Andrew Wee
Degree: PhD
Position: Professor
Affiliation: NUS – Department of Physics
Research Type: Experiment
Office: S13 03-12
Email: phyweets@nus.edu.sg
Contact: (65) 6516 6362
Website: http://www.physics.nus.edu.sg/~phyweets/
Research Interests
Scanning tunneling microscopy (STM) and synchrotron radiation studies of the molecule-substrate interface, graphene and 2D materials, and related device studies
CA2DM Publications:
2025 |
Liu, Xiongfang; Yang, Tong; Chen, Shanquan; Wu, Jing; Tang, Chi Sin; Ning, Yuanjie; Chen, Zuhuang; Dai, Liang; Sun, Mengxia; Chen, Mingyao; Han, Kun; Zhou, Difan; Zeng, Shengwei; Sun, Shuo; Li, Sensen; Yang, Ming; Breese, Mark B H; Cai, Chuanbing; Venkatesan, Thirumalai; Wee, Andrew T S; Yin, Xinmao Small polarons mediated near-room-temperature metal-insulator transition in vanadium dioxide and their hopping dynamics Journal Article APPLIED PHYSICS REVIEWS, 12 (1), 2025, ISSN: 1931-9401. @article{ISI:001403236800001, title = {Small polarons mediated near-room-temperature metal-insulator transition in vanadium dioxide and their hopping dynamics}, author = {Xiongfang Liu and Tong Yang and Shanquan Chen and Jing Wu and Chi Sin Tang and Yuanjie Ning and Zuhuang Chen and Liang Dai and Mengxia Sun and Mingyao Chen and Kun Han and Difan Zhou and Shengwei Zeng and Shuo Sun and Sensen Li and Ming Yang and Mark B H Breese and Chuanbing Cai and Thirumalai Venkatesan and Andrew T S Wee and Xinmao Yin}, doi = {10.1063/5.0236807}, times_cited = {0}, issn = {1931-9401}, year = {2025}, date = {2025-03-01}, journal = {APPLIED PHYSICS REVIEWS}, volume = {12}, number = {1}, publisher = {AIP Publishing}, address = {1305 WALT WHITMAN RD, STE 300, MELVILLE, NY 11747-4501 USA}, abstract = {Researchers pursuing advanced photoelectric devices have discovered near room-temperature metal-insulator transitions (MIT) in nonvolatile VO2. Despite theoretical investigations suggesting that polaron dynamics mediate the MIT, direct experimental evidence remains scarce. In this study, we present direct evidence of the polaron state in insulating VO2 through high-resolution spectroscopic ellipsometry measurements and first-principles calculations. We illustrate the complementary role of polaron dynamics in facilitating Peierls and Mott transitions, thereby contributing to the MIT processes. Furthermore, our observations and characterizations of conventional metallic and correlated plasmons in the respective phases of the VO2 film offer valuable insight into their electron structures. This investigation enhances comprehension of the MIT mechanism in correlated systems and underscores the roles of polarons, lattice distortions, and electron correlations in facilitating phase transition processes in strongly correlated systems. Additionally, the detailed detection of small polarons and plasmons serves as inspiration for the development of new device functionalities.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Researchers pursuing advanced photoelectric devices have discovered near room-temperature metal-insulator transitions (MIT) in nonvolatile VO2. Despite theoretical investigations suggesting that polaron dynamics mediate the MIT, direct experimental evidence remains scarce. In this study, we present direct evidence of the polaron state in insulating VO2 through high-resolution spectroscopic ellipsometry measurements and first-principles calculations. We illustrate the complementary role of polaron dynamics in facilitating Peierls and Mott transitions, thereby contributing to the MIT processes. Furthermore, our observations and characterizations of conventional metallic and correlated plasmons in the respective phases of the VO2 film offer valuable insight into their electron structures. This investigation enhances comprehension of the MIT mechanism in correlated systems and underscores the roles of polarons, lattice distortions, and electron correlations in facilitating phase transition processes in strongly correlated systems. Additionally, the detailed detection of small polarons and plasmons serves as inspiration for the development of new device functionalities. |
2024 |
