Graphene and graphene fluoride: synthesis, properties and applications

Open Access
Author:
Wang, Bei
Graduate Program:
Physics
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
October 05, 2012
Committee Members:
  • Jun Zhu, Dissertation Advisor
  • Jun Zhu, Committee Chair
  • Mauricio Terrones Maldonado, Committee Chair
  • Vincent Henry Crespi, Committee Member
  • Gerald Dennis Mahan, Committee Member
  • Joan Marie Redwing, Committee Member
Keywords:
  • graphene
  • graphene fluoride
  • GFET
Abstract:
This dissertation explores the properties and applications of graphene and graphene fluoride at the junction of physics, material science and electronics. We study the synthesis and characterization of these two novel carbon nanomaterials, and their potential applications in biochemical sensing and optoelectronics. We start with chemical vapor deposition (CVD) synthesis of large-scale single-layer graphene sheets, followed by material characterization and device fabrication. We demonstrate bio-sensor arrays based on ion-sensitive graphene field-effect transistors (ISGFETs). The ISGFETs are capable of operating in solution environment and show typical carrier mobilities around 5000 cm2/Vs for both electrons and holes. A pulsing gate technique is employed to suppress the hysteresis present in GFETs. Using a SiO2 layer as sensing surface, pH measurement with an average sensitivity of 46 mV/pH is achieved. The pH sensing mechanism of ISGFETs can be explained by a site-dissociation model. We further demonstrate the functionalization of the SiO2 surface with 3-aminopropyltrimethoxysilane (APTMS). Preliminary studies of DNA hybridization show the promise of the ISGFETs as biosensors. Band gap engineering of graphene is a potential way to extend its applications to optoelectronics. We explore a chemical approach to open a band gap in graphene using fluorination. Thermal fluorination of bulk graphite yields a stoichiometric chemical derivative, (CF)n. Photoluminescence from (CF)n with laser energies up to 5.08 eV is studied at a wide range of temperatures and reveals its six emission modes. The lineshape of these emission modes implies that they are associated with mid-gap states related to defects, possibly fluorine deficient sites. The band gap of (CF)n is likely beyond the highest excitation energy employed, which suggests that it is a wide gap insulator. Partial fluorination is studied to achieve a reduced band gap. We employ CF4 plasma to treat CVD graphene sheets. The fluorine-containing radicals react with graphene to form CFx (x = 1, 2, 3) functional groups. As plasma time or power increases, CFx shifts to larger x components and eventually turns into CF4 gas molecules. We find that the transport behavior in this regime is dominated by the inhomogeneity of fluorination, which is related with the structural features of CVD graphene, i.e.. multi-layer patches, folds and wrinkles. Tunneling current from fluorinated graphene is shown to decrease drastically as fluorination time increases, which is consistent with the increase of the band gap. We observe that at lower fluorination ratio, the tunneling current can be explained by the Poole-Frenkel mechanism. The results of our studies suggest a way to achieve carbon based two dimensional semiconductors for both fundamental research and applications.