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:
2024 |
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, 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 = {0}, issn = {1748-3387}, year = {2024}, date = {2024-04-19}, journal = {NATURE NANOTECHNOLOGY}, 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 = {0}, 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. |
Vu, Nam Thanh Trung; Loh, Leyi; Chen, Yuan; Wu, Qingyun; Verzhbitskiy, Ivan A A; Watanabe, Kenji; Taniguchi, Takashi; Bosman, Michel; Ang, Yee Sin; Ang, Lay Kee; Trushin, Maxim; Eda, Goki Single Atomic Defect Conductivity for Selective Dilute Impurity Imaging in 2D Semiconductors Journal Article ACS NANO, 17 (16), pp. 15648-15655, 2023, ISSN: 1936-0851. @article{ISI:001048113400001, title = {Single Atomic Defect Conductivity for Selective Dilute Impurity Imaging in 2D Semiconductors}, author = {Nam Thanh Trung Vu and Leyi Loh and Yuan Chen and Qingyun Wu and Ivan A A Verzhbitskiy and Kenji Watanabe and Takashi Taniguchi and Michel Bosman and Yee Sin Ang and Lay Kee Ang and Maxim Trushin and Goki Eda}, doi = {10.1021/acsnano.3c02758}, times_cited = {0}, issn = {1936-0851}, year = {2023}, date = {2023-08-11}, journal = {ACS NANO}, volume = {17}, number = {16}, pages = {15648-15655}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {Precisely controlled impurity doping is of fundamentalsignificancein modern semiconductor technologies. Desired physical propertiesare often achieved at impurity concentrations well below parts permillion level. For emergent two-dimensional semiconductors, developmentof reliable doping strategies is hindered by the inherent difficultyin identifying and quantifying impurities in such a dilute limit wherethe absolute number of atoms to be detected is insufficient for commonanalytical techniques. Here we report rapid high-contrast imagingof dilute single atomic impurities by using conductive atomic forcemicroscopy. We show that the local conductivity is enhanced by morethan 100-fold by a single impurity atom due to resonance-assistedtunneling. Unlike the closely related scanning tunneling microscopy,the local conductivity sensitively depends on the impurity energylevel, allowing minority defects to be selectively imaged. We furtherdemonstrate subsurface impurity detection with single monolayer depthresolution in multilayer materials.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Precisely controlled impurity doping is of fundamentalsignificancein modern semiconductor technologies. Desired physical propertiesare often achieved at impurity concentrations well below parts permillion level. For emergent two-dimensional semiconductors, developmentof reliable doping strategies is hindered by the inherent difficultyin identifying and quantifying impurities in such a dilute limit wherethe absolute number of atoms to be detected is insufficient for commonanalytical techniques. Here we report rapid high-contrast imagingof dilute single atomic impurities by using conductive atomic forcemicroscopy. We show that the local conductivity is enhanced by morethan 100-fold by a single impurity atom due to resonance-assistedtunneling. Unlike the closely related scanning tunneling microscopy,the local conductivity sensitively depends on the impurity energylevel, allowing minority defects to be selectively imaged. We furtherdemonstrate subsurface impurity detection with single monolayer depthresolution in multilayer materials. |
Zhu, Yiru; Lim, Juhwan; Zhang, Zhepeng; Wang, Yan; Sarkar, Soumya; Ramsden, Hugh; Li, Yang; Yan, Han; Phuyal, Dibya; Gauriot, Nicolas; Rao, Akshay; Hoye, Robert L Z; Eda, Goki; Chhowalla, Manish Room-Temperature Photoluminescence Mediated by Sulfur Vacancies in 2D Molybdenum Disulfide Journal Article ACS NANO, 17 (14), pp. 13545-13553, 2023, ISSN: 1936-0851. @article{ISI:001024802200001, title = {Room-Temperature Photoluminescence Mediated by Sulfur Vacancies in 2D Molybdenum Disulfide}, author = {Yiru Zhu and Juhwan Lim and Zhepeng Zhang and Yan Wang and Soumya Sarkar and Hugh Ramsden and Yang Li and Han Yan and Dibya Phuyal and Nicolas Gauriot and Akshay Rao and Robert L Z Hoye and Goki Eda and Manish Chhowalla}, doi = {10.1021/acsnano.3c02103}, times_cited = {0}, issn = {1936-0851}, year = {2023}, date = {2023-07-07}, journal = {ACS NANO}, volume = {17}, number = {14}, pages = {13545-13553}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {Atomic defects inmonolayer transition metal dichalcogenides (TMDs)such as chalcogen vacancies significantly affect their properties.In this work, we provide a reproducible and facile strategy to rationallyinduce