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2. Gate-tunable quantum pathways of high harmonic generation in graphene

Author

Cha, Soonyoung, Kim, Minjeong, Kim, Youngjae, Choi, Shinyoung, Kang, Sejong, Kim, Hoon, Yoon, Sangho, Moon, Gunho, Kim, Taeho, Lee, Ye Won, Cho, Gil Young, Park, Moon Jeong, Kim, Cheol-Joo, Kim, B. J., Lee, JaeDong, Jo, Moon-Ho, and Kim, Jonghwan

Subjects
Condensed Matter - Mesoscale and Nanoscale Physics, Condensed Matter - Materials Science, Physics - Optics
Abstract

Under strong laser fields, electrons in solids radiate high-harmonic fields by travelling through quantum pathways in Bloch bands in the sub-laser-cycle timescales. Understanding these pathways in the momentum space through the high-harmonic radiation can enable an all-optical ultrafast probe to observe coherent lightwave-driven processes and measure electronic structures as recently demonstrated for semiconductors. However, such demonstration has been largely limited for semimetals because the absence of the bandgap hinders an experimental characterization of the exact pathways. In this study, by combining electrostatic control of chemical potentials with HHG measurement, we resolve quantum pathways of massless Dirac fermions in graphene under strong laser fields. Electrical modulation of HHG reveals quantum interference between the multi-photon interband excitation channels. As the light-matter interaction deviates beyond the perturbative regime, elliptically polarized laser fields efficiently drive massless Dirac fermions via an intricate coupling between the interband and intraband transitions, which is corroborated by our theoretical calculations. Our findings pave the way for strong-laser-field tomography of Dirac electrons in various quantum semimetals and their ultrafast electronics with a gate control., Comment: 33 pages, 4 figures, Soonyoung Cha and Minjeong Kim contributed equally to this work, To whom correspondence should be addressed: jonghwankim@postech

Published
2022
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5. {\theta}-Tunable Photoluminescence from Interlayer Excitons in Twisted Bilayer Graphene

Author

Patel, Hiral, Huang, Lujie, Kim, Cheol-Joo, Park, Jiwoong, and Graham, Matt W.

Subjects
Condensed Matter - Mesoscale and Nanoscale Physics
Abstract

Using resonant 2-photon excitation of interlayer electrons in twisted bilayer graphene (tBLG), we resolve photoluminescence (PL) that tunes spectrally with stacking angle, {\theta}. This weak signal is 4- 5$\times$ larger than the non-resonant background and is emitted from the interlayer band anti-crossing regions traditionally associated with van Hove singularity resonances. However, our observation of resonant PL emission with delayed ~1 ps electronic thermalization suggests interlayer carriers may instead form bound-excitons. Using both the 2-photon PL and intraband transient absorption spectra, we observe bright and dark state peak-splitting associated with an interlayer exciton binding energy ranging from 0.5 to 0.7 eV for {\theta} = 8$^o$ to 17$^o$. These results support theoretical models showing interlayer excitons in tBLG are stabilized by a vanishing exciton-coupling strength to the metallic continuum states. This unexpected dual metal-exciton optical property of tBLG suggests possible {\theta}-tuneable control over carrier thermalization, extraction and emission in optical graphene-based devices.

Published
2018
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7. Atomically-thin Ohmic Edge Contacts Between Two-dimensional Materials

Author

Guimaraes, Marcos H. D., Gao, Hui, Han, Yimo, Kang, Kibum, Xie, Saien, Kim, Cheol-Joo, Muller, David A., Ralph, Daniel C., and Park, Jiwoong

Subjects
Condensed Matter - Mesoscale and Nanoscale Physics, Condensed Matter - Materials Science
Abstract

With the decrease of the dimensions of electronic devices, the role played by electrical contacts is ever increasing, eventually coming to dominate the overall device volume and total resistance. This is especially problematic for monolayers of semiconducting transition metal dichalcogenides (TMDs), which are promising candidates for atomically thin electronics. Ideal electrical contacts to them would require the use of similarly thin electrode materials while maintaining low contact resistances. Here we report a scalable method to fabricate ohmic graphene edge contacts to two representative monolayer TMDs - MoS2 and WS2. The graphene and TMD layer are laterally connected with wafer-scale homogeneity, no observable overlap or gap, and a low average contact resistance of 30 k$\Omega$ $\mu$m. The resulting graphene edge contacts show linear current-voltage (IV) characteristics at room temperature, with ohmic behavior maintained down to liquid helium temperatures.

