Ang Kah Wee
Degree: PhD
Position: Associate Professor
Affiliation: NUS – Department of Electrical and Computer Engineering
Research Type: Experiment
Office: E1-05-25
Email: eleakw@nus.edu.sg
Contact: (65) 6516 2575
Website: https://www.ece.nus.edu.sg/staff/bio/akw.html
Research Interests:
Nanoelectronics & Photonics Materials and Devices
Nanoscience and Nanotechnology
CA2DM Publications:
2022 |
Sahdan, Muhammad Fauzi; Arramel, ; Xiaodai, Sharon Lim; Wang, Hong; Birowosuto, Muhammad Danang; Haur, Sow Chorng; Ang, Kah-Wee; Wee, Andrew Thye Shen Metal-insulator transition switching in VOx-VSe2 heterojunctions Journal Article PHYSICAL REVIEW MATERIALS, 6 (1), 2022, ISSN: 2475-9953. @article{ISI:000747807700002, title = {Metal-insulator transition switching in VO_{\textit{x}}-VSe_{2} heterojunctions}, author = {Muhammad Fauzi Sahdan and Arramel and Sharon Lim Xiaodai and Hong Wang and Muhammad Danang Birowosuto and Sow Chorng Haur and Kah-Wee Ang and Andrew Thye Shen Wee}, doi = {10.1103/PhysRevMaterials.6.014003}, times_cited = {0}, issn = {2475-9953}, year = {2022}, date = {2022-01-19}, journal = {PHYSICAL REVIEW MATERIALS}, volume = {6}, number = {1}, publisher = {AMER PHYSICAL SOC}, address = {ONE PHYSICS ELLIPSE, COLLEGE PK, MD 20740-3844 USA}, abstract = {First-order metal-insulator transition (MIT) observed in strongly correlated systems such as vanadium dioxide (VO2) holds potential in electronics, energy, to optical applications. Starting from a vanadium diselenide (VSe2) bulk crystal, we demonstrated a direct surface conversion from VSe2 to VO2 via laser exposure in ambient condition. The process generates defects, and the heat from the laser promotes oxidation forming VOx. Raman spectra at room temperature suggest the resulting oxide formed is monoclinic (M1) VO2. Above the transition temperature (T-C), all the phonon modes are damped indicating formation of the rutile phase (metallic). Photoluminescence (PL) intensity enhancement and peak shifts observed at T-C suggest correlation to the band structure transformation. In addition, we observed electrically induced MIT in our lateral VSe2-VOx heterojunction device.}, keywords = {}, pubstate = {published}, tppubtype = {article} } First-order metal-insulator transition (MIT) observed in strongly correlated systems such as vanadium dioxide (VO2) holds potential in electronics, energy, to optical applications. Starting from a vanadium diselenide (VSe2) bulk crystal, we demonstrated a direct surface conversion from VSe2 to VO2 via laser exposure in ambient condition. The process generates defects, and the heat from the laser promotes oxidation forming VOx. Raman spectra at room temperature suggest the resulting oxide formed is monoclinic (M1) VO2. Above the transition temperature (T-C), all the phonon modes are damped indicating formation of the rutile phase (metallic). Photoluminescence (PL) intensity enhancement and peak shifts observed at T-C suggest correlation to the band structure transformation. In addition, we observed electrically induced MIT in our lateral VSe2-VOx heterojunction device. |
2021 |
Tan, Wee Chong; Ang, Kah-Wee Volatile Organic Compound Sensors Based on 2D Materials Journal Article ADVANCED ELECTRONIC MATERIALS, 7 (7), 2021, ISSN: 2199-160X. @article{ISI:000634537200001, title = {Volatile Organic Compound Sensors Based on 2D Materials}, author = {Wee Chong Tan and Kah-Wee Ang}, doi = {10.1002/aelm.202001071}, times_cited = {0}, issn = {2199-160X}, year = {2021}, date = {2021-03-29}, journal = {ADVANCED ELECTRONIC MATERIALS}, volume = {7}, number = {7}, publisher = {WILEY}, address = {111 RIVER ST, HOBOKEN 07030-5774, NJ USA}, abstract = {Due to their unique chemical and physical properties, 2D inorganic materials (2DMs) have attracted a lot of attention in recent years as an attractive platform for diverse applications in opto-electronics, energy generation and storage, and sensing. This article presents the progress being made in sensors designed for measuring volatile organic compounds (VOCs) in ambient air. It considers commercially available sensors, such as the amperometric and conductometric types, and the recent advancements in VOCs sensors employing 2DMs and their nanocomposites. Their performance is summarized in a graphical form and the challenges and possible mitigation methods for these types of VOC sensor are highlighted.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Due to their unique chemical and physical properties, 2D inorganic materials (2DMs) have attracted a lot of attention in recent years as an attractive platform for diverse applications in opto-electronics, energy generation and storage, and sensing. This article presents the progress being made in sensors designed for measuring volatile organic compounds (VOCs) in ambient air. It considers commercially available sensors, such as the amperometric and conductometric types, and the recent advancements in VOCs sensors employing 2DMs and their nanocomposites. Their performance is summarized in a graphical form and the challenges and possible mitigation methods for these types of VOC sensor are highlighted. |
Li, Sifan; Li, Bochang; Feng, Xuewei; Chen, Li; Li, Yesheng; Huang, Li; Fong, Xuanyao; Ang, Kah-Wee Electron-beam-irradiated rhenium disulfide memristors with low variability for neuromorphic computing Journal Article NPJ 2D MATERIALS AND APPLICATIONS, 5 (1), 2021. @article{ISI:000607110700001, title = {Electron-beam-irradiated rhenium disulfide memristors with low variability for neuromorphic computing}, author = {Sifan Li and Bochang Li and Xuewei Feng and Li Chen and Yesheng Li and Li Huang and Xuanyao Fong and Kah-Wee Ang}, doi = {10.1038/s41699-020-00190-0}, times_cited = {0}, year = {2021}, date = {2021-01-04}, journal = {NPJ 2D MATERIALS AND APPLICATIONS}, volume = {5}, number = {1}, publisher = {NATURE PORTFOLIO}, address = {HEIDELBERGER PLATZ 3, BERLIN, 14197, GERMANY}, abstract = {State-of-the-art memristors are mostly formed by vertical metal-insulator-metal (MIM) structure, which rely on the formation of conductive filaments for resistive switching (RS). However, owing to the stochastic formation of filament, the set/reset voltage of vertical MIM