Graphene Functionalization: a Route towards the Use of Graphene for Microelectronic Applications
Grafeen functionalisatie: een route naar het gebruik van grafeen voor micro-elektronische toepassingen
Nourbakhsh, Amirhasan; S0197655
Graphene is a gapless semiconductor with a high charge carrier mobility. In an attempt to apply this promising material for electrical and optical applications, functionalization of graphene is a prerequisite to introduce a bandgap.In this thesis, we present our findings on two major graphene functionalization methods: covalent and non-covalent functionalization of graphene in order to apply it for electronic and optical applications.Regarding the covalent functionalization, using oxygen plasma treatment we observed the occurrence of the metal-to-semiconductor transition in single layer graphene (SLG). Careful control of the plasma treatment allows reproducible production of semiconducting graphene. Semiconductivity is demonstrated by electrical and photoluminescence (PL) measurements. Opening of the bandgap as a result of the plasma treatment is explained in terms of graphene surface functionalization with chemisorbed oxygen atoms. Using ab initio calculations, we then present more details about the oxygen - graphene interaction and its effect on the graphene optoelectronic properties. A bandgap is indeed predicted by the calculation, when graphene is decorated with oxygen atoms in a specific chemisorption configuration. We then demonstrate Schottky rectifying junctions between semiconducting, modified SLG and a metal. The occurrence of a Schottky barrier between semiconducting graphene and metals with different work functions is investigated by electrically characterizing the as-fabricated junctions. We also compare the effects of the oxygen treatment on the structural, optical, and electrical properties of single-layer and bilayer graphene (BLG). We observe only photoluminescence in SLG, whereas the BLG remain optically unchanged. DFT calculations are carried on representative oxidized SLG and BLG models to predict electronic density of states and band structures. Sufficiently oxidized SLG shows a bandgap and thus loss of semimetallic behavior, while single-side oxidized BLG maintains its semimetallic behavior even at high oxygen density in agreement with the results of the PL experiments. DFT calculations confirm that the Fermi velocity in single-side oxidized BLG is remarkably comparable with that of pristine SLG, pointing to a similarity of electronic band structure. Regarding the non-covalent functionalization, we first investigated the effects of thermal annealing on SLG samples deposited on SiO2 supports. We found that heating SLG samples in inert atmosphere induces permanent changes in their electrical/optical properties, without affecting their structural properties. We believe that charge transfer from the SiO2 support is responsible for the occurrence of hole doping in our SLG samples. The Raman spectra measured on annealed SLG samples contain signatures of excess positive charge accumulated in graphene. The findings are further confirmed by electrical characterization performed on SLG-FETs. In a different approach, we show a procedure to reversibly tune the excess charge concentration in SLG, from p- to n-type, up to 1.2×10E13/cm2. The tuning is achieved by engineering the interaction between graphene and the underlying substrate with an amino group-terminated self-assembled monolayer, and subsequent rinsing in aqueous solutions at controlled pH. Raman spectroscopy and electrical measurements on treated graphene devices confirm the occurrence of doping. We found the field-effect mobility not to be significantly affected by the procedure.In last part of this thesis we demonstrate a technique to improve the Ion/I off ratio in bilayer graphene FET by asymmetrical doping of layers. Doping is achieved by n-doping the bottom layer by depositing bilayer graphene flakes on NH2-SAM and hole doping the top layer via coating the device with a film of F4TCNQ-containing polymer. Asymmetric surface doping of bilayer graphene can induce an electric field between both layers which results in opening of an electronic bandgap due to symmetry breaking. DFT modeling shows an effective electric field between the layers and field effect measurements show an increase of Ion/I off ratio in bilayer FET up to ~100 due to opening of the bandgap. The demonstrated Ion/I off ratio is the highest to date reported value for graphene based devices.