Method for producing passivated silicon nanoparticles

Abstract

A method for passivating silicon nanoparticles, the method comprising: synthesizing silicon nanoparticles containing free radicals in a plasma reactor; trapping the silicon nanoparticles in a trapping fluid composition comprising a trapping fluid and a free radical reactive compound under vacuum to form a trapped silicon nanoparticle composition comprising the trapping fluid, silicon nanoparticles, and the radical reactive compound; and reacting the radical reactive compound with the free radicals of the silicon nanoparticles to produce a passivation composition comprising passivated silicon nanoparticles and the trapping fluid.

Description

[Technical Field]

[0001] (CROSS-REFERENCE TO RELATED APPLICATIONS) none.

[0002] FIELD OF THE INVENTION The method of the present invention involves synthesizing silicon nanoparticles in a plasma reactor, trapping the silicon nanoparticles in a trapping fluid composition comprising a free radical reactive compound under vacuum, and reacting the free radical reactive compound with the free radicals on the silicon nanoparticles to produce a passivating composition comprising passivated silicon nanoparticles. [Background technology]

[0003] The emergence of nanotechnology is bringing about a paradigm shift in many technological fields because the properties of many materials change at nanoscale dimensions. For example, reducing the dimensions of some structures to the nanoscale can increase the surface area to volume ratio, thus resulting in changes in the electrical, magnetic, reactive, chemical, structural, and thermal properties of the material. Nanomaterials can already be found in commercial applications and will be present in a wide range of technologies in the coming decades, including computers, photovoltaics, optoelectronics, medicine / pharmaceuticals, building materials, military applications, and many others.

[0004] While early research efforts focused on porous silicon, much interest and effort is now shifting from porous silicon to silicon nanoparticles. A key feature of small (average size less than 10 nanometers (nm)) silicon nanoparticles is that they are photoluminescent in visible light when stimulated by a lower wavelength source (UV). This is thought to result from quantum confinement effects, which occur when the nanoparticle diameter is smaller than the exciton radius, resulting in bandgap bending (i.e., an increase in the gap). Researchers have shown that the bandgap energy (in electron volts) of nanoparticles varies as a function of the nanoparticle diameter.

[0005] Although silicon is an indirect bandgap semiconductor in bulk, silicon nanoparticles with diameters less than 10 nm can rival direct bandgap materials, made possible by interface trapping of excitons. Direct bandgap materials can be used in optoelectronic applications, making silicon nanoparticles potentially a promising material for future optoelectronic applications. Another intriguing property of nanomaterials is their reduced melting point, which follows from the surface phonon instability theory. Researchers have shown that the melting point of nanomaterials formed from nanoparticles varies as a function of the nanoparticle diameter.

[0006] Industry, universities, and research laboratories have invested significant effort in developing manufacturing methods and equipment that can be used to produce nanoparticles. Some of these techniques include microreactor plasmas, aerosol pyrolysis of silane, sonication of etched silicon, and laser ablation of silicon. Plasma discharge offers another opportunity to create nanoparticles at high temperatures from atmospheric plasmas or at approximately room temperature using low-pressure plasmas. High-temperature plasmas are also being investigated.

[0007] Low-pressure plasma has been investigated as a method for producing silicon nanoparticles since the 1990s. Nanocrystalline silicon particles have been produced using ultrahigh vacuum (UHV) and very high frequency (VHF, approximately 144 MHz) capacitively coupled plasma. This approach uses a VHF plasma cell attached to a UHV chamber to decompose silane with the plasma. Hydrogen or argon carrier gas is pulsed into the plasma cell, forcing nanoparticles formed in the plasma through an orifice into a UHV reactor, where they are deposited. The radio frequency allows for efficient coupling of radio frequency (RF) power to the discharge to create a high-ion density and ion energy plasma. Other researchers have used an inductively coupled plasma (ICP) reactor to create a 13.56 MHz RF plasma with high ion energy and density.

[0008] ICP reactors did not effectively produce nanoparticles and were replaced by capacitively coupled discharges. Capacitively coupled systems with ring electrodes could create plasma instabilities that produced a constricting plasma with much higher ion density and energy than ambient glow discharges. This instability rotates around the discharge tube, shortening the particle residence time in the high-energy region. Since residence time is approximately equal to the time conditions are favorable for nanoparticle nucleation, shorter residence times result in capacitively coupled systems producing smaller nanoparticles. Therefore, shortening the residence time reduces the amount of time available for particles to nucleate from dissociated precursor molecular fragments and provides a measure of control over particle size distribution. This method has produced nanocrystalline and luminescent silicon particles. However, the radio frequency power in capacitively coupled systems is not well coupled to the discharge. As a result, much of the input radio frequency power is reflected back to the power supply, requiring a relatively high input power (approximately 200 W) to provide a moderate amount of power (approximately 5 W) to the plasma. This significantly reduces the life of the power supply and reduces the cost-effectiveness of this technique for producing silicon nanoparticles.

[0009] Low-pressure, radio-frequency pulsed plasma reactors and direct fluid trapping of nanoparticles formed in the reactor have also been investigated. This method involves using a pressure gradient to project nanoparticles at supersonic speeds into a trapping fluid, minimizing particle size growth.

[0010] Potential applications of nanoparticles may require different absorbance and photoluminescence properties of the nanoparticles. For example, absorbance in the ultraviolet range may be more important than absorbance in a different range in sun protection applications. Also, greater photoluminescence may be desirable. While much research has focused on reactors and methods for effectively and efficiently producing nanoparticles, little research has been done to control the properties of the produced nanoparticles. Therefore, there is a need for a method to control the properties (i.e., absorbance and luminescence) of the produced nanoparticles. Summary of the Invention

[0011] The present invention is directed to a method for passivating silicon nanoparticles, the method comprising: synthesizing silicon nanoparticles comprising free radicals in a plasma reactor; trapping the silicon nanoparticles in a trapping-fluid composition comprising a trapping fluid and a free-radical-reactive compound under vacuum to form a trapped silicon nanoparticle composition comprising a trapping fluid, silicon nanoparticles, and a radical-reactive compound; and reacting the radical-reactive compound with the free radicals of the silicon nanoparticles to produce a passivation composition comprising passivated silicon nanoparticles and the trapping fluid.