chalcogen vacancies in monolayer MoS2 by annealingat 600 & DEG;C in an argon/hydrogen (95%/5%) atmosphere. SynchrotronX-ray photoelectron spectroscopy shows that a Mo 3d(5/2) corepeak at 230.1 eV emerges in the annealed MoS2 associatedwith nonstoichiometric MoS x (0 < x < 2), and Raman spectroscopy shows an enhancement ofthe & SIM;380 cm(-1) peak that is attributed tosulfur vacancies. At sulfur vacancy densities of & SIM;1.8 x10(14) cm(-2), we observe a defect peak at & SIM;1.72 eV (referred to as LXD) at room temperaturein the photoluminescence (PL) spectrum. The LXD peak isattributed to excitons trapped at defect-induced in-gap states andis typically observed only at low temperatures (& LE;77 K). Time-resolvedPL measurements reveal that the lifetime of defect-mediated LXD emission is longer than that of band edge excitons, bothat room and low temperatures (& SIM;2.44 ns at 8 K). The LXD peak can be suppressed by annealing the defective MoS2 in sulfur vapor, which indicates that it is possible to passivatethe vacancies. Our results provide insights into how excitonic anddefect-mediated PL emissions in MoS2 are influenced bysulfur vacancies at room and low temperatures.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Atomic defects inmonolayer transition metal dichalcogenides (TMDs)such as chalcogen vacancies significantly affect their properties.In this work, we provide a reproducible and facile strategy to rationallyinduce chalcogen vacancies in monolayer MoS2 by annealingat 600 & DEG;C in an argon/hydrogen (95%/5%) atmosphere. SynchrotronX-ray photoelectron spectroscopy shows that a Mo 3d(5/2) corepeak at 230.1 eV emerges in the annealed MoS2 associatedwith nonstoichiometric MoS x (0 < x < 2), and Raman spectroscopy shows an enhancement ofthe & SIM;380 cm(-1) peak that is attributed tosulfur vacancies. At sulfur vacancy densities of & SIM;1.8 x10(14) cm(-2), we observe a defect peak at & SIM;1.72 eV (referred to as LXD) at room temperaturein the photoluminescence (PL) spectrum. The LXD peak isattributed to excitons trapped at defect-induced in-gap states andis typically observed only at low temperatures (& LE;77 K). Time-resolvedPL measurements reveal that the lifetime of defect-mediated LXD emission is longer than that of band edge excitons, bothat room and low temperatures (& SIM;2.44 ns at 8 K). The LXD peak can be suppressed by annealing the defective MoS2 in sulfur vapor, which indicates that it is possible to passivatethe vacancies. Our results provide insights into how excitonic anddefect-mediated PL emissions in MoS2 are influenced bysulfur vacancies at room and low temperatures. |
Zollitsch, Christoph W; Khan, Safe; Nam, Vu Thanh Trung; Verzhbitskiy, Ivan A; Sagkovits, Dimitrios; O'Sullivan, James; Kennedy, Oscar W; Strungaru, Mara; Santos, Elton J G; Morton, John J L; Eda, Goki; Kurebayashi, Hidekazu Probing spin dynamics of ultra-thin van der Waals magnets via photon-magnon coupling Journal Article NATURE COMMUNICATIONS, 14 (1), 2023. @article{ISI:001001567500003, title = {Probing spin dynamics of ultra-thin van der Waals magnets via photon-magnon coupling}, author = {Christoph W Zollitsch and Safe Khan and Vu Thanh Trung Nam and Ivan A Verzhbitskiy and Dimitrios Sagkovits and James O'Sullivan and Oscar W Kennedy and Mara Strungaru and Elton J G Santos and John J L Morton and Goki Eda and Hidekazu Kurebayashi}, doi = {10.1038/s41467-023-38322-x}, times_cited = {0}, year = {2023}, date = {2023-05-05}, journal = {NATURE COMMUNICATIONS}, volume = {14}, number = {1}, publisher = {NATURE PORTFOLIO}, address = {HEIDELBERGER PLATZ 3, BERLIN, 14197, GERMANY}, abstract = {van der Waals magnetic materials, which retain magnetism down to a single two-dimensional layer of atoms, have great technological potential for spin-based information processing, however, typical approaches to measure their spin dynamics are often hampered by the small number of spins in a single atomic layer compared to three dimensional materials. Here, Zollitsch et al present a methodology for the detection of spin dynamics in van der Waals magnets via photon-magnon coupling between it and a superconducting resonator, with potential to resolve spin dynamics down to a single monolayer.}, keywords = {}, pubstate = {published}, tppubtype = {article} } van der Waals magnetic materials, which retain magnetism down to a single two-dimensional layer of atoms, have great technological potential for spin-based information processing, however, typical approaches to measure their spin dynamics are often hampered by the small number of spins in a single atomic layer compared to three dimensional materials. Here, Zollitsch et al present a methodology for the detection of spin dynamics in van der Waals magnets via photon-magnon coupling between it and a superconducting resonator, with potential to resolve spin dynamics down to a single monolayer. |
