Twistronics 2023 International Workshop
on twisted bilayer graphene and beyond
University of Seoul, Seoul, Korea, January 11th ~ 13th, 2023
Twistronics 2023 Poster Contribution
Poster Session is on Jan. 11 (Wed.) & Jan. 12 (Thu.), 16:30 ~ 17:30.
Poster Format:
- Size: max. 100cm for the width, 180cm for the height
Ferroelectric HfZrO2-based atomically-thin two-dimensional semiconductor MoS2field effect transistor working at low temperature of 80 K
Moonyoung Jung1, Hyo-Bae Kim2, Ji-Hoon Ahn2, Dongseok Suh1*
1Department of Energy Science, Sungkyunkwan University, Suwon 16419, Korea
2Department of Materials Science and Chemical Engineering, Hanyang University, Ansan 15588, Korea
HfO2-based ferroelectrics have various advantages, such as excellent compatibility with complementary metal-oxide-semiconductor (CMOS) processes and sustaining ferroelectricity even in thin films and at low temperatures. These days, ferroelectric-based random-access memory, low-power transistor, and neuromorphic device are actively under investigation. We fabricated a Zr-doped HfO2 (HfZrO2) based ferroelectric field effect transistor using an atomically thin two-dimensional MoS2 as a channel. The hysteresis in channel current flowing through MoS2 appeared as a function of gate voltage due to the ferroelectric gate dielectrics of HfZrO2. We checked the performance of this ferroelectric-gated MoS2 transistor working at a low temperature of 80 K. All device parameters, such as subthreshold swing and memory window as well as on/off ratio, clearly showed that it could be the memory device working well in cryogenic memory applications. This research was supported by the National Research Foundation of Korea with project no. NRF-2022M3I7A3051578.
Activating magnetoelectric optical properties by twisting antiferromagnetic bilayers
Kunihiro Yananose1
1 Center for Theoretical Physics, Department of Physics and Astronomy,
Seoul National University, Seoul 08826, Republic of Korea
kunihiro@snu.ac.kr
Twisting in bilayers introduces structural chirality with two enantiomers, i.e., left- and right-handed bilayers, depending on whether the twist is clockwise or counterclockwise. The interplay between this global chirality and additional degrees of freedom, such as magnetic ordering and the local octahedral chirality arising from the geometry of the bonds, can yield striking phenomena. In this work [1], we focus on antiferromagnetic CrI3 twisted bilayers, which are characterized by a staggered octahedral chirality in each monolayer. Using symmetry analysis, density functional theory, and tight-binding model calculations, we show that layer’s twisting can lower the structural and magnetic point-group symmetries, thus activating pyroelectricity and the magneto-optical Kerr effect, which would otherwise be absent in untwisted antiferromagnetic bilayers. Interestingly, both electric polarization and Kerr angle are controllable by the twist angle, and their sign is reversible by switching between left- and right-twist. These findings demonstrate that the interplay between twisting and octahedral chirality in magnetic bilayers represents an extraordinary resource for tailoring their physical properties for spintronic and optoelectronic applications.
[1] K. Yananose et al., Physical Review B 106, 184408 (2022).
Bio:
Kunihiro Yananose is a fresh Ph.D. in Physics who is going to get his degree in the coming February. He studies theoretical and computational condensed matter physics, using the density functional theory as the main methodology.
