Study of graphene layer growth on dielectric substrate in microwave plasma torch at atmospheric pressure

Highlights

• Decomposition of ethanol without hydrogen admixture was used to form few-layer graphene layer on dielectric substrate

• Density and size of graphene nanosheets was controlled by ethanol flow rate and microwave power

• High substrate temperature and high flux of C₂ species resulted in transition from horizontal to vertical graphene growth

Abstract

The initial stage of graphene layer deposition on silicon oxide substrate by ethanol decomposition in dual-channel microwave plasma torch at atmospheric pressure was studied in dependence on precursor flow rate and delivered microwave power. Depending on ethanol flow rate and substrate temperature, horizontally or vertically aligned graphene nanosheets with various density could be prepared directly on dielectric substrate. In the regime with high microwave power, above 400 W, mixture of amorphous carbon particles and graphene sheets was deposited on the substrate. Prepared layers were analyzed by scanning electron microscopy (SEM), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). The microwave plasma diagnostics was carried out using optical emission spectroscopy (OES). The sample analysis showed increasing density of horizontally aligned carbon nanosheets with increasing ethanol flow rate and their delamination and transition into vertically aligned graphene sheets with increasing substrate temperature. The Raman spectroscopy analysis of layers showed presence of D (1345 cm⁻¹), G (1585 cm⁻¹) and 2D (2685 cm⁻¹) peaks with 2D/G ratio of 1.59 and full width at half maximum (FWHM) of 2D peak was 42 cm⁻¹, corresponding to few layer graphene structure. In case of amorphous nanoparticles deposition, the D* peak at 1210 cm⁻¹ and D** at 1500 sccm⁻¹ was observed in Raman spectra with D/G ratio of 1.19 and C1s XPS spectra of carbon contained 20.4 at.% of sp³ carbon phase in comparison to 8.3 at.% in case of graphene nanosheets layer. High D/G ratio, up to 3.5, and low intensity 2D band was characteristic for vertically aligned graphene nanosheets layers. The possibility to influence density and size of graphene nanosheets on substrate represents promising alternative for future deposition of graphene on arbitrary substrate.

Introduction

Graphene belongs among the most promising materials in both, fundamental and applied science [1]. Today, exfoliation from graphite crystals and chemical vapor deposition belong to most widely used methods to prepare graphene layers on the substrate [2]. These methods produce high quality layers in limited amounts or require elaborate transfer techniques to dielectric substrates. Alternative approach is controlled decomposition of organic precursors using plasma enhanced chemical vapor deposition (PECVD) [3]. Such approach enables synthesis of good quality material in reasonable amounts. It is of great interest to grow graphene layers directly on dielectric substrates. There are two main approaches to achieve growth of graphene on the dielectric substrate. First approach uses solid carbon source deposited onto a desired dielectric substrate which can be transformed into graphene films by high temperature annealing. In order to accelerate the decomposition of solid carbon sources, capping metal layers such as Cu or Ni deposited on carbon sources can be used [4]. Second approach is direct CVD of graphene on dielectric substrate with introduction of metal vapors or use of plasma enhanced CVD process. While there has been some success in these methods, the graphene nucleation density, the growth rate on dielectric substrates as compared to metallic substrates and the size of graphene single crystal domains is usually small (about few micrometers) [5]. There was also reported growth of graphene directly on dielectric substrate by PECVD where the decomposition of carbon precursor enabled growth at lower temperature, however, until now this method resulted only in limited success (450–500 °C) and small domain sizes of graphene [6,7]. Another approach to grow graphene on dielectric substrate without metallic catalyst is so called vertically aligned few layer graphene (FLG) or carbon nanowalls. Such a growth was first reported by Malesevic et al. for synthesis of FLG from mixture of methane and hydrogen in microwave PECVD [8]. Overall, wide range of plasma sources was used for synthesis of FLG including direct current (DC), radio-frequency (RF) capacitive and inductively coupled to microwave (MW) power sources, and naturally, PECVD has become main method for FLG synthesis [9]. Typical growth pressure range is several Pascals for capacitive coupled systems to 10³–10⁴ Pa for DC and microwave powered setups. The growth temperature was limited by surface reaction kinetics and varies from 750 to 1300 K and all experiments, except MW setups, used external resistive heating for the elevation of substrate temperature. Among substrates used for deposition were silicon, quartz, metals or their combination and precursors were mixture of H₂ with hydrocarbon (CH₄, C₂H₂) or fluorocarbons, such as CF₄, CHF₃ or C₂F₆. To achieve better quality of the layer combination of oxygen containing precursors such as CO₂ and CO together with H₂ and CH₄ were used. It is worth noting that the systems used for growth of FLG are almost always low pressure systems and experiments at atmospheric pressure were only partially successful and resulted in mixture of various forms of carbon allotropes. Growth using atmospheric pressure DC normal glow discharge in Ar/CH₄ and Ar/H₂/ethanol was reported on Si, Cu and stainless steel substrate and studied by Bo et al. [10] and applied as gas sensor by Yu et al. [11]. Meško et al. [12] reported growth on Ni using atmospheric pressure DC glow PECVD in mixture of Ar/H₂/ethanol or hexane and reported growth rates as high as 100 nm per minute at 1000 K. This approach was in detail reviewed by Bo et al. [9] and recently by Vasell et al. [13] and Santhosh et al. [14]. Alternative method for the growth of graphene was developed by Dato et al. [15] concerning microwave plasma decomposition of ethanol or dimethyl ether for synthesis of graphene nanosheets in the gas phase. This method was further refined by Tatarova et al. [16] and Tsyganov et al. [17] also developed a theoretical model of the synthesis process and could simulate and experimentally map the particle and thermal fluxes in their plasma reactor. Melero et al. also showed that decomposition of ethanol in TIAGO torch at atmospheric pressure could be used for synthesis of carbon nanotubes and graphene nanosheets without use of catalyst [18,19] and efficient hydrogen generation [20]. This approach was recently extended to N-doping and the use of methane as precursor by Bundaleska et al. [21,22]. In our work we have showed that dual channel microwave plasma torch could be used for direct growth of carbon nanotubes and nanofibers on the substrate [23] and that these could be used as sensors [24] with comparable performance to graphene oxide [24,25]. In comparison with hydrocarbon precursors, the use of ethanol enables robust system for nucleation and assembly of graphene layer without the need to add another reactive gas, such as H₂, into deposition mixture. This is caused by ethanol structure of CH₃-CH₂OH, which is at high temperature, above 2000 K, decomposed predominantly into CO and H₂ molecules and C₂ species responsible for graphene nucleation and growth. This way simple, single precursor, system can be used for bottom-up synthesis of graphene with various properties. In this work we study formation of carbon based layers on dielectric substrate by decomposition of ethanol in dual-channel microwave plasma torch at atmospheric pressure. We study formation of basic building blocks directly on dielectric substrate (Si/SiO₂) in dependence on ethanol precursor flow rate, delivered microwave power and substrate temperature.