Search

This manuscript outlines current research activities based on ion beam science at GNS Science in New Zealand. Ion beam analysis methods include Rutherford backscattering and nuclear reaction analys...

Dipolar interactions among superparamagnetic nanoparticles lead to interesting phenomena such as superspin glass and superferromagnetism that are of interest in fabricating novel spintronic devices. This work investigates the occurrence of such phenomena in magnetic thin film stacks composed of permalloy-80 and ZnO thin films synthesized by ion beam sputtering. Particle induced X-ray emission was used to measure the composition of the thin film stacks. Atomic force microscopy revealed the changes in surface topography with composition. Detailed investigations were carried out to study the magnetic properties. Temperature-dependent magnetization measurement confirmed that the thin film stacks follow Bloch T3/2 law and the Curie temperature of the stacks was determined to lie between 620 and 670 K. Zero-field cooled and field cooled measurements identified the presence of superparamagnetic nanoparticles in the stack. Field dependent magnetization measurements were further carried out at different temperatures to identify and reveal the nature of dipolar interactions among the nanoparticles. The preliminary investigation carried out in this work confirms the possibility of fabricating a magnetic semiconductor system composed of interacting superparamagnetic nanoparticles by ion beam sputtering.

Room temperature implantation of 30 keV Co ions into an amorphous carbon film with a high fluence of 1.2 × 1017 Co/cm2 results in formation of magnetic nanostructures displaying multiple magnetic phases. Cross-sectional TEM images show formation of Co containing nanoparticles at the surface and near-surface regions of the implanted films. EDXS measurements suggest the nanoparticles to be composed primarily of Co and O at the surface and Co and C in deeper regions. These nanoparticles with differing compositions were observed to be segregated by a thin layer devoid of Co. Magnetic measurements reveal the presence of superparamagnetic behavior from small CoxC nanoclusters with a blocking temperature of 5 K. There is a small fraction of larger CoxC nanoclusters that show magnetic hysteresis even at room temperature. The saturation magnetic moment is as high as 0.51 μB/Co at 2 K and 0.32 μB/Co at room temperature. Spin-disorder is seen with a range of spin glass temperatures below ∼70 K. Our high fluence Co implantation into amorphous carbon has resulted in the formation of complex magnetic nanostructures composed of cobalt, oxygen, and carbon. These nanostructures give rise to multiple magnetic phases such as superparamagnetism, spin glass, ferromagnetism, and possibly antiferromagnetism.

New approaches to synthesize decorative black coatings on metallic surfaces are of significant research and commercial interest to manufacturing industries. In this work, decorative hard black coatings were deposited on a titanium surface by carbon ion implantation at ambient temperature. A 10 keV C+ beam was implanted on a Ti substrate to a fluence of 1–1.25 × 1018 C cm−2. Rutherford backscattering spectrometry and transmission electron microscopy results show that the implantation resulted in a bi-layered coating structure with a ~50 nm amorphous carbon layer at the surface followed by a ~50 nm amorphous titanium carbide intermixing layer deposited on top of a crystalline Ti surface. Raman spectroscopy confirms the formation of carbide intermixing layer and shows that the amorphous carbon layer has 15–20% sp3 content. Nanoindentation measurements show that the surface hardness of the implanted surface has increased by 72%, from 3.7 to 6.6 GPa, upon carbon implantation. Scratch tests further demonstrate a reduction in coefficient of friction in the implanted surface by 25%, signifying an improvement in wear-resistance of the coated materials. Colorimetry measurements reveal that carbon implantation reduces the luminosity of the Ti surface from 77 to 49 and chromaticity from 4.67 to 1.36, confirming incorporation of black color on Ti surface. The results demonstrate that C implantation onto a Ti surface at high fluence results in a black coating with high surface hardness and wear-resistance that can be employed in decorative surface applications for manufacturing industries.

