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Ion beam-induced shaping of Ni nanoparticles embedded in a silica matrix: from spherical to prolate shape.

Kumar H, Ghosh S, Avasthi DK, Kabiraj D, Mücklich A, Zhou S, Schmidt H, Stoquert JP - Nanoscale Res Lett (2011)

Bottom Line: The Ni NPs embedded in silica matrix have been prepared by atom beam sputtering technique and subsequent annealing.Irradiation induces a change from single crystalline nature of spherical particles to polycrystalline nature of elongated particles.Magnetization measurements indicate that changes in coercivity (Hc) and remanence ratio (Mr/Ms) are stronger in the ion beam direction due to the preferential easy axis of elongated particles in the beam direction.

View Article: PubMed Central - HTML - PubMed

Affiliation: Nanostech Laboratory, Department of Physics, Indian Institute of Technology Delhi, New Delhi 110016, India. hsehgal_007@yahoo.com.

ABSTRACT
Present work reports the elongation of spherical Ni nanoparticles (NPs) parallel to each other, due to bombardment with 120 MeV Au+9 ions at a fluence of 5 × 1013 ions/cm2. The Ni NPs embedded in silica matrix have been prepared by atom beam sputtering technique and subsequent annealing. The elongation of Ni NPs due to interaction with Au+9 ions as investigated by cross-sectional transmission electron microscopy (TEM) shows a strong dependence on initial Ni particle size and is explained on the basis of thermal spike model. Irradiation induces a change from single crystalline nature of spherical particles to polycrystalline nature of elongated particles. Magnetization measurements indicate that changes in coercivity (Hc) and remanence ratio (Mr/Ms) are stronger in the ion beam direction due to the preferential easy axis of elongated particles in the beam direction.

No MeSH data available.


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Simulations based on thermal spike model. (a) Schematic model for the thermal simulation of a Ni particle embedded in the SiO2 matrix irradiated by 120 MeV Au+9 ion. (b) Calculated radial profile of lattice temperature in the z = 0 plane of bulk SiO2 and seven different spherical Ni nanoparticles (2, 4, 6, 10, 15, 20, 30 nm) embedded in silica after 1 ps and (c) 10 ps of ion impact. The melting (TM) and vaporization (TV) temperature of SiO2 and Ni are also indicated
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Figure 4: Simulations based on thermal spike model. (a) Schematic model for the thermal simulation of a Ni particle embedded in the SiO2 matrix irradiated by 120 MeV Au+9 ion. (b) Calculated radial profile of lattice temperature in the z = 0 plane of bulk SiO2 and seven different spherical Ni nanoparticles (2, 4, 6, 10, 15, 20, 30 nm) embedded in silica after 1 ps and (c) 10 ps of ion impact. The melting (TM) and vaporization (TV) temperature of SiO2 and Ni are also indicated

Mentions: Figure 4a shows, schematically, a simplified two-dimensional model, in which a 120 MeV Au+9 ion passes through the center of a spherical Ni particle embedded in silica matrix. Figure 4b,c shows the simulated radial distribution of the lattice temperature within 1 and 10 ps of 120 MeV Au+9 ion impact, for bulk silica and Ni nanoparticles (diameter, 2-30 nm) embedded in a silica matrix. It is well studied that a latent track may result due to the rapid quenching of the molten lattice. Here in our case, the estimated molten region in silica is ~10 nm from simulation results and agrees well with the earlier published experimental results [30]. Thermal spike simulations cannot be applied to surface NPs which behave differently (temperature evolution and stress relaxation) from embedded NPs. The following observations are evident from Figure 4b, c: (1) For 0 < d ≤4 nm Ni particles temperature reaches up to its bulk vaporization temperature (Tv) throughout its volume, so Ni atoms dissolve in the cylindrical molten silica track and promote the growth of neighboring bigger nanoparticles (>4 nm) by a ripening process, (2) For 4 < d ≤10 nm Ni particles, the lattice temperature of both Ni and surrounding silica rises above their respective melting points, so both Ni and silica are in molten state. Because of high electron-lattice coupling constant and low thermal conductivity of silica as compared to Ni, the temperature rise in silica is high as compared to metallic Ni even though incident ion energy is deposited to Ni and then coupled to silica, but thermal evolution itself does not give full explanation for deformation of metal NPs and hence shear stress also needs to be included as another factor in shape deformation. (3) For Ni particles having diameter >10 nm, Ni particle and surrounding silica both have temperature below their respective melting point and so retain their original shape. However, from TEM micrographs it is observed that Ni particles of size <14 nm are deformed and shape deformation is completely suppressed for particles of size >14 nm. The diameter in experiments does not match exactly with those obtained from thermal spike model-based simulations as pressure-dependent variation of thermodynamic parameters is neglected and size dependent variation of thermodynamic parameters is unknown, and hence the bulk values for Ni are used in the present case. The outcome of thermal spike simulations is that the lattice temperature of smaller size Ni particles increases much higher than lattice temperature of relatively bigger particles and experimental results are explainable within errors.


