Publications by authors named "Leonid V Zhigilei"

23 Publications

  • Page 1 of 1

Correction: The effect of pulse duration on nanoparticle generation in pulsed laser ablation in liquids: insights from large-scale atomistic simulations.

Phys Chem Chem Phys 2020 Jul 2;22(27):15769. Epub 2020 Jul 2.

Department of Materials Science and Engineering, University of Virginia, 395 McCormick Road, Charlottesville, Virginia 22904-4745, USA.

Correction for 'The effect of pulse duration on nanoparticle generation in pulsed laser ablation in liquids: insights from large-scale atomistic simulations' by Cheng-Yu Shih et al., Phys. Chem. Chem. Phys., 2020, 22, 7077-7099, DOI: .
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http://dx.doi.org/10.1039/d0cp90142cDOI Listing
July 2020

Graphene reinforced carbon fibers.

Sci Adv 2020 Apr 24;6(17):eaaz4191. Epub 2020 Apr 24.

Department of Mechanical and Aerospace Engineering, University of Virginia, 122 Engineer's Way, Charlottesville, VA 22904, USA.

The superlative strength-to-weight ratio of carbon fibers (CFs) can substantially reduce vehicle weight and improve energy efficiency. However, most CFs are derived from costly polyacrylonitrile (PAN), which limits their widespread adoption in the automotive industry. Extensive efforts to produce CFs from low cost, alternative precursor materials have failed to yield a commercially viable product. Here, we revisit PAN to study its conversion chemistry and microstructure evolution, which might provide clues for the design of low-cost CFs. We demonstrate that a small amount of graphene can minimize porosity/defects and reinforce PAN-based CFs. Our experimental results show that 0.075 weight % graphene-reinforced PAN/graphene composite CFs exhibits 225% increase in strength and 184% enhancement in Young's modulus compared to PAN CFs. Atomistic ReaxFF and large-scale molecular dynamics simulations jointly elucidate the ability of graphene to modify the microstructure by promoting favorable edge chemistry and polymer chain alignment.
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http://dx.doi.org/10.1126/sciadv.aaz4191DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7182419PMC
April 2020

Effect of a liquid environment on single-pulse generation of laser induced periodic surface structures and nanoparticles.

Nanoscale 2020 Apr 24;12(14):7674-7687. Epub 2020 Mar 24.

Department of Materials Science and Engineering, University of Virginia, 395 McCormick Road, Charlottesville, Virginia 22904-4745, USA.

The effect of a liquid environment on the fundamental mechanisms of surface nanostructuring and generation of nanoparticles by single pulse laser ablation is investigated in a closely integrated computational and experimental study. A large-scale molecular dynamics simulation of spatially modulated ablation of Cr in water reveals a complex picture of the dynamic interaction between the ablation plume and water. Ablation plume is found to be rapidly decelerated by the water environment, resulting the formation and prompt disintegration of a hot metal layer at the interface between the ablation and water. A major fraction of the ablation plume is laterally redistributed and redeposited back to the target, forming smooth frozen surface features. Good agreement between the shapes of the surface features predicted in the simulation and the ones generated in single pulse laser ablation experiments performed for Cr in water supports the mechanistic insights revealed in the simulation. The results of this study suggest that the presence of a liquid environment can eliminate the sharp features of the surface morphology, reduce the amount of the material removed from the target by more than an order of magnitude, and narrow down the nanoparticle size distribution as compared to laser ablation under vacuum. Moreover, the computational predictions of the effective incorporation of molecules constituting the liquid environment into the surface region of the irradiated target and the generation of high vacancy concentrations, exceeding the equilibrium levels by more than an order of magnitude, suggest a potential for hyperdoping of laser-generated surfaces by solutes present in the liquid environment.
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http://dx.doi.org/10.1039/d0nr00269kDOI Listing
April 2020

The effect of pulse duration on nanoparticle generation in pulsed laser ablation in liquids: insights from large-scale atomistic simulations.

