Method for the synthesis of porous polycrystalline silicon-based materials

Abstract

The present invention relates to the preparation of porous polycrystalline silicon-based materials. The object of this invention is a method to prepare a porous polycrystalline silicon-based material using hydrogen-terminated silicon-based nanoparticles, comprising the steps: flowing a precursor gas mixture into a reaction chamber; applying an electromagnetic field to said precursor gas mixture, thereby forming a non-thermal plasma; forming a solid product comprising a plurality of hydrogen-terminated silicon-based nanoparticles; depositing said solid product on a surface, thereby forming a granular structure; exposing said granular structure to an oxidizing medium; subject said granular structure exposed to an oxidizing medium to a trigger mechanism such as a light emitted by a light source, thereby inducing the sintering of the nanoparticles through a chain reaction to form a porous polycrystalline silicon-based material. The proposed invention aims to overcome the high energy threshold needed for the sintering of silicon-based nanoparticles without the need for specialized equipment.

Description

[0001] DESCRIPTION

[0002] METHOD FOR THE SYNTHESIS OF POROUS POLYCRYSTALLINE SILICON-BASED MATERIALS

[0003] FIELD OF THE INVENTION

[0004] The present invention relates to nanostructured materials, in particular to the preparation method of porous polycrystalline silicon-based materials.

[0005] PRIOR ART

[0006] Silicon (Si) is a fundamental material for microelectronics, being widely used for decades now. Silicon nanocrystals (Si-NCs), which are crystalline silicon nanoparticles (Si-NPs), have emerged as a very interesting biologically compatible, nontoxic, and biodegradable alternative to toxic metal-based nanocrystals in optoelectronics, with the added benefit of being readily prepared using an abundant material (silicon is the second most abundant material in the earth crust, representing 27 % of it).

[0007] One drawback of Si-NCs is that Si, as an elemental group IV material, is hard to synthetize in crystalline form due to its covalent binding nature, with bond dissociation energy of 327 kJ mol1(Si-Si), which generally requires high temperatures to form crystalline nuclei, as well as its propensity to binding with oxygen with high dissociation energies of 798 kJ mol1(Si-O).

[0008] Sintering and annealing of Si-based materials is usually explored to change the structural, electronic, and mechanical properties of materials such as amorphous, nano and microcrystalline silicon, silicon carbide (SiC) nanopowders, doped Si nanoparticles, silicon dioxide (SiOz) nanoparticles, and polycrystalline silicon (poly-Si). Hydrogenated Porous silicon (P-Si), typically obtained from an expensive top-down approach of etching of Si wafers resulting in waste of material, has been found to have a strong explosive interaction with oxygen resulting in a high energy yield, superior to most common carbon-based explosives. Although the oxidation rate of silicon is usually limited by the slow diffusion of oxygen through the silicon dioxide (SiOz) layer at the Si surface, the extensive surface area of P-Si (up to 103m2cm'3) constituted by several nanocrystals, allows for a substantial increase in the oxidation reaction rates of silicon.

[0009] However, explosive oxidation of hydrogen-terminated P-Si layers so far has always followed the top-down approach of first creating pores in bulk Si material, followed by filling the pores of hydrogen-terminated P-Si with oxygen or other oxidizers such as perchlorates or nitrates at ambient or cryogenic temperatures. This allows oxygen to be adsorbed inside the hydrogen-terminated pores of P-Si upon reaction triggering, resulting in a strong explosion. Typically, a high energy threshold must be achieved to rupture Si-H bonds (298 kJ mol-1) in the surface Si layer so that oxygen can enter the bulk crystalline Si to allow for the explosive oxidation to begin. Thermal, electrical, optical, chemical, or mechanical mechanisms have been reported as viable triggers for the oxidation reaction in P-Si obtained from top-down approaches.

