keV Xenon Ion Research Articles

Dendrite Formation on the Poly(methyl methacrylate) Surface Reactively Sputtered with a Cesium Ion Beam.

The development of topography plays an important role when low-energy projectiles are used to modify the surface or analyze the properties of various materials. It can be a feature that allows one to create complex structures on the sputtered surface. It can also be a factor that limits depth resolution in ion-based depth profiling methods. In this work, we have studied the evolution of microdendrites on poly(methyl methacrylate) sputtered with a Cs 1 keV ion beam. Detailed analysis of the topography of the sputtered surface shows a sea of pillars with islands of densely packed pillars, which eventually evolve to fully formed dendrites. The development of the dendrites depends on the Cs fluence and temperature. Analysis of the sputtered surface by physicochemical methods shows that the mechanism responsible for the formation of the observed microstructures is reactive ion sputtering. It originates from the chemical reaction between the target material and primary projectile and is combined with mass transport induced by ion sputtering. The importance of chemical reaction for the formation of the described structures is shown directly by comparing the change in the surface morphology under the same dose of a nonreactive 1 keV xenon ion beam.

Nanostructuring Germanium Nanowires by In Situ TEM Ion Irradiation

AbstractOnce nanomaterials have been synthesized, inducing further structural modifications is challenging. However, being able to do so in a controlled manner is crucial. In this context, germanium nanowires are irradiated in situ within a transmission electron microscope (TEM) by a 300 keV xenon ion beam at temperatures ranging from room temperature (RT) to 500 °C. The ion irradiation is performed in situ and the evolution of nanowires during irradiation is monitored. At 300 °C and below, where the temperature is low enough to allow amorphization, the ion beam causes the formation of nanostructures within the nanowires. Formation of nanopores and swelling of nanowires is observed for a very low fluence of 2.2 × 1014 and up to 4.2 × 1015 ions cm−2. At higher fluences, the thickness of the nanowires decreases, the nanowires lose their wire‐like cylindrical shape and the nanostructuring caused by the ion beam becomes more complex. The nanostructures are observed to be stable upon crystallization when the nanowires are annealed at 530 °C. Furthermore, in situ imaging allows the growth of nanopores during irradiation to be followed at RT and at 300 °C providing valuable insights into the mechanism responsible for the nanostructuring.

Effect of aluminium concentration on phase formation and radiation stability of Cr2AlxC thin film

Near-stoichiometric and under-stoichiometric Cr2AlxC (x = 0.9 and 0.75) amorphous compositions were deposited onto a silicon substrate at 330 K in a layer-by-layer fashion using magnetron sputtering from elemental targets. The film thickness was found to be 0.9 µm and 1.2 µm for the near- and under-stoichiometric compositions respectively. A transmission electron microscope (TEM) heating holder was used to heat thin sample lamellae prepared using focused ion beam milling. Near-stoichiometric Cr2AlC thin films consisted of nano MAX phase after crystallization at 873 K. Under-stoichiometric Cr2AlxC (x = 0.75) thin films contained MAX phase along with nanocrystalline chromium aluminides after crystallization at 973 K. Irradiations with 320 keV xenon ions was performed at 623 K using a TEM with an in-situ ion irradiation (MIAMI) facility. Nanocrystalline films of near-stoichiometric Cr2AlC irradiated up to 83 displacements per atom (dpa) showed no observable changes. Also, irradiation of under-stoichiometric nanocrystalline thin films up to 138 dpa did not show any observable amorphization, and recrystallization was observed. This radiation resistance of near- and under-stoichiometric thin films is attributed to the known self-healing property of Cr2AlxC compositions further enhanced by nanocrystallinity.

Ion-beam-induced bending of semiconductor nanowires

The miniaturisation of technology increasingly requires the development of both new structures as well as novel techniques for their manufacture and modification. Semiconductor nanowires (NWs) are a prime example of this and as such have been the subject of intense scientific research for applications ranging from microelectronics to nano-electromechanical devices. Ion irradiation has long been a key processing step for semiconductors and the natural extension of this technique to the modification of semiconductor NWs has led to the discovery of ion beam-induced deformation effects. In this work, transmission electron microscopy with in situ ion bombardment has been used to directly observe the evolution of individual silicon and germanium NWs under irradiation. Silicon NWs were irradiated with either 6 keV neon ions or xenon ions at 5, 7 or 9.5 keV with a flux of 3 × 1013 ions cm−2 s−1. Germanium NWs were irradiated with 30 or 70 keV xenon ions with a flux of 1013 ions cm−2 s−1. These new results are combined with those reported in the literature in a systematic analysis using a custom implementation of the transport of ions in matter Monte Carlo computer code to facilitate a direct comparison with experimental results taking into account the wide range of experimental conditions. Across the various studies this has revealed underlying trends and forms the basis of a critical review of the various mechanisms which have been proposed to explain the deformation of semiconductor NWs under ion irradiation.

