My main research interest for my graduate studies was to grow graphene on a SiC substrate, to characterize its the physical properties and to modify its surface for more potential applications. For my Ph.D thesis work, I have investigated the temperature dependent structural changes of graphene layers on the SiC substrate, the modification of the gaphene layers using electron beam irradiation, and chemical contrast in imaging upon the graphene layer thickness by using scanning photoemission microscopy. The thesis consists of four chapters. Summaries of each chapter are as follows.
Temperature Dependent Structural Change of Graphene Layers on 6H-SiC(0001) surfaces
We investigated the electronic and structural properties of graphene layers grows on a 6H-SiC(Si-terminated) substrate by using photoemission spectroscopy(PES), low energy electron diffraction (LEED), and near edge X-ray absorption fine structure(NEXAFS). The angle between the plane of the graphene sheet and the SiC substrate was measured by monitoring the variation of the $\pi^*$ transition in the NEXAFS spectrum that has the thickness of the graphene layers. As the thickness of the graphene layers increased, the angle gradually decreased.
Functionalization of multi-layer graphene grown on 6H-SiC(0001) using 1 MeV electron beam irradiation
Graphene layers grown on 6H-SiC(0001) were irradiated with 1MeV electron beam to functionalize its surface. A surface analysis using atomic force microscopy(AFM), the near edge X-ray adsorption fine structure(NEXAFS) spectra of C K-edge and photoemission spectroscopy(PES) suggests that the electron beam irradiation in ambient condition can induce controlled oxidation of the graphene layer without aqueous detergent treatments.
Scanning Photoemission Microscopy of Graphene on $SiO_2$
We successfully recorded scanning photoemission microscopy(SPEM) images of a graphene flake as well as the C 1s core level spectra for the mono-layer and multi-layer graphene. The samples with lateral dimensions on a micrometer scale were prepared on a silicon dioxide surface by direct exfoliation of crystalline graphite. The thickness of graphene was identified by using a combination of optical microscopy and atomic force microscopy. A mono-layer graphene was distinguished from the multi-layer graphens through the chemical contrast images of SPEM.
SiC(0001) 표면위에 성장시킨 그라핀 층의 온도에 따른 구조변화
SiC(0001) 표면에 성장시긴 그라핀 층의 전자적·구조적 특성변화를 광전자분광학(Photoemission Electron Spectroscopy, PES), 저에너지 전자회절(Low Energy Electron Diffraction, LEED), 그리고 X-선 흡수 끝머리부근 미세구조(NEXAFS)를 이용하여 연구하였다. SiC 기판에 성장시킨 그라핀 층의 두께에 따른 방향성은 NEXAFS 스펙트럼의 $\pi^*$ 의 각도에 의존하는 세기변화를 측정하여 계산하였다. 그라핀 층의 두께가 증가할 수록 그라핀층의 흡착각은 감소하였다.
1 MeV 전자빔을 이용한 SiC(0001) 기판위에 성장시킨 그라핀 층의 기능화
SiC(0001) 기판위에 성장시킨 그라핀 층에 1 MeV의 전자빔을 조사하여 표면의 특성 변화를 연구하였다. 원자힘 현미경(Atomic Force Microscopy, AFM), X-선 흡수 끝머리부근 미세구조(NEXAFS) 그리고 광전자분광학(Photoemission Electron Spectroscopy, PES)를 이용하여 공기중에서 전자빔을 조사하였을 때 전자빔의 조사량을 조절하여 그라핀층의 산화정도를 제어할 수 있었다.
$SiO_2$ 위에 흡착된 그라핀의 주사형 광전자 현미경 실험
$SiO_2$ 기판에 흡착된 단층(mono-layer)과 다층(multi-layer)로 이루워진 그라핀을 주사형 광전자 현미경(scanning Photoemission Microscopy, SPEM)을 이용하여 영상을 얻었으며, 각 층에서 국부적으로 C 1s 스펙트럼을 측정하는데 성공하였다. 이 그라핀 시료는 $SiO_2$ 기판위에서 스카치 테이프를 그래파이트(graphite)를 벽개하는 방법을 이용하여 준비하였다. 단층과 다층에서 측정한 SPEM 영상은 각 층에 화학적 대비가 있음을 보여주었다.