Phys Chem Chem Phys 2020 Apr;22(13):7077-7099

Department of Materials Science and Engineering, University of Virginia, 395 McCormick Road, Charlottesville, Virginia 22904-4745, USA.

The generation of colloidal solutions of chemically clean nanoparticles through pulsed laser ablation in liquids (PLAL) has evolved into a thriving research field that impacts industrial applications. The complexity and multiscale nature of PLAL make it difficult to untangle the various processes involved in the generation of nanoparticles and establish the dependence of nanoparticle yield and size distribution on the irradiation parameters. Large-scale atomistic simulations have yielded important insights into the fundamental mechanisms of ultrashort (femtoseconds to tens of picoseconds) PLAL and provided a plausible explanation of the origin of the experimentally observed bimodal nanoparticle size distributions. In this paper, we extend the atomistic simulations to short (hundreds of picoseconds to nanoseconds) laser pulses and focus our attention on the effect of the pulse duration on the mechanisms responsible for the generation of nanoparticles at the initial dynamic stage of laser ablation. Three distinct nanoparticle generation mechanisms operating at different stages of the ablation process and in different parts of the emerging cavitation bubble are identified in the simulations. These mechanisms are (1) the formation of a thin transient metal layer at the interface between the ablation plume and water environment followed by its decomposition into large molten nanoparticles, (2) the nucleation, growth, and rapid cooling/solidification of small nanoparticles at the very front of the emerging cavitation bubble, above the transient interfacial metal layer, and (3) the spinodal decomposition of a part of the ablation plume located below the transient interfacial layer, leading to the formation of a large population of nanoparticles growing in a high-temperature environment through inter-particle collisions and coalescence. The coexistence of the three distinct mechanisms of the nanoparticle formation at the initial stage of the ablation process can be related to the broad nanoparticle size distributions commonly observed in nanosecond PLAL experiments. The strong dependence of the nanoparticle cooling and solidification rates on the location within the low-density metal-water mixing region has important implications for the long-term evolution of the nanoparticle size distribution, as well as for the ability to quench the nanoparticle growth or dope them by adding surface-active agents or doping elements to the liquid environment.
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http://dx.doi.org/10.1039/d0cp00608dDOI Listing
April 2020

Unveiling Carbon Ring Structure Formation Mechanisms in Polyacrylonitrile-Derived Carbon Fibers.

ACS Appl Mater Interfaces 2019 Nov 1;11(45):42288-42297. Epub 2019 Nov 1.

Department of Mechanical and Aerospace Engineering , University of Virginia , 122 Engineer's Way , Charlottesville , Virginia 22904 , United States.

As the demand for electric vehicles (EVs) and autonomous vehicles (AVs) rapidly grows, lower-cost, lighter, and stronger carbon fibers (CFs) are urgently needed to respond to consumers' call for greater EV traveling range and stronger safety structures for AVs. Converting polymeric precursors to CFs requires a complex set of thermochemical processes; a systematic understanding of each parameter in fiber conversion is still, to a large extent, lacking. Here, we demonstrate the effect of carbonization temperature on carbon ring structure formation by combining atomistic/microscale simulations and experimental validation. Experimental testing, as predicted by simulations, exhibited that the strength and ductility of PAN CFs decreased, whereas the Young's modulus increased with increasing carbonization temperature. Our simulations unveiled that high carbonization temperature accelerated the kinetics of graphitic phase nucleation and growth, leading to the decrease in strength and ductility but increase in modulus. The methodology presented herein using combined atomistic/microscale simulations and experimental validation lays a firm foundation for further innovation in CF manufacturing and low-cost alternative precursor development.
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http://dx.doi.org/10.1021/acsami.9b15833DOI Listing
November 2019

Femtosecond laser generation of microbumps and nanojets on single and bilayer Cu/Ag thin films.

Phys Chem Chem Phys 2019 Jun;21(22):11846-11860

University of Vienna, Department of Physical Chemistry, Vienna, Austria.

