PHD student

Oreste De Luca1

What is graphene?

Graphene is a single layer of carbon atoms arrayed in a 2D honeycomb structure

Andrej Gejm and Konstantin Novosëlov

“scotch tape method”

2010 Nobel Prize in Physics

2004 - Mechanical exfoliation of graphite (HOPG)

Graphene-like 2D materials:

• Graphene derivatives: hydrogenated graphene, graphene oxide, fluorographene

• Structural analogues of graphene: Silicene (hexagonal network of silicon atoms) germanene (a graphene-like network of germanium atoms), TGCN, ...

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Graphene lattice structure

lattice vectors

first -neighborsvectors

About graphene

a=1.42 Å

d=2.46 Å

Bonds between the atoms in the plane

Bond oriented in the vertical direction,out of the plane

The hybridized bonds form the 𝜋-band and 𝜋∗-bands, which characterize the peculiar electronic properties of graphene

𝜎 bond

𝜋 bond

1947 - Band structure for a single graphite layer was theoretically calculated by Wallace

2004 – first experimental isolation of graphene flakes

distance between the first neighbors

distance between C atoms of the unit cell

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Why graphene is so important?

Graphene has been identified asa promising material for severalapplications

• large theoretical specific surface area ( ≈2630 m2g-1)

• high intrinsic mobility (200000 cm2v-1s-1)

• high mechanical strength (Young modulus ≈1.0TPa)

• thermal conductivity (≈5000 Wm-1K-1)

• electrical properties in planar direction

(sheet resistance ≈280 Ω cm-2)

• chemical stability flexibility

• optical transmittance (≈97.7%).

Unique combinationof remarkable properties

• Electronic devices

• Energy conversion devices

• Solar cells (DSSCs, …)

• …

• Sensors:

• Electronic sensor

• Gas detection

• Organic sensor • …

• Catalysis process

• Realization of innovative membranes

Chemical or physical functionalization of graphene

improvement and/or the modification of graphene properties by subjecting the

Improve or modify its properties

Ability to extend or to optimize its potential applications

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Chemical Vapour Deposition

Industrial applicationsObjective: production of large-area uniformgraphene films of high structural quality at low cost

Synthesis procedures

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Common synthesis procedures

Mechanical exfoliation

Reduced graphene oxide

Epitaxial growth on SiC substrate

few steps (including the oxidation of graphite, dispersion of the graphene-oxide layers in a solvent, and reducing it back to graphene) allow to obtain good graphene sheets

Highly Ordered Pyrolytic Graphite (HOPG) is mechanically exfoliated in individual planes A controlled sublimation of silicon from

the SiC crystal at temperatures around 1500°C permit to produce large area graphene. This graphene is 1-2 atomic monolayers thick and can completely cover a wafer of several cm2.

No large area graphenecan be obtained!

Too expensive!

Chemical Vapour Deposition is the right choice!

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Chemical vapour deposition (CVD)

CVD approach for graphene growth on metal substrates:

• relatively simple process;

• scalable to industrial scale for large graphene film synthesis

• allows to get reasonable material quality

Ultra high vacuum (UHV), low or ambient pressure conditions

Metallic substrates (monocrystals, polycrystalline foils, film deposited, …)

Best growth conditions

(to obtain large graphene areas)

General growth conditions

Ambient pressure deposition

Copper foil catalyst (low carbon solubility)

Gas carbon sources: ethane, methane, ...

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Chemical vapour deposition (CVD)

Annealing step:

reduces the substrate native oxide

forms large metal grains on the surface

encourages carbon atoms deposition

Growth step:

hydrocarbon pyrolysis of carbon species on the catalyst surface

Dissociation of carbon species and grapheneformation on the substrate

Cooling step:

Fast or slow cooling and the proper gas mixture (H2 and/or H2) influence the deposition quality.

During the CVD process, thermodynamic and cinetic parameters play a key role on graphene properties

• Annealing temperature and time

• Proper gas flows mixture

• Cooling modalities and time

In our case, graphene is growth on copper foil substrates by CVD at ambient pressure

• No pumping system

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Optical microscope (OM): the first type of microscope which uses visible light combined with a system of lenses to magnify images

Microscopy techniques

Scanning electron microscope (SEM): a particular electron microscope that produces images of a sample by scanning it with a focused beam of electrons

The most common mode of detection is by secondary electrons emitted by atoms excited by the electron beam.

