The first order Raman spectrum of isotope
labelled nitrogen-doped reduced graphene oxide
Tobias Dahlberg
Tobias Dahlberg
February 19, 2016
i
Abstract
The topic of this thesis is the study of nitrogen functionalities in nitrogen-doped
reduced graphene oxide using Raman spectroscopy. Specifically, the project set
out to investigate if the Raman active nitrogen-related vibrational modes of
graphene can be identified via isotope labelling. Previous studies have used
Raman spectroscopy to characterise nitrogen doped graphene, but none has
employed the method of isotope labelling to do so. The study was conducted by
producing undoped, nitrogen-doped and nitrogen-15-doped reduced graphene
oxide and comparing the differences in the first-order Raman spectrum of the
samples. Results of this study are inconclusive. However, some indications
linking the I band, a band previously speculated to be nitrogen or sp3 carbon
related, to nitrogen functionalities are found. Also, a hypothetical Raman band
denoted I* possibly related to sp3 hybridised carbon is introduced in the same
spectral area as I. This indication of a separation of the I band into two bands,
each dependent on one of these factors could bring clarity to this poorly understood spectral area. As the results of this study are highly speculative, further
research is needed to confirm them and the work presented here serves as a
preliminary investigation.
Sammanfattning
Ämnet som behandlas i denna avhandling är studien av kvävefunktionaliteter i
kvävedopat reducerat grafenoxid med hjälp av Ramanspektroskopi. Specifikt så
var målet med projektet att undersöka om Ramanaktiva kväverelaterade vibrationsmoder i grafen kunde identifieras via isotopmärkning. I tidigare studier
har Ramanspektroskopi använts för att karakterisera kvävedopat grafen, men
isotopmärkning har aldrig tllämpats för detta ändamål. Studien genomfördes
genom framställning av odopat, kväve-dopat och kväve-15-dopat reducerat grafenoxid vilkas Ramanspektra sedan jämfördes. Resultaten av denna studie kan
inte ses som avgörande då då datamängden var för liten. Men vissa indikationer som förbinder I-bandet, ett band som tidigare spekluerats som relaterat till
kväve eller sp3 hybridiserat kol, till kvävefunktionaliteter hittades. Dessutom
introducerades ett hypotetisk Raman band, I*, i samma spektrala region som
I. Detta band anses möjligen vara relaterade till sp3 hybridiserat kol. Denna
indikation av en separation av I bandet till två band, vardera beroende av en
av dessa faktorer kan bringa klarhet till detta tidigare dåligt förstådda spektrala område. Eftersom resultaten av denna studie är högst spekulativa, behövs
det ytterligare studier för att bekräfta dem och det arbete som presenteras här
fungerar enbart som en förstudie for eventuellt fortsatt arbete.
Tobias Dahlberg
February 19, 2016
ii
Contents
1 Introduction
3
2 Theory
2.1 Graphene . . . . . . . . . . . . . . . . . . . .
2.1.1 Electronic Properties . . . . . . . . . .
2.1.2 Vibrational Properties . . . . . . . . .
2.1.3 Nitrogen Doping . . . . . . . . . . . .
2.1.4 Reduced Graphene Oxide . . . . . . .
2.2 Characterization Methods . . . . . . . . . . .
2.2.1 XPS . . . . . . . . . . . . . . . . . . .
2.2.2 Raman Spectroscopy . . . . . . . . . .
2.2.2.1 Raman Spectra of Graphene
2.2.2.2 Isotope Labelling . . . . . . .
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
5
5
6
6
8
9
10
10
12
13
15
3 Experimental
3.1 Synthesis . . . . . . . . . . . . . . . . . . . . .
3.1.1 Synthesis of GO . . . . . . . . . . . . .
3.1.2 Synthesis of N-rGO, N15-rGO and rGO
3.2 Characterization . . . . . . . . . . . . . . . . .
3.2.1 XPS . . . . . . . . . . . . . . . . . . . .
3.2.2 Raman Spectroscopy . . . . . . . . . . .
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
17
17
17
17
18
18
18
4 Results and Discussion
4.1 XPS . . . . . . . . .
4.2 Raman Analysis . .
4.2.1 D-Band . . .
4.2.2 I-Band . . . .
4.2.3 I*-Band . . .
4.2.4 D”-Band . .
4.2.5 G-Band . . .
4.2.6 D’-Band . . .
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
19
19
20
20
22
24
26
28
29
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
5 Summary and Outlook
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
31
1
Tobias Dahlberg
February 19, 2016
2
Chapter 1
Introduction
One of the most talked about and hyped groups of materials of the last decade
are the sp2 nanocarbons which contains well-known names such as graphene,
carbon nanotubes, fullerenes and graphite. The arguably most famous member of this group is graphene, which is a one-dimensional sheet comprised of
carbon atoms arranged in hexagonal rings. Graphene, which was first isolated
in 2004 by Novoselov et al. [21], is the youngest member of the sp2 nanocarbon family. Although being the youngest member of the family graphene is
sometimes referred to as the mother of all sp2 nanocarbons. The reason behind this moniker is that all other members of this family can be seen as being
constructed using graphene as a basic building block. For example, carbon nanotubes, being nanometer scale (diameter wise) tubes of hexagonal carbon rings,
can simply be seen as rolled up sheets of graphene. During the last decade, the
research community has seen a figurative explosion of graphene-related articles
being published [14]. What has drawn researchers to this area are the extraordinary properties of this material. For example, graphene exhibits both excellent
chemical, mechanical and electrical properties, making it attractive for many
different potential applications including; electronics [13], hydrogen storage [16]
and fuel cells [17]. However, for graphene to be properly useful, its properties
need to be carefully tuned to the particular application. One way of modifying both the chemical and electronic qualities of graphene is through doping,
the intentional inclusion of impurities. Being next to carbon in the periodic
table nitrogen is one of the most suitable dopant atoms for carbon structures
due to their similarities in size and electronic characteristics [18]. The resulting
properties of nitrogen-doped carbon depend on the exact nature and amount of
nitrogen inclusions present. As of yet, the only precise method of analysing the
structure of the nitrogen functionalities present in doped sp2 nanocarbon has
been through the use of X-Ray Photoelectron Spectroscopy (XPS) which is a
powerful but difficult and time-consuming method [19].