The formation mechanisms of microbumps and nanojets on films composed of single and double Cu/Ag layers deposited on a glass substrate and irradiated by a single 60 fs laser pulse are investigated experimentally and in atomistic simulations. The composition of the laser-modified bilayers is probed with the energy dispersive X-ray spectroscopy and used as a marker for processes responsible for the modification of the film morphology. For the bilayer with the top Ag layer facing the laser, the increase in fluence is found to result in a sequential appearance of a Ag microbump, the exposure of the Cu underlayer by removal of the Ag layer, a Cu microbump, and a frozen nanojet. The Cu on Ag bilayer exhibits a partial spallation of the top Cu film, followed by the generation of surface structures that mainly consist of Ag at higher fluences. The experimental observations are explained with atomistic simulations, which reveal that the stronger electron-phonon coupling of Cu results in the confinement of the deposited laser energy in the top Cu layer in the Cu on Ag case and channelling of the energy from the top Ag layer to the underlying Cu layer in the Ag on Cu case. This difference in the energy (re)distribution directly translates into differences in the morphology of the laser-modified bilayers. In all systems, the generation of microbumps and nanojets occurs in the molten state. It is driven by the dynamic relaxation of the laser-induced stresses and, at higher fluences, the release of vapor at the interface with the substrate. The resistance of the colder periphery of the laser spot to the ejection of spalled layers as well as the rapid solidification of the transient molten structures are largely defining the final shapes of the surface structures.
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http://dx.doi.org/10.1039/c9cp02174dDOI Listing
June 2019

Laser-Rewriteable Ferromagnetism at Thin-Film Surfaces.

ACS Appl Mater Interfaces 2018 May 17;10(17):15232-15239. Epub 2018 Apr 17.

Helmholtz-Zentrum Dresden-Rossendorf, Institut für Ionenstrahlphysik und Materialforschung , Bautzner Landstrasse 400 , D-01328 Dresden , Germany.

Manipulation of magnetism using laser light is considered as a key to the advancement of data storage technologies. Until now, most approaches seek to optically switch the direction of magnetization rather than to reversibly manipulate the ferromagnetism itself. Here, we use ∼100 fs laser pulses to reversibly switch ferromagnetic ordering on and off by exploiting a chemical order-disorder phase transition in FeAl, from the B2 to the A2 structure and vice versa. A single laser pulse above a threshold fluence causes nonferromagnetic B2 FeAl to disorder and form the ferromagnetic A2 structure. Subsequent laser pulsing below the threshold reverses the surface to B2 FeAl, erasing the laser-induced ferromagnetism. Simulations reveal that the order-disorder transition is regulated by the extent of surface supercooling; above the threshold for complete melting throughout the film thickness, the liquid phase can be deeply undercooled before solidification. As a result, the vacancy diffusion in the resolidified region is limited and the region is trapped in the metastable chemically disordered state. Laser pulsing below the threshold forms a limited supercooled surface region that solidifies at sufficiently high temperatures, enabling diffusion-assisted reordering. This demonstrates that ultrafast lasers can achieve subtle atomic rearrangements in bimetallic alloys in a reversible and nonvolatile fashion.
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http://dx.doi.org/10.1021/acsami.8b01190DOI Listing
May 2018

Two mechanisms of nanoparticle generation in picosecond laser ablation in liquids: the origin of the bimodal size distribution.

Nanoscale 2018 Apr;10(15):6900-6910

Department of Materials Science and Engineering, University of Virginia, 395 McCormick Road, Charlottesville, Virginia 22904-4745, USA.

