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:
2024 |
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 = {0}, 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 = {0}, 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 = {0}, 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 28 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 = {28}, 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 = {0}, 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 12 ACS APPLIED MATERIALS & INTERFACES, 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 = {12}, issn = {1944-8244}, year = {2023}, date = {2023-12-14}, journal = {ACS APPLIED MATERIALS & INTERFACES}, 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. |
Hu, Junxiong; Han, Yulei; Chi, Xiao; Omar, Ganesh Ji; Ezzi, Mohammed Mohammed Esmail Al; Gou, Jian; Yu, Xiaojiang; Andrivo, Rusydi; Watanabe, Kenji; Taniguchi, Takashi; Wee, Andrew Thye Shen; Qiao, Zhenhua; Ariando, A Tunable Spin-Polarized States in Graphene on a Ferrimagnetic Oxide Insulator Journal Article ADVANCED MATERIALS, 36 (8), 2023, ISSN: 0935-9648. @article{ISI:001113825300001, title = {Tunable Spin-Polarized States in Graphene on a Ferrimagnetic Oxide Insulator}, author = {Junxiong Hu and Yulei Han and Xiao Chi and Ganesh Ji Omar and Mohammed Mohammed Esmail Al Ezzi and Jian Gou and Xiaojiang Yu and Rusydi Andrivo and Kenji Watanabe and Takashi Taniguchi and Andrew Thye Shen Wee and Zhenhua Qiao and A Ariando}, doi = {10.1002/adma.202305763}, times_cited = {7}, issn = {0935-9648}, year = {2023}, date = {2023-12-06}, journal = {ADVANCED MATERIALS}, volume = {36}, number = {8}, publisher = {WILEY-V C H VERLAG GMBH}, address = {POSTFACH 101161, 69451 WEINHEIM, GERMANY}, abstract = {Spin-polarized two-dimensional (2D) materials with large and tunable spin-splitting energy promise the field of 2D spintronics. While graphene has been a canonical 2D material, its spin properties and tunability are limited. Here, this work demonstrates the emergence of robust spin-polarization in graphene with large and tunable spin-splitting energy of up to 132 meV at zero applied magnetic fields. The spin polarization is induced through a magnetic exchange interaction between graphene and the underlying ferrimagnetic oxide insulating layer, Tm3Fe5O12, as confirmed by its X-ray magnetic circular dichroism (XMCD). The spin-splitting energies are directly measured and visualized by the shift in their Landau-fan diagram mapped by analyzing the measured Shubnikov-de-Haas (SdH) oscillations as a function of applied electric fields, showing consistent fit with the first-principles and machine learning calculations. Further, the observed spin-splitting energies can be tuned over a broad range between 98 and 166 meV by field cooling. The methods and results are applicable to other 2D (magnetic) materials and heterostructures, and offer great potential for developing next-generation spin logic and memory devices.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Spin-polarized two-dimensional (2D) materials with large and tunable spin-splitting energy promise the field of 2D spintronics. While graphene has been a canonical 2D material, its spin properties and tunability are limited. Here, this work demonstrates the emergence of robust spin-polarization in graphene with large and tunable spin-splitting energy of up to 132 meV at zero applied magnetic fields. The spin polarization is induced through a magnetic exchange interaction between graphene and the underlying ferrimagnetic oxide insulating layer, Tm3Fe5O12, as confirmed by its X-ray magnetic circular dichroism (XMCD). The spin-splitting energies are directly measured and visualized by the shift in their Landau-fan diagram mapped by analyzing the measured Shubnikov-de-Haas (SdH) oscillations as a function of applied electric fields, showing consistent fit with the first-principles and machine learning calculations. Further, the observed spin-splitting energies can be tuned over a broad range between 98 and 166 meV by field cooling. The methods and results are applicable to other 2D (magnetic) materials and heterostructures, and offer great potential for developing next-generation spin logic and memory devices. |
