Lithium nanocluster formation in Li⁺ - ion implanted MgO

Abstract

Photon absorption (PA), Positron Beam Analysis (PBA) and Neutron Depth Profiling (NDP) is applied to study the relation between photon absorption behavior and the precipitates formed by ion implantation and thermal annealing. Monocrystals of MgO(100) were implanted with 1.0x10 16 6 Li ions cm -2 at an energy of 30 keV. The samples were thermally annealed in air in steps up to 1200 K. After each step Doppler broadening Positron Beam Analysis (PBA) was applied to monitor the depth profile of the implantation defects. The evolution of the depth profile of lithium was followed with the aid of NDP. During the annealing there is hardly any change in the location of the lithium implantation peak at 150 nm (peak concentration 2 at. %). Only after annealing to 1200 K the majority of the lithium has left the crystal and optical absorption effects have disappeared. During annealing at 750 K an absorption band develops between 400 and 600 nm; at 950 K the maximum absorption is centered at 450 nm corresponding to Mie absorption and scattering by lithium nanoclusters. Positron beam analysis shows a considerable increase of annihilations with low momentum electrons in the implanted zone. A positron method for measuring electron momentum distributions (2D-ACAR) coupled to an intense positron beam gave evidence for the presence of semi-coherent metallic lithium inclusions.

Positron confinement in embedded lithium nanoclusters

“…40 The presence of dislocation loops in MgO does not affect the S parameter significantly, but it does shorten the diffusion length compared to the bulk values. In the layer below the ion implantation layer ͑layer 3͒ there are not only dislocation loops but also a tail of implanted species as observed by NDP, 20 possibly because of channeling effects. It was estimated that about 6% of the implanted species end up in this range.…”

“…The results of these two methods have already been discussed in an earlier paper, which focused on the defect evolution during the annealing procedure. 20 In the present work we address the confinement of positrons in lithium nanoclusters, and therefore only the positron experimental techniques applied to the samples are presented in this paper.…”

“…The choice for this model is mainly based on defect analysis performed on previously ion-implanted MgO samples that were analyzed with techniques such as transmission electron microscopy, positron annihilation spectroscopy, and neutron depth profiling. 20,40,41 In particular, there is a subsurface layer containing the ion implanted species and the main implantation defects ͑layer 2͒. Dislocation loops are formed at smaller depths in the MgO top layer ͑layer 1͒ and below the ion implantation layer.…”

“…It was shown previously 20 that Doppler broadening positron beam analysis ͑PBA͒ and two-dimensional angular correlation of annihilation radiation ͑2D-ACAR͒ measurements show a very high fraction of positron annihilations in Li nanoclusters, indicating possible positron quantum confinement. Below we will first discuss the concepts of positron affinity and the positronic potential well.…”

Electronic structure and orientation relationship of Li nanoclusters embedded in MgO studied by depth-selective positron annihilation two-dimensional angular correlation

“…This model is consistent with the earlier defect analysis of ion implanted MgO materials that were analyzed with transmission electron microscopy, positron-anihilation spectroscopy, and neutron depth profiling. 10,34,35 In order to obtain more information on the Li implantation layer, three values of the positron implantation energy (E ϩ ) for the 2D-ACAR experiments were selected: 4 keV, 7.5 keV, and 11.5 keV. The energy corresponding to optimum implantation in the lithium layer ͑MgO:Li͒ is Ϸ4 keV if one uses the POSH beam, and about 3.5 keV if one uses the VEP beam.…”