On the Structural, Morphological, and Electrical Properties of Carbon Nanowalls Obtained by Plasma-Enhanced Chemical Vapor Deposition
In this study, we investigated the morphological, structural, and electrical properties of carbon nanowall (CNW) structures obtained by plasma-enhanced chemical vapour deposition (PECVD) and underlined the induced effects of argon/nitrogen (Ar/N2) postsynthesis plasma treatment on the electrical behaviour. The top view and cross-section scanning electron microscopy micrographs revealed that the fabricated samples are about 18 μm height, and the edges are less than 10 nm. The Raman analysis showed the presence of the specific peaks of graphene-based materials, i.e., D-band, G-band, D-band, 2D-band, and D+G-band. The average values of the electrical resistance of fabricated samples were evaluated by current-voltage characteristics acquired at room temperature, in the ranges of 0 V–0.2 V, and an increase was noticed with about 50% after the Ar/N2 postsynthesis plasma treatment compared to pristine samples. Moreover, the Hall measurements proved that the obtained CNW structures had p-type conductivity (Hall coefficient was 0.206 m3/C), and the concentration of charge carriers was cm-3, at room temperature.
Nowadays, almost all allotrope forms of carbon are used in fields such as electronics, energy, medicine, and pharmaceutics [1–3] because of their large variety of dimensionality and structure, e.g., diamond/graphite has three-dimensional (3D) architecture, carbon sheets are two-dimensional (2D), wires/tubes are one-dimensional (1D), and fullerenes are zero-dimensional (0D). In 2002, an innovative configuration is reported by Wu and his collaborators  which consists of nanostructured carbon sheets that stand vertically on the substrate [5, 6]; this architecture was denoted carbon nanowalls (CNWs). Like the case of other allotropes of carbon, the versatility of CNWs for various applications was proved by the customization of their physical and chemical properties, e.g., the morphology of samples can be easily controlled by the concentration of carbon atoms , and the surface can be tuned to be hydrophilic or hydrophobic by chemical and thermal treatments performed during the growth process . Due to their unique configuration of graphene-like layers vertically aligned on the substrate  and of the presence of curly sharp edges, CNWs have a high aspect ratio as around 100 and a large surface area of about 1000 m2/g [4, 10]. Also, because the graphene layers are interconnected, the CNWs are characterized by good electrical and thermal conductivity .
The first CNW synthesis reports were made by Wu and his collaborators , using microwave plasma-enhanced chemical vapour deposition (MWPCVD). Later, other types of plasma were used such as radiofrequency capacitive coupled plasma-enhanced chemical vapour deposition (RFCCPECVD) and direct current plasma-enhanced chemical vapour deposition (DCPECVD) [10, 12–17], and hot-wire chemical vapour deposition [18, 19]. Thus, one of the most frequently used methods to deposit CNWs is plasma-enhanced chemical vapour deposition (PECVD) because of its high degree of reproducibility, large-scale synthesis, good homogeneity of samples, and relatively low working temperature [20, 21]. For the majority of the CVD-based growth methods, the injecting gas is argon (Ar) because it has high excitation and ionization potential and it is inert, but oxygen, hydrogen, and aromatic hydrocarbons are also used . The presence of the last two components favours the increasing of both the density of carbon dimers and the degree of graphitization . Likewise, other reactive gases or mixtures have been reported for the growth of CNWs such as methane/hydrogen [23, 24], argon/methane/hydrogen , argon/acetylene/ammonia , or acetylene/ammonia .
In this paper, we report the morphological, structural, and electrical properties of fabricated CNW structures by PECVD either on individual platinum thin film or interdigitated platinum electrodes, and we discuss the induced effects of argon/nitrogen (Ar/N2) postsynthesis plasma treatment on their electrical behaviour.
While the electrical properties of such nanostructures are essential for various applications such as sensors  and supercapacitors , the number of reports is scarce or few because of one of the challenges that have to be overcome, namely, contacting the sample. In this paper, for the characteristics, one electrode has contacted platinum, and with the other, we contacted the top of the CNWs by a thin layer of gold.
2. Materials and Methods
Onto silicon oxide (SiO2) substrates covered with 5 nm titanium buffer layer (Ti), commercially available, a solid film of platinum (Pt) or Pt interdigitated (Ptdig) electrodes of 283 nm thickness were grown by direct current (DC) magnetron sputtering using a 108auto/SE Cressington system. The working parameters were 0.6 Pa working pressure; 40 mA and 220 V working current and voltage, respectively; and 24 min deposition time. The target had a 1-inch diameter and 99.999% purity (FHR Company). Prior to starting the platinum growth process, the base vacuum in the deposition chamber was Pa. Initially, the SiO2/Ti substrates were ultrasonically cleaned by acetone, methanol, and deionized water; each procedure was 10-minute long, and they were dried under nitrogen (N2) flow. The use of platinum as the electrode for various junctions with CNWs is frequent because the Pt/CNWs have a very stable chemical and physical interface . The SiO2 substrates have dimension, p-type electrical conductivity, and (100) preferential growth direction. By using a mask of , carbon nanowall (CNW) structures were grown by plasma-enhanced chemical vapour deposition (PECVD). The detailed growth procedure of the CNW structure can be found in , but for this study, the working time was 1 hour. Two types of CNW architectures denoted here CNWs-1050 and CNWs-1400 were fabricated onto the SiO2/Ti/Ptdig structure by using two different gas mixture ratios of Ar/H2/C2H2 as 1050/25/2 sccm and 1400/25/2 sccm, respectively. For the SiO2/Ti/Ptfilm structure, CNWs-1400 was deposited by the same working routine as for SiO2/Ti/Ptdig/CNW configuration, and a gold (Au) thin layer of 75 nm thickness completed the arrangement. The same growth conditions and the same equipment as for the platinum were used, but the working time was adjusted to obtain the appropriate thickness. The dimension of the gold target, its purity, and the provider company are identical to those of the platinum target. A schematic representation of the fabricated structures is presented in Figure 1.