Tuning of the electronic structure and magnetic properties of xenon ion implanted zinc oxide

A correlation between the electronic structure and magnetic properties of ZnO single crystals (ZnO SCs) and 300 keV xenon ion (Xe3+) implanted ZnO SCs has been studied using x-ray absorption near edge structure (XANES) spectroscopy, valence band photoemission spectroscopy (VB-PES), x-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and a superconducting quantum interference device-type magnetometer. The XANES studies revealed that the number of unoccupied p-states in the implanted ZnO SCs is higher than in the pristine ZnO SCs. In the implanted ZnO SCs, the binding energy of the Zn 2p3/2 core level peak shifted to higher energy which further confirmed the increase in the valence band maximum (VBM) energy level. The VBM of the xenon ion (Xe3+) implanted ZnO SCs increased from 3.17–3.49 eV, obtained from UPS (He-I) measurements. A VB-PES study revealed that the number of electrons in the valence band of the O 2p–Zn 4sp hybridized states of the implanted ZnO SCs is higher than in the pristine ZnO SCs. The magnetic M–H loops demonstrated enhanced room temperature ferromagnetism in the Xe implanted ZnO SCs, attributable to the increasing number of surface defects and native defect sites in oxygen vacancies and zinc interstitials.

Investigating the effect of heat treatment on the diffusion behaviour of xenon implanted in glassy carbon

The effect of sequential isochronal annealing on the diffusion behavior of implanted xenon in glassy carbon is reported. Glassy carbon substrates were implanted with 200 keV xenon ions to a fluence of 1 × 1016Xe+cm−2. The sample was annealed in vacuum at temperatures ranging from 300 °C to 1000 °C for 5 h in steps of 100 °C. The RBS depth profiles obtained at temperatures above 800 °C showed that some diffusion occurred. The broadening of the peaks was not accompanied with a loss of the implanted Xe. Microstructural changes in the glassy carbon substrate due to Xe bombardment and annealing were monitored using Raman spectroscopy. The Raman spectrum obtained after xenon bombardment showed that the glassy carbon substrate became amorphized. However, a slight recovery of the glassy carbon structure was noticed after heat treatment. The SEM micrographs of the glassy carbon substrate showed an increase in the surface roughness of the glassy carbon substrate after implantation. The increase in the roughness of the glassy carbon substrate was attributed to the sputtering of the loosely bonded carbon atoms along the polishing marks after implantation and annealing.

Residual stress in nano-structured stainless steel (AISI 316L) prompted by Xe+ ion bombardment at different impinging angles

The effect of low energy (<1 keV) xenon (Xe+) ion bombardment on the residual stress of polycrystalline iron alloy (AISI 316L steel) is reported. The results take into account the influence of the ion incident angle maintaining constant all other bombarding parameters (i.e., ion energy and current density, temperature, and doses). The bombarded surface topography shows that ions prompt the formation of nanometric regular patterns on the surface crystalline grains and stressing the structure. The paper focalizes on the study of the surface residual stress state stemming from the ion bombardment studied by means of the “sin2 ψ” and “Universal Plot” methods. The analysis shows the absence of shear stress in the affected material region and the presence of compressive in-plane residual biaxial stress (∼200 MPa) expanding up to ∼1 μm depth for all the studied samples. Samples under oblique bombardment present higher compressive stress values in the direction of the projected ion beam on the bombarded surface. The absolute value of the biaxial surface stress difference (σ11-σ22) increases on ion impinging angles, a phenomenon associated with the momentum transfer by the ions. The highest stress level was measured for ion impinging angles of 45° (σ11 = −380 ± 10 MPa and σ22 = −320 ± 10 MPa). The different stresses obtained in the studied samples do not affect significantly the formation of characteristic surface patterns.