memristors is difficult to control, which results in poor temporal and spatial switching uniformity. Here, a two-terminal lateral memristor based on electron-beam-irradiated rhenium disulfide (ReS2) is realized, which unveils a resistive switching mechanism based on Schottky barrier height (SBH) modulation. The devices exhibit a forming-free, stable gradual RS characteristic, and simultaneously achieve a small transition voltage variation during positive and negative sweeps (6.3%/5.3%). The RS is attributed to the motion of sulfur vacancies induced by voltage bias in the device, which modulates the ReS2/metal SBH. The gradual SBH modulation stabilizes the temporal variation in contrast to the abrupt RS in MIM-based memristors. Moreover, the emulation of long-term synaptic plasticity of biological synapses is demonstrated using the device, manifesting its potential as artificial synapse for energy-efficient neuromorphic computing applications.}, keywords = {}, pubstate = {published}, tppubtype = {article} } State-of-the-art memristors are mostly formed by vertical metal-insulator-metal (MIM) structure, which rely on the formation of conductive filaments for resistive switching (RS). However, owing to the stochastic formation of filament, the set/reset voltage of vertical MIM memristors is difficult to control, which results in poor temporal and spatial switching uniformity. Here, a two-terminal lateral memristor based on electron-beam-irradiated rhenium disulfide (ReS2) is realized, which unveils a resistive switching mechanism based on Schottky barrier height (SBH) modulation. The devices exhibit a forming-free, stable gradual RS characteristic, and simultaneously achieve a small transition voltage variation during positive and negative sweeps (6.3%/5.3%). The RS is attributed to the motion of sulfur vacancies induced by voltage bias in the device, which modulates the ReS2/metal SBH. The gradual SBH modulation stabilizes the temporal variation in contrast to the abrupt RS in MIM-based memristors. Moreover, the emulation of long-term synaptic plasticity of biological synapses is demonstrated using the device, manifesting its potential as artificial synapse for energy-efficient neuromorphic computing applications. |
Li, Yesheng; Ang, Kah-Wee Hardware Implementation of Neuromorphic Computing Using Large-Scale Memristor Crossbar Arrays Journal Article ADVANCED INTELLIGENT SYSTEMS, 3 (1), 2021. @article{ISI:000669799200003, title = {Hardware Implementation of Neuromorphic Computing Using Large-Scale Memristor Crossbar Arrays}, author = {Yesheng Li and Kah-Wee Ang}, doi = {10.1002/aisy.202000137}, times_cited = {0}, year = {2021}, date = {2021-01-01}, journal = {ADVANCED INTELLIGENT SYSTEMS}, volume = {3}, number = {1}, publisher = {WILEY}, address = {111 RIVER ST, HOBOKEN 07030-5774, NJ USA}, abstract = {Brain-inspired neuromorphic computing is a new paradigm that holds great potential to overcome the intrinsic energy and speed issues of traditional von Neumann based computing architecture. With the ability to perform vector-matrix multiplications and flexible tunable conductance, the memristor crossbar array (CBA) structure is one of the most promising candidates to realize neural cognitive systems. The boom in the development of memristive synapses and neurons has propelled the developments of artificial neural networks (ANNs) to emulate the highly hierarchically organized network of human brain in the past decade. To achieve this, realizing large scale, high-density memristive CBAs is a prerequisite to constructing complex ANNs. Herein, the stringent requirements in device performance and array parameters for hardware ANNs are analyzed, and the efforts in addressing the associated challenges are discussed. Recent progress on the experimental demonstration of neuromorphic computing systems (NCSs) is presented. Recommendations for further performance optimization at the device, circuit, and algorithm levels are proposed. This Report serves as a guide for the hardware implementation of NCS based on large-scale CBAs.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Brain-inspired neuromorphic computing is a new paradigm that holds great potential to overcome the intrinsic energy and speed issues of traditional von Neumann based computing architecture. With the ability to perform vector-matrix multiplications and flexible tunable conductance, the memristor crossbar array (CBA) structure is one of the most promising candidates to realize neural cognitive systems. The boom in the development of memristive synapses and neurons has propelled the developments of artificial neural networks (ANNs) to emulate the highly hierarchically organized network of human brain in the past decade. To achieve this, realizing large scale, high-density memristive CBAs is a prerequisite to constructing complex ANNs. Herein, the stringent requirements in device performance and array parameters for hardware ANNs are analyzed, and the efforts in addressing the associated challenges are discussed. Recent progress on the experimental demonstration of neuromorphic computing systems (NCSs) is presented. Recommendations for further performance optimization at the device, circuit, and algorithm levels are proposed. This Report serves as a guide for the hardware implementation of NCS based on large-scale CBAs. |
2020 |
Zhang, Panpan; Wang, Lin; Ang, Kah-Wee; Fong, Xuanyao Transition from trap-mediated to band-like transport in polycrystalline monolayer molybdenum disulfide memtransistors Journal Article APPLIED PHYSICS LETTERS, 117 (22), 2020, ISSN: 0003-6951. @article{ISI:000596026700001, title = {Transition from trap-mediated to band-like transport in polycrystalline monolayer molybdenum disulfide memtransistors}, author = {Panpan Zhang and Lin Wang and Kah-Wee Ang and Xuanyao Fong}, doi = {10.1063/5.0031799}, times_cited = {0}, issn = {0003-6951}, year = {2020}, date = {2020-11-30}, journal = {APPLIED PHYSICS LETTERS}, volume = {117}, number = {22}, publisher = {AIP Publishing}, address = {1305 WALT WHITMAN RD, STE 300, MELVILLE, NY 11747-4501 USA}, abstract = {Multi-terminal memtransistors using polycrystalline monolayer molybdenum disulfide (MoS2) have recently emerged as novel synaptic devices. Due to the coexistence of disorder and strong Coulomb carrier-carrier interactions in MoS2, localization and delocalization of carriers can come into play successively upon the relative strength of disorder and interactions, which can be tuned by the Fermi level (}, keywords = {}, pubstate = {published}, tppubtype = {article} } Multi-terminal memtransistors using polycrystalline monolayer molybdenum disulfide (MoS2) have recently emerged as novel synaptic devices. Due to the coexistence of disorder and strong Coulomb carrier-carrier interactions in MoS2, localization and delocalization of carriers can come into play successively upon the relative strength of disorder and interactions, which can be tuned by the Fermi level ( |
