One of the challenges in nucleation and growth of small atomic aggregates is to develop the novel research field of solid-state chemistry and material science using gas-phase-synthesized superatom nanoclusters. Among various superatoms, the greatest advantage of the central-atom-encapsulating binary superatoms, such as M@Si₁₆, is that their electronic structures can be designed by optimizing the central metal atom M while retaining the geometrical symmetry of the Si₁₆ cage [1], as shown in the figure below. We have systematically investigated the nucleation and growth of M@Si₁₆ superatoms with gas-phase mass spectrometry, and then performed the surface immobilization of the M@Si₁₆ cations and anions on a substrate monodispersively with the size-selective soft-landing. Initial products prepared via the surface immobilization of M@Si₁₆ superatoms on solid surfaces decorated with monolayer films of C₆₀ molecules were investigated using scanning tunneling microscopy (STM) [2] and X-ray photoelectron spectroscopy (XPS) [3]. Furthermore, we have developed a large-scale synthesis method for M@Si₁₆ (M = Ti and Ta) by scaling up the clean dry-process with a high-power impulse magnetron sputtering (HiPIMS, nanojima ^®) [4] and by a direct liquid embedded trapping (DiLET) method [5]. The spectroscopic results reveal that the structures of soft-landed and isolated M@Si₁₆ superatoms are the metal-encapsulating tetrahedral silicon-cage (METS) [5,6].

Furthermore, superatoms of group-5 metals (M = V, Nb, and Ta) encapsulating Si₁₆ cage nanoclusters (M@Si₁₆) can be efficiently generated to form assembled films [7]. Temperature-dependent current–voltage (I–V) characteristics of the M@Si₁₆ assembled films revealed that the electrical conduction mechanism is not band transport, but hopping transport with Efros–Shklovskii variable range hopping for all central M atoms [8]. The results show that electrons involved in conduction are strongly correlated to localized electronic states.

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[2] M. Nakaya, A. Nakajima, et al. Nanoscale, 6, 14702 (2014).

[3] M. Shibuta, A. Nakajima, et al. J. Am. Chem. Soc. 137, 14015 (2015).

[4] C. H. Zhang, A. Nakajima, et al. J. Phys. Chem. A 117, 10211 (2013).

[5] H. Tsunoyama, A. Nakajima, et al. J. Phys. Chem. C 121, 20507 (2017).

[6] H. Tsunoyama, A. Nakajima, et al. Acc. Chem. Res. 51, 1735 (2018).

[7] M. Shibuta, A. Nakajima, et al. J. Phys. Chem. C, 124, 28108 (2020).

[8] T. Yokoyama, A. Nakajima, et al. J. Phys. Chem. C, 125, 18420 (2021).