Article pubs.acs.org/JPCC
Environmentally Friendly Processing Technology for Engineering Silicon Nanocrystals in Water with Laser Pulses V. Svrcek,*,† D. Mariotti,‡ U. Cvelbar,§ G. Filipič,§ M. Lozac’h,† C. McDonald,†,‡ T. Tayagaki,† and K. Matsubara†
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Research Center for Photovoltaics, National Institute of Advanced Industrial Science and Technology (AIST), Central 2, Umezono 1-1-1, Tsukuba 305-8568, Japan ‡ Nanotechnology & Integrated Bio-Engineering Centre (NIBEC), University of Ulster, Coleraine BT52 1SA, United Kingdom § Department for Surface Engineering and Optoelectronics(F4), Jozef Stefan Institute, Jamova cesta 39, SI-1000 Ljubljana, Slovenia ABSTRACT: Herein, we demonstrate the customized, environmentally friendly tailoring of nanoparticles and their surface chemistry by short pulsed laser irradiation in liquids. This process allows for the formation of crystalline spherical particles exceeding several hundreds of nanometers in water from colloids of electrochemically etched silicon nanocrystals (Si-NCs), which exhibit quantum confinement effects and room-temperature stable luminescence. In particular, nanosecond (ns) pulsed laser irradiation of the Si-NC/water colloids causes the selective heating of the Si-NCs accompanied by the formation of spherical particles. In contrast, femtosecond (fs) laser pulsed irradiation induces the formation of colloidal Si-NCs with peculiar surface chemistry; in particular, fs pulses generate short-lived plasmas with more ionized species in water, which enable the surface engineering of quantum confined Si-NCs, thus limiting Si-NC agglomeration and enhancing their photoluminescent properties.
1. INTRODUCTION In the last two decades, silicon nanocrystals (Si-NCs) have been demonstrated to possess bright room-temperature photoluminescence (PL) as well as the opportunity for new physics that derives from quantum confinement and that could, for instance, considerably enhance the performance of solar cells (e.g., through quantum cutting effects and multiple exciton generation).1−7 The main advantages of using Si-NCs are a mature silicon technology and reliance on the environmentally friendly nature and abundance of silicon. This makes Si-NCs, in principle, an excellent candidate for their integration into existing technologies at low cost and with no environmental risk. For these reasons, Si-NCs have been researched widely and have been considered a prime candidate for photovoltaics, bioimaging, novel catalyst designs, and novel types of lightemitting diodes/lasers.3−15 Also, the use of laser-based technologies for processing Si-NCs offers good opportunities as such processes are well-established within manufacturing and their impact on the environment is relatively low.16 Laser ablation of solid bulk targets in vacuum and/or in liquid solutions has been widely reported, including for the synthesis of Si-NCs.17−22 In particular, laser processes carried out in liquid media (e.g., water) greatly simplify dust and nanoparticulate management, avoiding potential airborne pollution and limiting potential health risks.23−32 An aspect of paramount importance for Si-NCs is, however, represented by their surface characteristics. It was previously © 2016 American Chemical Society
demonstrated that the precise control of the surface characteristics of Si-NCs at quantum-confinement sizes (10 cm−1) in comparison to the peak originating from a crystalline Si wafer (Figure 4b, dotted line) and indeed also as compared to the peak observed in Figure 4a. This is due to the contribution of quantum confined Si-NCs and confirms their presence in the sample processed by the fs-laser. To assess separately the contribution of the Si-NCs, we produced drop-casted samples from the upper part of the colloid following 2 min sedimentation, in an attempt to remove the larger crystalline particles. This procedure did not allow forming homogeneous films, and therefore the drop-casted samples exhibited slightly different characteristics in different parts of the substrate. However, Raman analysis from these samples clearly showed typical peaks of quantum confined Si-NCs (Figure 4c). For instance, the Raman spectrum of Figure 4c (black solid line) shows a clear shift of the Si peak to 518 cm−1, which is attributed to quantum confinement. A broadening of the peak is also observed in comparison to that of the silicon wafer with a maximum located at 520 cm−1 (red dotted line, Figure 4c). The inset of Figure 4c reports the Raman spectrum taken at a different part of the substrate, which exhibit the maximum located at 511 cm−1. Although these results show some variability originating from the Si-NCs size distribution and sample preparation, the presence of crystalline Si-NCs with diameters varying between 2 and 4 nm is confirmed.57 Because the production of spherical particles by ns-laser processing has been previously discussed,32 we will now focus on the samples produced by the fs-laser treatment highlighting differences from the ns-process. TEM, SEM, and Raman analyses have shown that fs-laser processing produces both larger spherical particles and deagglomerated Si-NCs. Although deagglomerated Si-NCs appear to be the main product of the fs-laser process and only a few larger spherical particles are produced in this case (in comparison with ns-laser processing, see Figure 3a vs b), for completeness, we will first conduct further analysis to confirm the chemical composition and
3. RESULTS Figure 2a presents a typical SEM image of the starting material; while the size of the agglomerates exceeds several micrometers,
Figure 2. (a) Typical scanning electron microscopy image of electrochemically etched silicon nanocrystal (Si-NC) powder and corresponding (b) transmission electron microscopy image.
the TEM image in Figure 2b shows that these are made out of Si-NCs with diameters below quantum confinement size (