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 36 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 = {36}, 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. |
Subagyo, Riki; Maulida, Pramitha Y D; Kowal, Dominik; Hartati, Sri; Muslimawati, Rossyaila M; Zetra, Yulfi; Diguna, Lina J; Akhlus, Syafsir; Mahyuddin, Muhammad H; Zhang, Lei; Tang, Chi S; Diao, Caozheng; Wee, Andrew T S; Birowosuto, Muhammad D; Arramel, Andrivo; Rusydi, Andrivo; Kusumawati, Yuly Spectroscopic Evidence of Localized Small Polarons in Low-Dimensional Ionic Liquid Lead-Free Hybrid Perovskites Journal Article ACS APPLIED MATERIALS & INTERFACES, 15 (47), pp. 54677-54691, 2023, ISSN: 1944-8244. @article{ISI:001111123400001, title = {Spectroscopic Evidence of Localized Small Polarons in Low-Dimensional Ionic Liquid Lead-Free Hybrid Perovskites}, author = {Riki Subagyo and Pramitha Y D Maulida and Dominik Kowal and Sri Hartati and Rossyaila M Muslimawati and Yulfi Zetra and Lina J Diguna and Syafsir Akhlus and Muhammad H Mahyuddin and Lei Zhang and Chi S Tang and Caozheng Diao and Andrew T S Wee and Muhammad D Birowosuto and Andrivo Arramel and Andrivo Rusydi and Yuly Kusumawati}, doi = {10.1021/acsami.3c12889}, times_cited = {1}, issn = {1944-8244}, year = {2023}, date = {2023-11-15}, journal = {ACS APPLIED MATERIALS & INTERFACES}, volume = {15}, number = {47}, pages = {54677-54691}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {Rational design is an important approach to consider in the development of low-dimensional hybrid organic-inorganic perovskites (HOIPs). In this study, 1-butyl-1-methyl pyrrolidinium (BMP), 1-(3-aminopropyl)imidazole (API), and 1-butyl-3-methyl imidazolium (BMI) serve as prototypical ionic liquid components in bismuth-based HOIPs. Element-sensitive X-ray absorption spectroscopy measurements of BMPBiBr4 and APIBiBr(5) reveal distinct resonant excitation profiles across the N K-edges, where contrasting peak shifts are observed. These 1D-HOIPs exhibit a large Stokes shift due to the small polaron contribution, as probed by photoluminescence spectroscopy at room temperature. Interestingly, the incorporation of a small fraction of tin (Sn) into the APIBiBr(5) (Sn/Bi mole ratio of 1:3) structure demonstrates a strong spectral weight transfer accompanied by a fast decay lifetime (2.6 ns). These phenomena are the direct result of Sn-substitution in APIBiBr(5), decreasing the small polaron effect. By changing the active ionic liquid, the electronic interactions and optical responses can be moderately tuned by alteration of their intermolecular interaction between the semiconducting inorganic layers and organic moieties.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Rational design is an important approach to consider in the development of low-dimensional hybrid organic-inorganic perovskites (HOIPs). In this study, 1-butyl-1-methyl pyrrolidinium (BMP), 1-(3-aminopropyl)imidazole (API), and 1-butyl-3-methyl imidazolium (BMI) serve as prototypical ionic liquid components in bismuth-based HOIPs. Element-sensitive X-ray absorption spectroscopy measurements of BMPBiBr4 and APIBiBr(5) reveal distinct resonant excitation profiles across the N K-edges, where contrasting peak shifts are observed. These 1D-HOIPs exhibit a large Stokes shift due to the small polaron contribution, as probed by photoluminescence spectroscopy at room temperature. Interestingly, the incorporation of a small fraction of tin (Sn) into the APIBiBr(5) (Sn/Bi mole ratio of 1:3) structure demonstrates a strong spectral weight transfer accompanied by a fast decay lifetime (2.6 ns). These phenomena are the direct result of Sn-substitution in APIBiBr(5), decreasing the small polaron effect. By changing the active ionic liquid, the electronic interactions and optical responses can be moderately tuned by alteration of their intermolecular interaction between the semiconducting inorganic layers and organic moieties. |
