arXiv subjects

Shoaib Ahmad

Publications and source records attributed to Shoaib Ahmad.

At least 19 recordsLinked to original sources

arXiv2025

Experimental and Theoretical Aspects of the Fragmentation of Carbon's Single and Multi-Walled Nanotubes

Energetic ion irradiation is an effective method for studying how single and multi-shelled carbon nanotubes break apart. The energy from ions is dissipated through both linear and nonlinear processes in the nanotubes, leading to defect formation. Fragmentation occurs via atomic collision cascades and thermal spikes, each described by different theoretical models. Experiments with Cs-irradiated nanotubes support these models, and an information-theoretic approach further explains the fragmentation mechanisms. Sputtered species yield probability distributions, which are analyzed using Shannon entropy and fractal dimension to assess spatial characteristics. Kullback-Leibler divergence helps identify the diversity of emission mechanisms. Together, thermal and information-theoretic models clarify and distinguish the roles of collision cascades and thermal spikes in nanotube fragmentation.

cond-mat.mes-hall↗

arXiv2019

Dynamic emergence of fragmenting, exploding cages of irradiated single-walled carbon nanotubes and C60

Experimental results from irradiated-carbon nanostructures can be partially explained by selective applications of classical damage theories and ad-hoc thermal models by treating the binary atomic collision cascades and multi-atomic thermal spikes as energy-dissipating mechanisms. An information-theoretic model is developed by treating the irradiated single-walled carbon nanotubes and C60 as entropy-generating dissipative structures. The model is based on evaluating the experimental probability distribution functions of the sputtered constituents and components from the irradiated carbon nanostructures that yield their Shannon entropy or information. Three information-based functions of fractal dimension, relative entropy and dynamic emergence are defined and employed to characterize the profiles of self-organizing, information-generating dissipative structures. Fractal dimension determines the space-filling character, relative entropy establishes the distance between any two of the dissipative structures. Dynamic emergence function provides the profile of the emerging sequences. Results derived from the existing theories and thermal models are compared with the information-theoretic dynamic emergence model to comprehensively describe the nature of the dissipative structures in irradiated carbon nanostructures without making apriori assumptions about the energy-dissipating mechanisms.

cond-mat.mes-hall↗

arXiv2019

Multi scaling of Fractal Dimension and Relative Entropy as Diagnostic Tools for Irradiated Carbon Nano-tubes

Linear and nonlinear dissipative structures emerge in the irradiated single and multi walled carbon nano-tubes in the form of collision cascades and thermal spikes. These are diagnosed by the information theoretic tools of fractal dimension and relative entropy by the probabilistic description of dissipative structures using the measure that depends upon the energy of the irradiating ion and the incremental energy step. Multi scaling of the measure of the probability distributions is shown to induce variability of fractal dimension of the sputtered carbon atoms and clusters. Fractal dimension and relative entropy care shown to unambiguously distinguish and identify the information generating collision cascades and thermal spikes.

cond-mat.mes-hall↗

arXiv2018

Information-theoretic model of self-organizing fullerenes and the emergence of C60

An information-theoretic model describes the dissipative dynamical systems composed of ensembles of fragmenting, self-organizing fullerenes. The probabilities derived from the variations of the fullerene number densities that occur during cage-to-cage transformations are used to evaluate Shannon entropy for every carbon cage. Fractal dimension of the cages are calculated from the respective entropy. C60 is shown to emerges as the end-directed evolution of dynamical systems of four different ensembles of fullerenes. The information generating, transforming cages of carbon provide a perspective to evaluate the self-organizational behavior of dissipative structures.

cond-mat.mes-hall↗

arXiv2018

An information theoretic model for the linear and nonlinear dissipative structures in irradiated single-walled carbon nanotubes

Experiments with irradiated single-walled carbon nanotubes are shown to generate a set of probability distribution functions and to derive a set of information theoretic entropy-based parameters. Energetic Cs+ ions initiate linear collision cascades and nonlinear thermal spikes in single-walled carbon nanotubes. The probability distribution functions are constructed from the normalized experimental yields of the sputtered atoms and clusters. The information or Shannon entropy and fractal dimension are evaluated for each of the emitted species. Along with the fractal dimension, the information is used to identify and distinguish the energy dissipation processes that generate conditions for monatomic sputtering and clusters emissions.

