Superatoms. Группа авторов
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calculated by Sun et al. [23]. Clearly, its geometry is not spherical. However, the molecular orbitals of Na20 (Figure 2.5) show strong resemblance with that in the jellium model. The nondegenerate highest occupied molecular orbital (HOMO) is primarily a 2S orbital and HOMO‐q (q = 1–5) are d‐type, q = 6–8 are p‐type, and q = 9 is s‐type, just as the case in the jellium model. In addition, a HOMO–lowest unoccupied molecular orbital (LUMO) gap of 1.43 eV is indicative of a chemically inert behavior of Na20 cluster.
Figure 2.4 Ground‐state geometry of Na20.
Source: Sun et al. [23]. © American Chemical Society.
Figure 2.5 Molecular orbitals and energy levels of neutral Na20 cluster. The HOMO–LUMO energy gap is indicated (in green).
Source: Sun et al. [23]. © American Chemical Society.
To what extent can a jellium model describe the interaction between two real clusters was further investigated by Hakkinen and Manninen [24] by taking into account the geometries and electronic structure of clusters, explicitly. Using molecular dynamics and density functional theory, they considered a Na8 cluster in a variety of surroundings. In the gas phase, Na8 cluster was found to retain its geometry even up to 600 K. But, when two Na8 clusters are brought together (see Figure 2.6), they collapse forming a deformed Na16 cluster and the electronic shell structure is destroyed. They further showed that Na8 cluster forms an epitaxial layer (Figure 2.7) when supported on a Na (100) surface. This shows that Na8 is a magic cluster only when it is held in isolation.
Figure 2.6 Reaction between two Na8 clusters in vacuum. (a) Time evolution of the potential energy relative to its value in the initial configuration (solid curve, scale on the left) and the center‐of‐mass distance of the two clusters (dotted curve, scale on the right). The two snapshots indicate the initial configuration (left) and the configuration at 2.6 ps (right). (b) Time evolution of the Kohn‐Sham eigen values. The dotted curves indicate empty states.
Source: Hakkinen and Manninen [24]. © American Physical Society.
While Na8 was found to see its geometry destroyed when interacting with another Na8 cluster or when supported on a Na (100) surface, the result for Au20 is different. Note that according to the jellium model, Au20 is also a closed shell cluster. Although it has a pyramidal geometry (Figure 2.8), its molecular orbitals show the same pattern as in the jellium model [25]. As shown in Figure 2.9, Au20 maintains its pyramidal structure when deposited on a carbon substrate [26]. However, it is not clear if Au20 would continue to maintain its gas phase geometry when interacting with each other or when supported on an Au substrate? No studies have yet been done to make any conclusion. However, based on extensive studies of Al13, another free electron meal cluster [27–36], it is unlikely that Au20 would maintain its virgin geometry in the above situation.
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.