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Tuning defect density in single
layer graphene by ion
implantation technique
Lee Yan Long
A0111488E
National University of Singapore
Supervisor: Dr. Pattabiraman Santhana Raman
Co-Supervisor: A/Prof Jeroen Anton van Kan
A thesis submitted in partial fulfilment for the degree of
Bachelor of Science (Honours) in Physics
2017 April
Abstract
Current technologies in water desalination always face the issue of be-
ing hard to implement in less affluent countries due to the high cost
involving membrane replacement and power requirements for com-
mercial use. Single layer graphene membrane has been highlighted to
be a suitable substitute [1] for the current polymer membrane that is
lower in cost yet higher efficiency.
In this report, we attempt to adjust the defect density on single layer
graphene supported on silicon oxide by ion implantation in order to
investigate the defect density that the layer can support. We plan
to optimise different parameters such as energy and dosage of ions to
maximise the defect density. Such result can then be applied onto free
standing graphene on gold grids to help save cost. We then analyse
and characterise the sample after ion beam irradiation using Raman
spectroscopy.
They were later separated into their respective carbon amorphisation
regime and this is used to determine unwanted results (i.e. highly
amorphous carbon). The carbon regime is also used to calculate their
defect density to determine the optimised dosage and energy. Within
experimental limits, we observe that maximal defect density is about
10keV while remaining highly sp2 hybridised and that the upper limit
for the dosage is identified to be about 1016 ions/cm2. The trend of
the defect density was explained using stopping power in the linear
cascade and were checked using simulation from Kalypso. Overall
results align quite closely with theory but remain limited due to small
energy range used in this work.
Acknowledgements
Firstly, I would like to express my heartfelt thanks to my supervisors,
for the opportunity to work with one of the most interesting mate-
rial, graphene. It was mainly from Professor Jeroen and Dr Raman
oversight of the project that made it all work out in the end.
I would also like to express my greatest gratitude to Dr. Tanmoy Basu
who was with me throughout my entire lab work, clarifying my doubts
and being the role model researcher. The laboratory technicians Raj
and Mdm Ng Soo Ngo were also crucial in this project by assisting me
in clearing up the issues I have caused when using their equipment.
Of course, a call-out to all the other members of the CIBA lab; Huei
Min, Sarfraz and those who were not named for all their kind support
during this project. Thank you so much.
This project has also been made memorable with other Final Year
Project students that had to struggle through their own projects.
Andrew Seah, Zhang Danwei and Stella Ng whose presence had made
this journey more enjoyable than it would have been. I wish them all
the best for their projects.
Most importantly, I would like to thank my family members who were
with me through thick and thin throughout my entire student life.
Contents
List of Figures vi
List of Tables vii
1 Introduction 1
1.1 Motivations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1.1 Advantages of Single-Layer Graphene (SLG) . . . . . . . . 2
1.2 Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2 Fabrication methods 5
2.1 Methods of Fabrication . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2 Ion Beam Sputtering . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.3 Sample preparation . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3 Related Theories 11
3.1 Raman Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.1.1 Peak features . . . . . . . . . . . . . . . . . . . . . . . . . 13
3.2 Type of defects . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
3.2.1 Defect density calculation . . . . . . . . . . . . . . . . . . 18
4 Results & Analysis 20
4.1 Simulation method . . . . . . . . . . . . . . . . . . . . . . . . . . 20
4.2 Experimental result . . . . . . . . . . . . . . . . . . . . . . . . . . 23
4.3 Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
4.3.1 Ordering in graphene . . . . . . . . . . . . . . . . . . . . . 25
iv
CONTENTS
4.3.2 Defect density . . . . . . . . . . . . . . . . . . . . . . . . . 28
References 32
A List of Spectra peaks 35
A.1 Data table for all peaks of collected spectra . . . . . . . . . . . . 35
v
List of Figures
1.1 Schematic diagram of a conventional desalination membrane . . . 1
1.2 Diagram of water flow . . . . . . . . . . . . . . . . . . . . . . . . 2
1.3 Side-by-side comparison of Multilayer graphene and SLG . . . . . 3
1.4 Flowchart of the membrane control . . . . . . . . . . . . . . . . . 4
1.5 Image of expected final product . . . . . . . . . . . . . . . . . . . 4
2.1 Plasma irradiation of the graphene membrane . . . . . . . . . . . 5
2.2 ISTB in Centre of Ion Beam Application (CIBA) . . . . . . . . . 6
2.3 ISTB schematics (CIBA) . . . . . . . . . . . . . . . . . . . . . . . 7
2.4 ISTB interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.5 Film carrier method . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.6 TEM graphene holder . . . . . . . . . . . . . . . . . . . . . . . . 10
3.1 3 different type of scattering of photons in a material . . . . . . . 12
3.2 Classical illustration of Selection rules . . . . . . . . . . . . . . . 14
3.3 Band diagram of the transition for Raman peaks . . . . . . . . . . 15
3.4 Raman Spectrum of Graphene. . . . . . . . . . . . . . . . . . . . 16
3.5 Graphene defects . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
4.1 Kalypso simulation visualizer . . . . . . . . . . . . . . . . . . . . 20
4.2 Logarithm graph of the number of atoms to the atomic displace-
ment from origin in the z-direction . . . . . . . . . . . . . . . . . 21
4.3 Sputtering yield result . . . . . . . . . . . . . . . . . . . . . . . . 22
4.4 Typical stopping power during ion implantation . . . . . . . . . . 23
4.5 Stages or regime of graphite . . . . . . . . . . . . . . . . . . . . . 26
4.6 Images from AFM . . . . . . . . . . . . . . . . . . . . . . . . . . 28
vi
4.7 Defect density vs Energy . . . . . . . . . . . . . . . . . . . . . . . 29
4.8 Defect density vs Dosage . . . . . . . . . . . . . . . . . . . . . . . 29
List of Tables
4.1 Chart of D peak position, I(D)/I(G) ratio and 2D peak FWHM . 27
A.1 Table of intensity peaks for all the spectra . . . . . . . . . . . . . 35
A.2 Table of peak shifts for all the spectra . . . . . . . . . . . . . . . 36
vii
viii
1
Introduction
Most desalination filters are based on the idea of reverse osmosis based on polymer
films. It is made by having a large polyester base as a support and a polysulfone
layer as a substrate with a thin layer of polyamide layer that serves as the actual
filter.
