Slaven Garaj
Degree: PhD
Position: Assistant Professor
Affiliation: NUS – Department of Physics
Research Type: Experiment
Office: S13-02-04
Email: slaven@nus.edu.sg
Contact: (65) 6516 2164
CA2DM Publications:
2024 |
Ronceray, Nathan; Spina, Massimo; Chou, Vanessa Hui Yin; Lim, Chwee Teck; Geim, Andre K; Garaj, Slaven Elastocapillarity-driven 2D nano-switches enable zeptoliter-scale liquid encapsulation Journal Article NATURE COMMUNICATIONS, 15 (1), 2024. @article{ISI:001158425400020, title = {Elastocapillarity-driven 2D nano-switches enable zeptoliter-scale liquid encapsulation}, author = {Nathan Ronceray and Massimo Spina and Vanessa Hui Yin Chou and Chwee Teck Lim and Andre K Geim and Slaven Garaj}, doi = {10.1038/s41467-023-44200-3}, times_cited = {0}, year = {2024}, date = {2024-01-02}, journal = {NATURE COMMUNICATIONS}, volume = {15}, number = {1}, publisher = {NATURE PORTFOLIO}, address = {HEIDELBERGER PLATZ 3, BERLIN, 14197, GERMANY}, abstract = {Biological nanostructures change their shape and function in response to external stimuli, and significant efforts have been made to design artificial biomimicking devices operating on similar principles. In this work we demonstrate a programmable nanofluidic switch, driven by elastocapillarity, and based on nanochannels built fromlayered two-dimensional nanomaterials possessing atomically smooth surfaces and exceptional mechanical properties. We explore operational modes of the nanoswitch and develop a theoretical framework to explain the phenomenon. By predicting the switchingreversibility phase diagram-based on material, interfacial and wetting properties, as well as the geometry of the nanofluidic circuit-we rationally design switchable nano-capsules capable of enclosing zeptoliter volumes of liquid, as small as the volumes enclosed in viruses. The nanoswitch will find useful application as an active element in integrated nanofluidic circuitry and could be used to explore nanoconfined chemistry and biochemistry, or be incorporated into shape-programmable materials.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Biological nanostructures change their shape and function in response to external stimuli, and significant efforts have been made to design artificial biomimicking devices operating on similar principles. In this work we demonstrate a programmable nanofluidic switch, driven by elastocapillarity, and based on nanochannels built fromlayered two-dimensional nanomaterials possessing atomically smooth surfaces and exceptional mechanical properties. We explore operational modes of the nanoswitch and develop a theoretical framework to explain the phenomenon. By predicting the switchingreversibility phase diagram-based on material, interfacial and wetting properties, as well as the geometry of the nanofluidic circuit-we rationally design switchable nano-capsules capable of enclosing zeptoliter volumes of liquid, as small as the volumes enclosed in viruses. The nanoswitch will find useful application as an active element in integrated nanofluidic circuitry and could be used to explore nanoconfined chemistry and biochemistry, or be incorporated into shape-programmable materials. |
2023 |
Wang, Zuxin; Olvera-Vargas, Hugo; Martins, Marcos Vinicius Surmani; Garcia-Rodriguez, Orlando; Garaj, Slaven; Lefebvre, Olivier High performance and durable graphene-grafted cathode for electro-Fenton degradation of tetramethyldecynediol Journal Article CHEMICAL ENGINEERING JOURNAL, 455 , 2023, ISSN: 1385-8947. @article{ISI:000931159000001, title = {High performance and durable graphene-grafted cathode for electro-Fenton degradation of tetramethyldecynediol}, author = {Zuxin Wang and Hugo Olvera-Vargas and Marcos Vinicius Surmani Martins and Orlando Garcia-Rodriguez and Slaven Garaj and Olivier Lefebvre}, doi = {10.1016/j.cej.2022.140643}, times_cited = {0}, issn = {1385-8947}, year = {2023}, date = {2023-02-02}, journal = {CHEMICAL ENGINEERING JOURNAL}, volume = {455}, publisher = {ELSEVIER SCIENCE SA}, address = {PO BOX 564, 1001 LAUSANNE, SWITZERLAND}, abstract = {A novel reduced graphene oxide grafted carbon fiber (rGO-CF) cathode for electro-Fenton was prepared using a simple thermal treatment method and applied to the degradation of tetramethyldecynediol (TMDD), a massively used non-ionic surfactant. Electrochemical characterization demonstrated that the new cathode possessed high electroactive surface area (1387.1 cm2 g-1), low electron-transfer resistance (0.198 ohm) and high activity for oxygen reduction. With these excellent properties, rGO-CF achieved 122.1 mg L-1 of H2O2 accumulation, 45 % higher than with the unmodified cathode (84.2 mg L-1). Compared to the unmodified cathode, rGO-CF doubled the TMDD degradation rate and increased the 2-hour mineralization yield from 49.4 % to 84.0 %. Most importantly, rGO-CF exhibited excellent stability verified over 1000 cycles of cyclic voltammetry and 5 runs of electro-Fenton experiments. A degradation pathway for the oxidation of TMDD is proposed. With ease of fabrication, stability and affordable cost, the new electrode shows tremendous potential for practical applications.}, keywords = {}, pubstate = {published}, tppubtype = {article} } A novel reduced graphene oxide grafted carbon fiber (rGO-CF) cathode for electro-Fenton was prepared using a simple thermal treatment method and applied to the degradation of tetramethyldecynediol (TMDD), a massively used non-ionic surfactant. Electrochemical characterization demonstrated that the new cathode possessed high electroactive surface area (1387.1 cm2 g-1), low electron-transfer resistance (0.198 ohm) and high activity for oxygen reduction. With these excellent properties, rGO-CF achieved 122.1 mg L-1 of H2O2 accumulation, 45 % higher than with the unmodified cathode (84.2 mg L-1). Compared to the unmodified cathode, rGO-CF doubled the TMDD degradation rate and increased the 2-hour mineralization yield from 49.4 % to 84.0 %. Most importantly, rGO-CF exhibited excellent stability verified over 1000 cycles of cyclic voltammetry and 5 runs of electro-Fenton experiments. A degradation pathway for the oxidation of TMDD is proposed. With ease of fabrication, stability and affordable cost, the new electrode shows tremendous potential for practical applications. |
