444 results on '"Jamie H. Warner"'
Search Results
102. Postgrowth Substitutional Tin Doping of 2D WS
- Author
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Ren-Jie, Chang, Yuewen, Sheng, Gyeong Hee, Ryu, Nhlakanipho, Mkhize, Tongxin, Chen, Yang, Lu, Jun, Chen, Ja Kyung, Lee, Harish, Bhaskaran, and Jamie H, Warner
- Abstract
Doping of two-dimensional materials provides them tunable physical properties and widens their applications. Here, we demonstrate the postgrowth doping strategy in monolayer and bilayer tungsten disulfide (WS
- Published
- 2019
103. Postgrowth substitutional tin doping of 2D WS2 crystals using chemical vapor deposition
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Gyeong Hee Ryu, Ja Kyung Lee, Jamie H. Warner, Jun Chen, Tongxin Chen, Nhlakanipho Mkhize, Harish Bhaskaran, Yang Lu, Yuewen Sheng, and Ren-Jie Chang
- Subjects
Materials science ,Bilayer ,Doping ,Tungsten disulfide ,chemistry.chemical_element ,02 engineering and technology ,Chemical vapor deposition ,Tungsten ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,Tin doping ,0104 chemical sciences ,Condensed Matter::Soft Condensed Matter ,Condensed Matter::Materials Science ,chemistry.chemical_compound ,Chemical engineering ,chemistry ,Condensed Matter::Superconductivity ,Monolayer ,Condensed Matter::Strongly Correlated Electrons ,General Materials Science ,0210 nano-technology - Abstract
Doping of two-dimensional materials provides them tunable physical properties and widens their applications. Here, we demonstrate the postgrowth doping strategy in monolayer and bilayer tungsten disulfide (WS2) crystals, which utilizes a metal exchange mechanism, whereby Sn atoms become substitutional dopants in the W sites by energetically favorable replacement. We achieve this using chemical vapor deposition techniques, where high-quality grown WS2 single crystals are first grown and then subsequently reacted with a SnS precursor. Thermal control of the exchange doping mechanism is revealed, indicating that a sufficiently high enough temperature is required to create the S vacancies that are the initial binding sites for the SnS precursor and metal exchange occurrence. This results in a better control of dopant distribution compared to the tradition all-in-one approach, where dopants are added during the growth phase. The Sn dopants exhibit an n-type doping behavior in the WS2 layers based on the decreased threshold voltage obtained from transistor device measurements. Annular dark-field scanning transmission electron microscopy shows that in bilayer WS2 the Sn doping occurs only in the top layer, creating vertical heterostructures with atomic layer doping precision. This postgrowth modification opens up ways to selectively dope one layer at a time and construct mixed stoichiometry vertical heterojunctions in bilayer crystals.
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- 2019
104. Atomic Structure and Dynamics of Defects and Grain Boundaries in 2D Pd
- Author
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Jun, Chen, Gyeong Hee, Ryu, Sapna, Sinha, and Jamie H, Warner
- Abstract
We study the atomic structure and dynamics of defects and grain boundaries in monolayer Pd
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- 2019
105. Atomic structural catalogue of defects and vertical stacking in 2H/3R mixed polytype multilayer WS
- Author
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Gyeong Hee, Ryu, Jun, Chen, Yi, Wen, Si, Zhou, Ren-Jie, Chang, and Jamie H, Warner
- Abstract
We examine the atomic structure of chemical vapour deposition grown multilayer WS2 pyramids using aberration corrected annular dark field scanning transmission electron microscopy coupled with an in situ heating holder. The stacking orders and specific types of defects after partial degradation by S and W atomic loss at high temperature are resolved layer-by-layer. Our study of an individual WS2 pyramid with at least six layers, reveals a mixed 2H and 3R polytype stacking. Etching occurred both top and bottom of the WS2 pyramid, which aids in determining the exact vertical layer stacking configurations in the thicker regions. We provide an extensive catalogue of the contrast profiles associated with defects in WS2 as a function of layer number and stacking type, as imaged using ADF-STEM. These results provide extensive details about the identification of a wide range of defects in S2 layers, and the unique ADF-STEM contrast patterns that arise from complex multilayer stacking.
- Published
- 2019
106. Atomic structure and defect dynamics of monolayer lead iodide nanodisks with epitaxial alignment on graphene
- Author
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Taishan Zhu, Kyriakos Porfyrakis, Jamie H. Warner, Sapna Sinha, Yuewen Sheng, Jeffrey C. Grossman, and Arthur France-Lanord
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Materials science ,Band gap ,Science ,General Physics and Astronomy ,02 engineering and technology ,010402 general chemistry ,Epitaxy ,Two-dimensional materials ,01 natural sciences ,General Biochemistry, Genetics and Molecular Biology ,Article ,law.invention ,symbols.namesake ,law ,Vacancy defect ,Monolayer ,lcsh:Science ,Multidisciplinary ,Nanoscale materials ,Graphene ,Quantum dots ,Synthesis and processing ,General Chemistry ,021001 nanoscience & nanotechnology ,0104 chemical sciences ,Nanopore ,TA ,Chemical physics ,Quantum dot ,symbols ,lcsh:Q ,van der Waals force ,0210 nano-technology - Abstract
Lead Iodide (PbI2) is a large bandgap 2D layered material that has potential for semiconductor applications. However, atomic level study of PbI2 monolayer has been limited due to challenges in obtaining thin crystals. Here, we use liquid exfoliation to produce monolayer PbI2 nanodisks (30-40 nm in diameter and > 99% monolayer purity) and deposit them onto suspended graphene supports to enable atomic structure study of PbI2. Strong epitaxial alignment of PbI2 monolayers with the underlying graphene lattice occurs, leading to a phase shift from the 1 T to 1 H structure to increase the level of commensuration in the two lattice spacings. The fundamental point vacancy and nanopore structures in PbI2 monolayers are directly imaged, showing rapid vacancy migration and self-healing. These results provide a detailed insight into the atomic structure of monolayer PbI2, and the impact of the strong van der Waals interaction with graphene, which has importance for future applications in optoelectronics., Imaging liquid phase exfoliated nanosheets on suspended graphene via annular dark-field STEM can enable identification of various defects, vacancies and their migration. Here, the authors report matching of zigzag edges of monolayer PbI2 with graphene arm-chairs leading to a phase shift from 1 T to 1 H structure to maximize commensuration of the lattices.
- Published
- 2019
107. Atomically Sharp Dual Grain Boundaries in 2D WS
- Author
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Jun, Chen, Gang Seob, Jung, Gyeong Hee, Ryu, Ren-Jie, Chang, Si, Zhou, Yi, Wen, Markus J, Buehler, and Jamie H, Warner
- Abstract
It is shown that tilt grain boundaries (GBs) in bilayer 2D crystals of the transition metal dichalcogenide WS
- Published
- 2019
108. Anisotropic Fracture Dynamics Due to Local Lattice Distortions
- Author
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Gang Seob Jung, Markus J. Buehler, Shanshan Wang, Mohsen Danaie, Si Zhou, Zhao Qin, Angus I. Kirkland, and Jamie H. Warner
- Subjects
Void (astronomy) ,Materials science ,Condensed matter physics ,General Engineering ,Nucleation ,General Physics and Astronomy ,Fracture mechanics ,02 engineering and technology ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,Surface energy ,0104 chemical sciences ,Condensed Matter::Materials Science ,Monatomic ion ,Fracture toughness ,Brittleness ,General Materials Science ,0210 nano-technology ,Anisotropy - Abstract
A brittle material under loading fails by the nucleation and propagation of a sharp crack. In monatomic crystals, such as silicon, the lattice geometries front to the crack-tip changes the way of propagation even with the same cleavage surface. In general, however, crystals have multiple kinds of atoms and how the deformation of each atom affects the failure is still elusive. Here, we show that local atomic distortions from the different types of atoms causes a propagation anisotropy in suspended WS2 monolayers by combining annular dark-field scanning transmission electron microscopy and empirical molecular dynamics that are validated by first-principles calculations. Conventional conditions for brittle failure such as surface energy, elasticity, and crack geometry cannot account for this anisotropy. Further simulations predict the enhancement of the strengths and fracture toughness of the materials by designing void shapes and edge structures.
