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Core−Shell Nanomaterials for Microwave Absorption and
Electromagnetic Interference Shielding: A Review
Yudhajit Bhattacharjee and Suryasarathi Bose*
Cite This: ACS Appl. Nano Mater. 2021, 4, 949−972 Read Online
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ABSTRACT: Core−shell nanoparticles are a unique class of
nanostructured materials, which received significant attention in
the last two decades owing to their exciting properties and a wide
variety of applications. By judiciously tuning the “core” as well as
the “shell”, an assortment of “core−shell” nanostructures can be
obtained with tailorable properties which can play pivotal roles in
designing materials for electromagnetic interference (EMI)
shielding and microwave absorption. In recent times when the
use of high-end electronics has been on the rise, electromagnetic
pollution is inevitable and it affects every walk of life. Among
numerous e-pollutions, a recently highlighted domain that has far-
reaching consequences is electromagnetic interference. EMI is the disturbance created during an electronic device’s operation when
it is in the vicinity of an electromagnetic field caused by another electronic or electric device. Since EMI decreases the lifetime and
degrades the performance of the electronic instruments and even affects human health, it is crucial to protect sophisticated
instruments and components from EM interference. High-performance EMI shields capable of attenuating microwave propagation
efficiently have been developed in the past decade. Herein, in this review, we attempted to provide a logical guide to various “core−
shell” nanostructures (≤500 nm) that have been synthesized in the past decade for the application in microwave absorption and EMI
shielding. The prime focus of this review article is to highlight the fundamental concept of EMI shielding and microwave absorption
that have been reported for various systems in the literature along with various synthetic and fabrication strategies in designing
suitable broadband EM absorbers/screeners. Finally, we also made an effort to provide a holistic outlook and perspective in which
upcoming research will continue to flourish.
KEYWORDS: core−shell nanomaterials, EM pollution, electromagnetic interference shielding (EMI), nanocomposites,
microwave absorption
■ INTRODUCTION
Nanomaterials play a pivotal role in dealing with some of
humanity’s important scientific and engineering problems.
Their small size, higher surface area, scalability, and several
other favorable attributes have made them the most lucrative
materials in the 21st century. Nanomaterials can be of various
types, categorized mostly due to their shape and properties,
which in turn has led to various applications in a wide variety
of sectors across the world.1
Among all of the types that exist in
the family of nanomaterials, there lies one special kind called
“core−shell” nanomaterials, and in this article, we will focus on
this special kind.
In general, simple or (single-component) nanomaterials are
usually composed of only one material; whereas, as the name
implies, composite and “core−shell” nanomaterials are
composed of two or more materials with a core, which is the
inner structure and shell(s) outer layer. Historically (in the late
1980s), scientists observed that heterogeneous, concentric
semiconductor nanoparticles provide better efficiency than
their corresponding single-particle counterpart and, sometimes,
they even develop some new properties.2−4
More recently,
during the early 1990s, when researchers synthesized
concentric multilayer semiconductor nanoparticles for improv-
ing the property of such semiconductor materials, the term
core−shell was coined.5,6
With the advancement in this
particular field of nanomaterials, the definition of the core−
shell nanomaterials can be modified to describe the class of
materials that have two components, an inner core and single
or multiple outer shells having a distinct boundary separately
identifiable. In recent times, there has been a surge in research
activity because of the enormous call for an increased number
Received: January 28, 2021
Accepted: February 3, 2021
Published: February 11, 2021
Review
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949
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of advanced materials fueled by the demands of modern
technology. Concurrently, newly developed advanced charac-
terization techniques have also significantly helped to elucidate
these different core−shell nanostructures’ structures.7,8
Based
on the arrangements and compositions of components in the
system, core−shell nanomaterials can be categorized into
numerous classes which are a combination of inorganic and
organic components.7,9
As attention toward target-specific
nanostructured materials with multifunctional and/or im-
proved properties is increasing, core−shell nanostructures are
gradually attracting attention because of their versatile
compositions and structures to serve such purposes.10−13
Besides, these nanomaterials have properties which are
synergistic among the core and shell and/or exhibit new
properties owing to the interactions between the core and the
shell.14−18
The design of core−shell nanomaterials can be modified by
changing the constituent materials or by altering the thickness
ratio core and/or shell. As in the nanosystem, the core and
shell may exist in different forms, as shown in Figure 1. Various
structural morphologies such as the core may be a solo sphere
covered with a continuous shell (Figure 1a) or a continuous
multishell (Figure 1b). In some other cases, which is depicted
in Figure 1c is the yolk−shell nanostructure where the shell is
hollow and in which a small sphere is acting as a core.
Extension to this is multishell structures (Figure 1d). In some
unusual cases, core−shell structures can form smaller spheres
attached (acting as a shell) around the core19
(Figure 1e); in
another kind, instead of a continuous layer shell, it takes the
form of smaller discrete spheres attached to a core sphere in a
discontinuous manner19
(Figure 1f); also, in some situations,
the core may not be a single entity but an accumulation of
multiple entity/spheres (Figure 1g) within the continuous
shell. There are cases where core−shell nanostructures might
take the shape of rod, tube, or hexagonal morphologies (Figure
1h,i).20−22
Being a multi-interface in nature, some of the above mention
systems have particular advantages and disadvantages.
Considering the systems (Figure 1e,f) have a diffused shell
structure,19
the surface roughness of these systems will be
higher. Due to this, interaction with the polymer matrix will be
higher for these particles, and as a result, the mechanical
properties of the composite system can be enhanced by
carefully tuning the filler concentration.19
Considering the
system (Figure 1i) as the aspect ratio of these systems is higher
than that of spherical ones, the electrical percolation threshold
for these systems will be lower, which will help in enhanced
electrical conductivity and EM shielding at a lower
concentration.23
For systems like Figure 1a−d, being multi-
interface in nature, these systems generate significant interfacial
polarization loss and, being periodic in nature, these materials
would help in generating multiple scattering/reflections, which
helps in effective EM attenuation.24−26
Such distinctive,
advantageous, and tailorable structure and properties have
also advanced these materials as a significant class of emergent
materials for multifarious applications in, for instance,
bionanotechnology,27−29
enhanced optical devices,30−32
tail-
ored magnetic devices,33,34
bioimaging systems,35,36
energy
storage materials,37,38
genetic engineering and stem cells,39,40
solar cells,41,42
and many important catalytic processes.43−46
In the past decade, core−shell nanostructures have emerged
as significant yet less explored materials in the field of
designing EMI shielding/microwave absorbing materials. The
advent of the high-speed data transmission era (4G and 5Gs)
and the burgeoning advance in wearable electronic devices not
only improved significantly 21st-century modern lifestyle but
also brought a great deal of hazardous electromagnetic
pollution to human civilization. To counter this, microwave
absorbers find their place in various applications, e.g., achieving
electromagnetic interference (EMI) shielding,47−51
sophisti-
cated contact lenses,52
and improving detector sensitivity.53
In
light of the widespread use of numerous optoelectronic devices
such as wearable sensors and smartphones and the other
Figure 1. Schematic illustration of various core−shell architectures: (a, b) core−shell and core−double shell, (c, d) yolk−shell and yolk−multishell,
(e, f) core−diffused shell, (g) multicore−shell, (h) hexagonal core−shell, and (i) rod/tube-like core−shell architectures.
ACS Applied Nano Materials www.acsanm.org Review
950

