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Evidence for indirect band gap in BaSnO3 using angle-resolved photoemission spectroscopy
주범수,장영준,Luca Moreschini,Aaron Bostwick,Eli Rotenberg,한문섭 한국물리학회 2017 Current Applied Physics Vol.17 No.5
Transparent BaSnO3 thin films have been proposed as an alternative transparent conducting oxide (TCO). Although bulk synthesis and high-quality fabrication of epitaxial films are well established, there are still unsolved aspects about their electronic structure, such as the direct or indirect nature and the size of the band gap. We investigated the electronic structure of epitaxial BaSnO3 thin films using in situ angleresolved photoemission spectroscopy. We directly measured an indirect band gap of 3.7 eV, a value compatible with those of previous reports, but we also identified additional in-gap states at 1.6 eV below the conduction band minimum that we attribute to intrinsic defects, mainly oxygen vacancies.
Kim, Yong Su,Jeon, Sang Ho,Bostwick, Aaron,Rotenberg, Eli,Ross, Philip N.,Walter, Andrew L.,Chang, Young Jun,Stamenkovic, Vojislav R.,Markovic, Nenad M.,Noh, Tae Won,Han, Seungwu,Mun, Bongjin Simon The Royal Society of Chemistry 2013 Physical chemistry chemical physics Vol.15 No.43
<P>We report on the chemical adsorption mechanism of atomic oxygen on the Pt(111) surface using angle-resolved-photoemission spectroscopy (ARPES) and density functional calculations. The detailed band structure of Pt(111) from ARPES reveals that most of the bands near the Fermi level are surface-states. By comparing band maps of Pt and O/Pt, we identify that d<SUB><I>xz</I></SUB> (d<SUB><I>yz</I></SUB>) and d<SUB><I>z</I><SUP>2</SUP></SUB> orbitals are strongly correlated in the surface-states around the symmetry point M and K, respectively. Additionally, we demonstrate that the s- or p-orbital of oxygen atoms hybridizes preferentially with the d<SUB><I>xz</I></SUB> (d<SUB><I>yz</I></SUB>) orbital near the M symmetry point. This weak hybridization occurs with minimal charge transfer.</P> <P>Graphic Abstract</P><P>Using ARPES and DFT, the chemical bonding structure of atomic oxygen on the Pt(111) is explored. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c3cp53376j'> </P>
In situ investigation of conducting interface formation in LaAlO3/SrTiO3 heterostructure
유향근,Moreschini Luca,Bostwick Aaron,Walter Andrew L.,노태원,Rotenberg Eli,장영준 한국물리학회 2021 Current Applied Physics Vol.30 No.-
The high-mobility conducting interface (CI) between LaAlO3 (LAO) and SrTiO3 (STO) has revealed many fascinating phenomena, including exotic magnetism and superconductivity. But, the formation mechanism of the CI has not been conclusively explained. Here, using in situ angle-resolved photoemission spectroscopy, we elucidated the mechanisms for the CI formation. In as-grown samples, we observed a built-in potential (Vbi) proportional to the polar LAO thickness starting from the first unit cell (UC) with CI formation appearing above 3 UCs. However, we found that the Vbi is removed by synchrotron ultraviolet (UV)-irradiation; The built-in potential is recovered by oxygen gas (O2(g))-exposure. Furthermore, after UV-irradiation, the CI appears even below 3UC of LAO. Our results demonstrate not only the Vbi-driven CI formation in as-grown LAO/STO, but also a new route to control of the interface state by UV lithographic patterning or other surface modification.
