# Atomistic Modeling as a Key to Unraveling Ultra-Short Laser Nano-Synthesis and Nano-Treatment

Tatiana E Itina, Dmitry S Ivanov, Ilemona S Omeje

► To cite this version:

Tatiana E Itina, Dmitry S Ivanov, Ilemona S Omeje. Atomistic Modeling as a Key to Unraveling Ultra-Short Laser Nano-Synthesis and Nano-Treatment. Proceedings of SPIE, the International Society for Optical Engineering, 2025, Proceedings Volume 13352, Nanoscale and Quantum Materials: From Synthesis and Laser Processing to Applications 2025, 13352, pp.1335206. 10.1117/12.3049888. ujm-05032897

HAL Id: ujm-05032897

https://ujm.hal.science/ujm-05032897v1

Submitted on 14 Apr 2025

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# Atomistic Modeling as a Key to Unraveling Ultra-Short Laser Nano-Synthesis and Nano-Treatment

Tatiana E. Itina*, Dmitry S. Ivanov, Ilemona S. Omeje
Hubert Curien Laboratory, CNRS UMR 5516, UJM, Saint-Etienne, France

## ABSTRACT

The advent of short and ultrashort laser pulses has revolutionized micro- and nanostructuring, enabling precise nanoparticle synthesis, assembly, and fragmentation. These laser-induced material modifications have profound implications across catalysis, energy storage, medicine, and microelectronics. This study investigates the main mechanisms of these interactions through atomistic simulations. Our findings demonstrate that femtosecond laser pulses can induce a spectrum of non-equilibrium effects, from subtle structural modifications to explosive material disintegration. Critical phenomena, such as nanostructure formation and surface oxidation, are explored, particularly in the context of their impact on nano-catalysis and energy applications. We also highlight the potential of ultrashort pulses for tailoring surface properties, such as wettability, showcasing their utility in advanced material processing.

**Keywords:** Atomistic modeling, ultrashort laser pulses, nanoparticle synthesis, nano-catalysis, energy storage, molecular dynamics.

## 1. INTRODUCTION

The interaction of ultrashort laser pulses with materials has opened new frontiers in nanotechnology, offering unprecedented control at the atomic scale. These femtosecond (fs) and picosecond (ps) pulses facilitate rapid manipulation of matter, essential for applications ranging from nanoparticle synthesis to surface functionalization. High energy densities associated with ultrashort pulses enable localized heating and structural modifications, positioning laser-based processing as an invaluable tool in nanomaterials engineering [1, 2].

A critical aspect of laser-material interactions is the interplay between thermal and electronic excitations, driving phenomena such as surface melting, amorphization, phase transitions, and nanoparticle generation. Atomistic modeling, using molecular dynamics (MD) simulations coupled with the two-temperature model (TTM), provides a detailed understanding of these mechanisms, bridging theoretical predictions with experimental results [3, 4].

Recent studies show that at low laser fluences, materials undergo subtle modifications, such as thin-layer melting and amorphization, sensitive to crystalline orientation [5]. At higher fluences, explosive vaporization and nanoparticle formation dominate, with applications in catalysis, energy storage, and sensors [6].

This study employs atomistic simulations to elucidate ultrashort laser-induced processes, exploring their potential for advancing nanomaterials science and technology.

*tatiana.itina@univ-st-etienne.fr; phone 33 477915829; fax 33 477915781; laboratoirehubertcurien.fr
## 2. METHODS

### Molecular Dynamics and Two-Temperature Model

Molecular dynamics (MD) simulations were used to study ultrashort laser interactions with metals and semiconductors. These simulations solve Newton's equations of motion for atomic systems using interatomic potentials. The two-temperature model (TTM) captures energy exchange between the electron and lattice subsystems during ultrafast laser heating [7].

