Screening and antiscreening in fullerene-like cages dipole-eld amplication with ionic nanocages Pier Luigi Silvestrelli1S. Subashchandrabose12

2025-05-03 0 0 680.38KB 19 页 10玖币

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Screening and antiscreening in fullerene-like cages: dipole-ﬁeld

ampliﬁcation with ionic nanocages

Pier Luigi Silvestrelli1,∗S. Subashchandrabose1,2,

Abdolvahab Seif1, and Alberto Ambrosetti1

1) Dipartimento di Fisica e Astronomia,

Universit`a degli Studi di Padova, 35131 Padova, Italy and

2) Centre for Research and Development,

Department of Physics, PRIST Deemed University,

Thanjavur, Tamilnadu-613403, India

(Dated: October 12, 2022)

Abstract

The successful synthesis of endohedral complexes consisting of nanoscale carbon cages that can

encapsulate small molecules has been a remarkable accomplishment since these systems are ideal

models to investigate how conﬁnement eﬀects can induce changes in structural and electronic prop-

erties of encapsulated molecular species. We here investigate from ﬁrst principles screening eﬀects

observed when small molecules, characterized by a ﬁnite electronic dipole moment, such as HF,

LiF, NaCl, and H2O, are encapsulated into diﬀerent nanoscale cages: C60, C72 , B36N36, Be36O36,

Li36F36, Li36Cl36, Na36F36, Na36 Cl36, and K36Br36. Binding energies and electronic properties,

of these complexes have been computed. In particular, detailed analysis of the eﬀective dipole

moment of the complexes and of the electronic charge distribution suggests that screening eﬀects

crucially depend on the nature of the intramolecular bonds of the cage: screening is maximum

in covalent-bond carbon nanocages, while it is reduced in partially-ionic nanocages B36N36 and

Be36O36, being very small in the latter cage which turns out to be almost “electrically transpar-

ent”. Interestingly, in the case of the ionic-bond nanocages, an antiscreening eﬀect is observed: in

fact, due to the relative displacement of positive and negative ions, induced by the dipole moment

of the encapsulated molecule, these cages act as dipole-ﬁeld ampliﬁers. Our results open the way

to the possibility of tuning the dipole moment of nanocages and of generating electrostatic ﬁelds at

the nanoscale without the aid of external potentials. Moreover, we can expect some transferability

of the observed screening eﬀects also to nanotubes and 2D materials.

arXiv:2210.05324v1 [cond-mat.mes-hall] 11 Oct 2022

INTRODUCTION

Buckminsterfullerene (C60) is a carbon nanostructured allotrope with a cage-like fused-

ring structure (truncated icosahedron) made of 20 carbon hexagons and 12 carbon pentagons

where each carbon atom has three bonds. Since its discovery[1] this complex has received

intense study, also considering that, although C60 is the most stable and the most common

naturally occurring fullerene, many other cage-like nanostructures have been obtained and

can be hypothesized, by both considering diﬀerent numbers of C atoms and also replacing

carbons with other atoms. For instance, it has been natural to search for cages made by B

and N atoms, since the B-N pair is isoelectronic with a pair of C atoms; however, a fullerene

structure made by 60 B and N atoms is not optimal since the presence of pentagonal rings

does not allow a complete alternate sequence of B and N atoms. Fullerene-like alternate

B-N cages can be formed introducing isolated squares characterized by 4 B-N bonds with

alternate B and N atoms. In particular, a structure made by 36 B and 36 N atoms (B36N36),

with a relatively large energy gap between the highest molecular orbital (HOMO) and the

lowest molecular orbital (LUMO), has been found to be energetically very stable, both in

theoretical ﬁrst-principles studies and experimental investigations (see ref. 2 and references

therein).

Interestingly, by high-energy collisions of ionized fullerene species, harsh conditions of

high temperature and pressure, electric arc, or by organic synthesis methods (“molecular

surgery”), it is nowadays possible to produce C60 endohedral complexes with metal ions,

noble gases, and small molecules, such as H2, N2, H2O, and CH4(the ﬁrst organic molecule

to be encapsulated)[3–8]. Such recent achievements in the synthesis of endohedral fullerene

complexes have stimulated many experimental and theoretical investigations since the cav-

ity inside fullerenes provides a unique environment for the study of isolated atoms and

molecules. Moreover, these systems represent ideal models to study how conﬁnement eﬀects

can induce changes in structural and electronic properties of small molecular species and

also provide a possible way to alter the properties of the otherwise rather inert fullerenes.

