Environmentally Friendly Processing Technology for Engineering

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Environmentally friendly processing technology for engineering silicon nanocrystals in water with laser pulses Vladimir Svrcek, Davide Mariotti, Uros Cvelbar, Gregor Filipic, Mickael Lozach, Calum McDonald, Takeshi Tayagaki, and Koji Matsubara J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b04405 • Publication Date (Web): 04 Aug 2016 Downloaded from http://pubs.acs.org on August 10, 2016

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The Journal of Physical Chemistry

Environmentally Friendly Processing Technology for Engineering Silicon Nanocrystals in Water with Laser Pulses V. Svrcek,*a D. Mariotti, b U. Cvelbar, c G. Filipič,c M. Lozac’h,a C. McDonald, a, b T. Tayagaki, a K. Matsubara a a

Research Center for Photovoltaics, National Institute of Advanced Industrial Science and Technology (AIST), Central 2, Umezono 1-1-1, Tsukuba, 305-8568, Japan,

b c

Nanotechnology & Integrated Bio-Engineering Centre (NIBEC), University of Ulster, UK

Department for Surface Engineering and Optoelectronics(F4), Jozef Stefan Institute, Jamova

cesta 39, SI-1000 Ljubljana, Slovenia

Abstract: Herein we demonstrate the customized, enviromentally 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 enhance their photoluminescent properties. *Corresponding Author: Vladimir Svrcek, Research Center for Photovoltaics, National Institute of Advanced Industrial Science and Technology (AIST), Central 2, Umezono 1-1-1, Tsukuba, 305-8568, Japan, Phone: +81-29-861-5429; Fax: +81-29-861-3367; e-mail: [email protected]

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1. Introduction

In the last two decades, silicon nanocrystals (Si-NCs) have 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 SiNCs, 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, bio-imaging, novel catalyst designs and novel types of light emitting diodes/lasers3-15. Also, the use of laserbased technologies for processing Si-NCs offers good opportunities as such processes are well-established within manufacturing and their impact on the environment is relatively low16. Laser ablation of solid bulk targets in vacuum and/or in liquid solutions have been widely reported, including for the synthesis of Si-NCs17-22. In particular laser processes carried out in liquid media (e.g. water) greatly simplifies dust and nanoparticulate management, avoiding potential airborne pollution and limiting potential health risks23-32. An aspect of paramount importance for Si-NCs is however represented by their surface characteristics. It was previously demonstrated that the precise control of the surface characteristics of Si-NCs at quantum-confinement sizes ( 90 %) due to extensive filtration steps to remove the larger agglomerates50,53; (2) the difficulty of obtaining well-dispersed Si-NCs in water with efficient and stable PL at room temperature54. In our laser-based treatment, 2.5 mg of the electrochemically etched Si-NC powder was dissolved in 10 mL of water before being processed by the ns- or fs-laser.

Since the Si-NCs agglomerates are hydrophobic,

processing in water required adding a small amount of ethanol (10 drops or ~0.5 µL) to wet the surface of the Si-NC agglomerates; this produces a homogeneous colloidal dispersion in water55. In order to compare the pulsed laser processing of the Si-NC colloids, we used ns- and fs-laser with about the same wavelength and the same average pulse power per surface area as presented in Figure 1. In particular, the ns-laser process was conducted by irradiating the colloidal samples with a Kr:F ns pulsed laser (245 nm, 20 ns) and for the fs-laser process we used a pulsed laser (Libra Solo Ultrafast Optical Parametric Amplifier-OPerA 360 nm, 83 fs). In order to obtain the closest experimental conditions, namely the same average irradiation energy and comparable absorption, we selected the laser wavelength to be 360 nm for the fs laser and 245 nm for the ns laser. At these wavelengths, the absorption of silicon is sufficiently close, 1.7 x 10-6 cm-1 at 245 nm 4 ACS Paragon Plus Environment