Formation of Metastable Phases Observed in the Zr-Co-Nb Multilayers upon Ion Beam Irradiation

Six multilayered films with overall compositions of Co10Zr80Nb10, Co10Zr70Nb20, Co20Zr70Nb10, Co30Zr60Nb10, Co20Zr60Nb20and Co10Zr60Nb30were first prepared by alternatively depositing high pure metals onto the NaCl single crystal substrates and then irradiated by 160 keV xenon ions with the doses ranging from 2×1014Xe+/cm2to 9×1015Xe+/cm2. It turns out that several metastable phases, e.g. amorphous phase, were formed in these metallic multilayered films upon irradiation. Meanwhile, some interesting phase transformations were also observed with increasing irradiation doses. Besides, the phase formation and transformation mechanism of non-equilibrium alloy phases are discussed in terms of thermodynamics of solids and atomic collision theory. KEYWORDS: Zr-Co-Nb system, metastable alloys, phase transformation, ion beam irradiation

Synthesis of amorphous alloys and amorphous–crystalline composites in ternary Ni–Nb–Zr system by ion beam mixing

Six sets of Ni–Nb–Zr multilayered films were designed and prepared with the overall compositions of Ni63Nb28Zr9, Ni48Nb13Zr39, Ni29Nb60Zr11, Ni79Nb12Zr9, Ni31Nb31Zr38, and Ni31Nb11Zr58, and an ion beam mixing experiment was then conducted using 180 keV xenon ions. It is found that the Ni–Nb–Zr system is a readily glass-forming system, and both fully amorphous alloys and amorphous–crystalline composites could be synthesized. A detailed discussion was presented for the formation mechanism of the amorphous alloys and amorphous–crystalline composites. In addition, thermodynamic calculation predicted the glass-forming ability of the system, which is in good agreement with the IBM results. The experimental results and thermodynamic calculation obtained in present study are also supported e.g. by the observations of rapid solidification.

Different atomic structures observed from ternary Ni-Nb-Ta metallic glasses obtained by ion beam mixing

Four sets of ternary Ni-Nb-Ta multilayered samples with overall compositions of Ni69Nb8Ta23, Ni55Nb13Ta32, Ni42Nb16Ta42 and Ni29Nb18Ta53, respectively were prepared and subjected to 185 keV xenon ion beam mixing. The experimental results showed that in the four Ni-Nb-Ta multilayered samples, metallic glasses could all be obtained at appropriate doses, supporting the prediction directly from a proven realistic Ni-Nb-Ta interatomic potential through molecular dynamics simulations, and that two different atomic structures were observed, as in the corresponding selected area diffraction patterns, the locations of the diffused bands reflected from the metallic glass phases were observed at different angles for the Ni69Nb8Ta23 and Ni29Nb18Ta53 metallic glasses. Interestingly, Voronoi tellessation analysis indicated that the observed difference in atomic structures could be attributed to the distinct coordinate number spectra, i.e., the spectrum of the Ni69Nb8Ta23 metallic glass has its coordinate number (CN) equal to 13 as dominating atomic configuration (with a weight of about 27%), whereas for the Ni29Nb18Ta53 metallic glass, CN=14 is the dominating atomic configuration (also about 27%). Moreover, the distinct atomic configurations of the obtained Ni-Nb-Ta metallic glasses could be correlated to the structures of the constituent metals of the ternary Ni-Nb-Ta system, as the first neighbor of fcc is 12 and the sum of the first and second neighbors of bcc is 14, implying the structural heredity did play a role in metallic glass formation.

Modeling particle-induced electron emission in a simplified plasma Test Cell

Particle-induced electron emission (PIE) is modeled in a simplified, well-characterized plasma Test Cell operated at UCLA. In order for PIE to be a useful model in this environment, its governing equations are first reduced to lower-order models which can be implemented in a direct simulation Monte Carlo and Particle-in-Cell framework. These reduced-order models are described in full and presented as semi-empirical models. The models are implemented to analyze the interaction of low- and high-energy (∼1–2 keV) xenon ions and atoms with the stainless steel electrodes of the Test Cell in order to gain insight into the emission and transport of secondary electrons. Furthermore, there is a lack of data for xenon-stainless steel atom- and ion-surface interactions for similar environments. Using experimental data as a reference, both total yields and emitted electron energy distribution functions can be deduced by observing sensitivities of current collection results to these numerical models and their parameters.

