Large Area C60 Film Obtained by Microwave Oven Irradiation from an Organic Resin

In the present work the synthesis of fullerene thin film produced in a conventional microwave oven from the decomposition of terpenoid is reported. The polycrystalline structure of the sample was determined by X-ray diffraction (XRD); the sample showed several phases, and the main phase corresponds to fullerene ordered in a face-centered cubic structure (FCC), with a lattice parameter a = 14.16 Å, with two more structures: one is orthorhombic system with lattice parameters a = 9.53 Å, b = 8.87 Å, and c = 8.354 Å, and the other is the monoclinic system with lattice parameters a = 10.24 Å, b = 7.80 Å, c = 9.49 Å, and


Introduction
Carbon thin films are important for the development of applications in semiconductors, nano electronics, and aerospace industry due to the physical properties of their crystal structure. These properties are high electric conductivity or semiconductivity, photo conductivity, and optical nonlinearity [1]. Several methods are currently used for the preparation of carbon films [2][3][4][5]. In these methods the films are obtained in temperature conditions at ranges of 950-1250 ∘ C [6] with different energies from 100 to 1000 eV [7] at pressure from 1 to 5 × 10 −7 Torr [8][9][10] using inert atmospheres or carbon gases as control atmospheres with flowing in a continuous way to obtain small area films with thicknesses from 500 nm to 10 000 nm with a crystalline or amorphous structure [11], making this synthesis expensive. Comparing the carbon film precursors at present, the use of organic resins such as terpenoids has proven to be efficient in obtaining carbon films by using techniques such as CVD [12][13][14][15][16][17]. Comparing the chemical precursors used in the synthesis of carbon films, it was observed that organic resins present more advantages than the inorganic precursors because some of these resins are environment friendly [18]. It is important to mention that camphor resin has been successfully used in carbon nanomaterials synthesis and also in carbon films, graphene, carbon nanotubes, and other carbon allotropes [19,20]. However it must be mentioned that the sample amount obtained in these experiments is very small. Based on the previous information it is necessary to look for new synthesis methods which must be not only more effective but also cheaper [21,22]. Therefore the microwave assisted synthesis (MAOS) [23] is a cost-effective alternative technology which reduces the impact on the environment by saving energy, being able to produce materials and microstructures that cannot be performed by other methods [24]. The synthesis of carbon films using camphor has already some history with not very clear results about the crystal structure of the same and the method of synthesis [20,[25][26][27][28], and we believe that it is still a very attractive study and control method variety. The aim of this work was to find the synthesis and microstructural characterization of the carbon films by microwave radiation using the techniques such as X-ray diffraction, scanning electron microscope, high-resolution transmission electron microscopy, and atomic force microscopy.

Microwave Oven Preparation.
The plate was removed from the microwave oven, and the samples were placed in a position where the microwave radiation reaches the maximum. Determinations of maximum and minimum points were done as reported in the literature [24,[29][30][31][32][33]. Resin sample was located in one of the points where microwave radiation has one maximum.

Sample Preparation. For this work 250 mg of camphor
Sigma-Aldrich was placed in a Florence flask because it was observed that this glass's result better than of Pyrex glass under the same radiation condition. The flask volume was 250 mL, and the glass container with camphor was located inside a commercial SANYO microwave oven with a frequency of 2450 MHz. The sample was heat-treated to the maximum power (1480 Watts) for five minutes, until a carbon film was observed through the microwave oven windows. During the heat treatment, the temperature was measured by using an Infrared Thermometer Cole Palmer Mod. 800-323-4340 with LCD display, with a temperature range from −18 to 900 ∘ C.

Sample Characterization.
The film sample was characterized by X-ray diffraction in a Siemens D-500 diffractometer using CuK ( = 1.54Å). The sample was observed with two instruments a scanning electron microscope SEM/FIB NOVA 200 (with point resolution of 1.7Å) and highresolution transmission electron microscopy FEI Tecnai G-20 to 200 kV with resolution of 1.9Å. The micrographs were analyzed using Digital Micrograph Software version 3.7 for GMS 1.2 Gatan Company. Topography was also measured with AFM (JEOL 5200) using a standard scanner (10 × 10 microns in "XY" and 3 microns "Z") with a 20 nm platinumiridium coated silicon tip (Veeco SCM-PIT) with 5 N/m spring constant and 20 nm tip diameter.

XRD Patterns.
The diffraction pattern of carbon thin film is shown in Figure 1. In this pattern many phases were observed and they were identified using a reference database cards ICDD PDF-2 Release 2003. It was observed that the well-defined peaks in this pattern correspond to the highly ordered crystalline structures. In this pattern those peaks are thin and correspond to main phase of the sample which is C 60 fullerene molecule ordered in a face-centered cubic structure which is the phase of higher symmetry. In this pattern a broad peak, in the range between 20 and 26 degrees, can be observed, and this peak is crowned by other well-defined low intensity peaks, corresponding to lower symmetry phases C 60 ordered in orthorhombic and monoclinic structures. Another phase observed was the hexagonal 2H graphite phase. It can be noticed that the presence of these phases may be caused by the difference in temperature in the container and between the sample and glass substrate. A summary of the observed phases is shown in Table 1. Figure 2(a) the SEM electron micrograph of fullerene film is shown. Since graphite tape may cause confusion with the carbon film, which is commonly used to hold samples, the carbon film was supported on a copper tape. In Figure 2(b) it was observed that fullerene film consists of a series of stacked monolayers. The film thickness was measured using FEI Nova Nanolab analysis and imaging software. The film thickness varies from 140.8 to 523.3 nm. A qualitative chemical composition was performed (Figure 2(c)) by electron dispersive spectroscopy (EDS). The sample is mainly composed by carbon (93.88% at) and oxygen (6.12% at). Figure 3, bright field electron transmission micrograph of sample is observed. From this figure, it is easy to observe the crystalline behavior of cubic phase. Two interplanar distances were measured, using the Digital Micrograph program (D.M). The direction indices associated with those d spacings   were [4 0 0] y [3 3 3] and zone axis from plane (011). The buckyball molecule diameter was also measured using the D.M, It was found that molecule diameter value was 6.83Å and corresponds to C 60 molecule diameter.  (Figure 4(c)). In the sample surface, islands deposited crystals found in stages are observed ( Figure 5(a)), having a corresponding symmetry 43m crystal habit associated with the tetrahedron (Figure 5(b)), comprising deriving four faces of the octahedron in class 4/m32/m. And observing a height of the mounds of crystals in the range from 18 to 120.8 nm ( Figure 5(c)).

Conclusions
(i) In this work, it was possible to obtain a carbon thin film from the pyrolysis of camphor in a conventional microwave oven. The film is polycrystalline and consists of fullerenes arranged in different crystal structures, graphite 2H and amorphous carbon. This indicates that the the sample is formed within the furnace in a gradient of temperatures around 800 ∘ C working with maximum power of the oven. The main phase corresponds to fullerene ordered in a face-centered cubic structure and other phases such as C 60 orthorhombic, C 60 monoclinic.
(ii) The surface of the film consists of several monolayers of carbon molecules stacked carbon, even leading material of varying thickness from 140.8 to 523.3 nm, and the sample shows oxidation with 6.12% at.
(iii) The space group of the main phase is Fm3m. In this phase a crystalline tetrahedral habit was observed having 43m symmetry. The carbon thin film shows spiral-shaped tetrahedron single crystals of 535.03 to 345.32 nm and an angle of counterclockwise rotation ranging from 103.7 ∘ to −16.8 ∘ .