Samples can be observed in high vacuum, in low vacuum, in ambient conditions and controlled temperature

The electrons interact with atoms in the sample

Signals produced and detected contain information about the sample's surface

topography and composition

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It is a technique widely used to investigate the chemical composition of a surface and provides information about the occupied electronic states. The information gained from XPS analysis are related to the binding energies of the occupied electronic levels, intensities and shapes of the various structures

Schematization of a photoemission experiment XPS Chemical Shift, C 1s Shake-up process

Chemical shiftThe EK of the electrons emitted depends on the bonding of the atoms involved

X-ray photoelectron spectroscopy (XPS)

EB = hν - EK - eφBindingenergy

Kinetic energy

Work function

Energy of the incident radiation

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Raman spectroscopy

It is a spectroscopic technique used to observe vibrational, rotational, and other low-frequency modes in a particular system

This energy shift gives information about the vibrational modes in the system

Laser light interacts with molecular vibration (phonos or other excitations)

The energy of the laser photons being shifted up or down

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Aim of this work

Investigate the role of residual oxidizing contaminants on graphenesynthesis growth by CVD at ambient pressure

• Hydrogen gas amount during growth and during cooling step are varied• Two cooling rate (fast and slow) are used

The realquestion is

Can we obtain defect-free graphene islands through APCVD?

The presence and the effect of residual oxidizing contaminants are indirectly investigated through the suppression of H2 during the growth and/or cooling steps

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Experimental section: graphene growth

Copper foil (99.9% purity; 50-um thick) cleaning procedure (before deposition):

sonication in acetone and then in isopropanol for 15 min, and finally blown dry with nitrogen. Afterwards, treatment with acetic acid at 35 °C for 10 min and then blown dry again with nitrogen gas

After, the substrate is introduced in a quartz reactor, then placed in the hot furnace (at 700°)

Furnace temperature versus time graph for the synthesis process, with the corresponding Ar, H2, and CH4 flows

The cooling is performed under 500 sccm of Ar with or without H2, either rapidly (the tube is extracted manually outside the furnace) or slowly (5 C/min between 1000 C and 700 C, then rapid cooling between 700 C and room temperature)

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DepositionCH4 (0.5sccm)

AnnealingCH4

(0,5 sccm)

After deposition on copper foil, graphene is transferred onto SiO2 (300-nm-thick)/Si wafers by the usual method based on polymethyl methacrylate (PMMA)

1. A PMMA layer is spin-coated at 3000 rpm for 1 min

over the front side of the graphene/copper sample

and baked on a hot plate at 100 C for 5 min

2. After protecting the PMMA film, graphene grown

on the back face is removed by oxygen plasma

(50W for 5 min). Then, the copper foil is immersed

overnight in aqueous ammonium persulfate.

3. The floating PMMA/graphene film is rinsed in

distilled water, transferred to a SiO2/Si piece, and

left to dry in air

4. A second PMMA layer is spin-coated over the first

one under the same conditions

5. Finally, PMMA is removed by dipping the sample

into acetone

Graphene transfer

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Results: optical microscope analysis

graphene islands became visible on the substrates

Optical microscope is a simple and very fast technique to visualize graphene on copper foil

Optical pictures (60X magnification) of two samples grown with H2and cooled slowly with Ar (a) or Ar + H2 (b), before (top images) andafter treatment (bottom images), respectively, on the heating plate

Graphene protecs metalsurfaces against oxidation

Before thermaltreatment

After thermaltreatment

Red region: bare copper

“White” region: graphene-covered20 µm 20 µm

20 µm 20 µm

Small graphenedots

copper foils are baked on a heating plate at 150 °C for 5 min in air

Cu grainboundaries

Cu grainboundaries

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Grapheneis dramatically restrained without H2, since the presence of H2 during slow cooling should protect

Results: optical microscope analysis

Samples in which H2 is employed duringgrowth step and slow cooling process(100/10/S , 100/10/F respectively) or notemployed in fast cooling step (100/0/F)

(a)Sample grown with H2 and cooled rapidly, with H2 gas

20 µm 20 µm 20 µm

Small graphenedots

Large grapheneislands

Bare copper

(b) Sample grown without H2, and cooled slowly with H2

(b) Sample grown without H2, and cooled slowly without H2

No grapheneterraces or dots

OM investigation

Show large graphene terraces on Cu foil

Samples in which H2 is not used duringgrowth step and cooling process (100/0/S ,0/0/S , 0/10/S respectively)

Do not show large graphene areas,but at most small graphene dots

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Core level spectra are recorded from carbon (C 1s), oxygen (O 1s), and copper (Cu 2p).

The samples are not contaminated because containing oxygen, carbon, and copper structures

Results: XPS analysis

Typical XPS spectrum of graphene growth on copper foil

weak variabilityin the C coverage

wide spreads on the degreeof oxidation and on the C amount for the 100/0/S samples

concentration mean value

standard deviation

C average concentration is drastically reduced and O concentration increases nearly fourfold

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• C-C bond is predominant (~ 90%) ,with a small dispersion

• the remainder bonds (amorphous and oxidized carbon) could probably be associated to graphene flake edges, contaminations, etc.