Figure 1.1: Schematic diagram of a conventional desalination membrane
- The layer of polyamide grown through interfacial polymerisation (IP) [2]. Image
adapted from Sydney Water Corporation.
This membrane is then rolled into a tube where differential pressure is applied
from one end of the tube to the other. As the water passes through the membrane,
desalination occurs as salt were trapped and unable to permeate into the inner
columns. Water has to pass through several columns of such permeable membrane
into the inner column where the water is considered desalinated.
However, it has been discussed that a graphene based material will be better
suited for such an application instead.
1
1. INTRODUCTION
Figure 1.2: Diagram of water flow - The amount of portable water obtained
is shown in the stream of water at the tube centre. Salt is rejected away from the
core and remains stuck in the outer column. Image adapted from Sydney Water
Corporation.
1.1 Motivations
In a paper published in 2013, a Massachusetts Institute of Technology (MIT)
research team has completed their Density Functional Theory (DFT) calculation
and proved that it was possible for a graphene membrane to sustain average pores
of 7A ideal for desalination purpose [1].
1.1.1 Advantages of Single-Layer Graphene (SLG)
It is expected that the graphene membrane would improve conventional desalina-
tion processes since reverse osmosis requires high pressure which is not required
by the graphene filter [3]. Also, conventional methods requires high maintenance
since the membrane has to be changed regularly in every few months or even
weeks if desalination occurs in brackish water [2] due to the absorbance of salt
particle on the membrane.
There are also many other possible material such as a multilayer stacked
graphene oxide membrane which could also form nanochannels for filtration [4].
While such layers are easy to manufacture, this filtration technique is less
direct since the channels for water flow is from between the gap of each layer
that can potentially be equally hard to control. Furthermore, direct pores on
graphene is a much more efficient method since the layer is much thinner, leading
to a higher diffusion of water molecule.
2
1.2 Objectives
Figure 1.3: Side-by-side comparison of Multilayer graphene and SLG -
(a) shows the multilayer graphene while (b) shows SLG. The water molecules are
depicted by the red and white spheres were allowed to pass the membrane while
the rest of the larger ions such as Na+ (purple sphere) and Cl− (green sphere) are
impermeable. Image adapted from [4].
1.2 Objectives
As mentioned in the abstract, the key objective of this report is to mainly obtain
a maximised defect density of Single-Layer Graphene (SLG) on silicon substrate
and extend the result onto free standing graphene. The rationale is to minimize
cost since operating directing on the TEM grids for the free standing graphene can
be expensive and requires a high level of technical knowledge. After optimizing
the defect density, we will then proceed onto the TEM grid to evaluate the size
of the pores we create and fine-tune the pore through the use of other gas source.
While it is possible to use the alternative Ar+2 ions, the wien filter which is simply
a velocity selectoer did not have sufficient E/B field strength to filter through the
larger projectile.
Figure 1.4 shows a flowchart of the project.
Upon collecting the base data which optimised the defect density, we would
be characterising the type of defects under Transmission Electron Microscope
(TEM) that is observed from the ion beam irradiation and investigate the average
pore sizes across the graphene. Do note that Transmission Electron Microscope
is best performed under low voltages of 60kV to avoid reconstruction of the
3
1. INTRODUCTION
Figure 1.4: Flowchart of the membrane control - This is the rough sequence
of the project. First, we start trying to obtain a baseline comparison for the defect
density formed on the supported graphene before moving to further stages.
graphene surface as will be mentioned later in section 3.2. However, this will not
be explored in this phase of the project due to time constraint.
The final step before passing over the project to the engineers to improve
and implement the membrane is to achieve the ideal pore size and test the ion
transport across the membrane.