cond-mat.mes-hall↗

arXiv2018

Information generating, sharing and manipulating Source-Reservoir-Sink model of self-organizing dissipative structures

Information-theoretic description of the signal transmitter, the channel and receiver is extended to the network of self-organizing dissipative structures consisting of a source, a reservoir and a sink. The information generation by the source is subjected to controlled manipulation by the reservoir before being transmitted to the sink. The reservoir can have memory and variable capacity for information storage. The role of the reservoir in building the manipulative capacity for information storage and selective sharing is illustrated by the characteristic of asymmetric exchange between the reservoir and the sink. A box-model is used to develop the model to represent material, process and information sharing among the source, the reservoir and the sink. The model is applied to self-organizing carbon cages with the end-directed evolution of the buckyball.

cond-mat.stat-mech↗

arXiv2018

Space-filling, multi-fractal, localized thermal spikes in silicon, germanium and zinc oxide

The mechanism responsible for the emission of clusters from heavy ion irradiated solids is proposed to be thermal spikes. Collision cascade-based theories describe atomic sputtering but cannot explain the consistently observed experimental evidence for significant cluster emission. Statistical thermodynamic arguments for thermal spikes are employed here for qualitative and quantitative estimation of the thermal spike-induced cluster emission from silicon, germanium and zinc oxide. The evolving cascades and spikes in elemental and molecular semiconducting solids are shown to have fractal characteristics. Power law potential is used to calculate the fractal dimension.The fractal dimension is shown to be dependent upon the exponent of the power law interatomic potential. Each irradiating ion has the probability of initiating a space-filling, multi-fractal thermal spike that may sublime a localized region near the surface by emitting clusters in relative ratios that depend upon the energies of formation of respective surface vacancies.

cond-mat.mes-hall↗

arXiv2017

Space-Filling Fractal Description of Ion-induced Local Thermal Spikes in Molecular Solid of ZnO

Anions of the molecules ZnO, O2 and atomic Zn and O constitute mass spectra of the species sputtered from pellets of molecular solid of ZnO under Cs+ irradiation. Their normalized yields are independent of energy of the irradiating Cs+. Collision cascades cannot explain the simultaneous sputtering of atoms and molecules. We propose that the origin of the molecular sublimation, dissociation and subsequent emission is the result of localized thermal spikes induced by individual Cs+ ions. The fractal dimension of binary collision cascades of atomic recoils in the irradiated ZnO solid increases with reduction in the energy of recoils. Upon reaching the collision diameters of atomic dimensions, the space-filling fractal-like transition occurs where cascades transform into thermal spikes. These localized thermal spikes induce sublimation, dissociation and sputtering from the region. The calculated rates of the subliming and dissociating species due to localized thermal spikes agree well with the experimental results.

cond-mat.mtrl-sci↗

arXiv2016

Regenerative Soot-IX: C3 as the dominant, stable carbon cluster in high pressure sooting discharges

Results are presented that have been obtained while operating the graphite hollow cathode duoplasmatron ion source in dual mode under constant discharge current. This dual mode operation enabled us to obtain the mass and emission spectra simultaneously. In mass spectra C3 is the main feature but C4 and C5 are also prominent, whereas in emission spectra C2 is also there and its presence shows that it is in an excited state rather than in an ionic state. These facts provide evidence that C3 is produced due to the regeneration of a soot forming sequence and leave it in ionic state. C3 is a stable molecule and the only dominant species among the carbon clusters that survives in a regenerative sooting environment at high-pressure discharges.

physics.plasm-ph↗

arXiv2016

Thermal spikes induced sublimation of carbon nanotubes

We report and provide justification for the consistently observed four experimental facts of the mass spectrometric data of carbon cluster emission from the low-energy Cs irradiated single-walled carbon nanotubes . Firstly, the diatomic carbon C(2) is the most abundant sputtered species for Cs+ in the energy range 0.2 keV to 2.0 keV. Secondly, monatomic carbon C(1) is emitted with the least sputtering yield. Thirdly, at low cesium energies i the emitted species are C(2), C(3) and C(4). Lastly, as the irradiating Cs+ energy increases, the normalized yield of atomic carbon monotonically increases while clusters show gradual decrease and saturation. Sputtering of clusters is proved here to be due to thermal spikes. Binary collision cascade theory does not explain cluster sputtering. A statistical thermal model is developed to explain the experimentally observed data. The probability of a cluster C(x) to be emitted is shown to be proportional to that for the creation of an x-member vacancy with formation energy E at temperature T as {exp(E/kT)+1}^(-1). The energies of formation of single and double vacancies from DFT calculations and the ratio of normalized experimental yields have been used to estimate spike temperature. We show that by invoking thermal spikes, cluster emission from, and the multiple vacancy generation in, the Cs+-irradiated SWCNTs can be explained. We also suggest modifications to Monte Carlo type calculations of sputtering.