In this thesis, we try to expand this tool-kit also to include Raman spectroscopy. Raman spectroscopy is a fast, non-destructive and easy to use method,
and it is often used to characterise nanocarbons as they have a strong Raman
3
Tobias Dahlberg
February 19, 2016
activity [15]. Because of this, the Raman spectra of graphene has been intensely researched and studies have shown it to hold a great deal of structural
information [29, 30, 31, 32]. One method of singling out the effects of certain functionalities on the Raman spectra is through isotope labelling. In this
method instead of using, for example, the common earth abundant nitrogen-14,
the heavier nitrogen-15 is used. As Raman spectroscopy probes the vibrational
properties of a material, this mass change modifies the vibrations related to that
element and they can thus, easily be identified and characterised. Isotope labelling and Raman spectroscopy have been used before to study the vibrational
modes of sp2 carbon [33, 34, 12]. The results of one of these studies were that
the Raman modes related to the isotope labelled element were down-shifted in
frequency. The frequency down-shift was found to follow the same behaviour as
a classical harmonic oscillator [12]. This study observed these results in carbon13 labelled carbon nanotubes and similar effects should be observed while using
the heavier nitrogen-15. This method has not been employed before to investigate the nitrogen inclusion in sp2 carbon structures. We expect to be able
to distinguish the nitrogen related Raman vibrations in a more straightforward
manner by employing isotope-labelling during the synthesis process.
The study was conducted by systhesising undoped, nitrogen-doped and nitrogen15-doped reduced graphene oxide, a form of sp2 carbon similar to graphene with
the main difference being a higher degree of defectiveness, and then comparing
their respective Raman spectra. XPS was used to characterise the elemental and
chemical compositions of the materials, to act as a support for the observations
made in Raman spectroscopy. Raman spectra are often measured as single spot
measurements. This type of measurement provides spectral information that is
very spatially limited, making it unideal for some projects. Instead, the Raman
spectra in this study were measured as overview maps. These maps contained
around 1000 spectra spatially separated over large areas for each sample. This
method provides a greater amount of data points allowing for stronger statistical analysis while simultaneously ensuring that genuine representations of the
materials bulk properties are measured.
This thesis studied nitrogen inclusion in reduced graphene oxide by isotope
labelling and Raman spectroscopy. With the aim to expand the understanding
of vibrational modes related to nitrogen included in sp2 carbon.
4
Chapter 2
Theory
2.1
Graphene
The structure of the graphene lattice can be seen in Figure 2.1a. As we can
see, the structure is comprised of a hexagonal network of carbon atoms with
two unique atomic sites A and B. The atoms are separated by the C-C bond
distance ac−c = 0.142nm. In this figure, we can also see the unit cell (dotted
region) and the unit vectors of the cell. The unit vectors a1 and a2 are described
by Equation (2.1) [2, section 2.2.1]. This basic geometry also serves as the fundamental building blocks of other sp2 nanocarbons, such as carbon nanotubes,
leading to them having similar properties [2, section 1.1].
Figure 2.1: The real space unit cell of graphene showing unit vectors a1 and a2
and atomic sites A and B.[1]
√
a1 =
√
3 a
a,
,
2
2
a2 =
5
3
a
a, −
2
2
(2.1)
Tobias Dahlberg
2.1.1
February 19, 2016
Electronic Properties
In Figure 2.2 the electron dispersion relation, the relationship between momentum and energy of electrons, for the 1BZ (first Brillouin zone) of graphene is
shown, the high symmetry points K,K’,Γ and M are marked. One important
feature to note here is the degeneracy of the valence and conduction bands, the
lower and upper surfaces shown in Figure 2.2, at the K and K 0 high symmetry
points. With the Fermi level of graphene also located at these points, this makes
graphene a zero band gap semiconductor with properties similar to metals. If
we zoom in at the K point, another interesting feature is revealed about the electronic properties of graphene. Near these points, the dispersion relationship is
linearly dependent on the wave vector. This linear dependency is something usually seen in massless relativistic particles, and it gives the electrons in graphene
similar properties. From this, electrons and holes in graphene are effectively
massless, resulting in a material with remarkable transport properties. These
points are known as the Dirac points. [2, section 2.2]
Figure 2.2: The electron dispersion relationship of the 1BZ of graphene with high
symmetry points Γ, K, K 0 and M marked. Inset shows dispersion relationship
over directions KΓ, ΓM and M K. [2, p. 30]
2.1.2
Vibrational Properties
The phonon dispersion relation is shown in Figure 2.3. There we can see that
graphene has six phonon branches corresponding to different lattice vibrations.
Lattice vibrations or molecular vibrations are the oscillations of a material due
to the molecular bonds present bending or stretching, essentially acting as a
complex system of harmonic oscillators. The abbreviations in the figure stand
for iLO (in-plane longitudinal optical), iTO (in-plane transversal optical), oTO
(out-of-plane transversal optical), iLA (in-plane longitudinal acoustic), iTA (inplane transversal acoustic) and oTA (out-of-plane transversal acoustic). These
abbreviations explain how the atoms in the vibrations moves relative to the lattice. If we take a closer look at, for example, the iLO phonons the corresponding
6
Tobias Dahlberg
February 19, 2016
lattice vibration is shown in Figure 2.4a. As we can see, iLO corresponds to
atoms moving parallel to the graphene plane in the longitudinal direction (translates to up and down in the figure) with each atom moving out of phase to its
neighbours, which is the definition of an optical vibrational mode (with acoustic
modes refers to neighbours being in phase). Some other examples of vibrational
modes can also be seen in Figure 2.4 where vibrations near Γ Figure 2.4(a) and
K Figure 2.4(b) are shown. [2, section 3]
Figure 2.3: The phonon dispersion relationship of graphene over directions KΓ,
ΓM and M K. [2, p. 53]
7
Tobias Dahlberg
February 19, 2016
Figure 2.4: Vibrations in graphene and their related phonons near a) Γ b) K.
[4]
2.1.3
Nitrogen Doping
As stated before, nitrogen doping is a commonly used method for tuning the
properties of graphene. For example, the inclusion of nitrogen in graphene introduces chemically active sites into this otherwise highly inert structure. These
active sites give catalytic properties to graphene as they can participate in chemical reactions, such as the breaking of O-O bonds which is useful for fuel cell applications [10]. Nitrogen doping also alters the electronic properties of graphene,
due to nitrogen having one more valence electron than carbon. This introduction of donor atoms then shifts the Fermi level of the graphene, changing the
ordinarily conductive material into a semiconductor. The exact nature of the induced changes depends on the total number and types of nitrogen functionalities
present in the sample. Usually four types of nitrogen inclusions are considered in
Nitrogen-doped rGO (N-rGO) and these are called Npyridinic , Npyrrolic , Quaternary center Nitrogen (NQcenter ) and NQvalley . The Npyridinic arises when nitrogen
shares one p-electron to the π-system next to a vacancy, it can be seen in Figure 2.5 marked in blue. Npyrrolic which originates from nitrogen atoms that
share two p-electrons with the π system of graphene, resulting in five member
rings or six member rings bonded to hydrogen next to a vacancy, this is shown
in Figure 2.5 marked in green. NQcenter , also known as graphitic nitrogen, results from sp2 coordinated nitrogen that forms a direct in-plane substitution of
a carbon atom. Another variation of graphitic nitrogen is when nitrogen forms
a graphitic substitution near an edge or vacancy and it is known as NQvalley .