[0012] The method of the present invention produces silicon nanoparticles with improved photoluminescence. [Brief explanation of the drawings]

[0013] The following detailed description of the present invention can be best understood when read in conjunction with the following drawings, in which like structure is indicated with like reference numerals and in which: [Figure 1] FIG. 1 is a schematic diagram illustrating an exemplary embodiment of a low-pressure pulsed plasma reactor that can be used to prepare photoluminescent nanoparticles in accordance with embodiments of the present disclosure. [Figure 2] 2 shows the normalized photoluminescence emission spectra of a nanoparticle composition passivated with a free radical reactive compound in a trapping fluid according to the method of the present invention and a comparative example that was not so passivated. The top plot in FIG. 2 is the as-deposited spectrum, and the bottom plot is after aging at temperature and humidity conditions for 5 days. DETAILED DESCRIPTION OF THE INVENTION

[0014] 1. A method for passivating silicon nanoparticles, comprising: synthesizing silicon nanoparticles containing free radicals in a plasma reactor; entrapping the silicon nanoparticles in a trapping fluid composition comprising a trapping fluid and a free radical reactive compound under vacuum to form an entrapped silicon nanoparticle composition comprising a trapping fluid, silicon nanoparticles, and a free radical reactive compound; reacting a radical reactive compound with the free radicals of the silicon nanoparticles to form a passivation composition comprising passivated silicon nanoparticles and a scavenging fluid; A method comprising:

[0015] Referring initially to FIG. 1, photoluminescent silicon nanoparticles are prepared by providing at least a first reactive gas mixture to a plasma reactor system 5. The reactive gas mixture typically includes a first reactive precursor gas and an inert gas. Preferably, the first reactive precursor gas comprises about 0.1% to about 50% of the total volume of the reactive gas mixture. However, the first reactive precursor gas may comprise other volume percentages, such as about 1% to about 50% of the total volume of the reactive gas mixture.

[0016] Preferably, the first reactive precursor gas includes silicon. Typically, the first reactive precursor gas is selected from silane, disilane, halogen-substituted silanes, halogen-substituted disilanes, C1-C4 alkyl silanes, C1-C4 alkyl disilanes, and mixtures thereof. The reactive gas mixture may include silane comprising about 0.1 to about 2% of the total reactive gas mixture. However, the reactive gas mixture may also include other percentages of silane. Alternatively, the first reactive precursor gas may also include, but is not limited to, SiCl4, HSiCl3, and H2SiCl2.

[0017] The reaction gas mixture may optionally include an inert gas. Preferably, the inert gas includes argon. Alternatively, it is also contemplated that the inert gas may include xenon, neon, or a mixture of inert gases. When present in the reaction gas mixture, the inert gas may comprise from about 1% to about 99% of the total volume of the reaction gas mixture. However, other volume percentages of inert gas are also contemplated.

[0018] The reactive gas mixture may include a second precursor gas, which may itself comprise about 0.1 to about 49.9 volume percent of the reactive gas mixture. The second precursor gas may include BCl3, B2H6, PH3, GeH4, or GeCl4. Alternatively, the second precursor gas may include other gases containing carbon, germanium, boron, phosphorus, or nitrogen. Preferably, the combination of the first reactive precursor gas and the second precursor gas together comprise about 0.1 to about 50% of the total volume of the reactive gas mixture.

[0019] The reaction gas mixture may further include hydrogen gas. Preferably, the hydrogen gas is present in an amount of about 1% to about 10% of the total volume of the reaction gas mixture. However, it is contemplated that the reaction gas mixture may include other percentages of hydrogen gas.

[0020] Referring again to FIG. 1 , plasma reactor system 5 includes plasma generation chamber 11 having reactant gas inlet 21 and outlet 22 having opening or orifice 23 therein. Particle collection chamber 15 is in communication with plasma generation chamber 11. Particle collection chamber 15 contains trapping fluid composition 16 in container 31. Container 31 may be adapted to be stirred (by means not shown). For example, container 31 may be disposed on a rotatable support (not shown) or may include a stirring mechanism. Preferably, the trapping fluid composition is a liquid at the operating temperature of the system. Plasma reactor system 5 also includes vacuum source 17 in communication with particle collection chamber 15 and plasma generation chamber 11.

[0021] The plasma generation chamber 11 includes an electrode configuration 13 attached to a variable frequency rf amplifier 10. The plasma generation chamber 11 also includes a second electrode configuration 14. The second electrode configuration 14 is either grounded, DC biased, or operated in a push-pull configuration with respect to the electrode 13. The electrodes 13, 14 are used to couple very high frequency (VHF) power to the reactive gas mixture to ignite and sustain a glow discharge of plasma within the region shown as 12. The first reactive precursor gas (or gases) then dissociates in the plasma to provide charged silicon atoms that nucleate and form silicon nanoparticles having an average silicon core diameter of less than about 10 nm, preferably about 2.2 to about 4.7 nm. However, other discharge tube configurations are contemplated and may be used in practicing the methods disclosed herein.

[0022] The silicon nanoparticles contain free radicals. It will be understood that each silicon nanoparticle produced may or may not contain one or more free radicals, but some of the silicon nanoparticles produced in the process will contain free radicals.

[0023] Silicon nanoparticles are collected in the trapping fluid composition in the particle collection chamber 15. To control the diameter of the nanoparticles formed, the distance between the opening 23 in the outlet 22 of the plasma generation chamber 11 and the surface of the trapping fluid composition ranges from about 5 to about 50 aperture diameters. The inventors have found that placing the surface of the trapping fluid composition too close to the outlet of the plasma generation chamber can result in undesirable interactions between the plasma and the trapping fluid composition. Conversely, placing the surface of the trapping fluid composition too far from the aperture reduces particle collection efficiency. Because the collection distance is a function of the aperture diameter and the pressure drop between the plasma generation chamber and the collection chamber, the inventors have found that, based on the operating conditions described herein, an acceptable collection distance is about 1 to about 20 cm, preferably about 5 to about 10 cm. In other words, an acceptable collection distance is about 5 to about 50 aperture diameters.

