Press Release

March 25, 2022
Keio University

Success in fabricating aluminum superatom substrates—Development of novel functional nanostructured substrates for chemical
conversion and sensors—
Professor Atsushi Nakajima, Project Associate Professor Masahiro Shibuta (at the time ofresearch; currently Associate Professor at Osaka City University), and Assistant Professor
Tomoya Inoue et al., have succeeded in fabricating aluminum (Al) superatom※1 substrates by sizeselectively depositing※2 Al nanoclusters which were synthesized in the gas phase on an organic
substrate※3 pre-decorated with organic molecules, revealing that Al 13-mer superatoms were fixed
to the surfaces in a negative ion state.
In order to overcome global environmental problems, it is crucial that a functional nanostructured
substrate is developed to facilitate more efficient processes for chemical and energy conversion.
Nanocluster superatoms are novel functional nanoscale units which are composed of several toseveral tens of atoms because nanocluster superatoms exhibit novel physical and chemical
properties based on discrete electronic states mimicking that of an atom. However, there had been
an intrinsic problem that nanocluster superatoms could not be stabilized on substrates owing togeometric deformations and the change in charge states through a nanocluster-substrateinteraction, along with a methodological issue for the synthetic difficulty of efficient generation of
nanocluster superatoms.
In this research, anionic Al 13-mer superatoms of Al13− were synthesized abundantly in a gas
phase※4 and the Al13− superatoms were fixed onto a substrate orderly using the organic substrates
pre-decorated by organic molecules such as C60. In general, nanoscale aluminum-based materials
exhibit extreme chemical reactivity and are instantly oxidized. However, the research group foundthat the oxidation reactivity rates of Al-superatom substrates could be reduced by about two digits
by fabricating ordered assembled-films of Al13− superatoms. The results can be regarded highly
useful in terms of developing functional nanostructured substrates with Al13− superatoms, leading
to the creation of nanostructured functionalities toward the next generation chemical and energy
conversions.
The results of this research were published in the academic journal ‘‘Nature Communication’’ onMarch 14, 2022.
1. Main Points of Research・Nanocluster superatoms consisting of a specific number of atoms are known as novel functionalunits in atomic aggregates from several to several tens of atoms.
・The methodologies of synthesizing superatoms had not been established in terms of synthesizing
large quantities and non-destructive immobilization on substrates.
・Researchers achieved large scale synthesis of aluminum 13-mer superatoms by a gas-phase1/6

method, and immobilized anionic Al13− superatoms on well-defined substrates pre-decorated byorganic molecules.
・Observations of the electronic states and chemical reactivities revealed that the supported Al13−superatoms are stably immobilized through substrate interactions associated with pre-decorated
electron-donating molecules.
・Researchers expect that ordered functional nanostructures based on superatoms will facilitate
the creation of highly efficient chemical transformation catalysts and energy conversion
nanomaterials using common elements in the future.
2. Background of Research“Nanocluster superatoms” (hereinafter simply “superatoms”) consisting of a specific number ofatoms are known as novel functional units in atomic aggregates in the order of several to several
tens of atoms, and are expected to be functional nanomaterial sources for global society in the nextgeneration. For example, carbon 60-mer (C60; fullerene) is regarded as one of the representative
superatoms, and it has been utilized widely as a material for, among other things, solar cells. The
extensive search for new superatoms and the evaluations of their functionalities have been
investigated through gas phase syntheses, leading to immobilizing the superatoms on solidsubstrates. The superatom substrates are expected to be applicable to the chemical catalysts usingnovel nanostructured materials with inexpensive metal elements and to energy conversion
materials exhibiting high photoelectric conversion efficiency.
Above all, the anionic Al13− superatom, composed of 13 Al atoms and a negative charge, is one of
the most studied superatoms, which are greatly stabilized by simultaneous geometric andelectronic factors; closely-packed icosahedral structures in which an Al atom are symmetricallysurrounded by 12 Al atoms and electron shell closings with the total number of 40 valence
electrons (Fig. 1). The latter electronic stability of 40 electrons is attributed to completing electronshells by three valence electrons on each Al atom (39 electrons) together with a charge electron ofthe anion. Since the electronic feature in Al13 is similar to that of a fluorine or chlorine atom, Al13
is called a “halogen superatom.”

2P6
1F14
2S2
1D10
1P6
1S2
Al13 superatom（Icosahedron）

Superatomic orbital（shell-closing at 40 electrons）

Fig. 1. Geometric and electronic structures of Al 13-mer anion (Al13− superatom)
In terms of the application of novel functional nanomaterials using superatoms, the methodologies
had been required not only for synthesizing superatoms in a large-scale quantity but also forimmobilizing superatoms non-destructively on substrates. Specifically, there are a lot of issues
with superatom depositions, such as the structural deformation on a substrate surface,aggregation on a surface, and the loss of intrinsic natures of superatoms due to charge transfer
between superatoms and the substrate. Furthermore, experts considered it difficult to immobilize
Al13− superatoms on substrates because nanoscale aluminum-based materials exhibit extreme
chemical reactivity and are instantly oxidized.

