1

Master Thesis on

Synthesis of single-layer graphene and studying oxidation

behaviour of copper foil

By: Mouna Rafei

Supervisor: Hamid Barzegar

Examiner: Thomas Wågberg

Department of Physics, Umeå University

Master Thesis, 60 hp

Autumn 2020

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Abstract

The ultimate aim of the current study is to investigate the electron transfer from copper

(Cu) to single layer graphene through a thin Cu oxide layer. Therefore the project is divided

into two main parts. In the first part, single layer graphene is synthesized with chemical vapour

deposition technique on a Cu foil and the grown graphene is characterized by means of Raman

spectroscopy and scanning electron microscopy (SEM). We tune different experimental

parameters to grow high quality graphene. We show that a pre-annealing of the Cu foil, in

Varigon environment, modifies the Cu crystal grain size and that modifies the growth dynamic

of the graphene. Optimum annealing time in correlation with growth time results in high Ǵ/G

ratio and a narrow FWHM of Ǵ band in Raman spectrum. The second part of the project focuses

on controlling the surface oxidation of Cu foil with respect to the oxide layer thickness. The

surface and cross section of the oxidized Cu foil is examined by SEM and the presence of oxide

layer is confirmed via energy dispersive X-ray spectroscopy (EDS) analysis. We show that the

surface roughness of the oxide layer can be minimized by controlling the oxidation condition

with a minimum oxide layer thickness of 18 μm. In addition the grain size of the oxide layer is

getting larger with increasing the oxidation temperature. Furthermore, the oxygen

concentration in the oxide layer is controlled through a controlled reduction process which is

confirmed via EDS analysis. XPS spectroscopy is also used for elemental analysis as well as

revealing the chemical state of the Cu oxide.

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Contents

1. Introduction and Motivation

2. Theory

2.1 Graphene

2.2 Chemical Vapour Deposition (CVD)

2.3 CVD Synthesis of Graphene

2.3.1 Substrate

2.3.2 Temperature

2.3.3 Pressure

2.3.4 The precursor and additive gases

2.4 Oxidation of Copper (Cu)

2.5 Characterization

2.5.1 Raman Spectroscopy

2.5.1.1 Raman Instrumentation

2.5.2 Raman Characterization of Graphene

2.5.3 Scanning Electron Microscopy (SEM)

2.5.3.1 SEM Instrumentation

2.5.4 X-ray photoelectron spectroscopy (XPS)

2.5.4.1 XPS Instrumentation

3. Experimental and Measurement Setup

3.1 CVD Setup

3.2 Raman Spectroscopy

3.3 SEM

3.4 XPS

4. Experimental Process

4.1 CVD Synthesis of Graphene

4.2 Transfer to Si/SiO2 Wafer

4.3 Oxidation of Cu

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5. Results

5.1 Synthesis of Monolayer Graphene

5.1.1 Effect of Annealing Time

5.1.2 Effect of Pressure

5.1.3 Quality Evaluation of the Grown Graphene

5.2 Surface Oxidation of Copper Foil

5.2.1 Surface and Cross Section of Oxidized Cu

5.2.2 Effect of Temperature on Grain Size

5.2.3 XPS Analysis

5.3 Thermal Reduction of Cu Oxide

5.3.1 Cross section of Reduced Cu Oxide

6. Summary and Conclusion

6.1 Outlook

7. References

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1. Introduction and Motivation

The remarkable properties of monolayer graphene, a two dimensional crystalline sheet of

carbon atoms, makes it attractive material for future applications [1]. In spite of having an

atomic thickness, graphene exhibits an extraordinary strength with the Young’s modulus being

reported at 1 TPa [2]. In addition unique physical properties of single layer graphene are being

investigated for several applications including: supercapacitors, transistors, organic electrodes,

etc. [3, 4].

There are different techniques to obtain single layer graphene including mechanical

exfoliation, liquid phase exfoliation [5] and chemical vapour deposition (CVD). Single layer

graphene first was isolated by Novoselov et al. in 2004 via mechanical exfoliation of graphite

[6]. Although this method provides a high-quality graphene, controlling the number of

graphene’s layers remains a challenge. Therefore in order to get single layer , large area, high

quality and inexpensive graphene, CVD synthesis is the most promising method for growing

graphene [7]. In 2009 Li et al. reported their results on the CVD synthesis of graphene. They

grew single layer graphene using Cu foil as a substrate and methane as a carbon precursor.

Since then their procedure has become a standard way for growing single layer graphene

through CVD [8]. It is worth to mention that CVD synthesis of graphene consumes more

energy, compared to other techniques, since it requires high temperature (almost 1000 °C) [9,

10]. In addition the grown graphene on Cu foil needs to be transferred to different substrates

for further application or characterization. This is a challenging step of the experiment since it

may introduce defect and impurities to the grown graphene. The most common transfer

technique is known as wet transfer in which a thin polymer based support film is used to

transfer graphene (please see the experimental part for detail) [11, 12].

Several metal oxides have been explored to have semiconducting properties; like TiO2,

ZnO, Fe2O3, Bi2O3, Cu2O and CuO. Among them Cu2O and CuO with a small bandgap (2.1–

2.6 eV for Cu2O and 1.9–2.1 eV for CuO [13]) are known to be highly photoactive in the visible

region which make them good candidates for photoelectrochemical and photovoltaic

applications. Both phases of Cu oxide show p-type semiconductor properties and they are of

great interest due to non-toxicity, easy availability of the raw material and low cost [14].

Among the oxidation methods, thermal oxidation of Cu [15] is the most proper way for getting

thin oxide layer on Cu surface. It has been shown that properties like thickness of oxide layer,

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grain size and surface area can significantly affect the optical and electrical properties of oxide

layer [16, 17].

The ultimate goal of the current project is studying electron transfer from metal to single

layer graphene through a thin metal oxide layer. Figure 1 represents the possible electron

tunnelling through the oxide layer in a graphene/metal oxide layer/metal sandwiched structure.

