MULTI-LAYERED GRAPHENE AND
GRAPHENE-LIKE ultra THIN FILMS
in MICROWAVES
P. Kuzhir, K. Batrakov, A. Paddubskaya,
S. Voronovich, N. Valynets,
S. Maksimenko,
Research Institute of Nuclear Problems of BSU, Belarus
T. Kaplas and Yu. Svirko
University of Eastern Finland, Finland
Investigated films:
Pyrolytic carbon (PyC), 5-240 nm thick
A few-layer graphene, 5 nm thick (10 layers)
Sandwich structure: graphene monolayers + PMMA



All investigated films are deposited on 0.5 mm thick silica substrates



Synthesis

Characterization
Microwave probing (Ka-band, 26-37 GHz)
Comparative analysis of EMI SE provided by ultra-thin films
and conventional polymer composites filled with nanocarbon, and
carbon foam
MICROWAVE PROBING of
GRAPHENE-LIKE ultra THIN FILMS: challenge
Film thickness PyC = 5-240 nm
Film thickness multilayered graphene = 5 nm (10 layers)
Film thickness of sandwich structures =
2 or 3 graphene monolayer + PMMA (600-800 nm)
skin depth in microwave range = tens of microns (15-20 microns)
Film thickness  skin depth
graphene-like ultrathin films should be transparent for microwaves ?
http://www.microwaves101.com/encyclopedia/skindepth.cfm
Pyrolytic Carbon thin films
Pyrolitic Carbon thin films: methane-based chemical
vapor deposition

We employed CVD process with no continuous gas flow inside the
chamber to reduce gas consumption and to allow more time for
polyaromatic structures formation.

The loading of the clean quartz substrate into the CVD chamber
was followed by purge filling of the chamber with nitrogen (twice) and
then with hydrogen to ensure a clean process.

After that the chamber was filled with hydrogen up to the
pressure of 5.5 mBar and was heated up to the temperature of 700 °C at
the rate of 10 °C/min.

At 700 °C, the chamber was pumped down and the hydrogenmethane gas mixture was injected and heated up to the temperature of
1100 °C.

CH4:H2 gas mixture was kept at this temperature for 5 min and
then was cooled down to 700 °C. After that the chamber was pumped
down, filled with hydrogen at the pressure of 10 mBar and cooled down to
room temperature.
Parameters of the synthesized PyC films
Thickness
of
prepared
carbon films was measured
by a stylus profiler (Veeco
Instruments, Dektak 150)
with an accuracy of 1.5 nm.

The thickness of deposited film is dependent on methane
concentration.

Ultrathin PyC films of prescribed thickness in the range of
5-240 nm were synthesized.
Raman characterization
perfect graphite “G” peak
”D” (disorder) peak

Raman spectra of the manufactured PyC films by using Renishaw inVia Raman Microscope at
the excitation wavelength of 514 nm.

The Raman spectrum of perfect graphite dominates by “G” (graphite) peak, 1582 cm-1 and
indicates the presence of the crystalline graphite in the sample.

The disorder broadens the G-peak and usually results in ”D” (disorder) peak in the vicinity of
-1
1360 cm .

The ratio of these peaks I(D)/I(G) is conventionally used as a measure of graphite
crystallinity. In an ideal graphite, I(D)/I(G) = 0, while in highly disoriented PyC, this ratio is relatively
high, I(D)/I(G) ≥1.
Raman characterization
the ratio of the sp2-sp3 bonds
graphitic lattice defects
amount of amorphous carbon

In the thinnest films, I(D)/I(G) = 1.2, while in the thickest
films, I(D)/I(G) = 1.0.

Positions of D and G peaks were at 1357 cm-1 and 1595 cm1, respectively, and did not depend on the film thickness.

The G-peak situated in the vicinity of 1600 cm-1 indicates
that the film is mainly consists of sp2 bonded graphene
nanocrystallites, i. e. its morphological properties are close to
those of nanographite.