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3. Content of Research and Results◆ Surface immobilization of size-selected Al nanoclusters
This research group developed a gas-phase nanocluster generation apparatus (nanojima🄬🄬), ※ 5
enabling the synthesis of nanoclusters in a vacuum with large quantities and achieved massivesynthesis of pure Al13− superatoms reaching 1 billion to 10 billion ions per second. Using thenanocluster source under soft-landing conditions while controlling the collisional energy to asubstrate, Al13− superatoms are non-destructively deposited as a monolayer onto a substratesurface in a few hours.
Specifically, to realize the anionic charge state of Al13− superatoms on a substrate, the highlyordered organic substrate pre-decorated by coronene derivative molecules (HB-HBC)※6 was used
as an electron-donnor (p-type) organic substrate. For comparison, the organic substrate predecorated by an electron-acceptor (n-type) C60 was prepared for the substrate, and the electronicstates and chemical reactivities were evaluated when Al13− superatoms were deposited on eachorganic substrate.
◆ The stability of Al 13-mer anion (Al13− superatoms)
The Al13− superatom substrates were fabricated by the deposition of Al13− on the pre-decorated byn-type (C60) or p-type (HB-HBC) organic molecules in a well-ordered manner, and the Al13−
substrates were transported to an X-ray photoelectron spectroscopy (XPS) ※ 7 apparatus while
keeping ultra-high vacuum conditions, where the charge state and chemical reactivity wereevaluated. The oxidation state of the Al atom in Al13− superatoms deposited on n-type C60 organic
substrate was +3 state (Fig. 2a bottom), showing that Al13 immediately reacted after the depositionof Al13− superatoms with residual gas like oxygen or water molecules even under ultra-highvacuum conditions. In fact, XPS spectra negligibly changed with further oxygen gas dosing (Fig.
2a top), indicating that Al13 were easily oxidized at the deposition onto n-type C60 substrate (Fig.
2b).
On the other hand, the oxidation state of Al atom in Al13− superatoms deposited on p-type HBHBC organic substrate was metallic 0 valent state (Fig. 2c bottom), showing that individual Al13−
superatoms were immobilized on the substrate monodispersely without oxidations. Moreover, interms of the electronic state of p-type HB-HBC substrate, HB-HBC molecules were positivelycharged by donating an electron to Al13. The results show that, as well as halogen elements likefluorine and chlorine atoms, halogen superatoms of Al13 were electronically stabilized on a p-type
HB-HBC substrate as Al13− superatoms (Fig. 2d). The behavior can be rationalized by the localinteractions between superatoms and the substrate while controlling the charge state (cation oranion) by pre-decorating appropriate organic molecules with respect to optimizing the propertiesof the substrate. This study could reveal a new strategy for designing organic substrates to create
a functional substrates using superatoms as building blocks for nanostructured units.

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Al 2pb

Al13/C60

C60 fullerene(n-type)

After O2exposure

Easily oxidized
by O2 exposure
As-deposited+3Al 2p0d

Al13/HB-HBC
After O2exposure

aCoronene derivative
HB-HBC (p-type)
Side view+As-deposited+30Binding energy (electron volt)

Fig. 2. XPS spectra for Al 2p (a, c) and schematic images of Al13− superatoms supported on anorganic surface (b, d)◆ Specific stability of Al13− superatoms
To examine the specific stability of Al13− superatoms on substrates, Al nanoclusters with adifferent number of constituent atoms (Aln−; n is the number of constituent atoms, n = 7-24) weresimilarly synthesized and deposited on p-type HB-HBC organic substrates. The oxidative
reactivities on each size of Aln− were evaluated by XPS measurements for Aln− nanocluster
substrates before and after oxygen molecule exposure. In terms of the oxidative reactivities in Fig.
3, Al13− superatoms are about two digits more stable than other size Aln− nanoclusters. Thus, thesize-selective deposition of Aln− nanoclusters with nanojima🄬🄬 apparatus enables us to reveal theimportance of a specific number of atoms (13 Al atoms in this research) on a substrate including
the local interaction between superatoms and the molecular substrate.