Figure 1: Schematic representation of possible electron tunnelling through metal oxide layer in a

sandwiched structure (graphene/metal oxide layer/metal).

In the current project the sandwiched structure will be single layer graphene/Cu oxide

layer/Cu foil. Similar configuration can be used to form different types of graphene/metal oxide

layer/metal sandwiched structures.

In this work, the single layer graphene is synthesized through CVD technique and

transferred to Si/SiO2 wafer for further characterization. The optimum condition has been set

to get the high quality and single-layer graphene. Moreover the surface of the Cu foil is

thermally oxidized in a controlled environment to get very thin oxide layer. This is in direction

of the final goal of the project where thin oxide layer facilitate the electron transfer (as shown

in figure 1). The work also suggests different approaches for controlling the surface roughness

and concentration of oxygen in the oxide layer.

2. Theory

2.1 Graphene

Graphene is a two-dimensional (2D) material with a covalently bonded carbon atoms in a

hexagonal symmetry. Graphene is the building block of graphite in which graphene layers are

e −

Metal

Metal Oxide Layer

Single Layer Graphene

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hold together with a weak van der Waals interaction [18, 19]. Distinctive and extraordinary

properties of graphene make it different from graphite and creates huge interest among the

researchers, like high, in plane, electron mobility at room temperature (250,000 cm2/Vs) [6],

exceptional thermal conductivity (5000 W m-1K-1) [20], strong mechanical properties [21],

remarkable chemical properties [22], etc. Owing to these excellent properties, it is widely

accepted that graphene will be progressive material for the future applications. Figure 2

represents a honeycomb structure of an individual layered graphene in which the carbon atoms

are covalently bonded together.

Figure 2: Schematic of a graphene sheet.

2.2 Chemical Vapour Deposition (CVD)

CVD is a technique which is widely used for synthesis of nanomaterials such as graphene,

SWCNTs, TMDs, etc. The process involves passing a precursor (gas or a vapour) through the

reaction chamber which can react or decompose at the surface of a preselected substrate at an

appropriate temperature, as it is depicted in figure 3. CVD is a promising technique for

synthesis of nanomaterials because of several controllable parameters during the process such

as temperature, precursor, type of substrate and pressure of the reaction chamber [23]. In

addition versatility, quality, simplicity and productivity of CVD technique is interesting

characteristics [24]. In this work, low pressure CVD (pressures lower than the atmospheric

pressure; typically lower than 10-6 Pa) has been applied for synthesis of single layer graphene

[25].

Carbon atom

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Figure 3: Schematic representation of a typical CVD process.

2.3 CVD Synthesis of Graphene

Prior to the CVD growth of graphene, the substrate is “pre-cleaned” to remove the

impurities and possible oxide layer (as explained in experimental part). The substrate is then

further cleaned by annealing at high temperature inside the reaction chamber [26]. It has been

reported that impurities on the substrate may result in discontinuities of graphene growth,

which in turn results in formation of multilayer graphene or few layer graphene [27]. Therefore

cleaning of the substrate plays an essential role to get high quality graphene [28]. Moreover,

annealing at high temperature (in presence of hydrogen gas) may modify the surface and crystal

grain size of the substrate. After annealing, the graphene growth starts by introducing the

carbon precursor gas. It has also been reported that the type of substrate, growth temperature,

pressure inside the reaction chamber and type of the precursor and additive gases during CVD

synthesis of graphene have significant influence on graphene’s quality which will be discussed

later in the text. Different steps of graphene growth are schematically illustrated in figure 4.

Figure 4: Mechanism of graphene growth on the substrate.

CVD Oven

Reaction Chamber

In Flow Out Flow

Substrate

Ar/H2/CH4

CH4

C

H2

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In general the growth of graphene is explained as follow [29]:

1. Absorption of hydrocarbon molecules.

2. Decomposition of hydrocarbon molecules to form carbon atoms.

3. Propagation and attachment of carbon atoms to nucleation sites to form graphene film

on the substrate.

2.3.1 Substrate

Transition metals are the most common substrates for CVD growth of nanostructures.

Variety of transition metals are examined for graphene growth such as gold (Au), copper (Cu),

cobalt (Co), iridium (Ir), nickel (Ni), palladium (Pd), platinum (Pt), rhenium (Re),

and ruthenium (Ru) [30, 31]. Among them Fe, Co, Ni and Cu, are the most promising due to

their performance as well as low cost and availability. Depending on the distribution of

electrons in the d-shell, transition metals have different carbon solubility. For instance, Fe has

an asymmetrical electron distribution (3d6 4s2) in its d-shell, which results in high carbon

solubility [31]. Co and Ni with the orbital formation of 3d7 and 3d8 respectively, are commonly

used for formation of several layers graphene [30] and Cu is the most promising one to get high

quality single layer and large scale graphene because of filled d-shell which results in low

carbon solubility [32, 33].

2.3.2 Temperature

It has been shown that temperature has a significant effect on the nucleation and domain

size of graphene. The reported growth temperatures are mainly between 970-1050 °C [34, 35]

(below the melting point of Cu 1085 °C [31]). In general higher temperature results in low

density of nucleation sites which in turn results in larger domain size [35, 36]. However lower

temperatures hinders the formation of nucleation sites.

2.3.3 Pressure

Pressure is an important parameter during CVD synthesis of graphene and growth kinetics

of graphene is a function of pressure inside the reaction chamber. Therefore pressure will

significantly affect the uniformity of graphene [37]. Although there are fewer reports on the

graphene growth at ultra-high vacuum (UHV) CVD [38], it has been manifested that UHVCVD

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improves the quality of the grown graphene. In contrast, there have been several attempts to

synthesize graphene at atmospheric pressure due to simplicity and low cost. In 2013, Shin, Y.C.

and J. Kong et al. reported on the synthesis of graphene at atmospheric pressure with excluding

hydrogen in all the steps. Their results revealed a high quality graphene which is comparable

with low pressure CVD synthesis [39]. In this work, we will apply low pressure CVD

(pressures in the range of 20 mbar) to get single-layer graphene.

