AFM and Raman study of graphene deposited on silicon surfaces nanostructured by ion beam irradiation.

Nanoscale structures were produced on silicon surfaces by low-energy oxygen ion irradiation: periodic rippled or terraced patterns formed spontaneously, depending on the chosen combination of beam incidence angle and ion fluence. Atomic force microscopy image processing and analysis accurately described the obtained nanotopographies. Graphene monolayers grown by chemical vapour deposition were transferred onto the nanostructured silicon surfaces. The interfacial interaction between the textured surface and the deposited graphene governs the conformation of the thin carbon layer; the resulting different degree of regularity and conformality of the substrate-induced graphene corrugations was studied and it was related to the distinctive topographical features of the silicon nanostructures. Raman spectroscopy revealed specific features of the strain caused by the alternating suspension and contact with the underlying nanostructures and the consequent modulation of the silicon-graphene interaction. Lay Description In the field of nanosciences, nanotechnologies and advanced materials, it is pivotal to produce and integrate nanostructures in a controlled, cost-effective and possibly high-throughput manner. Currently, the surface nanopatterning by ion beam irradiation (IBI) is showing its potential in overcoming some of the limits that characterize the conventional lithographic techniques. IBI can produce self-organized and regular patterns of nanostructures, such as ripples, dots, and holes, having heights and lateral periodicities in a pre-defined range. The nanopatterns can develop over large surface areas of a broad class of materials and have been tested for different applications, e.g. microelectronic device fabrication, catalysis, nanoscale magnetism, surface-enhanced Raman scattering. Also, the ion-induced patterns allowed the control of physical properties such as wettability, reflectance, and photoluminescence. As a whole, this is a highly dynamic and continuously evolving field. Graphene was first obtained from bulk graphite by mechanical exfoliation; remarkable mechanical, thermal and optical properties came to light; they make graphene an ideal material for sensing. Also, the research on graphene paves the way for basic and applied studies on other ultrathin thickness materials, such as monolayer transition metal dichalcogenide or monolayer metal oxide, which also show peculiar properties as compared to their bulk version. Since graphene is a zero-gap semi-metal, a finite energy gap must be open to engineer graphene-based electronic devices. At this aim, researchers can take advantage of the mechanical properties of graphene, since its out-of-plane deformation can favourably modify its electronic structure. While the spontaneous, random self-folding of graphene to form wrinkles has thermodynamics reasons and is unavoidable above a certain material length, the interactions between graphene and substrates can generate, in principle, designed wrinkled structures. Therefore, a field of study has emerged, focused on the control and tuning of the geometry of graphene corrugations and their influence on specific physical and chemical properties. To the best of our knowledge, this work studies for the first time the geometry and strain in graphene wrinkles induced by silicon substrates that were suitably nanopatterned by ion beam irradiation.

On defects' role in enhanced perpendicular magnetic anisotropy in Pt/Co/Pt, induced by ion irradiation.

Modifications of magnetic and magneto-optical properties of Pt/Co(d Co )/Pt upon Ar+ irradiation (with energy 1.2, 5 and 30 keV) and fluence, F at the range from 2 · 1013-2 · 1016 Ar+ cm-2) were studied. Two 'branches' of increased perpendicular magnetic anisotropy (PMA) and enhanced magneto-optical response are found on 2D (d Co , F) diagrams. The difference in F between 'branches' is driven by ion energy. Structural features correlated with magnetic properties have been analysed thoroughly by x-ray diffraction, Rutherford backscattering spectrometry and positron annihilation spectroscopy. Experimental results are in agreement with TRIDYN numerical calculations of irradiation-induced layers intermixing. Our work discusses particularly structural factors related to crystal lattice defects and strain, created and modified by irradiation, co-responsible for the increase in the PMA.

Morphological and Functional Modifications of Optical Thin Films for Space Applications Irradiated with Low-Energy Helium Ions.

Future space missions will operate in increasingly hostile environments, such as those in low-perihelion solar orbits and Jovian magnetosphere. This exploration involves the selection of optical materials and components resistant to the environmental agents. The conditions in space are reproduced on ground through the use of ion accelerators. The effects of He particles coming from the solar wind impinging on a gold thin film have been systematically investigated, considering absorbed doses compatible with the duration of the European Space Agency Solar Orbiter mission. Structural and morphological changes have been proved to be dependent not only on the dose but also on the irradiation flux. A predictive model of the variation of thin film reflectance has been developed for the case of lower flux irradiation. The results are discussed regarding reliability and limitations of laboratory testing. The outcomes are important to address the procedures for the space qualification tests of optical coatings.

Self-Diffusion in Amorphous Silicon by Local Bond Rearrangements.

