Abstract
In this study, we describe the formation mechanism of web-like three-dimensional (3-D) titania nanofibrous structures during femtosecond laser ablation of titanium (Ti) targets in the presence of background air. First, we demonstrate the mechanism of ablation of Ti targets by multiple femtosecond laser pulses at ambient air in an explicit analytical form. The formulas for evaporation rates and the number of ablated particles, which is analogous to the deposition rate of the synthesized nanofibers, for the ablation by a single pulse and multiple pulses as a function of laser parameters, background gas, and material properties are predicted and compared to experimental results. Afterwards, the formation of nanofibrous structures is demonstrated by applying an existing simplified kinetic model to Ti targets and ambient conditions. The predicted theory provides nanofiber diameter dependency with the combination of laser parameters, target properties, and ambient gas characteristics. Experimental studies are then performed on titania nanofibrous structures synthesized by laser ablation of Ti targets using MHz repletion-rate femtosecond laser at ambient air. The models' predictions are then compared with the experimental results, where nanostructures with different morphologies are manufactured by altering laser parameters. Our results indicate that femtosecond laser ablation of Ti targets at air background yields crystalline titania nanostructures. The formation of crystalline titania nanostructures is preceded by thermal mechanism of nucleation and growth. The results point out that laser pulse repetition and dwell time can control the density, size, and pore size of the engineered nanofibrous structure. As the deposition rate of nanostructures is analogous to the ablation rate of the target, higher density of nanofibrous structure is seen at greater laser fluences. The predicted theory can be applied to predict ablation mechanism and nanofiber formation of different materials.
Highlights
Nanostructures have recently drawn great attention due to the exceptional physical, optical, and chemical properties they present, compared to those of their bulk counterparts
This study described the formation mechanism of interwoven 3-D nanofibrous structures during femtosecond laser ablation of Ti targets in ambient air
The ablation mechanism of Ti targets by successive pulses using femtosecond laser processing at background air was first demonstrated in analytical form
Summary
Nanostructures have recently drawn great attention due to the exceptional physical, optical, and chemical properties they present, compared to those of their bulk counterparts. A femtosecond laser pulse can heat any material to a soliddensity plasma state with temperature and pressure above the critical point This leads to the generation of high density plume which results in nanoparticle aggregation after plume condensation. We have introduced a single-step method to synthesize 3-D titania nanofibrous structures under high repetition femtosecond laser irradiation at ambient conditions.. Our previous results indicated that the morphology of the nanostructures could be controlled by altering laser parameters As it follows, this study demonstrates the ablation mechanism of Ti targets by multiple femtosecond laser pulses at ambient air in an explicit analytical form. Experimental studies are conducted on titania nanofibrous structures synthesized by laser ablation of Ti targets using MHz repletion-rate femtosecond laser at ambient air. The predictions of the models are compared with the experimental results where nanostructures with different morphologies are synthesized by altering laser parameters
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