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- 2019
109. Broadband transparent optical phase change materials for high-performance nonvolatile photonics
- Author
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Junying Li, Carlos Ríos, Zhuoran Fang, Christopher Roberts, Vladimir Liberman, Paul Robinson, Hongtao Lin, Yifei Zhang, Huikai Zhong, Si Zhou, Huashan Li, Kathleen Richardson, Myungkoo Kang, Bridget Bohlin, Mikhail Y. Shalaginov, Jamie H. Warner, Jeffrey B. Chou, Anupama Yadav, Tian Gu, Qingyang Du, and Juejun Hu
- Subjects
Materials science ,Science ,General Physics and Astronomy ,Physics::Optics ,FOS: Physical sciences ,02 engineering and technology ,Applied Physics (physics.app-ph) ,01 natural sciences ,Optical switch ,General Biochemistry, Genetics and Molecular Biology ,Article ,010309 optics ,Computer Science::Hardware Architecture ,Optical physics ,0103 physical sciences ,Broadband ,Optical materials and structures ,lcsh:Science ,Electronic circuit ,Condensed Matter - Materials Science ,Multidisciplinary ,Spatial light modulator ,business.industry ,Materials Science (cond-mat.mtrl-sci) ,Integrated optics ,Physics - Applied Physics ,General Chemistry ,021001 nanoscience & nanotechnology ,Coupling (electronics) ,Optoelectronics ,Contrast ratio ,lcsh:Q ,Photonics ,0210 nano-technology ,business ,Refractive index ,Materials for optics - Abstract
Optical phase change materials (O-PCMs), a unique group of materials featuring drastic optical property contrast upon solid-state phase transition, have found widespread adoption in photonic switches and routers, reconfigurable meta-optics, reflective display, and optical neuromorphic computers. Current phase change materials, such as Ge-Sb-Te (GST), exhibit large contrast of both refractive index (delta n) and optical loss (delta k), simultaneously. The coupling of both optical properties fundamentally limits the function and performance of many potential applications. In this article, we introduce a new class of O-PCMs, Ge-Sb-Se-Te (GSST) which breaks this traditional coupling, as demonstrated with an optical figure of merit improvement of more than two orders of magnitude. The first-principle computationally optimized alloy, Ge2Sb2Se4Te1, combines broadband low optical loss (1-18.5 micron), large optical contrast (delta n = 2.0), and significantly improved glass forming ability, enabling an entirely new field of infrared and thermal photonic devices. We further leverage the material to demonstrate nonvolatile integrated optical switches with record low loss and large contrast ratio, as well as an electrically addressed, microsecond switched pixel level spatial light modulator, thereby validating its promise as a platform material for scalable nonvolatile photonics., Comment: 16 pages, 6 figures
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- 2019
110. Direct imaging of photoswitching molecular conformations using individual metal atom markers
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Mihael A. Gerkman, Grace G. D. Han, Sapna Sinha, and Jamie H. Warner
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Materials science ,Photoswitch ,Graphene ,Intermolecular force ,General Engineering ,General Physics and Astronomy ,02 engineering and technology ,010402 general chemistry ,021001 nanoscience & nanotechnology ,Photochemistry ,01 natural sciences ,0104 chemical sciences ,law.invention ,symbols.namesake ,chemistry.chemical_compound ,Azobenzene ,chemistry ,law ,Scanning transmission electron microscopy ,symbols ,Molecule ,General Materials Science ,van der Waals force ,0210 nano-technology ,Isomerization - Abstract
Photoswitching behavior of individual organic molecules was imaged by annular dark-field scanning transmission electron microscopy (ADF-STEM) using a highly electron beam transparent graphene support. Photoswitching azobenzene derivatives with ligands at each end containing single transition-metal atoms (Pt) were designed (Pt-complex), and the distance between the strong ADF-STEM contrast from the two Pt atoms in each Pt-complex is used to track molecular length changes. UV irradiation was used to induce photoswitching of the Pt complex on graphene, and we show that the measured Pt-Pt distances within isolated molecules decrease from ∼2.1 nm to ∼1.4 nm, indicative of a trans-to- cis isomerization. Light illumination of the Pt-complex on the graphene support also caused their diffusion out from initial clusters to the surrounding area of graphene, indicating that the light-activated mobilization overcomes the intermolecular van der Waals interactions. This approach shows how individual isolated heavy metal atoms can be included as markers into complex molecules to track their structural changes using ADF-STEM on graphene supports, providing an effective method to study a diverse range of complex organic materials at the single molecule level.
- Published
- 2019
111. In situ high temperature atomic level dynamics of large inversion domain formations in monolayer MoS2
- Author
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Yang Lu, Jamie H. Warner, Gyeong Hee Ryu, Yi Wen, Si Zhou, Christopher S. Allen, Jun Chen, and Angus I. Kirkland
- Subjects
In situ ,Void (astronomy) ,Materials science ,02 engineering and technology ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,0104 chemical sciences ,Chemical physics ,Vacancy defect ,Metastability ,Lattice (order) ,Scanning transmission electron microscopy ,Monolayer ,General Materials Science ,Grain boundary ,0210 nano-technology - Abstract
Here we study the high-temperature formation and dynamics of large inversion domains (IDs) that form in monolayer MoS2 using atomic-resolution annular dark-field scanning transmission electron microscopy (ADF-STEM) with an in situ heating stage. We use temperatures above 700 °C to thermally activate rapid S vacancy migration and this leads to a formation mechanism of IDs that differs from the one at room temperature, where S vacancy migration is limited. We show that at high temperatures the formation of IDs occurs from intersected networks of long S vacancy line defects, whose strain fields are non-orthogonal and trigger large scale atomic reconstructions. Once formed, the IDs are influenced by the dynamic behaviour of nearby line defects and voids. With Mo and S atoms undergoing movement, the two types of ID grain boundaries can shift to allow further expansion of the ID area along the adjoining line defects. We reveal that IDs serve as metastable configurations between line defect rearrangements and eventual void formation under electron beam irradiation during heating. The formation of voids near to the IDs causes them to revert back to pristine lattice, which has the effect of restricting the ID domain size to a certain range (e.g. 3–5 nm in our observation) instead of continuously enlarging. This study provides insights into how the MoS2 IDs form and evolve at high temperature and can benefit the tailoring of electronic properties of two dimensional materials by structural manipulation.
- Published
- 2019
112. High photoresponsivity in ultrathin 2D lateral graphene:WS2:graphene photodetectors using direct CVD growth
- Author
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Yuewen Sheng, Linlin Hou, Qianyang Zhang, Xiaochen Wang, Ren-Jie Chang, Hefu Huang, Tongxin Chen, Jamie H. Warner, and Yingqiu Zhou
- Subjects
Materials science ,business.industry ,Graphene ,Schottky barrier ,Transistor ,Photodetector ,02 engineering and technology ,Chemical vapor deposition ,Orders of magnitude (numbers) ,010402 general chemistry ,021001 nanoscience & nanotechnology ,Epitaxy ,01 natural sciences ,0104 chemical sciences ,law.invention ,Responsivity ,law ,Optoelectronics ,General Materials Science ,0210 nano-technology ,business - Abstract
We show that reducing the degree of van der Waals overlapping in all 2D ultrathin lateral devices composed of graphene:WS2:graphene leads to significant increase in photodetector responsivity. This is achieved by directly growing WS2 using chemical vapor deposition (CVD) in prepatterned graphene gaps to create epitaxial interfaces. Direct-CVD-grown graphene:WS2:graphene lateral photodetecting transistors exhibit high photoresponsivities reaching 121 A/W under 2.7 × 105 mW/cm2 532 nm illumination, which is around 2 orders of magnitude higher than similar devices made by the layer-by-layer transfer method. The photoresponsivity of our direct-CVD-grown device shows negative correlation with illumination power under different gate voltages, which is different from similar devices made by the transfer method. We show that the high photoresponsivity is due to the lowering of effective Schottky barrier height by improving the contact between graphene and WS2. Furthermore, the direct CVD growth reduces overlapping sections of WS2:Gr and leads to more uniform lateral systems. This approach provides insights into scalable manufacturing of high-quality 2D lateral electronic and optoelectronic devices.
- Published
- 2019
113. Grain boundaries as electrical conduction channels in polycrystalline monolayer WS2
- Author
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Harish Bhaskaran, Yingqiu Zhou, Syed Ghazi Sarwat, Markus J. Buehler, Gang Seob Jung, and Jamie H. Warner
- Subjects
Kelvin probe force microscope ,Materials science ,Condensed matter physics ,Schottky barrier ,02 engineering and technology ,Substrate (electronics) ,Chemical vapor deposition ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,0104 chemical sciences ,Monolayer ,General Materials Science ,Electrical measurements ,Grain boundary ,Crystallite ,0210 nano-technology - Abstract
We show that grain boundaries (GBs) in polycrystalline monolayer WS2 can act as conduction channels with a lower gate onset potential for field-effect transistors made parallel, compared to devices made in pristine areas and perpendicular to GBs. Localized doping at the GB causes photoluminescence quenching and a reduced Schottky barrier with the metal electrodes, resulting in higher conductivity at lower applied bias values. Samples are grown by chemical vapor deposition with large domains of ∼100 μm, enabling numerous devices to be made within single domains, across GBs and at many similar sites across the substrate to reveal similar behaviors. We corroborate our electrical measurements with Kelvin probe microscopy, highlighting the nature of the doping-type in the material to change at the grain boundaries. Molecular dynamics simulations of the GB are used to predict the atomic structure of the dislocations and meandering tilt GB behavior on the nanoscale. These results show that GBs can be used to provide conduction pathways that are different to transport across GBs and in pristine area for potential electronic applications.