Li, Bochang; Li, Sifan; Wang, Han; Chen, Li; Liu, Liang; Feng, Xuewei; Li, Yesheng; Chen, Jingsheng; Gong, X; Ang, Kah-Wee An Electronic Synapse Based on 2D Ferroelectric CuInP2S6 Journal Article ADVANCED ELECTRONIC MATERIALS, 6 (12), 2020, ISSN: 2199-160X. @article{ISI:000587430400001, title = {An Electronic Synapse Based on 2D Ferroelectric CuInP_{2}S_{6}}, author = {Bochang Li and Sifan Li and Han Wang and Li Chen and Liang Liu and Xuewei Feng and Yesheng Li and Jingsheng Chen and X Gong and Kah-Wee Ang}, doi = {10.1002/aelm.202000760}, times_cited = {0}, issn = {2199-160X}, year = {2020}, date = {2020-11-09}, journal = {ADVANCED ELECTRONIC MATERIALS}, volume = {6}, number = {12}, publisher = {WILEY}, address = {111 RIVER ST, HOBOKEN 07030-5774, NJ USA}, abstract = {Memristors with biological synaptic behaviors and functions have been intensively studied as an important component for neuromorphic computing system, which hold promise to address the power consumption issue in modern computers based on von Neumann architecture. However, the resistive switching mechanism that relies on the stochastic formation of conductive filaments leads to poor cycle-to-cycle (temporal) and cell-to-cell (spatial) variations for filamentary memristors. The emergence of memristors based on 2D ferroelectric materials can potentially avoid these issues. Here, a vertical Au/CuInP2S6 (CIPS)/Ti diode is demonstrated using exfoliated ferroelectric CIPS flake. Through ferroelectric switching, the CIPS diode realizes resistive switching with a ratio larger than 6 x 10(3). The endurance measurement shows a small set and reset voltage variation of 5.3% and 9.1%, respectively. Key synaptic behaviors including spike-time-dependent plasticity, paired-pulse-facilitation, and paired-pulse-depression are successfully mimicked, manifesting the potential application of CIPS diode in a neuromorphic computing system. Moreover, pattern learning and memory behaviors are emulated using a 3 x 3 CIPS crossbar array.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Memristors with biological synaptic behaviors and functions have been intensively studied as an important component for neuromorphic computing system, which hold promise to address the power consumption issue in modern computers based on von Neumann architecture. However, the resistive switching mechanism that relies on the stochastic formation of conductive filaments leads to poor cycle-to-cycle (temporal) and cell-to-cell (spatial) variations for filamentary memristors. The emergence of memristors based on 2D ferroelectric materials can potentially avoid these issues. Here, a vertical Au/CuInP2S6 (CIPS)/Ti diode is demonstrated using exfoliated ferroelectric CIPS flake. Through ferroelectric switching, the CIPS diode realizes resistive switching with a ratio larger than 6 x 10(3). The endurance measurement shows a small set and reset voltage variation of 5.3% and 9.1%, respectively. Key synaptic behaviors including spike-time-dependent plasticity, paired-pulse-facilitation, and paired-pulse-depression are successfully mimicked, manifesting the potential application of CIPS diode in a neuromorphic computing system. Moreover, pattern learning and memory behaviors are emulated using a 3 x 3 CIPS crossbar array. |
Chen, Li; Yu, Zhi Gen; Liang, Dan; Li, Sifan; Tan, Wee Chong; Zhang, Yong-Wei; Ang, Kah-Wee Ultrasensitive and robust two-dimensional indium selenide flexible electronics and sensors for human motion detection Journal Article NANO ENERGY, 76 , 2020, ISSN: 2211-2855. @article{ISI:000571043100005, title = {Ultrasensitive and robust two-dimensional indium selenide flexible electronics and sensors for human motion detection}, author = {Li Chen and Zhi Gen Yu and Dan Liang and Sifan Li and Wee Chong Tan and Yong-Wei Zhang and Kah-Wee Ang}, doi = {10.1016/j.nanoen.2020.105020}, times_cited = {0}, issn = {2211-2855}, year = {2020}, date = {2020-10-01}, journal = {NANO ENERGY}, volume = {76}, publisher = {ELSEVIER}, address = {RADARWEG 29, 1043 NX AMSTERDAM, NETHERLANDS}, abstract = {To accurately detect human motion, a sensing material/device must be flexible, ultrasensitive to strain, and electrically and mechanically robust. However, existing electronics and sensors, which are either not flexible enough, or not ultrasensitive to strain, or suffering from poor on/off ratio and low charge mobility, do not meet these stringent requirements. Here, we demonstrate a flexible and ultrasensitive three-terminal strain sensor based on two-dimensional (2D) InSe for detecting human-motion activities. The 2D InSe exhibits a tunable bandgap and a large piezoresistive effect via strain engineering. Through electrostatic gating effect, the gauge factor of the sensor can be enhanced by 8-fold and 7-fold for a low tensile and compressive strain of only +/- 0.25%, respectively. Remarkably, the fabricated 2D InSe-based transistor achieves both a record high on/off ratio of similar to 10(8) and an electron mobility of similar to 383 cm(2)/V s, superior to all existing ones. Furthermore, flexible InSe logic inverters with a high voltage gain of similar to 10 and a large noise margin of similar to 0.7 x V-DD (supply voltage) are realized under various strain conditions. This work paves the way to enable simultaneous integration of high-performance flexible sensors and electronics based on a common 2D InSe material platform towards achieving a fully integrated sensing system.