Amorphous carbon and hydrogenated amorphous carbon layers were implanted at room temperature with Co ions to investigate the role of hydrogen on the Co distribution. Amorphous carbon (a:C) and hydrogenated amorphous carbon (a:C–H) films were prepared by mass selective ion beam deposition with a 5 kV acceleration voltage using C+ and C3H6+ ions, respectively. The typically 100 nm thin films were implanted with Co using a 30 kV acceleration voltage to a fluence of 4 × 1016 cm−2. Raman measurements showed that Co implantation in hydrogenated amorphous carbon causes increased sp2 clustering while in amorphous carbon there is significant rehybridisation of carbon from sp3 to sp2 bonding. High resolution Rutherford backscattering measurements indicated that in the absence of hydrogen in the base matrix, the implantation profile assumes a unimodal distribution as predicted by simulations. However, in the presence of hydrogen the effects of collision cascade enhanced diffusion are significant in altering the implantation profile resulting in a bimodal distribution. The difference in the Co depth distribution between a:C and a:C–H films is explained by the change in thermal conductivity of the carbon matrix in the presence of hydrogen. The ability to position Co (magnetic atoms) in the surface region of diamond-like carbon films offers great advantages for applications in novel magnetic devices.

Hydrogenated diamond-like carbon films produced by C 3 H 6 deposition at 5 kV and implanted at room temperature with 30 keV Co atoms to 12 at.% show not only a bimodal distribution of Co atoms but also a massive redistribution of hydrogen in the films. Resonant nuclear reaction analysis was used to measure the hydrogen depth profiles (15N-method). Depletion of hydrogen near the surface was measured to be as low as 7 at.% followed by hydrogen accumulation from 27 to 35 at.%. A model is proposed considering the thermal energy deposited by collision cascade for thermal insulators. In this model, sufficient energy is provided for dissociated hydrogen to diffuse out of the sample from the surface and diffuse into the sample towards the interface which is however limited by the range of the incoming Co ions. At a hydrogen concentration of ∼35 at.%, the concentration gradient of the mobile unbounded hydrogen atoms is neutralised effectively stopping diffusion towards the interface. The results point towards new routes of controlling the composition and distribution of elements at the nanoscale within a base matrix without using any heat treatment methods. Exploring these opportunities can lead to a new horizon of materials and device engineering needed for enabling advanced technologies and applications.

We report the effects of implantation-induced-sputtering in determining the true magnetisation of iron nitrides formed by nitrogen implantation into iron films and the changes subsequently observed in their magnetic anisotropy. Iron thin films (40 nm) capped with gold layers were synthesized by ion beam sputtering. Nitrogen was implanted onto the thin film to a dose of 7 × 1016 cm-2. Dynamic ion-solid interaction simulations calculate the average nitrogen concentration within the iron thin films to be 13 at.%. Ion beam analysis was used to evaluate the composition and depth profile of the thin films before and after implantation. Cross-sectional transmission electron microscopy helped in identifying the implantation induced effects at the surface and interface of the thin film structure. Magnetisation measurements show that nitrogen implantation enhances the magnetisation of thin films and reduces their in-plane magnetic anisotropy. High resolution TEM and magnetisation results suggest formation of α’/α”-Fe16N2 with large magnetic moments and perpendicular magnetic anisotropy. The enhancement in magnetisation of the thin film with and without considering the sputtering losses is determined to be 15% and 5%, respectively. Our review of literature shows that underestimating the sputtering losses from implantation leads to reporting of inconsistent and reduced magnetic moments for Fe16N2.

Amorphous atomically flat diamond-like carbon (DLC) coatings were produced by direct ion deposition using a system based on a Penning ion source, butane precursor gas and post acceleration. Hydrogen depth profiles of the DLC coatings were measured with the 15N R-NRA method using the resonant nuclear reaction 1 H( 15 N, αγ) 12 C ( E res = 6.385 MeV). The films produced at 3.0–10.5 kV acceleration voltage show two main effects. First, compared to average elemental composition of the film, the near-surface region is hydrogen depleted. The increase of the hydrogen concentration by 3% from the near-surface region towards the bulk is attributed to a growth model which favours the formation of sp 2 hybridised carbon rich films in the film formation zone. Secondly, the depth at which the maximum hydrogen concentration is measured increases with acceleration voltage and is proportional to the penetration depth of protons produced by the ion source from the precursor gas. The observed effects are explained by a deposition process that takes into account the contributions of ion species, hydrogen effusion and preferential displacement of atoms during direct ion deposition.