Chen, Yuan; Liang, Haidong; Loh, Leyi; Ho, Yiwei; Verzhbitskiy, Ivan; Watanabe, Kenji; Taniguchi, Takashi; Bosman, Michel; Bettiol, Andrew A; Eda, Goki Gate-Tunable Bound Exciton Manifolds in Monolayer MoSe2 Journal Article NANO LETTERS, 23 (10), pp. 4456-4463, 2023, ISSN: 1530-6984. @article{ISI:000984351100001, title = {Gate-Tunable Bound Exciton Manifolds in Monolayer MoSe2}, author = {Yuan Chen and Haidong Liang and Leyi Loh and Yiwei Ho and Ivan Verzhbitskiy and Kenji Watanabe and Takashi Taniguchi and Michel Bosman and Andrew A Bettiol and Goki Eda}, doi = {10.1021/acs.nanolett.3c00814}, times_cited = {0}, issn = {1530-6984}, year = {2023}, date = {2023-05-03}, journal = {NANO LETTERS}, volume = {23}, number = {10}, pages = {4456-4463}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {Two-dimensional (2D) semiconductors with point defects are predicted to host a variety of bound exciton complexes analogous to trions and biexcitons due to strong many-body effects. However, despite the common observation of defect mediated subgap emission, the existence of such complexes remains elusive. Here, we report the observation of bound exciton (BX) complex manifolds in monolayer MoSe2 with intentionally created monoselenium vacancies (VSe) using proton beam irradiation. The emission intensity of different BX peaks is found to exhibit contrasting dependence on electrostatic doping near the onset of free electron injection. The observed trend is consistent with the model in which free excitons exist in equilibrium with excitons bound to neutral and charged VSe defects, which act as deep acceptors. These complexes are more strongly bound than trions and biexcitons, surviving up to around 180 K, and exhibit moderate valley polarization memory, indicating partial free exciton character.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Two-dimensional (2D) semiconductors with point defects are predicted to host a variety of bound exciton complexes analogous to trions and biexcitons due to strong many-body effects. However, despite the common observation of defect mediated subgap emission, the existence of such complexes remains elusive. Here, we report the observation of bound exciton (BX) complex manifolds in monolayer MoSe2 with intentionally created monoselenium vacancies (VSe) using proton beam irradiation. The emission intensity of different BX peaks is found to exhibit contrasting dependence on electrostatic doping near the onset of free electron injection. The observed trend is consistent with the model in which free excitons exist in equilibrium with excitons bound to neutral and charged VSe defects, which act as deep acceptors. These complexes are more strongly bound than trions and biexcitons, surviving up to around 180 K, and exhibit moderate valley polarization memory, indicating partial free exciton character. |
Liu, Chaofei; Zhang, Xiuying; Wang, Xinyun; Wang, Ziying; Abdelwahab, Ibrahim; Verzhbitskiy, Ivan; Shao, Yan; Eda, Goki; Sun, Wanxin; Shen, Lei; Loh, Kian Ping Ferroelectricity in Niobium Oxide Dihalides NbOX2 (X = Cl, I): A Macroscopic- to Microscopic-Scale Study Journal Article ACS NANO, 17 (8), pp. 7170-7179, 2023, ISSN: 1936-0851. @article{ISI:000969846300001, title = {Ferroelectricity in Niobium Oxide Dihalides NbOX2 (X = Cl, I): A Macroscopic- to Microscopic-Scale Study}, author = {Chaofei Liu and Xiuying Zhang and Xinyun Wang and Ziying Wang and Ibrahim Abdelwahab and Ivan Verzhbitskiy and Yan Shao and Goki Eda and Wanxin Sun and Lei Shen and Kian Ping Loh}, doi = {10.1021/acsnano.2c09267}, times_cited = {0}, issn = {1936-0851}, year = {2023}, date = {2023-04-10}, journal = {ACS NANO}, volume = {17}, number = {8}, pages = {7170-7179}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {2D materials with ferroelectric and piezoelectric properties are of interest for energy harvesting, memory storage and electromechanical systems. Here, we present a systematic study of the ferroelectric properties in NbOX2 (X = Cl, I) across different spatial scales. The in-plane ferroelectricity in NbOX2 was investigated using transport and piezoresponse force microscopy (PFM) measurements, where it was observed that NbOCl2 has a stronger ferroelectric order than NbOI2. A high local field, exerted by both PFM and scanning tunneling microscopy (STM) tips, was found to induce 1D collinear ferroelectric strips in NbOCl2. STM imaging reveals the unreconstructed atomic structures of NbOX2 surfaces, and scanning tunneling spectroscopy was used to probe the electronic states induced at defect (vacancy) sites.