- Published
- 2019
114. Transfer of photosynthetic NADP+/NADPH recycling activity to a porous metal oxide for highly specific, electrochemically-driven organic synthesis
- Author
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Bhavin Siritanaratkul, Fraser A. Armstrong, Martin Winkler, Thomas G. Roberts, Jamie H. Warner, Thomas O. M. Samuels, Clare F. Megarity, and Thomas Happe
- Subjects
inorganic chemicals ,biology ,010405 organic chemistry ,Oxide ,Active site ,General Chemistry ,010402 general chemistry ,Photochemistry ,Photosynthesis ,Electrochemistry ,01 natural sciences ,Redox ,0104 chemical sciences ,Indium tin oxide ,Chloroplast ,chemistry.chemical_compound ,chemistry ,biology.protein ,Organic synthesis - Abstract
In a discovery of the transfer of chloroplast biosynthesis activity to an inorganic material, ferredoxin-NADP+ reductase (FNR), the pivotal redox flavoenzyme of photosynthetic CO2 assimilation, binds tightly within the pores of indium tin oxide (ITO) to produce an electrode for direct studies of the redox chemistry of the FAD active site, and fast, reversible and diffusion-controlled interconversion of NADP+ and NADPH in solution. The dynamic electrochemical properties of FNR and NADP(H) are thus revealed in a special way that enables facile coupling of selective, enzyme-catalysed organic synthesis to a controllable power source, as demonstrated by efficient synthesis of l-glutamate from 2-oxoglutarate and NH4+.
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- 2019
115. High-Performance WS
- Author
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Yuewen, Sheng, Tongxin, Chen, Yang, Lu, Ren-Jie, Chang, Sapna, Sinha, and Jamie H, Warner
- Abstract
The solid progress in the study of a single two-dimensional (2D) material underpins the development for creating 2D material assemblies with various electronic and optoelectronic properties. We introduce an asymmetric structure by stacking monolayer semiconducting tungsten disulfide, metallic graphene, and insulating boron nitride to fabricate numerous red channel light-emitting devices (LEDs). All the 2D crystals were grown by chemical vapor deposition (CVD), which has great potential for future industrial scale-up. Our LEDs exhibit visibly observable electroluminescence (EL) at both 5.5 V forward and 7.0 V backward biasing, which correlates well with our asymmetric design. The red emission can last for at least several minutes, and the success rate of the working device that can emit detectable EL is up to 80%. In addition, we show that sample degradation is prone to happen when a continuing bias, much higher than the threshold voltage, is applied. Our success of using high-quality CVD-grown 2D materials for red light emitters is expected to provide the basis for flexible and transparent displays.
- Published
- 2019
116. Grain Boundaries as Electrical Conduction Channels in Polycrystalline Monolayer WS
- Author
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Yingqiu, Zhou, Syed Ghazi, Sarwat, Gang Seob, Jung, Markus J, Buehler, Harish, Bhaskaran, and Jamie H, Warner
- Abstract
We show that grain boundaries (GBs) in polycrystalline monolayer WS
- Published
- 2019
117. MoS
- Author
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Jiwoong, Yang, Moon Kee, Choi, Yuewen, Sheng, Jaebong, Jung, Karen, Bustillo, Tongxin, Chen, Seung-Wuk, Lee, Peter, Ercius, Ji Hoon, Kim, Jamie H, Warner, Emory M, Chan, and Haimei, Zheng
- Abstract
Two dimensional (2D) materials have found various applications because of their unique physical properties. For example, graphene has been used as the electron transparent membrane for liquid cell transmission electron microscopy (TEM) due to its high mechanical strength and flexibility, single-atom thickness, chemical inertness, etc. Here, we report using 2D MoS
- Published
- 2019
118. MoS2 liquid cell electron microscopy through clean and fast polymer-free MoS2 transfer
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Karen Bustillo, Emory M. Chan, Tongxin Chen, Haimei Zheng, Yuewen Sheng, Jiwoong Yang, Jaebong Jung, Moon Kee Choi, Ji-Hoon Kim, Peter Ercius, Seung-Wuk Lee, and Jamie H. Warner
- Subjects
Materials science ,Bioengineering ,Nanotechnology ,02 engineering and technology ,Substrate (electronics) ,Epitaxy ,law.invention ,symbols.namesake ,Liquid cell electron microscopy ,law ,MD Multidisciplinary ,General Materials Science ,Nanoscience & Nanotechnology ,nanocrystal formation ,Graphene ,Mechanical Engineering ,Heterojunction ,General Chemistry ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,Membrane ,heterostructures ,Nanocrystal ,Transmission electron microscopy ,symbols ,polymer-free transfer ,van der Waals force ,MoS2 ,0210 nano-technology - Abstract
Two dimensional (2D) materials have found various applications because of their unique physical properties. For example, graphene has been used as the electron transparent membrane for liquid cell transmission electron microscopy (TEM) due to its high mechanical strength and flexibility, single-atom thickness, chemical inertness, etc. Here, we report using 2D MoS2 as a functional substrate as well as the membrane window for liquid cell TEM, which is enabled by our facile and polymer-free MoS2 transfer process. This provides the opportunity to investigate the growth of Pt nanocrystals on MoS2 substrates, which elucidates the formation mechanisms of such heterostructured 2D materials. We find that Pt nanocrystals formed in MoS2 liquid cells have a strong tendency to align their crystal lattice with that of MoS2, suggesting a van der Waals epitaxial relationship. Importantly, we can study its impact on the kinetics of the nanocrystal formation. The development of MoS2 liquid cells will allow further study of various liquid phenomena on MoS2, and the polymer-free MoS2 transfer process will be implemented in a wide range of applications.
- Published
- 2019
119. High Photoresponsivity in Ultrathin 2D Lateral Graphene:WS
- Author
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Tongxin, Chen, Yuewen, Sheng, Yingqiu, Zhou, Ren-Jie, Chang, Xiaochen, Wang, Hefu, Huang, Qianyang, Zhang, Linlin, Hou, and Jamie H, Warner
- Abstract
We show that reducing the degree of van der Waals overlapping in all 2D ultrathin lateral devices composed of graphene:WS
- Published
- 2019
120. Atomic Scale Imaging of Reversible Ring Cyclization in Graphene Nanoconstrictions
- Author
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G. Andrew D. Briggs, Gun-Do Lee, Ja Kyung Lee, Jamie H. Warner, Harry L. Anderson, Euijoon Yoon, and Sungwoo Lee
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Surface diffusion ,Materials science ,Graphene ,General Engineering ,General Physics and Astronomy ,02 engineering and technology ,010402 general chemistry ,021001 nanoscience & nanotechnology ,Ring (chemistry) ,01 natural sciences ,Atomic units ,0104 chemical sciences ,law.invention ,Molecular dynamics ,Tight binding ,Chemical physics ,law ,Molecule ,General Materials Science ,Density functional theory ,0210 nano-technology - Abstract
We present an atomic level study of reversible cyclization processes in suspended nanoconstricted regions of graphene that form linear carbon chains (LCCs). Before the nanoconstricted region reaches a single linear carbon chain (SLCC), we observe that a double linear carbon chain (DLCC) structure often reverts back to a ribbon of sp2 hybridized oligoacene rings, in a process akin to the Bergman rearrangement. When the length of the DLCC system only consists of ∼5 atoms in each LCC, full recyclization occurs for all atoms present, but for longer DLCCs we find that only single sections of the chain are modified in their bonding hybridization and no full ring closure occurs along the entire DLCCs. This process is observed in real time using aberration-corrected transmission electron microscopy and simulated using density functional theory and tight binding molecular dynamics calculations. These results show that DLCCs are highly sensitive to the adsorption of local gas molecules or surface diffusion impurities and undergo structural modifications.
- Published
- 2019
121. In situ high temperature atomic level dynamics of large inversion domain formations in monolayer MoS
- Author
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Jun, Chen, Si, Zhou, Yi, Wen, Gyeong Hee, Ryu, Christopher, Allen, Yang, Lu, Angus I, Kirkland, and Jamie H, Warner
- Abstract
Here we study the high-temperature formation and dynamics of large inversion domains (IDs) that form in monolayer MoS2 using atomic-resolution annular dark-field scanning transmission electron microscopy (ADF-STEM) with an in situ heating stage. We use temperatures above 700 °C to thermally activate rapid S vacancy migration and this leads to a formation mechanism of IDs that differs from the one at room temperature, where S vacancy migration is limited. We show that at high temperatures the formation of IDs occurs from intersected networks of long S vacancy line defects, whose strain fields are non-orthogonal and trigger large scale atomic reconstructions. Once formed, the IDs are influenced by the dynamic behaviour of nearby line defects and voids. With Mo and S atoms undergoing movement, the two types of ID grain boundaries can shift to allow further expansion of the ID area along the adjoining line defects. We reveal that IDs serve as metastable configurations between line defect rearrangements and eventual void formation under electron beam irradiation during heating. The formation of voids near to the IDs causes them to revert back to pristine lattice, which has the effect of restricting the ID domain size to a certain range (e.g. 3-5 nm in our observation) instead of continuously enlarging. This study provides insights into how the MoS2 IDs form and evolve at high temperature and can benefit the tailoring of electronic properties of two dimensional materials by structural manipulation.