}, keywords = {}, pubstate = {published}, tppubtype = {article} } To accurately detect human motion, a sensing material/device must be flexible, ultrasensitive to strain, and electrically and mechanically robust. However, existing electronics and sensors, which are either not flexible enough, or not ultrasensitive to strain, or suffering from poor on/off ratio and low charge mobility, do not meet these stringent requirements. Here, we demonstrate a flexible and ultrasensitive three-terminal strain sensor based on two-dimensional (2D) InSe for detecting human-motion activities. The 2D InSe exhibits a tunable bandgap and a large piezoresistive effect via strain engineering. Through electrostatic gating effect, the gauge factor of the sensor can be enhanced by 8-fold and 7-fold for a low tensile and compressive strain of only +/- 0.25%, respectively. Remarkably, the fabricated 2D InSe-based transistor achieves both a record high on/off ratio of similar to 10(8) and an electron mobility of similar to 383 cm(2)/V s, superior to all existing ones. Furthermore, flexible InSe logic inverters with a high voltage gain of similar to 10 and a large noise margin of similar to 0.7 x V-DD (supply voltage) are realized under various strain conditions. This work paves the way to enable simultaneous integration of high-performance flexible sensors and electronics based on a common 2D InSe material platform towards achieving a fully integrated sensing system. |
Li, Chen; Lin, Wang; Yue, Peng; Xuewei, Feng; Sarkar, Soumya; Sifan, Li; Bochang, Li; Liang, Liu; Kaizhen, Han; Xiao, Gong; Jingsheng, Chen; Yan, Liu; Genquan, Han; Ang, Kah-Wee A van der Waals Synaptic Transistor Based on Ferroelectric Hf0.5Zr0.5O2 and 2D Tungsten Disulfide Journal Article ADVANCED ELECTRONIC MATERIALS, 6 (6), 2020, ISSN: 2199-160X. @article{ISI:000530860500001, title = {A van der Waals Synaptic Transistor Based on Ferroelectric Hf_{0.5}Zr_{0.5}O_{2} and 2D Tungsten Disulfide}, author = {Chen Li and Wang Lin and Peng Yue and Feng Xuewei and Soumya Sarkar and Li Sifan and Li Bochang and Liu Liang and Han Kaizhen and Gong Xiao and Chen Jingsheng and Liu Yan and Han Genquan and Kah-Wee Ang}, doi = {10.1002/aelm.202000057}, times_cited = {0}, issn = {2199-160X}, year = {2020}, date = {2020-05-08}, journal = {ADVANCED ELECTRONIC MATERIALS}, volume = {6}, number = {6}, publisher = {WILEY}, address = {111 RIVER ST, HOBOKEN 07030-5774, NJ USA}, abstract = {Neuromorphic computing on the hardware level is promising for performing ever-increasing data-centric tasks owing to its superiority to conventional von Neumann architecture in terms of energy efficiency and learning ability. One key aspect to its implementation is the development of artificial synapses that can effectively emulate the multiple functionalities exhibited by their biological counterparts. Here, building on an inorganic ferroelectric gate stack integrated with a 2D layered semiconductor (WS2), a new type of ferroelectricity-based synaptic transistor that differs from those relying on interface traps or floating gate configuration is reported. By virtue of a 6 nm thick ferroelectric hafnium zirconium oxide by atomic layer deposition and postannealing treatment, the device shows a channel resistance change ratio above 10(5) corresponding to opposite ferroelectric polarization direction. Furthermore, by applying electrical stimulus to the gate, it demonstrates good capability to mimic various synaptic behaviors including long-term potentiation, long-term depression, spike-amplitude-dependent plasticity, and spike-rate-dependent plasticity. Given the inherent compatibility of the ferroelectric gate stack with existing fabrication technology, and the reliability of ferroelectricity engineering, this work paves the way toward practical implementation of synaptic devices in neuromorphic circuits.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Neuromorphic computing on the hardware level is promising for performing ever-increasing data-centric tasks owing to its superiority to conventional von Neumann architecture in terms of energy efficiency and learning ability. One key aspect to its implementation is the development of artificial synapses that can effectively emulate the multiple functionalities exhibited by their biological counterparts. Here, building on an inorganic ferroelectric gate stack integrated with a 2D layered semiconductor (WS2), a new type of ferroelectricity-based synaptic transistor that differs from those relying on interface traps or floating gate configuration is reported. By virtue of a 6 nm thick ferroelectric hafnium zirconium oxide by atomic layer deposition and postannealing treatment, the device shows a channel resistance change ratio above 10(5) corresponding to opposite ferroelectric polarization direction. Furthermore, by applying electrical stimulus to the gate, it demonstrates good capability to mimic various synaptic behaviors including long-term potentiation, long-term depression, spike-amplitude-dependent plasticity, and spike-rate-dependent plasticity. Given the inherent compatibility of the ferroelectric gate stack with existing fabrication technology, and the reliability of ferroelectricity engineering, this work paves the way toward practical implementation of synaptic devices in neuromorphic circuits. |
2019 |