[0010] The proposed invention aims to solve limitations of explosive oxidation of silicon-based materials. We demonstrate that we can simulate the needed hydrogen-terminated P-Si material through a bottom-up approach by using hydrogen-terminated silicon-based nanoparticles and that we can explosively sinter these silicon-based nanoparticles without adding oxidizers by using only the oxygen present in ambient air, creating an on-demand porous polycrystalline silicon-based material.

[0011] SUMMARY OF THE INVENTION

[0012] The object of this invention consists of a method to prepare a porous polycrystalline silicon-based material using hydrogen-terminated silicon-based nanoparticles, said method comprising the following steps:

[0013] Flowing a precursor gas mixture into a reaction chamber, said precursor gas mixture comprising at least one silicon-based gas;

[0014] Applying an electromagnetic field to said precursor gas mixture, thereby forming a non-thermal plasma; Forming a solid product in the plasma, wherein said solid product comprises a plurality of hydrogen-terminated silicon-based nanoparticles;

[0015] Depositing said solid product on a surface, thereby forming a granular structure comprised of an array of hydrogen-terminated silicon-based nanoparticles;

[0016] Exposing said granular structure to an oxidizing medium;

[0017] Subjecting said granular structure exposed to said oxidizing medium, to a trigger mechanism, thereby inducing the sintering of the plurality of hydrogen-terminated silicon-based nanoparticles through a chain reaction to form a porous polycrystalline silicon-based material, said porous polycrystalline silicon-based material having a grain size that is larger than the grain size of the hydrogen-terminated silicon-based nanoparticles.

[0018] In an advantageous aspect of the present invention, the free electrons present in the non-thermal plasma collide with the molecules of the silicon-based gas, causing dissociation and ionizing said molecules into reactive ions and radicals, ultimately forming nucleation ionic clusters that grow into hydrogen-terminated silicon-based nanoparticles with tight size distributions. Preferably, at least one silicon-based gas is selected from the group consisting of silane, tetrachlorosilane and mixtures thereof.

[0019] In addition to silicon-based gas, the precursor gas mixture comprises hydrogen and / or at least one noble gas selected from the group consisting of helium, argon and mixtures thereof. Moreover, by adding other precursor gases to the mixture such as methane or ethylene, phosphine, trimethylborane, or other hydride gases, it's possible to synthetize silicon-based composite materials with carbon, phosphorus, boron, among other elements, following the same reaction mechanism.

[0020] In an advantageous aspect of the present invention, the non-thermal plasma electrons negatively charge the formed hydrogen-terminated silicon-based nanoparticles, thus limiting agglomeration and the size of each nanoparticle, which in turn increases the overall compactness of the granular structure formed from the depositing hydrogen- terminated silicon-based nanoparticles. Free-standing hydrogen-terminated silicon-based nanoparticles are deposited on a surface inside the reaction chamber while controlling deposition rate and pressure to achieve a certain range of compactness of the resulting granular surface.

[0021] The trigger mechanism for the reaction can be thermal, electrical, optical, chemical, or mechanical, so long as this trigger mechanism is energetic enough to break the surface Si-H bonds in the silicon-based nanoparticles, thus initiating the chain reaction.

[0022] In a preferred embodiment of the present invention, the trigger mechanism to which the reaction is subjected to comprises an optical trigger consisting of light emitted from a light source, being said light irradiated onto the hydrogen-terminated silicon-based nanoparticles.

[0023] The light emitted by a light source induces the sintering of hydrogen-terminated silicon- based nanoparticles irradiated at the surface of the granular structure, in the presence or after exposure to an oxidizing medium, wherein said oxidizing medium is selected from ambient air, liquid oxygen, perchlorates, nitrates, and mixtures thereof.

[0024] In an advantageous aspect of the present invention, the explosive reaction is triggered immediately (millisecond range) upon irradiation of the hydrogen-terminated silicon- based nanoparticles with the light source.

