M.A. van Huis

Atomic Pillar-Based Nanoprecipitates Strengthen AlMgSi Alloys

Atomic-resolution electron microscopy reveals that pillarlike silicon double columns exist in the hardening nanoprecipitates of AlMgSi alloys, which vary in structure and composition. Upon annealing, the Si2 pillars provide the skeleton for the nanoparticles to evolve in composition, structure, and morphology. We show that they begin as tiny nuclei with a composition close to Mg2Si2Al7 and a minimal mismatch with the aluminum matrix. They subsequently undergo a one-dimensional growth in association with compositional change, becoming elongated particles. During the evolution toward the final Mg5Si6 particles, the compositional change is accompanied by a characteristic structural change. Our study explains the nanoscopic reasons that the alloys make excellent automotive materials.

Transformations of gold nanoparticles investigated using variable temperature high-resolution transmission electron microscopy.

Recently designed advanced in-situ specimen holders for transmission electron microscopy (TEM) have been used in studies of gold nanoparticles. We report results of variable temperature TEM experiments in which structural transformations have been correlated with specimen temperature, allowing general trends to be identified. Transformation to a decahedral morphology for particles in the size range 5-12nm was observed for the majority of particles regardless of their initial structure. Following in-situ annealing, decahedra were found to be stable at room temperature, confirming this as the equilibrium morphology, in agreement with recently calculated phase diagrams. Other transitions at low temperature in addition to surface roughening have also been observed and correlated with the same nanoscale phase diagram. Investigations of gold particles at high temperature have revealed evidence for co-existing solid and liquid phases. Overall, these results are important in a more precise understanding of the structure and action of catalytic gold nanoparticles and in the experimental verification of theoretical calculations.

Strong spin-orbit splitting and magnetism of point defect states in monolayer WS2

The spin-orbit coupling (SOC) effect has been known to be profound in onolayer pristine transition metal dichalcogenides (TMDs). Here we show that point defects, which are omnipresent in the TMD membranes, exhibit even stronger SOC effects and change the physics of the host materials drastically. In this article we chose the representative monolayer WS2 slabs from the TMD family together with seven typical types of point defects including monovacancies, interstitials, and antisites. We calculated the formation energies of these defects, and studied the effect of spin-orbit coupling (SOC) on the corresponding defect states. We found that the S monovacancy (VS) and S interstitial (adatom) have the lowest formation energies. In the case of VS and both of the WS and WS2 antisites, the defect states exhibit strong splitting up to 296 meV when SOC is considered. Depending on the relative position of the defect state with respect to the conduction band minimum (CBM), the hybrid functional HSE will either increase the splitting by up to 60 meV (far from CBM), or decrease the splitting by up to 57 meV (close to CBM). Furthermore, we found that both the WS and WS2 antisites possess a magnetic moment of 2 μB localized at the antisite W atom and the neighboring W atoms. The dependence of SOC on the orientation of the magnetic moment for the WS and WS2 antisites is discussed. All these findings provide insights in the defect behavior under SOC and point to possibilities for spintronics applications for TMDs.

A hot implantation study on the evolution of defects in He ion implanted MgO(1 0 0)

AbstractIon implantation at elevated temperature,so-called hot implantation,was used to study nucleation and thermalstability of the defects. In this work,MgO(100) single crystal samples were implanted with 30 keV He ions at variousimplantation temperatures. The implantation doses ranged from 10 14 to 10 16 cm 2 . The implantation introduced defectswere subsequently studied by thermal helium desorption spectroscopy (THDS) and Doppler broadening positron beamanalysis (PBA). The THDS study provides vital information on the kinetics of He release from the sample. PBAtechnique,being sensitive to the open volume defects,provides complementary information on cavity evolution. TheTHD study has shown that in most cases helium release is characterised by the activation energy of Q ¼ 4:7 0:5eVwith the maximum release temperature of T max ¼ 1830 K. By applying first order desorption model the pre-exponentfactor is estimated as m ¼ 4:3 10 11 s 1 . 2002 Published by Elsevier Science B.V. Keywords:MgO; He ion implantation; Thermal desorption

In situ mechanical, temperature and gas exposure treatments of materials combined with variable energy positron beam techniques

An overview is given of the extension of the Delft variable energy positron (VEP) beam facility with equipment for in situ heating, cooling, 4-point bending, hydrogen permeation and gas ad- and absorption of bulk materials, surfaces and interfaces.

Variable temperature investigation of the atomic structure of gold nanoparticles

The characterisation of nanoparticle structures is the first step towards understanding and optimising their utility in important technological applications such as catalysis. Using newly developed in-situ transmission electron microscopy (TEM) specimen holders, the temperature dependent atomic structure of gold nanoparticles in the size range 5?12 nm has been investigated. In this size interval, the decahedral morphology has been identified as the most favourable structure at or above room temperature, while particle surface roughening becomes evident above 600?C. An icosahedral transition has also been identified at low temperature in particles under 9 nm in diameter. These experimental results are consistent with recently published temperature dependent equilibrium phase maps for gold nanoparticles.