The synthesis of chemically clean and environmentally friendly nanoparticles through pulsed laser ablation in liquids has shown a number of advantages over conventional chemical synthesis methods and has evolved into a thriving research field attracting laboratory and industrial applications. The fundamental understanding of processes leading to the nanoparticle generation, however, still remains elusive. In particular, the origin of bimodal nanoparticle size distributions in femto- and picosecond laser ablation in liquids, where small nanoparticles (several nanometers) with narrow size distribution are commonly observed to coexist with larger (tens to hundreds of nanometers) ones, has not been explained so far. In this paper, joint computational and experimental efforts are applied to understand the mechanisms of nanoparticle formation in picosecond laser ablation in liquids and to explain the bimodal nanoparticle size distributions. The results of a large-scale atomistic simulation reveal the critical role of the dynamic interaction between the ablation plume and the liquid environment, leading to the generation of large nanoparticles through a sequence of hydrodynamic instabilities at the plume-liquid interface and a concurrent nucleation and growth of small nanoparticles in an expanding metal-liquid mixing region. The computational predictions are supported by a series of stroboscopic videography experiments showing the emergence of small satellite bubbles surrounding the main cavitation bubble generated in single pulse experiments. Carefully timed double pulse irradiation triggers expansion of secondary cavitation bubbles indicating, in accord with the simulation results, the presence of localized sites of laser energy deposition (possibly large nanoparticles) injected into the liquid at the early stage of the bubble formation.
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http://dx.doi.org/10.1039/C7NR08614HDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6637654PMC
April 2018

Generation of Subsurface Voids, Incubation Effect, and Formation of Nanoparticles in Short Pulse Laser Interactions with Bulk Metal Targets in Liquid: Molecular Dynamics Study.

J Phys Chem C Nanomater Interfaces 2017 Aug 1;121(30):16549-16567. Epub 2017 Jun 1.

Department of Materials Science and Engineering, University of Virginia, 395 McCormick Road, Charlottesville, Virginia 22904-4745, United States.

The ability of short pulse laser ablation in liquids to produce clean colloidal nanoparticles and unusual surface morphology has been employed in a broad range of practical applications. In this paper, we report the results of large-scale molecular dynamics simulations aimed at revealing the key processes that control the surface morphology and nanoparticle size distributions by pulsed laser ablation in liquids. The simulations of bulk Ag targets irradiated in water are performed with an advanced computational model combining a coarse-grained representation of liquid environment and an atomistic description of laser interaction with metal targets. For the irradiation conditions that correspond to the spallation regime in vacuum, the simulations predict that the water environment can prevent the complete separation of the spalled layer from the target, leading to the formation of large subsurface voids stabilized by rapid cooling and solidification. The subsequent irradiation of the laser-modified surface is found to result in a more efficient ablation and nanoparticle generation, thus suggesting the possibility of the incubation effect in multipulse laser ablation in liquids. The simulations performed at higher laser fluences that correspond to the phase explosion regime in vacuum reveal the accumulation of the ablation plume at the interface with the water environment and the formation of a hot metal layer. The water in contact with the metal layer is brought to the supercritical state and provides an environment suitable for nucleation and growth of small metal nanoparticles from metal atoms emitted from the hot metal layer. The metal layer itself has limited stability and can readily disintegrate into large (tens of nanometers) nanoparticles. The layer disintegration is facilitated by the Rayleigh-Taylor instability of the interface between the higher density metal layer decelerated by the pressure from the lighter supercritical water. The nanoparticles emerging from the layer disintegration are rapidly cooled and solidified due to the interaction with water environment, with a cooling rate of ∼2 × 10 K/s observed in the simulations. The computational prediction of two distinct mechanisms of nanoparticle formation yielding nanoparticles with different characteristic sizes provides a plausible explanation for the experimental observations of bimodal nanoparticle size distributions in laser ablation in liquids. The ultrahigh cooling and solidification rates suggest the possibility for generation of nanoparticles featuring metastable phases and highly nonequilibrium structures.
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http://dx.doi.org/10.1021/acs.jpcc.7b02301DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5545760PMC
August 2017

Atomistic modeling of nanoparticle generation in short pulse laser ablation of thin metal films in water.

J Colloid Interface Sci 2017 Mar 15;489:3-17. Epub 2016 Oct 15.