A finite magnitude of the D” peak suggests that the film
contains also amorphous carbon.

Crystallinity of the film can be evaluated from the full
width half maximum of the G-peak in the Raman spectrum
FWHMG ≈ 70 cm-1 : the size of graphite crystallites is to be
less than 5 nm.
Transmittance, optics
Transmittance spectra was measured
with
Perkin
Elmer
Lambda
9
spectrophotometer in the spectral
range from 200 nm to 2000 nm. The
transmittance was measured with
respect to a bare fused silica
substrate.
The transmittance is almost constant
in the visual and IR spectral range
having a minimum at wavelength of 270
nm.
The obtained featureless transmission
spectrum in visible spectral range is
typical for graphitic materials (e.g.
graphene).
Optical transmittance of the PyC films
as a function of wavelength.
It ensures no significant coloring, i.e.
the ultrathin carbon films are well
suited for transparent conductive
electrodes.
Sheet Resistance vs Optical Transmittance
A
V
Quartz

The sheet resistance Rs of the PyC films was measured with the conventional four-point
probe technique.

The setup was based on probe station (Signatone S-1160) with 2 mm probe spacing and
U = 5 V driving voltage.

Since the thickness of the samples was considerably less than the probe spacing, the sheet
resistance can be obtained as:
RS 
U 
I ln(2)
Sheet resistance versus optical transmittance is
comparable to chemically derived graphene.
Microwave probing in Ka-band (26-37.5 GHz)
The EM response of samples as ratios of transmitted/input (S21) and
reflected/input (S11) signals was measured within the 26-37 GHz frequency
range (Ka-band).
The waveguide cross-section was 7.2×3.4 mm.
Scalar analyzer R2-408R(VSWR and
Transmission Loss Meter R2-408R)
Microwave probing of PyC films

Reflectivity of PyC
films
of
relatively
small
thicknesses (up to 28 nm) is
almost the same as for bare
quartz substrate (these films
reflect 20-22 %).

Films with thickness
higher than 30 nm reflect from
27 to 34 % of EM radiation.

In the PyC films with
thickness lower than 75 nm,
the
absorption
increases
significantly
with
the
thickness.
28 GHz
Film thickness, nm

The thickest PyC films demonstrate significant EMI SE. Only 22, 18 and
16 % of microwave signal could penetrate through the PyC film with thickness of
75 nm, 110 nm and 241 nm, respectively, deposited on silica substrate.

Even 30 − 35 nm thick PyC films secure already about 60 % of EM
attenuation at 28 GHz mostly due to absorption of the microwave radiation. Thus,
fabricated PyC films, whose thickness is thousand times less than the skin depth
of conventional metals, provide a reasonably high EM attenuation.
Microwave probing of PyC films
Multi-layered graphene films
Multi-layered graphene films: catalytic CVD process

0.5 mm quartz sample was coated with copper thin film which was physically
deposited, i.e. by thermal evaporation in vacuum (10-5 mBar) condition, on the quartz
surface.

The used copper layer thickness was 300 nm which is thick enough for surviving of
dewetting in high temperature without making holes but thin enough for carbon atoms to
get through grain boundaries.

Before the process, the CVD chamber was cleansed with nitrogen (twice) and
hydrogen to remove oxygen remains from the chamber.

Next the Cu coated quartz sample was heated in hydrogen atmosphere (7 mBar) to
700 °C.

At this temperature hydrogen was pumped from the chamber and methanehydrogen gas mixture (1:1) was injected in the chamber until the pressure was ~10 mBar.

The temperature was then risen to 950 °C with rate of 10 °C/min.

After 5 min the chamber was cooled down to 700 °C during 60 min and the CH4-H2
atmosphere was replaced with hydrogen (7 mBar).

