Strain Engineering a 4a×3a4a\times\sqrt{3}a Charge Density Wave Phase in Transition Metal Dichalcogenide 1T-VSe2_2

We report a rectangular charge density wave (CDW) phase in strained 1T-VSe$_2$ thin films synthesized by molecular beam epitaxy on c-sapphire substrates. The observed CDW structure exhibits an unconventional rectangular 4a{\times}{\sqrt{3a}} periodicity, as opposed to the previously reported hexagonal $4a\times4a$ structure in bulk crystals and exfoliated thin layered samples. Tunneling spectroscopy shows a strong modulation of the local density of states of the same $4a\times\sqrt{3}a$ CDW periodicity and an energy gap of $2\Delta_{CDW}=(9.1\pm0.1)$ meV. The CDW energy gap evolves into a full gap at temperatures below 500 mK, indicating a transition to an insulating phase at ultra-low temperatures. First-principles calculations confirm the stability of both $4a\times4a$ and $4a\times\sqrt{3}a$ structures arising from soft modes in the phonon dispersion. The unconventional structure becomes preferred in the presence of strain, in agreement with experimental findings

주사형 터널링 현미경을 이용한 절연체 위의 다층 그래핀에 대한 연구

그래핀은, 탄소 원자들이 2차원 평면 위에 벌집모양 구조로 배열되어 결합하고 있는 단원자층의 탄소동소체를 뜻한다. 이를 층상구조로 쌓아 올리면 열역학적으로 안정된 구조를 갖는 3차원 물질인 흑연이 된다. 실제로 흑연으로부터 그래핀을 분리해내는 역학적 박리 방법이 성공하고, 그래핀의 전자가 상대론적 움직임을 따르는 디락 페르미온의 특성을 갖는다는 것이 실험적으로 증명된 것은 10년이 채 되지 않는다. 현재 기본적인 그래핀 전자구조가 규명된 이후, 다층 그래핀과 그래핀 모서리에 대한 연구는 그래핀의 부격자 대칭성의 깨짐으로 나타날 특성들로 인해 관심을 받고 있으나, 아직 그 국소적인 전자구조는 실험적으로 완벽하게 확인되지 않았다. 본 논문에서는, 화학기상퇴적 방법으로 대면적 그래핀을 구리 위에 성장시켜 유전체 위로 옮김으로써 만들어진, 단층과 다층 그래핀, 그래핀 모서리의 전자구조를 주사형 터널링 현미경을 이용하여 연구하였다. 먼저 그래핀의 전자구조가, 산화물이나 질화물로 이루어진 절연체 위에서 측정될 때에 비해, 얇은 층상 절연체인 질화붕소 위에서 측정될 때, 이론적으로 예측된 특성과 가장 유사함을 확인하였다. 이는 표면 분석 장비로 기판 위에 올려진 그래핀의 전기적 특성을 관측함에 있어서, 기판의 기하학적 굴곡이나 전하 웅덩이가 보려는 물질의 특성을 왜곡시킬 수 있음을 말해준다. 그리고 같은 방법으로 단층 그래핀을 여러 차례 같은 기판 위에 쌓아 올림으로써 인위적으로 생성된 다층 그래핀의 특성을 확인할 수 있었다. 두 층의 그래핀을 쌓은 경우, 위층과 아래층의 그래핀 면들의 정렬상태가 열적으로 가장 안정한 상태인 버널 스태킹에서 벗어나 존재할 수 있다. 이 때, 두 층의 규칙적인 공간상 전자 분포가 겹쳐지면서 무아레 패턴이 관측되었으며, 층 사이 간격이 열적 평형 상태에서 두 배 이상 증가하면 두 층은 각각 독립된 그래핀의 특성을 보일 수 있음을 확인하였다. 그래핀의 모서리 구조를 원자 레벨로 측정하기 위하여, 단층 그래핀을 성장시키는 과정에서 생성된, 균일하게 배열된 그래핀 조각들을 원하는 기판 위에 올려 주사형 터널링 현미경으로 관측하였다. 대부분의 그래핀의 모서리는 절연체 위에서 심한 굴곡을 보이며 불안정하게 존재하였다. 이들 중 지그재그 모양으로 잘려진 그래핀 모서리에서 페르미 에너지를 갖는 전자 밀도가 국소적으로 높게 나타나는 것을 확인하였다. 이는 평평한 전자 밴드가 페르미 레벨 근처에 존재하기 때문에 나타나는 현상으로서, 절연체 위의 그래핀 모서리에서 왜곡없이 실험적으로 관측된 것은 처음이다. 나아가 나노미터의 폭을 갖는 그래핀 리본을 만든다면, 양쪽 모서리에 모이는 전자들이 상호작용하면서 서로 반대인 스핀 상태로 정렬하여, 스핀에 따른 전하수송 특성을 밝힐 수 있을 것이다.Since the first successful isolation of graphene in 2004, it became one of the most studied materials by many scientists to explore new physics in a 2-dimensional nanostructure. After the 2010 Nobel Prize of physics was awarded to A. Geim and K. Novoselov for the discovery of graphene, many researchers rushed into graphene research to solve its physics and find ways of application. Bilayer graphene and graphene edges have become one of the important remaining issues in graphene. Bilayer graphene is distinguished from single layer graphene in Dirac Fermion picture due to coupling between two layers, resulting in anomalous quantum hall effect and possible application to electronic devices with a tunable band gap. As graphene edges show a unique electronic structure because of symmetry breaking, it is also drawing considerable attention to analyze finite-sized effects in graphene nanoribbon or nanopatch. In this thesis, most studies were performed on graphene grown on Cu foil by chemical vapor deposition (CVD) method and mechanically transferred on insulating substrates. A CVD system operated under high vacuum was constructed for graphene growth. Recently, CVD grown graphenes have been widely used in graphene research because large-sized samples can be obtained, contrary to the small-sized, mechanically exfoliated graphene. A Copper substrate has been used in the CVD growth of graphene since single layer graphene can be easily achieved due to the low carbon solubility. It