2022 |
Malhotra, Ritika; Halbig, Christian Eberhard; Sim, Yu Fan; Lim, Chwee Teck; Leong, David Tai; Neto, Castro A H; Garaj, Slaven; Rosa, Vinicius Cytotoxicity survey of commercial graphene materials from worldwide Journal Article NPJ 2D MATERIALS AND APPLICATIONS, 6 (1), 2022. @article{ISI:000852419000001, title = {Cytotoxicity survey of commercial graphene materials from worldwide}, author = {Ritika Malhotra and Christian Eberhard Halbig and Yu Fan Sim and Chwee Teck Lim and David Tai Leong and Castro A H Neto and Slaven Garaj and Vinicius Rosa}, doi = {10.1038/s41699-022-00330-8}, times_cited = {3}, year = {2022}, date = {2022-09-09}, journal = {NPJ 2D MATERIALS AND APPLICATIONS}, volume = {6}, number = {1}, publisher = {NATURE PORTFOLIO}, address = {HEIDELBERGER PLATZ 3, BERLIN, 14197, GERMANY}, abstract = {Graphene and other 2D materials are having a profound impact on science and technology. Unfortunately, progress in this area has not been followed by strict quality controls and toxicity benchmarks. Herein, we report a survey of the cytotoxicity of 36 products nominally labeled as "graphene." These are available from suppliers worldwide and synthesized through various techniques. Detailed characterization suggests that these products represent a heterogeneous class of materials with varying physicochemical properties and a noticeable quantity of contaminants. We demonstrate that the cellular toxicity of these products is not related to a particular characteristic of graphene; rather, it is fundamentally determined by the presence of impurities in the commercially available graphene family materials tested.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Graphene and other 2D materials are having a profound impact on science and technology. Unfortunately, progress in this area has not been followed by strict quality controls and toxicity benchmarks. Herein, we report a survey of the cytotoxicity of 36 products nominally labeled as "graphene." These are available from suppliers worldwide and synthesized through various techniques. Detailed characterization suggests that these products represent a heterogeneous class of materials with varying physicochemical properties and a noticeable quantity of contaminants. We demonstrate that the cellular toxicity of these products is not related to a particular characteristic of graphene; rather, it is fundamentally determined by the presence of impurities in the commercially available graphene family materials tested. |
Zhu, Houjuan; Zan, Wenyan; Chen, Wanli; Jiang, Wenbin; Ding, Xianguang; Li, Bang Lin; Mu, Yuewen; Wang, Lei; Garaj, Slaven; Leong, David Tai Defect-Rich Molybdenum Sulfide Quantum Dots for Amplified Photoluminescence and Photonics-Driven Reactive Oxygen Species Generation Journal Article ADVANCED MATERIALS, 34 (31), 2022, ISSN: 0935-9648. @article{ISI:000819976700001, title = {Defect-Rich Molybdenum Sulfide Quantum Dots for Amplified Photoluminescence and Photonics-Driven Reactive Oxygen Species Generation}, author = {Houjuan Zhu and Wenyan Zan and Wanli Chen and Wenbin Jiang and Xianguang Ding and Bang Lin Li and Yuewen Mu and Lei Wang and Slaven Garaj and David Tai Leong}, doi = {10.1002/adma.202200004}, times_cited = {0}, issn = {0935-9648}, year = {2022}, date = {2022-07-03}, journal = {ADVANCED MATERIALS}, volume = {34}, number = {31}, publisher = {WILEY-V C H VERLAG GMBH}, address = {POSTFACH 101161, 69451 WEINHEIM, GERMANY}, abstract = {Transition metal dichalcogenide (TMD) quantum dots (QDs) with defects have attracted interesting chemistry due to the contribution of vacancies to their unique optical, physical, catalytic, and electrical properties. Engineering defined defects into molybdenum sulfide (MoS2) QDs is challenging. Herein, by applying a mild biomineralization-assisted bottom-up strategy, blue photoluminescent MoS2 QDs (B-QDs) with a high density of defects are fabricated. The two-stage synthesis begins with a bottom-up synthesis of original MoS2 QDs (O-QDs) through chemical reactions of Mo and sulfide ions, followed by alkaline etching that creates high sulfur-vacancy defects to eventually form B-QDs. Alkaline etching significantly increases the photoluminescence (PL) and photo-oxidation. An increase in defect density is shown to bring about increased active sites and decreased bandgap energy; which is further validated with density functional theory calculations. There is strengthened binding affinity between QDs and O-2 due to lower gap energy ( increment E-ST) between S-1 and T-1, accompanied with improved intersystem crossing (ISC) efficiency. Lowered gap energy contributes to assist e(-)-h(+) pair formation and the strengthened binding affinity between QDs and O-3(2). Defect engineering unravels another dimension of material properties control and can bring fresh new applications to otherwise well characterized TMD nanomaterials.