamagnetic Susceptibility in 13‐Atom Platinum Nanocluster Superatoms

Abstract

Abstract:We are used to predict diamagnetic susceptibilities χ D to a good approximation by atomic increments since there is normally little dependence on the chemical environment. Surprisingly, we find from SQUID magnetization measurements that χ D per Pt atom of zeolite supported Pt 13 nanoclusters exceeds that of Pt 2+ ions by a factor of 37-50. The observation verifies an earlier theoretical prediction. The phenomenon is understood nearly quantitatively on the basis of a simple expression for diamagnetic susceptibility and the superatom nature of the 13-atom near-spherical cluster. The two main contributions come from ring currents in the delocalized hydride shell, the second one from cluster molecular orbitals hosting the Pt 5d and 6s electrons.Nanosize particles of metallic elements have attracted great attention since their properties often deviate markedly from those of the bulk metal. For example, micro-and nanosize platinum powder has been reported to become superconducting, [1][2][3] while bulk Pt does not. There are predictions that metal clusters with 10 2 -10 3 free carriers may become superconducting. [4] Furthermore, at low temperature, Pt nanoparticles and clusters show pronounced superparamagnetism, [5][6][7] whereas bulk Pt has a small positive magnetic susceptibility due to Pauli paramagnetism that is slightly temperature dependent. [8] Other work predicts a greatly enhanced diamagnetism in metal clusters, [9,10] but there has been no experimental verification of this. Diamagnetism is an important indicator of superconductivity as well, which raises the interest in this property for small clusters even further. However, since the London penetration depth of superconductors is normally on the order of 100 nm one should not expect to find an ideal diamagnetism with full exclusion of the magnetic field for objects of less than 1 nm diameter. Moreover, since magnetic properties reflect the wavefunction they are of general interest in physics as well as in chemistry.We have previously demonstrated that monodisperse icosahedral or cuboctahedral platinum nanoclusters with 13 ± 2 atoms can be prepared supported within the porous structure of NaY zeolite. [6] Although they are all of the same size they exist in three different magnetic states: 15-20% in a high-spin state with a magnetic moment of µ = 3.7 ± 0.4 and 3.0 ± 0.4 µ B for Pt 13 and Pt 13 H m , respectively, a very small fraction of <1% contributes to the spin-½ EPR signal, and the rest is diamagnetic. [7] The coexistence of different electronic states of the same cluster may be rationalized by different local environments, such as the presence of different numbers of zeolite framework Al atoms or extra framework cations, Na + or K + , in close proximity to the cluster, and the extent of hydrogen coverage will also play a role.Here we resume the subject of magnetism with these clusters using a specially synthesized iron-free L zeolite (Supporting Information) to avoid any interference of iron and ambiguities in the interpretation of the data...

Magnetic Properties and the Superatom Character of 13-Atom Platinum Nanoclusters

“…Here, however, depending on the hydrogen coverage, χD is larger by a factor 36-50 than the expected tabulated value of ca. −12 cm 3 •mol −1 [7]. This enhancement is spectacular and reveals a superatom effect that has been predicted for sodium clusters [6] but experimentally observed so far only for the Pt13 clusters [7].…”

“…As expected, based on the Curie law, the paramagnetism of the high-spin clusters dampens out quickly with increasing temperature. Surprisingly, at 300 K, a negative magnetization is observed that is proportional with the applied magnetic field ( Figure 6) [7]. The slope of the magnetization is the diamagnetic susceptibility, χD.…”

“…However, it is well known that ferromagnetism is greatly affected when the dimensionality of the material is reduced to thin films or monoatomic chains [1], and in nanoparticles, elements which are typically diamagnetic in the bulk, like Au, Pd, and Pt, develop an increasingly higher magnetic moment with decreasing size [2,3] and show high-spin states in well-defined clusters [4,5]. Less well known, although predicted [6] but observed experimentally for the first time only recently [7], is the fact that the diamagnetic moment per atom increases dramatically in small clusters. An understanding of these size-dependencies is of considerable interest for fundamental reasons.…”

ABCluster: the artificial bee colony algorithm for cluster global optimization

“…For instance, gold clusters can help building novel materials 3 and the icosahedral 13-atom platinum cluster has anomalous large diamagnetic susceptibility 4 . Clusters have attracted intense attention from scientific communities 1,2 due to their practical and theoretical interest.…”

Spectroscopy of metal “superatom” nanoclusters and high-Tcsuperconducting pairing

“…More fundamentally, one cannot do a resistance measurement, and the Meissner effect would be too weak for magnetometry or SternGerlach-type [36] beam deflection [37]. (In addition, a Larmor diamagnetic response would be exhibited by closed-shell clusters even in their normal state [40][41][42]. )…”