In particular, Kurotobi and Murata developed a synthetic route to surgically insert a single

water molecule into the most common fullerene C60[6], a remarkable achievement considering

that water under normal conditions prefers to exist in a hydrogen bond forming hydrophilic

environment. The water molecule, with its relatively large dipole moment (1.9 D), is ex-

pected to polarize the symmetric non-polar C60 cage. However, the theoretical study of such

polarization eﬀects has given rise to a scientiﬁc controversy. In fact, while Kurotobi and

Murata[6], and Bucher[9] estimated a surprisingly high value of the dipole moment of the

H2O@C60 complex (a value similar to that of the isolated water molecule), other theoretical

ﬁrst-principles studies[10–12] indicate that the dipole moment of H2O@C60 is instead much

lower (about 0.5 D) than that for the isolated water, thus suggesting that a substantial

counteracting dipole moment is induced in the C60 cage, which considerably screens the

electric ﬁeld produced by the dipole moment of the encapsulated water molecule. The resid-

ual dipole moment of H2O@C60 is still signiﬁcant, which could have interesting implications

for possible applications of fullerenes.

In this work, by adopting independent theoretical approaches, we conﬁrm our previous

conclusions[12] about the pronounced screening of the dipole moment of a water molecule

encapsulated into C60 and extend the study to the encapsulation of some linear diatomic

molecules, characterized by a dipole moment comparable (HF) to or even much larger (LiF

and NaCl) than that of water. We also investigate screening eﬀects in other cage-like nanos-

tructures, such as B36N36, Be36O36, C72 (a carbon fullerene with the same structure of

B36N36), and the hypothetical ionic-bond cages (again with the same structure of B36N36)

Li36F36, Li36Cl36, Na36F36, Na36 Cl36, and K36Br36.

In Figs. 1-3 we show some of the investigated nanocages, namely C72, B36N36, and Li36F36,

characterized by covalent-bonds, partially-ionic bonds, and predominantly-ionic bond, re-

spectively (the ﬁgures are also representative of the other considered systems). We also plot

the electron charge distribution to highlight the diﬀerent bonding character of the nanocages.

FIG. 1: C72 nanocage. The electron charge distribution corresponding to an isosurface of 1.0 e/˚

is also partially plotted (in a quarter of the ﬁgure).

Basically, our calculations of binding and electronic properties, and detailed analysis of

the eﬀective dipole moment of the complexes and the electronic charge distribution elucidate

the encapsulation eﬀects and suggest that the screening phenomenon crucially depends on

the nature of the intramolecular bonds of the cage: screening is maximum in covalent-bond

carbon nanocages, is reduced in partially-ionic ones, while in the case of the ionic-bond

nanocages, an antiscreening eﬀect is observed. Hence, the latter systems surprisingly act as

dipole-ﬁeld ampliﬁers.

METHODS

Our ﬁrst-principles simulations have been performed with the Quantum-ESPRESSO ab

initio package[13], within the framework of the Density Functional Theory (DFT). The

investigated systems are located in periodically repeated cubic supercells, suﬃciently large

(ﬁnite-size eﬀects have been carefully tested) to avoid signiﬁcant spurious interactions due

to periodic replicas: the lattice side ranges from 30 to 40 a.u., depending on the nanocage

diameter. As a consequence, the sampling of the Brillouin Zone has been restricted to the

Γ-point only. Electron-ion interactions were described using ultrasoft pseudopotentials and

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Screeningandantiscreeninginfullerene-likecages:dipole-eldamplicationwithionicnanocagesPierLuigiSilvestrelli1,S.Subashchandrabose1;2,AbdolvahabSeif1,andAlbertoAmbrosetti11)DipartimentodiFisicaeAstronomia,UniversitadegliStudidiPadova,35131Padova,Italyand2)CentreforResearchandDevelopment,Department...

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Screening and antiscreening in fullerene-like cages dipole-eld amplication with ionic nanocages Pier Luigi Silvestrelli1S. Subashchandrabose12

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