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Transition metal dichalcogenide (TMD) quantum dots (QDs) with defects have attracted interesting chemistry due to the contribution of vacancies to their unique optical, physical, catalytic, and electrical properties. Engineering defined defects into molybdenum sulfide (MoS2) QDs is challenging. Herein, by applying a mild biomineralization-assisted bottom-up strategy, blue photoluminescent MoS2 QDs (B-QDs) with a high density of defects are fabricated. The two-stage synthesis begins with a bottom-up synthesis of original MoS2 QDs (O-QDs) through chemical reactions of Mo and sulfide ions, followed by alkaline etching that creates high sulfur-vacancy defects to eventually form B-QDs. Alkaline etching significantly increases the photoluminescence (PL) and photo-oxidation. An increase in defect density is shown to bring about increased active sites and decreased bandgap energy; which is further validated with density functional theory calculations. There is strengthened binding affinity between QDs and O-2 due to lower gap energy ( increment E-ST) between S-1 and T-1, accompanied with improved intersystem crossing (ISC) efficiency. Lowered gap energy contributes to assist e(-)-h(+) pair formation and the strengthened binding affinity between QDs and O-3(2). Defect engineering unravels another dimension of material properties control and can bring fresh new applications to otherwise well characterized TMD nanomaterials. |
2021 |
Sharma, Rajesh Kumar; Agrawal, Ishita; Dai, Liang; Doyle, Patrick; Garaj, Slaven DNA Knot Malleability in Single-Digit Nanopores Journal Article NANO LETTERS, 21 (9), pp. 3772-3779, 2021, ISSN: 1530-6984. @article{ISI:000651773600009, title = {DNA Knot Malleability in Single-Digit Nanopores}, author = {Rajesh Kumar Sharma and Ishita Agrawal and Liang Dai and Patrick Doyle and Slaven Garaj}, doi = {10.1021/acs.nanolett.0c05142}, times_cited = {0}, issn = {1530-6984}, year = {2021}, date = {2021-03-04}, journal = {NANO LETTERS}, volume = {21}, number = {9}, pages = {3772-3779}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {Knots in long DNA molecules are prevalent in biological systems and serve as a model system for investigating static and dynamic properties of biopolymers. We explore the dynamics of knots in double-stranded DNA in a new regime of nanometer-scale confinement, large forces, and short time scales, using solid-state nanopores. We show that DNA knots undergo isomorphic translocation through a nanopore, retaining their equilibrium morphology by swiftly compressing in a lateral direction to fit the constriction. We observe no evidence of knot tightening or jamming, even for single-digit nanopores. We explain the observations as the malleability of DNA, characterized by sharp buckling of the DNA in nanopores, driven by the transient disruption of base pairing. Our molecular dynamics simulations support the model. These results are relevant not only for the understanding of DNA packing and manipulation in living cells but also for the polymer physics of DNA and the development of nanopore-based sequencing technologies.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Knots in long DNA molecules are prevalent in biological systems and serve as a model system for investigating static and dynamic properties of biopolymers. We explore the dynamics of knots in double-stranded DNA in a new regime of nanometer-scale confinement, large forces, and short time scales, using solid-state nanopores. We show that DNA knots undergo isomorphic translocation through a nanopore, retaining their equilibrium morphology by swiftly compressing in a lateral direction to fit the constriction. We observe no evidence of knot tightening or jamming, even for single-digit nanopores. We explain the observations as the malleability of DNA, characterized by sharp buckling of the DNA in nanopores, driven by the transient disruption of base pairing. Our molecular dynamics simulations support the model. These results are relevant not only for the understanding of DNA packing and manipulation in living cells but also for the polymer physics of DNA and the development of nanopore-based sequencing technologies. |
Lee, Hae Yeon; Ezzi, Mohammed Al M; Raghuvanshi, Nimisha; Chung, Jing Yang; Watanabe, Kenji; Taniguchi, Takashi; Garaj, Slaven; Adam, Shaffique; Gradecak, Silvija Tunable Optical Properties of Thin Films Controlled by the Interface Twist Angle Journal Article NANO LETTERS, 21 (7), pp. 2832-2839, 2021, ISSN: 1530-6984. @article{ISI:000641160500018, title = {Tunable Optical Properties of Thin Films Controlled by the Interface Twist Angle}, author = {Hae Yeon Lee and Mohammed Al M Ezzi and Nimisha Raghuvanshi and Jing Yang Chung and Kenji Watanabe and Takashi Taniguchi and Slaven Garaj and Shaffique Adam and Silvija Gradecak}, doi = {10.1021/acs.nanolett.0c04924}, times_cited = {0}, issn = {1530-6984}, year = {2021}, date = {2021-02-16}, journal = {NANO LETTERS}, volume = {21}, number = {7}, pages = {2832-2839}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {Control of materials properties has been the driving force of modern technologies. So far, materials properties have been modulated by their composition, structure, and size. Here, by using cathodoluminescence in a scanning transmission electron microscope, we show that the optical properties of stacked, >100 nm thick hexagonal boron nitride (hBN) films can be continuously tuned by their relative twist angles. Due to the formation of a moire superlattice between the two interface layers of the twisted films, a new moire ' sub-band gap is formed with continuously decreasing magnitude as a function of the twist angle, resulting in tunable luminescence wavelength and intensity increase of >40x. Our results demonstrate that moire ' phenomena extend beyond monolayer-based systems and can be preserved in a technologically relevant, bulklike material at room temperature, dominating optical properties of hBN films for applications in medicine, environmental, or information technologies.