After the CVD process, the remaining Cu was removed by ferric chloride (FeCl3)
solution and rinsed with water.
Mechanism
Multi-layers graphene films: characterization
For monolayer graphene the 2D peak position should be around 2680 cm-1
and the FWHM ~30 cm-1, while for multilayered graphene the 2D peak is
shifted close to 2700 cm-1 and FWHM is increased up to ~60 cm-1.
In our sample the 2D peak is located at ~2705 cm-1 and the FWHM is
about 65 cm-1 indicating that the sample is a multilayered graphene rather than
mono- or bi-layered graphene.
Fig. 2. Atomic force microscopy image of
graphene film.
Fig. 1. Typical Raman spectrum measured from nanocarbon
film indicates highly crystalline, multilayered graphene
material.
Microwave probing of multilayered graphene films
Fig . Reflection (R), transmission (T) and absorption (A) of
graphene film vs frequency.
In Ka-band we observed that multilayered graphene being thousands times
thinner than skin depth of conventional metals provide reasonably high EM
attenuation properties, caused by absorption of EM signal. EM absorption is
as high as 43% at 26 GHz for graphene film of 5 nm thickness.
Sandwich structures:
monolayer graphene + PMMA
Sandwich structures: monolayer
graphene + PMMA. Synthesis
 The first graphene monolayer was synthesized by chemical vapor
deposition (CVD) at 1000 C in methane atmosphere on the copper
catalyst
 and then spin coated by the 600-800 nm thick PMMA layer.
 Next, Cu was wet etched in ferric chloride.
 The obtained PMMA film with deposited graphene was washed in
distilled water and then placed on quartz substrate.
 After drying the next graphene layer was deposited on the top of
the PMMA.
 This procedure has allowed us to fabricate sandwich-like coatings
containing two – three graphene layers.
Transfer and deposition of several
graphene layers
Sandwich structures: monolayer
graphene + PMMA. Chatacterization
The absorption of one layer on top of
quartz should be about 1.9 - 2 %.
Here are the results that we observed
1 layer 800.1827 0.9828
2 layers 801.0099 0.9521
3 layers 799.3555 0.9238
According to Raman spectrum
investigations,
monolayer graphene is 90-95% of
monolayers
and 10-5% of bilayer graphene.
The absorption of 2 and 3 layers is a
bit more than ~2 % but this is because
that the “monolayer” is only about
90% monolayer.
Microwave probing of
sandwich graphene+PMMA films

Graphene monolayer provide almost 20% of absorption of microwave signal at
28 GHz.

2 layers of monolayered graphene separated by 600-800 nm of PMMA give 35
% of absorption of microwave power.

3 layers provide almost 50% of MW absorption.

As far as neat PMMA is transparent to microwaves we suppose to have the
same results for one-two and three monolayers of graphene (0.5 – 1.5 nm thick).

