Dr Nianhua Peng

An understanding of the effect of cumulative radiation damage on the integrity of ceramic wasteforms for plutonium and minor actinide disposition is key to the scientific case for safe disposal. Alpha recoil due to the decay of actinide species leads to the amorphisation of the initially crystalline host matrix, with potentially deleterious consequences such as macroscopic volume swelling and reduced resistance to aqueous dissolution. For the purpose of laboratory studies the effect of radiation damage can be simulated by various accelerated methodologies. The incorporation of short-lived actinide isotopes accurately reproduces damage arising from both alpha-particle and the heavy recoil nucleus, but requires access to specialist facilities. In contrast, fast ion implantation of inactive model ceramics effectively simulates the heavy recoil nucleus, leading to amorphisation of the host crystal lattice over very short time-scales. Although the resulting materials are easily handled, quantitative analysis of the resulting damaged surface layer has proved challenging.
In this investigation, we have developed an experimental methodology for characterisation of radiation damaged structures in candidate ceramics for actinide disposition. Our approach involves implantation of bulk ceramic samples with 2 MeV Kr+ ions, to simulate heavy atom recoil; combined with grazing incidence X-ray absorption spectroscopy (GI-XAS) to characterise only the damaged surface layer. Here we present experimental GI-XAS data acquired at the Ti and Zr K-edges of ion implanted zirconolite, as a function of grazing angle, demonstrating that this technique can be successfully applied to characterise only the amorphised surface layer. Comparison of our findings with data from metamict natural analogues provide evidence that heavy ion implantation reproduces the amorphous structure arising from naturally accumulated radiation damage.

The important problem of how to generate lateral order for ion implantation patterning of substrates is solved by using a nanoporous anodic alumina membrane as a mask. Co and Pt implantation is used at two implantation doses. In order to observe the achieved implantation zones free from artifacts, electron transparent thin nitride and oxide films are used as substrates, which allows the quality of pattern transfer from the mask to the thin film to be assessed by plan-view transmission electron microscopy. Characteristic density variations of implanted elements across projected pore-regions of the mask, such as ring and dome shapes, and corresponding variation of cluster size are discussed, and therefore the method also serves as a suitable test bed for ion beam focusing studies by cylindrical or conical pores. © 2013 the Owner Societies.

Spatially patterned ion beam implantation of 190keV Co + ions into a SiO 2 thin film on a Si substrate has been achieved by using nanoporous anodic aluminum oxide with a pore diameter of 125 nm as a mask. The successful synthesis of periodic embedded Co regions using pattern transfer is demonstrated for the first time using cross-sectional (scanning) transmission electron microscopy (TEM) in combination with analytical TEM. Implanted Co regions are found at the correct relative lateral periodicity given by the mask and at a depth of about 120nm. © 2012 IOP Publishing Ltd.

There are many technical challenges in the fabrication of devices from novel materials. The characterization of these materials is critical in the development of efficient photovoltaic systems. We show how the application of recent advances in MeV IBA, providing the self-consistent treatment of RBS (Rutherford backscattering) and PIXE (particle induced X-ray emission) spectra, makes a new set of powerful complementary depth profiling techniques available for all thin film technologies, including the chalcopyrite compound semiconductors. We will give and discuss a detailed analysis of a CuInAl metallic precursor film, showing how similar methods are also applicable to other films of interest.

Progress in ion implantation of metal ions into substrates of amorphous silica or Si-nitride with respect to lateral periodic patterning is presented. We use a 2D-nanoporous membrane of anodic aluminium oxide (AAO) as mask to conduct the Co ion implantations. The criteria for successful masked implantation and main problems are presented, including testing of the masks in a focused ion beam (FIB) system. It is proposed that electron transparent thin windows are the most suitable substrate for methods development, as TEM observation can be followed without any further sample milling. Co clusters are found to exhibit the same lateral order as the pores, and first annealing tests to achieve Co nanoparticles are shown using an in-situ heating TEM holder. © 2012 IEEE.

The electrical characteristics of gallium-doped zinc oxide (ZnO:Ga) thin films prepared by rf diode sputtering were altered via nitrogen implantation by performing two implants at 40 keV and 80 keV with doses of 1×1015 and 1×1016 cm?2 to achieve a p-type semiconductor. An implantation of 1×1015 cm?2 N+-ions yielded a p-type with hole concentrations 1017?1018 cm?3 in some as-implanted samples. The films annealed at temperatures above 200°C in O2 and above 400°C in N2 were n-type with electron concentrations 1017?1020 cm?3. The higher nitrogen concentration (confirmed by SRIM and SIMS), in the films implanted with a 1×1016 cm?2 dose, resulted in lower electron concentrations, respectively, higher resistivity, due to compensation of donors by nitrogen acceptors. The electron concentrations ratio n(1×1015)/n(1×1016) decreases with increasing annealing temperature. Hall measurements showed that 1×1016 cm?2 N-implanted films became p-type after low temperature annealing in O2 at 200°C and in N2 at 400°C with hole concentrations of 3.2×1017 cm?3 and 1.6×1019 cm?3, respectively. Nitrogen-implanted ZnO:Ga films showed a c-axes preferred orientation of the crystallites. Annealing is shown to increase the average transmittance (>80%) of the films and to cause bandgap widening (3.19?3.3 eV).