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Control of materials properties has been the driving force of modern technologies. So far, materials properties have been modulated by their composition, structure, and size. Here, by using cathodoluminescence in a scanning transmission electron microscope, we show that the optical properties of stacked, >100 nm thick hexagonal boron nitride (hBN) films can be continuously tuned by their relative twist angles. Due to the formation of a moire superlattice between the two interface layers of the twisted films, a new moire ' sub-band gap is formed with continuously decreasing magnitude as a function of the twist angle, resulting in tunable luminescence wavelength and intensity increase of >40x. Our results demonstrate that moire ' phenomena extend beyond monolayer-based systems and can be preserved in a technologically relevant, bulklike material at room temperature, dominating optical properties of hBN films for applications in medicine, environmental, or information technologies. |
2019 |
Basu, Tanmoy; Blaskovic, Milan; Tripathy, Sudhiranjan; Tian, Feng; Singh, Ranveer; Som, Tapobrata; Garaj, Slaven; van Kan, Jeroen Anton MATERIALS LETTERS, 256 , 2019, ISSN: 0167-577X. @article{ISI:000489719300044, title = {Local surface conductivity mapping of single-layer graphene subject to low energy argon bombardment: Energy loss mechanism and defect induction efficiency}, author = {Tanmoy Basu and Milan Blaskovic and Sudhiranjan Tripathy and Feng Tian and Ranveer Singh and Tapobrata Som and Slaven Garaj and Jeroen Anton van Kan}, doi = {10.1016/j.matlet.2019.126638}, times_cited = {0}, issn = {0167-577X}, year = {2019}, date = {2019-12-01}, journal = {MATERIALS LETTERS}, volume = {256}, publisher = {ELSEVIER}, address = {RADARWEG 29, 1043 NX AMSTERDAM, NETHERLANDS}, abstract = {An ion-beam exposure can be applied to form defects in supported graphene which can be tuned with energies and fluence. However, recent results show that nuclear energy loss process is the dominant mechanism in defect formation. In the present work, it is shown that in low energy regime (1-10 keV Ar+ ions), both nuclear and electronic energy losses play an important role in forming different types of defects. Here we show a linear relation between defect induction efficiency and energy loss. Conductive atomic force microscopy shows a significant reduction in the current through supported graphene after ion beam exposure. (C) 2019 Elsevier B.V. All rights reserved.}, keywords = {}, pubstate = {published}, tppubtype = {article} } An ion-beam exposure can be applied to form defects in supported graphene which can be tuned with energies and fluence. However, recent results show that nuclear energy loss process is the dominant mechanism in defect formation. In the present work, it is shown that in low energy regime (1-10 keV Ar+ ions), both nuclear and electronic energy losses play an important role in forming different types of defects. Here we show a linear relation between defect induction efficiency and energy loss. Conductive atomic force microscopy shows a significant reduction in the current through supported graphene after ion beam exposure. (C) 2019 Elsevier B.V. All rights reserved. |
Sharma, Rajesh Kumar; Agrawal, Ishita; Dai, Liang; Doyle, Patrick S; Garaj, Slaven Complex DNA knots detected with a nanopore sensor Journal Article NATURE COMMUNICATIONS, 10 , 2019, ISSN: 2041-1723. @article{ISI:000488485300008, title = {Complex DNA knots detected with a nanopore sensor}, author = {Rajesh Kumar Sharma and Ishita Agrawal and Liang Dai and Patrick S Doyle and Slaven Garaj}, doi = {10.1038/s41467-019-12358-4}, times_cited = {0}, issn = {2041-1723}, year = {2019}, date = {2019-10-02}, journal = {NATURE COMMUNICATIONS}, volume = {10}, publisher = {NATURE PUBLISHING GROUP}, address = {MACMILLAN BUILDING, 4 CRINAN ST, LONDON N1 9XW, ENGLAND}, abstract = {Equilibrium knots are common in biological polymers-their prevalence, size distribution, structure, and dynamics have been extensively studied, with implications to fundamental biological processes and DNA sequencing technologies. Nanopore microscopy is a high-throughput single-molecule technique capable of detecting the shape of biopolymers, including DNA knots. Here we demonstrate nanopore sensors that map the equilibrium structure of DNA knots, without spurious knot tightening and sliding. We show the occurrence of both tight and loose knots, reconciling previous contradictory results from different experimental techniques. We evidence the occurrence of two quantitatively different modes of knot translocation through the nanopores, involving very different tension forces. With large statistics, we explore the complex knots and, for the first time, reveal the existence of rare composite knots. We use parametrized complexity, in concert with simulations, to test the theoretical assumptions of the models, further asserting the relevance of nanopores in future investigation of knots.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Equilibrium knots are common in biological polymers-their prevalence, size distribution, structure, and dynamics have been extensively studied, with implications to fundamental biological processes and DNA sequencing technologies. Nanopore microscopy is a high-throughput single-molecule technique capable of detecting the shape of biopolymers, including DNA knots. Here we demonstrate nanopore sensors that map the equilibrium structure of DNA knots, without spurious knot tightening and sliding. We show the occurrence of both tight and loose knots, reconciling previous contradictory results from different experimental techniques. We evidence the occurrence of two quantitatively different modes of knot translocation through the nanopores, involving very different tension forces. With large statistics, we explore the complex knots and, for the first time, reveal the existence of rare composite knots. We use parametrized complexity, in concert with simulations, to test the theoretical assumptions of the models, further asserting the relevance of nanopores in future investigation of knots. |
