Reported values （0.2 MPa-7.0 GPa） of the interlayer shear strength （ISS） of graphite are very dispersed. The main challenge to obtain a reliable value of the ISS using conventional measuring methods was the unavai...Reported values （0.2 MPa-7.0 GPa） of the interlayer shear strength （ISS） of graphite are very dispersed. The main challenge to obtain a reliable value of the ISS using conventional measuring methods was the unavailability of sufficiently large single crystalline graphite. Here we present a novel experimental method to measure the ISS, and obtain the value as -0.14 GPa. Our result can serve as an important basis for understanding mechanical behavior of graphite or graphene-based materials.展开更多
Mechanical exfoliation is a widely used method to isolate high quality graphene layers from bulk graphite. In our recent experiments, some ordered microstructures, consisting of a periodic alternation of kinks and str...Mechanical exfoliation is a widely used method to isolate high quality graphene layers from bulk graphite. In our recent experiments, some ordered microstructures, consisting of a periodic alternation of kinks and stripes, were observed in thin graphite flakes that were mechanically peeled from highly oriented pyrolytic graphite. In this paper, a theoretical model is presented to attribute the formation of such ordered structures to the alternation of two mechanical processes during the exfoliation： （1） peeling of a graphite flake and （2） mechanical buckling of the flake being sub- jected to bending. In this model, the width of the stripes L is determined by thickness h of the flakes, surface energy Y, and critical buckling strain ecr. Using some appropriate values of y and ecr that are within the ranges determined by other inde- pendent experiments and simulations, the predicted relations between the stripe width and the flake thickness agree reason- ably well with our experimental measurements. Conversely, measuring the L-h relations of the periodic microstructures in thin graphite flakes could help determine the critical mechan- ical buckling strain εcr and the interface energy γ.展开更多
Reversible control of surface wettability has wide applications in lab-on-chip systems, tunable optical lenses, and microfluidic tools. Using a graphene sheet as a sam- ple material and molecular dynamic simulations, ...Reversible control of surface wettability has wide applications in lab-on-chip systems, tunable optical lenses, and microfluidic tools. Using a graphene sheet as a sam- ple material and molecular dynamic simulations, we demon- strate that strain engineering can serve as an effective way to control the surface wettability. The contact angles 0 of water droplets on a graphene vary from 72.5° to 106° under biaxial strains ranging from -10% to 10% that are applied on the graphene layer. For an intrinsic hydrophilic surface （at zero strain）, the variation of 0 upon the applied strains is more sensitive, i.e., from 0° to 74.8°. Overall the cosines of the contact angles exhibit a linear relation with respect to the strains. In light of the inherent dependence of the contact an- gle on liquid-solid interfacial energy, we develop an analytic model to show the cos 0 as a linear function of the adsorption energy Eads of a single water molecule over the substrate sur- face. This model agrees with our molecular dynamic results very well. Together with the linear dependence of Eads on bi- axial strains, we can thus understand the effect of strains on the surface wettability. Thanks to the ease of reversibly ap- plying mechanical strains in micro/nano-electromechanical systems, we believe that strain engineering can be a promis- ing means to achieve the reversibly control of surface wetta- bility.展开更多
Electrical double layer （EDL） capacitors based on recently emergent graphene materials have shown several folds performance improvement compared to conventional porous carbon materials, driving a wave of technology ...Electrical double layer （EDL） capacitors based on recently emergent graphene materials have shown several folds performance improvement compared to conventional porous carbon materials, driving a wave of technology breakthrough in portable and renewable energy storage. Accordingly, much interest has been generated to pursue a comprehensive understanding of the fundamental yet elusive double layer structure at file electrode^electrolyte interface. In this paper, we carried out comprehensive molecular dynamics simulations to obtain a com- prehensive picture of how ion type, solvent properties, and charging conditions affect the EDL structure at the graphene electrode surface, and thereby its contribution to capacitance. We show that different symmetrical monovalent aqueous electrolytes M^X- （M~ = Na~, K~, Rb＋, and Cs＋; X- = F-, CI-, and I ） indeed have distinctive EDL structures. Larger ions, such as, Rb＊, Cs＊, C1, and I, undergo partial dehydration and penetrate through the first water layer next to the graphene electrode surfaces under charging. As such, the electrical potential distribution through the EDL strongly depends on the ion type. Interestingly, we further reveal that the water can play a critical role in determining the capacitance value. The change of dielectric constant of water in different electrolytes largely cancels out the variance in electric potential drop across the EDL of different ion type. Our simulation sheds new lights on how the interplay between solvent molecules and EDL structure cooperatively contributes to capacitance, which agrees with our experimental results well.展开更多