Department of Materials Science and Engineering, University of Virginia, 395 McCormick Road, Charlottesville, VA 22904-4745, USA. Electronic address:

Laser ablation in liquids is actively used for generation of clean colloidal nanoparticles with unique shapes and functionalities. The fundamental mechanisms of the laser ablation in liquids and the key processes that control the nanoparticle structure, composition, and size distribution, however, are not yet fully understood. In this paper, we report the results of first atomistic simulations of laser ablation of metal targets in liquid environment. A model combining a coarse-grained representation of the liquid environment (parameterized for water), a fully atomistic description of laser interactions with metal targets, and acoustic impedance matching boundary conditions is developed and applied for simulation of laser ablation of a thin silver film deposited on a silica substrate. The simulations, performed at two laser fluences in the regime of phase explosion, predict a rapid deceleration of the ejected ablation plume and the formation of a dense superheated molten layer at the water-plume interface. The water in contact with the hot metal layer is brought to the supercritical state and transforms into an expanding low density metal-water mixing region that serves as a precursor for the formation of a cavitation bubble. Two distinct mechanisms of the nanoparticle formation are predicted in the simulations: (1) the nucleation and growth of small (mostly ⩽10nm) nanoparticles in the metal-water mixing region and (2) the formation of larger (tens of nm) nanoparticles through the breakup of the superheated molten metal layer triggered by the emergence of complex morphological features attributed to the Rayleigh-Taylor instability of the interface between at the superheated metal layer and the supercritical water. The first mechanism is facilitated by the rapid cooling of the growing nanoparticles in the supercritical water environment, resulting in solidification of the nanoparticles located in the upper part of the mixing region on the timescale of nanoseconds. The computational prediction of the two mechanisms of nanoparticle formation yielding nanoparticles with different characteristic sizes is consistent with experimental observations of two distinct nanoparticle populations appearing at different stages of the ablation process.
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http://dx.doi.org/10.1016/j.jcis.2016.10.029DOI Listing
March 2017

Growth Twinning and Generation of High-Frequency Surface Nanostructures in Ultrafast Laser-Induced Transient Melting and Resolidification.

ACS Nano 2016 07 12;10(7):6995-7007. Epub 2016 Jul 12.

Univ Lyon, UJM-Saint-Etienne, CNRS, IOGS, Laboratoire Hubert Curien UMR5516, F-42023 St-Etienne, France.

The structural changes generated in surface regions of single crystal Ni targets by femtosecond laser irradiation are investigated experimentally and computationally for laser fluences that, in the multipulse irradiation regime, produce sub-100 nm high spatial frequency surface structures. Detailed experimental characterization of the irradiated targets combining electron back scattered diffraction analysis with high-resolution transmission electron microscopy reveals the presence of multiple nanoscale twinned domains in the irradiated surface regions of single crystal targets with (111) surface orientation. Atomistic- and continuum-level simulations performed for experimental irradiation conditions reproduce the generation of twinned domains and establish the conditions leading to the formation of growth twin boundaries in the course of the fast transient melting and epitaxial regrowth of the surface regions of the irradiated targets. The observation of growth twins in the irradiated Ni(111) targets provides strong evidence of the role of surface melting and resolidification in the formation of high spatial frequency surface structures. This also suggests that the formation of twinned domains can be used as a sensitive measure of the levels of liquid undercooling achieved in short pulse laser processing of metals.
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http://dx.doi.org/10.1021/acsnano.6b02970DOI Listing
July 2016

The minimum amount of "matrix" needed for matrix-assisted pulsed laser deposition of biomolecules.

J Phys Chem B 2014 Nov 10;118(46):13290-9. Epub 2014 Nov 10.

Department of Materials Science and Engineering, University of Virginia , 395 McCormick Road, Charlottesville, Virginia 22904-4745, United States.