Tan, Eveline; Li, Ban Lin; Ariga, Katsuhiko; Lim, Chwee-Teck; Garaj, Slaven; Leong, David Tai Toxicity of Two-Dimensional Layered Materials and Their Heterostructures Journal Article 44 BIOCONJUGATE CHEMISTRY, 30 (9), pp. 2287-2299, 2019, ISSN: 1043-1802. @article{ISI:000487180000003, title = {Toxicity of Two-Dimensional Layered Materials and Their Heterostructures}, author = {Eveline Tan and Ban Lin Li and Katsuhiko Ariga and Chwee-Teck Lim and Slaven Garaj and David Tai Leong}, doi = {10.1021/acs.bioconjchem.9b00502}, times_cited = {44}, issn = {1043-1802}, year = {2019}, date = {2019-09-01}, journal = {BIOCONJUGATE CHEMISTRY}, volume = {30}, number = {9}, pages = {2287-2299}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {Two-dimensional layered materials (2D LMs) are taking the scientific world by storm. Graphene epitomizes 2D LMs with many interesting properties and corresponding applications. Following the footsteps of graphene, many other types of 2D LMs such as transition metal dichalcogenides, black phosphorus, and graphitic-phase C3N4 nanosheets are emerging to be equally interesting as graphene and its derivatives. Some of these applications such as nanomedicine do have a high probability of human exposure. This review focuses on the biological and toxicity effects of 2D LMs and their associated mechanisms linking their chemistries to their biological end points. This review aims to help researchers to predict and mitigate any toxic effects. With understanding, redesign of newer and safer 2D LMs becomes possible.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Two-dimensional layered materials (2D LMs) are taking the scientific world by storm. Graphene epitomizes 2D LMs with many interesting properties and corresponding applications. Following the footsteps of graphene, many other types of 2D LMs such as transition metal dichalcogenides, black phosphorus, and graphitic-phase C3N4 nanosheets are emerging to be equally interesting as graphene and its derivatives. Some of these applications such as nanomedicine do have a high probability of human exposure. This review focuses on the biological and toxicity effects of 2D LMs and their associated mechanisms linking their chemistries to their biological end points. This review aims to help researchers to predict and mitigate any toxic effects. With understanding, redesign of newer and safer 2D LMs becomes possible. |
Ke, Jian-An; Garaj, Slaven; Gradescak, Silvija Nanopores in 2D MoS2: Defect-Mediated Formation and Density Modulation Journal Article ACS APPLIED MATERIALS & INTERFACES, 11 (29), pp. 26228-26234, 2019, ISSN: 1944-8244. @article{ISI:000477787200064, title = {Nanopores in 2D MoS_{2}: Defect-Mediated Formation and Density Modulation}, author = {Jian-An Ke and Slaven Garaj and Silvija Gradescak}, doi = {10.1021/acsami.9b03531}, times_cited = {2}, issn = {1944-8244}, year = {2019}, date = {2019-07-24}, journal = {ACS APPLIED MATERIALS & INTERFACES}, volume = {11}, number = {29}, pages = {26228-26234}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {Oxidation is a scalable process for introducing nanopores in two-dimensional transitional metal dichalcogenides (TMDs) for membrane applications. The nanopore density is determined by the areal density of their nucleation sites; understanding the nature of the defects and their control would enable tailoring of TMD membranes for targeted applications. In this work, we show that the nanopore distribution is dramatically different in strained and unstrained MoS2 crystals. We correlate this spatial distribution to the underlying arrangement of dislocations in MoS2 crystals, in contrast to previously suggested sulfur vacancies. To control the nucleation density of MoS2 nanopores, we demonstrate that the pore density can be modulated by electron beam exposure prior to the nanopore formation. Raman analysis of electron beam-exposed samples indicates that hydrocarbon adsorption activates defect species other than dislocations, which significantly enhances the nanopore density in MoS2.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Oxidation is a scalable process for introducing nanopores in two-dimensional transitional metal dichalcogenides (TMDs) for membrane applications. The nanopore density is determined by the areal density of their nucleation sites; understanding the nature of the defects and their control would enable tailoring of TMD membranes for targeted applications. In this work, we show that the nanopore distribution is dramatically different in strained and unstrained MoS2 crystals. We correlate this spatial distribution to the underlying arrangement of dislocations in MoS2 crystals, in contrast to previously suggested sulfur vacancies. To control the nucleation density of MoS2 nanopores, we demonstrate that the pore density can be modulated by electron beam exposure prior to the nanopore formation. Raman analysis of electron beam-exposed samples indicates that hydrocarbon adsorption activates defect species other than dislocations, which significantly enhances the nanopore density in MoS2. |