Noviyanto, Alfian; Amalia, Ratih; Maulida, Pramitha Yuniar Diah; Dioktyanto, Mudzakkir; Arrosyid, Bagas Haqi; Aryanto, Didik; Zhang, Lei; Wee, Andrew T S; Arramel, Anomalous Temperature-Induced Particle Size Reduction in Manganese Oxide Nanoparticles Journal Article ACS OMEGA, 8 (47), pp. 45152-45162, 2023, ISSN: 2470-1343. @article{ISI:001110569800001, title = {Anomalous Temperature-Induced Particle Size Reduction in Manganese Oxide Nanoparticles}, author = {Alfian Noviyanto and Ratih Amalia and Pramitha Yuniar Diah Maulida and Mudzakkir Dioktyanto and Bagas Haqi Arrosyid and Didik Aryanto and Lei Zhang and Andrew T S Wee and Arramel}, doi = {10.1021/acsomega.3c08012}, times_cited = {3}, issn = {2470-1343}, year = {2023}, date = {2023-11-13}, journal = {ACS OMEGA}, volume = {8}, number = {47}, pages = {45152-45162}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {The intricate role of temperature in the structure-property relationship of manganese oxide nanoparticles (Mn3O4 NPs) remains an open question. In this study, we successfully synthesized Mn3O4 NPs using the hydrothermal method with two differing temperatures, namely, 90 and 150 degrees C. Interestingly, a smaller average particle size is found when Mn3O4 NPs are synthesized at 150 degrees C compared to 90 degrees C, corresponding to 46.54 and 63.37 nm, respectively. This was confirmed by the time variation of temperature setting of 150 degrees C where the size evolution was insignificant, indicating a competing effect of nucleation and growth particles. Under varying NaOH concentrations (2-6 M) at 150 degrees C, a reduction in the particle size is found at the highest NaOH concentration (6 M). The particle grows slightly, indicating that the growth state is dominant compared to the nucleation state at low concentrations of NaOH. This finding implies that the high nucleation rate originates from the excessive monomer supply in the high-temperature reaction. In terms of crystallinity order, the structural arrangement of Mn3O4 NPs (150 degrees C) is largely decreased; this is likely due to a facile redox shift to the higher oxidation state of manganese. In addition, the higher ratio of adsorbed oxygen and lattice oxygen in Mn3O4 NPs at 150 degrees C is indirectly due to the higher oxygen vacancy occupancies, which supported the crystallinity decrease. Our findings provide a new perspective on manganese oxide formation in hydrothermal systems.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The intricate role of temperature in the structure-property relationship of manganese oxide nanoparticles (Mn3O4 NPs) remains an open question. In this study, we successfully synthesized Mn3O4 NPs using the hydrothermal method with two differing temperatures, namely, 90 and 150 degrees C. Interestingly, a smaller average particle size is found when Mn3O4 NPs are synthesized at 150 degrees C compared to 90 degrees C, corresponding to 46.54 and 63.37 nm, respectively. This was confirmed by the time variation of temperature setting of 150 degrees C where the size evolution was insignificant, indicating a competing effect of nucleation and growth particles. Under varying NaOH concentrations (2-6 M) at 150 degrees C, a reduction in the particle size is found at the highest NaOH concentration (6 M). The particle grows slightly, indicating that the growth state is dominant compared to the nucleation state at low concentrations of NaOH. This finding implies that the high nucleation rate originates from the excessive monomer supply in the high-temperature reaction. In terms of crystallinity order, the structural arrangement of Mn3O4 NPs (150 degrees C) is largely decreased; this is likely due to a facile redox shift to the higher oxidation state of manganese. In addition, the higher ratio of adsorbed oxygen and lattice oxygen in Mn3O4 NPs at 150 degrees C is indirectly due to the higher oxygen vacancy occupancies, which supported the crystallinity decrease. Our findings provide a new perspective on manganese oxide formation in hydrothermal systems. |
Whitcher, T J; Fauzi, A D; Diao, C; Chi, X; Syahroni, A; Asmara, T C; Breese, M B H; Neto, Castro A H; Wee, A T S; Majidi, M A; Rusydi, A Reply to: Reassessing the existence of soft X-ray correlated plasmons Journal Article NATURE COMMUNICATIONS, 14 (1), 2023. @article{ISI:001089230100018, title = {Reply to: Reassessing the existence of soft X-ray correlated plasmons}, author = {T J Whitcher and A D Fauzi and C Diao and X Chi and A Syahroni and T C Asmara and M B H Breese and Castro A H Neto and A T S Wee and M A Majidi and A Rusydi}, doi = {10.1038/s41467-023-40652-9}, times_cited = {0}, year = {2023}, date = {2023-10-24}, journal = {NATURE COMMUNICATIONS}, volume = {14}, number = {1}, publisher = {NATURE PORTFOLIO}, address = {HEIDELBERGER PLATZ 3, BERLIN, 14197, GERMANY}, keywords = {}, pubstate = {published}, tppubtype = {article} } |
