Ion beam-induced shaping of Ni nanoparticles embedded in a silica matrix: from spherical to prolate shape
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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.
KeywordsFePt Easy Axis Silica Matrix Rutherford Backscattering Spectrometry Shape Deformation
Metal nanoparticles (NPs) embedded in transparent matrices are the subject of large scientific and technological interest as they show significantly different properties as compared to their bulk counterpart [1, 2]. The NP size and shape, orientation, interparticle separation and dielectric constant of the surrounding matrix are the crucial parameters which control their properties. Generally, the NP shape and orientation is difficult to control by synthesis parameters. One of the interesting aspects of shape anisotropy in noble metal NPs is the splitting of the surface plasmon resonance band [3, 4, 5, 6], which can be tuned from visible to infrared region. Prolate-shaped NPs/nanorods show new and improved photonic, optoelectronic, and sensing properties as compared to spherical NPs [3, 5]. On the other hand, an array of magnetic prolate-shaped NPs/nanorods with perpendicular magnetic anisotropy permits to overcome the problem of superparamagnetic instability arising due to the decrease in the particle size in magnetic recording media [6, 7, 8]. Another requirement for recording at high density with a minimum noise is to reduce the interaction between magnetic nanorods, which can be achieved by encapsulation of magnetic nanorods in a non-magnetic matrix. In literature, various methods are reported to prepare prolate-shaped NPs/nanorods, but the investigated methods yield randomly oriented structures (e.g., by chemical routes) , small areas (e.g., by electron or focused ion-beam lithography) [5, 7] or are limited to a specific class of materials (e.g., porous alumina template growth) [6, 8].
Swift heavy ion (SHI) irradiation is an important tool in the modification of materials and is extensively used to manipulate the matter at nanometer scale. One of the important effects of SHI irradiation is the anisotropic shape deformation of amorphous silica nanospheres to oblate shape [9, 10] and crystalline metallic NPs, e.g., Co [11, 12], Au [13, 14, 15, 16, 17], Ag [18, 19, 20, 21], Pt [22, 23], and FePt  embedded in silica matrix, to prolate shape. No shape deformation is observed for embedded Fe NPs in silica matrix by 120 MeV Au9+ ions at a fluence of 3 × 1013 ions/cm2, However, tilt of easy axis of magnetization [25, 26] was observed and explained by ion hammering effect. The deformation behavior of silica nanospheres, i.e., expansion in the direction perpendicular to ion beam and shrinkage in the direction parallel to ion beam, is known under the name "hammering effect" and explained by the viscoelastic thermal spike model [27, 28]. On the other hand, there is no consistent theory describing the shape deformation of metal NPs in amorphous silica matrix, but the suggested mechanisms include melting of NPs in thermal spike [29, 30, 31], creep deformation induced by an overpressure due to differences in volume expansion and compressibility of NP and silica matrix , and shear stress-driven deformation due to in-plane strain perpendicular to ion beam direction [14, 16, 22, 23].
In the present work, we report the elongation/anisotropic shape deformation of Ni NPs from spherical to prolate ones under 120 MeV Au+9 ion irradiation at fluence of 5 × 1013 ions/cm2, where shape deformation strongly depends on the initial Ni particle size. Further, to understand the shape deformation process, simulations based on thermal spike model [29, 30, 31] were carried out and the effect of irradiation on structural and magnetic properties is presented.
A set of thin films of silica containing Ni NPs (Ni-SiO2 nanogranular films) were synthesized by atom beam sputtering technique, as described elsewhere [32, 33, 34, 35]. Silica and Ni were co-sputtered on thermally oxidized Si substrates mounted on a rotating sample holder. The relative area of silica and Ni chips exposed to the atom beam determines the concentration and size of Ni particles. In this study, the area of Ni was maintained to obtain ~10 at% Ni in the films. Ni-SiO2 nanogranular films were annealed in Ar-H2 (5%) atmosphere at 850°C (1 h) for promoting the growth of Ni particles and labeled as pristine film thereafter. The pristine film was irradiated at room temperature and at normal incidence with 120 MeV Au+9 ions at a fluence of 5 × 1013 ions/cm2 in 15 UD Tandem Pelletron accelerator at the Inter University Accelerator Centre, New Delhi, India. The irradiation was performed in a high vacuum chamber with a base pressure of 2.8 × 10-6 Torr. The beam current was kept <0.5 pnA (particle nano ampere) during irradiation to avoid heating of the film. The ion beam was uniformly scanned over 1 × 1 cm2 area using an electromagnetic scanner. The range, electronic (Se) and nuclear (Sn) stopping powers of 120 MeV Au+9 ions in silica were calculated using SRIM 2006 code  and amount to ~15 μm, 14.7 keV/nm and 0.2 keV/nm, respectively. For such a large range, stopping powers can be considered constant over a film of few nanometers thickness. The composition and film thickness were measured by Rutherford backscattering spectrometry (RBS) using 1.7 MeV He+ ions at a scattering angle of 170°. Magnetization curves were measured using a Quantum Design MPMS SQUID magnetometer with a maximum field of 2 T applied parallel (out-plane measurement) and perpendicular (in-plane measurement) to the ion beam direction. TEM measurements were used to evaluate the size and shape evolution of Ni NPs before and after irradiation. TEM samples were prepared in cross-sectional geometry using the conventional techniques and were analyzed in FEI Titan 80-300 microscope working at accelerating voltage of 300 kV.
