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ABSTRACT: The nanomechanical properties, deformation, and shape recovery mechanism of NiTi nanowires (NWs) under torsion are studied using molecular dynamics simulations. The effects of loading rate, aspect ratio of NWs, and NW shape are evaluated in terms of atomic trajectories, potential energy, torque required for deformation, stress, shear modulus, centro-symmetry parameter, and radial distribution function. Simulation results show that dislocation nucleation starts from the surface and then extends to the interior along the {110} close-packed plane. For a high loading rate, the occurrence of torsional buckling of a NW is faster, and the buckling gradually develops near the location of the applied external loading. The critical torsional angle and critical buckling angle increase with aspect ratio of the NWs. Square NWs have better mechanical strength than that of circular NWs due to the effect of shape. Shape recovery naturally occurs before buckling.
Journal of Molecular Modeling 01/2013; · 1.80 Impact Factor
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ABSTRACT: The nanotribology of an alkanethiol self-assembled monolayer (SAM) under tilt contact with a scanning probe tip is studied using molecular dynamics (MD) simulations. The tilt contact is described in terms of the tilt angle and the magnitude of the specimen-tip separation. The effects of tilt angle and magnitude of the specimen-tip separation on the normal force, friction force, friction coefficient, shear strength of the tip-SAM junction, and self-recovery characteristics are evaluated during the scanning probe tip process at a temperature of 300K. The simulation results clearly show that the magnitudes and periods of the normal force and friction force increase with decreasing magnitude of the specimen-tip separation due to a large change of the tilt angle of the SAM chains during the deformation and recovery stages. For scanning and indentation processes, the effect of the tilt angle of the probe tip on the normal force is more significant than that on the friction force for the SAM. The behaviors of interfacial contact forces, friction coefficient, and shear strength strongly depend on the number of interacting atoms and the contact area, which increases with decreasing magnitude of the specimen-tip separation and increasing tilt angle of the probe tip. The self-recovery of SAM is significantly affected by the magnitude of the specimen-tip separation; the recovery ability of SAM is worse for magnitude of the specimen-tip separation below -0.9nm with a large tilt angle of the probe tip.
Micron 09/2012; · 1.53 Impact Factor
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ABSTRACT: Two-dimensional dip-pen nanolithography (DPN) combined with laser-assisted heating is studied using molecular dynamics (MDs) simulations. The effects of humidity, deposition temperature, heating rate (laser-assisted patterning), and cooling rate on ink molecules are evaluated in terms of molecular transference, alkanethiol meniscus characteristics, surface binding energy, number of transferred chains, pattern characteristics, and the diffusion coefficient of ink molecules. The simulation results clearly show that the number of molecules transferred significantly increases with increasing humidity, which leads to increases in meniscus size and pattern size. The surface binding energy decreases and the diffusion coefficient of ink molecules increases with increasing humidity and deposition temperature. The dwell stage has the largest number of molecules transferred and the largest diffusion distance of ink molecules. The number of vaporous water molecules increases when the temperature is above 300 K, which limits meniscus growth and leads to unstable deposition. The DPN transfer efficiency can be significantly enhanced by increasing the laser pulse energy/heating rate. The transfer efficiency improves as the system humidity increases to saturation (374 water molecules).
Journal of Colloid and Interface Science 04/2012; 372(1):170-5. · 3.07 Impact Factor
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ABSTRACT: The elasticity and nanomechanical response of Aspergillus niger spores determined using atomic force microscopy (AFM) and nanoindentation are discussed. The force-displacement curve of the spore surfaces shows that the average surface roughness of spores was approximately 33 nm and that the adhesion force ranged from 9 to 28 nN. The Young's modulus of the A. niger spores ranged from 0.1 to 21.4 GPa and the hardness ranged from 0.01 to 0.17 GPa. The critical buckling load of the spore membrane is 290 μN.
Micron 10/2011; 43(2-3):407-11. · 1.53 Impact Factor
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ABSTRACT: The two important controllable parameters in the dip-pen nanolithography (DPN) process, such as writing temperature and velocity, are used to investigate the related effects on mechanisms of transference of alkanethiol self-assembled monolayer (SAM), transfer number, gasification number, and nanowire formation using molecular dynamics simulations. The simulated results show that the molecular transport ability during the direct-write process from the tip to the substrate is dependent on writing temperature and velocity, because the molecules have high kinetic energy and undergo fast diffusion when the temperature is increased; high transfer ability occurs at a slow writing velocity. The nanowire thickness and length increase significantly with increasing writing temperature, and its length increases much faster than its thickness (height). When the writing temperature is increased, transfer number and gasification number of the molecules become dramatically larger. The transfer number of ink molecules increases with decreasing writing velocity.
