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Author

Bei Peng

Other affiliations: Northwestern University
Bio: Bei Peng is an academic researcher from University of Electronic Science and Technology of China. The author has contributed to research in topics: Capacitive sensing & Diamond. The author has an hindex of 19, co-authored 91 publications receiving 3203 citations. Previous affiliations of Bei Peng include Northwestern University.


Papers
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Journal ArticleDOI
TL;DR: Multiwalled carbon nanotubes with a mean fracture strength >100 GPa are reported, which exceeds earlier observations by a factor of approximately three and are in excellent agreement with quantum-mechanical estimates for nanot tubes containing only an occasional vacancy defect, and are approximately 80% of the values expected for defect-free tubes.
Abstract: The excellent mechanical properties of carbon nanotubes are being exploited in a growing number of applications from ballistic armour to nanoelectronics. However, measurements of these properties have not achieved the values predicted by theory due to a combination of artifacts introduced during sample preparation and inadequate measurements. Here we report multiwalled carbon nanotubes with a mean fracture strength >100 GPa, which exceeds earlier observations by a factor of approximately three. These results are in excellent agreement with quantum-mechanical estimates for nanotubes containing only an occasional vacancy defect, and are ∼80% of the values expected for defect-free tubes. This performance is made possible by omitting chemical treatments from the sample preparation process, thus avoiding the formation of defects. High-resolution imaging was used to directly determine the number of fractured shells and the chirality of the outer shell. Electron irradiation at 200 keV for 10, 100 and 1,800 s led to improvements in the maximum sustainable loads by factors of 2.4, 7.9 and 11.6 compared with non-irradiated samples of similar diameter. This effect is attributed to crosslinking between the shells. Computer simulations also illustrate the effects of various irradiation-induced crosslinking defects on load sharing between the shells. The mechanical properties of carbon nanotubes rarely match the values predicted by theory owing to a combination of artefacts introduced during sample preparation and inadequate measurements. However, by avoiding chemical treatments and using high-resolution imaging, it is possible to obtain values of the mean fracture strength that exceed previous values by approximately a factor of three.

1,038 citations

Journal ArticleDOI
TL;DR: The computational results demonstrate similar size dependence, complementing the experimental findings, and reveal that the observed size effect is an outcome of surface reconstruction together with long-range ionic interactions.
Abstract: Understanding the mechanical properties of nanowires made of semiconducting materials is central to their application in nano devices. This work presents an experimental and computational approach to unambiguously quantify size effects on the Young’s modulus, E, of ZnO nanowires and interpret the origin of the scaling. A micromechanical system (MEMS) based nanoscale material testing system is used in situ a transmission electron microscope to measure the Young’s modulus of [0001] oriented ZnO nanowires as a function of wire diameter. It is found that E increases from ∼140 to 160 GPa as the nanowire diameter decreases from 80 to 20 nm. For larger wires, a Young’s modulus of ∼140 GPa, consistent with the modulus of bulk ZnO, is observed. Molecular dynamics simulations are carried out to model ZnO nanowires of diameters up to 20 nm. The computational results demonstrate similar size dependence, complementing the experimental findings, and reveal that the observed size effect is an outcome of surface reconstr...

393 citations

Journal ArticleDOI
TL;DR: In this paper, the size effects on free-standing polycrystalline FCC thin films subjected to macroscopic homogeneous axial deformation have been investigated and it was shown that thickness plays a major role in deformation behavior and fracture.
Abstract: The membrane deflection experiment developed by Espinosa and co-workers was used to examine size effects on mechanical properties of free-standing polycrystalline FCC thin films. We present stress–strain curves obtained on films 0.2, 0.3, 0.5 and 1.0 μm thick including specimen widths of 2.5, 5.0, 10.0 and 20.0 μm for each thickness. Elastic modulus was consistently measured in the range of 53– 55 GPa for Au, 125– 129 GPa for Cu and 65– 70 GPa for Al. Several size effects were observed including yield stress variations with membrane width and film thickness in pure tension. The yield stress of the membranes was found to increase as membrane width and thickness decreased. It was also observed that thickness plays a major role in deformation behavior and fracture of polycrystalline FCC metals. A strengthening size scale of one over film thickness was identified. In the case of Au free-standing films, a major transition in the material inelastic response occurs when thickness is changed from 1 to 0.5 μm . In this transition, the yield stress more than doubled when film thickness was decreased, with the 0.5 μm thick specimen exhibiting a more brittle-like failure and the 1 μm thick specimen exhibiting a strain softening behavior. Similar plasticity size effects were observed in Cu and Al. Scanning electron microscopy performed on Au films revealed that the number of grains through the thickness essentially halved, from approximately 5 to 2, as thickness decreased. It is postulated that this feature affects the number of dislocations sources, active slip systems, and dislocation motion paths leading to the observed strengthening. This statistical effect is corroborated by the stress–strain data in the sense that data scatter increases with increase in thickness, i.e., plasticity activity. The size effects here reported are the first of their kind in the sense that the measurements were performed on free-standing polycrystalline FCC thin films subjected to macroscopic homogeneous axial deformation, i.e., in the absence of deformation gradients, in contrast to nanoindentation, beam deflection, and torsion, where deformation gradients occur. To the best of our understanding, continuum plasticity models in their current form cannot capture the observed size scale effects.

