# Self-Organization Regimes Induced by Ultrafast Laser on Surfaces in the Tens of Nanometer Scales

^{1}

^{2}

^{3}

^{*}

## Abstract

**:**

## 1. Introduction

## 2. Materials and Methods

#### 2.1. Material Preparation

^{3}cubes by using a wire saw. Two types of polishing were performed during experiments, the mechanical and electrochemical polishing. The automatic polishing was performed on “Buehler Automet 250” by using a coarse paper of P180, moving successively to P320, P600, P1200 and P2400 followed by a diamond 3 μm and 1 μm and vibratory polishing on “Buehler Vibromet 2” with the colloidal silica 0.05 μm for 17 h, prior to laser irradiations. Electropolishing was performed after automatic polishing on “Struers LectroPol-5” by using stainless steel electrolyte at 25 Volts for 60 s. Both polishing procedures assure mirror-zero scrach samples with an initial arithmetical mean surface roughness (Ra) below 5 nm. The Ra threshold Ra

_{th}for nanostructures formation was found to be 5 nm on the Atomic Force Microscopy (AFM), on a scan of 5 × 5 μm. Crystal orientations were checked by X-ray diffraction prior to laser irradiation to ensure the uniform cutting direction.

#### 2.2. Laser Setup

#### 2.3. Characterization

## 3. Results

#### 3.1. Advanced Surface Topography Control as a Function of Time Delay and Laser Fluence

#### 3.2. Wide Variety of Nanostructure Regimes

#### 3.3. Wide Variety Nanopatterns Morphologies

#### 3.4. Nanopatterns Control by Laser Dose

#### 3.5. Initial Roughness Effect on Nanostructures Formation

_{th}. Mechanical polishing was performed for sample 1 by using a coarse paper up to P2400 followed by a diamond polishing of 3 μm and 1 μm and vibratory polishing with colloidal silica of 0.05 μm for several hours, which guarantee a mirror surface with a very low arithmetic roughness. Kurtosis (Ku) statistical parameters was used to characterize the type of initial roughness and sharpness of surface spikes. If Ku > 3 the surface is considered “spiky” and if Ku < 3 the surface is considered “bumpy”. If Ku = 3, the surface has completely random surface roughness. For sample 1, the Ku = 7.54 which is considered spiky surface with a Ra < Ra

_{th}.

_{th}and a Ku of 2.84 which is considered bumpy surface.

## 4. Discussion

## 5. Conclusions

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Conflicts of Interest

## Abbreviations

LIPSS | Laser-Induced Periodic Surface Structures |

HSFL | High Spatial Frequency LIPSS |

SEM | Scanning Electron Microscopy |

AFM | Atomic Force Microscopy |

Ra | Arithmetic Roughness |

Ra_{th} | Arithmetic Roughness Threshold |

Ku | Kurtosis |

${P}_{r}$ | Prandtl Number |

${P}_{r}^{c}$ | Critical Prandtl Number |

${N}_{DPS}$ | Number of Double-Pulses Sequences |

$\Delta t$ | Time-Delay Between the Double-Pulses |

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**Figure 1.**2D AFM images of laser-induced nanopatterns formation on Ni(100) as a function of time delay and peak fluence at a fixed ${N}_{DPS}$ of 25. The different zones of interest are nanopeaks, nanobumps, nanohumps and nanocavities. They are created progressively at different doses: (0.18 J/cm${}^{2}$; 8 ps), (0.18 J/cm${}^{2}$; 10 ps), (0.18 J/cm${}^{2}$; 15 ps), (0.24 J/cm${}^{2}$; 25 ps).

**Figure 2.**(

**a**–

**d**) The 3D AFM images of the laser spot topography in the spallation regime and the principal nanostructures types (nanopeaks, nanobumps, nanohumps and nanocavities). (

**e**) Maximum nanostructures height as a function of time delay and laser fluence. The colored circles in (

**e**) (black, green, blue and orange) present respectively the regions of (

**a**–

**d**) nanostructures.

**Figure 3.**3D AFM images of the principal nanopatterns (

**a**–

**d**) at different scales, presenting the laser spot region. A scan profile was performed for each type of the principal nanostructures presenting the shape and the periodicity of each type.

**Figure 4.**(

**a**) 2D scanning electron microscopy (SEM) images presenting the influence of the ${N}_{DPS}$ in enhancing nanostructures at a fixed time delay. (

**b**) 2D SEM images of growing nanopatterns by decreasing the time delay at a fixed peak fluence of 0.24 J/cm${}^{2}$ and ${N}_{DPS}$ = 25. (

**c**) 2D SEM image of the gaussian laser spot at a fixed laser parameters (0.18 J/cm${}^{2}$; 10 ps; ${N}_{DPS}$ = 44). (

**d**) The left region of the gaussian laser spot, showing the significant role of laser fluence in controlling different types of nanostructures.

**Figure 5.**Initial surface topography of two different samples polished by different procedures (Mechanical polishing in (

**a**) and Electrochemical in (

**c**)). Arithmetic roughness (Ra) and Kurtosis (Ku) were measured and compared for both samples. (

**b**,

**d**) 2D and 3D SEM images of (

**a**,

**c**) respectively after laser irradiation. The crucial role of initial type of roughness is observed by comparing the nanostructures concentration in the SEM.

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**MDPI and ACS Style**

Nakhoul, A.; Maurice, C.; Agoyan, M.; Rudenko, A.; Garrelie, F.; Pigeon, F.; Colombier, J.-P.
Self-Organization Regimes Induced by Ultrafast Laser on Surfaces in the Tens of Nanometer Scales. *Nanomaterials* **2021**, *11*, 1020.
https://doi.org/10.3390/nano11041020

**AMA Style**

Nakhoul A, Maurice C, Agoyan M, Rudenko A, Garrelie F, Pigeon F, Colombier J-P.
Self-Organization Regimes Induced by Ultrafast Laser on Surfaces in the Tens of Nanometer Scales. *Nanomaterials*. 2021; 11(4):1020.
https://doi.org/10.3390/nano11041020

**Chicago/Turabian Style**

Nakhoul, Anthony, Claire Maurice, Marion Agoyan, Anton Rudenko, Florence Garrelie, Florent Pigeon, and Jean-Philippe Colombier.
2021. "Self-Organization Regimes Induced by Ultrafast Laser on Surfaces in the Tens of Nanometer Scales" *Nanomaterials* 11, no. 4: 1020.
https://doi.org/10.3390/nano11041020