Eda Goki
Degree: PhD
Position: Associate Professor
Affiliation: NUS – Department of Physics
Research Type: Experiment
Office: S13-02-05
Email: g.eda@nus.edu.sg
Contact: (65) 6516 2970
Website: http://www.physics.nus.edu.sg/~phyeda/members.html
Research Interests:
Photoluminescence spectroscopy, charge transport, electro-optics, solar energy conversion, crystal phase engineering- Investigate fundamental low dimensionality effects in nano-materials;
- Engineer the structure and chemistry of nano-materials;
- Control nano-materials assembly into macroscopic structures;
- Implement nano-materials into devices.
CA2DM Publications:
2025 |
Zhu, Yiru; Zhang, Zhepeng; Wang, Ye; Sarkar, Soumya; Li, Yang; Yan, Han; Ishibe-Veiga, Larissa; Bagri, Anita; Yang, Ziwei Jeffrey; Ramsden, Hugh; Eda, Goki; Hoye, Robert L Z; Wang, Yan; Chhowalla, Manish Effect of Substrate on Sulfur Vacancy Defect-Mediated Photoluminescence in Two-Dimensional MoS2 Journal Article JOURNAL OF PHYSICAL CHEMISTRY C, 129 (17), pp. 8294-8302, 2025, ISSN: 1932-7447. @article{ISI:001470681000001, title = {Effect of Substrate on Sulfur Vacancy Defect-Mediated Photoluminescence in Two-Dimensional MoS_{2}}, author = {Yiru Zhu and Zhepeng Zhang and Ye Wang and Soumya Sarkar and Yang Li and Han Yan and Larissa Ishibe-Veiga and Anita Bagri and Ziwei Jeffrey Yang and Hugh Ramsden and Goki Eda and Robert L Z Hoye and Yan Wang and Manish Chhowalla}, doi = {10.1021/acs.jpcc.4c08491}, times_cited = {0}, issn = {1932-7447}, year = {2025}, date = {2025-04-18}, journal = {JOURNAL OF PHYSICAL CHEMISTRY C}, volume = {129}, number = {17}, pages = {8294-8302}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {Chalcogen vacancy defects in monolayer transition metal dichalcogenides form in-gap states that can trap excitons, leading to defect-mediated photoluminescence (PL) emission. Here, we show that room-temperature (RT, 300 K) PL from sulfur vacancies in defective monolayer MoS2 is sensitive to doping from dielectric substrates such as SiO2 and HfO2. The defect-mediated PL is observed for monolayer MoS2 on untreated HfO2 but is quenched on untreated SiO2, which is attributed to electron doping of MoS2 on SiO2. Electron doping of MoS2 is confirmed by Raman and synchrotron X-ray photoelectron spectroscopy. Annealing of the SiO2 substrate modifies its surface states, which is reflected in the recovery of the defect-mediated PL emission. The role of substrate-induced doping on sulfur vacancy-mediated PL is further supported by gate-dependent PL measurements. Our results suggest that excess electrons fill the defect energy states from sulfur vacancies in MoS2, reducing the probability of photoexcited carrier occupation and subsequent defect-mediated emission.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Chalcogen vacancy defects in monolayer transition metal dichalcogenides form in-gap states that can trap excitons, leading to defect-mediated photoluminescence (PL) emission. Here, we show that room-temperature (RT, 300 K) PL from sulfur vacancies in defective monolayer MoS2 is sensitive to doping from dielectric substrates such as SiO2 and HfO2. The defect-mediated PL is observed for monolayer MoS2 on untreated HfO2 but is quenched on untreated SiO2, which is attributed to electron doping of MoS2 on SiO2. Electron doping of MoS2 is confirmed by Raman and synchrotron X-ray photoelectron spectroscopy. Annealing of the SiO2 substrate modifies its surface states, which is reflected in the recovery of the defect-mediated PL emission. The role of substrate-induced doping on sulfur vacancy-mediated PL is further supported by gate-dependent PL measurements. Our results suggest that excess electrons fill the defect energy states from sulfur vacancies in MoS2, reducing the probability of photoexcited carrier occupation and subsequent defect-mediated emission. |
Freeman, Charlie W F; Youel, Harry; Budniak, Adam K; Xue, Zekun; Libero, Henry De; Thomson, Thomas; Bosman, Michel; Eda, Goki; Kurebayashi, Hidekazu; Cubukcu, Murat Tunable Ultrastrong Magnon-Magnon Coupling Approaching the Deep-Strong Regime in a van der Waals Antiferromagnet Journal Article ACS NANO, 19 (16), pp. 16024-16031, 2025, ISSN: 1936-0851. @article{ISI:001469180000001, title = {Tunable Ultrastrong Magnon-Magnon Coupling Approaching the Deep-Strong Regime in a van der Waals Antiferromagnet}, author = {Charlie W F Freeman and Harry Youel and Adam K Budniak and Zekun Xue and Henry De Libero and Thomas Thomson and Michel Bosman and Goki Eda and Hidekazu Kurebayashi and Murat Cubukcu}, doi = {10.1021/acsnano.5c02576}, times_cited = {0}, issn = {1936-0851}, year = {2025}, date = {2025-04-17}, journal = {ACS NANO}, volume = {19}, number = {16}, pages = {16024-16031}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {Antiferromagnetic (AFM) magnons in van der Waals (vdW) materials offer substantial potential for applications in magnonics and spintronics. In this study, we demonstrate ultrastrong magnon-magnon coupling in the GHz regime within a vdW AFM, achieving a maximum coupling rate of 0.91. Our investigation shows the tunability of coupling strength through temperature-dependent magnetic anisotropies. We compare coupling strength values derived from the gap size from the measured spectrum with those calculated directly through the coupling parameter and show that the gap size as a measure of coupling strength is limited for the ultrastrong coupling regime. Additionally, analytical calculations show the possibility to reach the deep-strong coupling regime by engineering the magnetic anisotropy. These findings highlight the potential of vdW AFMs as a model case to study magnetization dynamics in low-symmetry magnetic materials.