After removing PMMA, we will have 1.5-2 nm thick graphene films providing
almost 50% of absorption of microwave power.
Conclusions. Graphene-like thin films in
microwaves
Graphene-like films being 100-1000 times thinner than skin depth provide
reasonably high EM attenuation properties in microwave frequency range,
caused by absorption mechanism.
EM absorption is as high as 50% for PyC film of 75 nm thickness and multilayered graphene, 1.5-2 nm thick.
EM attenuation provided by PyC film of 30 nm thickness is compatible with
EM attenuation of 1-2 wt.% commercial MWCNT embedded into epoxy,
1 mm thick
The extremely small thickness and weight of graphene-like films makes them especially
attractive for application in satellite and airplane communication systems.
Moreover, PyC films and graphene films can be deposited on both dielectric and metal
substrates of any shape or size by using conventional and inexpensive chemical vapor deposition
technology.
Thus, PyC and multi-layered graphene could be used as ultrathin, transparent, weightless, and
flexible EMI shield, which would be tremendously important for portable electronic devices,
transparent electronics and displays, and EM field isolation in 3D ICs.
Microwave probing: commercial multi-walled CNT
High-frequency polarizability, microwave and THz range, modeling:
Reasonable nanotubes parameters – 10-15 mkm length, 5-15 nm diameter, 7-10 walls.
Strong electromagnetic screening of inner shells take place in MW range (only 2-4 outer walls take part in
EM interaction in MW frequencies). No electromagnetic screening takes place in THz range, all metal
walls take part in EM interaction.
What should we expect from the theory of
EM interaction of composite based on MWNT in
MW range (10-100 GHz)?
THz range (1-10 THz)?
1) Some dependence on the mean
outer diameter;
2) Strong dependence on the length
of CNT
3) Quite strong screening of inner
shells
of
multiwalled
CNT
(starting from 5th).
1) No significant dependence on the
mean outer diameter
2) Strong dependence on the length
of CNT
3) No screening of inner shells of
multiwalled CNT (all metal walls
take part in EM interaction).
Therefore if we use CNT with small
number of walls (up to 12-15), as
thick and long as possible, we could
succeed with producing effective EM
material for MW range.
Therefore if we use CNT with as high
number of walls as possible, they
could be thin or thick, as long as
possible, we could succeed with
producing effective EM material for
THz range application.
The number of walls as well as
thickness of CNT can be controlled
by catalyst and synthesis conditions.
The optimal geometry of CNT to be used in MW and THz range could be different!
Microwave probing Ka-band (26-37,5 GHz): MWNT
HR TEM micrographs (JEM-2010) of MWNTs
samples with narrow distributions of average outer
diameter (a–c), (d) the histogram of distribution of
average outer diameter of these MWNTs.

MWNTs were synthesized by CVD
methods via catalytic pyrolysis of ethylene at
950K on FeCo-based catalysts.

MWNTs were investigated with average
outer diameter ~ 12-14 nm respectively.

The wall number has been estimated as
8-15 walls.
Materials used: Exfoliated graphite

EG was obtained by intercalation of natural graphite flakes,
subsequently submitted to a thermal shock.

Accordion-like particles were produced, leading to a material of low
packing density, around 3 g/L.

EG particles have the form of distorted accordion-like cylinders,
having a typical diameter within the range 0.3 - 0.5 microns, and a aspect
ratio around 20.
Constituting individual worms of raw expanded
graphite, as seen by SEM.
Distorted honeycomb microstructure as seen by SEM.
The constitutive graphite sheets may be seen
as disoriented thin discs.
A. Celzard, J.F. Marêché, G. Furdin, Progress in Materials Science 50 (2005) 93-179.
Microwave probing: Exfoliated graphite

High
electromagnetic
shielding efficiency in microwave
frequency range has been found
for epoxy/EG composites already
for 1 wt.% of EG fillers.

When the concentration
is below or equal to 1 wt.%,
reflection of microwave signal is
responsible for EMI shielding
ability. 2 wt.% of EG within epoxy
resin provides almost the same
contribution of absorption and
reflection mechanism into EM
attenuation.
S-parameters vs frequency for epoxy/EG composites with different
contents of EG inclusions (0.25-2 wt.%). Inset: Transmittance (T),
absorbance (A) and reflectance (R) provided by epoxy/EG at 30 GHz.

2 wt.% EG in epoxy is
not transparent to microwaves
because of 45% of absorption and
55 % of reflection.
One more advantage is that EG is easier dispersed in the epoxy resin.
Graphitic materials vs Carbon nanotubes
SWNT (Heji),
2 wt.%
1mm
MWNT (Heji),
2 wt.%
1mm
Transmission 30
% mostly due to
reflection
Transmission 45
% mostly due to
reflection
MWNT (thick,
relatively small
number of walls),
2 wt.%
1mm
Transmission
approx.10 %
mostly due to
reflection
CB of low
surface area,
2 wt.%
1 mm
EG, 2 wt.%
1mm
TG, 2 wt.%
1mm
Transmission 60
% mostly due to
reflection
Transmission
close to 0 due to
60% reflection
and 40%
absorption
Transmission 60
% mostly due to
reflection
Large surface area is extremely important
Dielectric permittivity of composite in
range 20Hz – 1MHz
The higher the structure (meaning
that the CB comprises chain-like
agglomerates,
whereas
less
structured materials have more
individual particles, not so much
associated), the higher is the
conductivity
30 GHz
EG being embedded into epoxy in 2 wt.%
is the best EM attenuator,
but this concentration seems to be
critical for mechanical properties
Carbon foams: Preparation
Compound
Amount (g)
Role
Tannin
Furfuryl alcohol
Diethyl ether
Water
Formaldehyde (37% water solution)
Para-toluene-4-sulphonic acid (65% water solution)
30
6 - 15
1.5 - 5
6
7.4
11
Base of the resin
Strengthener
Blowing agent
Solvent
Cross-linking agent
Catalyst
carbon foams

