Structure dependent electronic sputtering of a-C:H films by swift heavy ions

Electronic sputtering of a-C:H films by elastic recoil detection analysis technique is studied under 80 MeV Ni⁸⁺ and 150 MeV Ag¹³⁺ ion irradiations. These studies show that electronic sputtering yield of C and H from the films varies with film structure quite significantly. The structure of the films is analyzed from the characteristic graphitic (G) and disordered (D) modes of Raman vibration. Atomic force microscopy was performed on two films for grain size determination. The difference in electronic sputtering yields is discussed on the basis of structural influence of the films on swift heavy ion and solid interaction.     

Highly sensitive molecular detection with swift heavy ions

Various imaging techniques using microbeam have been applied in biology. Secondary ion mass spectrometry (SIMS) is one of the prominent tools for biological imaging; SIMS can provide data on molecular distribution in biological samples smaller than 1 μm. However, conventional SIMS has only low sensitivity for molecular ions; therefore there is a need for beams of more sensitive primary ions. Plasma desorption mass spectrometry (PDMS) is a method using high energy fission fragments from excitation of a ²⁵²Cf source, and it allows ionization of large molecules (typically up to 20 kDa) due to the dense electronic excitation. Although PDMS is not in use today because of the development of soft ionization methods, ionization induced by high energy ion collision still remains the only method which combines high spatial resolution and sensitive detection of large molecules. In this work, the secondary ion yield of amino acid and phospholipid was measured for 6 MeV Cu⁴⁺. The yields were compared to bismuth cluster ions, which achieve relatively high yield. It was confirmed that the swift heavy ion has a couple of hundred times higher yield for large molecules than bismuth cluster ions.     

Track creation after swift heavy ion irradiation of insulators

The dynamics of structural modifications of insulators irradiated with swift heavy ions were investigated theoretically applying a combination of Monte-Carlo method (MC), used to describe SHI penetration and following excitation and relaxation of the electronic subsystem, with Two Temperature Model (TTM) describing the heating of the lattice. This MC-TTM combination demonstrates that secondary ionizations play a very important role for the track formation process. They lead to an additional term in the heat diffusion equation related to energy stored in the hole subsystem. This storage of energy causes a significant delay of heating and prolongs the timescales up to tens of picoseconds.     

Amorphization kinetics under swift heavy ion irradiation: A cumulative overlapping-track approach

A simple illustrative physical model is presented to describe the kinetics of damage and amorphization by swift heavy ions (SHI) in LiNbO₃. The model considers that every ion impact generates initially a defective region (halo) and a full amorphous core whose relative size depends on the electronic stopping power. Below a given stopping power threshold only a halo is generated. For increasing fluences the amorphized area grows monotonically via overlapping of a fixed number N of halos. In spite of its simplicity the model, which provides analytical solutions, describes many relevant features of the kinetic behaviour. In particular, it predicts approximate Avrami curves with parameters depending on stopping power in qualitative accordance with experiment that turn into Poisson laws well above the threshold value.     

Effect of swift heavy ion irradiations in polycrystalline aluminum nitride

Thanks to its high thermal conductivity, aluminum nitride may be a serious candidate as fuel coating for the Gas Fast Reactor. However, its behavior under irradiation is not entirely well understood. In order to catch a glimpse of this behavior, specimens were irradiated with swift heavy ions of different energies then characterized by both thermally stimulated luminescence and optical absorption spectrophotometry. With these techniques, the native defects, as well as those affected by irradiation, were identified: thus, by comparison to the virgin sample, no new defect detectable by these techniques is created by irradiations. Eventually, the fact that these techniques complement each other allowed to understand the effect of irradiation parameters on the defect concentration.     

Density evolution in formation of swift heavy ion tracks in insulators

Swift heavy ion irradiation leaves a latent ion track around the ion path in many materials. Here we report computational molecular dynamics (MD) simulation results on track formation in several insulating materials, quartz, amorphous silica (a-SiO₂), zinc oxide and diamond, concentrating especially in mass transport leading to density variations in the track volume during the initial stages of track formation. These details are largely unobservable in experiments due to the picosecond timescale and very local nature, and also in many computational models of track formation. Earlier a low-density core - high-density shell fine structure has been observed in latent tracks in amorphous silica, and here we study if other materials than silica show similar behavior. The results highlight the dynamical nature of track formation, that includes competing effects of heat and mass transport, rapid quenching of the heated area and recrystallization.     