Two-dimensional materials with ferroelectric properties break the size effect of conventional ferroelectric materials and unlock unprecedented potentials of ferroelectric-related application at small length scales.Usi...Two-dimensional materials with ferroelectric properties break the size effect of conventional ferroelectric materials and unlock unprecedented potentials of ferroelectric-related application at small length scales.Using first-principles calculations,a sliding-induced ferroelectric-to-antiferroelectric behavior in bilayer group-IV monochalcogenides(MX,with M=Ge,Sn and X=S,Se)is discovered.Upon this mechanism,the top layer exhibits a reversible intralayer ferroelectric switching,leading to a reversible transition between the ferroelectric and antiferroelectric states in the bilayer MXs.Further results show that the interlayer van der Waals interaction,which is usually considered to be weak,can actually generate an in-plane lattice distortion and thus cause the breaking/forming of intralayer covalent bonds in the top layer,leading to the observed anomalous phenomenon.This unique property has advantages for energy harvesting over existing piezoelectric and triboelectric nanogenerators.The interlayer sliding-induced big polarization change(40μC cm^(−2))and ultrahigh polarization changing rate generate an open-circuit voltage two orders of magnitude higher than that of MoS_(2)-based nanogenerators.The theoretical prediction of power output for this bilayer MXs at a moderate sliding speed 1 m s^(−1)is four orders of magnitude higher than the MoS_(2)nanogenerator,indicating great potentials in energy harvesting applications.展开更多
Lateral heterostructures of two-dimensional(2D)materials,integrating different phases or materials into a single piece of nanosheet,have attracted intensive research interests for electronic devices.Extending the 2D l...Lateral heterostructures of two-dimensional(2D)materials,integrating different phases or materials into a single piece of nanosheet,have attracted intensive research interests for electronic devices.Extending the 2D lateral heterostructures to spintronics demands more diverse electromagnetic properties of 2D materials.In this paper,using density functional theory calculations,we survey all IV,V,and VI group transition metal dichalcogenides(TMDs)and discover that CrS_(2)has the most diverse electronic and magnetic properties:antiferromagnetic(AFM)metallic 1T phase,non-magnetic(NM)semiconductor 2H phase,and ferromagnetic(FM)semiconductor 1T′phase with a Curie temperature of~1000 K.Interestingly,we find that a tensile or compressive strain can turn the 1T′phase into a spin-up or spin-down half-metal.Such strain tunability can be attributed to the lattice deformation under tensile/compressive strain that selectively promotes the spin-up/spin-down VBM(valence band bottom)orbital interactions.The diverse electromagnetic properties and the strain tunability enable strain-controlled spintronic devices using a single piece of CrS_(2)nanosheet with improved energy efficiency.As a demo,a prototypical design of the spin-valve logic device is presented.It offers a promising solution to address the challenge of high energy consumption in miniaturized spintronic devices.展开更多
Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine(HMX) is one of the most widely used powerful explosives. The direct and selective detection of HMX, without the requirement of specialized equipment, remains a great ch...Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine(HMX) is one of the most widely used powerful explosives. The direct and selective detection of HMX, without the requirement of specialized equipment, remains a great challenge due to its extremely low volatility, unfavorable reduction potential and lack of aromatic rings. Here, we report the first chemical probe of direct identification of HMX at ppb sensitivity based on a designed metal-organic cage(MOC). The cage features two unsaturated dicopper units and four electron donating amino groups inside the cavity, providing multiple binding sites to selectively enhance host-guest events. It was found that compared to other explosive molecules the capture of HMX inside the cavity would strongly modulate the emissive behavior of the host cage, resulting in highly induced fluorescence “turn-on”(160 folds). Based on the density functional theory(DFT) simulation, the mutual fit of both size and binding sites between host and guest leads to the synergistic effects that perturb the ligand-to-metal charge-transfer(LMCT) process, which is probably the origin of such selective HMX-induced turn-on behavior.展开更多