Figure 1.5: Image of expected final product - The graphene layer is grafted
onto a polyester tube. Pressure will be exerted from the top and water will flow
into the tube of diameter 2R and filtered through pores of diameter 2a where the
ideal size of 2a is about 7A. Choice of polyester will be based on compatible surface
free energy with graphene. Image adapted from [1].
4
2
Fabrication methods
2.1 Methods of Fabrication
There were many potential methods meant for formation of such defects. Oxygen
plasma and beam irradiation are highlighted to be best methods.
For plasma, the key idea is to form a plasma above the sample and use the
oxygen to oxidise the carbon and liberate it from the sample.
Figure 2.1: Plasma irradiation of the graphene membrane - The above
schematics shows how the experiment was carried out. The blue material illustrates
SiN supported on Si. A circular hole of 2micron diameter is drilled using Focused
Ion Beam drilling before the graphene layer is grown on top of it. Image adapted
from [5].
While some samples were found to be water permeable with sufficiently ideal
pore sizes, the pores were found to be distributed fairly randomly and large
5
2. FABRICATION METHODS
spread of pore sizes was observed [5]. Furthermore, success rates was also low
as water permeability was only observed in 20% amongst 140 samples [5]. Due
to the irregularities in pore formation through such methods, there is a need of
exploring more concrete methods of pore formation.
Electron beam irradiation have also been tested in some research paper but
the observed pore size was observed to be too small (∼3A) [6]. It was believed
that the low mass of the electron was the cause.
Thus, for a controlled means of pore formation, the most direct method would
be ion sputtering. We would be using the Ion Source Test Bench (ISTB) which
operates in the high vacuum (∼10−6 mbar). This ion beam machine allows an
indirect control over the dosage via monitoring the current and has multiple
adjustable beam size set by the aperture within. The source of the beam is
generated through Radio Frequency (RF) voltages which requires lower pressure
to sustain a plasma.
Figure 2.2: ISTB in Centre of Ion Beam Application (CIBA) - Equipment
and setup was designed and assembled by CIBA.
6
2.2 Ion Beam Sputtering
2.2 Ion Beam Sputtering
To ensure proper alignment of the beam spot, we calibrate the beam spot for each
energy and aperture position on a sliver plate before performing the sputtering
on our sample. The energies of the Ar+ ions are filter by a wien filter (velocity
selector). Only ions of selected charge and velocity, controlled by the potential,
are allowed to pass into the aperture undeflected.
The beam then passes through 2 aperture that can be manipulated to align
and focus the beam. A copper foil is attached to the last aperture and blocks the
stray beams of argon from reaching the sample.
Figure 2.3: ISTB schematics (CIBA) - The beam focus is monitored by the
probe and extraction potential. The ion is then accelerated through the potential
column until the desired energy. The first aperture aligns the beam while the wien
filter deflect ions of different energy. The second aperture helps control the beam
size. Red line indicates the direction of beam travel.
We can also monitor the dosage via the use of the current. Dosage here refers
to total dosage per unit area (cm2) henceforth. Once the Ar+ ions collide into
7
2. FABRICATION METHODS
the graphene sample, it will transfer this charge onto it. Since the sample is
relatively conductive as the only non-conductive part of the sample is the thin
layer of silicon oxide layer, current will be able to flow from the charge integrator
to neutralize the charged sample. This current is monitored and calculated to
serve as our indirect measurement of the dosage. Sometimes, secondary electrons
could also be emitted from the sample during sputtering and thus a voltage of
∼+90V is applied on the sample to suppress the emission.
Figure 2.4: ISTB interface - This is the interface used to control all the pa-
rameters of the beam. The interface was designed by CIBA members. The photo
shows the parameters used for optimising the beam (maximising the current) for
15keV.
Energies of 5keV, 10keV, 15keV Ar+ ion beam are selected to obtain a rough
trend of the defect density. The choice of the energies is arbitrary and based on
the energy range of the ISTB. We expected that at high energies, we could get
higher defect density with the same dosage. The beam are all aligned to sputter
at the centre of the graphene where defects were observed to be minimal during
the transfer of graphene onto the substrate [7].
8
2.3 Sample preparation
2.3 Sample preparation
Although graphene samples are grown in the C2D centre, it has to be discussed
here since the graphene transfer method could potentially induce defect before
irradiation.
The graphene sample is grown via chemical vapor deposition (CVD) on typ-
ically copper substrate. This is chosen for its low solubility with carbon that
prevents diffusion of the carbon atom into the substrate. The substrate is pre-
treated with argon and hydrogen gas and later annealed. This ensures common
orientation that maximises homogeneous and defect-free growth. The thickness of
the substrate was also noted to be 25micron where layer-by-layer growth occurs.
In CVD growth of graphene, we used methane gas as precursor since it can
decompose at the lower temperatures (∼ 1200K) even without catalysis. The
product of hydrogen gas after adsorption will be carried out by the gas flow. The
growth occurs for 30minutes at about 1000K (copper serves as the catalysis as
well).