Electronic stopping power of swift heavy ions in carbon

The mean stopping power of He, Be, C and Al in carbon has been studied in a continuous energy ranging from 100 to 800 keV per nucleon using a modified time of flight-energy elastic recoil detection analysis set-up. In order to eliminate the calibration problems of Si detectors associated with heavy ions, the time of flight section was used to measure recoil energy both before entering and after passing through the stopping foil. Consequently, the energy dependence of the stopping power was achieved with high precision. The effect of the foil thickness on the mean stopping power was investigated by tilting the stopping foil at different angles to the incoming particles to increase the effective foil thickness up to 22%. The stopping curves obtained at different tilting angles show a similar energy dependence with scattering less than 0.8% from each other, and no obvious thickness dependence is observed. Both the stopping power and the energy dependence are in good agreement with the literature data. Comparing with semi-empirical SRIM prediction, considerable discrepancies up to 6%, 10% and 8% in stopping values for Be, C and Al, respectively, are observed.     

Total sputtering yields of solids under MeV-energy Si ion bombardment

Total sputtering yields have been measured for SiO₂ and Cu targets bombarded with Si ions at an incident energy between 500 keV and 5.0 MeV using a quartz crystal microbalance technique. In order to measure total yields accurately, we have developed a beam modulation technique to avoid the effect of thermal drift. In the MeV energy range, an ion penetrates through thin SiO₂ and Cu targets and is implanted into a quartz crystal. Therefore, the thickness of these layers deposited on quartz crystals was carefully controlled to avoid damage of quartz crystal by incident ions. As a result, total sputtering yields of SiO₂ increased with incident Si ion energy, while those of the Cu target decreased. The total yields of the SiO₂ target were represented well by a power low of the electronic stopping power.     

Unique capabilities of heavy ion elastic recoil detection with gas ionization detectors

Specific aspects of heavy ion elastic recoil detection (ERD) with gas ionization detectors have been studied using representative measurements. A particular strength of the technique is the detection and direct quantification of elements with atomic numbers in the range Z=2–8, which are often not accessible with other ion beam techniques. Within the wider spectrum of analytical techniques in materials science, heavy ion ERD has unique capabilities, when the particular problem requires in addition the analysis of heavy elements or hydrogen detection. Whenever only heavy element analysis or only hydrogen profiling is of interest, alternative techniques tend to be superior.     

Study of swift heavy ion irradiation effect on indium tin oxide coated electrode for the dye-sensitized solar cell application

Our long term objective is to study the swift heavy ion (SHI) irradiation effect on photoanode of dye-sensitized solar cell (DSSC) with the aim to investigate the stability of DSSC component in space irradiations and possibility of improvement in efficiency of DSSC due to ion induced effects in oxide layer. The DSSC photoanode consists of three layers viz; transparent conducting oxide (TCO), porous oxide with wide band gap and monolayer of dye molecule on top of corning glass substrate respectively. In the present study, procured radio frequency (RF) sputtered indium tin oxide (ITO) film on corning glass substrate were irradiated by SHI using 110 MeV Ni⁸⁺ ions at different fluences ranging from 3.0 × 10¹¹ ion/cm² to 1.0 × 10¹⁴ ion/cm². After irradiation significant changes have been observed in the structural, optical and electrical properties using X-ray diffraction (XRD), UV-Vis and Four Probe measurements, respectively. Overall there is 13% increase in optical transmittance which is favorable and moderate increase in sheet resistance from 8 Ω/□ to 18 Ω/□ which is still within acceptable limits for DSSC applications.     

高能重离子辐照制备碳氮化合物材料研究

室温下,先用100-120 keV的N离子注入类金刚石薄膜和石墨中,注入剂量5×1017至5×1018 cm-2,再用高能Xe、U、C60离子分别辐照注氮后的样品,然后用显微FTIR和Raman、XRD/XPS等手段进行分析表征,研究了实验样品中由辐照引起的新化学键和新相的产生.实验结果显示,高能重离子辐照可在所有样品中产生大量的CN键,高N浓度和大密度能量沉积导致sp3/sp2键比率的增加以及形成α-和β-C3N4必需的N-sp3C键的量的增加.C60离子辐照在注氮石墨样品中引起了ta-C、N=sp2C和N-sp3C键的形成;而高能离子辐照在注氮类金刚石薄膜样品中产生了α-和β-C3N4晶态夹杂物,其尺寸在1.4-3.6 nm之间.     

Swift heavy ion induced magnetic phase transition of Fe–Rh alloy

We report the effects of swift heavy ion irradiation on structure and magnetic properties of Fe–50at.%Rh alloys. The alloys are irradiated with 120–200 MeV heavy ions (Ni, Kr or Xe) at room temperature. Before and after the irradiations, the magnetization and the lattice parameter are measured by using superconducting quantum interference device (SQUID) and X-ray diffractometer (XRD), respectively. The lattice parameter at room temperature increases by about 0.3% and antiferromagnetic–ferromagnetic transition temperature decreases below 5 K by the irradiations. Effects of electronic excitation due to swift heavy ions on the change in magnetic properties and lattice structure are discussed.     