Lu, Bin; Niu, Yue; Chen, Qian; Wong, Ping Kwan Johnny; Guo, Qingjie; Jiang, Wei; Rath, Ashutosh; Pennycook, Stephen J; Wang, Lei; Xia, Ke; Zhai, Ya; Wee, Andrew Thye Shen; Zhang, Wen Is Semiconducting Transition-Metal Dichalcogenide Suitable for Spin Pumping? Journal Article NANO LETTERS, 25 (1), pp. 35-40, 2024, ISSN: 1530-6984. @article{ISI:001379965300001, title = {Is Semiconducting Transition-Metal Dichalcogenide Suitable for Spin Pumping?}, author = {Bin Lu and Yue Niu and Qian Chen and Ping Kwan Johnny Wong and Qingjie Guo and Wei Jiang and Ashutosh Rath and Stephen J Pennycook and Lei Wang and Ke Xia and Ya Zhai and Andrew Thye Shen Wee and Wen Zhang}, doi = {10.1021/acs.nanolett.4c03469}, times_cited = {1}, issn = {1530-6984}, year = {2024}, date = {2024-12-17}, journal = {NANO LETTERS}, volume = {25}, number = {1}, pages = {35-40}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {Spin pumping has been reported on interfaces formed with ferromagnetic metals and layered transition-metal dichalcogenides (TMDs), as signified by enhanced Gilbert damping parameters extracted from magnetodynamics measurements. However, whether the observed damping enhancement purely arises from the pumping effect has remained debatable, given that possible extrinsic disturbances on the interfaces cannot be excluded in most of the experiments. Here, we explore an atomically clean interface formed with CoFeB and atomically thin MoSe2, achieved by an all in situ growth strategy based on molecular beam epitaxy. Taking advantage of ferromagnetic resonance analysis, we find that the Gilbert damping of the CoFeB/MoSe2 interface closely resembles that of CoFeB/SiO2, suggesting the absence of spin pumping. With similar findings demonstrated on a few more representative interfaces, this work clarifies the unsuitability of semiconducting TMDs for spin pumping and suggests that the observed damping enhancement in the previous reports may be predominantly attributed to extrinsic contributions during the experimental process.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Spin pumping has been reported on interfaces formed with ferromagnetic metals and layered transition-metal dichalcogenides (TMDs), as signified by enhanced Gilbert damping parameters extracted from magnetodynamics measurements. However, whether the observed damping enhancement purely arises from the pumping effect has remained debatable, given that possible extrinsic disturbances on the interfaces cannot be excluded in most of the experiments. Here, we explore an atomically clean interface formed with CoFeB and atomically thin MoSe2, achieved by an all in situ growth strategy based on molecular beam epitaxy. Taking advantage of ferromagnetic resonance analysis, we find that the Gilbert damping of the CoFeB/MoSe2 interface closely resembles that of CoFeB/SiO2, suggesting the absence of spin pumping. With similar findings demonstrated on a few more representative interfaces, this work clarifies the unsuitability of semiconducting TMDs for spin pumping and suggests that the observed damping enhancement in the previous reports may be predominantly attributed to extrinsic contributions during the experimental process. |
Wang, Dingguan; Haposan, Tobias; Fan, Jinwei; Arramel, ; Wee, Andrew T S Recent Progress of Imaging Chemical Bonds by Scanning Probe Microscopy: A Review Journal Article ACS NANO, 18 (45), pp. 30919-30942, 2024, ISSN: 1936-0851. @article{ISI:001345125600001, title = {Recent Progress of Imaging Chemical Bonds by Scanning Probe Microscopy: A Review}, author = {Dingguan Wang and Tobias Haposan and Jinwei Fan and Arramel and Andrew T S Wee}, doi = {10.1021/acsnano.4c10522}, times_cited = {1}, issn = {1936-0851}, year = {2024}, date = {2024-10-30}, journal = {ACS NANO}, volume = {18}, number = {45}, pages = {30919-30942}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {In the past decades, the invention of scanning probe microscopy (SPM) as the versatile surface-based characterization of organic molecules has triggered significant interest throughout multidisciplinary fields. In particular, the bond-resolved imaging acquired by SPM techniques has extended its fundamental function of not only unraveling the chemical structure but also allowing us to resolve the structure-property relationship. Here, we present a systematical review on the history of chemical bonds imaged by means of noncontact atomic force microscopy (nc-AFM) and bond-resolved scanning tunneling microscopy (BR-STM) techniques. We first summarize the advancement of real-space imaging of covalent bonds and the investigation of intermolecular noncovalent bonds. Beyond the bond imaging, we also highlight the applications of the bond-resolved SPM techniques such as on-surface synthesis, the determination of the reaction pathway, the identification of molecular configurations and unknown products, and the generation of artificial molecules created via tip manipulation. Lastly, we discuss the current status of SPM techniques and highlight several key technical challenges that must be solved in the coming years. In comparison to the existing reviews, this work invokes researchers from surface science, chemistry, condensed matter physics, and theoretical physics to uncover the bond-resolved SPM technique as an emerging tool in exploiting the molecule/surface system and their future applications.