Ion beam-induced shaping of Ni nanoparticles embedded in a silica matrix: from spherical to prolate shape.

Kumar H, Ghosh S, Avasthi DK, Kabiraj D, Mücklich A, Zhou S, Schmidt H, Stoquert JP - Nanoscale Res Lett (2011)

Simulations based on thermal spike model. (a) Schematic model for the thermal simulation of a Ni particle embedded in the SiO2 matrix irradiated by 120 MeV Au+9 ion. (b) Calculated radial profile of lattice temperature in the z = 0 plane of bulk SiO2 and seven different spherical Ni nanoparticles (2, 4, 6, 10, 15, 20, 30 nm) embedded in silica after 1 ps and (c) 10 ps of ion impact. The melting (TM) and vaporization (TV) temperature of SiO2 and Ni are also indicated
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3211206&req=5

Figure 4: Simulations based on thermal spike model. (a) Schematic model for the thermal simulation of a Ni particle embedded in the SiO2 matrix irradiated by 120 MeV Au+9 ion. (b) Calculated radial profile of lattice temperature in the z = 0 plane of bulk SiO2 and seven different spherical Ni nanoparticles (2, 4, 6, 10, 15, 20, 30 nm) embedded in silica after 1 ps and (c) 10 ps of ion impact. The melting (TM) and vaporization (TV) temperature of SiO2 and Ni are also indicated
Mentions: Figure 4a shows, schematically, a simplified two-dimensional model, in which a 120 MeV Au+9 ion passes through the center of a spherical Ni particle embedded in silica matrix. Figure 4b,c shows the simulated radial distribution of the lattice temperature within 1 and 10 ps of 120 MeV Au+9 ion impact, for bulk silica and Ni nanoparticles (diameter, 2-30 nm) embedded in a silica matrix. It is well studied that a latent track may result due to the rapid quenching of the molten lattice. Here in our case, the estimated molten region in silica is ~10 nm from simulation results and agrees well with the earlier published experimental results [30]. Thermal spike simulations cannot be applied to surface NPs which behave differently (temperature evolution and stress relaxation) from embedded NPs. The following observations are evident from Figure 4b, c: (1) For 0 < d ≤4 nm Ni particles temperature reaches up to its bulk vaporization temperature (Tv) throughout its volume, so Ni atoms dissolve in the cylindrical molten silica track and promote the growth of neighboring bigger nanoparticles (>4 nm) by a ripening process, (2) For 4 < d ≤10 nm Ni particles, the lattice temperature of both Ni and surrounding silica rises above their respective melting points, so both Ni and silica are in molten state. Because of high electron-lattice coupling constant and low thermal conductivity of silica as compared to Ni, the temperature rise in silica is high as compared to metallic Ni even though incident ion energy is deposited to Ni and then coupled to silica, but thermal evolution itself does not give full explanation for deformation of metal NPs and hence shear stress also needs to be included as another factor in shape deformation. (3) For Ni particles having diameter >10 nm, Ni particle and surrounding silica both have temperature below their respective melting point and so retain their original shape. However, from TEM micrographs it is observed that Ni particles of size <14 nm are deformed and shape deformation is completely suppressed for particles of size >14 nm. The diameter in experiments does not match exactly with those obtained from thermal spike model-based simulations as pressure-dependent variation of thermodynamic parameters is neglected and size dependent variation of thermodynamic parameters is unknown, and hence the bulk values for Ni are used in the present case. The outcome of thermal spike simulations is that the lattice temperature of smaller size Ni particles increases much higher than lattice temperature of relatively bigger particles and experimental results are explainable within errors.

Bottom Line: The Ni NPs embedded in silica matrix have been prepared by atom beam sputtering technique and subsequent annealing.Irradiation induces a change from single crystalline nature of spherical particles to polycrystalline nature of elongated particles.Magnetization measurements indicate that changes in coercivity (Hc) and remanence ratio (Mr/Ms) are stronger in the ion beam direction due to the preferential easy axis of elongated particles in the beam direction.

View Article: PubMed Central - HTML - PubMed

Affiliation: Nanostech Laboratory, Department of Physics, Indian Institute of Technology Delhi, New Delhi 110016, India. hsehgal_007@yahoo.com.

ABSTRACT
Present work reports the elongation of spherical Ni nanoparticles (NPs) parallel to each other, due to bombardment with 120 MeV Au+9 ions at a fluence of 5 × 1013 ions/cm2. The Ni NPs embedded in silica matrix have been prepared by atom beam sputtering technique and subsequent annealing. The elongation of Ni NPs due to interaction with Au+9 ions as investigated by cross-sectional transmission electron microscopy (TEM) shows a strong dependence on initial Ni particle size and is explained on the basis of thermal spike model. Irradiation induces a change from single crystalline nature of spherical particles to polycrystalline nature of elongated particles. Magnetization measurements indicate that changes in coercivity (Hc) and remanence ratio (Mr/Ms) are stronger in the ion beam direction due to the preferential easy axis of elongated particles in the beam direction.

No MeSH data available.


Related in: MedlinePlus