[0024] The plasma generation chamber 11 also includes a power source. Power is supplied via a variable frequency radio frequency power amplifier 10, which is triggered by an arbitrary function generator to establish a high frequency pulsed plasma in the region 12. Preferably, the radio frequency power is capacitively coupled into the plasma using a ring electrode, parallel plate, or anode / cathode arrangement in the gas. Alternatively, the radio frequency power may be inductively coupled into the plasma using an RF coil arrangement around the discharge tube.

[0025] The plasma generation chamber 11 may also include a dielectric discharge tube. Preferably, the reactive gas mixture enters the dielectric discharge tube where the plasma is generated. Nanoparticles forming from the reactive gas mixture begin to nucleate as the first reactive precursor gas molecules dissociate in the plasma.

[0026] Vacuum source 17 typically includes a vacuum pump. Vacuum source 17 may comprise a mechanical pump, a turbomolecular pump, or a cryogenic pump, although other vacuum sources are contemplated as well.

[0027] The electrodes 13, 14 for the plasma source within the plasma generation chamber 11 typically comprise a flow-through showerhead design in which a VHF radio frequency biased upstream porous electrode plate 13 is separated from a downstream porous electrode plate 14 with the holes in the plates aligned with one another. The holes may be circular, rectangular, or any other desired shape. Alternatively, the plasma generation chamber 11 may surround an electrode 13 coupled to a VHF radio frequency power source and having a tip with a variable distance between the tip and a ground ring inside the chamber 11.

[0028] The VHF radio frequency power supply typically operates in a frequency range of about 30 to about 500 MHz. The tip 13 can be positioned a variable distance from a VHF radio frequency drive ring 14, which operates in a push-pull mode (180° out of phase). The electrodes 13, 14 can include an induction coil coupled to the VHF radio frequency power supply, such that radio frequency power is delivered to the reactant gas mixture by an electric field formed by the induction coil. A portion of the plasma generation chamber 11 is configured to operate at pressures ranging from 0.133 millipascals to 67 megapascals (1×10 -7 The electrode coupling configurations may be evacuated to vacuum levels ranging between 0.1 and 1.5 torr (up to 500 torr). However, other electrode coupling configurations are also contemplated for use with the methods disclosed herein.

[0029] The plasma in region 12 may be initiated using a radio frequency plasma via an RF power amplifier, such as, for example, an AR Worldwide Model KAA2O4O, or an Electronics and Innovation Model 3200L, or an EM Power RF Systems, Inc. Model BBS2E3KUT. The amplifier can be driven (or pulsed) by an arbitrary function generator (e.g., a Tektronix AFG3252 Function Generator) capable of producing up to 200 watts of power at 0.15 to 150 MHz. It may be possible for the arbitrary function to drive the power amplifier using pulse trains, amplitude modulation, frequency modulation, or different waveforms. Power coupling between the amplifier and the reactive gas mixture typically increases as the frequency of the RF power increases. The ability to drive power at higher frequencies may allow for more effective coupling between the power source and the discharge. The increased coupling may be expressed as a decrease in the voltage standing wave ratio (VSWR).

[0030]

number

[0031]

number

[0032] The power and frequency of the plasma system are typically preselected to create an optimal operating space for the formation of photoluminescent silicon nanoparticles. Preferably, both the power and frequency are adjusted to create an appropriate ion and electron energy distribution in the discharge that favors the dissociation of the silicon-containing reactive precursor gas molecules and the nucleation of nanoparticles. Appropriate control of both the power and frequency prevents the silicon nanoparticles from growing too large.

[0033] Referring again to Figure 1, one exemplary embodiment of a low-pressure, radio-frequency pulsed plasma reactor 5 is shown schematically. In the illustrated embodiment, a reactant gas mixture is introduced into a plasma generation chamber 11. The plasma reactor 5 can operate at a frequency range of 30 MHz to 150 MHz, a pressure in the plasma generation chamber 11 of 13 Pascals to 1.33 Megapascals (100 mTorr to 10 Torr), and a power of about 1 W to about 200 W. However, other powers, pressures, and frequencies for the plasma reactor 5 are also contemplated.

[0034] The pulsed plasma system shown in Figure 1 can be used to produce photoluminescent silicon nanoparticles. Plasma pulsing allows the operator to directly manage the residence time for particle nucleation, thereby controlling the particle size distribution and agglomeration rate in the plasma. The pulsing feature of the system allows for controlled adjustment of particle residence time within the plasma, which affects the size of the nanoparticles. By shortening the plasma "on" time, nucleated particles have less time to agglomerate, and therefore, the size of the nanoparticles may be reduced on average (e.g., the nanoparticle distribution may shift toward smaller diameter particle sizes).

[0035] Advantageously, operation of the plasma reactor system 5 in the higher frequency range and pulsing of the plasma provides the same conditions as conventional constricted / filamentary discharge techniques that use plasma instabilities to generate high ion energy / density, but with the added advantage that the user can control the operating conditions to select and generate nanoparticles with sizes that result in photoluminescent properties.

[0036] For pulsed injection, nanoparticle synthesis can be achieved using a pulsed energy source, such as pulsed ultrashort wave rf plasma, high frequency rf plasma, or a pulsed laser for pyrolysis. Preferably, the VHF radio frequency is pulsed at a frequency ranging from about 1 to about 50 kHz. However, it is contemplated that the VHF radio frequency can be pulsed at other frequencies as well.

[0037] Another method for transferring nanoparticles to the trapping fluid composition is to pulse the input of the reactive gas mixture while the plasma is ignited. For example, to synthesize Si nanoparticles, a plasma can be ignited in the presence of a first reactive precursor gas, in the presence of at least one other gas, such as an inert gas, to sustain the discharge. Nanoparticle synthesis is stopped when the flow of the first reactive precursor gas is stopped by a mass flow controller. Nanoparticle synthesis continues when the flow of the first reactive precursor gas is restarted. This generates a pulsed stream of nanoparticles. This technique can be used to increase the concentration of nanoparticles in the trapping fluid composition when the flux of nanoparticles impinging on the trapping fluid composition is greater than the rate at which the nanoparticles are absorbed into the trapping fluid composition.