2.3.4 The precursor and additive gases

During CVD process the type of the carbon precursor affects the surface reactions on Cu

foil. Basically, during CVD synthesis of graphene Ar and H2 together with a carbon precursor

are passed through the reaction chamber. Ar is usually used as a carrier gas however the

presence of H2 helps to clean the Cu surface and reduces the native metal oxides [40-42]. In

addition; size, shape, thickness and crystallinity of the graphene domains are affected by H2

[43, 44]. Different concentration of hydrogen has been used during CVD synthesis of graphene,

for instance Dong and co-authors introduced 20% H2 in Ar [45], Vlassiouk and co-workers

used very diluted hydrogen (2.5%) [46]. In contrast, in 2014, it has been reported that,

continuous and uniform graphene (with minimum oxidation sites) can be grown only with

purified methane and without having H2 in any of the growth stages [39, 47]. It is worth to

mention that in addition to hydrocarbon gases (such as methane [8, 48, 49], acetylene, ethylene

and propene [50]), solid (such as polymers) [51, 52] and liquid (hexane, ethanol, etc.) [53-55]

carbon precursors are also used for CVD synthesis of graphene.

2.4 Oxidation of Cu

In general, the surface of the Cu metal can be oxidized easily even at room temperature

and in the presence of ambient air. In this process a very thin native oxide layer can be formed

on the metal surface, however the oxidation procedure progress only within a few nanometer,

since at room temperature the energy of the oxygen atoms is too low for diffusion through the

native oxide layer. Further oxidation needs to be promoted by means of temperature of

chemical reactions. Oxidation process of metals is of high interest due to their environmental

stability and their efficiency in semiconductor based devices and catalytic properties. Therefore

it is of great interest to understand the kinetics of the oxidation process of metals and controls

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the morphology, size and formation of the metal oxide. Such information can be used to further

tune their physical and chemical properties.

The copper oxide thin films are well known to be used in various application due to their

remarkable physical and chemical properties. Copper oxides exist in two crystalline phases;

cuprous oxide or cuprite (Cu2O) and cupric oxide or tenorite (CuO). [13] Both phases exhibit

p-type semiconductor properties with a band gap 2.1-2.6 eV in the case of Cu2O and 1.9-2.1

eV in the case of CuO. [13, 56]. It has been also reported that CuO phases exhibit n-type

conductivity [57]. Copper oxides are widely used in photovoltaic devices [58], solar cells [59]

and catalysis [60]. It is worth to mention that despite of numerous studies, fabrication of single-

phase copper oxide is still a big challenge.

Synthesis of copper oxide follows two reactions as stated in equation (1) and (2):

4Cu + O2 = 2Cu2O (1)

2Cu2O + O2 = 4CuO (2)

Consequently, Cu2O is the first grown oxide type and further oxidation leads to the

formation of second type CuO (CuO/ Cu2O/Cu).

So far thermal oxidation of Cu has been done in the ambient oxygen or water vapour,

where the oxidation condition was controlled by varying the oxidation time and temperature

[61, 62]. In the current work we will focus on creating a thin oxide layer on the copper foil

through the controlled oxidation environment. We tune the Cu oxidation via controlling the

oxidation time, temperature, oxygen concentration and pressure during oxidation (between 40

to 50 mbar).

2.5 Characterization

2.5.1 Raman Spectroscopy

Inelastic scattering of light was first predicted by A.Smekal in 1923 and in 1928 was

observed experimentally by Raman and Krishnan [63, 64] and the phenomenon has been called

Raman scattering. Raman spectroscopy is based on Raman scattering and provides information

about the structures, chemical composition as well as physical properties of the materials [65].

Figure 5 demonstrates the process of Raman scattering. The interaction of electrons and

the incident light will excite the electrons to a virtual state. The virtual states are created through

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the interaction of light with the molecule and their energies are determined by the frequency of

the incident light. Depending on the frequency of an incident and scattered light, the electron

can go back to its original state (Rayleigh scattering), higher (Stocks) or lower energy states

(anti-Stocks). Among the three different process Rayleigh is the most probable process [65,

66].

Figure 5: Energy level diagram of Raman process.

2.5.1.1 Raman Instrumentation

Typically, a Raman instrument consists of five main components including: excitation

source, sample illumination systems and collection optics, wavelength selectors and separators,

detectors and recording device. The different components of Raman spectroscopy are

schematically shown in Figure 6.

Rayleigh Raman Stocks Raman Anti-Stocks

Ener

gy

Lowest vibrational level of

ground state

Excited vibrational level of

ground state

Virtual states

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Figure 6: Schematic of modern Raman spectroscopy [66].

Different laser sources can be utilized in Raman spectroscopy and the common sources

are Ar + (457, 488, 514 nm), He-Ne (633 nm) and Diode NIR (785 nm) [66]. After removing

Rayleigh scattering, separation of the scattered Raman radiation from the sample can be

performed by gratings which changes depending on the lasers. In addition detectors are based

on Charged-coupled devices (CCDs) that are silicon-based semiconductors and can generate

photoelectrons and store them as an electrical charge. Analog-to-digital converter reads each

pixel that arises from the stored charges [66].

2.5.2 Raman Characterization of Graphene

Raman scattering is widely used to distinguish the number of graphene’s layers [67],

structural defects [68], grain boundaries and strain in the structure [69]. Figure 7 shows a

typical Raman spectrum of a monolayer graphene.

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Figure 7: Raman spectrum of graphene with three distinct peaks.