associated complex electromagnetic instruments, EMI shield-
ing technology has been becoming even more essential in
recent years.26,54−61
An effective EM shielding material not only reduces
unwanted EM emissions but also protects electronic
components from stray signals. The primary requisite of any
EMI shield is to reflect EM radiation using mobile charge
carriers which interact explicitly with the incoming fields.62
Therefore, shield materials have to be electrically conductive.
However, with the recent progress in the field of microwave
absorbing materials, it has been clear that conductivity is not
the only criterion. There also follows a secondary mechanism
that requires absorption of EM signals due to the material’s
magnetic dipoles and/or electric dipole interacting with the
EM field. High electrical conductivity is the important variable
determining the reflection and absorption traits of the shield
materials.63
Furthermore, a third mechanism which in recent
days has emerged as the most prevalent mechanism to justify
many unusual results is multiple internal reflections. This
particular mechanism contributes significantly to EMI
shielding effectiveness, though they are still less explored, as
this parameter is difficult to quantify with the present-day
instrumentation facilities available to us. Multiple internal
reflections occur due to the presence of scattering centers,
interfaces, or defect sites within the shield.24
In earlier times,
metals and metallic architectures were the material of choice to
combat EM interference,25
but additional weight and
susceptibility to corrosion make metals less preferred in
various industries. In modern times with the state of the art
electronic equipment and devices, we are in the race for
slimmer and slicker devices which are easy to fabricate and
possess several other qualities such as being lightweight,
flexible, and high-strength. Polymer nanocomposites with
embedded conducting, magnetic, or hybrid nanostructures as
fillers have become a prevalent substitute for EM shield
because of better processability and enhanced scalability.55,64
Recent work in EMI shielding and radio frequency/extremely
low-frequency interference screening is in fact shifting toward
designing more and more hybrid functional materials which
have versatile properties in a single framework. Core−shell
nanocomposite materials are one of the lucrative and effective
hybrid materials to look for.
Herein, in this review article, we aim to focus on various
“core−shell” materials that have been designed and utilized as
EM screening materials over the past decade along with their
mechanistic insights toward shielding. Few other review
articles discuss various aspects of EMI shielding materials
and offer a comprehensive summary of various domains of
EMI shielding such as materials, methods, fabrications, and
theories.20,25,64−68
From the existing literature and perspec-
tives, we can say that core−shell materials emerge as one of the
key aspects of modern-day research in numerous fields7,8
and
also in the field of EMI shielding,24
and as far as we are aware,
this is the first review article which summarizes various aspects
of core−shell materials for designing EMI shielding materials.
■ SCOPE OF THE REVIEW
This particular review aims to focus on the progress of the
different types of core−shell nanoparticles toward the
application toward EMI shielding along with some fundamen-
tal concepts and factors related to this domain. We have
categorized these types based on the volume of the work done
in the past decade toward EMI shielding and adjusted the
sequence likewise. These are ones where (i) ferrites act as the
core (this includes primitive and non-primitive ferrites), (ii)
metal alloys act as the core, (iii) carbonaceous materials act as
the core, (iv) metals and metal derivatives act as the core, and
finally (v) other relevant systems (this being a miscellaneous
section containing different types of systems, viz., polymer,
metal carbides, fibers, etc.) act as the core material. We have
thoroughly discussed the synthesis of these core−shell
materials and discussed their mechanistic insight regarding
shielding performance. Finally, we have kept in mind the
importance of this class of material and how future research
will continue to flourish in this area.
■ FUNDAMENTALS OF EMI SHIELDING
The electromagnetic (EM) energy is represented by its two
components, static and/or time-varying electric and magnetic
force fields, also known as induction fields. These fields are
generated in the vicinity of the source and are radiated in
directions away from the source. Pertinent to the several
modes of field force, the choice of material in each case to
shield the relevant field is distinct, since the shielding
mechanism involved is different for each of the force fields.
The electromagnetic field can be distinguished as two regimes:
quasi-static energy (with very low frequency or very large
wavelength) stated as near-field and radiated far-field related to
high frequency or small wavelength electromagnetic energy.69
About time-varying fields, a wave impedance (intrinsic
impedance) parameter (Zw or Z0) can be defined for the
medium in terms of the electric (E) and magnetic (H) field
intensities. That is, Zw or Z0 =
E
H
Ω. When the source is a
capacitive (potential-dependent)-type dipole (as shown in
Figure 2), the associated E field is large, which leads to a high
wave impedance situation. On the other hand, a current-
dependent loop source offers an extensive H field and the
corresponding wave impedance of the medium tends to be
small.69
The mechanism of shielding is that the shielding medium of
appropriate material and geometry, when placed in the region
of an electromagnetic field, should offer a barrier impedance
ZM sufficiently large in comparison with the wave impedance
Zw in the case of an electric field dominant ambient so that the
wave is impeded sufficiently and shielded off from entering the
region beyond the shielding barrier, as illustrated in Figure 2.
Likewise, in the case of the magnetic field caused by a
current-dependent loop source, the effective shielding can be
achieved by a shield (of appropriate material and geometry)
with a barrier admittance YM more than Yw (Figure 3), where
Yw = 1/Zw.
Figure 2. Schematic illustration of shielding in electric field dominant
ambient.
951

The primary considerations of electromagnetic shielding for
time-varying fields are the proper choice of material and the
geometry of the shield to provide the necessary barrier-
immittance parameter. The material and the geometry
cohesively accomplish the shielding by the following
mechanisms:65,69
1. Absorption of electromagnetic energy in the shielding
medium (absorption loss)
2. Reflection of the electromagnetic energy back into the
source side with minimal transmission into the region
being shielded (reflection loss)
Shielding efficiency is defined as measuring how a material
obstructs the electromagnetic radiation/energy of a certain
frequency that is incident on it. When EM radiation falls on the
shield, then a fraction of incident radiation having power (Pi) is
reflected (PR) back, while some fraction of the incident power
is absorbed and dissipated in the form of heat energy, and the
residual fraction is transmitted (PT) through the shield.
Therefore, overall, three distinct processes contribute to the
whole attenuation, resulting in total shielding effectiveness
= = =
= + +
P
P
E
E
H
H
SE 10 log 20 log 20 log
SE SE SE
T
i
t
i
t
i
t
R A M (1)
In this equation, P, E, and H refer to power, magnetic, and
electric field intensities. Thus, SER refers to the total amount of
reflection and SEA represents shielding due to absorption. It is
advisible to note that contributions from secondary or multiple
reflections inside the shield (Figure 4a) only occur in a finite-
dimensional system and therefore SEM can be neglected in a
thicker medium, allowing the equation to be written as
= +
SE SE SE
T R A (2)
Loss in Energy due to Reflection. According to classical
theory, reflection is the most important and usually the
primary mechanism of shielding. Energy loss due to reflection
is mostly attributed to the relative mismatch of impedance
between the surface of the shield and the incoming
electromagnetic waves. The expression can be written as
σ
πμ
σ μ
= = + ∝
Z
Z f
SE 20 log
4
39.5 10 log
2
( / )
R
0
in (3)
Here σ is the conductivity, frequency is denoted by f, and μ is
the relative permeability. It can be inferred from this expression
that SER is directly proportional to the conductivity and
inversely related to the frequency and relative permeability,
and thus, for a constant σ and μ, reflection loss decreases with
increasing frequency. For reflection to occur from the
material’s surface, the material must have mobile charge
carriers (electron and hole).
Loss in Energy due to Absorption. Absorption is the
secondary mechanism of EM attenuation. According to plane
wave theory, the amplitude of the electromagnetic wave
reduces exponentially inside the material while passing through
it. Thus, loss due to absorption results from ohmic losses and
heating of the material owing to the currents induced inside
the medium. Absorption loss (SEA) in decibels (dB) for
conductive materials is written as
πσμ σμ
= = ∝ ∝
σ
f d ad
SE 20 log e 8.7( )
d
A
/ 1/2
(4)
here d represents thickness and α signifies the attenuation
constant of the shield material. The attenuation constant can
be defined as the extent to which the intensity of EM radiation
is reduced when it passes through a material, or in other words,
it can also be termed as consolidated loss in the system. We
can infer from the equation that absorption loss depends on
three main factors, viz., conductivity, sample thickness, and
permeability of materials. This equation’s applicable condition
is that materials have to be conductive; conductive current ≫
displacement current. These dependencies of absorption or
reflection loss (SEA/SER) on permeability and conductivity
show that, in magneto-conducting materials, shielding is
dominated absorption.
π
λ
=
a
n
4
0 (5)
In this equation, λ0 represents the wavelength in a vacuum and
n represents the refractive index, which can be written as
(εμ)1/2
; for nonmagnetic materials, μ = 1. Hence, eq 5 can be
written as
πε
λ
=
a
4 1/2
0 (6)
Attenuation due to Multiple-Reflection. When we
consider thin samples’ interaction with incoming EM radiation,
in those cases, EM waves can be trapped between two
boundaries, as waves reflect from the second boundary and
return to the first boundary. A part of the wave is absorbed,
and the rest is again incident on the secondary boundary. This
process occurs back and forth multiple times, as shown in
Figure 4, and ultimately, waves lose their power and get
trapped inside the material.
Figure 3. Schematic illustration of shielding in magnetic field
dominant ambient.
Figure 4. (a) Schematic Illustration of various mechanisms associated
with EM shielding on a 3D material; (b−e) various kinds of multiple
internal reflections and in the case of a multilayered structure, a
hollow structure, a multishell structure, and in a sphere.
952

= − δ
−
SE 20 log(1 e )
d
M
2 /
(7)
Here δ is the skin depth, which is defined as the minimum
thickness below which the incident field is attenuated to 1/e of
its initial power and is given by
δ πσμ
= −
f
( ) 1/2
(8)
As the EM waves are trapped inside the materials, attenuation
due to multiple internal reflections is closely related to
absorption and depends on the thickness (d) of the materials.
Hence, multiple internal reflections play an important role in
multilayered, porous structures, multi-interface materials
(core−shell), and several others.24,47,59,70
Figure 4 represents
multiple reflections inside various systems. In all of these
structures, a big surface area and solid structure exclusive of
vacant space provide additional active sites for scattering and
multiple reflections of EM waves. The porosity and the
hollowness of the structure lead to them having a higher
surface area, tailorable internal architecture, decreased density,
and tunable permeability. All of these properties in turn aid in
the improvement in EM shielding performance.
■ FACTORS INFLUENCING EMI SHIELDING
Permittivity and Permeability. To design an effective
EM shield, two crucial factors, viz., permittivity and
permeability, should be taken into account. The EM shielding
attribute is correlated with impedance matching (better match
leads to absorption and the mismatch leads to reflection), and
impedance matching is significantly influenced by the
permittivity and permeability of a system according to the
following equation71
α π μ ε μ ε μ μ ε ε
= ″ ″ − ′ ′ + ″ + ′ ′ + ″
fc 2( ( )( )
2 2 2 2
(9)
As far as the electrical shielding is concerned, conduction loss
and polarization loss are two pivotal parameters that account
for the dielectric loss. Polarization loss can be attributed to
electronic, ionic, dipole orientation, and interfacial polarization
(due to trapping of space charge). If we consider free-electron
theory, we can write the dielectric loss as ε″ =
σ
πε f
2 0
, where σ is
the electrical conductivity, which signifies that high electrical
conductivity, in turn, enhances ε″. Two phenomena, ionic and
electronic polarization, occur at the very high-frequency region
(above 1000 GHz). Dipole polarization occurs due to the
existence of defects and residual groups in the material.72,73
The interfacial polarization and other relaxations occur due to
confined space charges at the interfaces. In these situations, the
relaxation process can be explored via Cole−Cole plot, and in
it, the semicircle plot is obtained due to Debye dipolar
relaxation.74
The relationship between ε′ and ε″ can be
expressed as
ε ε ε ε ε
′ − + ″ = −
∞ ∞
( ) ( ) ( )
2 2
s
2
(10)
where εs and ε∞ denote the static and relative dielectric
permittivity, respectively, at higher frequencies. If polarization
relaxation occurs, a semicircle plot is observed while plotting
the Cole−Cole plot (ε″ vs ε′).
Anisotropy Field. Anisotropy plays an important role in
the EM shielding mechanism.75
Magnetic shielding occurs due
to several types of magnetic losses, viz., eddy current loss,
exchange resonance, and natural ferromagnetic resonance.
Natural resonance is related to the materials’ anisotropy field
Ha, and the expression is
γ
π
=
f
H
2
r
a
(11)
Here γ/2π is the gyromagnetic ratio. And Ha can be written as
= μ
H
K
M
a
2
0 s
, where K is the anisotropy constant and Ms is the
saturation magnetization.24,76
The above equation reduces to
γ
μ π
=
f
k
M
2
2
r
0 s (12)
Thus, it can be inferred from this equation that higher
saturation magnetization or a smaller anisotropy field will lead
to a decrease in resonance frequency or we can say that a
smaller anisotropic field will increase the absorption
bandwidth.
Eddy Current Loss. Eddy loss is one of the most important
losses in the semiconducting as well as conducting magnetic
system. Eddy current loss is the Joule loss due to the eddy
current induced by the alternating magnetic field. Eddy
currents are generated whenever there is a relative motion
between a conducting material and a varying external magnetic
field exists. Magnetic metals and their alloys (Co, Ni, Fe, FeCo,
CoNi, etc.) exhibit very high saturation magnetization and also
good magnetic permeability.77,78
The conducting behavior of
these materials is primarily responsible for eddy current
generation and subsequent loss, resulting in reduced
permeability at lower frequencies especially in the megahertz
region. Ferrites on the other hand are semiconducting and also
exhibit lower saturation magnetization compared to magnetic
metals and their alloys, and hence, the resonance frequency of
ferrites hovers around the lower GHz frequency range (1−2
GHz).79
This situation limits their usage in the higher GHz
region.24
Snoek’s Limit. Another important factor of designing the
EM shield is tuning the materials’ permeability and
manipulating Snoek’s limit which deals with the microwave
permeability spectrum in magnetic materials. Domain wall
resonance and spin-exchange resonance are the two different
mechanisms that contribute to the complex permeability,80−82
so the permeability is expressed as
μ ϖ χ ϖ χ ϖ
= + +
( ) 1 ( ) ( )
sr dw (13)
where
χ ϖ
ϖ ϖ
=
+ +
K
j
( )
1 ( )
sr
sr
sr (14)
and
χ ϖ
ϖ
ϖ ϖ βϖ
=
− +
K
j
( )
dw
dw dw
2
dw
2 2
(15)
In these equations, ϖ represents the radio frequency magnetic
field, ϖsr is the spin resonance, and ϖdw denotes the domain
wall motion frequency. Ksr and Kdw denote the static spin and
domain wall motion susceptibilities, and β denotes the domain
wall damping factor. It has been observed that, although the
spin rotation factor remains in the higher frequency, the
domain wall contribution diminishes. Therefore, above 100
MHz, complex permeability is the factor that is governed only
by the spin rotational component. In terms of magnetization,
the relation between Ksr and ϖsr can be written as
953