Figure 2.5: Film carrier method - Crystal size was found to have millimeter
single crystal domain in this method. However, quality is still inferior to the scotch-
tape method. Image adapted from [8].
Here, we use the carrier film method to transfer the film over from onto
silicon. We spin-coat poly(methyl methacrylate) (PMMA) onto the sample and
9
2. FABRICATION METHODS
later dissolve them into solution of iron chloride (FeCl3) to remove the copper
substrate. The graphene layer held by the PMMA is then further treated by
acetone to remove any copper residue and soft-baked onto the silicon substrate.
We then remove the PMMA with toluene. This method is the easiest by far
but the crucial downside to this method is that the PMMA residue cannot be
entirely removed, thus induces a lot of contamination. Nevertheless, we attempt
to conduct our spectroscopy in regions of low contamination as seen under optical
microscopy (see Figure 4.6). In addition, we do note that the Full Width Half
Maximum (FWHM) for 2D is quite close to theoretical value from the AFM,
indicating little defects during transport.
For the latter portion of the experiment, we had to analyse the size of the
pore through TEM and thus, the free-standing sample was prepared and placed
on gold grids instead for characterization.
Figure 2.6: TEM graphene holder - (a) shows the sample used in this work.
The faint contrast shows the graphene layer on the blue silicon substrate. The size
of the graphene is about 5mm by 5mm. Choice of substrate (Si with Thin layer
of SiO2) is primarily to enhance Interference Raman Scattering effect. (b) shows
the sample for TEM characterisation for future work. The TEM grid that holds
the graphene sample is at row 10. The holder is sealed and vacuumed to reduce
contamination. The grid sizes are 1micron placed 2micron apart while the entire
grid has a diameter of 3mm and is 250micron thick. Gold is chosen due to its inert
properties yet high conduction to suppress noise signal.
10
3
Related Theories
The primary aim of the project involves identifying the defect density from the
sample. Thus the most appropriate method would be Raman spectroscopy. This
method is non-destructive and would not affect the sample too much especially
at low power of 3mW for short durations (scan time: ∼300seconds) [9]. 514nm
wavelength was used instead of the standard 532nm for better resolution of the
peak [10].
In graphene, there are several features in the spectrum that could be observed.
To understand these spectra, we would need to first understand the mechanics of
Raman spectroscopy.
3.1 Raman Spectroscopy
Raman spectroscopy operates on the key principles of excitation of the molecules.
As the photon from the laser strikes the sample, it can scatter inelastically with
the various modes of the crystal array, resulting in higher or lower wavelength
based on the transition. For instance, when it interacts with the ground state, it
could excite it to a virtual state before de-exciting to a vibrational or rotational
state. This is known as the Stokes scatter and is the most commonly observed
transition since most of the atoms are in the ground state during characteriza-
tion. Fundamentally, only vibrational and rotational states that causes the dipole
moment to change will be pronounced in the spectrum. Thus, not all possible
11
3. RELATED THEORIES
states can be seen through this spectroscopy. Raman sensitive states are deter-
mined by Group Theory. (E2g and A1g seen in literature refers to the group of
the vibrational states.)
Figure 3.1: 3 different type of scattering of photons in a material -
Rayleigh scattering (in green) is the elastic scattering of the photon whereas Stokes
and Anti-Stokes scattering are inelastic. Only virtual states that correspond to
rotational and vibrational mode will be observed in the Raman spectrum. Image
adapted from wikipedia.
As such, the shift in the wavelength of the outgoing laser will correspond to
the energy between the energy level transition. However, the mechanism can be
much more complex. Ideally, in a perfect crystal, these transitions have to obey
selection rules based on the conservation of the energy and crystal momentum
with the crystal phonons of the optical branch shown as such:
k1 = k2 + q (3.1)
k1 = k2 + q1 + q2 (3.2)
Equation 3.1 and 3.2 show the selection rule for one phonon and two phonon
processes respectively where q refers to the phonon contribution. Note that there
are more than one two-phonon processes than can be allowed. For the one phonon
12
3.1 Raman Spectroscopy
process, the recombination must occur k1=k2 for the process to be Raman sensi-
tive, hence q is necessarily zero. However in the presence of a defect state, this is
no longer obeyed (q 6= 0).
3.1.1 Peak features
In short, the Raman spectra give the various energies of the vibrational and
rotational mode within the sample. Thus, if defects are introduced, these modes
will be disturbed and we can observe different modes.
When a laser acts on the sample, the photon from the laser excites and create
and electron-hole pair that emits phonons before recombining into a photon of
lower energy. This recombination must also occur at the initial position [11].
One of the common peak is known as the G peak. It reflects the stretching
mode of the C-C atom in the graphene and is activated by electrons near the
zone centre. For a perfect crystal, since there are no dispersion by the phonons
(q=0) at the zone centre due to the conservation of crystal momentum, this peak
is always fixed at 1580cm−1, independent of excitation wavelength. The width
and peak position of this peak is sensitive to the strain and deformation of the
zone centre and can be observed via the peak shift in G band [12].