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and 1.1 x 10-6 cm-1 at 360 nm56. In both cases the process was conducted at room temperature and for 55 minutes. The laser beam was shaped and focused onto a spot on the liquid surface by an optical lens with a focal length of 250 mm. The average laser energy delivered per pulse by each laser was set to be approximately 25 J/cm2 in both cases while using the repetition rate of 5 Hz for the ns-laser and of 1 kHz for fs-laser, respectively. During the irradiation the glass container was rotated in both cases. We acknowledge that many different combinations of the processing parameters are possible, however for this work we have prioritized delivering the same average energy and ensuring rather similar absorption profile in the two different laser-based processes. We should also note that technological limitations and accessible experimental conditions were also considered to make this study justified from an application point of view; therefore a trade-off between perfectly matching experimental conditions and affordable experimental set-up was required. After processing, the colloids were drop-casted on glass substrates for analysis with a scanning electron microscope (SEM; Hitachi S-4300) and on grids for transmission electron microscopy (TEM; JEOL JEM-2010). X-ray photoelectron spectroscopy (XPS) was conducted on an area of approximately 20 µm in diameter on samples of 1 cm × 1 cm size. The depth of analysis was approximately 1-5 nm, and the X-ray excitation was monochromatic from Mg Kα (1253.6 eV, 400 W) under low analysis angle (10°) on monocrystalline substrate. Fourier transform infrared spectroscopy (FTIR; Perkin Elmer Spectrum 2000) was used to determine the surface terminations of the nanoparticles. The Raman spectra were acquired at room temperature using RENISHAW system whereby the wavelength 532 nm was used for the excitation source. The photoluminescence (PL) was measured by a spectrometer (Spectrofluorometer, Horiba Jobin Yvon) at room temperature with excitation at 375 nm.

3. Results Fig. 2a presents a typical SEM image of the starting material; whilst the size of the agglomerates exceeds several micrometers, the TEM image in Fig. 2b shows that these are made out of Si-NCs with diameters below quantum confinement size (< 10 nm) and attached to each other through Van der Waal pair-wise attractive inter-particle forces or in 5 ACS Paragon Plus Environment


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most cases attached to the agglomerate by covalent bonds55. In some cases detached single Si-NCs could also be found. Electrochemically etched Si-NCs are then irradiated in the water-based colloid by ns- and fs-laser pulses. As a result of irradiation, their morphology is changed as seen from SEM in Fig. 3. Irradiation by ns pulsed laser produces a large number of spherical particles (Fig. 3a) with the mean diameter of approximately 350 nm and ranging up to a maximum of 500 nm. In the case of fs laser irradiation, the Si-NCs appear to be fragmented and only very few large spherical particles are found (Fig. 3b, image shows larger area over 300 µm). In order to assess the crystalline nature of spherical particles produced by the ns-laser process, we conducted Raman analysis of the ns-processed sample after drop-casting on a glass substrate. Figure 4a shows a typical Raman spectrum of the silicon spheres (Fig.4a, black line). A distinct peak at 520 cm-1 is detected which supports the formation of silicon crystalline spherical particles. A broad peak with a maximum located around 470 cm-1 is also observed. This peak is originating from the glass substrate as confirmed by the Raman spectrum of the glass substrate only (Fig.4a, red dot line). In the case of fs laser processing the Raman analysis also showed a distinct peak located at 520 cm-1 (Fig. 4 b, black line) corresponding to Si. However it can be easily seen that the peak is asymmetric and broader (> 10 cm-1) in comparison to the peak originating from a crystalline Si wafer (Fig. 4b, dotted line) and indeed also 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. In order 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 homogenous 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 maximum located at 520 cm-1 (red dotted line, Fig. 4c). The inset of Fig. 4c reports the Raman spectrum taken at a different part of 6 ACS Paragon Plus Environment