π
=
K
M
K
2
sr
s
2
1 (16)
and
ϖ γ
= ′
C
K
M
2
sr
1
s (17)
Here saturation magnetization is denoted by Ms and K1 is the
magnetocrystalline anisotropy, and γ is the gyro-magnetic ratio.
ϖ πγ
= ′
K C M
2
sr sr s (18)
At resonance frequency ϖsr = ϖr = 2πfr
ϖ μ − ∝ M
( 1)
r s (19)
This eq 19 is referred to as Snoek’s limit, which provides an
understanding of how to achieve control on permeability in the
case of ferrite.
Morphology and Size and Interfaces. Morphology and
particle size play a huge role in EMI shielding.75
The
dimensions of magnetic materials significantly influence their
permeability, as has been discussed in earlier sections. In
anisotropic materials, alteration in anisotropy energy can alter
the spin relaxation time.75,83
Apart from the bulk magnets,
permeability in nanomaterials is controlled by relaxation
mechanisms. The alteration in anisotropy energy alters the
spin relaxation time. Spin fluctuation remains very fast in the
superparamagnetic state owing to its small size; hence,
relaxation occurs at higher frequencies.84−87
Biswas et al.
reported the effect of shape anisotropy and size on the EMI
shielding properties in a polycarbonate (PC)/polyvinylidene
difluoride (PVDF) blend.83
Moreover, certain morphologies
that generate high porosity and larger surface area introduce
multiple interfaces that accrue bound charges at the interfaces,
causing the polarization effect known as Maxwell−Wagner
polarization.88
Arief et al. reported various nanoengineered
FeCo anisometric nanostructures in a poly(vinylidene
difluoride) matrix for EMI shielding where various morphol-
ogies and their effect were studied.83
Furthermore, several
surfaces hold unsaturated bonds that are primarily accountable
for dipolar polarization, and therefore, we can conclude that
multi-interface materials are advantageous for EM attenu-
ation.25
Bhattacharjee et al. studied the multi-interface nature
in a composite system and concluded that multiple interfaces
in a single system could synergistically help in effective EMI
shielding.24
Heat Treatment and Time. The classical understanding
of materials science tells us that heat treatment (annealing)
increases disorder and subsequently creates defects in
vacancies, substitution in the system, and generating dangling
bonds. These are observed in many ferrite systems where
reflection loss is altered by altering the annealing temper-
ature.89
These defects and other associated phenomena create
an additional energy level around the Fermi level, leading to a
significant change in attenuation. In some cases, reaction
conditions such as reaction time and temperature also
influence reflection loss,89
as altering reaction conditions will
lead to changes in structural features of the system as time and
temperature are altered. Iqbal et al. reported the thermal
annealing driven exceptional shielding performance of the
Ti3CNTx (Mxene) system. They have achieved significant
enhancement in EMI shielding when annealing was done at
350 °C in a 40 μm Mxene film.59
Wu et al. also studied the
effect of annealing temperature on microwave attenuation for
reduced graphene oxide. They have found that, with the
increase in the annealing temperature, oxygen functional
groups can be effectively removed from graphene oxide
(GO), especially the groups which are on the surface. The
ratio of oxygen functional groups has a direct effect on the EA
and EMI shielding performance of reduced graphene oxide.90
Sharma et al. studied the mechanism of growth for a hybrid
structure comprised of reduced graphene oxide (rGO) and
CuS microflowers as a function of time and investigated its
influence on EMI shielding performace.91
Thickness. Other than the specific significance of skin
depth over EMI shielding, thickness, in general, influences the
EM shielding performance. According to the quarter wave-
length theory, minimum reflection loss (RLmin) of the incident
microwave occurs when the thickness of the absorber is
approximately a quarter of the propagating wavelength and
multiplied by an odd number and this can be expressed as92
λ
=
t n
4
m
m
(20)
where n is any positive integer number (1, 2, 3, ...) so that n =
1 corresponds to the first dip at low frequency. The
transmitting wavelength inside the material (λm) is given by
λ
λ
μϵ
=
[ ]
m
0
1/2
(21)
The matching condition resulting in the exclusion of the
incident and reflected waves at the surface of the absorber
material, for example, the dips at t = 5 mm, occurred at (λm/4),
(3λm/4), and so on. Therefore, from this, we can conclude
that, with the increasing sample thickness, reflection peaks shift
toward the lower frequencies.
■ EM ABSORPTION IN COMPOSITES
Having discussed how magnetic and nonmagnetic materials
shield EM waves and the factors that are responsible for that,
we shall discuss composite systems in this section, that have
certain differences with respect to the previous scenario. In the
composite materials, heterogeneous micro/nanostructures
exhibit variation in the material’s local field, which works as
a polarization site.77,93
These polarization centers generate lag
in the displacement current relative to the conduction current.
In the nanocomposite, matrix and filler both play a very
important role in significantly influencing EM shielding
behavior. The composite matrix and filler both have different
electromagnetic properties, and in this condition, permittivity
and permeability can be treated as a complex entity and
substituted by effective permittivity (ε = ε′ + iε″) and
permeability (μ = μ″ + jμ″), where i is the imaginary number
and ε is a complex entity. Here, ε′ denotes the electrical
component’s energy storage capacity and ε″ is the dielectric
loss. On the other hand, μ′ and μ″ denote the magnetic energy
storage and various magnetic losses. These parameters
(complex permeability) possess a complex dependency on
the morphology, size, and conductivity which we discussed
earlier. As far as the application of composite is concerned, in
fields such as radar and stealth, the essential prerequisite is to
modulate (decrease the effective permittivity and increase the
effective permeability) the effective μ and ε in such a way so
that we can minimize the reflection amount and enhance the
absorption. This can be achieved by minimizing the impedance
954

mismatch among shielding materials and free space. According
to transmission line theory, intrinsic surface impedance can be
written as65
πμ
σ πε
=
| |
| |
=
+
Z
E
H
fj
fj
2
2
in
(22)
and reflection loss can be written as70
=
−
+
z z
z z
RL (dB) 20 log in 0
in 0 (23)
The maximum absorption of EM waves implies minimum
reflection loss (RLmin), and this happens when the impedance
of free space and composite material is matched. Impedance
matching conditions in the ideal situation is when Zin = Z0 =
377 Ω.20
Here impedance of air is denoted by Z0, and Zin
represents the input impedance of the composite (absorber
material). The above condition is satisfied at a definite
matching thickness (tm) and the corresponding matching
frequency (fm). An effective EM absorber is supposed to make
the absorption bandwidth as broad as possible, which is
controlled by the quarter-wavelength theory discussed earlier
and given by the equation92
εμ
=
t
nc
f
4
m
m (24)
Here n is the refractive index and c the speed of light.
■ DISCUSSION OF EMI SHIELDING/MICROWAVE
ABSORPTION OF VARIOUS CORE−SHELL
MATERIALS
1. Ferrites as a Core. 1.a. Primitive Ferrite (Fe3O4/Fe2O3)
as a Core. Iron oxide nanoparticles are one of the primitive
classes of magnetic nanomaterials which existed in the
literature. In the last two decades, various synthetic strategies
of its multidimensional shapes and sizes came into prominence
because of their multifarious applications.94
This system
(Fe3O4/Fe2O3) under an alternating magnetic field (AMF)
can generate heat which makes magnetic iron oxide nano-
particles an ideal heat source for biomedical applications
including cancer thermoablative therapy, tissue preservation,
and remote control of cell function.95−100
In various cases, this
particular system has been used as a nanofluid coolant for
modern engines.101
Due to the presence of magnetic dipoles in
the system, it generates a substantial amount of magnetic
heating when interacted with the external EM field.75,102
This
particular trait makes these systems lucrative for designing EM
screeners.24,75
In the following section, we will discuss about
various systems where these primitive ferrites act as a "core"
structure.
1.a.i. Carbon/Polymer as a Shell. Chen et al. reported a
facile three-step synthesis (hydrothermal followed by annealing
at 360 °C) strategy to make porous Fe3O4@C core−shell
nanorods having ferromagnetic characteristics at room temper-
ature. Here glucose was chosen as a source of carbon, which
acts as a shell. In this system, effective complementarities
among the dielectric loss that originates from carbon shells and
the magnetic loss that originates from the magnetic core enable
Figure 5. (a) Schematic illustration of the synthesis scheme of Fe3O4@C. (b) Electron micrograph of nanomaterials.104
(c) Reflection loss profile
of the composite. Reproduced with permission from ref 104. Copyright 2020 Elsevier.
955