We have studied the magnetization of vertically aligned graphene nanoflakes irradiated with nitrogen ions of 100 KeV energy and doses in the range 10¹¹?10¹w ions/cm². The non-irradiated graphene nanoflakes show a paramagnetic contribution, which is increased progressively by ion irradiation at low doses up to 10¹u/cm². However, further increase on implantation dose reduces the magnetic moment which coincides with the onset of amorphization as verified by both Raman and x-ray photoelectron spectroscopic data. Overall, our results demonstrate the absence of ferromagnetism on either implanted or unimplanted samples from room temperature down to a temperature of 5 K.

© 2015 American Physical Society.Here we study the magnetotransport properties of the ferropnictide crystals BaFe2As2 and BaFe1.985 Co0.015As2. These materials exhibit a high field linear magnetoresistance that has been attributed to the quantum linear magnetoresistance model. In this model, the linear magnetoresistance is dependent on the concentration of scattering centers in the material. By using proton-beam irradiation to change the defect scattering density, we find that the dependence of the magnitude of the linear magnetoresistance on scattering quite clearly contravenes this prediction. A number of other scaling trends in the magnetoresistance and high field Hall data are observed and discussed.

The nature of the electronic state of a metal depends strongly on its dimensionality. In a system of isolated conducting chains, the Fermi-liquid (quasiparticle) description appropriate for higher dimensions is replaced by the so-called Tomonaga-Luttinger liquid picture characterized by collective excitations of spin and charge. Temperature is often regarded as a viable tuning parameter between states of different dimensionality, but what happens once thermal broadening becomes comparable to the interchain hopping energy remains an unresolved issue, one that is central to many organic and inorganic conductors. Here we use the ratio of the thermal to electrical conductivities to probe the nature of the electronic state in PrBa 2 Cu 4 O 8 as a function of temperature. We find that despite the interchain transport becoming non-metallic, the charge carriers within the CuO chains appear to retain their quasiparticle nature. This implies that temperature alone cannot induce a crossover from Fermi-liquid to Tomonaga-Luttinger-liquid behaviour in quasi-one-dimensional metals.

Lateral ordered Co, Pt and Co/Pt nanostructures were fabricated in SiO and SiN substrates by high fluence metal ion implantation through periodic nanochannel membrane masks based on anodic aluminium oxides (AAO). The quality of nanopatterning transfer defined by various AAO masks in different substrates was examined by transmission electron microscopy (TEM) in both imaging and spectroscopy modes. © 2013 Elsevier B.V.

Lateral ordered Co, Pt and Co/Pt nanostructures were fabricated in SiO2 and Si3N4 substrates by high fluence metal ion implantation through periodic nanochannel membrane masks based on anodic aluminium oxides (AAO). The quality of nanopatterning transfer defined by various AAO masks in different substrates was examined by transmission electron microscopy (TEM) in both imaging and spectroscopy modes. © 2013 Elsevier B.V. All rights reserved.

Penetration of a nanochannel mask by 190keV Co+ ions is tested for the purpose of achieving laterally modulated ion implantation into a SiO2 thin film on a Si substrate. A 2D-nanoporous membrane of anodic aluminum oxide (AAO) is chosen as the mask. Criteria and challenges for designing the mask are presented. Implantation experiments through a mask with pore diameter of 125 nm and inter-pore distance of 260 nm are carried out. Cross-sectional TEM (XTEM) is shown as an ideal tool to assess depth distribution and lateral distribution of implanted ions at the same time, complemented by Rutherford backscattering spectroscopy. Using energy dispersive x-ray spectroscopy linescans, a Co distribution with lateral modulation is found at 120 nm below the oxide surface. First experiments in converting the atomic distribution of Co to discrete nanoparticles by in-situ TEM annealing are presented. © 2012 Materials Research Society.

Understanding the effect of radiation damage and noble gas accommodation in potential ceramic hosts
for plutonium disposition is necessary to evaluate their long-term behaviour during geological disposal.
Polycrystalline samples of Nd-doped zirconolite and Nd-doped perovskite were irradiated ex situ with
2 MeV Kr+ at a dose of 5 1015 ions cm2 to simulate recoil of Pu nuclei during alpha decay. The feasibility
of thin section preparation of both pristine and irradiated samples by Focused Ion Beam sectioning
was demonstrated. After irradiation, the Nd-doped zirconolite revealed a well defined amorphous region
separated from the pristine material by a thin (40?60 nm) damaged interface. The zirconolite lattice was
lost in the damaged interface, but the fluorite sublattice was retained. The Nd-doped perovskite contained
a defined irradiated layer composed of an amorphous region surrounded by damaged but still
crystalline layers. The structural evolution of the damaged regions is consistent with a change from
orthorhombic to cubic symmetry. In addition in Nd-doped perovskite, the amorphisation dose depended
on crystallographic orientation and possibly sample configuration (thin section or bulk). Electron Energy
Loss Spectroscopy revealed Ti remained in the 4+ oxidation state but there was a change in Ti coordination
in both Nd-doped perovskite and Nd-doped zirconolite associated with the crystalline to amorphous
transition.