Research · Materials Physics

Laser Direct-Write Polymerization of Ultrathin C₆₀ Films

A focused 532 nm laser both polymerizes evaporated C₆₀ films a few nanometers thick and reads the result out through Raman spectroscopy. It writes micron-scale patterns without a resist, and the reaction runs twice as fast in 10 nm films as in 2 nm films.

  • Concluded
  • Jan 2025 – 2026
  • Gutiérrez Lab, University of South Florida
  • Mentor: Dr. Humberto R. Gutiérrez
Poster (PDF) →
Research poster titled On-Demand Laser-Induced Polymerization of Evaporated C60 Thin Films: Direct-Write Micropatterning and Thickness-Dependent Kinetics, by James Watt, Yasinthara Wadumethsri, and Humberto R. Gutiérrez. Panels cover the abstract, introduction, methods, AFM and Raman characterization of the films, laser-written dot, line and rectangle patterns mapped by Raman peak position, single-exponential decay fits at 2, 3, 5 and 10 nm, the normalized timescale versus thickness, conclusions, and references.
Gutiérrez Lab · University of South FloridaDownload poster (PDF)

The question

Solid C₆₀ is a crystal of soccer-ball molecules held together only weakly, which limits how well it conducts. Light can link neighboring molecules covalently through a [2+2] cycloaddition, turning the film into polymerized C₆₀. I asked two questions: can that reaction be written on demand, exactly where you want it, in films only a few nanometers thick? And how does film thickness change how fast it happens?

What I did

  • Film growth. I deposited 2, 3, 5, and 10 nm C₆₀ films on Si/SiO₂ by high-vacuum thermal evaporation under closed-loop quartz-crystal thickness control, and confirmed the thicknesses with AFM step-edge profiles. Raman spectra of the as-deposited films showed all ten C₆₀ vibrational modes with no sign of pre-polymerization or oxidation.
  • One laser, two jobs. A focused 532 nm laser drives the polymerization and measures it. The key signature is the Ag(2) “pentagonal-pinch” Raman mode shifting from 1469 to 1460 cm⁻¹ as molecules bond.
  • Patterning. I wrote single spots, lines, and rastered rectangles, then mapped the Raman peak position across each region to check that only the targeted area had changed (map below).
  • Kinetics. I tracked the Ag(2) intensity with time-resolved Raman during irradiation at each thickness.
  • Devices. I fabricated field-effect transistor test structures on 5 and 10 nm films and measured transport before and after polymerization.

Results

  • Resist-free direct writing. The polymerized regions are sharply confined to where the laser was pointed, with no mask or photoresist. Spots match the ~6 µm laser spot, overlapping spots write ~20 µm lines, and rastering writes ~25 × 30 µm rectangles. Inside a pattern the Ag(2) mode sits within ±2 cm⁻¹ of 1460 cm⁻¹, while the film around it stays at ~1470 cm⁻¹. Raman peak-position maps of a C₆₀ film: (a) a laser-written line and (b) a laser-rastered rectangle. Polymerized regions appear dark blue (Ag(2) mode near 1460 cm⁻¹) against the orange pristine film (near 1469 cm⁻¹).

  • Thicker films react faster. At every thickness the conversion follows a clean single-exponential decay. The fitted timescale falls from τ ≈ 173 s at 2 nm to τ ≈ 84 s at 10 nm, so 10 nm films polymerize about twice as fast. Normalized to the 2 nm film, the timescales for 2 / 3 / 5 / 10 nm are 1.00 / 0.79 / 0.72 / 0.50.

  • Electronics: a useful null. The transistors showed no measurable change in transport between pristine and polymerized films. This marks a boundary on when photopolymerization can be used to “write” electronic properties in this geometry.

Normalized polymerization timescale versus C₆₀ film thickness: 1.00 at 2 nm, 0.79 at 3 nm, 0.72 at 5 nm, and 0.50 at 10 nm.

Status

I wrote the work up as a manuscript with Y. Wadumethsri and Dr. H. R. Gutiérrez. The project has concluded without a journal submission. The film-growth, patterning, and kinetics results above stand on their own.