- Published
- 2019
122. Addressing the isomer cataloguing problem for nanopores in two-dimensional materials
- Author
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Ananth Govind Rajan, Alex W. Robertson, Kevin S. Silmore, Michael S. Strano, Jacob L. Swett, Jamie H. Warner, and Daniel Blankschtein
- Subjects
Solid-state chemistry ,Physics::Biological Physics ,Materials science ,Graphene ,Nanoporous ,Mechanical Engineering ,02 engineering and technology ,General Chemistry ,010402 general chemistry ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,01 natural sciences ,0104 chemical sciences ,law.invention ,Nanopore ,Chemical graph theory ,Mechanics of Materials ,law ,Chemical physics ,Lattice (order) ,General Materials Science ,Chemical stability ,Kinetic Monte Carlo ,0210 nano-technology - Abstract
The presence of extended defects or nanopores in two-dimensional (2D) materials can change the electronic, magnetic and barrier membrane properties of the materials. However, the large number of possible lattice isomers of nanopores makes their quantitative study a seemingly intractable problem, confounding the interpretation of experimental and simulated data. Here we formulate a solution to this isomer cataloguing problem (ICP), combining electronic-structure calculations, kinetic Monte Carlo simulations, and chemical graph theory, to generate a catalogue of unique, most-probable isomers of 2D lattice nanopores. The results demonstrate remarkable agreement with precise nanopore shapes observed experimentally in graphene and show that the thermodynamic stability of a nanopore is distinct from its kinetic stability. Triangular nanopores prevalent in hexagonal boron nitride are also predicted, extending this approach to other 2D lattices. The proposed method should accelerate the application of nanoporous 2D materials by establishing specific links between experiment and theory/simulations, and by providing a much-needed connection between molecular design and fabrication. Nanopores in 2D materials have various possible lattice isomers, making relevant quantitative analysis difficult. An isomer-cataloguing framework is developed to address this problem, demonstrating remarkable agreement between simulated and experimental data.
- Published
- 2019
123. Symmetry-Controlled Reversible Photovoltaic Current Flow in Ultrathin All 2D Vertically Stacked Graphene/MoS
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Yingqiu, Zhou, Wenshuo, Xu, Yuewen, Sheng, Hefu, Huang, Qianyang, Zhang, Linlin, Hou, Viktoryia, Shautsova, and Jamie H, Warner
- Abstract
Atomically thin vertical heterostructures are promising candidates for optoelectronic applications, especially for flexible and transparent technologies. Here, we show how ultrathin all two-dimensional vertical-stacked type-II heterostructure devices can be assembled using only materials grown by chemical vapor deposition, with graphene (Gr) as top and bottom electrodes and MoS
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- 2019
124. Symmetry-controlled reversible photovoltaic current flow in ultrathin all 2D vertically stacked graphene/MoS2/WS2/graphene devices
- Author
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Yuewen Sheng, Linlin Hou, Wenshuo Xu, Hefu Huang, Yingqiu Zhou, Qianyang Zhang, Viktoryia Shautsova, and Jamie H. Warner
- Subjects
Photocurrent ,Materials science ,business.industry ,Graphene ,Doping ,Heterojunction ,02 engineering and technology ,Chemical vapor deposition ,Dielectric ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,7. Clean energy ,0104 chemical sciences ,law.invention ,Semiconductor ,Stack (abstract data type) ,law ,Optoelectronics ,General Materials Science ,0210 nano-technology ,business - Abstract
Atomically thin vertical heterostructures are promising candidates for optoelectronic applications, especially for flexible and transparent technologies. Here, we show how ultrathin all two-dimensional vertical-stacked type-II heterostructure devices can be assembled using only materials grown by chemical vapor deposition, with graphene (Gr) as top and bottom electrodes and MoS2/WS2 as the active semiconductor layers in the middle. Furthermore, we show that the stack symmetry, which dictates the type-II directionality, is the dominant factor in controlling the photocurrent direction upon light irradiation, whereas in homobilayers, photocurrent direction cannot be easily controlled because the tunnel barrier is determined by the doping levels of the graphene, which appears fixed for top and bottom graphene layers due to their dielectric environments. Therefore, the ability to direct photovoltaic current flow is demonstrated to be only possible using heterobilayers (HBs) and not homobilayers. We study the photovoltaic effects in more than 40 devices, which allows for statistical verification of performance and comparative behavior. The photovoltage in the graphene/transition-metal dichalcogenide-heterobilayer/graphene (Gr/TMD-HB (MoS2/WS2)/Gr) increases up to 10 times that generated in the monolayer TMD devices under the same optical illumination power, due to efficient charge transfer between WS2 and MoS2 and extraction to graphene electrodes. By applying external gate voltages ( Vg), the band alignment can be tuned, which in turn controls the photovoltaic effect in the vertical heterostructures. The tunneling-assisted interlayer charge recombination also plays a significant role in modulating the photovoltaic effect in the Gr/TMD-HB/Gr. These results provide important insights into how layer symmetry in vertical-stacked graphene/TMD/graphene ultrathin optoelectronics can be used to control electron flow directions during photoexcitation and open up opportunities for tandem cell assembly.
- Published
- 2019
125. Waterproof molecular monolayers stabilize 2D materials
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Wenshuo Xu, Zongyou Yin, Ju Li, Cong Su, Katsuya Teshima, Juejun Hu, Lei Sun, Tetsuya Yamada, Zegao Wang, Jamie H. Warner, Jing Kong, Qing-Bo Yan, Mircea Dincă, Xiang Ji, Gang Su, Mingdong Dong, Hongtao Lin, and Nobuyuki Zettsu
- Subjects
Multidisciplinary ,Chemical substance ,Materials science ,Passivation ,Annealing (metallurgy) ,02 engineering and technology ,engineering.material ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,0104 chemical sciences ,Corrosion ,symbols.namesake ,molecular monolayer stabilizer ,Coating ,Chemical engineering ,multilayer 2D materials ,Monolayer ,Physical Sciences ,symbols ,engineering ,Molecule ,van der Waals force ,0210 nano-technology ,anticorrosion - Abstract
Two-dimensional van der Waals materials have rich and unique functional properties, but many are susceptible to corrosion under ambient conditions. Here we show that linear alkylamines n-C(m)H(2m+1)NH(2), with m = 4 through 11, are highly effective in protecting the optoelectronic properties of these materials, such as black phosphorus (BP) and transition-metal dichalcogenides (TMDs: WS(2), 1T′-MoTe(2), WTe(2), WSe(2), TaS(2), and NbSe(2)). As a representative example, n-hexylamine (m = 6) can be applied in the form of thin molecular monolayers on BP flakes with less than 2-nm thickness and can prolong BP’s lifetime from a few hours to several weeks and even months in ambient environments. Characterizations combined with our theoretical analysis show that the thin monolayers selectively sift out water molecules, forming a drying layer to achieve the passivation of the protected 2D materials. The monolayer coating is also stable in air, H(2) annealing, and organic solvents, but can be removed by certain organic acids.
- Published
- 2019
126. Inhomogeneous strain release during bending of WS2 on flexible substrates
- Author
-
Jamie H. Warner, Yuewen Sheng, Martin Tweedie, Harish Bhaskaran, Wenshuo Xu, and Syed Ghazi Sarwat
- Subjects
Materials science ,Graphene ,Tungsten disulfide ,Heterojunction ,02 engineering and technology ,Substrate (electronics) ,Chemical vapor deposition ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,Flexible electronics ,0104 chemical sciences ,law.invention ,chemistry.chemical_compound ,chemistry ,law ,Boron nitride ,General Materials Science ,Composite material ,Thin film ,0210 nano-technology - Abstract
Two-dimensional (2D) materials hold great promise in flexible electronics, but the weak van der Waals interlayer bonding may pose a problem during bending, where easy interlayer sliding can occur. Furthermore, thin films of rigid materials are often observed to delaminate from soft substrates during straining. Here, we study the influence of substrate strain on some of the heterostructure configurations we expect to find in devices, composed of three common 2D materials: graphene, tungsten disulfide, and boron nitride. We used photoluminescence (PL) spectroscopy to measure changes in the heterostructures with strain applied in situ. All heterostructures were fabricated directly on polymer substrates, using materials synthesized by chemical vapor deposition. We observed an inhomogeneous release of strain in all structures, leading to a nonrecoverable broadening of the PL peak and shift of the bandgap. This suggests the need for preconditioning devices before service to ensure stable behavior. A gradual time-dependent relaxation of strain between strain cycles was characterized using time-dependent measurements—an effect which could lead to drift of device behavior during operation. Furthermore, possible degradation was assessed by performing the strain and relax the cycle up to 200 times, where we found little further change after the initial shifts had stabilized. These results have important ramifications for devices fabricated from these and other 2D materials, as they suggest extra processing steps and considerations that must be taken to achieve consistent and stable properties.