These inclusions are shown in Figure 2.5 marked in black and red, respectively.
8
Tobias Dahlberg
February 19, 2016
Figure 2.5: Illustration of different nitrogen inclusions in graphene. Npyrrolic
(green), Npyridinic (blue), N-Qcenter (black) and N-Qvalley (red). [8]
2.1.4
Reduced Graphene Oxide
Reduced graphene oxide (rGO) are reduced and exfoliated flakes of graphite
oxide (GO); this process is shown in Figure 2.6. The GO precursor used is
often synthesised using a modified Hummer’s method where graphite is mixed
with sulfuric acid, potassium permanganate and sodium nitrate [27]. These
flakes are similar to graphene in structure, with the primary difference being the
degree of defectiveness. As rGO is subjected to heavy oxidisation, the resulting
structures are massively defective and corrugated even after reduction. Despite
being defective, this material retains some of the properties of graphene while
also being much easier to synthesise with a high yield. It is worth emphasising
that the possibility to upscale this method is one of its most promising features
as it makes it suitable for industrial use. Also, the high degree of defectiveness
of rGO also makes it easier to functionalise than pristine graphene, another
attractive property. These two properties factor in to make rGO a promising
material for future applications. [20]
9
Tobias Dahlberg
February 19, 2016
Figure 2.6: Illustration of the transformation of graphite to reduced graphene
oxide.[22]
2.2
2.2.1
Characterization Methods
XPS
XPS is a spectroscopic technique that is used to measure the elemental and
chemical composition of a material. The basic principle of this technique is to
measure the number and energy of photoelectrons emitted by a sample during x-ray irradiation. As each element and each chemical state have electrons
with unique binding energies, the energy of emitted photoelectrons works as a
fingerprint indicating the composition of the material. Counting the number
of emitted electrons and correlating the count to their binding energy gives a
measure of the relative amount of that chemical species present. XPS is limited
to detecting surface properties as only the electrons emitted close to the surface
can easily make it to the detector. Electrons emitted further into the sample
have a significant chance of being recaptured or scattered by the material.
The relevant bands for the analysis of rGO and N-rGO are the N1s, O1s
and C1s bands shown in Figure 2.7. From these spectral regions the nitrogen,
oxygen and carbon content and their chemical states can be estimated.
10
Tobias Dahlberg
February 19, 2016
Figure 2.7: Example XPS spectra of N-rGO showcasing the relevant bands O1s,
N1s and C1s used to calculate oxygen, nitrogen and carbon content respectively.
The N1s band used for the analysis of nitrogen inclusion appear at a binding
energy of 400 eV. This band is composed of a varying number of sub-peaks.
These sub-peaks correspond to the different types of nitrogen functionalities
present in the sample as shown in Figure 2.8a. The first relevant band appear
around 398 eV and it is assigned to Npyridinic . Next a band appear around 399
eV and it is assigned to Npyrrolic . Then a band at 401 eV appear which originates
from N-Qcenter . Lastly, a band at 402 eV is observed and it is assigned to NQvalley .[8]
Appearing around 285 eV the C1s band is related to the types of carbon
found in a structure. A high-resolution spectrum of this band is shown in
Figure 2.8b where the sub-bands relevant to this study is highlighted. The first
important sub-band appears at 284.5 eV and is related to carbon bonded to
carbon in an sp2 configuration. At 285.5 eV the next relevant sub-band appears
11
Tobias Dahlberg
February 19, 2016
(a)
(b)
Figure 2.8: A high resolution XPS spectra of a) N1s and b) C1s band used to
characterise the amount and types of carbon and nitrogen present.
which is also related to carbon-carbon bonds but this time, to sp3 coordinated
carbon, which is the same type of carbon found in diamond. [9]
2.2.2
Raman Spectroscopy
Raman spectroscopy is a spectroscopic technique that is used to analyse the
molecular vibrational modes of a material. This method has its strengths in
that it is non-destructive, fast and requires little sample preparation. Relying
on the Raman effect, this method measures the inelastic scattering of photons
in a material to create a vibrational spectrum characteristic of that material. In
Figure 2.9, schematic energy level diagrams depicting the scattering processes
are shown. Firstly we have the elastic Rayleigh scattering that is not relevant to
Raman spectroscopy as it does not change the photon energy. In this process,
a photon excites the molecule to a virtual state with energy denoted as hv0 , a
short-lived excited state not related to any eigenfunction of the system. From
this virtual state, the molecule then relaxes back to the initial state, emitting a
photon. Then we have the first of the Raman scattering processes, Stokes scattering. In this process, the photon again excites the system to a virtual state
from the ground state E0 . The molecule then emits a phonon of energy hvm
becoming vibrationally excited before radiatively relaxing to E0 − hvm . This
phonon interaction causes a difference in energy of hvm between the absorbed
and emitted photon. This energy difference is the measured quantity in Raman
spectroscopy, known as the Raman shift. Lastly, we have the anti-stokes scat12
Tobias Dahlberg
February 19, 2016
Figure 2.9: Illustration of Rayleigh and Raman scattering processes for photons.
[23]
tering. In this process, the system, instead of generating a phonon, interacts
with an already excited phonon increasing the energy of the resulting photon.
Raman processes are not limited to these examples and can contain multiple
phonon and defect related scattering events. The number of scattering events
taking place in Raman processes indicates its order. As those listed in Figure 2.9
contain only one, they are considered first order processes. If the excited virtual
state coincides with a real electronic state, the process is called resonant and
the likelihood of the transitions occurring is greatly increased. [2, section 4]
The shape of Raman bands is often described using Gaussian or Lorentzian
functions. When deconvoluting Raman spectra containing many bands the
Voigt functions is often used instead. This function is a convolution of the
Gaussian and Lorentzian functions and, therefore, it can describe both shapes
simultaneously. [28]
2.2.2.1
Raman Spectra of Graphene
Raman spectroscopy is a commonly used technique to characterise both electronic and structural properties of graphene. To give a more detailed description
of the Raman processes in graphene, they are usually not illustrated as can be
seen in Figure 2.9. Instead, they are commonly shown at their respective location in the electron band structure, as can be seen in Figure 2.10. In this
section the Raman active bands present in graphene and their properties will
13
Tobias Dahlberg
February 19, 2016
be discussed. In Figure 2.11 an example of the first order Raman spectrum of
graphene is shown. There we can see the measured data (blue dots in the figure)
and its deconvolution; an estimation of the bands making up the spectrum). In
the following sections, a brief overview of the I, D, D”, G and D’ bands are
presented. Commonly the spectral area 1000-1200 cm−1 is only assigned to one
band, the I band. It should be noted that the regions 1000-1200 cm−1 and 13501550 cm−1 are poorly researched and there is no real consensus on their exact
nature. The I* band presented in the figure will be discussed in Section 4.2.3,
as it is unique to this study.