B@Al12−superatom

Al13 −
superatom

AlnAlnB

Number of Al atoms (n)Fig. 3. The relative reactivity of supported Al nanoclusters against oxygen molecules showing
the specific stabilities of Al13− and B@Al12− superatoms.
◆ Specific stability of boron encapsulating Al superatoms
The Al13- superatoms are geometrically stabilized by a closely-packed icosahedral structure inwhich 12 Al atoms symmetrically surround a central Al atom (Al@Al12 structure, @ means
encapsulation, [Fig. 1]). In this study, the geometric stability was examined by substituting thecentral Al atom for a boron (B) atom, which belongs to the same group in the periodic table,
because a smaller B atom was anticipated to enhance the geometric packing. B@Al12− superatoms
were generated using Al targets mixed with B elements and deposited on p-type HB-HBC
substrate. The reactivity measurements against oxygen molecules showed that B@Al12−

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superatoms exhibit high chemical stability equal to or better than that of Al13− superatoms andthat the B atom was encapsulated by the surrounding Al12 atoms. With respect to superatom
formation, a method of combining different elements can widen the synthesizable superatoms andthe controllability of chemical stability, while the number of constituent atoms is only a parameter
for a single element nanocluster. In addition to Al13− superatoms, the formation stable B@Al12−superatoms will be important milestones for creating functional nanomaterials using superatoms.
4. Future Developments
This study in a bottom-up approach has successfully paved the road to utilize Al13− superatoms
generated in the gas phase for functional nanomaterials, together with generating the know-how
on supporting substrates with ordered organic substrate. Moreover, the finding of binary B@Al12−
superatoms is promising in the exploration of research for novel functional nanostructures basedon Al based superatoms. Future research involving the interdisciplinary study of superatoms andsurface science based on this study is expected to pave the way for previously inconceivable nextgeneration functionalities of ordered nanostructure substrates that facilitate the creation of more
efficient chemical transformation catalysts and energy conservation nanomaterials using commonelements.
5. Special Notes※This work is supported in part by:・JSPS KAKENHI of Grants-in-Aid for Scientific Research (A) No. 19H00890 (A.N.)
・JSPS KAKENHI of Grants-in-Aid for Scientific Research (A) No. 18K04942 (M.S.)・JSPS KAKENHI of Grants-in-Aid for Challenging Research No. 17H06226 (A.N.)
・JSPS KAKENHI of Grants-in-Aid for Challenging Research No. 21K18939 (A.N.)
・MEXT KAKENHI of Grants-in-Aid for Transformative Research Areas (A) ‘‘Hyper-Ordered
Structure Science’’ (21H05573) (A.N.)＜Terms＞※1 superatoms
One of nanoclusters in which atomic aggregates enhance a rational property mimicking asingle atom in spite of aggregates of more than a few to tens of atoms.
※2 depositing
Immobilization of the nanoclusters or nanoparticles on the surfaces of solid or powder byadhesion.
※3 organic substrate
Highly-ordered substrate in which organic molecules are assembled as a thin film by vacuum
vaporization to decorate a base substrate such as graphite.

※4 gas phase synthesis
The method for synthesizing chemical species with ions or plasma in an inert gas such as
helium. Since the chemically pure species are generated under solvent-free conditions, low
impurities are contaminated, and the method is applicable to synthesizing nanostructured
species consisting of highly reactive metal elements. By combining a mass-selection technique,
it is possible to deposit nanoclusters (included superatoms) onto a substrate for preparing
atomically precise nanocluster assemblies.

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※5 gas-phase nanocluster generation apparatus (nanojima🄬🄬)
The apparatus for synthesizing pure nanoclusters without ligands efficiently in a gas phasewith high power impulse magnetron sputtering (HiPIMS) method. Coupled with thedeposition apparatus, nanoclusters can be deposited on substrates or powders in a nondestructive and monodisperse manner. The apparatus is called nanojima🄬🄬 by the trademark
registration of ayabo Co. Ltd.
※6 HB-HBC
Hexa-tert-butyl-hexa-peri-hexabenzocoronene (C66H66) is one of the polycyclic aromatichydrocarbons and a planar organic molecule which act as a p-type organic semiconductor.
※7 X-ray photoelectron spectroscopy (XPS)A spectroscopic method to measure the composition of sample material and the chemicalbinding environments of constituent elements by measuring the energy of photoelectrons
emitted from X-ray irradiation for a sample.
Details of Journal Article
Journal: Nature Communications
Paper title: ‘‘Al13− and B@Al12− superatoms on a molecularly decorated substrate’’Authors: Masahiro Shibuta1, Tomoya Inoue2, Toshiaki Kamoshida2, Toyoaki Eguchi3, and Atsushi
Nakajima1,2
1 Keio Institute of Pure and Applied Science, Keio University, 2 Department of Chemistry, Facultyof Science and Technology, Keio University, 3 Department of Physics, Graduate School of Science,Tohoku University
URL: https://www.nature.com/articles/s41467-022-29034-9
DOI: 10.1038/s41467-022-29034-9*Please direct any requests or inquires to the contact information provided below.
・Inquiries about research
Keio University Faculty of Science and Technology, Department of Chemistry,
Professor Atsushi Nakajima
Tel: +81-45-566-1712
Fax: +81-45-566-1697 Email: nakajima@chem.keio.ac.jp
・Inquiries about press release
Keio University Office of Communications and Public Relations (Mr. S. Sawano)
Tel: +81-3-5427-1541
Fax: +81-3-5441-7640 Email: m-pr@adst.keio.ac.jp
https://www.keio.ac.jp/

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