Graphene on metallic foils

Abstract

This work is focused on a simple method of graphene preparation. The method is based on thermal annealing of a carbon/metal structure, where cobalt and nickel were the tested metals. The technological process is based on an optimization of parameters of annealing (temperature and duration) and on a thickness of carbon layer. A multilayered graphene was prepared; for its preparation, annealing temperature within the range of 600–800°C was sufficient.

Introduction

Graphene is a promising material with excellent electrical and thermal properties. ¹ Many preparation methods for graphene have been developed like epitaxial growth on silicon carbide (SiC), ² synthesis from metal/SiC structure, ³ chemical vapor deposition process ⁴ and other. However, the high price of SiC plates and the application of dangerous gases together with high processing temperature are main disadvantages of mentioned methods. Presented work reports results concerning graphene preparation from the structure carbon/metal where cobalt and nickel thin foils were applied.

Sample preparation

The concerned method is a relatively simple way to prepare graphene layers. The preparation is based on commercially available metallic foils. The following foils were used: Ni foil, thickness 50 µm, purity 99.98% and Co foil, thickness 50 µm, purity 99.9%; both from GoodFellow, Huntingdon, England. The foils were first thoroughly cleaned in acetone and isopropylalcohol. Then, the deposition of a thin layer of amorphous C by the flash method followed; C was evaporated by burning off a C filament by flow of a large current (16 Φ#x0029; under vacuum. The structure C/metal prepared this way was annealed under vacuum at temperatures ranging from 600°C to 900°C, for a period from 1 min to 120 min. Annealing was carried out on the ceramic table (Boraelectric Heating Element HTR-1001, Momentive Performance Materials Quartz GmbH Lubeck, Germany). Next step was transfer of the graphene film onto a dielectric substrate (silicon dioxide (SiO₂)). The metallic foil with the graphene film was covered with a layer of gold, 1 µm thick. Then, the metal was etched off in the mixture of nitric acid (HNO₃)–water (H₂O) in the ratio 1:2 by volume. The etching time was approximately 20 min. The obtained foil of gold/graphene was carefully washed in H₂O and transferred onto the SiO₂/silicon (Si) substrate, where the oxide thickness was 300 nm. Subsequently, the substrate with the transferred foil was dried at the temperature of 80°C. The last step was etching off of the gold layer in the mixture of HNO₃–hydrochloric acid–H₂O in the ratio 1:3:8 by volume for the period of 5 min and final washing of the structure with water.

Graphene was analysed by means of Raman spectroscopy. Raman spectra were recorded in the wavelength range from 1000 cm⁻¹ to 3000 cm⁻¹ using the DXR Raman microscope (Thermo Fisher Scientific Inc., Waltham, Massachusetts, USA) spectrometer equipped with the 532 nm excitation laser and air-cooled charge coupled device detector. The maximal output of a beam was limited to 10 mW, and the recorded point was of 1 μm diameter. X-ray photoelectron spectroscopic (XPS) measurements were performed in ultra-high vacuum (10⁻⁸ Pa) using the ESCAProbe P apparatus (Omicron Nanotechnology Ltd, Germany) equipped with an aluminium anode as an X-ray source with energy 1486.7 eV. The X-ray source was monochromatic. The size of the analysed area was approximately 1 mm². A peak component fitting was performed using symmetric Gaussian–Lorentzian peaks.

For visual representation of surface profile, a scanning electron microscopy (SEM) technique was used (LYRA3 GMU; Tescan, Czech Republic). The applied voltage was 10 kV. The atomic force microscopic (AFM) analysis was conducted in the Veeco CP II (Carl Zeiss South Africa) apparatus in the tapping mode.

Results

In Figure 1, examples of Raman spectra are shown. The basic structure is the Co foil with evaporated layer of C, 46 nm thick. The Raman spectrum of the deposited layer showed that the layer is of amorphous C. The other spectra belong to the structures after annealing at various temperatures; the annealing time was 1 min (measured from the moment when the temperature reached the required value till switching off of the heating table). The spectrum obtained at the temperature of 700°C represents the case, when the crystallization of the C layer begins. By increasing the temperature to 750°C, substantial separation of the D and G bands takes place and the two-dimensional (2-D) band is being formed. The structure of the amorphous C converts into the crystallize state and is of the character of a multilayered graphene. As the size of the D and G bands is equal, it is possible to state that the layer is highly defective. ⁵ After annealing at the temperature of 800°C, no C was detected at the Co foil surface (the respective Raman spectrum has no characteristic C bands). C deposited at the surface of the metallic foil diffuses into a large depth at this temperature. After cooling down, no effective C segregation at the metallic surface takes place and no graphene layer is formed any more. The last spectrum belongs to graphene transferred on the surface of the SiO₂/Si substrate. This is a structure obtained by annealing at the temperature of 750°C for 1 min. The spectrum is very similar to that of the structure annealed at 750°C; the difference is that the size ratio of the D band to the G band is a half. This bears witness of lower defect rate of the transferred layer compared with the original graphene film formed on the Co foil. Reason of the case can be seen in small roughness of the SiO₂/Si substrate in comparison with the metal foil.