Published
2016
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9. Circular Dichroism Reflectance Anisotropy of Chiral Atomically Thin Films.

Author

Tomei, Ilaria, Pierucci, Filippo, Bonanni, Beatrice, Sgarlata, Anna, Fanfoni, Massimo, Yang, Seong-Jun, Kim, Cheol-Joo, and Goletti, Claudio

Subjects
CIRCULAR dichroism, THIN films, LINEAR dichroism, REFLECTANCE spectroscopy, OPTICAL properties
Abstract

Recently, a technical modification of a Reflectance Anisotropy Spectroscopy (RAS) spectrometer has been proposed to investigate the circular dichroism (CD) of samples instead of the normally studied linear dichroism. CD-RAS measures the anisotropy of the optical properties of a sample under right-handed and left-handed circularly polarized light. Here, we present the application of CD-RAS to measure the circular dichroism of a twisted bilayer of graphene, purposely prepared as a possible substrate for the adsorption of thin molecular layers, in air, in liquid or in a vacuum. This result demonstrates the performance of the apparatus and shows interesting perspectives for the investigation of chiral organic assemblies deposited in solid film. [ABSTRACT FROM AUTHOR]

Published
2024
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11. Single-Shot Multispectral Encoding: Advancing Optical Lithography for Encryption and Spectroscopy

Author

Shim, Hyewon, Park, Geonwoong, Yun, Hyunsuk, Ryu, Sunmin, Noh, Yong-Young, and Kim, Cheol-Joo

Abstract

Most modern optical display and sensing devices utilize a limited number of spectral units within the visible range, based on human color perception. In contrast, the rapid advancement of machine-based pattern recognition and spectral analysis could facilitate the use of multispectral functional units, yet the challenge of creating complex, high-definition, and reproducible patterns with an increasing number of spectral units limits their widespread application. Here, we report a technique for optical lithography that employs a single-shot exposure to reproduce perovskite films with spatially controlled optical band gaps through light-induced compositional modulations. Luminescent patterns are designed to program correlations between spatial and spectral information, covering the entire visible spectral range. Using this platform, we demonstrate multispectral encoding patterns for encryption and multivariate optical converters for dispersive optics-free spectroscopy with high spectral resolution. The fabrication process is conducted at room temperature and can be extended to other material and device platforms.

Published
2024
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15. Integrated 1D epitaxial mirror twin boundaries for ultrascaled 2D MoS2field-effect transistors

Author

Ahn, Heonsu, Moon, Gunho, Jung, Hang-gyo, Deng, Bingchen, Yang, Dong-Hwan, Yang, Sera, Han, Cheolhee, Cho, Hyunje, Yeo, Youngki, Kim, Cheol-Joo, Yang, Chan-Ho, Kim, Jonghwan, Choi, Si-Young, Park, Hongkun, Jeon, Jongwook, Park, Jin-Hong, and Jo, Moon-Ho

Abstract

In atomically thin van der Waals materials, grain boundaries—the line defects between adjacent crystal grains with tilted in-plane rotations—are omnipresent. When the tilting angles are arbitrary, the grain boundaries form inhomogeneous sublattices, giving rise to local electronic states that are not controlled. Here we report on epitaxial realizations of deterministic MoS2mirror twin boundaries (MTBs) at which two adjoining crystals are reflection mirroring by an exactly 60° rotation by position-controlled epitaxy. We showed that these epitaxial MTBs are one-dimensionally metallic to a circuit length scale. By utilizing the ultimate one-dimensional (1D) feature (width ~0.4 nm and length up to a few tens of micrometres), we incorporated the epitaxial MTBs as a 1D gate to build integrated two-dimensional field-effect transistors (FETs). The critical role of the 1D MTB gate was verified to scale the depletion channel length down to 3.9 nm, resulting in a substantially lowered channel off-current at lower gate voltages. With that, in both individual and array FETs, we demonstrated state-of-the-art performances for low-power logics. The 1D epitaxial MTB gates in this work suggest a novel synthetic pathway for the integration of two-dimensional FETs—that are immune to high gate capacitance—towards ultimate scaling.