[0025] The sintering reaction is characterized by the chain exothermal oxidation reaction in which several Si-H bonds are ruptured due to the energy transfer from the emitted light, resulting in several Si dangling bonds that ignite an explosive exothermal reaction with the oxidating gas, such as oxygen, that propagates self-sustained throughout the whole sample as the energy release from the explosive reaction breaks more Si-H surface bonds in its vicinity which result in new Si dangling bonds, that in turn react with more oxygen. The high number of Si dangling bonds favor the formation of Si-0 bonds, releasing energy. This chain reaction will continue to propagate until there are no more Si-H surface bonds in the reaction's vicinity. In a preferred embodiment of the present invention, the irradiation of the granular structure is performed immediately after the exposure of said granular structure to ambient air or a controlled oxidizing medium, to reduce loss of Si-H bonds due to natural oxidation and to maximize the amount of initial surface Si-H bonds to be ruptured upon irradiation, thereby initiating the sintering chain reaction.

[0026] The strong exothermic nature of this oxidation reaction leads to a large and localized increase in temperature of the Si nanocrystals of the granular structure, since their thermal conductivity is very low, and the temperature increase is sufficient to melt the silicon-based nanoparticles and fuse them together into a polycrystalline material.

[0027] In an advantageous aspect of the present invention, the explosive sintering of hydrogen- terminated silicon-based nanoparticles can be performed without the addition of oxygen or other oxidant substances, wherein oxygen present in air or in the controlled medium where the material is at standard atmospheric pressure is enough to trigger and sustain the sintering chain reaction. In addition, the method of the present invention does not require high temperatures or pressures, wherein the sintering of hydrogen-terminated silicon-based nanoparticles can be performed without external intentional heating, and in fact at any temperature, as the energy necessary to initiate the reaction is provided by the emitted light.

[0028] DESCRIPTION OF THE FIGURES

[0029] Figure 1 - Scanning Electron Microscopy (SEM) image of the porous polycrystalline silicon material obtained by the method of the present invention, at a scale of (a) 10 pm, (b) 2 pm and (c) 500 nm. The explosively sintered silicon nanoparticles form a polycrystalline material since they consist of crystalline silicon grains, connected by grain boundaries. The material is not continuously solid, being porous instead. The sintering process fuses the silicon nanoparticles into grains connected to each other that form percolation pathways through the material. Figure 2 - Scanning Transmission Electron Microscopy (STEM) bright-field image of the porous polycrystalline silicon material obtained by the method of the present invention, at a scale of 10 nm, with an inset of the corresponding fast Fourier transform (FFT) image in the top-right corner. The center of the bright-field image shows crystal lattices from the grains that became fused together as a result of the sintering process, as well as their grain boundaries. The 2 concentric rings in the FFT image and the presence of diffuse spots demonstrate that the sample is polycrystalline, as opposed to distinct spots in FFT images of monocrystalline samples [1],

[0030] Figure 3 - Evolution over time of the Fourier-Transform Infrared (FTIR) spectra of fresh hydrogen-terminated silicon nanoparticles exposed to ambient air.

[0031] Figure 4 - Fourier-Transform Infrared (FTIR) spectra of fresh hydrogen-terminated silicon nanoparticles, of sintered silicon nanoparticles after three days of exposure to ambient air, as well as of non-sintered silicon nanoparticles (dotted line) after three days of exposure to ambient air.

[0032] Figure 5 - Raman spectra of a sample of silicon nanoparticles (a) before sintering and (b) after sintering, (a) The silicon nanoparticle sample before sintering shows the same optical and vibrational modes resulting from under-coordination, bond length variation, and quantum effects that lower the vibrational frequencies of the incident optical phonons (LA, TOi and TO2 peaks) expected from hydrogen-terminated silicon nanoparticles and a LTO peak at 514.5 cm1with T of 15.2 cm1corresponding to the crystalline core of the hydrogen-terminated silicon nanoparticles [2], [3], [4], [5], [6], The Raman shift value of the LTO peak is correlated with a mean silicon nanoparticle diameter of d ~ 2.2 nm [7], [8], [9], (b) The sintering process clears the LA peak and a TO peak, blue-shifts the LTO peak to 520.1 cm1and increases its intensity by a degree of magnitude. Moreover, the T decreases from 15.2 cm1to 6.7 cm'1. These changes in Raman shift indicate the sintered silicon nanoparticle material has a larger grain size (> 10 nm)

[0010] , [5],

[0011] ,

[0012] , DETAILED DESCRIPTION

[0033] The more general and advantageous configurations of the present invention are described in the Summary of the Invention. Such configurations are detailed below in accordance with other advantageous and / or preferred embodiments of implementation of the present invention.