After the fabrication of SiO2/Ti/Ptdig/CNWs-1050 and SiO2/Ti/Ptdig/CNWs-1400 structures, the samples were subjected to a plasma treatment consisting of 100/10 sccm gas mixture of Ar/N2 for 30 min. The purpose of such a postsynthesis operation is the functionalization of CNWs by accommodating active radicals based on nitrogen.
The morphological features of CNW structures were investigated by scanning electron microscopy (SEM) by using an Apreo S ThermoFisher machine, with the maximum resolution of 0.7 nm. The working voltage and pressure were 10 kV and Pa, respectively. The structural characteristics were recorded by a Jobin Yvon Raman spectrometer (T 64000), with an excitation wavelength of 514 nm and the laser power of 100 mW. The electrical behaviour of the SiO2/Ti/Ptfilm/CNWs-1400/Au structure was evaluated by current-voltage characteristics () at room temperature in the ranges of -0.5 V–+0.5 V.
The experimental set-up consisted of a Keithley 2400 source meter and a Keithley 6517a electrometer, assisted by a computer. Moreover, for a complete electrical characterization, Hall effect measurements (6 A current and 0.7 T magnetic field, room temperature) were performed, and the Hall coefficient and the concentration of charge carriers were evaluated.
For the SiO2/Ti/Ptdig/CNWs-1050 and SiO2/Ti/Ptdig/CNWs-1400 architectures, characteristics in the ranges of 0 V–0.2 V were acquired at room temperature, before and after the Ar/N2 treatment, and the values of the electrical resistance were determined. Besides, by using resistance-voltage curves, the dark conductivity of pristine samples was evaluated and discussed.
3. Results and Discussion
The scanning electron microscopy (SEM) top view micrographs of pristine CNWs-1050, CNWs-1400, and CNWs-1400 covered by the gold thin layer are presented in Figure 2 together with the cross-section of the SiO2/Ti/Ptfilm/CNWs-1400/Au structure. The evaluated height of fabricated CNW structures was about 18 μm, while the width of edges is less than 10 nm.
By analyzing the morphological results, the determined side length of the CNWs-1050 structure is less than 1 μm, and for CNWs-1400, it easily exceeds 1.1 μm, while it is obvious that the latter configuration has a smaller density of edges than the first one. The increase of the gas mixture concentration leads to an increase of the pressure in the working chamber, so the plasma starts to become unstable and the CNW growth process is affected by a reduction of the deposition rate .
Moreover, one may easily notice that the gold thin film entirely covers the obtained CNWs-1400 structure, but despite this, the formation of clusters is reduced and a lot of individual edges can be still observed. This property is an intrinsic one, and it is related to the self-support feature of such architectures . As expected, the large value of the height of CNWs is strongly correlated with the working time . Due to high values of working temperature and growth power, i.e., 700°C and 300 W, respectively, the sheets of carbon are forced to curl up on a vertical form, and also, the deposition rate is high [7, 30].
The Raman spectra of fabricated CNWs-1400 are shown in Figure 3. Five peaks, D-band, G-band, D-band, 2D-band, and D+G-band, were identified for the obtained samples. Except for the 2D-band which is slightly shifted to the infrared region of the electromagnetic spectrum, all the other peaks have typical values for CNWs. The D-band, located at 1350 cm-1, is the signature of the inelastic scattering of transverse optical phonons on the structural point-like defects, and the G-band at 1580 cm-1 is specific to the ring structure of graphite , indicating the presence of C–C bonds .
The D-band, which appears as a shoulder of G-band, is associated with the finite size of graphite crystallites and, sometimes, with graphite edges [32–34]. The 2D-band was identified at about 2700 cm-1, and it is considered the second-order D-band; it proves that the structures are well oriented and contain also amorphous carbon. This assumption (the existence of 2nd-order resonance bands) was confirmed also by the presence of D+G-band at 2930 cm-1.
The ratio between and , calculated as the ratio of the area intensities of the respective peaks, gives information about the disorder degree of the structure [33–35]; if this ratio is larger than 1, then the density of defects is high. For this study, the evaluated value of was 1.46, while the value of was 0.16, which is less than 1, typical for multilayer graphene structures .
