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Graphene Preparation and Characterization
Generalities
Most of the performances of graphene based devices rely on its surface characteristics. However, required physical properties and the overall quality of the graphene films differ depending on the various applications. For example, graphene with a highly crystallized grain and minimized structural defects are favorable for graphene transistor while some surface defects are usually preferable for gas sensing applications. Hence preparing appropriate material is of great importance for a certain device application. In this chapter several commonly used synthesis methods are introduced among which, one method which was originally developed in LPICM [1]. Once synthesized, it is necessary that graphene films are characterized to understand its electrical and physical properties for further device applications. There are several well-known common techniques to characterize physical properties of 2D materials such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), Raman spectroscopy, photoemission spectroscopy (PES), optical microscopy, optical transmittance and so on.
SEM and AFM are choice techniques for investigating the surface of materials at nanoscale. They can reveal the morphology and the thickness of the material but it becomes not clear when the graphene sheet is homogeneous and continuous as we merely find structural deformations. TEM provides useful information about material crystallinity by investigating its cross-section. But it requires destructive pre-treatment of the sample and the measurement is expensive and time consuming. Raman spectroscopy is the most powerful technique for non-destructively characterizing graphene and a simple measurement brings fruitful information about graphene, especially about its quality. ARPES is commonly used for studying material‟s energy dispersion respect to its momentum. Optical microscopy can be simply used to locate graphene layers by comparing the image contrast and optical transmittance measurement. It is utilized, for example, for counting the number of layers. In this chapter we will closely look at some of characterization techniques used for investigating the properties of graphene used in the experiments.
Preparation
Exfoliated graphene
Structurally graphite allotrope is formed by stacking many layers of graphene sheets. In principle, each layer of graphite is stacked by week Van-der-Waals force and therefore graphene can be detached by breaking these bonds. Hence the exfoliation of graphite to produce graphene has been investigated by various experimental approaches including mechanical and chemical methods.
Mechanical exfoliation is the process where the best quality graphene, in terms of structural integrity and electronic performance, can be obtained. In this process, graphene is separated by peeling off a layer from commercially available Highly Oriented Pyrolytic Graphite (HOPG) sheet by using scotch tape then transferred onto the substrate. The schematic diagram of the mechanical exfoliation procedure by using scotch tape is represented in the Figure 2.1.. Thanks to its excellent quality with simple and easy process, many fundamental studies have been carried out on mechanically exfoliated graphene since its first isolation by Manchester group [2]. However, graphene produced in this way has a limitation in sheet size (~micrometers) and low production yield which are sometimes crucial drawbacks for device applications.
Figure 2.1. Micromechanical exfoliation of 2D crystals. (a) Adhessive tape is pressed against a 2D crystal so that the top few layers are attached to the tape (b). (c) The tape with crystals of layered material is pressed against a surface of choice. (d) Upon peeling off, the bottom layer is left on the substrate [3].
On the other hand, high production yield can be achieved by chemical exfoliation method (Figure 2.2.). In this process [8], graphite is first soaked in mixtures of sulfuric and nitric acid. Then the acid molecules penetrate into the graphite to form alternating layers called graphite intercalate compounds (GICs) that expands the interlayer spacing between graphene sheets. Rapid evaporation of these GICs at high temperature eventually produces exfoliated graphene sheets [4-6]. Although chemically exfoliated graphene promises high production rate, chemical treatment during the process generates structural defects [7] that may affect the intrinsic electronic properties of graphene as well.
Figure 2.2. Experimental procedure for preparation of graphene by liquid phase intercalation and exfoliation of graphite [8].
Epitaxial graphene
Graphene can also be epitaxially grown. In this case, the graphene layer is obtained by decomposing the hexagonal single crystal silicon carbide (SiC) surface layers to graphene at high temperature [9] as illustrated in the Figure 2.3.. On the {0001} surfaces of the hexagonal SiC single crystal, there are both Si-terminated (0001) and C-terminated (000 ̅ ) faces. Si-terminated face is usually preferable for obtaining better homogeneous graphene growth. Furthermore, a carbon buffer layer is produced through the epitaxial graphene growth on the Si-face which is electrically inert [10,11].
Figure 2.3. Basics of graphene growth by thermal decomposition of SiC, together with the structural model of bilayer graphene on SiC. Shown as the blue broken line is the buffer layer [12].
The epitaxial grown graphene on SiC has the primary advantage that it does not require transfer process since SiC itself serves as a good insulator. Graphene directly grown on insulating substrate becomes compatible with the Si-based standard microelectronic technologies for large integrated circuits with dense transistors [13].