There are three main peaks in figure 7 namely D (~ 1350 cm-1), G (~ 1580 cm-1) and Ǵ (~

2675 cm-1) band. D band is defect sensitive and it is originated from deviation form hexagonal

flat structure including structural defect, impurities or wrinkles. Therefore the intensity of D

band should be low for high quality and defect free graphene. G band is standing for stretching

mode of C-C band and it exists in all sp2 hybridized carbon molecules. 2D or Ǵ band is

approximately two times larger than D band and that is the reason to call it 2D band. It is always

showing up even in the absence of its first order, D band. Studying the ratio of intensities

between Ǵ and G is a common way for determining the number of layers. For single layer

graphene the ratio of Ǵ to G is greater than 2 [70, 71]. In addition the Full Width Half Maximum

(FWHM) of Ǵ peak itself can be used to estimate the number of layers and for monolayer

graphene it is reported somewhere between 25-45 cm-1 [71].

2.5.3 Scanning Electron Microscopy (SEM)

Scanning Electron Microscopy (SEM) is a type of Electron Microscopes (EM) which is

widely used to study the morphology and composition of nanostructures. SEM consist of

several components namely: electron beam, electromagnetic lenses, vacuum system, detectors

1500 2000 2500 3000

0,0

0,2

0,4

0,6

0,8

1,0

Inte

nsi

ty

Raman shift (cm-1)

D G

2D or Ǵ

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and CCD camera. SEM provides high-resolution image of the surface. The incident electron

beam scatters after interaction with the examined sample. The scattered electrons can be

classified into two categories: secondary electrons (SEs) as a result of inelastic scattering and

backscatter electrons (BSEs) as result of elastic scattering. Low energy secondary electrons are

used to create images and mainly originates from a few nanometers below the sample surface.

On the other hand, BSEs are a function of the atomic number of sample elements, darker and

brighter spots appear on the image which means that heavy elements appear brighter than the

light ones [72, 73]. Moreover, there is a possibility to identify the composition of sample with

energy dispersive X-ray spectroscopy (EDS) in the SEM. The interaction of the incident

electrons with the sample generates X-rays which carries qualitative and quantitative

information about the chemical composition of the sample. Figure 8 is a representation of

different out coming signals (SEs, BSEs and X-rays) in the scanning electron microscopy.

Figure 8: The effect of interaction of electron beam with the specimen [74].

2.5.3.1 SEM Instrumentation

Figure 9 displays the components of a typical SEM. As it is depicted, electron gun is placed

on the top which generates beam of electrons. The beam goes through the magnetic lenses

which create magnetic field to deflect the electrons to focus them onto the sample. All

compartments of SEM must be under vacuum environment [72].

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Figure 9: The components of scanning electron microscopy [72].

2.5.4 X-ray Photoelectron Spectroscopy (XPS)

X-ray photoelectron spectroscopy (XPS) was first developed in 1960s by Kai Siegbahn

and his research group at Uppsala University, Sweden and the first practical instrument was

manufactured in the early 1970s. In 1981s Nobel Prize for Physics was awarded to Siegbahn

for his work with XPS [75].

XPS is used for determination of elemental composition, chemical state of the elements

and contamination at the surface of the materials (1-10 nm usually). The surface of the materials

is irradiated with a beam of X-rays and at the same time ejected electrons from the surface are

collected by a detector which record the number of released electrons with respect to their

kinetic energy. The kinetic energy of the released electrons can be used to calculate the binding

energy of the electron in the sample as explained by equation 3:

E kinetic = E photon – (E binding + ɸ) (3)

From the equation, E photon is the energy of the incident X-ray, E kinetic is the kinetic energy

of the ejected electrons from the surface of the material and ɸ is the work function of the

instrument (minimum energy that is required for ejecting the electron from the atom) [75, 76].

The obtained information are then used to plot the binding energy versus intensity which gives

quantitative knowledge of sample composition.

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2.5.4.1 XPS Instrumentation

XPS is running under Ultra High Vacuum (UHV < 10-8 Torr). Common X-ray sources are

usually Mg Kα radiation (hν = 1253.6 eV) or Al Kα radiation (hν = 1486.6 eV). After

irradiating the sample with X-ray source, the ejected electrons from the surface move to the

electron energy analyser through the transfer lens. The transfer lens is placed between the

sample and energy analyser that separates the electrons according to their kinetic energy.

Among the several types of energy analysers, electrostatic hemispherical analyser is the

common one [77]. As depicted in figure 10, the detector counts the electrons and the computer

analyses the results and presents a spectrum of the intensity versus the binding energy of

electrons.

Figure 10: Schematic representation of X-ray photoelectron spectroscopy.

Electron energy analyser

Sample

X-ray source

Detector

Electrons

Computer readout

Inte

nsi

ty

Binding energy

Transfer lens

hν

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3. Experimental and Measurement Setup

3.1 CVD setup

Figure 11 is a photograph of our CVD setup.

(a) (b)

Figure 11: The CVD set-up consisting of 1(a) Mass Flow Controllers (MFCs), 2(a) Unit controller,

3(a) oven, 4(a) vacuum pump, 5(a) vacuum system controller. (b) is the vacuum system controller in

details; 1(b) pressure gauge, 2(b) pressure regulator and 3(b) vacuum valve.

The setup consists of a Mass Flow Controllers (MFCs) to control the flow of five different

gases including: hydrogen, methane, argon, Varigon and oxygen (argon and Varigon are

controlled with a single MFC). The flow meters are connected to the unit controller which can

be used to set and control the flow rates of gases through the reaction chamber. The reaction

chamber is place inside an oven and is connected to a vacuum system controller (including

pressure gauge, pressure regulator and vacuum pump). Different part of the setup are listed in

table 1.

1

2

3

5

4

3 1

2

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Table 1: List of components used for the CVD setup.

Component Manufacturer Notes

Oven Nabertherm Model: P480

Quartz tube AdValueTech 120 cm

Stainless steel tubing and

connections Swagelok American standard

Pressure gauge Edwards Model: P3

Pressure regulator Equilibar

Vacuum valve Pfeiffer Model: D-35614

Assar

Vacuum pump Edwards Model: 10i

Mass Flow Controllers MKS

Instruments Model: 1179C

Unit controller MKS

Instruments Model: 946

3.2 Raman Spectroscopy

The transferred graphene on Si/SiO2 wafer was characterized by Renishaw in-via confocal

Raman microscope at the ViSp facility at Umeå University to analyse the number of graphene’s

layers and quality.