Results and discussion
The measured film thickness is ~150 nm with an average Ni atomic concentration of 10.5 ± 1% as estimated from fitting of RBS spectra using RUMP simulation code .
Coercivity (Hc) and remanence ratio (Mr/Ms) measured at 5 K for the pristine and irradiated Ni-SiO2 nanogranular film with magnetic field parallel and perpendicular to the 120 MeV Au+9 ion beam direction.
H c (Oe)
M r / M s
H c (Oe)
M r / M s
Simulations based on thermal spike model
The fitted values of mass density (ρ), melting temperature (TM), vaporization temperature (TV), latent heat of fusion (LM), latent heat of vaporization (LV), lattice specific heat (Cl), lattice thermal conductivity (Kl) and electron-phonon coupling constant (g) for Ni and SiO2 used in the thermal spike simulations [30, 44, 45].
ρ (g cm-3)
2.62 (solid), 2.32 (liquid)
LM (J g-1)
LV (J g-1)
Cl (J g-1 K-1)
0.39 + 1.9 × 10-4T-3.3 × 10-8T2+3.8 × 10-11T3; (300 < T < Tm),
0.65 + 3.297 × 10-4T; (300 < T < TM),
0.62; (T > TM).
1.3-3 × 10-7T; (T > TM).
3.4-1.3 × 10-2T+2.12 × 10-5 T2- 1.5 × 10-8T3+3.6 × 10-12T4;
1 × 10-3 (T > 300)
(100 < T < TM), 0.5 (T > TM)
gl (W cm-3 K-1)
9.54 × 1011
1.25 × 1013
One may also think ion hammering as responsible mechanism for the elongation of Ni NPs. However, according to ion hammering mechanism, a large elongation is expected for smaller particles than relatively bigger particles, which contradicts our observation and hence rules out ion hammering effect. Klaumünzer et al.  also pointed out that hammering as an indirect mechanism alone is not sufficient to account for the observed deformation of solid NPs, i.e., the metallic NP must actively participate in the deformation process. In other words, elongation occurs only when the lattice temperatures of both metallic NP and the dielectric SiO2 exceed their respective individual melting temperatures, i.e., elongation of embedded NPs occur due to flow of metallic species into molten silica tracks [14, 22]. According to the viscoelastic thermal spike model [27, 28], the origin of anisotropic shape deformation of amorphous materials, e.g., silica is due to relaxation of shear stresses in the ion track region. These shear stresses are generated due to rapid thermal expansion of the ion-induced thermal spikes. The complete relaxation in the track region is assumed to take place when the ion track temperature exceeds a certain flow temperature (melting point). As for 4 < d ≤10 nm diameter Ni NPs this condition (melting of Ni as well as surrounding silica) is satisfied, so combination of stress effects in silica and thermal spike model gives a fairly good explanation for the elongation/shape deformation from spherical to prolate shape of Ni NPs.
In conclusion, we report the elongation of Ni NPs parallel to each other embedded in silica matrix by 120 MeV Au+9 ion irradiation at fluence of 5 × 1013 ions/cm2 with mean aspect ratio of ~2. Shape deformation is observed for particles <14 nm and is suppressed for particles >14 nm under studied beam parameters. Irradiation leads to formation of surface Ni particles without silica matrix and also not deformed, expected due to large electronic sputtering yield of silica. Large changes in coercivity (Hc) and remanence ratio (Mr/Ms) are observed in the direction parallel to Au+9 ion beam than in the perpendicular direction, which is due to the elongation/formation of prolate shape Ni particles in the beam direction. However, a macroscopic perpendicular magnetic anisotropy is not observed due to the existence of both spherical and deformed prolate shape Ni particles in the irradiated film. The experimental observations are well explained by thermal spike model-based simulations. Fabrication of pristine films with particles of average size in the range from 5 to 20 nm in order to control the macroscopic magnetic anisotropy with easy axis in the ion beam direction could be set as future perspective of this work.
One of the authors (H. Kumar) acknowledges CSIR India for financial support as SRF and Dr. D. C. Agarwal (research associate, IUAC Delhi) for his kind help during sample preparation. One of the authors (D.K.A.) is thankful to Department of Science and Technology (DST), India for providing the financial assistance for 'Atom beam source' under the project 'Nanostructuring by energetic ion beams' under Nano-mission. We also acknowledge the Pelletron group, IUAC Delhi for providing stable beam during irradiation experiment.
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