Field-driven Rugged Forest of 1D Ni-doped Au@FexOy Magnetoplasmonic Nanorods for Photoelectrochemical Catalyst
Mahendra Goddatiaa, Lemma Teshome Tufaaa, Jaebeom Leea,b*
aDepartment of Chemical Engineering and Applied Chemistry, Chungnam National University, Daejeon, 34134, Republic of Korea
bDepartment of Chemistry, Chungnam National University, Daejeon, 34134, Republic of Korea
*Corresponding author: Jaebeom Lee, nanoleelab@cnu.ac.kr
Abstract: A feasible nanoscale framework of plasmonic heterogeneous materials and proper surface engineering can enhance photoelectrochemical (PEC) water-splitting performance owing to increased light absorbance, efficient bulk carrier transport, and interfacial charge transfer. In this article, a new magnetoplasmonic (MagPlas) Ni-doped Au@FexOy nanorods (NRs) based material is introduced as a novel photoanode for PEC water splitting. The core-shell Ni/Au@FexOy MagPlas NRs were synthesized by a two-step process, i.e., one-pot solvothermal synthesis of Au@FexOy, where the hallow FexOy Nanotubes (NTs) is a hybrid of Fe2O3 and Fe3O4 and its sequential hydrothermal treatment for Ni doping. Then, a transverse magnetic field – induced assembly was adopted to decorate Ni/Au@FexOy on fluorine doped oxide (FTO) glass to be an artificially roughen morphologic surface, called rugged forest, which allows more light absorption. Then, its optical and surface characterization, COMSOL simulations were carried out. The core-shell Ni/Au@FexOy MagPlas NRs has achieved a photocurrent of 2.72 mA∙cm-2 at 1.23 VRHE benefiting from rugged morphology that provides more active sites and oxygen vacancies as the hole transfer medium, and high scattered light absorption to improve the interface charge transfer performance of the photoanode. It is probable that the current observation provides important insights about plasmonic photocatalytic hybrids as well as surface morphology into the fabrication of efficient photoanodes for PEC.
Keywords: 1D nanomaterial, rugged forest, surface scattering, magnetoplasmonic nanorods, photoanode, water splitting.
Reference:
- Nguyen, Huu-Quang, et al. “One-Pot Synthesis of Magnetoplasmonic Au@FexOy Nanowires: Bioinspired Bouligand Chiral Stack.”ACS nano16.4 (2022): 5795-5806.
- Lei, Bo, et al. “In situ synthesis of α-Fe2O3/Fe3O4 heterojunction photoanode via fast flame annealing for enhanced charge separation and water oxidation.” ACS Applied Materials & Interfaces 13.3 (2021): 4785-4795.
- Chen, Xu, et al. “Plasmonic gold nanorods decorated Ti3C2 MXene quantum dots-interspersed nanosheets for full-spectrum photoelectrochemical water splitting.” Chemical Engineering Journal 426 (2021): 130818.
Mechanical manipulation of Moire ferroelectric domain structures in twisted bilayer WSe2
Sang Hwa Park1, Ayoung Yuk1, Hyobin Yoo1, Sang Mo Yang1*
1Department of Physics, Sogang University (35, Baekbeom-ro, Mapo-gu, Seoul, Republic of Korea)
*Corresponding author: Sang Mo Yang, smyang@sogang.ac.kr
Two-dimensional materials with vertical ferroelectric and piezoelectric properties are highly required for the realization of ultrathin advanced electronic devices. In this viewpoint, Moiré superlattices in van der Waals heterostructures with periodic potential domains come in attractive next-generation nonvolatile memory material. Two layers of transition metal dichalcogenides (TMD, MX2 structure where M: transition metal and X: chalcogen) stacked with desirable angle give birth to a micro-to-nano scale Moiré ferroelectric superlattices, so-called MX and XM domains. This spatially new type of polarization changes its versatile structures by interlayer sliding, thereby enabling the manipulation of local ferroelectricity with ease.
Here, we’d like to introduce an ongoing study about the nanonewton(nN)-scale touching effect on these planar ferroelectricity performed by local functional atomic force microscopy (AFM). By taking a point-by-point snapshot of the nanometer size of plasticity in the domain structures, we present experimental approach to the key mechanical characteristics like line tension, defect formation and switches made of bistate topological defect configuration.
[1] Manuscript authors, Manuscript reference.
Bio:
Sang Hwa Park, graduate student in Sogang University, department of Physics. Mainly intersted in two-dimensional ferroelectric van der Waals materials.