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Antiferromagnetic (AFM) magnons in van der Waals (vdW) materials offer substantial potential for applications in magnonics and spintronics. In this study, we demonstrate ultrastrong magnon-magnon coupling in the GHz regime within a vdW AFM, achieving a maximum coupling rate of 0.91. Our investigation shows the tunability of coupling strength through temperature-dependent magnetic anisotropies. We compare coupling strength values derived from the gap size from the measured spectrum with those calculated directly through the coupling parameter and show that the gap size as a measure of coupling strength is limited for the ultrastrong coupling regime. Additionally, analytical calculations show the possibility to reach the deep-strong coupling regime by engineering the magnetic anisotropy. These findings highlight the potential of vdW AFMs as a model case to study magnetization dynamics in low-symmetry magnetic materials. |
Liang, Haidong; Chen, Yuan; Loh, Leyi; Cheng, Nicholas Lin Quan; Litvinov, Dmitrii; Yang, Chengyuan; Chen, Yifeng; Zhang, Zhepeng; Watanabe, Kenji; Taniguchi, Takashi; Koperski, Maciej; Quek, Su Ying; Bosman, Michel; Eda, Goki; Bettiol, Andrew Anthony Site-Selective Creation of Blue Emitters in Hexagonal Boron Nitride Journal Article ACS NANO, 19 (15), pp. 15130-15138, 2025, ISSN: 1936-0851. @article{ISI:001465855100001, title = {Site-Selective Creation of Blue Emitters in Hexagonal Boron Nitride}, author = {Haidong Liang and Yuan Chen and Leyi Loh and Nicholas Lin Quan Cheng and Dmitrii Litvinov and Chengyuan Yang and Yifeng Chen and Zhepeng Zhang and Kenji Watanabe and Takashi Taniguchi and Maciej Koperski and Su Ying Quek and Michel Bosman and Goki Eda and Andrew Anthony Bettiol}, doi = {10.1021/acsnano.5c03423}, times_cited = {0}, issn = {1936-0851}, year = {2025}, date = {2025-04-12}, journal = {ACS NANO}, volume = {19}, number = {15}, pages = {15130-15138}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {Hexagonal boron nitride (hBN) has been of great interest due to its ability to host several bright quantum emitters at room temperature. However, the identification of the observed emitters remains challenging due to spectral variability, as well as the lack of atomic defect structure information. In this work, we demonstrate the site-selective creation of blue emitters in exfoliated hBN flakes with high-energy ion irradiation. With the correlation analysis of cryogenic and temperature-dependent photoluminescence (PL) spectroscopy, we observe two zero phonon lines (ZPLs) at similar to 432.8 and 454.3 nm. Photoluminescence excitation (PLE) measurements further confirm the emission origins of the two prominent lines. Scanning transmission electron microscopy (STEM) reveals that the dominant defect structures present in ion-irradiated samples are vacancy-type (V x ) and adatom(intercalant)-type (A x ). Together with first-principles GW-BSE (Bethe-Salpeter equation) calculations, we deduce that the observed blue emissions are likely related to boron intercalants (Bint). Our results not only discover a group of blue emissions in hBN but also provide insights into the physical origin of the emissions with local atomic structures in hBN.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Hexagonal boron nitride (hBN) has been of great interest due to its ability to host several bright quantum emitters at room temperature. However, the identification of the observed emitters remains challenging due to spectral variability, as well as the lack of atomic defect structure information. In this work, we demonstrate the site-selective creation of blue emitters in exfoliated hBN flakes with high-energy ion irradiation. With the correlation analysis of cryogenic and temperature-dependent photoluminescence (PL) spectroscopy, we observe two zero phonon lines (ZPLs) at similar to 432.8 and 454.3 nm. Photoluminescence excitation (PLE) measurements further confirm the emission origins of the two prominent lines. Scanning transmission electron microscopy (STEM) reveals that the dominant defect structures present in ion-irradiated samples are vacancy-type (V x ) and adatom(intercalant)-type (A x ). Together with first-principles GW-BSE (Bethe-Salpeter equation) calculations, we deduce that the observed blue emissions are likely related to boron intercalants (Bint). Our results not only discover a group of blue emissions in hBN but also provide insights into the physical origin of the emissions with local atomic structures in hBN. |
