Growth and Characterization of Graphene on Texture-Controlled Platinum Films

Abstract

In this study, the primary purpose of this research is to grow high quality graphene on platinum (Pt) films, especially wrinkle-free graphene as a 2-dimensional membrane for transparent conductor and hydrophobic water-distillation applications by using texture-controlled Pt films that have incorporated oxygen atoms. In order to achieve the final goals, this research primary had been focused on analysis of abnormal Pt grains through annealing process and study of graphene growth kinetics through chemical vapor deposition process. Then, a new transfer method was applied to graphene transfer by reacting graphene/Pt interface, without incurring damages and unintentional doping. The wrinkle-free graphene was synthesized by using texture-controlled Pt films (200, 220) with giant grains (GGPt) via chemical vapor deposition (CVD). The Pt films on SiO2/Si substrates could be controlled by sputtering with Ar/O2 gas mixtures and abnormal grain growth was affected by the incorporated oxygen during post-annealing process. In order to analysis of graphene growth kinetics on GGPt, each films were heated at the CVD process temperature of ~975 ºC and maintained for 10 min under CH4/H2 gas mixture (5 and 50 sccm, respectively) without cleaning treatments. Enhanced surface perfectness and limited number of grain boundary (GB) of Pt induced homogeneous C-precipitation, thus the high-crystallized monolayer graphene sheets was formed. The transferred graphene shows wrinkle-free characteristics regardless of the orientation types of Pt, probably due to much lesser difference in thermal expansion coefficient (TEC, ~11 μm m-1K-1 at 1000 °C) to graphene. The wrinkles or ripples-free graphene films showed a high crystallinity and high carrier mobility at room-temperature up to ~8,500 cm2V-1s-1. To transfer graphene, a thermal-assisted transfer method was applied by a NaOH (1 M) aqueous solution at 90 °C. The thermal-assisted transfer method was only activated by the hydroxide (OH-) in NaOH solution to separate the graphene/Pt interface. The thermal-assisted transfer process allowed the complete transfer of large-scale graphene films onto arbitrary target substrates without incurring damages and unintentional doping. Compare to bare GGPt, graphene-free GGPt showed no contamination and degradation after the graphene transfer. The fact was demonstrated by XPS data, which showed almost same binding energy of Pt-4f5/2, Pt-4f7/2 (74±0.2 eV). On the basis of these results, a recycle ability of Pt was demonstrated. Also, the result of graphene on the recycled Pt showed almost same quality as the obtained graphene from 1st Pt. Furthermore, the transfer method could be applicable to the large-scale patterned graphene on Pt films with SiO2 regions. By comparing an electrochemical transfer method, the thermal-assisted transfer method have proved to be successfully transferred onto SiO2/Si substrate for the patterned Pt films. The reason is that the reaction between Si and Na+ took place in the boiled NaOH solution to react the SiO2 surface. Through a pre-annealing step in CVD process, the porous graphene membrane could be obtained from the porous Pt texture. The density and size of pore depended on the pre-annealing time in hydrogen gas. Especially, a dense pores of Pt films was obtained with controllable density (~2×105 cm-2) and ~2.5 μm of radius by pre-annealing for 5 min. Since an oxygen was inserted during Pt film sputtered as an adhesion layer between Pt and SiO2/Si substrate, Pt sintering has occurred by oxygen diffusion during pre-annealing step in H2 atmosphere. The porous graphene membrane were successfully transferred onto SiO2/Si substrate by thermal-assisted transfer method. Surprisingly, graphene was grown direct in the pores of Pt films. It was demonstrated through that Pt particles directly formed growth of the graphene. In summary, this study shows the wrinkle-free characteristic of graphene layer by using Pt thin films with preferred orientations and giant grains. In addition, large-scale and patterned graphene films can be successfully transferred onto arbitrary substrates via thermal-assisted transfer method and the Pt substrates can be repeatedly used for the proliferation of graphene applications. Furthermore, this transfer technique shows a high tolerance to variations in types and morphologies of underlying substrates, which is essential for the various applications proposed for graphene.