Xiang, Du; Liu, Tao; Wang, Junyong; Wang, Peng; Wang, Lin; Zheng, Yue; Wang, Yanan; Gao, Jing; Ang, Kah-Wee; Eda, Goki; Hu, Weida; Liu, Lei; Chen, Wei Anomalous Broadband Spectrum Photodetection in 2D Rhenium Disulfide Transistor Journal Article ADVANCED OPTICAL MATERIALS, 7 (23), 2019, ISSN: 2195-1071. @article{ISI:000501216800004, title = {Anomalous Broadband Spectrum Photodetection in 2D Rhenium Disulfide Transistor}, author = {Du Xiang and Tao Liu and Junyong Wang and Peng Wang and Lin Wang and Yue Zheng and Yanan Wang and Jing Gao and Kah-Wee Ang and Goki Eda and Weida Hu and Lei Liu and Wei Chen}, doi = {10.1002/adom.201901115}, times_cited = {0}, issn = {2195-1071}, year = {2019}, date = {2019-12-01}, journal = {ADVANCED OPTICAL MATERIALS}, volume = {7}, number = {23}, publisher = {WILEY-V C H VERLAG GMBH}, address = {POSTFACH 101161, 69451 WEINHEIM, GERMANY}, abstract = {2D transition metal dichalcogenide (TMD)-based phototransistors generally work under photoconductive, photovoltaic, or photogating mode, in which photocarriers are generated from band-to-band excitation. Nevertheless, due to the relatively large bandgap, most TMD phototransistors working under these modes are restricted in visible spectrum. Here, photodetection in 2D multilayer rhenium disulfide (ReS2) transistor via bolometric mode, which relies on light heating induced conductance change instead of band-to-band photoexcitation is reported, making it possible for sub-bandgap photon detection. The bolometric effect induced photoresponse is first revealed by an anomalous sign switching of photocurrent from positive to negative while increasing gate voltage under visible light, which is further validated by the temperature dependent electrical transport measurements. The phototransistor exhibits remarkable photoresponse under infrared regime, beyond the optical bandgap absorption edge of the ReS2 flake. Additionally, it demonstrates a low noise equivalent power, less than 5 x 10(-2) pW Hz(-1/2), which is very promising for ultra-weak light detection. Moreover, the response time is below 3 ms, nearly 3-4 orders of magnitude faster than previously reported ReS2 photodetectors. The findings promise bolometric effect as an effective photodetection mode to extend the response spectrum of large bandgap TMDs for novel and high-performance broadband photodetectors.}, keywords = {}, pubstate = {published}, tppubtype = {article} } 2D transition metal dichalcogenide (TMD)-based phototransistors generally work under photoconductive, photovoltaic, or photogating mode, in which photocarriers are generated from band-to-band excitation. Nevertheless, due to the relatively large bandgap, most TMD phototransistors working under these modes are restricted in visible spectrum. Here, photodetection in 2D multilayer rhenium disulfide (ReS2) transistor via bolometric mode, which relies on light heating induced conductance change instead of band-to-band photoexcitation is reported, making it possible for sub-bandgap photon detection. The bolometric effect induced photoresponse is first revealed by an anomalous sign switching of photocurrent from positive to negative while increasing gate voltage under visible light, which is further validated by the temperature dependent electrical transport measurements. The phototransistor exhibits remarkable photoresponse under infrared regime, beyond the optical bandgap absorption edge of the ReS2 flake. Additionally, it demonstrates a low noise equivalent power, less than 5 x 10(-2) pW Hz(-1/2), which is very promising for ultra-weak light detection. Moreover, the response time is below 3 ms, nearly 3-4 orders of magnitude faster than previously reported ReS2 photodetectors. The findings promise bolometric effect as an effective photodetection mode to extend the response spectrum of large bandgap TMDs for novel and high-performance broadband photodetectors. |
Feng, Xuewei; Li, Yida; Wang, Lin; Chen, Shuai; Yu, Zhi Gen; Tan, Wee Chong; Macadam, Nasiruddin; Hu, Guohua; Huang, Li; Chen, Li; Gong, Xiao; Chi, Dongzhi; Hasan, Tawfique; Thean, Aaron Voon-Yew; Zhang, Yong-Wei; Ang, Koh-Wee A Fully Printed Flexible MoS2 Memristive Artificial Synapse with Femtojoule Switching Energy Journal Article ADVANCED ELECTRONIC MATERIALS, 5 (12), 2019, ISSN: 2199-160X. @article{ISI:000486813700001, title = {A Fully Printed Flexible MoS_{2} Memristive Artificial Synapse with Femtojoule Switching Energy}, author = {Xuewei Feng and Yida Li and Lin Wang and Shuai Chen and Zhi Gen Yu and Wee Chong Tan and Nasiruddin Macadam and Guohua Hu and Li Huang and Li Chen and Xiao Gong and Dongzhi Chi and Tawfique Hasan and Aaron Voon-Yew Thean and Yong-Wei Zhang and Koh-Wee Ang}, doi = {10.1002/aelm.201900740}, times_cited = {6}, issn = {2199-160X}, year = {2019}, date = {2019-09-19}, journal = {ADVANCED ELECTRONIC MATERIALS}, volume = {5}, number = {12}, publisher = {WILEY}, address = {111 RIVER ST, HOBOKEN 07030-5774, NJ USA}, abstract = {Realization of memristors capable of storing and processing data on flexible substrates is a key enabling technology toward "system-on-plastics". Recent advancements in printing techniques show enormous potential to overcome the major challenges of the current manufacturing processes that require high temperature and planar topography, which may radically change the system integration approach on flexible substrates. However, fully printed memristors are yet to be successfully demonstrated due to the lack of a robust printable switching medium and a reliable printing process. An aerosol-jet-printed Ag/MoS2/Ag memristor is realized in a cross-bar structure by developing a scalable and low temperature printing technique utilizing a functional molybdenum disulfide (MoS2) ink platform. The fully printed devices exhibit an ultra-low switching voltage (0.18 V), a high switching ratio (10(7)), a wide range of tuneable resistance states (10-10(10) omega) for multi-bit data storage, and a low standby power consumption of 1 fW and a switching energy of 4.5 fJ per transition set. Moreover, the MoS2 memristor exhibits both volatile and non-volatile resistive switching behavior by controlling the current compliance levels, which efficiently mimic the short-term and long-term plasticity of biological synapses, demonstrating its potential to enable energy-efficient artificial neuromorphic computing.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Realization of memristors capable of storing and processing data on flexible substrates is a key enabling technology toward "system-on-plastics". Recent advancements in printing techniques show enormous potential to overcome the major challenges of the current manufacturing processes that require high temperature and planar topography, which may radically change the system integration approach on flexible substrates. However, fully printed memristors are yet to be successfully demonstrated due to the lack of a robust printable switching medium and a reliable printing process. An aerosol-jet-printed Ag/MoS2/Ag memristor is realized in a cross-bar structure by developing a scalable and low temperature printing technique utilizing a functional molybdenum disulfide (MoS2) ink platform. The fully printed devices exhibit an ultra-low switching voltage (0.18 V), a high switching ratio (10(7)), a wide range of tuneable resistance states (10-10(10) omega) for multi-bit data storage, and a low standby power consumption of 1 fW and a switching energy of 4.5 fJ per transition set. Moreover, the MoS2 memristor exhibits both volatile and non-volatile resistive switching behavior by controlling the current compliance levels, which efficiently mimic the short-term and long-term plasticity of biological synapses, demonstrating its potential to enable energy-efficient artificial neuromorphic computing. |
