N. Crespo-Monteiro

Reversible and Irreversible Laser Microinscription on Silver‐Containing Mesoporous Titania Films

Silver species adsorbed on colloidal titania have been known for a long time to exhibit photochromism. [ 1 ] They reversibly change color in response to light exposure. Under UV illumination, i.e. at photon energy greater than the band-gap of TiO 2 (3.2 eV), the photogenerated electrons of titania migrate towards the positively charged Ag(I) species and reduce them, leading to the formation of silver nanoparticles. [ 2 ] These particles exhibit a grey-brown color caused by an inhomogeneous broadening of their surface plasmon resonance, which depends on the nanoparticle size, shape and close environment. [ 3–5a ] Reversibly, under visible light the material can bleach or change color. This multicolour photochromism has been reported recently on nanoporous titania fi lms loaded with silver nanoparticles. [ 5a,b ]

Irradiance influence on the multicolor photochromism of mesoporous TiO2 films loaded with silver nanoparticles

The photochromism of heterogeneous silver nanoparticles grown thermally in mesoporous TiO2 films is investigated under visible laser illuminations with different irradiances. At low irradiance, a selective oxidation occurs that changes the film color in accordance with the illuminant wavelength. At higher irradiance, the oxidation of large nanoparticles is going with the formation of many small nanoparticles in their vicinity, which gives the films an orangy color whatever the wavelength. This color fades after long exposure times at a laser line. The laser polarization has also an influence on the dichroism of the colored films.

Self-organized growth of metallic nanoparticles in a thin film under homogeneous and continuous-wave light excitation

Using a monochromatic plane wave to generate periodic arrays of metallic nanoparticles with tunable features buried in thin films is the original work we report here. We focus on the way such waveguiding metallic photonic crystals can self-emerge from thin films homogeneously loaded with metallic precursors under continuous-wave and homogeneous laser excitation. This paper fully describes the conditions leading to the formation of periodic structures and highlights the role of several parameters in the underlying physical mechanisms. The laser exposure parameters, especially, fix the geometrical and optical properties of the generated structures. Grating lines are parallel to the laser polarization and the period is directly linked to the laser wavelength. Both electron resonances of metal nanoparticles and optical resonances of guided modes interact to form the periodic patterns under homogeneous exposure. A model, based on the coupled mode theory, can be proposed to predict the spontaneous generation of such periodic nanostructures. It concludes that the guided waves exponentially enhance during illumination due to a positive feedback loop with the ordered growth of particles. This process opens up new fabrication techniques for making optical devices and may find applications in various fields such as polarization imaging, displays, security or lighting.

Multicolor photochromism of silver-containing mesoporous films of amorphous or anatase TiO2

Mesoporous TiO2 films loaded with silver nanoparticles grown photocatalytically, which are initially brown, change their color under visible laser irradiations. In this article, we compare the multicolor photochromisms of amorphous and anatase phases of TiO2. The mesoporous films are impregnated with silver salt and then exposed to a low-intensity UV laser light to grow silver nanoparticles. The Ag–TiO2 films are then exposed to visible laser beams, and the influences of several exposure parameters on the photochromic behavior are examined. Most of the previous studies have reported a poor stability of the photoinduced colors under day light or even in the dark, and few of them demonstrated the ability to get various colors on the same sample. These inconveniences limit the application field of such materials. On the other hand, except in our previous studies, only crystalline TiO2 is generally used, in its anatase or rutile phase. In this article we show that mesoporous films of amorphous and anatase phases of TiO2 respond in an efficient manner to light excitation and that multiple colors can be obtained on both kinds of films. For the first time on such Ag–TiO2 films we show that the various photoinduced colors are stable over considerable months. Visible intensity is shown to have a significant influence on the film behavior, which was not identified in previous studies. The laser-induced spectral changes are also shown to depend on the incident laser polarization. The photochromic behaviors are characterized in terms of color changes and spectral variations. The reproducibility of the photochromic process along reduction/oxidation cycles is demonstrated, and the stability of different laser-induced colors is reported on 6-month-old samples.

Dichroic colored luster of laser-induced silver nanoparticle gratings buried in dense inorganic films

This paper deals with the colorimetric properties of silver nanoparticle gratings buried in a dense titania film that result from a continuous wave laser-induced self-organization process. The samples exhibit shining colors in the direction of the specular reflection, which are very sensitive to polarization. We show that a large color gamut and a tunable dichroism can be reached by varying the exposure conditions. We also discuss the physical meaning of the observed variations in the dichroism. This laser process produces a single pass marking with a micrometer resolution and could be useful for developing innovative solutions in fields like active color displays, security, polarization imaging, or design.