Carbon foams are made of 95% of natural, green and renewable precursor.

Tannin is extract of acacia/mimosa bark.

Tannin is cross-linked with a little of formaldehyde in presence of furfuryl, ether and
catalyst.

Carbonaceous counterpart is obtained by pyrolysis up to 900°C in high-purity nitrogen.

The density was simply adjusted from the amount of blowing agent (ether) in the initial
formulation, the higher this amount, the lower the density and the higher the average cell
size.
Carbon foams: Description, physical properties
Main characteristics
Skeletal density : 1.98 g/cm3
Bulk density : 0.025 – 0.20 g/cm3
SEM images
Relative density : 1.3 – 10.1 %
Porosity : 89.9 – 98.7 %
Estimated cost << 10 € / kg
Average coordination number : 12
Porosity mainly open
1 mm
500 µm
Bulk density : 0.067 g/cm3
Bulk density : 0.067 g/cm3
compression strength
elastic modulus
Electrical conductivity
thermal (k) conductivity
 = 0.25 – 2.25 MPa
E = 0.7 – 35.5 MPa
1.5 – 40 S/cm
k = 0.04 – 0.105 W/(mK)
100 µm
Carbon foam: microwave range

EMI shielding efficiency has been found to be relatively high for 2 mm thick carbon foams,
higher than 8 dB for samples with large pore size, the maximum at 23 dB EMI SE being obtained
in Ka-band for the most dense samples.

The reflectance ability provided by studied foams characterized by the lowest density are
not less than 7 dB in Ka-bands, which corresponds to approx 30 % reflection of incident signal.

Carbon foams with higher density reflect 75-50% of the microwave radiation.
Carbon foam: microwave range
Reflectance (R), absorbance (A) and transmission (T)
of the EM radiation on samples are connected with
the measured S – parameters in the following way:
R = S112, T = S212, A = 1 – R – T
Carbon foam vs nanocarbon/polymer composites
PyC,
Graphene,
5-100 nm
MWNT (Heji),
2 wt.% in epoxy
1mm
Transmission
20% mostly due
to absorption
Transmission
45% mostly due
to reflection
MWNT (thick,
relatively small
number of walls),
2 wt.% in epoxy
1mm
Transmission
approx.10 %
mostly due to
reflection
CB, 2 wt.% in
epoxy
1 mm
EG, 2 wt.% in
epoxy
1mm
Carbon foam
2mm
Transmission
60 % mostly due
to reflection
Transmission
close to 0 due to
60% reflection
and 40%
absorption
Depending on
density
transmission can
be from 15 % to
0, caused by
reflection/
absorption
Depending on properties and application field, different
materials can be used
Nano-Carbon based
polymer composites
Carbon foam

Could be produced in any shape,
but finally rigid.

Fire resistant

Low cost

Manufacturability, processibility,

Thickness

Flexibility

Light (7-25 times lighter than
convetional polymers)
Ultra-thin carbonaceous
films (PyC, graphene,
etc)

Flexibility

Thickness

Weight

Corrosion resistance

Easy to produce (cheap CVD
technology)
Thank you for attention!