}, keywords = {}, pubstate = {published}, tppubtype = {article} } In the past decades, the invention of scanning probe microscopy (SPM) as the versatile surface-based characterization of organic molecules has triggered significant interest throughout multidisciplinary fields. In particular, the bond-resolved imaging acquired by SPM techniques has extended its fundamental function of not only unraveling the chemical structure but also allowing us to resolve the structure-property relationship. Here, we present a systematical review on the history of chemical bonds imaged by means of noncontact atomic force microscopy (nc-AFM) and bond-resolved scanning tunneling microscopy (BR-STM) techniques. We first summarize the advancement of real-space imaging of covalent bonds and the investigation of intermolecular noncovalent bonds. Beyond the bond imaging, we also highlight the applications of the bond-resolved SPM techniques such as on-surface synthesis, the determination of the reaction pathway, the identification of molecular configurations and unknown products, and the generation of artificial molecules created via tip manipulation. Lastly, we discuss the current status of SPM techniques and highlight several key technical challenges that must be solved in the coming years. In comparison to the existing reviews, this work invokes researchers from surface science, chemistry, condensed matter physics, and theoretical physics to uncover the bond-resolved SPM technique as an emerging tool in exploiting the molecule/surface system and their future applications. |
Chen, Mingyao; Liu, Huimin; He, Xu; Li, Minjuan; Tang, Chi Sin; Sun, Mengxia; Koirala, Krishna Prasad; Bowden, Mark E; Li, Yangyang; Liu, Xiongfang; Zhou, Difan; Sun, Shuo; Breese, Mark B H; Cai, Chuanbing; Wang, Le; Du, Yingge; Wee, Andrew T S; Yin, Xinmao Uncovering an Interfacial Band Resulting from Orbital Hybridization in Nickelate Heterostructures Journal Article ACS NANO, 18 (40), pp. 27707-27717, 2024, ISSN: 1936-0851. @article{ISI:001324763700001, title = {Uncovering an Interfacial Band Resulting from Orbital Hybridization in Nickelate Heterostructures}, author = {Mingyao Chen and Huimin Liu and Xu He and Minjuan Li and Chi Sin Tang and Mengxia Sun and Krishna Prasad Koirala and Mark E Bowden and Yangyang Li and Xiongfang Liu and Difan Zhou and Shuo Sun and Mark B H Breese and Chuanbing Cai and Le Wang and Yingge Du and Andrew T S Wee and Xinmao Yin}, doi = {10.1021/acsnano.4c09921}, times_cited = {1}, issn = {1936-0851}, year = {2024}, date = {2024-09-26}, journal = {ACS NANO}, volume = {18}, number = {40}, pages = {27707-27717}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {The interaction of atomic orbitals at the interface of perovskite oxide heterostructures has been investigated for its profound impact on the band structures and electronic properties, giving rise to unique electronic states and a variety of tunable functionalities. In this study, we conducted an extensive investigation of the optical and electronic properties of epitaxial NdNiO3 synthesized on a series of single-crystal substrates. Unlike nanofilms synthesized on other substrates, NdNiO3 on SrTiO3 (NNO/STO) gives rise to a unique band structure featuring an additional unoccupied band situated above the Fermi level. Our comprehensive investigation, which incorporated a wide array of experimental techniques and density functional theory calculations, revealed that the emergence of the interfacial band structure is primarily driven by orbital hybridization between the Ti 3d orbitals of the STO substrate and the O 2p orbitals of the NNO thin film. Furthermore, exciton peaks have been detected in the optical spectra of the NNO/STO film, attributable to the pronounced electron-electron (e-e) and electron-hole (e-h) interactions propagating from the STO substrate into the NNO film. These findings underscore the substantial influence of interfacial orbital hybridization on the electronic structure of oxide thin films, thereby offering key insights into tuning their interfacial properties.