Synthesis and Characterization of Silver-Gold Bimetallic Nanoparticles for Random Lasing

Abstract

We developed rough silver-gold bimetallic nanoparticles for random lasing. Silver nanoparticles were synthesized based on a citrate-reduction method and the gold (III) chloride trihydrate was added to produce bimetallic nanoparticles. Gold atoms were deposited on the surface of the silver (Ag) through galvanic replacement reactions after the solution was stored at room temperature. Sample characterization and a spectrometry experiment were performed where bimetallic nanoparticles with nanogaps and the extinction of the nanoparticles were observed. The aim of this research is to synthesize nanoparticles for random dye laser in a weakly scattering regime. The novel bimetallic nanoparticles were added to Rhodamine 640 solution to produce random lasing. We found that random dye laser with bimetallic nanoparticles produced spectral narrowing and lasing threshold compared to random dye laser with silver nanoparticles. We attribute that to the localized surface plasmon effects which increase local electromagnetic field to provide sufficient optical gain for random lasing. The rough surface of bimetallic nanoparticles also contributes to the properties of random lasing. Thus, we suggest that the rough bimetallic nanoparticles can be used to develop random lasers.

Keywords: and random lasers; bimetallic; localized surface plasmon effects; nanomaterial; surface roughness.

Figure 1

The synthesis flow of the bimetallic nanoparticles.

Figure 2

TEM images of silver nanoparticles (1.1 × 10¹¹ cm⁻³) (a) before centrifugation with a scale bar of 100 nm and (b) after centrifugation for 1 h (10× dilution) with a scale bar 200 nm. From that Figure, the size of nanoparticles was estimated as ~30 ± 2 nm. (c) TEM images of rough silver-gold bimetallic nanoparticles (2.8 × 10¹⁰ cm⁻³) with a scale bar of 50 nm. The enlarged view of a silver nanoparticle and a rough bimetallic nanoparticle shown are also shown in the Figure. The green circles show the nanogaps. 40 μL of HAuCl₄ was used in the sample shown in Figure 2c.

Figure 3

TEM images of bimetallic nanoparticles with various amount of HAuCl₄; (a) 60 μL, (b) 80 μL, (c) 100 μL and (d) 120 μL with a scale bar of 100 nm. The aggregation is shown by blue circles. The highest aggregation of nanoparticles occurs after adding 120 μL of HAuCl₄ to the silver nanoparticles.

Figure 4

Size distribution of silver nanoparticles and silver-gold nanoparticles. 40 μL of HAuCl₄ was used in the sample.

Figure 5

Random laser measurement.

Figure 6

Extinction spectra of silver and bimetallic nanoparticles (nps) and absorption and fluorescence spectrum of Rhodamine 640: Extinction spectrum of silver (orange curve); extinction spectrum of rough bimetallic nanoparticles (red dash dot curve); absorption of Rhodamine 640 (purple dash curve) and fluorescence spectrum of Rhodamine 640 (dark blue dot curve). The vertical line (dark green) indicates the pump laser wavelength. Bimetallic nanoparticles were prepared by adding 40 μL of HAuCl₄.

Figure 7

Emission spectra of (a) Rh640 (1 × 10⁻⁴ M)/silver and (b) Rh640 (1 × 10⁻⁴ M)/rough bimetallic random lasers.

Figure 8

(a) Emission peak intensity and (b) Emission linewidth at 75% of emission peak intensity as a function of pump energy density for Rh640 (1 × 10⁻⁴ M)/silver and Rh640/bimetallic random lasers. The lasing threshold is estimated when the emission peak intensity increases substantially with the pump level or when the emission linewidth drops significantly as indicated by the brown line. Error bars show the fluctuation of the emission intensity and linewidth for ten readings from Ocean Optics Spectrometers.