• Sample is heavily oxidized and amorphized.

• C-C sp2 fraction is smaller than 50%

• Large dispersions in the concentrations of amorphous and oxidized carbon

Comparison between C1s typical spectra of 100/10/S and 100/0/S samples

Black line: acquired spectra

Results: XPS analysis

Coloured lines: fitting components

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Cu 2p spectrum of sample 100/0/S (blue line):

• shake-up satellite peaks (two overlapping peaks at 942.4 and 944.6 eV, and one at 963 eV).

• Moreover, the shoulder (BE 934.7 eV) on the Cu 2p3/2 peak centered at 932.6 eV is relevant to Cu(OH)2

Results: XPS analysis

Cu 2p spectra of samples 100/10/S and 100/0/S

Cu 2p spectrum of sample 100/10/S (red line):

• Cu 2p 3/2 (BE=932.7 eV) and a Cu 2p 1/2 peak about 20 eV higher in energy.

• No CuO occurrence (no high intensity shake-up peaks)

very similar to metallic

identification of the Cu compounds from the O

O peaks but it is not essential to

The small oxygen atomic concentration should plausibly be distributed between ‘‘CO’’ and ‘‘Cu(exposure of the copper surface to air after growth), even though the Cu 2p does not indicate signs of it.

XPS investigation

Shake-up structures

C1s spectra of graphene samples inwhich H2 is used in the growth processand in the cooling steps (fast or slow)

no oxidation or amorphization effect

oxidizing and etching action of the residual contaminants is apparentAll other C1s spectra

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Impurities in APCVD

Where do these contaminants come from?

A way to improve the process, without add a pumping system, is to properly use hydrogen flows in the CVD process

The presence of contaminants influences the structural quality of graphene

In such a rudimentary system, the occurrence of oxidizing species is inevitable?

No leakages in the gas circuit

Presence of water in the reactor is removed from high annealing temperature

Presence of air in the tube placed in the hot furnace (residual O2)

Trace amounts of oxidizing impurities in the gas cylinders of H2 and CH4 (99.9 % and

99.5% purity, respectively)

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The overall effect of H2 during the growth

and during the cooling steps is very beneficial

H2 plays an important role in the growth of highly pure graphene areas

• Acting as a co-catalyst promoting hydrocarbon decomposition and thereby graphene formation;

• reduce the oxidized copper surface to enable the growth of graphene

• etching reagent

Role of hydrogen in APCVD graphene

Preserve graphene layer against etching by the residual impurities at high temperatures

In the absence of H2, graphene formation isinhibited by the oxidizing impurities (mainly O2)

Remove the oxidizing species from the furnace, inhibiting their etching effect

growth slow cooling

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Results: SEM analysis

Copper foil substrate

5 µm

SEM investigations show the surface morphology of the samples

1 µm

(a) Scanning electron microscopy image of graphene hexagonal flakes on copper foil (Sample 100/10/F). (b) Zoom on a monolayer graphene hexagon

Graphenedomains

Graphene domain with the nucleation seed visible in the center

Monolayer graphene hexagon

Graphene structures have random orientations

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Results: Raman analysis

Raman spectroscopy is used to check the structural quality of the graphene hexagonal domains

The Raman measurements are limited to the hexagons with the weakest contrast

The disorder-related D band (~ 1350 cm-1) is not observable

Graphene domains of high quality with minimal defects

The non-perturbed G band is slightlyupshifted from 1582 cm-1 to 1590 cm-1)

This technique gives information on:

• Deposition quality

• Number of graphene layers

• Interaction between substrate and layer deposited

• Chemical doping of graphene islands

• ……

Residual strain from the copper substrate and unintentional doping

The light hexagons are monolayer graphene islands

G /G’ band ratio is upper than 1.5 Monolayer graphene

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Conclusions

Role of H2 during the growth or the cooling process

Growth of graphene on copper foils with CH4 by APCVD

Small amount of H2 or fast cooling

Slow cooling of the copper foil without H2

low carbon coverage with heavily oxidized and amorphized graphene

Appreciable graphenecoverage

Using a flow of H2 gas during graphene grown step and during cooling process monolayer graphene terraces can be obtained

residual oxidizing impurities are limited

Ambient Pressure Chemical Vapour Deposition allows to obtain, under the appropriate conditions, the formation of defect-free graphene islands

Growth process

Effects of oxidizing contaminants

Graphene growth step using H2

Mono - few layers graphene islands

Graphene growth step without H2 flow

No graphene large areas

Cooling step

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Thanks for your time!

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