[0034] In a preferred embodiment of the present invention, the hydrogen-terminated silicon- based nanoparticles have a grain size of 1 - 50 nm. The small size of the hydrogen- terminated silicon-based nanoparticles and the space between each other in the bulk of the granular structure allows the hydrogen-terminated silicon-based nanoparticles (hydrogen surface concentration of 1014-1015atoms cm'2, hydrogen volumetric concentration of packed nanoparticles 1015-1022atoms cm'3) to be close enough to experience the energy transfer from the exothermic reaction, but far enough so that oxygen and hydrogen species can be present between them to sustain the propagation of the reaction in all directions through the silicon-based nanoparticle layers to form the porous polycrystalline silicon material.

[0035] In a preferred embodiment of the present invention, the sintering of the plurality of hydrogen-terminated silicon-based nanoparticles is performed without intentional external heating and at standard atmospheric pressure, 760 Torr.

[0036] In a preferred embodiment of the present invention, the light source comprises a laser or a focused light from any other source. Preferably, the light emitted by the light source has a wavelength in the NIR-Vis-UV range, from 250 nm to 2100 nm. More preferably, the light source comprises a laser wherein the light emitted by the laser has a wavelength equal to or below 633 nm. In an advantageous aspect of the present invention, the light emitted by the laser with a wavelength equal to or below 633 nm leads to the rupture of the surface Si-H bonds, creating Si dangling bonds which ignite the explosive oxidation reaction, resulting in the sintered silicon-based material and the removal of hydrogen terminations from the hydrogen-terminated silicon-based nanoparticles. In another embodiment of the present invention, the electromagnetic field power density and frequency are all such power density and frequency that allow the generation of a non-thermal plasma inside the reaction chamber and to promote a high crystallinity of the produced hydrogen-terminated silicon-based nanoparticles, with a preferred power density between 10 W cm-3and 1000 W cm'3and a preferred frequency of 10 MHz to 100 MHz.

[0037] In a preferred embodiment of the present invention, the residence time of the precursor gas mixture in the plasma is less than 50 ms; being the precursor gas mixture is preferably discharged at 0.1 - 15 Torr. In an advantageous aspect of the present invention, the low flow rate of silicon-based gas coupled with a high reactor pressure above 2 Torr further promotes the formation of the crystal lattice of the hydrogen- terminated silicon-based nanoparticles, thus obtaining highly crystalline hydrogen- terminated silicon-based nanoparticles. In addition, when deposited at a pressure between below 15 Torr, hydrogen-terminated silicon-based nanoparticles achieve a certain range of compactness that allows the sintering reaction to propagate throughout the several layers of the deposited nanoparticles.

[0038] In a preferred embodiment of the present invention, the hydrogen-terminated silicon- based nanoparticles are crystalline in nature, therefore comprising a crystalline fraction of at least 60 %.

[0039] In another preferred embodiment of the present invention, the sintering process is triggered as soon as the hydrogen-terminated silicon-based nanoparticles are exposed to ambient air or a controlled oxidizing medium.

[0040] In a preferred embodiment of the present invention, the surface on which the solid product is deposited comprises a stainless-steel mesh, to facilitate the collection and transport of the deposited hydrogen-terminated silicon nanoparticles. It is also an object of the present invention a porous polycrystalline silicon-based material obtainable by the described method using hydrogen-terminated silicon-based nanoparticles, wherein said porous polycrystalline silicon-based material comprises a crystal lattice structure embedded in an amorphous and a porous structure, consisting of interconnected crystalline grains.

