The extraordinary properties of graphene make it one of the most interesting materials for future applications. Chemical vapor deposition (CVD) is the synthetic method that permits obtaining large areas of monolayer graphene. To achieve this, it is important to find the appropriate conditions for each experimental system. In our CVD reactor working at low pressure, important factors appear to be the pretreatment of the copper substrate, considering both its cleaning and its annealing before the growing process. The carbon precursor/hydrogen flow ratio and its modification during the growth are significant in order to obtain large area graphene crystals with few defects. In this work, we have focused on the study of the methane and the hydrogen flows to control the production of single layer graphene (SLG) and its growth time. In particular, we observe that hydrogen concentration increases during a usual growing process (keeping stable the methane/hydrogen flow ratio) resulting in etched domains. In order to balance this increase, a modification of the hydrogen flow results in the growth of smooth hexagonal SLG domains. This is a result of the etching effect that hydrogen performs on the growing graphene. It is essential, therefore, to study the moderated presence of hydrogen.
This paper aims to explore additional routes for graphene growth by chemical vapor deposition (CVD) [
Graphene consists of a single atomic layer of carbon with a honeycomb structure [
Graphene has attracted the attention because of its outstanding characteristics. High mechanical strength [
A conventional CVD growth method consists of a continuous flow of carbon gas precursor/H2/Ar mixture in order to generate the graphene growth [
According to the previous results of Li et al. [ Exposure of Cu to methane, argon, and hydrogen. Catalytic decomposition of methane on Cu to mainly form CH
Depending upon the temperature, methane pressure, methane, argon and hydrogen flow, and partial pressure, the Cu surface being obtained undersaturated, saturated, or supersaturated with CH
Formation of nuclei as a result of local supersaturation of CH
Nuclei grow to form graphene islands. Full Cu surface coverage by graphene under specific temperature flow rates and pressure conditions.
In this work we pursue to grow graphene without voids or defects forming large domains, by means of a modified method. Continuous graphene film without defects is desired in order for the material to maintain its exceptional properties and to be useful for applications.
The whole process was carried out in a suitable CVD oven (schematically drawn in Figure
Schematic illustration of GRAPHman reactor for DC plasma and CVD process of the FEMAN group at University of Barcelona.
Polycrystalline copper foil 75
CVD synthesis is performed after the plasma etching and the reduction of copper foil inside a quartz tube surrounded by an oven. Figure
All the steps of the process can be controlled by a computer program; moreover, in order to secure the high purity conditions under which the graphene growth is carried out, the reactor is pumped to high vacuum (down to 6
Various publications have reported the crucial impact of this temperature concerning the CVD synthesis of graphene on copper using methane as a carbon precursor [
During the hydrogen plasma treatment the hydrogen radicals react with the copper oxide reducing it to metallic copper [
Optical emission spectroscopy of hydrogen plasma during reduction of copper: after 1 minute (red line) and after 5 minutes (black line). The reduction of the OH radical peak intensity in the spectral range of 305–330 nm reveals the removal of the oxide layer.
(a) Polycrystalline copper foil before (up piece) and after (down piece) the hydrogen etching. The difference in the surface color indicates the deoxidation of it. The red line indicates 4 cm. EBSD maps of (b) the as-received polycrystalline copper foil (scale bar 50
As an additional tool to characterize the copper surface we used electron backscattered diffraction (EBSD). It is a large area imaging method which detects the different index copper facets. Here, EBSD is used in order to demonstrate the growth of the copper grains after the H2 plasma and the growth process, with respect to the as-received copper foil. In Figure
Scanning electron microscopy was used to evaluate the morphology of the obtained graphene. The micrographs of Figures
Scanning electron microscope pictures of graphene domains on copper foil after (a) 20′ growth. Four lobes with dendritic extension (blue circle) and butterfly-like (red circle) graphene islands are grown with a size of 40
In sample B (Figure
The white spots observed in the pictures of Figure
Taking into account these observations, we performed a graphene growth (sample C, Figure
Experimental conditions of different samples of growth graphene. The variation in the precursor flows, growth time, pressure, and substrate pretreatment are being demonstrated.