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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 nm and 4 nm is confirmed57. Because the production of spherical particles by ns-laser processing have been previously discussed32, we will now focus on the samples produced by the fs-laser treatment highlighting differences with the ns-process.TEM, SEM and Raman analysis has shown that fs-laser processing produces both larger spherical particles and de-agglomerated SiNCs. Although de-agglomerated 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. 3b), for completeness, we will first conduct further analysis to confirm the chemical composition and crystal structure of the larger spherical particles. In order to better assess the crystalline structure of particles produced by the fs-laser processe, we conducted TEM, transmission electron diffraction (TED) (Fig. 5a-b) and energy dispersive X-ray spectroscopy (EDS) (Fig. 5c) studies. Corresponding EDS results indicate that the particle is mainly made of silicon. Furthermore, the TED pattern (figure 5b) produced at the edge of a spherical particle (figure 5a) suggests that the particle contains crystalline regions. In particular, the TED pattern relates to silicon (111) lattice plane with distances of 3.14 Å (Fig. 5b). The characteristics of the de-agglomerated Si-NCs after fs-laser processing were initially analysed by TEM. The size distribution of the Si-NCs is a critical feature that can determine the outcome of the processing technique. Typical size distribution of the SiNCs after fs-laser treatment was determined by TEM analysis (Fig. 6). The Si-NCs present a relatively narrow size distribution between 1 nm and 8 nm with an average of 3.2 nm, consistent with the Raman results in figure 4c. In order to assess the surface composition of the particles after fs-laser process, FTIR spectra were measured (Fig. 7). Agglomerated Si-NCs produced by electrochemical etching were mostly terminated by hydrogen atoms52,58. This is observed from asprepared sample (black line in figure 7) where Si-Hx absorption is present between 600 cm−1 and 700 cm−1 and Si-H stretching specifically from Si-H2 is responsible for absorption at about 916 cm−1 59. The absorption coefficient is comparable for all the Si-Hx 7 ACS Paragon Plus Environment


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contributions, where the silicon dihydrides appears to be the most frequent type of termination when compared to Si-H and Si-H3. The absorption related to Si-O-Si bonds was also found between 1000 cm−1 and 1200 cm−1 , 60, 61 which suggested that a degree of oxidation is present. Interestingly significant surface modifications were induced under fs laser processing (Fig.7, red line). Namely, the absorption peaks related to Si-H bonds and other H surface terminations were replaced by Si-O-Si and Si-OH. This is observed from the corresponding FTIR spectrum (red line in figure 7) with the Si-O-Si peak around 1100 cm-1 and the broad peak related to OH in the region 2500-3500 cm−1. XPS analysis was then carried out in order to get a better understanding of the fs-treated samples. High resolution XPS spectra of the Si 2p and O 1s peaks are shown in Fig. 8a and 8b, respectively, which indicate surface oxidation of the particles. XPS analysis reveals extensive oxidation that goes beyond the XPS penetration depth and therefore can only support the presence of particles with a surface oxide thicker than several nanometres. However the surface OH-terminations are confirmed as observed in figure 8b. Overall the combined FTIR, XPS, TEM and Raman analysis suggest that the existence of quantum confined Si-NCs following fs-laser processing, which are formed by a crystalline Si-core with a thin oxide surface that include OH-terminations as well. Importantly, the fs-laser treatment brought significant changes in the PL properties of the colloids. This is presented in Fig. 9 where the PL for both laser treatments are presented. The figure features typical room temperature PL spectra of as-prepared Si-NCs in water (black line), then treated by ns- or fs-laser processing (red and green dashed lines, respectively) as well as the PL of the same samples treated by ns/fs laser processing after ageing for 24 h (open symbols). In the case of the ns-laser treatment, the colloid PL intensity decreased (after laser irradiation for 55 min), whereas for fs-laser treatment the PL intensity considerably increased by more than 6 times (green dashed line). The fs laser treatment also induced a blue shift of the PL maximum (~20 nm) and at the same time a PL below 450 nm, the so-called F-band,61,62,63 appeared (Fig.9, indicated by green arrow). After 24 h, the PL intensity increased for the fs-treated colloid (Fig 9, open symbols).