3.3 SEM

The morphology and cross section of the Cu foils were examined by Scanning Electron

Microscope (SEM) at Umeå Core Facility for Electron Microscopy (UCEM), using Carl Zeiss

Merlin FESEM instrument.

3.4 XPS

The oxidized Cu foils were analysed by XPS at KBC building, using AXIS Ultra DLD

electron spectrometer manufactured by KRATOS Analytical Ltd. (UK) for determination of

the chemical composition.

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4. Experimental Process

4.1 CVD Synthesis of Graphene

First the commercial Cu foil (25 μm thickness and 99.999% purity from Alfa Aesar) was

placed in acetic acid and deionized water (10 min each) followed by drying with pressurized

air. The cleaned Cu foil was then placed at the centre of the oven inside a quartz tube. After

pumping the system with vacuum pump to pressures as low as 4×10-2, the system was purged

with Ar to remove any impurities or moisture (the pumping/purging cycle was repeated few

times). The Cu foil was then heated up to the desired temperature in Varigon environment and

was kept at that temperature for a specific time to clean the surface of the Cu foil (here on we

refer to this step as “annealing”). Afterwards, the gaseous carbon source, CH4, was introduced

to perform the graphene growth. Finally, the furnace was cooled down to temperatures below

180 °C under the flow of Varigon environment and the sample was taken out.

During synthesis of graphene several parameters were changed in order to get high quality

single layer graphene and the effect of different parameters were examined by studying the

quality of the grown graphene. Different growth parameters are listed in table 2.

Table 2: The growth parameters for synthesis of graphene.

Temperature

(°C)

Time

(min)

Var

(sccm)

CH4

(sccm)

Pressure

(mbar)

Heating 1050 60 40 0 8 or 1013

Annealing 1050 15 to 60 40 0 8 or 1013

Growth 1050 30 40 20 20 or 1013

Cooling - - 40 0 8 or 1013

4.2 Transfer to Si/SiO2 Wafer

The grown graphene on the Cu foil was transferred to Si/SiO2 wafer (via a wet transfer

method) for further characterization. Figure 12 shows the sequence of wet chemical

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Figure 12: Steps during wet chemical transfer; a) Synthesized graphene on Cu, b) Ozone cleaned Cu,

c) The PMMA layer on graphene/Cu, d) Floated PMMA/graphene/Cu on the etching solvent, e) The

floated film after etching overnight, f) The transferred film on Si/SiO2 wafer, g) Sample in acetone bath

to remove PMMA residue.

Since both sides of Cu was covered with graphene the graphene from the bottom side was

removed via ozone cleaner (12(b)). Then as-grown graphene was spin coated (1500 RPM, 60

s) with PMMA in anisole (35 mg/ml) and baked on a hot plate for 2 min at 100 °C (12(c)). The

coated Cu foil was then gently put in sodium persulfate (Na2S2O8) solution (with concentration

of 6 mg/ml) overnight to etch the Cu foil (12(d)). After etching the Cu foil, the floated film

(PMMA/graphene) was extracted gently from the etching solvent with microscopic slide and

put in deionized water for a few hours to remove the etching solvent from the bottom side

(12(e)). Then, the film was picked up with Si/SiO2 wafer and baked on the hot plate at 100 °C

for 2 minutes (12(f)). Finally, the sample was placed in the acetone bath and left overnight to

remove PMMA residue (12(g)).

4.3 Oxidation of Cu

First the commercial Cu foil (250 μm thickness and 99.9999% purity from Alfa Aesar)

was pre-cleaned in the acetic acid and deionized water each for 10 minutes, followed by drying.

Then the cleaned Cu was placed at the centre of the oven inside a quartz tube. After sealing the

tube and pumping, the system was purged with Ar (the cycle was repeated few times). The

sample was then heated up to the desired temperature. Afterwards the sample was kept at the

a b d

e f

c

g

(

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same temperature in Varigon environment to remove the native surface oxide and then O2 and

Ar were introduced to perform the oxidation process. Finally, the sample was taken out after

cooling down the system to the temperature below 65 °C.

The oxidation of Cu was carried out with varying several parameters; such as oxidation

temperature, oxidation time, oxygen concentration and pressure of the reaction chamber. The

effect of oxidation temperature has been studied at three different temperatures (400 °C, 600

°C and 1050 °C). Further, the flow rates of gases and oxidation time have been changed to

obtain a thin oxide layer on Cu foil. Different experimental parameters are listed in table 3.

Table 3: Experimental parameters for oxidation of Cu foil.

Oxidation temperature

(°C)

Oxidation time

(sec)

Ar

(sccm)

O2

(sccm)

Pressure during oxidation

(mbar)

400, 600 and 1050 10-120 100-400 1-15 40-50

5. Results and Discussion

5.1 Synthesis of Monolayer Graphene

5.1.1 Effect of Annealing Time

In the first part of the study we were trying to optimize our experimental setup to grow a

high quality single layer graphene. According to the previous studies, increasing the annealing

time in hydrogen environment results in a reconstructed Cu surface and larger grain size [78].

Therefore, in our experiments we examined four different annealing time (15, 30, 45 and 60

min). Figure 13 represents SEM images of four different copper foil with different annealing

time. Apart from the annealing time, the growth parameters are those in table 2. From the figure

it is clear that longer annealing time leads to larger grain size.

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Figure 13: Effect of annealing time on crystal structure of Cu; (a) 15 min annealing, (b) 30 min

annealing, (c) 45 min annealing, (d) 60 min annealing.

To examine the structure and number of graphene layers, the samples were characterized

by Raman spectroscopy, using 514 nm laser as the excitation wavelength. For more accurate

analysis, random spots over the sample were examined and the average of five spectra were

used for analysis. From the Raman spectra the number of graphene layers were estimated by

calculating the intensity ratio of (Ǵ/G) as well as FWHM of Ǵ band. The results are

summarized in table 4.