Tang, Chi Sin; Zeng, Shengwei; Wu, Jing; Chen, Shunfeng; Naradipa, Muhammad A; Song, Dongsheng; Milosevic, M V; Yang, Ping; Diao, Caozheng; Zhou, Jun; Pennycook, Stephen J; Breese, Mark B H; Cai, Chuanbing; Venkatesan, Thirumalai; Ariando, Ariando; Yang, Ming; Wee, Andrew T S; Yin, Xinmao "Detection of two-dimensional small polarons at oxide interfaces by optical spectroscopy" (vol 10, 031406 2023) Journal Article APPLIED PHYSICS REVIEWS, 10 (3), 2023, ISSN: 1931-9401. @article{ISI:001062976900001, title = {"Detection of two-dimensional small polarons at oxide interfaces by optical spectroscopy" (vol 10, 031406 2023)}, author = {Chi Sin Tang and Shengwei Zeng and Jing Wu and Shunfeng Chen and Muhammad A Naradipa and Dongsheng Song and M V Milosevic and Ping Yang and Caozheng Diao and Jun Zhou and Stephen J Pennycook and Mark B H Breese and Chuanbing Cai and Thirumalai Venkatesan and Ariando Ariando and Ming Yang and Andrew T S Wee and Xinmao Yin}, doi = {10.1063/5.0173910}, times_cited = {0}, issn = {1931-9401}, year = {2023}, date = {2023-09-01}, journal = {APPLIED PHYSICS REVIEWS}, volume = {10}, number = {3}, publisher = {AIP Publishing}, address = {1305 WALT WHITMAN RD, STE 300, MELVILLE, NY 11747-4501 USA}, keywords = {}, pubstate = {published}, tppubtype = {article} } |
Tang, Chi Sin; Zeng, Shengwei; Wu, Jing; Chen, Shunfeng; Naradipa, Muhammad A; Song, Dongsheng; Milosevic, M V; Yang, Ping; Diao, Caozheng; Zhou, Jun; Pennycook, Stephen J; Breese, Mark B H; Cai, Chuanbing; Venkatesan, Thirumalai; Ariando, Ariando; Yang, Ming; Wee, Andrew T S; Yin, Xinmao Detection of two-dimensional small polarons at oxide interfaces by optical spectroscopy Journal Article APPLIED PHYSICS REVIEWS, 10 (3), 2023, ISSN: 1931-9401. @article{ISI:001038283300001, title = {Detection of two-dimensional small polarons at oxide interfaces by optical spectroscopy}, author = {Chi Sin Tang and Shengwei Zeng and Jing Wu and Shunfeng Chen and Muhammad A Naradipa and Dongsheng Song and M V Milosevic and Ping Yang and Caozheng Diao and Jun Zhou and Stephen J Pennycook and Mark B H Breese and Chuanbing Cai and Thirumalai Venkatesan and Ariando Ariando and Ming Yang and Andrew T S Wee and Xinmao Yin}, doi = {10.1063/5.0141814}, times_cited = {1}, issn = {1931-9401}, year = {2023}, date = {2023-09-01}, journal = {APPLIED PHYSICS REVIEWS}, volume = {10}, number = {3}, publisher = {AIP Publishing}, address = {1305 WALT WHITMAN RD, STE 300, MELVILLE, NY 11747-4501 USA}, abstract = {Two-dimensional (2D) perovskite oxide interfaces are ideal systems to uncover diverse emergent properties, such as the arising polaronic properties from short-range charge-lattice interactions. Thus, a technique to detect this quasiparticle phenomenon at the buried interface is highly coveted. Here, we report the observation of 2D small-polarons at the LaAlO3/SrTiO3 conducting interface using high-resolution spectroscopic ellipsometry. First-principles investigations show that interfacial electron-lattice coupling mediated by the longitudinal phonon mode facilitates the formation of these polarons. This study resolves the long-standing question by attributing the formation of interfacial 2D small polarons to the significant mismatch between experimentally measured interfacial carrier density and theoretical values. Our study sheds light on the complexity of broken periodic lattice-induced quasi-particle effects and its relationship with exotic phenomena at complex oxide interfaces. Meanwhile, this work establishes spectroscopic ellipsometry as a useful technique to detect and locate optical evidence of polaronic states and other emerging quantum properties at the buried interface.