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4. Discussion The results showed thatns-laser processing results in larger silicon crystalline spherical particles with diameter of several hundreds nanometrs (Fig. 3a). It is known in the literature from similar studies that ns-laser irradiation might induce selective heating in water and melts silicon particles to form crystalline spherical particles31,32,50. Since the main mechanism that allows the formation of large spherical particle is considered to be selective heating, the size of the particles dispersed in colloidal solution must be sufficiently small to allow for enough laser power absorption at the chosen laser wavelength50. In order to produce sufficiently small nanoparticles to allow for laser energy absorption, ball milling has been previously used50. Unfortunately, this mechanical pre-treatment makes the process relatively complicated and has shown to introduce metallic contaminants. The results in this work demonstrate that freshly prepared surfactant-free Si-NC agglomerates produced by electrochemical etching of high quality silicon wafers are a suitable starting material, which enable the formation of spherical particles through ns-laser irradiation, when they are dispersed in water (Fig. 3a). It is clearly seen that most of the electrochemically etched agglomerates which can exceed several micrometers in size, turn into large spherical particles after ns laser irradiation without any additional pre-treatment. Even though the size of some agglomerates is very large, the presence of the Si-NCs with quantum confinement size (< 10 nm) enhances the selective heating and allows the formation of spherical particles with diameters of several hundred nanometers. The growth of colloidal nanoparticles of different materials into sub-micrometer spheres under pulsed laser irradiation was already observed32,50, however the detailed mechanism of this process is yet to be clarified. One of the plausible explanations for the formation of larger spherical particles is based on the heating-melting-evaporation model where nanoparticle agglomeration was considered the dominant factor followed by melting and subsequent solidification32,50. It should be noted, however, that shock waves generated by ns-laser irradiation also lead to the detachment of the Si-NCs from the agglomerates35. Therefore the crystallization process to form the larger spherical particles cannot take place directly from the agglomerates and the surface characteristics of the Si-NCs contribute to the re-attachment 9 ACS Paragon Plus Environment


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of the Si-NCs after fragmentation from the agglomerates35. In other words, following the fragmentation of the Si-NC agglomerates by the laser-produced shock waves, the surface characteristics of the Si-NCs have to be preserved to promote re-attachment into newly formed agglomerates and form the spherical particles while selective heating and recrystallization take place. Therefore, melting and the crystallization process takes place directly from larger particles and re-agglomerated Si-NCs when the size allows for sufficient energy absorption and heating64,65. In this case, the NCs and particle surface oxidation in water is significantly reduced where heating-melting process after ns-laser interaction is more efficient. This process is schematically presented in Fig. 10a. Measurements by FTIR showed that as-prepared Si-NCs and agglomerates are mostly Hterminated. While shockwaves initially fragmented the large agglomerates (with diameter in the micrometre range, e.g. figure 2a), hydrogen terminations contributed to Si-NCs reagglomeration into large particles (with diameter in the range of hundreds of nanometres, e.g. figure 3a)52,66. By continuing the ns-laser process the large particles produced by the re-agglomeration of Si-NCs are then subject to selective heating which promotes coalescence and crystallization. This re-agglomeration, coalescence and crystallization is characterized by the gradual disappearing of Si-NCs from the colloid67. Since our initial colloids contained luminescent Si-NCs, the formation of large spherical particles (figure 3a) under ns-laser processing results in the suppression of the PL intensity as shown in Fig. 9. It is clear that partial oxidation of Si-NCs in water may influence the crystallization and particle growth. However, the structural analysis (Fig. 4a) suggests that most of the spherical particles produced during ns-laser processing are fully crystalline and most likely oxidation is prevented and too slow to occur between the fragmentation and re-agglomeration steps. In comparison to ns-laser irradiation, fs-laser irradiation is much less efficient in synthesizing the larger spherical particles, whereby most of the Si-NCs that are fragmented do not re-attach due to a modified surface chemistry. There are multiple reasons why the fs laser process preferentially modifies the Si-NC surface chemistry vs. preserving H-terminations which are conducive to NCs re-attachment and spherical particle formation.