Figure 2.10: Illustration of Raman processes of different bands where the cones
show the shape of the electron dispersion relationship close to the Dirac points
of graphene. a) G band, b) D band, c) D’ band. [2, p. 55]
I Band
The I band appears in the 1100-1200 cm−1 region, and little is known about this
feature. It has been reported that this band originates in C-C, C=C or sp2 -sp3
carbon bonds [3]. Alternatively, the band is seen as related to functionalised
graphene (such as nitrogen-doped graphene) or heavily disordered carbon [10].
G Band
Named so after the fact that it occurs in all graphene allotropes and the existence
of this band is a clear indicator of the presence of sp2 carbon. [2, p. 161] The G
band is a first order Raman band where a virtually excited electron inelastically
scatters with an iTO or iLO phonons near the Γ point; this process can be seen
in Figure 2.10a [4]. The G band is usually located around 1585 cm−1 .
D Band
The D band is named after its close relation to defects in graphenes sp2 structure.
14
Tobias Dahlberg
February 19, 2016
Figure 2.11: An example of the first order Raman spectrum of graphene (blue)
showcasing a deconvolution of its sub-bands.
As we can see in Figure 2.10b, this is a second order process where a virtually
excited electron near the K point scatters elastically from a defect to the K 0
point where it is inelastically scattered by an iTO phonon back to the initial
state near the K point. The D band is usually observed in the spectral region
around 1300 cm−1 and it intensifies and broadens with increased defectiveness.
Due to this relationship, the relative intensity of the D band is often used to
estimate the crystallinity of sp2 nanocarbons. [2, p. 215]
D” Band
The D” band is a broad peak observed in the region 1400-1550 cm−1 and is
another poorly understood band. This spectral feature is believed to be defect
related [11].
D’ Band
The D’ band is another defect related Raman band [3], it usually appears around
1615 cm−1 , the underlying mechanics can be seen in Figure 2.10c.
2.2.2.2
Isotope Labelling
Isotope labelling of a structure is to exchange some elements with heavier isotopes. Doing this induces changes in the vibrational properties of the material,
leaving the electronic characteristics unchanged. Molecular vibrations can, in
a simplified manner, be seen as harmonic oscillators whose frequency ω is de15
Tobias Dahlberg
February 19, 2016
scribed by Equation (2.2)
r
ω=
k
,
m
(2.2)
where m is the mass of the oscillator and k is the spring constant (corresponds
to the bond strength in a molecule). This means that, if we assume the spring
constant to remain unchanged during isotope labelling, the change in frequency
after a mass change can be expressed as Equation (2.3)
r
m
(2.3)
ω2 = ω
m(1 − x) + m2 x
where m2 , ω2 the isotope labelled mass and frequency and x is ration of atoms
in the structure which have been isotope exchanged (x = 1 all atoms are isotopes, x = 0 no atoms are isotopes). As we can see from the equation, the
oscillator frequency down-shifts with increased mass. The relationship shown in
Equation (2.3) have been shown to accurately describe the down-shift of Raman
bands in carbon-13 isotope labelled carbon nanotubes [12].
16
Chapter 3
Experimental
3.1
3.1.1
Synthesis
Synthesis of GO
The GO was synthesised using a modified Hummer’s method. This method
has more recently become known as Tours method and involves oxidation in
a concentrated sulfuric and phosphoric acid mixture together with potassium
permanganate. This method is safer and faster, yet yields similar products as
the conventional Hummers method. The starting graphite used was Alfa aesar
-100mesh natural flake graphite, briquetting grade.[25]
3.1.2
Synthesis of N-rGO, N15-rGO and rGO
rGO was chosen for this study because of it being easily functionalised and
synthesised. The three samples N-rGO, Nitrogen-15 doped rGO (N15-rGO)
and rGO were prepared using a method of simultaneous thermal reduction and
exfoliation similar to [26]. For the nitrogen-doped and isotope labelled samples,
100 mg of ammonium nitrate (Sigma-Aldrich > 99%) and 100 mg nitrogen-15
enriched ammonium nitrate (Sigma-Aldrich 98% N15) was mixed with 100 mg
of GO by stirring for 30 min in 10 ml of ethanol 99.5%. rGO was prepared using
the same method omitting the addition of ammonium nitrate. The mixture was
then dried while stirred at 60 ◦ C on a hotplate. After drying, the sample was
calcinated in an oven at 350 ◦ C for 1 h and then washed with ethanol and with
milli-Q water with repeated centrifugation. Finally, the samples were dried in
90 ◦ C until thoroughly dry (< 2 h).
17
Tobias Dahlberg
3.2
3.2.1
February 19, 2016
Characterization
XPS
The measurements were carried out using a Kratos Axis Ultra DLD electron
spectrometer using a monochromatic Al Kα source operating at 120 W .
3.2.2
Raman Spectroscopy
Three samples of rGO, N-rGO and N15-rGO, all in powder form, were flattened
onto a glass slide to ease the analysis. The samples were analysed using a
Renishaw inVia confocal Raman microscope with an excitation wavelength of
633nm. An overview map (an image of where each pixel contains a Raman
spectrum) of more than 1000 spectra were recorded for each of N15-rGO, NrGO and rGO respectively. A high amount of spatially separated measurement
points was acquired to ensure that the data obtained were representative of the
materials bulk characteristics. The acquired spectra were noise filtered using a
multivariate noise filter and baseline corrected using an asymmetric least square
fit. The resulting processed spectra were batch de-convoluted in MATLABTM
via a custom script. The script used 14 randomised starting guesses for each
spectrum to minimise any bias introduced via the choice of starting parameters
and all bands were fitted with Voigt functions. During the deconvolution, an
additional Raman band was added to the region 1100-1300 cm−1 as the data
indicated its existence. This hypothetical band will in the subsequent sections
be referred to as the I* band. Statistical analysis was conducted on the data
resulting from the deconvolution. The statistical analysis was used to estimate
the mean differences between the three samples. A one-way ANOVA (Analysis
Of Variance) test at a 5% significance level was used to achieve this.