Table 4: Effect of annealing time on ratio of Ǵ/G and FWHM of Ǵ band.

Annealing

time(min) 15 30 45 60 60

Growth time

(min) 30 30 30 30 15

Ratio of Ǵ/G

(cm-1) 4.5 3.8 2.8 2.7 3.8

FWHM (Ǵ)

(cm-1) 28.3 33.7 37.1 40.8 34.8

100 μm 100 μm

100 μm 100 μm

a

c

b

d

24

The results indicate that with increasing the annealing time from 15 min to 60 min, the

ratio of Ǵ/G follows a downward trend and at the same time the FWHM of Ǵ increases. Since

in all experiments (with different annealing time) the growth steps are the same, we can

conclude that annealing time plays a critical role on growth dynamic of graphene. In 2014, it

has been shown by Jin, Y., et al and Zhang, X., et al. that with long annealing time, H atoms

cover the surface and terminates the active graphene edges, thereafter the diffusion of active C

species into the area under the first layer graphene results in a formation of second layer [79,

80].

Comparing our results with the previous studies (as mentioned above) we conclude that a

long annealing time results in a faster growth rate for graphene and consequently the second

layer starts to growth earlier in time. This will lead to the growth of multilayer graphene in

shorter time compared to the experiment with shorter annealing time. To support our

hypothesis, we shorten the growth time from 30 min (the constant growth time for synthesis of

monolayer graphene in previous experiments) to 15 min to grow graphene on a Cu foil which

was annealed for 60 min (the results are summarized in last column of table 4). By comparing

the results, one can see that short growth time with long annealing (60 min) gives the same

values as we got for 30 min annealing. This results confirms that larger grain size accelerates

the growth rate of graphene.

5.1.2 Effect of Pressure

Synthesis of graphene was carried out both at low pressure (LPCVD) and atmospheric

pressure (APCVD) to examine the effect of pressure.

25

Figure 14: Raman spectra of synthesized graphene at LPCVD and APCVD.

Raman spectrum of APCVD synthesized graphene (figure 14) confirms that the grown

graphene is always multilayer. Even, with very short growth time and diluted CH4 we got

double-layer graphene with a broad Ǵ band (60.8 cm-1). The growth parameters are

summarized in table 5. Although APCVD requires a simple set up with no needs for vacuum

pump, LPCVD is necessary for our system to grow high quality single-layer graphene.

Table 5: The growth parameters for LPCVD and APCVD.

Temperature

(°C)

Annealing

time

Growth

time

Heating

(Var:CH4)

(sccm)

Annealing

(Var:CH4)

(sccm)

Growth

(Var:CH4)

(sccm)

Cooling

(Var:CH4)

(sccm)

LPCVD 1000 15 min 30 min 40:0 40:0 40:20 40:0

APCVD 1000 15 min 30 sec 100:0 100:0 100:2 100:0

1500 2000 2500 30000

500

1000

1500

2000

2500

3000

3500

4000

Inte

nsi

ty

Raman shift (cm-1)

Synthesis of graphene at LPCVD

Synthesis of graphene at APCVD

D

Ǵ

G

26

5.1.3 Quality Evaluation of the Grown Graphene

After optimizing our setup, we successfully grow high quality graphene. In order to

evaluate the quality of the growth, we tried to compare our results with high quality commercial

graphene (from GRAPHENE SUPERMARKET). The commercial monolayer graphene was

transferred to Si/SiO2 wafer following the same wet transferring method as described in section

4.2. Raman spectra of two monolayer graphene are depicted in figure 15.

Figure 15: Raman spectra of commercial and as-grown graphene.

The spectra of the two monolayer graphene are almost overlapping except that the D band

of the grown graphene is strong in comparison with the one from commercial graphene. This

can be related to the impurities that we got from our system. After synthesis of graphene on Cu

foil, the samples were scanned with EDS and we observed nanoparticles of Al.

5.2 Surface Oxidation of copper foil

In general surface oxidation process of Cu foil with nano- or micro-scale precision is a

challenging process. Therefore we performed several experiments to obtain mild oxidation

condition. Copper foil was oxidized at different conditions with respect to oxidation

1500 2000 2500 30000

500

1000

1500

2000

2500

3000

3500

4000

4500

Inte

nsi

ty

Raman shift (cm-1)

The Grown Graphene at 1000 °C

The Commercial GrapheneǴ

G D

27

temperature, oxidation time, oxygen concentration and pressure of the reaction chamber during

oxidation. Figure 16 presents the photograph of pristine Cu foil (16 a) together with five

oxidized Cu foils with different oxidation condition. The oxidation parameters are listed in

table 6. It is clear from the figure that, following the colour, one can evaluate the degree of

oxidation.

Figure 16: (a) pristine Cu, (b) mild oxidized Cu to (e) strong oxidized condition and (f)

destructed Cu foil according to the table 6.

Table 6: The summarized experimental parameters for Cu oxidation.

Oxidation temperature

(°C)

Ar

(sccm)

O2

(sccm)

Oxidation time

(sec)

Pressure during

oxidation

(mbar)

b 1050 250 1 10 40-50

c 1050 200 1 30 40-50

d 1050 100 1 30 40-50

e 1050 100 5 120 40-50

f 600 100 15 120 1013

The oxidation process is more aggressive at atmospheric pressure as it is shown in figure

16 (f). It can destroy the surface of the Cu foil which indicates that atmospheric pressure is not

suitable for controlled oxidation.

e d

c b a

f

28

5.2.1 Surface and Cross Section of Oxidized Cu

Surface and cross section of the Cu was examined after oxidation with Scanning Electron

Microscopy (SEM). Figure 17 represents the surface and cross section of the pristine Cu (a)

and two oxidized Cu foils (b and c) with two different experimental conditions, as explained

in table 7.

Figure 17: Surface and cross section of (a) pure copper, (b) mild oxidized Cu and (c) strong oxidized

Cu.

Table 7: The experimental parameters for oxidation of Cu.