The next two frequently observed peaks are known as D and 2D peaks. This
two peaks arise due to the breathing mode of the carbon rings. In the breathing
mode, carbon atoms in the hexagonal ring expands outwards and then inwards
periodically [13].
Graphene has a dirac cone structure when following the phonon dispersion
along K point to the K′ point. Likewise, the photon can produce an electron-hole
pair. If this electron emit a phonon along K to K′ point, another phonon will
be emitted to back-scatter that electron from K′ to K. Suppose the graphene is
defect-free, the total conservation of crystal momentum will have ensured that
the net momentum of the phonon must be zero (q=0) this is only allowed if the
phonon are of opposite wavevector. The shift in the wavenumber would then be
based on the initial amount of energy the electron has which is proportional to
the wavelength of the laser. This shift is about (∼2700cm−1) when using 514nm
green laser.
13
3. RELATED THEORIES
Figure 3.2: Classical illustration of Selection rules - The green line shows
the phonon scattered while the solid black arrow shows the electron path. The
lightning and explosion symbol represents electron-hole production and recombi-
nation respectively. All of these are two phonon processes (since we have 2 green
lines for each image). (i) demonstrated a process that happens when q1 + q2 6= 0
in a perfect crystal (the sum of the 2 phonon wavevector do not cancel). While
momentum is conserved, the electron hole pair cannot recombine to form the pho-
ton to emit for a wavelength shift to be detected (i.e. incoming laser 514nm, no
outgoing beam [recall: Raman shift needs a change in wavelength so you need both
an incoming and outgoing photons to detect the change]). (ii, iii and iv) demon-
strates the allowed transition. For (iv), the black spot illustrates a defect that can
scatter the electron or hole resulting in an allowed recomination even though net
momentum is not zero. Image adapted from [12].
14
3.1 Raman Spectroscopy
If we now consider a defect, the crystal structure is destroyed and crystal
momentum need not be conserved. This meant that the electron can scatter
a phonon that can be back scattered by a defect state (or vice versa). Since
the phonon momentum is not longer strictly observed as in Equation 3.1 and
3.2, more transitions are allowed [14]. These defect states manifest itself as the
D peak and with increasing defects, it further relaxes momentum conservation,
leading to a wider D peak.
However, note that not all the defect type will be reflected in the intensity
or width of the D peak but also the presence of another peak known as D’ peak
(∼1370cm−1) or additional shifts in the G peak (if it causes strain).
Figure 3.3: Band diagram of the transition for Raman peaks - Green and
red line illustrates absorption and emission of photons respectively while the solid
line illustrate the scattering by the electron. The electron transition must also
obey conservation of crystal momentum as seen from the G and 2D band where
the total phonon momentum is zero. While the energy transition can vary, only
those transition that correspond to an actual vibrational state will resonate. Thus,
only at some energy value, we see a Raman peak. However, this conservation is
not observed in the D band due to the defect state. Image adapted from Institute
of Optics, University of Rochester.
Core details are as follow: 2D peak will always be observed in both highly
symmetrical graphene or low defect graphene while the D peak will only be present
in defect graphene.
15
3. RELATED THEORIES
Figure 3.4: Raman Spectrum of Graphene. - The key features reflecting
the mechanism mentioned is shown here. D’ peak is the intravalley case of D peak
where the transition is only about K or K’ instead of K to K’ or vice versa. D’ peak
is also sensitive to several type defect that may not be obtained from the intensity
of the D peak such as the zigzag edge. Image adapted from William Marsh Rice
University.
3.2 Type of defects
Since the order of the pore size we are looking for is in the order of nm, the
defects are mostly localised thus in the this section, the primary focus would be
on point defects rather than 1D defects.
Graphene can easily reconstruct into non-hexangonal rings even when there
are no point defects. The most commonly known would be the Stone-Wales defect
where the rotation of one the C-C bond would cause 2 pentagons and 2 heptagons.
This defect is generally negligible at room temperature but it can be very stable
after ion beam once formed under ion beam irradiation [15]. Nevertheless, since
there are no dangling bonds or vacancies formed from such defect, it is not the
defect we are primarily concerned with since the end goal involves pore formation.
Specific to our case, for singly charged ions, most of the defects are expected
to be single or double vacancies [16]. For single vacancies, one atom is removed
from the lattice and the graphene is reconstructed to form a 5 membered ring
and a 9 membered ring. This is known as the V1(5-9) defect.
Even though the formation energy is high (∼7.5eV), the migration energy is
very low (∼1.5eV) and thus will be an important factor in the redistribution in
16
3.2 Type of defects
the pores formed. When 2 of such defect combines through migration, it can
lead to a double vacancy site where the graphene will reconstruct again to form 2
pentagons and 1 octagon [V2(5-8-5 defect)]. This defect has a formation energy
of 4eV per atom which meant this double vacancy is more favored than the single
vacancy. By rotating the bonds in the octagon, it can be transformed to the
V2(555-777) defect of 3 pentagons and 3 heptagon.