the porous core−shell nanorods to screen an electromagnetic
wave frequency region over 2−18 GHz. They achieved a
maximum reflection loss of 27.9 dB at 14.96 GHz for the
composite made with wax (55 wt % filler) with a thickness of 2
mm.103
Meng et al. reported a 3D foam using an Fe3O4@C
core−shell structure. They used resorcinol as a carbon source
in this work. Synthesis is done by in situ polymerization and
subsequent carbothermal reduction in a tube furnace, where
Fe2O3 was converted to Fe3O4, and the final foam-like
structure was obtained as illustrated in Figure 5a,b. Here
they have adjusted the amount of polymeric precursor to
perfectly control the outer carbon layer with an average
thickness range of 9.1−34.8 nm. Due to the synergistic
contribution of the core and shell, it helps in impedance
matching, which in turns produces a minimum RL value of
−55.5 dB with a 2.5 mm absorber thickness and 28.3 nm shell
thickness. Especially, on reduction of the thickness from 5.5 to
1.4 mm, the Fe3O4@C exhibits efficient absorption (RL ≤ −20
dB) in an ultrawide band (14.9 GHz), as illustrated in Figure
5c.104
Magnetic vortex core−shell (Fe3O4@C) nanorings were
synthesized by Wang et al. and composited with wax to make a
composite which showed a significant reflection loss of 61.54
dB, for 1.5 mm thickness. The outstanding attenuation was
mainly due to the eddy loss enhanced by a combination of
confinement vortex and strain-driven vortex.105
For the Fe3O4@PANI system, Bhattacharjee et al. reported a
PVDF-based flexible composite of it for EMI shielding
application. In this work, the author reported a core−shell
system with a core of 400 nm diameter and a PANI shell
thickness of 20−25 nm. Along with 3 wt % of carbon nanotube
and 10 wt % of core−shell filler materials when added to the
PVDF matrix, this composite showed superior performance of
total shielding effectiveness of 19.5 dB at a thickness of only
300 μm.24
Manna et al. reported a trilaminar (core−shell)
nanostructure comprised of Fe3O4@C@PANI. For ease of
understanding, this ternary composite was synthesized by
varying the anile and Fe3O4@C content, as illustrated in Figure
6a. They have reported a significantly high value of EMI
shielding, and it could be attributed to the presence of dual
interfaces and dielectric−magnetic integration Fe3O4@C@
PANI. The authors mentioned that higher dielectric loss
through interfacial polarization and spin relaxation in Fe3O4@
C@PANI could also contribute toward enhanced microwave
absorption ability of Fe3O4@C@PANI compared to Fe3O4@C
and Fe3O4/PANI binary composites. They have reported an
EMI shielding value of 65 dB in the 2−8 GHz band for the
composition (Fe3O4@C:aniline = 9:1).106
Wu et al. reported a
core−shell structure comprised of iron oxide and polypyrrole
(PPy). This core−shell nanostructure was prepared by
sequential etching, polymerization, and replication. They
varied the etching time to design various combinations to
study their effect systematically. Here, the author discussed
how ferric ions from Fe3O4 were released from the skin layer,
which helps oxidative pyrrole polymerization, as illustrated in
Figure 6b. A maximum reflection loss of −41.9 dB is obtained
at 13.3 GHz with a matching layer thickness of 2.00 mm and
an etching time of 30 min.107
1.a.ii. Dielectric Materials as a Shell. Dielectric coating on
magnetic nanoparticles can generate significant magneto-
dielectric loss. These magneto-dielectric systems are highly
desirable systems for EM attenuation due to their synergistic
property enhancement.
Using this idea, Bhattacharjee et al. designed a core−shell
system comprised of Fe3O4 as a core and SiO2 as a shell
material (Fe3O4@SiO2). They have synthesized this core−shell
nanostructure by using the hydrothermal method and followed
by SiO2 coating by Stober’s method. As a result, material, when
composited with PVDF along with carbon nanotube, gives rise
to significant shielding efficiency. The author reported an EMI
shielding effectiveness of 15 dB for a low thickness of 300
Figure 6. (a) Schematic representation of synthesis of core dual shell@Fe3O4@C@ PANI. (b) Schematic representation of synthesis of Fe3O4@
PPy.106,107
Reproduced with permission from refs 106 and 107. Copyright 2017 American Chemical Society.
956

μm.24
Guo et al. reported a different shell of SiO2 around the
Fe3O4 core. In this article, the author reported a design where
two layers of SiO2 shells were made by a different method. The
transitional nonporous silica layer was first coated via a sol−gel
process, and then, by a surfactant-assembly method, the
mesoporous silica structure was coated as the outer shell layer,
resulting in the formation of uniform core−shell mesoporous
silica FO@nSiO2@mSiO2. To examine the microwave-
absorption properties, epoxide resin was chosen as the matrix,
and to it, presynthesized FO, FO@nSiO2, and FO@nSiO2@
mSiO2 composite materials were dispersed in 1:3 weight ratio.
A very low maximum reflection loss of 5 dB was reported at 16
GHz frequency with a composite thickness of 2 mm.108
Bhattacharjee et al. reported a unique strategy to synthesis of a
core−multishell heterostructure where Fe3O4 was coated with
SiO2 and finally wrapped with a multiwalled carbon nanotube
(MWCNT), resulting in the formation of an Fe3O4@SiO2@
MWCNT heterostructure. In this heterostructure, SiO2 acts as
a spacer on which MWCNTs were effectively wrapped. When
this unique nanostructure (10 wt %) was composited with
PVDF along with additional MWCNT (3 wt %), the final
composite showed an excellent EMI shielding effectiveness of
around −40 dB for 600 μm thickness, and moreover, these
composites were fairly flexible in nature. The authors have
reached the commercial limit of 20 dB at only 100 μm
thickness. Multiple loss consisting of conduction loss, eddy
current loss, and contributing Snoek’s limit109,110
helps in
achieving high EMI shielding effectiveness.24,111
Yuan et al.
reported an rGO coated Fe3O4@SiO2@polypyrrole (FSPG)
core−shell nonwoven composite film for EMI shielding
application. In this work, electrospun Fe3O4@SiO2 nonwoven
fabrics act as the framework to fabricate Fe3O4@SiO2@
polypyrrole core−shell fibrous network structures by oxidation.
Subsequently, the graphene oxide sheets were coated on the
fibrous network to construct a sandwich structure film,
followed by the reduction with hydrazine hydrate. This
composite shows an average EMI shielding effectiveness of
32 dB (the specific EMI shielding effectiveness can be
∼12,608.4 dB cm2
g−1
) for a 0.27 mm thick film. The reason
for the shielding effectiveness can be attributed to the relatively
high electrical conductivity of FSPG films (0.71 S/cm) and
multiple internal reflections which were generated due to
interconnected core−shell and sandwich microstructure in the
free-standing films.112
Zhu et al. synthesized core−shell nanotubes, where dielectric
TiO2 acts as a shell around Fe3O4. In this synthetic route,
authors initially obtained an Fe2O3 nanotube and TiO2 coating
was done using Ti(SO4)2 as a Ti source by a wet chemical
approach. Initially, the thickness of the amorphous layers was
around 20 nm, but after the hydrogen deoxidation process, the
amorphous TiO2 was converted to a crystalline structure
comprised of TiO2 nanoparticles (with 2.5 nm average
diameter), and subsequently, the layer thickness was reduced
to 18 nm. At the same time, Fe2O3 was also transformed into
cubic Fe3O4. Authors have concluded that eddy current loss
was the main factor in reducing the anisotropy energy and
subsequently increasing the composite’s shielding efficiency.
They reported a reflection loss of 20.6 dB at 17.28 GHz with a
composite thickness of 5 mm.113
Yu et al. reported a synthesis
of a highly dispersed yolk−shell structure comprised of Fe3O4
and ZrO2 (Fe3O4@ZrO2). Here, a homogeneous shell has
been synthesized using a polymer surfactant [hydroxypropyl
cellulose (HPC)] assisted sol−gel method. They have
successfully achieved 25−30 nm uniform thickness over the
Fe3O4 core. SiO2 plays an important role in this synthesis
which provided a hollow intermediate shell of 500 nm, as
illustrated in Figure 7a,b. They have observed that at higher
temperatures a hollowing transformation occurs for Fe3O4
cores and a crystalline phase change occurs for the ZrO2
shell but particles are thermostable under temperatures of 700
Figure 7. (a, b) Transmission electron micrograph of various morphologies of Fe3O4@SiO2 and Fe3O4@ZrO2 core−shell nanostructures.114
Reproduced with permission from ref 114. Copyright 2014 Royal Society of Chemistry.
957