Published
2024
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17. Mxene-Based High Performance Microfluidic Ph Sensors for Electronic Tongue

Author

Kim, Hyuk Jin, primary, Lee, Chung Won, additional, Park, So Hyeon, additional, Choi, Sungkyun, additional, Park, Sung Hyuk, additional, Nam, Gi Baek, additional, Ryu, Jung-El, additional, Eom, Tae Hoon, additional, Kim, Byungsoo, additional, Kim, Cheol-Joo, additional, Kim, Soo Young, additional, and Jang, Ho Won, additional

Published
2023
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18. High‐Performance Solution‐Processed 2D P‐Type WSe 2 Transistors and Circuits through Molecular Doping

Author

Zou, Taoyu, primary, Kim, Hyun‐Jun, additional, Kim, Soonhyo, additional, Liu, Ao, additional, Choi, Min‐Yeong, additional, Jung, Haksoon, additional, Zhu, Huihui, additional, You, Insang, additional, Reo, Youjin, additional, Lee, Woo‐Ju, additional, Kim, Yong‐Sung, additional, Kim, Cheol‐Joo, additional, and Noh, Yong‐Young, additional

Published
2022
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20. Gate-tunable quantum pathways of high harmonic generation in graphene

Author

Cha, Soonyoung, primary, Kim, Minjeong, additional, Kim, Youngjae, additional, Choi, Shinyoung, additional, Kang, Sejong, additional, Kim, Hoon, additional, Yoon, Sangho, additional, Moon, Gunho, additional, Kim, Taeho, additional, Lee, Ye Won, additional, Cho, Gil Young, additional, Park, Moon Jeong, additional, Kim, Cheol-Joo, additional, Kim, B. J., additional, Lee, JaeDong, additional, Jo, Moon-Ho, additional, and Kim, Jonghwan, additional

Published
2022
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22. Large Memory Window of van der Waals Heterostructure Devices Based on MOCVD‐Grown 2D Layered Ge 4 Se 9

Author

Noh, Gichang, primary, Song, Hwayoung, additional, Choi, Heenang, additional, Kim, Mingyu, additional, Jeong, Jae Hwan, additional, Lee, Yongjoon, additional, Choi, Min‐Yeong, additional, Oh, Saeyoung, additional, Jo, Min‐kyung, additional, Woo, Dong Yeon, additional, Jo, Yooyeon, additional, Park, Eunpyo, additional, Moon, Eoram, additional, Kim, Tae Soo, additional, Chai, Hyun‐Jun, additional, Huh, Woong, additional, Lee, Chul‐Ho, additional, Kim, Cheol‐Joo, additional, Yang, Heejun, additional, Song, Senugwoo, additional, Jeong, Hu Young, additional, Kim, Yong‐Sung, additional, Lee, Gwan‐Hyoung, additional, Lim, Jongsun, additional, Kim, Chang Gyoun, additional, Chung, Taek‐Mo, additional, Kwak, Joon Young, additional, and Kang, Kibum, additional

Published
2022
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25. Wafer-Scale Programmed Assembly of One-Atom-Thick Crystals

Author

Yang, Seong-Jun, primary, Jung, Ju-Hyun, additional, Lee, Eunsook, additional, Han, Edmund, additional, Choi, Min-Yeong, additional, Jung, Daesung, additional, Choi, Shinyoung, additional, Park, Jun-Ho, additional, Oh, Dongseok, additional, Noh, Siwoo, additional, Kim, Ki-Jeong, additional, Huang, Pinshane Y., additional, Hwang, Chan-Cuk, additional, and Kim, Cheol-Joo, additional