18
Chapter 4
Results and Discussion
4.1
XPS
Table 4.1 and Table 4.2 shows a compilation of the relevant information obtained from the XPS measurements. There we can see the measured atomic
content of the different nitrogen inclusion and also the total nitrogen and oxygen content of the three samples. The most important result shown here is that
the two nitrogen doped samples, N-rGO and N15-rGO, were similar in total
and specific nitrogen inclusion content. We can also observe that the N15-rGO
sample contained a lower amount of sp2 carbon (peak at 284.5 eV) compared
to N-rGO. While both samples had almost identical sp3 carbon (peak at 285.5
eV) content. These results indicate that these samples are suitable for Raman
comparison, as the main differences are those introduced via isotope labelling.
Table 4.1: The results of the XPS analysis of the carbon content of the three
samples.
Sample
N-rGO
N15-rGO
rGO
Csp2 [at%]
43.14
38.85
48.54
Csp3 [at%]
18.36
18.45
15.3
Table 4.2: The results of the XPS study of the nitrogen content of the three
samples. The atomic percentages of the different nitrogen inclusions and the
total nitrogen content are shown.
Sample
N-rGO
N15-rGO
Npyridinic
[at%]
3.66
3.93
Npyrrolic
[at%]
4.38
4.61
NQcenter
[at%]
0.84
0.89
19
NQvalley
[at%]
0.51
0.61
Ntotal
[at%]
9.31
10.04
Tobias Dahlberg
4.2
February 19, 2016
Raman Analysis
In this section, the results of the Raman analysis is presented. All peak heights
mentioned in this section are normalized with respect to the G band
4.2.1
D-Band
The results of the measurements of the D band parameters of the three samples
are shown in Figure 4.1 and the results of the statistical analysis are shown in
Table 4.3. Firstly we can note that the mean center position of the D-band
in N-rGO and N15-rGO is down-shifted relative to rGO by 8.2 cm−1 and 7.5
cm−1 , for N-rGO and N15-rGO respectively. The results also show that there
is a statistically significant but weak up-shift of 0.7 cm−1 in the mean center
position of N15-rGO, compared to N-rGO. This result indicates that this band
does not directly originate in phonons related to nitrogen inclusions. Instead,
this can be explained by the increased defectiveness introduced by the nitrogen
inclusion in the structure. Knowing that the D band has been observed as
closely linked with defective sp2 structures this result is expected.
From the analysis of the peak height, all of which have been normalized
with respect to the G-band intensity, we can see that both N-rGO and N15rGO shows an intensity increase of 85% and 56% (relative rGO), respectively.
Also, it is apparent that the D band in the N15-rGO is significantly lower than
that of N-rGO, being decreased by 28.3%. From this, it can be reasoned that
the decline in intensity can be linked to the lower sp2 content in N15-rGO, as
revealed by the XPS analysis. Reports studying nanodiamond powders have
also related a reduction of D band intensity to increased sp3 content [7, p. 104].
This reason may also be an explanation of this behaviour as the N15-rGO sample
contains a higher sp3 to sp2 ratio than the N-rGO.
Widthwise this band seems to broaden with nitrogen doping as a width
increase (relative rGO) of −10.1 cm−1 and −5.3 cm−1 is observed for N-rGO and
N15-rGO. This behaviour is expected as the D bandwidth has been reported to
increase with increased defectiveness due to changes in the lifetime of related
phonons.
To summarise these results the D-band intensifies, broadens and down-shifts
with nitrogen doping while being almost unaffected by isotope labelling. These
findings indicate that the band is related to the degree of defectiveness present
in the sample and not necessarily nitrogen inclusion directly. As the D band is
well studied and has been confirmed to originate in defect activated vibrational
modes of sp2 carbon this behaviour is theoretically expected.
20
Tobias Dahlberg
February 19, 2016
Figure 4.1: D band center position, height relative to G band and width with
mean values marked by a star and the 95% confidence interval marked by solid
lines. The colors correspond to rGO (red), N-rGO (blue) and N15-rGO (black).
Table 4.3: Results of ANOVA on D band parameters for rGO, N-rGO and N15rGO. Results should be read as the difference between samples means i.e. rGO
vs N-rGO is equivalent to mean(rGO)-mean(N-rGO).
Sample
rGO vs N-rGO
rGO vs N15-rGO
N-rGO vs N15-rGO
Mean center shift [cm−1 ]
8.2 ± 0.1
7.5 ± 0.1
−0.7 ± 0.1
p value
1 ∗ 10−9
1 ∗ 10−9
1 ∗ 10−9
Sample
rGO vs N-rGO
rGO vs N15-rGO
N-rGO vs N15-rGO
Mean height shift [-]
−0.85 ± 0.01
−0.56 ± 0.01
0.283 ± 0.009
p value
1 ∗ 10−9
1 ∗ 10−9
1 ∗ 10−9
Sample
rGO vs N-rGO
rGO vs N15-rGO
N-rGO vs N15-rGO
Mean width shift [cm−1 ]
−10.1 ± 0.2
−5.3 ± 0.2
4.8 ± 0.2
p value
1 ∗ 10−9
1 ∗ 10−9
1 ∗ 10−9
21
Tobias Dahlberg
4.2.2
February 19, 2016
I-Band
The most important result shown here can be seen in Figure 4.2 and Table 4.4. It
is the significant (p-value << 0.05) down-shift of 11.7 cm−1 of the I band center
in N15-rGO relative N-rGO. As this change is both statistically significant and
larger than 10 cm−1 it is not unreasonable to correlate this down-shift to the
isotopic labelling. Knowing that isotope labelling a chemical species with heavier
isotopes should down-shift the related Raman bands, this correlation is further
supported. As mentioned before, the down-shift upon isotope labelling can be
described by Equation (2.3). By using this equation, and setting m1 = 14, m2 =
15 and x = 0.99 the expected down-shift can be estimated to 40 cm−1 ; four
times the measured value. This discrepancy between theory and experimental
results indicates that while this equation describes the behaviour of carbon-13
labelled sp2 carbon well, it does not accurately represent the more complicated
case of nitrogen-15 labelled carbon. As a nitrogen doped structures are more
defective and nitrogen inclusions form more complex and varied structures than
carbon-13 inclusions, this might serve as an explanation for this deviation. It
can also be noted that the mean I band center position up-shifts by 1.2 cm−1 in
N-rGO compared to rGO. However, as this change is relatively weak, it cannot
be correlated to nitrogen doping with any certainty.
If we move on to how the mean height of the peak differs between the samples
we can see that it is increased by 18.5% in N-rGO and 9.2% in N15-rGO,
compared to rGO. As this increase is strongly pronounced for both N-rGO and
N15-rGO, it can be related to nitrogen doping. Interestingly, we can notice that
the mean height of the peak in N15-rGO is lower than that in N-rGO. This
result can again be correlated to the fact that the N15-rGO, as revealed by the
XPS study, contained less sp2 carbon than the other samples.