is known that multiple layer graphene is routinely grown on a nickel substrate. However, the electronic structure of graphene on a copper does not resemble theoretically-predicted one near the Fermi level due to the hybridization with metallic states of substrates. In order to avoid the strong modification of electronic structures by metallic surfaces, several insulating layers were chosen as a noble substrate. Amorphous SiO2 and SiN with a certain thickness were used to easily distinguish graphene with naked eyes. But, the charged impurities and the corrugation of the substrates affected on the electronic structure of graphene. Though thin CuN and BN crystalline layers showed good insulating properties with large band gaps, BN were chosen for this study due to the thermal and chemical stability. As a new way of placing CVD graphenes on the substrates, a vacuum transfer method was developed so as to minimize chemical residues on graphene layers after several wet processes. Using a graphene mask and xy motors, the graphene was transferred cleanly on a prepared substrate under rough vacuum condition. Graphene samples were investigated using low temperature scanning tunneling microscopy (STM) and spectroscopy (STS). In the STM chamber, an e-beam heater is installed for in-situ cleaning of the tip and the sample, and a xy walker is prepared to cover the large scan area. The electronic-structure of a bilayer graphene may reveal spatial variation due to the coupling between the two layers as a function of stacking distance as well as lateral misalignment of the two layers. However, while a pristine Bernal (AB) stacked bilayer graphene is energetically stable and easily formed by mechanical exfoliation, growth on a SiC single crystal and epitaxial growth on metal substrates, double layer graphene out of AB staking has not been studied experimentally yet. We transferred two graphene layers successively onto thin insulating substrates, revealing variation of electronic structures on the top graphene layer relative to the bottom layer. Mechanically-stacked double layer graphene showed various Moíre patterns as determined by lateral misalignment in topographic images. The local density of states depended on the separation distance between two graphene layers and their corrugation. In this study, we were also able to approach to close-packed graphene patches to investigate graphene edges. By transferring partially-grown graphene patches on top of monolayer graphene or thin insulating layer, geometric and electronic edge structures could be studied on small islands of graphene. Corrugated structures of open edge were shown in topographic images, and local density of states was measured near various edges of graphene. Localized states on Fermi level, which were expected theoretically at zigzag edges, were also observed at open edges of graphene on insulating substrates in STS measurements for the first time.Docto

Enhanced Carrier Transport along Edges of Graphene Devices

The relation between macroscopic charge transport properties and microscopic carrier distribution is one of the central issues in the physics and future applications of graphene devices (GDs). We find strong conductance enhancement at the edges of GDs using scanning gate microscopy. This result is explained by our theoretical model of the opening of an additional conduction channel localized at the edges by depleting accumulated charge by the tip

Enhanced Carrier Transport along Edges of Graphene Devices

The relation between macroscopic charge transport properties and microscopic carrier distribution is one of the central issues in the physics and future applications of graphene devices (GDs). We find strong conductance enhancement at the edges of GDs using scanning gate microscopy. This result is explained by our theoretical model of the opening of an additional conduction channel localized at the edges by depleting accumulated charge by the tip