[0041] In another embodiment of the present invention, the porous polycrystalline silicon- based material comprises a grain size of 10 - 500 nm.

[0042] In a preferred embodiment of the present invention, the porous polycrystalline silicon- based material comprises a crystalline fraction above 80 %.

[0043] The preferred embodiments described above can, of course, be combined in various configurations, the present invention being not limited to the embodiments previously described.

[0044] EXAMPLES

[0045] Formation of hydrogen-terminated silicon nanoparticles

[0046] An Ar / SiH4 gas mixture is flown down the top through a tube connecting to a quartz tube, comprising a reaction chamber, where the plasma is formed at low pressures (1 Torr - 10 Torr). The quartz tube has two different diameters across its 30 cm length: a narrow diameter zone, with 9 mm outer diameter and stretching for 15 cm; and a wider diameter zone with 26 mm outer diameter and stretching for 14 cm. Between these two zones there is a 1 cm-long connection zone. The tube walls are 2 mm thick. The quartz tube was set up so that the hydrogen-terminated silicon nanoparticles synthesis occurred in the narrower zone. Two copper ring electrodes spaced 16 mm apart from each other are fitted to the quartz tube and connected to a radiofrequency source at 13.57 MHz to allow for the application of radiofrequency power at 80 W to the gases flowing inside the tube and generate the non-thermal plasma. From this, the volume inside the tube is calculated at 0.314 cm3between the two copper electrodes. The applied radiofrequency power was set at 80 W for all hydrogen-terminated silicon nanoparticles synthesis. Therefore, the power density applied to the gas mixture between the electrodes to form the plasma was 255 W cm'3. Hydrogen-terminated silicon nanoparticles form in the plasma through the electron impact dissociation of SiH4 and subsequent nucleation and clustering of the resulting species. The final hydrogen- terminated silicon nanoparticles, herein designated as Si-NPs, are collected downstream from the quartz tube on fine stainless steel mesh filters fitted onto KF40 vacuum inserts. After collection, the meshes containing the Si-NPs were removed from the reactor system under an argon atmosphere to prevent their oxidation.

[0047] The formulation for the formation of Si-NPs consisted of gas flow rates of 0.650 seem for SiH4 and 36 seem for Ar, with discharge pressure of 3.625 Torr and an RF power density of 255 W cm'3at 13.56 MHz. The calculated residence time of the gas mixture in the plasma will be 2.2 ms.

[0048] Oxidation of hydrogen-terminated silicon nanoparticles

[0049] Fourier-Transform Infrared Spectroscopy (FTIR) was used to analyze the evolution of oxidation of as-synthesized hydrogen-terminated silicon nanoparticles over time after exposure to ambient air. The FTIR graphs in Figure 3 show the evolution of the spectra of the fresh hydrogen-terminated silicon nanoparticles in the ranges of the Si-O-Si vibrational peak bond (left) and the surface silicon hydrides (Si4-n— Si— Hn) vibrational peaks (right) for different times of air exposure: 5 min, 263 min (4.5 h), 1504 min (25 h) and 4380 min (73 h). The Si-O-Si vibrational peak in the region 950 cm1- 1200 cm1has a strong increase in intensity, while the Si4-n— Si— Hnhydrides' bands located at 2050 cm1- 2150 cm1decrease in intensity as the air exposure time increases

[0012] ,

[0013] ,

[0014] , [6], [9],

[0015] , This confirms that the freshly deposited silicon nanoparticles are hydrogen- terminated upon deposition and gradually lose these terminations through natural oxidation

[0012] ,

[0013] , [6], [9],

[0015] , We can also verify that natural oxidation of the hydrogen-terminated silicon nanoparticles starts to occur immediately after air exposure

[0013] , [6], [9],

[0015] ,

[0050] Explosive sintering of hydrogen-terminated silicon nanoparticles Explosive sintering of the as-produced Si-NPs was triggered using a 633 nm HeNe Laser (Renishaw RL633) at 20 mW power, with the samples being focused with a Research Grade Leica DM2700 microscope using a Leica Nplan 50x objective (NA 0.75, WD 0.37 mm) to focus on a 1.03 pm2sized laser spot, achieving an intensity of 2.4 MW cm'2. The explosive sintering reaction propagated throughout the whole sample from the laser spot. All sintering processes were done at room temperature and ambient atmosphere with no additives.