°C. They have made several combinations ranging from a thick
integrated shell to an unintegrated shell and finally made one
composition where gold nanoparticles were dispersed between
ZrO2 shells and SiO2 shells. To assess the microwave
absorption property of the material as a function of
temperature, a series of materials at temperatures of 20, 250,
and 500 °C was taken into consideration. Composites were
made using epoxy resins (1:5 by weight), and RL = 26 dB was
reported for the composite where the filler has a gold
nanoparticle dispersion around the SiO2 shell at a 6 GHz
frequency range for a thickness of 2 mm at a varied
temperature from 20 to 500 °C.114
Meng et al. reported a
novel double core−shell (Fe3O4/ZnO)@C nanostructure
prepared through a four-step process (solution self-propagating
combustion, heat treatment, phenolic polymerization, and
subsequent carbothermal reduction). Different molar ratios of
Fe and Zn atoms were taken, and consequently, four different
samples were prepared. The idea behind designing this double
core−shell nanostructure for microwave absorption was that
multiple interfaces present inside the system lead to an
enhancement of multiple interface polarization behavior.
Precisely, since a considerable portion of Fe3O4 and ZnO is
gradually obtained in situ from ZnFe2O4, the subsequent
double cores can ensure effective uniform dispersion to
increase the mutual contact area, which provides good
interfacial polarization conditions. When the molar ratio of
Zn to Fe was taken is 1:1 and composited with wax, the
composite shows a minimum RL value of 40 dB at 15.31 GHz
with 2 mm thickness and has an effective wideband absorption
of 6.5 GHz and an efficient absorption (RL < 20 dB)
bandwidth of 3.4 GHz.115
Xu et al. reported different morphology nanostructures with
Fe3O4 and CuSiO3. They have synthesized ellipsoidal Fe3O4@
CuSiO3 nanorattles. Various templates having varied SiO2 layer
thickness and different aspect ratio have been designed on
which the CuSiO3 shell has been deposited, as illustrated in
Figure 8. Authors have investigated their microwave absorption
properties. The results demonstrated that the ellipsoidal
nanorattles with an aspect ratio of 3−4 exhibited about 20%
enhancement of microwave absorption intensity compared
with spherical Fe3O4@CuSiO3. There was a reflection loss
value of 25 dB at 3 GHz with a 4 mm thickness of composite
(made from 1:5 proportions of filler to epoxy ratio) having 1.2
mm thickness.22
1.a.iii. 2D Chalcogenides as a Shell. Transition metal
dichalcogenides (TMDCs) are semiconductor materials of the
type MX2, where M is a transition metal atom (such as Mo or
W) and X is a chalcogen atom (such as S, Se, or Te), that
provide a likely substitute to 2D graphene materials and their
derivatives. Due to its robustness, MoS2 is the most explored
material in this family. TMDCs exhibit a unique blend of
several important properties such as atomic-scale thickness,
direct bandgap, very strong spin−orbit coupling, fairly good
mechanical and electrical properties, and these attributes make
TMDCs extremely lucrative for applications in high-end
electronics, optoelectronics, flexible electronics, DNA sequenc-
ing, personalized medicine, and energy storage.116−118
More-
over, the laminar structure of MoS2, which has a similarity with
graphene, can generate strong electronic polarization that is
useful for enhanced dielectric loss and efficient microwave
absorption.119−121
Pan et al. reported a synthesis of porous coin-like Fe2O3@
MOS2 microsheets which was done via a solvothermal route
followed by heat treatment (annealing) at different temper-
atures. The morphology and structure analysis indicates that
MoS2 nanosheets are coated on the surface uniformly to form
Figure 8. (a) Schematic representation of the synthesis route for Fe3O4@CuSiO3 nanorattles. (b) Electron micrograph of Fe3O4@CuSiO3
nanorattles.22
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the core−shell structure. The authors investigated their
microwave absorption properties after the filler was compos-
ited with wax, and from the permeability study, the authors
concluded that efficient microwave absorption properties are
shown by the composite with a maximum effective absorption
bandwidth (reflection loss below −10 dB) of about 4.73 GHz
(11.13−15.86 GHz) for a composite thickness of 2.0 mm,
accompanied by the minimum reflection loss of about −37.02
dB. When they vary the calcination temperature and increase it
to 650 °C, the effective microwave absorption frequency band
shifts to the lower frequency (9.91−14.19 GHz) with the
minimum reflection loss −35.5 dB under the similar thickness
of 2.0 mm. Magnetic resonance frequency loss and impedance
matching between layers are understood to be a predominant
factor for microwave absorption.122
Li et al. showed the
excellent synergy between the Fe3O4 and MoS2 systems by
theoretical calculations. To effectively utilize the synergistic
effect and morphology, Fe3O4@MoS2 and MoS2@Fe3O4
nanocomposites (NCs) were constructed. They have shown
by controlling the reaction time that different morphologies
can be designed. They have measured the EM attenuation of
the composites made with 30 wt % of the filler with paraffin.
The composite shows the optimal RL values (∼50 dB) with an
absorption bandwidth (5.0 GHz) and high chemical stability.
Impressively, the Fe3O4@MoS2 obtained at 200 °C exhibited a
minimum RL value of −50.75 dB with a matching thickness of
2.90 mm. Here, enhanced impedance matching was respon-
sible for the absorption.123
Table 1 lists the EMI shielding/microwave absorption
properties of all of the primitive ferrite-based core−shell
materials discussed above.
1.b. Other Non-Primitive Transitional Metal Ferritic
Structures as a Core. As the traditional microwave absorption
materials, hexagonal ferrite materials have great application
prospects due to their magneto-crystalline anisotropy in the C-
axis direction124
such as M-type ferrite125,126
and Z-type
Table 1. EMI Shielding/Microwave Absorption of Various Primitive-Ferrite-Based Core−Shell Materials
Sn filler materials matrix
EMI SE/RL
(dB)
thickness
(d, mm)
bands
(GHz) ref
1. Fe3O4@C core−shell nanorods wax (55 wt % filler) 27.9 2 14.96 103
2. 3D Fe3O4@C foam wax (50 wt % filler) 55.5 2.5 8.2 104
3. Fe3O4@C nanorings wax (25 wt % filler) 61.5 1.50 16.8 105
4. Fe3O4@PANI PVDF (Fe3O4; 10 wt % + MWCNT 3 wt %) 19.5 0.3 12−18 24
5. Fe3O4@C@PANI wax (50 wt % filler) 65 1 2−8 106
6. Fe3O4@C@PPy wax (50 wt % filler) 41.9 2 13.3 107
7. Fe3O4@SiO2 PVDF (Fe3O4@SiO2; 10 wt % + MWCNT 3 wt %) 15 0.3 12−18 24
8. Fe3O4@nSiO2@mSiO2 epoxy (1:3) filler to matrix 5 2 16 108
9. Fe3O4@SiO2@MWCNT PVDF (Fe3O4@SiO2@MWCNT; 10 wt % +
MWCNT 3 wt %)
40 0.6 12−18 24
10. Fe3O4@SiO2@PPy@rGo 32 0.27 8−12 112
11. Fe3O4@TiO2 wax (50 wt % filler) 20.6 5 17.3 113
12. Fe3O4@SiO2−Au@ZrO2 epoxy (1:5) filler to matrix 26 2 6 114
13. (Fe3O4/ZnO)@C wax (50 wt % filler) 40 2 15.3 115
14. Fe3O4@CuSiO3 ellipsoidal
nanorattle
epoxy (1:5) filler to matrix 25 4 3 22
15. Fe2O3@MoS2 wax (50 wt % filler) 37 2 12.8 122
16. Fe3O4@MoS2 wax (30 wt % filler) 50.7 2.9 12 123
Figure 9. (a, b) Schematic representation of the synthesis route for hollow core−shell ZnFe2O4@C130
and core−shell CoFe2O4@1T/2H-MoS2.131
(c) Microwave absorption profile with respect to frequency. Reproduced with permission from refs 130 and 131. Copyright 2020 Elsevier and
American Chemical Society.
959

ferrite.127
Similarly tetragonal ferrites and other ferritic
structures are also widely studied for various applications.128
Liu et al. reported a porous core−shell nanostructure
comprised of BaFe2O4 as a core and a carbon layer as a shell.
They have synthesized this core−shell system by a hydro-
thermal process, and glucose was used to coat the ferrite. They
have tuned the hydrothermal temperatures (185, 190, 195, and
200 °C) as well as barium ferrite and glucose mass ratio (1:1
and 1:2, respectively). The authors investigated the microwave
absorption properties of the composite made out of this
nanostructure which showed an optimum reflection loss of
−73.42 dB which was synthesized at 200 °C hydrothermal
temperature and with 1:2 proportions having a thickness of
1.40 mm at 17.84 GHZ in the frequency bandwidth of 3.80
GHz. For 1:1 proportion, the RL value reached about 87 dB at
10 GHz with a thickness of 2.95 mm and a bandwidth of 3.80
GHz.129
Huang et al. reported a hollow core−shell structure
consisting of ZnFe2O4 as the core and a carbon layer as the
shell. They have opted for a simple and efficient hydrothermal
procedure followed by precursor coating using resorcinol and
finally by calcinating it at a higher temperature to get the
hollow ZnFe2O4@C core−shell nanostructure, as illustrated in
Figure 9a. The composites show excellent microwave
absorption properties in terms of the minimum reflection
Table 2. EMI Shielding/Microwave Absorption of Various Transition-Metal-Ferrite-Based Core−Shell Materials
Sn filler materials matrix EMI SE/RL (dB) thickness (d, mm) bands (GHz) ref
1. BaFe2O4@C wax (30 wt % filler) 87 3 10 129
2. ZnFe2O4@C wax (50 wt % filler) 51.4 2 15.4 130
3. CoFe2O4@1T/2H-MoS2 wax (40 wt % filler) 68.5 1.8 13.2 131
4. Co0.6Fe2.4O4@MoS2 wax (50 wt % filler) 79.9 2.7 11.2 132
Figure 10. (a) Schematic illustration of the synthesis route and (b) microscopic characterization for CoNi@GC. (c) Schematic illustration of the
mechanism of shielding in the system.135
Reproduced with permission from ref 135. Copyright 2019 American Chemical Society.
960