[0038] Generally, nanoparticles can be synthesized through VHF radio frequency low-pressure plasma discharges at increased plasma residence times relative to the residence times of precursor gas molecules. Alternatively, crystalline nanoparticles can be synthesized at lower plasma residence times under the same operating conditions of discharge drive frequency, drive amplitude, discharge tube pressure, chamber pressure, plasma power density, gas molecule residence time through the plasma, and collection distance from the plasma source electrode. The average particle size of the nanoparticles can be controlled by controlling the plasma residence time, and the high ion energy / density region of the VHF radio frequency low-pressure glow discharge can be controlled relative to the residence time of at least one precursor gas molecule through the discharge.

[0039] The size distribution of nanoparticles can also be controlled by controlling the plasma residence time, the high ion energy / density region of a VHF radio frequency low pressure glow discharge, relative to the residence time of the at least one precursor gas molecule through the discharge. Typically, the shorter the plasma residence time of the VHF radio frequency low pressure glow discharge relative to the gas molecule residence time, the smaller the average nanoparticle diameter at steady state operating conditions. The operating conditions may be defined by the discharge drive frequency, drive amplitude, discharge tube pressure, chamber pressure, plasma power density, precursor mass flow rate, and collection distance from the plasma source electrode. However, other operating conditions are contemplated as well. For example, as the plasma residence time of the VHF radio frequency low pressure glow discharge increases relative to the gas molecule residence time, the average nanoparticle diameter decreases as the VHF radio frequency low pressure glow discharge decreases, relative to the gas molecule residence time, as the average nanoparticle diameter decreases, as expressed by the formula: y = y0 - exp(-t r / C), where y is the average nanoparticle diameter, y0 is the offset, and t r is the plasma residence time and C is a constant. The particle size distribution can also increase with increasing plasma residence time under otherwise constant operating conditions.

[0040] The average particle size (and nanoparticle size distribution) of the nucleated nanoparticles can be controlled by controlling the mass flow rate of at least one precursor gas in the VHF radio frequency low-pressure glow discharge. For example, as the mass flow rate of the precursor gas(es) is increased in the VHF radio frequency low-pressure plasma discharge, the average synthesized nanoparticle diameter increases with increasing y = y for steady-state operating conditions. o +exp(-MFR / C'), where y is the average nanoparticle diameter, y0 is the offset, MFR is the precursor mass flow rate, and C' is a constant. Typical operating conditions may include discharge drive frequency, drive amplitude, discharge tube pressure, chamber pressure, plasma power density, gas molecule residence time through the plasma, and collection distance from the plasma source electrode. The particle size distribution of the synthesized average core nanoparticles can also be calculated as y = y for steady-state operating conditions. oIt may also be reduced as an exponential decay model of the form +exp(-MFR / K), where y is the average nanoparticle diameter, y is an offset, MFR is the precursor mass flow rate, and K is a constant.

[0041] As previously described, nucleated nanoparticles formed in plasma generation chamber 11 are transferred to particle collection chamber 15, which contains trapping fluid composition 16. Preferably, the charged nanoparticles can be evacuated from chamber 11 to particle collection chamber 15 by cycling the plasma to a low ion energy state or by turning the plasma off. Upon transfer to particle collection chamber 15, the nucleated nanoparticles are absorbed into the trapping fluid composition.

[0042] The nucleated nanoparticles can be transferred from the plasma generation chamber 11 to the particle collection chamber 15 containing the capture fluid composition through an opening or orifice 23 that creates a pressure differential. It is contemplated that the pressure differential between the plasma generation chamber 11 and the particle collection chamber 15 can be controlled by various means. In one configuration, the inner diameter of the discharge tube of the plasma generation chamber 11 is much smaller than the inner diameter of the particle collection chamber 15, thus creating a pressure drop. In another configuration, a grounded physical opening or orifice may be placed between the discharge tube and the collection chamber 15, which forces the plasma to reside partially inside the orifice, based on the Debye length of the plasma and the size of the chamber 15. Another configuration includes using a variable electrostatic opening, where a concentrated positive charge is generated that forces the negatively charged plasma through the opening 23.

[0043] The scavenging fluid composition comprises a scavenging fluid and a free radical reactive compound.

[0044] The trapping fluid of the trapping fluid composition is typically selected so that the trapping fluid composition can be used for silicon nanoparticle trapping and, desirably, as a material handling and storage medium. The trapping fluid can be selected from any fluid that disperses the nanoparticles in the trapping fluid composition as they are collected, inhibits particle-particle interactions, and thereby forms a dispersion or suspension of the nanoparticles in the trapping fluid composition. Thus, the nanoparticles and the trapping fluid are miscible. The trapping fluid can be a mixture of miscible fluids, at least one of which can be a free-radical reactive compound.

[0045] The vapor pressure of the trapping fluid is desirably lower than the operating pressure in the plasma reactor. Preferably, the operating pressure in the reactor and collection chamber 15 ranges from 0.133 Pascals to 0.667 Pascals (1 to 5 mTorr). Other operating pressures are contemplated as well.

[0046] The trapping fluid may desirably comprise or consist of a silicone fluid, a hydrocarbon fluid, and / or a halocarbon fluid. The silicone fluid, hydrocarbon fluid, and / or halocarbon fluid may contain a substituent. Substituents for the trapping fluid include, but are not limited to, unsaturated hydrocarbyls having 2 to 12 carbon atoms, borate, boronic acid, amide, azide, azo, amino, carbodiimide, imine, isocyanate, nitrile, nitro, alcohol, aldehyde, carboxylic acid, epoxy, ester, ether, hydroxyl, keto, peroxy, phosphate, phosphine, phosphine oxide, phosphinite, phosphite, phosphonate, phosphonite, disulfide, thioether, thiol, halo, silanol, or hydroxyl, amine, or vinyl. The substituents typically modify the absorbance and photoluminescence of the silicon nanoparticles. The trapping fluid may be a single material or a mixture of two or more trapping fluids. When the trapping fluid is a mixture, none or all of the trapping fluid may be substituted.