Figure 3.5: Graphene defects - Structures of reconstructed graphene (a)
V2(5-8-5) (b) V2(555-777) (c) V2(5555-6-7777) This defect is formed via ro-
tating a bond from a heptagon in V2(555-777) (d) V1(5-9). Image Adapted from
[17].
When many atoms are removed at once, such as during sputtering, the re-
moval of surface will also led to pores of dangling unsaturated bonds. However,
investigation of the type of defects would not be possible without the use of TEM
and hence this portion is mainly a build-up for the future progression of the
project.
Nevertheless, such information also meant that at visible light range (∼ 2eV)
and at low power (∼ 2mW), defects are only able to migrate at best. This has
been supported by other papers [15][17].
17
3. RELATED THEORIES
3.2.1 Defect density calculation
Since each of the modes can be reflected on the spectrum due to resonance from
the highly symmetrical band structure, the general consensus have been to use
those peak intensities to evaluate the corresponding defect density.
As the sample becomes increasingly amorphous, a D peak appears with in-
creasing I(D)/I(G) ratio where I(X) refer to the intensity of the X peak. D′ peak
slowly appears as the peaks starts to broaden due to less stringent constraint on
the selection rule due to increasing defects [18]. At low defect regime, we can
interpret I(D) as the average defect probed by the laser over the area of the laser
while I(G) is proportional to the area itself.
I(D)
I(G)∝ (Ll)
2/(Ld)2
(Ll)2=C(λ)
Ld
2
(3.3)
Ld refers to the average length between defects and Ll refers to the length
covered by the laser while C(λ) is some constant related to the excitation energy.
This equation is known as the Tuinstra Koenig relation [19].
At higher disorder to the state of amorphous carbon, I(D) will start to drop
as the amount of sp2 hexagonal rings began to be distorted while I(G) stays the
same since the relative motion of the sp2 atoms remains mostly unchanged [18].
I(D)
I(G)= C ′(λ) ∗ L2
d (3.4)
The values of the constant has been computationally determined [20] and
fitted to many different paper [21][12][1][19]. We did not managed to verify such
proportionality ratio due to the lack of excitation energies we could use since
we have only characterised our sample under 514nm. After substituting the
excitation wavelength into (3.3) and (3.4), it reduces the former equation into
this:
For low defect regime (Ld > 10nm)
L2d(nm
2) = 126× [I(D)
I(G)]−1 (3.5)
n2D(cm−2) = 2.5 ∗ 1011 × [
I(D)
I(G)] (3.6)
18
3.2 Type of defects
For high defect regime (2.5nm < Ld < 10nm)
L2d(nm
2) = 183× I(D)
I(G)(3.7)
n2D(cm−2) = 1.7 ∗ 1013 × [
I(D)
I(G)]−1 (3.8)
nD refer to the defect density which is related to the length between defect
(Ld). While the actual equation is much harder to derive, this simpler model
fitting should do well to fit the regime. However, even the high defect regime will
no hold (Ld ∼ 2nm) [14].
19
4
Results & Analysis
4.1 Simulation method
In order to attain some expectation of defects formed expected from the various
parameter, we used Kalypso [22] to simulate the sputtering on the sample based
on the energies and species of the ion. Kalypso is a molecular dynamics (MD)
simulation that operates on the leapfrog algorithm for non-equilibrium conditions
such as sputtering and collision calculation in the classical framework.
Figure 4.1: Kalypso simulation visualizer - The diagram shows the time
evolution in one of the runs at 15keV. The timestamp demonstrates the sputtering
from t=0.65fs onwards. The sample was viewed from the sides for the top two
image while the last image was from the top view.
20
4.1 Simulation method
Kalypso is used to calculate the sputtering yield we expected to obtain for the
different energies that we are sampling. Using the coulomb potential obtained
from Li [23], we explore the sputtering yield of a singly charge argon atom on
a free standing graphene. One key assumption in doing so is that the Ar+ ions
interact independently between one another. Yet it saves computational time.
The target is a graphene monolayer of 1×105 atoms at 300K. 1000 trials were
recorded.
All the potential used are empirical potential (i.e. potential based on ex-
perimental fitting). The repulsive potential between the C-Ar, C-C interactions
follows the Ziegler-Biersack-Littmark (ZBL) while that of the attractive potential
follows the Brenner potential [23].
When using the program we also varied the timestep based on each parameter
used in order to allow the layer to achieve local energy minimization. Total time
for each simulation was also obtained by based on the timestep. The sputtering
yield was obtained by analyzing the final position of all the atom cumulatively and
the average value of the 1000 trials for each energy assigned is used to represent
the yield.
Figure 4.2: Logarithm graph of the number of atoms to the atomic
displacement from origin in the z-direction - This is the final record position
of all atoms across all run for 15keV. As seen, most of the atoms remains within
4.5A which is used as an indicator. Any atoms beyond 4.5A will be deemed to
have been sputtered and liberated. There is an increasing trend in number of atom
between 1.5A and 4.5A due to the dislocation perpendicular to the surface caused
by the target.