Liu, Tao; Liu, Song; Tu, Kun-Hua; Schmidt, Hennrik; Chu, Leiqiang; Xiang, Du; Martin, Jens; Eda, Goki; Ross, Caroline A; Garaj, Slaven Crested two-dimensional transistors Journal Article NATURE NANOTECHNOLOGY, 14 (3), pp. 223-+, 2019, ISSN: 1748-3387. @article{ISI:000460300900013, title = {Crested two-dimensional transistors}, author = {Tao Liu and Song Liu and Kun-Hua Tu and Hennrik Schmidt and Leiqiang Chu and Du Xiang and Jens Martin and Goki Eda and Caroline A Ross and Slaven Garaj}, doi = {10.1038/s41565-019-0361-x}, times_cited = {0}, issn = {1748-3387}, year = {2019}, date = {2019-03-01}, journal = {NATURE NANOTECHNOLOGY}, volume = {14}, number = {3}, pages = {223-+}, publisher = {NATURE PUBLISHING GROUP}, address = {MACMILLAN BUILDING, 4 CRINAN ST, LONDON N1 9XW, ENGLAND}, abstract = {Two-dimensional transition metal dichalcogenide (TMD) materials, albeit promising candidates for applications in electronics and optoelectronics(1-3), are still limited by their low electrical mobility under ambient conditions. Efforts to improve device performance through a variety of routes, such as modification of contact metals(4) and gate dielectrics(5-9) or encapsulation in hexagonal boron nitride(10), have yielded limited success at room temperature. Here, we report a large increase in the performance of TMD field-effect transistors operating under ambient conditions, achieved by engineering the substrate's surface morphology. For MoS2 transistors fabricated on crested substrates, we observed an almost two orders of magnitude increase in carrier mobility compared to standard devices, as well as very high saturation currents. The mechanical strain in TMDs has been predicted to boost carrier mobility(11), and has been shown to influence the local bandgap(12,13) and quantum emission properties(14) of TMDs. With comprehensive investigation of different dielectric environments and morphologies, we demonstrate that the substrate's increased corrugation, with its resulting strain field, is the dominant factor driving performance enhancement. This strategy is universally valid for other semiconducting TMD materials, either p-doped or n-doped, opening them up for applications in heterogeneous integrated electronics.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Two-dimensional transition metal dichalcogenide (TMD) materials, albeit promising candidates for applications in electronics and optoelectronics(1-3), are still limited by their low electrical mobility under ambient conditions. Efforts to improve device performance through a variety of routes, such as modification of contact metals(4) and gate dielectrics(5-9) or encapsulation in hexagonal boron nitride(10), have yielded limited success at room temperature. Here, we report a large increase in the performance of TMD field-effect transistors operating under ambient conditions, achieved by engineering the substrate's surface morphology. For MoS2 transistors fabricated on crested substrates, we observed an almost two orders of magnitude increase in carrier mobility compared to standard devices, as well as very high saturation currents. The mechanical strain in TMDs has been predicted to boost carrier mobility(11), and has been shown to influence the local bandgap(12,13) and quantum emission properties(14) of TMDs. With comprehensive investigation of different dielectric environments and morphologies, we demonstrate that the substrate's increased corrugation, with its resulting strain field, is the dominant factor driving performance enhancement. This strategy is universally valid for other semiconducting TMD materials, either p-doped or n-doped, opening them up for applications in heterogeneous integrated electronics. |
Zhang, Pei; Xiang, Haiyan; Tao, Li; Dong, Hongjie; Zhou, Yige; Hu, Travis Shihao; Chen, Xuli; Liu, Song; Wang, Shuangyin; Garaj, Slaven Chemically activated MoS2 for efficient hydrogen production Journal Article NANO ENERGY, 57 , pp. 535-541, 2019, ISSN: 2211-2855. @article{ISI:000458419000055, title = {Chemically activated MoS_{2} for efficient hydrogen production}, author = {Pei Zhang and Haiyan Xiang and Li Tao and Hongjie Dong and Yige Zhou and Travis Shihao Hu and Xuli Chen and Song Liu and Shuangyin Wang and Slaven Garaj}, doi = {10.1016/j.nanoen.2018.12.045}, times_cited = {0}, issn = {2211-2855}, year = {2019}, date = {2019-03-01}, journal = {NANO ENERGY}, volume = {57}, pages = {535-541}, publisher = {ELSEVIER}, address = {RADARWEG 29, 1043 NX AMSTERDAM, NETHERLANDS}, abstract = {Two-dimensional layered molybdenum disulfide (MoS2) is a promising catalyst for hydrogen evolution reaction (HER), and a good replacement for platinum (Pt) in elelctrochemical water splitting. Most transition metal dichalcogenides (TMDs) show excellent catalytic activity, which stems from their active sites located along the edges. However, small density of the active sites in the basal plane, largely limits TMDs performance. To enhance the HER catalysis activity of MoS2, we developed an efficient and scalable approach to significantly increase the overall electrochemically active sites using mild sodium hypochlorite (NaClO) solution anisotropic etching. The effect is further enhanced by oxygen-plasma pretreatment of the material, which - upon chemical etching - leads to highly porous and highly reactive structure. The resulting chemically activated MoS2 (ca-MoS2) was systematically characterized and optimized. The optimized ca-MoS2 powder exhibits enhanced HER performance with an overpotential of 0.34 V at a current density of 0.5 mA cm(-2) in this experiment due to the increasing active sites, and the Tafel slope also smaller than other samples. This chemical etching method provides new ways to design atomic structure modification, including controlling layered TMD electrochemical property and new type of transistor fabrication.