[0047] Examples of silicone fluids that may be included in the capture fluid include, but are not limited to, polydimethylsiloxane, mixed phenylmethyl-dimethylcyclosiloxane, tetramethyltetraphenyltrisiloxane, and pentaphenyltrimethyltrisiloxane, all of which are suitable for use as a capture fluid.

[0048] Examples of hydrocarbon fluids that may be included in the capture fluid include, but are not limited to, branched and straight chain hydrocarbons having 20 to 40 carbon atoms, or refined petroleum distillates solvent refined paraffins.

[0049] The trapping fluid has a viscosity sufficient to trap the silicon nanoparticles and agitate to prevent the particles from agglomerating, alternatively between 1 and 500, alternatively between 10 and 100 millipascal-seconds (centipoise). One skilled in the art would know how to measure the viscosity of a fluid. Viscosity is measured using a Brookfield LV viscometer, spindle 12, at 12 rpm and 25°C.

[0050] Those skilled in the art will understand how to make and / or obtain the capture fluids of the present invention. Many of these fluids are commercially available. The capture fluids can generally be reused multiple times in the processes of the present invention. To reuse the capture fluid, it is common to isolate the silicon nanoparticles from the capture fluid (e.g., by filtration or centrifugation followed by decantation) and then use the recovered capture fluid again to capture additional silicon nanoparticles.

[0051] The silicon nanoparticles synthesized according to the method of the present invention contain free radicals.

[0052] A free radical reactive compound is any compound that reacts with radicals of silicon nanoparticles and / or non-radical reactive sites on the silicon nanoparticles. The reaction of free radicals and / or other reactive sites with the free radical reactive compound produces nanoparticles that have greater photoluminescence than would be possible without the free radical reactive compound. An example of a non-radical reactive site is a hydride group.

[0053] The free radical reactive compound can be an organic compound, or a hydrocarbon having 1 to 25 carbon atoms that reacts with the free radicals of the silicon nanoparticles, or an organic compound having 1 to 25 carbon atoms and unsaturation, or an organic compound having 1 to 25 carbon atoms and an alkenyl or alkynyl group, or an alkenyl group, or an organic compound having 1 to 25 carbon atoms and an ester functional group. The free radical reactive compound can be represented by formula (I): (I)R 1 C(=O)R 2 (In the formula, R 1 is a hydrocarbyl having 1 to 20 carbon atoms, alternatively 1 to 12 carbon atoms; R 2 is a hydrocarbyl having 1 to 6 carbon atoms, and R 1 and R 2 one of R has a carbon-carbon double or triple bond; 2 The free radical reactive compound may be a C 1~25 Alkenyl alkanoate, or aryl alkanoate, or aryl (C 6~12 ) alkanoate or allyl decanoate.

[0054] The free radical reactive compound and the trapping fluid may be the same material, in which case the trapping fluid contains a free radical reactive group. The free radical reactive group contained in the trapping fluid is an organic group, or a hydrocarbyl group having 1 to 25 carbon atoms that reacts with the free radicals of the silicon nanoparticles, or an organic group having 1 to 25 carbon atoms and unsaturation, or an organic group having 1 to 25 carbon atoms and an alkenyl or alkynyl functionality, or an alkenyl functionality, or an organic group having 1 to 25 carbon atoms and an ester functionality. The free radical reactive group is represented by the formula (II): (I)R 3 C(=O)R 4 (In the formula, R 3 is a hydrocarbylene linking group having 1 to 20 carbon atoms, alternatively 1 to 12 carbon atoms, and R 2 is a hydrocarbyl having 1 to 6 carbon atoms, and R 3 and R 4 one of R has a carbon-carbon double or triple bond; 4 may be by virtue of having a carbon-carbon double or triple bond.

[0055] The silicon nanoparticles can be exposed to the free radical reactive compound after they are synthesized and before the nanoparticles are exposed to oxygen or any other passivating agent or compound, such as nitrogen or hydrogen, or before surface oxidation occurs. The free radical reactive compound can be present when the silicon nanoparticles are trapped in the trapping fluid composition, or the free radical reactive compound is added to the trapping fluid and the silicon nanoparticles after the silicon nanoparticles are trapped in the trapping fluid to form the trapping fluid composition.

[0056] The silicon nanoparticles react with the free radical reactive compounds in the capture fluid composition to form a passivating composition comprising passivated silicon nanoparticles and the capture fluid.

[0057] Passivated silicon nanoparticles are silicon nanoparticles that have been reacted with a free radical reactive compound. The reaction can involve reaction of free radical or free radical and non-radical reactive sites on the silicon nanoparticles with the free radical reactive compound. The reaction of the free radical reactive compound with the radical or non-radical reactive sites on the silicon nanoparticles prevents or removes defects from the silicon nanoparticles.

[0058] The passivating composition also includes a scavenging fluid and reaction by-products of the free radical reactive compound with the silicon nanoparticle free radicals or other reactive groups.

[0059] The capture fluid composition is desirably agitated during direct capture of nanoparticles. Acceptable contemplated forms of agitation include stirring, rotation, inversion, and other suitable means. If a higher rate of nanoparticle absorption into the capture fluid is desired, more vigorous forms of agitation are contemplated. For example, one such contemplated method of vigorous agitation is sonication.

[0060] After dissociation of the first reactive precursor gas in the plasma generation chamber 11, silicon nanoparticles are formed and entrained in the gas phase. The distance between the nanoparticle synthesis site and the surface of the capture fluid composition must be short enough to prevent unwanted functionalization while the nanoparticles are entrained. If the particles interact in the gas phase, agglomerates of many individual small particles are formed and entrapped in the capture fluid composition. If excessive interactions occur in the gas phase, the particles may sinter together, forming particles with diameters greater than 10 nm. The collection distance is defined as the distance from the outlet of the plasma generation chamber to the surface of the capture fluid composition. The collection distance is typically in the range of about 5 to about 50 aperture diameters.

[0061] Alternatively stated, the collection distance is in the range of about 1 to about 20 cm. The collection distance may more usually be in the range of about 6 to about 12 cm, and preferably about 5 to about 10 cm. However, other collection distances are contemplated.