Transmission electron microscopy of the amorphization of copper indium diselenide by in situ ion irradiation

Copper indium diselenide (CIS), along with its derivatives Cu(In,Ga)(Se,S)2, is a prime candidate for use in the absorber layers of photovoltaic devices. Due to its ability to resist radiation damage, it is particularly well suited for use in extraterrestrial and other irradiating environments. However, the nature of its radiation hardness is not well understood. In this study, transmission electron microscopy (TEM) with in situ ion irradiation was used to monitor the dynamic microstructural effects of radiation damage on CIS. Samples were bombarded with 400 keV xenon ions to create large numbers of atomic displacements within the thickness of the TEM samples and thus explore the conditions under which, if any, CIS could be amorphized. By observing the impact of heavily damaging radiation in situ—rather than merely the end-state possible in ex situ experiments—at the magnifications allowed by TEM, it was possible to gain an understanding of the atomistic processes at work and the underlying mechanism that give rise to the radiation hardness of CIS. At 200 K and below, it was found that copper-poor samples could be amorphized and copper-rich samples could not. This difference in behavior is linked to the crystallographic phases that are present at different compositions. Amorphization was found to progress via a combination of one- and two-hit processes. The radiation hardness of CIS is discussed in terms of crystallographic structures/defects and the consequences these have for the ability of the material to recover from the effects of displacing radiation.

Thermodynamic prediction of metastable phases of Ni-Ti system formed by ion beam mixing

By taking into account the valence electron number and periodic number of constituting metals, a new method is first proposed to calculate the structural enthalpy in the Miedema’s model and the modified Miedema’s model is then used to predict the formation of metastable phases in Ni-Ti system. To testify the relevance of the present prediction, the multilayered films of Ni1−xTix (x=27.3, 30.5, 42.4, 83.1, 89) are prepared and irradiated by 200 keV xenon ions. Experiment results reveal that uniform amorphous phases are obtained in the Ni72.7Ti27.3, Ni69.5Ti30.5, and Ni57.6Ti42.4 films by increasing the irradiation dose. While for the Ni16.9Ti83.1 and Ni11Ti89 films, an hcp Ti-based solid solution phase and a bcc Ti-rich solid solution phase coexist upon irradiation dose higher than 6×1014 Xe+/cm2. The predictions of relative stabilities of metastable phases in Ni-Ti system by the modified Miedema’s model match well with IBM experiments, thus justifying the modification proposed in the present study.

Synthesis of metallic glasses and metallic glass based composites in the Cu-Mo-Hf system by ion beam mixing

Single-phase and dual-phase metallic glasses as well as metallic glass based composites were synthesized in the Cu-Mo-Hf ternary metal system by 200 keV xenon ion beam mixing of far-from-equilibrium. It was found that Mo-Hf-based and Cu-Mo-based single-phase metallic glasses could be obtained at compositions around Cu17Mo20Hf63 and Cu34Mo57Hf9, respectively. Interestingly, at the nearly equal-atomic stoichiometry of Cu38Mo31Hf31, a dual-phase Cu-Mo-Hf metallic glass, consisting both of the Mo-Hf-based and Cu-Mo-based phases, was first obtained at relatively low irradiation doses ranging from (1–5)×1015 Xe+/cm2, and a single-phase metallic glass was eventually obtained at a dose of 7×1015 Xe+/cm2. In addition, two glass-based composites were obtained at the compositions of Cu14Mo62Hf24 and Cu77Mo14Hf9, and they consisted of the Mo-Hf based and Cu-Mo based metallic glasses, dissolved with some uniformly distributed BCC Mo-based and FCC Cu-based crystalline solid solutions, respectively. The formation mechanism of the above described non-equilibrium alloy phases was also discussed in terms of the atomic collision theory.

Non-equilibrium alloy phase formation and transformation driven by ion beam mixing in the Fe-Hf-Nb multilayers

In the ion beam mixing experiments, eight Fe-Hf-Nb multilayered films, with overall compositions of Fe67Hf22Nb11, Fe67Hf11Nb22, Fe54Hf38Nb8, Fe54Hf30Nb16, Fe54Hf11Nb35, Fe25Hf67Nb8, Fe25Hf50Nb25 and Fe25Hf11Nb64, were irradiated by 200 keV xenon ions to doses ranging from 3×1014 Xe+/cm2 to 7×1015 Xe+/cm2. The results showed that unique amorphous phases were obtained at designed alloy compositions, falling in the favored glass-forming region deduced from three binary metal sub-systems. Interestingly, at some alloy compositions, the crystal-amorphous-crystal transformations were observed back and forth while varying the irradiation doses. In addition, at the alloy composition of Fe25Hf67Nb8, a metastable FCC phase was formed through an HCP-FCC structural phase transformation and it had a large lattice constant identified to be a=4.51 A. Besides, the formation mechanism of non-equilibrium alloy phases was also discussed in terms of thermodynamics of solids and atomic collision theory.