}, keywords = {}, pubstate = {published}, tppubtype = {article} } 2D materials with ferroelectric and piezoelectric properties are of interest for energy harvesting, memory storage and electromechanical systems. Here, we present a systematic study of the ferroelectric properties in NbOX2 (X = Cl, I) across different spatial scales. The in-plane ferroelectricity in NbOX2 was investigated using transport and piezoresponse force microscopy (PFM) measurements, where it was observed that NbOCl2 has a stronger ferroelectric order than NbOI2. A high local field, exerted by both PFM and scanning tunneling microscopy (STM) tips, was found to induce 1D collinear ferroelectric strips in NbOCl2. STM imaging reveals the unreconstructed atomic structures of NbOX2 surfaces, and scanning tunneling spectroscopy was used to probe the electronic states induced at defect (vacancy) sites. |
Khela, Maya; Dabrowski, Maciej; Khan, Safe; Keatley, Paul S; Verzhbitskiy, Ivan; Eda, Goki; Hicken, Robert J; Kurebayashi, Hidekazu; Santos, Elton J G Laser-induced topological spin switching in a 2D van der Waals magnet Journal Article NATURE COMMUNICATIONS, 14 (1), 2023. @article{ISI:001029839500027, title = {Laser-induced topological spin switching in a 2D van der Waals magnet}, author = {Maya Khela and Maciej Dabrowski and Safe Khan and Paul S Keatley and Ivan Verzhbitskiy and Goki Eda and Robert J Hicken and Hidekazu Kurebayashi and Elton J G Santos}, doi = {10.1038/s41467-023-37082-y}, times_cited = {0}, year = {2023}, date = {2023-03-13}, journal = {NATURE COMMUNICATIONS}, volume = {14}, number = {1}, publisher = {NATURE PORTFOLIO}, address = {HEIDELBERGER PLATZ 3, BERLIN, 14197, GERMANY}, abstract = {Two-dimensional (2D) van der Waals (vdW) magnets represent one of the most promising horizons for energy-efficient spintronic applications because their broad range of electronic, magnetic and topological properties. However, little is known about the interplay between light and spin properties in vdW layers. Here we show that ultrafast laser excitation can not only generate different type of spin textures in CrGeTe3 vdW magnets but also induce a reversible transformation between them in a topological toggle switch mechanism. Our atomistic spin dynamics simulations and wide-field Kerr microscopy measurements show that different textures can be generated via high-intense laser pulses within the picosecond regime. The phase transformation between the different topological spin textures is obtained as additional laser pulses are applied to the system where the polarisation and final state of the spins can be controlled by external magnetic fields. Our results indicate laser-driven spin textures on 2D magnets as a pathway towards reconfigurable topological architectures at the atomistic level.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Two-dimensional (2D) van der Waals (vdW) magnets represent one of the most promising horizons for energy-efficient spintronic applications because their broad range of electronic, magnetic and topological properties. However, little is known about the interplay between light and spin properties in vdW layers. Here we show that ultrafast laser excitation can not only generate different type of spin textures in CrGeTe3 vdW magnets but also induce a reversible transformation between them in a topological toggle switch mechanism. Our atomistic spin dynamics simulations and wide-field Kerr microscopy measurements show that different textures can be generated via high-intense laser pulses within the picosecond regime. The phase transformation between the different topological spin textures is obtained as additional laser pulses are applied to the system where the polarisation and final state of the spins can be controlled by external magnetic fields. Our results indicate laser-driven spin textures on 2D magnets as a pathway towards reconfigurable topological architectures at the atomistic level. |
