Ion Channeling Research Articles

Correlation between density and hydrogen content in vertically aligned carbon nanotube forests by ion beam analysis

Interactions of vertically aligned multiwall carbon nanotubes (CNTs) with high energy He+ beams were studied using elastic recoil detection analysis and ion beam channeling. The relationship between the elastic recoil of hydrogen, the depth of He–H interactions, and the number of carbon atoms per volume (denoted as effective density) was calculated. Ion channeling was observed in CNT forests shorter than 40 μm. It was found that the effective density and hydrogen content were inversely correlated with the CNT height. In compliance with channeling and density calculations, the authors propose that this effect is due to the weakening of Van-der-Waals forces in taller CNT forests. The methodology suggested in this work may be extended to assessing densities of thin, highly porous materials.

Defect structure of oxygen-vacancy clusters in O18 and self ion implanted Fe(100) crystal by ion channeling and ab-initio study

Lattice site position of O18 in the presence of vacancy defects in Fe(100) crystal is measured by ion channeling and the corresponding oxygen-vacancy cluster configuration is predicted with Density Functional Theory (DFT) calculations. Energy dependent dechanneling and Slow Positron Doppler broadening analysis show the defects to be vacancy dislocation loops. In O18 ion implanted Fe, O18 is found to be in tetrahedral interstitial position. If self ion is irradiated together with O18 to increase vacancy defects, O18 is found to be trapped at 1.2 Å away from the lattice position along <111>. From DFT calculations, similar lattice site of O is predicted with a displacement of 1.1 Å along <111> for the interaction of oxygen with ½ <111> vacancy loop structure. Upon increasing vacancy defects further by increasing the fluence of self ions, O18 is found to shift near octahedral interstitial site displaced by 0.6 Å along <100>. DFT calculations show a similar displacement of 0.46 Å along <100> for the interaction of oxygen with <100> vacancy loop structure. Our combined experimental and DFT studies provide strong evidence of O trapping at vacancy dislocation loops and O trapping site is sensitive to ½ <111> and <100> type vacancy dislocation loops.

Fine features of parametric X-ray radiation by relativistic electrons and ions

In present work within the frame of dynamic theory for parametric X-ray radiation in two-beam approximation we have presented detailed studies on parametric radiation emitted by relativistic both electrons and ions at channeling in crystals that is highly requested at planned experiments. The analysis done has shown that the intensity of radiation at relativistic electron channeling in Si (110) with respect to the conventional parametric radiation intensity has up to 5% uncertainty, while the error of approximate formulas for calculating parametric X-ray radiation maxima does not exceed 1.2%. We have demonstrated that simple expressions for the Fourier components of Si crystal susceptibility χ0 and χgσ could be reduced, as well as the temperature dependence for radiation maxima in Si crystal (diffraction plane (110)) within Debye model. Moreover, for any types of channeled ions it is shown that the parametric X-ray radiation intensity is proportional to z2−b(Z,z)/z with the function b(Z,z) depending on the screening parameter and the ion charge number z=Z−Ze.

Normal and reverse defect annealing in ion implanted II-VI oxide semiconductors

Post-implantation annealing is typically used to remove structural defects and electrically activate implanted dopants in semiconductors. However, ion-induced defects and their interaction with dopants in group II oxide semiconductors are not fully understood. Here, we study defect evolution in the course of annealing in CdO and ZnO materials implanted with nitrogen which is one of the most promising candidates for p-type doping. The results of photoluminescence and ion channeling measurements revealed a striking difference in defect behavior between CdO and ZnO. In particular, the defect annealing in CdO exhibits a two stage behavior, the first stage accounting for efficient removal of point defects and small defect clusters, while the second one involves gradual disappearance of extended defects where the sample decomposition can play a role. In contrast, a strong reverse annealing occurs for ZnO with a maximum defect concentration around 900 °C. This effect occurs exclusively for nitrogen ions and is attributed to efficient growth of extended defects promoted by the presence of nitrogen.

