Search

Shape modification of embedded nanoparticles can be achieved by means of swift heavy ion irradiation. During irradiation, the particles elongate and align with the direction of the ion beam, presumably due to nanometer-scale phase transitions induced by individual ion impacts. However, the details of this transformation are not fully understood. The shape of metal nanoparticles embedded in dielectric matrices defines the non-linear optical properties of the composite material. Therefore, understanding the transformation process better is beneficial for producing materials with the desired optical properties. We study the elongation mechanism of gold nanoparticles using atomistic simulations. Here we focus on long-timescale processes and adhesion between the nanoparticle and the matrix. Without the necessity of ad-hoc assumptions used earlier, our simulations show that, due to adhesion with the oxide, the nanoparticles can grow in aspect ratio while in the molten state even after silicon dioxide solidifies. Moreover, they demonstrate the active role of the matrix: Only explicit simulations of ion impacts around the embedded nanoparticle provide the mechanism for continuous elongation up to experimental values of aspect ratio. Experimental transmission electron microscopy micrographs of nanoparticles after high-fluence irradiation support the simulations. The elongated nanoparticles in experiments and their interface structures with silica, as characterized by the micrographs, are consistent with the simulations. These findings bring ion beam technology forward as a precise tool for shaping embedded nanostructures for various optical applications. [ABSTRACT FROM AUTHOR]

Quantitative photoacoustic tomography aims at estimating optical parameters from photoacoustic images that are formed utilizing the photoacoustic effect caused by the absorption of an externally introduced light pulse. This optical parameter estimation is an ill-posed inverse problem, and thus it is sensitive to measurement and modeling errors. In this work, we propose a novel way to solve the inverse problem of quantitative photoacoustic tomography based on the perturbation Monte Carlo method. Monte Carlo method for light propagation is a stochastic approach for simulating photon trajectories in a medium with scattering particles. It is widely accepted as an accurate method to simulate light propagation in tissues. Furthermore, it is numerically robust and easy to implement. Perturbation Monte Carlo maintains this robustness and enables forming gradients for the solution of the inverse problem. We validate the method and apply it in the framework of Bayesian inverse problems. The simulations show that the perturbation Monte Carlo method can be used to estimate spatial distributions of both absorption and scattering parameters simultaneously. These estimates are qualitatively good and quantitatively accurate also in parameter scales that are realistic for biological tissues. [ABSTRACT FROM AUTHOR]

Nano-porosity in amorphous Ge formed by swift heavy ion irradiation displays a peculiar self-organisation process. Initially almost randomly distributed pores grow with increasing irradiation fluence and segregate in layers orientated parallel to the sample surface. This self-organisation mechanism depends on the ion energy, thickness of the amorphous Ge layer and the angle of ion incidence and shows a characteristic length depending on ion energy and irradiation angle. Molecular dynamics simulations of individual ion tracks show that voids form due to the transition from the low-density amorphous to the high-density liquid phase, which also gives rise to a flow directed away from large pores and surfaces. The flow results in a characteristic distance from surfaces and larger pores, below which new voids do not form, and supports the formation of voids at the amorphous/crystalline interface. Simulations also demonstrate that, while direct impacts can reposition small voids, partial or nearby impacts promote their growth at the same location. These processes are plausible drivers for the self-organization. [Display omitted] [ABSTRACT FROM AUTHOR]

Concentrated solid solution alloys have attracted rapidly increasing attention due to their potential for designing materials with high tolerance to radiation damage. To tackle the effects of chemical complexity in defect dynamics and radiation response, we present a computational study on swift heavy ion induced effects in Ni and equiatomic Ni -based alloys (Ni 50 Fe 50 , Ni 50 Co 50 ) using two-temperature molecular dynamics simulations (2T-MD). The electronic heat conductivity in the two-temperature equations is parameterized from the results of first principles electronic structure calculations. A bismuth ion (1.542 GeV) is selected and single impact simulations performed in each target. We study the heat flow in the electronic subsystem and show that alloying Ni with Co or Fe reduces the heat dissipation from the impact by the electronic subsystem. Simulation results suggest no melting or residual damage in pure Ni while a cylindrical region melts along the ion propagation path in the alloys. In Ni 50 Co 50 the damage consists of a dislocation loop structure (d = 2 nm) and isolated point defects, while in Ni 50 Fe 50 , a defect cluster (d = 4 nm) along the ion path is, in addition, formed. The simulation results are supported by atomic-level structural and defect characterizations in bismuth-irradiated Ni and Ni 50 Fe 50 . The significance of the 2T-MD model is demonstrated by comparing the results to those obtained with an instantaneous energy deposition model without consideration of e-ph interactions in pure Ni and by showing that it leads to a different qualitative behavior. [ABSTRACT FROM AUTHOR]