- Published
- 2018
127. Graphene as a flexible template for controlling magnetic interactions between metal atoms
- Author
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Euijoon Yoon, Dong-Wook Kim, Jaejun Yu, Suklyun Hong, Alex W. Robertson, Gun-Do Lee, Sungwoo Lee, Jisoon Ihm, and Jamie H. Warner
- Subjects
Physics ,Condensed matter physics ,Spintronics ,Magnetic moment ,Graphene ,Doping ,02 engineering and technology ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,01 natural sciences ,law.invention ,Atomic orbital ,law ,0103 physical sciences ,Physics::Atomic and Molecular Clusters ,General Materials Science ,Density functional theory ,010306 general physics ,0210 nano-technology ,Bilayer graphene ,Graphene nanoribbons - Abstract
Metal-doped graphene produces magnetic moments that have potential application in spintronics. Here we use density function theory computational methods to show how the magnetic interaction between metal atoms doped in graphene can be controlled by the degree of flexure in a graphene membrane. Bending graphene by flexing causes the distance between two substitutional Fe atoms covalently bonded in graphene to gradually increase and these results in the magnetic moment disappearing at a critical strain value. At the critical strain, a carbon atom can enter between the two Fe atoms and blocks the interaction between relevant orbitals of Fe atoms to quench the magnetic moment. The control of interactions between doped atoms by exploiting the mechanical flexibility of graphene is a unique approach to manipulating the magnetic properties and opens up new opportunities for mechanical-magnetic 2D device systems.
- Published
- 2018
128. Inhomogeneous Strain Release during Bending of WS
- Author
-
Martin E P, Tweedie, Yuewen, Sheng, Syed Ghazi, Sarwat, Wenshuo, Xu, Harish, Bhaskaran, and Jamie H, Warner
- Abstract
Two-dimensional (2D) materials hold great promise in flexible electronics, but the weak van der Waals interlayer bonding may pose a problem during bending, where easy interlayer sliding can occur. Furthermore, thin films of rigid materials are often observed to delaminate from soft substrates during straining. Here, we study the influence of substrate strain on some of the heterostructure configurations we expect to find in devices, composed of three common 2D materials: graphene, tungsten disulfide, and boron nitride. We used photoluminescence (PL) spectroscopy to measure changes in the heterostructures with strain applied in situ. All heterostructures were fabricated directly on polymer substrates, using materials synthesized by chemical vapor deposition. We observed an inhomogeneous release of strain in all structures, leading to a nonrecoverable broadening of the PL peak and shift of the bandgap. This suggests the need for preconditioning devices before service to ensure stable behavior. A gradual time-dependent relaxation of strain between strain cycles was characterized using time-dependent measurements-an effect which could lead to drift of device behavior during operation. Furthermore, possible degradation was assessed by performing the strain and relax the cycle up to 200 times, where we found little further change after the initial shifts had stabilized. These results have important ramifications for devices fabricated from these and other 2D materials, as they suggest extra processing steps and considerations that must be taken to achieve consistent and stable properties.
- Published
- 2018
129. Atomic Structure and Dynamics of Self-Limiting Sub-Nanometer Pores in Monolayer WS
- Author
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Gyeong Hee, Ryu, Arthur, France-Lanord, Yi, Wen, Si, Zhou, Jeffrey C, Grossman, and Jamie H, Warner
- Abstract
We reveal a self-limiting mechanism during the formation of a specific type of circular nanopore in monolayer WS
- Published
- 2018
130. High-Performance Two-Dimensional Schottky Diodes Utilizing Chemical Vapour Deposition-Grown Graphene-MoS
- Author
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Hefu, Huang, Wenshuo, Xu, Tongxin, Chen, Ren-Jie, Chang, Yuewen, Sheng, Qianyang, Zhang, Linlin, Hou, and Jamie H, Warner
- Abstract
Heterostructures based on two-dimensional (2D) materials have attracted enormous interest as they display unique functionalities and have potential to be applied in next-generation electronics. In this report, we fabricated three types of heterostructures based on chemical vapor deposition-grown graphene and MoS
- Published
- 2018
131. Hollow Electron Ptychographic Diffractive Imaging
- Author
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Biying Song, Hidetaka Sawada, Angus I. Kirkland, Zhiyuan Ding, Peng Wang, Jamie H. Warner, Xiaoqing Pan, Fucai Zhang, and Christopher S. Allen
- Subjects
Diffraction ,Materials science ,business.industry ,Electron energy loss spectroscopy ,Detector ,Phase (waves) ,General Physics and Astronomy ,02 engineering and technology ,Electron ,021001 nanoscience & nanotechnology ,01 natural sciences ,Data set ,Optics ,Atomic resolution ,0103 physical sciences ,010306 general physics ,0210 nano-technology ,business - Abstract
We report a method for quantitative phase recovery and simultaneous electron energy loss spectroscopy analysis using ptychographic reconstruction of a data set of "hollow" diffraction patterns. This has the potential for recovering both structural and chemical information at atomic resolution with a new generation of detectors.
- Published
- 2018
132. Porous Graphene Layers on Pt Catalyst for Long-Term Stability of Fuel Cell Electrode
- Author
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Sang Ouk Kim, Heeyeon Kim, Alex W. Robertson, and Jamie H. Warner
- Subjects
Engineering ,business.industry ,Porous graphene ,Electrode ,Fuel cells ,Nanotechnology ,business ,Term (time) ,Catalysis - Abstract
With the development of various energy technologies, much interest is focused on the feasibility and efficiency study of energy devices such as fuel cells, batteries, supercapacitors and water electrolysis systems. Among them, polymer electrolyte fuel cell (PEFC), which convert hydrogen into electric energy with zero emission of pollutants, is one of the most promising environmentally friendly technologies. Despite innumerable studies for more than half a century, degradation of the key component, membrane electrode assembly (MEA), is still a big obstacle for the commercialization of PEFC system. For the high performance and long-term stability of Pt/C electrode catalyst, the agglomeration of Pt particles and dissolution or detachment of Pt particles from carbon support have to be improved. For this purpose, we adopted a new shape of nano-carbon material for the surface modification of Pt catalyst. Graphene has been an attractive two-dimensional carbon allotrope having large surface area and electronic conductivity. Graphene also shows high flexibility and mechanical strength so that it can be used for large number of applications such as flexible display or printable electronics, etc [1-2]. In most cases, people need large-area graphene films with little or no defect for high thermal and electronic conductivity. Also, people need to transfer the as-prepared graphene film for each application. All of these processes are not easy or simple, and they are also labor-intensive processes. However, in the case of catalysis, porous graphene films can be synthesized via very simple one-step process and can be used as effective protective layers for Pt catalysts. In this study, we developed porous graphene films in order to improve the long-term stability of Pt catalysts maintaining the high performance of them. The graphene films were synthesized by single-step vaporization process, where the number of graphene layers and the defects in their structure are manipulated by temperature and composition of the precursors. In this process, the amounts of structural defects, pyridine was simultaneously introduced to the vaporization process, which is much easier and cost-effective compared to the conventional NH3-treatment at high temp [3]. Consequently, our Pt/C catalysts coated with porous graphene films showed similar initial activity compared with the commercial catalysts (Pt 40wt%, Johnson Matthey) showing more than 150% higher long-term stability [4]. [1] A.K. Geim and K. S. Novoselov, Nature Mater. 6 (2007) 183. [2] X. L. Li, G. Y. Zhang, X. D. Bai, X. M. Sun, X. R. Wang, E. Wang and H. J. Dai, Nature Nanotech. 3 (2008) 538. [3] Y. Wang, Y. Shao, D. W. Matson, J. Li and Y. Lin, ACS nano 4(4) (2010) 1790. [4] H. Kim, A. Robertson, S. O. Kim, J. M. Kim and J. H. Warner, ACS nano 9(6) (2015) 5947.
- Published
- 2016
133. Photoinduced Schottky Barrier Lowering in 2D Monolayer WS 2 Photodetectors
- Author
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Haijie Tan, Xiaochen Wang, Youmin Rong, Yingqiu Zhou, Ye Fan, and Jamie H. Warner
- Subjects
Photocurrent ,Materials science ,Equivalent series resistance ,business.industry ,Schottky barrier ,Schottky diode ,Photodetector ,02 engineering and technology ,Chemical vapor deposition ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,Atomic and Molecular Physics, and Optics ,0104 chemical sciences ,Electronic, Optical and Magnetic Materials ,Semiconductor ,Monolayer ,Optoelectronics ,0210 nano-technology ,business - Abstract
Arrays of metal–semiconductor–metal (MSM) photodetectors are fabricated using chemical vapor deposition (CVD) grown 2D monolayer WS2 as the absorbing semiconductor (WS2) with gold electrodes. A study of the channel length dependence (0.2–6.4 μm) on the photoresponsivity and gain show substantial increase in performance is achieved when the length is reduced to 200 nm. A large gain factor of up to 480 is measured for 200 nm length devices and attributed to lowering of the Schottky barriers due to the filling of trapped states between the metal contact and WS2 by photogenerated carriers. Only photoexcited carriers close to the interface contribute to filling trap states and lowering the Schottky barrier and therefore increasing channel length only adds series resistance to the device that reduces performance. These results reveal detailed insights regarding the mechanisms for photocurrent generation in lateral MSM photodetectors that employ CVD grown monolayer WS2 material, which has important consequences for the commercial applications and large scale development of 2D imaging arrays.