An Overview on the Formation and Processing of Nitrogen-Vacancy Photonic Centers in Diamond by Ion Implantation

Nitrogen-vacancy (NV) in diamond possesses unique properties for the realization of novel quantum devices. Among the possibilities in the solid state, a NV defect center in diamond stands out for its robustness—its quantum state can be initialized, manipulated, and measured with high fidelity at room temperature. In this paper, we illustrated the formation kinetics of NV centers in diamond and their transformation from one charge state to another. The controlled scaling of diamond NV center-based quantum registers relies on the ability to position NV defect centers with high spatial resolution. Ion irradiation technique is widely used to control the spatial distribution of NV defect centers in diamond. This is addressed in terms of energetics and kinetics in this paper. We also highlighted important factors, such as ion struggling, ion channeling, and surface charging, etc. These factors should be considered while implanting energetic nitrogen ions on diamond. Based on observations of the microscopic structure after implantation, we further discussed post-annealing treatment to heal the damage produced during the ion irradiation process. This article shows that the ion implantation technique can be used more efficiently for controlled and efficient generation of NV color centers in diamond, which will open up new possibilities in the field of novel electronics and computational engineering, including the art of quantum cryptography, data science, and spintronics.

Interactions of moving charged particles with triple-walled carbon nanotubes

We study plasmon excitations and channeling trajectories of charged particles in triple-walled carbon nanotubes (TWNTs) based on a semi-classical kinetic model combined with the Molecular Dynamics method. Numerical results show that the outer and inner tubes of a TWNT exert strong influence on the peak structures of the self-energy (or the image potential) and the stopping power curves for the channeling ion, resulting in one or two narrow peaks in the low speed region. In addition, the radial dependencies of the total potential, which includes the image potential due to dynamic polarization of the electron gas in nanotubes and a reactive empirical bond order potential for atomic interactions, are compared for single-walled carbon nanotubes (SWNTs), double-walled carbon nanotubes (DWNTs) and TWNTs. By comparing the ion channeling trajectories in those types of nanotubes, we conclude that the variation of the total energy of ions with their channeling distance along the nanotube axis is related to the types of channeling trajectories, exhibiting smooth helical shapes in TWNTs and a succession of sharp reflections off the wall in SWNTs and DWNTs.

Dependence of the structure of ion-modified NiTi single crystal layers on the orientation of irradiated surface

The composition and structure of Si layers implanted into titanium nickelide single crystals with different orientations relative to the ion beam propagation direction have been studied using Auger electron spectroscopy and transmission electron microscopy. The role of the “soft” [111]B2 and “hard” [001]B2 NiTi orientations in the formation of the structure of ion-modified surface layer, as well as the defect structure of the surface layers of the single crystals, has been revealed. Orientation effects of selective sputtering and channeling of ions, which control the composition and thickness of the oxide and amorphous layers being formed, ion and impurity penetration depth, as well as the concentration profile of the Ni distribution over the surface, have been detected.

Crystallographic Orientation Maps Obtained from Ion and Backscattered Electron Channeling Contrast

Crystallographic Orientation Maps Obtained from Ion and Backscattered Electron Channeling Contrast

Dose-rate dependence of damage buildup in 3C-SiC

The influence of the defect generation rate on radiation damage processes in SiC remains poorly understood. Here, we use a combination of ion channeling and transmission electron microscopy to systematically study the dose-rate dependence of damage buildup in 3C-SiC bombarded in the temperature range of 25–200 °C with 500 keV Ar ions. The results reveal a pronounced dose-rate effect, whose magnitude increases close-to-linearly with temperature. When ion dose and temperature are held constant, the dose-rate dependence of the damage level is nonlinear, with saturation at high dose rates. Electron microscopy reveals that the average size of stable defect clusters increases with increasing dose rate. These findings have important implications for understanding and predicting radiation damage in SiC.