We use molecular dynamics (MD) simulations to study the effects of vacancies on tube diameters and interwall spacings of multi-walled carbon nanotubes (MWCNTs). Two types of vacancies, double vacancy and three danglin...We use molecular dynamics (MD) simulations to study the effects of vacancies on tube diameters and interwall spacings of multi-walled carbon nanotubes (MWCNTs). Two types of vacancies, double vacancy and three dangling-bond (3DB) single vacancy, are identified to have opposite effects on the tube size change, which explains the inconsistency of the experimentally measured interwall spacings of MWCNTs after electron beam irradiation. A theoretical model to quantitatively predict the shrunk structures of the irradiated MWCNTs is further developed. We also discuss the fabrications of prestressed MWCNTs, in which reduced interwall spacings are desired to enhance the overall elastic modulus and strength.展开更多
基金supported by the National Natural Science Foundation of China(10832005)the National Basic Research Program of China (2007CB936803)
文摘Reported values （0.2 MPa-7.0 GPa） of the interlayer shear strength （ISS） of graphite are very dispersed. The main challenge to obtain a reliable value of the ISS using conventional measuring methods was the unavailability of sufficiently large single crystalline graphite. Here we present a novel experimental method to measure the ISS, and obtain the value as -0.14 GPa. Our result can serve as an important basis for understanding mechanical behavior of graphite or graphene-based materials.
基金financia support from NSFC(Grant 10832005)the National Basic Research Program of China(Grant 2007CB936803)+1 种基金the National 863 Project(Grant2008AA03Z302)the support from the engineering faculty of Monash University through seed grant 2014
文摘Mechanical exfoliation is a widely used method to isolate high quality graphene layers from bulk graphite. In our recent experiments, some ordered microstructures, consisting of a periodic alternation of kinks and stripes, were observed in thin graphite flakes that were mechanically peeled from highly oriented pyrolytic graphite. In this paper, a theoretical model is presented to attribute the formation of such ordered structures to the alternation of two mechanical processes during the exfoliation： （1） peeling of a graphite flake and （2） mechanical buckling of the flake being sub- jected to bending. In this model, the width of the stripes L is determined by thickness h of the flakes, surface energy Y, and critical buckling strain ecr. Using some appropriate values of y and ecr that are within the ranges determined by other inde- pendent experiments and simulations, the predicted relations between the stripe width and the flake thickness agree reason- ably well with our experimental measurements. Conversely, measuring the L-h relations of the periodic microstructures in thin graphite flakes could help determine the critical mechan- ical buckling strain εcr and the interface energy γ.
基金supported by the National Natural Science Foundation of China(11172149)the financial support from the IBM World Community Grid project "Computing for Clean Water"+2 种基金the Boeing-Tsinghua Joint Research Project "New Air Filtration Materials"grant 2012 from engineering faculty of Monash Universitysupported by an award under the Merit Allocation Scheme on the Australia NCI National Facility at the ANU
文摘Reversible control of surface wettability has wide applications in lab-on-chip systems, tunable optical lenses, and microfluidic tools. Using a graphene sheet as a sam- ple material and molecular dynamic simulations, we demon- strate that strain engineering can serve as an effective way to control the surface wettability. The contact angles 0 of water droplets on a graphene vary from 72.5° to 106° under biaxial strains ranging from -10% to 10% that are applied on the graphene layer. For an intrinsic hydrophilic surface （at zero strain）, the variation of 0 upon the applied strains is more sensitive, i.e., from 0° to 74.8°. Overall the cosines of the contact angles exhibit a linear relation with respect to the strains. In light of the inherent dependence of the contact an- gle on liquid-solid interfacial energy, we develop an analytic model to show the cos 0 as a linear function of the adsorption energy Eads of a single water molecule over the substrate sur- face. This model agrees with our molecular dynamic results very well. Together with the linear dependence of Eads on bi- axial strains, we can thus understand the effect of strains on the surface wettability. Thanks to the ease of reversibly ap- plying mechanical strains in micro/nano-electromechanical systems, we believe that strain engineering can be a promis- ing means to achieve the reversibly control of surface wetta- bility.