Another interesting feature is that the width decreases by 5.7 cm−1 and by
15.0 cm−1 relative rGO, in N-rGO and N15-rGO respectively. This observation
shows that the band narrows with isotope labelling that again could indicate a
link to nitrogen functionalities.
To sum up these results they indicate that the I band is firmly related to
nitrogen doping in graphene as we can see that it both intensifies and narrows
after nitrogen doping. This connection is further supported by the significant
down-shift of the band after isotope labelling.
22
Tobias Dahlberg
February 19, 2016
Figure 4.2: I band center position, height relative to G band and width with
mean values marked by a star and the 95% confidence interval marked by solid
lines. The colors correspond to rGO (red), N-rGO (blue) and N15-rGO (black).
Table 4.4: Results of ANOVA on I band parameters for rGO, N-rGO and N15rGO. Results should be read as the difference between samples means i.e. rGO
vs N-rGO is equivalent to mean(rGO)-mean(N-rGO).
Comparison
rGO vs N-rGO
rGO vs N15-rGO
N-rGO vs N15-rGO
Mean center shift [cm−1 ]
−1.2 ± 0.5
10.5 ± 0.5
11.7 ± 0.5
p value
8 ∗ 10−9
1 ∗ 10−9
1 ∗ 10−9
Comparison
rGO vs N-rGO
rGO vs N15-rGO
N-rGO vs N15-rGO
Mean height shift [-]
−0.18.5 ± 0.002
−0.092 ± 0.002
0.093 ± 0.002
p value
1 ∗ 10−9
1 ∗ 10−9
1 ∗ 10−9
Comparison
rGO vs N-rGO
rGO vs N15-rGO
N-rGO vs N15-rGO
Mean width shift [cm−1 ]
5.7 ± 0.5
15.0 ± 0.5
9.2 ± 0.5
p value
1 ∗ 10−9
1 ∗ 10−9
1 ∗ 10−9
23
Tobias Dahlberg
4.2.3
February 19, 2016
I*-Band
The results of the measured hypothetical I* band parameters of rGO, N-rGO
and N15-rGO is shown in Figure 4.3 and Table 4.5. As we can see from these
results, the mean center position of the I* band is significantly down-shifted by
6.9 cm−1 and 7.1 cm−1 (relative to rGO) for N-rGO and N15-rGO respectively.
Whereas, only a 0.2 cm−1 mean difference is seen between N-rGO and N15rGO. As this change is small and barely significant, having a p-value (strength
of evidence, should be below 0.05 for statistical significance) of 0.0248, this
band cannot be said to be affected by isotope labelling. From this, it can be
speculated that as this band down-shifts as a result of nitrogen doping but is
not affected by the isotope labelling, the band is probably defect related.
Figure 4.3: I* band center position, height relative to G band and width with
mean values marked by a star and the 95% confidence interval marked by solid
lines. The colors correspond to rGO (red), N-rGO (blue) and N15-rGO (black).
In the statistical analysis, we also see a significant increase in peak height
relative rGO of 25.2% for N-rGO and 25.5% for N15-rGO. As the p-value of the
test between the mean heights of N-rGO and N15-rGO is 0.1395, no significant
difference in mean height can be established between the two. Following the
same reasoning as in Section 4.2.1 the height of all band linked to sp2 carbon
should be decreased in the N15-rGO sample. However, as this is not the case
for this band, it can be speculated to be associated with sp3 carbon. Drawing
24
Tobias Dahlberg
February 19, 2016
this conclusion is not unreasonable as this spectral region has been reported as
connected to sp3 carbon in previous studies [6]. As we saw in the XPS analysis,
the sp3 carbon content in both N-rGO and N15-rGO are at similar levels. This
result would mean that a Raman band related to this species would likely have
a comparable intensity in both samples, which further supports this conclusion.
Table 4.5: Results of ANOVA on I* band parameters for rGO, N-rGO and N15rGO. Results should be read as the difference between samples means i.e. rGO
vs N-rGO is equivalent to mean(rGO)-mean(N-rGO).
Sample
rGO vs N-rGO
rGO vs N15-rGO
N-rGO vs N15-rGO
Mean center shift [cm−1 ]
6.9 ± 0.2
7.1 ± 0.2
0.2 ± 0.2
p value
1 ∗ 10−9
1 ∗ 10−9
0.0248
Sample
rGO vs N-rGO
rGO vs N15-rGO
N-rGO vs N15-rGO
Mean height shift [-]
−0.252 ± 0.004
−0.255 ± 0.004
−0.003 ± 0.004
p value
1 ∗ 10−9
1 ∗ 10−9
0.1395
Sample
rGO vs N-rGO
rGO vs N15-rGO
N-rGO vs N15-rGO
Mean width shift [cm−1 ]
2.5 ± 0.4
−4.4 ± 0.4
−7.0 ± 0.3
p value
1 ∗ 10−9
1 ∗ 10−9
1 ∗ 10−9
25
Tobias Dahlberg
4.2.4
February 19, 2016
D”-Band
The results relevant to the D” band are presented in Figure 4.4 and Table 4.6.
We can see that this band displays a similar behaviour to that observed in
the D band. The mean height is strongly increased by nitrogen doping being
shifted relative rGO by 31.7% in N-rGO and 24.8% in N15-rGO. Again the
band intensity is lowered in N15-rGO compared to N-rGO. Similarly to the D
Figure 4.4: D” band center position, height relative to G band and width with
mean values marked by a star and the 95% confidence interval marked by solid
lines. The colors correspond to rGO (red), N-rGO (blue) and N15-rGO (black).
band, the mean center position shows a dependence on nitrogen doping but not
strongly on isotope labelling, being down-shifted relative rGO by 8.2 cm−1 in NrGO and 10.3 cm−1 in N15-rGO. The difference between N-rGO and N15-rGO
here is stronger than that observed for the D band but still relatively weak. As
stated before, changes of this magnitude cannot be assigned significance owing
to the inherently inhomogeneous nature of reduced graphene oxide.
Interestingly it can be noted that these results indicate that the width of
this band is not different between the N-rGO and N15-rGO. This result is not
consistent with the behaviour exhibited by the D-band and the reason behind
this is unclear.
The results presented here seems to indicate that this band is defect related,
as it shows a behaviour similar to the D band. This observation is in line with
26
Tobias Dahlberg
February 19, 2016
the results of previous studies.
Table 4.6: Results of ANOVA on D” band parameters for rGO, N-rGO and
N15-rGO. Results should be read as the difference between samples means i.e.
rGO vs N-rGO is equivalent to mean(rGO)-mean(N-rGO).
