FR | EN

Theme 08

Functionalized surfaces: From organized relief to enhanced functionality

A self-organized nanostructure is not an end in itself. The pulse that writes it never writes one thing: it fixes the geometry of the relief, the microstructure of the first few hundred nanometres, and the chemical state of the first few nanometres, all at once. A property reads a combination of those three registers, almost never one alone. This theme is ordered by which register carries the property, from the case where the relief is sufficient to the case where it decides almost nothing.

With Anthony Nakhoul, Mathilde Prudent, Priya Dominic, Djafar Iabbaden, Benjamin Dusser, Noémie Lebrun, Florence Garrelie, Jean-François Pierson, Philippe Steyer, Alixe Dreano and Frédéric Christien.

01Three registers, one pulse

What an application specifies is rarely a shape. It is a wetting angle, a reflectance, a permeation flux, a bacterial adhesion rate. The relief is only one of the things that answer, and the same irradiation that selects the morphology also refines the grains underneath and rewrites the oxide on top. Reading a laser-textured surface as a relief alone is what makes functional results look irreproducible from one laboratory to the next.

The useful question is therefore not which morphology to write, but which register the property is actually written in. The sections below answer it case by case, and the answer changes from one function to the next.

02Colour without pigment

03Can a texture keep hydrogen out?

Hydrogen embrittlement is decided at the entry, not in the bulk: what matters is how much hydrogen crosses the first nanometres. Two textures were written on a Fe-Cr alloy, low spatial frequency ripples (LSFL) and high spatial frequency ripples (HSFL), then compared to a mirror-polished reference by electrochemical permeation.

The reduction is large, 89% for LSFL and 95% for HSFL, and it is not a corrugation effect. Dissolving the oxide in a citric acid bath brings the uptake back up on both textures, which places the barrier squarely in the chemical register. What the pulse leaves behind is a 2 to 5 nm film with a duplex stacking of iron and chromium oxides, partly crystalline on HSFL, quite unlike the native oxide that grows on a polished surface.

Topography is not irrelevant, it is second order: at comparable oxide thickness, a relief with high skewness and high kurtosis still cuts permeation by a further 40%. The specification for a hydrogen-resistant surface is therefore a pair, an oxide stacking and a height distribution, both fixed by the same shot. Whether a two-dimensional pattern, written by crossing polarizations, can push the effect further is the obvious next test.

04Chemistry rearranged by the pulse

Metallic glasses make the chemical register visible, because they have no grains to hide it. On a Zr₅₀Cu₅₀ thin film, the crest of a ripple is no longer the alloy that was deposited: a copper-rich cap about 60 nm thick sits on a zirconium and oxygen layer about 130 nm thick, and the matrix now holds monoclinic zirconia crystallites and face-centred cubic copper nanoparticles.

On Zr₆₅Cu₃₅ the same irradiation opens a dense array of nanowells about 20 nm across, with a flared profile well suited to holding a molecule once it is in. The wells are not written by the light. They nucleate on the nanometric interstices left between the columns of the sputtered film, so their density and size are set by the deposition rather than by the beam. Cavitation localizes at those defects while the pressure is still confined, and the local oxidation that follows converts the glass into a composite of monoclinic zirconia in an amorphous ZrCu matrix.

One shot delivers a container and a new material around it: a topographic function and a chemical function written together.

05What decides how a droplet sits?

Wetting is where the registers refuse to be separated, so it has to be attacked with two half-experiments. Fix the geometry and change the chemistry, then fix the chemistry and change the geometry.

The first half is done on tungsten. High spatial frequency ripples of the same period and the same sub-20 nm amplitude are written in ambient air and at 10^-7 mbar. The relief is identical; the chemistry is not. In air, the surface crosses the 1273 to 600 K window in 7.2 ns, long enough for oxygen to diffuse about 2.8 nm and for a crystalline monoclinic WO₃ layer roughly 2 nm thick to grow. In vacuum, only the native amorphous oxide is there.

The contact angle is near 0 degrees on the ambient ripples and about 58 degrees on the vacuum ones. Crystalline WO₃ carries the higher surface energy, so it wets better than the amorphous native film. Over 84 days both surfaces drift towards larger angles as hydrocarbons adsorb, and the two curves never cross. On a fixed relief, the chemical register alone spans almost the entire hydrophilic range.

Put side by side, the two halves bracket the problem. Geometry alone, at fixed chemistry, spans 54 to 91 degrees. Chemistry alone, at fixed geometry, spans 0 to 58 degrees. Neither register is sufficient and neither is negligible, which is why a wetting specification cannot be met by scanning laser parameters and measuring afterwards. It has to be posed as a target in both registers at once, and that is a modelling problem.

06Why this closes the loop

← Back to research themes | All publications →