Superatoms. Группа авторов
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Название: Superatoms
Автор: Группа авторов
Издательство: John Wiley & Sons Limited
Жанр: Химия
isbn: 9781119619567
isbn:
With 39 valence electrons and an electronic configuration of 1S2 1P6 1D10 2S2 1F14 2P5, Al13 is known to mimic the chemistry of a halogen atom. Indeed, its electron affinity of 3.57 eV is almost identical to that of the Cl atom. It was theoretically predicted [30] and experimentally verified [31] that KAl13 is an ionically bonded cluster where an electron is transferred from K to Al13. Evidence that Al13 behaves like a halogen also came from an experiment of Bergeron et al. who showed that Al13I2 − can be viewed as Al13 −.2I, making it look like a triiodide (I3 −) ion [32]. Similarly, Al14I3 − can be viewed as Al14 2+.3I− with Al14 behaving like an alkaline earth element. From the above results, the authors concluded that Al13 and Al14 exhibit a new form of superatom chemistry in which superatoms behave like atoms when they react with other atoms/molecules. However, a different conclusion was reached by Han and Jung who examined whether Al n clusters exhibit multiple atomic characteristics depending upon n by studying halogenated Al n (n = 11–15) complexes and plotted the charge (Q) distribution in MX and MX2 systems (M = Al11–Al15, X = F, Cl, Br, I) vs electronegativity, η of X [33, 34]. The results are presented in Figure 2.10. Noting that the charge transfer Q(M) is nearly independent of n in Al n clusters in both the systems, the authors concluded that “there is no evidence of an alkaline earth superatom in the Al14 clusters” and that “there are no theoretical grounds to regard Al13I2 − as Al13 −.2I.”
Figure 2.7 The initial (left) and the final (right) configuration of the collapse of Na8 on Na (100). Note the epitaxial arrangement of the adatoms at the end of the run (at 2.8 ps). Both side and top views of the two configurations are shown.
Source: Hakkinen and Manninen [24]. © American Physical Society.
Figure 2.8 (a) Structure and super atomic‐molecule models of Au20 (TAu4). (b) Schematic representation for the superatom−atom D3S−s bonding of Au20 (TAu4).
Source: Cheng et al. [25]. © Royal Society of Chemistry.
Figure 2.9 Direct atomic imaging and dynamical fluctuations of the tetrahedral Au20 cluster soft‐landed on amorphous carbon substrate.
Source: Adapted with permission from Wang et al. [26]. © Royal Society of Chemistry.
Figure 2.10 Q(M) versus η (X = F, Cl, Br, I) for (a) MX (M = Al11–Al15, Al, halogen atoms) and (b) MX2 (M = Al11–Al15, Al, Si, alkaline earth atoms). The data of Al11 through Al15 basically coincide without revealing any exceptions for Al13 or Al14.
Source: Han and Jung [33]. © American Chemical Society.
For superatoms to be used as building blocks of cluster‐assembled materials, it is important that not only they be stable and mimic the chemistry of atoms but also they should remain in their virgin form when forming a crystal. Liu et al. [35] studied the stability of a KAl13 crystal confined to the CsCl structure. The hypothesis was that Al13 − clusters will stay apart from each other due to the negative charge they carry. On the contrary, they found that the Al atoms in neighboring Al13 clusters interact and KAl13 as a crystal was unstable. To determine whether changing the cation from a metal to a nonmetallic one would result in stabilizing the Al13 − icosahedral geometry, Huang et al. [36] recently studied the stability of [(CH3)4N+][Al13 −] crystal. Note that the ionization potential of (CH3)4N+ is 3.27 eV, which is even smaller than that of a K atom, namely, 4.34 eV. In addition, the diameter of (CH3)4N+ is 4.2 Å, which is comparable to the diameter of Al13, namely 5.3 Å. The binding energy of [(CH3)4N+][Al13 −] cluster, namely 2.68 eV, is also larger than that of KAl13 cluster, which is 2.49 eV. Expecting that a crystal of [(CH3)4N+][Al13 −] may be stable with both the cation and the anion maintaining their individual geometry, Huang et al. confined the initial crystal structure to three forms – (i) body‐centered‐cubic, (ii) rock salt, and (iii) zinc‐blende phases, which are common crystal structures of binary salts (e.g., CsCl, NaCl, and ZnO) (see Figure 2.11). After optimization, the results show that while the (CH3)4N maintains its structure in all of the above systems, Al13 clusters coalesce, ceasing to remain as individual clusters. It is, thus, safe to conclude that stable metallic clusters with electronic closed shells are not good candidates for cluster‐assembled materials.
Figure 2.11 Initial crystal structure for (a) body‐centered‐cubic (bcc), (b) rock salt (rs), and (c) zinc‐blende (zb) phases of (CH3)4N+Al13− bulk. (d) Optimized structures of (CH3)4N+Al13− show the coalescence of Al13 clusters when forming a bulk material.
Source: Huang [36]. © American Chemical Society.
On the contrary, jellium shell closure rule has been effectively used to explain the stability of ligated metal clusters, particularly ligated Cu, Ag, СКАЧАТЬ