- Published
- 2016
134. Low-Temperature Chemical Vapor Deposition Synthesis of Pt-Co Alloyed Nanoparticles with Enhanced Oxygen Reduction Reaction Catalysis
- Author
-
Sang Ouk Kim, Jamie H. Warner, Heeyeon Kim, Dong Sung Choi, and Alex W. Robertson
- Subjects
Materials science ,Mechanical Engineering ,Inorganic chemistry ,Alloy ,Dispersity ,Nanoparticle ,chemistry.chemical_element ,02 engineering and technology ,Chemical vapor deposition ,engineering.material ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,Nanomaterial-based catalyst ,0104 chemical sciences ,Catalysis ,Metal ,chemistry ,Mechanics of Materials ,visual_art ,engineering ,visual_art.visual_art_medium ,General Materials Science ,0210 nano-technology ,Platinum - Abstract
Novel Pt-Co alloyed nanocatalysts are generated via chemical vapor deposition-assisted facile one-pot synthesis. The method guarantees highly monodisperse Pt-Co alloy nanoparticles with precise control of metallic compositions within 1 at%. A significant features is that a perfectly alloyed single-crystal structure is obtained at temperatures as low as 500 °C, which is much lower than conventional alloying temperatures.
- Published
- 2016
135. Generalized Mechanistic Model for the Chemical Vapor Deposition of 2D Transition Metal Dichalcogenide Monolayers
- Author
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Daniel Blankschtein, Ananth Govind Rajan, Jamie H. Warner, and Michael S. Strano
- Subjects
Chemistry ,General Engineering ,General Physics and Astronomy ,Thermodynamics ,Nanotechnology ,02 engineering and technology ,Chemical vapor deposition ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,Isothermal process ,Transition metal dichalcogenide monolayers ,0104 chemical sciences ,chemistry.chemical_compound ,Monolayer ,General Materials Science ,Kinetic Monte Carlo ,Bond energy ,0210 nano-technology ,Molybdenum disulfide ,Phase diagram - Abstract
Transition metal dichalcogenides (TMDs) like molybdenum disulfide (MoS2) and tungsten disulfide (WS2) are layered materials capable of growth to one monolayer thickness via chemical vapor deposition (CVD). Such CVD methods, while powerful, are notoriously difficult to extend across different reactor types and conditions, with subtle variations often confounding reproducibility, particularly for 2D TMD growth. In this work, we formulate the first generalized TMD synthetic theory by constructing a thermodynamic and kinetic growth mechanism linked to CVD reactor parameters that is predictive of specific geometric shape, size, and aspect ratio from triangular to hexagonal growth, depending on specific CVD reactor conditions. We validate our model using experimental data from Wang et al. (Chem. Mater. 2014, 26, 6371-6379) that demonstrate the systemic evolution of MoS2 morphology down the length of a flow CVD reactor where variations in gas phase concentrations can be accurately estimated using a transport model (CSulfur = 9-965 μmol/m(3); CMoO3 = 15-16 mmol/m(3)) under otherwise isothermal conditions (700 °C). A stochastic model which utilizes a site-dependent activation energy barrier based on the intrinsic TMD bond energies and a series of Evans-Polanyi relations leads to remarkable, quantitative agreement with both shape and size evolution along the reactor. The model is shown to extend to the growth of WS2 at 800 °C and MoS2 under varied process conditions. Finally, a simplified theory is developed to translate the model into a "kinetic phase diagram" of the growth process. The predictive capability of this model and its extension to other TMD systems promise to significantly increase the controlled synthesis of such materials.
- Published
- 2016
136. Mechanisms of monovacancy diffusion in graphene
- Author
-
Jamie H. Warner, Angus I. Kirkland, Elena Besley, Alexander Markevich, Alex W. Robertson, and Jack D. Wadey
- Subjects
Range (particle radiation) ,Condensed matter physics ,Graphene ,Chemistry ,Diffusion pathway ,General Physics and Astronomy ,02 engineering and technology ,Activation energy ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,0104 chemical sciences ,law.invention ,law ,Saddle point ,Vacancy defect ,Density functional theory ,Physical and Theoretical Chemistry ,Diffusion (business) ,0210 nano-technology - Abstract
A comprehensive investigation of monovacancy diffusion in graphene has been carried out with the use of density functional theory and the climbing image nudged elastic band method. An out-of-plane spiro structure is found for the first-order saddle point, which defines the transition state in the vacancy diffusion pathway. The obtained activation energy for diffusion is significantly lower than the reported values for the in-plane saddle point structures. The time between consecutive vacancy jumps in graphene is estimated to be in the range of 100–200 s at room temperature in a good agreement with experimental observations.
- Published
- 2016
137. Redox-Dependent Franck–Condon Blockade and Avalanche Transport in a Graphene–Fullerene Single-Molecule Transistor
- Author
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Hatef Sadeghi, Kyriakos Porfyrakis, Jan A. Mol, Sara Sangtarash, Panagiotis Dallas, Chit Siong Lau, Jamie H. Warner, Gregory Rogers, G. Andrew D. Briggs, and Colin J. Lambert
- Subjects
Fullerene ,Bioengineering ,02 engineering and technology ,010402 general chemistry ,01 natural sciences ,law.invention ,symbols.namesake ,law ,Physics::Atomic and Molecular Clusters ,General Materials Science ,Physics::Chemical Physics ,Spectroscopy ,Quantum tunnelling ,Condensed matter physics ,Graphene ,Chemistry ,Mechanical Engineering ,Relaxation (NMR) ,Transistor ,Molecular scale electronics ,General Chemistry ,Condensed Matter::Mesoscopic Systems and Quantum Hall Effect ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,0104 chemical sciences ,TA ,Chemical physics ,symbols ,0210 nano-technology ,Raman spectroscopy - Abstract
We report transport measurements on a graphene–fullerene single-molecule transistor. The device architecture where a functionalized C60 binds to graphene nanoelectrodes results in strong electron–vibron coupling and weak vibron relaxation. Using a combined approach of transport spectroscopy, Raman spectroscopy, and DFT calculations, we demonstrate center-of-mass oscillations, redox-dependent Franck–Condon blockade, and a transport regime characterized by avalanche tunnelling in a single-molecule transistor.
- Published
- 2015
138. In Situ Observations of Free-Standing Graphene-like Mono- and Bilayer ZnO Membranes
- Author
-
Jiong Zhao, Gianaurelio Cunniberti, Jamie H. Warner, Huy Ta Quang, Frank Ortmann, Jürgen Eckert, Mark H. Rümmeli, Arezoo Dianat, and Alicja Bachmatiuk
- Subjects
Materials science ,Graphene ,Electron energy loss spectroscopy ,Bilayer ,General Engineering ,General Physics and Astronomy ,Nanotechnology ,law.invention ,Membrane ,Chemical engineering ,law ,Quantum dot ,General Materials Science ,Nanorod ,Thin film ,Wurtzite crystal structure - Abstract
ZnO in its many forms, such as bulk, thin films, nanorods, nanobelts, and quantum dots, attracts significant attention because of its exciting optical, electronic, and magnetic properties. For very thin ZnO films, predictions were made that the bulk wurtzite ZnO structure would transit to a layered graphene-like structure. Graphene-like ZnO layers were later confirmed when supported over a metal substrate. However, the existence of free-standing graphene-like ZnO has, to the best of our knowledge, not been demonstrated. In this work, we show experimental evidence for the in situ formation of free-standing graphene-like ZnO mono- and bilayer ZnO membranes suspended in graphene pores. Local electron energy loss spectroscopy confirms the membranes comprise only Zn and O. Image simulations and supporting analysis confirm that the membranes are graphene-like ZnO. Graphene-like ZnO layers are predicted to have a wide band gap and different and exciting properties as compared to other ZnO structures.
- Published
- 2015
139. 2D layered noble metal dichalcogenides (Pt, Pd, Se, S) for electronics and energy applications
- Author
-
Wenshuo Xu, J. Chen, E. Chen, and Jamie H. Warner
- Subjects
Materials science ,Mechanical Engineering ,Tungsten disulfide ,Transition metal dichalcogenides (TMDs) ,chemistry.chemical_element ,Nanotechnology ,engineering.material ,Diselenide ,Metal ,chemistry.chemical_compound ,Two-dimensional (2D) materials ,chemistry ,Transition metal ,Electronic properties ,visual_art ,Electronic devices ,lcsh:TA401-492 ,visual_art.visual_art_medium ,engineering ,lcsh:Materials of engineering and construction. Mechanics of materials ,General Materials Science ,Noble metal ,Platinum ,Molybdenum disulfide ,Palladium - Abstract
The layered materials, two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted increasing attention because of their unique properties in the atomic structures, bandgaps, electronic properties, electrochemistry activities, and the potential applications in devices. Till now, most research has focused on the group VIB metal TMDs, like molybdenum disulfide (MoS2) and tungsten disulfide (WS2), whereas TMDs composed of other groups metals are not widely explored. Herein, we present and summarize the group 10 transition metal platinum (Pt) and palladium (Pd) dichalcogenides, like platinum diselenide (PtSe2), palladium diselenide (PdSe2), platinum disulfide (PtS2) and palladium disulfide (PdS2) to discuss their special points as the published discussion on group VIB metal TMDs, and find the similarities and the differences between the metals. This review will focus on the group 10 noble metal dichalcogenides of their atomic structure, synthetic approaches, defects and dopants, layer dependent band structure, electronic properties, electrochemistry activities and the broadband opto-electronic devices.