Figure 1.

Raman spectra of graphene films prepared on Co foil.

In the course of the experiments, influence of thickness of the evaporated C layer (10, 46 and 72 nm) was followed as well. Figure 2 shows Raman spectra of three graphene films, which are differed by the thickness of C layers deposited on the Co foil (annealed at 750°C for 5 min). At the smallest thickness, we failed to prepare the graphene layers – the spectrum represents only very thin amorphous C layer. Graphene films that have been prepared from the structures with thicker C layer (46 and 72 nm) are very similar. At the C layer thickness of 72 nm, the peak ratio I_2-D/I_G is slightly higher; the graphene film consists of smaller number of C monolayers. When extending the annealing time of the metallic foils, the optimal annealing temperature shifted towards lower values. At annealing time within 30–60 min, graphene was formed at the Co foil surface already at the temperature of 600°C. Similar dependences were found when experimenting with Ni foil, too.

Figure 2.

Raman spectra of graphene films with different thickness of C films deposited on Co foil.

For verification of formation of graphene layers on metallic foils, XPS spectroscopy was used. Figure 3 shows detailed C 1 s peaks obtained from XPS spectra and with fitted components according to the study by Biesinger et al. ⁶ There is illustrated the spectrum of Co foil before annealing (1), spectrum of graphene layer obtained by annealing of the C(46)/Co structure at the temperature of 750°C for 1 min (2) and that of the same structure annealed at the temperature of 800°C for 1 min, where there was no C detected at the surface by Raman spectroscopy (3). The spectrum of the Co foil before annealing contains two components: B with binding energy of 284.88 eV and C with binding energy of 289.17 eV. The B component belongs to normal C compounds adsorbed at the surface of materials, such as the bonds C–H, C–C and C=C; the C component then corresponds to bonds of C with oxygen. ⁷ The spectrum of the structure annealed at the temperature of 750°C can be decomposed into three components: A with binding energy of 284.50 eV, B with binding energy of 285.18 eV and C with binding energy of 288.83 eV. The A component with relatively low full width at half maximum value under 1 eV corresponds to graphitic C. ⁷ The B and C components are equal to the respective ones detected in the spectrum of unannealed Co. The spectrum of the structure annealed at the temperature of 800°C again contains the B and C components together with D component with binding energy 285.95 eV. The last component has probably the same origin as the C component. From these results, it is evident that the graphitic component of C was detected by the XPS spectroscopy only for the structure, for which graphene was detected by the Raman spectroscopy.

Figure 3.

Details of C 1 s peaks from XPS spectra.

An important feature of graphene films is their morphology. In Figure 4, parts of surface of a graphene film prepared at 750°C for 300 s (the thickness of evaporated C is 46 nm) are visualized by various techniques. The figure represents the graphene film that has been transferred on SiO₂ (300 nm)/Si substrate. Figure 4(a) shows a selected section visualized by the SEM method. The area is approximately 150 × 150 μm² large. The surface is covered by defects, which are produced by the transfer of graphene and by imperfections in the process of graphene growth. Figure 4(b) then shows a surface section of about 2 × 2 μm² size, visualized by the AFM method. Left part of the snap represents the surface of SiO₂ (300 nm)/Si substrate without graphene, while right part represents the surface of graphene. The roughness of the SiO₂ surface is R _a = 1.2 nm and the surface shows small tips. On the other hand, the graphene film surface shows globular character with R _a = 2.2 nm and looks like to cover the tips of the SiO₂. The thickness of the transferred graphene film has been obtained from the height of a leap between surface parts with and without graphene; the measurement has been done several times and we have gained the value 8 ± 2 nm. Number of layers in prepared graphene can be calculated by dividing the obtained value by the thickness of one monolayer of C. One monolayer of C in graphite is 0.335 nm thick. In graphene layers, however, the distance of individual layers varies from 0.55 nm to 0.70 nm. ⁸ Taking these data into consideration, the number of C monolayers is varying from 11 to 14.

Figure 4.

Surface morphology of the graphene film transferred on the SiO₂/Si substrate: (a) SEM picture and (b) AFM scan.

Conclusion

This article describes a simple method of graphene preparation, which does not require any expensive technological equipment, costly substrate materials or high temperatures. The method is based on thermal annealing of a C/metal structure, where Co and Ni were the tested metals. At annealing, C diffused into the metal and graphene is prepared by C segregation on the metal surface during cooling down of the structure. A multilayered graphene was prepared; for its preparation, annealing temperature within the range of 600–800°C was sufficient, and the basic structures with evaporated C layers of thickness 46 and 72 nm produced graphene films with similar parameters. Formation of graphene was confirmed by the XPS analysis. The surface morphology of graphene films is influenced by their transfer on the SiO₂/Si substrate. Thickness of prepared graphene films can be estimated from AFM measurement to the value of 8 ± 2 nm.