Oxidation temperature

(°C)

Ar

(sccm)

O2

(sccm)

Oxidation time

(sec)

Pressure during

oxidation

(mbar)

b 600 400 1 10 42

c 600 100 5 60 45

In cross section view the lighter contrast, close to the surface, indicate the thickness of the

oxide layer (note that the lighter contrast in (a) originates from the surface of the Cu foil). It is

clear from the figures that more aggressive oxidation condition forms a thicker oxide layer.

The thickness of the oxide layer is measured in different part of the image and the average is

used for analysis which reveals 18.7 µm and 47.6 µm thickness for (b) and (c) respectively.

In order to confirm the nature of the oxide layer, EDS was employed to scan the cross

section of the oxidized Cu foil with respect to oxygen and copper and determine their atomic

concentration. Figure 18(a) is a cross section view of an oxidized Cu foil. The elemental

mapping, the area shown by rectangle in (a), are shown in figures 18 (b) and 18 (c) with respect

a c

18.7 μm 100 μm

1

100 μm

100 μm 100 μm

100 μm

47.6 μm

100 μm

b c

29

to oxygen and copper respectively. While Cu mapping shows a uniform concentration all

around the cross section, the higher concentration of oxygen near the surface confirms that the

light contrast close to the surface indicates the thickness of the oxide layer. Furthermore, the

numerical analysis of EDS results revealed that oxygen concentration is 10.7% on the selected

area.

Figure 18: Energy dispersive X-ray analysis on the cross section of oxidized Cu with respect to

the concentration of O and Cu which confirms higher concentration of O near the surface.

5.2.2 Effect of Temperature on Grain Size

During the experiments we realized that the oxidation temperature affects the surface

roughness and the crystal grain size. Figure 19 depicts the SEM images of three different

samples, oxidized at different temperatures namely 400 °C, 600 °C and 1050 °C. The SEM

images show how the grain size is getting larger by increasing the oxidation temperature,

however to confirm this effect further analysis of the crystal structures, such as XRD are

necessary.

100 μm

a b c

30

Figure 19: Effect of oxidation temperature on surface roughness and crystal grain size.

5.2.3 XPS Analysis

The surface of the oxidized Cu foil was studied by XPS to analyse the chemical

composition. The XPS analysis of the pristine Cu foil show certain amount of Cu metal and Cu

oxide (native oxide) phases at the surface.

Figure 20 (a) shows the wide range XPS spectrum of the oxidized Cu foil at 1050 ºC

indicating Cu 2p, O 1s, C 1s, Cu 3s and Cu 3p. High resolution spectrum of the O 1s pick

(between 529.1-531.9 eV) is shown in figure 20 (b). The O 1s peak can be deconvoluted into

three main peaks with binding energies corresponding to oxygen atoms in CuO, Cu2O or

Cu(OH)2 and COOH phases.

Figure 20 (a): Complete X-ray photoelectron spectrum of oxidized Cu at 1050 ºC.

a (400 °C) b (600 °C) c (1050 °C)

10 μm 10 μm 10 μm

1 μm 1 μm 1 μm

0 200 400 600 800 1000

0

200000

400000

600000

800000

Inte

nsi

ty(c

ps)

Binding Energy(eV)

Cu 2p

Cu LMM Auger

O 1s

C 1s

Cu 3p

Cu 3s

31

Figure 20 (b): High resolution X-ray photoelectron spectrum of O 1s for oxidized Cu.

Comparison of the XPS analysis on three different samples (oxidized at different

temperatures according to table 8), revealed that Cu atoms at the surface of the foil are

completely oxidized and metallic phase of Cu was not observed after oxidation. XPS analysis

also confirms that the main oxidation phase in all three sample is cuprous oxide or Cu2O.

Table 8: Summarized XPS analysis for oxidized Cu at 400 ºC, 600 ºC and 1050 ºC.

Oxidized samples

Binding energy of O 1s (eV) AC, at.%

CuO Cu2O CuO Cu2O

400 ºC 529.2 530.1 2.41 18.21

600 ºC 529.2 530.1 1.78 12.56

1050 ºC 529.1 531.1 2.67 37.13

O 1s

524 526 528 530 532 534 536

0

5000

10000

15000

20000

25000

Inte

nsi

ty(c

ps)

Binding Energy(eV)

Experimental Data

Fitted Data

CuO

Cu2O, Cu(OH)2

COOH

32

5.3 Thermal Reduction of Cu Oxide

Through the oxidation process, even at very mild oxidation condition, we obtained

minimum thickness of 18 µm for the oxide layer which is still too high for ultimate goal of the

project, i. e to measure charge transfer through the metal oxide layer. Therefore we tried to

decrease the oxide layer thickness via a reverse thermal reduction process in hydrogen

environment. The reduction process is done in Varigon environment at different temperatures

and time. The experiments are divided in two different sets: In the first set the hydrogen

introduced from the beginning of the process when the sample was heated up. In the second set

hydrogen was introduced when the oven reached to the desired reduction temperature.

Figure 21: Surface and cross section of the reduced Cu oxides according to the parameters in the

table 9.

1 μm

a

100 μm

At%: Cu (96.6), O (3.4)

1 μm

b

At%: Cu (96.9), O (3.1)

100 μm

1 μm

At%: Cu (96.7), O (3.3)

c

100 μm

d

At%: Cu (96.7), O (3.3)

100 μm

1 μm

33

Table 9: Experimental parameters for thermal reduction of Cu oxide.

Reduction

temperature

(°C)

Var

(sccm)

Reduction time

(min) Pressure during reduction

(mbar)

a 600 50 1 11.5

b 600 50 10 11.5

c 1050 50 10 11.5

d 1050 50 30 1013

When hydrogen is present from the beginning, almost all Cu oxides return to Cu state.

EDS analysis (figure 21) reveal that the percentage of oxygen is almost the same for different

reduction conditions (the reduction parameters are summarized in table 9). The percentages of

oxygen in the reduced Cu foils, with different temperatures, treatment time and pressure, are

around 3%, even less than pristine Cu with native oxide layer (4.6%), for all the conditions.