Huang, Xin; Feng, Xuewei; Chen, Li; Wang, Lin; Tan, Wee Chong; Huang, Li; Ang, Kah-Wee Fabry-Perot cavity enhanced light-matter interactions in two-dimensional van der Waals heterostructure Journal Article NANO ENERGY, 62 , pp. 667-673, 2019, ISSN: 2211-2855. @article{ISI:000474636100071, title = {Fabry-Perot cavity enhanced light-matter interactions in two-dimensional van der Waals heterostructure}, author = {Xin Huang and Xuewei Feng and Li Chen and Lin Wang and Wee Chong Tan and Li Huang and Kah-Wee Ang}, doi = {10.1016/j.nanoen.2019.05.090}, times_cited = {5}, issn = {2211-2855}, year = {2019}, date = {2019-08-01}, journal = {NANO ENERGY}, volume = {62}, pages = {667-673}, publisher = {ELSEVIER}, address = {RADARWEG 29, 1043 NX AMSTERDAM, NETHERLANDS}, abstract = {Despite monolayer transition metal dichalcogenide (TMD) shows a direct band gap property, its atomic thickness causes poor light absorption that severely limits its practical applications. For improving the optical gain of TMD, however, many approaches were proposed such as complicated fabrication process that compromises the stability and reliability of two-dimensional (2D) materials, which further limits the device scalability. In this work, a simple method is reported to engineer the light-matter interactions in few-layer molybdenum disulfide (MoS2) and tungsten diselenide (WSe2) via an asymmetric Fabry-Perot cavity (FPc). The cavity is based on the hybrid integration of TMD/hexagonal boron nitride (h-BN)/Au/SiO2 heterostructure realized through layer-by-layer stacking. By modulating the underlying h-BN thickness, constructive resonant absorption can be achieved by multiple internal reflections, which significantly increases the Raman and optical absorption of MoS2 and WSe2. Leveraging on the enhanced light-matter interactions, we further integrate this asymmetric Fabry-Perot cavity into WSe2/MoS2 van der Waals heterostructure (vdWH) to realize high performance photodiode and photovoltaic devices, leading to a similar to 5 folds increase in photodiode responsivity and a peak external quantum efficiency (EQE) of 7.5%. This work demonstrates an effective way towards hybrid integration of Fabry-Perot cavity with 2D materials, which could offer a potential pathway for enabling novel optoelectronic devices, such as 2D light-emitting diodes (LEDs) and solar cells.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Despite monolayer transition metal dichalcogenide (TMD) shows a direct band gap property, its atomic thickness causes poor light absorption that severely limits its practical applications. For improving the optical gain of TMD, however, many approaches were proposed such as complicated fabrication process that compromises the stability and reliability of two-dimensional (2D) materials, which further limits the device scalability. In this work, a simple method is reported to engineer the light-matter interactions in few-layer molybdenum disulfide (MoS2) and tungsten diselenide (WSe2) via an asymmetric Fabry-Perot cavity (FPc). The cavity is based on the hybrid integration of TMD/hexagonal boron nitride (h-BN)/Au/SiO2 heterostructure realized through layer-by-layer stacking. By modulating the underlying h-BN thickness, constructive resonant absorption can be achieved by multiple internal reflections, which significantly increases the Raman and optical absorption of MoS2 and WSe2. Leveraging on the enhanced light-matter interactions, we further integrate this asymmetric Fabry-Perot cavity into WSe2/MoS2 van der Waals heterostructure (vdWH) to realize high performance photodiode and photovoltaic devices, leading to a similar to 5 folds increase in photodiode responsivity and a peak external quantum efficiency (EQE) of 7.5%. This work demonstrates an effective way towards hybrid integration of Fabry-Perot cavity with 2D materials, which could offer a potential pathway for enabling novel optoelectronic devices, such as 2D light-emitting diodes (LEDs) and solar cells. |
Wang, Lin; Chen, Li; Wong, Swee Liang; Huang, Xin; Liao, Wugang; Zhu, Chunxiang; Lim, Yee-Fun; Li, Dabing; Liu, Xinke; Chi, Dongzhi; Ang, Koh-Wee Electronic Devices and Circuits Based on Wafer-Scale Polycrystalline Monolayer MoS2 by Chemical Vapor Deposition Journal Article ADVANCED ELECTRONIC MATERIALS, 5 (8), 2019, ISSN: 2199-160X. @article{ISI:000479319100017, title = {Electronic Devices and Circuits Based on Wafer-Scale Polycrystalline Monolayer MoS_{2} by Chemical Vapor Deposition}, author = {Lin Wang and Li Chen and Swee Liang Wong and Xin Huang and Wugang Liao and Chunxiang Zhu and Yee-Fun Lim and Dabing Li and Xinke Liu and Dongzhi Chi and Koh-Wee Ang}, doi = {10.1002/aelm.201900393}, times_cited = {0}, issn = {2199-160X}, year = {2019}, date = {2019-08-01}, journal = {ADVANCED ELECTRONIC MATERIALS}, volume = {5}, number = {8}, publisher = {WILEY}, address = {111 RIVER ST, HOBOKEN 07030-5774, NJ USA}, abstract = {2D layered materials such as graphene and transition-metal dichalcogenides (TMDCs) have emerged as promising candidates for next-generation nanoelectronic applications due to their atomically thin thicknesses and unique electronic properties. Among TMDCs, molybdenum disulfide (MoS2) has been extensively investigated as a channel material for field-effect transistor (FET) and circuit realization. However, to date most reported works have been limited to exfoliated MoS2 nanosheets