Experiments on self-diffusion in amorphous silicon (Si) were performed at temperatures between 460 to 600° C. The amorphous structure was prepared by Si ion implantation of single crystalline Si isotope multilayers epitaxially grown on a silicon-on-insulator wafer. The Si isotope profiles before and after annealing were determined by means of secondary ion mass spectrometry. Isothermal diffusion experiments reveal that structural relaxation does not cause any significant intermixing of the isotope interfaces whereas self-diffusion is significant before the structure recrystallizes. The temperature dependence of self-diffusion is described by an Arrhenius law with an activation enthalpy Q=(2.70±0.11) eV and preexponential factor D_{0}=(5.5_{-3.7}^{+11.1})×10^{-2} cm^{2} s^{-1}. Remarkably, Q equals the activation enthalpy of hydrogen diffusion in amorphous Si, the migration of bond defects determining boron diffusion, and the activation enthalpy of solid phase epitaxial recrystallization reported in the literature. This close agreement provides strong evidence that self-diffusion is mediated by local bond rearrangements rather than by the migration of extended defects as suggested by Strauß et al. (Phys. Rev. Lett. 116, 025901 (2016)PRLTAO0031-900710.1103/PhysRevLett.116.025901).

Plasmonic nanoparticles embedded in single crystals synthesized by gold ion implantation for enhanced optical nonlinearity and efficient Q-switched lasing.

We report on the synthesis of embedded gold (Au) nanoparticles (NPs) in Nd:YAG single crystals using ion implantation and subsequent thermal annealing. Both linear and nonlinear absorption of the Nd:YAG crystals have been enhanced significantly due to the embedded Au NPs, which is induced by the surface plasmon resonance (SPR) effect in the visible light wavelength band. Particularly, through a typical Z-scan system excited by a femtosecond laser at 515 nm within the SPR band, the nonlinear absorption coefficients of crystals with Au NPs have been observed to be nearly 5 orders of magnitude larger than that without Au NPs. This giant enhancement of nonlinear absorption properties is correlated with the saturable absorption (SA) effect, which is the basis of passive Q-switching or mode-locking for pulsed laser generation. In addition, the linear and nonlinear absorption enhancement could be tailored by varying the fluence of implanted Au+ ions, corresponding to the NP size and concentration modulation. Finally, the Nd:YAG wafer with embedded Au NPs has been applied as a saturable absorber in a Pr:LuLiF4 crystal laser cavity, and efficient pulsed laser generation at 639 nm has been realized, which presents superior performance to the MoS2 saturable absorber based system. This work opens an avenue to enhance and modulate the nonlinearities of dielectrics by embedding plasmonic Au NPs for efficient pulsed laser operation.

Polyatomic ions from a high current ion implanter driven by a liquid metal ion source.

High current liquid metal ion sources are well known and found their first application as field emission electric propulsion thrusters in space technology. The aim of this work is the adaption of such kind of sources in broad ion beam technology. Surface patterning based on self-organized nano-structures on, e.g., semiconductor materials formed by heavy mono- or polyatomic ion irradiation from liquid metal (alloy) ion sources (LMAISs) is a very promising technique. LMAISs are nearly the only type of sources delivering polyatomic ions from about half of the periodic table elements. To overcome the lack of only very small treated areas by applying a focused ion beam equipped with such sources, the technology taken from space propulsion systems was transferred into a large single-end ion implanter. The main component is an ion beam injector based on high current LMAISs combined with suited ion optics allocating ion currents in the µA range in a nearly parallel beam of a few mm in diameter. Different types of LMAIS (needle, porous emitter, and capillary) are presented and characterized. The ion beam injector design is specified as well as the implementation of this module into a 200 kV high current ion implanter operating at the HZDR Ion Beam Center. Finally, the obtained results of large area surface modification of Ge using polyatomic Bi2+ ions at room temperature from a GaBi capillary LMAIS will be presented and discussed.

Effects of low-dosed imidacloprid pulses on the functional role of the caged amphipod Gammarus roeseli in stream mesocosms.

Effects of two series of imidacloprid pulses on caged amphipods (Gammarus roeseli) and their shredder efficiency for litter decomposition were studied for 70 days as part of a comprehensive stream mesocosm experiment. The duration of each imidacloprid pulse of 12µgL(-1) was 12h. About 250mL cages with an initial stock of 10 adult gammarids together with different conditioned litter substrates were used. Beside alder leaves (Alnus glutinosa), straw (× Triticosecale) was also used in different trials and tested for its suitability to serve as litter substrate. Results from tracer and microprobe measurements approved the suitability of the test system under low-flow condition of 10cms(-1) in the surrounding stream water. Population development followed a logistic growth function with a carrying capacity of 200 Ind cage(-1) for alder and 161 for straw. In the course of the study, the F1 generation reached sexual maturity and F2 offspring appeared. Increased nitrogen contents of gammarid-free trials compared to stocked ones after 70 days indicated that biofilm on both substrates was an important food source for G. roeseli. However, increased shredding activity by gammarids was only detected for alder during the second pulse series. During the remaining time and also for straw, losses of coarse particular organic matter were quite constant and slow indicating the dominance of transport limited decomposition processes on the litter surfaces. No effect of imidacloprid pulses on population levels and litter decomposition could be detected. However, the number of brood carrying females was reduced in the treatments compared to the control groups in the last 3 weeks of the study. In conclusion, repeated low-level and short-term exposition may have adverse long-term effects on G. roeseli in the field with regard to both the population size and the functional role as key shredder.

