A. Meldrum

Radiation Effects in Zircon

The widespread distribution of zircon in the continental crust, its tendency to concentrate trace elements, particularly lanthanides and actinides, its use in age-dating, and its resistance to chemical and physical degradation have made zircon the most important accessory mineral in geologic studies. Because zircon is highly refractory, it also has important industrial applications, including its use as a lining material in high-temperature furnaces. However, during the past decade, zircon has also been proposed for advanced technology applications, such as a durable material for the immobilization of plutonium or, when modified by ion-beam irradiation, as an optic waveguide material. In all of these applications, the change in properties as a function of increasing radiation dose is of critical importance. In this chapter, we summarize the state-of-knowledge on the radiation damage accumulation process in zircon.

Nanocomposites formed by ion implantation: Recent developments and future opportunities

Abstract Ion implantation is a versatile and powerful technique for forming many types of nanocrystalline precipitates embedded in the near-surface region of a wide variety of crystalline and amorphous host materials. The unique optical, electronic and magnetic properties of these nanocomposites has stimulated considerable recent research interest. In this review, we discuss recent developments in the field as well as some of the problems that currently hinder the potential applications of nanocomposites formed by ion implantation.

A transient liquid-like phase in the displacement cascades of zircon, hafnon and thorite

The study of radiation effects in solids is important for the development of ‘radiation-resistant’ materials for fission-reactor applications. The effects of heavy-ion irradiation in the isostructural orthosilicates zircon (ZrSiO4), hafnon (HfSiO4) and thorite (ThSiO4) are particularly important because these minerals are under active investigation for use as a waste form for plutonium-239 resulting from the dismantling of nuclear weapons. During ion irradiation, localized ‘cascades’ of displaced atoms can form as a result of ballistic collisions in the target material, and the temperature inside these regions may for a short time exceed the bulk melting temperature. Whether these cascades do indeed generate a localized liquid state has, however, remained unclear. Here we investigate the irradiation-induced decomposition of zircon and hafnon, and find evidence for formation of a liquid-like state in the displacement cascades. Our results explain the frequent occurrence of ZrO2 in natural amorphous zircon. Moreover, we conclude that zircon-based nuclear waste forms should be maintained within strict temperature limits, to avoid potentially detrimental irradiation-induced amorphization or phase decomposition of the zircon.

Ion beam synthesis of gold nanoclusters in SiO2: Computer simulations versus experiments

A kinetic 3D lattice Monte-Carlo (KLMC) method is applied to describe nucleation and growth of nanoclusters from a space- and time-dependent supersaturated solid solution of impurity atoms formed by high-dose ion implantation. Within the framework of homogeneous nucleation, the dependence of the nanocluster size distribution on implantation temperature Timp and ion current j is shown. The simulation results are consistent with size and depth distributions of gold nanoclusters in SiO2 observed after implantation at different Timp. The influence of changes of Timp or j during implantation on the size distribution is simulated and the results are discussed with respect to corresponding implantations.

Refractometric sensing with fluorescent-core microcapillaries.

Capillaries present a promising structure for microfluidic refractive index sensors. We demonstrate a capillary-type fluorescent core microcavity sensor based on whispering gallery mode (WGM) resonances. The device consists of a microcapillary having a layer of fluorescent silicon quantum dots (QDs) coated on the channel surface. The high effective index of the QD layer confines the electric field near the capillary channel and causes the development of WGM resonances in the fluorescence spectrum. Solutions consisting of sucrose dissolved in water were pumped through the capillary while the fluorescence WGMs were measured with a spectrometer. The device showed a refractometric sensitivity of 9.8 nm/RIU (up to 13.8 nm/RIU for higher solution refractive index) and a maximum detection limit of ~7.2 x 10(-3) RIU. Modeling the field inside the capillary structure, which is analogous to a layered hollow ring resonator, shows that sensitivities as high as 100 nm/RIU and detection limits as low as ~10(-5) RIU may be achievable by optimizing the QD film thickness.

A transmission electron microscopy investigation of sulfide nanocrystals formed by ion implantation

Ion implantation was used to form compound semiconductor nanocrystal precipitates of ZnS, CdS, and PbS in both glass and crystalline matrices. The precipitate microstructures and size distributions were investigated by cross-sectional transmission electron microscopy techniques. Several unusual features were observed, including strongly depth-dependent size variations of the ZnS precipitates and central void features in the CdS nanocrystals. The morphology and crystal structure of the nanocrystal precipitates could be controlled by selection of the host material. The size distribution and microstructural complexity were significantly reduced by implanting a low concentration of ions into a noncrystalline host, and by using multi-energy implants to give a flat concentration profile of the implanted elements. (c) 1999 Materials Research Society.

Formation of CdS and CdSe nanocrystals by sequential implantation

Abstract Nanocrystals of CdS and CdSe have been formed in SiO2, Al2O3, and Si by sequential ion implantation and annealing. In SiO2, the nanocrystals have the hexagonal wurtzite structure and are randomly oriented. In Al2O3 and Si, nanocrystals are three dimensionally oriented. They have the cubic zincblende structure in Si and can be produced with either structure in Al2O3 by controlling the implantation conditions. In SiO2 and Al2O3, nanocrystals exhibit strong optical absorption and photoluminescence (PL). Evidence for quantum confinement is observed, and the PL results for CdSe are strongly temperature dependent.

Nonresonant carrier tunneling in arrays of silicon nanocrystals

Silicon nanocrystals are of interest in the nascent field of silicon microphotonics, with potential applications as waveguide amplifiers, light-emitting diodes, and silicon-based lasers. Comparing computational simulations and experiment, it is shown that nonresonant carrier tunneling in ensembles of silicon nanocrystals is a controlling factor in the luminescence. In thin film silicon nanocrystal composites, only the larger particles can be luminescent as a result of rapid carrier tunneling, suggesting that these applications may only be achieved for well-isolated nanocrystals or for arrays with a narrow distribution of sizes.

Hollow Bragg waveguides fabricated by controlled buckling of Si/SiO 2 multilayers

We describe integrated air-core waveguides with Bragg reflector claddings, fabricated by controlled delamination and buckling of sputtered Si/SiO2 multilayers. Thin film deposition parameters were tailored to produce a desired amount of compressive stress, and a patterned, embedded fluorocarbon layer was used to define regions of reduced adhesion. Self-assembled air channels formed either spontaneously or upon heating-induced decomposition of the patterned film. Preliminary optical experiments confirmed that light is confined to the air channels by a photonic band-gap guidance mechanism, with loss ~5 dB/cm in the 1550 nm wavelength region. The waveguides employ standard silicon processes and have potential applications in MEMS and lab-on-chip systems.

Buried superconducting layers comprised of magnesium diboride nanocrystals formed by ion implantation

Superconducting layers of MgB2 were formed on Si substrates using techniques that are widely used and accepted in the semiconductor industry. Mg ions were implanted into boron films deposited on Si or Al2O3 substrates. After a thermal processing step, buried superconducting layers comprised of MgB2 nanocrystals were obtained which exhibit the highest Tc reported so far for MgB2 on silicon (Tconset≈33.6 K, ΔTc=0.5 K, as measured by current transport). These results show that our approach is clearly applicable to the fabrication of superconducting devices that can be operated at much higher temperatures (≈20 K) than the current Nb technology (≈6 K) while their integration with silicon structures remains straight-forward.