[0051] Fourier-Transform Infrared Spectroscopy (FTIR) was used to analyze the outcome of the explosive sintering mechanism and compare it to the spectra of the freshly synthesized SiNPs before being sintered, and of synthetized SiNPs exposed to ambient air, as can be seen on Figure 4. The two graphs in Figure 4 compare the spectra in the Si-O-Si vibrational range (left) and in the range of the surface Si4-n- Si- Hnhydrides vibrational peaks (right) measured for the fresh Si-NPs with time exposure of 5 min and for the explosively sintered Si-NPs, herein designated as s-Si-NPs, which were exposed to air after sintering for about 3 days. There is no change in the FTIR spectra for the s-Si-NP material over time as it is a stable material. The fresh SiNPs show a very small Si-O-Si stretching band in the region 950 cm1- 1200 cm1and a strong band in the region 2050 cm1- 2150 cm1due to Si4-n- Si- Hnhydrides' bonds, whereas for the s-Si-NPs, a strong Si-O-Si band is observed at 1050 cm1in conjunction with virtually no peaks due to silicon hydrides

[0012] ,

[0013] ,

[0016] ,

[0017] , [6],

[0018] , This shows that the illumination with the laser leads to the rupture of the surface Si-H bonds, creating Si dangling bonds which ignite the explosive oxidation reaction, resulting in the sintered Si material and a complete removal of hydrogen terminations (Si4-n— Si— Hnbonds)

[0019] ,

[0020] , The graphs also show the spectra measured for an "island" on the sample with the s-Si-NP material that was not sintered and after 3 days of air exposure. Here, peaks of both Si-O-Si and Si4-n— Si- Hnbonds are observed

[0012] ,

[0013] ,

[0016] ,

[0017] , [6],

[0018] , The spectrum is similar to what was observed for the free-standing SiNPs after around 3 days of air exposure shown in Figure 3. This comparison confirms that the explosive sintering process leads to instantaneous and explosive oxidation of the SiNP samples into the porous polycrystalline s-Si-NP material seen in SEM (Figure 1) without the need for additional treatment of the SiNPs and at ambient conditions.

[0052] Crystallinity and size of non-sintered and sintered silicon nanoparticles

[0053] Raman spectroscopy was used to assess the material both before and after sintering, as seen on Figure 5 (a) and (b), respectively.

[0054] The crystalline fraction (Xc) of non-sintered and sintered samples of silicon nanoparticles was determined from the corresponding Raman spectra of Figure 5, by employing the following equation

[0021] , [7],

[0022] : where / cand IA are the integrated intensities of the crystalline phase (LTO peak) and of the amorphous phase (LA, TOi, and TO2 bands), respectively, and the factor y = 0.1 accounts for the high-level scattering cross-section of the amorphous phase relative to the crystalline phase of the material

[0021] ,

[0022] ,

[0055] Similarly, the particle diameter was estimated by comparing the peak center of the LTO peak of the samples to that of bulk crystalline silicon (521 cm1) using the Bond Polarizability model [8], [9]: where Zlcu is the Raman peak shift, 6 is the constant value 47.41 cm , a is the lattice parameter of Si (a=0.543 nm), d is the particle diameter, and y is the constant value 1.44 [8], [9],

[0056] The peak values and calculated Xc and d values for non-sintered and sintered SiNPs are presented in Table 1. Table 1 - Crystalline fraction determination ofSiNPs before and after sintering, from the Raman spectra of Figure 5.