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The average power per pulse of the fs-laser is 301 GW/cm2 (25 J cm-2 in 83 fs), which is approximately 5 orders of magnitude higher than the average power per pulse delivered by the ns-laser (1.3 MW/cm2; 25 J cm-2 in 20 ns). The fs-laser pulse durations is responsible for high intensity power capable of producing highly charged ions by multiphoton ionization but with pulses are too short to heat the plasma significantly. As a consequence, a highly reactive and cold plasma is created with ion states determined almost solely by the peak laser intensity, independent of the thermal properties, and that results in reduced agglomerates generation. The fs-laser is known to be highly effective in the dissociation and ionization of organic and inorganic molecules into the constituent atomic ions while the ns-laser generally induces only partial dissociation.68 Furthermore, the electron number densities in the liquid solution are considerably enhanced with the fs pulses.69 Irradiation of semiconductors and dielectrics with fs pulses produces plasmas by ionization at early stages and more rapidly than in the case of ns-laser irradiation.70 These intense fs pulses are able to deliver the energy which exceeds the binding energy of the NCs, which are therefore detached from the agglomerates (Fig. 10b), whereas electrons produced interact with photons and absorb laser energy. It follows that the most important mechanisms deal with electron impact and multi-photon ionization.69,70 The relative importance of these mechanisms depends non-linearly on irradiation intensity, and when high enough, the importance of multi-photons processes becomes higher and agglomerates are ionized within a few tens of fs. When ionization is complete, the plasmas are formed in the skin layer of the agglomerates, where very high electron densities of about 1x1023 cm-3 are reached69. If the electron energy exceeds the work function of the silicon material, they can be detached from the agglomerates. Therefore, the fragmentation of the Si-NCs via fs-laser pulses relies on mechanisms different from the shock waves generated by the ns-laser Another important factor that differentiates the two processes is also the chemistry induced at the surface of the Si-NCs after their detachment from the agglomerates (see Fig. 10). The electron number density is increased within a few micrometer in depth from the surface around the plasma spot, whilst the plasma is typically generated beneath the surface (~1 mm) of the colloid. A high concentration of electrons injected within the liquid from the plasma creates a highly non-equilibrium zone with electrons that are 11 ACS Paragon Plus Environment


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driving reaction kinetics and induce the removal or replacement of the hydrogen atoms located at the Si-NC surface. In particular, the fs-laser plasma is expected to generate OH radicals and ions in the aqueous colloid which are highly reactive and can quickly replace the hydrogenated silicon surfaces with Si-OH. Condensation of Si-OH at the surface of the Si-NCs can produce an oxide layer which prevents further oxidation to occur and importantly it transforms the hydrophobic Si-NCs into NCs with hydrophilic surfaces. At least in qualitative terms, the FTIR spectra (see FTIR in Figure 7, red line) confirmed that fs-laser processing mainly induced a different type of terminations compared to asprepared Si-NCs and the products of the ns-laser process. The large O-H absorption above 3500 cm−1 could be due to Si-OH terminations on the detached Si-NCs which have not condensed. However, it is most likely that this could correspond to adsorbed water molecules on the surface of the Si-NCs which have become hydrophilic. This provides an explanation for the FTIR signal of Figure 7 and is consistent with the large Si-O-Si absorption peak; the oxide peak around 1100 cm-1 is also characterized by a broad absorption typical of a non-stochiometric oxide and therefore supports the presence of an oxide layer at the interface with the core of the Si-NC. XPS results corroborate the formation of a SiO2 layer and some OH terminations. Although XPS reveals extensive oxidation, the TEM and Raman results support the presence of Si-NCs with an unoxidized crystalline core with quantum confinement size < 8 nm. This surface and structural analysis is further supported by the optical properties of the Si-NCs. The fslaser irradiation induced a passivated and hydrophilic surface chemistry on the Si-NCs which prevented for most part the re-attachment and therefore the coalescence of the SiNCs into larger spherical particles.47 Compared with ns-laser processing, reagglomeration and coalescence of Si-NCs in water are considerably reduced. Subsequently, the formation of an oxide layer via condensation of the OH-terminations contributed to the stabilization of the PL properties (Fig. 9, blue line). As a result, a small blue shift of the PL maxima is justified due to the reduction of diameter of the Si-NC core due to the oxide growth; also, the appearance of the so-called F-band (< 500 nm, Fig.9, indicated by arrow) is linked to defect states in a surface sub-oxide, which confirms again the formation of the oxide shell.71,72