[0062] The nanoparticles may include silicon alloys. Silicon alloys that can be formed include, but are not limited to, silicon carbide, silicon germanium, silicon boron, silicon phosphorus, and silicon nitride. The silicon alloy may be formed by mixing at least one first precursor gas with a second precursor gas, or by using precursor gases containing different elements. However, other methods of forming alloyed nanoparticles are also contemplated.

[0063] Silicon nanoparticles can undergo additional doping processes. Preferably, silicon nanoparticles undergo gas-phase doping in a plasma, in which a second precursor gas dissociates and is incorporated into the silicon nanoparticles as they nucleate. Alternatively, silicon nanoparticles may undergo gas-phase doping downstream of nanoparticle generation but before the silicon nanoparticles are trapped in a liquid. Furthermore, doped silicon nanoparticles may also be generated in the trapping fluid composition, in which a dopant is preloaded into the trapping fluid composition and interacts with the nanoparticles after they are trapped. Doped nanoparticles can be formed by contact with organosilicon gases or liquids, including, but not limited to, trimethylsilane, disilane, and trisilane. Gas-phase dopants can include, but are not limited to, BCl3, B2H6, PH3, GeH4, or GeCl4.

[0064] The inclusion of a free radical reactive compound in the trapping fluid composition and the direct liquid entrapment of nanoparticles in the trapping fluid composition impart unique properties to the composition. Silicon nanoparticles directly entrapped in the trapping fluid composition exhibit visible photoluminescence when removed from the system and excited by exposure to ultraviolet light. By using the method of the present invention, in which silicon nanoparticle free radicals react with the free radical reactive compound in the trapping fluid, the photoluminescence of the silicon nanoparticles is increased compared to nanoparticles entrapped in a trapping fluid that does not contain the free radical reactive compound. Also, depending on the average diameter of the nanoparticles, they can be photoluminescent at any wavelength within the visible spectrum and may visually appear red, orange, green, blue, purple, or any other color within the visible spectrum. The inclusion of different groups and / or compounds in the trapping fluid also affects quantum luminescence efficiency and absorbance. Photoluminescent silicon nanoparticles produced according to the present invention and entrapped directly in the trapping fluid composition typically exhibit a photoluminescence of at least 1×10 at an excitation wavelength of about 365 nm. 6 Directly trapped photoluminescent silicon nanoparticles typically have a quantum efficiency of at least 4% at an excitation wavelength of approximately 395 nm when measured on an Ocean Optics spectrophotometer equipped with an integrating sphere that has an absorption of greater than 10% of the incident photons.

[0065] Furthermore, both the photoluminescence intensity and luminescence quantum efficiency of the direct capture composition continue to increase over time when the nanoparticle-containing capture fluid composition is exposed to air. The maximum emission wavelength of nanoparticles directly trapped in a fluid tends to shift to shorter wavelengths over time when exposed to oxygen.

[0066] Photoluminescence intensity can be altered or increased, for example, by capturing silicon nanoparticles in a capture fluid composition according to the methods of the present invention. Photoluminescence intensity can be altered, or preferably increased, by at least 2, or at least 5, or at least 15 normalized units, where "normalized units" refers to the photoluminescence intensity value measured on the same day and normalized to the maximum photoluminescence intensity of a control sample (i.e., using the same method but without the free radical reactive compound). Changes in photoluminescence intensity can be evident even when measured after aging under passivation conditions. Passivation conditions include temperatures of 30 degrees Celsius (°C) or higher, or 50°C or higher, or even 60°C or higher, but not exceeding 100°C, or not exceeding 70°C, or even 60°C, and relative humidity of 50% or higher, or not exceeding 70%, or not exceeding 80%, or even not exceeding 85%, but not exceeding 100%, or not exceeding 95%, or even not exceeding 90%, for 5 days. Photoluminescence measured on the same day can increase by more than 15 normalized units for the passivated composition.

[0067] The peak luminescence emission intensity wavelength (nm), which is the point of maximum luminescence of the composition when excited and is approximately 365 nanometers (nm), typically shifts, alternatively by at least 10, alternatively by at least 20, alternatively by 10-150 nm, for passivated compositions containing passivated silicon nanoparticles prepared by the methods of the present invention compared to a control sample measured on the same day where the method of preparation does not include a free radical reactive compound in the capture fluid composition. The difference in peak luminescence emission intensity wavelength can be observed on the same day as produced, preferably after aging for 5 days at 60°C and 85% relative humidity.

[0068] The passivated composition may be exposed to UV before aging the sample, in which case exposure to UV light changes both the normalized photoluminescence intensity and the peak luminescence emission wavelength. [Example]

[0069] The following examples are set forth to illustrate preferred embodiments of the present invention. Those skilled in the art will appreciate that the techniques disclosed in the examples below are techniques discovered by the inventors to function well in the practice of the invention, and can therefore be considered to constitute preferred modes of practicing the invention. However, in light of the present disclosure, those skilled in the art will appreciate that many changes can be made in the specific embodiments disclosed and still obtain like or similar results without departing from the spirit and scope of the present invention. All percentages are by weight unless otherwise noted.

[0070] [Table 1]

[0071] Basic plasma nanoparticle synthesis procedure: Silicon nanoparticles were synthesized using an ultra-high frequency, low-pressure plasma system. Ultra-high purity precursor gases (Ar, H, and SiH) were metered into a quartz tube at specific ratios and pressures via mass flow controllers. Typical pressures in the quartz discharge tube ranged from 133 Pa to 667 Pa (1 to 5 Torr). The gases were then dissociated via an ultra-high frequency plasma discharge (100 to 150 MHz). The frequency was selected to maximize plasma coupling while simultaneously minimizing the drive amplitude of the function generator, which provided a sinusoidal signal to a class A radio frequency amplifier.