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Two-dimensional (2D) perovskite oxide interfaces are ideal systems to uncover diverse emergent properties, such as the arising polaronic properties from short-range charge-lattice interactions. Thus, a technique to detect this quasiparticle phenomenon at the buried interface is highly coveted. Here, we report the observation of 2D small-polarons at the LaAlO3/SrTiO3 conducting interface using high-resolution spectroscopic ellipsometry. First-principles investigations show that interfacial electron-lattice coupling mediated by the longitudinal phonon mode facilitates the formation of these polarons. This study resolves the long-standing question by attributing the formation of interfacial 2D small polarons to the significant mismatch between experimentally measured interfacial carrier density and theoretical values. Our study sheds light on the complexity of broken periodic lattice-induced quasi-particle effects and its relationship with exotic phenomena at complex oxide interfaces. Meanwhile, this work establishes spectroscopic ellipsometry as a useful technique to detect and locate optical evidence of polaronic states and other emerging quantum properties at the buried interface. |
Hu, Junxiong; Tan, Junyou; Ezzi, Mohammed Al M; Chattopadhyay, Udvas; Gou, Jian; Zheng, Yuntian; Wang, Zihao; Chen, Jiayu; Thottathil, Reshmi; Luo, Jiangbo; Watanabe, Kenji; Taniguchi, Takashi; Wee, Andrew Thye Shen; Adam, Shaffique; Ariando, A Controlled alignment of supermoire lattice in double-aligned graphene heterostructures Journal Article 11 NATURE COMMUNICATIONS, 14 (1), 2023. @article{ISI:001029450400007, title = {Controlled alignment of supermoire lattice in double-aligned graphene heterostructures}, author = {Junxiong Hu and Junyou Tan and Mohammed Al M Ezzi and Udvas Chattopadhyay and Jian Gou and Yuntian Zheng and Zihao Wang and Jiayu Chen and Reshmi Thottathil and Jiangbo Luo and Kenji Watanabe and Takashi Taniguchi and Andrew Thye Shen Wee and Shaffique Adam and A Ariando}, doi = {10.1038/s41467-023-39893-5}, times_cited = {11}, year = {2023}, date = {2023-07-12}, journal = {NATURE COMMUNICATIONS}, volume = {14}, number = {1}, publisher = {NATURE PORTFOLIO}, address = {HEIDELBERGER PLATZ 3, BERLIN, 14197, GERMANY}, abstract = {The supermoire lattice, built by stacking two moire patterns, provides a platform for creating flat mini-bands and studying electron correlations. An ultimate challenge in assembling a graphene supermoire lattice is in the deterministic control of its rotational alignment, which is made highly aleatory due to the random nature of the edge chirality and crystal symmetry. Employing the so-called "golden rule of three", here we present an experimental strategy to overcome this challenge and realize the controlled alignment of double-aligned hBN/graphene/hBN supermoire lattice, where the twist angles between graphene and top/bottom hBN are both close to zero. Remarkably, we find that the crystallographic edge of neighboring graphite can be used to better guide the stacking alignment, as demonstrated by the controlled production of 20 moire samples with an accuracy better than similar to 0.2 degrees. Finally, we extend our technique to low-angle twisted bilayer graphene and ABC-stacked trilayer graphene, providing a strategy for flat-band engineering in these moirematerials.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The supermoire lattice, built by stacking two moire patterns, provides a platform for creating flat mini-bands and studying electron correlations. An ultimate challenge in assembling a graphene supermoire lattice is in the deterministic control of its rotational alignment, which is made highly aleatory due to the random nature of the edge chirality and crystal symmetry. Employing the so-called "golden rule of three", here we present an experimental strategy to overcome this challenge and realize the controlled alignment of double-aligned hBN/graphene/hBN supermoire lattice, where the twist angles between graphene and top/bottom hBN are both close to zero. Remarkably, we find that the crystallographic edge of neighboring graphite can be used to better guide the stacking alignment, as demonstrated by the controlled production of 20 moire samples with an accuracy better than similar to 0.2 degrees. Finally, we extend our technique to low-angle twisted bilayer graphene and ABC-stacked trilayer graphene, providing a strategy for flat-band engineering in these moirematerials. |