C60薄膜在快重离子辐照下的结构稳定性研究

用傅立叶变换红外光谱、X射线衍射谱、X射线光电子谱和拉曼散射技术分析了能量为GeV量级的S、Fe、Xe、和U离子,以及120keV的H离子在室温下辐照多层堆积C60薄膜的结构稳定性,即快重离子在C60薄膜中由高密度电子激发引起的效应,主要包括C60分子的聚合、分子结构的损伤、新的高温-高压相的形成和晶态向非晶态的转变.     

Selective disorder in the CuO basal planes of YBa2Cu3O7−y by swift heavy ion induced secondary electrons

In situ temperature dependent resistivity, ρ(T) study on c-axis oriented YBa₂Cu₃O_7−y thin films irradiated with 200 MeV Ag ions at 79 K is shown to induce point defects in addition to amorphous ion tracks. Annealing characteristics of these defects indicate that the point defects are basically oxygen disorder selectively created in the CuO basal planes of YBa₂Cu₃O_7−y structure by secondary electrons emanating from the path of 200 MeV Ag ions. These electrons are shown to create defects by inelastic interaction process. Contrary to the general expectation, we show that the superconducting transition temperature, T_c is suppressed at a rate two orders of magnitude faster at extremely low fluences where ion tracks are far apart from each other than at high fluences where tracks tend to overlap. The transition width on the other hand remains unaffected while resistivity shows a large increase at high fluences. At high fluences, a two-step superconducting transition emerged, which indicate the evolution of two types of superconducting regions with distinctly different T_cs.     

Characterization of swift heavy ion tracks in CaF2 by scanning force and transmission electron microscopy

Single crystals of fluorite (CaF₂) were exposed to various swift heavy ions (Ca up to U) of energy 1–11.1 MeV per nucleon, covering a large range of electronic stopping power S_e between 4.6 and 35.5 keV/nm. The irradiated (1 1 1) cleaved surfaces were investigated by means of scanning force microscopy in tapping mode. Nanometric hillocks produced by the ion projectiles were analyzed in terms of creation efficiency E_eff, diameter and height values, and diameter–height correlation. Hillock formation appears with a low efficiency above a S_e threshold of 5 keV/nm. The mean height of these hillocks is approximately constant (1 nm) between 5 and 10 keV/nm and increases linearly with S_e above 10 keV/nm reaching 12.5 nm for the largest S_e value investigated. Similarly, the efficiency grows versus S_e achieving 100% for S_e > 13 keV/nm where each projectile produces an individual hillock. Above 13 keV/nm, the hillock height and diameter are strongly correlated. The diameter was deduced by graphical deconvolution of the scanning-tip curvature that is determined experimentally for each set of measurements. In the entire S_e regime, the mean diameter exhibits a constant value of 13 nm, which is significantly larger than 6 nm wide tracks observed by transmission electron microscopy.     

Band gap controlled H loss from passivated Hg1−xCdxTe (MCT) wafers under intense electronic excitations

We report here loss of H monitored by on-line elastic recoil detection analysis (ERDA) technique from passivated Hg_1−xCd_xTe (MCT) wafers due to irradiation by 80 MeV Ni⁹⁺, 120 MeV Au¹⁵⁺ and 200 MeV Ag¹⁰⁺. The loss of H is more in case of the wafer irradiated by Ag ions as compared to other two because of higher electronic energy loss (S_e). For same S_e value, H loss is more in case of the wafer having x = 0.29 as compared to the one having x = 0.204. This is due to higher band gap of the former as compared to the later, which is an important data for proper use of these materials as IR detector in intense radiation zone. These results are explained on the basis of thermal spike model of ion-solid interaction.     

Study of 160 MeV Ni ion irradiation effects on electrodeposited polypyrrole films

Conducting polymer polypyrrole thin films doped with LiCF₃SO₃, [CH₃(CH₂)₃]₄NBF₄ and [CH₃(CH₂)₃]₄NPF₆ have been electrodeposited potentiodynamically on ITO coated glass substrate. The polymer films are irradiated with 160 MeV Ni¹²⁺ ions at three different fluences of 5 × 10¹⁰, 5 × 10¹¹ and 3 × 10¹² ions cm⁻². An increase in dc conductivity of polypyrrole films from 100 S/cm to 170 S/cm after irradiation with highest fluence is observed in four-probe measurement. X-ray diffractogram shows increase in the crystallinity of the polypyrrole films upon SHI irradiation, which goes on increasing with the increase in fluence. Absorption intensity increase in the higher wavelength region is observed in the UV–Vis spectra. The SEM studies show that the cauliflower like flaky microstructure of the surface of polypyrrole films turns globular upon SHI irradiation at fluence 5 × 10¹¹ ions cm⁻² and becomes smooth and dense at the highest fluence used. The cyclic voltammetry studies exhibit that the redox properties of the polypyrrole films do not change much on SHI irradiation.