Ho, Yi Wei; Chen, Mingjun; Wong, Cheng Quan; Grzeszczyk, Magdalena; Budniak, Adam K; Viana-Gomes, Jose C; Li, Xinwei; Koperski, Maciej; Eda, Goki Imaging the Neel Vector in Few-Layer CrPS4 with Second-Harmonic Generation Journal Article NANO LETTERS, 25 (14), pp. 5624-5630, 2025, ISSN: 1530-6984. @article{ISI:001453704200001, title = {Imaging the Neel Vector in Few-Layer CrPS_{4} with Second-Harmonic Generation}, author = {Yi Wei Ho and Mingjun Chen and Cheng Quan Wong and Magdalena Grzeszczyk and Adam K Budniak and Jose C Viana-Gomes and Xinwei Li and Maciej Koperski and Goki Eda}, doi = {10.1021/acs.nanolett.4c06237}, times_cited = {0}, issn = {1530-6984}, year = {2025}, date = {2025-03-26}, journal = {NANO LETTERS}, volume = {25}, number = {14}, pages = {5624-5630}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {Second-harmonic generation (SHG) is a powerful technique for probing the crystallographic symmetry. Recently, SHG has also been shown to exhibit sensitivity to time-reversal symmetry, allowing studies of magnetic order in layered antiferromagnetic (AFM) materials. Here, we report SHG investigation of chromium thiophosphate (CrPS4), a layered A-type AFM material with a finite/zero net magnetization for odd/even-layer-number crystals, respectively. The SHG intensity of bulk CrPS4 across its Neel temperature reveals that both crystallographic and magnetic symmetry breaking contribute to SHG at low temperatures. From the polarization analysis, we identify the tensor element linked to the magnetic order. Interestingly, the polarization response is sensitive to the history of the magnetic field applied to the even-number-layer crystals, suggesting that the SHG is sensitive to the Neel vector direction. Polarized SHG microscopy of a terraced CrPS4 shows that the AFM order of even-number-layer regions is determined by the ferromagnetic order of the neighboring odd-number-layer regions.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Second-harmonic generation (SHG) is a powerful technique for probing the crystallographic symmetry. Recently, SHG has also been shown to exhibit sensitivity to time-reversal symmetry, allowing studies of magnetic order in layered antiferromagnetic (AFM) materials. Here, we report SHG investigation of chromium thiophosphate (CrPS4), a layered A-type AFM material with a finite/zero net magnetization for odd/even-layer-number crystals, respectively. The SHG intensity of bulk CrPS4 across its Neel temperature reveals that both crystallographic and magnetic symmetry breaking contribute to SHG at low temperatures. From the polarization analysis, we identify the tensor element linked to the magnetic order. Interestingly, the polarization response is sensitive to the history of the magnetic field applied to the even-number-layer crystals, suggesting that the SHG is sensitive to the Neel vector direction. Polarized SHG microscopy of a terraced CrPS4 shows that the AFM order of even-number-layer regions is determined by the ferromagnetic order of the neighboring odd-number-layer regions. |
Dabrowski, Maciej; Haldar, Sumit; Khan, Safe; Keatley, Paul S; Sagkovits, Dimitros; Xue, Zekun; Freeman, Charlie; Verzhbitskiy, Ivan; Griepe, Theodor; Atxitia, Unai; Eda, Goki; Kurebayashi, Hidekazu; Santos, Elton J G; Hicken, Robert J Ultrafast thermo-optical control of spins in a 2D van der Waals semiconductor Journal Article NATURE COMMUNICATIONS, 16 (1), 2025. @article{ISI:001449775100029, title = {Ultrafast thermo-optical control of spins in a 2D van der Waals semiconductor}, author = {Maciej Dabrowski and Sumit Haldar and Safe Khan and Paul S Keatley and Dimitros Sagkovits and Zekun Xue and Charlie Freeman and Ivan Verzhbitskiy and Theodor Griepe and Unai Atxitia and Goki Eda and Hidekazu Kurebayashi and Elton J G Santos and Robert J Hicken}, doi = {10.1038/s41467-025-58065-1}, times_cited = {1}, year = {2025}, date = {2025-03-21}, journal = {NATURE COMMUNICATIONS}, volume = {16}, number = {1}, publisher = {NATURE PORTFOLIO}, address = {HEIDELBERGER PLATZ 3, BERLIN, 14197, GERMANY}, abstract = {Laser pulses provide one of the fastest means of manipulating electron spins in magnetic compounds and pave the way to ultrafast operation within magnetic recording, quantum computation and spintronics. However, effective management of the heat deposited during optical excitation is an open challenge. Layered two-dimensional (2D) van der Waals (vdW) materials possess unique thermal properties due to the highly anisotropic nature of their chemical bonding. Here we show how to control the rate of heat flow, and hence the magnetization dynamics, induced by an ultrafast laser pulse within the 2D ferromagnet Cr2Ge2Te6. Using time-resolved beam-scanning magneto-optical Kerr effect microscopy and microscopic spin modelling calculations, we show that by reducing the thickness of the magnetic layers, an enhancement of the heat dissipation rate into the adjacent substrate leads to a substantial reduction in the timescale for magnetization recovery from several nanoseconds down to a few hundred picoseconds. Finally, we demonstrate how the low thermal conductivity across vdW layers may be used to obtain magnetic domain memory behaviour, even after exposure to intense laser pulses. Our findings reveal the distinctive role of vdW magnets in the ultrafast control of heat conduction, spin dynamics and non-volatile memory.