Enhanced tunability of two-dimensional electron gas on SrTiO3 through heterostructuring
유향근,Luca Moreschini,Aaron Bostwick,Andrew L. Walter,노태원,Eli Rotenberg,장영준 한국물리학회 2020 Current Applied Physics Vol.20 No.11
Two-dimensional electron gases (2DEGs) on the SrTiO3 (STO) surface or in STO-based heterostructures have exhibited many intriguing phenomena, which are strongly dependent on the 2DEG-carrier density. We report that the tunability of the 2DEG-carrier density is significantly enhanced by adding a monolayer LaTiO3 (LTO) onto the STO. Ultraviolet (UV) irradiation induced maximum carrier density of the 2DEG in LTO/STO is increased by a factor of ~4 times, compared to that of the bare STO. By oxygen gas exposure, it becomes 10 times smaller than that of the bare STO. This enhanced tunability is attributed to the drastic surface property change of a polar LTO layer by UV irradiation and O2 exposure. This indicates that the 2DEG controllability in LTO/STO is more reliable than that on the bare STO driven by defects, such an oxygen vacancy.
Evidence for absence of metallic surface states in BiO2-terminated BaBiO3 thin films
오지섭,김민우,김기덕,이한결,유향근,신수빈,장영준,한문섭,Chris Jozwiak,Aaron Bostwick,Eli Rotenberg,김형도,노태원 한국물리학회 2018 Current Applied Physics Vol.18 No.6
We investigated the atomic configuration and the electronic structure of a BaBiO3 (BBO) thin film and its (001) surface. It was theoretically predicted that two-dimensional electron gases would be formed when the film is BiO2-terminated. We deduced depth-profile information for a BBO thin film using angle-dependent X-ray photoemission spectroscopy. Analysis of the spectral weights of the Ba 3d and Bi 4f core levels confirmed that the BBO film should have a BiO2-terminated topmost layer. We used in-situ angle-resolved photoemission spectroscopy to experimentally determine the electronic structure of a BBO thin film and found no metallic surface state. We distinguished surface states from bulk states by evaporating potassium atoms in-situ on the surface. A surface state near the bottom of the topmost bulk valence band, which was predicted by the DFT calculations, was identified. However, other surface states well separated from the bulk states were not observed. Our results provided evidence that descriptions of the BBO electronic structure require more detailed and elaborate approaches.
Electronic structure and charge-density wave transition in monolayer VS2
김혁진,최병기,이인학,김민재,천승현,Jozwiak Chris,Bostwick Aaron,Rotenberg Eli,Chang Young Jun 한국물리학회 2021 Current Applied Physics Vol.30 No.-
Vanadium disulfide (VS2) attracts elevated interests for its charge-density wave (CDW) phase transition, ferromagnetism, and catalytic reactivity, but the electronic structure of monolayer has not been well understood yet. Here we report synthesis of epitaxial 1T VS2 monolayer on bilayer graphene grown by molecular-beam epitaxy (MBE). Angle-resolved photoemission spectroscopy (ARPES) measurements reveal that Fermi surface with six elliptical pockets centered at the M points shows gap opening at low temperature. Temperature-dependence of the gap size suggests existence of CDW phase transition above room temperature. Our observations provide important evidence to understand the strongly correlated electron physics and the related surface catalytic properties in two-dimensional transition-metal dichalcogenides (TMDCs).
Direct visualization and control of SrOx segregation on semiconducting Nb doped SrTiO3 (100) surface
유향근,Schwarz Daniel,Ulstrup Søren,Kim Woojin,Jozwiak Chris,Bostwick Aaron,노태원,Rotenberg Eli,Chang Young Jun 한국물리학회 2022 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.80 No.11
We investigated how SrOx segregates on a Nb doped SrTiO3 (100) surface by in air annealing. Using atomic force and photoemission electron microscopes, we can directly visualize the morphology and the electronic phase changes with SrOx segregation. SrOx islands less than 2 μm in size and 1–5 unit cells thick nucleate frst and grow in a labyrinth domain pattern. After prolonged annealing, SrOx forms a ~ 10 nm thick flm. We show that the domain pattern can be controlled by introducing a surface miscut angle of SrTiO3. Additionally, the segregated SrOx has a lower work function, compared to that of SrTiO3. These results suggest that the control and tunability of SrOx segregation is applicable to the design of a new functional electronic devices in the semiconducting SrTiO3 based heterostructure.