(wihf213@sogang.ac.kr)
Moiré flat bands and interfacial charge polarization in lattice relaxed twisted bilayer hexagonal boron nitride
Fengping Li1, Dongkyu Lee1,2, Nicolas Leconte1, Jiaqi An1, Srivani Javvaji1, and Jeil Jung1, 2
1Department of Physics, University of Seoul, Seoul 02504, Korea
2Department of Smart Cities, University of Seoul, Seoul 02504, Korea
We study the electronic structure and interfacial charge polarization of twisted bilayer hexagonal boron nitride (t2BN) as a function of twist angle and perpendicular electric fields. Our calculations rely on exact exchange and random phase approximation fitted force fields for the atomistic relaxations, and on first principles calculations informed intralayer and interlayer tight-binding hopping terms for the electronic structure. The sizeable interfacial charge polarization for h-BN bilayers near 0° parallel alignment can be understood from the maximization of the local interlayer dipoles forming at AB and BA stacking sites that carry most of the bandgap states, while this polarization is generally suppressed near 60° antiparallel alignment using a similar argument. Perpendicular electric fields can further be leveraged to tune the size of these AB and BA stacking areas as well as their interlayer distance, thus yielding an additional control knob to tune the band gap size, the width of the flat bands and the distribution of the interlayer charges that are associated with the nearly flat low-energy bands. Maximum flatness is achieved for any angle smaller than 1.08° and 1.5° for parallel alignment and anti-parallel alignment respectively.
Atomic-relaxation effects on electronic structure and spectral functions in twisted monolayer-bilayer graphene (tMBG)
Youngju Park1, Nicolas Leconte1 and Jeil Jung*1
1Department of Physics, University of Seoul,
163 Siripdaero, Dongdaemun-gu, Seoul 02504
yjpark39@gmail.com
We obtain the atomic and electronic structure for twisted monolayer-bilayer graphene (tMBG) using the LAMMPS molecular dynamics code on the one hand and a two-center approximation tight-binding (TB) approach informed by density functional theory (DFT) on the other hand[1]. The smaller twist angle’s more substantial atomic relaxation effects lead to a significant total energy reduction. We observe a minimum bandwidths W= ~5 meV and a maximum secondary band gap δS(V/C) = ~40 meV for θ = ~1.2° twist angle. Also, we calculate the layer-resolved spectral functions as a function of energy near the two Dirac points where K corresponds to the bilayer and K’ to monolayer graphene. We analyze the spectral functions for constant energy cuts to understand how relaxation affects the spectral signatures, thus providing guidance at interpreting possible experiments.
[1] Nicolas Leconte et al., Phys. Rev. B 106, 115410 (2022).
[2] Youngju Park, Nicolas Leconte, and Jeil Jung, in preparation.
Acknowledgments:
This work was supported by the Korean NRF through the Grants No. 2021R1A6A3A13045898 (Y.P.), No. 2020R1A5A1016518 (N. L), and Samsung Science and Technology Foundation Grant No. SSTF-BA1802-06 (J.J.).
Hubbard magnetic phases in magic-angle twisted bilayer graphene and self-consistent extended Hubbard parameters
Dongkyu Lee1,2, Jeil Jung1,2
1Department of Physics, University of Seoul, Seoul 02504, Korea
2Department of Smart Cities, University of Seoul, Seoul 02504, Korea
The DFT+U or mean-field Hubbard model approach improves the precision of the electronic band structure calculations in describing localized, strongly correlated electronic properties. However, The Hubbard U correction has the limitation of empirically determining the U value and ignoring the interaction between sites that affect the hopping parameters. To solve this, a research group has proposed a pseudo-hybrid density functional approach for the extended Hubbard model and reported that the errors of bandgaps and magnetic moments are significantly improved compared to the LDA and GGA for several materials[1]. On the other hand, it has been reported that local magnetization appears in twisted bilayer graphene with a lower Hubbard U value than in monolayer graphene[2]. Because of this, it is an increasingly important issue to obtain realistic values of the graphene hopping parameters and Hubbard model parameters.
In this research, we report realistic estimates of on-site Hubbard U and inter-site Hubbard V1 and V2 for the π bands of bilayer graphene based on the Agapito-Curtarolo-Buongiorno-Nardelli(ACBN0) pseudohybrid functional method[1]. In addition, the hopping parameters with self-energy correction using the extended Hubbard parameters are reported. We confirmed that the intersite Hubbard interaction corrected hopping term is larger than previously reported values and agrees well with the experimentally measured fermi velocity. Finally, we show the magnetic phase diagram and band structures of magic-angle twisted bilayer graphene using Hubbard model.
[1] Lee, Sang-Hoon et al., Physical Review Research, 2(4) (2020).
[2] V. Vahedi et al., SciPost Phys., 11, 083 (2021).