Electronic stopping power of slow-light channeling ions in ZnTe from first principles

Nonadiabatic dynamics simulations are performed to investigate the electronic stopping power of a helium ion moving through ZnTe crystalline thin films under channeling conditions. Using ab initio time-dependent density-functional theory, we found by direct simulation that electronic stopping power versus projectile velocity deviates from velocity proportionality and displays a transition between two velocity regimes for helium ions channeling along middle crystalline axes in $\ensuremath{\langle}100\ensuremath{\rangle}$ and $\ensuremath{\langle}111\ensuremath{\rangle}$ channels and also in a $\ensuremath{\langle}110\ensuremath{\rangle}$ channel with low-impact parameters. This transition causes a change in the slope of the energy loss versus ion velocity curve at a characteristic velocity related to the impact parameter and the lattice plane spacing. It may be an indication of extra energy loss channel beyond the electron-hole excitation. To analyze it, we checked the charge transfer between the moving projectiles and host atoms. It is found that the soft transition between two velocity regimes can be attributed to the resonant coherent excitation stimulated by the time-periodic potential experienced by the channeling ion and also the charge exchange in close encounters between Helium ion and host atoms.

Nitrogen-vacancy centers created by N+ ion implantation through screening SiO2 layers on diamond

We report on an ion implantation technique utilizing a screening mask made of SiO2 to control both the depth profile and the dose. By appropriately selecting the thickness of the screening layer, this method fully suppresses the ion channeling, brings the location of the highest nitrogen-vacancy (NV) density to the surface, and effectively reduces the dose by more than three orders of magnitude. With a standard ion implantation system operating at the energy of 10 keV and the dose of 1011 cm2 and without an additional etching process, we create single NV centers close to the surface with coherence times of a few tens of μs.

Swift heavy ion track formation in SrTiO3 and TiO2 under random, channeling and near-channeling conditions

Conditions for ion track formation in single crystal SrTiO3 and TiO2 (rutile) after irradiations using swift heavy ion beams with specific energies below 1 MeV/amu were investigated in this work. Rutherford backscattering spectroscopy in channeling was used to measure ion tracks in the bulk, while atomic force microscopy was used for observation of ion tracks on the surfaces. Variations in the ion track sizes and respective thresholds were observed after irradiations under random, channeling and near-channeling conditions close to normal incidence. These variations are attributed to the specifics of the electronic stopping power of swift heavy ions under the investigated conditions. In the case of ion channeling, electronic stopping power is reduced and observed ion tracks are smaller. The opposite was found under the near-channeling conditions when lowering of the ion track formation threshold was observed. We attribute this finding to the oscillating electronic stopping power with large peak values. For both materials, thresholds for bulk and surface ion track formation were found to be surprisingly close, around 10 keV nm−1. Obtained results are compared with predictions of the analytical thermal spike model.

Structural heteroepitaxy during topochemical transformation of silicon to silicon carbide

Silicon carbide samples synthesized from silicon by topochemical substitution of atoms are studied by the ion channeling method. The results of the analysis unambiguously demonstrate the occurrence of structural heteroepitaxy. The lattice of synthesized silicon carbide of hexagonal polytype 6H is epitaxially matched in the 〈0001〉 direction with the lattice grating grid array network of an initial substrate silicon in the 〈111〉 direction. The main features of structural self-coupling matching in this epitaxial heterocomposite are revealed. Despite the very large silicon carbide and silicon lattice parameter mismatch, the misfit dislocation density at the interface is low, which is a feature of the topochemical substitution method leading to comparable structures.

Publisher's Note: Large fraction of crystal directions leads to ion channeling [Phys. Rev. B 94 , 214109 (2016)

Publisher's Note: Large fraction of crystal directions leads to ion channeling [Phys. Rev. B 94 , 214109 (2016)

The Importance of REST for Development and Function of Beta Cells.

Beta cells are defined by the genes they express, many of which are specific to this cell type, and ensure a specific set of functions. Beta cells are also defined by a set of genes they should not express (in order to function properly), and these genes have been called forbidden genes. Among these, the transcriptional repressor RE-1 Silencing Transcription factor (REST) is expressed in most cells of the body, excluding most populations of neurons, as well as pancreatic beta and alpha cells. In the cell types where it is expressed, REST represses the expression of hundreds of genes that are crucial for both neuronal and pancreatic endocrine function, through the recruitment of multiple transcriptional and epigenetic co-regulators. REST targets include genes encoding transcription factors, proteins involved in exocytosis, synaptic transmission or ion channeling, and non-coding RNAs. REST is expressed in the progenitors of both neurons and beta cells during development, but it is down-regulated as the cells differentiate. Although REST mutations and deregulation have yet to be connected to diabetes in humans, REST activation during both development and in adult beta cells leads to diabetes in mice.