文摘Electrical double layer （EDL） capacitors based on recently emergent graphene materials have shown several folds performance improvement compared to conventional porous carbon materials, driving a wave of technology breakthrough in portable and renewable energy storage. Accordingly, much interest has been generated to pursue a comprehensive understanding of the fundamental yet elusive double layer structure at file electrode^electrolyte interface. In this paper, we carried out comprehensive molecular dynamics simulations to obtain a com- prehensive picture of how ion type, solvent properties, and charging conditions affect the EDL structure at the graphene electrode surface, and thereby its contribution to capacitance. We show that different symmetrical monovalent aqueous electrolytes M^X- （M~ = Na~, K~, Rb＋, and Cs＋; X- = F-, CI-, and I ） indeed have distinctive EDL structures. Larger ions, such as, Rb＊, Cs＊, C1, and I, undergo partial dehydration and penetrate through the first water layer next to the graphene electrode surfaces under charging. As such, the electrical potential distribution through the EDL strongly depends on the ion type. Interestingly, we further reveal that the water can play a critical role in determining the capacitance value. The change of dielectric constant of water in different electrolytes largely cancels out the variance in electric potential drop across the EDL of different ion type. Our simulation sheds new lights on how the interplay between solvent molecules and EDL structure cooperatively contributes to capacitance, which agrees with our experimental results well.
基金The authors gratefully acknowledge the support of NSFC(Grant Nos.11974269,51728203)the support by 111 project 2.0(Grant No.BP0618008)+2 种基金J.D.also thanks the support of the National Key R&D Program of China(Grant No.2018YFB1900104)J.Z.L.acknowledges the support from ARC discovery projects(DP180101744)and HPC from National Computational Infrastructure from AustraliaThis work is also supported by State Key Laboratory for Mechanical Behavior of Materials.
文摘Two-dimensional materials with ferroelectric properties break the size effect of conventional ferroelectric materials and unlock unprecedented potentials of ferroelectric-related application at small length scales.Using first-principles calculations,a sliding-induced ferroelectric-to-antiferroelectric behavior in bilayer group-IV monochalcogenides(MX,with M=Ge,Sn and X=S,Se)is discovered.Upon this mechanism,the top layer exhibits a reversible intralayer ferroelectric switching,leading to a reversible transition between the ferroelectric and antiferroelectric states in the bilayer MXs.Further results show that the interlayer van der Waals interaction,which is usually considered to be weak,can actually generate an in-plane lattice distortion and thus cause the breaking/forming of intralayer covalent bonds in the top layer,leading to the observed anomalous phenomenon.This unique property has advantages for energy harvesting over existing piezoelectric and triboelectric nanogenerators.The interlayer sliding-induced big polarization change(40μC cm^(−2))and ultrahigh polarization changing rate generate an open-circuit voltage two orders of magnitude higher than that of MoS_(2)-based nanogenerators.The theoretical prediction of power output for this bilayer MXs at a moderate sliding speed 1 m s^(−1)is four orders of magnitude higher than the MoS_(2)nanogenerator,indicating great potentials in energy harvesting applications.