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5. Conclusions In this work, we have demonstrated that hydrogen-terminated Si-NC agglomerates with sizes that exceed several micrometers lead to very different results when irradiated by short ns- or fs-laser pulses. Our investigations showed that ns laser pulses are more efficient for the selective heating and formation of larger spherical crystalline particles. Modest changes in the surface characteristics of the Si-NCs under ns-laser irradiation were the main reason for re-attachment and for the growth of Si particles with larger diameters. Since Si-NCs are essentially transformed in large (not quantum confined) Si spherical particles, the PL of the colloid is decreased as a function of ns-laser irradiation time. On the contrary, the fs-laser irradiation is rather different, and leads to significant changes in the surface characteristics of the Si-NCs initially through the replacement of H-terminations with OH groups. Moreover, the plasma generated by fs-laser irradiation leads to the stabilization of the optical properties of the Si-NCs with quantumconfinement sizes. In the case of fs laser irradiation, the surface oxide of the Si-NCs allows for a good particle dispersion and prevents the re-agglomeration process from occurring. As a result, the PL of the colloid increases more than 6 times and is chemically stabilized. Our results revealed that fs-laser irradiation at short wavelengths were successfully employed for surfactant-free Si-NC surface functionalization and enhancement of PL properties of Si-NCs in water (Fig 9, dashed lines). All our investigations clearly pointed out that the short laser processing may represent a very simple and environmentally friendly technology for the synthesis of either large spherical particles or for surface engineering of Si-NCs at the nanoscale by narrowing of the pulse width. Overall, this laser-based approach at low intensities has direct impact on energy saving. Moreover, the work actively promote the use environmentally friendly materials (e.g. silicon) by extending their functionalities over a wider scale to include surface characteristics of quantum confined systems as well as large particles that are difficult and energy-demanding to achieve with other methods. The technique proposed here also employ water-based chemistries that combined to physical mechanisms allow for unprecedented results. We clearly show and suggest that synergies between physical and chemical processes can broaden the potential of green and sustainable chemistry. This 13 ACS Paragon Plus Environment


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proposed technology is able at once to replace commonly used top-down synthesis of nanocrystals that use high energy budgets and a range of other surface functionalization technologies which use toxic chemicals.

Acknowledgements This work was partially supported by a NEDO Project (Japan). This work was partially supported by EPSRC (award n. EP/K022237/1 and n. EP/M024938/1). This work has been also partially funded by the Leverhulme International Network n.IN-2012-136 (UK), Slovenian Research Agency (ARRS) project L2-7667. The authors also acknowledge the activities of the EU COST action TD1208 for useful discussions.


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Figure captions

Figure 1. Schematic representation of the experimental setup with laser system.

Figure 2. a) Typical scanning electron microscopy image of electrochemically etched silicon nanocrystal (Si-NC) powder and corresponding b) transmission electron microscopy image.

Figure 3. Typical SEM images of products after ns- (a) and after fs- (b) laser irradiation of silicon nanocrystals powder in water. Inset in (a) shows submicron spherical particles in details.

Figure 4. a) Raman spectrum of silicon spherical particles (black line) prepared by nslaser processing and measured at 532 nm. Dotted line represents the Raman spectrum of the glass substrate and is shown for comparison. b) Raman spectrum of samples (black line) after fs-laser processing. Dotted line represents the spectrum of the silicon wafer and is shown for comparison. c) Corresponding Raman spectrum of Si-NCs (black line) prepared by fs-laser processing. Dotted line represents the spectrum of the silicon wafer and is shown for comparison. Inset in (c) represents the Raman spectrum taken in another part of the sample.