cond-mat.mes-hall↗

arXiv2016

Formation and transformation of low density, onion-like carbon cages

A novel growth technique for low density, non-icosahedral carbon onion-like structures on Cu surface is described. The technique differs with the formation of carbon onions inside the C1+ implanted metal surfaces at high temperatures with densities ~ 2 g cm-3 in the form of multishelled icosahedral fullerenes wrapped around C60. We report spheroidal cage formation of shell with variable shell thickness, radii and curvature on C1+-irradiated Cu surfaces by the accumulating C atoms which emerge out of the edges of the implanted sheets and grids. Atom-by-atom C accreting structures grow along the edges, corners and crevices. These, in turn, form the open and closed multiple-shelled cages. Variable curvature is the most common feature of the growing structures. The growth at room temperature does not ensure sphericity of the cages due to the absence of high temperature annealing. These cages have very low density, hollow interiors and often exhibit the conspicuous cage-inside-cage structures. Another feature of the as-grown, non-spherical onions is the existence of spherical protrusions with different local radii within the same cage. Formation of cages with sizes ~ few nanometer to hundreds of nanometer depends upon the irradiation rate of C1+. In situ formation and transformations of the cages observed under 120 keV electron beam irradiation is described by using a nanoelastic model.

cond-mat.mes-hall↗

arXiv2016

Amorphous carbon films in direct current magnetron sputtering from regenerative sooting discharge

We present results of carbon coatings on metal substrates in cylindrical hollow cathode (CHC) direct current magnetron sputtering. This is a new technique of making amorphous carbon film in CHC magnetron sputtering from regenerative sooting discharge. The carbon films are deposited on Cu and Al substrates in Ne atmosphere and compared with the films of carbon soot on the same materials produced from conventional arc discharge between graphite electrodes at 80 Angstrom in He background. The films are characterized using online emission, Raman, and Fourier transform infrared spectroscopy; X-ray diffraction (XRD) and Scanning electron microscopy (SEM). Raman spectroscopy reveals the existence of graphite and diamond like structures from arc discharge while in CHC magnetron sputtering, graphite like structures are dominant. XRD pattern from arc discharge show precipitates of Al4C3 of rhombohedral and hexagonal types in nanometer ranges for aluminum sample and probable formation of diamond and hexagonal carbon in copper whilst in magnetron sputtering we get amorphous carbon films. SEM images of surface show collection of loose agglomerates of carbon particles in arc discharge whereas for magnetron sputtering structures are regular with smooth edges and fine grains.

cond-mat.mtrl-sci↗

arXiv2016

Dynamics of fragmentation and multiple vacancy generation in irradiated single-walled carbon nanotubes

The results from mass spectrometry of clusters sputtered from Cs+ irradiated single-walled carbon nano-tubes (SWCNTs) as a function of energy and dose identify the nature of the resulting damage in the form of multiple vacancy generation. For pristine SWCNTs at all Cs+ energies, C2 is the most dominant species, followed by C3, C4 and C1. The experiments were performed in three stages: in the first stage, Cs+ energy E(Cs+) was varied. During the second stage, the nanotubes were irradiated continuously at E(Cs+) = 5 keV for 1,800 s. Afterwards, the entire sequence of irradiation energies was repeated to differentiate between the fragmentation patterns of the pristine and of heavily irradiated SWCNTs. The sputtering and normalized yields identify the quantitative and relative extent of the ion-induced damage by creating double, triple and quadruple vacancies; the single vacancies are least favored. Sputtering from the heavily irradiated SWCNTs occurs not only from the damaged and fragmented nanotubes, but also from the inter-nanotube structures that are grown due to the accumulation of the sputtered clusters. Similar irradiation experiments were performed with the multi-walled carbon nanotubes; the results confirmed the dominant C2 followed by C3, C4 and C1.