Abdelwahab, Ibrahim; Tilmann, Benjamin; Zhao, Xiaoxu; Verzhbitskiy, Ivan; Berte, Rodrigo; Eda, Goki; Wilson, William L; Grinblat, Gustavo; de Menezes, Leonardo S; Loh, Kian Ping; Maier, Stefan A Highly Efficient Sum-Frequency Generation in Niobium Oxydichloride NbOCl2 Nanosheets Journal Article ADVANCED OPTICAL MATERIALS, 11 (7), 2023, ISSN: 2195-1071. @article{ISI:000928804500001, title = {Highly Efficient Sum-Frequency Generation in Niobium Oxydichloride NbOCl_{2} Nanosheets}, author = {Ibrahim Abdelwahab and Benjamin Tilmann and Xiaoxu Zhao and Ivan Verzhbitskiy and Rodrigo Berte and Goki Eda and William L Wilson and Gustavo Grinblat and Leonardo S de Menezes and Kian Ping Loh and Stefan A Maier}, doi = {10.1002/adom.202202833}, times_cited = {0}, issn = {2195-1071}, year = {2023}, date = {2023-02-05}, journal = {ADVANCED OPTICAL MATERIALS}, volume = {11}, number = {7}, publisher = {WILEY-V C H VERLAG GMBH}, address = {POSTFACH 101161, 69451 WEINHEIM, GERMANY}, abstract = {Parametric infrared (IR) upconversion is a process in which low-frequency IR photons are upconverted into high-frequency ultraviolet/visible photons through a nonlinear optical process. It is of paramount importance for a wide range of security, material science, and healthcare applications. However, in general, the efficiencies of upconversion processes are typically extremely low for nanometer-scale materials due to the short penetration depth of the excitation fields. Here, parametric IR upconversion processes, including frequency doubling and sum-frequency generation, are studied in layered van der Waals NbOCl2. An upconversion efficiency of up to 0.004% is attained for the NbOCl2 nanosheets, orders of magnitude higher than previously reported values for nonlinear layered materials. The upconverted signal is sensitive to layer numbers, crystal orientation, excitation wavelength, and temperature, and it can be utilized as an optical cross-correlator for ultrashort pulse characterization.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Parametric infrared (IR) upconversion is a process in which low-frequency IR photons are upconverted into high-frequency ultraviolet/visible photons through a nonlinear optical process. It is of paramount importance for a wide range of security, material science, and healthcare applications. However, in general, the efficiencies of upconversion processes are typically extremely low for nanometer-scale materials due to the short penetration depth of the excitation fields. Here, parametric IR upconversion processes, including frequency doubling and sum-frequency generation, are studied in layered van der Waals NbOCl2. An upconversion efficiency of up to 0.004% is attained for the NbOCl2 nanosheets, orders of magnitude higher than previously reported values for nonlinear layered materials. The upconverted signal is sensitive to layer numbers, crystal orientation, excitation wavelength, and temperature, and it can be utilized as an optical cross-correlator for ultrashort pulse characterization. |
Guo, Qiangbing; Qi, Xiao-Zhuo; Zhang, Lishu; Gao, Meng; Hu, Sanlue; Zhou, Wenju; Zang, Wenjie; Zhao, Xiaoxu; Wang, Junyong; Yan, Bingmin; Xu, Mingquan; Wu, Yun-Kun; Eda, Goki; Xiao, Zewen; Yang, Shengyuan A; Gou, Huiyang; Feng, Yuan Ping; Guo, Guang-Can; Zhou, Wu; Ren, Xi-Feng; Qiu, Cheng-Wei; Pennycook, Stephen J; Wee, Andrew T S Ultrathin quantum light source with van der Waals NbOCl2 crystal Journal Article NATURE, 613 (7942), pp. 53-+, 2023, ISSN: 0028-0836. @article{ISI:000955711300008, title = {Ultrathin quantum light source with van der Waals NbOCl_{2} crystal}, author = {Qiangbing Guo and Xiao-Zhuo Qi and Lishu Zhang and Meng Gao and Sanlue Hu and Wenju Zhou and Wenjie Zang and Xiaoxu Zhao and Junyong Wang and Bingmin Yan and Mingquan Xu and Yun-Kun Wu and Goki Eda and Zewen Xiao and Shengyuan A Yang and Huiyang Gou and Yuan Ping Feng and Guang-Can Guo and Wu Zhou and Xi-Feng Ren and Cheng-Wei Qiu and Stephen J Pennycook and Andrew T S Wee}, doi = {10.1038/s41586-022-05393-7}, times_cited = {9}, issn = {0028-0836}, year = {2023}, date = {2023-01-05}, journal = {NATURE}, volume = {613}, number = {7942}, pages = {53-+}, publisher = {NATURE PORTFOLIO}, address = {HEIDELBERGER PLATZ 3, BERLIN, 14197, GERMANY}, abstract = {Interlayer electronic coupling in two-dimensional materials enables tunable and emergent properties by stacking engineering. However, it also results in significant evolution of electronic structures and attenuation of excitonic effects in two-dimensional semiconductors as exemplified by quickly degrading excitonic photoluminescence and optical nonlinearities in transition metal dichalcogenides when monolayers are stacked into van der Waals structures. Here we report a van der Waals crystal, niobium oxide dichloride (NbOCl2), featuring vanishing interlayer electronic coupling and monolayer-like excitonic behaviour in the bulk form, along with a scalable second-harmonic generation intensity of up to three orders higher than that in monolayer WS2. Notably, the strong second-order nonlinearity enables correlated parametric photon pair generation, through a spontaneous parametric down-conversion (SPDC) process, in flakes as thin as about 46 nm. To our knowledge, this is the first SPDC source unambiguously demonstrated in two-dimensional layered materials, and the thinnest SPDC source ever reported. Our work opens an avenue towards developing van der Waals material-based ultracompact on-chip SPDC sources as well as high-performance photon modulators in both classical and quantum optical technologies(1-4).