Published
2022
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30. Graphene and boron nitride lateral heterostructures for atomically thin circuitry

Author

Levendorf, Mark P., Kim, Cheol-Joo, Brown, Lola, Huang, Pinshane Y., Havener, Robin W., Muller, David A., and Park, Jiwoong

Subjects
Graphene -- Magnetic properties -- Electric properties -- Properties -- Research, Boron nitride -- Magnetic properties -- Electric properties -- Properties -- Research, Environmental issues, Science and technology, Zoology and wildlife conservation
Abstract

Precise spatial control over the electrical properties of thin films is the key capability enabling the production of modern integrated circuitry. Although recent advances in chemical vapour deposition methods have enabled the large-scale production of both intrinsic and doped graphene (1-6), as well as hexagonal boron nitride (h-BN) (7-10), controlled fabrication of lateral heterostructures in these truly atomically thin systems has not been achieved. Graphene/h-BN interfaces are of particular interest, because it is known that areas of different atomic compositions may coexist within continuous atomically thin films (5,10) and that, with proper control, the bandgap and magnetic properties can be precisely engineered (11-13). However, previously reported approaches for controlling these interfaces have fundamental limitations and cannot be easily integrated with conventional lithography (14-16) Here we report a versatile and scalable process, which we call 'patterned regrowth', that allows for the spatially controlled synthesis of lateral junctions between electrically conductive graphene and insulating h-BN, as well as between intrinsic and substitutionally doped graphene. We demonstrate that the resulting films form mechanically continuous sheets across these heterojunctions. Conductance measurements confirm laterally insulating behaviour for h-BN regions, while the electrical behaviour of both doped and undoped graphene sheets maintain excellent properties, with low sheet resistances and high carrier mobilities. Our results represent an important step towards developing atomically thin integrated circuitry and enable the fabrication of electrically isolated active and passive elements embedded in continuous, one-atom-thick sheets, which could be manipulated and stacked to form complex devices at the ultimate thickness limit., Figure 1a summarizes our approach for the patterned regrowth and characterization. After growing the first film of graphene ([G.sup.1], superscript representing the growth order), we then deposit a protective layer [...]

Published
2012

37. ��-Tunable Photoluminescence from Interlayer Excitons in Twisted Bilayer Graphene

Author

Patel, Hiral, Huang, Lujie, Kim, Cheol-Joo, Park, Jiwoong, and Graham, Matt W.

Subjects
Condensed Matter::Materials Science, Mesoscale and Nanoscale Physics (cond-mat.mes-hall), FOS: Physical sciences, Condensed Matter::Mesoscopic Systems and Quantum Hall Effect
Abstract

Using resonant 2-photon excitation of interlayer electrons in twisted bilayer graphene (tBLG), we resolve photoluminescence (PL) that tunes spectrally with stacking angle, ��. This weak signal is 4- 5$\times$ larger than the non-resonant background and is emitted from the interlayer band anti-crossing regions traditionally associated with van Hove singularity resonances. However, our observation of resonant PL emission with delayed ~1 ps electronic thermalization suggests interlayer carriers may instead form bound-excitons. Using both the 2-photon PL and intraband transient absorption spectra, we observe bright and dark state peak-splitting associated with an interlayer exciton binding energy ranging from 0.5 to 0.7 eV for �� = 8$^o$ to 17$^o$. These results support theoretical models showing interlayer excitons in tBLG are stabilized by a vanishing exciton-coupling strength to the metallic continuum states. This unexpected dual metal-exciton optical property of tBLG suggests possible ��-tuneable control over carrier thermalization, extraction and emission in optical graphene-based devices.

Published
2018
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41. Atomically Smooth Graphene‐Based Hybrid Template for the Epitaxial Growth of Organic Semiconductor Crystals.

