Enhanced positron trapping by Ag nanoclusters at low temperatures: A challenge of positron sensitivity to quantum dots

Abstract

Microstructure evolution of three Al-Ag alloys with different Ag contents (1 wt. % Ag, 5 wt. % Ag, and 15 wt. % Ag) was studied by positron annihilation spectroscopy during the aging process. In situ measurements of the positron lifetime and Doppler broadening of annihilation radiation indicate the fast formation of Ag-rich clusters during natural aging of the alloys. The formation of Ag-rich clusters was further confirmed by coincidence Doppler broadening measurements. The Ag signal reflected by the Coincidence Doppler broadening spectrum increases with increasing Ag content and is further enhanced after subsequent artificial aging at 140 °C. This might be due to the increase in the size of Ag clusters. The temperature dependence of the Doppler broadening spectra between 10 K and 290 K was measured for the Al-Ag alloys after natural and artificial aging. Detrapping of positrons from Ag clusters with increasing temperature was observed for all the three Al-Ag alloys after natural aging and for the Al-1 wt. % Ag after artificial aging. This indicates that Ag clusters act as shallow positron trapping centers. The thermal detrapping of positrons becomes ambiguous with increasing Ag content in the alloy and is nearly invisible in the artificially aged Al-5 wt. % Ag and Al-15 wt. % Ag. The positron binding energy of the Ag cluster is roughly estimated to be about 18.8 meV and 50 meV in the Al-1 wt. % Ag sample after natural aging and artificial aging at 140 °C, respectively, which suggests that the confinement of positrons in the quantum-dot like state depends on the size or chemical composition of clusters. Theoretical calculations confirm positron trapping by Ag nanoclusters, and the confinement of positrons is enhanced with increasing Ag cluster size.

Study of Interaction Mechanism between Positrons and Ag Clusters in Dilute Al–Ag Alloys at Low Temperature

“…Based on such temperature-dependent behavior, we may deduce that the Ag clusters can act as shallow positron trapping sites to trap positrons, and the trapping ability increases with the decrease in temperature. The conclusion is consistent with the results of B. Zou et al [ 11 ].…”

“…A CDB ratio curve can reflect the momentum distribution of core electrons of certain elements annihilated with positrons, due to the significantly enhanced signal-to-noise ratio of at least 10 5 :1 for a typical CDB spectrum. As shown in Figure 5 , there is a narrow peak at around 11 × 10 −3 m 0 c in the CDB ratio curve of pure Ag, which is consistent with the experimental results of Y. Nagai [ 32 ] and B. Zou [ 11 ]. The curves in Figure 5 show a bump at 11 × 10 −3 m 0 c at low temperatures, which disappears at high temperatures.…”

“…As regards the temperature dependence, it is clear that an increased temperature facilities the detrapping of positrons from trapping sites for all the binding energies except E b = 1 eV. It was reported that the positron binding energy for Ag clusters is lower than 40 meV [ 11 ]; therefore, Ag clusters could only act as shallow trapping defects for positrons, and the low temperature enhancement of the effect of Ag on positron trapping is thereby understandable.…”

“…Nagai et al [ 10 ] claimed that the chemical composition of Ag-rich nanoclusters is AlAg 3 in aged Al-2.0 at.% Ag from experimental and calculated two-dimensional angular correlation of positron annihilation radiation (2D-ACAR) anisotropy results. In a recent study, B. Zou et al [ 11 ] reported the precipitation process of Ag nanoclusters in three Al–Ag alloys with different Ag contents (Al-1 wt.%Ag, Al-5 wt.%Ag, and Al-15 wt.%Ag). However, in the above studies, little attention was paid to the Al–Ag alloys with a very low Ag concentration (less than 0.5 wt.%).…”

“…This indicated that the Ag atom has a weak but extended attractive potential to positrons in Al–Ag alloys. Zou et al [ 11 ] found that the Ag clusters in Al–Ag alloys can act as positron trapping centers and that the confinement of positrons is enhanced with the increase in the size of Ag clusters. Moreover, Hugenschmidt et al [ 24 ] found that coincidence doppler broadening (CDB) is sensitive to the detection of small precipitates in aluminum, and even a 0.1 nm thin Sn layer embedded underneath a 200 nm Al coating leads to a significant and unambiguous fingerprint in the CDB spectrum.…”