Figure 5. a) Transmission electron microscope image of spherical particles produced by fs-laser irradiation and corresponding b) diffraction pattern and c) energy dispersive Xray spectroscopy.

Figure 6. Histogram of silicon nanocrystals (Si-NCs) size distribution after fs-laser processing obtained from transmission electron microscopy analysis.

Figure 7. Fourier transform infrared (FTIR) spectra of the fs-laser (red line) treated silicon nanocrystals (Si-NCs) in water. FTIR spectrum of as-prepared Si-NC agglomerates (black line) is shown for comparison. (Color online) 22 ACS Paragon Plus Environment

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Figure 8. High resolution peaks from X-ray photoelectron spectroscopy of a) Si 2p and b) O 1s from the samples after fs laser irradiation of the colloids.

Figure 9. Typical room temperature photoluminescence (PL) spectra of silicon nanocrystals in colloids as-prepared (black lines), and treated by ns- (solid lines) and fslaser irradiation (dashed line) respectively. In the figure open symbols represents PL of the colloids after 24 hours. In all cases the excitation wavelength was 375 nm.

Figure 10: Schematic representation of the chemical process and mechanisms taking place in the water-based colloids with silicon nanocrystals (Si-NCs) during ns- or fs-laser irradiation.

TOC. Environmentally-friendly short pulsed laser spherical submicron shaping and surface engineering of photoluminescent silicon nanocrystals in water.



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Figures

Figure 1. Schematic representation of the experimental setup with laser system.


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Figure 2. a) Typical scanning electron microscopy image of electrochemically etched silicon nanocrystal (Si-NC) powder and corresponding b) transmission electron microscopy image.



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Figure 3. Typical SEM images of products after ns- (a) and after fs- (b) laser irradiation of silicon nanocrystals powder in water. Inset in (a) shows submicron spherical particles in details.


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Figure 4. a) Raman spectrum of silicon spherical particles (black line) prepared by nslaser processing and measured at 532 nm. Dotted line represents the Raman spectrum of the glass substrate and is shown for comparison. b) Raman spectrum of samples (black line) after fs-laser processing. Dotted line represents the spectrum of the silicon wafer and is shown for comparison. c) Corresponding Raman spectrum of Si-NCs (black line) prepared by fs-laser processing. Dotted line represents the spectrum of the silicon wafer and is shown for comparison. Inset in (c) represents the Raman spectrum taken in another part of the sample.



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Figure 5. a) Transmission electron microscope image of spherical particles produced by fs-laser irradiation and corresponding b) diffraction pattern and c) energy dispersive Xray spectroscopy.


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Figure 6. Histogram of silicon nanocrystals (Si-NCs) size distribution after fs-laser processing obtained from transmission electron microscopy analysis.



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Figure 7. Fourier transform infrared (FTIR) spectra of the fs-laser (red line) treated silicon nanocrystals (Si-NCs) in water. FTIR spectrum of as-prepared Si-NC agglomerates (black line) is shown for comparison. (Color online)


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Figure 8. High resolution peaks from X-ray photoelectron spectroscopy of a) Si 2p and b) O 1s from the samples after fs laser irradiation of the colloids.



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Figure 9. Typical room temperature photoluminescence (PL) spectra of silicon nanocrystals in colloids as-prepared (black lines), and treated by ns- (solid lines) and fslaser irradiation (dashed line) respectively. In the figure open symbols represents PL of the colloids after 24 hours. In all cases the excitation wavelength was 375 nm.


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Figure 10: Schematic representation of the chemical process and mechanisms taking place in the water-based colloids with silicon nanocrystals (Si-NCs) during ns- or fs-laser irradiation.



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Environmentally Friendly Processing Technology for Engineering

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