- Published
- 2020
140. Transparent ultrathin all-two-dimensional lateral Gr:WS2:Gr photodetector arrays on flexible substrates and their strain induced failure mechanisms
- Author
-
Yuewen Sheng, Xiaochen Wang, Chit Siong Lau, Martin Tweedie, Jamie H. Warner, and Linlin Hou
- Subjects
Flexible electronics ,Photocurrent ,Fabrication ,Plasma etching ,Materials science ,business.industry ,Mechanical Engineering ,Photoconductivity ,TMDs ,Substrate (electronics) ,Chemical vapor deposition ,2D materials ,Transistors ,Strain engineering ,lcsh:TA401-492 ,Optoelectronics ,lcsh:Materials of engineering and construction. Mechanics of materials ,General Materials Science ,business - Abstract
Herein, we detail developments in the scalable fabrication of all-two-dimensional (2D) flexible and transparent photodetectors. These devices were fabricated from chemical vapour deposition derived graphene and tungsten disulphide on poly(ethylene naphthalate) (PEN), a material commonly used as a substrate in flexible electronics. Alternative patterning approaches that are necessitated by the more thermally and chemically sensitive polymer substrates are explored, resulting in several modifications to conventional, silicon-substrate device fabrication approaches. In particular, the relatively new lift-off patterning approach for 2D materials is applied as an alternative to plasma etching due to the deleterious effect of plasma on the PEN substrates. This enabled the observation of a strain-modulated enhancement of photocurrent of up to one order of magnitude, demonstrating the utility of strain engineering to the field of 2D devices—provided it can be controllably applied. Further application of strain led in all cases to permanent failure and loss of photoconductivity, the details of which are explored using scanning electron microscopy. These results provide important details relevant to the fabrication of large-scale arrays of flexible and transparent 2D devices on polymer substrates.
- Published
- 2020
141. In Situ Atomic-Level Studies of Gd Atom Release and Migration on Graphene from a Metallofullerene Precursor
- Author
-
Kyriakos Porfyrakis, Yuewen Sheng, Sapna Sinha, Jamie H. Warner, Ian M. Griffiths, Neil P. Young, Si Zhou, and Angus I. Kirkland
- Subjects
Materials science ,Hydrogen ,General Physics and Astronomy ,chemistry.chemical_element ,02 engineering and technology ,010402 general chemistry ,01 natural sciences ,Nanoclusters ,law.invention ,Metal ,chemistry.chemical_compound ,law ,Monolayer ,Scanning transmission electron microscopy ,QD ,General Materials Science ,QC ,Graphene ,General Engineering ,021001 nanoscience & nanotechnology ,0104 chemical sciences ,chemistry ,Chemical engineering ,Transmission electron microscopy ,visual_art ,Metallofullerene ,visual_art.visual_art_medium ,0210 nano-technology - Abstract
We show how gadolinium (Gd)-based metallofullerene (Gd3N@C80) molecules can be used to create single adatoms and nanoclusters on a graphene surface. An in situ heating holder within an aberration-corrected scanning transmission electron microscope is used to track the adhesion of endohedral metallofullerenes (MFs) to the surface of graphene, followed by Gd metal ejection and diffusion across the surface. Heating to 900 °C is used to promote adatom migration and metal nanocluster formation, enabling direct imaging of the assembly of nanoclusters of Gd. We show that hydrogen can be used to reduce the temperature of MF fragmentation and metal ejection, enabling Gd nanocluster formation on graphene surfaces at temperatures as low as 300 °C. The process of MF fragmentation and metal ejection is captured in situ and reveals that after metal release, the C80 cage opens further and fuses with the surface monolayer carbon glass on graphene, creating a highly stable carbon layer for further Gd adatom adhesion. Small voids and defects (∼1 nm) in the surface carbon glass act as trapping sites for Gd atoms, leading to atomic self-assembly of 2D monolayer Gd clusters. These results show that MFs can adhere to graphene surfaces at temperatures well above their bulk sublimation point, indicating that the surface bound MFs have strong adhesion to dangling bonds on graphene surfaces. The ability to create dispersed single Gd adatoms and Gd nanoclusters on graphene may have impact in spintronics and magnetism.
- Published
- 2018
142. Controlling Photoluminescence Enhancement and Energy Transfer in WS 2 :hBN:WS 2 Vertical Stacks by Precise Interlayer Distances
- Author
-
Yingqiu Zhou, Yuewen Sheng, Tian Jiang, Wenshuo Xu, Yizhi Wang, Michael S. Strano, Jamie H. Warner, and Daichi Kozawa
- Subjects
Electron density ,Materials science ,Photoluminescence ,business.industry ,Doping ,Heterojunction ,02 engineering and technology ,General Chemistry ,Chemical vapor deposition ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,7. Clean energy ,0104 chemical sciences ,Biomaterials ,Monolayer ,Optoelectronics ,General Materials Science ,Quantum efficiency ,Photonics ,0210 nano-technology ,business ,Biotechnology - Abstract
2D semiconducting transition metal dichalcogenides (TMDs) are endowed with fascinating optical properties especially in their monolayer limit. Insulating hBN films possessing customizable thickness can act as a separation barrier to dictate the interactions between TMDs. In this work, vertical layered heterostructures (VLHs) of WS2 :hBN:WS2 are fabricated utilizing chemical vapor deposition (CVD)-grown materials, and the optical performance is evaluated through photoluminescence (PL) spectroscopy. Apart from the prohibited indirect optical transition due to the insertion of hBN spacers, the variation in the doping level of WS2 drives energy transfer to arise from the layer with lower quantum efficiency to the other layer with higher quantum efficiency, whereby the total PL yield of the heterosystem is increased and the stack exhibits a higher PL intensity compared to the sum of those in the two WS2 constituents. Such doping effects originate from the interfaces that WS2 monolayers reside on and interact with. The electron density in the WS2 is also controlled and subsequent modulation of PL in the heterostructure is demonstrated by applying back-gated voltages. Other influential factors include the strain in WS2 and temperature. Being able to tune the energy transfer in the VLHs may expand the development of photonic applications in 2D systems.
- Published
- 2019
143. Atomically sharp interlayer stacking shifts at anti-phase grain boundaries in overlapping MoS
- Author
-
Si, Zhou, Shanshan, Wang, Zhe, Shi, Hidetaka, Sawada, Angus I, Kirkland, Ju, Li, and Jamie H, Warner
- Abstract
When secondary domains nucleate and grow on the surface of monolayer MoS2, they can extend across grain boundaries in the underlying monolayer MoS2 and form overlapping sections. We present an atomic level study of overlapping antiphase grain boundaries (GBs) in MoS2 monolayer-bilayers using aberration-corrected annular dark field scanning transmission electron microscopy. In particular we focus on the antiphase GB within a monolayer and track its propagation through an overlapping bilayer domain. We show that this leads to an atomically sharp interface between 2H and 3R interlayer stacking in the bilayer region. We have studied the micro-nanoscale "meandering" of the antiphase GB in MoS2, which shows a directional dependence on the density of 4 and 8 member ring defects, as well as sharp turning angles 90°-100° that are mediated by a special 8-member ring defect. Density functional theory has been used to explore the overlapping interlayer stacking around the antiphase GBs, confirming our experimental findings. These results show that overlapping secondary bilayer MoS2 domains cause atomic structure modification to underlying anti-phase GB sites to accommodate the van der Waals interactions.
- Published
- 2018
144. Ultralong 1D Vacancy Channels for Rapid Atomic Migration during 2D Void Formation in Monolayer MoS
- Author
-
Qu, Chen, Huashan, Li, Si, Zhou, Wenshuo, Xu, Jun, Chen, Hidetaka, Sawada, Christopher S, Allen, Angus I, Kirkland, Jeffrey C, Grossman, and Jamie H, Warner
- Abstract
High-energy irradiation of materials can lead to void formation due to the aggregation of vacancies, reducing the local stress in the system. Studying void formation and its interplay with vacancy clusters in bulk materials at the atomic level has been challenging due to the thick volume of 3D materials, which generally limits high-resolution transmission electron microscopy. The thin nature of 2D materials is ideal for studying fundamental material defects such as dislocations and crack tips and has potential to reveal void formation by vacancy aggregation in detail. Here, using atomic-resolution in situ transmission electron microscopy of 2D monolayer MoS
- Published
- 2018
145. Ultralong 1D vacancy channels for rapid atomic migration during 2D void formation in monolayer MoS2
- Author
-
Angus I. Kirkland, Jamie H. Warner, Hidetaka Sawada, Wenshuo Xu, Jun Chen, Si Zhou, Huashan Li, Jeffrey C. Grossman, Christopher S. Allen, and Qu Chen
- Subjects
Void (astronomy) ,Materials science ,General Engineering ,General Physics and Astronomy ,02 engineering and technology ,010402 general chemistry ,021001 nanoscience & nanotechnology ,Thermal diffusivity ,01 natural sciences ,0104 chemical sciences ,In situ transmission electron microscopy ,Chemical physics ,Transmission electron microscopy ,Vacancy defect ,Monolayer ,General Materials Science ,Irradiation ,0210 nano-technology - Abstract
High-energy irradiation of materials can lead to void formation due to the aggregation of vacancies, reducing the local stress in the system. Studying void formation and its interplay with vacancy clusters in bulk materials at the atomic level has been challenging due to the thick volume of 3D materials, which generally limits high-resolution transmission electron microscopy. The thin nature of 2D materials is ideal for studying fundamental material defects such as dislocations and crack tips and has potential to reveal void formation by vacancy aggregation in detail. Here, using atomic-resolution in situ transmission electron microscopy of 2D monolayer MoS2, we capture rapid thermal diffusion of S vacancies into ultralong (∼60 nm) 1D S vacancy channels that initiate void formation at high vacancy densities. Strong interactions are observed between the 1D channels and void growth, whereby Mo and S atoms are funneled back and forth between the void edge and the crystal surface to enable void enlargement. Preferential void growth up to 100 nm is shown to occur by rapid digestion of 1D S vacancy channels as they make contact. These results reveal the atomistic mechanisms behind void enlargement in 2D materials under intense high-energy irradiation at high temperatures and the existence of ultralong 1D vacancy channels. This knowledge may also help improve the understanding of void formation in other systems such as nuclear materials, where direct visualization is challenging due to 3D bulk volume.