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Two-dimensional layered molybdenum disulfide (MoS2) is a promising catalyst for hydrogen evolution reaction (HER), and a good replacement for platinum (Pt) in elelctrochemical water splitting. Most transition metal dichalcogenides (TMDs) show excellent catalytic activity, which stems from their active sites located along the edges. However, small density of the active sites in the basal plane, largely limits TMDs performance. To enhance the HER catalysis activity of MoS2, we developed an efficient and scalable approach to significantly increase the overall electrochemically active sites using mild sodium hypochlorite (NaClO) solution anisotropic etching. The effect is further enhanced by oxygen-plasma pretreatment of the material, which - upon chemical etching - leads to highly porous and highly reactive structure. The resulting chemically activated MoS2 (ca-MoS2) was systematically characterized and optimized. The optimized ca-MoS2 powder exhibits enhanced HER performance with an overpotential of 0.34 V at a current density of 0.5 mA cm(-2) in this experiment due to the increasing active sites, and the Tafel slope also smaller than other samples. This chemical etching method provides new ways to design atomic structure modification, including controlling layered TMD electrochemical property and new type of transistor fabrication. |
Ding, Xianguang; Peng, Fei; Zhou, Jun; Gong, Wenbin; Slaven, Garaj; Loh, Kian Ping; Lim, Chwee Teck; Leong, David Tai Defect engineered bioactive transition metals dichalcogenides quantum dots Journal Article NATURE COMMUNICATIONS, 10 , 2019, ISSN: 2041-1723. @article{ISI:000454757200006, title = {Defect engineered bioactive transition metals dichalcogenides quantum dots}, author = {Xianguang Ding and Fei Peng and Jun Zhou and Wenbin Gong and Garaj Slaven and Kian Ping Loh and Chwee Teck Lim and David Tai Leong}, doi = {10.1038/s41467-018-07835-1}, times_cited = {0}, issn = {2041-1723}, year = {2019}, date = {2019-01-03}, journal = {NATURE COMMUNICATIONS}, volume = {10}, publisher = {NATURE PUBLISHING GROUP}, address = {MACMILLAN BUILDING, 4 CRINAN ST, LONDON N1 9XW, ENGLAND}, abstract = {Transition metal dichalcogenide (TMD) quantum dots (QDs) are fundamentally interesting because of the stronger quantum size effect with decreased lateral dimensions relative to their larger 2D nanosheet counterparts. However, the preparation of a wide range of TMD QDs is still a continual challenge. Here we demonstrate a bottom-up strategy utilizing TM oxides or chlorides and chalcogen precursors to synthesize a small library of TMD QDs (MoS2, WS2, RuS2, MoTe2, MoSe2, WSe2 and RuSe2). The reaction reaches equilibrium almost instantaneously (similar to 10-20 s) with mild aqueous and room temperature conditions. Tunable defect engineering can be achieved within the same reactions by deviating the precursors' reaction stoichiometries from their fixed molecular stoichiometries. Using MoS2 QDs for proof-of-concept biomedical applications, we show that increasing sulfur defects enhanced oxidative stress generation, through the photodynamic effect, in cancer cells. This facile strategy will motivate future design of TMDs nanomaterials utilizing defect engineering for biomedical applications.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Transition metal dichalcogenide (TMD) quantum dots (QDs) are fundamentally interesting because of the stronger quantum size effect with decreased lateral dimensions relative to their larger 2D nanosheet counterparts. However, the preparation of a wide range of TMD QDs is still a continual challenge. Here we demonstrate a bottom-up strategy utilizing TM oxides or chlorides and chalcogen precursors to synthesize a small library of TMD QDs (MoS2, WS2, RuS2, MoTe2, MoSe2, WSe2 and RuSe2). The reaction reaches equilibrium almost instantaneously (similar to 10-20 s) with mild aqueous and room temperature conditions. Tunable defect engineering can be achieved within the same reactions by deviating the precursors' reaction stoichiometries from their fixed molecular stoichiometries. Using MoS2 QDs for proof-of-concept biomedical applications, we show that increasing sulfur defects enhanced oxidative stress generation, through the photodynamic effect, in cancer cells. This facile strategy will motivate future design of TMDs nanomaterials utilizing defect engineering for biomedical applications. |
2018 |
Bogaert, Kevin; Liu, Song; Liu, Tao; Guo, Na; Zhang, Chun; Gradecak, Silvija; Garaj, Slaven Two-Dimensional MoxW1-xS2 Graded Alloys: Growth and Optical Properties Journal Article SCIENTIFIC REPORTS, 8 , 2018, ISSN: 2045-2322. @article{ISI:000442870300067, title = {Two-Dimensional Mo\textit{_{x}}W_{1-\textit{x}}S_{2} Graded Alloys: Growth and Optical Properties}, author = {Kevin Bogaert and Song Liu and Tao Liu and Na Guo and Chun Zhang and Silvija Gradecak and Slaven Garaj}, doi = {10.1038/s41598-018-31220-z}, times_cited = {0}, issn = {2045-2322}, year = {2018}, date = {2018-08-27}, journal = {SCIENTIFIC REPORTS}, volume = {8}, publisher = {NATURE PORTFOLIO}, address = {HEIDELBERGER PLATZ 3, BERLIN, 14197, GERMANY}, abstract = {Two-dimensional (2D) transition metal dichalcogenides can be alloyed by substitution at the metal atom site with negligible effect on lattice strain, but with significant influence on optical and electrical properties. In this work, we establish the relationship between composition and optical properties of the MoxW1-xS2 alloy by