21
4. RESULTS & ANALYSIS
From this simulation, we can observe how the yield varies at different energy
per atom basis.
Figure 4.3: Sputtering yield result - Plot of the number of sputtered atoms
per graphene atom at the various energies.
In Figure 4.2, we obtain a trend of the atomic position of all the atoms after
each run. However, the atoms would be displaced from the z-axis and only at a
sufficient far distance from the plane would the atom be considered removed or
fully dislodged. The final yield over all the energies are then collated and plotted
in Figure 4.3.
At low energies, nuclear stopping is much more dominant and that causes the
most amount of damage in graphene. However, that starts to drop as electronic
stopping starts to dominate. This is embedded within the ZBL potential [24].
For our simulation, this peaks at 5keV where the damage is maximised. While
stopping power of Ar+ in such setup were not found in known studies, Stopping
Ranges of Ion in Matter (SRIM) program [10] demonstrates that the stopping
power at lower energies for Ar+ projectile on amorphous C is mostly nuclear in
nature (Se/Sn ∼0.2).
22
4.2 Experimental result
Figure 4.4: Typical stopping power during ion implantation -
This figure demonstrates how the stopping power for a heavy element be-
haves empirically. Since the speed of our ions are slow, nuclear stop-
ping is more dominant then electronic stopping. Image adapted from:
http://www.jhaj.net/jasjeet/tcad/Learn3/l3d.htm.
4.2 Experimental result
23
4. RESULTS & ANALYSIS
24
4.3 Analysis
The above spectra are results that were obtained with cooperation from In-
stitute of Material Research and Engineering (IMRE) and sorted based on the
different energy of the ion.
They show the general trend of increasing disorder in the graphene across
the different energies. It can be seen across the peaks that the disorder start to
increase at increased dosages, the D peak appears and D′ peak appears later as all
the peaks broadens. The peaks broaden due to the increased in allowed modes of
phonon as previously discussed. This continues as the G and D’ peak converges
since they became so wide that join as a combined peak at about 1600cm−1 [12].
4.3 Analysis
4.3.1 Ordering in graphene
In carbon related materials, A.C. Ferrari and J. Robertson has classified the
stages of the material into 3 stages as shown in Figure 4.5.
We also need to check that the sample remains largely monolayer even after
sputtering for application purposes. As mentioned, the 2D peak Full Width Half
Maximum (FWHM) can be used to monitor the thickness of the carbon layer
since increasing the thickness allows more modes, resulting in a wider, shorter
peak of higher frequency. Furthermore, width of any peaks are also defect induced
so it can signal defect states as well. Since most of the peaks for 2D are single
peaks, using the FWHM would be sufficient as an indication of layer number.
Alternative method would be to use the actual intensity of G peak [21]. However,
the 2D peak is much more useful for this case since could also be used to deduce
the level of hybridisation since the peak arises mainly due to the breathing mode
(indicating hexagonal-arranged atoms).
Literature record [11] reveals monolayers to have FWHM of 30cm −1 while
double layers to be about 50cm−1. Despite having a larger FWHM as compared
to literature value, Atomic Force Microscopy (AFM) analysis has shown that it
is largely monolayer even at 38cm−1. The step edge is found to be about 0.84nm
[26] which is about the step edge of a graphene monolayer. Figure 4.6 shows
supporting images.
25
4. RESULTS & ANALYSIS
Figure 4.5: Stages or regime of graphite - The notation used: NC, a and
ta refers to nanocrystalline, amorphous and tetrahedral amorphous. This classi-
fication has not only been used in graphite but also in graphene [14]. Stage 1 is
also considered the low defect regime while Stage 2 is the high defect regime for
Equation 3.6 and 3.8 respectively. Image adapted from Ferrari and Robertson.
26
4.3 Analysis
Dose Energy G peak position I(D)/I(G) 2D FWHM
ions/cm2 (keV) (cm−1) (cm−1)
0 0 1586.8 0 38.1
1E13 5 1596.2 0.07 39.6
1E14 5 1580.6 0.40 47.7
1E15 5 1589.9 0.68 not observed
1E13 10 1593.1 0.19 36.2
1E14 10 1586.8 1.17 43.4
1E15 10 1589.9 2.86 60.7
1E16 10 1577.5 0.95 983.5
1E13 15 1577.5 0.12 not observed
1E14 15 1586.1 0.51 36.6
5E14 15 1583.7 0.85 35.3
1E15 15 1596.2 0.72 39.9
Table 4.1: Chart of D peak position, I(D)/I(G) ratio and 2D peak
FWHM) - The values are obtained from curve fitting using the Origin program.
At 10keV and 1014 dose, there is an anomaly where I(D)/I(G) exceed 2. This
anomaly has also been observed in other works [25]. The large FWHM at 10keV
of 1016 dosage indicates many spots having 2 or more layers graphene are stacked
from sputtering. To fully investigate it, we would need to deconvolve the peak.
