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Journal ArticleDOI

Absorption spectra of alkali-C60nanoclusters

24 Sep 2014-Physical Chemistry Chemical Physics (The Royal Society of Chemistry)-Vol. 16, Iss: 40, pp 22399-22408
TL;DR: It is shown that in few cases the absorption spectra depend on the arrangement of the alkali atoms over the fullerene, though sometimes the absorption Spectra do not allow us to distinguish between different configurations.
Abstract: We investigate the absorption spectra of alkali-doped C60 nanoclusters, namely C60Nan, C60Kn, and C60Lin, with n = 1, 2, 6, 12, in the framework of the time-dependent density-functional theory (TDDFT). We study the dependence of the absorption spectra on the nature of the alkali. We show that in few cases the absorption spectra depend on the arrangement of the alkali atoms over the fullerene, though sometimes the absorption spectra do not allow us to distinguish between different configurations. When only one or two alkali atoms are adsorbed on the fullerene, the optical response of alkali-doped C60 is similar to that of the anion C60− with a strong response in the UV domain. In contrast, for higher concentration of alkali, a strong optical response is predicted in the visible range, particularly when metal–metal bonds are formed. The weak optical response of the Ih-symmetry C60Li12 is proposed to be used as a signature of its structure.

Summary (2 min read)

1. Introduction

  • The authors calculations have confirmed that lithium atoms coat homogeneously the fullerene on the C60 surface via pentagonal sites (at least up to 12 alkali atoms), contrary to sodium and potassium atoms which prefer to form 4-atom islands on the surface [24, 25].
  • In the following section, the authors give some details of calculations, and then they will present their results.

2. Computational details

  • The cluster geometries were taken from their previous works [24, 25].
  • In the present work, the authors have only considered the most representative configurations among the lowest-energy isomers for each complex.
  • To give an accurate description of the charge-transfer excited states [35], the authors have used a long-range corrected hybrid functional namely CAM-B3LYP [36].
  • The performance of the present level of calculation was evaluated with several tests (see Supplementary information).
  • To obtain a good description of the ionic bonding between alkali atoms and C60 fullerene, the authors should have correct values of ionization potential of alkali atoms and electron affinity of C60.

3. Results and discussions

  • 1 Absorption Spectra of C60 and C60- Calculated absorption spectra of both neutral C60 and anionic C60 - fullerenes are given in Figure 1.
  • In Figure 2, the authors show the absorption spectra of C60Li, C60Na, and C60K clusters for the two lowest-energy configurations.
  • For h configuration, the main band presents two maxima centered at 4.15 and 4.60 eV respectively.
  • The peaks at low energy, 1.36 and 1.40 eV for C60K, are due to excitations of the extra electron transferred from the alkali atom to the fullerene.

3.3 Absorption Spectra of C60Li2 and C60Na2

  • The authors consider now the absorption of two lithium or sodium atoms on C60, the adsorption of two K is not investigated since it was not considered in the previous study on structural properties [25].
  • The authors present only spectra for three of the lowest-energy isomers labeled respectively hhc, hhd, hhe for C60Na2, and hpc, ppa, ppc for C60Li2 .
  • The five configurations in which the two alkali atoms are both on h sites are labeled hh followed by a letter a,b,c,d,e, going from a for the adjacent sites to e for the opposite sites.
  • The spectra have a main band centered at 4.0 eV and two weak peaks below 2 eV for which both the exact positions and the energy gap depend on the configuration.
  • Therefore the ppa configuration can be distinguished from other.

3.4 Absorption Spectra of C60Li6, C60Na6 and C60K6

  • Previous DFT calculations have shown that the adsorption of six alkali atoms on the C60 molecule favor a relatively large distance with no strong metallic bonding nor a formation of a metallic droplet [17, 24,25].
  • In the last structure, labeled p6, the metal atoms are located above a pentagonal ring of which the positions are as close to one another as possible.
  • But the spectra strongly depend on the nature of the alkali.
  • For a given excitation, the authors have evaluated the 9 spatial overlap between the occupied and virtual orbitals using the Λ diagnostic test proposed by Tozer [58].

3.5 Absorption Spectra of C60Li12, C60Na12 and C60K12

  • For higher concentration of alkali (12 alkali atoms), the behavior of sodium and potassium atoms differs from that of lithium atoms since lithium atoms homogeneously cover the surface of the C60 with twelve atoms on the twelve pentagonal sites, while sodium and potassium atoms prefer to form 4-atom islands [24, 25].
  • Calculated spectra, given in Figure 7, are found to depend on both the nature of the alkali and the arrangement of the metal atoms over the fullerene.
  • It is clear that the optical response of the p12 structure differs from those of the other configurations, and can be used as a signature of its structure.
  • Finally, the main band centered at 2.55 eV is due to a collective excitation implying all carbon and sodium atoms.