基金The authors gratefully acknowledge the support of NSFC(Grants Nos.11974269,51728203,51621063,51601140,51701149,51671155,91963111)the support by 111 project 2.0(Grant No.BP0618008)+3 种基金the National Science Basic Research Plan in the Shaanxi Province of China(2018JM5168)Innovation Capability Support Program of Shaanxi(Nos.2018PT-28,2017KTPT-04)J.D.also thanks to the Fundamental Research Funds for the Central Universities.J.Z.L.acknowledges the support from ARC discovery projects(DP180101744)HPC from National Computational Infrastructure from Australia.This work is also supported by the State Key Laboratory for Mechanical Behavior of Materials and HPC platform of Xi’an Jiaotong University.The authors would also like to thank Mr.F.Yang and Dr.X.D.Zhang at the Network Information Center of Xi’an Jiaotong University for support of the HPC platform.
文摘Lateral heterostructures of two-dimensional(2D)materials,integrating different phases or materials into a single piece of nanosheet,have attracted intensive research interests for electronic devices.Extending the 2D lateral heterostructures to spintronics demands more diverse electromagnetic properties of 2D materials.In this paper,using density functional theory calculations,we survey all IV,V,and VI group transition metal dichalcogenides(TMDs)and discover that CrS_(2)has the most diverse electronic and magnetic properties:antiferromagnetic(AFM)metallic 1T phase,non-magnetic(NM)semiconductor 2H phase,and ferromagnetic(FM)semiconductor 1T′phase with a Curie temperature of~1000 K.Interestingly,we find that a tensile or compressive strain can turn the 1T′phase into a spin-up or spin-down half-metal.Such strain tunability can be attributed to the lattice deformation under tensile/compressive strain that selectively promotes the spin-up/spin-down VBM(valence band bottom)orbital interactions.The diverse electromagnetic properties and the strain tunability enable strain-controlled spintronic devices using a single piece of CrS_(2)nanosheet with improved energy efficiency.As a demo,a prototypical design of the spin-valve logic device is presented.It offers a promising solution to address the challenge of high energy consumption in miniaturized spintronic devices.
基金the National Natural Science Foundation of China (NSFC, Nos. 21773135, 22032003, 21821001)the Ministry of Science and Technology (MOST, No. 2017YFA0204501)the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation, No. TRR61)。
文摘Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine(HMX) is one of the most widely used powerful explosives. The direct and selective detection of HMX, without the requirement of specialized equipment, remains a great challenge due to its extremely low volatility, unfavorable reduction potential and lack of aromatic rings. Here, we report the first chemical probe of direct identification of HMX at ppb sensitivity based on a designed metal-organic cage(MOC). The cage features two unsaturated dicopper units and four electron donating amino groups inside the cavity, providing multiple binding sites to selectively enhance host-guest events. It was found that compared to other explosive molecules the capture of HMX inside the cavity would strongly modulate the emissive behavior of the host cage, resulting in highly induced fluorescence “turn-on”(160 folds). Based on the density functional theory(DFT) simulation, the mutual fit of both size and binding sites between host and guest leads to the synergistic effects that perturb the ligand-to-metal charge-transfer(LMCT) process, which is probably the origin of such selective HMX-induced turn-on behavior.
基金Project supported by the National Basic Research Program (973) of China (No 2007CB936803)the National High-Tech R&D Program (863) of China (No 2008AA03Z302)+1 种基金the National Natural Science Foundation of China (No 10832005)the Joint Research Scheme of the National Natural Science Foundation of China and Research Grants Council of Hong Kong (No 50518003)
文摘We use molecular dynamics (MD) simulations to study the effects of vacancies on tube diameters and interwall spacings of multi-walled carbon nanotubes (MWCNTs). Two types of vacancies, double vacancy and three dangling-bond (3DB) single vacancy, are identified to have opposite effects on the tube size change, which explains the inconsistency of the experimentally measured interwall spacings of MWCNTs after electron beam irradiation. A theoretical model to quantitatively predict the shrunk structures of the irradiated MWCNTs is further developed. We also discuss the fabrications of prestressed MWCNTs, in which reduced interwall spacings are desired to enhance the overall elastic modulus and strength.