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Laser pulses provide one of the fastest means of manipulating electron spins in magnetic compounds and pave the way to ultrafast operation within magnetic recording, quantum computation and spintronics. However, effective management of the heat deposited during optical excitation is an open challenge. Layered two-dimensional (2D) van der Waals (vdW) materials possess unique thermal properties due to the highly anisotropic nature of their chemical bonding. Here we show how to control the rate of heat flow, and hence the magnetization dynamics, induced by an ultrafast laser pulse within the 2D ferromagnet Cr2Ge2Te6. Using time-resolved beam-scanning magneto-optical Kerr effect microscopy and microscopic spin modelling calculations, we show that by reducing the thickness of the magnetic layers, an enhancement of the heat dissipation rate into the adjacent substrate leads to a substantial reduction in the timescale for magnetization recovery from several nanoseconds down to a few hundred picoseconds. Finally, we demonstrate how the low thermal conductivity across vdW layers may be used to obtain magnetic domain memory behaviour, even after exposure to intense laser pulses. Our findings reveal the distinctive role of vdW magnets in the ultrafast control of heat conduction, spin dynamics and non-volatile memory. |
Verzhbitskiy, Ivan A; Mishra, Abhishek; Mitra, Sanchali; Zhang, Zhepeng; Das, Sarthak; Lau, Chit Siong; Lee, Rainer; Huang, Ding; Eda, Goki; Ang, Yee Sin; Goh, Kuan Eng Johnson Low-Temperature Contacts and the Coulomb Blockade Effect in Layered Nanoribbons with In-Plane Anisotropy Journal Article ACS NANO, 19 (11), pp. 10878-10888, 2025, ISSN: 1936-0851. @article{ISI:001445756900001, title = {Low-Temperature Contacts and the Coulomb Blockade Effect in Layered Nanoribbons with In-Plane Anisotropy}, author = {Ivan A Verzhbitskiy and Abhishek Mishra and Sanchali Mitra and Zhepeng Zhang and Sarthak Das and Chit Siong Lau and Rainer Lee and Ding Huang and Goki Eda and Yee Sin Ang and Kuan Eng Johnson Goh}, doi = {10.1021/acsnano.4c15086}, times_cited = {0}, issn = {1936-0851}, year = {2025}, date = {2025-03-13}, journal = {ACS NANO}, volume = {19}, number = {11}, pages = {10878-10888}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {One-dimensional (1D) nanoribbons (NRs) constitute rapidly advancing nanotechnology with significant potential for emerging applications such as quantum sensing and metrology. TiS3 nanoribbons exhibit strong in-plane crystal anisotropy, enabling robust 1D confinement and resilience to edge disorder. Nevertheless, charge transport in 1D TiS3 remains relatively unexplored, particularly at low temperatures, where high contact resistance impacts device performance and fundamentally limits its applications. Here, we engineer electrical contacts between a bulk metal and a 1D NR and explore the low-temperature characteristics of the 1D field-effect devices. We report ohmic contacts for 1D TiS3 with temperature-independent contact resistances as low as 2.7 +/- 0.3 k Omega keywords = {}, pubstate = {published}, tppubtype = {article} } One-dimensional (1D) nanoribbons (NRs) constitute rapidly advancing nanotechnology with significant potential for emerging applications such as quantum sensing and metrology. TiS3 nanoribbons exhibit strong in-plane crystal anisotropy, enabling robust 1D confinement and resilience to edge disorder. Nevertheless, charge transport in 1D TiS3 remains relatively unexplored, particularly at low temperatures, where high contact resistance impacts device performance and fundamentally limits its applications. Here, we engineer electrical contacts between a bulk metal and a 1D NR and explore the low-temperature characteristics of the 1D field-effect devices. We report ohmic contacts for 1D TiS3 with temperature-independent contact resistances as low as 2.7 +/- 0.3 k Omega |
Loh, Leyi; Ning, Shoucong; Kieczka, Daria; Chen, Yuan; Yang, Jianmin; Wang, Zhe; Pennycook, Stephen J; Eda, Goki; Shluger, Alexander L; Bosman, Michel Electron Ptychography for Atom-by-Atom Quantification of 1D Defect Complexes in Monolayer MoS2 Journal Article ACS NANO, 19 (6), pp. 6195-6208, 2025, ISSN: 1936-0851. @article{ISI:001416560700001, title = {Electron Ptychography for Atom-by-Atom Quantification of 1D Defect Complexes in Monolayer MoS_{2}}, author = {Leyi Loh and Shoucong Ning and Daria Kieczka and Yuan Chen and Jianmin Yang and Zhe Wang and Stephen J Pennycook and Goki Eda and Alexander L Shluger and Michel Bosman}, doi = {10.1021/acsnano.4c14988}, times_cited = {1}, issn = {1936-0851}, year = {2025}, date = {2025-02-07}, journal = {ACS NANO}, volume = {19}, number = {6}, pages = {6195-6208}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {Defect complexes can induce beneficial functionalities in two-dimensional (2D) semiconductors. However, understanding their formation mechanism with single-atom sensitivity has proven to be challenging for light elements using conventional transmission electron microscopy (TEM) techniques. Here, we demonstrate the atom-resolved formation of various one-dimensional (1D) defect complexes-consisting of rhenium dopants, sulfur interstitials, and sulfur vacancies-in monolayer MoS2 by applying electron ptychography to our four-dimensional scanning transmission electron microscopy (4D-STEM) data sets. Our image resolution of 0.35 angstrom and a