loss, thickness, and effective absorption bandwidth. The
minimum reflection loss of hollow ZnFe2O4@C composites
is −51.38 dB at 15.39 GHz with a thickness of 2.0 mm, and the
absorption bandwidth below −10 dB is up to 4.10 GHz
(12.56−16.66 GHz) with a thickness of 2.2 mm.130
Wang et al. reported a facile synthesis of a core−shell
structure containing CoFe2O4 as a core and the 1T/2H phase
of MoS2 as a shell. In this framework, CoFe2O4 nanospheres
were completely embedded in a special three-dimensional
(3D) nest-like 1T/2H phase MoS2, as illustrated in Figure 9b.
They also mentioned that, in comparison with CoFe2O4
nanospheres, the coercivities of the synthesized CoFe2O4@
1T/2H-MoS2 core−shell system greatly increase. Furthermore,
they mentioned 1T/2H-MoS2 exhibits ferromagnetism super-
imposed onto large diamagnetism. The authors tuned the
microwave absorption properties by adjusting the content of
1T/2H-phase MoS2. The combination of 1T/2H-MoS2 with
CoFe2O4 helps adjust the permittivity and optimize the
composites’ impedance matching. The microwave absorption
study revealed that a reflection loss (RLmin) of −68.5 dB for
the composites with a thickness of 1.81 mm is at 13.2 GHz, as
illustrated in Figure 9c.131
On a similar note, Long et al. reported similar ferrite systems
with different stoichiometric amounts. Co0.6Fe2.4O4@MoS2
nanocomposites were made by a two-step hydrothermal
method. The result showed that Co0.6Fe2.4O4@MoS2 nano-
composites showed greater EM absorption performances with
low RLmin value (79.9 dB at 11.2 GHz) and broad absorption
bandwidth at very low matching thicknesses (2.34 and 2.98
mm).132
properties of all of the transition metal ferrite-based core−shell
materials discussed above.
2. Metal Alloys as a Core. Lately, binary ferromagnetic
alloys are proved to be a prevalent choice for EMI shielding
materials. A few recent advances were reported discussing
CoNi and FeNi alloy-based soft composites for significantly
high reflection loss and shielding effectiveness.49,111,133
Among
them, FeCo and CoNi nanoalloys are very promising because
of their remarkably high magnetic saturation. The excellent
tunability of these nanostructures in the wet chemical approach
makes them suitable for various other applications as well.83,111
Some of the early reports include where Wang et al. reported
CoNi@C nanocapsules as an efficient microwave absorber. An
arc-discharge technique prepared these nanocapsules. They
have selected an ingot with an atomic Co:Ni ratio of 60:40.
This is because of the proper magneto-crystalline anisotropy
possessed by nanocapsules with such core compositions which
lead to generation of magnetic resonances in the 2−18 GHz
frequency range. They have varied dielectric loss of the
composite made from wax and CoNi@C nanocapsules as filler,
responsible for microwave absorption. They reported an
optimal RL value of 34 dB at 16.2 GHz for the composite
Figure 11. (a) Illustration of the preparation of 3D urchin-like CoNi@G@NCNT. (b) Microwave absorption profile for 3D urchin-like CoNi@
G@NCNT. (c) Structural characterizations of 3D urchin-like CoNi@G@NCNT.136
Reproduced with permission from ref 136. Copyright 2020
American Chemical Society.
961

thickness of 2 mm.134
Liu et al. reported the synthesis of a
core−shell structure where CoNi was decorated with graphitic
carbon (CoNi@graphitic carbon) decorated on B, N co-doped
hollow carbon polyhedra. This synthesis has been done using
pyrolysis of the metal−organic framework as a precursor. The
B, N co-doped hollow carbon polyhedra, originated from the
calcination of Co-Ni-ZIF-67, are composed of carbon
nanocages and BN domains, and graphitic carbon layers
encapsulate the CoNi alloy, as illustrated in Figure 10a,b. A
filler loading of 30 wt % provides the absorber with a maximum
RL of −62.8 dB at 7.2 GHz at a thickness of 3 mm, and the
effective absorption bandwidth below −10 dB remarkably
reaches as strong as 8 GHz when the thickness is only 2 mm.
The possible mechanism of shielding is depicted in Figure
10c.135
Guo et al. reported three-dimensional (3D) urchin-like
amorphous nitrogen-doped carbon nanotube (NCNT) arrays
with embedded cobalt−nickel@graphene core−shell nano-
particles (NPs) in the inner parts of NCNTs (CoNi@G@
NCNTs). The CoNi NPs are covered with about seven layers
of graphene shell, resulting in the formation of CoNi@G
core−shell structures. CoNi@G core−shell NPs are further
encapsulated with carbon nanotube (NCNTs), as illustrated in
Figure 11a,c. This unique 3D architecture helps in generating
multiple scattering of incoming microwaves. Due to the
synergistic effect between magnetic and dielectric loss and
additional interfacial polarization, the 3D urchin-like CoNi@
G@NCNTs exhibit excellent microwave (MW) energy
attenuation ability. 3D urchin-like CoNi@G@NCNTs exhibit
excellent MW attenuation property with a reflection loss of
−54.0 dB at 5.2 GHz with a composite thickness of 2.5 mm, as
illustrated in Figure 11b.136
Wang et al. reported a synthesis of
FeCo@C core−shell structure with FeCo Prussian blue
analogues (FeCo PBAs) as the nucleation sites and polymerize
dopamine on the surface to produce core−shell FeCo PBAs@
polydopamine (PDA) nanocubes. Finally, high-temperature
pyrolysis transforms the precursor into desirable magnetic
carbon-based composites with the microstructure feature of
core−shell FeCo@C nanoparticles encapsulated in PDA-
derived carbon nanocages (FeCo@C@CNGs). The strongest
reflection loss was observed to be 67.8 dB at 15.8 GHz, and the
effective absorption bandwidth covers 11.0−16.3 GHz with a
thickness of 2.00 mm.137
Zhu et al. reported a similar material
having a similar dielectric spacer but altered the shell to TiO2.
They got some interesting results using it.138
Kuang et al.
reported a similar composition where the dielectric spacer is
altered and amorphous carbon was introduced. They have
observed high RL values of 79.2 for 2 mm thickness.139
Jiang et al. reported a study of core−double shell FeCo/C/
(x)BaTiO3 nanocomposites on microwave absorption in the
1−18 GHz frequency range. They showed that enhanced
relative permittivity provided better impedance matching of
the core−double shell FeCo@C@BaTiO3 than that of an
FeCo@C and BaTiO3 mixture. Reflection loss (RL) values
observed for the FeCo@C@BaTiO3 composite (20 wt %)
were almost double those of the FeCo@C composite at the
matching absorber thickness from 2 to 7.5 mm. This was
attributed to the enhanced interfacial effects. They have
reported an RL value of the composite of ∼41.7 dB at 11.3
GHz at the absorbent thickness of 2 mm.140
Guo et al.
reported a synthesis of multistrata core−shell structure
comprised of a tricomponent ceramic alloy FeSiAl as a core
and aluminum and silicon dioxide as a shell (FeSiAl@Al2O3@
SiO2) by a plasma-induced method. These dense ceramic
layers effectually decrease the permittivity of FeSiAl without
losing permeability. Furthermore, these microstructures were
enriched with multiple interfaces to favor the interfacial
polarization and multiple internal reflection probabilities. Due
to the strong synergistic magneto-dielectric contribution, the
multistrata core−shell structure showed minimum reflection
loss of −46.29 dB at 16.93 GHz for the composite thickness of
2 mm.141
Table 3 lists the EM absorption/shielding properties of all of
the metal-alloy-based core−shell materials discussed earlier.
3. Carbonaceous Materials as a Core. Carbonaceous
derivatives in the last decades have generated tremendous
attention toward the design of functional composites due to
their tunable surface properties, ultrathin structural feature,
significantly high strength-to-weight ratio, anticorrosion
properties, superior carrier mobility (∼200,000 cm2
/V·s),
and very high thermal conductivity (∼5300 W/m·K).142−147
Hybrid composites containing carbonaceous inclusions often
act as better shielding materials for their light weight, high
specific surface area, and flexibility along with previously
mentioned electronic traits.25,93
Carbon nanospheres, carbon
nanotubes, graphene, and graphene derivatives are widely
studied,67,148−151
and here in this section, we will discuss how
in core−shell systems they influence EM absorption/EMI
shielding properties.
Wang et al. report a carbon-nanosphere (CNS)-based core−
shell composite where CNS acts as the core and MnO2
nanoflakes uniformly distributed around the CNS to coat it
as a shell material. These nanostructures were synthesized by a
water-bathing method. The incorporation of the CNS with
MnO2 enhances the electrical conductivity. Furthermore, here
in this work, impendence matching of EM waves has been
significantly improved. In addition, increasing interfaces
between the CS and MnO2 facilitate the EMI shielding
performance. A high dielectric loss value and electromagnetic
Table 3. EMI Shielding/Microwave Absorption of Various Metal-Alloy-Based Core−Shell Materials
1. CoNi@C wax (50 wt % filler) 34 2 16.2 134
2. CoNi@graphitic carbon wax (30 wt% filler) 62.8 3 7.2 135
3. CoNi@C@NCNT wax (45 wt % filler) 54 2.5 5.2 136
4. FeCo@C@CNGs wax (50 wt % filler) 67.8 2 15.8 137
5. FeCo@C@CNT wax (20 wt % filler) 79.2 2 14 139
6. FeCo@SiO2 wax (30 wt % filler) 38.49 2 11 138
7. FeCo@TiO2 wax (30 wt % filler) 36.53 5 3 138
8. FeCo@SiO2@TiO2 wax (30 wt % filler) 33.7 3 5.8 138
9. FeCo@C@BaTiO3 wax (20 wt % filler) 41.7 11.3 2 140
10. FeSi@Al2O3@SiO2 wax (80 wt % filler) 46.29 2 16.93 141
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shielding effectiveness of 16−23 dB were observed for the
CS@MnO2 in the X and Ku band, which is mainly ascribed to
the improved absorption loss.152
Bhattacharjee et al. reported
several core−shell architectures where carbon nanospheres
(CNS) acts as the core material and dielectric silicon dioxide
(SiO2), magnetic iron oxide (Fe3O4), and a combination of
both SiO2 and Fe3O4 act as the shell materials. These
nanostructured materials were synthesized by hydrothermal
and solvothermal methods. Three different core−shell systems,
such as, CNS@SiO2 (conducto-dielectric), CNS@SiO2@
Fe3O4 (conducto-dielectric-magnetic), as illustrated in Figure
12a, and another type CNS@ Fe3O4 (conducto-magnetic),
showed different properties, which lead to different shielding
effectiveness when composited with PVDF along with
multiwalled carbon nanotubes. They have reported a maximum
total shielding effectiveness of ∼ −42 dB in the X and Ku band
for the CNS@SiO2@Fe3O4 heterostructure for the signifi-
cantly low composite thickness of 600 μm, as illustrated in
Figure 12b. Here in this article, they have also conducted real
life antenna tests that reveal that this composite attenuates EM
and is an effective coating on an antenna. It does not hamper
antenna performance.19
Jio et al. reported a two-step facile and scalable method
(including pyrolysis and magnetron sputtering) to synthesize a
core−shell nanostructure consisting of cotton-derived carbon
fibers (CDCFs) as the core and nanocopper as the shell. They
have used a low-cost and commonly available biomass material
(cotton cloth) as raw material. In this case, the cotton fibers
were pyrolyzed into a conductive and hydrophobic cotton-
derived carbon fiber, and then, a thin layer of nanocopper was
deposited on the surface of carbon fibers by magnetron
sputtering. This composite with high electrical conductivity
induces interfacial polarization and conduction loss, which in
turn helps them achieve a high electromagnetic interference
(EMI) shielding effectiveness of 29.3 dB in the X band.153
Lu
et al. reported the synthesis of carbon-nanotube-based core−
shell materials where carbon nanotubes (MWCNTs) were
coated with CdS nanocrystals (CdS-MWCNTs) and
MWCNTs were coated with different-thickness CdS sheaths.
These syntheses were done through a mild solution-process
technique. The influence of CdS amount, external temperature,
loading concentration, and sample thickness was investigated
on EM absorption performance. The composite with 6 vol %
CdS-MWCNTs exhibited the strongest absorption of −47 dB
at 473 K with an absorber thickness of 2.6 mm in the X band.
Absorption performance is ascribed to effective impedance
matching, benefiting from interfacial polarization loss from the
CdS nanocrystals and the tunable dielectric property at higher
temperature.154
Zhang et al. reported a promising core−shell
nanostructure where the carbon nanosphere acts as a core and
the MoS2 sheet wrapped around it acts as a shell. This material
showed interesting results which exhibited 52.6 GHz for 1.4
mm thickness at 15.2 GHz, showing an effective absorption
bandwidth of 6.2 GHz.155
Wang et al. replaced the core with
Figure 12. (a) Synthesis scheme for CNS@SiO2@Fe3O4
19
. (b) Three-dimensional depiction of the total shielding effectiveness (SET) w.r.t.
thickness of PVDF composites at the Ku band.19
963