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The interaction of atomic orbitals at the interface of perovskite oxide heterostructures has been investigated for its profound impact on the band structures and electronic properties, giving rise to unique electronic states and a variety of tunable functionalities. In this study, we conducted an extensive investigation of the optical and electronic properties of epitaxial NdNiO3 synthesized on a series of single-crystal substrates. Unlike nanofilms synthesized on other substrates, NdNiO3 on SrTiO3 (NNO/STO) gives rise to a unique band structure featuring an additional unoccupied band situated above the Fermi level. Our comprehensive investigation, which incorporated a wide array of experimental techniques and density functional theory calculations, revealed that the emergence of the interfacial band structure is primarily driven by orbital hybridization between the Ti 3d orbitals of the STO substrate and the O 2p orbitals of the NNO thin film. Furthermore, exciton peaks have been detected in the optical spectra of the NNO/STO film, attributable to the pronounced electron-electron (e-e) and electron-hole (e-h) interactions propagating from the STO substrate into the NNO film. These findings underscore the substantial influence of interfacial orbital hybridization on the electronic structure of oxide thin films, thereby offering key insights into tuning their interfacial properties. |
Wu, Wenjun; Sun, Shuo; Tang, Chi Sin; Wu, Jing; Ma, Yu; Zhang, Lingfeng; Cai, Chuanbing; Zhong, Jianxin; Milosevic, Milorad V; Wee, Andrew T S; Yin, Xinmao Realization of a 2D Lieb Lattice in a Metal-Inorganic Framework with Partial Flat Bands and Topological Edge States Journal Article ADVANCED MATERIALS, 36 (40), 2024, ISSN: 0935-9648. @article{ISI:001296568000001, title = {Realization of a 2D Lieb Lattice in a Metal-Inorganic Framework with Partial Flat Bands and Topological Edge States}, author = {Wenjun Wu and Shuo Sun and Chi Sin Tang and Jing Wu and Yu Ma and Lingfeng Zhang and Chuanbing Cai and Jianxin Zhong and Milorad V Milosevic and Andrew T S Wee and Xinmao Yin}, doi = {10.1002/adma.202405615}, times_cited = {1}, issn = {0935-9648}, year = {2024}, date = {2024-08-23}, journal = {ADVANCED MATERIALS}, volume = {36}, number = {40}, publisher = {WILEY-V C H VERLAG GMBH}, address = {POSTFACH 101161, 69451 WEINHEIM, GERMANY}, abstract = {Flat bands and Dirac cones in materials are the source of the exotic electronic and topological properties. The Lieb lattice is expected to host these electronic structures, arising from quantum destructive interference. Nevertheless, the experimental realization of a 2D Lieb lattice remained challenging to date due to its intrinsic structural instability. After computationally designing a Platinum-Phosphorus (Pt-P) Lieb lattice, it has successfully overcome its structural instability and synthesized on a gold substrate via molecular beam epitaxy. Low-temperature scanning tunneling microscopy and spectroscopy verify the Lieb lattice's morphology and electronic flat bands. Furthermore, topological Dirac edge states stemming from pronounced spin-orbit coupling induced by heavy Pt atoms are predicted. These findings convincingly open perspectives for creating metal-inorganic framework-based atomic lattices, offering prospects for strongly correlated phases interplayed with topology.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Flat bands and Dirac cones in materials are the source of the exotic electronic and topological properties. The Lieb lattice is expected to host these electronic structures, arising from quantum destructive interference. Nevertheless, the experimental realization of a 2D Lieb lattice remained challenging to date due to its intrinsic structural instability. After computationally designing a Platinum-Phosphorus (Pt-P) Lieb lattice, it has successfully overcome its structural instability and synthesized on a gold substrate via molecular beam epitaxy. Low-temperature scanning tunneling microscopy and spectroscopy verify the Lieb lattice's morphology and electronic flat bands. Furthermore, topological Dirac edge states stemming from pronounced spin-orbit coupling induced by heavy Pt atoms are predicted. These findings convincingly open perspectives for creating metal-inorganic framework-based atomic lattices, offering prospects for strongly correlated phases interplayed with topology. |