physics.atm-clus↗

arXiv2016

Formation of carbon nano and micro structures on C1+ irradiated copper surfaces

A series of experiments has identified mechanisms of carbon nano- and micro-structure formation at room temperature, without catalyst and in the environment of immiscible metallic surroundings. The structures include threaded nano fibres, graphitic sheets and carbon onions. Copper as substrate was used due to its immiscibility with carbon. Energetic carbon ions of 0.2-2.0 MeV irradiated Cu targets. Cu substrates, apertures and 3 mm diameter Transmission Electron Microscope copper grids were implanted with the carbon. We observed wide range of micrometer-size structures formed on Cu grids and along the edges of the irradiated apertures. These are shown to be threaded nano fibers (TNF) of few micrometer thicknesses with lengths varying from 10 to 3000 micrometer. Secondary electron microscopy (SEM) identifies the micrometer-size structures while Confocal microscopy was used to learn about the mechanisms by which C1+ irradiated Cu provides the growth environment. Huge carbon onions of diameters ranging from hundreds of nm to μm were observed in the as-grown and annealed samples. Transformations of the nanostructures were observed under prolonged electron irradiations of SEM and TEM. One of the immediate outcomes of our present study is relevant for the efforts to fabricate carbon onions with specific properties.

cond-mat.mes-hall↗

arXiv2016

Irradiation induced vacancy creation in single walled carbon nanotubes

Single walled carbon nanotubes of 2 nm diameter and 3 to 13 micro meter length were compressed in Cu bullets and irradiated with positive ions of Cs with energies from 0.2 to 2.0 keV and subjected to successively increasing Cs dose. From the mass spectra of the sputtered carbon atoms and clusters as a function of Cs energy the monitoring of trend of the relative number densities of the fragmenting species indicates the accumulating damage. Irradiation induced fragmentation provide clues to the structural changes as a result of creation of vacancies due to the sputtering of monatomic, diatomic, triatomic and higher species. Monitoring of the the irradiated target electrical conductivity provides information of the sequences of the transformation that may be occurring in the structures of the single walled carbon nanotube as a function of Cs energy and dose.

cond-mat.mes-hall↗

arXiv2016

Cs+ sputtered clusters from multi-walled carbon nanotubes and graphite

Experiments with multiwalled carbon nanotubes and graphite as targets in a source of negative ions with cesium sputtering have shown that nanotubes with nanometer radii and micrometer length can be compared with micrometer size graphite grains to understand the irradiation effects that include the formation, sputtering of carbon clusters and the resulting structural changes. The simultaneous adsorption of cesium on the surface and bombardment by energetic cesium ions is shown to play its role in the cluster formation and sputtering of carbon atoms and clusters and the cesium substituted carbon clusters as anions. Qualitative and quantitative sputtered species outputs are related to their respective structures. Structural changes are shown to occur in MWCNTs and seen in SEM micrographs. The individual identity of the heavily bombarded MWCNTs may have given way to the merged structures while effects on the structure of heavily irradiated graphite grains size needs to be further investigated

cond-mat.mtrl-sci↗

arXiv2016

Transition from the C3-dominated discharge to the sooting plasma

Mass spectrometry and photoemission spectroscopy of a graphite hollow cathode source identify the parameters of the transition from the C(3) dominated discharge to the sooting plasma. The transition is a function of the shape and profile of a special cusp magnetic field which is a function of radius and azimuth angle, the geometry of the source, the discharge current, and pressure. Characteristic atomic and molecular emission lines and bands in the C(3) discharge transform into broad bands emitted by the excited soot. We identify four prominent emission bands between 300 to 400 nm to be the hallmark of the sooting plasma.

cond-mat.mtrl-sci↗

arXiv2016

Regenerative Soot-VIII: Sputtering and formation of C1 and C2

Photoemission spectroscopy of the regenerative soot in neon's glow discharge plasma reveals the contributions from the sputtered atomic and molecular carbon species. We present the pattern of sputtering and the formation of monatomic C1 and diatomic C2 as a function of the discharge current, the support gas pressure and the number densities of the excited and ionized neon as the active constituents of the carbonaceous plasma.

cond-mat.mtrl-sci↗