}, keywords = {}, pubstate = {published}, tppubtype = {article} } Interlayer electronic coupling in two-dimensional materials enables tunable and emergent properties by stacking engineering. However, it also results in significant evolution of electronic structures and attenuation of excitonic effects in two-dimensional semiconductors as exemplified by quickly degrading excitonic photoluminescence and optical nonlinearities in transition metal dichalcogenides when monolayers are stacked into van der Waals structures. Here we report a van der Waals crystal, niobium oxide dichloride (NbOCl2), featuring vanishing interlayer electronic coupling and monolayer-like excitonic behaviour in the bulk form, along with a scalable second-harmonic generation intensity of up to three orders higher than that in monolayer WS2. Notably, the strong second-order nonlinearity enables correlated parametric photon pair generation, through a spontaneous parametric down-conversion (SPDC) process, in flakes as thin as about 46 nm. To our knowledge, this is the first SPDC source unambiguously demonstrated in two-dimensional layered materials, and the thinnest SPDC source ever reported. Our work opens an avenue towards developing van der Waals material-based ultracompact on-chip SPDC sources as well as high-performance photon modulators in both classical and quantum optical technologies(1-4). |
2022 |
Liang, Haidong; Chen, Yuan; Yang, Chengyuan; Watanabe, Kenji; Taniguchi, Takashi; Eda, Goki; Bettiol, Andrew A High Sensitivity Spin Defects in hBN Created by High-Energy He Beam Irradiation Journal Article ADVANCED OPTICAL MATERIALS, 11 (1), 2022, ISSN: 2195-1071. @article{ISI:000873866700001, title = {High Sensitivity Spin Defects in hBN Created by High-Energy He Beam Irradiation}, author = {Haidong Liang and Yuan Chen and Chengyuan Yang and Kenji Watanabe and Takashi Taniguchi and Goki Eda and Andrew A Bettiol}, doi = {10.1002/adom.202201941}, times_cited = {0}, issn = {2195-1071}, year = {2022}, date = {2022-10-27}, journal = {ADVANCED OPTICAL MATERIALS}, volume = {11}, number = {1}, publisher = {WILEY-V C H VERLAG GMBH}, address = {POSTFACH 101161, 69451 WEINHEIM, GERMANY}, abstract = {Recently the negatively charged boron vacancies (V-B(-)) in hexagonal boron nitride (hBN) have been shown as spin defects that have great potential in quantum sensing. However, so far the sensitivity is limited by either photoluminescence (PL) intensity or the optically detected magnetic resonance (ODMR) contrast, and linewidth. In this work, the generation of these spin defects is demonstrated using high-energy helium ion beams, and ODMR measurements with different laser and microwave powers are performed. The spin defects generated by high-energy helium ions exhibit a high PL intensity and ODMR contrast while keeping a small linewidth, hence a good sensitivity. By comparing different fluences of helium irradiations, an optimal fluence is determined which is sufficient in creating spin defects without damaging the overall crystal lattice structure. With this optimal fluence, a high signal-to-noise ratio ODMR spectrum can be obtained with an accurate measurement of zero-field splitting frequency, and a best sensitivity as approximate to 2.55 mu T/root Hz. Moreover, with a focused beam, such spin defects can be created deterministically with nanometer precision.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Recently the negatively charged boron vacancies (V-B(-)) in hexagonal boron nitride (hBN) have been shown as spin defects that have great potential in quantum sensing. However, so far the sensitivity is limited by either photoluminescence (PL) intensity or the optically detected magnetic resonance (ODMR) contrast, and linewidth. In this work, the generation of these spin defects is demonstrated using high-energy helium ion beams, and ODMR measurements with different laser and microwave powers are performed. The spin defects generated by high-energy helium ions exhibit a high PL intensity and ODMR contrast while keeping a small linewidth, hence a good sensitivity. By comparing different fluences of helium irradiations, an optimal fluence is determined which is sufficient in creating spin defects without damaging the overall crystal lattice structure. With this optimal fluence, a high signal-to-noise ratio ODMR spectrum can be obtained with an accurate measurement of zero-field splitting frequency, and a best sensitivity as approximate to 2.55 mu T/root Hz. Moreover, with a focused beam, such spin defects can be created deterministically with nanometer precision. |