primarily due to the difficulty in synthesizing large-area and high-quality MoS2 thin film. A demonstration of wafer-scale monolayer MoS2 synthesis is reported by chemical vapor deposition (CVD), enabling transistors, memristive memories, and integrated circuits to be realized simultaneously. Specifically, building on top-gated FETs with a high-kappa gate dielectric (HfO2), Boolean logic circuits including inverters and NAND gates are successfully demonstrated using direct-coupled FET logic technology, with typical inverters exhibiting a high voltage gain of 16, a large total noise margin of 0.72 V-DD at V-DD = 3 V, and perfect logic-level matching. Additionally, resistive switching is demonstrated in a MoS2-based memristor, indicating that they have great potential for the development of resistive random-access memory. By virtue of scalable CVD growth capability, the way toward practical and large-scale electronic applications of MoS2 is indicated.}, keywords = {}, pubstate = {published}, tppubtype = {article} } 2D layered materials such as graphene and transition-metal dichalcogenides (TMDCs) have emerged as promising candidates for next-generation nanoelectronic applications due to their atomically thin thicknesses and unique electronic properties. Among TMDCs, molybdenum disulfide (MoS2) has been extensively investigated as a channel material for field-effect transistor (FET) and circuit realization. However, to date most reported works have been limited to exfoliated MoS2 nanosheets primarily due to the difficulty in synthesizing large-area and high-quality MoS2 thin film. A demonstration of wafer-scale monolayer MoS2 synthesis is reported by chemical vapor deposition (CVD), enabling transistors, memristive memories, and integrated circuits to be realized simultaneously. Specifically, building on top-gated FETs with a high-kappa gate dielectric (HfO2), Boolean logic circuits including inverters and NAND gates are successfully demonstrated using direct-coupled FET logic technology, with typical inverters exhibiting a high voltage gain of 16, a large total noise margin of 0.72 V-DD at V-DD = 3 V, and perfect logic-level matching. Additionally, resistive switching is demonstrated in a MoS2-based memristor, indicating that they have great potential for the development of resistive random-access memory. By virtue of scalable CVD growth capability, the way toward practical and large-scale electronic applications of MoS2 is indicated. |
Wang, Lin; Liao, Wugang; Wong, Swee Hang; Yu, Zhi Gen; Li, Sifan; Lim, Yee-Fun; Feng, Xuewei; Tan, Wee Chong; Huang, Xin; Chen, Li; Liu, Liang; Chen, Jingsheng; Gong, Xiao; Zhu, Chunxiang; Liu, Xinke; Zhang, Yong-Wei; Chi, Dongzhi; Ang, Kah-Wee Artificial Synapses Based on Multiterminal Memtransistors for Neuromorphic Application Journal Article ADVANCED FUNCTIONAL MATERIALS, 29 (25), 2019, ISSN: 1616-301X. @article{ISI:000472552900004, title = {Artificial Synapses Based on Multiterminal Memtransistors for Neuromorphic Application}, author = {Lin Wang and Wugang Liao and Swee Hang Wong and Zhi Gen Yu and Sifan Li and Yee-Fun Lim and Xuewei Feng and Wee Chong Tan and Xin Huang and Li Chen and Liang Liu and Jingsheng Chen and Xiao Gong and Chunxiang Zhu and Xinke Liu and Yong-Wei Zhang and Dongzhi Chi and Kah-Wee Ang}, doi = {10.1002/adfm.201901106}, times_cited = {0}, issn = {1616-301X}, year = {2019}, date = {2019-06-01}, journal = {ADVANCED FUNCTIONAL MATERIALS}, volume = {29}, number = {25}, publisher = {WILEY-V C H VERLAG GMBH}, address = {POSTFACH 101161, 69451 WEINHEIM, GERMANY}, abstract = {Neuromorphic computing, which emulates the biological neural systems could overcome the high-power consumption issue of conventional von-Neumann computing. State-of-the-art artificial synapses made of two-terminal memristors, however, show variability in filament formation and limited capacity due to their inherent single presynaptic input design. Here, a memtransistor-based arti?cial synapse is realized by integrating a memristor and selector transistor into a multiterminal device using monolayer polycrys-talline-MoS2 grown by a scalable chemical vapor deposition (CVD) process. Notably, the memtransistor offers both drain- and gate-tunable nonvolatile memory functions, which efficiently emulates the long-term potentiation/depression, spike-amplitude, and spike-timing-dependent plasticity of biological synapses. Moreover, the gate tunability function that is not achievable in two-terminal memristors, enables significant bipolar resistive states switching up to four orders-of-magnitude and high cycling endurance. First-principles calculations reveal a new resistive switching mechanism driven by the diffusion of double sulfur vacancy perpendicular to the MoS2 grain boundary, leading to a conducting switching path without the need for a filament forming process. The seamless integration of multiterminal memtransistors may offer another degree-of-freedom to tune the synaptic plasticity by a third gate terminal for enabling complex neuromorphic learning.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Neuromorphic computing, which emulates the biological neural systems could overcome the high-power consumption issue of conventional von-Neumann computing. State-of-the-art artificial synapses made of two-terminal memristors, however, show variability in filament formation and limited capacity due to their inherent single presynaptic input design. Here, a memtransistor-based arti?cial synapse is realized by integrating a memristor and selector transistor into a multiterminal device using monolayer polycrys-talline-MoS2 grown by a scalable chemical vapor deposition (CVD) process. Notably, the memtransistor offers both drain- and gate-tunable nonvolatile memory functions, which efficiently emulates the long-term potentiation/depression, spike-amplitude, and spike-timing-dependent plasticity of biological synapses. Moreover, the gate tunability function that is not achievable in two-terminal memristors, enables significant bipolar resistive states switching up to four orders-of-magnitude and high cycling endurance. First-principles calculations reveal a new resistive switching mechanism driven by the diffusion of double sulfur vacancy perpendicular to the MoS2 grain boundary, leading to a conducting switching path without the need for a filament forming process. The seamless integration of multiterminal memtransistors may offer another degree-of-freedom to tune the synaptic plasticity by a third gate terminal for enabling complex neuromorphic learning. |