spatial precision of 2 pm allow us to achieve accurate matching between experimental structures and density functional theory (DFT) simulations at the atomic level. Additionally, we utilize out-of-focus ptychography to observe defect formation processes at dose rates comparable to those used in conventional TEM imaging, while maintaining a large field of view. This study demonstrates the systematic application of electron ptychography to extensive 4D-STEM data sets for quantitative defect imaging in 2D materials. We provide direct, atomically precise evidence that critical defect densities govern the formation of extended 1D defect complexes. For instance, we show that sulfur single-vacancy lines form when the vacancy density reaches 5 x 1013 cm-2 and transform into double-vacancy lines beyond 8 x 1013 cm-2. Rhenium-dopant lines emerge at a dopant concentration higher than 3 x 1013 cm-2, where metastable sulfur interstitial-vacancy lines also form as the cumulative electron dose reaches 3 x 105 e/angstrom 2, initiating a local nucleation of the 1T ' phase. Our results highlight the potential of electron ptychography for high-precision defect characterization and engineering in ultrathin 2D materials.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Defect complexes can induce beneficial functionalities in two-dimensional (2D) semiconductors. However, understanding their formation mechanism with single-atom sensitivity has proven to be challenging for light elements using conventional transmission electron microscopy (TEM) techniques. Here, we demonstrate the atom-resolved formation of various one-dimensional (1D) defect complexes-consisting of rhenium dopants, sulfur interstitials, and sulfur vacancies-in monolayer MoS2 by applying electron ptychography to our four-dimensional scanning transmission electron microscopy (4D-STEM) data sets. Our image resolution of 0.35 angstrom and a spatial precision of 2 pm allow us to achieve accurate matching between experimental structures and density functional theory (DFT) simulations at the atomic level. Additionally, we utilize out-of-focus ptychography to observe defect formation processes at dose rates comparable to those used in conventional TEM imaging, while maintaining a large field of view. This study demonstrates the systematic application of electron ptychography to extensive 4D-STEM data sets for quantitative defect imaging in 2D materials. We provide direct, atomically precise evidence that critical defect densities govern the formation of extended 1D defect complexes. For instance, we show that sulfur single-vacancy lines form when the vacancy density reaches 5 x 1013 cm-2 and transform into double-vacancy lines beyond 8 x 1013 cm-2. Rhenium-dopant lines emerge at a dopant concentration higher than 3 x 1013 cm-2, where metastable sulfur interstitial-vacancy lines also form as the cumulative electron dose reaches 3 x 105 e/angstrom 2, initiating a local nucleation of the 1T ' phase. Our results highlight the potential of electron ptychography for high-precision defect characterization and engineering in ultrathin 2D materials. |
2024 |
Loh, Leyi; Ho, Yi Wei; Xuan, Fengyuan; del Aguila, Andres Granados; Chen, Yuan; Wong, See Yoong; Zhang, Jingda; Wang, Zhe; Watanabe, Kenji; Taniguchi, Takashi; Pigram, Paul J; Bosman, Michel; Quek, Su Ying; Koperski, Maciej; Eda, Goki Nb impurity-bound excitons as quantum emitters in monolayer WS2 Journal Article NATURE COMMUNICATIONS, 15 (1), 2024. @article{ISI:001360396900001, title = {Nb impurity-bound excitons as quantum emitters in monolayer WS_{2}}, author = {Leyi Loh and Yi Wei Ho and Fengyuan Xuan and Andres Granados del Aguila and Yuan Chen and See Yoong Wong and Jingda Zhang and Zhe Wang and Kenji Watanabe and Takashi Taniguchi and Paul J Pigram and Michel Bosman and Su Ying Quek and Maciej Koperski and Goki Eda}, doi = {10.1038/s41467-024-54360-5}, times_cited = {2}, year = {2024}, date = {2024-11-20}, journal = {NATURE COMMUNICATIONS}, volume = {15}, number = {1}, publisher = {NATURE PORTFOLIO}, address = {HEIDELBERGER PLATZ 3, BERLIN, 14197, GERMANY}, abstract = {Point defects in crystalline solids behave as optically addressable individual quantum systems when present in sufficiently low concentrations. In two-dimensional (2D) semiconductors, such quantum defects hold potential as versatile single photon sources. Here, we report the synthesis and optical properties of Nb-doped monolayer WS2 in the dilute limit where the average spacing between individual dopants exceeds the optical diffraction limit, allowing the emission spectrum to be studied at the single-dopant level. We show that these individual dopants exhibit common features of quantum emitters, including narrow emission lines (with linewidths <1 meV), strong spatial confinement, and photon antibunching. These emitters consistently occur within a narrow spectral range across multiple samples, distinct from common quantum emitters in van der Waals (vdW) materials that show large ensemble broadening. Analysis of the Zeeman splitting reveals that they can be attributed to bound exciton complexes comprising dark excitons and negatively charged Nb.