27
4. RESULTS & ANALYSIS
Figure 4.6: Images from AFM - (a) shows an image from the optical micro-
scope where the blue contrast on the left is the graphene layer and silicon oxide on
the right. Note that there are few contaminant (black specks). This is the graphene
sample before ion beam irradiation. (b) is the phase image for the material. The
contrast shows the differing material. (c) shows the height profile of the silicon
dioxide and graphene layer. These are averaged and used to calculate layer depth.
The weak colour contrast shows that the height difference is minimal, indicating
the thin graphene layer does not have much height difference.
Almost all the above result lies within stage 1 and stage 2 amorphisation
which meant very low sp3 hybridisation involved. Only at 1016 dosage and 10keV
demonstrates the likelihood of that sample to be at stage 3 amorphisation based
on the exceptionally broad FWHM and along with combined result from the G
peak position and I(G)/I(D). Thus it can be mostly concluded that any dosage
on the order of 1016 ions/cm2 should not be applied on the sample since it would
be mostly sp3 hybridised.
4.3.2 Defect density
Using equation 3.6 and equation 3.8, the defect density are calculated and plotted
on a graph.
Dosage has been demonstrated to have increased the amount of defect density
until 1016 dosage until dosage above has resulted in unwanted amorphisation as
shown in Table 4.1. The peak in energy was expected to be about 5keV but
actual data reveals a higher energy peak of the defect density. This in fact was
also observed in other experiments [27].
28
4.3 Analysis
Figure 4.7: Defect density vs Energy - Data were collected from different
spectrum and collated into the graph. The peak defect density for the lower energies
seem to be maximal at ∼10keV
Figure 4.8: Defect density vs Dosage - The general trend shows the increasing
dosage increases the defect density
29
4. RESULTS & ANALYSIS
Some papers [28] has advised that this deviation could be due to the effects of
a substrate as presence of the substrate could backscatter the projectile, further
allowing interaction or damage with the graphene sample. At higher energy,
the substrate atoms can even be liberated and damage the graphene further.
However, displacement energy for graphene carbon about is quite high (23.6eV)
[29], making this seems less unlikely. Further testing is required see if we can
reproduce this deviation.
From Figure 4.8, we noted that there is a saturation in the dosage where
defect density at lower energies of 5keV and 10keV indicates signs of saturation.
Nevertheless, more data should be obtained before anything conclusive can be
mentioned.
We also observed an outlier in the 1015 dose of 5keV. This is mainly attributed
to the different stage of graphene amorphisation the sample is in. Calculation for
this point performed was at the higher regime as compared to the rest which were
at the lower regime. This meant that the transition will result in very different
defect profile especially at higher energies. However, this do not actually show
that higher energy will increase the density of defects since we are comparing dif-
ferent regimes. We were also reminded that higher energies of the same dosage do
not necessarily conform to a higher carbon amorphisation regime as experiments
in the MeV range has demonstrated stage 1 amorphisation [28]. Nevertheless,
the energy range between the 15keV to the 1MeV range should be investigated
as defect density in stage 2 ought to yield higher defect density.
Within the lower defect regime and stage 1 amorphisation, ∼ 10keV would be
the optimised energy but for stage 2 amorphisation, this data alone is insufficient
to tell.
Overall, one deduction based on our observation is that the overall dose should
not exceed 1016 or stage 3 amorphisation would occur. Within the experimental
limits, 10keV is the optimised beam energy for stage 1 amorphisation but more
experiments is needed to confirm this. Higher energies also have to be investigated
to further maximise defect density at possibly higher (stage 2) defect regime.
30
4.4 Further works/improvement
Currently, the pore sizes are being determined through TEM samples using sim-
ilar parameter at 15keV ion beam since the focus for the beam was better com-
pared to 10keV.
Similar to the suggestion of Prof. Jeroen Anton van Kan, alternative use of
cluster instead of singly charged ions could also be used to stimulate larger pore
size since the cluster would be more likely to create larger pores although it will
be much harder to control and manipulate due to the lower stability of a cluster.
One future work that has to be done is to test for the ion transport across
the graphene pores. This can be done by transferring the graphene layer onto
a membrane support and monitor the potential of the membrane to test for ion
transportation and selectivity of the membrane towards NaCl.
5
Conclusion
In this work, I have managed to learn how to operate a sputtering machine and
became aware of how a general ion beam sputtering machine work even though
some of these learning experiences are derived from damaging the equipment and
samples. By manipulating the beam properties, I was able to induce different
densities of defects on supported graphene and characterize the various sample
using techiques such as Raman spectroscopy and AFM. The defect density was
then calculated and found to be optimized at around 10keV. To further verify
the result, simulations on Kalypso was done and result is found be to be close to
experiments.
31
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34
Appendix A
List of Spectra peaks
A.1 Data table for all peaks of collected spectra
Table A.1: Table of intensity peaks for all the spectra
35
A. LIST OF SPECTRA PEAKS
Table A.2: Table of peak shifts for all the spectra
36