Abstract: Swift heavy ions (SHI), of specific kinetic energies in the excess of 1MeV/u, can create cylindrical regions of structural transformation in SiO2 targets, also known as SHI tracks. Recent measurements of the track cross-sections in -quartz show significant and consistent discrepancies across different experimental techniques used. In particular, the track radii obtained from channelling experiments based on the Rutherford Backscattering Spectrometry (RBS-c) method increase monotonically with the electronic stopping power, whereas the track radii obtained from the Small Angle X-ray scattering (SAXS) saturate past a certain stopping power threshold. We perform a systematic study of the structure of the -quartz tracks obtained from the molecular dynamics (MD) simulations incorporating a time-dependent energy deposition based on the inelastic thermal spike model, which allows us to discuss the possible origins of these experimental discrepancies. [Copyright &y& Elsevier]

Abstract: We have studied the elongation of Au nanoclusters embedded in amorphous SiO2 using MD simulations. The effect of swift heavy ions (SHI) was implemented using instantaneous energy deposition with a radial profile that was obtained from the inelastic thermal spike model. During the first impact on the cluster, the clusters (d =9–11nm) gained about 20% in length due to thermal stress within the cluster, which is released by a longitudinal flow of Au atoms to the ion track in silica. Our simulations also show that high temperatures at the track core lead to the formation of silica vapor, whose pressure may flatten the cluster. [Copyright &y& Elsevier]

Highly energetic ions have been previously used to modify the shape of metal nanoparticles embedded in an insulating matrix. In this work, we demonstrate that under suitable conditions, energetic ions can be used not only for shape modification but also for manipulation of nanorod orientation. This observation is made by imaging the same nanorod before and after swift heavy ion irradiation using a transmission electron microscope. Atomistic simulations reveal a complex mechanism of nanorod re-orientation by an incremental change in its shape from a rod to a spheroid and further back into a rod aligned with the beam. [ABSTRACT FROM AUTHOR]

• Bayesian framework enables accurate estimates and reliability assessment. • Modeling errors can be compensated using Bayesian methods. • Modeling of the errors improves the reliability of the credibility estimates. In quantitative photoacoustic tomography, the aim is to reconstruct distributions of optical parameters of an imaged target from an initial pressure distribution obtained from ultrasound measurements. In order to obtain accurate and quantitative information on the optical parameters, modeling light transport in the target is required. Utilizing an approximative model for light transport would be favorable to reduce the computational cost, but the modeling errors of the approximative model can result in significant errors in the reconstructions. In this work, we approach the image reconstruction problem of quantitative photoacoustic tomography in the Bayesian framework. We utilize the Bayesian approximation error method to compensate for the modeling errors between the diffusion approximation and Monte Carlo model for light transport. The approach is studied with two-dimensional numerical simulations with varying optical parameters and noise levels. The results show that Bayesian approximation error method can be used to reduce the effects of the modeling errors in quantitative photoacoustic tomography in a wide range of optical parameters. [ABSTRACT FROM AUTHOR]

We study the effects of local heating due to electron-phonon coupling in cumulative 10 MeV Ni ion irradiation on pre-damaged pure Ni and equiatomic NiFe alloy. In particular, we estimate the defect recovery in NiFe compared to pure Ni due to the chemical disorder. The simulations are done using the two-temperature molecular dynamics approach that includes the electronic stopping and electron-phonon interactions. A total of 50% of the damage that consists of isolated point defects and small defect clusters is recovered in NiFe, whereas no noticeable recovery is observed in elemental Ni. We have also demonstrated strain relaxation associated with defect recombination based on the results from both simulation and X-ray diffraction measurements in NiFe. The damage recovery primarily depends on pre-existing damage level, defect configuration, ion energy (stopping powers) and electron-phonon coupling parameter of the materials of interest. [ABSTRACT FROM AUTHOR]