[0057] From Table 1, we can see that both the crystalline fraction and the particle diameter increase after sintering.

[0058] Grain size of the porous polycrystalline silicon material

[0059] The size of the s-Si-NPs in the SEM image of Figure 1 was assessed using the ImageJ software (version 1.53t). These grains present sizes between 30 nm and 85 nm, with a mean size of 48.9 ± 1.7 nm, and a median size of 45.2 nm (Table 2) which agrees with the expected grain size from the Raman shift of Figure 5 (b) (> 10 nm). This is a substantial size increase (~25x) from the as-synthesized 2 - 3 nm Si-NPs collected in the stainless steel meshes with the aforementioned deposition conditions, shown on the STEM image of Figure 2.

[0060] Table 2 - Descriptive statistics of the porous polycrystalline silicon material's (s-Si-NP's) grains from the SEM image in Figure 1. REFERENCES

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Claims

CLAIMS1. A method to prepare a porous polycrystalline silicon-based material using hydrogen-terminated silicon-based nanoparticles, said method comprising the following steps:Flowing a precursor gas mixture into a reaction chamber, said precursor gas mixture comprising at least one silicon-based gas;Applying an electromagnetic field to said precursor gas mixture, thereby forming a non-thermal plasma;Forming a solid product in the plasma, wherein said solid product comprises a plurality of hydrogen-terminated silicon-based nanoparticles;Depositing said solid product on a surface, thereby forming a granular structure comprised of an array of hydrogen-terminated silicon-based nanoparticles layers;Exposing said granular structure to an oxidizing medium;Subjecting said granular structure exposed to said oxidizing medium, to a trigger mechanism, thereby inducing the sintering of the plurality of hydrogen-terminated silicon-based nanoparticles through a chain reaction to form a porous polycrystalline silicon-based material, said porous polycrystalline silicon-based material having a grain size that is larger than the grain size of the hydrogen-terminated silicon-based nanoparticles.

2. A method according to claim 1 wherein the at least one silicon-based gas is selected from the group consisting of silane, tetrachlorosilane and mixtures thereof.

3. A method according to any of the previous claims wherein the precursor gas mixture comprises hydrogen and / or at least one noble gas selected from the group consisting of helium, argon and mixtures thereof.

4. A method according to any of the previous claims wherein the oxidizing medium is selected from ambient air, liquid oxygen, perchlorates, nitrates, and mixtures thereof.

5. A method according to any of the previous claims wherein said hydrogen- terminated silicon-based nanoparticles comprise a grain size of 1 - 50 nm.

6. A method according to any of the previous claims wherein the sintering of the plurality of hydrogen-terminated silicon-based nanoparticles is performed without external heating, and at standard atmospheric pressure, 760 Torr.

7. A method according to any of the previous claims wherein the trigger mechanism comprises an optical trigger consisting of light emitted from a light source, being said light irradiated onto the hydrogen-terminated silicon-based nanoparticles.

8. A method according to claim 7 wherein the light source comprises a laser or a focused light from any other source.

9. A method according to any of claims 7 - 8 wherein the light emitted by the light source has a wavelength in the NIR-Vis-UV range, from 250 nm to 2100 nm.

10. A method according to any of the previous claims wherein the residence time of the precursor gas mixture in the plasma is less than 50 ms; being the precursor gas mixture is preferably at 0.1 - 15 Torr.

11. A method according to any of the previous claims wherein the hydrogen- terminated silicon-based nanoparticles comprise a crystalline fraction of at least 60 %.

12. Porous polycrystalline silicon-based material, obtainable by method according to claims 1 - 11, comprising a crystal lattice structure embedded in an amorphous and a porous structure.

13. Porous polycrystalline silicon-based material according to claim 12 comprising a grain size of 10 - 500 nm.

14. Porous polycrystalline silicon-based material according any of the claims 12 - 13 comprising a crystalline fraction above 80 %.

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