Copper Nanoparticles Embedded in Natural Plagioclase Mineral Crystals: In Situ Formation and Third-Order Nonlinearity

Abstract

Copper nanoparticles are low-cost plasmonic materials with the ability of manipulating light at the nanometer scale. The exploration of copper nanoparticles toward photonic applications suffers instability due to the easy prone oxidation. In this paper, we developed the in situ formation of copper nanoparticles with a diameter between 40 and 50 nm in a natural plagioclase mineral crystal matrix by controlling the Cu+–Na+ ion exchange process in the temperature range of 1050–1200 °C. By combining quantitative element distribution characterization and spectroscopic measurement, we illustrate the in situ formation process and mechanism of Cu nanoparticles in natural plagioclase mineral crystals. Moreover, we determined the nonlinear optical properties of the materials by using the picosecond Z-scan technique. The third-order nonlinear optical susceptibility χ(3) at 1064 and 532 nm was estimated to be (1.046 ± 0.213) × 10–10 esu and (3.236 ± 0.198) × 10–10 esu, respectively. The natural plagioclase mineral crystals embedded with copper nanoparticles are believed to hold great potential to serve as plasmonic materials for a variety of practical applications, especially in nonlinear optics.

High-Temperature Diffusion of Cu2+ Ions into Natural Plagioclase Crystals: The Role of Li2O

“…To highlight the changes in plagioclase crystals after Cu–Li diffusion, the Cu diffusion-treated red plagioclase crystal was introduced as a comparison. The red color is attributed to the typical SPR absorption peak of copper nanoparticles (Cu 0 ) in plagioclase crystals . On the contrary, when Li 2 O was added to the diffusant (the molar ratio of Cu to Li is 1:1), the plagioclase crystal became blue–green after the Cu–Li diffusion.…”

“…The red color is attributed to the typical SPR absorption peak of copper nanoparticles (Cu 0 ) in plagioclase crystals. 20 On the contrary, when Li 2 O was added to the diffusant (the molar ratio of Cu to Li is 1:1), the plagioclase crystal became blue−green after the Cu−Li diffusion. A broad absorption peak near 750 nm was observed in the absorption spectrum, which is caused by the presence of Cu 2+ .…”

“…Li 2 CuO 2 is usually formed by heating the mixture of CuO and Li 2 O at 700 °C with oxygen for 16 h; 34,35 therefore, in the above description of high-temperature Cu−Li diffusion, 700 °C is taken as the dividing point between stage I and stage II. The newly formed Li 2 CuO 2 in the diffusant is thermodynamically S5a, the red plagioclase crystal containing Cu + and copper nanoparticles was first prepared in the Cu diffusion experiment; 20 then, the red plagioclase crystal was planted in the mixed diffusant of Li 2 O and ZrO 2 , and the Li diffusion was carried out at 1000 °C for 3 h. The red plagioclase crystal did not turn blue−green (Figure S5b) after Li diffusion. The above results mean that Li + diffused into the plagioclase crystal later does not lead to the transformation of prediffused Cu + into Cu 2+ .…”

“…In terms of the doping of copper and other precious-metal ions in aluminosilicate materials, current research reports mostly focus on the typical ion-exchange technique of soda-lime glass, − whose chemical composition is similar to that of plagioclase crystals. However, the ion-exchange technique of soda-lime glass does not fully apply to the plagioclase crystal owing to the relatively lower diffusion temperature (around 600 °C) of soda-lime glass. − In our previous study, the in situ formation of copper nanoparticles in the plagioclase crystals was revealed by using a high-temperature copper diffusion process . The Cu in the diffusant exists stably in the form of Cu + at high temperatures (1050–1200 °C) in the air atmosphere.…”

“…17−19 In our previous study, the in situ formation of copper nanoparticles in the plagioclase crystals was revealed by using a high-temperature copper diffusion process. 20 The Cu in the diffusant exists stably in the form of Cu + at high temperatures (1050−1200 °C) in the air atmosphere. After diffusing into the crystal by Cu + −Na + ion exchange, a part of Cu + ions were converted into Cu 0 by capturing the hot electrons in the crystal.…”