Diamond-like carbon (DLC) coatings were deposited with a new direct ion deposition system using a novel 360 degree ion source operating at acceleration voltage between 4 and 8 kV. Cross-sectional TEM images show that the coatings have a three layered structure which originates from changes in the deposition parameters taking into account ion source condition, ion current density, deposition angles, ion sputtering and ion source movement. Varying structural growth conditions can be achieved by tailoring the deposition parameters. The coatings show good promise for industrial use due to their high hardness, low friction and excellent adhesion to the surface of the samples.

We report the structural and optical properties of ZnS thin films fabricated by ion-beam sputtering. X-ray diffraction (XRD) and transmission electron microscopy (TEM) results revealed a polycrystalline ZnS film with zinc blende phase as manifested by diffraction from the (111), (220) and (311) planes. Annealing resulted in the appearance of a metastable wurtzite phase with a concentration up to 26.6%. An energy bandgap, estimated from absorption spectra, was found to vary between 3.32 and 3.40 eV. The lower energy of this bandgap, as compared to bulk ZnS, is associated with the structural point defects along with mixed zinc blende and wurtzite phases of the polycrystalline ZnS films. Ion beam sputtering deposition can be used to tune the optical bandgap for potential applications in optoelectronic materials.

Modern computing technology is based on silicon. To date, a cost-effective and easy to implement method to obtain isotopically pure silicon is highly desirable for attaining efficient heat dissipation in microelectronic devices and for hosting spin qubits in quantum computing. We propose that it is possible to use a 28Si+ ion beam to obtain an isotopically pure near-surface region in wafer silicon. However, this requires a highly stable, high current, and isotopically pure 28Si ion beam. This work presents and discusses the instrumentation details and experimental parameters involved in generating this required ion beam. Silane is used as the precursor gas and is decomposed in a Penning ion source to generate a 28Si+ ion beam. The influence of key ion source parameters such as the gas flow rate, magnetic field strength, and anode voltage is presented. An isotopically pure 28Si+ ion beam with 10 ± 0.5 μA current on the target is obtained at the GNS Science 40 kV ion implanter. The beam was observed to be stable for at least 8 h and contains less than 700 ppm of other Si isotopes. This high current and high purity provides opportunities to explore efficient modification of the isotopic distribution in a native Si substrate at ambient temperature. The results highlight opportunities offered by using Penning ion source based low energy ion implanters for the synthesis of isotopically modified Si surface regions-a technique also applicable to other materials such as diamonds and diamond-like carbon.

Micrometre thick diamond-like carbon (DLC) coatings produced by direct ion deposition were implanted with 30 keV Ar+and transition metal ions in the lower percentage (+and Cu+ions, whereas a smoothing of the surface from 5.2 to 2.7 nm and a grain size reduction from 175 to 93 nm are measured for Ag+implanted coatings with a fluence of1.24×1016 at. cm−2. Calculated hydrogen and carbon depth profiles showed surprisingly significant changes in concentrations in the near-surface region of the DLC coatings, particularly when implanted with Ag+ions. Hydrogen accumulates up to 32 at.% and the minimum of the carbon distribution is shifted towards the surface which may be the cause of the surface smoothing effect. The ion implantations caused an increase in electrical conductivity of the DLC coatings, which is important for the development of solid-state gas sensors based on DLC coatings.

We report the observation of ferromagnetic order in diamond-like carbon (DLC) films made by mass selective ion beam deposition and after low energy implantation with Co ions. Different Co fluences were studied with a peak concentration of up to 25% at an average Co implantation depth of 30 nm. The saturation moment per Co atom (0.2–0.3 μ B) was found to be strongly dependent on temperature and it was significantly lower than that reported in bulk cobalt or cobalt nanoparticles (1.67 μ B per Co atom). The observed magnetic moment cannot be attributed to ferromagnetic nanoparticles as no evidence for superparamagnetism was detected. The magnetic order observed may be due to Co bonding in DLC possibly leading to dilute ferromagnetic semiconductor behaviour with an inhomogeneous distribution of cobalt atoms. Raman spectroscopy measurements showed that Co implantation resulted in an increase in the sp2 clustering with increasing Co fluence. Thus, our results show that Co implantation into DLC films increases the graphitic properties of the film and leads to magnetic order at room temperature.