Sample | Methane |
Hydrogen flow (sccm) | Total time |
Pressure (Pa) | Pretreatment | Crystal size ( | |
---|---|---|---|---|---|---|---|
A | 5 | 20 | 20 | 20 | Hydrogen plasma etching | ~50 | |
B | 5 | 20 | 40 | 20 | Hydrogen plasma etching | ~60 | |
C | 5 | First 20 min, 20 sccm | Last 20 min, 0 sccm | 40 | 20 | Hydrogen plasma etching | ~60 (full cover) |
D | 5 | 20 | 20 | 20 | Chemical etching | Bad shaped |
The flows of methane and hydrogen are demonstrated as a function of the growth time. Methane flow is stable. Hydrogen flow is stable for sample B. In sample C a 2-step recipe is followed. In the second step the hydrogen flow is stopped. This moderates the hydrogen etching effect, resulting in increase of the graphene surface coverage from 25% to 99% of the total surface.
Raman spectroscopy results, performed with a Jobin-Yvon LabRam HR 800 system, are shown in Figure
(a) Raman spectra of graphene as growth on copper foil (red line), after transfer to glass (black line) (with thermally grown quartz on top of the glass) and in the back side of the foil (green line). After the transfer a small D peak appears in the spectra, probably a result of defects introduced during the transfer. In the spectra obtained directly in the copper foil the background noise of the copper has been removed. (b) This spectrum corresponds to graphene after the transfer on top of SiO2/Si. The characteristics
Although, as the spectrum is taken directly from graphene deposited on the copper without any transfer process, some background noise in the signal appears because of photoluminescence from the copper substrate, a background subtraction during the data analysis was performed to evidence the signal coming from graphene [
The data of the Raman spectra regarding the G and 2D position, the intensity ratio between them, and the
Sample | G position (cm−1) | 2D position (cm−1) |
|
2D |
---|---|---|---|---|
Graphene on copper | 1578 | 2671 | 1,62 | 48 |
After transfer to SiO2 | 1582 | 2679 | 2,5 | 45 |
After transfer to quartz | 1587 | 2679 | 1,74 | 48 |
AFM image of the graphene transferred on top of Si/SiO2. The lobes of the crystal can be distinguished. The roughness reveals good homogeneity and absence of second layer growth. Some cracks are introduced by the transfer process. The white spots are impurities introduced by the PMMA, difficult to get completely cleaned. In the up part we see the profile of a 1
The important role of hydrogen plasma, used for etching the copper foil, was revealed when the growth of graphene was carried out under the same conditions as described for sample A, without any previous ion pretreatment in the copper foil (sample D). This time a precleaning process in a HNO3/distilled water solution is being performed as described in the work of Kim et al. [
Scanning electron microscope picture of graphene grown on copper foil that has not been pretreated with hydrogen plasma etching (sample D). The growth time is 20 minutes. The scale bar has 100
The ability of more sufficient control in the growth of graphene is being explored. The hydrogen flow appears to perform an etching in the graphene domains which affects their morphology and uniformity. The results reveal that a sufficient graphene growth is possible when we optimize the switching of the carbon precursor/hydrogen flow ratio during the process. This reduces the etching effect that the hydrogen is performing, allowing the growth of graphene and the full cover of the substrate by it. Large area graphene is necessary when it comes to large scale applications followed by a careful transfer, to avoid the damaging of the graphene film. The Raman spectroscopy ensures us about the single layer thickness of the graphene film. Performing the Raman measurements on top of different substrates (copper foil, glass, and SiO2) does not affect significantly the shape and ratio of the characteristic graphene peaks. In addition the important role of the substrate pretreatment is being investigated. Performing hydrogen plasma before the growth reduces efficiently and faster the native copper oxide layer so that large graphene crystals can be grown.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The author is supported by the Greek State Scholarships Foundation. The author would like to thank the CCiTUB for the help with the structural and morphological characterization. This work was developed in the frame of the Project 2014SGR984 of AGAUR from the Generalitat de Catalunya and the Projects MAT2010-20468 and ENE2014-56109-C3-1-R of MICINN from Spanish Government.