CNT instead of the carbon nanosphere and got slightly better
shielding effectiveness.156
Table 4 lists the EMI shielding/
microwave absorption properties of all carbonaceous-material-
based core−shell structures discussed above.
4. Metals and Metal Derivatives (Oxide, Sulfide,
Selenide) as a Core. Metals and metallic architectures are
considered as the oldest known EM screening materials. We
have discussed in the earlier sections that metals, which have
abundant mobile charge carriers and thus have good electrical
conductivity, interact explicitly with the incoming fields and
show high electromagnetic shielding performance.157−159
Wang et al. reported core−shell cobalt (Co)@cobalt oxide
(CoO) nanomaterials directly synthesized by annealing of Co
nanocrystals in the air at 300 °C. Here, the authors synthesized
flake-shaped assemblies composed of Co nanocrystals and
annealed them at 300 °C in the air to produce mixtures of
CoO and Co3O4 on the Co nanocrystals. The insulated CoOx
shells improved the electrical resistivity and subsequently
generate eddy current loss, which helps in effective suppression
of EM interference in X band and Ku band. Besides, the
appropriate thickness of CoOx shells can adjust the
permittivity of Co−CoO composites by inducing interfacial
polarization. Nanocomposite powders were dispersed in wax to
make composites and microwave absorption properties of the
prepared composite were investigated in the frequency range
of 2−18 GHz. The sample which was annealed for 1 h shows a
maximum reflection loss of ∼30.5 dB. At the same thickness,
the sample annealed for 3 h shows a maximum reflection loss
of ∼24 dB.160
Kuchi et al. reported a synthesis of honeycomb-
like core−shell nickel@copper silicate (Ni@CuSiO3) nano-
structure with controlled shell (CuSiO3) thickness to modulate
impedance matching for enhancing microwave absorption
properties. The nanospheres with a shell thickness of about 30
nm significantly enhanced microwave absorption performance
when composited with wax. The wax composite showed strong
reflection loss of −39.5 dB at 9.6 GHz for the absorber
thickness of 2.5 mm, having an effective absorbing bandwidth
of 4.8 GHz (7.8−12.6 GHz).161
Liu et al. reported hierarchical
hollow CuS@CoS2 nanoboxes with double shells. These
unique nanostructures were designed by a template-assisted
process inspired by Pearson’s hard and soft acid−base (HSAB)
principle. In this case, the hollow nanoboxes were composed of
a CuS core and a CoS2 nanosheet outer shell. The formation of
outer Co(OH)2 nanosheets is key to achieving double shells by
using “coordinating etching and precipitating”, and the etching
of the cubic Cu2O precursor plays an important role in
stabilizing the hollow nanoboxes, as illustrated in Figure 13a,b.
When EM absorption of this composite was investigated, this
hierarchical hollow nanobox exhibited strong microwave
absorption attenuation due to the synergistic effects of dipolar
and interfacial polarization, multiple scatterings, and matched
impedance between interfaces. Typically, the minimum
Table 4. EMI Shielding/Microwave Absorption of Various Carbonaceous-Material-Based Core−Shell Nanostructures
Sn filler materials Matrix EMI SE/RL (dB) thickness (d, mm) bands (GHz) ref
1. CNS@MnO2 wax (30 wt % filler) 23 2 18 152
2. CNS@SiO2 PVDF (10 wt % filler + 3 wt % MWCNT) 30 0.6 12−18 19
3. CNS@Fe3O4 PVDF (10 wt % filler + 3 wt % MWCNT) 39 0.6 12−18 19
4 CNS@SiO2@Fe3O4 PVDF (10 wt % filler + 3 wt % MWCNT) 42 0.6 12−18 19
5. CDCF@nanoCu 29.3 2 8−12 153
6. CdS@MWCNT wax (30 wt % filler) 47 2.6 8−12 154
7. CNTs@MoS2 wax (40 wt % filler) 54.75 1.49 2−18 156
8. CS@MoS2 wax (40 wt % filler) 52.6 1.40 15.2 155
Figure 13. (a) Schematic illustration for the formation process of the hierarchical hollow CuS@CoS2 nanoboxes. (b) SEM and TEM images of
Cu2O nanocubes.21
Reproduced with permission from ref 21. Copyright 2020 Elsivier.
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reflection loss is up to 58.6 dB at 2.5 mm when the filler
loading is 20 wt %.21
Zhu et al. reported a paramagnetic core−shell composite
comprised of CoS2 as a core and MoS2 as a shell (CoS2@
MoS2) and graphene oxide. This 3D structure is formed with
an rGO nanosheet coated on the surface of CoS2@MoS2.
When composited with wax with 20% filler loading, it exhibits
58 dB of minimum RL for 2.4 mm thickness.162
Zhang et al.
reported a core−shell nanostructure comprised of NiS2 as a
core and MoS2 as a shell (NiS2@MoS2). These nanospheres
were synthesized by a hydrothermal route. Finally, these
nanostructures were composited with PVDF, and the micro-
wave absorption performance was investigated. This is mainly
attributed to a characteristic impedance match, dipole
polarization, interface polarization, and multiple reflections.
They have reported a minimum reflection loss of 41.05 dB at
12.08 GHz, and the absorption bandwidth exceeding 10 dB is
4.4 GHz with a composite thickness of 2.2 mm.163
Singh et al.
reported CdSe@V2O5 core−shell quantum dots with reduced
graphene oxide (rGO-V-CdSe) for the designing of lightweight
electromagnetic wave shielding material. A solvothermal
approach was used to synthesize this core−shell system. The
high dielectric loss and shielding effectiveness of this system
was attributed to various defects which were present in it
which led to dipolar and interfacial polarization. An overall
Table 5. EMI Shielding/Microwave Absorption of Various Metal- and Metal-Derivative-Based Core−Shell Nanostructures
1. Co@CoO wax (50 wt % filler) 30.5 1.7 14.5 160
2. Ni@CuSiO3 wax (30 wt % filler) 39.5 2.5 9.6 161
3. CuS@CoS2 wax (20 wt % filler) 58.6 2.5 7.1 21
4. CoS2@MoS2@rGO wax (20 wt % filler) 58 2.4 11 162
5. NiS2@MoS2 PVDF (20 wt % filler) 41 2.2 12 163
6. CdSe@V2O5 38 1 8.2−12.4 164
Figure 14. (a) Schematics and mechanism showing the preparation of HSS by a biological template.166
(b) Frequency dependence of EMI SE for
the doped BF/PANI, BF/PANI0.5, and the mixture. (c) Schematic EMI shielding mechanism for the BF/PANI composite.167
Reprinted with
permission from refs 166 and 167. Copyright 2020 and 2017 Elsevier and American Chemical Society, respectively.
965