Wang, Lin; Liao, Wugang; Xu, Shengqiang; Gong, Xiao; Zhu, Chunxiang; Ang, Kah-Wee Unipolar n-Type Conduction in Black Phosphorus Induced by Atomic Layer Deposited MgO Journal Article IEEE ELECTRON DEVICE LETTERS, 40 (3), pp. 471-474, 2019, ISSN: 0741-3106. @article{ISI:000460664000028, title = {Unipolar n-Type Conduction in Black Phosphorus Induced by Atomic Layer Deposited MgO}, author = {Lin Wang and Wugang Liao and Shengqiang Xu and Xiao Gong and Chunxiang Zhu and Kah-Wee Ang}, doi = {10.1109/LED.2019.2895678}, times_cited = {7}, issn = {0741-3106}, year = {2019}, date = {2019-03-01}, journal = {IEEE ELECTRON DEVICE LETTERS}, volume = {40}, number = {3}, pages = {471-474}, publisher = {IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC}, address = {445 HOES LANE, PISCATAWAY, NJ 08855-4141 USA}, abstract = {Two-dimensional black phosphorus (BP) holds great promise for future nanoscale field-effect transistors owing to its finite bandgap and expected high carrier mobility. Nevertheless, while most studies have reported unipolar p-type or ambipolar BP transistors with a hole conduction dominance, excellent n-type transport in BP has been a critical challenge. Here, we report unipolar n-type BP transistors realized by a MgO capping layer via atomic layer deposition. By coveragewith 20-nm-thick-MgO, remarkable electron conduction is obtained in transistors fabricated on BP flakes of thicknesses from 2.6 to 7.6 nm, accompanied by fully suppressed hole transport. Furthermore, the unipolar electron transport is found to retain (or even improve) after a period of six months, with the highest extrinsic electron mobility reaching 135.9 cm(2)/Vs. The effective approach to realizing unipolar n-FETs of BP demonstrated in this letter paves the way to the implementation of BP-basedCMOS digital logic circuits.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Two-dimensional black phosphorus (BP) holds great promise for future nanoscale field-effect transistors owing to its finite bandgap and expected high carrier mobility. Nevertheless, while most studies have reported unipolar p-type or ambipolar BP transistors with a hole conduction dominance, excellent n-type transport in BP has been a critical challenge. Here, we report unipolar n-type BP transistors realized by a MgO capping layer via atomic layer deposition. By coveragewith 20-nm-thick-MgO, remarkable electron conduction is obtained in transistors fabricated on BP flakes of thicknesses from 2.6 to 7.6 nm, accompanied by fully suppressed hole transport. Furthermore, the unipolar electron transport is found to retain (or even improve) after a period of six months, with the highest extrinsic electron mobility reaching 135.9 cm(2)/Vs. The effective approach to realizing unipolar n-FETs of BP demonstrated in this letter paves the way to the implementation of BP-basedCMOS digital logic circuits. |
Tan, Wee Chong; Wang, Lin; Feng, Xuewei; Chen, Li; Huang, Li; Huang, Xin; Ang, Kah-Wee Recent Advances in Black Phosphorus-Based Electronic Devices Journal Article ADVANCED ELECTRONIC MATERIALS, 5 (2), 2019, ISSN: 2199-160X. @article{ISI:000459622700029, title = {Recent Advances in Black Phosphorus-Based Electronic Devices}, author = {Wee Chong Tan and Lin Wang and Xuewei Feng and Li Chen and Li Huang and Xin Huang and Kah-Wee Ang}, doi = {10.1002/aelm.201800666}, times_cited = {0}, issn = {2199-160X}, year = {2019}, date = {2019-02-01}, journal = {ADVANCED ELECTRONIC MATERIALS}, volume = {5}, number = {2}, publisher = {WILEY}, address = {111 RIVER ST, HOBOKEN 07030-5774, NJ USA}, abstract = {The rediscovery of graphene in the recent past has propelled the rapid development of exfoliation and other thin layer processing techniques, leading to a renewed interest in black phosphorus (BP). Since 2014, BP has been extensively studied due to its superior electronic, photonic, and mechanical properties. In addition, the unique intrinsic anisotropic characteristics resulting from its puckered structure can be utilized for designing new functional devices. In retrospect, significant efforts have been directed toward the synthesis, basic understanding, and applications of BP in the fields of nanoelectronics, ultrafast optics, nanophotonics, and optoelectronics. Here, the recent development of BP-based devices, such as nanoribbon field-effect transistors, complementary logic circuits, memory devices, and the progress made in meeting the challenges associated with contact resistance, in-plane anisotropy, and advanced gate stack, are reviewed. Finally, the prospects of 2D materials in meeting the International Technology Roadmap for Semiconductor requirements for the year 2030 are discussed.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The rediscovery of graphene in the recent past has propelled the rapid development of exfoliation and other thin layer processing techniques, leading to a renewed interest in black phosphorus (BP). Since 2014, BP has been extensively studied due to its superior electronic, photonic, and mechanical properties. In addition, the unique intrinsic anisotropic characteristics resulting from its puckered structure can be utilized for designing new functional devices. In retrospect, significant efforts have been directed toward the synthesis, basic understanding, and applications of BP in the fields of nanoelectronics, ultrafast optics, nanophotonics, and optoelectronics. Here, the recent development of BP-based devices, such as nanoribbon field-effect transistors, complementary logic circuits, memory devices, and the progress made in meeting the challenges associated with contact resistance, in-plane anisotropy, and advanced gate stack, are reviewed. Finally, the prospects of 2D materials in meeting the International Technology Roadmap for Semiconductor requirements for the year 2030 are discussed. |