Hierarchical Laser Structuring of Polymer Surfaces | #Nanophotonics #Biomaterials #SurfaceWettability #AdhesionControl #SmartSurfaces #FunctionalMaterials

Laser-Assisted Fabrication of Hierarchical Micro/Nanostructures on Polymer Films via LIPSS and Fresnel Diffraction

1. Introduction to Hierarchical Structuring

Hierarchical structuring refers to creating surface patterns at multiple length scales—typically micro- and nanoscale—on the same surface. In nature, such structures are found in lotus leaves (superhydrophobicity) and insect eyes (anti-reflectivity). Reproducing these effects synthetically, especially on polymers, allows for customized surface behavior.


2. Materials and Methods

Polymer Selection

  • Material used: Poly(bisphenol A carbonate) (PBAC)

  • Preparation: Thin films spin-coated on silicon wafers, ~150 nm thick

Laser Parameters

  • Laser type: Q-switched Nd:YAG (266 nm, UV)

  • Pulse duration: 8 ns

  • Fluence: ~7 mJ/cm²

  • Mask: TEM copper grid with 90 µm square holes

This setup allows simultaneous irradiation and patterning, where nanostructures (LIPSS) form due to laser-surface interaction, and microstructures are defined by Fresnel diffraction through the grid.

3. Formation of Micro- and Nanostructures

LIPSS Formation

  • Formed due to interference between incident laser light and surface-scattered waves

  • Aligned with laser polarization

  • Period ~180–240 nm

  • Depth varies with pulse number and fluence

Microscale Features

  • Induced by Fresnel diffraction through the mask

  • Produces square-patterned regions (~90 µm) on the polymer

  • Local intensity variations lead to gradients in structure depth and spacing

Combined Effect

  • Hierarchical zones with both ripple-like nanostructures and large-scale patterns

  • Edge regions show increased structure depth due to diffraction-enhanced fluence

4. Surface Property Modifications

Wettability

  • LIPSS alone: Increases hydrophilicity due to increased surface area

  • Combined structures: Result in increased hydrophobicity due to hierarchical roughness (similar to lotus leaf effect)

Adhesion

  • Measured via colloidal probe AFM

  • Increased adhesion near structured edges

  • Minimal change in flat/nano-only areas

Mechanical Response

  • Nanomechanical mapping shows variation in stiffness and modulus

  • Affected by surface morphology and possible local chemical changes

5. Characterization Techniques

  • AFM (Atomic Force Microscopy): Topography, structure depth, periodicity

  • Colloidal Probe AFM: Adhesion force measurements

  • Contact Angle Goniometry: Surface wettability analysis

  • FTIR Spectroscopy: Surface chemical composition before/after irradiation

6. Advantages of the One-Step Laser Structuring Approach

  • No chemical etching or masks required (aside from reusable TEM grid)

  • Scalable and reproducible

  • Simultaneous micro- and nano-patterning

  • Compatible with diverse polymers

  • Environmentally friendly (dry process, no solvents needed post-spin-coating)

7. Applications

This method enables surface functionalization in diverse areas:

  • Biomedicine: Cell guidance, antibacterial coatings

  • Optics: Anti-reflective surfaces, diffraction gratings

  • Microfluidics: Wettability-tuned channels

  • Nanoelectronics: Patterned templates for device integration

  • Sensors: Enhanced surface area for detection

8. Future Directions

  • Tuning periodicity and orientation with beam shaping or rotating polarization

  • Exploring other polymer systems

  • Integration into roll-to-roll systems for industrial scale-up

  • Multifunctional surfaces combining mechanical, optical, and chemical functionality

#Nanophotonics #Biomaterials #SurfaceWettability #AdhesionControl #SmartSurfaces #FunctionalMaterials

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