Sun, Mengxia; He, Xu; Chen, Mingyao; Tang, Chi Sin; Liu, Xiongfang; Dai, Liang; Liu, Jishan; Zeng, Zhigang; Sun, Shuo; Breese, Mark B H; Cai, Chuanbing; Wang, Le; Du, Yingge; Wee, Andrew T S; Yin, Xinmao Tunable Collective Excitations in Epitaxial Perovskite Nickelates Journal Article ACS PHOTONICS, 11 (6), pp. 2324-2334, 2024, ISSN: 2330-4022. @article{ISI:001235219700001, title = {Tunable Collective Excitations in Epitaxial Perovskite Nickelates}, author = {Mengxia Sun and Xu He and Mingyao Chen and Chi Sin Tang and Xiongfang Liu and Liang Dai and Jishan Liu and Zhigang Zeng and Shuo Sun and Mark B H Breese and Chuanbing Cai and Le Wang and Yingge Du and Andrew T S Wee and Xinmao Yin}, doi = {10.1021/acsphotonics.4c00210}, times_cited = {0}, issn = {2330-4022}, year = {2024}, date = {2024-05-29}, journal = {ACS PHOTONICS}, volume = {11}, number = {6}, pages = {2324-2334}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {The formation of plasmons through the collective excitation of charge density has generated intense discussions, offering insights into fundamental sciences and potential applications. While the underlying physical principles have been well-established, the effects of many-body interactions and orbital hybridization on plasmonic dynamics remain understudied. In this work, we present the observation of conventional metallic and correlated plasmons in epitaxial La1-xSrxNiO3 (LSNO) films with varying Sr doping concentrations (x = 0, 0.125, 0.25), unveiling their intriguing evolution. Unlike samples at other doping concentrations, the x = 0.125 intermediate doping sample does not exhibit the correlated plasmons despite showing high optical conductivity. Through a comprehensive experimental investigation using spectroscopic ellipsometry and X-ray absorption spectroscopy, the O2p-Ni3d orbital hybridization for LSNO with a doping concentration of x = 0.125 is found to be significantly enhanced, alongside a considerable weakening of its effective correlation U*. These factors account for the absence of correlated plasmons and the high optical conductivity observed in LSNO (0.125). Our results underscore the profound impact of orbital hybridization on the electronic structure and the formation of plasmons in strongly correlated systems. This in turn suggests that LSNO could serve as a promising alternative material in optoelectronic devices.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The formation of plasmons through the collective excitation of charge density has generated intense discussions, offering insights into fundamental sciences and potential applications. While the underlying physical principles have been well-established, the effects of many-body interactions and orbital hybridization on plasmonic dynamics remain understudied. In this work, we present the observation of conventional metallic and correlated plasmons in epitaxial La1-xSrxNiO3 (LSNO) films with varying Sr doping concentrations (x = 0, 0.125, 0.25), unveiling their intriguing evolution. Unlike samples at other doping concentrations, the x = 0.125 intermediate doping sample does not exhibit the correlated plasmons despite showing high optical conductivity. Through a comprehensive experimental investigation using spectroscopic ellipsometry and X-ray absorption spectroscopy, the O2p-Ni3d orbital hybridization for LSNO with a doping concentration of x = 0.125 is found to be significantly enhanced, alongside a considerable weakening of its effective correlation U*. These factors account for the absence of correlated plasmons and the high optical conductivity observed in LSNO (0.125). Our results underscore the profound impact of orbital hybridization on the electronic structure and the formation of plasmons in strongly correlated systems. This in turn suggests that LSNO could serve as a promising alternative material in optoelectronic devices. |