shielding effectiveness up to ∼38 dB has been recorded in the
X band.164
Table 5 lists EMI shielding/microwave absorption proper-
ties of metal- and metal-derivative-based core−shell materials
discussed in this section.
5. Other Relevant Systems. Apart from the various
systems we have discussed until now, there are some minor yet
important systems that are left, which mostly consist of
polymers, fibers, and some carbide systems. We will discuss
these systems in this section.
Dong et al. reported a synthesis of core−shell SiC
whiskers@carbon heterostructure via chemical vapor deposi-
tion followed by a hydrothermal and carbonization process
using glucose as a carbon precursor. These heterostructures’
EM absorption properties concerning thickness were evaluated
in the X band and also in the Ku band. The authors have
modulated the thickness of the carbon shell from 10 to 100 nm
just by regulating glucose concentration. A significant
enhancement in absorption performance concerning pristine
SiCw was realized after the SiCw was coated with carbon
shells. The minimum reflection loss of −5.3 dB was obtained
for SiCw coated with about 10 nm carbon shell and −61.2 dB
for SiCw coated with about 70 nm carbon shell and to −59.4
dB when SiCw was coated with about 100 nm carbon shell.
SiCw@C heterostructures coated with about 70 or 100 nm
carbon cover the whole X band and Ku band just by regulating
the sample thickness.165
Wei et al. reported nano-Ni coated
hollow silicon carbide core−shell spheres where β-SiC with
yeast morphology was prepared by sol−gel and carbon thermal
reduction methods, as illustrated in Figure 14a. After that, the
magnetic nickel nanoparticle layer was coated on the surface
using Pd activated alkaline electroless plating. When these
particles were composited with paraffin, it was observed that
the structure possesses excellent microwave absorbing proper-
ties at the frequency bandwidth of 2−18 GHz, where a
maximum reflection loss of −50.75 dB was obtained at the
absorber layer thickness of 4.2 mm.166
Zhang et al. reported a
facile route for synthesizing a polyaniline (PANI) coated
bagasse fiber (BF) via a one-step in situ polymerization of
aniline in the dispersed system of BF. The BF core plays a
significant part by inducing a high degree of doping and a high
crystalline phase of PANI shell in the final composite, and as a
result, the electrical conductivity of the composite was higher
than that of the pristine PANI. The composite exhibits a
shielding effectiveness of 28.8 dB for a thickness of only 0.4
mm, as illustrated in Figure 14b; the mechanism of shielding is
depicted in Figure 14c. Shielding traits are mainly attributed to
the increased electrical conductivity and the effective multiple
reflections induced by core−shell structure.167
properties of various core−shell nanostructures discussed in
this section.
■ MECHANISTIC INSIGHTS OF EMI SHIELDING
USING CORE−SHELL MATERIALS: GENERALIZED
UNDERSTANDING
In this review article, we have seen various core−shell
nanostructures tuned and tailored in a way to absorb EM
radiations. These nanostructures attenuate EM radiation by
different mechanistic pathways. In this section, we will try to
provide a generalist understanding that will help us in
designing newer materials.
The most important factor which is responsible for EM
absorption in core−shell materials is the creation of multiple
interfaces to generate multiple scattering/reflection among an
individual nanostructure, which in turn helps in reducing and
ultimately attenuating the power of the incoming EM waves,
which attributed as EM absorption. Multiple reflection occurs
when the particle size is bigger than that of the wavelength of
EM wave, and if the dimensions of a particle are comparable
with, or smaller than, the wavelength, scattering occurs. In a
polymer composite system, it is no longer possible to separate
the contributions of reflection and scattering. So, both
phenomena occur and overlap. The next important factor for
EM attenuation/absorption is impedance matching/mismatch-
ing among core and shell layers. The basic underlying principle
is more impedance matching; a higher fraction of EM waves
will penetrate through the system and ultimately dies down
inside after multiple scattering and internal reflections, leading
to higher absorption phenomenon. In core−shell and multi-
layered system, impedance matching leads to strong absorption
of EM waves which in turn suggests that there is loss due to
absorption. On the other hand, impedance mismatch between
two layers facilitates reflection in between layers, which in turn
helps in generating loss due to multiple reflection/scattering.
Another mechanism of EM absorption which is prevalent is
interfacial polarization. As we know, more layers and interfaces
maximize the interfacial polarization loss (a type of dielectric
loss). From highly conducting (MWCNT) to partially
conducting, magnetic/semiconducting materials (Fe3O4/
CdS/SiC, etc.) contribute to an important loss mechanism
known as eddy current loss. Eddy current loss is the Joule loss
due to the eddy current induced by the alternating magnetic
field. Eddy currents are generated whenever there is a relative
motion between a conducting material and a varying external
magnetic field exists.168,169
As we know, when an alternating
magnetic field is applied to a magnetic system (conducting in
nature), an emf is induced in the material itself according to
Faraday’s law of electromagnetic induction. As magnetic
materials are conducting in nature, these EMFs circulate
current within the body of the material. These circulating
currents are called eddy currents. This will occur when the
conductor experiences a changing magnetic field. In other
words, to counter the change in the magnetic field and
maintain the symmetry in the system, materials try to generate
resistance to nullify the changing magnetic field and
subsequently a circular loop of currents (known as eddy
currents) is generated. These currents are not responsible for
doing any useful work, and it produces a loss (I2
R loss) in the
Table 6. EMI Shielding/Microwave Absorption of Various Core−Shell Nanostructures
1. SiC whiskers@carbon wax (30 wt % filler) 59.4 2.5 14.2 165
2. SiC spheres@nano Ni wax (50 wt % filler) 50.75 4.2 2−18 166
3. Bagasse fiber@PANI wax (40 wt % filler 28.8 0.4 8.2−12.4 167
966

magnetic material known as an eddy current loss. Due to this
(resistance), a significant amount of heat is generated which
translated to EM loss (absorption) in a system. The eddy
current density can be expressed as J = σE, where σ is the
electric conductivity and E is the electric field induced by the
alternating H field. One final mechanism which is prevalent in
most magnetic materials is natural/ferromagnetic resonance.
Magnetic resonance losses also have contributions to micro-
wave heating of some metal oxides such as ferrites and other
magnetic materials. Such losses are induced by electron spin
resonance in the GHz frequency and domain wall resonance in
lower frequencies.170,171
From the literature, we know that
residual losses are originating from various magnetic
resonances and relaxations, which occur mainly as rotational
resonance and domain wall resonance.172
Accordingly, the
magnetic resonance loss can be regarded as the residual loss or
its components. All of these loss factors cumulatively or
individually contribute to the shielding efficiency for different
materials. A schematic depiction of different contributing
factors (various losses) are provided below (Figure 15).
■ OUTLOOK AND PERSPECTIVES
This review has introduced various synthesis methods and
fabrication techniques to prepare core−shell heterostructures
for designing highly efficient EM absorbing materials. These
heterostructures have emerged as one of the most powerful
and strategic materials for attenuating EM waves in the past
decade. However, still, not much effort has been made to
attenuate EM signals using these architectures. Among all
divisions we have discussed here, ferrite- and metal-alloy-based
core−shell structures are the most researched division. The
bulk amount of work deals with making composites with wax
where the filler material content is sometimes as high as 60−
80%, which makes them mechanically unstable and not fit for
various applications. Another persisting problem with wax
composites is that they are mostly thick (2−6 mm) and this
restricts them for use in high-end wearable electronics.
However, in some literature, we also encounter that high
shielding effectiveness with very little filler content (around
10−15%) along with heterogeneous conducting filler (viz.,
carbon nanotube, graphene oxide, etc.). These techniques, in
our view, upcoming material designers should focus on to
develop a multipronged coherent strategy to design custom-
ized solutions. Tweaking the structures and making them more
conducting and incorporation of magnetic domains in the
polymer matrix will hopefully enable us to design thin and
high-performance EM shielding materials. In several cases
throughout the article, we have seen how interfaces play a
pivotal role in EM absorption and shielding, as these interfaces
generate a significant amount of interfacial polarization and
matching and mismatching of different impedances at different
levels will help in attenuation. Thus, generating multiple
interfaces will play a pivotal part in designing modern-day
shielding materials. Therefore, we anticipate that the develop-
ment of various core−shell composite architectures will open
up a new direction soon for conventional and new-age
materials for ultraefficient EMI shielding.
■ AUTHOR INFORMATION
Corresponding Author
Suryasarathi Bose − Department of Materials Engineering,
Indian Institute of Science, Bangalore 560012, India;
orcid.org/0000-0001-8043-9192; Email: sbose@
iisc.ac.in
Figure 15. Schematic depiction of various losses in materials responsible for EM absorption.
967

Author
Yudhajit Bhattacharjee − Department of Materials
Engineering, Indian Institute of Science, Bangalore 560012,
India; orcid.org/0000-0001-9079-7516
Complete contact information is available at:
https://pubs.acs.org/10.1021/acsanm.1c00278
Notes
The authors declare no competing ﬁnancial interest.
■ ACKNOWLEDGMENTS
Y.B. and S.B. would like to thank DST, India (DST/NM/NS/
2018/157), for ﬁnancial support. The authors would also like
to thank Ms. Suravi Sengupta for her contribution in graphics.
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