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Point defects in crystalline solids behave as optically addressable individual quantum systems when present in sufficiently low concentrations. In two-dimensional (2D) semiconductors, such quantum defects hold potential as versatile single photon sources. Here, we report the synthesis and optical properties of Nb-doped monolayer WS2 in the dilute limit where the average spacing between individual dopants exceeds the optical diffraction limit, allowing the emission spectrum to be studied at the single-dopant level. We show that these individual dopants exhibit common features of quantum emitters, including narrow emission lines (with linewidths <1 meV), strong spatial confinement, and photon antibunching. These emitters consistently occur within a narrow spectral range across multiple samples, distinct from common quantum emitters in van der Waals (vdW) materials that show large ensemble broadening. Analysis of the Zeeman splitting reveals that they can be attributed to bound exciton complexes comprising dark excitons and negatively charged Nb. |
Khan, Safe; Aw, Eva S Y; Nagle-Cocco, Liam A V; Sud, Aakanksha; Ghosh, Sukanya; Subhan, Mohammed K B; Xue, Zekun; Freeman, Charlie; Sagkovits, Dimitrios; Gutierrez-Llorente, Araceli; Verzhbitskiy, Ivan; Arroo, Daan M; Zollitsch, Christoph W; Eda, Goki; Santos, Elton J G; Dutton, Sian E; Bramwell, Steven T; Howard, Chris A; Kurebayashi, Hidekazu Spin-Glass States Generated in a van der Waals Magnet by Alkali-Ion Intercalation Journal Article ADVANCED MATERIALS, 36 (36), 2024, ISSN: 0935-9648. @article{ISI:001273247900001, title = {Spin-Glass States Generated in a van der Waals Magnet by Alkali-Ion Intercalation}, author = {Safe Khan and Eva S Y Aw and Liam A V Nagle-Cocco and Aakanksha Sud and Sukanya Ghosh and Mohammed K B Subhan and Zekun Xue and Charlie Freeman and Dimitrios Sagkovits and Araceli Gutierrez-Llorente and Ivan Verzhbitskiy and Daan M Arroo and Christoph W Zollitsch and Goki Eda and Elton J G Santos and Sian E Dutton and Steven T Bramwell and Chris A Howard and Hidekazu Kurebayashi}, doi = {10.1002/adma.202400270}, times_cited = {2}, issn = {0935-9648}, year = {2024}, date = {2024-07-22}, journal = {ADVANCED MATERIALS}, volume = {36}, number = {36}, publisher = {WILEY-V C H VERLAG GMBH}, address = {POSTFACH 101161, 69451 WEINHEIM, GERMANY}, abstract = {Tuning magnetic properties in layered van der Waals (vdW) materials has captured significant attention due to the efficient control of ground states by heterostructuring and external stimuli. Electron doping by electrostatic gating, interfacial charge transfer, and intercalation is particularly effective in manipulating the exchange and spin-orbit properties, resulting in a control of Curie temperature (TC) and magnetic anisotropy. Here, an uncharted role of intercalation is discovered to generate magnetic frustration. As a model study, Na atoms are intercalated into the vdW gaps of pristine Cr2Ge2Te6 (CGT) where generated magnetic frustration leads to emerging spin-glass states coexisting with a ferromagnetic order. A series of dynamic magnetic susceptibility measurements/analysis confirms the formation of magnetic clusters representing slow dynamics with a distribution of relaxation times. The intercalation also modifies other macroscopic physical parameters including the significant enhancement of TC from 66 to 240 K and the switching of magnetic easy-hard axis direction. This study identifies intercalation as a unique route to generate emerging frustrated spin states in simple vdW crystals.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Tuning magnetic properties in layered van der Waals (vdW) materials has captured significant attention due to the efficient control of ground states by heterostructuring and external stimuli. Electron doping by electrostatic gating, interfacial charge transfer, and intercalation is particularly effective in manipulating the exchange and spin-orbit properties, resulting in a control of Curie temperature (TC) and magnetic anisotropy. Here, an uncharted role of intercalation is discovered to generate magnetic frustration. As a model study, Na atoms are intercalated into the vdW gaps of pristine Cr2Ge2Te6 (CGT) where generated magnetic frustration leads to emerging spin-glass states coexisting with a ferromagnetic order. A series of dynamic magnetic susceptibility measurements/analysis confirms the formation of magnetic clusters representing slow dynamics with a distribution of relaxation times. The intercalation also modifies other macroscopic physical parameters including the significant enhancement of TC from 66 to 240 K and the switching of magnetic easy-hard axis direction. This study identifies intercalation as a unique route to generate emerging frustrated spin states in simple vdW crystals. |