- Published
- 2018
146. Facile Fabrication of Large-Area Atomically Thin Membranes by Direct Synthesis of Graphene with Nanoscale Porosity
- Author
-
Qu Chen, Jamie H. Warner, Jing Kong, Sui Zhang, Rohit Karnik, Giang D. Nguyen, An-Ping Li, and Piran R. Kidambi
- Subjects
Materials science ,Fabrication ,Nanoporous ,Graphene ,Mechanical Engineering ,Nanotechnology ,02 engineering and technology ,Permeance ,Chemical vapor deposition ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,0104 chemical sciences ,law.invention ,Nanopore ,Membrane ,Mechanics of Materials ,law ,General Materials Science ,0210 nano-technology ,Nanoscopic scale - Abstract
Direct synthesis of graphene with well-defined nanoscale pores over large areas can transform the fabrication of nanoporous atomically thin membranes (NATMs) and greatly enhance their potential for practical applications. However, scalable bottom-up synthesis of continuous sheets of nanoporous graphene that maintain integrity over large areas has not been demonstrated. Here, it is shown that a simple reduction in temperature during chemical vapor deposition (CVD) on Cu induces in-situ formation of nanoscale defects (≤2-3 nm) in the graphene lattice, enabling direct and scalable synthesis of nanoporous monolayer graphene. By solution-casting of hierarchically porous polyether sulfone supports on the as-grown nanoporous CVD graphene, large-area (>5 cm2 ) NATMs for dialysis applications are demonstrated. The synthesized NATMs show size-selective diffusive transport and effective separation of small molecules and salts from a model protein, with ≈2-100× increase in permeance along with selectivity better than or comparable to state-of-the-art commercially available polymeric dialysis membranes. The membranes constitute the largest fully functional NATMs fabricated via bottom-up nanopore formation, and can be easily scaled up to larger sizes permitted by CVD synthesis. The results highlight synergistic benefits in blending traditional membrane casting with bottom-up pore creation during graphene CVD for advancing NATMs toward practical applications.
- Published
- 2018
147. Atomic structure of defects and dopants in 2D layered transition metal dichalcogenides
- Author
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Jamie H. Warner, Alex W. Robertson, and Shanshan Wang
- Subjects
Materials science ,Condensed matter physics ,Graphene ,Electron energy loss spectroscopy ,02 engineering and technology ,General Chemistry ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,0104 chemical sciences ,law.invention ,law ,Transmission electron microscopy ,Vacancy defect ,Atom ,Scanning transmission electron microscopy ,Grain boundary ,0210 nano-technology ,MXenes - Abstract
Layered transition metal dichalcogenides (TMDs) offer monolayer 2D systems with diverse properties that extend beyond what graphene alone can achieve. The properties of TMDs are heavily influenced by the atomic structure and in particular imperfects in the crystallinity in the form of vacancy defects, grain boundaries, cracks, impurity dopants, ripples and edge terminations. This review will cover the current knowledge of the detailed structural forms of some of the most intensively studied 2D TMDs, such as MoS2, WSe2, MoTe2, WTe2, NbSe2, PtSe2, and also covers MXenes. The review will utilize results achieved using state-of-the-art aberration corrected transmission electron microscopy, including annular dark-field scanning transmission electron microscopy (ADF-STEM) and electron energy loss spectroscopy (EELS), showing how elemental discrimination can be achieved to understand structure at a deep level. The review will also cover the impact of single atom substitutional dopants, such as Cr, V and Mn, and electron energy loss spectroscopy used to understand the local bonding configuration. It is expected that this review will provide an atomic level understanding of 2D TMDs with a connection to imperfections that can arise from chemical vapour deposition synthesis, intentional doping, rips and tears, dislocations, strain, polycrystallinity and confinement to nanoribbons.
- Published
- 2018
148. Blister-based-laser-induced-forward-transfer: a non-contact, dry laser-based transfer method for nanomaterials
- Author
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Eleanor E. B. Campbell, Alexander V. Bulgakov, Mitsuhiro Okada, Jamie H. Warner, S Y Heng, Hisanori Shinohara, Andrei Gromov, Nathan Goodfriend, Oleg Nerushev, Wenshuo Xu, and Ryo Kitaura
- Subjects
Materials science ,Nanoparticle ,Bioengineering ,02 engineering and technology ,Chemical vapor deposition ,010402 general chemistry ,01 natural sciences ,law.invention ,Nanomaterials ,law ,Monolayer ,medicine ,General Materials Science ,Electrical and Electronic Engineering ,business.industry ,Mechanical Engineering ,Blisters ,General Chemistry ,021001 nanoscience & nanotechnology ,Laser ,0104 chemical sciences ,Surface coating ,Mechanics of Materials ,Femtosecond ,Optoelectronics ,medicine.symptom ,0210 nano-technology ,business - Abstract
We show that blister-based-laser-induced forward transfer (BB-LIFT) can be used to cleanly desorb and transfer nano- and micro-scale particles between substrates without exposing the particles to the laser radiation or to any chemical treatment that could damage the intrinsic electronic and optical properties of the materials. The technique uses laser pulses to inducethe rapid formation of a blister on a thin metal layer deposited on glass via ablation at the metal/glass interface. Femtosecond laser pulses are advantageous for forming beams of molecules or small nanoparticles with well-defined velocity and narrow angular distributions. Both fs and ns laser pulses can be used to cleanly transfer larger nanoparticles including relatively fragile monolayer 2D transition metal dichalcogenide crystals and for direct transfer of nanoparticles from chemical vapour deposition (CVD) growth substrates, although the mechanisms for inducing blister formation are different.
- Published
- 2018
149. Preferential Pt Nanocluster Seeding at Grain Boundary Dislocations in Polycrystalline Monolayer MoS
- Author
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Shanshan, Wang, Hidetaka, Sawada, Xiaoyu, Han, Si, Zhou, Sha, Li, Zheng Xiao, Guo, Angus I, Kirkland, and Jamie H, Warner
- Abstract
We show that Pt nanoclusters preferentially nucleate along the grain boundaries (GBs) in polycrystalline MoS
- Published
- 2018
150. Utilizing interlayer excitons in bilayer WS2 for increased photovoltaic response in ultrathin graphene vertical cross-bar photodetecting tunneling transistors
- Author
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Yuewen Sheng, Haijie Tan, Jamie H. Warner, Wenshuo Xu, Ye Fan, and Yingqiu Zhou
- Subjects
Materials science ,business.industry ,Graphene ,Photoconductivity ,Exciton ,Bilayer ,General Engineering ,General Physics and Astronomy ,Heterojunction ,02 engineering and technology ,Dielectric ,010402 general chemistry ,021001 nanoscience & nanotechnology ,7. Clean energy ,01 natural sciences ,0104 chemical sciences ,law.invention ,law ,Monolayer ,Optoelectronics ,General Materials Science ,0210 nano-technology ,business ,Quantum tunnelling - Abstract
Here we study the layer-dependent photoconductivity in Gr/WS2/Gr vertical stacked tunneling (VST) cross-bar devices made using two-dimensional (2D) materials all grown by chemical vapor deposition. The larger number of devices (>100) enables a statistically robust analysis on the comparative differences in the photovoltaic response of monolayer and bilayer WS2, which cannot be achieved in small batch devices made using mechanically exfoliated materials. We show a dramatic increase in photovoltaic response for Gr/WS2(2L)/Gr compared to monolayers because of the long inter- and intralayer exciton lifetimes and the small exciton binding energy (both interlayer and intralayer excitons) of bilayer WS2 compared with that of monolayer WS2. Different doping levels and dielectric environments of top and bottom graphene electrodes result in a potential difference across a ∼1 nm vertical device, which gives rise to large electric fields perpendicular to the WS2 layers that cause band structure modification. Our results show how precise control over layer number in all 2D VST devices dictates the photophysics and performance for photosensing applications.
- Published
- 2018
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