Ion channeling measurements of β-FeSi&lt;sub&gt;2&lt;/sub&gt; films epitaxially grown on Si(111) and their analysis by multiple scattering theory

Ion channeling measurements of β-FeSi&lt;sub&gt;2&lt;/sub&gt; films epitaxially grown on Si(111) and their analysis by multiple scattering theory

About mechanisms of radiation-induced effect of nanostructurization of near-surface volumes of metals

Mechanisms of the radiation-induced development of nanostructures in subsurface metal regions have been analyzed based on field-ion microscopy data. It is concluded that the modification of near-surface metal regions on a nanometer scale as a result of the interaction with Ar+ ion beams proceeds by several mechanisms. In particular, for a fluence of F = 1016 ion/cm2 (at an ion energy of E = 30 keV), the main contribution is due to the ion channeling. A tenfold increase in the ion fluence leads to prevailing deformation mechanism in nanostructure formation in the subsurface metal regions.

Large fraction of crystal directions leads to ion channeling

It is well established that when energetic ions are moving in crystals, they may penetrate much deeper if they happen to be directed in some specific crystal directions. This `channeling' effect is utilized for instance in certain ion beam analysis methods and has been described by analytical theories and atomistic computer simulations. However, there have been very few systematic studies of channeling in directions other than the principal low-index ones. We present here a molecular dynamics-based approach to calculate ion channeling systematically over all crystal directions, providing ion `channeling maps' that easily show in which directions channeling is expected. The results show that channeling effects can be quite significant even at energies below 1 keV, and that in many cases, significant planar channeling occurs also in a wide range of crystal directions between the low-index principal ones. In all of the cases studied, a large fraction ($\ensuremath{\sim}20--60%$) of all crystal directions show channeling. A practical implication of this is that modern experiments on randomly oriented nanostructures will have a large probability of channeling. It also means that when ion irradiations are carried out on polycrystalline samples, channeling effects on the results cannot a priori be assumed to be negligible. The maps allow for easy selection of good `nonchanneling' directions in experiments or alternatively finding wide channels for beneficial uses of channeling. We implement channeling theory to also give the fraction of channeling directions in a manner directly comparable to the simulations. The comparison shows good qualitative agreement. In particular, channeling theory is very good at predicting which channels are active at a given energy. This is true down to sub-keV energies, provided the penetration depth is not too small.

Effects of ion bombardment on bulk GaAs photocathodes with different surface-cleavage planes

Photocathodes with (110) cleave plane are shown to be the least susceptible to ion damage. The observed susceptibility is explained by enhanced ion channeling.

Effects of Fe concentration on the ion-irradiation induced defect evolution and hardening in Ni-Fe solid solution alloys

Understanding alloying effects on the irradiation response of structural materials is pivotal in nuclear engineering. To systematically explore the effects of Fe concentration on the irradiation-induced defect evolution and hardening in face-centered cubic Ni-Fe binary solid solution alloys, single crystalline Ni-xFe (x = 0–60 at%) alloys have been grown and irradiated with 1.5 MeV Ni ions. The irradiations have been performed over a wide range of fluences from 3 × 1013 to 3 × 1016 cm−2 at room temperature. Ion channeling technique has shown reduced damage accumulation with increasing Fe concentration in the low fluence regime, which is consistent to the results from molecular dynamic simulations. No irradiation-induced compositional segregation was observed in atom probe tomography within the detection limit, even in the samples irradiated with high fluence Ni ions. Transmission electron microscopy analyses have further demonstrated that the defect size significantly decreases with increasing Fe concentration, indicating a delay in defect evolution. Furthermore, irradiation induced hardening has been measured by nanoindentation tests. Ni and the Ni-Fe alloys have largely different initial hardness, but they all follow a similar trend for the increase of hardness as a function of irradiation fluence.