The ability of matrix-assisted pulsed laser evaporation (MAPLE) technique to transfer and deposit high-quality thin organic, bioorganic, and composite films with minimum chemical modification of the target material has been utilized in numerous applications. One of the outstanding problems in MAPLE film deposition, however, is the presence of residual solvent (matrix) codeposited with the polymer material and adversely affecting the quality of the deposited films. In this work, we investigate the possibility of alleviating this problem by reducing the amount of matrix in the target. A series of coarse-grained molecular dynamics simulations are performed for a model lysozyme-water system, where the water serves the role of volatile "matrix" that drives the ejection of the biomolecules. The simulations reveal a remarkable ability of a small (5-10 wt %) amount of matrix to cause the ejection of intact bioorganic molecules. The results obtained for different laser fluences and water concentrations are used to establish a "processing map" of the regimes of molecular ejection in matrix-assisted pulsed laser deposition. The computational predictions are supported by the experimental observation of the ejection of intact lysozyme molecules from pressed lysozyme targets containing small amounts of residual water. The results of this study suggest a new approach for deposition of thin films of bioorganic molecules with minimum chemical modification of the molecular structure and minimum involvement of solvent into the deposition process.
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http://dx.doi.org/10.1021/jp508284nDOI Listing
November 2014

What determines MALDI ion yields? A molecular dynamics study of ion loss mechanisms.

Anal Bioanal Chem 2012 Mar 3;402(8):2511-9. Epub 2011 Jul 3.

Tofwerk AG, Uttigenstrasse 22, 3600 Thun, Switzerland.

Ion recombination in matrix-assisted laser desorption/ionization (MALDI) is as important as any ion formation process in determining the quantity of ions observed but has received comparatively little attention. Molecular dynamics simulations are used here to investigate some models for recombination, including a Langevin-type model, a soft threshold model and a tunneling model. The latter was found to be superior due to its foundations in a widespread physical phenomenon, and its lack of excessive sensitivity to parameter choice. Tunneling recombination in the Marcus inverted region may be a major reason why MALDI is a viable analytical method, by allowing ion formation to exceed ion loss on the time scale of the plume expansion. Ion velocities, photoacoustic transients and pump-probe measurements might be used to investigate the role of recombination in different MALDI matrices, and to select new matrices.
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http://dx.doi.org/10.1007/s00216-011-5194-xDOI Listing
March 2012

Selective ablation of Xe from silicon surfaces: molecular dynamics simulations and experimental laser patterning.

J Phys Chem A 2011 Jun 22;115(23):6250-9. Epub 2011 Apr 22.

Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem 91904, Israel.

The mechanism of laser-induced removal of Xe overlayers from a Si substrate has been investigated employing MD simulations and evaluated by buffer layer assisted laser patterning experiments. Two distinct regimes of overlayer removal are identified in the simulations of a uniform heating of the Si substrate by a 5 ns laser pulse: The intensive evaporation from the surface of the Xe overlayer and the detachment of the entire Xe overlayer driven by explosive boiling in the vicinity of the hot substrate. Simulations of selective heating of only a fraction of the silicon substrate suggest that the lateral heat transfer and bonding to the unheated, colder regions of the Xe overlayer is very efficient and suppresses the separation of a fraction of the overlayer from the substrate. Interaction with surrounding cold Xe is responsible for significant increase in the substrate temperature required for achieving the spatially selective ablation of the overlayer. The predictions of the MD simulations are found to be in a qualitative agreement with the results of experimental measurements of the threshold laser power required for the removal of Xe overlayers of different thickness and the shapes of metallic stripes generated by buffer-assisted laser patterning.
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http://dx.doi.org/10.1021/jp111658wDOI Listing
June 2011

Structural stability of carbon nanotube films: the role of bending buckling.

ACS Nano 2010 Oct;4(10):6187-95

Department of Materials Science and Engineering, University of Virginia, 395 McCormick Road, Charlottesville, Virginia 22904-4745, United States.