[0072] The precursor gas was dissociated in a very high frequency plasma discharge through a number of reactions that followed Maxwell-Boltzmann statistics (due to the non-equilibrium nature of the plasma discharge). Silicon atoms coalesced, nucleated, and grew in the plasma to form silicon nanocrystals. The power of the plasma discharge controlled the temperature of the individual particles, allowing for control of the crystallinity of the particles. High power resulted in crystalline particles, while low power produced amorphous particles. The concentration of silicon atoms in the plasma and the residence time of the atoms controlled the size of the nanoparticles. Once the nanoparticles exited the plasma through an orifice located at the bottom of the quartz plasma chamber, they no longer grew. The particles formed SiH on their surface. x The particles exited the plasma, carrying with them (x<4), radicals (dangling bonds), and / or halogen species (if present in the quartz tube). The particles were expelled through an orifice into the deposition chamber by a large pressure drop. The pressure in the deposition chamber was 1.33 millipascals (1×10 -5 The pressure was less than 1000 psi (less than 1000 psi) (generated by a high vacuum pump, i.e., turbomolecular, cryogenic, or diffusion pump). This large pressure drop generated a supersonic jet of particles that escaped from the plasma chamber. The supersonic jet minimized any gas-phase particle-particle interactions, thereby keeping the particles monodisperse in the gas stream.

[0073] Less than 1.33 millipascals (<1×10 -5 A stirring fluid (trapping fluid), a low-viscosity liquid (viscosity less than 0.2 Pascal-seconds) at a pressure of 1000 psi (2000 psi), was placed in the cup and used to trap particles at low pressure. The surface of the trapping fluid was located within a distance of the orifice where the particles remained dispersed in the supersonic jet. The low viscosity of the trapping fluid allowed particles to be injected into the trapping fluid without forming a film on the fluid's surface. Fluid agitation was used to regenerate the surface of the trapping fluid and move the trapped particles away from the center of the orifice.

[0074] Once the silicon nanoparticles were trapped in the trapping fluid, the fluid and particle dispersion were removed from the vacuum chamber and the photoluminescence spectrum was measured. This measurement was performed on a Horiba FL3 spectrofluorometer equipped with a 450-watt xenon source. The excitation monochromator was set to 365 nm with a 2-nm slit width. A 400-nm edge filter was placed in the beam path to the emission monochromator downstream of the sample. The sample (silicon nanoparticles dispersed in the trapping fluid) was placed in a cuvette (either quartz or methyl acrylate) with a 1-cm path length.

[0075] Emission spectra were measured perpendicular to the excitation beam (or head-on at an angle of 22.5° if the sample was not transparent enough to allow measurements at a perpendicular angle). The slit width of the emission monochromator was 2 nm, and measurements were performed with an integration time of 0.1 s per wavelength, measuring every 1 nm. Spectra were corrected for the quantum yield of the emission detector. The emission data were then calculated using the adjusted R 2 The spectrum was then fitted to a distribution (typically a Gaussian distribution for silicon nanoparticles) with a ρ > 0.98. From this fit, the emission maximum wavelength, full width at half maximum (FWHM), and emission intensity were obtained. The emission spectrum was then converted to a diameter spectrum via a formula fitted to the data generated by the silicon quantum dot model developed by Luo, Stradins, and Zunger:

[0076]

number

[0077] The silicon nanoparticle and entrapment fluid dispersion is placed in a temperature-humidity oven (typically 60 °C and 85% relative humidity) for 5-6 days to form a diffusion-limited oxide (SiOx , x<2), this passivation typically blue-shifts the emission spectrum and increases the photoluminescence intensity via passivation of exciton trap states, effectively turning on more particles.

[0078] The absorbance of the silicon nanoparticle / trapping fluid dispersion was measured using a Shimadzu UV 1800 UV-vis spectrophotometer. This is a double-beam spectrophotometer with a measurement range of 190-1100 nm and a bandwidth of 1 nm. The undiluted trapping fluid and nanoparticle / trapping fluid dispersion were placed in quartz cuvettes fitted with a 1 cm path length. The absorbance spectra of the samples were then measured, and the spectrum of the trapping fluid (reference sample) was subtracted.

[0079] Example 2 1-3; Comparative Example 1 Table 1 shows the silicon nanoparticle dispersions used in these examples. The trapping fluid used in these experiments was Diffoil Ultra 20, a hydrocarbon vacuum pump oil manufactured by Kurt J. Lesker. The ligand used to functionalize the silicon nanoparticles in vacuo was allyl decanoate (C 13 H 24O2, CAS No.: 57856-81-2). For Sample B (Example 1) and Sample C (Example 2), allyl decanoate was dispersed at 1 wt. % in Diffoil Ultra 20 fluid prior to introduction into the system. Sample C (Example 2) received an additional 3 hours of UV light exposure treatment after the silicon nanoparticle trapping fluid dispersion was removed from the reactor and prior to the aging procedure. Samples A and D had only Diffoil Ultra 20 as the trapping fluid. Sample A is Comparative Example 1. Sample D (Example 3) had 1 wt. % (based on the mass of Diffoil Ultra 20) allyl decanoate dispersed in the silicon nanoparticle / Diffoil Ultra 20 dispersion by ultrasonic agitation followed by 3 hours of UV light exposure prior to the aging procedure. Table 2 describes the experimental conditions used in this study. The gas precursor values ​​are gas volume percentages. The frequency is the radio frequency of the plasma. P F is the forward power, and P R is the reflected power, and P C is the combined power, and P eff is the power efficiency.

[0080] Figure 2 shows normalized photoluminescence emission spectra of examples and comparative examples, in which different silicon nanoparticles were entrapped in Diffoil Ultra 20 hydrocarbon fluid dispersions. The top plot in Figure 2 is the as-deposited spectrum of the particle dispersion measured on the day of synthesis, and the bottom plot is the spectrum of the particle dispersion after aging at temperature and humidity conditions (60°C and 85% relative humidity) for 5 days. Sample A is Comparative Example 1, and all spectra are normalized to the maximum photoluminescence emission intensity of Sample A (Comparative Example 1) as deposited. 1 wt. % allydodecanoate was dispersed in Diffoil Ultra 20 before silicon nanoparticle deposition for Samples B and C (Example 2) and after deposition for Sample D (Example 3). Samples C (Example 2) and D (Example 3) were then exposed to UV light for 3 hours, after which the samples were aged at temperature and humidity.

