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Duvjir, Ganbat,Choi, Byoung Ki,Jang, Iksu,Ulstrup, Søren,Kang, Soonmin,Thi Ly, Trinh,Kim, Sanghwa,Choi, Young Hwan,Jozwiak, Chris,Bostwick, Aaron,Rotenberg, Eli,Park, Je-Geun,Sankar, Raman,Kim, Ki-Seo American Chemical Society 2018 NANO LETTERS Vol.18 No.9
<P>Emergent phenomena driven by electronic reconstructions in oxide heterostructures have been intensively discussed. However, the role of these phenomena in shaping the electronic properties in van der Waals heterointerfaces has hitherto not been established. By reducing the material thickness and forming a heterointerface, we find two types of charge-ordering transitions in monolayer VSe<SUB>2</SUB> on graphene substrates. Angle-resolved photoemission spectroscopy (ARPES) uncovers that Fermi-surface nesting becomes perfect in ML VSe<SUB>2</SUB>. Renormalization-group analysis confirms that imperfect nesting in three dimensions universally flows into perfect nesting in two dimensions. As a result, the charge-density wave-transition temperature is dramatically enhanced to a value of 350 K compared to the 105 K in bulk VSe<SUB>2</SUB>. More interestingly, ARPES and scanning tunneling microscopy measurements confirm an unexpected metal-insulator transition at 135 K that is driven by lattice distortions. The heterointerface plays an important role in driving this novel metal-insulator transition in the family of monolayer transition-metal dichalcogenides.</P> [FIG OMISSION]</BR>
Novel polymorphic phase of two-dimensional VSe2: the 1T′ structure and its lattice dynamics
Duvjir, Ganbat,Choi, Byoung Ki,Ly, Trinh Thi,Lam, Nguyen Huu,Chun, Seung-Hyun,Jang, Kyuha,Soon, Aloysius,Chang, Young Jun,Kim, Jungdae The Royal Society of Chemistry 2019 Nanoscale Vol.11 No.42
<P>Polymorphisms allowing multiple structural phases are among the most fascinating properties of transition metal dichalcogenides (TMDs). Herein, the polymorphic 1T′ phase and its lattice dynamics for bilayer VSe2 grown on epitaxial bilayer graphene are investigated <I>via</I> low temperature scanning tunneling microscopy (STM). The 1T′ structure, mostly observed in group-6 TMDs, is unexpected in VSe2, which is a group-5 TMD. Emergence of the 1T′ structure in bilayer VSe2 suggests the important roles of interface and layer configurations, providing new possibilities regarding the polymorphism of TMDs. Detailed topographical analysis elucidates the microscopic nature of the 1T′ structure, confirming that Se-like and V-like surfaces can be resolved depending on the polarity of the sample bias. In addition, bilayer VSe2 can transit from a static state of the 1T′ phase to a dynamic state consisting of lattice vibrations, triggered by tunneling current from the STM tip. Topography also shows hysteretic behavior during the static-dynamic transition, which is attributed to latent energy existing between the two states. The observed lattice dynamics involve vibrational motion of the Se atoms and the middle V atoms. Our observations will provide important information to establish in-depth understanding of the microscopic nature of 1T′ structures and the polymorphism of two-dimensional TMDs.</P>
Lattice Dynamics Driven by Tunneling Current in 1T′ Structure of Bilayer VSe2
Duvjir Ganbat,Kim Jungdae,Tsermaa Baatarchuluun,Choi Byoung Ki,Chang Young Jun 한국물리학회 2020 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.77 No.11
Interesting lattice dynamics were recently reported in 1T` structure of bilayer VSe2 attributed to the vibration of Se atoms, but it was limited to topographic observation using scanning tunneling microscopy (STM). We conduct further systematic STM study to understand the origin of lattice dynamics in bilayer VSe2. Time spectroscopy of tunneling current is measured with the feedback loop disabled, while holding the position of the STM tip over a single Se atom. The time spectroscopy indeed shows random telegraph signals that tunneling current fluctuates between high and low currents. Such fluctuations of current reflect the vibrational motion of the Se atom. Statistical analysis described by the Markov process provides average residence time (τ) for high and low current states, and transition rate (R = 1/τ). Interestingly, the transition rate as a function of tunneling current (I) follows a power law of R = IN (N = constant), and the N value is close to 1. This linearity of transition rate indicates that the lattice dynamics are mostly induced by tunneling current. Our result confirms the role of tunneling current that drives the observed lattice dynamics in the bilayer VSe2.
재구조된 Si(5 5 12)-2×1 형판 위에 결이 맞는 CaF 나노선의 성장
김희동,DUVJIR GANBAT,Otgonbayar Dugerjav,Huiting Li,서재명 한국물리학회 2011 새물리 Vol.61 No.12
The commensurate nanowire grown by thermal deposition of CaF₂ on Si(5 5 12)-2×1 at 500℃ has been studied by using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). The CaF₂molecules deposited on this substrate are dissociated to form one-dimensional (1D) nanowires composed of CaF (113) and CaF (111) facets. With increasing deposited amount of CaF₂, the density of the nanowires increases,but the original period of Si(5 5 12)-2×1 is maintained. The CaF/Si(5 5 12) system is expected to have applications to the etching of semiconductors. 주사 터널링 현미경과 주사 터널링 분광법을 이용하여 초고진공 하에서Si(5 5 12)-2×1 기판 위에 CaF₂를 증착하여 기판과 결이맞는 일차원 나노선의 형성 방법을 연구하였다. 본 연구를 통하여 500℃의 기판 위에서 CaF₂는 해리되어 CaF (113) 패싯과 CaF (111) 패싯으로 이루어진 일차원 나노선을 형성하는 사실을 발견하였다. 이 나노선의 밀도는 CaF₂증착량으로 조절할 수 있으며, 또한 증착도중 Si(5 5 12)-2×1의 원래 주기가 유지되는 것을 확인하였다. 그리하여 CaF/Si(5 5 12) 계가 반도체 에칭에 응용될 가능성을확인하였다.