In films, mats, buckypaper, and other materials composed of carbon nanotubes (CNTs), individual CNTs are bound together by van der Waals forces and form entangled networks of bundles. Mesoscopic dynamic simulations reproduce the spontaneous self-assembly of CNTs into continuous networks of bundles and reveal that the bending buckling and the length of CNTs are the two main factors responsible for the stability of the network structures formed by defect-free CNTs. Bending buckling of CNTs reduces the bending energy of interconnections between bundles and stabilizes the interconnections by creating effective barriers for CNT sliding. The length of the nanotubes is affecting the ability of van der Waals forces of intertube interactions to counterbalance the internal straightening forces acting on curved nanotubes present in the continuous networks. The critical length for the formation of stable network structures is found to be ∼120 nm for (10,10) single-walled CNTs. In the simulations where the bending buckling is artificially switched off, the network structures are found to be unstable against disintegration into individual bundles even for micrometer-long CNTs.
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http://dx.doi.org/10.1021/nn1015902DOI Listing
October 2010

Scaling laws and mesoscopic modeling of thermal conductivity in carbon nanotube materials.

Phys Rev Lett 2010 May 28;104(21):215902. Epub 2010 May 28.

University of Virginia, Department of Materials Science and Engineering, 395 McCormick Road, Charlottesville, Virginia 22904-4745, USA.

The scaling laws describing the thermal conductivity in random networks of straight conducting nanofibers are derived analytically and verified in numerical simulations. The applicability of the scaling laws to more complex structures of interconnected networks of bundles in carbon nanotube (CNT) films and mats is investigated in mesoscopic simulations. The heat transfer in CNT materials is found to be strongly enhanced by self-organization of CNTs into continuous networks of bundles. The thermal conductivity of CNT films varies by orders of magnitude depending on the length of the nanotubes and their structural arrangement in the material.
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http://dx.doi.org/10.1103/PhysRevLett.104.215902DOI Listing
May 2010

Molecular dynamics simulations of MALDI: laser fluence and pulse width dependence of plume characteristics and consequences for matrix and analyte ionization.

J Mass Spectrom 2010 Apr;45(4):333-46

Novartis Institutes for Biomedical Research, Tofwerk AG, Uttigenstrasse 22, 3600 Thun, Switzerland.

Molecular dynamics simulations of matrix-assisted laser desorption/ionization were carried out to investigate laser pulse width and fluence effects on primary and secondary ionization process. At the same fluence, short (35 or 350 ps) pulses lead to much higher initial pressures and ion concentrations than longer ones (3 ns), but these differences do not persist because the system relaxes toward local thermal equilibrium on a nanosecond timescale. Higher fluences accentuate the initial disparities, but downstream differences are not substantial. Axial velocities of ions and neutrals are found to span a wide range, and be fluence dependent. Total ion yield is only weakly dependent on pulse width, and consistent with experimental estimates. Secondary reactions of matrix cations with analyte neutrals are efficient even though analyte ions are ablated in clusters of matrix.
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http://dx.doi.org/10.1002/jms.1732DOI Listing
April 2010

Making molecular balloons in laser-induced explosive boiling of polymer solutions.

Phys Rev Lett 2007 May 22;98(21):216101. Epub 2007 May 22.

Department of Materials Science and Engineering, University of Virginia, 395 McCormick Road, Charlottesville, Virginia 22904-4745, USA.

The effect of the dynamic molecular rearrangements leading to compositional segregation is revealed in coarse-grained molecular dynamics simulations of short pulse laser interaction with a polymer solution in a volatile matrix. An internal release of matrix vapor at the onset of the explosive boiling of the overheated liquid is capable of pushing polymer molecules to the outskirts of a transient bubble, forming a polymer-rich surface layer enclosing the volatile matrix material. The results explain unexpected "deflated balloon" structures observed in films deposited by the matrix-assisted pulsed laser evaporation technique.
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http://dx.doi.org/10.1103/PhysRevLett.98.216101DOI Listing
May 2007

Kinetic limit of heterogeneous melting in metals.

Phys Rev Lett 2007 May 9;98(19):195701. Epub 2007 May 9.

Department of Materials Science and Engineering, University of Virginia, 395 McCormick Road, Charlottesville, VA 22904-4745, USA.