[0081] [Table 2]

[0082] Silicon nanoparticle / hydrocarbon oil dispersions, Sample B (Example 1) and Sample C (Example 2), to which arylidedecanoate ligands were added prior to nanoparticle deposition in a low-pressure plasma reactor, exhibit significantly higher initial photoluminescence emission intensities compared to Comparative Example 1 (Sample A), which is silicon nanoparticles dispersed in hydrocarbon oil alone. Sample C (Example 2) was subjected to an additional 3 hours of UV light exposure after removal from the vacuum chamber to further promote hydrosilylation functionalization of the nanoparticles. Sample D (Example 3) was a dispersion with ligand molecules (arylidedecanoate) dispersed in solution after removal from the vacuum system and 3 hours of UV light exposure. It is clear from the top plot of Figure 2 that in situ functionalization (i.e., silicon nanoparticles injected into a hydrocarbon capture fluid with functionalizing ligands present at low pressure in a plasma reactor) significantly increased the photoluminescence emission intensity of the silicon nanoparticles. Sample B (Example 1) and Sample C (Example 2) have peak intensities 21.7 times and 19.3 times higher than the as-deposited control sample, respectively. Sample D (Example 3) after deposition functionalization had a photoluminescence emission intensity that was only 0.65 times higher than the control sample (Sample A). Compared to the functionalized sample, Sample A (Comparative Example 1) was slightly blue-shifted. Peak emission intensity wavelengths on the day of deposition: Sample A (Comparative Example 1) = 714 nm, Sample B (Example 1) = 729 nm, Sample C (Example 2) = 721 nm, and Sample D (Example 3) = 736 nm.

[0083] The bottom plot in Figure 2 shows the normalized photoluminescence emission spectra (normalized to the peak emission intensity of as-deposited Sample A) of the silicon nanoparticle dispersions used in this study after aging for 5 days at 60 °C and 85% relative humidity. In all cases, the samples exhibited increased emission intensity relative to the as-deposited samples, indicating that non-emissive trap states on the surface of the silicon nanoparticles were passivated via functionalizing ligands and / or oxidation, thus turning on more particles in the ensemble. Comparative Example 1 (Sample A) exhibited a 22.1-fold increase in peak photoluminescence intensity. The peak emission intensities of the in situ functionalized arylidedecanoate samples (Sample B (Example 1) and Sample C (Example 2)) increased by 23.8-fold and 24.9-fold, respectively, relative to the peak intensity of the control sample on the day of deposition. These two samples only showed a slight increase during the aging process: Sample B (Example 1) increased from 21.7x on day 0 to 23.8x after aging, and Sample C (Example 2) increased from 19.3x on day 0 to 24.9x after aging. This slight increase indicates that the particles were sufficiently passivated by the in situ allylidedecanoate functionalization, with only a few non-radiative surface trap states passivated via oxidation. The post-treated functionalized sample, Sample D (Example 3), showed a 13.5-fold increase in emission intensity compared to Comparative Example 1 (Sample A as deposited), indicating that traditional wet chemical hydrosilylation worked. In all cases, the peak emission wavelength shifted to the blue due to the reduction of the silicon core of the nanoparticles by oxidation. The control sample had the largest blue shift in emission wavelength (λ = 58 nm), while Sample C (Example 2), the in situ functionalized sample with post-deposition UV treatment, had the smallest shift (λ = 39 nm).

[0084] Visual observation of the samples was performed immediately after deposition under white light illumination. Samples with arylidedecanoate dispersed in Dilloil Ultra 20 prior to silicon nanoparticle deposition, Examples 1 and 2 (Samples B and C, respectively), produce clear nanoparticle / hydrocarbon fluid dispersions. Samples without arylidedecanoate or in which arylidedecanoate was mixed after silicon nanoparticle dispersion, Examples 3 (Sample D) and Comparative Example 1 (Sample A), show cloudy suspensions. After aging for 5 days at 60°C and 85% relative humidity, the samples were excited from below with 365 nm light. Samples in Examples 1 and 2 (Samples B and C, respectively), in which the functionalizing ligand arylidedecanoate was mixed with the trapping fluid prior to silicon nanoparticle deposition, show good dispersion of the luminescent silicon particles. Samples in which no functionalizing molecules were added for in situ functionalization, Comparative Example 1 (Sample A) and Example 3 (Sample D), show silicon nanoparticles settling out of the fluid.

Claims

1. 1. A method for passivating silicon nanoparticles, comprising: synthesizing silicon nanoparticles containing free radicals in a plasma reactor; entrapping the silicon nanoparticles in a trapping fluid composition comprising a trapping fluid and a free radical reactive compound under vacuum to form an entrapped silicon nanoparticle composition comprising the trapping fluid, the silicon nanoparticles, and the free radical reactive compound; reacting the radical reactive compound with the free radicals of the silicon nanoparticles to form a passivation composition comprising passivated silicon nanoparticles and the capture fluid.

2. The method of claim 1 , wherein the radically reactive group of the radically reactive compound is an unsaturated hydrocarbyl group.

3. 3. The method of claim 2, wherein the free radical reactive compound is an unsaturated carboxylate having from 4 to 20 carbon atoms.

4. 4. The method of claim 3, wherein the free radical reactive compound is prop-2-enyl decanoate.

5. The method of any one of claims 1 to 4, wherein the capture fluid is a hydrocarbon, a paraffinic hydrocarbon, a polysiloxane, or a fluorocarbon.

6. The method of any one of claims 1 to 5, wherein the hydrocarbon, the polysiloxane, and the fluorocarbon comprise absorption-modifying groups.

7. The method of any one of claims 1 to 6, wherein the entrapped silicon nanoparticle composition is aged under passivating conditions.

8. 8. The method of claim 7, wherein the passivation conditions are a temperature of 45 to 80° C. and a humidity of 50 to 95% relative humidity.

9. 9. The method of claim 1, wherein the silicon nanoparticles further comprise non-radical reactive sites, and the radical reactive compound reacts with the non-radical reactive sites of the silicon nanoparticles.

10. The method of any one of claims 1 to 9, wherein the entrapped silicon nanoparticle composition is treated with ultraviolet light.

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