Modifications of Electronic and Structural Properties of Graphene by Using Alkali Metal Ions

다국어 초록 (Multilingual Abstract)

Graphene has been widely studied in recent years due to its superior electronic properties as well as potential applications for graphene-based next-generation devices. As an effort to utilize its unique structural and electronic properties of graphene, we have investigated the modifications of its properties by doping low-energy alkali metal ions (Li + and Na+ ) on single layer graphene (SLG) and also on bilayer graphene (BLG) grown on SiC(0001) substrate. We have utilized the synchrotron-based angle-resolved photoemission spectroscopy (ARPES) and high-resolution core-level spectroscopy (HRCLS) as main experimental tools to understand any changes in electronic properties of the alkali iondoped graphene system.
As a first system, we have studied the band gap engineering of SLG formed on SiC(0001) by using slow Li+ ions of energy 5 eV. In order to utilize the superb electronic properties of graphene in future electronic nano-devices, a dependable means of controlling the transport properties of its Dirac electrons has to be devised by forming a tunable band gap. We find the opening of a sizable and tunable band gap up to 0.85 eV, which depends on the Li+ ion dose () together with thermal treatment. This band gap of 0.85 eV turns out to be the largest band gap in the π-band of SLG by any means reported so far. Our Li 1s core-level data together with the valence band data suggest that Li+ ions do not intercalated below the topmost graphene layer, but cause a significant charge asymmetry between the carbon AB-sublattices of SLG to drive the opening of the band gap. We thus provide a route to producing a tunable graphene band gap by doping Li+ ions, which may play a pivotal role in utilizing graphene in future graphene-based electronic nano-devices.
We have investigated modification of thermal and electronic properties of BLG by using Na+ ions as our second system in this thesis. BLG has an extensive list of industrial applications in graphene-based nano-devices such as energy storage devices, flexible displays, and thermoelectric devices. By doping slow Na+ ions on Li-intercalated BLG, which is formed by intercalation of neutral Li atoms below SLG on SiC(0001) substrate, we find significantly improved thermal and electronic properties of BLG by using ARPES and HRCLS with synchrotron photons. Our HRCLS data reveal that the adsorbed Na+ ions on a BLG produced by Li-intercalation through SLG spontaneously intercalate below the BLG and substitute Li atoms bound to silicon atoms in the Si-C bilayer of 6H-SiC(0001) to form Na-Si bonds at the SiC interface while preserving the same phase of BLG. This is in sharp contrast with no intercalation of Na+ ions on SLG though neutral Na atoms intercalate. The Na+ -induced BLG is found to be stable upon heating up to T=400 °C, but returns to SLG when heated at T d =500 °C. The evolution of the πbands upon doping the Na+ ions followed by thermal annealing show that the carrier concentration of the π-band may be artificially controlled within an order of magnitude without damaging the Dirac nature of the π-electrons. The doubled desorption temperature from that (T d =250 °C) of the Na-intercalated SLG together with the electronic stability of the Na+ -intercalated BLG may find more practical and effective applications in advancing graphene-based thermoelectric devices and anode materials for rechargeable batteries.

목차 (Table of Contents)

• 1. Introduction ……………… 1
• 2. Theoretical Background of Graphene 4
• 2.1 Introduction ………………………………………… 4
• 2.2 Electronic structure of single layer graphene ……… 4
• 2.3 Electronic structure of bilayer graphene ………… 10
• 2.4 Graphene functionalization for opening a band gap …12
• 2.4.1 Band gap in single layer graphene ……………… 14
• 2.4.2 Asymmetric band gap in bilayer graphene ………… 16
• 2.5 Graphene epitaxy on 6H-SiC(0001) substrate ……… 17
• 3. Experimental Methods …… 20
• 3.1 Photoemission Spectroscopy (PES) …………… 20
• 3.1.1 Angle Resolved Photoemission Spectroscopy (ARPES) …………… 27
• 3.1.2 High Resolution Core Level Spectroscopy (HRCLS) ……………… 28
• 3.2 Low Energy Electron Diffraction (LEED) ………… 29
• 3.3 Experimental Environment ………………… 32
• 4. Band gap engineering for single-layer graphene by using slow Li + ions ……………… 35
• 4.1 Introduction …………………… 35
• 4.2 Experimental Details …………………… 37
• 4.3 Results and Discussions …………………… 39
• 4.4 Summary ……………………………………………… 47
• 5. Modification of thermal and electronic properties of bilayer graphene by using slow Na + ions ……………… 48
• 5.1 Introduction …………………… 48
• 5.2 Experimental Details ………………………… 50
• 5.3 Results and Discussions ………………… 53
• 5.4 Summary ………………………………………… 61