In 2007, Zhou et al. reported an interesting characteristic of graphene epitaxially grown on SiC substrate. In that case, the graphene-substrate interaction caused a band gap opening of up to ~0.26eV with less than 4 layers [14]. This band gap enables graphene to be useful as an electronic material especially for field effect transistors (FETs). However, because of the process complexity difficulties such as determining the face and edge-termination (silicon or carbon) of the substrate and controlling the decomposition rate, precise control of growth conditions for graphene on SiC substrate remain up to day a very important challenges.
CVD graphene
Single or few layer graphene can also be synthesized on the surface of transition metal (e.g., nickel, copper, cobalt, palladium) substrates by chemical vapor deposition (CVD) process. In this process, carbon-containing gases, usually methane (CH4), are used as precursors. At high temperature (e.g., at ~1000 ˚C for Ni), compared with room temperature, relatively large amount of carbon atoms can be dissolved into catalyst metal. Then during the cooling step, excessive dissolved carbon atoms will diffuse out to the surface of metal substrate which results in graphene layers by surface segregation of carbon [15]. On the other hand, generally, higher quality of single layer graphene is expected to be grown on the Cu substate rather than Ni substrate because of its larger grain size with low carbon solubility compared with Ni subsrate [16]. Low carbon solubility of Cu (0.001~0.008 wt.& at 1,084 ˚C [17]) allows only the surface reaction with gases thus a single graphene (SLG) layer is formed by self-limiting mechanism on the substrate. Brief illustration of the CVD process with Cu substrate [17] is represented in Figure 2.4..
Figure 2.4. Schematic illustration of three main stages of CVD process for graphene growth:
(a) copper foil with native oxide; (b) the exposure of the copper foil to CH4/H2 atmosphere at 1000 ˚C leading to the nucleation of graphene islands; (c) enlargement of the graphene flakes with different lattice orientations [17].
Depending on various process conditions such as temperature, gas flow rate, metal substrate and cooling rate, the quality and the properties of synthesized graphene vary significantly. For example, SLG is usually synthesized on Cu substrate because of its self-limiting characteristic where the growth stops after one graphene layer is formed at the surface [16]. The Figure 2.5. shows the Raman spectra of graphene synthesized on Ni substrate with different cooling rates studied by Qungkai Yu et al. [18]. They investigated three different cooling rates corresponding to fast (20 ˚C/s), medium (10 ˚C/s) and slow (0.1 ˚C/s). The result shows that the optimized cooling rate exists for obtaining better quality graphene by CVD process where the medium cooling rate is the case for their study.
Figure 2.5. Raman spectra of CVD graphene on Ni substrate depending on different cooling rate [18].
PECVD
Thanks to its process advantages, especially possible large area and low-cost production, CVD process for graphene synthesis is actively studied by many researchers. However, high process temperature near 1,000 ˚C is a critical issue in terms of industrial aspects. Hence, like for CNTs, there have been many efforts to lower the process temperature by employing plasma enhanced CVD (PECVD) process. The process temperature has been lowered to around 500 ˚C. Many groups are still working on it to optimize and simplify the process conditions and steps with the improvement of graphene quality [19-21].
Transfer of Graphene
Even though large area graphene layers can be grown by CVD process, they are still on the catalytic metal substrate surface. Once graphene is synthesized on this metal substrate, it is supposed to be transferred onto the final insulating substrate required for most of electronic device applications. The Figure 2.6. [22] shows the typical procedures of this transfer technique. Graphene synthesized by CVD process on Cu metal layer is covered by a sacrificial polymethyl methacrylate (PMMA) and annealed at first. Cu layer is subsequently removed by wet etching and the remaining graphene attached to the PMMA is rinsed by deionized (DI) water. Then it is put together onto the target insulating substrate and subsequently annealed. Finally, PMMA is selectively removed by acetone and only graphene remains on the insulating substrate [22-24].
Figure 2.6. Transfer of graphene [22]
However, this transfer process has many drawbacks such as surface contamination originating from PMMA [23,24], morphological and structural deformation (e.g., wrinkles, cracks, defects) of graphene layer [25] and it is even quite sophisticated process that requires appreciable time and cost [26]. For these reasons, direct growth of graphene on insulating substrate is desirable in terms of device applications for many researchers. LPICM, by using home-made triode type PECVD system, could directly grow graphene on the insulating substrates. This will be explained in detail in Chapter 3.
Characterization techniques
Scanning Electron Microscopy (SEM)
Scanning electron microscopy is one of simple and fast techniques to verifying the quality of the graphene on the substrate. It is a non-destructive technique so that the characterized graphene can later be used for device applications. The image taken by SEM provides useful information especially for polycrystalline graphene such as its location on the substrate, grain boundaries and structural deformations including wrinkles, vacancies, cracks, and so on.