Author

Nguyen, Nguyen Ngan, Lee, Hyo Chan, Baek, Kangkyun, Yoo, Min Seok, Lee, Hansol, Lim, Hyungsub, Choi, Shinyoung, Kim, Cheol‐Joo, Nam, SungWoo, and Cho, Kilwon

Abstract

Atomically thin 2D materials are good templates to grow organic semiconductor thin films with desirable features. However, the 2D materials typically exhibit surface roughness and spatial charge inhomogeneity due to nonuniform doping, which can affect the uniform assembly of organic thin films on the 2D materials. A hybrid template is presented for preparation of highly crystalline small‐molecule organic semiconductor thin film that is fabricated by transferring graphene onto a highly ordered self‐assembled monolayer. This hybrid graphene template has low surface roughness and spatially uniform doping, and it yields highly crystalline fullerene thin films with grain sizes >300 nm, which is the largest reported grain size for C60 thin films on 2D materials. A graphene/fullerene/pentacene phototransistor fabricated directly on the hybrid template has five times higher photoresponsivity than a phototransistor fabricated on a conventional graphene template supported by a SiO2 wafer. [ABSTRACT FROM AUTHOR]

Published
2021
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43. Unconventional roles of metal catalysts in chemical-vapor syntheses of single-crystalline nanowires.

Author

Kang, Kibum, Kim, Cheol-Joo, and Jo, Moon-Ho

Subjects
*CRYSTAL whiskers, *NANOWIRES, *EUTECTICS, *NUCLEATION, *CONDENSATION
Abstract

In this invited contribution at the 29th International Conference on the Physics of semiconductors (ICPS 2008), we review two examples of solid-catalytic nanowire (NW) growth in parallel comparisons to the NW growth from the eutectic liquid catalyst. First, we demonstrated the Cu-catalyzed Ge NW growth using GeH4 vapor precursor at 200 °C, which is far below the Cu–Ge eutectic temperature of 644 °C, with a relatively uniform diameter distribution directly templated from that of the catalysts. We provide evidence that the formation of solid Cu3Ge catalysts and Ge diffusion across the catalysts are responsible for such low-temperature growth of Ge NWs in a size-deterministic manner. Second, we show the spontaneous silicidation of NiSix NWs on continuous Ni bulks using SiH4 vapor precursor at 400 °C. This growth is particularly marked in that NiSix NWs are formed in a self-organized manner without employing the nanocluster catalysts. We discuss this spontaneous growth of NiSix NWs within the frame of the nucleation kinetics in the low supersaturation limit in analogous with the earlier examples of the vapor-condensation at the low vapor pressures. [ABSTRACT FROM AUTHOR]

Published
2009
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49. Ultrahigh Electrical Conductivity in p-Type CsPbI2Br Perovskite Thin Films by Modulation Doping

Author

Wang, Teng, Ha, Jangwoo, Mo, Taejoon, Heo, Seung Hwae, Hwang, Hyein, Han, Hyunjin, Kim, Cheol-Joo, Dickey, Michael D., Jin, Hyungyu, and Son, Jae Sung

Abstract

The widespread adoption of halide perovskites for application in thermoelectric devices, DC power generators, and lasers is hindered by their low charge carrier concentration. In particular, increasing their charge carrier concentration is considered the main challenge to serve as a promising room-temperature thermoelectric material. Efforts have been devoted to enhancing the charge carrier concentration by doping and composition engineering. However, the coupling between charge carrier concentration and mobility, along with the poor stability of these materials, impedes their development for thermoelectric applications. Herein, we demonstrate the successful increase in the charge carrier concentration of CsPbI2Br by forming a heterojunction structure with Cu2S via a facile spin-coating method. The excellent band alignment between two materials combined with a charge-transfer mechanism realizes the modulation doping, resulting in 8 orders of magnitude increase in carrier concentration from 1012to 1020cm–3without detrimental effect on the carrier mobility of CsPbI2Br. The thermoelectric power factor of the heterostructured CsPbI2Br reached 6.6 μW/m·K2, which is 330 times higher than that of pristine CsPbI2Br. Furthermore, these films showed higher humidity stability than the control films. This study offers a promising avenue for increasing the charge carrier concentration of halide perovskites, thereby enhancing their potential for various applications.

Published
2024
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