A possible metastable MoPd 2 BCC phase in the Mo–Pd system obtained by ion beam mixing

A new metastable crystalline phase of bcc structure was obtained in the Mo–Pd system by 200 keV xenon ion beam mixing in the Mo 34Pd 66 and Mo 30Pd 70 multilayered thin films at relevant irradiation stages and its lattice constant was identified to be a BCC = 4.13 Å. The real composition of the bcc phase was then estimated to be around MoPd 2. The newly formed intermetallic compound with bcc structure gives evidence that MoPd 2 could be an ordered intermetallic phase in the binary phase diagram. The possible formation mechanism of the metastable MoPd 2 bcc phase was also discussed in terms of the atomic collision theory.

Synthesis of amorphous alloys and amorphous-crystalline composites in the Cu-Nb-Hf system by ion beam mixing

Seven sets of Cu-Nb-Hf multilayered films were designed and prepared with the overall compositions of Cu21Nb65Hf14, Cu33Nb49Hf18, Cu34Nb34Hf32, Cu34Nb10Hf56, Cu50Nb23Hf27, Cu58Nb10Hf32, and Cu70Nb8Hf22, and an ion beam mixing experiment was then conducted using 200 keV xenon ions. It is found that the Cu-Nb-Hf system is a metallic glass forming one, and the single amorphous alloys could be synthesized in the Cu-Nb-based alloys with less than 18 at.% of Hf as a third addition. Also, when the Hf concentration is greater than 18 at.%, i.e., at the compositions of Cu34Nb34Hf32, Cu34Nb10Hf56, Cu50Nb23Hf27, Cu58Nb10Hf32, and Cu70Nb8Hf22, ion beam mixing resulted in the formation of amorphous-crystalline composites, which might have better mechanical properties than single-phase glassy alloys. In addition, a detailed discussion was presented for the formation mechanism of the amorphous alloys and amorphous-crystalline composites.

Fractal patterns in Au–Ta multilayer films upon ion beam mixing

Metallic glass has been formed in Au50Ta50 multilayer films upon 200 keV xenon ion mixing at an irradiation dose of 1 × 1015 Xe+/cm2. Electron diffraction and high-resolution transmission electron microscopy analysis confirm that the metallic glass consists of two amorphous phases, evolved from Au and Ta lattices, respectively, and also reveal the formation of a fractal morphology with a fractal dimension of 1.73 ± 0.05 in the dual-phase glass features. In similar Au65Ta35 multilayer films, a fractal pattern is also observed at a dose of 1 × 1015 Xe+/cm2, while the pattern, with a fractal dimension of 1.74 ± 0.05, is composed in this case of a crystalline Au-based solid solution and a Ta-based amorphous phase. Interestingly, the fractal dimensions of the two irradiation-induced fractal patterns match quite well with that expected on a cluster diffusion-limited aggregation model.

Formation of non-equilibrium phases and associated structural transition in the Pd-Ta system induced by ion beam mixing

For the Pd-Ta system characterized by a negative heat of formation of −78 kJ/mol, 200 keV xenon ion beam mixing with nano-sized Pd-Ta multilayered films was conducted to study the non-equilibrium phase formation. The results showed that uniform amorphous alloys can be formed within a composition range of 25 at%–78 at% Ta, which falls in the maximum possible amorphization range of 22 at%–80 at% Ta predicted by the empirical model. Moreover, two metastable crystalline phases both of FCC structure, yet with different lattice constants were obtained. Interestingly, a self-assembled fractal pattern was observed in the Pd52Ta48 multilayered films after irradiation to a dose of 1×1015 Xe+/cm2 and its dimension was determined to be 1.75±0.05. The possible mechanisms for the formation of amorphous and metastable crystalline phases as well as for the growth of the fractal pattern were discussed.

Proposed truncated Cu–Hf tight-binding potential to study the crystal-to-amorphous phase transition

Proposed truncated Cu–Hf tight-binding potential was constructed by fitting the physical properties of Cu, Hf, and their stable compounds, i.e., Cu5Hf, Cu8Hf3, Cu10Hf7, and CuHf2. Based on the constructed potentials, molecular dynamics simulations were carried out to compare the relative stability of the crystalline solid solution and the disordered state. Simulation results not only reveal that the physical origin of crystal-to-amorphous transition is the crystalline lattice collapsing when the solute atoms exceeding the critical concentration, but also predict that the glass forming range (GFR) of the Cu–Hf system is 21–77 at. % Cu, which covers the GFRs determined by various metallic glass-producing techniques. Ion beam mixing experiments of the Cu–Hf system were conducted using 200 keV xenon ions and the results show that a uniform amorphous phase can be obtained in the Cu23Hf77 sample, matching well with the GFR determined by the interatomic potential, which, in turn, provides additional evidence to the relevance of the constructed Cu–Hf potential.