Current Nanoscience 09/2011; 7(5):830-837. · 1.78 Impact Factor
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ABSTRACT: The pattern transfer mechanism of an alkanethiol self-assembled monolayer (SAM) on various rough surfaces during the dip-pen nanolithography (DPN) process and pattern characterizations are studied using molecular dynamics (MD) simulations. The mechanisms of molecular transference, alkanethiol meniscus characteristics, surface adsorbed energy, number of molecular transfer, contact angle and pattern characteristics are evaluated during the DPN process at room temperature. The simulation results clearly show that the molecular transfer ability in DPN is optimum for deposition on a smooth surface, because surface defects create a potential diffusion barrier for the control of the spreading of excess ink molecules. The adsorbed area of SAMs, number of molecular transfer and pattern size are significantly inversely proportional to the degrees of roughness of a substrate. The adsorbed area of SAMs is increased by the pull-off process and the growth rate of adsorbed area is about 11–38%. The effect of surface roughness on the DPN process can be decreased by increasing the indentation depth of a tip.
Modelling and Simulation in Materials Science and Engineering 08/2011; 19(6):065008. · 2.30 Impact Factor
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ABSTRACT: The pattern transfer mechanism of an alkanethiol self-assembled monolayer (SAM) with different chain lengths during the dip-pen nanolithography (DPN) process and pattern characterizations are studied using molecular dynamics (MD) simulations. The mechanisms of molecular transference, alkanethiol meniscus characteristics, surface adsorbed energy, transfer number, and pattern formation are evaluated during the DPN process at room temperature. The simulation results clearly show that the molecular transfer ability in DPN is strongly dependent on the chain length. Shorter molecules have significantly better transport and diffusion abilities between the meniscus and substrate surface, and the transport period can be maintained longer. The magnitude of adsorbed energy increases with chain length, so many more molecules can be transferred to the surface when shorter molecules are used. After deposition, the magnitude of the adsorbed area and pattern height decrease with increasing chain length.
Journal of Colloid and Interface Science 05/2011; 361(1):316-20. · 3.07 Impact Factor
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ABSTRACT: The temperature-dependent mechanical properties of polyethylene terephthalate (PET) polymers are investigated using force-distance curves, adhesion force, and atomic force microscope (AFM) nanolithography combined the heating techniques. The results show that the width of grooves on the polymers at 20-60 °C were in the range of 14-363 nm. The wear depth of the polymers increased with increasing heating temperature. A volume of 251.85-2422.66 μm(3) at a load of 30-50 nN with heating to 30-60 °C was removed, as compared to that of 26.60-70.30 μm(3) obtained at room temperature. The contact forces of PET started increasing at 9 nN, whereas the size of the holes was average at a pressure. The results may be of importance in explaining the heating relationship among adhesion force, volume removal rate, and pressure.
Micron 02/2011; 42(5):492-7. · 1.53 Impact Factor
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ABSTRACT: Molecular dynamics simulations are used to investigate the mechanisms of molecular transference, pattern formation, and mechanical behavior in the dip-pen nanolithography (DPN) process. The effects of deposition temperature were studied using molecular trajectories, the meniscus characteristic, surface absorbed energy, and pattern formation analysis. At the first transferred stage (at the initial indentation depth), the conformation of SAM molecules lies almost on the substrate surface. The molecules start to stand on the substrate due to the pull and drag forces at the second transferred stage (after the tip is pulled up). According to the absorbed energy behavior, the second transferred stage has larger transferred amounts and the transfer rate is strongly related to temperature. When molecules were deposited at low temperature (e.g., room temperature), the pattern shape was more highly concentrated. The pattern shape at high temperatures expanded and the area increased because of good molecular diffusion.
Langmuir 10/2009; 26(5):3237-41. · 4.19 Impact Factor
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ABSTRACT: A multiscale particle method for coupling continuum and molecular models is described. In this method, the continuum model was assumed to be in a lattice form and can be applied in noncharacteristic areas or far away regions from the large deformations to save computational time. Defining a series of critical energies for different lattice sizes is convenient for lattice refinement. In the thermal equilibrium case, the efficiency is around 6 times higher than that of a classical molecular dynamics (MD) simulation; in addition, great numerical precision is achieved. To test the connection at the molecular/continuum interface, a large deformation case and a surface friction case were studied in the nanocontact and the nanosliding processes, respectively. The results were compared with the MD simulation and showed great precision. The deviation could be further reduced through a moderate adjustment of critical energies on the lattices, showing that this method is a seamless treatment technology.
Journal of Nanoscience and Nanotechnology 06/2009; 9(5):3295-300. · 1.56 Impact Factor
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ABSTRACT: The multiscale method has become a promising tool that could make a significant contribution to the fields of nano-science and technology. The objective of the multiscale method makes it possible to treat ever larger systems.Current study proposes a new model that combines the atomistic and continuum approaches. The continuum model is built on the lattice view and its critical deformation extent is defined for refinement (broken down into smaller lattices or atoms). Both the lattice and atoms can be calculated together using atomistic and coarse potentials without a calculation interface. This coarse particle model was applied to a nanocontact study and the results were compared to a traditional contact model.
