8 Electronic Properties of Transition Metal-Benzene Sandwich Clusters
343
molecule that yields V n Bz m
0/− (m = n – 1 and n). Furthermore, we described our
recent research that studied a new family of manganese-benzene cluster anions,
Mn n Bz n
− , which were found to exhibit unprecedented multiple-decker structures
with a tilted Mn-Bz stacking and a monotonically increasing behavior of their highspin multiplicities.
Lastly, we herein refer to several studies that are aimed at the controlled assembly
of transition metal-benzene sandwich clusters. For the functionalization of such
clusters, a promising route is to assemble the clusters on surfaces with certain
hierarchical nanostructures and tailored dimensionality. However, the fabrication
of cluster-based assemblies on surfaces requires a deposition technique that allows
clusters to be deposited softly onto a surface, as clusters may be dissociated
upon impact. In addition, a deep understanding of the cluster-surface interaction
is crucial because cluster structures are often perturbed by a strong interaction
with surfaces. To this end, Judai et al. [60, 118] adopted a soft-landing technique
to deposit VBz 2
+ cluster cations into a low-temperature Ar matrix with precise
control of deposition energy. The Ar matrix was chosen as it is known to serve
as a buffer dissipating the kinetic energy of the projectile clusters and keeping
the cluster largely intact [119–120]. Nagaoka et al. later deposited VBz 2
+ [121],
V 2 Bz 3
+ [122], and Cr(aniline) 2
+ cations [123] onto alkanethiolate self-assembled
monolayers (SAMs). Infrared reflection absorption spectroscopy (IRAS) confirmed
that, on both the substrates, the deposited clusters are neutralized and maintain
sandwich structures. While the use of Ar matrix is limited to cryogenic temperature
conditions, SAMs can support the sandwich clusters even at room temperature.
In addition, the sandwich clusters can be highly oriented on the SAMs. Very
recently, Huttmann et al. [124] synthesized europium-cyclooctatetraene (Eu-COT)
nanowires on a graphene substrate, where the nanowires are found to be lying
parallel to each other. Scanning tunneling spectroscopy (STS) and low-temperature
X-ray magnetic circular dichroism (XMCD) revealed that the Eu-COT wire is
a ferromagnetic insulator [125]. These immobilization methods, together with
spectroscopic applications and theoretical approaches, can readily provide a direct
route to investigate the low-dimensional functionality of transition metal-benzene
sandwich clusters on a substrate. As such, functionalized sandwich clusters will
open up possibilities for exploiting themselves as new building blocks in future
molecular electronics and spintronics.
Acknowledgments We are grateful to Prof. S. Yabushita (Keio University), Dr. T. Iwasa
(Hokkaido University), and Prof. K. Kanoda (The University of Tokyo) for fruitful discussion.
This work is partly supported by the program of Exploratory Research for Advanced Technology
(ERATO) in Japan Science and Technology Agency (JST) entitled with “Nakajima Designer
Nanocluster Assembly Project” and by JSPS KAKENHI of Grant-in-Aids for Scientific Research
(A) Grant Number 15H02002. The computations were partly performed using Research Center for
Computational Science, Okazaki, Japan.
343
molecule that yields V n Bz m
0/− (m = n – 1 and n). Furthermore, we described our
recent research that studied a new family of manganese-benzene cluster anions,
Mn n Bz n
− , which were found to exhibit unprecedented multiple-decker structures
with a tilted Mn-Bz stacking and a monotonically increasing behavior of their highspin multiplicities.
Lastly, we herein refer to several studies that are aimed at the controlled assembly
of transition metal-benzene sandwich clusters. For the functionalization of such
clusters, a promising route is to assemble the clusters on surfaces with certain
hierarchical nanostructures and tailored dimensionality. However, the fabrication
of cluster-based assemblies on surfaces requires a deposition technique that allows
clusters to be deposited softly onto a surface, as clusters may be dissociated
upon impact. In addition, a deep understanding of the cluster-surface interaction
is crucial because cluster structures are often perturbed by a strong interaction
with surfaces. To this end, Judai et al. [60, 118] adopted a soft-landing technique
to deposit VBz 2
+ cluster cations into a low-temperature Ar matrix with precise
control of deposition energy. The Ar matrix was chosen as it is known to serve
as a buffer dissipating the kinetic energy of the projectile clusters and keeping
the cluster largely intact [119–120]. Nagaoka et al. later deposited VBz 2
+ [121],
V 2 Bz 3
+ [122], and Cr(aniline) 2
+ cations [123] onto alkanethiolate self-assembled
monolayers (SAMs). Infrared reflection absorption spectroscopy (IRAS) confirmed
that, on both the substrates, the deposited clusters are neutralized and maintain
sandwich structures. While the use of Ar matrix is limited to cryogenic temperature
conditions, SAMs can support the sandwich clusters even at room temperature.
In addition, the sandwich clusters can be highly oriented on the SAMs. Very
recently, Huttmann et al. [124] synthesized europium-cyclooctatetraene (Eu-COT)
nanowires on a graphene substrate, where the nanowires are found to be lying
parallel to each other. Scanning tunneling spectroscopy (STS) and low-temperature
X-ray magnetic circular dichroism (XMCD) revealed that the Eu-COT wire is
a ferromagnetic insulator [125]. These immobilization methods, together with
spectroscopic applications and theoretical approaches, can readily provide a direct
route to investigate the low-dimensional functionality of transition metal-benzene
sandwich clusters on a substrate. As such, functionalized sandwich clusters will
open up possibilities for exploiting themselves as new building blocks in future
molecular electronics and spintronics.
Acknowledgments We are grateful to Prof. S. Yabushita (Keio University), Dr. T. Iwasa
(Hokkaido University), and Prof. K. Kanoda (The University of Tokyo) for fruitful discussion.
This work is partly supported by the program of Exploratory Research for Advanced Technology
(ERATO) in Japan Science and Technology Agency (JST) entitled with “Nakajima Designer
Nanocluster Assembly Project” and by JSPS KAKENHI of Grant-in-Aids for Scientific Research
(A) Grant Number 15H02002. The computations were partly performed using Research Center for
Computational Science, Okazaki, Japan.