Lu, Bin; Xia, Yuze; Ren, Yuqian; Xie, Miaomiao; Zhou, Liguo; Vinai, Giovanni; Morton, Simon A; Wee, Andrew T S; van der Wiel, Wilfred G; Zhang, Wen; Wong, Ping Kwan Johnny When Machine Learning Meets 2D Materials: A Review Journal Article 53 ADVANCED SCIENCE, 11 (13), 2024. @article{ISI:001149703000001, title = {When Machine Learning Meets 2D Materials: A Review}, author = {Bin Lu and Yuze Xia and Yuqian Ren and Miaomiao Xie and Liguo Zhou and Giovanni Vinai and Simon A Morton and Andrew T S Wee and Wilfred G van der Wiel and Wen Zhang and Ping Kwan Johnny Wong}, doi = {10.1002/advs.202305277}, times_cited = {53}, year = {2024}, date = {2024-01-26}, journal = {ADVANCED SCIENCE}, volume = {11}, number = {13}, publisher = {WILEY}, address = {111 RIVER ST, HOBOKEN 07030-5774, NJ USA}, abstract = {The availability of an ever-expanding portfolio of 2D materials with rich internal degrees of freedom (spin, excitonic, valley, sublattice, and layer pseudospin) together with the unique ability to tailor heterostructures made layer by layer in a precisely chosen stacking sequence and relative crystallographic alignments, offers an unprecedented platform for realizing materials by design. However, the breadth of multi-dimensional parameter space and massive data sets involved is emblematic of complex, resource-intensive experimentation, which not only challenges the current state of the art but also renders exhaustive sampling untenable. To this end, machine learning, a very powerful data-driven approach and subset of artificial intelligence, is a potential game-changer, enabling a cheaper - yet more efficient - alternative to traditional computational strategies. It is also a new paradigm for autonomous experimentation for accelerated discovery and machine-assisted design of functional 2D materials and heterostructures. Here, the study reviews the recent progress and challenges of such endeavors, and highlight various emerging opportunities in this frontier research area.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The availability of an ever-expanding portfolio of 2D materials with rich internal degrees of freedom (spin, excitonic, valley, sublattice, and layer pseudospin) together with the unique ability to tailor heterostructures made layer by layer in a precisely chosen stacking sequence and relative crystallographic alignments, offers an unprecedented platform for realizing materials by design. However, the breadth of multi-dimensional parameter space and massive data sets involved is emblematic of complex, resource-intensive experimentation, which not only challenges the current state of the art but also renders exhaustive sampling untenable. To this end, machine learning, a very powerful data-driven approach and subset of artificial intelligence, is a potential game-changer, enabling a cheaper - yet more efficient - alternative to traditional computational strategies. It is also a new paradigm for autonomous experimentation for accelerated discovery and machine-assisted design of functional 2D materials and heterostructures. Here, the study reviews the recent progress and challenges of such endeavors, and highlight various emerging opportunities in this frontier research area. |
2023 |
Tian, Mingming; Chen, Qian; Wong, Ping Kwan Johnny; Liu, Ruobai; Silly, Fabien; Silly, Mathieu G; Ohresser, Philippe; You, Biao; Du, Jun; Wee, Andrew T S; Rojas-Sanchez, Juan-Carlos; Huang, Zhaocong; Zhang, Wen; Zhai, Ya Modulation of magneto-dynamic properties of permalloy/holmium heterostructures with antiferromagnetic coupled interface Journal Article APPLIED PHYSICS LETTERS, 123 (26), 2023, ISSN: 0003-6951. @article{ISI:001133695400003, title = {Modulation of magneto-dynamic properties of permalloy/holmium heterostructures with antiferromagnetic coupled interface}, author = {Mingming Tian and Qian Chen and Ping Kwan Johnny Wong and Ruobai Liu and Fabien Silly and Mathieu G Silly and Philippe Ohresser and Biao You and Jun Du and Andrew T S Wee and Juan-Carlos Rojas-Sanchez and Zhaocong Huang and Wen Zhang and Ya Zhai}, doi = {10.1063/5.0169500}, times_cited = {3}, issn = {0003-6951}, year = {2023}, date = {2023-12-25}, journal = {APPLIED PHYSICS LETTERS}, volume = {123}, number = {26}, publisher = {AIP Publishing}, address = {1305 WALT WHITMAN RD, STE 300, MELVILLE, NY 11747-4501 USA}, abstract = {Magneto-dynamics and its interfacial modulation have attracted much attention in energy-efficient and nonvolatile spintronic devices. In particular, the antiferromagnetic coupling at the interface plays a crucial role in spin dynamic behaviors. In this work, we utilize rare-earth holmium (Ho) to interface with transition-metal alloy Ni80Fe20(Py) and achieve a naturally formed antiferromagnetic coupling between Py and interfacial Ho via the magnetic proximity effect, as confirmed by element-specific synchrotron radiation x-ray magnetic circular dichroism hysteresis loops. Importantly, the antiferromagnetic coupled interface is preserved even at a low temperature of 4.2 K, which is below the Curie temperature of Ho. Using ferromagnetic resonance analysis, we reveal that the Gilbert damping and the interfacial spin mixing conductance of the Py/Ho bilayers are much larger than those of the Py/Pt and Py/Pd, suggesting a superior spin transparent efficiency on such an interface with an antiferromagnetic coupling. More importantly, upon the insertion of 2-nm-thick Cu, the antiferromagnetic coupling disappears, associated with the evident suppression of Gilbert damping. This strengthens the critical role of the antiferromagnetic coupled interface in the magneto-dynamics of the transition-metal/rare-earth bilayers and provides a promising way of magneto-dynamics modulation in antiferromagnet-based devices.