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Surface chemistry of gold nanoparticles produced by pulsed laser ablation in liquids method is investigated by X-ray Photoelectron Spectroscopy (XPS). The presence of surface oxide expected on these systems is investigated using synchrotron radiation in conditions close to their original state in solvent, but free from substrate or solvent effects which could affect the interpretation of spectroscopic observations. For that purpose, we performed the experiment on a controlled free-standing nanoparticle beam produced by the combination of an atomizer and an aerodynamic lens system. These results are compared with those obtained by the standard situation of deposited nanoparticles on silicon substrate. An accurate analysis based on Bayesian statistics concludes that the existence of oxide in the free-standing conditions cannot be solely confirmed by the recorded core-level 4f spectra. If present, our data indicate an upper limit of 2.15 ± 0.68 % of oxide. However, a higher credence to the hypothesis of its existence is brought by the structureless valence profile of the free-standing beam. Moreover, the cross-comparison with the deposited nanoparticles case clearly evidences an important misleading substrate effect. Experiment with free-standing nanoparticles is then demonstrated to be the right way to further investigate oxidation states on Au-NP.

The synthesis of variable composition Au x Ag (1−x) alloyed nanoparticles prepared by laser ablation in water is reported. The nanoalloys exhibited a single characteristic surface plasmon resonance peak, whose spectral position was lying between the surface plasmon resonance peaks of neat Ag and Au nanoparticles, at about 400 and 530 nm respectively, depending directly on the composition of the alloyed nanoparticles. The nonlinear optical response of the nanoalloys was studied in details under 532 nm (visible) 35 ps and 4 ns laser excitation and it was found to be significant and greatly influenced by the position of the plasmonic band relative to the laser excitation wavelength. In this respect, increase of the Au molar fraction resulted in shifting of the nanoalloy plasmon band toward the location of the plasmon band of neat Au nanoparticles, i.e., at about 530 nm, closer to the laser excitation wavelength, hence causing more efficient resonance enhancement of the nonlinear optical response. Moreover, the nanoalloys were found to exhibit strong saturable absorption behavior when excited by ps or ns pulses, in the latter case this behavior changing to reverse saturable absorption at high laser intensity. The origin, the magnitude and the sign of the observed optical nonlinearities of the alloyed nanoparticles are explained and discussed in terms of the hot-electron and the interband contributions taking place under laser excitation in such nanostructures. The possibility of controlling the magnitude and the sign of the nonlinear optical response of Au x Ag (1−x) nanoalloys through their composition provides an attractive and efficient way to tailor the optical nonlinearities of noble metal nanoalloys, making them very useful for various emerging photonic, biophotonic, and optoelectronic applications.

Laser interaction with solids is routinely used for functionalizing materials' surfaces. In most cases, the generation of patterns/structures is the key feature to endow materials with specific properties like hardening, superhydrophobicity, plasmonic color‐enhancement, or dedicated functions like anti‐counterfeiting tags. A way to generate random patterns, by means of generation of wrinkles on surfaces resulting from laser melting of amorphous Ge‐based chalcogenide thin films, is presented. These patterns, similar to fingerprints, are modulations of the surface height by a few tens of nanometers with a sub‐micrometer periodicity. It is shown that the patterns' spatial frequency depends on the melted layer thickness, which can be tuned by varying the impinging laser fluence. The randomness of these patterns makes them an excellent candidate for the generation of physical unclonable function tags (PUF‐tags) for anti‐counterfeiting applications. Two specific ways are tested to identify the obtained PUF‐tag: cross‐correlation procedure or using a neural network. In both cases, it is demonstrated that the PUF‐tag can be compared to a reference image (PUF‐key) and identified with a high recognition ratio on most real application conditions. This paves the way to straightforward non‐deterministic PUF‐tag generation dedicated to small sensitive parts such as, for example, electronic devices/components, jewelry, or watchmak.

Nanodiamonds (NDs) and carbon-dots (CDs) suspensions exhibit significant NLO response under both ps and ns laser excitation. NDs exhibit important optical limiting action under nanosecond visible (532 nm) and infrared (1064 nm) laser excitation.

The chemical and geometrical structure of free-standing carbon dots (Cdots) prepared from the pyrolysis of N-hydroxysuccinimide (NHS) have been characterized using X-ray photoelectron spectroscopy (XPS). An aerodynamic lens system was used to generate a sufficient particle density of monodispersed Cdots for XPS studies at the PLEIADES beamline at the SOLEIL synchrotron facility. Varying the X-ray excitation energy between 315 and 755 eV allows probing of the Cdots from the surface toward their core, owing to the kinetic energy dependence of the photoelectron inelastic mean free path. The C1s, O1s and N1s core-levels were recorded with high spectral resolution to identify their main chemical components and branching ratios. While high-resolution transmission electron microscopy (HRTEM) reveals a defective graphitic core, the C1s spectrum evidences two main peaks similar to those measured from the solid NHS. Their relative abundance as a function of the probing depth is strongly related to the chemical composition of the ligand shell that does not vary substantially over the first 3.4 nm. Combining the depth-resolved XPS and HRTEM studies, it was concluded that the Cdots possess a graphitic core surrounded by a relatively homogeneous shell of at least 3.4 nm thickness with a composition similar to that of the solid NHS.