In Situ Contact Angle Tuning of Silver Nanostructures by Laser Heating in Different Fluid Ambient

Abstract

It has been theoretically suggested by Yan et al. (Colloids Surf., A 2013, 429, 142–148) that the contact angle θc of a liquid droplet on any given surface can be controlled by immersing it in an appropriate surrounding environment. Here, we report the first experimental demonstration of such an in situ contact angle tuning of silver (Ag) nanoparticles on quartz substrates via nanosecond pulsed laser heating under various fluid ambients, like air, water, and glycerol. Nanosphere lithography (NSL) was used to deposit Ag nanopyramids on quartz substrates. This system was subsequently melted by laser heating inside the various fluids to form nanoparticles. By using a combination of top and side view scanning electron microscopy (SEM) imaging, we show that the contact angle of Ag nanoparticles could be increased by going from heating in air to heating under fluids, with a near hemispherical shape (θc ∼ 99°) under air irradiation to nearly spherical (θc ∼ 167°) under glycerol irradiation. The mechanism of this contact angle change could be explained qualitatively by the changing interfacial energies of the substrate and metal in the various fluids. Similar contact angle control was also achieved for nanoparticles created by dewetting of Ag thin films in the various fluids. One practical implication of this contact angle tunability is the ability to change the intensity ratio of the quadrupolar to dipolar localized surface plasmon resonances in the Ag nanoparticles. This work also has implications to those applications in which nanoparticles on a substrate are heated in various gas or fluid ambients, such as in catalysis, as the ensuing shape change can modify properties.

Nanosecond switchable localized surface plasmons through resettable contact angle behavior in silver nanoparticles

“…We have built our switching idea based on a newly discovered effect in which the shape of the particle can be changed based on the conditions under when the particle is irradiated. In a recent work by our group [24], we demonstrated that Ag nanoparticles fabricated by laser melting in air, water and glycerol environments yielded significantly different shapes, with the glycerol leading to a near-spherical shape, while air produced near-hemispherical shape particles. This shape difference was attributed to different interfacial energies involved, with the glycerol-substrate and glycerol-Ag interfacial energies being larger than the air-substrate and air-Ag interfacial energy, this leading to higher contact angles for the glycerol case, as predicted by Young's equation [32].…”

“…The single pulse irradiation was performed at normal incidence by the linearly polarized laser beam of area 1 mm 2 . This technique was developed recently to synthesize nearspherical nanoparticles on quartz substrates via a dewetting process, as reported in reference [24]. The resulting nanostructures were then investigated by high resolution imaging using a ZEISS Merlin scanning electron microscope (SEM) in plan and side view orientations.…”

“…The magnitude of these changes depend upon the particle size and volume fraction in the dielectric media consisting of the quartz contact surface and surrounding air. Recently, it was experimentally demonstrated by Prasad et al [24] that it is possible to change the contact angle of Ag nanoparticles by nanosecond pulsed laser irradiation in various fluid ambient like air, water, and glycerol. Here, we have used glycerol environment to first synthesize near-spherical shaped Ag nanoparticle arrays using UV pulsed laser irradiation.…”