Temperature rise during laser‐induced self‐organization of nanoparticle gratings revealed by R aman microspectroscopy and electron microscopy

Abstract

Self‐organization of metallic nanoparticles (NPs) has recently been reported upon visible continuous‐wave (cw) laser exposure [1]. The self‐organized structures are Ag NP gratings embedded in a thin TiO 2 film deposited on glass. Such composite structures exhibit singular visual effects that can find applications in secured traceability. The related optical properties directly depend on the NP size distribution, the average grating period, the organization rate and the TiO 2 thickness and refractive index. These sample features appear to be largely controlled by the temperature rise that occurs during the laser‐induced self‐organization process. The aim of the present contribution is to estimate the plasmon‐induced temperature rise which appears to be strongly influenced by the laser scanning speed. To do so, Raman microspectroscopy and various modes of transmission electron microscopy (TEM) are used. The latter allow accurate information to be acquired on the NP size distributions resulting from different temperature rises, on their localization in the film and on the phase and chemical changes that occur in the film and the substrate surrounding NPs. Finally, we show how such thermal effects can be considerably decreased when using femtosecond (fs) laser pulses to initiate the NP self‐organization. The TiO 2 thin layer used in this work is initially mesoporous and amorphous and contains small silver NP of 1‐3 nm as described in a previously published article [1]. The self‐organized growth of silver NPs is implemented by scanning a laser beam focused on the sample surface at a constant speed. Post mortem Raman microspectroscopy shows that TiO 2 remains amorphous or adopt successively anatase, both anatase and rutile or only rutile crystalline forms for increasing laser scanning speeds, which was confirmed by high resolution TEM micrographs. Further in situ Raman microspectrocopy characterizations also attest an increase in temperature from 200°C to 750°C from low speed to higher speed in a range where anatase is formed; This increase of the temperature when the scanning speed increases was totally unexpected. In addition to TEM crystallographic characterization, scanning electron microscopy (SEM) appeared to be useful to identify different morphologies for anatase and rutile nanocrystals and to study changes in the nanocrystal density as a function of speed. Scanning TEM (STEM) micrographs and electron energy loss spectroscopy (EELS) analysis of sample cross‐sections prepared by focused ion beam (FIB) give further interesting information about the in‐depth structure of samples. Ag nanoparticles are located below the TiO 2 film (Fig. 1a) made of TiO 2 nanocrystals immersed in a Si‐based amorphous phase, in a new interfacial thin amorphous layer mixing both Ti from the initial film and Si from the glass substrate (Fig. 1b). A three‐dimensional reconstruction of the film sample from a series of FIB‐SEM experiments confirms that all Ag NPs are rather spherical and located in a single plane just below the nanocrystalized TiO 2 layer. High angle annular dark field scanning TEM (HAADF STEM) imaging was used to study systematically non‐monotonous changes in the NP size distribution with the temperature rise for many samples. All studies that we have performed so far point out that the temperature rise can be considered as a drawback since it affects the integrity of the supporting material; we present here few results obtained with fs laser pulses in order to investigate a way to self‐organize metallic NPs without high temperature rise in order to preserve the substrate and give the ability to work on softer substrates like plastic ones. Self‐organization can successfully be obtained without altering the substrate top surface (Fig. 1 c‐d). Ag NPs remain localized in the TiO 2 films, which is only locally crystallized around the grown NPs, as attested by STEM‐diffraction maps recorded in TEM. To conclude, this paper demonstrates the interest of a multimodal application of TEM techniques in order to provide a thorough study of the 3D nanostructure and chemical composition of complex samples made of Ag NP gratings embedded in a nanocrystallized TiO 2 film, which result from laser‐induced self‐organization processes. It provides crucial information on thermal effects that drive the laser‐induced self‐organization process.