The role of defects on electron behavior in graphene materials

Graphene-based materials exhibit many unique physical properties that are intriguing for both fundamental science and application purposes. This thesis describes three systems of sp2 bonded carbon: graphite, graphene and fullerene, and studies the electron behavior in these materials and how it is affected by the presence of defects. It is shown here that by inducing specific defects, phenomena such as ferromagnetism and superconductivity can arise in these systems. Graphite and its structural defects are studied by scanning tunneling microscopy and spectroscopy in chapter 2. This chapter represents the first detailed analysis of the structural and electronic properties of grain boundaries in graphite. Grain boundaries are the most common defects in highly oriented pyrolytic graphite (HOPG) because of its polycrystalline character. They form periodic arrays of point defects that are arranged in planes perpendicular to the graphene planes. On the graphite surface, grain boundaries manifest themselves as one-dimensional chains of point defects with a width around 1 nm and a length up to several micrometers. The periodic structure within a single grain boundary displays only two possible distances between point defects. This periodicity was found to be 0.5-10 nm in different grain boundaries. Atomically resolved STM images showed that grain boundaries are tilt grain boundaries, which are created between two rotated graphite grains. A new proposed structural model of grain boundaries based on periodically repeated point defects could explain all observed periodicities in STM. The electronic structure of grain boundaries has been studied locally with scanning tunneling spectroscopy (STS). Grain boundaries possess enhanced charge densities and localized electron states in comparison to the bare graphite surface. These states extend up to 4 nm from grain boundaries. Two localized electron states have been observed on grain boundaries having small periodicities (<4 nm), while a single localized state at the Fermi energy has been measured for larger periodicities, indicating a long-range interaction among point defects within a grain boundary. An unexpected phenomenon in carbon-based materials, ferromagnetism, is studied in chapter 3. Ferromagnetic signals have been observed in HOPG locally by magnetic force microscopy (MFM), and in the bulk magnetization measurements using a superconducting quantum interference device magnetometer (SQUID) at room temperature. In MFM, the ferromagnetic signals have been detected specifically at line defects of the graphite surface. The magnetic moments in these defects pointed out of the graphite surface. SQUID magnetization measurements in HOPG revealed anisotropic ferromagnetic-like hysteresis loops at both 5 K and 300 K. The saturation magnetization reached 1 × 10-2 emu/g along the basal plane of graphite, while it was an order of magnitude smaller in the direction parallel to graphene planes. Magnetic impurities have been excluded as the origin of the magnetic signal after careful analysis, supporting an intrinsic magnetic behavior of carbon-based materials. The observed ferromagnetism has been attributed to originate from unpaired sp- electron spins, localized at defects sites of grain boundaries. It was pointed out that the structure of defects within grain boundaries cause sublattice unbalance, which is a sufficient condition for formation of local magnetic moments in graphene lattice according to Lieb’s theorem. Because of the unique structure of grain boundaries, defects are formed on the same sublattice and therefore the magnetic coupling between the magnetic moments is always ferromagnetic. The ferromagnetism in graphite sustains unexpectedly high temperatures, where the Curie temperature is well above room temperature. Such a high Curie temperature could be explained on the basis of the 2D anisotropic Heisenberg model using self-consistent spin-wave theories, which gave rise to TC = 764 K. In the future, a controlled way of producing defects in graphite could lead to production of light and high temperature carbon ferromagnets. Moreover, grain boundaries in graphite can find applications in the field of spintronics as spin-polarized guides. Chapter 4 is devoted to a single layer of graphite, graphene, grown on SiC(0001). Graphene has shown a number of unexpected physical properties in the last years, which makes it a promising candidate for future electronic devices. Graphene possess a high quality two dimensional electron gas with extremely high mobility at room temperature, where charge carriers can be tuned between electrons and holes by gate. The system of graphene on SiC seems to be the most interesting platform for application purposes and for large scale production. However, the quality of the 2D electron gas in graphene on SiC is much lower than for a free standing graphene or graphene supported on SiO2 substrates. For this reason, the main focus in this chapter was devoted to the understanding the influence of the SiC interface on the electronic properties of a graphene monolayer. The successful formation of a few-layer graphene (1-3) on SiC(0001) has been performed by a heating procedure in ultra high vacuum. The resulting graphene layers have been studied by scanning tunneling microscopy (STM) and spectroscopy (STS). STM topography and STS measurements have shown that a single graphene layer grown on a SiC(0001) substrate is still affected by the electronic structure of the interface layer of SiC. The graphene monolayer demonstrated transparency at bias voltages > 100 mV in STM. At these voltages, localized states belonging to the underlying interface layer were observed on the first graphene layer. Inelastic electron tunneling spectroscopy (IETS) has revealed an extremely strong inelastic phonon contribution for the out of plane acoustic phonon (70 meV) of graphene, reaching a gigantic 50% intensity of the IETS peaks. This inelastic contribution has been enhanced particularly on the places with localized electron states of the interface layer. Surprisingly, STS spectra on single layer graphene have shown a gap-like feature at the Fermi level, which was pinned between the inelastic phonon contributions at ±70 meV. This gap-like feature is probably due to charge modulations from graphene electrons interacting with localized interlayer electron states, indicating that electron correlation effects play an important role for the charge carrier behavior at the Fermilevel. Undoped graphene is a semi-metal, but several ideas have been proposed how graphene can become superconducting by doping. Here is reported that a few layers of epitaxial grown graphene shows a transition to two-dimensional fluctuating superconductivity. The underlying mechanism is based on strong electron-phonon coupling between graphene electrons interacting with localized electron states formed at the SiC(0001) substrate/graphene interface and z- acoustic phonons of graphene. Finally, chapter 5 deals with curved graphene systems, fullerenes, for which a new wet deposition technique was successfully developed to produce ultra thin fullerene films. This technique could be especially useful for fullerene derivatives, which cannot sustain the high temperatures needed to evaporate these molecules in ultra high vacuum. It uses a special nebulizer to spray coat fullerenes dissolved in toluene or carbon disulfide onto a sample surface under ambient conditions. Spray coating of C60 has been successfully tested on graphite and gold surfaces. Monolayer thick C60 films have been formed on both surfaces at particular deposition parameters as confirmed by AFM and STM. The structural and electronic properties of spray coated C60 films on Au(111) have shown comparable results to thermally evaporated C60. The only difference was that solvent residues remained attached to the gold surface and could not be removed. However, the solvent residues have not modified the electronic structure of C60 on Au(111) in the case of CS2.