Real-Time Imaging of Laser-Induced Nanowelding of Silver Nanoparticles in Solution

Nanowelding of metallic nanoparticles
induced by laser illumination
is of particular interest because it provides convenient and controlled
means for shape-conversion of nanoparticles and fabrication of nanodevices.
However, the kinetics of the laser-induced nanoparticle nanowelding
remains largely unexplored. Herein, we exploited fluorescence microscopy
to directly image the real-time nanowelding kinetics of silver nanoparticles
(AgNPs) when illuminated with a continuous wave laser at 405 nm. We
observed that the laser illumination induced the AgNPs to form higher-order
branched structures or assemblies. More importantly, we quantified
the sizes of the laser-induced assemblies and found that the dependence
of the average size (A̅) of the assemblies
on the illumination time t followed A̅ ∝ 1 – e–t/τ. An analytical model based on simple polymerization was developed
to predict and understand the measured kinetics. We experimentally
verified the model by varying the laser power and the concentration
of AgNPs. Furthermore, we improved the model by taking into account
the merging of assemblies and predicted that the laser-induced assembling
kinetics was diffusion-limited, which was then verified experimentally
with AgNPs in 50% glycerol. Lastly, in contrast to the single-phased
ohmic nanocontact produced by the laser-induced nanowelding, we found
that the formed higher-order structures were separated into different
photoluminescent domains and different regions of the same laser-induced
assembly showed asynchronous, uncorrelated blinking behaviors. This
work is expected to facilitate the development of better nanowelding
strategies of metallic nanoparticles for broader applications.