4. Conclusion

  • The authors have presented the absorption spectra of the alkali-doped fullerene C60Mn with M=Li, Na, K, and n = 1, 2, 6, 12, in the framework of the time-dependent density-functional theory using the range-separated hybrid density functional CAM-B3LYP.
  • In few cases the absorption spectra were showed to depend on the arrangement of the alkali atoms over the fullerene, though sometimes the absorption spectra do not allow to distinguish between different configurations.
  • When metal-metal bonds are formed, the metal-C60 complexes present a strong optical response in infrared and visible domains in contrast with the isolated C60 for which dipole-allowed excitations are only found in the UV range.
  • This is especially evident for sodium- and potassium-doped C60 for which the onset of metal-metal bonding appears earlier than in lithium-doped C60.
  • For C60Li12, the Ih-symmetry structure presents a particular absorption spectrum, almost without peaks in the visible, which differs from spectra calculated for the other isomers, and then could be used as a signature of its structure since a measurement of the absorption spectrum of C60Li12 could validate the symmetry of the cluster.

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Absorption spectra of alkali-C-60 nanoclusters
Franck Rabilloud
To cite this version:
Franck Rabilloud. Absorption spectra of alkali-C-60 nanoclusters. Physical Chemistry Chemical
Physics, Royal Society of Chemistry, 2014, 16, pp.22399-22408. �10.1039/c4cp03352c�. �hal-02309839�

1
Absorption spectra of alkali-C
60
nanoclusters
Franck Rabilloud
Institut Lumière Matière, UMR5306 Université Lyon 1 CNRS,
Université de Lyon, 69622 Villeurbanne Cedex, France;
franck.rabilloud@univ-lyon1.fr
tel. 33 4 72 43 29 31
fax. 33 4 72 43 15 07
Date: September 3
th
, 2014
Abstract:
We investigate the absorption spectra of alkali-doped C
60
nanoclusters, namely C
60
Na
n
, C
60
K
n
,
and C
60
Li
n
, with n=1, 2, 6, 12, in the framework of the Time-Dependent Density-Functional
Theory (TDDFT). We study the dependence of the absorption spectra on the nature of the alkali.
We show that in few cases the absorption spectra depend on the arrangement of the alkali atoms
over the fullerene, though sometimes the absorption spectra do not allow to distinguish between
different configurations. When only one or two alkali atoms are adsorbed on the fullerene, the
optical response of alkali-doped C
60
is similar to that of the anion C
60
-
with a strong response
in the UV domain. In contrast, for higher concentration of alkali, a strong optical response is
predicted in the visible range, particularly when metal-metal bonds are formed. The weak
optical response of the I
h
-symmetry C
60
Li
12
is proposed to be used as a signature of its structure.
Keywords:
alkali-C
60
, C
60
, fullerenes, alkali, Absorption spectra, TDDFT

2
1. Introduction
The discovery of C
60
by Smalley and co-workers [1] gave birth to new fields of research
ranging from molecular chemistry to materials and condensed matter physics. In particular, the
interaction between fullerenes and alkali atoms has attracted a lot of attention since the
discovery of superconductivity in the potassium-C
60
[2] and rubidium-C
60
[3] fullerides. More
recently, the doping of alkali metal atoms on fullerenes was shown to cause a remarkable
enhancement in the hydrogen adsorption capacity and was suggested as a possible route toward
hydrogen storage materials [4-11]. The charge transfer from the alkali atom to the fullerene
cage leaves the alkali atom in a cationic state which can then bind H
2
molecules due to
polarization forces. However, when several alkali atoms lie a fullerene, the hydrogen adsorption
capacity greatly depends on the wetting or nonwetting of the fullerene surface with alkali atoms:
an homogenous coating of the fullerene by the alkali atoms is expected to be much more
efficient than the growth of a metallic droplet not wetting the fullerene surface which could
drastically limit the amount of stored hydrogen.
Many experimental and theoretical studies have been devoted to the electronic and
structural properties of C
60
M
n
clusters, with M=Li, Na, K [12-28]. Very early, the strong
stability of C
60
Li
12
was interpreted as a signature of a homogeneous coating of the fullerene by
all lithium atoms above the twelves pentagonal faces of the C
60
molecule resulting in an
icosahedral arrangement [12-14]. In contrast, coating the fullerene with sodium or potassium
atoms led to an even - odd alternation in the mass spectra, suggesting the onset of metallic
bonding. Following previous theoretical studies [13,17,19-22] achieved with some limitations
or constraints (for examples, use of a many-body force field without explicit electronic structure
or first principles calculations assuming a rigid C
60
cage), we have recently performed geometry
optimizations in the framework of the density-functional theory (DFT), without any constraint,
of selected initial configurations ranging from homogeneous covering of the fullerene to
complete segregation in which alkali atoms form a droplet not wetting the fullerene surface,
and also intermediate situations [24-25]. The structures of Li
n
-, Na
n
- and K
n
- coated C
60
fullerene with n=1, 2, 6, 12 were investigated. The optimization process involved the fullerene
structure. Our calculations have confirmed that lithium atoms coat homogeneously the fullerene
on the C
60
surface via pentagonal sites (at least up to 12 alkali atoms), contrary to sodium and
potassium atoms which prefer to form 4-atom islands on the surface [24, 25]. However, in some