Closer to reality--the influence of toxicity test modifications on the sensitivity of Gammarus roeseli to the insecticide imidacloprid.

Laboratory toxicity test designs are far from reality and therefore extrapolations to field situations may be more difficult. In laboratory experiments with the amphipod Gammarus roeseli exposed to the insecticide imidacloprid it was investigated if test conditions closer to reality influences its sensitivity and if it is possible to extrapolate results from these laboratory tests to results from a stream mesocosm study. Experiments were run by varying medium, temperature, size, and seasonal origin of gammarids. Age and seasonal aspects had strongest effects with juveniles and animals taken from a spring population being most sensitive with an EC50 (96 h) of 14.2 µg L⁻¹ imidacloprid. The test designs closest to the conditions in the stream mesocosms reflected best the results in mesocosms study on basis of LOEC values. However, the EC(x) extrapolation failed to predict the effects of short term imidacloprid pulses in the field.

Set-up and calibration of a triple ionization chamber system for dosimetry in mixed neutron/photon fields.

The aim of this study is to introduce a triple ionization chamber system to separate dose components of mixed neutron/photon fields. Fast and thermal neutron dose components have a different biological effectiveness than gamma dose components. If boron neutron capture is used to enhance the dose in certain areas of a patient, the precise knowledge of the thermal neutron flux is essential. A tissue equivalent and two magnesium ionization chambers have been prepared for use in a triple chamber system for this purpose. One of the magnesium chambers is coated with (10)B on the inside to enhance its response to thermal neutrons. All three chambers have been calibrated at a cobalt source, medical linear accelerators and several neutron sources. The chambers have been studied in Monte Carlo simulations and the results are compared with measurements.

Measurement of the fluence response of the GSI neutron ball dosemeter in the energy range from thermal to 19 MeV.

At high-energy particle accelerators, area monitoring needs to be performed in a wide range of neutron energies. In principle, neutrons occur from thermal energies up to the energy of the accelerated ions, which is for the present GSI (Gesellschaft für Schwerionenforschung) accelerator facility approximately 1-2 GeV per nucleon. There are no passive dosemeters available, which are designed for the use at high-energy accelerators. At GSI, a neutron dosemeter was developed, which is suitable for the measurement of high-energy neutron radiation by the insertion of a lead layer around Thermoluminescence (TL) detection elements (pairs of TL 600/700) at the centre of the dosemeter. The design of the sphere was derived from the construction of the extended range rem-counters for the measurement of ambient dose equivalent H(10). In this work, the dosemeter fluence response was measured in the quasi-monoenergetic neutron fields of the accelerator facility of the PTB in Braunschweig and in the thermal neutron field of the GKSS research reactor FRG-1 in Geesthacht. For the accelerator measurements, the reactions (7)Li(p,n)(7)Be, (3)H(p,n)(3)He and (2)H(d,n)(3)He were used to produce neutron fields with energy peaks between 144 keV and 19 MeV. The measured fluence responses are 27% too low for thermal energies and show an agreement with approximately 14% for the accelerator produced neutron fields related to the computed fluence responses (MCNP, FLUKA calculations). The measured as well as the computed fluence responses of the dosemeter are compared with the corresponding conversion coefficients.

Application of different TL detectors for the photon dosimetry in mixed radiation fields used for BNCT.

Different approaches for the measurement of a relatively small gamma dose in strong fields of thermal and epithermal neutrons as used for Boron Neutron Capture Therapy (BNCT) have been studied with various thermoluminescence detectors (TLDs). CaF(2):Tm detectors are insensitive to thermal neutrons but not tissue-equivalent. A disadvantage of applying tissue-equivalent (7)LiF detectors is a strong neutron signal resulting from the unavoidable presence of (6)Li traces. To overcome this problem it is usual to apply pairs of LiF detectors with different (6)Li content. The experimental determination of the thermal neutron response ratio of such a pair at the Geesthacht Neutron Facility (GeNF) operated by PTB enables measurement of the photon dose. In the experimental mixed field of thermal neutrons and photons of the TRIGA reactor at Mainz the photon dose measured with different types of (7)LiF/(nat)LiF TLD pairs agree within a standard uncertainty of 6% whereas the CaF(2):Tm detectors exhibit a photon dose by more than a factor of 2 higher. It is proposed to determine suitable photon energy correction factors for CaF(2):Tm detectors with the help of the (7)LiF/(nat)LiF TLD pairs in the radiation field of interest.

Quasi-monoenergetic neutron reference fields in the energy range from thermal to 200 MeV.

Well-characterised neutron fields are a prerequisite for the investigation of neutron detectors. Partly in collaboration with external partners, the PTB neutron metrology group makes available for other users neutron reference fields covering the full energy range from thermal to 200 MeV. The specification of the neutron fluence in these beams is traceable to primary standard cross sections.