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Magneto-dynamics and its interfacial modulation have attracted much attention in energy-efficient and nonvolatile spintronic devices. In particular, the antiferromagnetic coupling at the interface plays a crucial role in spin dynamic behaviors. In this work, we utilize rare-earth holmium (Ho) to interface with transition-metal alloy Ni80Fe20(Py) and achieve a naturally formed antiferromagnetic coupling between Py and interfacial Ho via the magnetic proximity effect, as confirmed by element-specific synchrotron radiation x-ray magnetic circular dichroism hysteresis loops. Importantly, the antiferromagnetic coupled interface is preserved even at a low temperature of 4.2 K, which is below the Curie temperature of Ho. Using ferromagnetic resonance analysis, we reveal that the Gilbert damping and the interfacial spin mixing conductance of the Py/Ho bilayers are much larger than those of the Py/Pt and Py/Pd, suggesting a superior spin transparent efficiency on such an interface with an antiferromagnetic coupling. More importantly, upon the insertion of 2-nm-thick Cu, the antiferromagnetic coupling disappears, associated with the evident suppression of Gilbert damping. This strengthens the critical role of the antiferromagnetic coupled interface in the magneto-dynamics of the transition-metal/rare-earth bilayers and provides a promising way of magneto-dynamics modulation in antiferromagnet-based devices. |
Koh, See Wee; Rekhi, Lavie; Arramel, ; Birowosuto, Muhammad Danang; Trinh, Quang Thang; Ge, Junyu; Yu, Wei; Wee, Andrew T S; Choksi, Tej S; Li, Hong Tuning the Work Function of MXene via Surface Functionalization Journal Article 24 ACS APPLIED MATERIALS & INTERFACES, 16 (49), pp. 66826-66836, 2023, ISSN: 1944-8244. @article{ISI:001158819600001, title = {Tuning the Work Function of MXene via Surface Functionalization}, author = {See Wee Koh and Lavie Rekhi and Arramel and Muhammad Danang Birowosuto and Quang Thang Trinh and Junyu Ge and Wei Yu and Andrew T S Wee and Tej S Choksi and Hong Li}, doi = {10.1021/acsami.3c11857}, times_cited = {24}, issn = {1944-8244}, year = {2023}, date = {2023-12-14}, journal = {ACS APPLIED MATERIALS & INTERFACES}, volume = {16}, number = {49}, pages = {66826-66836}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {MXenes, a class of two-dimensional materials, have garnered significant attention due to their versatile surface chemistry and customizable properties. In this study, we investigate the work function (WF) tuning capabilities of MXene Ti3C2Tx, where T-x denotes the surface termination, synthesized via both conventional hydrogen fluoride-etched and recently reported molten salt-etched routes. When MXene samples are subjected to gas phase reactions, WF variations exceeding 0.6 eV are achieved, highlighting the potential for precise WF control. Notably, the WF increases from similar to 4.23 eV (in N-doped MXene etched using molten salt) to similar to 4.85 eV (N-doped MXene etched using HF). Complementary density functional theory (DFT) calculations reveal WF tuning across a >1 eV range via modification of the surface with different terminal groups (bare metal, F*, O*, N*, and Cl*). These changes in WF are attributed to surface termination modifications and the formation of TiO2 and TiN phases during annealing. DFT calculations further unveil an inverse correlation between the WF and the electron affinity of surface terminations. The findings from this comprehensive study provide insights into the tunable WF of MXenes, paving the way for their potential applications as interfacial layers in photovoltaic, energy conversion, and storage technologies.