Sample
rGO vs N-rGO
rGO vs N15-rGO
N-rGO vs N15-rGO
Mean center shift [cm−1 ]
8.2 ± 0.5
10.3 ± 0.5
2.1 ± 0.4
p value
1 ∗ 10−9
1 ∗ 10−9
1 ∗ 10−9
Sample
rGO vs N-rGO
rGO vs N15-rGO
N-rGO vs N15-rGO
Mean height shift [-]
−0.317 ± 0.005
−0.248 ± 0.005
0.07 ± 0.004
p value
1 ∗ 10−9
1 ∗ 10−9
1 ∗ 10−9
Sample
rGO vs N-rGO
rGO vs N15-rGO
N-rGO vs N15-rGO
Mean width shift [cm−1 ]
−8.0 ± 0.3
−7.7 ± 0.3
0.3 ± 0.3
p value
1 ∗ 10−9
1 ∗ 10−9
0.1008
27
Tobias Dahlberg
4.2.5
February 19, 2016
G-Band
As we can see from both Figure 4.5 and the statistical analysis in Table 4.7 the
G band is weakly affected by both nitrogen doping and isotope labelling. All
changes are on the order of 1 cm−1 making these results hard to use to draw any
conclusions.
Figure 4.5: G band center position and width with mean values marked by a
star and the 95% confidence interval marked by solid lines. The height has been
omitted as it has been normalized to 1 for all samples. The colors correspond
to rGO (red), N-rGO (blue) and N15-rGO (black).
28
Tobias Dahlberg
February 19, 2016
Table 4.7: Results of ANOVA on G band parameters for rGO, N-rGO and N15rGO. Results should be read as the difference between samples means i.e. rGO
vs N-rGO is equivalent to mean(rGO)-mean(N-rGO).
Sample
rGO vs N-rGO
rGO vs N15-rGO
N-rGO vs N15-rGO
Mean center shift [cm−1 ]
2.7 ± 0.2
3.7 ± 0.2
1.0 ± 0.1
p value
1 ∗ 10−9
1 ∗ 10−9
1 ∗ 10−9
Sample
rGO vs N-rGO
rGO vs N15-rGO
N-rGO vs N15-rGO
Mean width shift [cm−1 ]
−6.2 ± 0.3
−7.5 ± 0.3
−1.3 ± 0.3
p value
1 ∗ 10−9
1 ∗ 10−9
1 ∗ 10−9
4.2.6
D’-Band
In Figure 4.6 and Table 4.8 we see that this band exhibits similar behaviour to
the D band, intensifying, broadening and down-shifting with nitrogen doping.
Also, we can see that the down-shift of this band in the N15-rGO relative NrGO is quite small, being less than 5 cm−1 which makes it hard to draw any
conclusions, as stated before. This result is in line with the expected behaviour
of this band as it has been linked to defects. Again the trend of a lowered
intensity of the band in N15-rGO can be observed.
Table 4.8: Results of ANOVA on D’ band parameters for rGO, N-rGO and
N15-rGO. Results should be read as the difference between samples means i.e.
rGO vs N-rGO is equivalent to mean(rGO)-mean(N-rGO).
Sample
rGO vs N-rGO
rGO vs N15-rGO
N-rGO vs N15-rGO
Mean center shift [cm−1 ]
−0.9 ± 0.2
3.6 ± 0.2
4.5 ± 0.2
p value
1 ∗ 10−9
1 ∗ 10−9
1 ∗ 10−9
Sample
rGO vs N-rGO
rGO vs N15-rGO
N-rGO vs N15-rGO
Mean height shift [-]
−0.340 ± 0.009
−0.264 ± 0.009
0.075 ± 0.009
p value
1 ∗ 10−9
1 ∗ 10−9
1 ∗ 10−9
Sample
rGO vs N-rGO
rGO vs N15-rGO
N-rGO vs N15-rGO
Mean width shift [cm−1 ]
−8.8 ± 0.2
−8.2 ± 0.2
0.6 ± 0.2
p value
1 ∗ 10−9
1 ∗ 10−9
1 ∗ 10−9
29
Tobias Dahlberg
February 19, 2016
Figure 4.6: D’ band center position, height relative to G band and width with
mean values marked by a star and the 95% confidence interval marked by solid
lines. The colors correspond to rGO (red), N-rGO (blue) and N15-rGO (black).
30
Chapter 5
Summary and Outlook
This study set out to investigate if isotope labelling in conjunction with Raman
spectroscopy could be used to characterise nitrogen functionalities in graphene.
Raman spectroscopy is a fast, easy and non-destructive method that probes
materials molecular vibrational modes. These qualities make it an attractive
method for characterisation. A strong understanding of the effects that nitrogen
inclusions have on the Raman spectrum of graphene could potentially enable
the characterisation of these functionalities through this method alone.
In this thesis, rGO doped with both nitrogen (N-rGO) and nitrogen-15 (N15rGO) was synthesised, and their Raman spectra were analysed. rGO was chosen as it is easier to both functionalise and synthesise compared to ordinary
graphene. From the study, potential evidence linking the I band to functionalised graphene was found, as it responded strongly to isotope labelling. Also,
tentatively a new band denoted I*, possibly related to sp3 carbon, was identified. Before this, the spectral region 1000-1300 cm−1 has only been thought to
contain the I band and it has been speculated to be connected to functionalised
graphene or sp2-sp3 carbon bonds. As this study indicates a separation of the I
band into two bands, each dependent on one of these factors, it could bring clarity to this poorly understood area. If these results can be replicated, and backed
up by more measurements, they could lead to a better future understanding of
the Raman spectrum of graphene.
The results of this thesis are speculative and further studies would be needed
to confirm them. Firstly, more data from more samples from each category (undoped, N-doped and N15-doped) would be necessary to gather, as one sample
from each class only gives an idea of the variations present in those specific samples and not the variations between the different sample categories. Gathering
more data would result in the possibility to use stronger statistical methods and
also, most importantly, the ability to accurately judge which observations can
be deemed significant.
Besides this, a study of the Raman spectra for N-rGO doped with different
levels of nitrogen could be used to identify clearly the nitrogen dependency of
the Raman bands. This process would likewise be interesting to conduct for
31
Tobias Dahlberg
February 19, 2016
varying degrees of isotope content, which would serve as further support of the
observation made in this thesis.
Further, the Raman spectrum of graphene is not only limited to the spectral
features discussed in this thesis. Thus additional information could be found by
studying, for example, the higher order Raman bands in the region 2300-3300
cm−1 .
To gain insight into the impact of specific nitrogen functionalities on the Raman spectrum, and possibly identify their vibrational modes. An experiment
where samples containing varying amounts of particular types of nitrogen inclusion could be synthesised. Such an experiment could, for example, be carried
out via heat treatment of N-rGO which would transform less stable nitrogen
functionalities in more stable ones [8]. Also, another possibility would be to
tweak synthesis parameters to promote different types of nitrogen inclusions, as
suggested by Indrawirawan et al.[26].