Wang, Zhe; Kalathingal, Vijith; Trushin, Maxim; Liu, Jiawei; Wang, Junyong; Guo, Yongxin; Ozyilmaz, Barbaros; Nijhuis, Christian A; Eda, Goki Upconversion electroluminescence in 2D semiconductors integrated with plasmonic tunnel junctions Journal Article NATURE NANOTECHNOLOGY, 19 (7), 2024, ISSN: 1748-3387. @article{ISI:001205711600001, title = {Upconversion electroluminescence in 2D semiconductors integrated with plasmonic tunnel junctions}, author = {Zhe Wang and Vijith Kalathingal and Maxim Trushin and Jiawei Liu and Junyong Wang and Yongxin Guo and Barbaros Ozyilmaz and Christian A Nijhuis and Goki Eda}, doi = {10.1038/s41565-024-01650-0}, times_cited = {6}, issn = {1748-3387}, year = {2024}, date = {2024-04-19}, journal = {NATURE NANOTECHNOLOGY}, volume = {19}, number = {7}, publisher = {NATURE PORTFOLIO}, address = {HEIDELBERGER PLATZ 3, BERLIN, 14197, GERMANY}, abstract = {Plasmonic tunnel junctions are a unique electroluminescent system in which light emission occurs via an interplay between tunnelling electrons and plasmonic fields instead of electron-hole recombination as in conventional light-emitting diodes. It was previously shown that placing luminescent molecules in the tunneling pathway of nanoscopic tunnel junctions results in peculiar upconversion electroluminescence where the energy of emitted photons exceeds that of excitation electrons. Here we report the observation of upconversion electroluminescence in macroscopic van der Waals plasmonic tunnel junctions comprising gold and few-layer graphene electrodes separated by a similar to 2-nm-thick hexagonal boron nitride tunnel barrier and a monolayer semiconductor. We find that the semiconductor ground exciton emission is triggered at excitation electron energies lower than the semiconductor optical gap. Interestingly, this upconversion is reached in devices operating at a low conductance (<10(-6) S) and low power density regime (<10(2) W cm(-2)), defying explanation through existing proposed mechanisms. By examining the scaling relationship between plasmonic and excitonic emission intensities, we elucidate the role of inelastic electron tunnelling dipoles that induce optically forbidden transitions in the few-layer graphene electrode and ultrafast hot carrier transfer across the van der Waals interface.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Plasmonic tunnel junctions are a unique electroluminescent system in which light emission occurs via an interplay between tunnelling electrons and plasmonic fields instead of electron-hole recombination as in conventional light-emitting diodes. It was previously shown that placing luminescent molecules in the tunneling pathway of nanoscopic tunnel junctions results in peculiar upconversion electroluminescence where the energy of emitted photons exceeds that of excitation electrons. Here we report the observation of upconversion electroluminescence in macroscopic van der Waals plasmonic tunnel junctions comprising gold and few-layer graphene electrodes separated by a similar to 2-nm-thick hexagonal boron nitride tunnel barrier and a monolayer semiconductor. We find that the semiconductor ground exciton emission is triggered at excitation electron energies lower than the semiconductor optical gap. Interestingly, this upconversion is reached in devices operating at a low conductance (<10(-6) S) and low power density regime (<10(2) W cm(-2)), defying explanation through existing proposed mechanisms. By examining the scaling relationship between plasmonic and excitonic emission intensities, we elucidate the role of inelastic electron tunnelling dipoles that induce optically forbidden transitions in the few-layer graphene electrode and ultrafast hot carrier transfer across the van der Waals interface. |
2023 |
Wang, Zhe; Kalathingal, Vijith; Eda, Goki; Nijhuis, Christian A Engineering the Outcoupling Pathways in Plasmonic Tunnel Junctions via Photonic Mode Dispersion for Low-Loss Waveguiding Journal Article ACS NANO, 18 (1), pp. 1149-1156, 2023, ISSN: 1936-0851. @article{ISI:001139483700001, title = {Engineering the Outcoupling Pathways in Plasmonic Tunnel Junctions via Photonic Mode Dispersion for Low-Loss Waveguiding}, author = {Zhe Wang and Vijith Kalathingal and Goki Eda and Christian A Nijhuis}, doi = {10.1021/acsnano.3c10832}, times_cited = {2}, issn = {1936-0851}, year = {2023}, date = {2023-12-26}, journal = {ACS NANO}, volume = {18}, number = {1}, pages = {1149-1156}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {Outcoupling of plasmonic modes excited by inelastic electron tunneling (IET) across plasmonic tunnel junctions (TJs) has attracted significant attention due to low operating voltages and fast excitation rates. Achieving selectivity among various outcoupling channels, however, remains a challenging task. Employing nanoscale antennas to enhance the local density of optical states (LDOS) associated with specific outcoupling channels partially addressed the problem, along with the integration of conducting 2D materials into TJs, improving the outcoupling to guided modes with particular momentum. The disadvantage of such methods is that they often involve complex fabrication steps and lack fine-tuning options. Here, we propose an alternative approach by modifying the dielectric medium surrounding TJs. By employing a simple multilayer substrate with a specific permittivity combination for the TJs under study, we show that it is possible to optimize mode selectivity in outcoupling to a plasmonic or a photonic-like mode characterized by distinct cutoff behaviors and propagation length. Theoretical and experimental results obtained with a SiO2-SiN-glass multilayer substrate demonstrate high relative coupling efficiencies of (62.77 +/- 1.74)% and (29.07 +/- 0.72)% for plasmonic and photonic-like modes, respectively. The figure-of-merit, which quantifies the tradeoff between mode outcoupling and propagation lengths (tens of mu m) for both modes, can reach values as high as 180 and 140. The demonstrated approach allows LDOS engineering and customized TJ device performance, which are seamlessly integrated with standard thin film fabrication protocols. Our experimental device is well-suited for integration with silicon nitride photonics platforms.