From this results, it can be concluded that the reduction starts from low temperatures (around

200 °C). Furthermore, SEM analysis on the surface of the samples clearly indicate that the

surface of the reduced Cu oxide is getting smoother by increasing the temperature and the

annealing.

Second set of the reduction experiments were done exactly with the same parameters in

table 9. In these experiments hydrogen was introduced after reaching the desired temperature

just for a certain time as stated in table 9. Figure 22 shows the SEM analysis on the surface and

cross section of these samples. One can see that the percentage of the oxygen varies in different

experiments whereas it is not easy to evaluate if the thickness of the oxide layer has changed

by reduction. In other words, it seems that oxygen is extracted through the whole oxide layer

during reduction and not only from the surface of the oxidized Cu.

34

Figure 22: Surface and cross section of the reduced Cu oxides according to the parameters in the

table 9 (hydrogen was introduced after reaching the desired temperature).

Moreover, one of the critical issues for our final aim (studying electron transfer) is the

surface of the sample which should be smooth enough for transferring graphene. Figure 22

shows that the surfaces of the samples are rough in comparison with the first set of experiments

(figure 21). To make the surface smoother, we decided to do heat-treatment on the reduced

sample (figure 22 c) for 30 minutes in Ar environment (50 sccm), high temperature (1050 ºC)

and atmospheric pressure. After heat-treatment the SEM analysis show the same roughness as

before and one can conclude that a combination of hydrogen, temperature and long treatment

results in a smooth surface. Figure 23 demonstrates the surface and cross section of reduced

Cu oxide before and after heat treatment.

1 μm

At%: Cu (88.9), O (11.1)

100 μm

a b

1 μm

100 μm

At%: Cu (92.0), O (8.0)

c

At%: Cu (96.6), O (3.4)

1 μm

100 μm

d

At%: Cu (96.4), O (3.6)

1 μm

100 μm

35

Figure 23: Surface and cross section of reduced Cu oxide before and after heat-treatment in Ar

environment.

5.3.1 Cross section of Reduced Cu Oxide

EDS was employed to evaluate the amount of oxygen in cross section of the reduced Cu

foil. Figure 24 shows the comparison between reduced Cu foil and Pristine Cu together with

their elemental mapping with respect to oxygen and copper.

Figure 24: Cross section of reduced Cu oxide and pristine Cu with respect to the concentration of

O and Cu.

10 μm

b) After heat-treatment

10 μm

a) Before heat-treatment

100 μm 100 μm

At%: Cu (95.5), O (4.5)

Reduced Cu oxide

100 μm

At%: Cu (95.4), O (4.6)

Pristine Cu

100 μm

36

Although the contrast can be seen on the cross section of the reduced Cu oxide, the

elemental mapping of oxygen and copper shows uniform distribution along the selected

rectangle area. This means that after reduction the oxidized Cu get back to the Cu metal but the

crystal structure of the Cu changes during reduction and oxidation process which requires more

crystal structure analysis such as XRD analysis.

37

6. Summary and Conclusion

In the current work, the optimum condition has been found and tested for synthesis of

high quality monolayer graphene as well as controlled oxidation of Cu foil. The single-

layer graphene was successfully synthesized and the surface of the Cu foil was oxidized

with micrometre precision.

In the first part of the project, the effect of annealing time and pressure have been

identified. Results from Raman spectroscopy reveal that long annealing time results in a

larger grain size and faster growth rate and at the same time formation of second layer

graphene is getting most probable. This means that second layer graphene starts to growth

before the first layer completely covers the surface. Furthermore, it is also manifested that

low pressure Chemical Vapour Deposition (CVD) is necessary for our system to get high

quality and single layer graphene.

In the second part of this work, the surface of the Cu foil was oxidized with respect to

the oxide layer thickness and the effect of the temperature, oxygen concentration, time and

pressure have been tested. Through the mild oxidation condition (low oxygen concentration

and very short oxidation time) the minimum thickness of the oxide layer was around 18

μm. In addition, the presence of oxide layer on the surface was confirmed with EDS reports.

SEM images also show that increasing temperature from 400 ºC to 1050 ºC results in a

larger grain size of oxide layer.

The behaviour of oxidized Cu under thermal reduction process has been studied by

SEM and EDS analysis and it is shown that reduction starts from low temperatures (around

200 ºC). Furthermore, the concentration of oxygen has been changed under the controlled

reduction environment and through the strong reduction condition the percentage of oxygen

even less than pristine Cu. Through the EDS analysis, the concentration of oxygen is

uniform both for reduced Cu oxide and pristine Cu but the SEM contrast at the surface of

the reduced Cu was still detectable. This reveals that after oxidation and reduction crystal

structure of the sample changes which is different in comparison with pristine Cu. To sum

up, it can be conclude that oxidation and reduction procedure affect the crystal structure of

Cu foil. On the other hand, the surface of the reduced Cu oxide became rough by time and

further treatment was applied to make the surface smoother which was unsuccessful. This

results confirms that combination of hydrogen, long treatment and high temperature leads

to get smooth surface.

38

6.1 Outlook

In further work the quality of single layer graphene can be improved by making

impurity and defect free graphene layer on the substrate.

Synthesis of graphene on the oxidized and reduced Cu foil is a part of future works.

Especially investigating the behaviour of graphene on the oxidized and reduced Cu foil.

Furthermore, the crystal structure of the oxidized and reduced Cu foil can be evaluated

by means of X-ray Diffraction (XRD) technique.

39

Acknowledgment

First and foremost I would like to express my gratitude to Professor Thomas Wågberg

and my supervisor Dr. Hamid Barzegar for their supports and advices during my master

thesis project.

I would like to thank everyone at the Department of Physics because of this nice

educational system and pleasant working environment.

Finally I owe my deepest gratitude to my lovely family, my dear husband Reza

Abbaspour and my friends for their kind support and encouragement during this journey.

40

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