To conclude, we can see that the results of this thesis are intriguing but
vague. However, they clearly indicate what to expect in similar experiments
and they provide a sound foundation for further studies.
32
Bibliography
[1] M. Huang, Studies of mechanically deformed single walled carbon nanotubes and graphene by optical spectroscopy, Columbia University, (2009)
[2] A. Jorio, M. Dresselhaus, R. Saito and G. Dresselhaus, Raman Spectroscopy in Graphene Related Systems, Wiley-VCH, Germany, (2011) .
[3] A. Sadezky, H. Muckenhuber b, H. Grothe, R. Niessner and U. Pöschl,
Carbon 43, 1731, (2005)
[4] R. Beams, L.G. Cancado and L. Novotny, Journal of Physics: Condensed
Matter J. Phys.: Condens. Matter 27, (2015)
[5] L.M. Malard, M.A. Pimenta, G. Dresselhaus, M.S. Dresselhaus, Physics
Reports 473,51, (2009)
[6] Z. Zhu, F. Cheng and J. Chen, J. Mater. Chem. A, 1, 9484, (2013)
[7] O.A. Williams, P. O’Brien, R. Nuzzo, Nanodiamond, Royal Society of
Chemistry, (2014).
[8] T. Sharifi, G. Hu, X. Jia and T. Wagberg, ACS Nano, 10, 8904,(2012).
[9] Z. Fan, W. Kai, J. Yan, T. Wei, L. Zhi, J. Feng, Y. Ren, L. Song and F.
Wei, ACS Nano, 5, 191, (2011).
[10] T. Sharifi, F. Nitze, H.R. Barzegar, C. Tai, M. Mazurkiewicz, A. Malolepszy, L. Stobinski and T. Wagberg, Carbon, 50, 3535, (2012).
[11] S. Claramunt, A. Varea, D. Lopez-Diaz, M. Velazquez fixa texten, A. Cornet and A. Cirera, Carbon, . Phys. Chem. C 119, 10123, (2015)
[12] S.D. Costa, C. Fantini, A. Righi, A. Bachmatiuk, M.H. Rummeli, R. Saito
and M.A. Pimenta , 49, 4719, (2011)
[13] J. Lee, T. Ha, H. Li, K.N. Parrish, M. Holt, A. Dodabalapur, R.S. Ruoff
and D. Akinwande, ACS Nano, 7, 7744, (2013)
[14] K.S. Novoselov, V.I. Falko, L. Colombo, P.R. Gellert, M.G. Schwab and K.
Kim, Nature ,490, 192, (2012)
33
Tobias Dahlberg
February 19, 2016
[15] M.A. Pimenta, G. Dresselhaus, M.S. Dresselhaus, L.G. Cancado, A. Jorio
and R. Saito, Phys.Chem.Chem.Phys., 9, 1276, (2007)
[16] S. Zhu, T. Li, ACS Nano, 8, 2864, (2014)
[17] S. Hu, M. Lozada-Hidalgo, F.C. Wang, A. Mishchenko, F. Schedin, R.R.
Nair, E.W. Hill, D.W. Boukhvalov, M.I. Katsnelson, R.A.W. Dryfe, I.V.
Grigorieva, H.A. Wu and A.K. Geim, Nature, 516, 227, (2014)
[18] H. Wang, T. Maiyalagan, and X. Wang, ACS Catal, 2, 781, (2012)
[19] K.A. Wepasnick, B.A. Smith, J.L. Bitter and D.H. Fairbrother, Anal
Bioanal Chem, 396, 1003, (2010)
[20] F. Perrozzi, S. Prezioso and L. Ottaviano, J. Phys.: Condens. Matter, 27,
21, (2015)
[21] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V.
Dubonos, I.V.Grigorieva, A.A. Firsov, Science, 306, 666,(2004)
[22] rGO synthesis, illustration, viewed 4 January 2016, ”https:
//www.utu.fi/en/units/sci/units/chemistry/research/mcca/
PublishingImages/GO%20rGO.jpg”.
[23] Raman scattering processes, illustration, viewed 4 January 2016,”http:
//bwtek.com/wp-content/uploads/2012/06/R-F-04.jpg”
[24] S. undu, T.C. Nagaiah, W. Xia, Y. Wang, S.V. Dommele, J.H. Bitter, M.
Santa, G. Grundmeier, M. Bron and W. Schuhmann, J. Phys. Chem. C,
113, 14302, (2009)
[25] D.C. Marcano, D.V. Kosynkin, J.M. Berlin, A. Sinitskii, Z. Sun, A. Slesarev, L.B. Alemany, W. Lu, and J.M. Tour, ACS Nano, 4, 4806, (2010)
[26] S. Indrawirawan, H Sun, X. Duana and S. Wang, J. Mater. Chem. A, 3,
3432, (2015)
[27] W.S. Hummers, R.E. Offeman, 80 (6), 1339, (1958)
[28] M. Bradley, Thermo Fisher Scientific Inc., (2007)
[29] T.M.G Mohiuddin, A. Lombardo, R.R. Nair, A. Bonetti, G. Savini, R. Jalil,
N. Bonini, D.M. Basko, C. Galiotis, N. Marzari, K.S. Novoselov, A.K. Geim
and A.C. Ferrari, Phys. Rev. B, 79, 205433, (2009)
[30] A. Jorio, A.G. Souza Filho, G. Dresselhaus, M.S. Dresselhaus, A.K. Swan,
M.S. nl, B. Goldberg, M.A. Pimenta, J.H. Hafner, C.M. Lieber and R.
Saito,Phys. Rev. B, 65, 155412, (2002)
[31] A. Das, S. Pisana, B. Chakraborty, S. Piscanec, S.K. Saha, U.V. Waghmare,
K.S. Novoselov, H.R. Krishnamurthy, A.K. Geim, A.C. Ferrari and A.K.
Sood, Nat. Nanotech., 3, 210, (2008)
34
Tobias Dahlberg
February 19, 2016
[32] M.M. Lucchese, F. Stavale, E.H. Ferreira, C. Vilane, M.V.O. Moutinho,
R.B. Capaz, C.A. Achete, A. Jorio, Carbon, 48, 1592, (2010)
[33] X. Li, W. Cai, L. Colombo and R.S. Ruoff, Nano Letters, 9, 4268, (2009)
[34] F. Simon, C. Kramberger, R. Pfeiffer, H. Kuzmany, V. Zlyomi, J. Krti,
P.M. Singer and H. Alloul, Physical Review Letters, 95, 017401, (2005)
35