investigating the effect of continuously-varying composition on photoluminescence intensity. We developed a new process for growth of two-dimensional MoxW1-xS2 alloys that span nearly the full composition range along a single crystal, thus avoiding any sample-related heterogeneities. The graded alloy crystals were grown using a diffusion-based chemical vapor deposition (CVD) method that starts by synthesizing a WS2 crystal with a graded point defect distribution, followed by Mo alloying in the second stage. We show that point defects promote the diffusion and alloying, as confirmed by Raman and photoluminescence measurements, density functional theory calculations of the reaction path, and observation that no alloying occurs in CVD-treated exfoliated crystals with low defect density. We observe a significant dependence of the optical quantum yield as a function of the alloy composition reaching the maximum intensity for the equicompositional Mo0.5W0.5S2 alloy. Furthermore, we map the growth-induced strain distribution within the alloyed crystals to decouple composition and strain effects on optical properties: at the same composition, we observe significant decrease in quantum yield with induced strain. Our approach is generally applicable to other 2D materials as well as the optimization of other composition-dependent properties within a single crystal.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Two-dimensional (2D) transition metal dichalcogenides can be alloyed by substitution at the metal atom site with negligible effect on lattice strain, but with significant influence on optical and electrical properties. In this work, we establish the relationship between composition and optical properties of the MoxW1-xS2 alloy by investigating the effect of continuously-varying composition on photoluminescence intensity. We developed a new process for growth of two-dimensional MoxW1-xS2 alloys that span nearly the full composition range along a single crystal, thus avoiding any sample-related heterogeneities. The graded alloy crystals were grown using a diffusion-based chemical vapor deposition (CVD) method that starts by synthesizing a WS2 crystal with a graded point defect distribution, followed by Mo alloying in the second stage. We show that point defects promote the diffusion and alloying, as confirmed by Raman and photoluminescence measurements, density functional theory calculations of the reaction path, and observation that no alloying occurs in CVD-treated exfoliated crystals with low defect density. We observe a significant dependence of the optical quantum yield as a function of the alloy composition reaching the maximum intensity for the equicompositional Mo0.5W0.5S2 alloy. Furthermore, we map the growth-induced strain distribution within the alloyed crystals to decouple composition and strain effects on optical properties: at the same composition, we observe significant decrease in quantum yield with induced strain. Our approach is generally applicable to other 2D materials as well as the optimization of other composition-dependent properties within a single crystal. |
2017 |
Esfandiar, A; Radha, B; Wang, F C; Yang, Q; Hu, S; Garaj, S; Nair, R R; Geim, A K; Gopinadhan, K Size effect in ion transport through angstrom-scale slits Journal Article SCIENCE, 358 (6362), pp. 511-513, 2017, ISSN: 0036-8075. @article{ISI:000413757500042, title = {Size effect in ion transport through angstrom-scale slits}, author = {A Esfandiar and B Radha and F C Wang and Q Yang and S Hu and S Garaj and R R Nair and A K Geim and K Gopinadhan}, doi = {10.1126/science.aan5275}, times_cited = {0}, issn = {0036-8075}, year = {2017}, date = {2017-10-27}, journal = {SCIENCE}, volume = {358}, number = {6362}, pages = {511-513}, publisher = {AMER ASSOC ADVANCEMENT SCIENCE}, address = {1200 NEW YORK AVE, NW, WASHINGTON, DC 20005 USA}, abstract = {In the field of nanofluidics, it has been an ultimate but seemingly distant goal to controllably fabricate capillaries with dimensions approaching the size of small ions and water molecules. We report ion transport through ultimately narrow slits that are fabricated by effectively removing a single atomic plane from a bulk crystal. The atomically flat angstrom-scale slits exhibit little surface charge, allowing elucidation of the role of steric effects. We find that ions with hydrated diameters larger than the slit size can still permeate through, albeit with reduced mobility. The confinement also leads to a notable asymmetry between anions and cations of the same diameter. Our results provide a platform for studying the effects of angstrom-scale confinement, which is important for the development of nanofluidics, molecular separation, and other nanoscale technologies.}, keywords = {}, pubstate = {published}, tppubtype = {article} } In the field of nanofluidics, it has been an ultimate but seemingly distant goal to controllably fabricate capillaries with dimensions approaching the size of small ions and water molecules. We report ion transport through ultimately narrow slits that are fabricated by effectively removing a single atomic plane from a bulk crystal. The atomically flat angstrom-scale slits exhibit little surface charge, allowing elucidation of the role of steric effects. We find that ions with hydrated diameters larger than the slit size can still permeate through, albeit with reduced mobility. The confinement also leads to a notable asymmetry between anions and cations of the same diameter. Our results provide a platform for studying the effects of angstrom-scale confinement, which is important for the development of nanofluidics, molecular separation, and other nanoscale technologies. |