380 citations

Journal ArticleDOI
TL;DR: Molecular dynamic simulations, using the Buckingham potential, were used to examine failure mechanisms in nanowires with diameters up to 20 nm and revealed a stress-induced phase transformation from wurtzite phase to a body-centered tetragonal phase at approximately 6% strain.
Abstract: An experimental and computational approach is pursued to investigate the fracture mechanism of [0001] oriented zinc oxide nanowires under uniaxial tensile loading. A MEMS-based nanoscale material testing stage is used in situ a transmission electron microscope to perform tensile tests. Experiments revealed brittle fracture along (0001) cleavage plane at strains as high as 5%. The measured fracture strengths ranged from 3.33 to 9.53 GPa for 25 different nanowires with diameters varying from 20 to 512 nm. Molecular dynamic simulations, using the Buckingham potential, were used to examine failure mechanisms in nanowires with diameters up to 20 nm. Simulations revealed a stress-induced phase transformation from wurtzite phase to a body-centered tetragonal phase at approximately 6% strain, also reported earlier by Wang et al. (1) The transformation is partial in larger nanowires and the transformed nanowires fail in a brittle manner at strains as high as 17.5%. The differences between experiments and computations are discussed in the context of (i) surface defects observed in the ZnO nanowires, and (ii) instability in the loading mechanism at the initiation of transformation.

200 citations

Journal ArticleDOI
TL;DR: In this article, an analytical model based on the energy method in both small deformation and finite kinematics (large deformation) regimes is used to interpret the measurements of the deflection of the cantilever under electrostatic actuation.
Abstract: In this paper, carbon nanotube-based nanoelectromechanical systems (NEMS) are nanofabricated and tested. In-situ scanning electron microscopy measurements of the deflection of the cantilever under electrostatic actuation are reported. In particular, a cantilever nanotube suspended over an electrode (nanoswitch), or two symmetric cantilever nanotubes (nanotweezers), from which a differential in electrical potential is imposed, are studied. The finite deformation regime investigated here is the first of its kind. An analytical model based on the energy method in both small deformation and finite kinematics (large deformation) regimes is used to interpret the measurements. The theory overcomes limitations of prior analysis reported in the literature towards the prediction of the structural behavior of NEMS. Some of the simplifying hypotheses have been removed. Furthermore, the theory takes into account the cylindrical shape of the deflected nanotube in the evaluation of its electrical capacitance, the influence of the van der Waals forces as well as finite kinematics. In addition, tip charge concentration and a quantum correction of the electrical capacitance are also considered. The energy-based method is used to predict the structural behavior and instability of the nanotube, corresponding to the on/off states of the nanoswitch, or to the open/close states of the nanotweezers—at the so-called pull-in voltage. Accuracy of the derived formulas is assessed by comparison of the theoretical prediction and experimental data in both small deformation and finite kinematics regimes. The results reported in this work are particularly useful in the characterization of the electromechanical properties of nanotubes as well as in the optimal design of nanotube-based NEMS devices.

189 citations


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01 May 1993
TL;DR: Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems.
Abstract: Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

29,323 citations

Journal ArticleDOI
01 Feb 2013-Science
TL;DR: Although not yet providing compelling mechanical strength or electrical or thermal conductivities for many applications, CNT yarns and sheets already have promising performance for applications including supercapacitors, actuators, and lightweight electromagnetic shields.
Abstract: Worldwide commercial interest in carbon nanotubes (CNTs) is reflected in a production capacity that presently exceeds several thousand tons per year. Currently, bulk CNT powders are incorporated in diverse commercial products ranging from rechargeable batteries, automotive parts, and sporting goods to boat hulls and water filters. Advances in CNT synthesis, purification, and chemical modification are enabling integration of CNTs in thin-film electronics and large-area coatings. Although not yet providing compelling mechanical strength or electrical or thermal conductivities for many applications, CNT yarns and sheets already have promising performance for applications including supercapacitors, actuators, and lightweight electromagnetic shields.

4,596 citations

Journal ArticleDOI
TL;DR: With the discovery of hexagonal boron nitride as an ideal dielectric, the materials are now in place to advance integrated flexible nanoelectronics, which uniquely take advantage of the unmatched portfolio of properties of two-dimensional crystals, beyond the capability of conventional thin films for ubiquitous flexible systems.
Abstract: The unique electrical, mechanical and physical properties of two-dimensional materials make them attractive candidates in flexible nanoelectronic systems. Here Akinwande et al. review the literature on two-dimensional materials in flexible nanoelectronics, and highlight barriers to their full implementation.

1,575 citations

Journal ArticleDOI
TL;DR: This work demonstrates the vertical and lateral integration of ZnO nanowires into arrays that are capable of producing sufficient power to operate real devices and uses the vertically integrated nanogenerator to power a nanowire pH sensor and a Nanowire UV sensor, thus demonstrating a self-powered system composed entirely of nanowiring.
Abstract: The lateral and vertical integration of ZnO piezoelectric nanowires allows for voltage and power outputs sufficient to power nanowire-based sensors.

1,465 citations

Journal ArticleDOI
TL;DR: A comprehensive overview of the progress that has been made within the context of 1D ZnO nanostructures synthesized via wet chemical methods can be found in this paper, where the synthetic methodologies and corresponding growth mechanisms, different structures, doping and alloying, position-controlled growth on substrates, and finally, their functional properties as catalysts, hydrophobic surfaces, sensors, and in nanoelectronic, optical, optoelectronics, and energy harvesting devices.
Abstract: One-dimensional (1D) ZnO nanostructures have been studied intensively and extensively over the last decade not only for their remarkable chemical and physical properties, but also for their current and future diverse technological applications. This article gives a comprehensive overview of the progress that has been made within the context of 1D ZnO nanostructures synthesized via wet chemical methods. We will cover the synthetic methodologies and corresponding growth mechanisms, different structures, doping and alloying, position-controlled growth on substrates, and finally, their functional properties as catalysts, hydrophobic surfaces, sensors, and in nanoelectronic, optical, optoelectronic, and energy harvesting devices.

1,247 citations