Xiao, Han; Zhuang, Wenzhuo; Loh, Leyi; Liang, Tao; Gayen, Anabil; Ye, Peng; Bosman, Michel; Eda, Goki; Wang, Xuefeng; Xu, Mingsheng Van der Waals Epitaxial Growth of 2D Layered Room-Temperature Ferromagnetic CrS2 Journal Article ADVANCED MATERIALS INTERFACES, 9 (30), 2022, ISSN: 2196-7350. @article{ISI:000853537300001, title = {Van der Waals Epitaxial Growth of 2D Layered Room-Temperature Ferromagnetic CrS_{2}}, author = {Han Xiao and Wenzhuo Zhuang and Leyi Loh and Tao Liang and Anabil Gayen and Peng Ye and Michel Bosman and Goki Eda and Xuefeng Wang and Mingsheng Xu}, doi = {10.1002/admi.202201353}, times_cited = {0}, issn = {2196-7350}, year = {2022}, date = {2022-09-13}, journal = {ADVANCED MATERIALS INTERFACES}, volume = {9}, number = {30}, publisher = {WILEY}, address = {111 RIVER ST, HOBOKEN 07030-5774, NJ USA}, abstract = {2D magnetic materials have been attracting enormous interest for both fundamental research and potential spintronic applications. Here, this work demonstrates the van der Waals epitaxial growth of air-stable 2D magnetic CrS2 on mica substrate by chemical vapor deposition (CVD). The layered nature, high crystallinity, stoichiometry, and high-purity 1T phase of the individual CrS2 crystallites are verified by a series of complementary characterization techniques including atomic force microscope, scanning transmission electron microscope and energy dispersive X-ray spectroscopy. The single-crystalline 1T-phase CrS2 crystallites exhibit a metallic nature with a moderate conductivity of 1.78 x 10(2) S m(-1) and a room-temperature ferromagnetism with a coercive field of 28.7 Oe. The CVD growth of the air-stable CrS2 crystallites with 1T phase and high crystallinity offer a new platform for the further physics research and spintronic device applications.}, keywords = {}, pubstate = {published}, tppubtype = {article} } 2D magnetic materials have been attracting enormous interest for both fundamental research and potential spintronic applications. Here, this work demonstrates the van der Waals epitaxial growth of air-stable 2D magnetic CrS2 on mica substrate by chemical vapor deposition (CVD). The layered nature, high crystallinity, stoichiometry, and high-purity 1T phase of the individual CrS2 crystallites are verified by a series of complementary characterization techniques including atomic force microscope, scanning transmission electron microscope and energy dispersive X-ray spectroscopy. The single-crystalline 1T-phase CrS2 crystallites exhibit a metallic nature with a moderate conductivity of 1.78 x 10(2) S m(-1) and a room-temperature ferromagnetism with a coercive field of 28.7 Oe. The CVD growth of the air-stable CrS2 crystallites with 1T phase and high crystallinity offer a new platform for the further physics research and spintronic device applications. |
Zhang, Zhepeng; Liang, Haidong; Loh, Leyi; Chen, Yifeng; Chen, Yuan; Watanabe, Kenji; Taniguchi, Takashi; Quek, Su Ying; Bosman, Michel; Bettiol, Andrew A; Eda, Goki Optically Active Chalcogen Vacancies in Monolayer Semiconductors Journal Article ADVANCED OPTICAL MATERIALS, 10 (23), 2022, ISSN: 2195-1071. @article{ISI:000852786000001, title = {Optically Active Chalcogen Vacancies in Monolayer Semiconductors}, author = {Zhepeng Zhang and Haidong Liang and Leyi Loh and Yifeng Chen and Yuan Chen and Kenji Watanabe and Takashi Taniguchi and Su Ying Quek and Michel Bosman and Andrew A Bettiol and Goki Eda}, doi = {10.1002/adom.202201350}, times_cited = {0}, issn = {2195-1071}, year = {2022}, date = {2022-09-12}, journal = {ADVANCED OPTICAL MATERIALS}, volume = {10}, number = {23}, publisher = {WILEY-V C H VERLAG GMBH}, address = {POSTFACH 101161, 69451 WEINHEIM, GERMANY}, abstract = {Defect engineering of atomically thin semiconducting crystals is an attractive route to developing single-photon sources and valleytronic devices. For these applications, defects with well-defined optical characteristics need to be generated in a precisely controlled manner. However, defect-induced optical features are often complicated by the presence of multiple defect species, hindering the identification of their structural origin. Here, we report systematic generation