}, keywords = {}, pubstate = {published}, tppubtype = {article} } MXenes, a class of two-dimensional materials, have garnered significant attention due to their versatile surface chemistry and customizable properties. In this study, we investigate the work function (WF) tuning capabilities of MXene Ti3C2Tx, where T-x denotes the surface termination, synthesized via both conventional hydrogen fluoride-etched and recently reported molten salt-etched routes. When MXene samples are subjected to gas phase reactions, WF variations exceeding 0.6 eV are achieved, highlighting the potential for precise WF control. Notably, the WF increases from similar to 4.23 eV (in N-doped MXene etched using molten salt) to similar to 4.85 eV (N-doped MXene etched using HF). Complementary density functional theory (DFT) calculations reveal WF tuning across a >1 eV range via modification of the surface with different terminal groups (bare metal, F*, O*, N*, and Cl*). These changes in WF are attributed to surface termination modifications and the formation of TiO2 and TiN phases during annealing. DFT calculations further unveil an inverse correlation between the WF and the electron affinity of surface terminations. The findings from this comprehensive study provide insights into the tunable WF of MXenes, paving the way for their potential applications as interfacial layers in photovoltaic, energy conversion, and storage technologies. |
Zhang, Wen; Fu, Qiang; Li, Jiahui; Lian, Beilei; Xia, Yuze; Zhou, Liguo; Wee, Andrew Thye Shen; Wong, Ping Kwan Johnny Probing van der Waals magnetic surface and interface via circularly polarized X-rays Journal Article 40 APPLIED PHYSICS REVIEWS, 10 (4), 2023, ISSN: 1931-9401. @article{ISI:001111315300002, title = {Probing van der Waals magnetic surface and interface via circularly polarized X-rays}, author = {Wen Zhang and Qiang Fu and Jiahui Li and Beilei Lian and Yuze Xia and Liguo Zhou and Andrew Thye Shen Wee and Ping Kwan Johnny Wong}, doi = {10.1063/5.0164400}, times_cited = {40}, issn = {1931-9401}, year = {2023}, date = {2023-12-01}, journal = {APPLIED PHYSICS REVIEWS}, volume = {10}, number = {4}, publisher = {AIP Publishing}, address = {1305 WALT WHITMAN RD, STE 300, MELVILLE, NY 11747-4501 USA}, abstract = {Advances in research of magnetic two-dimensional van der Waals (2D vdW) materials have opened up new opportunities in miniaturization of spintronic devices at the atomically thin limit. One major research thrust, which is the subject of this review, is that the magnetism of 2D vdW materials and their derived hetero-interface may be significantly affected by the local atomic geometry and environment. As a result, there is a pressing need for powerful advanced technique to characterize magnetic vdW surface and interface. Circularly polarized X-rays from synchrotron radiation light sources are key to this endeavor, as they can probe the microscopic magnetism of a specific element, for their atomic sensitivity, element-specificity, core-level excitation as well as capability of separation for the spin and orbit moments. We review a series of representative experimental achievements, exploiting circularly polarized X-rays, in (1) 2D vdW magnet, (2) magnetic metal/vdW interface, (3) molecule/vdW interface as well as (4) vdW/2D interface. On the basis of the review, advantages of probing magnetic vdW surface and interface by circularly polarized X-rays are presented, and challenges and opportunities are also discussed.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Advances in research of magnetic two-dimensional van der Waals (2D vdW) materials have opened up new opportunities in miniaturization of spintronic devices at the atomically thin limit. One major research thrust, which is the subject of this review, is that the magnetism of 2D vdW materials and their derived hetero-interface may be significantly affected by the local atomic geometry and environment. As a result, there is a pressing need for powerful advanced technique to characterize magnetic vdW surface and interface. Circularly polarized X-rays from synchrotron radiation light sources are key to this endeavor, as they can probe the microscopic magnetism of a specific element, for their atomic sensitivity, element-specificity, core-level excitation as well as capability of separation for the spin and orbit moments. We review a series of representative experimental achievements, exploiting circularly polarized X-rays, in (1) 2D vdW magnet, (2) magnetic metal/vdW interface, (3) molecule/vdW interface as well as (4) vdW/2D interface. On the basis of the review, advantages of probing magnetic vdW surface and interface by circularly polarized X-rays are presented, and challenges and opportunities are also discussed. |