The principle of this technique lies on the interpretation of secondary electrons generated by the interaction between incident electrons and the target sample. Incident electrons are significantly scattered and therefore, SEM operates in two different modes depending on the scattering type: elastic or inelastic.
First mode, based on the electron inelastic scattering, is the most commonly used way to investigate the surface morphology. SEM collects low-energy secondary electrons coming from the k-orbitals of only few atomic layers [27-29]. Second mode employs the electron elastic scattering where the electrons undergo no energy loss which is called back scattered electron imaging. SEM collects high-energy secondary electrons coming from the nuclei of bulk atoms. This enables us to have an image contrast as a function of elemental composition which is for investigating the existing elements but not for surface morphology. The Figure 2.7. is an example of SEM image for graphene-supported Pt nanoparticles taken as both modes [30]. A typical low-energy secondary electron image is represented in the Figure 2.7.(a) and a corresponding back scattered electron image on the same spot is demonstrated in the Figure 2.7.(b). In the Figure 2.7.(b), Pt nanoparticles are highlighted by brighter regions which are not clearly observable in the low-energy secondary electron image (Figure 2.7.(a)).
Table of contents :
Acknowledgement
List of Abbreviation
Résumé en Français
Chapter I.Introduction
1.1 Overview
1.2 Graphene: Carbon allotropes, Properties and Applications
1.2.1 Carbon Allotropes
1.2.2 Graphene properties
1.2.2.1 Electronic band structure of graphene
1.2.2.2 Electrical Properties
1.2.3 Graphene for device applications
1.2.3.1 Graphene based electrode
1.2.3.2 Graphene based transistor
1.2.3.3 Graphene for Battery
1.2.3.4 Graphene for photonic devices
1.2.3.5 Graphene for gas sensing applications
1.3 Functionalized graphene
1.3.1 Covalent functionalization
1.3.2 Non-covalent functionalization
1.4 Conclusions
References
Chapter II. Graphene Preparation and Characterization
2.1 Generalities
2.2 Preparation
2.2.1 Exfoliated graphene
2.2.2 Epitaxial graphene
2.2.3 CVD graphene
2.2.3.1 PECVD
2.2.3.2 Transfer of Graphene
2.3 Characterization techniques
2.3.1 Scanning Electron Microscopy (SEM)
2.3.2 Atomic Force Microscopy (AFM)
2.3.3 Raman spectroscopy
2.3.4 Photoemission spectroscopy (PES)
2.3.4.1 Generalities
2.3.4.2 X-ray Photoelectron Spectroscopy (XPS)
2.3.4.3 Ultraviolet Photoelectron Spectroscopy (UPS)
2.3.4.4 Angle-resolved photoemission spectroscopy (ARPES)
2.3.4.5 Auger electron spectroscopy (AES)
2.3.4.6 Electron energy loss spectroscopy (EELS)
2.4 Conclusions
Reference
Chapter III. Prepared Graphene
3.1 Prepared graphene
3.1.1 PECVD interfacial graphene growth (LPICM)
3.1.2 Epitaxial graphene (LPN)
3.2 Graphene Characterization
3.3 Conclusions
Reference
Chapter IV. Electrical Characterization
4.1 Device fabrication
4.1.1 Generalities
4.1.2 Graphene (interaction) on the substrate
4.1.3 Contact electrode deposition
4.1.3.1 Inkjet printing
4.1.3.2 Photo-lithography
4.1.3.3 Shadow-mask metal Evaporation
4.2 Four-electrode electrical measurements
4.2.1 Four-probe measurement
4.2.2 Four-electrode measurement
4.3 O2, H2O desorption
4.4 Light exposure response
4.5 Humidity response
4.5.1 Humidity response of the device
4.5.2 Vacuum annealing effect
4.5.3 Surface analysis
4.5.4 Mechanisms of humidity response
4.6 Conclusions
Reference
Chapter V. Functionalization
5.1 Generalities
5.2 Ruthenium Complex(Blue)
5.3 Functionalization of the device
5.3.1 Functionalization of the device
5.3.2 Photoresponse of the functionalized device
5.4 Double interfaces responses
5.4.1 Inversed response.
5.4.1.1 Intensity dependency of the inversed response under vacuum
5.4.1.2 Duty cycle dependency of the inversed response under vacuum
5.4.2 Charge transfer
5.5 Conclusions
Reference
VI. Conclusions & Perspectives