Depth-Selective 2D-ACAR and Coincidence Doppler Investigation of Embedded Au Nanocrystals in MgO

We present a depth-selective 2D-ACAR and two-detector Doppler broadening study on Au nanocrystals in monocrystalline MgO(100), produced in sub-surface layers by ion implantation and subsequent thermal annealing to temperatures beyond the stability range of vacancy clusters in MgO. In contrast to the case of Li nanocrystals, it was found that positrons do not trap inside the Au nanocrystals, but only in defects at the nanocrystal-to-host interface (attached vacancy clusters). This is interpreted in terms of the positron affinity of Au, MgO and the defects. Introduction Metallic nanoprecipitates embedded in crystalline host materials are promising materials for optical applications because of their special linear and non-linear optical properties [1]. Previous positron coincidence Doppler and lifetime studies showed a clear correlation between the presence of vacancy clusters attached to Au nanoclusters in MgO and an important change in surface plasmon frequency [2-4]. In this study, gold nanoclusters in epi-polished MgO(100) single crystals of size 10×10×1 mm 3 were created by means of implantation of 1 MeV Au ions at a dose of 1×10 16 Au ions/cm 2 at room temperature [5]. The implantation was followed by annealing at 1473 K for a period of 22 hrs. The gold nanoclusters were investigated by optical absorption spectroscopy [5], high-resolution X-ray diffraction (XRD) measurements [6] and cross-sectional transmission electron microscopy (X-TEM) [6]. They are present in a layer formed at a depth between ~140 and ~300 nm below the surface. Their size varies from 2-14 nm with a mean size of 4.6 nm. Furthermore, a cubeon-cube orientation relationship between the Au nanoclusters and the MgO host matrix is observed by X-TEM. Contrary to the positron studies in [2-4], we extended the thermal annealing to beyond the stability range of vacancy clusters in MgO. Previous 2D-ACAR studies on Li nanocrystals in MgO [7, 8] have shown that it is possible to probe structural and electronic structure properties of nanocrystals. Therefore depth-selective positron annihilation experiments were carried out, employing S parameter, two-detector Doppler coincidence and 2D-ACAR measurements. Results and discussion Figure 1 shows the S parameter versus the positron implantation energy for an as-grown (reference) MgO(100) sample, and a sample that was Au + implanted and annealed in ambient air at 773 K, 1273 K and 1473 K, respectively. After Au implantation the S parameter increases relative to the low value for as-grown MgO, and a maximum (A) is formed at about 3 keV. This indicates that vacancies are produced under the surface. The S parameter also increases for depths beyond the Rp-range of the Au ions, i.e. >300 nm, indicating that in that range vacancies are produced due to damage by low-mass knock-on ions. After the implanted sample is annealed at 773 K the S parameter increases substantially (peak B in Fig. 1) indicating that vacancies produced by implantation have aggregated into larger clusters. Further annealing in steps of 100 K up to 1273 K causes a reduction of the S parameter showing that part of the vacancies and/or small clusters of vacancies have been removed. At 1273 K the S parameter exhibits two peaks (C and D) positioned at about 3 and 8 keV, respectively. This type of profile has also been observed by Xu et al. [2-4]. Annealing the implanted MgO sample beyond this stage to 1473 K for 22 hours causes a drastic drop in the S parameter, indicating that the vacancies and vacancy clusters in the MgO matrix have been effectively removed. However, the S parameter still exhibits a weak maximum (F) that could be attributed to trapping in Au nanoclusters. Indeed, X-TEM measurements [6] showed the presence of Materials Science Forum Vols. 445-446 (2004) pp. 398-400 online at http://www.scientific.net

In‐situ TEM Observation of Gold Nanocluster Nucleation, Coarsening and Refining in Au Implanted MgO(100) Foils

The evolution of thin MgO(100) foils during in‐situ ion implantation and thermal heating was followed with transmission electron microscopy (TEM) in the IVEM‐TANDEM facility of Argonne National Laboratory (USA). A pre‐thinned MgO foil was implanted with 100 keV Au ions at an elevated temperature of 700 K to a fluence of 1.8×1016 Au cm−2 leading to the spontaneous formation of small (2–3 nm) Au nanoclusters. Post‐implantation thermal annealing at 1100 K led to coarsening of the precipitates reaching sizes of ∼9 nm. Subsequent irradiation with 600 keV Au ions (passing completely through the foil) led to refining of the precipitates, a reversed ripening process. After irradiation with 1×1016 Au ions cm−2, the cluster size decreases to less than ∼3 nm. In another experiment, during 300 keV Kr ion irradiation, where the ions pass through the foil, a dense network of cubically shaped nanovoids was observed after a dose of 4×1016 Kr ions cm−2.

Defects and nanocluster engineering in MgO

The optical properties of MgO crystals are known to change after introduction of nanosize metal precipitates. In this work the formation of metallic nanoclusters in the presence of nanosize rectangular shaped cavities was studied. The rectangular cavities were formed by 30 keV He+ implantation followed by 1273 K annealing. The formation of cavities and their location was established by Positron Beam Analysis (PBA). The rectangular shape and their alignment in (100) direction was observed by X-TEM. Subsequently, the samples were implanted with 600 keV Ag and 1000 keV Au in order to introduce the metal ions in the vicinity of the cavities. The samples were then annealed to provide the formation of nanoclusters. The evolution of the implantation induced defects was monitored by PBA. The optical properties were studied by light absorption measurements.

Stability and Geometry of Silica Nano-Ribbons (SNRs): : A First-Principles Study

Silica based materials are attractive because of their versatility and their unique structures and properties, which have led to numerous applications of silica in a range of fields. Recently, various low-dimensional silica materials have been synthesized experimentally. Here we present a first-principles study on the geometry and stability of novel low-dimensional silica nano-ribbons (SNRs) using density-functional theory (DFT) with van der Waals interactions (optB88-vdW). SNRs of various widths with different surface groups, and with the geometry of hexagonal rings and squares, were taken into consideration. An atomically flat ribbon with mixing squares and rings is also included. The calculations showed high stability for the single layer and bilayer silica ribbons, both containing hexagonal rings. The calculations also revealed a high flexibility of silica chains. The local structure and chemical bonding were carefully analyzed. Electronic band structure calculations showed an insulating nature of the SNRs with energy gaps of about 5.0 to 6.0 eV, which are determined by nonbonding and anti-bonding O 2p states.