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Outcoupling of plasmonic modes excited by inelastic electron tunneling (IET) across plasmonic tunnel junctions (TJs) has attracted significant attention due to low operating voltages and fast excitation rates. Achieving selectivity among various outcoupling channels, however, remains a challenging task. Employing nanoscale antennas to enhance the local density of optical states (LDOS) associated with specific outcoupling channels partially addressed the problem, along with the integration of conducting 2D materials into TJs, improving the outcoupling to guided modes with particular momentum. The disadvantage of such methods is that they often involve complex fabrication steps and lack fine-tuning options. Here, we propose an alternative approach by modifying the dielectric medium surrounding TJs. By employing a simple multilayer substrate with a specific permittivity combination for the TJs under study, we show that it is possible to optimize mode selectivity in outcoupling to a plasmonic or a photonic-like mode characterized by distinct cutoff behaviors and propagation length. Theoretical and experimental results obtained with a SiO2-SiN-glass multilayer substrate demonstrate high relative coupling efficiencies of (62.77 +/- 1.74)% and (29.07 +/- 0.72)% for plasmonic and photonic-like modes, respectively. The figure-of-merit, which quantifies the tradeoff between mode outcoupling and propagation lengths (tens of mu m) for both modes, can reach values as high as 180 and 140. The demonstrated approach allows LDOS engineering and customized TJ device performance, which are seamlessly integrated with standard thin film fabrication protocols. Our experimental device is well-suited for integration with silicon nitride photonics platforms. |
Ren, Tianhua; Wang, Junyong; Han, Kaizhen; Kang, Yuye; Kumar, Annie; Zhang, Gong; Wang, Zhe; Oulton, Rupert F; Eda, Goki; Gong, Xiao Optical Gain Spectrum and Confinement Factor of a Monolayer Semiconductor in an Ultrahigh-Quality Cavity Journal Article NANO LETTERS, 23 (24), pp. 11601-11607, 2023, ISSN: 1530-6984. @article{ISI:001133459300001, title = {Optical Gain Spectrum and Confinement Factor of a Monolayer Semiconductor in an Ultrahigh-Quality Cavity}, author = {Tianhua Ren and Junyong Wang and Kaizhen Han and Yuye Kang and Annie Kumar and Gong Zhang and Zhe Wang and Rupert F Oulton and Goki Eda and Xiao Gong}, doi = {10.1021/acs.nanolett.3c03357}, times_cited = {0}, issn = {1530-6984}, year = {2023}, date = {2023-12-08}, journal = {NANO LETTERS}, volume = {23}, number = {24}, pages = {11601-11607}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {Two-dimensional (2D) semiconductors have attracted great attention as a novel class of gain materials for low-threshold, on-chip coherent light sources. Despite several experimental reports on lasing, the underlying gain mechanism of 2D materials remains elusive due to a lack of key information, including modal gain and the confinement factor. Here, we demonstrate a novel approach to directly determine the absorption coefficient of monolayer WS2 by characterizing the whispering gallery modes in a van der Waals microdisk cavity. By exploiting the cavity's high intrinsic quality factor of 2.5 x 10(4), the absorption coefficient spectrum and confinement factor are experimentally resolved with unprecedented accuracy. The excitonic gain reduces the WS2 absorption coefficient by 2 x 10(4) cm(-1) at room temperature, and the experimental confinement factor is found to agree with the theoretical prediction. These results are essential for unveiling the gain mechanism in emergent, low-threshold 2D-semiconductor-based laser devices.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Two-dimensional (2D) semiconductors have attracted great attention as a novel class of gain materials for low-threshold, on-chip coherent light sources. Despite several experimental reports on lasing, the underlying gain mechanism of 2D materials remains elusive due to a lack of key information, including modal gain and the confinement factor. Here, we demonstrate a novel approach to directly determine the absorption coefficient of monolayer WS2 by characterizing the whispering gallery modes in a van der Waals microdisk cavity. By exploiting the cavity's high intrinsic quality factor of 2.5 x 10(4), the absorption coefficient spectrum and confinement factor are experimentally resolved with unprecedented accuracy. The excitonic gain reduces the WS2 absorption coefficient by 2 x 10(4) cm(-1) at room temperature, and the experimental confinement factor is found to agree with the theoretical prediction. These results are essential for unveiling the gain mechanism in emergent, low-threshold 2D-semiconductor-based laser devices. |