Electronic flat bands in twisted two-interface bilayer-monolayer-bilayer and monolayer-bilayer-monolayer graphene
Youngju Park1, Jiseon Shin1, Jisoo Woo1, Yoonsung Lee1 and Jeil Jung*1
1Department of Physics, University of Seoul,
163 Siripdaero, Dongdaemun-gu, Seoul 02504
woojisoo97@naver.com
We explore electronic flat bands in twisted monolayer-bilayer-monolayer graphene (tMBMG) and twisted bilayer-monolayer-bilayer (tBMBG) using an effective continuum model by R. Bistritzer et al. [1]. We calculate bandwidths (W) and band gaps (δP/S)within the parameter space of twist angles and interlayer potential differences. Both tMBMG and tBMBG have a bandwidth minimum near the twist angle of 1.5°, similar to twisted trilayer graphene (t3G). Still, the band isolation requires sufficiently large interlayer potential differences greater than ~0.05eV. By estimating the ratio U/W of an effective Coulomb potential U and the bandwidths, we observe that the AABC and ABBBC stacking in the tMBMG and tBMBG have a higher probability to observe isolated flat bands that satisfy U/W > 1. Also, we analyze the localization and topological properties of the flatband states.
[1] Rafi Bistritzer and Allan H. MacDonald, Phys. Rev. B 81, 245412 (2010).
[2] Youngju Park, Jiseon Shin, Jisoo Woo, Yoonsung Lee, and Jeil Jung, in preparation.
Acknowledgments:
This work was supported by the Korean NRF through the Grants No. 2021R1A6A3A13045898 (Y.P.), No. 2021R1A6A3A01087281 (J.S.), No. 2020R1A2C3009142 (J.W., Y.L.) and Samsung Science and Technology Foundation Grant No. SSTF-BA1802-06 (J.J.).
Stacking-dependent electronic structures of alternating-twist tetralayer graphene
Jiseon Shin1, Yoonsung Lee1, Yoonsung Lee1 and Jeil Jung*1
1Department of Physics, University of Seoul, Seoul 02504
1Department of Smart Cities, University of Seoul, Seoul 02504, Korea
yslee1108@g.uos.ac.kr
We study the single-particle of the electronic structure of the alternating-twist tetralayer graphene at θ = 1.75° using the continuum model for three different commensurate stackings, AA, AB, and SP at the middle moire interface, regarding the model as two sets of the twisted bilayer graphene as the interlayer potential difference varies. We present numerical analysis in the single-particle framework on bandwidths, bandgaps, and K-valley Chern numbers of the lowest-energy valence and conduction bands as a function of the twist angle and the interlayer potential difference. We analyze the linear longitudinal interband optical absorptions as a function of photon energy and the absorption map in moire Brillouin zone for specific transition energies. We estimate the ratio of the Coulomb interaction to the bandwidth as a function of the twist angle and the interlayer potential difference in a bid to give an insight to find the parameter set for substantial many-body effects in electronic structures together with quasiparticle band structures for the three commensurate stackings.
[1]Y. Cao, V. Fatemi, S. Fang, K. Watanabe, T. Taniguchi, E. Kaxiras, and P. Jarillo-Herrer, Nature 556, 43(2018).
[2] Y. Cao, V. Fatemi, A. Demir, S. Fang, S. L. Tomarken, J. Y. Luo, J. D. Sanchez-Yamagishi, K. Watanabe, T. Taniguchi, E. Kaxiras, R. C. Ashoori, and P. Jarillo Herrero, Nature 556, 80(2018).
[3] R. Bistritzer and A. H. MacDonald, PNAS 108, 12233(2011).
[4] J. Jung, A. Raoux, Z. Qiao, and A. H. MacDonald, Phys Rev. B 89, 205414 (2014).
Acknowledgments:
This work was supported by Samsung Science and Technology Foundation Grant No. SSTF-BA1802-06 (J.S.), Korean NRF through the Grants No.2021R1A6A3A01087281 (J.S.), No. 2020R1A2C3009142(J.J.). We acknowledge computational support from KISTI Grant No. KSC-2021_CRE-0389 and by the computing resources of Urban Big data and AI Institute(UBAI) at UOS