IEEE Nanotechnology Magazine 04/2009;
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ABSTRACT: Molecular dynamics simulations using tight-binding many body potential are carried out to study the roller imprint process of a gold single crystal. The effect of the roller tooth's taper angle, imprint depth, imprint temperature, and imprint direction on the imprint force, adhesion, stress distribution, and strain are investigated. A two-stage roller imprint process was obtained from an imprint force curve. The two-stage imprint process included the imprint forming with a rapid increase of imprint force and the unloading stage combined with the adhesion stage. The results show that the imprint force and adhesion rapidly increase with decreasing taper angle and increasing imprint depth. The magnitude of the maximum imprint force and the time at which this maximum occurs are proportional to the imprint depth, but independent of the taper angle. In a comparison of the imprint mechanisms with a vertical imprint case, while high stress and strain regions are concentrated below the mold for vertical imprint, they also occur around the mold in the case of roller imprint. The regions were only concentrated on the substrate atoms underneath the mold in vertical imprint. Plastic flow increased with increasing imprint temperature.
Nanoscale Research Letters 01/2009; 4(8):913-920. · 2.73 Impact Factor
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ABSTRACT: A multiscale particle method for coupling continuum and molecular models is described. In this method, the continuum model
was assumed to be a lattice form and can be applied in non-characteristic areas or far-away regions from the large deformations
to save computational time. Defining a series of critical energies for different lattice sizes is convenient for lattice refinement.
In the thermal equilibrium case, the efficiency is around six times higher than that of a classical molecular dynamics (MD)
simulation; in addition, great numerical precision is achieved. To test the connection at the molecular/continuum interface,
a large-deformation case was studied in a nanoimprinting process. The results were compared with the MD simulation and it
was found that the deviation could be reduced through a moderate adjustment of the critical energy in the lattices. This is
good evidence that this method is a seamless treatment technology.
Applied Physics A 01/2008; 91(2):273-279. · 1.63 Impact Factor
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ABSTRACT: The frictional and indented behavior of a diamond asperity on a diamond plate was carried out using a molecular dynamics (MD) and experiments. The contact load, contact area, dynamic frictional force, and dynamic frictional coefficient increased as the contact interference increased at a constant loading velocity. The microcontact and frictional behavior can be evaluated between a rigid smooth hemisphere to a deformable rough flat plane by combined the deformed behavior of the asperity obtained from MD results with the fractal and statistic parameters. The comparison and the discrepancy of simulated results and nanoindentation and scratching experimental results will be discussed.
Current Applied Physics.
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ABSTRACT: The mechanical characteristics of a single-walled carbon nanotube (SWCNT) filled with C60 fullerene subject to nanoindentation is studied using molecular dynamics (MD) simulations. The effects of temperature, indentation velocity, adhesion, and tip sizes were evaluated. The simulated results clearly show that the exerted load, Young’s modulus, elastic energy, and plastic energy decrease significantly with increasing temperature and decreasing indentation velocity and tip size. C60 fullerenes can effectively increase the mechanical strength of a SWCNT because they act as a “barrier” to resist the radial deformation, as well as an inner wall in a double-walled carbon nanotube. With the same indentation depth, the ratio of elastic energy to plastic energy for a material gradually increases with the increase in the radius of the tip. This indicates that the elastic recovery of a material is better when the tip has a larger radius.
Carbon. 49(6):2053-2061.
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ABSTRACT: The frictional mechanisms of a self-assembled monolayer (SAM) during nano-scale sliding are studied using molecular dynamics (MD) simulation. The MD model consists of a gold slider and gold substrate with n-hexadecanethiol SAM chemisorbed to the substrate. The trajectory, tilt angles, normal forces, frictional forces, friction coefficients and potential energies per molecular chain of the SAM molecules are evaluated during the frictional process for various parameters including as sliding height, sliding direction (i.e. pro- or anti- the SAM tilt angle), sliding velocity and system temperature. The various parameters are discussed with regard to frictional forces, mechanisms and SAM structural transition. Results show that stick-slip occurs and is related to the sliding period and tilt angle of the SAM molecules. Amplitude of the stick-slip cycle increases with decreasing sliding height until reaching a critical sliding height, which is characterized such that sliding below the critical height causes irreversible changes in the SAM molecular organization and cumulative loss of SAM lubricating efficiency. Different SAM recovery mechanisms were found for different sliding directions relative to SAM tilt angle (pro- or anti-tilt). In both cases, minimum friction occurred during the SAM tilt-angle recovery phase. The friction force curves for these two cases also showed a regular phase shift above the critical height. For stick-slip sliding above the critical height, anti-tilt sliding had significantly lower average friction, but this trend inverted below the critical height. Sliding lower than the critical height cause progressive disorder of the SAM structure and the characteristic differences between pro- and anti-tilt sliding were progressively lost.
Computational Materials Science. 39(4):808-816.