### Simulation Details

For bulk targets, we employed TTM-MD to calculate spatial and temporal evolutions of electron temperature. For nanoparticles, only temporal variations were considered due to rapid spatial homogenization at small scales. Metals were modeled with the embedded atom method (EAM) potential, while silicon used the Stillinger-Weber (SW) potential, both validated for ultrafast phenomena [8, 9]. Reactive systems employed the ReaxFF potential, crucial for studying oxidation and chemical reactions.

Periodic boundary conditions simulated bulk materials, with femtosecond laser pulses modeled as Gaussian distributions. Laser fluences ranged from below the ablation threshold to nanoparticle formation levels

## 3. SIMULATION RESULTS

### Non-Equilibrium Melting and Ablation

Our TTM-MD simulations reveal distinct regimes of material response to femtosecond laser pulses. At low fluences, silicon and metals exhibit non-equilibrium melting (Figure 1). Contrary to metals where commonly melting starts upon the drop of pressure, in Si melting rapidly propagates inside the target and starts upon the rise in pressure.

<figure><img src="image_2.png" /><figcaption>Figure 1: Atomic configurations after femtosecond laser interaction, illustrating thin-layer melting at low fluences, and explosive ablation with nanoparticle ejection at higher fluences. The anisotropic melting behavior of single-crystal silicon is highlighted, with deeper melting observed along the (001) plane. Below, lattice temperature and pressure are shown.</figcaption></figure>

For single-crystal silicon, anisotropic behavior was noted, with the (100) plane showing deeper melting than (111), consistent with experimental observations [10].
At higher fluences, explosive ablation and nanoparticle ejection occur, as depicted in Figure 2. One can see the formation of nanoparticles and nanostructures.

<figure><img src="image_3.png" /><figcaption>Figure 2: Atomic configurations after femtosecond laser interaction of Al target at sufficient laser fluence.</figcaption></figure>

## Surface Oxidation

Simulations of titanium oxidation under femtosecond laser irradiation demonstrate that oxide layer growth depends on time and temperature. The oxide thickness follows:

$$x(t, T) = \sqrt{\{k_{p,0} \cdot t \cdot \exp(-\{E_a\}/\{R \cdot T\})\}}$$

where $E_a$ is the activation energy, R is the gas constant, and $k_{p,0}$ is a pre-exponential factor. These results align with experimental trends and underline the role of precise thermal control in laser processing [11].

<figure><img src="image_4.png" /><figcaption>Figure 3: Calculated oxide film thickness formed on Ti surface and change in the number of molecules.</figcaption></figure>
## Surface Texturing and Wettability

Laser-induced surface texturing significantly influences wettability. Simulations show that surface roughness parameters, such as height (h), width (w), and spacing (s), dictate the spreading behavior of water nanodroplets, as illustrated in Figure 4. These findings offer pathways to design surfaces for biomedical and energy applications [12].

<figure><img src="image_5.png" /><figcaption>Figure 4: Snapshots of the side and front views of the spreading process of a water nanodroplet on Ti surfaces with different surface structure scales as a function of time [12].</figcaption></figure>

## Conclusions

In this study, we have employed atomistic simulations to investigate ultrafast laser interactions with metallic and silicon materials. The results highlight the diverse strongly non-equilibrium phenomena induced by ultrashort pulses, from subtle modifications to nanoparticle formation and surface oxidation. Key insights include:

1. The critical role of material structure and electron-lattice coupling in determining material responses.

2. Non-equilibrium processes, such as melting, ablation, oxidation can be simulated both at low and high lattice temperatures.

3. The sensitivity of surface modifications to laser fluence and to ambient environment.

4. The potential for tailoring material properties through controlled laser parameters.

Thus, atomistic modeling are particularly valuable in advancing femtosecond laser-based technologies and provide a framework for future research into nanofabrication and material treatment.

## ACKNOWLEDGEMENTS

We acknowledge the financial support of the French National Research Agency (ANR) under LAMORSIM project [Project-ANR-23-CE08-0029], and CINES of France for the computational resources needed for large-scale simulations.
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