3
cases, several configurations were found to compete for the lowest-energy isomers and to be
degenerate. The degeneracy of the most stable structures were found to result from a balance
between the electrostatic repulsion, due to the electronic charge transfer from the metal atoms
to C
60
versus the residual metallic bonding.
The goal of the present study is to characterize the absorption spectra of the most stable
configurations of C
60
alkali complexes. We will show that in few cases the absorption spectra
depend on the nature of alkali and also on the arrangement of the alkali atoms over the fullerene,
though sometimes the absorption spectra do not allow to distinguish between different
configurations. Complexes investigated here include C
60
M
n
with M = Li, Na, K, and n = 1, 2,
6, 12. In the following section, we give some details of calculations, and then we will present
our results.
2. Computational details
The cluster geometries were taken from our previous works [24, 25]. They were optimized
by use of the hybrid B3LYP functional [29, 30] with Gaussian basis sets of double zeta valence
quality. In the optimization process of cluster geometries, a number of structures were tested
for each size. Only selected initial geometries in which alkali atoms in contact with C
60
are
above either the center of a pentagonal ring or a hexagonal ring were considered, since the top-
or bridge-site type were previously shown to be unfavorable. Of course all the possible
configurations had not been explored due to the prohibitive cost of calculations. However, the
studied configurations were selected in order to provide different situations ranging from the
homogeneous covering of the fullerene to complete segregation and also intermediate
situations. All optimizations were carried out without symmetry constraints (C
1
point group).
In the present work, we have only considered the most representative configurations among the
lowest-energy isomers for each complex. The relative energies of each isomer are given in
Table 1, and the structures are showed in the figures below.
Absorption spectra were calculated with the GAUSSIAN09 program package [31] in the
framework of the time-dependent density-functional theory [32-34] (TDDFT). To give an
accurate description of the charge-transfer excited states [35], we have used a long-range
corrected hybrid functional namely CAM-B3LYP [36]. Calculations were achieved in the linear
combination of atomic orbitals scheme. All atoms were described with the 6-31G(d) basis sets

4
[37-39]. The absorption spectra showed in the next section give the oscillator strength as a
function of the excitation energy together with a curve obtained by a Lorentzian broadening
with a full width at half maximum (fwhm) of 0.05 eV. For each species, we present spectra
including calculated excitation energies up to the vertical ionization potential (IP) of the lowest-
energy isomer. IPs, given in Table 1, are calculated at CAM-B3LYP/6-31G(d) level. The
performance of the present level of calculation was evaluated with several tests (see
Supplementary information). Spectra of C
60
Li
2
and C
60
Na
2
calculated with a basis set enriched
by more diffuse functions, namely 6-31+G(d), are found to be very similar to those obtained
with the 6-31G(d) basis set. Several exchange and correlation functionals have been tested
including the hybrid B3LYP and PBE0 [40], and the long-range-corrected ones ωB97x [41],
LC-M06L(ω=0.33) [42,43,35], LC-ωPBE(ω=0.40) [44]. Spectra of C
60
Na
2
and
C
60
Na
6
are
given in Supplementary information. They are somewhat similar to those calculated using
CAM-B3LYP, but the main band obtained at B3LYP and PBE0 levels is slightly redshifted,
while it is blueshifted by 0.3-0.4 eV at ωB97x, LC-M06L, LC-ωPBE levels. Pre- and
postprocessing operations were performed with the graphical interface GABEDIT [45].
The present level of calculation is a good compromise between accuracy and cost. To
obtain a good description of the ionic bonding between alkali atoms and C
60
fullerene, we
should have correct values of ionization potential of alkali atoms and electron affinity of C
60
.
Present IPs are calculated at 5.59, 5.36, and 4.43 eV for Li, Na, and K atoms respectively, in
good agreement with the experimental data of 5.39 [46], 5.14 [47], and 4.34 [48] eV
respectively. The electroaffinity of C
60
fullerene is calculated at 1.87 eV while the experimental
value ranges from 1.62 to 2.68 eV [49, 50]. Otherwise, the vertical IP of C
60
is calculated at
7.60 eV, it is exactly the experimental value [51]. To our knowledge, the only available
experimental IP of C
60
-alkali concerns that of C
60
K for which the value would be in the 5.0-6.4
eV range [51]; the present calculated value is 4.64 eV.
3. Results and discussions
3.1 Absorption Spectra of C
60
and C
60
-
Calculated absorption spectra of both neutral C
60
and anionic C
60
-
fullerenes are given
in Figure 1. The absorption spectra of neutral C
60
is calculated up to 7 eV, but we also show the

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Frequently Asked Questions (1)
Q1. What contributions have the authors mentioned in the paper "Absorption spectra of alkali-c-60 nanoclusters" ?

The authors investigate the absorption spectra of alkali-doped C60 nanoclusters, namely C60Nan, C60Kn, and C60Lin, with n=1, 2, 6, 12, in the framework of the Time-Dependent Density-Functional Theory ( TDDFT ). The authors study the dependence of the absorption spectra on the nature of the alkali. The authors show that in few cases the absorption spectra depend on the arrangement of the alkali atoms over the fullerene, though sometimes the absorption spectra do not allow to distinguish between different configurations.