of optically active atomic defects in monolayer MoS2, WS2, MoSe2, and WSe2 via proton-beam irradiation. Defect-induced emissions are found to occur approximate to 100 to 200 meV below the neutral exciton peak, showing typical characteristics of localized excitons such as saturation at high-excitation rates and long lifetime. Using scanning transmission electron microscopy, it is shown that freshly created chalcogen vacancies are responsible for the localized exciton emission. Density functional theory and ab initio GW plus Bethe-Salpeter-equation calculations reveal that the observed emission can be attributed to transitions involving defect levels of chalcogen vacancy and the valence band edge state.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Defect engineering of atomically thin semiconducting crystals is an attractive route to developing single-photon sources and valleytronic devices. For these applications, defects with well-defined optical characteristics need to be generated in a precisely controlled manner. However, defect-induced optical features are often complicated by the presence of multiple defect species, hindering the identification of their structural origin. Here, we report systematic generation of optically active atomic defects in monolayer MoS2, WS2, MoSe2, and WSe2 via proton-beam irradiation. Defect-induced emissions are found to occur approximate to 100 to 200 meV below the neutral exciton peak, showing typical characteristics of localized excitons such as saturation at high-excitation rates and long lifetime. Using scanning transmission electron microscopy, it is shown that freshly created chalcogen vacancies are responsible for the localized exciton emission. Density functional theory and ab initio GW plus Bethe-Salpeter-equation calculations reveal that the observed emission can be attributed to transitions involving defect levels of chalcogen vacancy and the valence band edge state. |
Verzhbitskiy, Ivan; Eda, Goki Electrostatic control of magnetism: Emergent opportunities with van der Waals materials Journal Article APPLIED PHYSICS LETTERS, 121 (6), 2022, ISSN: 0003-6951. @article{ISI:000838442100024, title = {Electrostatic control of magnetism: Emergent opportunities with van der Waals materials}, author = {Ivan Verzhbitskiy and Goki Eda}, doi = {10.1063/5.0107329}, times_cited = {0}, issn = {0003-6951}, year = {2022}, date = {2022-08-08}, journal = {APPLIED PHYSICS LETTERS}, volume = {121}, number = {6}, publisher = {AIP Publishing}, address = {1305 WALT WHITMAN RD, STE 300, MELVILLE, NY 11747-4501 USA}, abstract = {Since the first reports on the observation of magnetic order in atomically thin crystals of FePS3, CrI3, and CrGeTe3 in 2016 and 2017, there has been a greatly renewed interest in the magnetism of van der Waals (vdW) layered magnets. Due to their dimensionality and structure, ultrathin vdW magnets offer tantalizing prospects for electrostatic control of magnetism for energy-efficient spintronic logic and memory devices. Recent demonstrations revealed unusually high susceptibility of some vdW magnets to electrostatic fields and shed light on a path to room temperature devices, a long-standing goal in spintronics research. In this Perspective, we discuss the potential of different classes of vdW magnets for electrostatic control of magnetism by comparing their properties with those of non-vdW magnets such as dilute magnetic III-V semiconductors and perovskite manganites that have been intensively studied in the past two decades. Published under an exclusive license by AIP Publishing.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Since the first reports on the observation of magnetic order in atomically thin crystals of FePS3, CrI3, and CrGeTe3 in 2016 and 2017, there has been a greatly renewed interest in the magnetism of van der Waals (vdW) layered magnets. Due to their dimensionality and structure, ultrathin vdW magnets offer tantalizing prospects for electrostatic control of magnetism for energy-efficient spintronic logic and memory devices. Recent demonstrations revealed unusually high susceptibility of some vdW magnets to electrostatic fields and shed light on a path to room temperature devices, a long-standing goal in spintronics research. In this Perspective, we discuss the potential of different classes of vdW magnets for electrostatic control of magnetism by comparing their properties with those of non-vdW magnets such as dilute magnetic III-V semiconductors and perovskite manganites that have been intensively studied in the past two decades. Published under an exclusive license by AIP Publishing. |