The velocity and nanoscale shape of the melting front are investigated in a model that combines the molecular dynamics method with a continuum description of the electron heat conduction and electron-phonon coupling. The velocity of the melting front is strongly affected by the local drop of the lattice temperature, defined by the kinetic balance between the transfer of thermal energy to the latent heat of melting, the electron heat conduction from the overheated solid, and the electron-phonon coupling. The maximum velocity of the melting front is found to be below 3% of the room temperature speed of sound in the crystal, suggesting a limited contribution of heterogeneous melting under conditions of fast heating.
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http://dx.doi.org/10.1103/PhysRevLett.98.195701DOI Listing
May 2007

Molecular dynamics model of ultraviolet matrix-assisted laser desorption/ionization including ionization processes.

J Phys Chem B 2005 Dec;109(48):22947-57

Novartis Institutes for Biomedical Research, 4056 Basel, Switzerland.

A molecular dynamics model of UV-MALDI including ionization processes is presented. In addition to the previously described breathing sphere approach developed for simulation of laser ablation/desorption of molecular systems, it includes radiative and nonradiative decay, exciton hopping, two pooling processes, and electron capture. The results confirm the main conclusions of the continuum model of Knochenmuss, Anal. Chem. 2003, 75, 2199, but provide a much more detailed description of the interaction between ablation/desorption and ionization processes in the critical early time regime. Both desorption and ablation regimes generate free ions, and yields are in accordance with experiment. The first molecular ions are emitted at high velocities shortly before neutral desorption begins, because of surface charging caused by electron escape from the top of the sample. Later ions are entrained and thermalized in the plume of neutral molecules and clusters. Clusters are found to be stable on a nanosecond time scale, so the ions in them will be released only slowly, if at all. Exciton hopping rate and the mean radius for ion recombination are shown to be key parameters that can have a significant effect on net ion yield.
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http://dx.doi.org/10.1021/jp052945eDOI Listing
December 2005

Limit of overheating and the threshold behavior in laser ablation.

Phys Rev E Stat Nonlin Soft Matter Phys 2003 Oct 1;68(4 Pt 1):041501. Epub 2003 Oct 1.

Department of Chemistry, 152 Davey Laboratory, Penn State University, University Park, Pennsylvania 16802, USA.

Constant temperature and pressure molecular-dynamics simulations in conjunction with constant pressure and enthalpy simulations, designed to examine the threshold behavior in laser ablation, demonstrate that the rate of homogeneous nucleation (explosive boiling) increases sharply in a very narrow temperature range at approximately 90% of the critical temperature. Moreover, the homogeneous nucleation is sufficiently rapid to prevent the superheated liquid from entering the spinodal region at densities greater than the critical density.
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http://dx.doi.org/10.1103/PhysRevE.68.041501DOI Listing
October 2003

Effect of pressure relaxation on the mechanisms of short-pulse laser melting.

Phys Rev Lett 2003 Sep 4;91(10):105701. Epub 2003 Sep 4.

Department of Materials Science & Engineering, University of Virginia, 116 Engineer's Way, Charlottesville, Virginia 22904-4745, USA.

The kinetics and microscopic mechanisms of laser melting of a thin metal film are investigated in a computational study that combines molecular dynamics simulations with a continuum description of the laser excitation and subsequent relaxation of the conduction band electrons. Two competing melting mechanisms, homogeneous nucleation of liquid regions inside the crystalline material and propagation of melting fronts from external surfaces, are found to be strongly affected by the dynamics of the relaxation of the laser-induced pressure.
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http://dx.doi.org/10.1103/PhysRevLett.91.105701DOI Listing
September 2003

Computer simulations of laser ablation of molecular substrates.

Chem Rev 2003 Feb;103(2):321-48

Department of Materials Science and Engineering, 116 Engineer's Way, University of Virginia, Charlottesville, Virginia 22904, USA.

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http://dx.doi.org/10.1021/cr010459rDOI Listing
February 2003