The electrical behaviour of the prepared SiO2/Ti/Ptfilm/CNWs-1400/Au structure was evaluated by current-voltage () characteristics in the ranges of -0.5 V–+0.5 V, at room temperature, and the results are presented in Figure 4. Taking into account that the work function of graphene or other graphite derivative materials is higher than the work function of the silicon substrate or silicon oxide substrate, at the Ptfilm/CNWs-1400 interface, a Schottky barrier limiting the transfer of electrons to anode is created. On the other hand, the CNWs-1400/Au interface behaves like an Ohmic contact. By analyzing the characteristics from Figure 4, the semiconductor behaviour of the whole SiO2/Ti/Ptfilm/CNWs-1400/Au junction can easily be understand, despite the fact that usually, the characteristics of graphene are similar with those of metals  because the bandgap is zero, and the Fermi level coincides with the valence band; the common point of the valence band and conduction band is called the Dirac point .
Hall measurements were conducted to determine the electrical characteristics of fabricated SiO2/Ti/Ptfilm/CNWs-1400/Au structures. The Hall coefficient, , was calculated to be 0.206 m3/C, and the carrier concentration was cm-3. The positive value of indicated that the fabricated samples have p-type electrical conduction, at room temperature. Even though the values are slightly smaller, these results are similar to others from literature; the buffer layer was changed as indium tin oxide  or titanium nitride .
In order to evaluate the effects induced by the Ar/N2 postsynthesis plasma treatment on the electrical behaviour of SiO2/Ti/Ptdig/CNWs-1050 and SiO2/Ti/Ptdig/CNWs-1400 architectures, the current-voltage characteristics in the ranges of 0 V–0.2 V were acquired at room temperature, and the obtained results are shown in Figure 5.
One can observe that after the Ar/N2 plasma treatment, the average values of the electrical resistance increases, for CNWs-1050 from 2.7–2.9 Ω to 4.1–4.2 Ω and for CNWs-1400 from 5.2–5.3 Ω to 12.2–12.3 Ω. This observation should be correlated with other obtained results which proved that one of the effects induced by Ar/N2 plasma treatment is the thinning of the walls , so the increase of the overall electrical resistance. The evaluated values of average electrical resistance for both SiO2/Ti/Ptdig/CNWs-1050 and SiO2/Ti/Ptdig/CNWs-1400 pristine structures are similar to other results from literature, i.e., Itoh et al. reported 4.05 Ω average electrical resistance for a structure of about 1 μm height .
The dependence of the electrical resistance on the applied voltage for SiO2/Ti/Ptdig/CNWs-1050 and SiO2/Ti/Ptdig/CNWs-1400 samples is presented in Figure 6.
To better understand the electrical properties of such structures, the dark conductivity of SiO2/Ti/Ptdig/CNWs-1050 and SiO2/Ti/Ptdig/CNWs-1400 pristine structures was calculated, and the average values were 30.9 S/cm and 25.7 S/cm, respectively. Similar results were found by Itoh et al. and Jun Cho et al.
Carbon nanowall structures were fabricated by plasma-enhanced chemical vapour deposition either on individual platinum thin film or on platinum interdigitated electrodes, and their morphological, structural, and electrical properties were discussed. For SiO2/Ti/Ptdig/CNW configuration, two different Ar/H2/C2H2 gas mixture ratios were used: 1050/25/2 sccm and 1400/25/2 sccm, respectively. The scanning electron microscopy cross-section micrograph revealed that the height of prepared samples was about 18 μm, and the width of edges was less than 10 nm. The Raman spectra showed the presence of five peaks, D-band, G-band, D-band, 2D-band, and D+G-band, typical for CNW structures. The presence of the 2D-band and D+G-band confirmed the incomplete graphitization during the growth process and the presence of amorphous carbon. Moreover, because the value of the ratio was higher than 1, we admit that the density of structural defects is high and it can be assumed as an effect of the large height.
The electrical behaviour of CNW structures was evaluated by current-voltage characteristics acquired at room temperature, and for SiO2/Ti/Ptfilm/CNWs-1400/Au structure, a diode-like response was observed, while for SiO2/Ti/Ptdig/CNWs-1050 and SiO2/Ti/Ptdig/CNWs-1400 samples, a graphene-like behaviour. Also, in order to evaluate the effects induced by postsynthesis Ar/N2 plasma treatment, the average values of the electrical resistance were determined, and for both cases, an increase of these was observed. The Hall measurements acquired for SiO2/Ti/Ptfilm/CNWs-1400/Au architecture showed a p-type electrical conductivity and a carriers’ concentration of cm-3.
Data is available from http://xa-s.fizica.unibuc.ro/bb/Bogdan/List.php.
Conflicts of Interest
The authors declare that there is no conflict of interest regarding the publication of this paper.
Financial support from the Executive Unit for Financing Higher Education, Research, Development and Innovation (UEFISCDI) through PN-III-P1-1.1-PD-2019-0540/103PD/2020, grant no. PN-III-P1-1.2-PCCDI-2017-0637/33PCCDI/2018, PN-III-P1-1.2-PCCDI-2017-0172/15PCCDI/2018, and grant no. 40PCCDI/2018 is acknowledged.
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