Photo-directed organization of silver nanoparticles in mesostructured silica and titania films

This paper quantifies the migration of silver contained within mesostructured hybrid silica films and mesoporous titania films under exposure to modulated light. This quantization allows to demonstrate that all of the initial silver salt can be concentrated and reduced in domains accumulating the higher photonic energy. Entirely reduced in the form of nanoparticles of few nanometers size embedded in the silica matrix, silver is then quite stable even under subsequent homogeneous exposures. It is also shown that thanks to the relatively slow nanoparticle growth, successive multiple exposures can be used to create complex 3D microstructures within silica films using a simple dual beam interferometer. In mesoporous titania films, the UV photo-growth of silver nanoparticles remains limited to the vicinity of the film interface because of the matrix absorption and cannot provide deep 3D patterns of silver nanoparticles. However, 2D refractive index patterns can be obtained under UV exposure, erased with visible light and updated thanks to a reversible photochromic behavior. In such films, opposite migration flows of silver species are proven under UV intensity gradient and homogeneous visible exposure.

Updatable Random Texturing of Ag/TiO2 Films for Goods Authentication

The photochromic properties of Ag/TiO2 films are used to successively photoprint, erase, and update random textures for authentication purpose. The textures result from exposure to UV speckle patterns scattered by optically rough surfaces, and can be erased under homogeneous exposure with visible laser light. When kept out of laser light, the textures are stable over time. A cross-correlation analysis of images of various sets of random textures establishes the ability to rewrite independent textures at the same place on items, as well as the differential character of the photoprinted random textures and their usefulness for goods authentication.

Permanent dichroic coloring of surfaces by laser-induced formation of chain-like self-organized silver nanoparticles within crystalline titania films

Laser-induced periodic surface structures, also known as ripples, have been investigated for a long time on various materials. They are self-organized grating-like structures that form spontaneously upon irradiation with a single laser beam; their formation requiring a high temperature rise, it generally results from interactions with ultrashort laser pulses. Recently, similar phenomena leading to periodic changes in the morphology of metallic nanoparticles embedded in polymer films have been reported under femtosecond laser exposure. Here, we demonstrate that chain-like self-organized silver nanoparticles can be grown, from an ionic silver precursor, within titania films under continuous wave visible laser beam. The nanoparticle chains are periodically spaced with a period that depends on the wavelength and are oriented parallel to the laser polarization. The samples present no significant surface modulation since all of the grown nanoparticles are located at the substrate-film interface, protected by a crystallized TiO2 film. SEM, TEM, HRTEM and EDX characterizations of the film nanostructuring are shown. Due to the coupling of incident light to a guided mode of the TiO2 film by the first diffraction order of the NP grating, such samples exhibit a strong dichroism whose characteristics depend on the laser exposure conditions. Color changes and spectral variations with polarization are measured and interpreted. The high stability of such colored films under high temperature rises or high intensity UV or visible exposures is demonstrated; it makes them good candidates for colored data storage.

Microstructuring of Mesoporous Titania Films Loaded with Silver Salts to Enhance the Photocatalytic Degradation of Methyl Blue under Visible Light

The microstructuring of the distribution of silver nanoparticles (NPs) in mesoporous titania films loaded with silver salts, using two-beam interference lithography leading to 1 Dimension (1D) grating, induces variations in the photocatalytic efficiency. The influence of the structuration was tested on the degradation of methyl blue (MB) under ultraviolet (UV) and visible illumination, giving rise to a significant improvement of the photocatalytic efficiency. The periodic distribution of the NPs was characterized by transmission electron microscopy (TEM), high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and scanning electron microscopy (SEM).

Role of silver nanoparticles in the laser-induced reversible colour-marking and controlled crystallization of mesoporous titania films

Due to their surface plasmon resonance silver nanoparticles are known to absorb visible light and give glasses various colors. Grown in mesoporous titania films, they give the material a photochromic behaviour that can be used to produce rewritable data carriers. On the one hand, UV light forms silver nanoparticles thanks to the photo-induced generation of electrons by titania matrix. On the other hand, visible light oxidizes the silver nanoparticles via the photoexcitation of electrons on Ag and their stabilization by oxygen molecules. The well controlled porosity of the mesoporous films allows to tune the nanoparticles size and to obtain, under UV illumination, homogenous distributions of small nanoparticles embedded within the titania matrix, which color the films. As all nanoparticles absorb light similarly, the film can then be completely bleached under exposure to a visible laser beam whose wavelength falls in the SPR band of the particles. Therefore, CW UV and visible focused-laser radiations, respectively, can repeatedly print and completely erase colored micropatterns within TiO2/Ag films. The paper shows patterns printed under different conditions, deals with the reproducibility of the process and the inscription stability, and explains the nanoscale mechanisms, including silver migration during exposures, leading to the reversible color changes on the basis of TEM, SEM, absorption spectroscopy and Raman micro-spectroscopy characterizations. This paper also evidences that CW laser illuminations at higher intensity locally crystallize the titania matrix and investigates the influence of the absorption-induced heating around nanoparticles.